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Publisher: AGU   (Total: 17 journals)   [Sort by number of followers]

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Geochemistry, Geophysics, Geosystems     Full-text available via subscription   (Followers: 24, SJR: 2.56, h-index: 69)
Geophysical Research Letters     Full-text available via subscription   (Followers: 74, SJR: 3.493, h-index: 157)
Global Biogeochemical Cycles     Full-text available via subscription   (Followers: 8, SJR: 3.239, h-index: 119)
J. of Advances in Modeling Earth Systems     Open Access   (Followers: 3, SJR: 1.944, h-index: 7)
J. of Geophysical Research : Atmospheres     Partially Free   (Followers: 66)
J. of Geophysical Research : Biogeosciences     Full-text available via subscription   (Followers: 12)
J. of Geophysical Research : Earth Surface     Partially Free   (Followers: 33)
J. of Geophysical Research : Oceans     Partially Free   (Followers: 28)
J. of Geophysical Research : Planets     Full-text available via subscription   (Followers: 26)
J. of Geophysical Research : Solid Earth     Full-text available via subscription   (Followers: 32)
J. of Geophysical Research : Space Physics     Full-text available via subscription   (Followers: 27)
Paleoceanography     Full-text available via subscription   (Followers: 4, SJR: 3.22, h-index: 88)
Radio Science     Full-text available via subscription   (Followers: 16, SJR: 0.959, h-index: 51)
Reviews of Geophysics     Full-text available via subscription   (Followers: 25, SJR: 9.68, h-index: 94)
Space Weather     Full-text available via subscription   (Followers: 8, SJR: 1.319, h-index: 19)
Tectonics     Full-text available via subscription   (Followers: 8, SJR: 2.748, h-index: 85)
Water Resources Research     Full-text available via subscription   (Followers: 71, SJR: 2.189, h-index: 121)
Journal Cover Journal of Geophysical Research : Oceans
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   Published by AGU Homepage  [17 journals]
  • Asymmetric oceanic response to a hurricane: Deepwater observations during
           Hurricane Isaac
    • Authors: Laura J. Spencer; Steven F. DiMarco, Zhankun Wang, Joseph J. Kuehl, David A. Brooks
      Abstract: The eye of Hurricane Isaac passed through the center of an array of six deepwater water‐column current meter moorings deployed in the northern Gulf of Mexico. The trajectory of the hurricane provided for a unique opportunity to quantify differences in the full water‐column oceanic response to a hurricane to the left and right of the hurricane trajectory. Prior to the storm passage, relative vorticity on the right side of the hurricane was strongly negative; while on the left, relative vorticity was positive. This resulted in an asymmetry in the near inertial frequencies oceanic response at depth and horizontally. A shift in the response to a slightly larger inertial frequencies ∼1.11f was observed and verified by theory. Additionally, the storm passage coincided with an asymmetric change in relative vorticity in the upper 1000 m, which persisted for ∼15 inertial periods. Vertical propagation of inertial energy was estimated at 29 m/day, while horizontal propagation at this frequency was approximately 5.7 km/day. Wavelet analysis showed two distinct sub‐inertial responses, one with a period of 2‐5 days and another with a period of 5‐12 days. Analysis of the sub‐inertial bands reveals that the spatial and temporal scales are shorter and less persistent than the near‐inertial variance. As the array is geographically located near the site of the Deepwater Horizon oil spill, the spatial and temporal scales of response have significant implications for the fate, transport, and distribution of hydrocarbons following a deepwater spill event. This article is protected by copyright. All rights reserved.
      PubDate: 2016-07-18T03:57:13.921395-05:
      DOI: 10.1002/2015JC011560
       
  • Impact of climate variability on the tidal oceanic magnetic signal ‐
           a model based sensitivity study
    • Authors: J. Saynisch; J. Petereit, C. Irrgang, A. Kuvshinov, M. Thomas
      Abstract: ESA's satellite magnetometer mission Swarm is supposed to lower the limit of observability for oceanic processes. While periodic magnetic signals from ocean tides are already detectable in satellite magnetometer observations, changes in the general ocean circulation are yet too small or irregular for a successful separation. An approach is presented that utilizes the good detectability of tidal magnetic signals to detect changes in the oceanic electric conductivity distribution. Ocean circulation, tides and the resultant magnetic fields are calculated with a global general ocean circulation model coupled to a 3D electromagnetic induction model. For the decay of the meridional overturning circulation, as an example, the impact of climate variability on tidal oceanic magnetic signals is demonstrated. Total overturning decay results in anomalies of up to 0.7 nT in the radial magnetic M2 signal at sea level. The anomalies are spatially heterogeneous and reach in extended areas 30% or more of the unperturbed tidal magnetic signal. The anomalies should be detectable in long time series from magnetometers on land or at the ocean bottom. The anomalies at satellite height (430 km) reach 0.1 nT and pose a challenge for the precision of the Swarm mission. Climate variability induced deviations in the tide system (e.g., tidal velocities and phases) are negligible. Changes in tidal magnetic fields are dominated by changes in sea water salinity and temperature. Therefore, it is concluded that observations of tidal magnetic signals could be used as a tool to detect respective state changes in the ocean. This article is protected by copyright. All rights reserved.
      PubDate: 2016-07-18T03:40:30.059138-05:
      DOI: 10.1002/2016JC012027
       
  • New perspectives for noble gases in oceanography
    • Authors: Werner Aeschbach
      Abstract: Conditions prevailing in regions of deep water formation imprint their signature in the concentrations of dissolved noble gases, which are conserved in the deep ocean. Such “recharge conditions” including temperature, salinity, and interactions with sea ice are important in view of ocean‐atmosphere CO2 partitioning. Noble gases, especially the temperature sensitive Kr and Xe, are well‐established tracers to reconstruct groundwater recharge conditions. In contrast, tracer oceanography has traditionally focused on He isotopes and the light noble gases Ne and Ar, which could be analyzed at the required high precision. Recent developments of analytical and data interpretation methods now provide fresh perspectives for noble gases in oceanography. This article is protected by copyright. All rights reserved.
      PubDate: 2016-07-15T04:16:47.656331-05:
      DOI: 10.1002/2016JC012133
       
  • Assessment of net community production and calcification of a coral reef
           using a boundary layer approach
    • Authors: Yuichiro Takeshita; Wade McGillis, Ellen M. Briggs, Amanda Carter, Emily Donham, Todd R. Martz, Nichole N. Price, Jennifer E. Smith
      Abstract: Coral reefs are threatened worldwide, and there is a need to develop new approaches to monitor reef health under natural conditions. Because simultaneous measurements of net community production (NCP) and net community calcification (NCC) are used as important indicators of reef health, tools are needed to assess them in situ. Here, we present the Benthic Ecosystem and Acidification Measurement System (BEAMS), to provide the first fully autonomous approach capable of sustained, simultaneous measurements of reef NCP and NCC under undisturbed, natural conditions on timescales ranging from tens of minutes to weeks. BEAMS combines the chemical and velocity gradient in the benthic boundary layer to quantify flux from the benthos for a variety of parameters to measure NCP and NCC. Here, BEAMS was used to measure these rates from two different sites with different benthic communities on the western reef terrace at Palmyra Atoll for two weeks in September, 2014. Measurements were made every ∼15 minutes. The trends in metabolic rates were consistent with the benthic communities between the two sites with one dominated by fleshy organisms and the other dominated by calcifiers (degraded and healthy reefs, respectively). This demonstrates the potential utility of BEAMS as a reef health monitoring tool. NCP and NCC were tightly coupled on timescales of minutes to days, and light was the primary driver for the variability of daily integrated metabolic rates. No correlation between CO2 levels and daily integrated NCC was observed, indicating that NCC at these sites were not significantly affected by CO2. This article is protected by copyright. All rights reserved.
      PubDate: 2016-07-15T04:16:39.299754-05:
      DOI: 10.1002/2016JC011886
       
  • Impact of the Antarctic bottom water formation on the Weddell G/yre and
           its northward propagation characteristics in GFDL model
    • Authors: Liping Zhang; Thomas L. Delworth
      Abstract: The impact of Antarctic bottom water (AABW) formation on the Weddell Gyre and its northward propagation characteristics are studied using a 4000‐yr long control run of the GFDL CM2.1 model as well as sensitivity experiments. In the control run, the AABW cell and Weddell Gyre are highly correlated when the AABW cell leads the Weddell Gyre by several years, with an enhanced AABW cell corresponding to a strengthened Weddell Gyre and vice versa. An additional sensitivity experiment shows that the response of the Weddell Gyre to AABW cell changes is primarily attributed to interactions between the AABW outflow and ocean topography, instead of the surface wind stress curl and freshwater anomalies. As the AABW flows northward, it encounters topography with steep slopes that induce strong downwelling and negative bottom vortex stretching. The anomalous negative bottom vortex stretching induces a cyclonic barotropic streamfunction over the Weddell Sea, thus leading to an enhanced Weddell Gyre. The AABW cell variations in the control run have significant meridional coherence in density space. Using passive dye tracers, it is found that the slow propagation of AABW cell anomalies south of 35oS corresponds to the slow tracer advection time scale. The dye tracers escape the Weddell Sea through the western limb of the Weddell Gyre and then go northwestward to the Argentine Basin through South Sandwich Trench and Georgia Basin. This slow advection by deep ocean currents determines the AABW cell propagation speed south of 35oS. North of 35oS the propagation speed is determined both by advection in the deep western boundary current and through Kelvin waves. This article is protected by copyright. All rights reserved.
      PubDate: 2016-07-15T04:16:19.028248-05:
      DOI: 10.1002/2016JC011790
       
  • A snapshot of internal waves and hydrodynamic instabilities in the
           Southern Bay of Bengal
    • Abstract: Measurements conducted in the southern Bay of Bengal (BoB) as a part of the ASIRI‐EBoB Program portray the characteristics of high‐frequency internal waves in the upper pycnocline as well as the velocity structure with episodic events of shear instability. A 20‐hour time series of CTD, ADCP and acoustic backscatter profiles down to 150 m as well as temporal CTD measurements in the pycnocline at z = 54 m were taken to the east of Sri Lanka. Internal waves of periods ∼ 10 to 40 min were recorded at all depths below a shallow (∼ 20 – 30 m) surface mixed layer in the background of an 8‐m amplitude internal tide. The absolute values of vertical displacements associated with high‐frequency waves followed the Nakagami distribution with a median value of 2.1 m and a 95% quintile 6.5 m. The internal wave amplitudes are normally distributed. The tails of the distribution deviate from normality due to episodic high‐amplitude displacements. The sporadic appearance of internal waves with amplitudes exceeding ∼ 5 m usually coincided with patches of low Richardson numbers, pointing to local shear instability as a possible mechanism of internal‐wave induced turbulence. The probability of shear instability in the summer BoB pycnocline based on an exponential distribution of the inverse Richardson number, however, appears to be relatively low, not exceeding 4% for Ri 
      PubDate: 2016-07-15T04:16:15.146988-05:
      DOI: 10.1002/2016JC011697
       
  • Timing and regional patterns of snowmelt on Antarctic sea ice from passive
           microwave satellite observations
    • Authors: Stefanie Arndt; Sascha Willmes, Wolfgang Dierking, Marcel Nicolaus
      Abstract: An improved understanding of the temporal variability and the spatial distribution of snowmelt on Antarctic sea ice is crucial to better quantify atmosphere‐ice‐ocean interactions, in particular sea‐ice mass and energy budgets. It is therefore important to understand the mechanisms that drive snowmelt, both at different times of the year and in different regions around Antarctica. In this study, we combine diurnal brightness temperature differences (dTB(37GHz)) and ratios (TB(19GHz)/TB(37GHz)) to detect and classify snowmelt processes. We distinguish temporary snowmelt from continuous snowmelt to characterize dominant melt patterns for different Antarctic sea ice regions from 1988/89 to 2014/15. Our results indicate four characteristic melt types. On average, 38.9±6.0% of all detected melt events are diurnal freeze‐thaw cycles in the surface snow layer, characteristic of temporary melt (Type A). Less than 2% reveal immediate continuous snowmelt throughout the snowpack, i.e. strong melt over a period of several days (Type B). In 11.7±4.0%, Type A and B take place consecutively (Type C), and for 47.8±6.8% no surface melt is observed at all (Type D). Continuous snowmelt is primarily observed in the outflow of the Weddell Gyre and in the northern Ross Sea, usually 17 days after the onset of temporary melt. Comparisons with Snow Buoy data suggest that also the onset of continuous snowmelt does not translate into changes in snow depth for a longer period but might rather affect the internal stratigraphy and density structure of the snowpack. Considering the entire data set, the timing of snowmelt processes does not show significant temporal trends. This article is protected by copyright. All rights reserved.
      PubDate: 2016-07-14T09:35:28.432404-05:
      DOI: 10.1002/2015JC011504
       
  • Effects of model physics on hypoxia simulations for the northern Gulf of
           Mexico: A model intercomparison
    • Abstract: A large hypoxic zone forms every summer on the Texas‐Louisiana Shelf in the northern Gulf of Mexico due to nutrient and freshwater inputs from the Mississippi/Atchafalaya River System. Efforts are underway to reduce the extent of hypoxic conditions through reductions in river nutrient inputs, but the response of hypoxia to such nutrient load reductions is difficult to predict because biological responses are confounded by variability in physical processes. The objective of this study is to identify the major physical model aspects that matter for hypoxia simulation and prediction. In order to do so we compare three different circulation models (ROMS, FVCOM and NCOM) implemented for the northern Gulf of Mexico, all coupled to the same simple oxygen model, with observations and against each other. By using a highly simplified oxygen model we eliminate the potentially confounding effects of a full biogeochemical model and can isolate the effects of physical features. In a systematic assessment we found that 1) model‐to‐model differences in bottom water temperatures result in differences in simulated hypoxia because temperature influences the uptake rate of oxygen by the sediments (an important oxygen sink in this system), 2) vertical stratification does not explain model‐to‐model differences in hypoxic conditions in a straightforward way, and 3) the thickness of the bottom boundary layer, which sets the thickness of the hypoxic layer in all three models, is key to determining the likelihood of a model to generate hypoxic conditions. These results imply that hypoxic area, the commonly used metric in the northern Gulf which ignores hypoxic layer thickness, is insufficient for assessing a model's ability to accurately simulate hypoxia, and that hypoxic volume needs to be considered as well. This article is protected by copyright. All rights reserved.
      PubDate: 2016-07-14T09:15:42.233144-05:
      DOI: 10.1002/2015JC011577
       
  • Synoptic forcing of wind relaxations at Pt. Conception, California
    • Authors: Melanie R Fewings; Libe Washburn, Clive E. Dorman, Christopher Gotschalk, Kelly Lombardo
      Abstract: Over the California Current upwelling system in summer, the prevailing upwelling‐favorable winds episodically weaken (relax) or reverse direction for a few days. Near Pt. Conception, California, the wind usually does not reverse, but wind relaxation allows poleward oceanic coastal flow with ecological consequences. To determine the offshore extent and synoptic forcing of these wind relaxations, we formed composite averages of wind stress from the QuikSCAT satellite and atmospheric pressure from the North American Regional Reanalysis (NARR) using 67 wind relaxations during summer 2000–2009. Wind relaxations at Pt. Conception are the third stage of an event sequence that repeatedly affects the west coast of North America in summer. First, 5–7 dy before the wind weakens near Pt. Conception, the wind weakens or reverses off Oregon and northern California. Second, the upwelling‐favorable wind intensifies along central California. Third, the wind relaxes at Pt. Conception, and the area of weakened winds extends poleward to northern California over 3–5 dy. The NARR underestimates the wind stress within ∼200 km of coastal capes by a factor of 2. Wind relaxations at Pt. Conception are caused by offshore extension of the desert heat low. This synoptic forcing is related to event cycles that cause wind reversal as in Halliwell and Allen [1987] and Mass and Bond [1996], but includes weaker events. The wind relaxations extend ∼600 km offshore, similarly to the California‐scale hydraulic expansion fan shaping the prevailing winds, and ∼1000 km alongshore, limited by an opposing pressure gradient force at Cape Mendocino. This article is protected by copyright. All rights reserved.
      PubDate: 2016-07-14T09:15:36.011722-05:
      DOI: 10.1002/2016JC011699
       
  • An observation and model‐based analysis of meridional transports in
           the South Atlantic
    • Authors: Sudip Majumder; Claudia Schmid, George Halliwell
      Abstract: A three dimensional velocity field constructed from Argo observations and sea surface heights (called Argo & SSH, hereinafter) is used to estimate meridional overturning volume transport and meridional heat transport (MHT) across 20°S, 25°S, 30°S, and 35°S for the years 2000 to 2014 in the South Atlantic. Volume transport in the upper branch of Meridional Overturning Circulation (MOC) and MHT from the observations are consistent with the previous observations, but are higher than the estimates derived from three data assimilative ocean models, at some of the latitudes. Both the observations and models show strong correlations between the strength of MOC and MHT at all the latitudes. The corresponding change in MHT for 1 Sv change of MOC strength, in the observations, increases from 0.046 PW in 25°S, 30°S and 35°S to 0.056 PW across 20°S. A comparison of model‐based transports at 35°S at the boundaries and in the interior with those from Argo & SSH shows significant differences between them with respect to the contributions in the three segments of the section. In addition, the contributions also vary greatly between the different models. An analysis of the seasonality of MOC in the models and in the observations reveals that MOC anomalies in the models mostly show strong annual cycles at all the latitudes, whereas those derived from Argo & SSH exhibit annual cycles at three latitudes (35°S, 30°S and to a lesser extent at 25°S) and a semiannual cycle at 20°S This article is protected by copyright. All rights reserved.
      PubDate: 2016-07-13T10:42:56.986488-05:
      DOI: 10.1002/2016JC011693
       
  • West Florida shelf upwelling: Origins and pathways
    • Authors: Robert H. Weisberg; Lianyuan Zheng, Yonggang Liu
      Abstract: Often described as oligotrophic, the west Florida continental shelf supports abundant fisheries, experiences blooms of the harmful algae, Karenia brevis and exhibits subsurface chlorophyll maxima evident in shipboard and glider surveys. Renewal of inorganic nutrients by the upwelling of deeper ocean water onto the shelf may account for this, but what are the origins and pathways by which such new water may broach the shelf break and advance toward the shoreline? We address these questions via numerical model simulations of pseudo‐Lagrangian, isopycnic water parcel trajectories. Focus is on 2010, when the west Florida shelf was subjected to an anomalously protracted period of upwelling caused by Gulf of Mexico Loop Current interactions with the shelf slope. Origins and pathways are determined by integrating trajectories over successive 45 day intervals, beginning from different locations along the shelf break and at various locations and depths along the shelf slope. Waters upwelling across the shelf break are found to originate from relatively shallow depths along the shelf slope. Even for the anomalous 2010 year, much of this upwelling occurs from about 150 m and above, although waters may broach the shelf break from 300 m depth, particularly in the Florida Panhandle. Such inter‐annual renewal of west Florida shelf waters appears to have profound effects on west Florida shelf ecology. This article is protected by copyright. All rights reserved.
      PubDate: 2016-07-13T10:42:55.654529-05:
      DOI: 10.1002/2015JC011384
       
  • Stable reconstruction of Arctic sea level for the 1950–2010 period
    • Authors: Peter Limkilde Svendsen; Ole B. Andersen, Allan Aasbjerg Nielsen
      Abstract: Reconstruction of historical Arctic sea level is generally difficult due to the limited coverage and quality of both tide gauge and altimetry data in the area. Here a strategy to achieve a stable and plausible reconstruction of Arctic sea level from 1950 to today is presented. This work is based on the combination of tide gauge records and a new 20‐year reprocessed satellite altimetry derived sea level pattern. Hence the study is limited to the area covered by satellite altimetry (68ºN and 82ºN). It is found that timestep cumulative reconstruction as suggested by Church and White (2000) may yield widely variable results and is difficult to stabilize due to the many gaps in both tide gauge and satellite data. A more robust sea level reconstruction approach is to use datum adjustment of the tide gauges in combination with satellite altimetry, as described by (Ray and Douglas, 2011). In this approach, a datum‐fit of each tide gauges is used and the method takes into account the entirety of each tide gauge record. This makes the Arctic sea level reconstruction much less prone to drifting. From our reconstruction, we found that the Arctic mean sea level trend is around 1.5 mm +/‐ 0.3 mm/y for the period 1950 to 2010, between 68ºN and 82ºN. This value is in good agreement with the global mean trend of 1.8 +/‐ 0.3 mm/y over the same period as found by Church and White (2004). This article is protected by copyright. All rights reserved.
      PubDate: 2016-07-13T10:42:53.776801-05:
      DOI: 10.1002/2016JC011685
       
  • Anomalous Java cooling at the initiation of positive IOD events
    • Authors: Andrew S. Delman; Janet Sprintall, Julie L. McClean, Lynne D. Talley
      Abstract: Anomalous sea surface temperature (SST) cooling south of Java, initiated during May‐July, is an important precursor to positive Indian Ocean Dipole (pIOD) events. As shown previously, the Java SST anomalies are spatially and temporally coincident with seasonal upwelling induced locally by southeasterly trade winds. However, we confirm earlier findings that interannual variability of the Java cooling is primarily driven by remote wind forcing from coastal Sumatra and the equatorial Indian Ocean (EqIO); we also find an inuence from winds along the Indonesian Throughow. The wind forcing in the EqIO and along coastal Sumatra does not initiate SST cooling locally due to a deep thermocline and thick barrier layer, but can force upwelling Kelvin waves that induce substantial surface cooling once they reach the seasonally shallower thermocline near the coast of Java. Satellite altimetry is used to obtain a Kelvin wave coefficient that approximates Kelvin wave amplitude variations along the equator. All pIOD years in the satellite record have anomalous levels of upwelling Kelvin wave activity along the equator during April‐June, suggesting that upwelling waves during this season are necessary for pIOD event development. However, a change to wind‐forced downwelling Kelvin waves during July‐August can abruptly terminate cool Java SST anomalies and weaken the pIOD event. Upwelling Kelvin wave activity along the equator and wind stress anomalies west of Sumatra are both robust predictors of the IOD index later in the calendar year, while values of the Kelvin wave coefficient are the most reliable predictor of pIOD events specifically. This article is protected by copyright. All rights reserved.
      PubDate: 2016-07-13T10:42:51.050031-05:
      DOI: 10.1002/2016JC011635
       
  • Winter water properties and the Chukchi Polynya
    • Authors: C. Ladd; C. W. Mordy, S. A. Salo, P. J. Stabeno
      Abstract: Water properties from moored measurements (2010 – 2015) near Icy Cape on the eastern Chukchi shelf have been examined in relation to satellite observations of ice cover. Atlantic Water (AW), with temperature > ‐1°C and salinity > 33.6, has been observed to upwell from deeper than 200 m in the Arctic Basin onto the Chukchi Shelf via Barrow Canyon. Most previous observations of AW on the Chukchi shelf have been in or near Barrow Canyon; observations of AW farther onto the shelf are rare. Despite mooring location on the shelf ∼225 km from the head of Barrow Canyon, five AW events have been observed at mooring C1 (70.8°N, 163.2°W) in four years of data. All but one of the events occurred under openings in the sea ice cover (either a polynya or the ice edge). No events were observed during the winter of 2011/2012, a year with little polynya activity in the region. In addition to changes in temperature and salinity, the AW events are typically associated with southwestward winds and currents, changes in sea‐ice cover, and increased nutrient concentrations in the bottom water. Estimates of heat content associated with the AW events suggest that the Chukchi Polynya can often be classified as a hybrid sensible heat/wind‐driven polynya. This article is protected by copyright. All rights reserved.
      PubDate: 2016-07-13T10:42:48.885569-05:
      DOI: 10.1002/2016JC011918
       
  • Float observations of an anticyclonic eddy off Hokkaido
    • Authors: Ryuichiro Inoue; Vincent Faure, Shinya Kouketsu
      Abstract: To clarify the formation process of the salinity minimum in the Kuroshio–Oyashio mixed water region and understand the mechanism of meridional heat transport between the subtropical and subpolar gyres, 16 profiling floats were deployed within a warm‐core anticyclonic eddy off Hokkaido from June 2012 to December 2013. Then, the evolution of an anticyclonic eddy was examined using time series of the water properties. The largest fluctuations in water properties were observed in April and May 2013, when the anticyclonic eddy first moved south to interact with a warm front, then back north. Salinity in the salinity minimum layer increased during the interaction. After the eddy detached from the frontal structure, low‐salinity water was again observed with small intrusive structures, which eventually converged to a smooth zigzag structure in the potential temperature‐salinity diagram, suggesting that both vertical mixing and vertical heaving played a role in the temporal changes observed after the eddy detached from the front. Since the salinity variation during the interaction event was about half the total salinity change during the whole experimental period, the interaction of an eddy with a front might be important for modifying the water properties of the eddy, and, therefore, for the meridional transport of heat and fresh water. This article is protected by copyright. All rights reserved.
      PubDate: 2016-07-13T10:42:36.911384-05:
      DOI: 10.1002/2016JC011698
       
  • Using multifractals to evaluate oceanographic model skill
    • Abstract: We are in an era of unprecedented data volumes generated from observations and model simulations. This is particularly true from satellite Earth Observations (EO) and global scale oceanographic models. This presents us with an opportunity to evaluate large scale oceanographic model outputs using EO data. Previous work on model skill evaluation has led to a plethora of metrics. The paper defines two new model skill evaluation metrics. The metrics are based on the theory of universal multifractals and their purpose is to measure the structural similarity between the model predictions and the EO data. The two metrics have the following advantages over the standard techniques: a) they are scale‐free, b) they carry important part of information about how model represents different oceanographic drivers. Those two metrics are then used in the paper to evaluate the performance of the FVCOM model in the shelf seas around the south‐west coast of the UK. This article is protected by copyright. All rights reserved.
      PubDate: 2016-07-13T10:42:30.280829-05:
      DOI: 10.1002/2016JC011741
       
  • Changes and influencing factors in biogenic opal export productivity in
           the Bering Sea over the last 4.3 Ma: Evidence from the records at IODP
           Site U1340
    • Authors: Qiang Zhang; Muhong Chen, Lanlan Zhang, Xiang Su, Rong Xiang
      Abstract: We reconstructed changes in biogenic opal export productivity (BOEP) in the southern Bering Sea (BS) over the last 4.3Ma, based on mass accumulation rate (MAR) of biogenic opal from Integrated Ocean Drilling Program (IODP) Site U1340. The results show that the BOEP in the BS was high and variable between ∼4.3Ma and ∼1.9Ma, extremely low and relatively stable from ∼1.9Ma to ∼1.1Ma, and then fluctuated frequently (generally high during interglacials and low during glacials) during the last ∼1.1Ma. One interval of enhanced BOEP from 4.3Ma∼3.2Ma is a response to the Late Miocene–Early Pliocene “Biogenic Bloom Event”. Another interval from 2.8Ma∼1.9Ma correlates with global opal burial shifting from high‐latitude oceans to upwelling‐influenced regions following the intensification of the Northern Hemisphere Glaciation (NHG). Whereas, the increase in BS opal export productivity during the last 1.1Ma tends to be a “local” phenomenon. Overall, the BOEP shows a similar trend and good correspondence to the input of the Alaskan Stream (AS), which can be traced using the Na2O/K2O ratio. We thus conclude that the AS may be the direct, and primary factor on BOEP variability in the BS during the last ∼4.3Ma. In addition, although the poor correlation between opal MAR and volcanic glass suggests that BOEP variability was not controlled by long‐term variations in the volcanism or ash abundance, increased ash abundance indicated by high contents of volcanic glasses was also a possible reason for enhanced BOEP during the period from ∼4.3Ma to ∼3.2Ma and the last ∼0.5Ma. This article is protected by copyright. All rights reserved.
      PubDate: 2016-07-13T10:42:25.363438-05:
      DOI: 10.1002/2016JC011750
       
  • Mixing of dissolved oxygen in Chesapeake Bay driven by the interaction
           between wind‐driven circulation and estuarine bathymetry
    • Authors: Malcolm Scully
      Abstract: Field observations collected in Chesapeake Bay demonstrate how wind‐driven circulation interacts with estuarine bathymetry to control when and where the vertical mixing of dissolved oxygen occurs. In the across‐Bay direction, the lateral Ekman response to along‐Bay wind forcing contributes to the vertical mixing of dissolved oxygen in two ways. First, the lateral tilting of the pycnocline/oxycline, consistent with the thermal wind relationship, advects the region of high vertical gradient into the surface and bottom boundary layers where mixing can occur. Secondly, upwelling of low oxygen water to the surface enhances the atmospheric influx. In the along‐Bay direction, the abrupt change in bottom depth associated with Rappahannock Shoal results in surface convergence and downwelling, leading to localized vertical mixing. Water that is mixed on the shoal is entrained into the up‐Bay residual bottom flow resulting in increases in bottom dissolved oxygen that propagate up the system. The increases in dissolved oxygen are often associated with increases in temperature and decreases in salinity, consistent with vertical mixing. However, the lagged arrival moving northward suggests that the propagation of this signal up the Bay is due to advection. This article is protected by copyright. All rights reserved.
      PubDate: 2016-07-13T10:26:04.135407-05:
      DOI: 10.1002/2016JC011924
       
  • Offshore forcing on the “pressure point” of the West Florida
           shelf: Anomalous upwelling and its influence on harmful algal blooms
    • Authors: Yonggang Liu; Robert H. Weisberg, Jason M. Lenes, Lianyuan Zheng, Katherine Hubbard, John J. Walsh
      Abstract: Gulf of Mexico Loop Current (LC) interactions with the West Florida Shelf (WFS) slope play an important role in shelf ecology through the upwelling of new inorganic nutrients across the shelf break. This is particularly the case when the LC impinges upon the shelf slope in the southwest portion of the WFS near the Dry Tortugas. By contacting shallow water isobaths at this “pressure point” the LC forcing sets the entire shelf into motion. Characteristic patterns of LC interactions with the WFS and their occurrences are identified using unsupervised neural network, Self‐Organizing Map, from 23 years (1993 – 2015) of altimetry data. The duration of the occurrences of such LC patterns is used as an indicator of offshore forcing of anomalous upwelling. Consistency is found between the altimetry‐derived offshore forcing and the occurrence and severity of WFS coastal blooms of the toxic dinoflagellate, Karenia brevis: years without major blooms tend to have prolonged LC contact at the “pressure point,” whereas years with major blooms tend not to have prolonged offshore forcing. Resetting the nutrient state of the shelf by the coastal ocean circulation in response to deep‐ocean forcing demonstrates the importance of physical oceanography in shelf ecology. A satellite altimetry‐derived seasonal predictor for major K. brevis blooms is also proposed. This article is protected by copyright. All rights reserved.
      PubDate: 2016-07-13T10:21:39.573799-05:
      DOI: 10.1002/2016JC011938
       
  • A permanent N2O sink in the Nordic seas and its strength and possible
           variability over the past four decades
    • Authors: Liyang Zhan; Liqi Chen, Jiexia Zhang, Yan Jinpei, Yuhong Li, Man Wu
      Abstract: The Nordic Seas have been assumed to be a net sink of the ozone‐depleting greenhouse gas N2O; however, few studies have been conducted in this region. The N2O profile data obtained during the 5th Chinese National Arctic Research Expedition demonstrate that the N2O distribution pattern in the Nordic Seas differs from that of most other oceans. The N2O sink characteristics of this region are confirmed by the undersaturation of N2O in the water column. The distributions of N2O in three subbasins of the Nordic Seas vary in the upper 1000 m but are homogenous below 1000 m due to a shared origin in the Greenland Basin (GB). Air‐sea exchange and vertical convection are thought to be the dominant factors in the N2O distribution in the GB, resulting in a distribution pattern that correlates significantly with the atmospheric mixing ratio variation over the past 40 years. Although recent studies have shown that weakened convection and/or enhanced Arctic outflow below the mid‐depth has occurred, our results show that these variations have yet to significantly affect the above relationship. The distribution could be considered a “historical record” that can be used to evaluate the air‐to‐sea flux over the past 40 years in the GB. The annual amount of N2O absorbed by the GB is ∼0.016‐0.029 Tg N, which is equal to 0.4‐0.8% of the world ocean emissions. This amount should not be simply neglected because it is absorbed by a region whose area accounts for only 0.03% of the world ocean area. This article is protected by copyright. All rights reserved.
      PubDate: 2016-07-13T10:21:24.855803-05:
      DOI: 10.1002/2016JC011925
       
