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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: 26, SJR: 2.439, h-index: 91)
Geophysical Research Letters     Full-text available via subscription   (Followers: 107, SJR: 3.323, h-index: 185)
Global Biogeochemical Cycles     Full-text available via subscription   (Followers: 15, SJR: 3.22, h-index: 136)
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J. of Geophysical Research : Oceans     Partially Free   (Followers: 49)
J. of Geophysical Research : Planets     Full-text available via subscription   (Followers: 107)
J. of Geophysical Research : Solid Earth     Full-text available via subscription   (Followers: 44)
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Paleoceanography     Full-text available via subscription   (Followers: 5, SJR: 3.067, h-index: 100)
Radio Science     Full-text available via subscription   (Followers: 37, SJR: 1.072, h-index: 59)
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Water Resources Research     Full-text available via subscription   (Followers: 79, SJR: 2.661, h-index: 144)
Journal Cover Journal of Geophysical Research : Oceans
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   Published by AGU Homepage  [17 journals]
  • Dense Water Formation in the North-Western Mediterranean area during
           HyMeX-SOP2 in 1/36° ocean simulations: Ocean-atmosphere coupling impact
    • Authors: Cindy Lebeaupin Brossier; Fabien Léger, Hervé Giordani, Jonathan Beuvier, Marie-Noëlle Bouin, Véronique Ducrocq, Nadia Fourrié
      Abstract: The north-western Mediterranean Sea is a key location for the thermohaline circulation of the basin. The area is characterized by intense air-sea exchanges favoured by the succession of strong northerly and north-westerly wind situations (mistral and tramontane) in autumn and winter. Such meteorological conditions lead to significant evaporation and ocean heat loss that are well known as the main triggering factor for the Dense Water Formation (DWF) and winter deep convection episodes.During the HyMeX second field campaign (SOP2, 1 February to 15 March 2013), several platforms were deployed in the area in order to document the DWF and the ocean deep convection, as the air-sea interface conditions.This study investigates the role of the ocean-atmosphere coupling on DWF during winter 2012-2013. The coupled system, based on the NEMO-WMED36 ocean model (1/36° resolution) and the AROME-WMED atmospheric model (2.5 km-resolution), was run during two months covering the SOP2 and is compared to an ocean-only simulation forced by AROME-WMED real-time forecasts and to observations collected in the north-western Mediterranean area during the HyMeX SOP2.The comparison shows small differences in terms of net heat, water and momentum fluxes. On average, DWF is slightly sensitive to air-sea coupling. However fine-scale ocean processes, such as shelf DWF and export or eddies and fronts at the rim of the convective patch are significantly modified. The wind-current interactions constitute an efficient coupled process at fine scale, acting as a turbulence propagating vectors, producing large mixing and convection at the rim of the convective patch.
      PubDate: 2017-05-25T17:26:12.158651-05:
      DOI: 10.1002/2016JC012526
       
  • Real-time monitoring of nutrients in the Changjiang Estuary reveals
           short-term nutrient-algal bloom dynamics
    • Authors: Kui Wang; Jianfang Chen, Xiaobo Ni, Dingyong Zeng, Dewang Li, Haiyan Jin, Patricia M. Glibert, Wenxian Qiu, Daji Huang
      Abstract: The Changjiang Estuary is a large-river estuary ecosystem in the East China Sea, and its plume, the Changjiang Diluted Water (CDW), transports a large mass of nutrients (NO3-+NO2-, PO43-, SiO32-) to the shelf sea, leading to substantial eutrophication; the CDW also supports high primary production. However, relationships between nutrient delivery and phytoplankton responses have been difficult to establish, as many nutrient delivery events and algal blooms are episodic, and the CDW may expand or become detached with changing winds. To study the relationship between nutrient delivery events, algal blooms and estuarine metabolism dynamics, a buoy system was deployed in the CDW from 9 September to 10 October, 2013, with measurements of chlorophyll a and dissolved nutrients. Day-to-day nutrient increases co-varied with salinity decreases, regulated by both the spring-neap tidal cycle and wind events. Several specific nutrient injection periods were detected, each followed by nutrient draw-down and chlorophyll a accumulation (algal blooms). Each algal bloom had its own unique pattern of nutrient uptake based on change in nutrient ratios (ΔN:ΔP; ΔN:ΔSi) and appeared to be dominated by different algal groups. These events occurred under weak wind and stable hydrodynamic conditions. Ecosystem metabolism based on net community production (NCP) showed that the upper estuarine ecosystem was autotrophic when chlorophyll a accumulated, but heterotrophic when wind-induced mixing strengthened, and upwelling brought organic-rich water to the near surface. In spite of several short-lived algal blooms, the average NCPdaily was negative during the observation period, indicating a net source of CO2 to the atmosphere.
      PubDate: 2017-05-25T17:20:35.654708-05:
      DOI: 10.1002/2016JC012450
       
  • The effects of oceanic barrier layer on the upper ocean response to
           tropical cyclones
    • Authors: Youfang Yan; Li Li, Chunzai Wang
      Abstract: Previous studies of the barrier layer (BL) and tropical cyclone (TC) intensification show controversial results. Some studies suggest that the BL is favorable for TC intensification, whereas others demonstrate that the BL has little impact. The present paper reconciles these previous results by showing that the influence of the BL on TC intensification is complex dependent on the stages, TC intensity, TC forcing time and ocean stratification. The influence can be divided into three different stages. First, when a TC is weak or the TC-induced forcing can not break through the mixed layer (ML), the presence of the BL leads to a thin ML, results in a large ML cooling, and reduces the effective TC heat potential (ETCHP) by distributing the air-sea heat loss to the thin ML, thus suppresses TC growth. Second, when TC strengthens or wind forcing increases, the forcing becomes strong enough to break through the ML and penetrates into the BL. The entrainment of BL water, which is warmer than the cooled ML, compensates the surface heat loss and leads to an increase of the ML temperature, supporting TC growth. Third, when the TC-induced forcing is strong enough to break through the BL base and penetrates to the thermocline, the BL reduces the magnitudes of cooling temperature and of decreasing ETCHP compared to those without the BL, which is also favorable for TC intensification. Thus, whether the BL intensifies TCs depends the conditions associated with the upper layer ocean and TCs.
      PubDate: 2017-05-25T17:20:31.630247-05:
      DOI: 10.1002/2017JC012694
       
  • Phytoplankton bloom triggered by an anticyclonic eddy: The combined effect
           of eddy-Ekman pumping and winter mixing
    • Authors: Qingyou He; Haigang Zhan, Yiping Shuai, Shuqun Cai, Qian P. Li, Gaolong Huang, Junmin Li
      Abstract: Recent composite based researches reveal that anticyclonic eddies facilitate the growth of phytoplankton in the subtropical gyres. Two dynamical mechanisms, eddy-Ekman pumping and winter mixing, have been examined individually, but their relative and combined effects remain unclear. Using satellite observations and model simulations, this study investigated the process of a distinct phytoplankton bloom generated in an anticyclonic eddy in the nutrient-depleted southeastern Indian Ocean. The bloom propagated westward along with the eddy for more than 600 km from late April to August in 2010. The peak of surface chlorophyll concentration in the eddy is 2.2 times larger than the mean value of the ambient. The development of the bloom is dominated by the winter deepening of mixed layer, whose velocity in vertical nutrient flux is on average 3 times larger than that of eddy-Ekman pumping. The results of a 1-D physical-biogeochemical model demonstrate that the role of eddy-Ekman pumping is also indispensable, because it not only transports extra nutrients into the mixed layer, but also results in significant chlorophyll enrichment in subsurface water. The superposition of eddy-Ekman pumping on winter mixing triples the chlorophyll enrichment both at the surface and in the upper layer, and the entrainment of subsurface phytoplankton into the mixed layer contributes significantly to the surface bloom, especially in its initial stage. Both of the satellite observations and model simulation show that eddy-Ekman pumping can lead to an early occurrence of the bloom for more than two weeks.
      PubDate: 2017-05-25T17:20:28.999248-05:
      DOI: 10.1002/2017JC012763
       
  • OSSE quantitative assessment of rapid-response pre-storm ocean surveys to
           improve coupled tropical cyclone prediction
    • Authors: G. R. Halliwell; M. Mehari, L. K. Shay, V. H. Kourafalou, H. Kang, H-S Kim, J. Dong, R. Atlas
      Abstract: Ocean fields that initialize coupled TC prediction models must accurately represent the dynamics of mesoscale features and the associated distribution of upper-ocean temperature and salinity. They must also provide unbiased realizations of upper-ocean heat content and stratification. Ocean Observing System Simulation Experiments (OSSEs) are performed for three storms [Isaac (2012), Edouard (2014), Gonzalo (2014)] to assess the impact of rapid-response pre-storm ocean profile surveys on improving ocean model initialization. Two types of surveys are evaluated: airborne deployments of expendable profilers and deployments of in-situ thermistor chains along lines intersecting predicted storm paths. Assimilation of the existing ocean observing system substantially constrains mesoscale structure in dynamical fields, primarily because of the four available altimeters. However, these observations only modestly constrain mesoscale structure and bias in upper-ocean thermal fields. Adding rapid-response airborne surveys to these observing systems produces substantial additional correction in thermal fields, but minimal additional correction in dynamical fields. Without altimetry assimilation, rapid-response profiles produce large additional correction in both dynamical and thermal fields. Airborne CTDs sampling temperature and salinity over 1000 m versus XBTs sampling temperature over 400 m produce additional correction for dynamical fields, but not for upper-ocean thermal fields. Airborne surveys are generally more effective than thermistor chain deployments because they can sample a larger area at higher horizontal resolution, and because the latter only measures temperature over the upper ∼100m. Both airborne profile surveys and thermistor chain deployments effectively reduce upper-ocean thermal biases.
      PubDate: 2017-05-25T17:20:25.59152-05:0
      DOI: 10.1002/2017JC012760
       
  • Circulation, hydrography, and transport over the summit of Axial Seamount,
           a deep volcano in the Northeast Pacific
    • Authors: G. Xu; J. W. Lavelle
      Abstract: A numerical model of ocean flow, hydrography, and transport is used to extrapolate observations of currents and hydrography and infer patterns of material flux in the deep ocean around Axial Seamount, a destination node of NSF's Ocean Observatories Initiative's Cabled Array. Using an inverse method, the model is made to approximate measured deep ocean flow around this site during a 35-day time period in the year 2002. The model is then used to extract month-long mean patterns and examine smaller-scale spatial and temporal variability around Axial. Like prior observations, model month-long mean currents flow anti-cyclonically around the seamount's summit in toroidal form with maximum speeds at 1500m depth of 10-11 cm/s. As a time mean, the temperature (salinity) anomaly distribution takes the form of a cold (briny) dome above the summit. Passive tracer material continually released at the location of the ASHES vent field exits the caldera primarily through its southern open end before filling the caldera. Once outside the caldera, the tracer circles the summit in clockwise fashion, fractionally re-entering the caldera over lower walls at its north end, while gradually bleeding southwestward during the modeled time period into the ambient ocean. A second tracer release experiment using a source of only two-day duration inside and near the CASM vent field at the northern end of the caldera suggests a residence time of the fluid at that locale of 8-9 days.
      PubDate: 2017-05-24T07:25:28.151435-05:
      DOI: 10.1002/2016JC012464
       
  • Photophysiological and light absorption properties of phytoplankton
           communities in the river-dominated margin of the northern Gulf of Mexico
    • Authors: Sumit Chakraborty; Steven E. Lohrenz, Kjell Gundersen
      Abstract: Spatial and temporal variability in photophysiological properties of phytoplankton were examined in relationship to phytoplankton community composition in the river-dominated continental margin of the northern Gulf of Mexico (NGOM). Observations made during five research cruises in the NGOM included phytoplankton photosynthetic and optical properties and associated environmental conditions and phytoplankton community structure. Distinct patterns of spatial and temporal variability in photophysiological parameters were found for waters dominated by different phytoplankton groups. Photophysiological properties for locations associated with dominance by a particular group of phytoplankton showed evidence of photoacclimation as reflected by differences in light absorption and pigment characteristics in relationship to different light environments. The maximum rate of photosynthesis normalized to chlorophyll (PmaxB) was significantly higher for communities dominated (> 60% biomass) by cyanobacteria+prochlorophyte (cyano+prochl). The initial slope of the photosynthesis-irradiance (P-E) curve normalized to chlorophyll (αB) was not clearly related to phytoplankton community structure and no significant differences were found in PmaxB and αB between different geographic regions. In contrast, maximum quantum yield of carbon fixation in photosynthesis (Φcmax) differed significantly between regions and was higher for diatom-dominated communities. Multiple linear regression models, specific for the different phytoplankton communities, using a combination of environmental and bio-optical proxies as predictor variables showed considerable promise for estimation of the photophysiological parameters on a regional scale. Such an approach may be utilized to develop size-class or phytoplankton group-specific primary productivity models for the NGOM.
      PubDate: 2017-05-24T07:20:38.868577-05:
      DOI: 10.1002/2016JC012092
       
  • Warm layer and cool skin corrections for bulk water temperature
           measurements for air-sea interaction Studies
    • Authors: Denny P. Alappattu; Qing Wang, Ryan Yamaguchi, Richard J. Lind, Mike Reynolds, Adam J. Christman
      Abstract: The sea surface temperature (SST) relevant to air-sea interaction studies is the temperature immediately adjacent to the air, referred to as skin SST. Generally, SST measurements from ships and buoys are taken at depths varies from several centimeters to five meters below the surface. These measurements, known as bulk SST, can differ from skin SST up to O(1°C).Shipboard bulk and skin SST measurements were made during the Coupled Air-Sea Processes and Electromagnetic ducting Research east coast field campaign (CASPER-East). An Infrared SST Autonomous Radiometer (ISAR) recorded skin SST, while R/V Sharp's Surface Mapping System (SMS) provided bulk SST from one-meter water depth. Since the ISAR is sensitive to sea spray and rain, missing skin SST data occurred in these conditions. However, SMS measurement is less affected by adverse weather and provided continuous bulk SST measurements. It is desirable to correct the bulk SST to obtain a good representation of the skin SST, which is the objective of this research.Bulk-skin SST difference has been examined with respect to meteorological factors associated with cool skin and diurnal warm layers. Strong influences of wind speed, diurnal effects and net longwave radiation flux on temperature difference are noticed. A three-step scheme is established to correct for wind effect, diurnal variability and then for dependency on net longwave radiation flux. Scheme is tested and compared to existing correction schemes. This method is able to effectively compensate for multiple factors acting to modify bulk SST measurements over the range of conditions experienced during CASPER-East.
      PubDate: 2017-05-24T07:20:29.369419-05:
      DOI: 10.1002/2017JC012688
       
  • Seasonal and interannual variability in along-slope oceanic properties off
           the US West Coast: Inferences from a high-resolution regional model
    • Authors: A. B. Kurapov; N. A. Pelland, D. L. Rudnick
      Abstract: A 6-year, 2009-2014 simulation using a 2-km horizontal resolution ocean circulation model of the Northeast Pacific coast is analyzed with focus on seasonal and interannual variability in along-slope subsurface oceanic properties. Specifically, the fields are sampled on the isopycnal surface σ = 26.5 kg m−3 that is found between depths of 150 and 300 m below the ocean surface over the continental slope. The fields analyzed include the depth z26.5, temperature T26.5, along-slope currents ν26.5 and the average potential vorticity PV between σ = 26.5 and 26.25 kg m−3. Each field is averaged in the cross-shore direction over the continental slope and presented as a function of the alongshore coordinate and time. The seasonal cycle in z26.5 shows a coherent upwelling-downwelling pattern from Mexico to Canada propagating to the north with a speed of 0.5 m s−1. The anomalously deep (−20 m) z26.5 displacement in spring-summer 2014 is forced by the southern boundary condition at 24°N as a manifestation of an emerging strong El Niño. The seasonal cycle in T26.5 is most pronounced between 36-53°N indicating that subarctic waters are replaced by warmer Californian waters in summer with the speed close 0.15 m s−1, which is consistent with earlier estimates of the undercurrent speed and also present ν26.5 analyses. The seasonal patterns and anomalies in z26.5 and T26.5 find confirmation in available long-term glider and ship-borne observations. The PV seasonality over the slope is qualitatively different to the south and north of the southern edge of Heceta Bank (43.9°N).
      PubDate: 2017-05-24T07:20:24.900531-05:
      DOI: 10.1002/2017JC012721
       
  • Standing infragravity waves over an alongshore irregular rocky bathymetry
    • Authors: G. Winter; R.J. Lowe, G. Symonds, J.E. Hansen, A.R. van Dongeren
      Abstract: Rocky reef coastlines typically feature highly variable and often abrupt cross- and alongshore changes in bathymetry. The effects of this irregular rocky bathymetry on the dynamics of infragravity waves are largely unknown. Most models of infragravity wave dynamics have been developed and validated on smooth alongshore-uniform bathymetries, which may break down over these highly variable bathymetries. A two-week field experiment was conducted on a rocky reef-fringed beach to investigate how the variable bathymetry affects the spatial and temporal variability of infragravity waves. The height of short (sea-swell) waves decreased over the shallow reef due to breaking, whereas the height of infragravity waves increased towards the shoreline. Both during a storm event (Hm0 = 2.3 m) and under moderate wave conditions (Hm0 = 1.0-1.8 m), the infragravity waves formed a persistent cross-shore standing wave pattern along the entire shoreline, despite the irregular bathymetry. In addition, the alongshore components of infragravity waves refracted by the presence of the nearshore reef, were observed to propagate in opposite directions up and down the coast resulting in a local alongshore standing wave pattern. Thus, the presence of highly variable nearshore bathymetry, which commonly occurs along rocky reef coastlines, may produce both cross- and alongshore standing wave patterns.
      PubDate: 2017-05-23T05:15:25.144911-05:
      DOI: 10.1002/2016JC012242
       
  • On the dynamics of flow past a cylinder: Implications for CTD package
           motions and measurements
    • Authors: D. R. Munday; M. P. Meredith
      Abstract: During the collection of ship-based observations, heaving of the vessel may lead to variation in the descent rate of a CTD package. This can result in the package being pulled upwards through previously sampled water, leading to difficult to quantify errors due to the complex wake. To reduce this problem to one of manageable stature, we use the simple paradigm of two dimensional flow past a cylinder. By using a tracer with a gradient along the flow, we quantify the effect of the cylinder on its distribution and the impact of post processing. At high Reynolds numbers, over 200, uniform translation leads to a small error in the tracer value. This error is likely negligible at the much higher Reynolds number of the ocean. When the flow is oscillated longitudinally, there are two main sources of error; attached vortices may propagate around the cylinder and/or shed vortices may translate into the path of the cylinder. Post processing by removing records from previous pressure levels removes much of the first error, due to it occurring as the package ascends. However, the second source of error is more difficult to remove, due to it occurring when the package is once again descending. In general, results indicate that long period oscillations are preferable. Whilst the magnitude of the errors are comparable to those from short period oscillations, they are spread farther apart in time and space and the overall effect is to localise the errors in small regions of the final depth profile.
      PubDate: 2017-05-22T05:40:47.920308-05:
      DOI: 10.1002/2017JC012708
       
  • On the dynamics of the Mouth of Columbia River: Results from a
           three-dimensional fully coupled wave-current interaction model
    • Authors: Çiğdem Akan; Saeed Moghimi, H. Tuba Özkan-Haller, John Osborne, Alexander Kurapov
      Abstract: Numerical simulations were performed using a 3D ocean circulation model (ROMS) two-way coupled to a phase-averaged wave propagation model (SWAN), in order to expand our understanding of the dynamics of wave-current interactions at the Mouth of the Columbia River (MCR). First, model results are compared with water elevations, currents, temperature, salinity and wave measurements obtained by the U.S. Army Corp of Engineers during the Mega-Transect Experiment in 2005. We then discuss the effects of the currents on the waves and vice versa. Results show that wave heights are intensified notably at the entrance of the mouth in the presence of the tidal currents, especially in ebb flows. We also find non-local modifications to the wave field because of wave focusing processes that re-direct wave energy towards the inlet mouth from adjacent areas, resulting in the presence of a tidal signatures in areas where local currents are weak. The model also suggests significant wave amplification at the edge of the expanding plume in the later stages of ebb, some tens of kilometers offshore of the inlet mouth, with potential implications for navigation safety. The effect of waves on the location of the plume is also analyzed, and results suggest that the plume is shifted in the down-wave direction when wave effects are considered, and that this shift is more pronounced for larger waves, and consistent with the presence of alongshore advection terms in the salt advection equation, which are related to the Stokes velocities associated with waves.
      PubDate: 2017-05-22T05:40:34.398229-05:
      DOI: 10.1002/2016JC012307
       
  • Land adjacency effects on MODIS Aqua top-of-atmosphere radiance in the
           shortwave infrared: Statistical assessment and correction
    • Authors: Lian Feng; Chuanmin Hu
      Abstract: Satellite measurements of coastal or inland waters near land/water interfaces suffer from land adjacency effects (LAEs), particularly in the short wave infrared (SWIR) wavelengths. Here, a statistical method was developed to quantify the LAEs as the ratio of top-of-atmosphere (TOA) total radiance (Lt, W m−2 µm−1 sr−1) between near-shore pixels and LAE-free offshore pixels (>12 pixels away from land). The calculations were conducted using MODIS Aqua images between 2003 and 2012 over the Madagascar Island, with results showing the dependency of LAEs on different environmental and observational factors. The LAEs decrease dramatically with increasing distance from shoreline, and increase with decreasing aerosol optical thickness at 869 nm (τ869). The nearby land surface albedo also plays a role in modulating the LAEs, but the impact is only prominent under low-aerosol conditions. Based on these observations, a look-up-table (LUT) to formulate a correction scheme was established. Tests of the correction scheme using satellite observations over the Hawaii Islands and using in situ measurements in the Chesapeake Bay show significant improvements in Lt (LAEs much closer to 1 than uncorrected data) and retrieved surface chlorophyll-a concentration (Chl-a, mg m−3), respectively. Furthermore, the number of Chl-a retrievals within the range of 0 – 64 mg m−3 also increases by >60%. While the ultimate solution of correcting the LAEs for coastal/inland water applications still requires further work, these preliminary results suggest that the method proposed here deserves further tests in other estuaries and lakes.
      PubDate: 2017-05-22T05:40:32.150019-05:
      DOI: 10.1002/2017JC012874
       
  • Eddy-induced sea surface temperature gradients in Eastern Boundary Current
           Systems
    • Authors: Yeping Yuan; Renato M. Castelao
      Abstract: Many processes are known to influence the distribution of sea surface temperature (SST) fronts in Eastern Boundary Current Systems (EBCS), including wind forcing, mesoscale activity, flow instabilities and flow-topography interactions. Here, we used satellite observations to quantify the relative importance of one of these processes, mesoscale eddy activity, on the distribution of SST gradients in EBCS. Eddies are characterized by large SST and SST gradient anomalies in all EBCS. At the end of the upwelling season, eddy-induced SST gradients are dominant components of the full SST gradient offshore of 300 km from the coast, especially in regions of high eddy activity. Comparisons between eddy-induced and full SST gradients in the California Current System indicate that the offshore migration of mesoscale eddies plays a significant role on the seasonal widening of the region of high frontal activity. SST gradients associated with anticyclonic eddies are largest within 400 km from shore, while the signature of cyclones can extend for up to 700 km from the coast.
      PubDate: 2017-05-22T05:35:32.822335-05:
      DOI: 10.1002/2017JC012735
       