  • Evolution of a Canada Basin ice‐ocean boundary layer and mixed layer
           across a developing thermodynamically forced marginal ice zone
    • Authors: Shawn G. Gallaher; Timothy P. Stanton, William J. Shaw, Sylvia T. Cole, John M. Toole, Jeremy P. Wilkinson, Ted Maksym, Byongjun Hwang
      Abstract: A comprehensive set of autonomous, ice‐ocean measurements were collected across the Canada Basin to study the summer evolution of the ice‐ocean boundary layer (IOBL) and ocean mixed layer (OML). Evaluation of local heat and freshwater balances and associated turbulent forcing reveals that melt ponds (MP's) strongly influence the summer IOBL‐OML evolution. Areal expansion of MP's in mid‐June start the upper ocean evolution resulting in significant increases to ocean absorbed radiative flux (19 Wm−2 in this study). Buoyancy provided by MP drainage shoals and freshens the IOBL resulting in a 39 MJm−2 increase in heat storage in just 19 days (52% of the summer total). Following MP drainage, a near‐surface fresh layer deepens through shear‐forced mixing to form the summer mixed layer (sML). In late summer, basal melt increases due to stronger turbulent mixing in the thin sML and the expansion of open water areas due in part to wind forced divergence of the sea ice. Thermal heterogeneities in the marginal ice zone (MIZ) upper ocean led to large ocean‐to‐ice heat fluxes (100‐200 Wm−2) and enhanced basal ice melt (3‐6 cm‐day−1), well away from the ice edge. Calculation of the upper ocean heat budget show that local radiative heat input accounted for at least 89% of the observed latent heat losses and heat storage (partitioned 0.77/0.23). These results suggest that the extensive area of deteriorating sea ice observed away from the ice edge during the 2014 season, termed the “thermodynamically forced MIZ,” was driven primarily by local radiative forcing. This article is protected by copyright. All rights reserved.
      PubDate: 2016-07-13T10:18:02.46016-05:0
      DOI: 10.1002/2016JC011778
       
  • Changes in summer sea ice, albedo, and portioning of surface solar
           radiation in the Pacific sector of Arctic Ocean during 1982‐2009
    • Abstract: SSM/I sea ice concentration and CLARA black‐sky composite albedo were used to estimate sea ice albedo in the region 70°–82°N, 130°–180°W. The long‐term trends and seasonal evolutions of ice concentration, composite albedo, and ice albedo were then obtained. In July–August 1982–2009, the linear trend of the composite albedo and the ice albedo was −0.069 and −0.046 units per decade, respectively. During 1 June to 19 August, melting of sea ice resulted in an increase of solar heat input to the ice‐ocean system by 282 MJ·m−2 from 1982 to 2009. However, because of the counter‐balancing effects of the loss of sea ice area and the enhanced ice surface melting, the trend of solar heat input to the ice was insignificant. The summer evolution of ice albedo matched the ice surface melting and ponding well at basin scale. The ice albedo showed a large difference between the multiyear and first‐year ice because the latter melted completely by the end of a melt season. At the SHEBA geolocations, a distinct change in the ice albedo has occurred since 2007 because most of the multiyear ice has been replaced by first‐year ice. A positive polarity in the Arctic Dipole Anomaly could be partly responsible for the rapid loss of summer ice within the study region in the recent years by bringing warmer air masses from the south and advecting more ice toward the north. Both these effects would enhance ice‐albedo feedback. This article is protected by copyright. All rights reserved.
      PubDate: 2016-07-11T10:46:10.915699-05:
      DOI: 10.1002/2016JC011831
       
  • The effects of Antarctic iceberg calving‐size distribution in a
           global climate model
    • Authors: A.A. Stern; A. Adcroft, O. Sergienko
      Abstract: Icebergs calved from the Antarctic continent act as moving sources of freshwater while drifting in the Southern Ocean. The lifespan of these icebergs strongly depends on their original size during calving. In order to investigate the effects (if any) of the calving size of icebergs on the Southern Ocean, we use a coupled general circulation model with an iceberg component. Iceberg calving length is varied from 62 m up to 2.3 km, which is the typical range used in climate models. Results show that increasing the size of calving icebergs leads to an increase in the westward iceberg freshwater transport around Antarctica. In simulations using larger icebergs, the reduced availability of meltwater in the Amundsen and Bellingshausen Seas suppresses the sea‐ice growth in the region. In contrast, the increased iceberg freshwater transport leads to increased sea‐ice growth around much of the East Antarctic coastline. These results suggest that the absence of large tabular icebergs with horizontal extent of tens of kilometers in climate models may introduces systematic biases in sea‐ice formation, ocean temperatures and salinities around Antarctica. This article is protected by copyright. All rights reserved.
      PubDate: 2016-07-11T10:46:05.674207-05:
      DOI: 10.1002/2016JC011835
       
  • Modeling the influence of deep water application of dispersants on the
           surface expression of oil: A sensitivity study
    • Authors: Jeremy Testa; E. Eric Adams, Elizabeth W. North, Ruoying He
      Abstract: Although the effects of chemical dispersants on oil droplet sizes and ascent speeds are well‐known, the fate and transport of dispersed oil droplets of different sizes under varying hydrodynamic conditions can be difficult to assess with observations alone. We used a particle tracking model to evaluate the effect of changes in droplet sizes due to dispersant application on the short‐term transport and surface expression of oil released under conditions similar to those following the June 3, 2010 riser cutting during the Deepwater Horizon event. We used simulated injections of oil droplets of varying size and number under conditions associated with no dispersant application and with dispersant application at 50% and 100% efficiency. Due to larger droplet sizes in the no‐dispersant scenario, all of the simulated oil reached the surface within 7 hrs, while only 61% and 28% of the oil reached the surface after 12 hs in the 50% and 100% dispersant efficiency cases, respectively. The length of the surface slick after 6 hrs was ∼2 km in the no‐dispersant case whereas there was no surface slick after 6 hrs in the 100% dispersant case, because the smaller oil droplets which resulted from dispersant application had not yet reached the surface. Model results suggest that the application of dispersants at the well head had the following effects: (1) less oil reached the surface in the 6‐12 hrs after application, (2) oil had a longer residence time in the water‐column, and (3) oil was more highly influenced by sub‐surface transport. This article is protected by copyright. All rights reserved.
      PubDate: 2016-07-11T10:40:32.871846-05:
      DOI: 10.1002/2015JC011571
       
  • Deep ventilation in the Okinawa Trough induced by Kerama Gap overflow
    • Abstract: Near‐bottom water flowing over the Kerama Gap's sills is thought to ventilate the deep water below ∼1100 m depth in the Okinawa Trough and then upwell with 5‒10 years residence time. The present study follows up on this phenomenon, using comprehensive profile data of temperature, salinity, dissolved oxygen, currents and turbulence obtained by intensive shipboard observations performed in June 2013 and June 2014 in the region. Strong near‐bottom sub‐tidal flow with speeds exceeding 0.5 m s−1 was observed within a layer of about 100 m thickness over the western side of the peak of the main sill. Temperature and salinity sections along the Kerama Gap indicated some depressions and overturns of the deep water downstream of the strong overflow, suggesting the existence of breaking internal gravity waves and hydraulic jumps. Associated vertical diffusivities, estimated using the Thorpe scale and the buoyancy frequency, were three to four orders of magnitude larger than typical values observed in the thermocline of the open ocean (∼10−5 m2 s−1). The dissolved oxygen section also indicated strong vertical mixing and associated upwelling with the entrainment of the near‐bottom overflow water into the lower thermocline beneath the Kuroshio in the Okinawa Trough. The present study not only supports the previous conceptual model but also provides new evidence that the Okinawa Trough is an upwelling location where nutrient rich Philippine Sea intermediate water is sucked up into the lower thermocline below the Kuroshio. This article is protected by copyright. All rights reserved.
      PubDate: 2016-07-11T10:40:31.449209-05:
      DOI: 10.1002/2016JC011822
       
  • Mixing and phytoplankton dynamics in a submarine canyon in the West
           Antarctic Peninsula
    • Authors: Filipa Carvalho; Josh Kohut, Matthew J. Oliver, Robert M. Sherrell, Oscar Schofield
      Abstract: Bathymetric depressions (canyons) exist along the West Antarctic Peninsula shelf and have been linked with increased phytoplankton biomass and sustained penguin colonies. However, the physical mechanisms driving this enhanced biomass are not well understood. Using a Slocum glider dataset with over 30,000 water column profiles, we evaluate the relationship between mixed layer depth (MLD, estimated using the depth of maximum buoyancy frequency) and phytoplankton vertical distribution. We use the glider deployments in the Palmer Deep region to examine seasonal and across canyon variability. Throughout the season, the ML becomes warmer and saltier, as a result of vertical mixing and advection. Shallow ML and increased stratification due to sea ice melt are linked to higher chlorophyll concentrations. Deeper mixed layers, resulting from increased wind forcing, show decreased chlorophyll, suggesting the importance of light in regulating phytoplankton productivity. Spatial variations were found in the canyon head region where local physical water column properties were associated with different biological responses, reinforcing the importance of local canyon circulation in regulating phytoplankton distribution in the region. Observations show that the intrusion of warm, nutrient enriched modified Upper Circumpolar Deep Water (mUCDW) plays a smaller role in explaining the elevated productivity observed over the canyon than was initially hypothesized. This article is protected by copyright. All rights reserved.
      PubDate: 2016-07-05T04:20:30.400382-05:
      DOI: 10.1002/2016JC011650
       
  • High resolution modeling of dense water formation in the
           north‐western Mediterranean during winter 2012‐2013: Processes
           and budget
    • Abstract: The evolution of the stratification of the north‐western Mediterranean between summer 2012 and the end of winter 2013 was simulated and compared with different sets of observations. A summer cruise and profiler observations were used to improve the initial conditions of the simulation. This improvement was crucial to simulate winter convection. Variations of some parameters involved in air ‐ sea exchanges (wind, coefficient of transfer used in the latent heat flux formulation, and constant additive heat flux) showed that the characteristics of water masses and the volume of dense water formed during convection cannot be simply related to the time‐integrated buoyancy budget over the autumn ‐ winter period. The volume of dense water formed in winter was estimated to be about 50,000 km3 with a density anomaly larger than 29.113 kg m−3. The effect of advection and air/sea fluxes on the heat and salt budget of the convection zone was quantified during the preconditioning phase and the mixing period. Destratification of the surface layer in autumn occurs through an interaction of surface and Ekman buoyancy fluxes associated with displacements of the North Balearic front bounding the convection zone to the south. During winter convection, advection stratifies the convection zone: from December to March, the absolute value of advection represents 58% of the effect of surface buoyancy fluxes. This article is protected by copyright. All rights reserved.
      PubDate: 2016-07-04T10:40:28.024325-05:
      DOI: 10.1002/2016JC011935
       
  • Evaluation of the structure function method to compute turbulent
           dissipation within boundary layers using numerical simulations
    • Authors: Aidin Jabbari; Amirreza Rouhi, Leon Boegman
      Abstract: Well‐resolved numerical simulations of turbulent open channel flows are analyzed to evaluate the accuracy of the 2nd order structure function method (SFM) in estimating the rate of dissipation of turbulent kinetic energy within boundary layers. The objective is to assess the variation in the 2/3 Kolmogorov constants due to flow anisotropy with distance from the wall. Comparison of the dissipation calculated directly from the numerical data, with that from the SFM shows that usage of the canonical constants, based on the assumption of local isotropy, can result in considerable error (>50%) in the prediction of dissipation when using the vertical or spanwise velocity components. From the numerically calculated dissipation, optimal Kolmogorov 2/3 constants were obtained and empirical relations, which account for near‐wall effects, were proposed. Usage of the optimal constants will improve estimation of the dissipation rate when the SFM is applied to compute dissipation in geophysical boundary‐layer flows. This article is protected by copyright. All rights reserved.
      PubDate: 2016-07-04T10:36:04.749997-05:
      DOI: 10.1002/2015JC011608
       
  • A process study of the Adriatic‐Ionian System baroclinic dynamics
    • Authors: M. Reale; A. Crise, R. Farneti, R. Mosetti
      Abstract: The driving mechanisms behind the decadal reversal of the Ionian Sea upper layer circulation recently sparked a considerable discussion in the Mediterranean scientific community. It has been suggested that the reversal can be driven by variations in wind stress curl over the basin, baroclinic dynamics acting within the Adriatic‐Ionian System (AISys) or baroclinic dynamics driven by thermohaline properties at the AISys eastern boundary. Here, we perform numerical simulations in order to assess the relative importance of remote forcings (wind stress, thermohaline fluxes, thermohaline open boundary conditions) on the vorticity and energy budget of the Ionian Sea. A mechanistic understanding of the AISys dynamics is achieved with an approach based on an increasing complexity in the model forcings and domain. Our experiments suggest that wind stress does not play a leading role in the vorticity and energy budgets of the Ionian Sea. Wind stress can reinforce or weaken the circulation but it is not able to reverse its sign. Its role becomes dominant only in the absence of inflows through the Antikythira Strait and Cretan Passage. Instead, reversals in the upper layer circulation of the Ionian Sea take place only in the presence of an active boundary on the Aegean Sea/Levantine Basin side and appear to be correlated with substantial exchanges of Availalble Potential Energy between the two basins (as observed at the end of the Eastern Mediterranean Transient). From an energetic point of view, AISys can be explained therefore only if the role of the Aegean Sea is explicitly condidered. This article is protected by copyright. All rights reserved.
      PubDate: 2016-07-04T10:35:58.603421-05:
      DOI: 10.1002/2016JC011763
       
  • Evaluation of entrainment formulations for liquid/gas plumes from
           underwater blowouts
    • Authors: Luciana de Freitas Tessarolo; Valdir Innocentini
      Abstract: A numerical model using the Lagrangian approach developed to simulate the fate of liquid/gas blowouts in deepwater is presented, and three entrainment formulations are tested: HOULT, JETLAG and CORJET parameterizations, given by Hoult et al. (1969), Lee and Cheung (1990) and Jirka (2004), respectively. The results are discussed and compared with field and laboratory observations. These formulations differ both in shear and forced contributions to the entrainment. As expected, the qualitative analysis of the dynamics of a liquid plume shows that the entrainment of ambient water decreases the acceleration due to buoyancy, and the plume and ambient momentums become increasingly similar over time. However, simulations of field and laboratory cases, where different plumes (gas, liquid and gas/liquid) were discharged into environments with different ambient stratifications and cross‐flows, show that the JETLAG parameterization provides the best results, while HOULT (CORJET) overestimates (underestimates) the entrainment. Additional numerical experiments applying only the JETLAG formulation are performed, considering different plume composition, ambient condition, nozzle diameter and initial discharge. For all the studied cases, the simulated results are in good agreement with the observations. Especially noteworthy were field experiments with gas released at depth of 50‐60 m. The vertical plume velocity decreased during the ascending motion, but after a certain level, the velocity increased. This feature was simulated by the JETLAG parameterization, and a closer analysis reveals the increase of buoyancy due to gas expansion exceeding the decrease caused by the entrainment. These results encourage the use of this model in realistic and complex situations. This article is protected by copyright. All rights reserved.
      PubDate: 2016-07-04T10:35:22.690812-05:
      DOI: 10.1002/2016JC011735
       
  • Small meanders of the Kuroshio Extension and associated northward
           spreading of warm water: Three‐vessel simultaneous observations and
           an eddy‐resolving ocean model simulation
    • Abstract: Small meanders of the Kuroshio Extension (KE) were examined in the three vessel simultaneous observations carried out across the KE along 143°E between 2 and 7 July 2012 and the hindcast simulation by a high‐resolution ocean model for the Earth Simulator (OFES). The observations captured the crest of the small meander that passed 143°E. Behind the meander crest, the meander showed the northward separation of the front from the meandering jet. It was suggested that the meander spread the low‐density KE water to the north above the main pycnocline there. The OFES showed similar characteristics of small meanders. We focused on three major small meanders. In the upstream KE region west of 143°E, these meanders were subject to northeastward ageostrophic advection, by which the low‐density KE water was carried to the north and their fronts moved away from the KE jet above the main pycnocline. This resulted in the KE water and the front extending to the west behind the crest of the downstream‐propagating meanders. This feature was particularly obvious for the two cases of the meander and was consistent with the observations. The displaced front formed a shallow geostrophic jet to the north of the main KE jet, which induced geostrophic advection along the jet and advected the front further northward and downstream near the crest of the meandering front. The KE water spread by the small meanders then mostly evolved into warm core rings or warm streamers. This article is protected by copyright. All rights reserved.
      PubDate: 2016-06-30T03:45:34.726128-05:
      DOI: 10.1002/2016JC011969
       
  • An abrupt shift in the Labrador Current system in relation to winter NAO
           events
    • Authors: Zeliang Wang; David Brickman, Blair J.W. Greenan, Igor Yashayaev
      Abstract: The behavior of the Labrador Current during the period from 1990 to 2007 is investigated with an eddy‐resolving circulation model for the North Atlantic Ocean. An EOF analysis of the model output suggests that the variability in the Labrador Current can be partitioned into a western Labrador Current (WLC; from the 300‐2500 m isobaths), and an eastern Labrador Current (ELC; from the 2500‐3300 m isobaths). The model results demonstrate that the WLC transport experienced an abrupt increase during 2000‐2002, consistent with data. This differed significantly from the ELC transport which was strong during the high winter NAO (North Atlantic Oscillation) years (1990‐95) and then steadily declined. This ELC trend is consistent with changes in the modelled Atlantic Meridional Overturning Circulation and convection depth. Our study proposes that the change in the WLC is due to a southwestward shift of the atmospheric circulation pattern starting in 2001, coincident with a change in the 2001 NAO index, and also in a westward shift of the action centers of the winter NAO events. This article is protected by copyright. All rights reserved.
      PubDate: 2016-06-30T03:45:22.931321-05:
      DOI: 10.1002/2016JC011721
       
  • Fine‐scale thermohaline ocean structure retrieved with 2‐D
           Prestack full‐waveform inversion of multichannel seismic data:
           Application to the Gulf of Cadiz (SW Iberia)
    • Abstract: This work demonstrates the feasibility of 2D time‐domain, adjoint‐state acoustic full‐waveform inversion (FWI) to retrieve high‐resolution models of ocean physical parameters such as sound speed, temperature and salinity. The proposed method is first described and then applied to pre‐stack multi‐channel seismic (MCS) data acquired in the Gulf of Cadiz (SW Iberia) in 2007 in the framework of the Geophysical Oceanography project. The inversion strategy flow includes specifically‐designed data pre‐conditioning for acoustic noise reduction, followed by the inversion of sound speed in the shotgather domain. We show that the final sound speed model has a horizontal resolution of ∼ 70m, which is two orders of magnitude better than that of the initial model constructed with coincident eXpendable Bathy Thermograph (XBT) data, and close to the theoretical resolution of O(λ). Temperature (T) and salinity (S) are retrieved with the same lateral resolution as sound speed by combining the inverted sound speed model with the thermodynamic equation of seawater and a local, depth‐dependent T‐S relation derived from regional conductivity‐temperature‐depth (CTD) measurements of the National Oceanic and Atmospheric Administration (NOAA) database. The comparison of the inverted T and S models with XBT and CTD casts deployed simultaneously to the MCS acquisition shows that the thermohaline contrasts are resolved with an accuracy of 0.18oC for temperature and 0.08 PSU for salinity. The combination of oceanographic and MCS data into a common, pseudo‐automatic inversion scheme allows to quantitatively resolve submeso‐scale features that ought to be incorporated into larger‐scale ocean models of oceans structure and circulation. This article is protected by copyright. All rights reserved.
      PubDate: 2016-06-29T10:55:50.123671-05:
      DOI: 10.1002/2016JC011844
       
  • Horizontal variability of high‐frequency nonlinear internal waves in
           Massachusetts Bay detected by an array of seafloor pressure sensors
    • Authors: J. A. Thomas; J. A. Lerczak, J. N. Moum
      Abstract: A two‐dimensional array of fourteen seafloor pressure sensors was deployed to measure properties of tidally‐generated, nonlinear, high‐frequency internal waves over a 14‐km by 12‐km area west of Stellwagen Bank in Massachusetts Bay during summer 2009. Thirteen high‐frequency internal wave packets propagated through the region over 6.5 days (one packet every semidiurnal cycle). Propagation speed and direction of wave packets were determined by triangulation, using arrival times and distances between triads of sensor locations. Wavefront curvature ranged from straight to radially spreading, with wave speeds generally faster to the south. Waves propagated to the southwest, rotating to more westward with shoreward propagation. Linear theory predicts a relationship between kinetic energy and bottom pressure variance of internal waves that is sensitive to sheared background currents, water depth, and stratification. By comparison to seafloor acoustic Doppler current profiler measurements, observations nonetheless show a strong relationship between kinetic energy and bottom pressure variance. This is presumably due to phase‐locking of the wave packets to the internal tide that dominates background currents and to horizontally uniform and relatively constant stratification throughout the study. This relationship was used to qualitatively describe variations in kinetic energy of the high‐frequency wave packets. In general, high‐frequency internal wave kinetic energy was greater near the southern extent of wavefronts and greatly decreased upon propagating shoreward of the 40‐m isobath. This article is protected by copyright. All rights reserved.
      PubDate: 2016-06-28T03:26:18.771975-05:
      DOI: 10.1002/2016JC011866
       
  • Intensification and poleward shift of subtropical western boundary
           currents in a warming climate
    • Authors: Hu Yang; Gerrit Lohmann, Wei Wei, Mihai Dima, Monica Ionita, Jiping Liu
      Abstract: A significant increase in sea surface temperature (SST) is observed over the mid‐latitude western boundary currents (WBCs) during the past century. However, the mechanism for this phenomenon remains poorly understood due to limited observations. In the present paper, several coupled parameters (i.e., sea surface temperature (SST), ocean surface heat fluxes, ocean water velocity, ocean surface winds and sea level pressure (SLP)) are analyzed to identify the dynamic changes of the WBCs. Three types of independent data sets are used, including reanalysis products, satellite‐blended observations and climate model outputs from the fifth phase of the Climate Model Intercomparison Project (CMIP5). Based on these broad ranges of data, we find that the WBCs (except the Gulf Stream) are intensifying and shifting toward the poles as long‐term effects of global warming. An intensification and poleward shift of near‐surface ocean winds, attributed to positive annular mode‐like trends, are proposed to be the forcing of such dynamic changes. In contrast to the other WBCs, the Gulf Stream is expected to be weaker under global warming, which is most likely related to a weakening of the Atlantic Meridional Overturning Circulation (AMOC). However, we also notice that the natural variations of WBCs might conceal the long‐term effect of global warming in the available observational data sets, especially over the Northern Hemisphere. Therefore, long‐term observations or proxy data are necessary to further evaluate the dynamics of the WBCs. This article is protected by copyright. All rights reserved.
      PubDate: 2016-06-27T15:00:03.047003-05:
      DOI: 10.1002/2015JC011513
       
  • Predictability of wind‐induced sea surface transport in coastal
           areas
    • Authors: A. Cucco; G. Quattrocchi, A. Satta, F. Antognarelli, F. de Biasio, E. Cadau, G. Umgiesser, S. Zecchetto
      Abstract: In this work we investigated the predictability of the wind induced sea surface transport in coastal areas. The wind fields predicted by two state‐of‐the‐art meteorological models, namely ECMWF and SKIRON, were used as forcing for a hydrodynamic and particles tracking model applied to reproduce a set of observed drifters trajectories in a coastal area of the Mediterranean Sea. A set of anemometric data derived by in situ measurements was also adopted as model forcing to reproduce the observed drifter paths. This approach provided a baseline that was used as a reference for evaluating the effects of the predicted wind accuracy on the numerical model solution. The accuracy of the simulation results obtained using, as model forcing, the observed wind data was fair and suitable for most of the operational oceanographic purposes. It decreased when using the wind data predicted by the two meteorological models. In particular, the results obtained using ECMWF data were about 3 times more accurate than the ones obtained using SKIRON ones. The uncertainties were strongly dependent on the range of observed wind speed classes with a different behavior depending on the type of adopted wind data. Finally the amplification of the errors in predicting the sea surface transport generated by the inaccuracies of the predicted wind fields was quantified. This article is protected by copyright. All rights reserved.
      PubDate: 2016-06-27T03:40:58.367251-05:
      DOI: 10.1002/2016JC011643
       
  • Vertical distribution of buoyant Microcystis blooms in a Lagrangian
           particle tracking model for short‐term forecasts in Lake Erie
    • Authors: M. D. Rowe; E. J. Anderson, T. T. Wynne, R. P. Stumpf, D. L. Fanslow, K. Kijanka, H. A. Vanderploeg, J. R. Strickler, T. W. Davis
      Abstract: Cyanobacterial harmful algal blooms (CHABs) are a problem in western Lake Erie, and in eutrophic fresh waters worldwide. Western Lake Erie is a large (3000 km2), shallow (8 m mean depth), freshwater system. CHABs occur from July to October, when stratification is intermittent in response to wind and surface heating or cooling (polymictic). Existing forecast models give the present location and extent of CHABs from satellite imagery, then predict two‐dimensional (surface) CHAB movement in response to meteorology. In this study, we simulated vertical distribution of buoyant Microcystis colonies, and 3D advection, using a Lagrangian particle model forced by currents and turbulent diffusivity from the Finite Volume Community Ocean Model (FVCOM). We estimated the frequency distribution of Microcystis colony buoyant velocity from measured size distributions and buoyant velocities. We evaluated several random‐walk numerical schemes to efficiently minimize particle accumulation artifacts. We selected the Milstein scheme, with linear interpolation of the diffusivity profile in place of cubic splines, and varied the time step at each particle and step based on the curvature of the local diffusivity profile to ensure that the Visser time step criterion was satisfied. Inclusion of vertical mixing with buoyancy significantly improved model skill statistics compared to an advection‐only model, and showed greater skill than a persistence forecast through simulation day 6, in a series of 26 hindcast simulations from 2011. The simulations and in‐situ observations show the importance of subtle thermal structure, typical of a polymictic lake, along with buoyancy in determining vertical and horizontal distribution of Microcystis. This article is protected by copyright. All rights reserved.
      PubDate: 2016-06-27T03:40:47.944411-05:
      DOI: 10.1002/2016JC011720
       
  • Horizontal mixing in the Southern Ocean from Argo float trajectories
    • Authors: Christopher J. Roach; Dhruv Balwada, Kevin Speer
      Abstract: We provide the first observational estimate of the circumpolar distribution of cross‐stream eddy diffusivity at 1000m in the Southern Ocean using Argo float trajectories. We show that Argo float trajectories, from the float surfacing positions, can be used to estimate lateral eddy diffusivities in the ocean and that these estimates are comparable to those obtained from RAFOS floats, where they overlap. Using the Southern Ocean State Estimate (SOSE) velocity fields to advect synthetic particles with imposed behaviour that is “Argo like” and “RAFOS like” diffusivity estimates from both sets of synthetic particles agreed closely at the three dynamically very different test sites: the Kerguelen Island region, the Southeast Pacific Ocean and the Scotia Sea, and support our approach. Observed cross‐stream diffusivities at 1000m, calculated from Argo float trajectories, ranged between 300 and 2500 m2s−1, with peaks corresponding to topographic features associated with the Scotia Sea, the Kerguelen Plateau, the Campbell Plateau and the Southeast Pacific Ridge. These observational estimates agree with previous regional estimates from the Diapycnal and Isopycnal Mixing Experiment in the Southern Ocean (DIMES) near the Drake Passage, and other estimates from natural tracers (helium), inverse modelling studies and current meter measurements. These estimates are also compared to the suppressed eddy diffusivity in the presence of mean flows. The comparison suggests that away from regions of strong topographic steering suppression explains both the structure and magnitude of eddy diffusivity, but that eddy diffusivities in the regions of topographic steering are greater than what would be theoretically expected and the ACC experiences localized enhanced cross‐stream mixing in these regions. This article is protected by copyright. All rights reserved.
      PubDate: 2016-06-27T03:30:40.87102-05:0
      DOI: 10.1002/2015JC011440
       
  • Contributions of different tidal interactions to fortnightly variation in
           tidal duration asymmetry
    • Authors: Wenyun Guo; Dehai Song, Xiao Hua Wang, Pingxing Ding, Jianzhong Ge
      Abstract: The general framework for identifying tidal duration asymmetry proposed by Song et al. (2011) is extended to express fortnightly variability in duration asymmetry. The extended metrics are verified and studied using observed sea‐level data at 481 stations worldwide. The results reveal that fortnightly variability is universal and that duration asymmetry can be stronger during neap tide than during spring tide. The fortnightly variability in duration asymmetry is primarily induced by three types of tidal interactions: interactions within the principal tidal constituents, interactions between high‐frequency and principal tidal constituents, and interactions between long‐period and principal tidal constituents. Among these interactions, the first type is most important at most of the stations and is related to the form number F. The contributions of different interactions can be quantified using their frequencies, amplitudes and phases. Global patterns of the fortnightly variation are illustrated using TOPEX/Poseidon altimetry data. The findings show that remarkable fortnightly variation in the tidal duration asymmetry occurs in most open oceans and is significant around an amphidromic point. The metrics derived in this study can be used to examine any time‐varying characteristics in tidal asymmetry (not limited to duration asymmetry) by selecting a suitable frequency threshold. This article is protected by copyright. All rights reserved.
      PubDate: 2016-06-27T03:30:38.876762-05:
      DOI: 10.1002/2016JC011689
       
  • Surface layer temperature inversion in the Bay of Bengal: Main
           characteristics and related mechanisms
    • Authors: Pankajakshan Thadathil; I. Suresh, S. Gautham, S. Prasanna Kumar, Matthieu Lengaigne, R.R. Rao, S. Neetu, Akshay Hegde
      Abstract: Surface Layer Temperature Inversion (SLTI), a warm layer sandwiched between surface and subsurface colder waters, have been reported to frequently occur in conjunction with barrier layers in the Bay of Bengal (BoB), with potentially commensurable impacts on climate and post‐monsoon tropical cyclones. Lack of systematic measurements from the BoB in the past prevented a thorough investigation of the SLTI spatio‐temporal variability, their formation mechanisms and their contribution to the surface temperature variations. The present study benefits from the recent Research Moored Array for African‐Asian‐Australian Monsoon Analysis and Prediction (RAMA) buoys located in BoB along 90°E at 4°N, 8°N, 12°N, and 15oN over the 2006‐2014 period. Analysis of data from these RAMA buoys indicates that SLTI forms after the summer monsoon, and becomes fully developed during winter (December – February). SLTI exhibits a strong geographical dependency, with more frequent (80% times during winter) and intense inversions (amplitude, ΔT ∼ 0.7oC) occurring only in the northern BoB compared to central and southern Bay. SLTI also exhibits large interannual and intraseasonal variations, with intraseasonal amplitude significantly larger (ΔT ∼ 0.44oC) than the interannual amplitude (∼ 0.26oC). Heat budget analysis of the mixed layer reveals that the net surface heat loss is the most dominant process controlling the formation and maintenance of SLTI. However, there are instances of episodic advection of cold, low‐saline waters over warm‐saline waters leading to the formation of SLTI as in 2012‐2013. Vertical processes contribute significantly to the mixed layer heat budget during winter, by warming the surface layer through entrainment and vertical diffusion. This article is protected by copyright. All rights reserved.
      PubDate: 2016-06-27T03:30:35.650938-05:
      DOI: 10.1002/2016JC011674
       
  • Distinct characteristics of the intermediate water observed off the east
           coast of Korea during two contrasting years
    • Abstract: The intermediate water known as ‘East Sea Intermediate Water' found south of the Subpolar Front (SF) is known to be formed in the northern East/Japan Sea (EJS), and its physical properties are determined by wintertime air‐sea interaction north of the SF. Hydrographic data collected off the Korean coast show significant decadal oscillations in spiciness (π) following isopycnals of intermediate layer, which are explained by the Arctic Oscillation (AO) and consequent cold‐air outbreaks. During positive AO phases, the cold‐air outbreak and water formation are more active and the intermediate water having the same π reaches higher density (higher π following the same isopycnals). At interannual timescale, however, the π variability is well beyond the relationship with the AO. Especially, significantly lower π (both fresher and colder) intermediate water was observed in spring of 2010 than 2001 under the same surface net‐heat flux peaks in the northern EJS in the two winters. Such contrasting characteristics of intermediate water between 2001 and 2010 are consistent with the HYCOM reanalysis results which indicate widespread extension of low‐ (high‐) π intermediate water in the southwestern EJS in 2010 (2001). A clear contrast in circulation pattern is suggested to derive the distinctly different characteristics of the intermediate water. Northward penetration of the East Korea Warm Current (EKWC) inhibited the southward extension of the intermediate water in 2001 off the east coast of Korea. On the other hand, the EKWC that poorly developed in 2010 allowed low‐π intermediate water to prevail in the southwestern EJS. This article is protected by copyright. All rights reserved.
      PubDate: 2016-06-27T03:30:34.163977-05:
      DOI: 10.1002/2015JC011593
       
  • Dense water formation in the north‐western mediterranean area during
           HyMeX‐SOP2 in 1/36° ocean simulations: Sensitivity to initial
           conditions
    • Abstract: The north‐western Mediterranean Sea is a key location where intense air‐sea exchanges occur in autumn and winter. The succession of strong mistral and tramontane situations, leading to significant evaporation and ocean heat loss, is well known as the controlling factor in the dense water formation (DWF) with deep convection episodes. During HyMeX‐SOP2 (1 February to 15 March 2013), several platforms sampled the area in order to document DWF and air‐sea exchanges. This study investigates the ability of the NEMO‐WMED36 ocean model (1/36°‐resolution), driven in surface by the hourly air‐sea fluxes from the AROME‐WMED forecasts (2.5km‐resolution), to represent DWF during HyMeX‐SOP2 and focuses on the sensitivity to initial conditions. After a short evaluation of the atmospheric forcing, the high‐resolution oceanic simulations using three different datasets as initial and boundary conditions are compared to observations collected during the field campaign. It evidences that using regional model outputs may lead to unrealistic thermohaline characteristics for the intermediate and deep waters, which degrade the simulated new dense water formed. Using ocean analyses built from observations, permits to obtain more realistic characteristics of the Western Mediterranean Dense Water. However, a low stratification favors an overestimation of the convective area and of the DWF rate. The DWF chronology is also impacted. Nevertheless, in every run, SOP2 is characterized by the production of water denser than 29.11 kg.m– 3 with a peak during the strong mistral event of 23‐25 February followed by a period of restratification, before a last event of bottom convection on 13‐15 March. This article is protected by copyright. All rights reserved.
      PubDate: 2016-06-27T03:30:31.358546-05:
      DOI: 10.1002/2015JC011542
       
  • The combined effect of transient wind‐driven upwelling and eddies on
           vertical nutrient fluxes and phytoplankton dynamics along Ningaloo Reef,
           Western Australia
    • Authors: Zhenlin Zhang; Ryan Lowe, Greg Ivey, Jiangtao Xu, James Falter
      Abstract: We investigate the influence of wind stresses, stratification and coastal mesoscale eddies on upwelling intensity, vertical nutrient fluxes, and phytoplankton biomass on the continental shelf off Ningaloo Reef in northwestern Australia during an austral spring‐summer period. A three‐dimensional (3D) hydrodynamic model, ROMS (Regional Ocean Modeling System), was coupled with a four‐component nitrogen‐based biogeochemical NPZD model (Nitrogen Phytoplankton Zooplankton Detritus) to resolve the shelf circulation as well as the coupled nutrient and phytoplankton dynamics within a broad shelf region surrounding Ningaloo Reef. The simulated currents, temperatures and chlorophyll a concentrations generally agreed well with both the remotely‐sensed satellite images and observational data collected during a field experiment from September to November 2010. Scenario tests for an individual wind‐driven upwelling event under a variety of hypothetical physical forcing conditions showed that shelf currents and biogeochemical variables were largely a function of wind stress and stratification. However, the functional relationships derived from this single wind event could not be extrapolated to other periods of the upwelling season, due to the additional influence of 3D mesoscale processes on the shelf. The presence, intensification and propagation of a coastal anti‐cyclonic eddy during the study period strongly influenced the spatial and temporal variations in nutrient profiles, which in turn caused fluctuations in vertical nutrient fluxes that were largely independent of wind stress. These results emphasize that it is necessary to fully capture the 3D details of the mesoscale and sub‐mesoscale coastal dynamics to properly predict upwelling‐induced coastal phytoplankton dynamics in eddy‐intensive shelf regions such as Ningaloo Reef. This article is protected by copyright. All rights reserved.
      PubDate: 2016-06-27T03:30:22.508578-05:
      DOI: 10.1002/2016JC011791
       
  • On theories dealing with the interaction of surface waves and ocean
           circulation
    • Authors: George Mellor
      Abstract: The classic theory for the interaction of surface gravity waves and the general ocean circulation entails the so‐called wave radiation stress terms in the phase‐averaged momentum equation. The equations of motion are for the combined Eulerian current and Stokes drift. On the other hand, a more recent approach includes the so‐called vortex force term in the momentum equation wherein the only wave property is Stokes drift. The equations of motion are for the Eulerian current. The idea has gained traction in the ocean science community, a fact that motivates this paper. A question is: can both theories be correct? This paper answers the question in the negative and presents arguments in favor of the wave radiation theory. The vortex force approach stems from an interesting mathematical construct, but it does stand up to physical or mathematical scrutiny as described in this paper. Although not the primary focus of the paper, some discussion of Langmuir circulation is included since the vortex force was first introduced as the basis of this oceanic cellular phenomenon. Finaly the paper explains the difference in the derivation of the radiation stress theory and the vortex force theory: the later theory entails errors related to its use of curl and reverse‐curl (or uncurl) processes. This article is protected by copyright. All rights reserved.
      PubDate: 2016-06-25T03:05:20.607719-05:
      DOI: 10.1002/2016JC011768
       
  • Forcing of the Overturning Circulation across a Circumpolar Channel by
           Internal Wave Breaking
    • Authors: Maria B. Broadbridge; Alberto C. Naveira Garabato, A. J. George Nurser
      Abstract: The hypothesis that the impingement of mesoscale eddy flows on small‐scale topography regulates diapycnal mixing and meridional overturning across the deep Southern Ocean is assessed in an idealised model. The model simulates an eddying circumpolar current coupled to a double‐celled meridional overturning with properties broadly resembling those of the Southern Ocean circulation, and represents lee wave‐induced diapycnal mixing using an online formulation grounded on wave radiation theory. The diapycnal mixing generated by the simulated eddy field is found to play a major role in sustaining the lower overturning cell in the model, and to underpin a significant sensitivity of this cell to wind forcing. The vertical structure of lower overturning is set by mesoscale eddies, which propagate the effects of near‐bottom diapycnal mixing by displacing isopycnals vertically. This article is protected by copyright. All rights reserved.
      PubDate: 2016-06-24T03:27:19.676648-05:
      DOI: 10.1002/2015JC011597
       
  • Vertical fluxes of nitrate in the seasonal nitracline of the Atlantic
           sector of the Arctic Ocean
    • Abstract: This study compiles co‐located oceanic observations of high‐resolution vertical profiles of nitrate concentration and turbulent microstructure around the Svalbard shelf slope, covering both the permanently ice‐free Fram Strait and the pack ice north of Svalbard. The authors present an overview over the seasonal evolution of the distribution of nitrate and its relation to upper ocean stratification. The average upward turbulent diffusive nitrate flux across the seasonal nitracline during the Arctic summer season is derived, with average values of 0.3 and 0.7 mmolm−2,d−1 for stations with and without ice cover, respectively. The increase under ice‐free conditions is attributed to different patterns of stratification under sea ice versus open water. The nitrate flux obtained from microstructure measurements lacked a seasonal signal. However, bottle incubations indicate that August nitrate uptake was reduced by more than an order of magnitude relative to the May values. It remains inconclusive whether the new production was limited by an unidentified factor other than NO3− supply in late summer, or the uptake was underestimated by the incubation method. This article is protected by copyright. All rights reserved.
      PubDate: 2016-06-23T03:50:29.871292-05:
      DOI: 10.1002/2016JC011779
       
  • Bromide and chloride distribution across the snow‐sea
           ice‐ocean interface: A comparative study between an Arctic coastal
           marine site and an experimental sea ice mesocosm
    • Authors: Wen Xu; Mario Tenuta, Feiyue Wang
      Abstract: During springtime in the Arctic, bromine explosion events occur when high concentrations of reactive bromine species are observed in the boundary layer with the concurrence of ozone depletion events and mercury depletion events. While a variety of substrates such as snow, sea ice, frost flowers and aerosols have been proposed to be the substrate and/or source of bromine activation in the Arctic, recent studies have highlighted the role of snow. Here we report concentration profiles of halides (Br− and Cl−), Na+, and oxidized mercury across the snow‐sea ice‐seawater interface at a coastal marine site in the Canadian Arctic Archipelago in March and June 2014, as well as in an experimental sea ice mesocosm in Winnipeg in January 2014. The occurrence of bromine activation at the Arctic site in March was indicated by the high mercury concentrations in snowpack. At both the Arctic and mesocosm sites, the molar ratios of Br−/Na+ were nearly constant throughout the sea ice depth, but highly variable in the upper layer of the overlying snowpack, revealing that bromine activation takes place in the sunlit snow instead of sea ice. This is supported by calculations showing that the loss of Br– from the upper layer of the snowpack is large enough to produce the observed concentrations of reactive bromine in the atmospheric boundary layer. However, the upper layer of the Arctic snowpack tends to be generally enriched in Br– due to the net addition of Br–‐enriched gases and non‐sea‐salt aerosols. This article is protected by copyright. All rights reserved.
      PubDate: 2016-06-22T18:55:42.980452-05:
      DOI: 10.1002/2015JC011409
       
  • A comparative study of wave‐current interactions over the eastern
           Canadian shelf under severe weather conditions using a coupled
           wave‐circulation model
    • Authors: Pengcheng Wang; Jinyu Sheng
      Abstract: A coupled wave‐circulation model is used to examine interactions between surface gravity waves and ocean currents over the eastern Canadian shelf and adjacent deep waters during three severe weather events. The simulated significant wave heights (SWHs) and peak wave periods reveal the importance of wave‐current interactions (WCI) during and after the storm. In two fast‐moving hurricane cases, the maximum SWHs are reduced by more than 11% on the right‐hand side of the storm track and increased by about 5% on the left‐hand side due to different WCI mechanisms on waves on two sides of the track. The dominate mechanisms of the WCI on waves include the current‐induced modification of wind energy input to the wave generation, and current‐induced wave advection and refraction. In the slow‐moving winter storm case, the effect of WCI decreases the maximum SWHs on both sides of the storm track due to different results of the current‐induced wave advection, which is affected greatly by the storm translation speed. The simulated sea surface temperature (SST) cooling induced by hurricanes and SST warming induced by the winter storm are also enhanced (up to 1.2oC) by the WCI mechanisms on circulation and hydrography. The 3D wave forces can affect water columns up to 200 m in all three storm cases. By comparison, the effect of breaking wave‐induced mixing in the ocean upper layer is more important under strong stratification conditions in two hurricane cases than under weak stratification conditions in the winter storm case. This article is protected by copyright. All rights reserved.
      PubDate: 2016-06-20T11:00:36.990861-05:
      DOI: 10.1002/2016JC011758
       
  • Assessing the impact of vertical land motion on 20th century global mean
           sea level estimates
    • Authors: B.D. Hamlington; P. Thompson, W. C. Hammond, G. Blewitt, R.D. Ray
      Abstract: Near‐global and continuous measurements from satellite altimetry have provided accurate estimates of global mean sea level in the past two decades. Extending these estimates further into the past is a challenge using the historical tide gauge records. Not only is sampling non‐uniform in both space and time, tide gauges are also affected by vertical land motion (VLM) that creates a relative sea level change not representative of ocean variability. To allow for comparisons to the satellite altimetry estimated global mean sea level (GMSL), typically the tide gauges are corrected using glacial isostatic adjustment (GIA) models. This approach, however, does not correct other sources of VLM that remain in the tide gauge record. Here, we compare Global Positioning System (GPS) VLM estimates at the tide gauge locations to VLM estimates from GIA models, and assess the influence of non‐GIA related VLM on GMSL estimates. We find that the tide gauges, on average, are experiencing positive VLM (i.e. uplift) after removing the known effect of GIA, resulting in an increase of 0.24 +/‐ 0.08 mm year−1 in GMSL trend estimates from 1900 to present when using GPS‐based corrections. While this result is likely dependent on the subset of tide gauges used and the actual corrections used, it does suggest that non‐GIA VLM plays a significant role in 20th century estimates of GMSL. Given the relatively short GPS records used to obtain these VLM estimates, we also estimate the uncertainty in the GMSL trend that results from limited knowledge of non‐GIA related VLM. This article is protected by copyright. All rights reserved.
      PubDate: 2016-06-20T11:00:27.282718-05:
      DOI: 10.1002/2016JC011747
       
  • Relevance of infragravity waves in a wave‐dominated inlet
    • Authors: Xavier Bertin; Maitane Olabarrieta
      Abstract: This study investigates the relevance of infragravity (IG) waves at Albufeira Lagoon Inlet, a shallow wave‐dominated inlet located on the Western Coast of Portugal. A field experiment carried out in September 2010 revealed the occurrence of low‐frequency oscillations (i.e. 25 to 300 s) in water levels and current velocities. While these fluctuations were present over the ebb‐ tidal delta along the whole tidal cycle, they only appeared between the beginning of the flood and up to two hours after high tide inside the lagoon. The XBeach modeling system was applied to Albufeira Lagoon Inlet and reproduced the generation and propagation of IG waves and their blocking during the ebb. This behavior was explained by blocking due to opposing tidal currents reaching 2.5 m.s−1 in shallow water depths. Numerical results suggest that the breakpoint mechanism and the long bound wave shoaling mechanisms contributed significantly to the generation of IG waves in the inlet. IG waves induced fluctuations in flood currents inside the lagoon reaching temporarily 100% of their magnitude. The fact that these fluctuations occur mostly at flood and not at ebb could promote flood dominance in the lagoon. This hypothesis will have to be verified, namely under storm wave conditions. This article is protected by copyright. All rights reserved.
      PubDate: 2016-06-20T10:55:28.529948-05:
      DOI: 10.1002/2015JC011444
       
  • A numerical model for the entire Wadden Sea: skill assessment and analysis
           of hydrodynamics
    • Abstract: A baroclinic three‐dimensional numerical model for the entire Wadden Sea of the German Bight in the southern North Sea is first assessed by comparison to field data for surface elevation, current velocity, temperature and salinity at selected stations and then used to calculate fluxes of volume and salt inside the Wadden Sea and the exchange between the Wadden Sea and the adjacent North Sea through the major tidal inlets. The model is simulating the reference years 2009‐2011. An overview of tidal prisms and residual volume fluxes of the main inlets and their variability is given. In addition, data from an intensive observational campaign in a tidal channel south of the island of Spiekeroog as well as satellite images and observations of sea surface properties from a ship of opportunity are used for the skill assessment. Finally, the intensity of estuarine overturning circulation and its variability in the tidal gullies are quantified and analyzed as function of gravitational and wind straining using various estimates including Total Exchange Flow (TEF). Regional differences between the gullies are assessed and drivers of the estuarine circulation are identified. For some inlets, the longitudinal buoyancy gradient dominates the exchange flow, for some others wind straining is more important. Also the intensity of tidal straining (scaled covariance of eddy viscosity and vertical shear) depends on buoyancy gradient and wind forcing in different ways, depending on local topography, orientation towards the main wind direction and influence by freshwater run‐off inside or outside the tidal basin. This article is protected by copyright. All rights reserved.
      PubDate: 2016-06-16T03:30:34.650685-05:
      DOI: 10.1002/2016JC011655
       
  • Sunlight‐induced chlorophyll fluorescence in the near‐infrared
           spectral region in natural waters: Interpretation of the spectral
           reflectance peak around 761 nm
    • Authors: Yingcheng Lu; Linhai Li, Chuanmin Hu, Lin Li, Minwei Zhang, Shaojie Sun, Chunguang Lv
      Abstract: Sunlight‐induced chlorophyll‐a fluorescence (SICF) can be used as a probe to estimate chlorophyll‐a concentrations (Chl) and infer phytoplankton physiology. SICF at ∼685 nm has been widely applied to studies of natural waters. SICF around 740 nm has been demonstrated to cause a spectral peak at ∼761 nm in the reflectance spectra of terrestrial vegetation. This spectral peak has also been observed in the reflectance spectra of natural waters, but its mechanism and applications have not yet been investigated and it has often been treated as measurement artifacts. In this study, we aimed to interpret this reflectance peak at ∼761 nm and discuss its potential applications for remotely monitoring natural waters. A derivative analysis of the spectral reflectance suggests that the 761‐nm peak is due to SICF. It was also found that the fluorescence line height (FLH) at 761 nm significantly and linearly correlates with Chl. FLH(761 nm) showed a tighter relationship with Chl than the relationship between FLH(∼685 nm) and Chl mainly due to weaker perturbations by non‐algal materials around 761 nm. While it is not conclusive, a combination of FLH(761 nm) and FLH(∼685 nm) might have some potentials to discriminate cyanobacteria from other phytoplankton due to their variable fluorescence at the two wavelengths. It was further found that reflectance spectra with a 5‐nm spectral resolution are adequate to capture the spectral SICF feature at ∼761 nm. These preliminary results suggest that FLH(761 nm) need to be explored more for future applications in optically complex coastal and inland waters. This article is protected by copyright. All rights reserved.
      PubDate: 2016-06-16T03:26:18.597519-05:
      DOI: 10.1002/2016JC011797
       
  • Estuarine Circulation vs Tidal Pumping: Sediment Transport in a
           Well‐Mixed Tidal Inlet
    • Abstract: High‐resolution water column observations have been carried out in the Wadden Sea to understand suspended particulate mater (SPM) transport in well‐mixed tidal channels . These observations include more than 4000 consecutive CTD‐, micro‐structure shear and turbidity profiles from a free‐falling micro‐structure probe, as well as velocity data from an ADCP and SPM samples for calibration. A horizontal density gradient was established by a landward temperature gradient built up during an extraordinarily warm and calm spring season. Tidal averaging along σ‐layers (relative depth) provides the first direct observations of along‐channel estuarine circulation in the Wadden Sea, with net inflow near the bottom and outflow near the surface. Increased westerly (up‐estuary) winds during the second part of the campaign weakened and eventually even reversed estuarine circulation and yielded a net barotropic eastward transport. SPM concentrations showed a strong quarter‐diurnal signal with maxima near full flood and full ebb and were generally lower during the calm period and increased during the windy period, mainly due to wave‐related resuspension over nearby inter‐tidal flats. The sediment flux analysis was based on a decomposition of the vertically integrated SPM flux into a barotropic advective component, an estuarine circulation component and a tidal pumping component. As a result, tidal pumping (due to ebb‐dominance weakly seaward) dominated the SPM flux during calm conditions, whereas barotropic advection dominated the strong landward SPM flux during the windy period. Along‐channel estuarine circulation is found to be of minor importance for the net SPM‐transport in such well‐mixed systems. This article is protected by copyright. All rights reserved.
      PubDate: 2016-06-16T03:26:15.426205-05:
      DOI: 10.1002/2016JC011640
       
  • The spectral albedo of sea ice and salt crusts on the tropical ocean of
           Snowball Earth: II. Optical modeling
    • Authors: Regina C. Carns; Bonnie Light, Stephen G. Warren
      Abstract: During the Snowball Earth events of the Neoproterozoic, tropical regions of the ocean could have developed a precipitated salt lag deposit left behind by sublimating sea ice. The major salt would have been hydrohalite, NaCl•2H2O. The crystals in such a deposit can be small and highly scattering, resulting in an allwave albedo similar to that of snow. The snow‐free sea ice from which such a crust could develop has a lower albedo, around 0.5, so the development of a crust would substantially increase the albedo of tropical regions on Snowball Earth. Hydrohalite crystals are much less absorptive than ice in the near‐infrared part of the solar spectrum, so their presence at the surface would increase the overall albedo as well as altering its spectral distribution. In this paper, we use laboratory measurements of the spectral albedo of a hydrohalite lag deposit, in combination with a radiative transfer model, to infer the inherent optical properties of hydrohalite as functions of wavelength. Using this result, we model mixtures of hydrohalite and ice representing both artificially created surfaces in the laboratory and surfaces relevant to Snowball Earth. The model is tested against sequences of laboratory measurements taken during the formation and the dissolution of a lag deposit of hydrohalite. We present a parameterization for the broadband albedo of cold, sublimating sea ice as it forms and evolves a hydrohalite crust, for use in climate models of Snowball Earth. This article is protected by copyright. All rights reserved.
      PubDate: 2016-06-16T03:25:59.963095-05:
      DOI: 10.1002/2016JC011804
       
  • Estimating the recharge properties of the deep ocean using noble gases and
           helium isotopes
    • Authors: Brice Loose; William J. Jenkins, Roisin Moriarty, Peter Brown, Loic Jullion, Alberto C. Naveira Garabato, Sinhue Torres Valdes, Mario Hoppema, Chris Ballentine, Michael P. Meredith
      Abstract: The distribution of noble gases and helium isotopes in the dense shelf waters of Antarctica reflect the boundary conditions near the ocean surface: air‐sea exchange, sea ice formation and subsurface ice melt. We use a non‐linear least‐squares solution to determine the value of the recharge temperature and salinity, as well as the excess air injection and glacial meltwater content throughout the water column and in the precursor to Antarctic Bottom Water. The noble gas‐derived recharge temperature and salinity in the Weddell Gyre are ‐1.95 °C and 34.95 psu near 5500 m; these cold, salty recharge values are a result of surface cooling as well as brine rejection during sea ice formation in Antarctic polynyas. In comparison, the global value for deep water recharge temperature is ‐0.44 °C at 5500 m, which is 1.5 °C warmer than the southern hemisphere deep water recharge temperature, reflecting the contribution from the north Atlantic. The contrast between northern and southern hemisphere recharge properties highlight the impact of sea ice formation on setting the gas properties in southern sourced deep water. Below 1000 m, glacial meltwater averages 3.5 ‰ by volume and represents greater than 50% of the excess neon and argon found in the water column. These results indicate glacial melt has a non‐negligible impact on the atmospheric gas content of Antarctic Bottom Water. This article is protected by copyright. All rights reserved.
      PubDate: 2016-06-16T03:25:53.371533-05:
      DOI: 10.1002/2016JC011809
       
  • Nonlinear and minor ocean tides in the Bay of Biscay from the strain tides
           observed by two geodetic laser strainmeters at Canfranc (Spain)
    • Authors: Antonella Amoruso; Luca Crescentini
      Abstract: This work presents and discusses the tidal analysis of about 2.5 years of strain data recorded by two 70‐m‐long high‐resolution laser strainmeters, which are operating in the Canfranc underground laboratory (Central Pyrenees, Spain). Spectra show clear tidal peaks whose frequencies range from the diurnal band to at least 8 cycles per day; strain amplitudes (relative change in length for each strainmeter) range from few 10−12 to 10−8. The reliability of observations and corrections for local distortions are testified by the excellent agreement between measurements and computations for all the diurnal and semi‐diurnal tides included in the TPXO8 and FES2012 tidal atlases, with the exception of L2. Observed higher‐frequency strain tides are mostly attributed to loading by nonlinear shallow‐water constituents in the Bay of Biscay, more than 120 km from the measurement site. The signals are quantitatively compared with computations using TPXO8 (MN4, M4, and MS4) and FES2012 (M3, N4, MN4, M4, MS4, and M6). Computations fully agree with M4 observations for one strainmeter, overestimate M4 by about 30% for the other strainmeter and M6 by about a factor of two, and underestimate the other tides. This work shows that data from high‐sensitivity strainmeters installed in high‐quality sites may provide valuable additional information for studying the nonlinear tidal dynamics and energetics of coastal waters and minor ocean tides, at spatial resolutions of tens to thousands square kilometers, depending on the strainmeter location with respect to the coastline. This article is protected by copyright. All rights reserved.
      PubDate: 2016-06-16T03:25:46.002114-05:
      DOI: 10.1002/2016JC011733
       
  • The spectral albedo of sea ice and salt crusts on the tropical ocean of
           Snowball Earth: I. Laboratory measurements
    • Authors: Bonnie Light; Regina C. Carns, Stephen G. Warren
      Abstract: The ice‐albedo feedback mechanism likely contributed to global glaciation during the Snowball Earth events of the Neoproterozoic era (1 Ga to 544 Ma). This feedback results from the albedo contrast between sea ice and open ocean. Little is known about the optical properties of some of the possible surface types that may have been present, including sea ice that is both snow‐free and cold enough for salts to precipitate within brine inclusions. A proxy surface for such ice was grown in a freezer laboratory using the single salt NaCl and kept below the eutectic temperature (−21.2°C) of the NaCl – H2O binary system. The resulting ice cover was composed of ice and precipitated hydrohalite crystals (NaCl · 2H2O). As the cold ice sublimated, a thin lag‐deposit of salt formed on the surface. To hasten its growth in the laboratory, the deposit was augmented by addition of a salt‐enriched surface crust. Measurements of the spectral albedo of this surface were carried out over 90 days as the hydrohalite crust thickened due to sublimation of ice, and subsequently over several hours as the crust warmed and dissolved, finally resulting in a surface with puddled liquid brine. The all‐wave solar albedo of the subeutectic crust is 0.93 (in contrast to 0.83 for fresh snow and 0.67 for melting bare sea ice). Incorporation of these processes into a climate model of Snowball Earth will result in a positive salt‐albedo feedback operating between −21°C and −36°C. This article is protected by copyright. All rights reserved.
      PubDate: 2016-06-16T03:25:44.751464-05:
      DOI: 10.1002/2016JC011803
       
  • Effects of rotation on turbulent buoyant plumes in stratified environments
    • Abstract: We numerically investigate the effects of rotation on the turbulent dynamics of thermally driven buoyant plumes in stratified environments at the large Rossby numbers characteristic of deep oceanic releases. When compared to non‐rotating environments, rotating plumes are distinguished by a significant decrease in vertical buoyancy and momentum fluxes leading to lower and thicker neutrally buoyant intrusion layers. The primary dynamic effect of background rotation is the concentration of entraining fluid into a strong cyclonic flow at the base of the plume resulting in cyclogeostrophic balance in the radial momentum equation. The structure of this cyclogeostrophic balance moving upward from the well head is associated with a net adverse vertical pressure gradient producing an inverted hydrostatic balance in the mean vertical momentum budgets. The present simulations reveal that the primary response to the adverse pressure gradient is an off‐axis deflection of the plume that evolves into a robust, organized anticyclonic radial precession about the buoyancy source. The off‐axis evolution is responsible for the weaker inertial overshoots, the increased thickness of lateral intrusion layers and the overall decrease in the vertical extent of rotating plumes at intermediate Rossby numbers compared to the non‐rotating case. For inlet buoyancy forcings and environmental Rossby numbers consistent with those expected in deepwater blowout plumes, the speed of the organized precession is found to be as large as typical oceanic crossflow speeds. This article is protected by copyright. All rights reserved.
      PubDate: 2016-06-16T03:25:40.110743-05:
      DOI: 10.1002/2016JC011737
       
  • Carbon export fluxes and export efficiency in the central Arctic during
           the record sea‐ice minimum in 2012: A joint 234Th/238U and
           210Po/210Pb study
    • Abstract: The Arctic sea‐ice extent reached a record minimum in September 2012. Sea‐ice decline increases the absorption of solar energy in the Arctic Ocean, affecting primary production and the plankton community. How this will modulate the sinking of particulate organic carbon (POC) from the ocean surface remains a key question. We use the 234Th/238U and 210Po/210Pb radionuclide pairs to estimate the magnitude of the POC export fluxes in the upper ocean of the central Arctic in summer 2012, covering time scales from weeks to months. The 234Th/238U proxy reveals that POC fluxes at the base of the euphotic zone were very low (2 ± 2 mmol C m−2 d−1) in late summer. Relationships obtained between the 234Th export fluxes and the phytoplankton community suggest that prasinophytes contributed significantly to the downward fluxes, likely via incorporation into sea‐ice algal aggregates and zooplankton‐derived material. The magnitude of the depletion of 210Po in the upper water column over the entire study area indicates that particle export fluxes were higher before July/August than later in the season. 210Po fluxes and 210Po‐derived POC fluxes correlated positively with sea‐ice concentration, showing that particle sinking was greater under heavy sea‐ice conditions than under partially ice‐covered regions. Although the POC fluxes were low, a large fraction of primary production (>30%) was exported at the base of the euphotic zone in most of the study area during summer 2012, indicating a high export efficiency of the biological pump in the central Arctic. This article is protected by copyright. All rights reserved.
      PubDate: 2016-06-16T03:25:32.385617-05:
      DOI: 10.1002/2016JC011816
       
  • Statistical properties of the surface velocity field in the northern Gulf
           of Mexico sampled by GLAD drifters
    • Abstract: The Grand LAgrangian Deployment (GLAD) used multi‐scale sampling and GPS technology to observe time series of drifter positions with initial drifter separation of O(100 m) to O(10 km), and nominal 5 minute sampling, during the summer and fall of 2012 in the northern Gulf of Mexico. Histograms of the velocity field and its statistical parameters are non‐Gaussian; most are multi‐modal. The dominant periods for the surface velocity field are 1‐2 days due to inertial oscillations, tides, and the sea breeze; 5‐6 days due to wind forcing and submesoscale eddies; 9‐10 days and two weeks or longer periods due to wind forcing and mesoscale variability, including the period of eddy rotation. The temporal e‐folding scales of a fitted drifter velocity autocorrelation function are bimodal with time scales, 0.25‐0.50 days and 0.9‐1.4 days, and are the same order as the temporal e‐folding scales of observed winds from nearby moored National Data Buoy Center stations. The Lagrangian integral time scales increase from coastal values of 8 hours to offshore values of approximately 2 days with peak values of 3‐4 days. The velocity variance is large, O(1) m2/s2, the surface velocity statistics are more anisotropic, and increased dispersion is observed at flow bifurcations. Horizontal diffusivity estimates are O(103) m2/s in coastal regions with weaker flow to O(105) m2/s in flow bifurcations, a strong jet, and during the passage of Hurricane Isaac. The Gulf of Mexico surface velocity statistics sampled by the GLAD drifters are a strong function of the feature sampled, topography, and wind forcing. This article is protected by copyright. All rights reserved.
      PubDate: 2016-06-16T03:25:24.931259-05:
      DOI: 10.1002/2015JC011569
       
  • Evidence of rising and poleward shift of storm surge in western North
           Pacific in recent decades
    • Authors: Leo Oey; Simon Chao
      Abstract: Recently, there has been considerable interest in examining how sea‐level extremes due to storm surge may be related to climate change. Evidence of how storm‐surge extremes have evolved since the start of the most recent warming of mid‐1970s to early 1980s has not been firmly established however. Here we use 64 years (1950‐2013) of observations and model simulations, and find evidence of a significant rise in the intensity as well as poleward‐shifting of location of typhoon surges in the western North Pacific after 1980s. The rising and poleward‐shifting trends are caused by the weakening of the steering flow in the tropics, which is related to climate warming, resulting in slower‐moving and longer‐lasting typhoons which had shifted northward. This article is protected by copyright. All rights reserved.
      PubDate: 2016-06-14T17:56:04.421055-05:
      DOI: 10.1002/2016JC011777
       