  • Rapid shelf-wide cooling response of a stratified coastal ocean to
           hurricanes
    • Authors: Greg Seroka; Travis Miles, Yi Xu, Josh Kohut, Oscar Schofield, Scott Glenn
      Abstract: Large uncertainty in the predicted intensity of tropical cyclones (TCs) persists compared to the steadily improving skill in the predicted TC tracks. This intensity uncertainty has its most significant implications in the coastal zone, where TC impacts to populated shorelines are greatest. Recent studies have demonstrated that rapid ahead-of-eye-center cooling of a stratified coastal ocean can have a significant impact on hurricane intensity forecasts. Using observation-validated, high-resolution ocean modeling, the stratified coastal ocean cooling processes observed in two U.S. Mid-Atlantic hurricanes were investigated: Hurricane Irene (2011)—with an inshore Mid-Atlantic Bight (MAB) track during the late summer stratified coastal ocean season—and Tropical Storm Barry (2007)—with an offshore track during early summer. For both storms, the critical ahead-of-eye-center depth-averaged force balance across the entire MAB shelf included an onshore wind stress balanced by an offshore pressure gradient. This resulted in onshore surface currents opposing offshore bottom currents that enhanced surface to bottom current shear and turbulent mixing across the thermocline, resulting in the rapid cooling of the surface layer ahead-of-eye-center. Because the same baroclinic and mixing processes occurred for two storms on opposite ends of the track and seasonal stratification envelope, the response appears robust. It will be critical to forecast these processes and their implications for a wide range of future storms using realistic 3D coupled atmosphere-ocean models to lower the uncertainty in predictions of TC intensities and impacts and enable coastal populations to better respond to increasing rapid intensification threats in an era of rising sea levels.
      PubDate: 2017-05-20T11:20:26.414635-05:
      DOI: 10.1002/2017JC012756
       
  • Large-scale experimental observations of sheet flow on a sandbar under
           skewed-asymmetric waves
    • Authors: Ryan S. Mieras; Jack A. Puleo, Dylan Anderson, Daniel T. Cox, Tian-Jian Hsu
      Abstract: A novel large wave flume experiment was conducted on a fixed, barred beach with a sediment pit on the sandbar, allowing for the isolation of small-scale bed response to large-scale forcing. Concurrent measurements of instantaneous sheet layer sediment concentration profiles and near-bed velocity profiles were obtained on a sandbar for the first time. Two sediment distributions were used with median grain diameters, d50, of 0.17 mm and 0.27 mm. Sheet flow occurred primarily under wave crests, where sheet thickness increased with increasing wave height. A proportionality constant, Λ, was used to relate maximum Shields parameter to maximum sheet thickness (normalized by d50), with bed shear stress computed using the quadratic drag law. An enhanced sheet layer thickness was apparent for the smaller sediment experiments (Λ = 18.7), when directly compared to closed-conduit oscillatory flow tunnel data (Λ = 10.6). However, Λ varied significantly (5 
      PubDate: 2017-05-20T11:15:25.389222-05:
      DOI: 10.1002/2016JC012438
       
  • Modification of sea surface temperature by chlorophyll concentration in
           the Atlantic upwelling systems
    • Authors: O. Hernandez; J. Jouanno, V. Echevin, O. Aumont
      Abstract: The influence of the chlorophyll on the upper Tropical Atlantic ocean is investigated with long term (1979-2012) regional oceanic simulations with 1/4° horizontal resolution based on the NEMO3.6 model. The model solar radiation penetration scheme depends on the chlorophyll concentration. Simulations with time and spatially varying concentrations obtained from satellite ocean color observations are compared with a simulation forced with constant chlorophyll concentration of 0.05 mg m−3, representative of chlorophyll depleted waters. Results indicate that regions of the Tropical Atlantic with chlorophyll concentrations larger than in the reference simulation (i.e. [chl] > 0.05 mg m−3) get warmer at the surface, with the exception of the main upwelling regions where high chlorophyll concentrations are associated with a significant cooling of the sea surface (∼1°C in the Benguela upwelling). The analysis of the model heat balance shows that the biological differential heating causes negative temperature anomalies in subsurface source waters prior to their upwelling at the coast. The shallow mixed-layer in the eastern equatorial and tropical Atlantic favors the persistence of these subsurface anomalies and may explain why the Benguela is particularly sensitive to the biological differential heating. In spite of the presence of high chlorophyll concentrations in the upwelling regions, both the larger amount of shortwave radiation captured in the surface layers and the modifications of the horizontal and vertical advection at the coast are found to play a secondary role in the SST change in the upwelling region.
      PubDate: 2017-05-17T03:26:22.749651-05:
      DOI: 10.1002/2016JC012330
       
  • Role for Atlantic inflows and sea ice loss on shifting phytoplankton
           blooms in the Barents Sea
    • Authors: L. Oziel; G. Neukermans, M. Ardyna, C. Lancelot, J-L. Tison, P. Wassmann, J. Sirven, D. Ruiz-Pino, J. C. Gascard
      Abstract: Phytoplankton blooms in the Barents Sea are highly sensitive to seasonal and interannual changes in sea ice extent, water mass distribution, and oceanic fronts. With the ongoing increase of Atlantic Water inflows, we expect an impact on these blooms. Here, we use a state-of-the-art collection of in situ hydro-geochemical data for the period 1998-2014, which includes ocean color satellite-derived proxies for the biomass of calcifying and non-calcifying phytoplankton. Over the last 17 years, sea ice extent anomalies were evidenced having direct consequences for the spatial extent of spring blooms in the Barents Sea. In years of minimal sea ice extent, two spatially distinct blooms were clearly observed: one along the ice edge and another in ice-free water. These blooms are thought to be triggered by different stratification mechanisms: heating of the surface layers in ice-free waters and melting of the sea ice along the ice edge. In years of maximal sea ice extent, no such spatial delimitation was observed. The spring bloom generally ended in June when nutrients in the surface layer were depleted. This was followed by a stratified and oligotrophic summer period. A coccolithophore bloom generally developed in August, but was confined only to Atlantic Waters. In these same waters, a late summer bloom of non-calcifying algae was observed in September, triggered by enhanced mixing, which replenishes surface waters with nutrients. Altogether, the 17-year time-series revealed a northward and eastward shift of the spring and summer phytoplankton blooms.
      PubDate: 2017-05-16T05:21:01.331179-05:
      DOI: 10.1002/2016JC012582
       
  • Morphological characterization of coral reefs by combining LiDAR and MBES
           data: A case study from Yuanzhi Island, South China Sea
    • Authors: Kai Zhang; Fanlin Yang, Hande Zhang, Dianpeng Su, QianQian Li
      Abstract: The correlation between seafloor morphological features and biological complexity has been identified in numerous recent studies. This research focused on the potential for accurate characterization of coral reefs based on high-resolution bathymetry from multiple sources. A standard deviation (STD) based method for quantitatively characterizing terrain complexity was developed that includes robust estimation to correct for irregular bathymetry and a calibration for the depth-dependent variablity of measurement noise. Airborne LiDAR and shipborne sonar bathymetry measurements from Yuanzhi Island, South China Sea, were merged to generate seamless high-resolution coverage of coral bathymetry from the shoreline to deep water. The new algorithm was applied to the Yuanzhi Island surveys to generate maps of quantitive terrain complexity, which were then compared to in situ video observations of coral abundance. The terrain complexity parameter is significantly correlated with seafloor coral abundance, demonstrating the potential for accurately and efficiently mapping coral abundance through seafloor surveys, including combinations of surveys using different sensors.
      PubDate: 2017-05-16T05:20:53.81525-05:0
      DOI: 10.1002/2016JC012507
       
  • The sea-level budget along the Northwest Atlantic coast: GIA, mass changes
           and large-scale ocean dynamics
    • Authors: Thomas Frederikse; Karen Simon, Caroline A. Katsman, Riccardo Riva
      Abstract: Sea-level rise and decadal variability along the northwestern coast of the North Atlantic Ocean are studied in a self-consistent framework that takes into account the effects of solid-earth deformation and geoid changes due to large-scale mass redistribution processes. Observations of sea and land level changes from tide gauges and GPS are compared to the cumulative effect of GIA, present-day mass redistribution, and ocean dynamics over a 50-year period (1965-2014). GIA explains the majority of the observed sea-level and land motion trends, as well as almost all inter-station variability. Present-day mass redistribution resulting from ice melt and land hydrology causes both land uplift and sea-level rise in the region.We find a strong correlation between decadal steric variability in the Subpolar Gyre and coastal sea level, which is likely caused by variability in the Labrador Sea that is propagated southward. The steric signal explains the majority of the observed decadal sea-level variability and shows an upward trend and a significant acceleration, which are also found along the coast.The sum of all contributors explains the observed trends in both sea level rise and vertical land motion in the region, as well as the decadal variability. The sum of contributors also explains the observed acceleration within confidence intervals. The sea-level acceleration coincides with an accelerating density decrease at high latitudes.
      PubDate: 2017-05-16T05:18:04.544712-05:
      DOI: 10.1002/2017JC012699
       
  • Large Eddy Simulation (LES) of wind-driven circulation in a peri-alpine
           lake: Detection of turbulent structures and implications of a complex
           surrounding orography
    • Authors: Marco A. Santo; Marco Toffolon, Giulia Zanier, Lorenzo Giovannini, Vincenzo Armenio
      Abstract: We investigate wind-driven circulation in a peri-alpine lake (Lake Ledro - Italy) using LES-COAST. Lake Ledro is interesting because its own dimensions are suited for LES and it is surrounded by complex orography, affecting wind distribution. We consider the winter condition when stratification is nearly absent. Two types of time-varying wind stress are used: spatially homogeneous and spatially inhomogeneous respectively.The analysis of the eddy viscosities shows substantial differences with respect to the ocean case characterized by absence of coastal boundaries and homogeneous, steady wind. The quantities exhibit a noticeable inhomogeneous behavior: the horizontal eddy viscosity is larger in the water body far from the boundaries, whereas the vertical one is larger close to the lateral boundaries due to the presence of a boundary layer. The energetic bottom boundary layer, typically occurring in lakes, is not present. This because of the intrinsic unsteadiness of the thermal wind blowing over the lake and due to the absence of large amplitude internal waves, the latter present only in case of stable stratification. In the inhomogeneous wind case, up-welling and down-welling areas are not confined along the shoreline only, but are also generated in the water body due to substantial horizontal velocity divergence, and turbulent mixing, quantified by eddy viscosities, TKE and its dissipation rate, appears enhanced with respect to the homogeneous wind case. Finally, downwelling/upwelling areas along the windward/leeward coastline respectively were observed, whose quantitative estimation may give explanation for the bloom of cyanobacteria at the lake surface observed in winter.
      PubDate: 2017-05-15T20:45:46.237516-05:
      DOI: 10.1002/2016JC012284
       
  • Modeling modified Circumpolar Deep Water intrusions onto the Prydz Bay
           continental shelf, East Antarctica
    • Authors: Chengyan Liu; Zhaomin Wang, Chen Cheng, Ruibin Xia, Bingrui Li, Zelin Xie
      Abstract: An eddy-resolving coupled regional ocean-sea ice-ice shelf model is employed to locate the hot spots where modified Circumpolar Deep Water (mCDW) intrudes onto the continental shelf within Prydz Bay, and locate the paths through which mCDW is transported to the Amery Ice Shelf (AIS) calving front. Evaluation of the model output is with satellite, hydrographic and borehole data. Two critical windows responsible for mCDW intrusions are identified. The first is the eastern branch of the cyclonic Prydz Bay gyre (PBG) that carries mCDW to the ice front line, accounting for an annual mean heat transport of ∼8.7 ×1011 J s−1. The second is located to the east of the Four Ladies Bank (FLB) where mCDW is channeled through submarine troughs, accounting for an annual mean heat transport of ∼16.2 ×1011 J s−1. The eddy-induced heat transport accounts for ∼23% in the path of the PBG and ∼52% in the path of the eastern coastal current, with respect to their total onshore heat transport. The seasonal pulsing of mCDW intrusions is greatly dependent on the seasonal cycle of the Antarctic Slope Current (ASC) that peaks with a maximum of ∼29.3 Sv at 75°E in June. In austral winter, mCDW is allowed to access the eastern flank of the AIS calving front with potential consequences for the basal mass balance of the AIS. The dynamic effects of small-scale troughs on the longshore ASC play an important role in the onshore mCDW transport.
      PubDate: 2017-05-15T20:45:38.28924-05:0
      DOI: 10.1002/2016JC012336
       
  • Hydraulics and mixing in a laterally divergent channel of a highly
           stratified estuary
    • Authors: W. Rockwell Geyer; D.K. Ralston, R.C. Holleman
      Abstract: Estuarine mixing is often intensified in regions where topographic forcing leads to hydraulic transitions. Observations in the salt-wedge estuary of the Connecticut River indicate that intense mixing occurs during the ebb tide in regions of supercritical flow that is accelerated by lateral expansion of the channel. The zones of mixing are readily identifiable based on echo-sounding images of large-amplitude shear instabilities. The gradient Richardson number (Ri) averaged across the mixing layer decreases to a value very close to 0.25 during most of the active mixing phase. The along-estuary variation in internal Froude number and interface elevation are roughly consistent with a steady, inviscid, two-layer hydraulic representation, and the fit is improved when a parameterization for interfacial stress is included. The analysis indicates that the mixing results from lateral straining of the shear layer, and that the rapid development of instabilities maintains the overall flow near the mixing threshold value of Ri=0.25, even with continuous, active mixing. The entrainment coefficient can be estimated from salt conservation within the interfacial layer, based on the finding that the mixing maintains Ri=0.25. This approach leads to a scaling estimate for the interfacial mixing coefficient based on the lateral spreading rate and the aspect ratio of the flow, yielding estimates of turbulent dissipation within the pycnocline that are consistent with estimates based on turbulence-resolving measurements.
      PubDate: 2017-05-15T20:41:01.837936-05:
      DOI: 10.1002/2016JC012455
       
  • Synoptic-to-planetary scale wind variability enhances phytoplankton
           biomass at ocean fronts
    • Authors: D. B. Whitt; J. R. Taylor, M. Lévy
      Abstract: In nutrient limited conditions, phytoplankton growth at fronts is enhanced by winds, which drive upward nutrient fluxes via enhanced turbulent mixing and upwelling. Hence, depth-integrated phytoplankton biomass can be ten times greater at isolated fronts. Using theory and two-dimensional simulations with a coupled physical-biogeochemical ocean model, this paper builds conceptual understanding of the physical processes driving upward nutrient fluxes at fronts forced by unsteady winds with timescales of 4-16 days. The largest vertical nutrient fluxes occur when the surface mixing layer penetrates the nutricline, which fuels phytoplankton in the mixed layer. At a front, mixed layer deepening depends on the magnitude and direction of the wind stress, cross-front variations in buoyancy and velocity at the surface, and potential vorticity at the base of the mixed layer, which itself depends on past wind events. Consequently, mixing layers are deeper and more intermittent in time at fronts than outside fronts. Moreover, mixing can decouple in time from the wind stress, even without other sources of physical variability. Wind-driven upwelling also enhances depth-integrated phytoplankton biomass at fronts; when the mixed layer remains shallower than the nutricline, this results in enhanced subsurface phytoplankton. Oscillatory along-front winds induce both oscillatory and mean upwelling. The mean effect of oscillatory vertical motion is to transiently increase subsurface phytoplankton over days to weeks, whereas slower mean upwelling sustains this increase over weeks to months. Taken together, these results emphasize that wind-driven phytoplankton growth is both spatially and temporally intermittent and depends on a diverse combination of physical processes.
      PubDate: 2017-05-15T20:40:55.450153-05:
      DOI: 10.1002/2016JC011899
       
  • The formation processes of phytoplankton growth and decline in mesoscale
           eddies in the western North Pacific Ocean
    • Authors: Yu-Lin Chang; Yasumasa Miyazawa, Lie-Yauw Oey, Tsubasa Kodaira, Shihming Huang
      Abstract: In this study, we investigate the processes of phytoplankton growth and decline in mesoscale eddies in the western North Pacific Ocean based on the in-situ chlorophyll data obtained from 52 cruises conducted by the Japan Meteorological Agency together with idealized numerical simulations. Both the observation and model results suggest that chlorophyll/phytoplankton concentrations are higher in cold than in warm eddies in near-surface water (z > –70 m). In the idealized simulation, the isopycnal movements associated with upwelling/downwelling transport biomass to different vertical depths during eddy formation (stage A). Biomass in cold eddies is transported toward shallower waters while those in warm eddies move toward deeper waters. In the period after the eddy has formed (stage B), sunlight and initially upwelled nutrients together promote the growth of phytoplankton in cold eddies. Phytoplankton in warm eddies decays due to insufficient sunlight in deeper waters. In stage B, upwelling and downwelling coexist in both warm and cold eddies, contributing nearly equally to vertical displacement. The upwelling/downwelling-induced nitrate flux accounts for a small percentage (∼3%) of the total nitrate flux in stage B. The vertical velocity caused by propagating eddies, therefore, is not the primary factor causing differences in phytoplankton concentrations between stage-B warm and cold eddies.
      PubDate: 2017-05-15T20:40:38.214171-05:
      DOI: 10.1002/2017JC012722
       
  • Biological and physical controls on the flux and characteristics of
           sinking particles on the Northwest Atlantic margin
    • Authors: Jeomshik Hwang; Steven J. Manganini, JongJin Park, Daniel B. Montluçon, John M. Toole, Timothy I. Eglinton
      Abstract: Biogenic matter characteristics and radiocarbon contents of organic carbon (OC) were examined on sinking particle samples intercepted at three nominal depths of 1000 m, 2000 m, and 3000 m (∼50 m above the seafloor) during a 3-year sediment trap program on the New England slope in the Northwest Atlantic. We have sought to characterize the sources of sinking particles in the context of vertical export of biogenic particles from the overlying water column and lateral supply of resuspended sediment particles from adjacent margin sediments. High aluminum (Al) abundances and low OC radiocarbon contents indicated contributions from resuspended sediment which was greatest at 3000 m but also significant at shallower depths. The benthic source (i.e., laterally supplied resuspended sediment) of opal appears negligible based on the absence of a correlation with Al fluxes. In comparison, CaCO3 fluxes at 3000 m showed a positive correlation with Al fluxes. Benthic sources accounted for 42 ∼ 63% of the sinking particle flux based on radiocarbon mass balance and the relationship between Al flux and CaCO3 flux. Episodic pulses of Al at 3000 m were significantly correlated with the near-bottom current at a nearby hydrographic mooring site, implying the importance of current variability in lateral particle transport. However, Al fluxes at 1000 m and 2000 m were coherent but differed from those at 3000 m, implying more than one mode of lateral supply of particles in the water column.
      PubDate: 2017-05-15T20:40:32.337226-05:
      DOI: 10.1002/2016JC012549
       
  • Open-ocean convection process: A driver of the winter nutrient supply and
           the spring phytoplankton distribution in the Northwestern Mediterranean
           Sea
    • Authors: Tatiana Severin; Faycal Kessouri, Mathieu Rembauville, Elvia Denisse Sánchez-Pérez, Louise Oriol, Jocelyne Caparros, Mireille Pujo-Pay, Jean-François Ghiglione, Fabrizio D'Ortenzio, Vincent Taillandier, Nicolas Mayot, Xavier Durrieu De Madron, Caroline Ulses, Claude Estournel, Pascal Conan
      Abstract: This study was a part of the DeWEX project (Deep Water formation EXperiment), designed to better understand the impact of dense water formation on the marine biogeochemical cycles. Here, nutrient and phytoplankton vertical and horizontal distributions were investigated during a deep open-ocean convection event and during the following spring bloom in the Northwestern Mediterranean Sea (NWM). In February 2013, the deep convection event established a surface nutrient gradient from the center of the deep convection patch to the surrounding mixed and stratified areas. In the center of the convection area, a slight but significant difference of nitrate, phosphate and silicate concentrations was observed possibly due to the different volume of deep waters included in the mixing or to the sediment resuspension occurring where the mixing reached the bottom. One of this process, or a combination of both, enriched the water column in silicate and phosphate, and altered significantly the stoichiometry in the center of the deep convection area. This alteration favored the local development of microphytoplankton in spring, whereas nanophytoplankton dominated neighboring locations where the convection reached the deep layer but not the bottom. This study shows that the convection process influences both winter nutrients distribution and spring phytoplankton distribution and community structure. Modifications of the convection spatial scale and intensity (i.e. convective mixing depth) is likely to have strong consequences on phytoplankton community structure and distribution in the NWM, and thus on the marine food web.
      PubDate: 2017-05-12T10:45:44.795637-05:
      DOI: 10.1002/2016JC012664
       
  • Storm-induced changes in pCO2 at the sea surface over the northern South
           China Sea during Typhoon Wutip
    • Authors: Haijun Ye; Jinyu Sheng, Danling Tang, Eko Siswanto, Muhsan Ali Kalhoro, Yi Sui
      Abstract: In-situ oceanographic measurements were made before and after the passage of Typhoon Wutip in September 2013 over the northern South China Sea. The surface geostrophic circulation over this region inferred from satellite altimetry data features a large-size anti-cyclonic eddy, a small-size cyclonic eddy, and smaller-size eddies during this period. Significant typhoon-induced changes occurred in the partial pressure of CO2 at the sea surface (pCO2sea) during Wutip. Before the passage of Wutip, pCO2sea was about 392.92±1.83, 390.31±0.50, and 393.04±4.31 μatm over the cyclonic eddy water, the anti-cyclonic eddy water, and areas outside two eddies, respectively. The entire study region showed a carbon source (1.31±0.46 mmol CO2 m−2 d−1) before Wutip. In the cyclonic eddy water after Wutip, high sea surface salinity (SSS), low sea surface temperature (SST), and high pCO2sea (413.05±7.56 μatm) made this area to be a carbon source (3.30±0.75 mmol CO2 m−2 d−1). In the anti-cyclonic eddy water after Wutip, both the SSS and SST were lower, pCO2sea was also lower (383.03±3.72 μatm), and this area became a carbon sink (-0.11±0.55 mmol CO2 m−2 d−1), in comparison with the pre-typhoon conditions. The typhoon-induced air-sea CO2 flux reached about 0.03 mmol CO2 m−2 d−1. Noticeable spatial variations in pCO2sea were affected mainly by the typhoon-induced mixing/upwelling and vertical stratifications. This study suggests that the local air-sea CO2 flux in the study region was affected significantly by oceanographic conditions during the typhoon.
      PubDate: 2017-05-12T10:40:50.280544-05:
      DOI: 10.1002/2016JC012643
       
  • Small-scale sea ice deformation during N-ICE2015: From compact pack ice to
           marginal ice zone
    • Authors: Annu Oikkonen; Jari Haapala, Mikko Lensu, Juha Karvonen, Polona Itkin
      Abstract: We studied small scale (50 m to 5 km) sea ice deformation from ship radar images recorded during the N-ICE2015 campaign. The campaign consisted of 4 consecutive drifting ice stations (Floes 1 to 4) north of Svalbard, with a total duration of nearly 5 months. Deformation was calculated using 5 different time intervals from 10 min to 24 h, and the deformation rate was found to depend strongly on the time scale. Floes 1 to 3 had a mean deformation rate within the range of 0.06 to 0.07 h– 1 with the interval of 10 min, and 0.03 to 0.04 h– 1 with the interval of 1 h. Floe 4 represented marginal ice zone (MIZ) with very high deformation rate, 0.14/0.08 h– 1 with the interval of 10 min/1 h. Deep in the ice pack, high deformation rates occurred only with high wind and drift speed, while in MIZ they were found also during calm conditions. The deformation rates were found to follow power law scaling with respect to length and time scale even on this small scales and in small domain (15 km × 15 km). The length scale dependence of deformation rate depends on the time scale: the power law scaling exponent β of the whole study period decreases from 0.82 to 0.52 with the time interval increasing from 10 min to 24 h. Ship radar images reveal the importance of the deformation history of the ice pack, since the deformation events were initialized along the lines of previous damages.
      PubDate: 2017-05-09T18:45:26.413209-05:
      DOI: 10.1002/2016JC012387
       