  • Using CryoSat‐2 altimeter data to evaluate M2 internal tides
           observed from multisatellite altimetry
    • Authors: Zhongxiang Zhao
      Abstract: This paper evaluates M2 internal tides observed from multisatellite altimetry (MultiSat20yr) using CryoSat‐2 altimeter data. MultiSat20yr is constructed using 20 years of sea surface height measurements made by multisatellite altimeters from 1992–2012. Here it is demonstrated that M2 internal tides can also be extracted using 4 years of CryoSat‐2 data from 2011–2014 (CryoSat4yr) by the same plane wave fit method. MultiSat20yr and CryoSat4yr are in good agreement in the central North Pacific, although they are from satellite data of different sampling patterns (2008 vs. 10688 tracks) and different observational periods (20 vs. 4 years). Further comparisons are carried out for three isolated components. MultiSat20yr and CryoSat4yr agree very well for both Hawaiian components, suggesting that the Hawaiian Ridge is a relatively stable generation site. In contrast, the Aleutian Ridge is a relatively unstable source in that the M2 amplitudes MultiSat20yr and CryoSat4yr are very different. With respect to MultiSat20yr, the M2 internal tide in 2011–2014 propagates slower (faster) to the south (north) of Hawaii, respectively, suggesting that the internal tide's propagation speed is subject to significant interannual variability. This feature is supported by M2 internal tides observed using multisatellite altimeter data in 2005 (MultiSat2005) and Argo measured upper ocean temperature profiles. MultiSat20yr is used to correct M2 internal tides in the CryoSat‐2 data. Significant and efficient variance reduction suggests that MultiSat20yr is a reliable internal tide model. A phase‐adjusted MultiSat20yr is built to account for the interannual variations, and it works better in internal tide correction. This article is protected by copyright. All rights reserved.
      PubDate: 2016-06-14T17:56:01.92579-05:0
      DOI: 10.1002/2016JC011805
       
  • Exploiting coastal altimetry to improve the surface circulation scheme
           over the Central Mediterranean Sea
    • Abstract: This work is the first study exploiting along track altimetry data to observe and monitor coastal ocean features over the transition area between the Western and Eastern Mediterranean Basins. The relative performances of both the AVISO and the X‐TRACK research regional altimetric datasets are compared using in situ observations. Both products are cross‐validated with tide gauge records. The altimeter‐derived geostrophic velocities are also compared with observations from a moored Acoustic Doppler Current Profiler. Results indicate the good potential of satellite altimetry to retrieve dynamic features over the area. However, X‐TRACK shows a more homogenous data coverage than AVISO, with longer time series in the 50 km coastal band. The seasonal evolution of the surface circulation is therefore analyzed by conjointly using X‐TRACK data and remotely sensed sea surface temperature observations. This combined dataset clearly depicts different current regimes and bifurcations, which allows us to propose a new seasonal circulation scheme for the Central Mediterranean. The analysis shows variations of the path and temporal behavior of the main circulation features: the Atlantic Tunisian Current, the Atlantic Ionian Stream, the Atlantic Libyan Current and the Sidra Gyre. The resulting bifurcating veins of these currents are also discussed, and a new current branch is observed for the first time. This article is protected by copyright. All rights reserved.
      PubDate: 2016-06-14T17:55:42.076191-05:
      DOI: 10.1002/2016JC011961
       
  • Kuroshio subsurface water feeds the wintertime Taiwan Warm Current on the
           inner East China Sea shelf
    • Authors: Ergang Lian; Shouye Yang, Hui Wu, Chengfan Yang, Chao Li, James T. Liu
      Abstract: The Taiwan Warm Current (TWC) has an overwhelming influence on the heat, salt and nutrients balance on one of the broadest shelf in the world, the East China Sea shelf. In winter, the TWC flows in an unusual upwind direction and reaches the Changjiang (Yangtze River) Estuary, but its origin and pathway are intensely debated. Here, combined evidences from current measurement, hydrographic and stable isotopic data all suggest that the wintertime TWC intrusion off the Changjiang Estuary mainly originates from the Kuroshio subsurface water northeast of Taiwan, rather than from the Taiwan Strait warm water. The Kuroshio‐branched water northeast of Taiwan can intrude into the inner shelf near the Zhe‐Min Coast via bottom layer, manifesting by a pronounced boundary at 50 m isobath around 28°N, and thereby feeds the TWC intrusion into the Changjiang Estuary. The intrusion complicates the hydrological process in the estuary and shelf sea, and its impact on marine environment deserves more research attentions. This article is protected by copyright. All rights reserved.
      PubDate: 2016-06-14T17:55:32.301215-05:
      DOI: 10.1002/2016JC011869
       
  • Interannual variability of winter‐spring temperature in the Middle
           Atlantic Bight: Relative contributions of atmospheric and oceanic
           processes
    • Abstract: Relative contributions between the local atmospheric and oceanic processes on the interannual variability of winter‐spring shelf temperature in the Middle Atlantic Bight (MAB) are investigated based on a regional ocean model. The model demonstrates sufficient capability to realistically simulate the interannual temperature changes during 2003‐2014. On interannual time scales, the mean winter/spring temperature in the MAB is determined by the combination of the initial temperature at the beginning of the season and the mean cumulative air‐sea flux, while the mean cumulative ocean advective flux plays a secondary role. In spite of the overall importance of air‐sea flux in determining the winter and spring temperature, the relative contributions between air‐sea flux and ocean advective flux on the evolution of the temperature anomaly in each individual year varies. The predictability of spring (April‐June) temperature based on winter (January‐March) temperature is weak because the temporal decorrelation time scale changes significantly from year to year. Both the highly variable shelf temperature and its decorrelation time scale are affected by the changes in the relative contributions between the air‐sea flux and ocean advective flux. This article is protected by copyright. All rights reserved.
      PubDate: 2016-06-13T09:36:55.390086-05:
      DOI: 10.1002/2016JC011646
       
  • Revisiting the Cause of the Eastern Equatorial Atlantic Cold Event in 2009
    • Abstract: An extreme cold sea surface temperature event occurred in the Atlantic cold tongue region in boreal summer 2009. It was preceded by a strong negative Atlantic meridional mode event associated with north‐westerly wind anomalies along the equator from March to May. Although classical equatorial wave dynamics suggest that westerly wind anomalies should be followed by a warming in the eastern equatorial Atlantic, an abrupt cooling took place. In the literature two mechanisms‐meridional advection of subsurface temperature anomalies and planetary wave reflection‐are discussed as potential causes of such an event. Here, for the first time we use in situ measurements in addition to satellite and reanalysis products to investigate the contribution of both mechanisms to the 2009 cold event. Our results suggest that meridional advection is less important in cold events than in corresponding warm events, and, in particular, did not cause the 2009 cold event. Argo float data confirm previous findings that planetary wave reflection contributed to the onset of the 2009 cold event. Additionally, our analysis suggests that higher baroclinic modes were involved. This article is protected by copyright. All rights reserved.
      PubDate: 2016-06-13T03:25:43.030623-05:
      DOI: 10.1002/2016JC011719
       
  • Basal melt, seasonal water mass transformation, ocean current variability,
           and deep convection processes along the Amery Ice Shelf calving front,
           East Antarctica
    • Abstract: Despite the Amery Ice Shelf (AIS) being the third largest ice shelf in Antarctica, the seasonal variability of the physical processes involved in the AIS‐ocean interaction remains undocumented and a robust observational, oceanographic‐based basal melt rate estimate has been lacking. Here, we use year‐long time series of water column temperature, salinity and horizontal velocities measured along the ice shelf front from 2001 to 2002. Our results show strong zonal variations in the distribution of water masses along the ice shelf front: modified Circumpolar Deep Water (mCDW) arrives in the east, while in the west, Ice Shelf Water (ISW) and Dense Shelf Water (DSW) formed in the Mackenzie polynya dominate the water column. Baroclinic eddies, formed during winter deep convection (down to 1100 m), drive the inflow of DSW into the ice shelf cavity. Our net basal melt rate estimate is 57.4 ± 25.3 Gt year−1 (1 ± 0.4 m year−1), larger than previous modeling‐ and glaciological‐based estimates, and results from the inflow of DSW (0.52 ± 0.38 Sv; 1 Sv = 106 m3 s−1) and mCDW (0.22 ± 0.06 Sv) into the cavity. Our results highlight the role of the Mackenzie polynya in the seasonal exchange of water masses across the ice shelf front, and the role of the ISW in controlling the formation rate and thermohaline properties of DSW. These two processes directly impact on the ice shelf mass balance, and on the contribution of DSW/ISW to the formation of Antarctic Bottom Water. This article is protected by copyright. All rights reserved.
      PubDate: 2016-06-13T03:25:38.35386-05:0
      DOI: 10.1002/2016JC011858
       
  • Impact of current speed on mass flux to a model flexible seagrass blade
    • Authors: Jiarui Lei; Heidi Nepf
      Abstract: Seagrass and other freshwater macrophytes can acquire nutrients from surrounding water through their blades. This flux may depend on the current speed (U), which can influence both the posture of flexible blades (reconfiguration) and the thickness of the flux‐limiting diffusive layer. The impact of current speed (U) on mass flux to flexible blades of model seagrass was studied through a combination of laboratory flume experiments, numerical modeling and theory. Model seagrass blades were constructed from low‐density polyethylene (LDPE), and 1, 2‐dichlorobenzene was used as a tracer chemical. The tracer mass accumulation in the blades was measured at different unidirectional current speeds. A numerical model was used to estimate the transfer velocity (K) by fitting the measured mass uptake to a one‐dimensional diffusion model. The measured transfer velocity was compared to predictions based on laminar and turbulent boundary layers developing over a flat plate parallel to flow, for which K ∝ U0.5 and ∝U, respectively. The degree of blade reconfiguration depended on the dimensionless Cauchy number, Ca, which is a function of both the blade stiffness and flow velocity. For large Ca, the majority of the blade was parallel to the flow, and the measured transfer velocity agreed with laminar boundary layer theory, K ∝ U0.5. For small Ca, the model blades remained upright, and the flux to the blade was diminished relative to the flat‐plate model. A meadow‐scale analysis suggests that the mass exchange at the blade scale may control the uptake at the meadow scale. This article is protected by copyright. All rights reserved.
      PubDate: 2016-06-13T03:20:34.54638-05:0
      DOI: 10.1002/2016JC011826
       
  • Regional influence of climate patterns on the wave climate of the
           southwestern Pacific: The New Zealand region
    • Authors: Victor A. Godoi; Karin R. Bryan, Richard M. Gorman
      Abstract: This work investigates how the wave climate around New Zealand and the southwest Pacific is modulated by the Pacific Decadal Oscillation (PDO), El Niño‐Southern Oscillation (ENSO), Indian Ocean Dipole (IOD), Zonal Wave‐number‐3 Pattern (ZW3), and Southern Annular Mode (SAM) during the period 1958–2001. Their respective climate indices were correlated with modeled mean wave parameters extracted from a 45 year (1957–2002) wave hindcast carried out with the WAVEWATCH III model using the wind and ice fields from the ERA‐40 reanalysis project. The correlation was performed using the Pearson's correlation coefficient and the wavelet spectral analysis. Prior to that, mean annual and interannual variabilities and trends in significant wave height (Hs) were computed over 44 years (1958–2001). In general, higher annual and interannual variabilities were found along the coastline, in regions dominated by local winds. An increasing trend in Hs was found around the country, with values varying between 1 and 6 cm/decade at the shoreline. The greatest Hs trends were identified to the south of 48°S, suggesting a relationship with the positive trend in the SAM. Seasonal to decadal time scales of the SAM strongly influenced wave parameters throughout the period analyzed. In addition, larger waves were observed during extreme ENSO and IOD events at interannual time scale, while they were more evident at seasonal and intraseasonal time scales in the correlations with the ZW3. Negative phases of the ZW3 and ENSO and positive phases of the IOD, PDO, and SAM resulted in larger waves around most parts of New Zealand.
      PubDate: 2016-06-12T08:55:45.193909-05:
      DOI: 10.1002/2015JC011572
       
  • Regulation of South China Sea throughflow by pressure difference
    • Authors: Huiling Qin; Rui Xin Huang, Weiqiang Wang, Huijie Xue
      Abstract: Sea Surface Height (SSH) data from the European Centre for Medium‐Range Weather Forecasts‐Ocean Reanalysis System 4 (ECMWF‐ORAS4) are used to determine the pressure difference in connection with variability of the South China Sea ThroughFlow (SCSTF) from 1958 to 2007. Two branches of SCSTF, the Karimata‐Sunda Strait ThroughFlow (KSSTF) and the Mindoro Strait ThroughFlow (MSTF), are examined. Using the ensemble empirical mode decomposition method (EEMD), time series of pressure difference and volume transport are decomposed into intrinsic mode functions and trend functions, with the corresponding variability on different time scales. Pressure difference agrees with the KSSTF volume transport on decadal time scale; while for the MSTF, pressure difference varies similarly with volume transport on interannual time scale. Separating the dynamic height difference into the thermal and haline terms, for the KSSTF more than half of the dynamic height difference (32 cm) is due to the thermal contributions; while the remaining dynamic height difference (23 cm) is due to the haline contributions. For the MSTF, the dynamic height difference (29 cm) is primarily due to the thermal contribution (26 cm).
      PubDate: 2016-06-12T08:55:32.333646-05:
      DOI: 10.1002/2015JC011177
       
  • Light beam attenuation and backscattering properties of particles in the
           Bohai Sea and Yellow Sea with relation to biogeochemical properties
    • Authors: Shengqiang Wang; Zhongfeng Qiu, Deyong Sun, Xiaojing Shen, Hailong Zhang
      Abstract: This study reports the first results of the variability in light beam attenuation and the backscattering properties of particles and their controlling factors during the summer in the Bohai Sea (BS) and Yellow Sea (YS), which are two typical shallow and semienclosed seas. We observe large variations in the particulate beam attenuation (cp) and backscattering coefficients (bbp); such variations are mainly attributed to changes in the total suspended matter, while the cross‐sectional area concentration shows tighter relationships with both cp and bbp. The mass‐specific beam attenuation ( cp*) and backscattering coefficients ( bbp*) vary more widely over about two orders of magnitude. The attenuation (Qce) and backscattering efficiencies (Qbbe) are important factors that control cp* and bbp*, which clearly separate all the samples into two types. Type 1 samples show low Qce and Qbbe and contain relatively high proportions of organic or large particles, while type 2 samples have high Qce and Qbbe and mainly contain relatively small mineral particles. The majority of the variability in cp* and bbp* within each type is related to the inverse of the product of particle apparent density (ρa) and mean diameter (DA); ρa plays a major role, while DA exerts only a slight impact. Overall, this study provides general knowledge of particulate beam attenuation and the backscattering properties in the BS and YS, which may improve our understanding of underwater radiative transfer processes, marine biogeochemical processes and ocean color algorithms.
      PubDate: 2016-06-10T02:35:39.821742-05:
      DOI: 10.1002/2016JC011727
       
  • Malvinas Current variability from Argo floats and satellite altimetry
    • Abstract: The Malvinas Current (MC) is an offshoot of the Antarctic Circumpolar Current (ACC). Downstream of Drake Passage, the northern fronts of the ACC veer northward, cross over the North Scotia Ridge (NSR) and the Malvinas Plateau and enter the Argentine Basin. We investigate the variations of the MC circulation between the NSR and 41°S and their possible relations with the ACC circulation using data from Argo floats and satellite altimetry. The data depict meandering and eddy‐shedding of the northern ACC jets as they cross the NSR. The altimetry fields show that these eddies are trapped, break down and dissipate over the Malvinas Plateau, suggesting that this region is a hot spot for dissipation of mesoscale variability. Variations of sea level anomalies (SLA) across the NSR do not impact the MC further north, except for intra‐seasonal variability associated with coastal trapped waves. Altimetry and float trajectories show events during which a large fraction of the MC is cut off from the ACC. Blocking events at around 48.5°S are a recurrent feature of the MC circulation. Over the 23 year altimetry record, we detected 26 events during which the MC surface transport at 48.5°S was reduced to less than half its long term mean. Blocking events last from 10 to 35 days and do not present any significant trend. These events were tracked back to positive SLA that built up over the Argentine Abyssal Plain. Future work is needed to understand the processes responsible for these blocking events. This article is protected by copyright. All rights reserved.
      PubDate: 2016-06-09T18:42:49.291438-05:
      DOI: 10.1002/2016JC011889
       
  • Interannual variability in net community production at the Western
           Antarctic Peninsula region (1997‐2014)
    • Authors: Zuchuan Li; Nicolas Cassar, Kuan Huang, Hugh Ducklow, Oscar Schofield
      Abstract: In this study, we examined the interannual variability of Net Community Production (NCP) in the Western Antarctic Peninsula (WAP) using in situ O2/Ar‐NCP estimates (2008‐2014) and satellite data (SeaWiFS and MODIS‐Aqua) from 1997 to 2014. We found that NCP generally first peaks offshore and follows sea‐ice retreat from offshore to inshore. Annually‐integrated NCP (ANCP) displays an onshore to offshore gradient, with coastal and shelf regions up to eight times more productive than offshore regions. We examined potential drivers of interannual variability in the ANCP using an Empirical Orthogonal Function (EOF) analysis. The EOF's first mode explains ∼50% of the variance, with high interannual variability observed seaward of the shelf break. The first principal component is significantly correlated with the day of sea‐ice retreat (R=‐0.58, p
      PubDate: 2016-06-09T18:42:46.011865-05:
      DOI: 10.1002/2015JC011378
       
  • Quantifying uncertainty in Gulf of Mexico forecasts stemming from
           uncertain initial conditions
    • Abstract: Polynomial Chaos (PC) methods are used to quantify initial conditions uncertainties in oceanic forecasts of the Gulf of Mexico circulation. Empirical Orthogonal Functions are used as initial conditions perturbations with their modal amplitudes considered as uniformly distributed uncertain random variables. These perturbations impact primarily the Loop Current system and several frontal eddies located in its vicinity. A small ensemble is used to sample the space of the modal amplitudes and to construct a surrogate for the evolution of the model predictions via a non‐intrusive Galerkin projection. The analysis of the surrogate yields verification measures for the surrogate's reliability and statistical information for the model output. A variance analysis indicates that the sea surface height predictability in the vicinity of the Loop Current is limited to about 20 days. This article is protected by copyright. All rights reserved.
      PubDate: 2016-06-09T04:42:30.689122-05:
      DOI: 10.1002/2015JC011573
       
  • Tropical and extratropical‐origin storm wave types and their
           influence on the East Australian longshore sand transport system under a
           changing climate
    • Authors: Ian D. Goodwin; Thomas R. Mortlock, Stuart Browning
      Abstract: Tropical expansion is potentially an amplifier of coastal change in the subtropics, through directional wave climate shifts. The storm wave climate and directional wave power distribution along the Southeast Australian Shelf (SEAS) is investigated with respect to tropical extent. Forty years of storm wave observations from nine mid‐shelf wave buoys are evaluated using synoptic storm wave typing. A robust latitudinal and along‐shelf gradient in storm wave types and wave propagation patterns exists. The tropical origin storms produce a shore‐normal propagation pattern along the SEAS, reduce the connectivity of coastal compartments through minor headland bypassing events or episodically reversing the net northward transport. In contrast, the extratropical origin storms produce a shore‐oblique propagation pattern from the Southern Tasman to the Coral Sea, and are an important control on the connectivity of regional longshore sand transport through episodic major headland bypassing events between compartments, and the maintenance of down‐drift coastlines in dynamic equilibrium. Future climate change projections indicate that the recent trend in the expansion of the latitudinal extent of the tropics in the south‐west Pacific region will continue throughout this century. The combined impacts of a projected 2.5° poleward shift on the storm wave climate is a significant reduction in net northward longshore sand transport and the efficiency of headland bypassing events. On the North and Central Coasts of New South Wales we project a ∼30% reduction in longshore sand transport for the dominant extratropical‐origin storm events, together with a ∼5% increase in reversed (net southward) longshore sand transport for tropical‐origin storm events. This article is protected by copyright. All rights reserved.
      PubDate: 2016-06-09T04:35:01.964305-05:
      DOI: 10.1002/2016JC011769
       
  • Wintertime water dynamics and moonlight disruption of the acoustic
           backscatter diurnal signal in an ice‐covered Northeast Greenland
           fjord
    • Abstract: Six and a half month records from three ice‐tethered Acoustic Doppler Current Profilers deployed in October 2013 in Young Sound fjord in Northeast Greenland are used to analyze the acoustic backscatter signal. The acoustic data suggest a systematic diel vertical migration (DVM) of scatters below the land‐fast ice during polar night. The scatters were likely comprised of zooplankton. The acoustic signal pattern typical to DVM persisted in Young Sound throughout the entire winter including the period of civil polar night. However, polynya‐enhanced estuarine‐like cell circulation that occurred during winter disrupted the DVM signal favoring zooplankton to occupy the near‐surface water layer. This suggests that zooplankton avoided spending additional energy crossing the interface with a relatively strong velocity gradient comprised by fjord inflow in the intermediate layer and outflow in the subsurface layer. Instead, the zooplankton tended to remain in the upper 40 m layer where relatively warmer water temperatures associated with upward heat flux during enhanced estuarine‐like circulation could be energetically favorable. Furthermore, our data show moonlight disruption of DVM in the subsurface layer and weaker intensity of vertical migration beneath snow covered land‐fast ice during polar night. Finally, by using existing models for lunar illuminance and light transmission through sea ice and snow cover, we estimated under ice illuminance and compared it with known light sensitivity of Arctic zooplankton species. This article is protected by copyright. All rights reserved.
      PubDate: 2016-06-09T04:32:01.105542-05:
      DOI: 10.1002/2016JC011703
       
  • Numerical modeling of intrinsically and extrinsically forced seasonal
           circulation in the China Seas: A kinematic study
    • Authors: Jianping Gan; Zhiqiang Liu, Linlin Liang
      Abstract: We developed a new three‐dimensional, high‐resolution ocean circulation model for the entire China Seas (CS) region. The model considered the linked physics associated with the western boundary current, monsoonal wind and tidal forcings, and topography in both the CS and the adjacent oceans. From this well validated model, we derived new insights into the three‐dimensional seasonal circulation of the CS in response to the intrinsic forcing of monsoonal winds and extrinsic forcing of flow exchange with adjacent oceans through the straits and over the slope around the periphery of the CS. Besides the East Asian monsoon forcing, we found that the extrinsic forcings interact coherently with each other and with the interior circulation to jointly shape the CS circulation. Specifically, we revealed rotating layered circulation in the CS. The circulation in the South China Sea has a vertical cyclonic‐anticyclonic‐cyclonic pattern in the upper‐middle‐lower layers, which we relate to the inflow‐outflow‐inflow transport in those layers in the Luzon Strait. The circulation in the East China Sea (ECS) is characterized by a vertically variable cyclonically rotating flow, and the circulation in the Yellow Sea (YS) is represented by a cyclonic movement in the upper layer and an anticyclonic movement in the lower layer. We attribute the cross‐shelf variation of the along‐shelf current to the ECS circulation pattern, while the vertically variable intrusive current at the central trough, together with the seasonally varied west and east coastal currents, shape the two‐layer circulation in the YS. This article is protected by copyright. All rights reserved.
      PubDate: 2016-06-09T04:24:55.545561-05:
      DOI: 10.1002/2016JC011800
       
  • The eddy kinetic energy budget in the Red Sea
    • Authors: Peng Zhan; Aneesh C. Subramanian, Fengchao Yao, Aditya R. Kartadikaria, Daquan Guo, Ibrahim Hoteit
      Abstract: The budget of eddy kinetic energy (EKE) in the Red Sea, including the sources, redistributions and sink, is examined using a high‐resolution eddy‐resolving ocean circulation model. A pronounced seasonally varying EKE is identified, with its maximum intensity occurring in winter, and the strongest EKE is captured mainly in the central and northern basins within the upper 200 m. Eddies acquire kinetic energy from conversion of eddy available potential energy (EPE), from transfer of mean kinetic energy (MKE), and from direct generation due to time‐varying (turbulent) wind stress, the first of which contributes predominantly to the majority of the EKE. The EPE‐to‐EKE conversion occurs almost in the entire basin, while the MKE‐to‐EKE transfer appears mainly along the shelf boundary of the basin (200 miso‐bath) where high horizontal shear interacts with topography. The EKE generated by the turbulent wind stress is relatively small and limited to the southern basin. All these processes are intensified during winter, when the rate of energy conversion is about four to five times larger than that in summer. The EKE is redistributed by the vertical and horizontal divergence of energy flux and the advection of the mean flow. As a main sink of EKE, dissipation processes is ubiquitously found in the basin. The seasonal variability of these energy conversion terms can explain the significant seasonality of eddy activities in the Red Sea. This article is protected by copyright. All rights reserved.
      PubDate: 2016-06-09T00:35:26.257736-05:
      DOI: 10.1002/2015JC011589
       
  • Seasonal and interannual variations of mixed layer salinity in the
           southeast tropical Indian Ocean
    • Authors: Ningning Zhang; Ming Feng, Yan Du, Jian Lan, Susan E. Wijffels
      Abstract: In this study, seasonal and interannual variations of the mixed layer salinity (MLS) in the southeast tropical Indian Ocean (SETIO) are analyzed using satellite observations, historical datasets and data‐assimilating ocean model outputs. On the seasonal cycle, the MLS in the SETIO becomes fresher in austral winter and saltier in austral summer: between the Java‐Lesser Sunda coast and the South Equatorial Current (SEC, 12°S), where positive entrainment and fresh advections counterbalance each other, the annual cycle of the MLS closely follows the variation of the air‐sea freshwater forcing; off the northwest and west Australian coasts, the MLS variations are influenced by the annual cycles of the Indonesian throughflow (ITF) and Leeuwin Current (LC) transports as well as the air‐sea freshwater forcing, with eddy fluxes acting to freshen the MLS along the SEC, the Eastern Gyral Current and the LC. On the interannual scale, El Niño (La Niña) events are typically associated with saltier (fresher) MLS in the SETIO. Composite and budget analyses reveal that interannual variations in precipitations drive the MLS anomalies off the Java‐Lesser Sunda coast; between 12°S and the northwest Australian coast, the MLS variations are influenced by both advection anomalies and local precipitation anomalies; whereas anomalous meridional currents contribute to the MLS variations off the west Australian coast. Both enhanced local precipitations and the ITF transport anomalies have substantial contributions to the drastic freshening of the Indonesian‐Australian Basin between the Java‐Lesser Sunda coast and the northwest Australian coast during the extended La Niña events in 1999‐2001 and 2010‐2012. This article is protected by copyright. All rights reserved.
      PubDate: 2016-06-08T10:51:27.013412-05:
      DOI: 10.1002/2016JC011854
       
  • Potential new production in two upwelling regions of the western Arabian
           Sea: Estimation and comparison
    • Authors: Xiaomei Liao; Haigang Zhan, Yan Du
      Abstract: Using satellite‐derived and in‐situ data, the wind‐driven potential new production (nitrate supply) for the 300‐km‐wide coastal band in two upwelling regions of the western Arabian Sea (AS) during the southwest monsoon is estimated. The upward nitrate flux to the euphotic zone is generally based on the physical processes of coastal transport (Ekman transport and geostrophic transport) and offshore Ekman pumping. The coastal geostrophic current in the western AS influences the upwelling intensity and latitudinal distributions of nitrate supply. The Oman and Somalia upwelling regions have similar level of potential new production (nitrate supply) during the summer monsoon, while the satellite estimates of primary production off Oman are two times greater than those off Somalia. The much higher potential f‐ratio in the Somalia upwelling region indicates that the primary production could be limited by availability of other macronutrients (e.g., silicate). The correlation analysis of the primary production and the aerosol optical thickness shows that the Oman upwelling region displays a stronger coupling between the atmospheric deposition and the phytoplankton abundance. The high summertime dust levels in the atmosphere are suggested to contribute to the high primary production in the Oman upwelling region. This article is protected by copyright. All rights reserved.
      PubDate: 2016-06-06T19:21:12.016173-05:
      DOI: 10.1002/2016JC011707
       
  • Climatology and linear trends of seasonal water temperature and heat
           budget in a semienclosed sea connected to the Kuroshio region
    • Authors: Eisuke Tsutsumi; Xinyu Guo
      Abstract: The climatology and linear trend of seasonal water temperature and heat budget in a semi‐enclosed sea connected to the Kuroshio region (Seto Inland Sea: SIS) are investigated by constructing and analyzing a gridded dataset of water temperature, salinity and air‐sea heat fluxes. In the SIS, winter‐time water temperature and ocean heat content (OHC) showed a significant increasing trend while those in summer‐time exhibited no significant change. By analyzing the heat budget between the tendency of ocean heat content ∂OHC/∂t and air‐sea net heat flux Qnet, the driving factor of the winter‐time warming trend in the SIS was identified as autumn Qnet, while the unchanged summer‐time OHC is mainly due to decreased spring‐time oceanic heat transport to the SIS from the Kuroshio region. In addition, we showed that the decreased spring‐time heat transport could have been induced by upwelling due to wind curl and wind speed in the shelf‐slope region, heat transport by the Kuroshio south of Shikoku Island, and freshwater input to the SIS from rivers. The importance of both oceanic and atmospheric forcing mechanisms for explaining variation in the water temperature in coastal areas is demonstrated. This article is protected by copyright. All rights reserved.
      PubDate: 2016-06-06T19:21:09.668175-05:
      DOI: 10.1002/2016JC011748
       
  • Swell impact on wind stress and atmospheric mixing in a regional coupled
           atmosphere‐wave model
    • Abstract: Over the ocean, the atmospheric turbulence can be significantly affected by swell waves. Change in the atmospheric turbulence affects the wind stress and atmospheric mixing over swell waves. In this study, the influence of swell on atmospheric mixing and wind stress are introduced into an atmosphere‐wave coupled regional climate model, separately and combined. The swell influence on atmospheric mixing is introduced into the atmospheric mixing length formula by adding a swell‐induced contribution to the mixing. The swell influence on the wind stress under wind‐following swell, moderate‐range wind, and near‐neutral and unstable stratification conditions is introduced by changing the roughness length. Five‐year simulation results indicate that adding the swell influence on atmospheric mixing has limited influence, only slightly increasing the near‐surface wind speed; in contrast, adding the swell influence on wind stress reduces the near‐surface wind speed. Introducing the wave influence roughness length has a larger influence than does adding the swell influence on mixing. Compared with measurements, adding the swell influence on both atmospheric mixing and wind stress gives the best model performance for the wind speed. The influence varies with wave characteristics for different sea basins. Swell occurs infrequently in the studied area, and one could expect more influence in high‐swell‐frequency areas (i.e., low‐latitude ocean). We conclude that the influence of swell on atmospheric mixing and wind stress should be considered when developing climate models. This article is protected by copyright. All rights reserved.
      PubDate: 2016-06-06T10:30:47.650292-05:
      DOI: 10.1002/2015JC011576
       
  • Turbulent plumes from a glacier terminus melting in a stratified ocean
    • Authors: Samuel J. Magorrian; Andrew J. Wells
      Abstract: The melting of submerged faces of marine‐terminating glaciers is a key contributor to the glacial mass budget via direct thermodynamic ablation and the impact of ablation on calving. This study considers the behaviour of turbulent plumes of buoyant meltwater in a stratified ocean, generated by melting of either near‐vertical calving faces or sloping ice shelves. We build insight by applying a turbulent plume model to describe melting of a locally planar region of ice face in a linearly stratified ocean, in a regime where subglacial discharge is insignificant. The plumes rise until becoming neutrally buoyant, before intruding into the ocean background. For strong stratifications, we obtain leading‐order scaling laws for the flow including the height reached by the plume before intrusion, and the melt rate, expressed in terms of the background ocean temperature and salinity stratifications. These scaling laws provide a new perspective for parameterising glacial melting in response to a piecewise‐linear discretisation of the ocean stratification. This article is protected by copyright. All rights reserved.
      PubDate: 2016-06-06T10:30:43.010198-05:
      DOI: 10.1002/2015JC011160
       
  • Pathways and mechanisms of offshore water intrusions on the Espírito
           Santo Basin shelf (18°‐22°S, Brazil)
    • Abstract: The pathways and physical mechanisms associated with intrusions of cold, nutrient–rich South Atlantic Central Water (SACW) on the continental shelf of the Espírito Santo Basin (ESB), off southeast Brazil (18°S‐22°S), are investigated. To this end, a set of process–oriented, Primitive–Equation (PE) numerical models are used, together with an independent and more complete PE model, available observations and simple theoretical ideas. SACW enters the model ESB shelf mostly through two preferential pathways along the Tubarão Bight (TB, 19.5°S‐22°S). These pathways are found to be locations where an equatorward along–isobath pressure gradient force (PGFy*) of O(10−6m s−2) develops in response to steady wind forcing. This equatorward PGFy* is essentially in geostrophic balance, inducing onshore flow across the shelf edge and most of the shelf proper. The Brazil Current (BC) imparts an additional periodic (in the along–shelf direction) PGFy* on the shelf. The intrinsic pycnocline uplifting effect of the BC in making colder water available at the shelf edge is quantified. The BC also induces local intrusions by inertially overshooting the shelf edge, consistent with estimated Rossby numbers of ≈0.3–0.5. In addition, the planetary β–effect is related to a background equatorward PGFy*. A modified Arrested Topographic Wave model is shown to be a plausible rationalization for the shelf–wide spreading of the pressure field imparted by the BC at the shelf edge. The deep–ocean processes examined here are found to enhance the onshore transport of SACW, while wind forcing is found to dominate it at leading order. This article is protected by copyright. All rights reserved.
      PubDate: 2016-06-06T10:25:42.082206-05:
      DOI: 10.1002/2015JC011468
       