  • Short-term variability of aragonite saturation state in the central
           Mid-Atlantic Bight
    • Authors: Yuan-Yuan Xu; Wei-Jun Cai, Yonghui Gao, Rik Wanninkhof, Joseph Salisbury, Baoshan Chen, Janet J. Reimer, Stephen Gonski, Najid Hussain
      Abstract: The uptake of anthropogenic carbon dioxide (CO2) from the atmosphere has resulted in a decrease in seawater aragonite saturation state (Ωarag), which affects the health of carbonate-bearing organisms and the marine ecosystem. A substantial short-term variability of surface water Ωarag, with an increase of up to 0.32, was observed in the central Mid-Atlantic Bight off the Delaware and the Chesapeake Bays over a short period of 10 days in summer 2015. High-frequency underway measurements for temperature, salinity, percentage saturation of dissolved oxygen, oxygen to argon ratio, pH, fCO2, and measurements based on discrete samples for pH, dissolved inorganic carbon and total alkalinity are used to investigate how physical and biogeochemical processes contribute to the changes of Ωarag. Quantitative analyses show that physical advection and mixing processes are the dominant forces for higher Ωarag in slope waters while biological carbon removal and CO2 degassing contribute to increased Ωarag in shelf waters.
      PubDate: 2017-05-09T18:35:36.704397-05:
      DOI: 10.1002/2017JC012901
       
  • Evaluation of anthropogenic influences on the Luhuitou fringing reef via
           spatial and temporal analyses (from Isotopic Values)
    • Authors: D. Cao; W. Cao, K. Yu, G. Wu, J. Yang, X. Su, F. Wang
      Abstract: Coral reefs have suffered remarkable declines worldwide. Nutrient over-enrichment is considered to be one of the primary local causes. The Luhuitou fringing reef in southern China is a well-known tourist destination that is subject to enormous coastal renovation. The mean δ13C, δ15N value and carbon over nitrogen ratio (C/N) of particulate organic matter were - 21.56 ± 1.94 ‰, 7.04 ± 3.81 ‰ and 5.81 ± 1.86, respectively, suggesting mixed sources of carbon and nitrogen. The IsoError calculations suggested that marine phytoplankton and marine benthic algae dominated the majority of carbon sources, while anthropogenic and terrestrial organic nitrogen dominated the nitrogen sources. A tendency towards greater terrestrial detritus and anthropogenic-derived discharges was found during dry seasons, and greater marine-derived organic matter during wet seasons. These results demonstrated the existence of anthropogenic influences and high dissolved inorganic nitrogen concentrations and C/N ratios. Anthropogenic nutrient discharge moderated nitrogen limitation, whereas phosphorus became more important to the reef ecosystem. Despite the marine carbon sources dominated, freshwater and terrestrial derived organic carbon sources were also very important. Meanwhile, anthropogenic and terrestrial organic nitrogen sources were dominant. Therefore, pollution from more extensive region and anthropogenic activities from riverine sewage discharges adjacent to reefs should be focused to effectively reduce human-derived nutrients on reefs.
      PubDate: 2017-05-09T18:35:29.699504-05:
      DOI: 10.1002/2017JC012871
       
  • On the seasonal variability of the Canary Current and the Atlantic
           meridional overturning circulation
    • Authors: Pedro Vélez-Belchí; M. Dolores Pérez-Hernández, María Casanova-Masjoan, Luis Cana, Alonso Hernández-Guerra
      Abstract: The Atlantic meridional overturning circulation (AMOC) is continually monitored along 26°N by the RAPID-MOCHA array. Measurements from this array show a 6.7 Sv seasonal cycle for the AMOC, with a 5.9 Sv contribution from the upper-mid-ocean. Recent studies argue that the dynamics of the eastern Atlantic is the main driver for this seasonal cycle; specifically, Rossby waves excited south of the Canary Islands. Using inverse modelling, hydrographic, mooring and altimetry data, we describe the seasonal cycle of the ocean mass transport around the Canary Islands and at the eastern boundary, under the influence of the African slope, where eastern component of the RAPID-MOCHA array is situated. We find a seasonal cycle of -4.1±0.5 Sv for the oceanic region of the Canary Current, and +3.7±0.4 Sv at the eastern boundary. This seasonal cycle along the eastern boundary is in agreement with the seasonal cycle of the AMOC, that requires the lowest contribution to the transport in the upper-mid-ocean to occur in fall. However, we demonstrate that the linear Rossby wave model used previously to explain the seasonal cycle of the AMOC is not robust, since it is extremely sensitive to the choice of the zonal range of the wind stress curl and produces the same results with a Rossby wave speed of zero. We demonstrate that the seasonal cycle of the eastern boundary is due to the recirculation of the Canary Current and to the seasonal cycle of the poleward flow that characterizes the eastern boundaries of the oceans.
      PubDate: 2017-05-08T05:18:24.484006-05:
      DOI: 10.1002/2017JC012774
       
  • How well does wind speed predict air-sea gas transfer in the sea ice zone?
           A synthesis of radon deficit profiles in the upper water column of the
           Arctic Ocean
    • Authors: B. Loose; R. P. Kelly, A. Bigdeli, W. Williams, R. Krishfield, M. Rutgers van der Loeff, S. B. Moran
      Abstract: We present 34 profiles of radon-deficit from the ice-ocean boundary layer of the Beaufort Sea. Including these 34, there are presently 58 published radon-deficit estimates of air-sea gas transfer velocity (k) in the Arctic Ocean; 52 of these estimates were derived from water covered by 10% sea ice or more. The average value of k collected since 2011 is 4.0 ± 1.2 m d−1. This exceeds the quadratic wind speed prediction of weighted kws = 2.85 m d−1 with mean-weighted wind speed of 6.4 m s−1. We show how ice cover changes the mixed-layer radon budget, and yields an “effective gas transfer velocity.” We use these 58 estimates to statistically evaluate the suitability of a wind speed parameterization for k, when the ocean surface is ice covered. Whereas the six profiles taken from the open ocean indicate a statistically good fit to wind speed parameterizations, the same parameterizations could not reproduce k from the sea ice zone. We conclude that techniques for estimating k in the open ocean cannot be similarly applied to determine k in the presence of sea ice. The magnitude of k through gaps in the ice may reach high values as ice cover increases, possibly as a result of focused turbulence dissipation at openings in the free surface. These 58 profiles are presently the most complete set of estimates of k across seasons and variable ice cover; as dissolved tracer budgets they reflect air-sea gas exchange with no impact from air-ice gas exchange.
      PubDate: 2017-05-05T00:30:33.732599-05:
      DOI: 10.1002/2016JC012460
       
  • Bay of Bengal salinity stratification and Indian summer monsoon
           intraseasonal oscillation: 1. Intraseasonal variability and causes
    • Authors: Yuanlong Li; Weiqing Han, M. Ravichandran, Wanqiu Wang, Toshiaki Shinoda, Tong Lee
      Abstract: The huge freshwater flux of the Indian summer monsoon (ISM; May-October) gives rise to strong salinity stratification in the Bay of Bengal (BoB), causing a shallow mixed layer and a thick barrier layer, which potentially affects intraseasonal oscillations of the monsoon (MISOs). In this study intraseasonal variability of the mixed layer depth (MLD) and barrier layer thickness (BLT) is investigated using in-situ observations from Argo floats and moored buoys and an ocean general circulation model (OGCM). The average MLD in the BoB is typically 20-30 m during the ISM, while the BLT increases from ∼10 m in May-June to 20-40 m in September-October. MISOs induce in-phase variations in MLD and isothermal layer depth (ILD), both of which are deepened by 8-15 m during MISO active phase, while the change of BLT is small and within the error range of Argo data sampling. In the northern (southern) bay, BLT increases by ∼5 m (2 m) during MISOs owing to a larger deepening of ILD than MLD. OGCM experiments are performed to understand the underlying mechanism. In the BoB intraseasonal variations of MLD, ILD and BLT arise largely from ocean internal instability, whereas those induced by MISOs are weaker. The in-phase variations of MLD and ILD during MISOs are induced by different processes. The MLD deepening is primarily caused by wind stress forcing, while the ILD deepening is driven by surface heat fluxes via surface cooling. The limited variability of BLT is due to the offsetting of different forcing processes.
      PubDate: 2017-05-04T17:49:41.530302-05:
      DOI: 10.1002/2017JC012691
       
  • Tropical cyclone sensitivity to ocean coupling in the ECMWF coupled model
    • Authors: Kristian S. Mogensen; Linus Magnusson, Jean-Raymond Bidlot
      Abstract: We present an investigation of the performance of the ECMWF coupled atmosphere-waves-ocean model for different ocean and atmosphere resolutions on a series of tropical cyclones in the Western Pacific with the aim to better understand the coupled feedback mechanisms in these exterme conditions.For some of the test cases, we only find little impact of coupling the atmosphere to the ocean, while in others, we observe a very large impact. To further understand these differences, we have selected two tropical cyclones (TCs) as case studies: TC Haiyan (with small impact of coupling) and TC Neoguri (with large impact of coupling). The comparison between these two cases suggests that the upper ocean stratification is the key in determining the strength of the coupled feedback. A strong coupled feedback is found whenever the ocean heat content of the upper layer is low while a very weak coupled feedback is found whenever the ocean has a thick warm mixed layer.The oceanographic response to tropical cyclones for the two storms has been compared to sea surface temperature and derived surface currents from drifting buoys and to subsurface observations from Argo and ship launched XBT's. These comparisons show that we are able to realistically reproduce the atmospheric and oceanographic interaction during tropical cyclone conditions which gives us confidence that the coupled modelling system is physically sound.
      PubDate: 2017-05-04T17:49:09.323699-05:
      DOI: 10.1002/2017JC012753
       
  • A submesoscale coherent vortex in the Ligurian Sea: From dynamical
           barriers to biological implications
    • Authors: Anthony Bosse; Pierre Testor, Nicolas Mayot, Louis Prieur, Fabrizio D'Ortenzio, Laurent Mortier, Hervé Le Goff, Claire Gourcuff, Laurent Coppola, Héloïse Lavigne, Patrick Raimbault
      Abstract: In June 2013, a glider equipped with oxygen and fluorescence sensors has been used to extensively sample an anticyclonic Submesoscale Coherent Vortex (SCV) in the Ligurian Sea (NW Mediterranean Sea). Those measurements are complemented by full-depth CTD casts (T, S, Oxygen) and water samples documenting nutrients and phytoplankton pigments within the SCV and outside. The SCV has a very homogeneous core of oxygenated waters between 300 m and 1200 m formed 4.5 months earlier during the winter deep convection event. It has a strong dynamical signature with peak velocities at 700 m depth of 13.9 cm s-1 in cyclogeostrophic balance. The eddy has a small radius of 6.2 km corresponding to high Rossby number of −0.45. The vorticity at the eddy center reaches −0.8f. Cross-stream isopycnic diffusion of tracers between the eddy core and the surroundings is found to be very limited due to dynamical barriers set by the SCV associated with a diffusivity coefficient of about 0.2 m2 s-1. The deep core is nutrients-depleted with concentrations of nitrate, phosphate and silicate 13–18% lower than the rich surrounding waters. However, the nutriclines are shifted of about 20–50 m toward the surface thus increasing the nutrients availability for phytoplankton. Chlorophyll-a concentrations at the deep chlorophyll maximum are subsequently about twice bigger as compared to outside. Pigments further reveal the predominance of nanophytoplankton inside the eddy and an enhancement of the primary productivity. This study demonstrate the important impact of post-convective SCVs on nutrients distribution and phytoplankton community, as well as on the subsequent primary production and carbon sequestration.
      PubDate: 2017-05-04T17:47:09.545253-05:
      DOI: 10.1002/2016JC012634
       
  • Impact of typhoons on the Changjiang plume extension in the Yellow and
           East China Seas
    • Authors: Joon Ho Lee; Il-Ju Moon, Jae-Hong Moon, Sung-Hun Kim, Yeong Yun Jeong, Jun-Ho Koo
      Abstract: It is well known that river discharges, winds, ocean currents, and tides are major dynamical factors that determine the distribution and extension of the Changjiang plume (CP) in the Yellow and East China Seas (YECS). Using observations and numerical experiments, this study demonstrates that, in addition to these factors, typhoons in the YECS also play a crucial role in the extension of the CP during the summer season. The hydrographic data observed at the Ieodo Ocean Research Station (IORS) and by a research vessel during the period of Typhoons Ewiniar (0603) and Dianmu (1004) showed that the typhoon-induced strong vertical mixing modified spatial distribution of the CP significantly, resulting in the delay of the CP's extension by as much as up to 20 days. A series of numerical experiments for Typhoon Dianmu also showed that the typhoon plays a blocking role for the extension of CP for up to 17 days through the vertical mixing process and the change of background winds. In particular, it is found that the delay due to Dianmu in 2010 contributed to the avoidance of potential mass mortality of marine life by preventing the low-salinity water from spreading to the aquaculture regions near Jeju Island.
      PubDate: 2017-05-04T17:45:48.608816-05:
      DOI: 10.1002/2017JC012754
       
  • Role of interannual Kelvin wave propagations in the equatorial Atlantic on
           the Angola Benguela curent system
    • Authors: Rodrigue Anicet Imbol Koungue; Serena Illig, Mathieu Rouault
      Abstract: The link between equatorial Atlantic Ocean variability and the coastal region of Angola-Namibia is investigated at interannual time scales from 1998 to 2012. An index of equatorial Kelvin wave activity is defined based on Prediction and Research Moored Array in the Tropical Atlantic (PIRATA). Along the equator, results show a significant correlation between interannual PIRATA monthly dynamic height anomalies, altimetric monthly Sea Surface Height Anomalies (SSHA) and SSHA calculated with an Ocean Linear Model. This allows us to interpret PIRATA records in terms of equatorial Kelvin waves. Estimated phase speed of eastward propagations from PIRATA equatorial mooring remains in agreement with the linear theory, emphasizing the dominance of the second baroclinic mode. Systematic analysis of all strong interannual equatorial SSHA shows that they precede by 1-2 months extreme interannual Sea Surface Temperature Anomalies along the African coast, which confirms the hypothesis that major warm and cold events in the Angola-Benguela current system are remotely forced by ocean atmosphere interactions in the equatorial Atlantic. Equatorial wave dynamics is at the origin of their developments. Wind anomalies in the Western Equatorial Atlantic force equatorial downwelling and upwelling Kelvin waves that propagate eastward along the equator and then polewards along the African coast triggering extreme warm and cold events respectively. A proxy index based on linear ocean dynamics appears to be significantly more correlated with coastal variability than an index based on wind variability. Results show a seasonal phasing, with significantly higher correlations between our equatorial index and coastal SSTA in October-April season.
      PubDate: 2017-05-04T17:45:42.578339-05:
      DOI: 10.1002/2016JC012463
       
  • Importance of Ekman transport and gyre circulation change on seasonal
           variation of surface dissolved iron in the western subarctic North Pacific
           
    • Authors: Takuya Nakanowatari; Tomohiro Nakamura, Keisuke Uchimoto, Jun Nishioka, Humio Mitsudera, Masaaki Wakatsuchi
      Abstract: Iron (Fe) is an essential nutrient for marine phytoplankton and it constitutes an important element in the marine carbon cycle in the ocean. This study examined the mechanisms controlling seasonal variation of dissolved Fe (dFe) in the western subarctic North Pacific (WSNP), using an ocean general circulation model coupled with a simple biogeochemical model incorporating a dFe cycle fed by two major sources (atmospheric dust and continental shelf sediment). The model reproduced the seasonal cycle of observed concentrations of dFe and macro nutrients at the surface in the Oyashio region with maxima in winter (February–March) and minima in summer (July–September), although the simulated seasonal amplitudes are a half of the observed values. Analysis of the mixed-layer dFe budget indicated that both local vertical entrainment and lateral advection are primary contributors to the wintertime increase in dFe concentration. In early winter, strengthened northwesterly winds excite southward Ekman transport and Ekman upwelling over the western subarctic gyre, transporting dFe-rich water southward. In mid- to late winter, the southward western boundary current of the subarctic gyre and the outflow from the Sea of Okhotsk also bring dFe-rich water to the Oyashio region. The contribution of atmospheric dust to the dFe budget is several times smaller than these ocean transport processes in winter. These results suggest that the westerly wind-induced Ekman transport and gyre circulation systematically influence the seasonal cycle of WSNP surface dFe concentration.
      PubDate: 2017-05-03T05:50:50.719547-05:
      DOI: 10.1002/2016JC012354
       
  • Intraseasonal-to-semiannual variability of sea-surface height in the
           eastern, equatorial Indian Ocean and southern Bay of Bengal
    • Authors: Xuhua Cheng; Julian P. McCreary, Bo Qiu, Yiquan Qi, Yan Du
      Abstract: Intraseasonal-to-semiannual variability of sea-surface height (SSH) in the eastern, equatorial Indian Ocean (EEIO) and southern Bay of Bengal (BoB) is investigated using altimetric data, and solutions to 1½−layer (first baroclinic mode) and linear, continuously stratified (LCS; multi-baroclinic-mode) models. The amplitude and dominant period of SSH variability differ regionally. Large-amplitude variability is found along the west coast of Sumatra, in a zonal band across the BoB centered along 5°N, east of Sri Lanka, and in the northwestern BoB, respectively. Along the Sumatran west coast, SSH variability peaks at 30 − 60 days, 90 days, and 180 days. Along 5°N and east of Sri Lanka, the 30 − 60-day variability is dominant.Sensitivity experiments using a nonlinear version of the 1½−layer model forced by realistic winds reproduce the observed patterns of intraseasonal variability in the southern BoB. At 30 − 60 days, the solutions show that eddies (nonlinear Rossby waves) propagating from the east, rather than local wind forcing, account for most of the variance east of Sri Lanka; furthermore, they demonstrate that the variance is significantly enhanced by the nonlinear transfer of 90 − 120-day energy into the intraseasonal band of 30 − 60 days. The LCS solutions show that the first two baroclinic modes explain most of the SSH variance at 90 − 180 days. The second baroclinic mode dominates the SSH variance at 180 days, a consequence of basin resonance and strong wind forcing.
      PubDate: 2017-04-27T18:30:59.730536-05:
      DOI: 10.1002/2016JC012662
       
  • Altered inherent optical properties and estimates of the underwater light
           field during an Arctic under ice bloom of Phaeocystis pouchetii
    • Authors: Alexey K. Pavlov; Torbjørn Taskjelle, Hanna M. Kauko, Børge Hamre, Stephen R. Hudson, Philipp Assmy, Pedro Duarte, Mar Fernández-Méndez, C.J. Mundy, Mats A. Granskog
      Abstract: In spring 2015, we observed an extensive phytoplankton bloom of Phaeocystis pouchetii, with chlorophyll a concentrations up to 7.5 mg m−3, under compact snow-covered Arctic sea ice at 80-81˚N during the Norwegian young sea ICE (N-ICE2015) expedition. We investigated the influence of the under-ice bloom on inherent optical properties (IOPs) of the upper ocean. Absorption and scattering in the upper 20 m of the water column at visible wavebands increased threefold and tenfold, respectively, relative to pre-bloom conditions. The scattering-to-absorption ratio during the Phaeocystis under-ice bloom was higher than in previous Arctic studies investigating diatom blooms. During the bloom, absorption by colored dissolved organic matter (at 375 nm), seemingly of autochthonous origin, doubled. Total absorption by particles (at 440 nm), dominated by phytoplankton (> 90%), increased tenfold. Measured absorption and scattering in the water were used as inputs for a 1D coupled atmosphere-ice-ocean radiative transfer model (AccuRT) to investigate effects of altered IOPs on the under-ice light field. Multiple scattering between sea ice and phytoplankton in the ocean led to an increase in scalar irradiance in the photosynthetically active radiation range (Eo(PAR)) at the ice-ocean interface by 6–7% compared to pre-bloom situation. This increase could have a positive feedback on ice-algal and under-ice phytoplankton productivity. The ratio between Eo(PAR) and downwelling planar irradiance (Ed(PAR)) below sea ice reached 1.85. Therefore, the use of Ed(PAR) might significantly underestimate the amount of PAR available for photosynthesis underneath sea ice. Our findings could help to improve light parameterizations in primary production models.
      PubDate: 2017-04-27T18:30:54.434852-05:
      DOI: 10.1002/2016JC012471
       
  • Tide-induced residual circulation in a bay with laterally asymmetric depth
    • Authors: Bong-Gwan Kim; Yang-Ki Cho
      Abstract: The tide-induced residual current is a crucial determinant of the distribution of sediment in a tidally dominated bay. There have been only a few studies of residual currents in bays with laterally asymmetric depth, although the residual circulation in bays with symmetric lateral depth variation has been studied extensively. Gomso Bay is a typical bay with laterally asymmetric depth. The bay is characterized by a deep channel to the north and a wide shoal to the south. This study reports the tide-induced residual current in Gomso Bay and explains its dynamics using numerical and analytical models. The results from the models are consistent with observations. Residual flows show a clockwise gyre inside the bay and counter-clockwise circulation near the mouth. Three physical causes induce residual circulation: 1) bottom friction coupled with a tidal height, 2) advection causing inward residual transport, and 3) a pressure gradient due to a surface slope induced by subtidal flow. The maximum inward residual current is shifted toward the wider shoal. This is caused by across-channel advection due to the laterally asymmetric depth.
      PubDate: 2017-04-27T18:28:29.163187-05:
      DOI: 10.1002/2016JC012473
       
  • Amundsen and Bellingshausen Seas simulation with optimized ocean, sea ice,
           and thermodynamic ice shelf model parameters
    • Authors: Y. Nakayama; D. Menemenlis, M. Schodlok, E. Rignot
      Abstract: Recent studies suggest that the thickness of Winter Water (WW), that is, water with potential temperature below ∼-1°C located below Antarctic Surface Water and above Circumpolar Deep Water (CDW) is critical in determining the ice shelf melt rate, especially for the Pine Island Glacier (PIG). Existing model studies, however, misrepresent WW thickness and properties in the Amundsen Sea (AS). Here, we adjust a small number of model parameters in a regional Amundsen and Bellingshausen Seas configuration of the Massachusetts Institute of Technology general circulation model in order to reproduce properties and thickness of WW and CDW close to observations, with significant improvement for WW compared to previous studies. The cost, which is defined as weighted model-data difference squared, is reduced by 23%. Although a previous modeling study points out that the local surface heat loss upstream from Pine Island Polynya could be the reason for the observed 2012 PIG melt decline and WW thickening, they did not show WW freshening, which was observed at the same time. Model sensitivity experiments for surface heat loss, PIG melt rate, and precipitation fail to replicate WW freshening concurrent with PIG melt decline, implying that these processes can not fully explain the observed PIG melt decrease.
      PubDate: 2017-04-25T10:50:46.06782-05:0
      DOI: 10.1002/2016JC012538
       
  • Spatial patterns of mixing in the Solomon Sea
    • Authors: M. S. Alberty; J. Sprintall, J. MacKinnon, A. Ganachaud, S. Cravatte, G. Eldin, C. Germineaud, A. Melet
      Abstract: The Solomon Sea is a marginal sea in the southwest Pacific that connects subtropical and equatorial circulation, constricting transport of South Pacific Subtropical Mode Water and Antarctic Intermediate Water through its deep, narrow channels. Marginal sea topography inhibits internal waves from propagating out and into the open ocean, making these regions hot spots for energy dissipation and mixing. Data from two hydrographic cruises and from Argo profiles are employed to indirectly infer mixing from observations for the first time in the Solomon Sea. Thorpe and finescale methods indirectly estimate the rate of dissipation of kinetic energy (ϵ) and indicate that it is maximum in the surface and thermocline layers and decrease by 2-3 orders of magnitude by 2000 m depth. Estimates of diapycnal diffusivity from the observations and a simple diffusive model agree in magnitude but have different depth structures, likely reflecting the combined influence of both diapycnal mixing and isopycnal stirring. Spatial variability of ϵ is large, spanning at least two orders of magnitude within isopycnal layers. Seasonal variability of ϵ reflects regional monsoonal changes in large-scale oceanic and atmospheric conditions with ϵ increased in July and decreased in March. Finally, tide power input and topographic roughness are well correlated with mean spatial patterns of mixing within intermediate and deep isopycnals but aren't clearly correlated with thermocline mixing patterns.
      PubDate: 2017-04-25T10:45:44.74654-05:0
      DOI: 10.1002/2016JC012666
       