  • Time series of temperature in Fram Strait determined from the
           2008‐2009 DAMOCLES acoustic tomography measurements and an ocean
           model
    • Abstract: A pilot acoustic tomography program in Fram Strait during 2008‐2009 measured a year‐long record of acoustic travel times along a 130km range acoustic path crossing the West Spitsbergen Current. Individual ray arrivals were not observed. Rather, the arrival patterns consisted of a single, stable, broad arrival pulse of about 100 ms duration. Travel time variations of ±0.15 s recorded the vigorous mesoscale environment of the region and the seasonal cycle. To estimate ocean temperature from the tomography data an inverse scheme employed a high‐resolution ocean model for Fram Strait as the reference ocean. The information from the tomographic measurements is primarily average temperature. Estimated temperatures, averaged over 0{1000 m depth and over range, had a mean of 1.11°C and variations of ±0.33°C; the uncertainty of the tomography estimates was about 60m°C. Agreement with an alternate inverse approach based on EOFs and a Markov Chain Monte Carlo inversion scheme relying on a matched‐peak approach was excellent, indicating a robust estimate for ocean temperature. The inverse estimates for average temperature agreed with the equivalent estimates from hydrographic sections obtained along the acoustic path at the start and end of the program. Among other deficiencies, the ocean model greatly underestimated the intensity of the mesoscale fluctuations and exhibited a warm bias of about 0.38°C in section‐averaged temperature. Tomographic measurements in Fram Strait offer unique large‐scale temperature constraints for ocean models through data assimilation. It is anticipated that these constraints will lead to more accurate estimates of the circulation and transports in Fram Strait. This article is protected by copyright. All rights reserved.
      PubDate: 2016-06-03T10:35:37.552553-05:
      DOI: 10.1002/2015JC011591
       
  • Changes in anthropogenic carbon storage in the Northeast Pacific in the
           last decade
    • Authors: Sophie N. Chu; Zhaohui Aleck Wang, Scott C. Doney, Gareth L. Lawson, Katherine A. Hoering
      Abstract: In order to understand the ocean's role as a sink for anthropogenic carbon dioxide (CO2), it is important to quantify changes in the amount of anthropogenic CO2 stored in the ocean interior over time. From August‐September 2012, an ocean acidification cruise was conducted along a portion of the P17N transect (50°N 150°W to 33.5°N 135°W) in the Northeast Pacific. These measurements are compared with data from the previous occupation of this transect in 2001 to estimate the change in the anthropogenic CO2 inventory in the Northeast Pacific using an extended multiple linear regression (eMLR) approach. Maximum increases in the surface waters were 11 μmol kg−1 over 11 years near 50°N. Here, the penetration depth of anthropogenic CO2 only reached ∼300 m depth, whereas at 33.5°N, penetration depth reached ∼600 m. The average increase of the depth‐integrated anthropogenic carbon inventory was 0.41 ± 0.12 mol m−2 yr−1 across the transect. Lower values down to 0.20 mol m−2 yr−1 were observed in the northern part of the transect near 50°N and increased up to 0.55 mol m−2 yr−1 towards 33.5°N. This increase in anthropogenic carbon in the upper ocean resulted in an average pH decrease of 0.002 ± 0.0003 pH units yr−1 and a 1.8 ± 0.4 m yr−1 shoaling rate of the aragonite saturation horizon. An average increase in apparent oxygen utilization of 13.4 ± 15.5 μmol kg−1 centered on isopycnal surface 26.6 kg m−3 from 2001 to 2012 was also observed. This article is protected by copyright. All rights reserved.
      PubDate: 2016-06-03T10:35:25.237232-05:
      DOI: 10.1002/2016JC011775
       
  • Saw‐tooth modulation of the deep water thermohaline properties in
           the southern Adriatic Sea
    • Abstract: In this study, we investigate the dynamics of the bottom layer of the southern Adriatic Sea (eastern Mediterranean basin) by merging experimental measurements and numerical simulations. We hypothesize that the recently observed continuous density decrease over time, which was basically related to a temperature increase, and the following sudden density rise, which was caused by the intrusion of very dense water masses (cold but relatively fresh), constitute one cycle of a general “saw‐tooth” pattern: the alternation of long‐lasting and almost linear density decreases (mixing phases) and sudden density increases (dense water intrusion phases). The model results, which provide a basin‐scale view of the process, corroborate this theory because they satisfactorily reproduced the observed oceanographic features. We describe the almost linear density decrease in terms of local mixing fostered by the advection of flow instabilities that originate from the large‐scale quasi‐permanent cyclonic circulation. Conversely, diffusive processes play a minor role in determining the bottom layer thermohaline variability. The interpretation of the experimental findings, supported by the numerical simulations, suggests that similar dynamics might be observed in other basins characterized by similar bathymetric and hydrodynamic features. This article is protected by copyright. All rights reserved.
      PubDate: 2016-06-02T10:32:47.168338-05:
      DOI: 10.1002/2015JC011522
       
  • Arctic Ocean stability: The effects of local cooling, oceanic heat
           transport, freshwater input, and sea ice melt with special emphasis on the
           Nansen Basin
    • Authors: Bert Rudels
      Abstract: The Arctic loses energy to space and heat is transported northward in the atmosphere and ocean. The largest transport occurs in the atmosphere. The oceanic heat flux is significantly smaller, and the warm water that enters the Arctic Ocean becomes covered by a low salinity surface layer, which reduces the heat transfer to the sea surface. This upper layer has two distinct regimes. In most of the deep basins it is due to the input of low salinity shelf water, ultimately conditioned by net precipitation and river runoff. The Nansen Basin is different. Here warm Atlantic water is initially in direct contact with and melts sea ice, its upper part being transformed into less dense surface water. The characteristics and depth of this layer are determined as functions of the temperature of the Atlantic water and for different energy losses using a one‐dimensional energy balance model. The amount of transformed Atlantic water is estimated for two different sea ice melt rates and the assumption of a buoyant boundary outflow. To create the upper layer sea ice formed elsewhere has to drift to the Nansen Basin. With reduced ice cover this ice drift might weaken and the ice could disappear by the end of winter. The surface buoyancy input would disappear, and the upper layer might eventually convect back into the Atlantic water, reducing the formation of less dense Polar water. The created ice free areas would release more heat to the atmosphere and affect the atmospheric circulation. This article is protected by copyright. All rights reserved.
      PubDate: 2016-06-02T10:28:04.937074-05:
      DOI: 10.1002/2015JC011045
       
  • Studies of the Canadian Arctic Archipelago water transport and its
           relationship to basin‐local forcings: Results from AO‐FVCOM
    • Authors: Yu Zhang; Changsheng Chen, Robert C. Beardsley, Guoping Gao, Zhigang Lai, Beth Curry, Craig M. Lee, Huichan Lin, Jianhua Qi, Qichun Xu
      Abstract: A high‐resolution (up to 2 km), unstructured‐grid, fully coupled Arctic sea ice‐ocean Finite‐Volume Community Ocean Model (AO‐FVCOM) was employed to simulate the flow and transport through the Canadian Arctic Archipelago (CAA) over the period 1978‐2013. The model‐simulated CAA outflow flux was in reasonable agreement with the flux estimated based on measurements across Davis Strait, Nares Strait, Lancaster Sound and Jones Sounds. The model was capable of re‐producing the observed interannual variability in Davis Strait and Lancaster Sound. The simulated CAA outflow transport was highly correlated with the along‐strait and cross‐strait sea surface height (SSH) difference. Compared with the wind forcing, the sea level pressure (SLP) played a dominant role in establishing the SSH difference and the correlation of the CAA outflow with the cross‐strait SSH difference can be explained by a simple geostrophic balance. The change in the simulated CAA outflow transport through Davis Strait showed a negative correlation with the net flux through Fram Strait. This correlation was related to the variation of the spatial distribution and intensity of the slope current over the Beaufort Sea and Greenland shelves. The different basin‐scale surface forcings can increase the model uncertainty in the CAA outflow flux up to 15%. The daily adjustment of the model elevation to the satellite‐derived SSH in the North Atlantic region outside Fram Strait could produce a larger North Atlantic inflow through west Svalbard and weaken the outflow from the Arctic Ocean through east Greenland. This article is protected by copyright. All rights reserved.
      PubDate: 2016-06-02T10:27:36.799776-05:
      DOI: 10.1002/2016JC011634
       
  • Atmospheric forcing of the upper ocean transport in the Gulf of Mexico:
           From seasonal to diurnal scales
    • Authors: Falko Judt; Shuyi S. Chen, Milan Curcic
      Abstract: The 2010 Deepwater Horizon oil spill in the Gulf of Mexico (GoM) was an environmental disaster that highlighted the urgent need to predict the transport and dispersion of hydrocarbon. Although the variability of the atmospheric forcing plays a major role in the upper ocean circulation and transport of the pollutants, the air‐sea interaction on various time scales is not well understood. This study provides a comprehensive overview of the atmospheric forcing and upper ocean response in the GoM from seasonal to diurnal time scales, using climatologies derived from long‐term observations, in situ observations from two field campaigns, and a coupled model. The atmospheric forcing in the GoM is characterized by a striking seasonality. In the summer, the time‐average large‐scale forcing is weak, despite of occasional extreme winds associated with hurricanes. In the winter, the atmospheric forcing is much stronger, and dominated by synoptic variability on time scales of 3‐7 days associated with winter storms and cold air outbreaks. The diurnal cycle is more pronounced during the summer, when sea breeze circulations affect the coastal regions and nighttime wind maxima occur over the offshore waters. Realtime predictions from a high‐resolution atmosphere‐wave‐ocean coupled model were evaluated for both summer and winter conditions during the Grand LAgrangian Deployment (GLAD) in July‐August 2012 and the Surfzone Coastal Oil Pathways Experiment (SCOPE) in November‐December 2013. The model generally captured the variability of atmospheric forcing on all scales, but suffered from some systematic errors. This article is protected by copyright. All rights reserved.
      PubDate: 2016-06-02T10:27:30.892283-05:
      DOI: 10.1002/2015JC011555
       
  • On the long‐term stability of the Lofoten Basin Eddy
    • Abstract: In recent years several studies have identified an area of intense anticyclonic activity about 500 km straight west of the Lofoten Islands at 70°N in the northern Norwegian Sea. Now recognized as the coherent Lofoten Basin Eddy (LBE), it is maintained by a supply of anticyclonic eddies that break away from the Norwegian Atlantic Current. Here we show from ship‐based surveys of its velocity field that it is quite stable with a central core in solid body rotation ∼1000 m deep, ∼8 km radius and a relative vorticity close to its theoretical limit –f. The surveys also show the LBE typically has a >60 km radius with maximum swirl velocities at 17‐20 km radius. From the velocity field we estimate the dynamic height amplitude at the surface to be about ∼0.21±0.03 dyn. m. Second, altimetry from the last 20 years shows the extremum in sea surface height relative to the surrounding waters to be about the same, 0.2 dyn. m. Third, a float trapped in the LBE for many months reveals a clear cyclonic wandering of the eddy over the deepest parts of the basin. Lastly, three hydrographic sections from the 1960s show the dynamic height signal to be virtually the same then as it is now. From these observations we conclude that the LBE is a permanent feature of the Nordic Seas and plays a central role in maintaining the pool of warm water in the western Lofoten Basin. This article is protected by copyright. All rights reserved.
      PubDate: 2016-06-02T10:27:29.029712-05:
      DOI: 10.1002/2016JC011726
       
  • Mesoscale resolution capability of altimetry: Present and future
    • Abstract: Wavenumber spectra of along‐track Sea Surface Height from the most recent satellite radar altimetry missions (Jason‐2, Cryosat‐2 and SARAL/Altika) are used to determine the size of ocean dynamical features observable with the present altimetry constellation. A global analysis of the along‐track 1D mesoscale resolution capability of the present‐day altimeter missions is proposed, based on a joint analysis of the spectral slopes in the mesoscale band and the error levels observed for horizontal wavelengths lower than 20km. The global sea level spectral slope distribution provided by Xu and Fu (2012) with Jason‐1 data is revisited with more recent altimeter missions, and maps of altimeter error levels are provided and discussed for each mission. Seasonal variations of both spectral slopes and altimeter error levels are also analyzed for Jason‐2. SARAL/Altika, with its lower error levels, is shown to detect smaller structures everywhere. All missions show substantial geographical and temporal variations in their mesoscale resolution capabilities, with variations depending mostly on the error level change but also on slight regional changes in the spectral slopes. In western boundary currents where the signal to noise ratio is favorable, the alongtrack mesoscale resolution is approximately 40 km for SARAL/AltiKa, 45 km for Cryosat‐2, and 50 km for Jason‐2. Finally, a prediction of the future 2D mesoscale sea level resolution capability of the Surface Water and Ocean Topography (SWOT) mission is given using a simulated error level. This article is protected by copyright. All rights reserved.
      PubDate: 2016-06-02T10:27:17.895174-05:
      DOI: 10.1002/2015JC010904
       
  • On subsurface cooling associated with the Biobio River Canyon (Chile)
    • Abstract: Submarine canyons cutting across the continental shelf can modulate the cross‐shelf circulation being effective pathways to bring water from the deep ocean onto the shelf. Here, we use 69 days of moored array observations of temperature and ocean currents collected during the spring of 2013 and winter‐spring 2014, as well as shipboard hydrographic surveys and sea‐level observations to characterize cold, oxygen poor and nutrient‐rich upwelling events along the Biobio Submarine Canyon (BbC). The BbC is located within the Gulf of Arauco at 36° 50'S in the Central Chilean Coast. The majority of subtidal temperature at 150 m depth is explained by subtidal variability in alongshore currents on the canyon with a lag of less than a day (r2=0.65). Using the vertical displacement of the 10° and 10.5°C isotherms, we identified nine upwelling events, lasting between 20 hours to 4.5 days, that resulted in vertical isothermal displacements ranging from 29 to 137 m. The upwelled water likely originated below 200 m. Majority of the cooling events were related with strong northwards (opposite Kelvin wave propagation) flow and low pressure at the coast. Most of these low pressure events occur during relatively weak local wind forcing conditions, and were instead related with Coastal Trapped Waves (CTWs) propagating southwards from lower latitudes. These cold, high‐nutrient, low‐oxygen waters may be further upwelled and advected into the Gulf of Arauco by wind forcing. Thus, canyon upwelling may be a key driver of biological productivity and oxygen conditions in this Gulf. This article is protected by copyright. All rights reserved.
      PubDate: 2016-05-30T03:41:39.147813-05:
      DOI: 10.1002/2016JC011796
       
  • The role of bubbles during air‐sea gas exchange
    • Authors: Steven Emerson; Seth Bushinsky
      Abstract: The potential for using the air‐sea exchange rate of oxygen as a tracer for net community biological production in the ocean is greatly enhanced by recent accuracy improvements for in situ measurements of oxygen on unmanned platforms. A limiting factor for determining the exchange process is evaluating the air‐sea flux contributed by bubble processes produced by breaking waves, particularly during winter months under high winds. Highly accurate measurements of noble gases (Ne, Ar & Kr) and nitrogen, N2, in seawater are tracers of the importance of bubble process in the surface mixed layer. We use measured distributions of these gases in the ventilated thermocline of the North Pacific and an annual time series of N2 in the surface ocean of the NE Subarctic Pacific to evaluate four different air‐water exchange models chosen to represent the range of model interpretation of bubble processes. We find that models must have an explicit bubble mechanism to reproduce concentrations of insoluble atmospheric gases, but there are periods when they all depart from observations. The recent model of Liang et al. (2013) stems from a highly resolved model of bubble plumes and categorizes bubble mechanisms into those that are small enough to collapse and larger ones that exchange gases before they resurface, both of which are necessary to explain the data. This article is protected by copyright. All rights reserved.
      PubDate: 2016-05-27T18:45:55.644248-05:
      DOI: 10.1002/2016JC011744
       
  • Quantifying mesoscale eddies in the Lofoten Basin
    • Abstract: The Lofoten Basin is the most eddy rich region in the Norwegian Sea. In this paper the characteristics of these eddies are investigated from a comprehensive database of nearly two decades of satellite altimeter data (1995‐2013) together with Argo profiling floats and surface drifter data. An automated method identified 1695/1666 individual anticyclonic/cyclonic eddies in the Lofoten Basin from more than 10000 altimeter‐based eddy observations. The eddies is found to be predominantly generated and residing locally. The spatial distributions of lifetime, occurance, generation sites, size, intensity and drift of the eddies are studied in detail. The anticyclonic eddies in the Lofoten Basin are the most long‐lived eddies (> 60 days), especially in the western part of the basin. We reveal two hotspots of eddy‐occurance on either side of the Lofoten Basin. Furthermore we infer a cyclonic drift of eddies in the western Lofoten Basin. Barotropic energy conversion rates reveals energy transfer from the slope current to the eddies during winter. An automated colocation of surface drifters trapped inside the altimeter‐based eddies are used to coroborate the orbital speed of the anticyclonic and cyclonic eddies. Moreover, the vertical structure of the altimeter‐based eddies is examined using colocated Argo profiling float profiles. Combination of altimetry, Argo floats and surface drifter data is therefore considered to be a promising observation‐based approach for further studies of the role of eddies in transport of heat and biomass from the slope current to the Lofoten Basin. This article is protected by copyright. All rights reserved.
      PubDate: 2016-05-27T18:45:51.062718-05:
      DOI: 10.1002/2016JC011637
       
  • Arctic sea surface height variability and change from satellite radar
           altimetry and GRACE, 2003‐2014
    • Authors: Thomas W. K. Armitage; Sheldon Bacon, Andy L. Ridout, Sam F. Thomas, Yevgeny Aksenov, Duncan J. Wingham
      Abstract: Arctic sea surface height (SSH) is poorly observed by radar altimeters due to the poor coverage of the polar oceans provided by conventional altimeter missions and because large areas are perpetually covered by sea ice, requiring specialised data processing. We utilise SSH estimates from both the ice‐covered and ice‐free ocean to present monthly estimates of Arctic Dynamic Ocean Topography (DOT) from radar altimetry south of 81.5°N and combine this with GRACE ocean mass to estimate steric height. Our SSH and steric height estimates show good agreement with tide gauge records and geopotential height derived from Ice‐Tethered Profilers. The large seasonal cycle of Arctic SSH (amplitude ∼5cm) is dominated by seasonal steric height variation associated with seasonal freshwater fluxes, and peaks in October‐November. Overall, the annual mean steric height increased by 2.2±1.4cm between 2003‐12 before falling to ca. 2003 levels between 2012‐14 due to large reductions on the Siberian shelf seas. The total secular change in SSH between 2003‐14 is then dominated by a 2.1±0.7cm increase in ocean mass. We estimate that by 2010 the Beaufort Gyre had accumulated 4,600km3 of freshwater relative to the 2003‐06 mean. Doming of Arctic DOT in the Beaufort Sea is revealed by Empirical Orthogonal Function analysis to be concurrent with regional reductions in the Siberian Arctic. We estimate that the Siberian shelf seas lost ∼180km3 of freshwater between 2003‐14, associated with an increase in annual mean salinity of 0.15psu yr– 1. Finally, ocean storage flux estimates from altimetry agree well with high‐resolution model results, demonstrating the potential for altimetry to elucidate the Arctic hydrological cycle. This article is protected by copyright. All rights reserved.
      PubDate: 2016-05-27T18:45:46.076039-05:
      DOI: 10.1002/2015JC011579
       
  • The Sicily Channel surface circulation revisited using a neural clustering
           analysis of a high‐resolution simulation
    • Abstract: The Sicily Channel surface circulation is investigated by analyzing the outputs of a high‐resolution ocean model MED12 forced during 46 years by the ARPERA atmospheric fields. Applying a neural network classifier, we show that the surface circulation in the Sicily Channel can be decomposed into 8 modes characterizing the major patterns of that circulation, particularly the Algerian Current separation at the entrance to the Sicily Channel, the features of the Atlantic Tunisian Current and of the Atlantic Ionian Stream. These modes reflect the variability of the circulation in space and time at seasonal and inter‐annual scales. Some modes preferably occur in winter whilst others are characteristic of summer. The mode sequence presents an inter‐annual variability in good agreement with observations. The topography of the Sicily Channel sill plays a major role in steering the circulation. In particular the summer upwelling along the southern coast of Sicily, which is present in several modes, could be explained by a large‐scale density forcing. A combination of barotropic/baroclinic double Kelvin waves generated on both sides of the sill provides a mechanism for explaining the complexity of the surface circulation advecting the surface waters from the Western Mediterranean towards the Eastern Mediterranean, the most salient features of which are the Atlantic Tunisian Current, the Atlantic Ionian Stream and the Tyrrhenian Sicilian Current which is a new feature highlighted by the present study. This article is protected by copyright. All rights reserved.
      PubDate: 2016-05-27T18:45:39.873262-05:
      DOI: 10.1002/2015JC011472
       
  • The impact of horizontal resolution of density field on the calculation of
           the Atlantic meridional overturning circulation at 34°S
    • Authors: Vladimir N. Stepanov; Doroteaciro Iovino, Simona Masina, Andrea Storto, Andrea Cipollone
      Abstract: The Atlantic meridional overturning circulation and meridional heat transport (hereafter the AMOC and MHT) at 34°S as simulated by global 1/16° eddy‐rich (henceforth GLOB16) and ¼° eddy‐permitting (GLOB4) models are compared with observational estimates. Three different methods are used for calculating the modelled AMOC: the first method (MOCmod) is based on simulated velocity fields, while the second (MOCob) relies on the same assumptions as available observed‐derived estimates. The third method (MOCob2) is also based on hydrostatic and geostrophic relationships, but relative to a barotropic circulation instead of the definition of velocity at a specific reference depth. All methods correctly reproduce the time‐mean GLOB16 AMOC strength, but the value of the non‐Ekman component of the GLOB16 AMOC is only about 75% of the observed‐derived estimate. The GLOB16 MHT is also significantly less than observation value (slightly more than 60% of the observed). However, the mean AMOC and MHT values at 34°S obtained with coarser resolution GLOB4 model are comparable with the observed‐derived estimates. Possible causes for the differences between the eddy‐rich model and observational data are studied. It is shown that the density field from the eddy‐rich model has high temporal variability along 34°S with spatial scale of about two hundred km that can be due to mesoscale variations, caused by Agulhas “leakage”. This results in the decrease of the mean meridional geostrophic velocity, which leads to smaller values of the AMOC and MHT in GLOB16: subsampling GLOB16 density on ¼° or ½° longitude grid along 34°S for MOCob calculation significantly increases the AMOC values. The findings in this paper provide guidance in understanding AMOC and MHT dissimilarities between observation‐based estimates and eddy‐rich ocean models at 34°S. This article is protected by copyright. All rights reserved.
      PubDate: 2016-05-27T18:45:32.231784-05:
      DOI: 10.1002/2015JC011505
       
  • Field observations of turbulent dissipation rate profiles immediately
           below the air‐water interface
    • Authors: Binbin Wang; Qian Liao
      Abstract: Near surface profiles of turbulence immediately below the air‐water interface were measured with a free‐floating Particle Image Velocimetry (PIV) system on Lake Michigan. The surface‐following configuration allowed the system to measure the statistics of the aqueous‐side turbulence in the topmost layer immediately below the water surface (z ≈ 0 ∼ 15$cm, z points downward with 0 at the interface). Profiles of turbulent dissipation rate (ε) were investigated under a variety of wind and wave conditions. Various methods were applied to estimate the dissipation rate. Results suggest that these methods yield consistent dissipation rate profiles with reasonable scattering. In general, the dissipation rate decreases from the water surface following a power law relation in the top layer, ε ∼ z−0.7, i.e., the slope of the decrease was lower than that predicted by the wall turbulence theory, and the dissipation was considerably higher in the top layer for cases with higher wave ages. The measured dissipation rate profiles collapse when they were normalized with the wave speed, wave height, water‐side friction velocity, and the wave age. This scaling suggests that the enhanced turbulence may be attributed to the additional source of turbulent kinetic energy (TKE) at the “skin layer” (likely due to micro‐breaking), and its downward transport in the water column. This article is protected by copyright. All rights reserved.
      PubDate: 2016-05-27T18:45:28.067299-05:
      DOI: 10.1002/2015JC011512
       
  • Propagation of uncertainty and sensitivity analysis in an integral
           oil‐gas plume model
    • Authors: Shitao Wang; Mohamed Iskandarani, Ashwanth Srinivasan, W. Carlisle Thacker, Justin Winokur, Omar M. Knio
      Abstract: Polynomial Chaos expansions are used to analyze uncertainties in an integral oil‐gas plume model simulating the Deepwater Horizon oil spill. The study focuses on six uncertain input parameters—two entrainment parameters, the gas to oil ratio, two parameters associated with the droplet‐size distribution, and the flow rate—that impact the model's estimates of the plume's trap and peel heights, and of its various gas fluxes. The ranges of the uncertain inputs were determined by experimental data. Ensemble calculations were performed to construct polynomial chaos‐based surrogates that describe the variations in the outputs due to variations in the uncertain inputs. The surrogates were then used to estimate reliably the statistics of the model outputs, and to perform an analysis of variance. Two experiments were performed to study the impacts of high and low flow rate uncertainties. The analysis shows that in the former case the flow rate is the largest contributor to output uncertainties, whereas in the latter case, with the uncertainty range constrained by aposteriori analyses, the flow rate's contribution becomes negligible. The trap and peel heights uncertainties are then mainly due to uncertainties in the 95% percentile of the droplet size and in the entrainment parameters.
      PubDate: 2016-05-27T06:51:08.387987-05:
      DOI: 10.1002/2015JC011365
       
  • Carbon exchange between a shelf sea and the ocean: The Hebrides Shelf,
           west of Scotland
    • Abstract: Global mass balance calculations indicate the majority of particulate organic carbon (POC) exported from shelf seas is transferred via downslope exchange processes. Here we demonstrate the downslope flux of POC from the Hebrides Shelf is approximately 3‐to‐5‐fold larger per unit length/area than the global mean. To reach this conclusion we quantified the offshore transport of particulate and dissolved carbon fractions via the “Ekman Drain”, a strong downwelling feature of the NW European Shelf circulation, and subsequently compared these fluxes to simultaneous regional air‐sea CO2 fluxes and on‐shore wind‐driven Ekman fluxes to constrain the carbon dynamics of this shelf. Along the shelf break we estimate a mean offshelf total carbon (dissolved + particulate) flux of 4.2 tonnes C m−1 d−1 compared to an onshelf flux of 4.5 tonnes C m−1 d−1. Organic carbon represented 3.3% of the onshelf carbon flux but 6.4% of the offshelf flux indicating net organic carbon export. Dissolved organic carbon represented 95% and POC 5% of the exported organic carbon pool. When scaled along the shelf break the total offshelf POC flux (0.007 Tg C d−1) was found to be three times larger than the regional air‐sea CO2 ingassing flux (0.0021 Tg C d−1), an order of magnitude larger than the particulate inorganic carbon flux (0.0003 Tg C d−1) but far smaller than the DIC (2.03 Tg C d−1) or DOC (0.13 Tg C d−1) fluxes. Significant spatial heterogeneity in the Ekman drain transport confirms that offshelf carbon fluxes via this mechanism are also spatially heterogeneous. This article is protected by copyright. All rights reserved.
      PubDate: 2016-05-26T05:31:16.637103-05:
      DOI: 10.1002/2015JC011599
       
  • Bottom pressure torque and the vorticity balance from observations in
           Drake Passage
    • Authors: Yvonne L. Firing; Teresa K. Chereskin, D. Randolph Watts, Matthew R. Mazloff
      Abstract: The vorticity balance of the Antarctic Circumpolar Current in Drake Passage is examined using four years of observations from current‐ and pressure‐recording Inverted Echo Sounders. The time‐varying vorticity, planetary and relative vorticity advection, and bottom pressure torque are calculated in a two‐dimensional array in the eddy‐rich Polar Frontal Zone (PFZ). Bottom pressure torque is also estimated at sites across Drake Passage. Mean and eddy nonlinear relative vorticity advection terms dominate over linear advection in the local (50‐km‐scale) vorticity budget in the PFZ, and are balanced to first order by the divergence of horizontal velocity. Most of this divergence comes from the ageostrophic gradient flow, which also provides a second‐order adjustment to the geostrophic relative vorticity advection. Bottom pressure torque is approximately one‐third the size of the local depth‐integrated divergence. Although the cDrake velocity fields exhibit significant turning with depth throughout Drake Passage even in the mean, surface vorticity advection provides a reasonable representation of the depth‐integrated vorticity balance. Observed near‐bottom currents are strongly topographically steered, and bottom pressure torques grow large where strong near‐bottom flows cross steep topography at small angles. Upslope flow over the northern continental slope dominates the bottom pressure torque in cDrake, and the mean across this Drake Passage transect, 3 to 4 × 10−9 m s−2, exceeds the mean wind stress curl by a factor of 15 to 20. This article is protected by copyright. All rights reserved.
      PubDate: 2016-05-26T05:30:37.400937-05:
      DOI: 10.1002/2016JC011682
       
  • Examining features of enhanced phytoplankton biomass in the Bay of Bengal
           using a coupled physical‐biological model
    • Abstract: A coupled bio‐physical ocean model is used to describe areas of enhanced phytoplankton biomass, seen remotely sensed observations, in the otherwise oligotrophic environment of the Bay of Bengal. The model is based on the Naval Coastal Ocean Model (NCOM), which is one‐way coupled to the 13‐component Carbon, Silicate, and Nitrogen Ecosystem (CoSiNE) model and configured for the Indian Ocean. Model results are compared and evaluated against a set of in situ shipboard observations as well as ocean color data acquired from several remote sensing platforms. The model is shown to successfully simulate the seasonal cycle of phytoplankton, the markedly contrasting scenarios of phytoplankton distribution in the north versus the south Bay of Bengal, and the biological impact from the 1997/1998 Indian Ocean Dipole (IOD) event. The model simulation provides us with vertical cross sections of phytoplankton biomass from summer and winter blooms in the southwest of the bay; information not found in remotely sensed data. It also successfully reproduces the timing of the onset of the blooms and their spatial extent, thereby providing a measure of its potential for augmenting in situ and remotely sensed observations to improve understanding of the dynamics of primary producers and carbon cycling in one of the most poorly sampled regions of the world's oceans. This article is protected by copyright. All rights reserved.
      PubDate: 2016-05-23T11:14:46.032803-05:
      DOI: 10.1002/2015JC011508
       
  • A coupling modulation model of capillary waves from gravity waves:
           Theoretical analysis and experimental validation
    • Authors: Pengzhen Chen; Xiaoqing Wang, Li Liu, Jinsong Chong
      Abstract: According to Bragg theory, capillary waves are the predominant scatterers of high‐frequency band (such as Ka‐band) microwave radiation from the surface of the ocean. Therefore, understanding the modulation mechanism of capillary waves is an important foundation for interpreting high‐frequency microwave remote sensing images of the surface of the sea. In our experiments, we discovered that modulations of capillary waves are significantly larger than the values predicted by the classical theory. Further, analysis shows that the difference in restoring force results in an inflection point while the phase velocity changes from gravity waves region to capillary waves region, and this results in the capillary waves being able to resonate with gravity waves when the phase velocity of the gravity waves is equal to the group velocity of the capillary waves. Consequently, we propose a coupling modulation model in which the current modulates the capillary wave indirectly by modulating the resonant gravity waves, and the modulation of the former is approximated by that of the latter. This model very effectively explains the results discovered in our experiments. Further, based on Bragg scattering theory and this coupling modulation model, we simulate the modulation of normalized radar cross‐section (NRCS) of typical internal waves and show that the high‐frequency bands are superior to the low‐frequency bands because of their greater modulation of and better radiometric resolution. This result provides new support for choice of radar band for observation of wave‐current modulation oceanic phenomena such as internal waves, fronts, and shears. This article is protected by copyright. All rights reserved.
      PubDate: 2016-05-19T10:56:03.785957-05:
      DOI: 10.1002/2015JC011048
       
  • Modeling the effect of wave‐vegetation interaction on wave setup
    • Authors: A.A. van Rooijen; R.T. McCall, J.S.M. van Thiel de Vries, A.R. van Dongeren, A.J.H.M. Reniers, J.A. Roelvink
      Abstract: Aquatic vegetation in the coastal zone attenuates wave energy and reduces the risk of coastal hazards, e.g. flooding. Besides the attenuation of sea‐swell waves, vegetation may also affect infragravity‐band (IG) waves and wave setup. To date, knowledge on the effect of vegetation on IG waves and wave setup is lacking, while they are potentially important parameters for coastal risk assessment. In this study, the storm impact model XBeach is extended with formulations for attenuation of sea‐swell and IG waves, and wave setup effects in two modes: the sea‐swell wave phase‐resolving (non‐hydrostatic) and the phase‐averaged (surfbeat) mode. In surfbeat mode a wave shape model is implemented to capture the effect of nonlinear wave‐vegetation interaction processes on wave setup. Both modeling modes are verified using data from two flume experiments with mimic vegetation and show good skill in computing the sea‐swell and IG wave transformation, and wave setup. In surfbeat mode, the wave setup prediction greatly improves when using the wave shape model, while in non‐hydrostatic mode (nonlinear) intra‐wave effects are directly accounted for. Subsequently, the model is used for a range of coastal geomorphological configurations by varying bed slope and vegetation extent. The results indicate that the effect of wave‐vegetation interaction on wave setup may be relevant for a range of typical coastal geomorphological configurations (e.g. relatively steep to gentle slope coasts fronted by vegetation). This article is protected by copyright. All rights reserved.
      PubDate: 2016-05-19T10:55:47.001577-05:
      DOI: 10.1002/2015JC011392
       
  • Atlantic sea surface height and velocity spectra inferred from satellite
           altimetry and a hierarchy of numerical simulations
    • Authors: Stavroula Biri; Nuno Serra, Martin G. Scharffenberg, Detlef Stammer
      Abstract: Frequency and wavenumber spectra of sea surface height (SSH) and surface geostrophic velocity are presented, as they result for the Atlantic Ocean from a 23‐year long altimeter data set and from a hierarchy of ocean model simulations with spatial resolutions of 16km, 8km, and 4km. SSH frequency spectra follow a spectral decay of roughly f−1 on long periods; toward higher frequencies a spectral decay close to f−2 is found. For geostrophic velocity spectra a somewhat similar picture emerges, albeit with flatter spectral relations. In terms of geostrophic velocity wavenumber spectra, we find a general relation close to k−3 in the high‐resolution model results. Outside low‐energy regions all model spectra come close to observed spectra at low frequencies and wavenumbers in terms of shape and amplitude. However, the highest model resolution appears essential for reproducing the observed spectra at high frequencies and wavenumbers. This holds especially for velocity spectra in mid and high latitudes, suggesting that eddy resolving ocean models need to be run at a resolution of 1/24° or better if one were to fully resolve the observed mesoscale eddy field. Causes for remaining discrepancies between observed and simulated results can be manifold. At least partially, they can be rationalized by taking into account an aliasing effect of unresolved temporal variability in the altimetric observations occurring on periods smaller than the 20‐days Nyquist period of the altimetric data, thereby leading to an overestimate of variability in the altimetric estimates, roughly on periods below 100 days. This article is protected by copyright. All rights reserved.
      PubDate: 2016-05-19T10:55:34.363174-05:
      DOI: 10.1002/2015JC011503
       
  • Spatial scales of optical variability in the coastal ocean: Implications
           for remote sensing and in situ sampling
    • Authors: Wesley J. Moses; Steven G. Ackleson, Johnathan W. Hair, Chris A. Hostetler, W. David Miller
      Abstract: Use of ocean color remote sensing to understand the effects of environmental changes and anthropogenic activities on estuarine and coastal waters requires the capability to measure and track optically detectable complex biogeochemical processes. An important remote sensor design consideration is the minimum spatial resolution required to resolve key ocean features of physical and biological significance. The spatial scale of variability in optical properties of coastal waters has been investigated using continuous, along‐track measurements collected using instruments deployed from ships, aircraft, and satellite. We defined the average coefficient of variance, CV‐a, within an image pixel as the primary statistical measure of sub‐pixel variability and investigated how CV‐a changes as a function of the Ground Sampling Distance (GSD). In general, dCV‐a /dGSD is positive, indicating that the sub‐pixel variability increases with GSD. The relationship between CV‐a and GSD is generally non‐linear and the greatest rate of change occurs at small spatial scales. Points of distinct transition in the relationship between CV‐a and GSD are evident between 75 m and 600 m, varying depending on the location and the optical parameter, and representing the GSD above which most of the spatial variability due to small‐scale features is subsumed within a pixel. At GSDs greater than the transition point, most of the small‐scale variability occurs at sub‐pixel scales and, therefore, cannot be resolved. On average, the transition GSD is around 200 m. The results have application in both sensor design and in situ sampling strategy in support of coastal remote sensing operations. This article is protected by copyright. All rights reserved.
      PubDate: 2016-05-18T11:22:36.285608-05:
      DOI: 10.1002/2016JC011767
       
  • Alongshore momentum transfer to the nearshore zone from energetic ocean
           waves generated by passing hurricanes
    • Authors: Ryan P. Mulligan; Jeffrey L. Hanson
      Abstract: Wave and current measurements from a cross‐shore array of nearshore sensors in Duck, NC, are used to elucidate the balance of alongshore momentum under energetic wave conditions with wide surf zones, generated by passing hurricanes that are close to and far from to the coast. The observations indicate that a distant storm (Hurricane Bill, 2009) with large waves has low variability in directional wave characteristics resulting in alongshore currents that are driven mainly by the changes in wave energy. A storm close to the coast (Hurricane Earl, 2010), with strong local wind stress and combined sea and swell components in wave energy spectra, has high variability in wave direction and wave period that influence wave breaking and nearshore circulation as the storm passes. During both large wave events the horizontal current shear is strong and radiation stress gradients, bottom stress, wind stress, horizontal mixing and cross‐shore advection contribute to alongshore momentum at different spatial locations across the nearshore region. Horizontal mixing during Hurricane Earl, estimated from rotational velocities, was particularly strong suggesting that intense eddies were generated by the high horizontal shear from opposing wind‐driven and wave‐driven currents. The results provide insight into the cross‐shore distribution of the alongshore current and the connection between flows inside and outside the surf zone during major storms, indicating that the current shear and mixing at the interface between the surf zone and shallow inner shelf is strongly dependent on the distance from the storm center to the coast. This article is protected by copyright. All rights reserved.
      PubDate: 2016-05-18T11:22:16.878425-05:
      DOI: 10.1002/2016JC011706
       
  • Crude oil jets in crossflow: Effects of dispersant concentration on plume
           behavior
    • Authors: David W. Murphy; Xinzhi Xue, Kaushik Sampath, Joseph Katz
      Abstract: This study investigates the effects of premixing oil with chemical dispersant at varying concentrations on the flow structure and droplet dynamics within a crude oil jet transitioning into a plume in a crossflow. It is motivated by the need to determine the fate of subsurface oil after a well blowout. The laboratory experiments consist of flow visualizations, in situ measurements of the time evolution of droplet size distributions using holography, and particle image velocimetry to characterize dominant flow features. Increasing the dispersant concentration dramatically decreases the droplet sizes and increases their number, and accordingly, reduces the rise rates of droplets and the upper boundary of the plume. The flow within the plume consists primarily of a pair of counter‐rotating quasi‐streamwise vortices (CVP) that characterize jets in crossflows. It also involves generation of vertical wake vortices that entrain small droplets under the plume. The evolution of plume boundaries is dominated by interactions of droplets with the CVP. The combined effects of vortex‐induced velocity and significant quiescent rise velocity of large (∼5 mm) droplets closely agrees with the rise rate of the upper boundary of the crude oil plume. Conversely, the much lower rise velocity of the smaller droplets in oil‐dispersant mixtures results in plume boundaries rising at rates that are very similar to those of the CVP center. The size of droplets trapped by the CVP is predicted correctly using a trapping function, which is based on a balance of forces on a droplet located within a horizontal eddy. This article is protected by copyright. All rights reserved.
      PubDate: 2016-05-18T11:21:38.118635-05:
      DOI: 10.1002/2015JC011574
       
  • Plankton patchiness investigated using simultaneous nitrate and
           chlorophyll observations
    • Authors: Simon van Gennip; Adrian P. Martin, Meric A. Srokosz, John T. Allen, Rosalind Pidcock, Stuart C. Painter, Mark C. Stinchcombe
      Abstract: The complex patterns observed in marine phytoplankton distributions arise from the interplay of biological and physical processes, but the nature of the balance remains uncertain centuries after the first observations. Previous observations have shown a consistent trend of decreasing variability with decreasing length‐scale. Influenced by similar scaling found for the properties of the water that the phytoplankton inhabit, ‘universal' theories have been proposed that simultaneously explain the variability seen from meters to hundreds of kilometers. However, data on the distribution of phytoplankton alone has proved insufficient to differentiate between the many causal mechanisms that have been suggested. Here we present novel observations from a cruise in the North Atlantic in which fluorescence (proxy for phytoplankton), nitrate and temperature were measured simultaneously at scales from 10 m to 100 km for the first time in the open ocean. These show a change in spectra between the small scale (10–100 m) and the mesoscale (10–100 km) which is different for the three tracers. We discuss these observations in relation to the current theories for phytoplankton patchiness. This article is protected by copyright. All rights reserved.
      PubDate: 2016-05-17T03:31:00.933022-05:
      DOI: 10.1002/2016JC011789
       
  • On the stability and spatiotemporal variance distribution of salinity in
           the upper ocean
    • Authors: Terence J. O'Kane; Didier P. Monselesan, Christophe Maes
      Abstract: Despite recent advances in ocean observing arrays and satellite sensors, there remains great uncertainty in the large scale spatial variations of upper ocean salinity on the interannual to decadal timescales. Consonant with both broad‐scale surface warming and the amplification of the global hydrological cycle, observed global multidecadal salinity changes typically have focussed on the linear response to anthropogenic forcing but not on salinity variations due to changes in the static stability and or variability due to the intrinsic ocean or internal climate processes. Here, we examine the static stability and spatio‐temporal variability of upper ocean salinity across a hierarchy of models and reanalyses. In particular, we partition the variance into time bands via application of singular spectral analysis, considering sea surface salinity (SSS), the Brunt Väisälä frequency (N2) and the ocean salinity stratification in terms of the stabilizing effect due to the haline part of N2 over the upper 500m. We identify regions of significant coherent SSS variability, either intrinsic to the ocean or in response to the interannually varying atmosphere. Based on consistency across models (CMIP5 and forced experiments) and reanalyses, we identify the stabilizing role of salinity in the tropics – typically associated with heavy precipitation and barrier layer formation, and the role of salinity in destabilizing upper ocean stratification in the subtropical regions where large scale density compensation typically occurs. This article is protected by copyright. All rights reserved.
      PubDate: 2016-05-13T10:40:53.622939-05:
      DOI: 10.1002/2015JC011523
       
  • The life cycle of a coherent Lagrangian Agulhas ring
    • Abstract: We document the long‐term evolution of an Agulhas ring detected from satellite altimetry using a technique from nonlinear dynamical systems that enables objective (i.e., observer‐independent) eddy framing. Such objectively detected eddies have Lagrangian (material) boundaries that remain coherent (unfilamented) over the detection period. The ring preserves a quite compact material entity for a period of about 2 years even after most initial coherence is lost within 5 months after detection. We attribute this to the successive development of short‐term coherent material boundaries around the ring. These boundaries provide effective short‐term shielding for the ring, which prevents a large fraction of the ring's interior from being mixed with the ambient turbulent flow. We show that such coherence regain events cannot be inferred from Eulerian analysis. This process is terminated by a ring‐splitting event which marks the ring demise, near the South American coast. The genesis of the ring is characterized by a ring‐merging event away from the Agulhas retroflection, followed by a 4‐month‐long partial coherence stage, scenario that is quite different than a current occlusion and subsequent eddy pinch off. This article is protected by copyright. All rights reserved.
      PubDate: 2016-05-12T04:56:07.598925-05:
      DOI: 10.1002/2015JC011620
       
  • Simulation of phytoplankton distribution and variation in the
           Bering‐Chukchi Sea using a 3‐D physical‐biological model
           
    • Authors: Haoguo Hu; Jia Wang, Hui Liu, Joaquim Goes
      Abstract: A three dimensional physical‐biological model has been used to simulate seasonal phytoplankton variations in the Bering and Chukchi Seas with a focus on understanding the physical and biogeochemical mechanisms involved in the formation of the Bering Sea Green Belt (GB) and the Subsurface Chlorophyll Maxima (SCM). Model results suggest that the horizontal distribution of the GB is controlled by a combination of light, temperature, and nutrients. Model results indicated that the SCM, frequently seen below the thermocline, exists because of a rich supply of nutrients and sufficient light. The seasonal onset of phytoplankton blooms is controlled by different factors at different locations in the Bering Sea. In the off‐shelf central region of the Bering Sea, phytoplankton blooms are regulated by available light. On the Bering Sea shelf, sea ice through its influence on light and temperature plays a key role in the formation of blooms, whereas in the Chukchi Sea, bloom formation is largely controlled by ambient seawater temperatures. A numerical experiment conducted as part of this study revealed that plankton‐sinking is important for simulating the vertical distribution of phytoplankton and the seasonal formation of the SCM. An additional numerical experiment revealed that sea ice algae account for 14.3∼36.9% of total phytoplankton production during the melting season, and it cannot be ignored when evaluating primary productivity in the Arctic Ocean. This article is protected by copyright. All rights reserved.
      PubDate: 2016-05-12T04:25:37.936594-05:
      DOI: 10.1002/2016JC011692
       
  • Downwelling wind, tides, and estuarine plume dynamics
    • Authors: Zhigang Lai; Ronghua Ma, Mingfen Huang, Changsheng Chen, Yong Chen, Congbin Xie, Robert C. Beardsley
      Abstract: The estuarine plume dynamics under a downwelling‐favorable wind condition were examined in the windy dry season of the Pearl River Estuary (PRE) using the PRE primitive‐equation Finite‐Volume Community Ocean Model (FVCOM). The wind‐ and tide‐driven estuarine circulation had a significant influence on the plume dynamics on both local and remote scales. Specifically, the local effect of downwelling‐favorable winds on the plume was similar to the theoretical descriptions of coastal plumes, narrowing the plume width and setting up a vertically‐uniform downstream current at the plume edge. Tides tended to reduce these plume responses through local turbulent mixing and advection from upstream regions, resulting in an adjustment of the isohalines in the plume and a weakening of the vertically‐uniform downstream current. The remote effect of downwelling‐favorable winds on the plume was due to the wind‐induced estuarine sea surface height (SSH), which strengthened the estuarine circulation and enhanced the plume transport accordingly. Associated with these processes, tide‐induced mixing tended to weaken the SSH gradient and thus the estuarine circulation over a remote influence scale. Overall, the typical features of downwelling‐favorable wind‐driven estuarine plumes revealed in this study enhanced our understanding of the estuarine plume dynamics under downwelling‐favorable wind conditions. This article is protected by copyright. All rights reserved.
      PubDate: 2016-05-12T04:25:28.179574-05:
      DOI: 10.1002/2015JC011475
       
  • Southern Ocean deep convection in global climate models: A driver for
           variability of subpolar gyres and Drake Passage transport on decadal time
           scales
    • Authors: Erik Behrens; Graham Rickard, Olaf Morgenstern, Torge Martin, Annette Osprey, Manoj Joshi
      Abstract: We investigate the individual and joint decadal variability of Southern Ocean state quantities, such as the strength of the Ross and Weddell Gyres, Drake Passage transport, and sea ice area, using the National Institute of Water and Atmospheric Research UK Chemistry and Aerosols (NIWA‐UKCA) model and CMIP5 models. Variability in these quantities is stimulated by strong deep reaching convective events in the Southern Ocean, which produce an Antarctic Bottom Water‐like water mass and affect the large‐scale meridional density structure in the Southern Ocean. An increase in the (near) surface stratification, due to freshwater forcing, can be a pre‐condition for subsequent strong convection activity. The combination of enhanced‐gyre driven sea ice and freshwater export, as well as ongoing subsurface heat accumulation, lead to a time lag between changes in oceanic freshwater and heat content. This causes an ongoing weakening of the stratification until sudden strong mixing events emerge and the heat is released to the atmosphere. We find that strong convection reduces sea ice cover, weakens the subpolar gyres, increases the meridional density gradient and subsequently results in a positive Drake Passage transport anomaly. Results of available CMIP5 models confirm that variability in sea ice, Drake Passage transport and the Weddell Gyre strength is enhanced if models show strong open ocean convective events. Consistent relationships between convection, sea ice, Drake Passage transport and Ross Gyre strength variability are evident in most models, whether or not they host open‐ocean convection. This article is protected by copyright. All rights reserved.
      PubDate: 2016-05-10T03:45:46.120138-05:
      DOI: 10.1002/2015JC011286
       
  • Effects of swell on transport and dispersion of oil plumes within the
           ocean mixed layer
    • Authors: Bicheng Chen; Di Yang, Charles Meneveau, Marcelo Chamecki
      Abstract: The transport in the ocean mixed layer (OML) of oil plumes originated from deep water blowouts is studied using large eddy simulations. In particular, we focus on the effects of swell on the modulation of turbulence in the OML and its impact on oil transport. Results show that when the wind‐swell misalignment between the wind and the swell propagation is small, Langmuir cells develop and significantly enhance the vertical dilution of the oil plume. Conversely, when the misalignment is large, vertical dilution is suppressed when compared to the no‐swell case. Due to the strong directional shear of the mean flow within the OML, plume depth significantly impacts mean transport direction. The size of oil droplets in the plume also plays an important role in vertical dilution and mean transport direction. This article is protected by copyright. All rights reserved.
      PubDate: 2016-05-09T18:25:23.793879-05:
      DOI: 10.1002/2015JC011380
       
  • Observations of seasonal subduction at the Iceland‐Faroe front
    • Authors: N. L. Beaird; P. B. Rhines, C.C. Eriksen
      Abstract: The polar front in the North Atlantic is bound to the ridge between Iceland and the Faroe Islands, where about one‐half of the northward transport of warm Atlantic Water into the Nordic Seas occurs, as well as about one sixth of the equatorward dense overflow. We find a low salinity water mass at the surface of the Iceland‐Faroe Front (IFF), which in wintertime subducts along outcropping isopycnals and is found in much modified form on the Atlantic side of the Iceland‐Faroe Ridge (IFR) crest. The features found on the Atlantic side of the crest at depth have temperature and salinity characteristics which are clearly traceable to the surface outcrop of the IFF. The presence of coherent low salinity layers on the Atlantic side of the IFR crest has not been previously reported. Warm waters above the IFR primarily feed the Faroe Current, and injection of a low salinity water mass may play an early role in the water mass transformation taking place in the Nordic Seas. The seasonality of the intrusive features suggests a link between winter convection, mixed layer instability and deep frontal subduction. These low salinity anomalies (as well as a low oxygen water mass from the Iceland Basin) can be used as tracers of the intermediate circulation over the IFR. This article is protected by copyright. All rights reserved.
      PubDate: 2016-05-06T20:10:35.768821-05:
      DOI: 10.1002/2015JC011501
       
  • Analytical solution for the vertical profile of daily production in the
           ocean
    • Abstract: Photosynthesis parameters are routinely estimated from in vitro measurements of primary production under constant light reaching each incubation bottle, by fitting a photosynthesis‐irradiance function to the measurements. Here we take one such function and integrate it in time for variable light input, similar to natural conditions, to obtain the analytical solution for the vertical profile of daily phytoplankton production in the field. This solution is then fitted to in situ measurements of primary production profiles in the same manner as a photosynthesis‐irradiance function is fitted to in vitro measurements under controlled and constant light conditions to retrieve the photosynthesis‐irradiance parameters. The method is tested on the Hawaii Ocean Time‐series data set. The solution explained 97.88% of the variance in measured normalized production at individual depths. The recovered parameters were then used to model the normalized daily water‐column production. The model explained 99.21% of variance in normalized watercolumn production of the entire data set. The seasonal cycle of the photosynthesis parameters recovered with the analytical solution was further studied for the Hawaii Ocean Time‐series. With respect to the photosynthesis parameter determination, the solution bridges the gap between classical photosynthesis‐irradiance measurements under controlled light conditions and in situ measurements which are made under natural, variable light conditions. It presents a new tool for the estimation of photosynthesis parameters from in situ measurements of primary production. This article is protected by copyright. All rights reserved.
      PubDate: 2016-05-05T03:51:53.346164-05:
      DOI: 10.1002/2015JC011293
       
  • Surface waves affect frontogenesis
    • Abstract: This paper provides a detailed analysis of momentum, angular momentum, vorticity, and energy budgets of a submesoscale front undergoing frontogenesis driven by an upper‐ocean, submesoscale eddy field in a Large Eddy Simulation (LES). The LES solves the wave‐averaged, or Craik‐Leibovich, equations in order to account for the Stokes forces that result from interactions between nonbreaking surface waves and currents, and resolves both submesoscale eddies and boundary layer turbulence down to 4.9m × 4.9m × 1.25m grid scales. It is found that submesoscale frontogenesis differs from traditional frontogenesis theory due to four effects: Stokes forces, momentum and kinetic energy transfer from submesoscale eddies to frontal secondary circulations, resolved turbulent stresses, and unbalanced torque. In the energy, momentum, angular momentum, and vorticity budgets for the frontal overturning circulation, the Stokes shear force is a leading‐order contributor, typically either the second or third largest source of frontal overturning. These effects violate hydrostatic and thermal wind balances during submesoscale frontogenesis. The effect of the Stokes shear force becomes stronger with increasing alignment of the front and Stokes shear and with a nondimensional scaling. The Stokes shear force and momentum transfer from submesoscale eddies significantly energize the frontal secondary circulation along with the buoyancy. This article is protected by copyright. All rights reserved.
      PubDate: 2016-05-05T03:51:34.208592-05:
      DOI: 10.1002/2015JC011563
       
  • Vertical variations of coral reef drag forces
    • Authors: Shai Asher; Stephan Niewerth, Katinka Koll, Uri Shavit
      Abstract: Modeling flow in a coral reef requires a closure model that links the local drag force to the local mean velocity. However, the spatial flow variations make it difficult to predict the distribution of the local drag. Here we report on vertical profiles of measured drag and velocity in a laboratory reef that was made of 81 Pocillopora Meandrina colony skeletons, densely arranged along a tilted flume. Two corals were CT‐scanned, sliced horizontally and printed using a 3D printer. Drag was measured as a function of height above the bottom by connecting the slices to drag sensors. Profiles of velocity were measured in‐between the coral branches and above the reef. Measured drag of whole colonies shows an excellent agreement with previous field and laboratory studies; however these studies never showed how drag varies vertically. The vertical distribution of drag is reported as a function of flow rate and water level. When the water level is the same as the reef height, Reynolds stresses are negligible and the drag force per unit fluid mass is nearly constant. However, when the water depth is larger, Reynolds stress gradients become significant and drag increases with height. An excellent agreement was found between the drag calculated by a momentum budget and the measured drag of the individual printed slices. Finally, we propose a modified formulation of the drag coefficient that includes the normal dispersive stress term and results in reduced variations of the drag coefficient at the cost of introducing an additional coefficient. This article is protected by copyright. All rights reserved.
      PubDate: 2016-05-05T03:45:53.853014-05:
      DOI: 10.1002/2015JC011428
       
  • Dynamical analysis of a satellite‐observed anticyclonic eddy in the
           northern Gulf of Anadyr of the Bering Sea
    • Authors: Yineng Li; Xiaofeng Li, Jia Wang, Shiqiu Peng
      Abstract: The characteristics and evolution of a satellite‐observed anticyclonic eddy in the northern Gulf of Anadyr (northwestern Bering Sea) during March and April 1999 are investigated using a three‐dimensional Princeton Ocean Model (POM). The anticyclonic‐like current pattern and asymmetric feature of the eddy were clearly seen in the synthetic aperture radar (SAR), sea surface temperature, and ocean color images in April, 1999. The results from model simulation reveal the three‐dimensional structure of the anticyclonic eddy, its movement and dissipation. Energy analysis indicates that the barotropic instability (BTI) is the main energy source for the growth of the anticyclonic eddy. The momentum analysis further reveals that the larger magnitude of the barotropic pressure gradient in the meridional direction causes the asymmetry of the anticyclonic eddy in the zonal and meridional directions, while the different magnitudes of the meridional baroclinic pressure gradient are responsible for the different intensity of currents between the northern and southern parts of the anticyclonic eddy. This article is protected by copyright. All rights reserved.
      PubDate: 2016-05-05T03:45:50.457246-05:
      DOI: 10.1002/2015JC011586
       
  • A semianalytical algorithm for quantitatively estimating sediment and
           atmospheric deposition flux from MODIS‐derived sea ice albedo in the
           Bohai Sea, China
    • Authors: Zhantang Xu; Shuibo Hu, Guifen Wang, Jun Zhao, Yuezhong Yang, Wenxi Cao, Peng Lu
      Abstract: Quantitative estimates of particulate matter (PM) concentration in sea ice using remote sensing data is helpful for studies of sediment transport and atmospheric dust deposition flux. In this study, the difference between the measured dirty and estimated clean albedo of sea ice was calculated and a relationship between the albedo difference and PM concentration was found using field and laboratory measurements. A semi‐analytical algorithm for estimating PM concentration in sea ice was established. The algorithm was then applied to MODIS data over the Bohai Sea, China. Comparisons between MODIS derived and in situ measured PM concentration showed good agreement, with a mean absolute percentage difference of 31.2%. From 2005 to 2010, the MODIS derived annual average PM concentration was approximately 0.025 g/L at the beginning of January. After a month of atmospheric dust deposition, it increased to 0.038 g/L. Atmospheric dust deposition flux was estimated to be 2.50 t/km2/month, similar to 2.20 t/km2/month reported in a previous study. The result was compared with on‐site measurements at a nearby ground station. The ground station was close to industrial and residential areas, where larger dust depositions occurred than in the sea, but although there were discrepancies between the absolute magnitudes of the two datasets, they demonstrated similar trends. This article is protected by copyright. All rights reserved.
      PubDate: 2016-05-02T18:48:30.369902-05:
      DOI: 10.1002/2015JC011067
       
  • Invigorating ocean boundary current systems around Australia during
           1979‐2014 ‐ as simulated in a near‐global
           eddy‐resolving ocean model
    • Authors: Ming Feng; Xuebin Zhang, Peter Oke, Didier Monselesan, Matthew Chamberlain, Richard Matear, Andreas Schiller
      Abstract: Ocean boundary currents, transporting water masses and marine biota along the coastlines, are important for regional climate and marine ecosystem functions. In this study, we review the dominant multi‐decadal trends of ocean boundary currents around Australia. Using an eddy‐resolving global ocean circulation model, this study has revealed that the major ocean boundary current systems around Australia, the East Australian Current (EAC), the Indonesian Throughflow (ITF), the Leeuwin Current, the South Australian Current and the Flinders Current, have strengthened during 1979‐2014, consistent with existing observations. Eddy energetics in the EAC, the ITF/South Equatorial Current in the southeast Indian Ocean, and the Leeuwin Current have also enhanced during the same period. The multi‐decadal strengthening of the ocean boundary current systems are primarily driven by large scale wind patterns associated with the dominant modes of climate variability and change – the phase shift of the Inter‐decadal Pacific Oscillation/Pacific Decadal Oscillation strengthens the ITF and the Leeuwin Current/South Australian Current; and the poleward shift and strengthening of surface winds in the subtropical gyres reinforce the EAC and the Flinders Current. The invigorating ocean boundary current systems have induced extreme oceanographic conditions along the Australian coastlines in recent years, including the poleward shift of marine ecosystems off the east coast of Australia and the consecutive Ningaloo Niño – marine heatwave events off the west coast during 2011‐2013. Understanding long‐term trends and decadal variations of the ocean boundary currents is crucial to project future changes of the coastal marine systems under the influence of human‐induced greenhouse gas forcing. This article is protected by copyright. All rights reserved.
      PubDate: 2016-05-02T18:48:18.576588-05:
      DOI: 10.1002/2016JC011842
       
  • Sensing of upslope passages of frontal bores across the trench slope break
           of the Japan Trench
    • Authors: Yoshio Fukao; Hiroko Sugioka, Aki Ito, Hajime Shiobara, Jerome M. Paros, Ryo Furue
      Abstract: The circum‐Pacific trench slope system is one of the most spectacular topographic features of the Earth. We report the relatively frequent occurrence of upslope passages (∼50 times a year) of internal bores in the Japan Trench system. Observations were made using a triaxial accelerometer, a tiltmeter, and a current meter at the seafloor, ∼3400 m deep, during 10 months from 2013 to 2014. We detected 42 tilt events from the records of the accelerometer and confirmed their consistency with the tiltmeter records. The tilt occurred always landward by 1–10 μ radian with a rise time on the order of 1000 s and return to the original with a much longer recovery time. The current and temperature records available for the first two events indicated that landward tilting was associated with an upslope current with a speed of several cm/s and a temperature drop of several tens of m°C. The temperature remained cold even after the current had diminished. All of these observations implied the frontal passages of upslope advancing bores, which caused the instrument to be tilted landward. The observational site was located at the trench slope break, separating the trench slope (sloping supercritically or near‐critically for the M2 internal tide) from the forearc basin (sloping subcritically). This unique locality suggested the internal tide origin of the observed bores, although other possibilities could not be precluded. Bores generated on the trench slope may play a role for upslope transportation of suspended sediments stirred up by deep submarine landslides. This article is protected by copyright. All rights reserved.
      PubDate: 2016-05-02T18:47:52.087473-05:
      DOI: 10.1002/2015JC011432
       
  • Interannual variability of the South Indian Countercurrent
    • Authors: Viviane V. Menezes; Helen E. Phillips, Marcio L. Vianna, Nathaniel L. Bindoff
      Abstract: In the present work, we investigate the interannual variability of the South Indian Countercurrent (SICC), a major and still understudied current of the Indian Ocean circulation. To characterize the interannual variability of the SICC, four different datasets (altimetry, GLORYS, OFAM3 and SODA) are analysed using multiple tools, which include Singular Spectrum Analysis and wavelet methods. The quasi‐biennial band dominates the SICC low frequency variance, with the main peak in the 1.5‐1.8 yr interval. A secondary peak (2.1‐2.5 yr) is only found in the western basin. Interannual and decadal‐type modulations of the quasi‐biennial signal are also identified. In addition, limitations of SODA before the 1960's in the SICC region are revealed. Within the quasi‐biennial band the SICC system presents two main patterns with a multiple jet structure. One pattern is characterized by a robust northern jet, while in the other the central jet is well developed and northern jet is weaker. In both patterns, the southern jet has always a strong signature. When the northern SICC jet is stronger, the northern cell of the subtropical gyre has a triangular shape, with its southern limb having a strong equatorward slant. The quasi‐biennial variability of the SICC is probably related to the Indian Ocean tropical climate modes that are known to have a strong biennial characteristic. This article is protected by copyright. All rights reserved.
      PubDate: 2016-05-02T18:47:25.604294-05:
      DOI: 10.1002/2015JC011417
       