  • On the spatial coherence of the Atlantic Water inflow across the Nordic
           Seas
    • Authors: Christophe Herbaut; Marie-Noëlle Houssais, Sally Close, Anne-Cécile Blaizot
      Abstract: The co-variability of the Atlantic Water (AW) branches in the Nordic Seas is investigated over the period 1979-2012 using an eddy permitting model. A noticeable circulation change is found in the mid-1990s. Prior to the mid-1990s, the leading mode of variability defines a large scale pattern, with concomitant variations in the Atlantic Water (AW) inflow in the Faroe-Shetland Channel (FSC), the Norwegian Atlantic Slope Current (NwASC), the AW inflow to the Barents Sea and the West Spitsbergen Current (WSC). After the mid-1990s, the co-variability between the NwASC and the AW inflow in both the FSC and the WSC is lost. Consequently, the northern Barents Sea circulation anomaly pattern, which is present throughout the full period, becomes the leading mode of circulation in the northern Nordic Seas after the mid-1990s. The circulation change of the mid-1990s appears to be linked to a weakening of the southwesterly wind anomalies in Norwegian Sea, as the northern center of action of the first mode of sea level pressure (NAO) weakens. Passive tracer experiments suggest that this circulation change may be accompanied by increased heat transfer from the AW current to the interior Nordic Seas. This in turn may have limited the influence of the recently observed AW warming in the Iceland-Scotland Passage on the NwASC downstream.
      PubDate: 2017-04-25T10:45:41.849468-05:
      DOI: 10.1002/2016JC012566
       
  • Targeted ocean sampling guidance for tropical cyclones
    • Authors: Sue Chen; James A. Cummings, Jerome M. Schmidt, Elizabeth R. Sanabia, Steven R. Jayne
      Abstract: A 3D variational ocean data assimilation adjoint approach is used to examine the impact of ocean observations on coupled tropical cyclone (TC) model forecast error for three recent hurricanes: Isaac (2012), Hilda (2015), and Matthew (2016). In addition, this methodology is applied to develop an innovative ocean observation targeting tool validated using TC model simulations that assimilate ocean temperature observed by Airborne eXpendable Bathy Thermographs and Air-Launched Autonomous Micro-Observer floats. Comparison between the simulated targeted and real observation data assimilation impacts reveals a positive maximum mean linear correlation of 0.53 at 400- 500 m, which implies some skill in the targeting application. Targeted ocean observation regions from these three hurricanes, however, show that the largest positive impacts in reducing the TC model forecast errors are sensitive to the initial pre-storm ocean conditions such as the location and magnitude of pre-existing ocean eddies, storm-induced ocean cold wake, and model track errors.
      PubDate: 2017-04-25T10:45:35.498412-05:
      DOI: 10.1002/2017JC012727
       
  • Isopycnal eddy mixing across the Kuroshio extension: Stable versus
           unstable states in an eddying model
    • Authors: Ru Chen; Sarah T. Gille, Julie L. McClean
      Abstract: The Kuroshio Extension (KE) jet transitions between stable and unstable states on interannual time scales. Cross-jet eddy mixing in the two states is contrasted in the KE region (28° – 40°N, 125 – 165°E), using a global eddying 0.1° configuration of the Parallel Ocean Program with online numerical particles. The four-year period chosen (June 1994 to May 1998) covers a full cycle of the stable state, unstable state and the transition period. Large values of cross-jet eddy diffusivities within the KE jet are concentrated in the upper 1000m. In the upper ocean, elevated cross-jet mixing within the KE jet is mainly concentrated in the downstream part of the KE jet, where the jet is weak but eddy activity is strong. The simulated time-mean KE jet is more intense and extends further east in the stable state than in the unstable state. Consequently, strong cross-jet mixing within the KE jet is located west of 150oE during June 1996-May 1997 (a typical unstable state), but east of 150oE during June 1995-May 1996 (a typical stable state). However, average mixing within the KE jet is indistinguishable in the typical stable and unstable states. In the deep ocean, mixing is strongly influenced by topography, and thus their horizontal structures have less inter-annual variability than in the upper ocean. One caveat is that results here cover one representative cycle of the two states. To obtain the climate mean mixing structures for the stable or unstable state, one would need numerical output covering a period much longer than four years.
      PubDate: 2017-04-24T10:21:20.133761-05:
      DOI: 10.1002/2016JC012164
       
  • Biases of five latent heat flux products and their impacts on mixed-layer
           temperature estimates in the South China Sea
    • Authors: Xin Wang; Rong-Wang Zhang, Jian Huang, Li-Li Zeng, Fei Huang
      Abstract: Five latent heat flux (LHF) products are evaluated based on in situ observations in the South China Sea (SCS), including the ECWMF ERA-Interim (ERA-I), the NCEP2, the Objectively Analyzed air-sea Fluxes (OAFlux), the Japanese 55-year Reanalysis (JRA55), and the TropFlux datasets. The results show that there are good correlations between the LHF products and observations, ranging from 0.68 to 0.74. However, mean biases of -8 to 40 W m−2 exist in the LHF products with respect to the observations. For root-mean-square errors, the OAFlux dataset is the closest to the observations, followed by ERA-I and TropFlux, while the NCEP2 dataset shows significant overestimation. It is found that the biases in the near-surface specific humidity are most correlated with the biases in the LHF products, followed by the biases in the near-surface wind speed, air temperature and sea surface temperature. The biases in the LHF products have a prominent seasonal variation that is 25 W m−2 higher in boreal winter than in summer. Using the thermal equation, it is shown that the tendency errors of the mixed-layer temperature estimated by the biases in the LHF products vary from -2.0 to 3.5°C/month in the SCS. When all of the products are averaged, the errors are reduced to a range of -0.7 to 1.5°C/month. It is noteworthy that the errors in summer are more obvious than those in winter, since a thinner mixed layer in the summer can amplify the effect of even a small bias in the LHF.
      PubDate: 2017-04-24T10:21:13.962667-05:
      DOI: 10.1002/2016JC012332
       
  • Coastal-trapped behavior of the diurnal internal tide at O ‘ahu,
           Hawai‘i
    • Authors: Katharine A. Smith; Mark A. Merrifield, Glenn S. Carter
      Abstract: The influence of rotation on the structure and propagation of internal tides around O‘ahu, Hawai‘i is investigated using in situ observations and a tidally forced, primitive equation model with realistic bathymetry and stratification. Particular attention is given to the diurnal internal tide, which largely has been de-emphasized in previous studies of the region because of the dominance of the semidiurnal internal tide but has been determined by recent studies to be a significant contributor to baroclinic variability. Though both diurnal and semidiurnal internal tides are generated primarily over Ka‘ena Ridge to the northwest of the island, the diurnal internal tide propagates clockwise around the island as an imperfectly trapped wave, while the semidiurnal internal tide propagates away from the ridge, unaffected by rotation. The diurnal and semidiurnal internal tides fall into the superinertial frequency range; however, the diurnal frequency apparently is sufficiently close to inertial (∼ 1.4f) for rotation to affect internal tide propagation. The in situ observations support the model finding that diurnal trapping provides the primary source of baroclinic variability along the eastern coast of the island, a stretch of coastline otherwise sheltered from the internal tide energy generated over the Hawaiian Ridge. The findings in Hawai‘i suggest that coastal trapping of superinertial internal tides may be a significant source of variability and mixing in other nearshore systems around the world.
      PubDate: 2017-04-24T10:21:12.698717-05:
      DOI: 10.1002/2016JC012436
       
  • Warm water pathways toward Nioghalvfjerdsfjorden Glacier, Northeast
           Greenland
    • Authors: Janin Schaffer; Wilken-Jon von Appen, Paul A. Dodd, Coen Hofstede, Christoph Mayer, Laura de Steur, Torsten Kanzow
      Abstract: Nioghalvfjerdsfjorden Glacier (79NG) is the largest of three marine-terminating outlet glaciers draining the Northeast Greenland Ice Stream. In order to understand how Atlantic waters supply waters in the cavity beneath the floating 79NG, we analyse historic and recent bathymetric, hydrographic, and velocity observations obtained on the Northeast Greenland continental shelf. The bathymetry is characterized by a trough system, consisting of the Westwind Trough and the Norske Trough in the northern and southern part of the continental shelf, respectively. Atlantic waters recirculating in Fram Strait cross the shelf break and enter the trough system at its south-eastern inlet towards the inner shelf. Warm Atlantic Intermediate Water (AIW) present below 200 m in the Norske Trough shows large contributions of the recirculating Atlantic water. We found that the bathymetry is sufficiently deep to provide a direct subsurface pathway for warm AIW between the shelf break and the 79NG cavity via the Norske Trough. Likewise, based on the hydrographic data, we show that the Norske Trough supplies AIW warmer than 1°C to the 79NG, which is not present in the Westwind Trough. Our moored and lowered velocity measurements indicate that a boundary current carries warm AIW along the northeastern slope of Norske Trough towards the 79NG. We suggest that warm anomalies in Atlantic water temperatures in Fram Strait could reach 79NG within less than 1.5 years. Thereby, they may cause increased basal melt rates at the 79NG.
      PubDate: 2017-04-24T10:20:37.862482-05:
      DOI: 10.1002/2016JC012462
       
  • Spatial variability of upper ocean POC export in the Bay of Bengal and the
           Indian Ocean determined using particle-reactive 234Th
    • Authors: S. Subha Anand; R. Rengarajan, V. V. S. S. Sarma, A. K. Sudheer, R. Bhushan, S. K. Singh
      Abstract: The northern Indian Ocean is globally significant for its seasonally reversing winds, upwelled nutrients, high biological production and expanding oxygen minimum zones. The region acts as sink and source for atmospheric CO2. However, the efficiency of the biological carbon pump to sequester atmospheric CO2 and export particulate organic carbon from the surface is not well known. To quantify the upper ocean carbon export flux and to estimate the efficiency of biological carbon pump in the Bay of Bengal and the Indian Ocean, seawater profiles of total 234Th were measured from surface to 300 m depth at 13 stations from 19.9°N to 25.3°S in a transect along 87°E, during spring intermonsoon period (March – April 2014). Results showed enhanced in situ primary production in the equatorial Indian Ocean and the central Bay of Bengal, and varied from 13.2 mmol C m−2 d−1 to 173.8 mmol C m−2 d−1. POC export flux in this region varied from 0 to 7.7 mmol C m−2 d−1. Though high carbon export flux was found in the equatorial region, remineralization of organic carbon in the surface and sub-surface waters considerably reduced organic carbon export in the Bay of Bengal. Annually recurring anticyclonic eddies enhanced organic carbon utilization and heterotrophy. Oxygen minimum zone developed due to stratification and poor ventilation was intensified by sub-surface remineralization. 234Th-based carbon export fluxes were not comparable with empirical statistical model estimates based on primary production and temperature. Region-specific refinement of model parameters is required to accurately predict POC export fluxes.
      PubDate: 2017-04-24T10:20:35.832231-05:
      DOI: 10.1002/2016JC012639
       
  • Stable isotopes in surface waters of the Atlantic Ocean: Indicators of
           ocean-atmosphere water fluxes and oceanic mixing processes
    • Authors: M. Benetti; G. Reverdin, G. Aloisi, Sveinbjörnsdóttir Árny Erla
      Abstract: The surface ocean hydrological cycle is explored based on ∼300 new δ18O and δD measurements from surface waters of the Atlantic Ocean and the Mediterranean Sea over the period 2010-2016. Our approach combines these surface observations with salinity (S) and stable isotope measurements of atmospheric water vapor. The distinct regional S-δ distributions are used to identify different surface water masses and their horizontal advection. Moreover, based on assumptions on the δ-S characteristics of seawater sources and the isotope composition of the evaporative (δe) and meteoric water (δMW) fluxes, the δ-S distribution is used to indicate the relative importance of evaporation (E) and meteoric water inputs (MW). Here, δe is estimated from the Craig and Gordon's equation using 120 days of measurements of the ambient air above the Atlantic Ocean collected during three cruises. To provide quantitative estimates of the E:MW ratio, we use the box model from Craig and Gordon (1965). This identifies the subtropical gyre as a region where E:MW ∼2 and the tropical ocean as a region were MW:E ∼2. Finally, we show that the δ18O-δD distribution is better represented by a linear fit than the δ-S relationship, even in basins governed by different hydrological processes. We interpret the δ18O-δD distribution considering the kinetic fractionation processes associated with evaporation. In the tropical region where MW exceeds E, the δ18O-δD distribution identifies the MW inputs from their kinetic signature, whereas in regions where E exceeds MW, the δ18O-δD distribution traces the humidity at the sea surface.
      PubDate: 2017-04-24T10:20:32.60819-05:0
      DOI: 10.1002/2017JC012712
       
  • Energy-optimal path planning in the coastal ocean
    • Authors: Deepak N. Subramani; Patrick J. Haley, Pierre F.J. Lermusiaux
      Abstract: We integrate data-driven ocean modeling with the stochastic Dynamically Orthogonal (DO) level-set optimization methodology to compute and study energy-optimal paths, speeds, and headings for ocean vehicles in the Middle-Atlantic Bight (MAB) region. We hindcast the energy-optimal paths from among exact time-optimal paths for the period 28 August 2006 to 9 September 2006. To do so, we first obtain a data-assimilative multiscale re-analysis, combining ocean observations with implicit two-way nested multi-resolution primitive-equation simulations of the tidal-to-mesoscale dynamics in the region. Second, we solve the reduced-order stochastic DO level-set partial differential equations (PDEs) to compute the joint probability of minimum arrival-time, vehicle-speed time-series, and total energy utilized. Third, for each arrival time, we select the vehicle-speed time-series that minimize the total energy utilization from the marginal probability of vehicle-speed and total energy. The corresponding energy-optimal path and headings are obtained through the exact particle backtracking equation. Theoretically, the present methodology is PDE-based and provides fundamental energy-optimal predictions without heuristics. Computationally, it is three- to four-orders of magnitude faster than direct Monte Carlo methods. For the missions considered, we analyze the effects of the regional tidal currents, strong wind events, coastal jets, shelfbreak front, and other local circulations on the energy-optimal paths. Results showcase the opportunities for vehicles that intelligently utilize the ocean environment to minimize energy usage, rigorously integrating ocean forecasting with optimal control of autonomous vehicles.
      PubDate: 2017-04-21T10:04:56.255266-05:
      DOI: 10.1002/2016JC012231
       
  • Impact of sea-level rise on tidal range in Chesapeake and Delaware Bays
    • Authors: Serena Blyth Lee; Ming Li, Fan Zhang
      Abstract: Coastal inundation is affected not only by rising mean sea level but also by changing tides. A numerical model is developed to investigate how sea-level rise and coastline changes may impact tides in two coastal-plain estuaries, Chesapeake Bay and Delaware Bay. Despite their different tidal characteristics, the two estuaries display similar responses to the sea-level rise and shoreline management scenarios. When hypothetic sea walls are erected at the present coastline to prevent low-lying land from flooding, tidal range increases, with greater amplification in the upper part of the two estuaries. When low-lying land is allowed to become permanently inundated by higher sea level, however, tidal range in both estuaries decreases. Analyses of the tidal energy budget show that the increased dissipation over the shallow water and newly inundated areas compensates for the reduced dissipation in deep water, leading to smaller tidal range. The changes in the tidal range are not proportional to the changes in the mean sea level, indicating a nonlinear tidal response to sea-level rise. The ratio of tidal range change to sea-level rise varies between -0.05 and 0.1 in Chesapeake Bay and between -0.2 and 0.25 in Delaware Bay. The model results suggest a potential adaptation strategy that uses inundation over low-lying areas to reduce tidal range at up-estuary locations.
      PubDate: 2017-04-21T10:04:52.016757-05:
      DOI: 10.1002/2016JC012597
       
  • Bio-optical properties of Arctic drift ice and surface waters north of
           Svalbard from winter to spring
    • Authors: Piotr Kowalczuk; Justyna Meler, Hanna Kauko, Alexey K. Pavlov, Monika Zabłocka, Ilka Peeken, Christine Dybwad, Giulia Castellani, Mats A. Granskog
      Abstract: We have quantified absorption by CDOM, aCDOM(λ), particulate matter, ap(λ), algal pigments, aph(λ) and detrital material, aNAP(λ) coincident with chlorophyll a in sea ice and surface waters in winter and spring 2015 in the Arctic Ocean north of Svalbard. The aCDOM(λ) was low in contrast to other regions of the Arctic Ocean, while ap(λ) has the largest contribution to absorption variability in sea ice and surface waters. ap(443) was 1.4 to 2.8 times and 1.3 to 1.8 times higher than aCDOM(443) in surface water and sea ice, respectively. aph(λ) contributed 90% and 81% to ap(λ), in open leads and under ice waters column, and much less (53% to 74%) in sea ice, respectively. Both aCDOM(λ) and ap(λ) followed closely the vertical distribution of chlorophyll a in sea ice and the water column. We observed a 10-fold increase of the chlorophyll a concentration and nearly 2-fold increase in absorption at 443 nm in sea ice from winter to spring. The aCDOM(λ) dominated the absorption budget in the UV both in sea ice and surface waters. In the visible range, absorption was dominated by aph(λ), which contributed more than 50% and aCDOM(λ), which contributed 43% to total absorption in water column. Detrital absorption contributed significantly (33%) only in surface ice layer. Algae dynamics explained more than 90% variability in ap(λ) and aph(λ) in water column, but less than 70% in the sea ice. This study presents detailed absorption budget that is relevant for modeling of radiative transfer and primary production.
      PubDate: 2017-04-21T10:00:43.343308-05:
      DOI: 10.1002/2016JC012589
       
  • Sea ice growth rates from tide-driven visible banding
    • Authors: Kate E. Turner; Inga J. Smith, Jean-Louis Tison, Véronique Verbeke, Mark McGuinness, Malcolm Ingham, Ross Vennell, Joe Trodahl
      Abstract: In this paper, periodic tide-current-driven banding in a sea-ice core is demonstrated as a measure of the growth rate of first-year sea ice at congelation-ice depths. The study was performed on a core from the eastern McMurdo Sound, exploiting the well-characterised tidal pattern at the site. It points the way to a technique for determining early-season ice growth rates from late-season cores, in areas where under-ice currents are known to be tidally-dominated and the ice is landfast, thus providing data for a time of year when thin ice prevents direct thickness (and therefore growth rate) measurements. The measured results were compared to the growth-vs.-depth predicted by a thermodynamic model.
      PubDate: 2017-04-21T10:00:33.280048-05:
      DOI: 10.1002/2016JC012524
       
  • Modeling temperature inversion in southeastern Yellow Sea during winter
           2016
    • Authors: Ig-Chan Pang; Jae-Hong Moon, Joon-Ho Lee, Ji-Seok Hong, Sung-Jun Pang
      Abstract: A significant temperature inversion with temperature differences larger than 3°C was observed in the southeastern Yellow Sea (YS) during February 2016. By analyzing in-situ hydrographic profiles and results from a regional ocean model for the YS, this study examines the spatiotemporal evolution of the temperature inversion and its connection with wind-induced currents in winter. Observations reveal that in winter, when the northwesterly wind prevails over the YS, the temperature inversion occurs largely at the frontal zone southwest of Korea where warm/saline water of a Kuroshio origin meets cold/fresh coastal water. Our model successfully captures the temperature inversion observed in the winter of 2016 and suggests a close relation between northwesterly wind bursts and the occurrence of the large inversion. In this respect, the strong northwesterly wind drove cold coastal water southward in the upper layer via Ekman transport, which pushed the water mass southward and increased the sea level slope in the frontal zone in southeastern YS. The intensified sea level slope propagated northward away from the frontal zone as a shelf wave, causing a northward upwind flow response along the YS trough in the lower layer, thereby resulting in the large temperature inversion. Diagnostic analysis of the momentum balance shows that the westward pressure gradient, which developed with shelf wave propagation along the YS trough, was balanced with the Coriolis force in accordance with the northward upwind current in and around the inversion area.
      PubDate: 2017-04-21T10:00:31.206323-05:
      DOI: 10.1002/2017JC012718
       
  • Ocean surface currents from satellite data
    • Authors: Kathleen Dohan
      Abstract: The atmosphere drives entire ocean motions, and yet the exchange of momentum between the atmosphere and ocean occurs in the thin layer where they meet, involving the smallest scales of turbulence. The Ocean Surface Current Analyses Real-time (OSCAR) project attempts to better understand this exchange using satellite observations with simplified physics to calculate global ocean currents. The goal is to continually improve the physics in OSCAR and more accurately model the currents. The theoretical study will help coupled ocean-atmosphere modeling efforts whereas the societal benefits of measuring ocean currents are broad, e.g., fish larval dispersion, heat transport, commercial shipping, and search and rescue.
      PubDate: 2017-04-20T13:01:07.681099-05:
      DOI: 10.1002/2017JC012961
       
  • Indian Ocean sources of Agulhas leakage
    • Authors: Jonathan V. Durgadoo; Siren Rühs, Arne Biastoch, Claus W.B. Böning
      Abstract: We examine the mean pathways, transit timescales, and transformation of waters flowing from the Pacific and the marginal seas through the Indian Ocean (IO) on their way toward the South Atlantic within a high-resolution ocean/sea-ice model. The model fields are analysed from a Lagrangian perspective where water volumes are tracked as they enter the IO. The IO contributes 12.6 Sv to Agulhas leakage, which within the model is 14.1 ± 2.2 Sv, the rest originates from the South Atlantic. The Indonesian Through-flow constitutes about half of the IO contribution, is surface bound, cools and salinificates as it leaves the basin within 10–30 years. Waters entering the IO south of Australia are at intermediate depths and maintain their temperature-salinity properties as they exit the basin within 15–35 years. Of these waters, the contribution from Tasman leakage is 1.4 Sv. The rest stem from recirculation of Subantarctic Mode Water formed within the IO. The marginal seas export 1.0 Sv into the Atlantic within 15–40 years, and the waters cool and freshen on-route. However, the model's simulation of waters from the Gulfs of Aden and Oman are too light and hence overly influenced to upper ocean circulations. In the Cape Basin, Agulhas leakage is well mixed. On-route, temperature-salinity transformations occur predominantly in the Arabian Sea and within the greater Agulhas Current region. Overall, the IO exports at least 7.9 Sv from the Pacific to the Atlantic, thereby quantifying the strength of the upper cell of the global conveyor belt.
      PubDate: 2017-04-13T15:20:39.896108-05:
      DOI: 10.1002/2016JC012676
       
  • Seagrass blade motion under waves and its impact on wave decay
    • Authors: M. Luhar; E. Infantes, H. Nepf
      Abstract: The hydrodynamic drag generated by seagrass meadows can dissipate wave energy, causing wave decay. It is well known that this drag depends on the relative motion between the water and the seagrass blades, yet the impact of blade motion on drag and wave energy dissipation remains to be fully characterized. In this experimental study, we examined the impact of blade motion on wave decay by concurrently recording blade posture during a wave cycle and measuring wave decay over a model seagrass meadow. We also identified a scaling law that predicts wave decay over the model meadow for a range of seagrass blade density, wave period, wave height, and water depth scaled from typical field conditions. Blade flexibility led to significantly lower drag and wave decay relative to theoretical predictions for rigid, upright blades. To quantify the impact of blade motion on wave decay, we employed an effective blade length, le, defined as the rigid blade length that leads to equivalent wave energy dissipation. We estimated le directly from images of blade motion. Consistent with previous studies, these estimates showed that the effective blade length depends on the dimensionless Cauchy number, which describes the relative magnitude of the wave hydrodynamic drag and the restoring force due to blade rigidity. As the hydrodynamic forcing increases, the blades exhibit greater motion. Greater blade motion leads to smaller relative velocities, reducing drag and wave energy dissipation (i.e. smaller le).
      PubDate: 2017-04-13T15:20:38.376602-05:
      DOI: 10.1002/2017JC012731
       
  • Small-scale open-ocean currents have large effects on wind-wave heights
    • Authors: Fabrice Ardhuin; Sarah T. Gille, Dimitris Menemenlis, Cesar B. Rocha, Nicolas Rascle, Bertrand Chapron, Jonathan Gula, Jeroen Molemaker
      Abstract: Tidal currents and large-scale oceanic currents are known to modify ocean wave properties, causing extreme sea states that are a hazard to navigation. Recent advances in the understanding and modeling capability of open ocean currents have revealed the ubiquitous presence of eddies, fronts and filaments at scales 10 to 100∼km. Based on realistic numerical models, we show that these structures can be the main source of variability in significant wave heights at scales less than 200 km, including important variations down to 10 km. Model results are consistent with wave height variations along satellite altimeter tracks, resolved at scales larger than 50 km. The spectrum of significant wave heights is found to be of the order of 70〈Hs〉2/(g2〈Tm0,-1〉2) times the current spectrum, where〈Hs〉 is the spatially-averaged significant wave height, 〈Tm0,-1〉 is the energy-averaged period, and g is the gravity acceleration. This variability induced by currents has been largely overlooked in spite of its relevance for extreme wave heights and remote sensing.
      PubDate: 2017-04-13T15:20:33.416558-05:
      DOI: 10.1002/2016JC012413
       
  • Mixing rates and vertical heat fluxes north of Svalbard from Arctic winter
           to spring
    • Authors: Amelie Meyer; Ilker Fer, Arild Sundfjord, Algot K. Peterson
      Abstract: Mixing and heat flux rates collected in the Eurasian Basin north of Svalbard during the N-ICE2015 drift expedition are presented. The observations cover the deep Nansen Basin, the Svalbard continental slope and the shallow Yermak Plateau from winter to summer. Mean quiescent winter heat flux values in the Nansen Basin are 2Wm−2 at the ice-ocean interface, 3Wm−2in the pycnocline and 1Wm−2 below the pycnocline. Large heat fluxes exceeding 300Wm−2 are observed in the late spring close to the surface over the Yermak Plateau. The data consisting of 588 microstructure profiles and 50 days of high-resolution under-ice turbulence measurements are used to quantify the impact of several forcing factors on turbulent dissipation and heat flux rates. Wind forcing increases turbulent dissipation seven times in the upper 50 m, and doubles heat fluxes at the ice-ocean interface. Presence of warm Atlantic Water close to the surface increases the temperature gradient in the water column, leading to enhanced heat flux rates within the pycnocline. Steep topography consistently enhances dissipation rates by a factor of four and episodically increases heat flux at depth. It is, however, the combination of storms and shallow Atlantic Water that leads to the highest heat flux rates observed: Ice-ocean interface heat fluxes average 100Wm−2 during peak events and are associated with rapid basal sea ice melt, reaching 25 cm, day−1.
      PubDate: 2017-04-13T15:20:27.14333-05:0
      DOI: 10.1002/2016JC012441
       
  • Flow and turbulence structure in a hypertidal estuary with the world's
           biggest tidal bore
    • Authors: Junbiao Tu; Daidu Fan
      Abstract: Turbulent and flow structure associated with breaking tidal bores are deliberately investigated on the basis of field measurements. High-resolution velocity and hydrographic data are collected in the middle Qiantang Estuary by a vertical array of acoustic Doppler velocimeters and optical backscatter sensors, collaborated with a bottom-mounted acoustic Doppler current profiler. Besides obvious variations in diurnal and spring-neap tidal cycles, the estuarine dynamics is featured by extreme asymmetry in flood and ebb tides. The flood tide is abnormally accelerated to generate tidal bores at the first 10 minutes or more, with breaking or undular configurations at the front. The occurrence of peak flow velocity, turbulent kinetic energy (TKE) and TKE dissipation rate (ε) is definitely associated with breaking bores, with their values several times to 2 orders of magnitude larger than the corresponding secondary peak values during the maximum ebb flows. Flow and turbulence dynamics are significantly affected by the tidal-bore Froude number. A sandwich ε structure is clear exhibited with the maximum value at the surface, secondary maximum near the bed, and the minimum at the intermediate. Dual TKE sources are indicated by an approximate local balance between shear production and dissipation near the bottom, and a top-down TKE dissipation using the modified Froude scaling in the vertical water column. The highly elevated dissipation by breaking bores is comparable to that by intense breaking waves in the surf zone, and the former potentially penetrates the entire water column to produce extreme sediment-resuspension events in combination with intense bottom shear stress.
      PubDate: 2017-04-13T15:15:42.427087-05:
      DOI: 10.1002/2016JC012120
       
  • Revised transport estimates of the Denmark Strait overflow
    • Authors: Kerstin Jochumsen; Martin Moritz, Nuno Nunes, Detlef Quadfasel, Karin M. H. Larsen, Bogi Hansen, Hedinn Valdimarsson, Steingrimur Jonsson
      Abstract: The major export route of dense water from the Nordic Seas into the North Atlantic is in the deep channel in Denmark Strait. Here, currents have been monitored with one or two moored Acoustic Doppler Current Profilers (ADCPs) since 1996. Volume transport estimates of the Denmark Strait Overflow Water (DSOW) so far based on these data, which were regressed to the total transport of dense water in a numerical model. The resulting transport has been used in many publications.Here, we present results from an extended five mooring array deployed in 2014/15, that included measurements outside the swift overflow core. This array provided the basis for new calculations to estimate the DSOW transports. Furthermore, a correction is proposed for biases detected on some ADCPs, which led to earlier underestimation of the flow in the lower part of the plume. Using the new method, the mean DSOW transport is estimated to be 3.2∼Sv in the period 1996-2016, without a significant trend. Uncertainties are typically ±0.5Sv.Beyond variations on the eddy scale, an empirical orthogonal functions (EOF) analysis of the velocity field reveals three dominant modes of variability: the first mode is roughly barotropic and corresponds to pulsations of the plume, the second mode represents the laterally shifting component of the plume's core position, and the third mode indicates the impact of the varying overflow thickness. Finally, DSOW transports are compared to the Faroe Bank Channel overflow transports, but no clear relationship is found.
      PubDate: 2017-04-13T15:15:37.148677-05:
      DOI: 10.1002/2017JC012803
       
  • Fjord light regime: Bio-optical variability, absorption budget, and
           hyperspectral light availability in Sognefjord and Trondheimsfjord, Norway
           
    • Authors: V. J. Mascarenhas; D. Voß, J. Wollschlaeger, O. Zielinski
      Abstract: Optically active constituents (OACs) in addition to water molecules attenuate light via processes of absorption and scattering and thereby determine underwater light availability. An analysis of their optical properties helps in determining the contribution of each of these to light attenuation. With an aim to study the bio-optical variability, absorption budget and 1% spectral light availability, hydrographical (temperature and salinity) and hyperspectral optical (downwelling irradiance and upwelling radiance) profiles were measured along fjord transects in Sognefjord and Trondheimsfjord, Norway. Optical water quality observations were also performed using Secchi disc and Forel-Ule scale. In concurrence, water samples were collected and analyzed via visible spectrophotometry, fluorometry, and gravimetry to quantify and derive inherent optical properties of the water constituents. An absorption model (R2=0.91, n=36, p
      PubDate: 2017-04-13T15:15:34.055025-05:
      DOI: 10.1002/2016JC012610
       
  • Turbulent kinetic energy of the ocean winds over the Kuroshio Extension
           from QuikSCAT winds (1999-2009)
    • Authors: Kai Yu; Changming Dong, Gregory P. King
      Abstract: We investigate mesoscale turbulence (10-1000 km) in the ocean winds over the Kuroshio Extension (28°N-40°N, 140°E-180°E) using the QuikSCAT dataset (November 1999 - October 2009). We calculate the second- (Djj) and third-order structure functions (Djjj) and the spatial variance (Vj) as a function of scale r (j = L,T denotes, respectively, the longitudinal (divergent) and transverse (vortical) component). The most interesting results of the analysis follow.Although both Vj(r) and Djj(r) measure the turbulent kinetic energy (TKE), we find that Vj(r) is the more robust measure. The spatial variance density has a broad peak near 450 km (close to the midlatitude Rossby radius of deformation). On interannual time scales, TKE correlates well with the El Niño 3.4 index.According to turbulence theory, the kinetic energy cascades downscale (upscale) if DLLL(r) (also skewness SL =DLLL/DL  L3/2) is negative (positive). Our results for the Kuroshio Extension are consistent with a downscale cascade (indicating convergence dominates). Furthermore, classical turbulence theory predicts that SL = -0.3 and independent of r; however, we find SL varies strongly with r, from -4 at small scales to -0.3 at large scales. This non-classical behaviour implies strong scale interaction, which we attribute to the rapid, and sometimes explosive, growth of storms in the region through baroclinic instability. Finally, we find that ST (a measure of cyclonic/anti-cyclonic asymmetry) is positive (cyclonic) and also varies strongly with r, from 4 at small scales to 0.5 at large scales. New turbulence models are needed to explain these results, and that will benefit Weather Prediction and climate modeling.
      PubDate: 2017-04-13T15:15:31.605384-05:
      DOI: 10.1002/2016JC012404
       
  • Short-crested waves in the surf zone
    • Authors: Zhangping Wei; Robert A. Dalrymple, Munan Xu, Roland Garnier, Morteza Derakhti
      Abstract: This study investigates short-crested waves in the surf zone by using the mesh-free Smoothed Particle Hydrodynamics model, GPUSPH. The short-crested waves are created by generating intersecting wave trains in a numerical wave basin with a beach. We first validate the numerical model for short-crested waves by comparison with large-scale laboratory measurements. Then short-crested wave breaking over a planar beach is studied comprehensively. We observe rip currents as discussed in Dalrymple [1975] and undertow created by synchronous intersecting waves. The wave breaking of the short-crested wave field created by the nonlinear superposition of intersecting waves and wave-current interaction result in the formation of isolated breakers at the ends of breaking wave crests. Wave amplitude diffraction at these isolated breakers gives rise to an increase in the alongshore wave number in the inner surf zone. Moreover 3D vortices and multiple circulation cells with a rotation frequency much lower than the incident wave frequency are observed across the outer surf zone to the beach. Finally, we investigate vertical vorticity generation under short-crested wave breaking and find that breaking of short-crested waves generates vorticity as pointed out by Peregrine [1998]. Vorticity generation is not only observed under short-crested waves with a limited number of wave components, but also under a directional wave spectra.
      PubDate: 2017-04-13T15:10:54.943308-05:
      DOI: 10.1002/2016JC012485
       
  • Characterizing frontal eddies along the East Australian Current from HF
           radar observations
    • Authors: Amandine Schaeffer; A. Gramoulle, M. Roughan, A. Mantovanelli
      Abstract: The East Australian Current (EAC) dominates the ocean circulation along south-eastern Australia, however, little is known about the submesoscale frontal instabilities associated with this western boundary current. One year of surface current measurements from HF radars, in conjunction with mooring and satellite observations, highlight the occurrence and propagation of meanders and frontal eddies along the inshore edge of the EAC. Eddies were systematically identified using the geometry of the high spatial resolution (∼1.5 km) surface currents, and tracked every hour. Cyclonic eddies were observed irregularly, on average every 7 days, with inshore radius ∼10 km. Amongst various forms of structures, frontal eddies associated with EAC meanders were characterized by poleward advection speeds of ∼0.3-0.4 m/s, migrating as far as 500 km south, based on satellite imagery. Flow field kinematics show that cyclonic eddies have high Rossby numbers (0.6-1.9) and enhance particle dispersion. Patches of intensified surface divergence at the leading edge of the structures are expected to generate vertical uplift. This is confirmed by subsurface measurements showing temperature uplift of up to 55 m over 24 hours and rough estimates of vertical velocities of 10s of meters per day. While frontal eddies propagate through the radar domain independently of local wind stress, upfront wind can influence their stalling and growth, and can also generate large cold core eddies through intense shear. Such coherent structures are a major mechanism for the transport and entrainment of nutrient rich coastal or deep waters, influencing physical and biological dynamics, and connectivity over large distances.
      PubDate: 2017-04-13T15:10:45.278734-05:
      DOI: 10.1002/2016JC012171
       
  • On the relationship between the meridional overturning circulation,
           alongshore wind stress, and United States East Coast sea level in the
           Community Earth System Model Large Ensemble
    • Authors: Christopher M. Little; Christopher G. Piecuch, Rui M. Ponte
      Abstract: By the late 21st century, climate models project enhanced dynamic sea level (DSL) rise along the western boundary of the North Atlantic associated with a decline in the Atlantic Meridional Overturning Circulation (AMOC). In contrast, coastal DSL variability over the last few decades has been driven largely by local winds, with limited evidence for coupling to AMOC strength. The unclear forcing- and timescale-dependence of relationships between local winds, AMOC strength, and DSL obscures: 1) the validity of tide gauge-derived DSL gradients as a proxy of AMOC strength and 2) the assessment of climate model reliability. Here, we analyze these relationships in the Community Earth System Model Large Ensemble (CESM-LE) over the 1920-2100 period. In CESM-LE, the amplitude of interannual to multidecadal DSL variability, and its along-coast correlation, are comparable to detrended annual mean tide gauge records. A “crossover timescale” of approximately 5-15 years partitions a local wind-driven coastal DSL regime from an overturning-related regime. Processes unrelated to either AMOC strength and local winds are important at interannual to decadal timescales. As external forcing increases in strength over the 21st century, DSL variability associated with the overturning circulation becomes dominant. While the largely externally-forced, AMOC-associated, component explains only 29±12% of DSL variance over the 1920-2010 period, it explains 89±3% of the variance in the 2011-2100 period. We discuss the implications of these results on the reliability of climate model projections of regional DSL, the use of coastal DSL as a proxy for AMOC, and the origins of multidecadal DSL variability.
      PubDate: 2017-04-13T15:10:33.178379-05:
      DOI: 10.1002/2017JC012713
       
  • Strongly nonlinear internal solitons: Models and applications
    • Authors: L. A. Ostrovsky; V. G. Irisov
      Abstract: A strongly nonlinear evolution equation based on an approximate Hamiltonian is suggested and applied to the description of non-stationary processes of formation and evolution of strong internal solitons and their groups (solibores). Comparison with experimental data shows that such a simplified approach can be effectively applied to the description and prediction of strong internal waves' evolution in the upper ocean.
      PubDate: 2017-04-13T15:10:28.266667-05:
      DOI: 10.1002/2017JC012762
       
  • Lagrangian and Eulerian characterization of two counterrotating
           submesoscale eddies in a western boundary current
    • Authors: Alessandra Mantovanelli; Shane Keating, Lucy R. Wyatt, Moninya Roughan, Amandine Schaeffer
      Abstract: In recent decades, high-spatial resolution ocean radar and satellite imagery measurements have revealed a complex tangle of submesoscale filaments and eddies, in the surface velocity, temperature and chlorophyll-a fields. We use a suite of high resolution data to characterize two counter-rotating, short-lived eddies formed at the front between the warm East Australian Current (EAC) and temperate coastal waters (30°S, Eastern Australia). In this region, submesoscale filaments and short-lived eddies are dynamically generated and decay at time scales of hours to days. Dominant cyclonic filaments of O(1) Rossby number formed along frontal jets and eddy boundaries, generating localized ageostrophic circulations at the submesoscale. Measurements of over-ocean wind direction and surface currents from high-frequency radars reveal the influence of the short-term, small-scale wind forcing on the surface circulation, enhancement of the horizontal shear, frontal jet destabilization and the generation and decay of the cyclonic eddy. By contrast, the anticyclonic eddy formation was most likely associated with EAC mesoscale instability and anticyclonic vorticity. Lagrangian tracks show that surface particles can be temporarily trapped in the eddies and frontal convergent zones, limiting their transport. Mixing between EAC-derived and coastal waters was increased along the frontal regions, and particles starting at the divergent regions around the eddies experienced significant dispersion at submesoscales. The cyclonic cold-core eddy entrained high chlorophyll-a shelf waters on its convergent side, suggesting spiral eddy cyclogenesis.
      PubDate: 2017-04-13T15:05:29.858331-05:
      DOI: 10.1002/2016JC011968
       
  • Nonbreaking wave-induced mixing in upper ocean during tropical cyclones
           using coupled hurricane-ocean-wave modeling
    • Authors: S. Aijaz; M. Ghantous, A. V. Babanin, I. Ginis, B. Thomas, G. Wake
      Abstract: The effects of turbulence generated by non-breaking waves have been investigated by testing and evaluating a new non-breaking wave parameterisation in a coupled hurricane-ocean-wave model. The MPI version of the Princeton Ocean Model (POM) with hurricane forcing is coupled with the WAVEWATCH-III (WW3) surface wave model. Hurricane Ivan is chosen as the test case due to its extreme intensity and availability of field data during its passage. The model results are validated against field observations of wave heights and sea surface temperatures (SST) from the National Data Buoy Centre (NDBC) during Hurricane Ivan and against limited in-situ current and bottom temperature data. A series of numerical experiments is set-up to examine the influence of the non-breaking wave parameterisation on the mixing of upper-ocean. The SST response from the modelling experiments indicates that the non-breaking wave-induced mixing leads to significant cooling of the SST and deepening of the mixed layer. It was found that the non-dimensional constant b1 in the non-breaking wave parameterisation has different impacts on the weak and the strong sides of the storm-track. A constant value of b1 leads to improved predictions on the strong side of the storm while a steepness dependent b1 provides a better agreement with in-situ observations on the weak side. A separate simulation of the intense tropical cyclone Olwyn in North-West Australia revealed the same trend for b1 on the strong side of the tropical cyclone.
      PubDate: 2017-04-13T15:00:29.269258-05:
      DOI: 10.1002/2016JC012219
       
  • Erosion of a California Undercurrent eddy by bottom topography
    • Authors: Hector S. Torres; Jose Gomez-Valdes
      Abstract: Subsurface eddies are ubiquitous features in eastern boundary current systems. These phenomena tend to modulate the across-shore distribution of heat and biogeochemical tracers. A California Undercurrent eddy was observed by shipboard observations in October 2009 off the northern Baja California continental slope. The spatiotemporal variation in the California Undercurrent eddy is investigated by using a mesoscale-resolving hindcast ocean simulation. A poleward coastal current that is driven by an upwelling-wind relaxation event and the coastline geometry instigated the separation of the California Undercurrent from the slope, forming a meander-like structure, which evolved as a mesoscale eddy-like structure. The latter structure evolved as a subsurface eddy with a warm anomaly core, a distinctive feature of eddies that form from the California Undercurrent. During the initial stage, the subsurface eddy presented a cone-shape form, with the maximum amplitude of the relative vorticity in the upper section. The inviscid effect of the irregular bottom topography altered both the initial direction of propagation and the initial eddy shape: the propagation direction of the eddy changed from north-south to southwest, and the shape changed from a cone shape to a subsurface lens shape. The strong eddy-topography interactions triggered a sign change in the heat flux direction from the environment towards the eddy and vice versa through the horizontally-divergent component of the velocity field, which accelerated the eddy's decay. This study shows the relevance of the synoptic wind stress events and the irregularity of the bottom topography on the mesoscale eddy activity in the southern portion of the California Current.
      PubDate: 2017-04-07T15:15:02.314329-05:
      DOI: 10.1002/2016JC011870
       
  • The role of ocean-atmosphere interaction in Typhoon Sinlaku (2008) using a
           regional coupled data assimilation system
    • Authors: Akiyoshi Wada; Masaru Kunii
      Abstract: For improving analyses of tropical cyclone (TC) and sea surface temperature (SST) and thereby TC simulations, a regional mesoscale strongly coupled atmosphere-ocean data assimilation system was developed with the local ensemble transform Kalman filter (LETKF) implemented with the Japan Meteorological Agency's nonhydrostatic model (NHM) coupled with a multilayer ocean model and the third-generation ocean wave model. The NHM-LETKF coupled data assimilation system was applied to Typhoon Sinlaku (2008) along with the original NHM-LETKF system to investigate the sensitivity of Sinlaku to SST assimilation with the Level 2 Pre-processed (L2P) standard product of satellite SST. SST calculated in the coupled-assimilation experiment with the coupled data assimilation system and the satellite SST (CPL) showed a better correlation with Optimally Interpolated SST than SST used in the control experiment with the original NHM-LETKF (CNTL) and SST calculated in the succession experiment with the coupled system without satellite SST (SUCC). The time series in the CPL experiment well captured the variation in the SST observed at the Kuroshio Extension Observation buoy site. In addition, TC-induced sea surface cooling was analyzed more realistically in the CPL experiment than that in the CNTL and SUCC experiments. However, the central pressure analyzed in each three experiments was overestimated compared with the Regional Specialized Meteorological Center Tokyo best track central pressure, mainly due to the coarse horizontal resolution of 15 km. The 96-h TC simulations indicated that the CPL experiment provided more favorable initial and boundary conditions than the CNTL experiment to simulate TC tracks more accurately.
      PubDate: 2017-04-07T15:05:52.109071-05:
      DOI: 10.1002/2017JC012750
       
  • Sea ice draft observations in Nares Strait from 2003 to 2012
    • Authors: Patricia A. Ryan; Andreas Münchow
      Abstract: Time series observation of sea ice draft and velocity from Nares Strait between 2003 and 2012 provide new insights on the statistical properties of sea ice leaving the Arctic for the Atlantic Oceans. Median ice draft is 0.8 m, but it varies annually from 1.5 m in 2007-08 to 0.5 m in 2008-09. Probability density distributions of sea ice draft depend on location across the channel with thicker ice near Canada and thinner ice near Greenland. Nevertheless, sea ice motion stops seasonally due to arching land-fast ice that spans the 30-40 km wide channel for up to 190 days per year such as during the 2011-12 winter. In contrast the 2006 to 2010 period exhibits a single ice arch lasting 47 days in April/May of 2008. Hence sea ice statistics are weighted by space not time using sea ice velocities estimated from co-located velocity observations. Multi-year sea ice with drafts exceeding 5 m constitute between 9% (2003-04) and 16% (2007-08) of the observed sea ice. The probability g(D) of this thick, ridged, multi-year ice decays exponentially with draft D at an e-folding scale D0 of 3.0 ± 0.2 m. The trend of D0 with time is statistically indistinguishable from zero. This observation suggests a steady export of multi-year sea ice at decadal time scales. We speculate that our observations document the draining of the last reservoir of thick ice from the Arctic Ocean found to the north of Ellesmere Island and Greenland.
      PubDate: 2017-04-04T08:20:37.524803-05:
      DOI: 10.1002/2016JC011966
       
  • Boussinesq modeling of wave-induced hydrodynamics in coastal wetlands
    • Authors: Agnimitro Chakrabarti; Steven R. Brandt, Qin Chen, Fengyan Shi
      Abstract: In this paper, an improved formulation of the vegetation drag force, applicable for the fully non-linear Boussinesq equations and based on the use of the depth varying, higher order expansion of the horizontal velocity, in the quadratic vegetation drag law has been presented. The model uses the same numerical schemes as FUNWAVE TVD but is based on the CACTUS framework. The model is validated for wave height and setup, against laboratory experiments with and without vegetation cover. The wave attenuation results using the improved formulation were compared with those using the first order reference velocity as well as with analytical solutions using linear wave theory. The analytical solution using the depth varying velocity, predicted by the linear wave theory, was shown to match the model results with the fully expanded velocity approach very well for all wave cases, except under near emergent and emergent conditions (when the ratio of stem height to water depth is greater than 0.75) and when the Ursell (Ur) number is less than 5. Simulations during peak storm waves, during Hurricane Isaac, showed that vegetation is very effective in reducing setup on platforms and in reducing the wave energy within the first few hundred meters.
      PubDate: 2017-04-04T08:20:36.022584-05:
      DOI: 10.1002/2016JC012093
       