  • Spatiotemporal distribution of seawater pH in the North Pacific subpolar
           region by using the parameterization technique
    • Authors: Bofeng Li; Yutaka W. Watanabe, Azusa Yamaguchi
      Abstract: We provided parameterizations to speculate changes in the vertical distributions of the seawater total alkalinity (TA), dissolved inorganic carbon (DIC), and pH by using all available observations of dissolved oxygen (DO, μmol kg−1), water temperature (T, ºC), and salinity (S) data from previous high quality datasets of ocean hydrographic properties. All datasets were collected in the North Pacific subpolar region during the period of 2000 to 2010, for 40–400 m depth, and between 40°N–56°N, 145°E–130°W). Root mean square errors for the parameterizations with 7.4 μmol kg−1 for TA, 7.1 μmol kg−1 for DIC, and 0.02 for pH were obtained with R2 of more than 0.95. To evaluate the validity of these parameterizations, we compared measured TA, DIC, and pH with predicted values. The differences were almost within 10 μmol kg−1 for TA and DIC, and 0.05 for pH. Applying our parameterizations to the climatological datasets of DO, T, and S data from World Ocean Atlas 2009, we reconstructed seasonal and monthly vertical distributions of seawater TA, DIC, and pH. Furthermore, by substituting our parameterizations into detailed vertical distributions of DO, T, and S data measured bi‐weekly by automatic ocean hydrographic sensors from Argo profiling floats, we described the detailed vertical spatiotemporal distributions of seawater TA, DIC, and pH. In the North Pacific subpolar region, in the case that there is no time‐series of ocean carbon species, our parameterizations allowed us to elucidate the dynamics of ocean carbon chemistry from two–week scale to decadal scale. This article is protected by copyright. All rights reserved.
      PubDate: 2016-05-02T18:40:57.605065-05:
      DOI: 10.1002/2015JC011615
       
  • Remote sensing of nearshore wave interference
    • Authors: P.B. Smit; R. Bland, T.T. Janssen, B. Laughlin
      Abstract: Wave focusing of energetic swell fields can result in small‐scale variations associated with coherent interference that can be important for nearshore circulation and beach dynamics. However, coherent interference is difficult to measure with conventional in‐situ instruments and is not accounted for in operational wave models. As a result, such effects are generally ignored. In this work we analyze X‐band radar observations collected at Ocean Beach, San Francisco using a Wigner‐Ville or coupled‐mode spectrum, to show how long‐dwell remote sensing technology allows us to identify coherent wave interference. Our analysis demonstrates that during energetic swell events, the nearshore wave field consists of two non‐collinear, but coherent, swell patterns that originate from the same offshore source but are directionally separated due to refraction over the San Francisco Bar. The length scale of the associated alongshore wave height variability (200m) is consistent with the wavenumber separation obtained from the coupled mode analysis. This confirms that the small‐scale variability is primarily due to coherent interference. In addition, our analysis shows that the shoreline exhibits a strong localized response near the radar site on the 200 m scale, which suggests that coherent interference effects can affect wave‐driven nearshore transport processes and localized erosion. This article is protected by copyright. All rights reserved.
      PubDate: 2016-04-30T03:30:59.620038-05:
      DOI: 10.1002/2016JC011705
       
  • Ocean chemistry, ocean warming, and emerging hypoxia: Commentary
    • Authors: Peter G. Brewer; Edward T. Peltzer
      Abstract: For 50 years ocean scientists have represented deep sea biogeochemical rates as a temperature independent function of depth with form R = R0e‐αz where z is depth in km. We show this resembles, but is not an identity for, a form of the classical Arrhenius equation K = Ae‐Ea/RT where T is temperature in Kelvins, R is the gas constant (8.314 JK−1mol−1) and A is a pre‐exponential factor. For a deep Sargasso Sea data set we find oxygen consumption rates are accurately represented by an Arrhenius process with apparent activation energy of 86.5 kJ mol−1, and Q10 = 3.63. This article is protected by copyright. All rights reserved.
      PubDate: 2016-04-30T03:26:43.959822-05:
      DOI: 10.1002/2016JC011651
       
  • An optical system for detecting and describing major algal blooms in
           coastal and oceanic waters around India
    • Authors: Elamurugu Alias Gokul; Palanisamy Shanmugam
      Abstract: An optical system is developed with the aim to detect and monitor three major algal blooms (including harmful algal blooms ‘HABs') over ecologically relevant scales around India and to strengthen algal forecasting system. This system is designed to be capable of utilizing remote sensing, in‐situ and radiative transfer techniques to provide species‐specific data necessary for increasing capabilities of an algal forecasting system. With the ability to measure in‐water optical properties by means of remote sensing and in‐situ observations, the optical system developed infers the desired phytoplankton signal from spectral distributions and utilize these data in a numerical classification technique to generate species‐specific maps at given spatial and temporal scales. A simple radiative transfer model is adopted for this system to provide a means to optimally interpolate to regions with sparse in‐situ observation data and to provide a central component to generate in‐water optical properties from remotely sensed data. For a given set of inherent optical properties along with surface and bottom boundary conditions, the optical system potentially provides researchers and managers coverage at different locations and depths for tracking algal blooms in the water column. Three major algal blooms focused here include Noctiluca scintillans/miliaris, Trichodesmium erythraeum, and Cochlodinium polykrikoides which are recurring events in coastal and oceanic waters around India. Because satellite sensors provide a synoptic view of the ocean, both spatially and temporally, our initial efforts tested this optical system using several MODIS‐Aqua images and ancillary data. Validation of the results with coincident in‐situ data obtained from either surface samples or depth samples demonstrated the robustness and potential utility of this approach, with an accuracy of eighty‐ninety percent for delineating the presence of all three blooms in a heterogeneous phytoplankton community. Despite its limitation in detecting specific species during their pre‐bloom phases and indicating whether a particular bloom is toxic or harmful, the proposed optical system will provide managers with the specific phytoplankton bloom maps to structure monitoring efforts and a powerful tool for studying the dynamics of algal blooms at various temporal and spatial scales. This article is protected by copyright. All rights reserved.
      PubDate: 2016-04-30T03:26:21.523515-05:
      DOI: 10.1002/2015JC011604
       
  • Internal tides recorded at ocean bottom off the coast of Southeast Taiwan
    • Abstract: An ocean‐bottom experiment consisting of an array of four ocean‐bottom seismometers (OBS) was conducted off the coast of southeast Taiwan during May‐July, 2011. We develop comprehensive analyses of the space‐time kinematics of the tidal signals recorded in the compact high‐sensitivity temperature loggers (CHTL) and the OBS geophones at the ocean bottom with depths ranging from 1,254 – 1,610 meters. The evidence suggests that internal tides are responsible for the recorded signals: baroclinic internal waves (mainly the M2 tide) are generated by barotropic tidal currents in the Luzon Strait. The internal tides exhibit gradual phase changing and irregularly fluctuating strength, leaving signatures in the CHTL as ambient temperature variations, signifying low‐mode wave motions within the stratified water layers; and in OBS geophones as intermittent “tremor” agitations, signifying high‐mode turbulent flows on the seafloor. The M2 internal tides across our array are found to propagate in the northeast direction at speeds ranging from 1 to 2+ m s−1. Furthermore, the internal tides are identified at the ocean‐bottom based on an operational hydrodynamic hindcast/forecast model. The simulations show good agreement with the observed temperature variation on the seafloor and substantiate the vertical velocity and displacement of the water parcel driven by the internal tides. The joint detection of the temperature and tremor signals provides further information about the interactions of internal tides with the seafloor topography and the associated energy dissipation. Our results elucidate the space‐time ubiquity of the internal tides at the ocean bottom, which is an important interface of dynamic oceanography. This article is protected by copyright. All rights reserved.
      PubDate: 2016-04-30T03:26:08.1473-05:00
      DOI: 10.1002/2015JC011370
       
  • Surface dynamics of crude and weathered oil in the presence of
           dispersants: Laboratory experiment and numerical simulation
    • Abstract: Marine oil spills can have dire consequences for the environment. Research on their dynamics is important for the well‐being of coastal communities and their economies. Propagation of oil spills is a very complex physical‐chemical process. As seen during the Deepwater Horizon event in the Gulf of Mexico during 2010, one of the critical problems remaining for prediction of oil transport and dispersion in the marine environment is the small‐scale structure and dynamics of surface oil spills. The laboratory experiments conducted in this work were focused on understanding the differences between the dynamics of crude and weathered oil spills and the effect of dispersants. After deposition on the still water surface, a drop of crude oil quickly spread into a thin slick; while at the same time, a drop of machine (proxy for weathered) oil did not show significant evolution. Subsequent application of dispersant to the crude oil slick resulted in a quick contraction or fragmentation of the slick into narrow wedges and tiny drops. Notably, the slick of machine oil did not show significant change in size or topology after spraying dispersant. An advanced multi‐phase, volume of fluid computational fluid dynamics model, incorporating capillary forces, was able to explain some of the features observed in the laboratory experiment. As a result of the laboratory and modeling experiments, the new interpretation of the effect of dispersant on the oil dispersion process including capillary effects has been proposed, which is expected to lead to improved oil spill models and response strategies. This article is protected by copyright. All rights reserved.
      PubDate: 2016-04-30T03:25:53.295862-05:
      DOI: 10.1002/2015JC011533
       
  • Data and numerical analysis of astronomic tides, wind‐waves, and
           hurricane storm surge along the northern Gulf of Mexico
    • Authors: M.V. Bilskie; S.C. Hagen, S.C. Medeiros, A.T. Cox, M. Salisbury, D. Coggin
      Abstract: The northern Gulf of Mexico (NGOM) is a unique geophysical setting for complex tropical storm‐induced hydrodynamic processes that occur across a variety of spatial and temporal scales. Each hurricane includes its own distinctive characteristics and can cause unique and devastating storm surge when it strikes within the intricate geometric setting of the NGOM. While a number of studies have explored hurricane storm surge in the NGOM, few have attempted to describe storm surge and coastal inundation using observed data in conjunction with a single large‐domain high‐resolution numerical model. To better understand the oceanic and nearshore response to these tropical cyclones, we provide a detailed assessment, based on field measurements and numerical simulation, of the evolution of wind waves, water levels, and currents for Hurricanes Ivan (2004), Dennis (2005), Katrina (2005), and Isaac (2012), with focus on Mississippi, Alabama, and the Florida Panhandle coasts. The developed NGOM3 computational model describes the hydraulic connectivity among the various inlet and bay systems, Gulf Intracoastal Waterway, coastal rivers and adjacent marsh, and built infrastructure along the coastal floodplain. The outcome is a better understanding of the storm surge generating mechanisms and interactions among hurricane characteristics and the NGOM's geophysical configuration. The numerical analysis and observed data explain the ∼2 m/s hurricane‐induced geostrophic currents across the continental shelf, a 6 m/s outflow current during Ivan, the hurricane‐induced coastal Kelvin wave along the shelf, and for the first time a wealth of measured data and a detailed numerical simulation was performed and was presented for Isaac. This article is protected by copyright. All rights reserved.
      PubDate: 2016-04-26T10:15:50.672861-05:
      DOI: 10.1002/2015JC011400
       
  • Interannual modulation of eddy kinetic energy in the northeastern South
           China Sea as revealed by an eddy‐resolving OGCM
    • Authors: Zhongbin Sun; Zhiwei Zhang, Wei Zhao, Jiwei Tian
      Abstract: Interannual modulation of eddy kinetic energy (EKE) in the northeastern South China Sea (NE‐SCS) is investigated based on outputs of an eddy‐resolving oceanic general circulation model between 1980–2014. The EKE displays distinct interannual modulations with periods between 1.5 and 7 years. The maximum peak‐to‐trough amplitude of the interannual modulation occurred during period 2004–2005, which was about 1.5‐fold the time‐mean EKE level. Further analysis suggested that interannual variability of EKE in the NE‐SCS is primarily modulated by the Luzon Strait transport (LST). During high‐EKE years, the LST increases corresponding to a strengthened Kuroshio intrusion. The strengthened Kuroshio intrusion enhances the baroclinic instability of current in the NE‐SCS and thus leads to a strong EKE. The reverse is true during low‐EKE years when LST is smaller. Influences of ENSO and Pacific mesoscale eddies on the interannual modulation of LST are also discussed in this study. This article is protected by copyright. All rights reserved.
      PubDate: 2016-04-22T04:55:31.47693-05:0
      DOI: 10.1002/2015JC011497
       
  • Internal swells in the tropics: Near‐inertial wave energy fluxes and
           dissipation during CINDY
    • Authors: S. M. Soares; A. Natarov, K. J. Richards
      Abstract: A developing MJO event in the tropical Indian Ocean triggered wind disturbances that generated inertial oscillations in the surface mixed layer. Subsequent radiation of near‐inertial waves below the mixed layer produced strong turbulence in the pycnocline. Linear plane wave dynamics and spectral analysis are used to explain these observations, with the ultimate goal of estimating the wave energy flux in relation to both the energy input by the wind and the dissipation by turbulence. The results indicate that the wave packets carry approximately 30‐40% of the wind input of inertial kinetic energy, and propagate in an environment conducive to the occurrence of a critical level set up by a combination of vertical gradients in background relative vorticity and Doppler shifting of wave frequency. Turbulent kinetic energy dissipation measurements demonstrate that the waves lose energy as they propagate in the transition layer as well as in the pycnocline, where approaching this critical level may have dissipated approximately 20% of the wave packet energy in a single event. Our analysis, therefore, supports the notion that appreciable amounts of wind‐induced inertial kinetic energy escape the surface boundary layer into the interior. However, a large fraction of wave energy is dissipated within the pycnocline, limiting its full depth penetration and transfer to the abyssal ocean. This article is protected by copyright. All rights reserved.
      PubDate: 2016-04-21T19:29:18.876105-05:
      DOI: 10.1002/2015JC011600
       
  • The influence of environmental parameters on active and maturing oceanic
           whitecaps
    • Authors: B. Scanlon; B. Ward
      Abstract: High resolution images of the ocean surface are examined using digital processing, achieving quantifications of actively breaking (WA), maturing (WB) and total (WT=WA+WB) whitecaps. The images are selected from two datasets of the North Atlantic and Southern Ocean to sample a maximal range of environmental conditions. A total of 125,860 images were processed and averaged to establish 622 10‐minute periods. Parameterizing WA, WB and WT with wind speed achieved modest correlations while also displaying large variabilities. Parameterizing WT with wind speed and specific Reynolds numbers achieved correlation coefficients ranging from 0.76 to 0.79. The filtering of WT into its active stage of evolution WA and subsequent fittings with wind speed and specific Reynolds numbers achieved reduced correlation coefficients ranging from 0.62 to 0.66. Unresolved scatter was revealed for low and mid‐range Reynolds numbers. Thus Reynolds numbers may not suitable parameters for quantifying WA. We suggest that the contribution of WB serves to conceal and thus underestimate the variability of actively breaking waves. This article is protected by copyright. All rights reserved.
      PubDate: 2016-04-21T19:27:28.367005-05:
      DOI: 10.1002/2015JC011230
       
  • Modeling surfzone to inner‐shelf tracer exchange
    • Abstract: A near‐shoreline, continuous dye release at an approximately alongshore‐uniform beach (IB09 experiment) is simulated with the wave‐resolving Boussinesq model funwaveC. The model generates surfzone eddies and transient rip currents but does not resolve inner‐shelf vertical variation or stratification. The funwaveC odel reproduces well the observed surfzone and inner‐shelf dye observations over roughly 350 m cross‐shore and 2000 m alongshore. Dye is advected alongshore by wave‐ and wind‐driven currents similarly in the observations and model. Near‐shoreline mean dye concentration decays downstream as a power law with similar observed (‐0.33) and modeled (‐0.38) exponents. Observed and modeled cross‐shore mean dye profiles are similar, though modeled inner‐shelf dye is somewhat elevated. Observed and modeled alongshore dye transports agree, though with compensating surfzone and inner‐shelf errors later in the release. For times
      PubDate: 2016-04-21T19:20:56.361575-05:
      DOI: 10.1002/2015JC011530
       
  • Effect of subseabed salt domes on tidal residual currents in the Persian
           Gulf
    • Authors: Hossein Mashayekh Poul; Jan Backhaus, Ali Dehghani, Udo Huebner
      Abstract: Geological studies in the Persian Gulf (PG) have revealed the existence of sub‐seabed salt‐domes. With suitable filtering of a high‐resolution PG seabed topography, it is seen that the domes leave their signature in the seabed, i.e. numerous hills and valleys with amplitudes of several tens of meters and radii from a few up to tens of kilometers. It was suspected that the 'shark skin' of the PG seabed may affect the tidal residual flow. The interaction of tidal dynamics and these obstacles was investigated in a non‐linear hydrodynamic numerical tidal model of the PG. The model was first used to characterize flow patterns of residual currents generated by a tidal wave passing over symmetric, elongated and tilted obstacles. Thereafter it was applied to the entire PG. The model was forced at its open boundary by the four dominant tidal constituents residing in the PG. Each tidal constituent was simulated separately. Results, i.e. tidal residual currents in the PG, as depicted by Lagrangian trajectories reveal a stationary flow that is very rich in eddies. Each eddy can be identified with a topographic obstacle. This confirms that the tidal residual flow field is strongly influenced by the nonlinear interaction of the tidal wave with the bottom relief which, in turn, is deformed by salt‐domes beneath the seabed. Different areas of maximum residual current velocities are identified for major tidal constituents. The pattern of trajectories indicates the presence of two main cyclonic gyres and several adjacent gyres rotating in opposite directions and a strong coastal current in the northern PG. This article is protected by copyright. All rights reserved.
      PubDate: 2016-04-21T19:20:36.78354-05:0
      DOI: 10.1002/2015JC011421
       
  • Eddy properties in the western Mediterranean Sea from satellite altimetry
           and a numerical simulation
    • Authors: Romain Escudier; Lionel Renault, Ananda Pascual, Pierre Brasseur, Dudley Chelton, Jonathan Beuvier
      Abstract: Three different eddy detection and tracking methods are applied to the outputs of a high‐resolution simulation in the Western Mediterranean Sea in order to extract mesoscale eddy characteristics. The results are compared with the same eddy statistics derived from satellite altimetry maps over the same period. Eddy radii are around 30 km in altimetry maps whereas, in the model, they are around 20 km. This is probably due to the inability of altimetry maps to resolve the smaller mesoscale in the region. About 30 eddies are detected per day in the basin with a very heterogeneous spatial distribution and relatively short lifespans (median life around 13 days). Unlike other areas of the open ocean, they do not have a preferred direction of propagation but appear to be advected by mean currents. The number of detected eddies seems to present an annual cycle when separated according to their lifespan. With the numerical simulation, we show that anticyclones extend deeper in the water column and have a more conic shape than cyclones. This article is protected by copyright. All rights reserved.
      PubDate: 2016-04-20T18:15:32.636372-05:
      DOI: 10.1002/2015JC011371
       
  • Sea ice circulation around the Beaufort Gyre: The changing role of wind
           forcing and the sea ice state
    • Abstract: Sea ice drift estimates from feature tracking of satellite passive microwave data are used to investigate seasonal trends and variability in the ice circulation around the Beaufort Gyre, over the multi‐decadal period 1980–2013. Our results suggest an amplified response of the Beaufort Gyre ice circulation to wind forcing, especially during the late 2000s. We find increasing anticyclonic ice drift across all seasons, with the strongest trend in autumn, associated with increased ice export out of the southern Beaufort Sea (into the Chukchi Sea). A flux gate analysis highlights consistency across a suite of drift products. Despite these seasonal anticyclonic ice drift trends, a significant anticyclonic wind trend occurs in summer only, driven, in‐part, by anomalously anticyclonic winds in 2007. Across all seasons, the ice drift curl is more anticyclonic than predicted from a linear relationship to the wind curl in the 2000s, compared to the 1980s/1990s. The strength of this anticyclonic ice drift curl amplification is strongest in autumn and appears to have increased since the 1980s (up to 2010). In spring and summer, the ice drift curl amplification occurs mainly between 2007 and 2010. These results suggest non‐linear ice interaction feedbacks (e.g. a weaker, more mobile sea ice pack), enhanced atmospheric drag, and/or an increased role of the ocean. The results also show a weakening of the anticyclonic wind and ice circulation since 2010. This article is protected by copyright. All rights reserved.
      PubDate: 2016-04-20T18:11:46.344743-05:
      DOI: 10.1002/2015JC010903
       
  • A tale of three islands: Downstream natural iron fertilization in the
           Southern Ocean
    • Authors: J. Robinson; E.E. Popova, M. Srokosz, A. Yool
      Abstract: Iron limitation of primary productivity prevails across much of the Southern Ocean but there are exceptions; in particular, the phytoplankton blooms associated with the Kerguelen Plateau, Crozet Islands and South Georgia. These blooms occur annually, fertilized by iron and nutrient‐rich shelf waters that are transported downstream from the islands. Here we use a highresolution (1/12°) ocean general circulation model and Lagrangian particle tracking to investigate whether inter‐annual variability in the potential lateral advection of iron, could explain the inter‐annual variability in the spatial extent of the blooms. Comparison with ocean color data, 1998 to 2007, suggests that iron fertilization via advection can explain the extent of each island's annual bloom, but only the inter‐annual variability of the Crozet bloom. The area that could potentially be fertilized by iron from Kerguelen was much larger than the bloom, suggesting that there is another primary limiting factor, potentially silicate, that controls the inter‐annual variability of bloom spatial extent. For South Georgia, there are differences in the year‐to‐year timing of advection and consequently fertilization, but no clear explanation of the inter‐annual variability observed in the bloom's spatial extent has been identified. The model results suggest that the Kerguelen and Crozet blooms are terminated by nutrient exhaustion, probably iron and or silicate, whereas the deepening of the mixed layer in winter terminates the South Georgia bloom. Therefore, iron fertilization via lateral advection alone can explain the annual variability of the Crozet bloom, but not fully that of the Kerguelen and South Georgia blooms. This article is protected by copyright. All rights reserved.
      PubDate: 2016-04-19T10:36:13.665908-05:
      DOI: 10.1002/2015JC011319
       
  • Decadal‐scale thermohaline variability in the Atlantic sector of the
           Southern Ocean
    • Authors: K. Hutchinson; S. Swart, A. Meijers, I. Ansorge, S. Speich
      Abstract: An enhanced Altimetry Gravest Empirical Mode (AGEM), including both adiabatic and diabatic trends, is developed for the Antarctic Circumpolar Current (ACC) south of Africa using updated hydrographic CTD sections, Argo data, and satellite altimetry. This AGEM has improved accuracy compared to traditional climatologies and other proxy methods. The AGEM for the Atlantic Southern Ocean offers an ideal technique to investigate the thermohaline variability over the past two decades in a key region for water mass exchanges and transformation. In order to assess and attribute changes in the hydrography of the region, we separate the changes into adiabatic and diabatic components. Integrated over the upper 2000 dbar of the ACC south of Africa, results show mean adiabatic changes of 0.16 ± 0.11°C.decade−1 and 0.006 ± 0.014 decade−1, and diabatic differences of ‐0.044 ± 0.13°C.decade−1 and ‐0.01 ± 0.017 .decade−1 for temperature and salinity, respectively. The trends of the resultant AGEM, that include both adiabatic and diabatic variability (termed AD‐AGEM), show a significant increase in the heat content of the upper 2000dbar of the ACC with a mean warming of 0.12 ± 0.087°C.decade−1. This study focuses on the Antarctic Intermediate Water (AAIW) mass where negative diabatic trends dominate positive adiabatic differences in the Subantarctic Zone (SAZ), with results indicating a cooling (‐0.17°C.decade−1) and freshening (‐0.032 decade−1) of AAIW in this area, whereas south of the SAZ positive adiabatic and diabatic trends together create a cumulative warming (0.31°C.decade−1) and salinification (0.014 decade−1) of AAIW. This article is protected by copyright. All rights reserved.
      PubDate: 2016-04-19T10:36:00.035638-05:
      DOI: 10.1002/2015JC011491
       
  • Wind‐induced variability of estuarine circulation in a tidally
           energetic inlet with curvature
    • Authors: Kaveh Purkiani; Johannes Becherer, Knut Klingbeil, Hans Burchard
      Abstract: In numerous studies, the functioning of estuarine circulation has been investigated, under idealized conditions, by means of numerical models. This has led to a deep understanding of the theory of estuarine residual flows. However, the question as to how estuarine circulation is established in real estuaries, in response to their topographical and forcing characteristics, remains. The present study uses a highly accurate three‐dimensional numerical model simulation to calculate estuarine circulation in a curved, tidally‐energetic channel of the Wadden Sea in the southeastern North Sea.The specific momentum balance of this curved inlet shows an approximate pressure‐gradient ‐ frictional balance in the longitudinal direction and a pressure gradient ‐ centrifugal balance in the lateral direction. A local Wedderburn number is introduced to quantify the varying contributions of wind stress and gravitational forcing on estuarine circulation. A total exchange flow (TEF) analysis is combined with an analysis of the intensity of the vertical overturning circulation to understand the dynamics of estuarine exchange in this inlet. The results show how established forcing mechanisms of residual circulation, such as horizontal buoyancy gradients and wind stress, act in a combined way. In general, the strength of estuarine circulation is always positively correlated with wind stress, with frequent reversals of residual flow for wind stress directed towards higher buoyancy. Only during calm weather conditions are longitudinal and lateral estuarine circulation highly correlated with the respective buoyancy gradients. This article is protected by copyright. All rights reserved.
      PubDate: 2016-04-15T18:56:02.606185-05:
      DOI: 10.1002/2015JC010945
       
  • Understanding Stokes forces in the wave‐averaged equations
    • Abstract: The wave‐averaged, or Craik‐Leibovich, equations describe the dynamics of upper ocean flow interacting with non‐breaking, not steep, surface gravity waves. This paper formulates the wave effects in these equations in terms of three contributions to momentum: Stokes advection, Stokes Coriolis force, and Stokes shear force. Each contribution scales with a distinctive parameter. Moreover, these contributions affect the turbulence energetics differently from each other such that the classification of instabilities is possible accordingly. Stokes advection transfers energy between turbulence and Eulerian mean‐flow kinetic energy, and its form also parallels the advection of tracers such as salinity, buoyancy, and potential vorticity. Stokes shear force transfers energy between turbulence and surface waves. The Stokes Coriolis force can also transfer energy between turbulence and waves, but this occurs only if the Stokes drift fluctuates. Furthermore, this formulation elucidates the unique nature of Stokes shear force and also allows direct comparison of Stokes shear force with buoyancy. As a result, the classic Langmuir instabilities of Craik and Leibovich, wave‐balanced fronts and filaments, Stokes perturbations of symmetric and geostrophic instabilities, the wavy Ekman layer, and the wavy hydrostatic balance are framed in terms of intuitive physical balances. This article is protected by copyright. All rights reserved.
      PubDate: 2016-04-13T08:26:07.420753-05:
      DOI: 10.1002/2015JC011566
       
  • Collision of oil droplets with marine aggregates: Effect of droplet size
    • Authors: Ruth A. Lambert; Evan A. Variano
      Abstract: Interactions between oil droplets and marine particle aggregates, such as marine snow, may effect the behavior of oil spills. Marine snow is known to scavenge fine particles from the water column, and has the potential to scavenge oil droplets in the same manner. To determine the degree to which such a process is important in the evolution of oil spills, we quantify the collision of oil droplets and marine aggregates using existing collision rate equations. Results show that interaction of drops and aggregates can substantially influence the drop size distribution, but like all such processes this result is sensitive to the local concentration of oil and aggregates. The analysis also shows that as the size distribution of oil droplets shifts towards larger droplets, a greater fraction of the total oil volume collides with marine aggregates. This result is robust to a variety of different assumptions in the collision model. Results also show that there is a not always a dominant collision mechanism. For example, when droplets and aggregates are both close to 10 micrometers in radius, shear and differential settling contribute nearly equally to the collision rate. This overlap suggests that further research on the interaction of shear and differential settling could be useful. This article is protected by copyright. All rights reserved.
      PubDate: 2016-04-08T13:03:22.209515-05:
      DOI: 10.1002/2015JC011562
       
  • Major variations in subtropical North Atlantic heat transport at short (5
           day) time scales and their causes
    • Abstract: Variability in the North Atlantic ocean heat transport at 26.5°N on short (5‐day) timescales is identified and contrasted with different behaviour at monthly intervals using a combination of RAPID/MOCHA/WBTS measurements and the NEMO‐LIM2 1/12° ocean circulation/sea ice model. Wind forcing plays the leading role in establishing the heat transport variability through the Ekman transport response of the ocean and the associated driving atmospheric conditions vary significantly with timescale. We find that at 5‐day timescales the largest changes in the heat transport across 26.5°N coincide with north‐westerly airflows originating over the American land mass that drive strong southward anomalies in the Ekman flow. During these events the northward heat transport reduces by 0.5‐1.4 PW. In contrast, the Ekman transport response at longer monthly timescales is smaller in magnitude (up to 0.5 PW) and consistent with expected variations in the leading mode of North Atlantic atmospheric variability, the North Atlantic Oscillation. The north‐westerly airflow mechanism can have a prolonged influence beyond the central 5‐day timescale and on occasion can reduce the accumulated winter ocean heat transport into the North Atlantic by ∼40%. This article is protected by copyright. All rights reserved.
      PubDate: 2016-04-08T12:57:51.10661-05:0
      DOI: 10.1002/2016JC011660
       
  • Characterizing storm water dispersion and dilution from small coastal
           streams
    • Authors: Leonel Romero; David A. Siegel, James C. McWilliams, Yusuke Uchiyama, Charles Jones
      Abstract: Characterizing the dispersion and dilution of stormwater from small coastal creeks is important for understanding the importance of land‐derived subsidies to nearby ecosystems and the management of anthropogenic pollutants. In Southern California, creek runoff is episodic, intense and short‐lived while the plumes are buoyant, all of which make the field sampling of freshwater plumes challenging. Numerical modeling offers a viable way to characterize these systems. The dilution and dispersion of freshwater from two creeks that discharge into the Santa Barbara Channel, California is investigated using Regional Ocean Modeling System (ROMS) simulations with a horizontal resolution of 100 m. Tight coupling is found among precipitation, hydrologic discharge, wind forcing and submesoscale flow structures which all contribute to plume evolution. During flooding, plumes are narrow and attached to the coast, due to downwelling/onshore wind forcing and intense vorticity filaments lying parallel to the shelf. As the storm passes, the winds typically shift to offshore/upwelling favorable conditions and the plume is advected offshore which enhances its dilution. Plumes reach the bottom nearshore while they form thin layers a few meters thick offshore. Dilution field of passive tracers released with the runoff is strongly anisotropic with stronger cross‐shelf gradients than along‐shelf. Dispersion analysis of statistical moments of the passive tracer distribution results in scale dependent diffusivities consistent with the particle‐pair analysis of Romero et al. (2013). Model validation, the roles of submesoscale processes and wind forcing on plume evolution and application to ecological issues and marine resource management are discussed. This article is protected by copyright. All rights reserved.
      PubDate: 2016-04-07T17:45:49.961528-05:
      DOI: 10.1002/2015JC011323
       
  • Attenuation coefficient of usable solar radiation of the global oceans
    • Authors: Junfang Lin; Zhongping Lee, Michael Ondrusek, Mati Kahru
      Abstract: Usable solar radiation (USR) represents spectrally integrated solar energy in the spectral range of 400‐560 nm, a domain where photons penetrate the most in oceanic waters and thus contribute to photosynthesis and heating at deeper depths. Through purely numerical simulations [Lee et al., 2014], it was found that the diffuse attenuation coefficient of downwelling USR (Kd(USR), m−1) is nearly a constant vertically in the upper water column for clear waters and most turbid waters. Subsequently an empirical model was developed to estimate Kd(USR) based on the diffuse attenuation coefficient at 490 nm (Kd(490), m−1). We here evaluate this relationship using data collected from a wide range of oceanic and coastal environments and found that the relationship between Kd(490) and Kd(USR) developed via the numerical simulation is quite robust. We further refined this relationship to extend the applicability to “clearest” natural waters. This refined relationship was then used to produce sample distribution of Kd(USR) of global oceans. As expected, extremely low Kd(USR) (∼ 0.02 m−1) was observed in ocean gyres, while significantly higher Kd(USR) (∼ 5.2 m−1) was found in very turbid coastal regions. A useful application of Kd(USR) is to easily and accurately propagate surface USR to deeper depths, potentially to significantly improve the estimation of basin scale primary production and heat fluxes in the upper water column. This article is protected by copyright. All rights reserved.
      PubDate: 2016-03-24T03:31:04.148584-05:
      DOI: 10.1002/2015JC011528
       