  • North Atlantic storm driving of extreme wave heights in the North Sea
    • Authors: R. J. Bell; S. L. Gray, O. P. Jones
      Abstract: The relationship between storms and extreme ocean waves in the North Sea is assessed using a long-period wave dataset and storms identified in the Interim ECMWF Re-Analysis (ERA-Interim). An ensemble sensitivity analysis is used to provide information on the spatial and temporal forcing from mean sea-level pressure and surface wind associated with extreme ocean wave height responses. Extreme ocean waves in the central North Sea arise due to intense extratropical cyclone winds from either the cold conveyor belt (northerly-wind events) or the warm conveyor belt (southerly-wind events). The largest wave heights are associated with northerly-wind events which tend to have stronger wind speeds and occur as the cold conveyor belt wraps rearwards round the cyclone to the cold side of the warm front. The northerly-wind events provide a larger fetch to the central North Sea to aid wave growth. Southerly-wind events are associated with the warm conveyor belts of intense extratropical cyclones that develop in the left upper-tropospheric jet exit region. Ensemble sensitivity analysis can provide early warning of extreme wave events by demonstrating a relationship between wave height and high pressure to the west of the British Isles for northerly-wind events 48 hours prior. Southerly-wind extreme events demonstrate sensitivity to low pressure to the west of the British Isles 36 hours prior.
      PubDate: 2017-04-04T08:20:30.289512-05:
      DOI: 10.1002/2016JC012501
       
  • Impacts of oil spills on altimeter waveforms and radar backscatter
           cross-section
    • Authors: Yongcun Cheng; Jean Tournadre, Xiaofeng Li, Qing Xu, Bertrand Chapron
      Abstract: Ocean surface films can damp short capillary-gravity waves, reduce the surface mean square slope, and induce “sigma0 blooms” in satellite altimeter data. No study has ascertained the effect of such film on altimeter measurements due to lack of film data. The availability of Environmental Response Management Application (ERMA) oil cover, daily oil spill extent and thickness data acquired during the Deepwater Horizon (DWH) oil spill accident provides a unique opportunity to evaluate the impact of surface film on altimeter data. In this study, the Jason-1/2 passes nearest to the DWH platform are analyzed to understand the waveform distortion caused by the spill as well as the variation of σ0 as a function of oil thickness, wind speed and radar band. Jason-1/2 Ku-band σ0 increased by 10 dB at low wind speed (
      PubDate: 2017-04-04T08:20:25.989153-05:
      DOI: 10.1002/2016JC012568
       
  • Equatorial Stokes drift and Rossby rip currents
    • Authors: Jan Erik H. Weber
      Abstract: The Stokes drift in long baroclinic equatorial Rossby waves is investigated theoretically by using eigenfunction expansions in the vertical. These waves are non-dispersive and propagate westward along the equator. Particular attention is paid to the first baroclinic, first meridional Rossby wave mode which has been observed in the equatorial Pacific. It is demonstrated that the Stokes drift depends very much on the depth-variation of the Brunt-Väisälä frequency. Even more importantly, it is found that, for arbitrary stable stratification, the total zonal Stokes volume transport induced by the Rossby wave mode (1,1) is identically zero. The eastward drift due reflected wave energy in the form of internal equatorial Kelvin waves is also addressed. Due to the very long period of the incident Rossby wave mode (1,1), the reflected equatorial Kelvin wave must at least be a 2. mode component in the vertical. The corresponding Stokes drift only induces a minor change near the surface of the total westward drift velocity at the equator. The implication for the existence of compensating Rossby rip currents along the equator is discussed.
      PubDate: 2017-03-31T13:51:24.609701-05:
      DOI: 10.1002/2016JC012653
       
  • Mechanism of seasonal eddy kinetic energy variability in the eastern
           equatorial Pacific Ocean
    • Authors: Minyang Wang; Yan Du, Bo Qiu, Xuhua Cheng, Yiyong Luo, Xiao Chen, Ming Feng
      Abstract: Enhanced mesoscale eddy activities or tropical instability waves (TIWs) exist along the northern front of the cold tongue in the eastern equatorial Pacific Ocean. In this study, we investigate seasonal variability of eddy kinetic energy (EKE) over this region and its associated dynamic mechanism using a global, eddy-resolving ocean general circulation model (OGCM) simulation, the equatorial mooring data and satellite altimeter observations. The seasonal-varying enhanced EKE signals are found to expand westward from 100°W in June to 180°W in December between 0°−6°N. This westward expansion in EKE is closely connected to the barotropically-baroclinically unstable zonal flows that are in thermal-wind balance with the seasonal-varying thermocline trough along 4°N. By adopting an 1½-layer reduced-gravity model, we confirm that the seasonal perturbation of the thermocline trough is dominated by the anti-cyclonic wind stress curl forcing, which develops due to southerly winds along 4°N from June to December.
      PubDate: 2017-03-31T13:51:22.889052-05:
      DOI: 10.1002/2017JC012711
       
  • An idealized study of near equatorial river plumes
    • Authors: Elbio D. Palma; Ricardo P. Matano
      Abstract: The dynamics of near-equatorial river plumes (NERPs) are investigated using a highly idealized model. The spreading of a NERP from an eastern boundary is characterized by a continuous shedding of westward propagating eddies. This process transfers the bulk of the freshwater discharge to the deep ocean, thus distinguishing NERPs from their mid-latitude counterparts. In the long-term limit a NERP can be rationalized as a β-plume emanating from a coastal source. The evolution of NERPs in an unstratified basin is quite different from that in a strafied one. The spin-up in an unstratified basin is characterized by the formation of an anticyclonic bulge, which spreads westward thus creating a density stratification that favors the subsequent development of smaller and faster moving secondary eddies. The collision of the secondary eddies with the leading bulge arrests the effects of mixing thus allowing the further spreading of the buoyancy anomaly. In a stratified basin the generation of anticyclonic eddies is accompanied by a concurrent generation of cyclones, which pump saltier waters to the surface hence leading to smaller sea surface salinity (SSS) anomalies. NERPs are sensitive to variations of the freshwater flux (Qfw) and the geomorphological setting. Larger Qfw generates bigger eddies, which spread at a rate proportional to the square root of the normalized flux. Wide shelves allow the interaction of the eddies with the bottom, thus promoting a cyclonic shift of the axis of the eddy train. The inclination of the coast affects the dynamical balance controlling the near-field behaviour of NERPs.
      PubDate: 2017-03-31T13:51:21.62319-05:0
      DOI: 10.1002/2016JC012554
       
  • Modeling postconvective submesoscale coherent vortices in the northwestern
           Mediterranean Sea
    • Authors: P. Damien; A. Bosse, P. Testor, P. Marsaleix, C. Estournel
      Abstract: For the first time, the formation of Submesoscale Coherent Vortices (SCVs) during intermediate and deep convection events is documented in a realistic high resolution (1 km) numerical simulation of the oceanic circulation in the northwestern Mediterranean Sea. Winter intermediate and deep convection leads to the formation of anticyclonic and cyclonic eddies with lifetimes exceeding one year. By focusing on three typical eddies, the main characteristics of such vortices are discussed. The anticyclonic eddies are typical of SCVs observed in deep convection areas so far. They are characterized by a small radius (∼ 6.5 km) and orbital peak velocities of about 7 cm/s located at great depth (∼ 1500 m) or intermediate depth (∼ 500 m). The cyclonic vortices show very similar characteristics, such as a high Rossby number (∼ 0.4), but with surface-intensified structures. The long lifetimes of both anticyclonic and cyclonic eddies reflect very slow diffusive processes between their core and their surroundings and a strong resistance to external perturbations. These long-lived eddies are found to participate in the spreading of a significant portion (from 15 to 35%) of the convected waters in the Gulf of Lions and contribute to the ventilation of the deep basin.
      PubDate: 2017-03-31T13:51:16.311955-05:
      DOI: 10.1002/2016JC012114
       
  • A statistical study on the subthermocline submesoscale eddies in the
           northwestern Pacific Ocean based on Argo data
    • Authors: Cheng Li; Zhiwei Zhang, Jiwei Tian, Wei Zhao
      Abstract: In this study, statistical characteristics and spatiotemporal variability of the cold-core subthermocline eddies (STEs) discovered by Zhang et al. [2015] in the northwestern Pacific are investigated with 14 years of Argo temperature/salinity (T/S) data between 2002–2015. By applying an objective identification procedure to Argo T/S profiles, a total of 337 lens-like STEs that trap cold and fresh homogeneous water are obtained. The STEs' cores are located between 26.2–27.0σ0 and their shapes share a uniform Gaussian structure in vertical. Vertical and horizontal scale of the STEs is generally between 100–200 m and 5–15 km, respectively, suggesting that they are in the category of submesoscale processes. According to their different densities and seasonalities, the STEs are classified into upper and lower types with density interface of 26.7σ0. The number of upper-type STEs is much larger in spring and summer than in autumn and winter while that of lower-type ones show relatively uniform seasonal distributions. Spatially, the STEs are primarily distributed within the region between the Kuroshio Extension (KE) and subarctic front (SAF) in meridional and 140–155˚E in zonal direction. Based on STEs' spatial distributions and vertical locations, we propose to name these STEs as KE intermediate-layer eddies (Kiddies). Water property analysis suggests that the Kiddies most likely originated from the SAF region. For the upper-type Kiddies, they are probably generated through subduction associated with mixed-layer frontogenesis, which at least contribute to 0.25 Sv annual subduction rate for the layers 26.2–26.7σ0.
      PubDate: 2017-03-31T13:51:11.83112-05:0
      DOI: 10.1002/2016JC012561
       
  • Wind-driven ocean dynamics impact on the contrasting sea-ice trends around
           West Antarctica
    • Authors: Sang-Ki Lee; Denis L. Volkov, Hosmay Lopez, Woo Geun Cheon, Arnold L. Gordon, Yanyun Liu, Rik Wanninkhof
      Abstract: Since late 1978, Antarctic sea-ice extent in the East Pacific has retreated persistently over the Amundsen and Bellingshausen Seas in warm seasons, but expanded over the Ross and Amundsen Seas in cold seasons, while almost opposite seasonal trends have occurred in the Atlantic over the Weddell Sea. By using a surface-forced ocean and sea-ice coupled model, we show that regional wind-driven ocean dynamics played a key role in driving these trends. In the East Pacific, the strengthening Southern Hemisphere (SH) westerlies in the region enhanced the Ekman upwelling of warm upper Circumpolar Deep Water and increased the northward Ekman transport of cold Antarctic surface water. The associated surface ocean warming south of 68°S and the cooling north of 68°S directly contributed to the retreat of sea ice in warm seasons and the expansion in cold seasons, respectively. In the Atlantic, the poleward shifting SH westerlies in the region strengthened the northern branch of the Weddell Gyre, which in turn increased the meridional thermal gradient across it as constrained by the thermal wind balance. Ocean heat budget analysis further suggests that the strengthened northern branch of the Weddell Gyre acted as a barrier against the poleward ocean heat transport, and thus produced anomalous heat divergence within the Weddell Gyre and anomalous heat convergence north of the gyre. The associated cooling within the Weddell Gyre and the warming north of the gyre contributed to the expansion of sea ice in warm seasons and the retreat in cold seasons, respectively.
      PubDate: 2017-03-31T13:51:09.142908-05:
      DOI: 10.1002/2016JC012416
       
  • Physical and biogeochemical controls of the phytoplankton blooms in
           North-Western Mediterranean Sea: A multiplatform approach over a complete
           annual cycle (2012–2013 DEWEX experiment)
    • Authors: Nicolas Mayot; Fabrizio D'Ortenzio, Vincent Taillandier, Louis Prieur, Orens Pasqueron de Fommervault, Hervé Claustre, Anthony Bosse, Pierre Testor, Pascal Conan
      Abstract: The North Western Mediterranean Sea exhibits recurrent and significant autumnal and spring phytoplankton blooms. The existence of these two blooms coincide with typical temperate dynamics. To determine the potential control of physical and biogeochemical factors on these phytoplankton blooms, data from a multiplatform approach (combining ships, Argo and BGC-Argo floats, and bio-optical gliders) were analyzed in association with satellite observations in 2012-2013. The satellite framework allowed a simultaneous analysis over the whole annual cycle of in situ observations of mixed layer depth, photosynthetical available radiation, particle backscattering, nutrients (nitrate and silicate) and chlorophyll-a concentrations. During the year 2012-2013, satellite ocean color observations, confirmed by in situ data, have revealed the existence of two areas (or bioregions) with comparable autumnal blooms but contrasting spring blooms. In both bioregions, the ratio of the euphotic zone (defined as the isolume 0.415 mol photons m−2 d−1, Z0.415) and the MLD identified the initiation of the autumnal bloom, as well as the maximal annual increase in [Chl-a] in spring. In fact, the autumnal phytoplankton bloom might be initiated by mixing of the summer shallowing deep chlorophyll maximum, while the spring restratification (when Z0.415/MLD ratio became > 1) might induce surface phytoplankton production that largely overcomes the losses. Finally, winter deep convection events that took place in one of the bioregions induced higher net accumulation rate of phytoplankton in spring associated with a diatom-dominated phytoplankton community principally. We suggest that very deep winter MLD lead to an increase in surface silicates availability, which favored the development of diatoms.
      PubDate: 2017-03-31T13:50:32.70281-05:0
      DOI: 10.1002/2016JC012052
       
  • Observation-based parameterization of air-sea fluxes in terms of wind
           speed and atmospheric stability under low-to-moderate wind conditions
    • Authors: Zhongshui Zou; Dongliang Zhao, Bin Liu, Jun A Zhang, Jian Huang
      Abstract: This study explores the behavior of the exchange coefficients for wind stress (CD), sensible heat flux (CH), and water vapor flux (CE) as functions of surface wind speed (U10) and atmospheric stability using direct turbulent flux measurements obtained from a platform equipped with fast-response turbulence sensors in a low-to-moderate wind region. Turbulent fluxes are calculated using the eddy correlation method with extensive observations. The total numbers of quality-controlled 30-min flux runs are 12,240, 5,813, and 5,637 for estimation of CD, CH, and CE, respectively. When adjusted to neutral stability using the Monin-Obukhov similarity theory (MOST), we found that CDN, CHN, and CEN decrease with neutral-adjusted wind speed when wind speed is less than 5 m/s. CDN is constant over the range 5 m/s 
      PubDate: 2017-03-31T13:50:27.588045-05:
      DOI: 10.1002/2016JC012399
       
  • Effect of a fast-moving tropical storm Washi on phytoplankton in the
           northwestern South China Sea
    • Authors: Hui Zhao; Jiayi Pan, Guoqi Han, Adam T. Devlin, Shuwen Zhang, Yijun Hou
      Abstract: Tropical cyclones may augment nutrients in the ocean surface layer through mixing, entrainment, and upwelling, triggering phytoplankton blooms in oligotrophic waters such as the South China Sea (SCS). Previous studies focused mainly on responses of marine environments to strong or slow-moving typhoons in the SCS. In this study, we analyze variations of chlorophyll-a (Chl-a) and oceanic conditions in the continental shelf region east of Hainan Island during the fast-moving tropical storm Washi, and investigate its influences on phytoplankton bloom and related dynamic mechanisms. Results indicate that there was significant variation of Chl-a concentration in the continental shelf region, with low values (about 0.1 mg m−3) before the storm and a 30% increase after the storm. This increase was spatially variable, much larger nearshore than offshore. Power spectral analysis of Acoustic Doppler Current Profiler (ADCP) data at a shelf site near the study region reveals strong near-inertial oscillations (NIOs) in the upper layer, with a period of about 36 hrs, close to the local inertial period. The NIOs intensified mixing and modified the stratification of the upper layer, inducing uplift of nutrients and Chl-a into the mixed layer from below, and leading to surface Chl-a increase. The relatively shallow nutricline and thermocline in the continental shelf region before the storm were favorable for upwelling of nutrients and generation of NIOs. Advection of nutrients from enhanced runoff during and after the storm may be responsible for the larger increase of the Chl-a nearshore.
      PubDate: 2017-03-31T13:50:24.346271-05:
      DOI: 10.1002/2016JC012286
       
  • Turbulent entrainment in a strongly stratified barrier layer
    • Authors: H. T. Pham; S. Sarkar
      Abstract: Large-eddy simulation (LES) is used to investigate how turbulence in the wind-driven ocean mixed layer erodes the stratification of barrier layers. The model consists of a stratified Ekman layer that is driven by a surface wind. Simulations at a wide range of N0/f are performed to quantify the effect of turbulence and stratification on the entrainment rate. Here, N0 is the buoyancy frequency in the barrier layer and f is the Coriolis parameter. The evolution of the mixed layer follows two stages: a rapid initial deepening and a late-time growth at a considerably slower rate. During the first stage, the mixed layer thickens to the depth that is proportional u*/fN0 to where u* is the frictional velocity. During the second stage, the turbulence in the mixed layer continues to entrain further into the barrier layer, and the turbulent length scale is shown to scale with u*/N0, independent of f. The late-time entrainment rate E follows the law of E= 0.035Ri*−1/2 where Ri* is the Richardson number. The exponent is identical and the coefficient of 0.035 is much smaller relative to the corresponding power law in the non-rotating boundary layer. Simulations using the KPP model (version applicable to this simple case without additional effects of Langmuir turbulence or surface buoyancy flux) also yield the entrainment scaling of E∝Ri*−1/2; however, the proportionality coefficient varies with the stratification. The structure of the Ekman current is examined to illustrate the strong effect of stratification in the limit of large N0/f. This article is protected by copyright. All rights reserved.
      PubDate: 2017-03-29T12:19:35.71541-05:0
      DOI: 10.1002/2016JC012357
       
  • The role of the Agulhas in the Benguela Current System: A numerical
           modelling approach
    • Authors: Jennifer A. Veitch; Pierrick Penven
      Abstract: A modelling approach is used to investigate the influence of the Agulhas on the southern Benguela system. Two climatological ROMS simulations are run that are identical except that in one of them the effect of the Agulhas is removed. Comparing their output allows for a clear indication of the role of the Agulhas on both the large-scale and shelf dynamics. About 15 Sv of the mean transport of the Benguela Current is shown to be contributed by Agulhas influx, with the most intense flow being associated with extreme turbulence and its meandering nature is a reflection of the passage of Agulhas Rings. The injection of warm water is particularly evident beyond the shelf-edge, producing a perennially intense cross-shelf density front that is enhanced during upwelling season. This gradient drives a jet that is fastest at the shelf-edge but that extends from the mid-shelf to at least 100 km beyond it and is associated with dynamic uplift via vortex squashing. Similarly generated is the Good Hope Jet that extends northwestward from the western Agulhas Bank. Turbulence associated with Agulhas leakage increases rapidly beyond the shelf-edge causing the upwelling front in the southern Benguela to be subject to intense mixing, leaving a relatively uniform front there. Locally generated regions of high turbulence exist in the vicinity of the shelf-edge jets as well as further north in association with the large filaments that originate from the perennial Lüderitz upwelling cell, with or without the influence of the Agulhas. This article is protected by copyright. All rights reserved.
      PubDate: 2017-03-28T12:40:58.065354-05:
      DOI: 10.1002/2016JC012247
       
  • Transport and thermohaline variability in Barrow Canyon on the
           northeastern chukchi Sea Shelf
    • Authors: Thomas J. Weingartner; Rachel A. Potter, Chase A. Stoudt, Elizabeth L. Dobbins, Hank Statscewich, Peter R. Winsor, Todd D. Mudge, Keath Borg
      Abstract: We used a 5-year time series of transport, temperature, and salinity from moorings at the head of Barrow Canyon to describe seasonal variations and construct a 37-year transport hindcast. The latter was developed from summer/winter regressions of transport against Bering-Chukchi winds. Seasonally, the regressions differ due to baroclinicity, stratification, spatial and seasonal variations in winds and/or the surface drag coefficients. The climatological annual cycle consists of summer downcanyon (positive and toward the Arctic Ocean) transport of ∼0.45 Sv of warm, fresh waters; fall (Oct. – Dec.) upcanyon transport of ∼-0.1 Sv of cooler, saltier waters; and negligible net winter (Jan. – Apr.) mass transport when shelf waters are saline and near-freezing. Fall upcanyon transports may modulate shelf freeze-up, and negligible winter transports could influence winter water properties. Transport variability is largest in fall and winter. Daily transport probability density functions are negatively skewed in all seasons and seasonal variations in kurtosis are a function of transport event durations. The latter may have consequences for shelf-basin exchanges. The climatology implies that the Chukchi shelf circulation re-organizes annually: in summer ∼40% of the summer Bering Strait inflow leaves the shelf via Barrow Canyon, but from fall through winter all of it exits via the western Chukchi or Central Channel. We estimate a mean transport of ∼0.2 Sv; ∼50% less than estimates at the mouth of the canyon. Transport discrepancies may be due to inflows from the Beaufort shelf and the Chukchi shelfbreak, with the latter entering the western side of the canyon. This article is protected by copyright. All rights reserved.
      PubDate: 2017-03-28T12:40:55.641332-05:
      DOI: 10.1002/2016JC012636
       
  • A SST-dependent Ku-band geophysical model function for RapidScat
    • Authors: Zhixiong Wang; Ad Stoffelen, Chaofang Zhao, Jur Vogelzang, Anton Verhoef, Jeroen Verspeek, Mingsen Lin, Ge Chen
      Abstract: A new Ku-band Geophysical Model Function (GMF) which includes Sea Surface Temperature (SST) dependence, named NSCAT-5, is developed for improved RapidScat wind retrieval. The RapidScat scatterometer instrument mounted on the International Space Station (ISS) provides near-global wind data over the oceans. Starting from the existing NSCAT-4 GMF, the variation of σ0 is approximated as a second-order Taylor expansion in sea surface temperature T for each given wind speed V, and the fitting coefficients are obtained from both observed and simulated radar cross sections, using ASCAT winds, of either vertical or horizontal polarizations. Furthermore, an intercalibration is performed which aligns the distribution of RapidScat wind speeds to that of ASCAT. NSCAT-5 is obtained by correcting NSCAT-4 with SST dependencies and intercalibration information. Validation of the new RapidScat wind products retrieved using NSCAT-5 shows clear improvements over those obtained with NSCAT-4: Wind inversion residuals no longer depend on SST, and the wind speed Probability Density Functions (PDFs) are closely overlapping with those of ASCAT. Also, RapidScat NSCAT-5 minus ASCAT wind speed differences show no SST dependence. The work presented here opens a door for further improving the quality of wind products from Ku-band backscatter measurements, and helps to build a long-term and consistent essential Climate Data Record (CDR) of scatterometer winds. This article is protected by copyright. All rights reserved.
      PubDate: 2017-03-28T12:40:51.561116-05:
      DOI: 10.1002/2016JC012619
       