  • An oceanic heat transport pathway to the Amundsen Sea Embayment
    • Authors: Angelica R. Rodriguez; Matthew R. Mazloff, Sarah T. Gille
      Abstract: The Amundsen Sea Embayment (ASE) on the West Antarctic coastline has been identified as a region of accelerated glacial melting. A Southern Ocean State Estimate (SOSE) is analyzed over the 2005–2010 time period in the Amundsen Sea region. The SOSE oceanic heat budget reveals that the contribution of parameterized small‐scale mixing to the heat content of the ASE waters is small compared to advection and local air‐sea heat flux, both of which contribute significantly to the heat content of the ASE waters. Above the permanent pycnocline the local air‐sea flux dominates the heat budget and is controlled by seasonal changes in sea ice coverage. Overall, between 2005 and 2010, the model shows a net heating in the surface above the pycnocline within the ASE. Sea water below the permanent pycnocline is isolated from the influence of air‐sea heat fluxes, and thus, the divergence of heat advection is the major contributor to increased oceanic heat content of these waters. Oceanic transport of mass and heat into the ASE is dominated by the cross‐shelf input and is primarily geostrophic below the permanent pycnocline. Diagnosis of the time‐mean SOSE vorticity budget along the continental shelf slope indicates that the cross‐shelf transport is sustained by vorticity input from the localized wind‐stress curl over the shelf break. This article is protected by copyright. All rights reserved.
      PubDate: 2016-03-23T17:51:06.851762-05:
      DOI: 10.1002/2015JC011402
       
  • Potential and timescales for oxygen depletion in coastal upwelling
           systems: A box‐model analysis
    • Authors: C. S. Harrison; B. Hales, S. Siedlecki, R. M. Samelson
      Abstract: A simple box model is used to examine oxygen depletion in an idealized ocean‐margin upwelling system. Near‐bottom oxygen depletion is controlled by a competition between flushing with oxygenated offshore source waters and respiration of particulate organic matter produced near the surface and retained near the bottom. Upwelling‐supplied nutrients are consumed in the surface box, and some surface particles sink to the bottom where they respire, consuming oxygen. Steady states characterize the potential for hypoxic near‐bottom oxygen depletion; this potential is greatest for faster sinking rates, and largely independent of production timescales except in that faster production allows faster sinking. Timescales for oxygen depletion depend on upwelling and productivity differently, however, as oxygen depletion can only be reached in meaningfully short times when productivity is rapid. Hypoxia thus requires fast production, to capture upwelled nutrients, and fast sinking, to deliver the respiration potential to model bottom waters. Combining timescales allows generalizations about tendencies toward hypoxia. If timescales of sinking are comparable to or smaller than the sum of those for respiration and flushing, the steady state will generally be hypoxic, and results indicate optimal timescales and conditions exist to generate hypoxia. For example, the timescale for approach to hypoxia lengthens with stronger upwelling, since surface particle and nutrient are shunted off‐shelf, in turn reducing subsurface respiration and oxygen depletion. This suggests that if upwelling winds intensify with climate change the increased forcing could offer mitigation of coastal hypoxia, even as the oxygen levels in upwelled source waters decline. This article is protected by copyright. All rights reserved.
      PubDate: 2016-03-23T17:50:52.097507-05:
      DOI: 10.1002/2015JC011328
       
  • An inverse modeling study of circulation in the Eastern Bering Sea during
           2007‐2010
    • Authors: Gleb Panteleev; Max Yaremchuk, Oceana Francis, Philis J. Stabeno, T. Weingartner, J. Zhang
      Abstract: A two‐way nested 4d‐variational data assimilation system is implemented in the Eastern Bering Sea (EBS) to investigate changes in circulation and thermodynamic state for a 3.8‐year period. Assimilated observations include data from 19 moorings deployed on the shelf and in the Bering Strait, 1705 hydrographic stations occupied during eight surveys, and remotely sensed sea surface temperature and sea surface height (SSH) data. Validation of the presented 4dVar reanalysis against the output of two sequential data‐assimilative systems (the Bering Ecosystem Study ice‐ocean Modeling and Assimilation System (BESTMAS) and the Arctic Cap Nowcast‐Forecast System (ACNFS)) has shown that the product is more consistent with the observed transports in the Bering Strait and in the EBS interior both in terms of their magnitude and time variability. Analysis of the data‐optimized solution quantifies a sequence of wind‐forced events that resulted in the anomalous heat and freshwater transports through the Bering Strait, including a 28‐day long flow reversal that occurred in November of 2009 and carried Siberian Coastal Current water down to the Gulf of Anadyr. Lagrangian study of the Arctic‐bound Pacific waters indicates the extreme importance of the cross‐shelf exchange along the path of the Bering Slope Current and quantifies the spectrum of residence times for the waters entering EBS through Unimak Pass and through Aleutian passages. Residence times in the EBS cold pool are diagnosed to be 2‐3 times longer than those in the surrounding waters. This article is protected by copyright. All rights reserved.
      PubDate: 2016-02-22T18:03:09.359725-05:
      DOI: 10.1002/2015JC011287
       
  • Issue Information
    • Pages: 2857 - 2859
      PubDate: 2016-06-23T01:28:12.914059-05:
      DOI: 10.1002/jgrc.21405
       
  • Editorial: Proposing a Special Section
    • Authors: Peter G. Brewer
      Pages: 2860 - 2861
      PubDate: 2016-05-14T21:30:23.650573-05:
      DOI: 10.1002/2016JC011913
       
  • Ocean bottom pressure records from the Cascadia array and short surface
           gravity waves
    • Authors: Charles Peureux; Fabrice Ardhuin
      Pages: 2862 - 2873
      Abstract: The ocean bottom pressure records from eight stations of the Cascadia array are used to investigate the properties of short surface gravity waves with frequencies ranging from 0.2 to 5 Hz. It is found that the pressure spectrum at all sites is a well‐defined function of the wind speed U10 and frequency f, with only a minor shift of a few dB from one site to another that can be attributed to variations in bottom properties. This observation can be combined with the theoretical prediction that the ocean bottom pressure spectrum is proportional to the surface gravity wave spectrum E(f) squared, times the overlap integral I(f) which is given by the directional wave spectrum at each frequency. This combination, using E(f) estimated from modeled spectra or parametric spectra, yields an overlap integral I(f) that is a function of the local wave age f/fPM=fU10/0.13 g. This function is maximum for f∕fPM = 8 and decreases by 10 dB for f∕fPM = 2 and f∕fPM = 30. This shape of I(f) can be interpreted as a maximum width of the directional wave spectrum at f∕fPM = 8, possibly equivalent to an isotropic directional spectrum, and a narrower directional distribution toward both the dominant low frequencies and the higher capillary‐gravity wave frequencies.
      PubDate: 2016-05-01T10:11:24.136792-05:
      DOI: 10.1002/2015JC011580
       
  • Antarctic tides from GRACE satellite accelerations
    • Pages: 2874 - 2886
      Abstract: The extended length of the GRACE data time series (now 13.5 years) provides the unique opportunity to estimate global mass variations due to ocean tides at large (∼300 km) spatial scales. State‐of‐the‐art global tide models rely heavily on satellite altimetry data, which are sparse for latitudes higher than 66°. Thus, the performance of the models is typically worse at higher latitudes. GRACE data, alternately, extend to polar latitudes and therefore provide information for both model validation and improvement at the higher latitudes. In this work, 11 years of GRACE inter‐satellite range‐acceleration measurements are inverted to solve for corrections to the amplitudes and phases of the major solar and lunar ocean tidal constituents (M2, K1, S2, and O1) from the GOT4.7 ocean tide model at latitudes south of 50°S. Two independent inversion and regularization methods are employed and compared against one another. Uncertainty estimates are derived by subtracting two independent solutions, each spanning a unique 5.5 years of data. Features above the noise floor in the derived solutions likely represent errors in GOT4.7. We find the GOT4.7 amplitudes to be generally too small for M2 and K1, and too large for S2 and O1, and to spatially correlate with geographic regions where GOT4.7 predicts the largest tidal amplitudes. In particular, we find GOT4.7 errors to be dominant over the Patagonia shelf (M2), the Filchner‐Ronne Ice Shelf (M2 and S2), the Ross Ice Shelf (S2), and the Weddell and Ross Seas (K1 and O1).
      PubDate: 2016-05-04T01:13:11.716305-05:
      DOI: 10.1002/2015JC011488
       
  • The role of heating, winds, and topography on sea level changes in the
           North Atlantic
    • Authors: Jinting Zhang; Kathryn A. Kelly, LuAnne Thompson
      Pages: 2887 - 2900
      Abstract: Seasonal and interannual‐to‐decadal variations of large‐scale altimetric sea surface height (SSH) owing to surface heating and wind forcing in the presence of topography are investigated using simplified models. The dominant forcing mechanisms are time scale dependent. On the seasonal time scale, locally forced thermosteric height explains most of the SSH variance north of 18°N. First‐mode linear long baroclinic Rossby waves forced by changes in the winds and eastern boundary conditions explain most of the variance between 10°N and 15°N and are also important east of Greenland. On interannual‐to‐decadal time scales, local thermosteric height remains important at several locations in the middle and high latitudes. A topographic Sverdrup response explains interannual‐to‐decadal SSH between 53°N and 63°N east of Greenland. Farther south, the linear Rossby wave model explains SSH variations on interannual‐to‐decadal time scales between 30°N and 50°N from mid‐basin to the eastern boundary. Propagation of the eastern boundary condition into the interior dominates the interannual‐to‐decadal SSH signals south of 30°N. The effect from NAO‐related heat flux on SSH is small, but forcing the topographic Sverdrup models with NAO‐regressed winds gives slightly better agreement with the observed SSH in the subpolar gyre on interannual‐to‐decadal time scales than using the full winds.
      PubDate: 2016-05-04T01:11:38.071912-05:
      DOI: 10.1002/2015JC011492
       
  • Relationship between optimal precursory disturbances and optimally growing
           initial errors associated with ENSO events: Implications to target
           observations for ENSO prediction
    • Authors: Junya Hu; Wansuo Duan
      Pages: 2901 - 2917
      Abstract: By superimposing initial sea temperature disturbances in neutral years, we determine the precursory disturbances that are most likely to evolve into El Niño and La Niña events using an Earth System Model. These precursory disturbances for El Niño and La Niña events are deemed optimal precursory disturbances because they are more likely to trigger strong ENSO events. Specifically, the optimal precursory disturbance for El Niño exhibits negative sea surface temperature anomalies (SSTAs) in the central‐eastern equatorial Pacific. Additionally, the subsurface temperature component exhibits negative anomalies in the upper layers of the eastern equatorial Pacific and positive anomalies in the lower layers of the western equatorial Pacific. The optimal precursory disturbance for La Niña is almost opposite to that of El Niño. The optimal precursory disturbances show that both El Niño and La Niña originate from precursory signals in the subsurface layers of the western equatorial Pacific and in the surface layers of the eastern equatorial Pacific. We find that the optimal precursory disturbances for El Niño and La Niña are particularly similar to the optimally growing initial errors associated with El Niño prediction that have been presented in previous studies. The optimally growing initial errors show that the optimal precursor source areas represent the sensitive areas for target observations associated with ENSO prediction. Combining the optimal precursory disturbances and the optimally growing initial errors for ENSO, we infer that additional observations in these sensitive areas can reduce initial errors and be used to detect precursory signals, thereby improving ENSO predictions.
      PubDate: 2016-05-04T01:11:32.290691-05:
      DOI: 10.1002/2015JC011386
       
  • Sources of dissolved inorganic carbon to the Canada Basin halocline: A
           multitracer study
    • Pages: 2918 - 2936
      Abstract: We examine the dissolved inorganic carbon maximum in the Canada Basin halocline using a suite of geochemical tracers to gain insight into the factors that contribute to the persistence of this feature. Hydrographic and geochemical samples were collected in the upper 500 m of the southwestern Canada Basin water column in the summer of 2008 and fall of 2009. These observations were used to identify conservative and nonconservative processes that contribute dissolved inorganic carbon to halocline source waters, including shelf sediment organic matter remineralization, air‐sea gas exchange, and sea‐ice brine export. Our results indicate that the remineralization of organic matter that occurs along the Bering and Chukchi Sea shelves is the overwhelming contributor of dissolved inorganic carbon to Pacific Winter Water that occupies the middle halocline in the southwestern Canada Basin. Nonconservative contributions from air‐sea exchange and sea‐ice brine are not significant. The broad salinity range associated with the DIC maximum, compared to the narrow salinity range of the nutrient maximum, is due to mixing between Pacific and Atlantic water and not abiotic addition of DIC.
      PubDate: 2016-05-04T01:11:58.17051-05:0
      DOI: 10.1002/2015JC011535
       
  • A global surface drifter data set at hourly resolution
    • Authors: Shane Elipot; Rick Lumpkin, Renellys C. Perez, Jonathan M. Lilly, Jeffrey J. Early, Adam M. Sykulski
      Pages: 2937 - 2966
      Abstract: The surface drifting buoys, or drifters, of the Global Drifter Program (GDP) are predominantly tracked by the Argos positioning system, providing drifter locations with O(100 m) errors at nonuniform temporal intervals, with an average interval of 1.2 h since January 2005. This data set is thus a rich and global source of information on high‐frequency and small‐scale oceanic processes, yet is still relatively understudied because of the challenges associated with its large size and sampling characteristics. A methodology is described to produce a new high‐resolution global data set since 2005, consisting of drifter locations and velocities estimated at hourly intervals, along with their respective errors. Locations and velocities are obtained by modeling locally in time trajectories as a first‐order polynomial with coefficients obtained by maximizing a likelihood function. This function is derived by modeling the Argos location errors with t location‐scale probability distribution functions. The methodology is motivated by analyzing 82 drifters tracked contemporaneously by Argos and by the Global Positioning System, where the latter is assumed to provide true locations. A global spectral analysis of the velocity variance from the new data set reveals a sharply defined ridge of energy closely following the inertial frequency as a function of latitude, distinct energy peaks near diurnal and semidiurnal frequencies, as well as higher‐frequency peaks located near tidal harmonics as well as near replicates of the inertial frequency. Compared to the spectra that can be obtained using the standard 6‐hourly GDP product, the new data set contains up to 100% more spectral energy at some latitudes.
      PubDate: 2016-05-04T01:11:04.794914-05:
      DOI: 10.1002/2016JC011716
       
  • Sea ice production variability in Antarctic coastal polynyas
    • Authors: Takeshi Tamura; Kay I. Ohshima, Alexander D. Fraser, Guy D. Williams
      Pages: 2967 - 2979
      Abstract: Enhanced sea ice production (SIP) in Antarctic coastal polynyas forms dense shelf water (DSW), leading to Antarctic Bottom Water (AABW) formation that ultimately drives the lower limb of the meridional overturning circulation. Some studies suggest that the variability of SIP in Antarctic coastal polynyas is driven by the influence of atmospheric forcing, i.e., surface winds and air temperature. Our previous mapping of SIP in 13 major Antarctic coastal polynyas from 1992 to 2007, using a heat flux calculation with ice thickness data derived from satellite data, is extended here to examine the interannual and seasonal variability of SIP from 1992 to 2013. The interannual variability of total ice production correlates more strongly with polynya extent than with atmospheric forcing, with the exception of the Shackleton Polynya, which correlates well with wind. There is no coherent signal in the interannual variability between the major Antarctic coastal polynyas. We find that stochastic changes to the coastal “icescape,” i.e., ice shelves, floating glaciers, fast ice, together with offshore first‐year ice, are also important factors driving SIP variability on multiyear time scales. Both the Ross Ice Shelf Polynya and Mertz Glacier Polynya experienced a significant reduction in SIP due to calving events and the repositioning of icebergs and fast ice. Our results also show opposing trends between polynya‐based SIP and sea ice extent in key regions of Antarctic sea ice change. Close monitoring of coastal icescape dynamics and change is essential to better understand the long‐term impact of coastal polynya variability and its influence on regional AABW production.
      PubDate: 2016-05-04T01:09:30.601279-05:
      DOI: 10.1002/2015JC011537
       
  • Turbulent flow field and air entrainment in laboratory plunging breaking
           waves
    • Pages: 2980 - 3009
      Abstract: This paper presents laboratory measurements of turbulent flow fields and void fraction in deep‐water plunging breaking waves using imaging and optical fiber techniques. Bubble‐size distributions are also determined based on combined measurements of velocity and bubble residence time. The most excited mode of the local intermittency measure of the turbulent flow and its corresponding length scale are obtained using a wavelet‐based method and found to correlate with the swirling strength and vorticity. Concentrated vortical structures with high intermittency are observed near the lower boundaries of the aerated rollers where the velocity shear is high; the length scale of the deduced eddies ranges from 0.05 to 0.15 times the wave height. The number of bubbles with a chord length less than 2 mm demonstrates good correlation with the swirling strength. The power‐law scaling and the Hinze scale of the bubbles determined from the bubble chord length distribution compare favorably with existing measurements. The turbulent dissipation rate, accounting for void fraction, is estimated using mixture theory. When void fraction is not considered, the turbulent dissipation rate is underestimated by more than 70% in the initial impinging and the first splash‐up roller. A significant discrepancy of approximately 67% between the total energy dissipation rate and the turbulence dissipation rate is found. Of this uncounted dissipation, 23% is caused by bubble‐induced dissipation.
      PubDate: 2016-05-13T01:26:41.887105-05:
      DOI: 10.1002/2015JC011377
       
  • Infragravity waves on fringing reefs in the tropical Pacific: Dynamic
           setup
    • Authors: J. M. Becker; M. A. Merrifield, H. Yoon
      Pages: 3010 - 3028
      Abstract: Cross‐shore pressure and current observations from four fringing reefs of lengths ranging from 135 to 420 m reveal energetic low‐frequency (∼0.001–0.05 Hz) motions. The spatial structure and temporal amplitudes of an empirical orthogonal function analysis of the pressure measurements suggest the dominant low‐frequency variability is modal. Incoming and outgoing linear flux estimates also support partially standing modes on the reef flat during energetic events. A cross‐covariance analysis suggests that breakpoint forcing excites these partially standing modes, similar to previous findings at other steep reefs. The dynamics of Symonds et al. (1982) with damping are applied to a step reef, with forcing obtained by extending a point break model of Vetter et al. (2010) for breaking wave setup to the low‐frequency band using the shoaled envelope of the incident free surface elevation. A one parameter, linear analytical model for the reef flat free surface elevation is presented, which describes between 75% and 97% of the variance of the observed low‐frequency shoreline significant wave height for all reefs considered over a range of conditions. The linear model contains a single dimensionless parameter that is the ratio of the inertial to dissipative time scales, and the observations from this study exhibit more low‐frequency variability when the dissipative time scale is greater than the inertial time scale for the steep reefs considered.
      PubDate: 2016-05-13T01:26:34.425471-05:
      DOI: 10.1002/2015JC011516
       
  • Probabilistic assessment of erosion and flooding risk in the northern Gulf
           of Mexico
    • Authors: Thomas Wahl; Nathaniel G. Plant, Joseph W. Long
      Pages: 3029 - 3043
      Abstract: We assess erosion and flooding risk in the northern Gulf of Mexico by identifying interdependencies among oceanographic drivers and probabilistically modeling the resulting potential for coastal change. Wave and water level observations are used to determine relationships between six hydrodynamic parameters that influence total water level and therefore erosion and flooding, through consideration of a wide range of univariate distribution functions and multivariate elliptical copulas. Using these relationships, we explore how different our interpretation of the present‐day erosion/flooding risk could be if we had seen more or fewer extreme realizations of individual and combinations of parameters in the past by simulating 10,000 physically and statistically consistent sea‐storm time series. We find that seasonal total water levels associated with the 100 year return period could be up to 3 m higher in summer and 0.6 m higher in winter relative to our best estimate based on the observational records. Impact hours of collision and overwash—where total water levels exceed the dune toe or dune crest elevations—could be on average 70% (collision) and 100% (overwash) larger than inferred from the observations. Our model accounts for non‐stationarity in a straightforward, non‐parametric way that can be applied (with little adjustments) to many other coastlines. The probabilistic model presented here, which accounts for observational uncertainty, can be applied to other coastlines where short record lengths limit the ability to identify the full range of possible wave and water level conditions that coastal mangers and planners must consider to develop sustainable management strategies.
      PubDate: 2016-05-13T01:26:22.297285-05:
      DOI: 10.1002/2015JC011482
       
  • Assessing climate impacts and risks of ocean albedo modification in the
           Arctic
    • Authors: N. Mengis; T. Martin, D. P. Keller, A. Oschlies
      Pages: 3044 - 3057
      Abstract: The ice albedo feedback is one of the key factors of accelerated temperature increase in the high northern latitudes under global warming. This study assesses climate impacts and risks of idealized Arctic Ocean albedo modification (AOAM), a proposed climate engineering method, during transient climate change simulations with varying representative concentration pathway (RCP) scenarios. We find no potential for reversing trends in all assessed Arctic climate metrics under increasing atmospheric CO2 concentrations. AOAM only yields an initial offset during the first years after implementation. Nevertheless, sea ice loss can be delayed by 25(60) years in the RCP8.5(RCP4.5) scenario and the delayed thawing of permafrost soils in the AOAM simulations prevents up to 40(32) Pg of carbon from being released by 2100. AOAM initially dampens the decline of the Atlantic Meridional Overturning and delays the onset of open ocean deep convection in the Nordic Seas under the RCP scenarios. Both these processes cause a subsurface warming signal in the AOAM simulations relative to the default RCP simulations with the potential to destabilize Arctic marine gas hydrates. Furthermore, in 2100, the RCP8.5 AOAM simulation diverts more from the 2005–2015 reference state in many climate metrics than the RCP4.5 simulation without AOAM. Considering the demonstrated risks, we conclude that concerning longer time scales, reductions in emissions remain the safest and most effective way to prevent severe changes in the Arctic.
      PubDate: 2016-05-13T01:26:20.356746-05:
      DOI: 10.1002/2015JC011433
       
  • On the nature of cross‐isobath energy fluxes in topographically
           modified barotropic semidiurnal Kelvin waves
    • Authors: Tianyi Zhang; Alexander E Yankovsky
      Pages: 3058 - 3074
      Abstract: Continental shelf topography modifies a Kelvin wave into a hybrid Kelvin‐edge wave with a nonzero across‐isobath velocity and a phase speed that decreases with increasing wave number while the group velocity reaches a minimum at intermediate wave numbers. We model the modified semidiurnal Kelvin wave adjustment to alongshore changes in the shelf width. The model domain consists of two alongshore‐uniform continental shelves of different widths adjoined through a 150 km long transition zone. The continental shelf and slope are adjacent to an ocean of a constant depth, allowing radiation of Poincaré waves. We consider three shelf widths of 150, 250, and 300 km, where properties of a zero mode at semidiurnal frequency change from Kelvin wave like to edge wave like. For each shelf width, a zero wave mode has its distinctive alongshore energy flux structure on the shelf. As the incident wave encounters a variable shelf width, the alongshore energy flux converges (diverges) on the shelf resulting in an offshore (onshore) energy flux over the continental slope. Furthermore, for a strongly convergent alongshore energy flux, the incident wave mode scatters into radiating Poincaré waves. On sufficiently wide shelves, a strong across‐isobath energy flux comparable with the incident wave energy flux can be triggered even by relatively modest changes of shelf width. An energy flux divergence parameter De is defined, which scales with magnitude and direction of the energy flux across the continental slope. More than 50% of the incident energy flux scatters into modes radiating offshore when De is −1 or less.
      PubDate: 2016-05-13T01:26:15.848384-05:
      DOI: 10.1002/2015JC011617
       
  • The vertical structure of upper ocean variability at the Porcupine Abyssal
           Plain during 2012–2013
    • Authors: Gillian M. Damerell; Karen J. Heywood, Andrew F. Thompson, Umberto Binetti, Jan Kaiser
      Pages: 3075 - 3089
      Abstract: This study presents the characterization of variability in temperature, salinity and oxygen concentration, including the vertical structure of the variability, in the upper 1000 m of the ocean over a full year in the northeast Atlantic. Continuously profiling ocean gliders with vertical resolution between 0.5 and 1 m provide more information on temporal variability throughout the water column than time series from moorings with sensors at a limited number of fixed depths. The heat, salt and dissolved oxygen content are quantified at each depth. While the near surface heat content is consistent with the net surface heat flux, heat content of the deeper layers is driven by gyre‐scale water mass changes. Below ∼150m, heat and salt content display intraseasonal variability which has not been resolved by previous studies. A mode‐1 baroclinic internal tide is detected as a peak in the power spectra of water mass properties. The depth of minimum variability is at ∼415m for both temperature and salinity, but this is a depth of high variability for oxygen concentration. The deep variability is dominated by the intermittent appearance of Mediterranean Water, which shows evidence of filamentation. Susceptibility to salt fingering occurs throughout much of the water column for much of the year. Between about 700–900 m, the water column is susceptible to diffusive layering, particularly when Mediterranean Water is present. This unique ability to resolve both high vertical and temporal variability highlights the importance of intraseasonal variability in upper ocean heat and salt content, variations that may be aliased by traditional observing techniques.
      PubDate: 2016-05-13T01:26:00.354526-05:
      DOI: 10.1002/2015JC011423
       
  • Seasonal variability of the meridional overturning circulation in the
           South China Sea and its connection with inter‐ocean transport based
           on SODA2.2.4
    • Authors: Yaohua Zhu; Guohong Fang, Zexun Wei, Yonggang Wang, Fei Teng, Tangdong Qu
      Pages: 3090 - 3105
      Abstract: We have proposed a five‐layer‐scheme to investigate the volume transport through the South China Sea (SCS) based on the updated Simple Ocean Data Assimilation (SODA2.2.4) product. By demonstrating horizontal transport in each layer, we have revealed different formation mechanisms for the meridional overturning circulation (MOC) in winter and summer in the SCS. Our analysis suggests three meridional circulation systems in the SCS: (1) the seasonal monsoon‐driven circulation in the surface layer, i.e., southward circulation in winter and northward in summer, (2) the compensatory transport‐induced seasonal intermediate MOC in the central SCS, and (3) the persistent deep MOC in the southern SCS all year round. By examining vertical velocity distribution, we have identified that the major overturning process of the intermediate MOC is located along the continental slope east and southeast of Vietnam, while the major overturning process of the deep MOC is located along the continental slope northwest of Borneo. The downwelling in the intermediate MOC in winter and upwelling in the deep MOC all year round bring different water masses to the intermediate and subintermediate layers to be mixed in the SCS. We found no evidence to suggest that the strength and extent of the MOC south of 18°N are related to inter‐ocean volume transport. The surface layer transport in the Luzon Strait has been decreasing since the 1960s. However, the causes of the meridionally staggered and interdecadal alternating acceleration/slowdown of the meridional stream function difference are unknown.
      PubDate: 2016-05-13T01:26:14.288566-05:
      DOI: 10.1002/2015JC011443
       
  • Cadmium in the waters off South Morocco: Nature of particles hosting Cd
           and insights into the mechanisms fractionating Cd from phosphate
    • Pages: 3106 - 3120
      Abstract: In this study, we report the distributions of total dissolvable cadmium and particulate cadmium from 27 stations in southern Moroccan coastal waters (22°N–30°N), which is part of the North‐West African upwelling system. These distributions were predominantly controlled by upwelling of the North Atlantic Central Waters (NACWs) and uptake by primary production. Atmospheric inputs and phosphogypsum slurry inputs from the phosphate industry at Jorf Lasfar (33°N), recently estimated as an important source of dissolved cadmium (240 t Cd yr−1), are at best of minor importance for the studied waters. Our study provides new insights into the mechanisms fractionating cadmium from phosphate. In the upper 30 m, the anomalies observed in terms of Cd:P ratios in both the particulate and total dissolvable fractions were related to an overall preferential uptake of phosphate. We show that the type of phytoplanktonic assemblage (diatoms versus dinoflagellates) is also a determinant of the fractionation intensity. In subsurface waters (30–60 m), a clear preferential release of P (versus Cd) was observed indicating that remineralization in Oxygen Minimum Zones is a key process in sequestering Cd.
      PubDate: 2016-05-13T01:25:27.535457-05:
      DOI: 10.1002/2016JC011688
       
  • Observations of wave transformation over a fringing coral reef and the
           importance of low‐frequency waves and offshore water levels to
           runup, overwash, and coastal flooding
    • Authors: Olivia M. Cheriton; Curt D. Storlazzi, Kurt J. Rosenberger
      Pages: 3121 - 3140
      Abstract: Many low‐lying tropical islands are susceptible to sea level rise and often subjected to overwash and flooding during large wave events. To quantify wave dynamics and wave‐driven water levels on fringing coral reefs, a 5 month deployment of wave gauges and a current meter was conducted across two shore‐normal transects on Roi‐Namur Island in the Republic of the Marshall Islands. These observations captured two large wave events that had waves with maximum heights greater than 6 m with peak periods of 16 s over the fore reef. The larger event coincided with a peak spring tide, leading to energetic, highly skewed infragravity (0.04–0.004 Hz) and very low frequency (0.004–0.001 Hz) waves at the shoreline, which reached heights of 1.0 and 0.7 m, respectively. Water surface elevations, combined with wave runup, reached 3.7 m above the reef bed at the innermost reef flat adjacent to the toe of the beach, resulting in flooding of inland areas. This overwash occurred during a 3 h time window that coincided with high tide and maximum low‐frequency reef flat wave heights. The relatively low‐relief characteristics of this narrow reef flat may further drive shoreline amplification of low‐frequency waves due to resonance modes. These results (1) demonstrate how the coupling of high offshore water levels with low‐frequency reef flat wave energetics can lead to large impacts along fringing reef‐lined shorelines, such as island overwash, and (2) lend support to the hypothesis that predicted higher sea levels will lead to more frequent occurrences of these extreme events, negatively impacting coastal resources and infrastructure.
      PubDate: 2016-05-13T01:26:08.465943-05:
      DOI: 10.1002/2015JC011231
       
  • A dynamical model of Kara Sea land‐fast ice
    • Authors: Einar Olason
      Pages: 3141 - 3158
      Abstract: This paper introduces modifications to the traditional viscous‐plastic sea‐ice dynamical model, which are necessary to model land‐fast ice in the Kara Sea in a realistic manner. The most important modifications are an increase in the maximum viscosity from the standard value of ζmax=(2.5×108s)P to ζmax=(1013s)P, and to use a solver for the momentum equation capable of correctly solving for small ice velocities (the limit here is set to 10−4 m/s). Given these modifications, a necessary condition for a realistic fast‐ice simulation is that the yield curve give sufficient uniaxial compressive strength. This is consistent with the idea that land‐fast ice in the Kara Sea forms primarily via static arching. The modified model is tested and tuned using forcing data and observations from 1997 and 1998. The results show that it is possible to model land‐fast ice using this model with the modifications mentioned above. The model performs well in terms of modeled fast‐ice extent, but suffers from unrealistic break‐ups during the start and end of the fast‐ice season. The main results are that fast ice in the Kara Sea is supported by arching of the ice, the arches footers resting on a chain of islands off shore.
      PubDate: 2016-05-13T01:25:35.890507-05:
      DOI: 10.1002/2016JC011638
       
  • Dissipation processes in the Tongue of the Ocean
    • Authors: James A. Hooper; Molly O. Baringer, Louis C. St. Laurent, William K. Dewar, Doug Nowacek
      Pages: 3159 - 3170
      Abstract: The Tongue of the Ocean (TOTO) region located within the Bahamas archipelago is a relatively understudied region in terms of both its biological and physical oceanographic characteristics. A prey‐field mapping cruise took place in the fall between 15 September 2008 and 1 October 2008, consisting of a series of transects and “clovers” to study the spatial and temporal variability. The region is characterized by a deep scattering layer (DSL), which is preyed on by nekton that serves as the food for beaked whale and other whale species. This study marks the first of its kind where concurrent measurements of acoustic backscatter and turbulence have been conducted for a nekton scattering layer well below the euphotic zone. Turbulence data collected from a Deep Microstructure Profiler are compared to biological and shear data collected by a 38 kHz Simrad EK 60 echo sounder and a hydrographic Doppler sonar system, respectively. From these measurements, the primary processes responsible for the turbulent production in the TOTO region are assessed. The DSL around 500 m and a surface scattering layer (SSL) are investigated for raised ε values. Strong correlation between turbulence levels and scattering intensity of prey is generally found in the SSL with dissipation levels as large as ∼10−7 W kg−1, 3 orders of magnitude above background levels. In the DSL and during the diel vertical migration, dissipation levels ∼10−8 W kg−1 were observed.
      PubDate: 2016-05-14T21:30:31.0139-05:00
      DOI: 10.1002/2015JC011165
       
 
 
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