  • Dynamics of Andaman Sea circulation and its role in connecting the
           equatorial Indian Ocean to the Bay of Bengal
    • Authors: Abhisek Chatterjee; D. Shankar, J. P. McCreary, P. N. Vinayachandran, A. Mukherjee
      Abstract: Circulation in the Bay of Bengal (BoB) is driven not only by local winds, but are also strongly forced by the reflection of equatorial Kelvin waves (EKWs) from the eastern boundary of the Indian Ocean. The equatorial influence attains its peak during the monsoon transition period when strong eastward currents force the strong EKWs along the equator. The Andaman Sea, lying between the Andaman and Nicobar island chains to its west and Indonesia, Thailand, and Myanmar to the south, east, and north, is connected to the equatorial ocean and the BoB by three primary passages, the southern (6°N), middle (10°N), and northern (15°N) channels. We use ocean circulation models, together with satellite altimeter data, to study the pathways by which equatorial signals pass through the Andaman Sea to the BoB and associated dynamical interactions in the process. The mean coastal circulation within the Andaman Sea and around the islands is primarily driven by equatorial forcing, with the local winds forcing a weak sea-level signal. On the other hand, the current forced by local winds is comparable to that forced remotely from the equator. Our results suggest that the Andaman and Nicobar Islands not only influence the circulation within the Andaman Sea, but also significantly alter the circulation in the interior bay and along the east coast of India, implying that they need to be represented accurately in numerical models of the Indian Ocean. This article is protected by copyright. All rights reserved.
      PubDate: 2017-03-28T12:40:49.224538-05:
      DOI: 10.1002/2016JC012300
       
  • Ekman circulation in the Arctic Ocean: Beyond the Beaufort Gyre
    • Authors: Barry Ma; Michael Steele, Craig M. Lee
      Abstract: Data derived from satellite based observations, with buoy-based observations and assimilations, are used to calculate ocean Ekman layer transport and evaluate long-term trends in the Arctic Ocean over the period 1979–2014. The 36-year mean of upwelling (downwelling) is 3.6 ± 2.0 (−4.0 ± 2.2) Sv for the entire Arctic Basin, with ∼0.4 Sv net downwelling contributed mostly by the Canadian region. With regard to long-term trends, the annual mean upwelling (downwelling) over the entire Arctic Basin is increasing at a linear rate of 0.89 (−0.96) Sv/decade. The Canada/Alaska coasts and Beaufort and Laptev seas are regions of greatest Ekman transport intensification. The central Arctic Ocean and Lincoln Sea also have an increasing trend in transport. The Canadian and Eurasian regions each account for about half the total vertical Ekman variations in the Arctic Basin. This article is protected by copyright. All rights reserved.
      PubDate: 2017-03-28T12:40:29.84646-05:0
      DOI: 10.1002/2016JC012624
       
  • The wave-driven current in coastal canopies
    • Authors: Maryam Abdolahpour; Magnus Hambleton, Marco Ghisalberti
      Abstract: Wave-driven flows over canopies of aquatic vegetation (such as seagrass) are characterized by the generation of a strong, shoreward mean current near the top of the canopy. This shoreward drift, which is observed to be up to 75% of the RMS above-canopy orbital velocity, can have a significant impact on residence times within coastal canopies. There have been limited observations of this current and an accurate formulation of its magnitude is still lacking. Accordingly, this study aims to develop a practical relationship to describe the strength of this current as a function of both wave and canopy characteristics. A simple model for the Lagrangian drift velocity indicates that the magnitude of the wave-driven current increases with the above-canopy oscillatory velocity, the vertical orbital excursion at the top of the canopy and the canopy density. An extensive laboratory study, using both rigid and (dynamically-scaled) flexible model vegetation, was carried out to evaluate the proposed model. Experimental results reveal a strong agreement between predicted and measured current velocities over a wide and realistic range of canopy and wave conditions. The validity of this model is also confirmed through available field measurements. Characterization of this wave-induced mean current will allow an enhanced capacity for predicting residence time, and thus key ecological processes, in coastal canopies. This article is protected by copyright. All rights reserved.
      PubDate: 2017-03-28T12:35:42.763999-05:
      DOI: 10.1002/2016JC012446
       
  • Flow Variability within the Alaska Coastal Current in winter
    • Authors: Ewa Jarosz; David Wang, Hemantha Wijesekera, W. Scott Pegau, James N. Moum
      Abstract: Coastal circulation off Kayak Island in the northern Gulf of Alaska was explored in wintertime (October 2012-March 2013) by deploying nine moorings within the Alaska Coastal Current (ACC). Hydrographic, bottom pressure, and velocity observations depicted well the winter variability of the ACC. Atmospheric observations showed a net loss of heat, 30 W m−2 or more, from the ocean to the atmosphere and indicated that storms with downwelling-favorable winds over 10 m s−1 frequently passed over the area. Due to vigorous mixing during storms, the waters were well-mixed or weakly stratified whereas bottom-pressure anomalies were mainly related to surface-elevation fluctuations and indicated that there was also a cross-shelf surface-elevation gradient. Current observations showed along-shelf nearly-barotropic subtidal flow of 40 cm s−1 or more throughout the water column. They also indicated that along-shelf flow was primarily driven by the cross-shelf pressure gradient resulting from the cross-shelf surface-elevation gradient and not by wind stress. Analyses suggested that flow dynamics within the ACC in winter were well-described by vertically-averaged momentum equations and showed a dominance of the cross-shelf pressure gradient that was mainly balanced by the Coriolis term. Observations also showed that when winds relaxed, cold low-salinity waters moved offshore and stratification was reestablished. Consequently, near-shore waters were less dense, i.e., cooler and fresher than offshore waters resulting in the cross-shelf density gradient that may have contributed to the along-shelf flow by generating near-surface currents of ∼ 20 cm s−1. This article is protected by copyright. All rights reserved.
      PubDate: 2017-03-28T12:35:29.147699-05:
      DOI: 10.1002/2016JC012102
       
  • Projected changes in significant wave height towards the end of the 21st
           century - Northeast Atlantic
    • Authors: Ole Johan Aarnes; Magnar Reistad, Øyvind Breivik, Elzbieta Bitner-Gregersen, Lars Ingolf Eide, Odin Gramstad, Anne Karin Magnusson, Bent Natvig, Erik Vanem
      Abstract: Wind field ensembles from six CMIP5 models force wave model time slices of the Northeast Atlantic over the last three decades of the 20th and the 21st centuries. The future wave climate is investigated by considering the RCP4.5 and RCP8.5 emission scenarios. The CMIP5 model selection is based on their ability to reconstruct the present(1971-2000) extratropical cyclone activity, but increased spatial resolution has also been emphasized. In total, the study comprises 35 wave model integrations, each about 30 years long, in total more than 1,000 years. Here, annual statistics of significant wave height are analyzed, including mean parameters and upper percentiles. There is general agreement among all models considered that the mean significant wave height is expected to decrease by the end of the 21st century. This signal is statistically significant also for higher percentiles, but less evident for annual maxima. The RCP8.5 scenario yields the strongest reduction in wave height. The exception to this is the north western part of the Norwegian Sea and the Barents Sea, where receding ice cover gives longer fetch and higher waves. The upper percentiles are reduced less than the mean wave height, suggesting that the future wave climate has higher variance than the historical period. This article is protected by copyright. All rights reserved.
      PubDate: 2017-03-28T12:35:23.282733-05:
      DOI: 10.1002/2016JC012521
       
  • A simulation of small to giant Antarctic iceberg evolution: Differential
           impact on climatology estimates
    • Authors: Thomas Rackow; Christine Wesche, Ralph Timmermann, Hartmut H. Hellmer, Stephan Juricke, Thomas Jung
      Abstract: We present a simulation of Antarctic iceberg drift and melting that includes small, medium-sized, and giant tabular icebergs with a realistic size distribution. For the first time, an iceberg model is initialized with a set of nearly 7000 observed iceberg positions and sizes around Antarctica. The study highlights the necessity to account for larger and giant icebergs in order to obtain accurate melt climatologies. We simulate drift and lateral melt using iceberg-draft averaged ocean currents, temperature, and salinity. A new basal melting scheme, originally applied in ice shelf melting studies, uses in situ temperature, salinity, and relative velocities at an iceberg's bottom. Climatology estimates of Antarctic iceberg melting based on simulations of small (≤ 2.2 km), 'small-to-medium'-sized (≤ 10 km), and small-to-giant icebergs (including icebergs > 10 km) exhibit differential characteristics: successive inclusion of larger icebergs leads to a reduced seasonality of the iceberg meltwater flux and a shift of the mass input to the area north of 58 ∘S, while less meltwater is released into the coastal areas. This suggests that estimates of meltwater input solely based on the simulation of small icebergs introduce a systematic meridional bias; they underestimate the northward mass transport and are, thus, closer to the rather crude treatment of iceberg melting as coastal runoff in models without an interactive iceberg model. Future ocean simulations will benefit from the improved meridional distribution of iceberg melt, especially in climate change scenarios where the impact of iceberg melt is likely to increase due to increased calving from the Antarctic ice sheet. This article is protected by copyright. All rights reserved.
      PubDate: 2017-03-23T18:50:40.453698-05:
      DOI: 10.1002/2016JC012513
       
  • Rip currents and alongshore flows in single channels dredged in the surf
           zone
    • Authors: Melissa Moulton; Steve Elgar, Britt Raubenheimer, John C. Warner, Nirnimesh Kumar
      Abstract: To investigate the dynamics of flows near non-uniform bathymetry, single channels (on average 30-m wide and 1.5-m deep) were dredged across the surf zone at five different times, and the subsequent evolution of currents and morphology was observed for a range of wave and tidal conditions. In addition, circulation was simulated with the numerical modeling system COAWST, initialized with the observed incident waves and channel bathymetry, and with an extended set of wave conditions and channel geometries. The simulated flows are consistent with alongshore flows and rip current circulation patterns observed in the surf zone. Near the offshore-directed flows that develop in the channel, the dominant terms in modeled momentum balances are wave breaking accelerations, pressure gradients, advection, and the vortex force. The balances vary spatially, and are sensitive to wave conditions and the channel geometry. The observed and modeled maximum offshore-directed flow speeds are correlated with a parameter based on the alongshore gradient in breaking-wave-driven-setup across the non-uniform bathymetry (a function of wave height and angle, water depths in the channel and on the sandbar, and a breaking threshold) and the breaking-wave-driven alongshore flow speed. The offshore-directed flow speed increases with dissipation on the bar and reaches a maximum (when the surf zone is saturated) set by the vertical scale of the bathymetric variability. This article is protected by copyright. All rights reserved.
      PubDate: 2017-03-23T18:39:12.110131-05:
      DOI: 10.1002/2016JC012222
       
  • The 2014 Lake Askja rockslide-induced tsunami: Optimization of numerical
           tsunami model using observed data
    • Authors: Sigríðhur Sif Gylfadóttir; Jihwan Kim, Jón Kristinn Helgason, Sveinn Brynjólfsson, Ármann Höskuldsson, Tómas Jóhannesson, Carl Bonnevie Harbitz, Finn Løvholt
      Abstract: A large rockslide was released from the inner Askja caldera into Lake Askja, Iceland on 21 July 2014. Upon entering the lake it caused a large tsunami that traveled about ∼3 km across the lake and inundated the shore with vertical run-up measuring up to 60–80 m. Following the event, comprehensive field data were collected, including GPS measurements of the inundation and multibeam echo soundings of the lake bathymetry. Using this exhaustive data set, numerical modeling of the tsunami has been conducted using both a nonlinear shallow water model and a Boussinesq-type model that includes frequency dispersion. To constrain unknown landslide parameters, a global optimization algorithm, Differential Evolution, was employed, resulting in a parameter set that minimized the deviation from measured inundation. The tsunami model of Lake Askja is the first example where we have been able to utilize field data to show that frequency dispersion is needed to explain the tsunami wave radiation pattern and that shallow water theory falls short. We were able to fit the trend in tsunami run-up observations around the entire lake using the Boussinesq model. In contrast, the shallow water model gave a different run-up pattern and produced pronounced offsets in certain areas. The well-documented Lake Askja tsunami thus provided a unique opportunity to explore and capture the essential physics of landslide tsunami generation and propagation through numerical modeling. Moreover, the study of the event is important because this dispersive nature is likely to occur for other subaerial impact tsunamis. This article is protected by copyright. All rights reserved.
      PubDate: 2017-03-23T18:39:08.377151-05:
      DOI: 10.1002/2016JC012496
       
  • Physical linkages between an offshore canyon and surf zone morphologic
           change
    • Authors: Jeff E. Hansen; Britt Raubenheimer, Steve Elgar, Jeffrey H. List, Thomas C. Lippmann
      Abstract: The causes of surf zone morphologic changes observed along a sandy beach onshore of a submarine canyon were investigated using field observations and a numerical model (Delft3D/SWAN). Numerically simulated morphologic changes using four different sediment transport formulae reproduce the temporal and spatial patterns of net cross-shore integrated (between 0- and 6.5-m water depths) accretion and erosion observed in a ∼300-m alongshore region, a few hundred meters from the canyon head. The observations and simulations indicate that the accretion or erosion results from converging or diverging alongshore currents driven primarily by breaking waves and alongshore pressure gradients. The location of convergence or divergence depends on the direction of the offshore waves that refract over the canyon, suggesting that bathymetric features on the inner shelf can have first-order effects on short-term nearshore morphologic change. This article is protected by copyright. All rights reserved.
      PubDate: 2017-03-23T18:39:06.499398-05:
      DOI: 10.1002/2016JC012319
       
  • Bio-optical anomalies in the world's oceans: An investigation on the
           diffuse attenuation coefficients for downward irradiance derived from
           biogeochemical Argo float measurements
    • Authors: Emanuele Organelli; Hervé Claustre, Annick Bricaud, Marie Barbieux, Julia Uitz, Fabrizio D'Ortenzio, Giorgio Dall'Olmo
      Abstract: Identification of oceanic regions characterized by particular optical properties is extremely important for ocean color applications. The departure from globally established bio-optical models introduces uncertainties in the retrieval of biogeochemical quantities from satellite observations. Thanks to an array of 105 Biogeochemical Argo floats acquiring almost daily downward irradiance measurements at selected wavelengths in the UV and blue region of the spectrum, we re-examined the natural variability of the spectral diffuse attenuation coefficients, Kd(λ), among the world's oceans and compared them to previously established bio-optical models. The analysis of 2847 measurements of Kd(λ) at 380 and 490 nm, within the first optical depth, provided a classification of the examined regions into three groups. The first one included the Black Sea, a water body characterized by very high colored dissolved organic matter (CDOM) content. The second group was essentially composed by the subtropical gyres (Atlantic and Pacific Oceans), with optical properties consistent with previous models (i.e., no anomalies). High latitude (North Atlantic and Southern oceans) and temperate (Mediterranean Sea) seas formed the third group, in which optical properties departed from existing bio-optical models. Annual climatologies of the Kd(380)/Kd(490) ratio evidenced a persistent anomaly in the Mediterranean Sea, that we attributed to a higher-than-average CDOM contribution to total light absorption. In the North Atlantic subpolar gyre, anomalies were observed only in wintertime and were also attributed to high CDOM concentrations. In the Southern Ocean, the anomaly was likely related to high phytoplankton pigment packaging rather than to CDOM. This article is protected by copyright. All rights reserved.
      PubDate: 2017-03-23T18:38:58.314139-05:
      DOI: 10.1002/2016JC012629
       
  • The impact of storms and stratification on sediment transport in the Rhine
           region of freshwater influence
    • Authors: Raúl P. Flores; Sabine Rijnsburger, Alexander R. Horner-Devine, Alejandro J. Souza, Julie D. Pietrzak
      Abstract: We present measurements of along and across-shore sediment transport in a region of the Dutch coast 10 kilometers north of the Rhine River mouth. This section of the coast is characterized by strong vertical density stratification because it is within the mid-field region of the Rhine region of freshwater influence, where processes typical of the far-field, such as tidal straining, are modified by the passage of distinct freshwater lenses at the surface. The experiment captured two storms, and a wide range of wind, wave, tidal and stratification conditions. We focus primarily on the mechanisms leading to cross-shore sediment flux at a mooring location in 12m of water, which are responsible for the exchange of sediment between the near-shore and the inner shelf. Net transport during storms was directed offshore and influenced by cross-shelf winds, while net transport during spring tides was determined by the mean state of stratification. Tidal straining dominated during neap tides; however, cross-shore transport was negligible due to small sediment concentrations. The passage of freshwater lenses manifested as strong pulses of offshore transport primarily during spring tides. We observe that both barotropic and baroclinic processes are relevant for cross-shore transport at depth and, since transport rates due to these competing processes were similar, the net transport direction will be determined by the frequency and sequencing of these modes of transport. Based on our observations, we find that wind- and wave-driven transport during storms tends move fine sediment offshore, while calmer, more stratified conditions move it back onshore. This article is protected by copyright. All rights reserved.
      PubDate: 2017-03-23T18:38:34.093644-05:
      DOI: 10.1002/2016JC012362
       
  • A downslope propagating thermal front over the continental slope
    • Authors: Hans van Haren; Phil J. Hosegood
      Abstract: In the ocean, internal frontal bores above sloping topography have many appearances, depending on the local density stratification, and on the angle and source of generation of the carrier wave. However, their common characteristics are a backwards breaking wave, strong sediment resuspension and relatively cool (denser) water moving more or less upslope underneath warm (less dense) water. In this paper, we present a rare example of a downslope moving front of cold water moving over near-bottom warm water. Large backscatter is observed in the downslope moving front's trailing edge, rather than the leading edge as is common in upslope moving fronts. Time series observations have been made during a fortnight in summer, using a 101-m long array of high-resolution temperature sensors moored with an acoustic Doppler current profiler at 396 m depth in near-homogeneous waters, near a small canyon in the continental slope off the Malin shelf (West-Scotland, UK). Occurring between fronts that propagate upslope with tidal periodicity, the rare downslope propagating one resembles a gravity current and includes strong convective turbulence coming from the interior rather than the more usual frictionally-generated turbulence arising from interaction with the seabed. Its turbulence is 3-10 times larger than that of more common upslope propagating fronts. As the main turbulence is in the interior with a thin stratified layer close to the bottom, little sediment is resuspended by a downslope propagating front. The downslope propagating front is suggested to be generated by oblique propagation of internal (tidal) waves and flow over a nearby upstream promontory. This article is protected by copyright. All rights reserved.
      PubDate: 2017-03-23T03:55:58.485204-05:
      DOI: 10.1002/2017JC012797
       
  • Interannual surface salinity in Northwest Atlantic shelf
    • Authors: Semyon A. Grodsky; Nicolas Reul, Bertrand Chapron, James A. Carton, Frank O. Bryan
      Abstract: Sea surface salinity (SSS) from the Aquarius and SMOS satellite missions displays a steady increase of ∼1psu over the entire northwestern Atlantic shelf south of Nova Scotia during the 2011-2015. Put in the context of longer ocean profile data the results suggest that mixed layer salinity and temperature north of the Gulf Stream experience positively correlated shelf-wide interannual oscillations (1psu/2degC). Salty and warm events occur coincident with anomalous easterly-southeasterly winds and Ekman transport counteracting the mean southwestward shelf currents. They are coincident with the weakening of both branches of the Scotian Shelf Current (SSC), but only moderately correlate with shifts of the Gulf Stream North Wall. This suggests that salt advection by anomalous SSC acting on the mean salinity gradient is the primary driver regulating the transport of fresh/cold water from high latitudes. The advection mechanism imposes a connectedness of the larger-scale interannual variability in this region and its tie to atmospheric oscillations.In the second part, an analysis of 15-year long numerical simulations is presented which show 8 interannual salinity oscillations (positive and negative). Six of these are driven by the horizontal advection by slow varying currents (>2 months), while 2 events are driven by the horizontal eddy advection (
      PubDate: 2017-03-23T03:55:51.640341-05:
      DOI: 10.1002/2016JC012580
       
  • New insights on tsunami genesis and energy source
    • Authors: Y. Tony Song; Ali Mohtat, Solomon C. Yim
      Abstract: Conventional tsunami theories suggest that earthquakes with significant vertical motions are more likely to generate tsunamis. In tsunami models, the vertical seafloor elevation is directly transferred to the sea-surface as the only initial condition. However, evidence from the 2011 Tohoku earthquake indicates otherwise; the vertical seafloor uplift was only 3∼5 meters, too small to account for the resultant tsunami. Surprisingly, the horizontal displacement was undeniably larger than anyone's expectation; about 60 meters at the frontal wedge of the fault plate, the largest slip ever recorded by in-situ instruments. The question is whether the horizontal motion of seafloor slopes had enhanced the tsunami to become as destructive as observed. In this study, we provide proof: (1) Combining various measurements from the 2011 Tohoku event, we show that the earthquake transferred a total energy of 3.1e+15 joule to the ocean, in which the potential energy (PE) due to the vertical seafloor elevation (including seafloor uplift/subsidence plus the contribution from the horizontal displacement) was less than a half, while the kinetic energy (KE) due to the horizontal displacement velocity of the continental slope contributed a majority portion; (2) Using two modern state-of-the-art wave flumes and a three-dimensional tsunami model, we have reproduced the source energy and tsunamis consistent with observations, including the 2004 Sumatra event. Based on the unified source energy formulation, we offer a competing theory to explain why some earthquakes generate destructive tsunamis, while others do not. This article is protected by copyright. All rights reserved.
      PubDate: 2017-03-20T11:20:40.567509-05:
      DOI: 10.1002/2016JC012556
       
  • Subregional characterization of mesoscale eddies across the
           Brazil-Malvinas Confluence
    • Authors: Evan Mason; Ananda Pascual, Peter Gaube, Simón Ruiz, Josep L. Pelegrí, Antoine Delepoulle
      Abstract: Horizontal and vertical motions associated with coherent mesoscale structures, including eddies and meanders, are responsible for significant global transports of many properties, including heat and mass. Mesoscale vertical fluxes also influence upper ocean biological productivity by mediating the supply of nutrients into the euphotic layer, with potential impacts on the global carbon cycle. The Brazil-Malvinas Confluence (BMC) is a western boundary current region in the South Atlantic with intense mesoscale activity. This region has an active role in the genesis and transformation of water masses and thus is a critical component of the Atlantic meridional overturning circulation. The collision between the Malvinas and Brazil Currents over the Patagonian shelf/slope creates an energetic front that translates offshore to form a vigorous eddy field. Recent improvements in gridded altimetric sea level anomaly fields allow us to track BMC mesoscale eddies with high spatial and temporal resolutions using an automated eddy tracker. We characterize the eddies across fourteen 5°×5° subregions. Eddy-centric composites of tracers and geostrophic currents diagnosed from a global reanalysis of surface and in situ data reveal substantial subregional heterogeneity. The in situ data are also used to compute the evolving quasi-geostrophic vertical velocity (QG-ω) associated with each instantaneous eddy instance. The QG- ω eddy composites have the expected dipole patterns of alternating upwelling/downwelling, however the magnitude and sign of azimuthally-averaged vertical velocity varies among subregions. Maximum eddy values are found near fronts and sharp topographic gradients. In comparison with regional eddy composites, subregional composites provide refined information about mesoscale eddy heterogeneity. This article is protected by copyright. All rights reserved.
      PubDate: 2017-03-20T11:20:39.23999-05:0
      DOI: 10.1002/2016JC012611
       
  • Generation of exchange flows in estuaries by tidal and gravitational eddy
           viscosity-fshear covariance (ESCO)
    • Authors: Yoeri M. Dijkstra; Henk M. Schuttelaars, Hans Burchard
      Abstract: We present a systematic analysis of generation mechanisms for exchange flows in partially stratified estuaries using water column (1DV) and width-averaged (2DV) numerical models. We focus on exchange flows generated by eddy viscosity - shear covariance (ESCO). We identify two distinctly different physical mechanisms. The first, tidal ESCO circulation, results from interactions between the barotropic tide and temporal variations of the eddy viscosity. While this flow is mostly generated by direct interactions between the tide and eddy viscosity variations at the main tidal frequency, a similarly important contribution can be attributed to indirect interactions. These are more complex interactions involving eddy viscosity variations at other frequencies than the main tidal frequency (e.g. M4). The second mechanism is called gravitational ESCO circulation. This results from an amplification of the gravitational circulation through indirect interactions between the gravitational circulation and temporal variations of the eddy viscosity at any time-scale. Tidal and gravitational ESCO circulation are generated by different mechanisms and have a different dependency on the phase and frequency of eddy viscosity variations and the density gradient.The relative contributions of gravitational circulation and tidal and gravitational ESCO circulation to the exchange flow are typically 1/3 each in tidally energetic well-mixed or partially stratified estuaries. The results are generalised using an idealised width-averaged model of the Scheldt River estuary. This model confirms the results of the water column model and additionally shows that temporal variations of turbulence not captured in the water column model have a significant effect on the exchange flow. This article is protected by copyright. All rights reserved.
      PubDate: 2017-03-20T11:15:37.45909-05:0
      DOI: 10.1002/2016JC012379
       
  • Large-scale laboratory study of breaking wave hydrodynamics over a fixed
           bar
    • Authors: Dominic A. van der A; Joep van der Zanden, Tom O'Donoghue, David Hurther, Iván Cáceres, Stuart J. McLelland, Jan S. Ribberink
      Abstract: A large-scale wave flume experiment has been carried out involving a T=4s regular wave with H=0.85 m wave height plunging over a fixed barred beach profile. Velocity profiles were measured at twelve locations along the breaker bar using LDA and ADV. A strong undertow is generated reaching magnitudes of 0.8 m/s on the shoreward side of the breaker bar. A circulation pattern occurs between the breaking area and the inner surf zone. Time-averaged turbulent kinetic energy (TKE) is largest in the breaking area on the shoreward side of the bar where the plunging jet penetrates the water column. At this location, and on the bar crest, TKE generated at the water surface in the breaking process reaches the bottom boundary layer. In the breaking area TKE does not reduce to zero within a wave cycle which leads to a high level of “residual” turbulence and therefore lower temporal variation in TKE compared to previous studies of breaking waves on plane beach slopes. It is argued that this residual turbulence results from the breaker bar-trough geometry, which enables larger length- and time-scales of breaking generated vortices and which enhances turbulence production within the water column compared to plane beaches. Transport of TKE is dominated by the undertow-related flux, whereas the wave-related and turbulent fluxes are approximately an order of magnitude smaller. Turbulence production and dissipation are largest in the breaker zone and of similar magnitude, but in the shoaling zone and inner surf zone production is negligible and dissipation dominates. This article is protected by copyright. All rights reserved.
      PubDate: 2017-03-20T11:15:36.069476-05:
      DOI: 10.1002/2016JC012072
       
  • Assessment of western Indian Ocean SST bias of CMIP5 models
    • Authors: Ibnu Fathrio; Satoshi Iizuka, Atsuyoshi Manda, Yasu-Masa Kodama, Sachinobu Ishida, Qoosaku Moteki, Hiroyuki Yamada, Yoshihiro Tachibana
      Abstract: The western Indian Ocean sea surface temperature (SST) is among the key factors that affect precipitation over India and East Africa. This study examined the western Indian Ocean SST biases among the Coupled Model Intercomparison Project phase 5 (CMIP5) models. It was found that the multi-model ensemble-mean SST biases over the western equatorial Indian Ocean are warmer than the observations during the summer monsoon season. However, about half the models show positive SST biases, whereas negative ones in the other half. The models with warmer SST biases exhibit a pattern similar to the Indian Ocean Dipole, with stronger equatorial easterly wind biases during fall and a deeper thermocline in the western equatorial Indian Ocean. In the models with cooler SST biases, negative SST biases are observed over the entire tropical Indian Ocean throughout the year and the wind biases over the equatorial Indian Ocean are southeasterly during summer and fall. Heat budget analysis revealed the importance of ocean currents in forming the early summer development of SST biases over the western equatorial Indian Ocean. The formation of SST biases is related to surface current biases induced by the weaker biases of southwesterly monsoon winds and SST biases over the southwestern equatorial Indian Ocean, which are advected by the East African Coastal Currents. On the other hand, almost of all the CMIP5 models show prominent cold SST biases over the northern Arabian Sea during the pre-monsoon season. The SST biases are induced by excess surface cooling during the winter monsoon season. This article is protected by copyright. All rights reserved.
      PubDate: 2017-03-20T11:15:29.324806-05:
      DOI: 10.1002/2016JC012443
       
  • Influence of the phytoplankton community structure on the spring and
           annual primary production in the North-Western Mediterranean Sea
    • Authors: Nicolas Mayot; Fabrizio D'Ortenzio, Julia Uitz, Bernard Gentili, Joséphine Ras, Vincenzo Vellucci, Melek Golbol, David Antoine, Hervé Claustre
      Abstract: Satellite ocean color observations revealed that unusually deep convection events in 2005, 2006, 2010 and 2013 led to an increased phytoplankton biomass during the spring bloom over a large area of the North-Western Mediterranean Sea (NWM). Here we investigate the effects of these events on the seasonal phytoplankton community structure, we quantify their influence on primary production, and we discuss the potential biogeochemical impact. For this purpose, we compiled in situ phytoplankton pigment data from five ship surveys performed in the NWM and from monthly cruises at a fixed station in the Ligurian Sea. We derived primary production rates from a light-photosynthesis model applied to these in situ data. Our results confirm that the maximum phytoplankton biomass during the spring bloom is larger in years associated with intense deep convection events (+ 51%). During these enhanced spring blooms, the contribution of diatoms to total phytoplankton biomass increased (+ 33%), as well as the primary production rate (+ 115%). The occurrence of a highly productive bloom is also related to an increase in the phytoplankton bloom area (+ 155%), and in the relative contribution of diatoms to primary production (+ 63%). Therefore, assuming that deep convection in the NWM could be significantly weakened by future climate changes, substantial decreases in the spring production of organic carbon and of its export to deep waters can be expected. This article is protected by copyright. All rights reserved.
      PubDate: 2017-03-17T19:15:43.10201-05:0
      DOI: 10.1002/2016JC012668
       
  • A new look at ocean ventilation timescales and their uncertainties
    • Authors: Rana A. Fine; Synte Peacock, Mathew E. Maltrud, Frank O. Bryan
      Abstract: A suite of eddy-resolving ocean transient tracer model simulations are first compared to observations. Observational and model pCFC-11 ages agree quite well, with the eddy-resolving model adding detail. The CFC ages show that the thermocline is a barrier to interior ocean exchange with the atmosphere on timescales of 45 years, the measureable CFC transient, although there are exceptions. Next, model simulations are used to quantify effects on tracer ages of the spatial dependence of internal ocean tracer variability due to stirring from eddies and biases from non-stationarity of the atmospheric transient when there is mixing. These add to tracer age uncertainties and biases, which are large in frontal boundary regions, and small in subtropical gyre interiors. These uncertainties and biases are used to re-interpret observed temporal trends in tracer-derived ventilation timescales taken from observations more than a decade apart, and to assess whether interpretations of changes in tracer ages being due to changes in ocean ventilation hold water. For the southern hemisphere subtropical gyres, we infer that the rate of ocean ventilation 26-27.2 σθ increased between the mid-1990s and the decade of the 2000s. However, between the mid-1990s and the decade of the 2010s, there is no significant trend ‒ perhaps except for South Atlantic. Observed age/AOU/ventilation changes are linked to a combination of natural cycles and climate change, and there is regional variability. Thus, for the future it is not clear how strong or steady in space and time ocean ventilation changes will be. This article is protected by copyright. All rights reserved.
      PubDate: 2017-03-17T11:50:31.875048-05:
      DOI: 10.1002/2016JC012529
       
  • On nonhydrostatic coastal model simulations of shear instabilities in a
           stratified shear flow at high Reynolds number
    • Authors: Zheyu Zhou; Xiao Yu, Tian-Jian Hsu, Fengyan Shi, W. Rockwell Geyer, James T. Kirby
      Abstract: The non-hydrostatic surface and terrain-following coastal model NHWAVE is utilized to simulate a continually forced stratified shear flow in a straight channel, which is a generic problem to test the existing non-hydrostatic coastal models' capability in resolving shear instabilities in the field-scale. The resolved shear instabilities in the shear layer has a Reynolds number of about 1.4 × 106, which is comparable to field observed value. Using the standard Smagorinsky closure with a grid size close to the Ozmidov length scale, simulation results show that the resolved energy cascade exceeds one order of magnitude and the evolution and turbulent mixing characteristics are predicted well. Two different approaches are used to estimate the turbulent dissipation rate, namely using the resolved turbulent energy spectrum and the parameterized sub-grid turbulent dissipation rate, and the predicted results provide the upper and lower bounds that encompass the measured values. Model results show significantly higher turbulence in braids of shear instabilities, which is similar to field observations while both the subgrid turbulent dissipation rate and resolved vorticity field can be used as surrogates for measured high acoustic backscatter signals. Simulation results also reveal that the surface velocity divergence/convergence is an effective identifier for the front of the density current and the shear instabilities. To guide future numerical studies in more realistic domains, an evaluation on the effects of different grid resolutions and subgrid viscosity on the resolved flow field and subgrid dissipation rate are discussed. This article is protected by copyright. All rights reserved.
      PubDate: 2017-03-17T11:50:26.483557-05:
      DOI: 10.1002/2016JC012334
       
  • Surface wave breaking over sheared currents: Observations from the Mouth
           of the Columbia River
    • Authors: Seth Zippel; Jim Thomson
      Abstract: Measurements of waves and currents from freely drifting buoys are used to evaluate wave breaking parameterizations at the Mouth of the Columbia River, where breaking occurs in intermediate depths and in the presence of vertically sheared currents. Breaking waves are identified using images collected with cameras onboard the buoys, and the breaking activity is well-correlated with wave steepness. Vertical shear in the currents produces a frequency-dependent effective current that modifies the linear dispersion relation. Accounting for these sheared currents in the wavenumber spectrum is essential in calculating the correct wave steepness; without this, wave steepness can be over (under) estimated on opposing (following) currents by up to 20%. The observed bulk steepness values suggest a limiting value of 0.4. The observed fraction of breaking waves is in good agreement with several existing models, each based on wave steepness. Further, a semi-spectral model designed for all depth regimes also compares favorably with measured breaking fractions. In this model, the majority of wave breaking is predicted to occur in the higher frequency bands (i.e., short waves). There is a residual dependance on directional spreading, in which wave breaking decreases with increasing directional spread. This article is protected by copyright. All rights reserved.
      PubDate: 2017-03-17T11:45:25.17049-05:0
      DOI: 10.1002/2016JC012498
       
  • Thin ice and storms: Sea ice deformation from buoy arrays deployed during
           N-ICE2015
    • Authors: Polona Itkin; Gunnar Spreen, Bin Cheng, Martin Doble, Fanny Girard-Ardhuin, Jari Haapala, Nick Hughes, Lars Kaleschke, Marcel Nicolaus, Jeremy Wilkinson
      Abstract: Arctic sea ice has displayed significant thinning as well as an increase in drift speed in recent years. Taken together this suggests an associated rise in sea ice deformation rate. A winter and spring expedition to the sea ice covered region north of Svalbard – the Norwegian young sea ICE 2015 expedition (N-ICE2015) - gave an opportunity to deploy extensive buoy arrays and to monitor the deformation of the first- and second-year ice now common in the majority of the Arctic Basin. During the 5-month long expedition, the ice cover underwent several strong deformation events, including a powerful storm in early February that damaged the ice cover irreversibly. The values of total deformation measured during N-ICE2015 exceed previously measured values in the Arctic Basin at similar scales: At 100 km scale, N-ICE2015 values averaged above 0.1, day−1, compared to rates of 0.08 day −1 or less for previous buoy arrays. The exponent of the power law between the deformation length scale and total deformation developed over the season from 0.37 to 0.54 with an abrupt increase immediately after the early February storm, indicating a weakened ice cover with more free drift of the sea ice floes. Our results point to a general increase in deformation associated with the younger and thinner Arctic sea ice and to a potentially destructive role of winter storms. This article is protected by copyright. All rights reserved.
      PubDate: 2017-03-17T11:40:37.589288-05:
      DOI: 10.1002/2016JC012403
       
  • Effects of sea-ice and biogeochemical processes and storms on under ice
           water fCO2 during the winter-spring transition in the high Arctic Ocean:
           Implications for sea-air CO2 fluxes
    • Authors: Agneta Fransson; Melissa Chierici, Ingunn Skjelvan, Are Olsen, Philipp Assmy, Algot K. Peterson, Gunnar Spreen, Brian Ward
      Abstract: We performed measurements of carbon dioxide fugacity (fCO2) in the surface water under Arctic sea ice from January to June 2015 during the Norwegian young sea ICE (N-ICE2015) expedition. Over this period, the ship drifted with four different ice floes and covered the deep Nansen Basin, the slopes north of Svalbard and the Yermak Plateau. This unique winter-to-spring dataset includes the first winter-time under-ice water fCO2 observations in this region. The observed under-ice fCO2 ranged between 315 µatm in winter and 153 µatm in spring, hence was undersaturated relative to the atmospheric fCO2. Although the sea ice partly prevented direct CO2 exchange between ocean and atmosphere, frequently occurring leads and breakup of the ice sheet promoted sea-air CO2 fluxes. The CO2 sink varied between 0.3 and 86 mmol C m−2 d−1, depending strongly on the open-water fractions (OW) and storm events. The maximum sea-air CO2 fluxes occurred during storm events in February and June. In winter, the main drivers of the change in under-ice water fCO2 were dissolution of CaCO3 (ikaite) and vertical mixing. In June, in addition to these processes, primary production and sea-air CO2 fluxes were important. The cumulative loss due to CaCO3 dissolution of 0.7 mol C m−2 in the upper 10 m played a major role in sustaining the undersaturation of fCO2 during the entire study. The relative effects of the total fCO2 change due to CaCO3 dissolution was 38%, primary production 26%, vertical mixing 16%, sea-air CO2 fluxes 16%, and temperature and salinity insignificant. This article is protected by copyright. All rights reserved.
      PubDate: 2017-03-16T11:06:18.658027-05:
      DOI: 10.1002/2016JC012478
       
  • Seasonal cycle of cross-equatorial flow in the central Indian Ocean
    • Authors: Yi Wang; Michael J. McPhaden
      Abstract: This study investigates the seasonal cycle of meridional currents in the upper layers of central equatorial Indian Ocean using acoustic Doppler current profiler (ADCP) and other data over the period 2004-2013. The ADCP data set collected along 80.5°E is the most comprehensive collection of direct velocity measurements in the central Indian Ocean to date, providing new insights into the meridional circulation in this region. We find that mean volume transport is southwards across the equator in the central Indian Ocean in approximate Sverdrup balance with the wind stress curl. In addition, mean westerly wind stress near the equator drives convergent Ekman flow in the surface layer and subsurface divergent geostrophic flow in the thermocline at 50-150 m depths. In response to a mean northward component of the surface wind stress, the maximum surface layer convergence is shifted off the equator to 0.75°N. Evidence is also presented for the existence of a shallow equatorial roll consisting of a northward wind-driven surface drift overlaying the southward-directed subsurface Sverdrup transport. Seasonal variations are characterized by cross equatorial transports flowing from the summer to the winter hemisphere in quasi-steady Sverdrup balance with the wind stress curl. In addition, semi-annually varying westerly monsoon transition winds lead to semi-annual enhancements of surface layer Ekman convergence and geostrophic divergence in the thermocline. These results quantify expectations from ocean circulation theories for equatorial Indian Ocean meridional circulation patterns with a high degree of confidence given the length of the data records. This article is protected by copyright. All rights reserved.
      PubDate: 2017-03-16T11:00:47.586481-05:
      DOI: 10.1002/2016JC012537
       
  • A joint active/passive physical model of sea surface microwave signatures
    • Authors: William J. Plant; Vladimir Irisov
      Abstract: Active and passive microwave signatures of the ocean can only depend on the ocean wave spectrum if bound and breaking waves are neglected. However, history has not been kind to attempts to explain both radiometer brightness temperatures (Tb) and normalized radar cross sections (σo) of the sea using the same ocean wave spectrum without bound and breaking waves. In this paper, we show that if bound and breaking waves are included in physical models of radiometer and scatterometer microwave signatures of the ocean, a single wave spectrum can explain both Tb and σo to reasonable accuracy. Bound waves are the roughness produced by gently breaking, or crumpling, waves that travel near the speed of the parent wave. Bound wave modeling is based on earlier work by Plant [1997] but using additional information about the slope probability distributions of the bound waves' parents. Breaking wave and foam modeling both build on the function Λ(cb) introduced by Phillips [1985], which describes the average length of breaking wave fronts on the ocean per unit area as a function of breaker velocity, cb. We model Λ(cb) as documented in our previous work [Irisov and Plant, 2016] where we show that the wave spectrum completely determines Λ(cb). We thus show here that the result of including bound and breaking waves in radiometer and scatterometer models of oceanic signatures is a much closer fit to data over a wide range of microwave frequencies and incidence angles using a single wave spectrum. This article is protected by copyright. All rights reserved.
      PubDate: 2017-03-14T03:35:34.766044-05:
      DOI: 10.1002/2017JC012749
       
  • Downstream evolution of the Kuroshio's time-varying transport and velocity
           structure
    • Authors: M. Andres; V. Mensah, S. Jan, M.-H. Chang, Y.-J. Yang, C.M. Lee, B. Ma, T.B. Sanford
      Abstract: Observations from two companion field programs—Origins of the Kuroshio and Mindanao Currents (OKMC) and Observations of Kuroshio Transport Variability (OKTV)—are used here to examine the Kuroshio's temporal and spatial evolution. Kuroshio strength and velocity structure were measured between June 2012 and November 2014 with pressure-sensor equipped inverted echo sounders (PIESs) and upward-looking acoustic Doppler current profilers (ADCPs) deployed across the current northeast of Luzon, Philippines and east of Taiwan with an eight-month overlap in the two arrays' deployment periods. The time-mean net (i.e., integrated from the surface to the bottom) absolute transport increases downstream from 7.3 Sv (±4.4 Sv standard error) northeast of Luzon to 13.7 Sv (±3.6 Sv) east of Taiwan. The observed downstream increase is consistent with the return flow predicted by the simple Sverdrup relation and the mean wind stress curl field over the North Pacific (despite the complicated bathymetry and gaps along the North Pacific western boundary). Northeast of Luzon, the Kuroshio–bounded by the 0 m s−1 isotach–is shallower than 750 dbar, while east of Taiwan areas of positive flow reach to the seafloor (3000 m). Both arrays indicate a deep counterflow beneath the poleward-flowing Kuroshio (-10.3 ±2.3 Sv by Luzon and -12.5 ±1.2 Sv east of Taiwan). Time-varying transports and velocities indicate the strong influence at both sections of westward propagating eddies from the ocean interior. Topography associated with the ridges east of Taiwan also influences the mean and time-varying velocity structure there. This article is protected by copyright. All rights reserved.
      PubDate: 2017-03-11T03:45:30.45504-05:0
      DOI: 10.1002/2016JC012519
       
  • Bay of Bengal Salinity Stratification and Indian Summer Monsoon
           Intraseasonal Oscillation: 2. Impact on SST and convection
    • Authors: Yuanlong Li; Weiqing Han, Wanqiu Wang, M. Ravichandran, Tong Lee, Toshiaki Shinoda
      Abstract: The Indian summer monsoon intraseasonal oscillations (MISOs) induce pronounced intraseasonal sea surface temperature (SST) variability in the Bay of Bengal (BoB), which has important feedbacks to atmospheric convection. An ocean general circulation model (OGCM) is employed to investigate the upper-ocean processes affecting intraseasonal SST variability and its feedback to the MISO convection. In the BoB the MISO induces intraseasonal SST variability predominantly through surface heat flux forcing with comparable contributions from shortwave radiation and turbulent heat flux, and to a much smaller extent through wind-driven ocean mixed layer entrainment. The ocean salinity stratification, represented by mixed layer depth (MLD) and barrier layer thickness (BLT), has a strong control on SST but weak impact on convection of the MISO. The MLD is critical for the amplitude of SST response to various forcing processes, while the BLT mainly affects entrainment by determining the temperature difference between the mixed layer and the water below. From May to mid-June, the shallow MLD and thin barrier layer greatly enhance intraseasonal SST anomalies, which can amplify convection fluctuations of the MISO through air-sea interaction and leads to intense but short-duration post-convection break spells. When either the MLD or the BLT is large, intraseasonal SSTs tend to be weak. Further investigation reveals that freshwater flux of the monsoon gives rise to the shallow MLD and thick barrier layer, and its overall effect on intraseasonal SSTs is a 20% enhancement. These results provide implications for improving the simulation and forecast of the MISO in climate models. This article is protected by copyright. All rights reserved.
      PubDate: 2017-03-11T03:30:26.898227-05:
      DOI: 10.1002/2017JC012692
       
  • Behavior of a wave-driven buoyant surface jet on a coral reef
    • Authors: Liv M. M. Herdman; James L. Hench, Oliver Fringer, Stephen G. Monismith
      Abstract: A wave-driven surface buoyant jet exiting a coral reef was studied in order to quantify the amount of water re-entrained over the reef crest. Both moored observations and Lagrangian drifters were used to study the fate of the buoyant jet. To investigate in detail the effects of buoyancy and along-shore flow variations, we developed an idealized numerical model of the system. Consistent with previous work, the ratio of along-shore velocity to jet-velocity and the jet internal Froude number were found to be important determinants of the fate of the jet. In the absence of buoyancy, the entrainment of fluid at the reef crest, creates a significant amount of retention, keeping 60% of water in the reef system. However, when the jet is lighter than the ambient ocean-water, the net effect of buoyancy is to enhance the separation of the jet from shore, leading to a greater export of reef water. Matching observations, our modeling predicts that buoyancy limits retention to 30% of the jet flow for conditions existing on the Moorea reef. Overall, the combination of observations and modeling we present here shows that reef-ocean temperature gradients can play an important role in reef-ocean exchanges. This article is protected by copyright. All rights reserved.
      PubDate: 2017-03-07T19:40:28.076378-05:
      DOI: 10.1002/2016JC011729
       
  • Observed basin-wide propagation of Mediterranean water in the Black Sea
    • Authors: Anastasia Falina; Artem Sarafanov, Emin Özsoy, Ufuk Utku Turunҫoğlu
      Abstract: Mediterranean water entering the Black Sea through the Bosphorus Strait forms mid-depth intrusions that contribute to the salt, heat, and volume balances of the sea, ventilate its water column at intermediate depths and restrain the upward flux of hydrogen sulfide from deeper layers. Despite the importance for the Black Sea environment, the circulation of Mediterranean-origin water in the basin is fundamentally underexplored. Here we use hydrographic data collected from ships and Argo profiling floats to identify pathways of the Mediterranean intrusions in the general circulation system of the sea. While earlier the intrusions were observed primarily near the Bosphorus Strait, we present an evidence for their intermittent extensive propagation throughout the basin. We find that the main conduit for the intrusions is the southern limb of the Rim Current that carries the intruded water from the Bosphorus Strait eastwards. A part of this eastward flow recirculates cyclonically into the interior of the sea, where traces of the intrusions gradually disappear because of mixing. We put forward the hypothesis that the formation of the most prominent intrusions is associated with strong cyclonic storms over the Bosphorus Strait, which lead to abnormally large influx of Mediterranean water. This article is protected by copyright. All rights reserved.
      PubDate: 2017-03-07T19:30:30.516987-05:
      DOI: 10.1002/2017JC012729
       
  • A modeling study of the impact of major storms on seabed shear stress and
           sediment transport on the Grand Banks of Newfoundland
    • Authors: Michael Z. Li; Yongsheng Wu, Guoqi Han, Robert H. Prescott, Charles C. L. Tang
      Abstract: Waves, current and sediment transport processes in major storms on the Grand Banks of Newfoundland were simulated using integrated wave, three-dimensional tide and circulation, and combined-flow sediment transport models. While the tidal and non-tidal currents are generally low and cause little sediment transport, storm-induced waves and currents enhance bed shear velocity by more than 5 times and cause significant sediment transport over the entire Grand Banks. The impact of storms on shear stress and transport strongly depends on water depths and the greatest impact occurs over the bathymetric highs on southeastern Grand Bank where the maximum shear velocity reaches 15 cm s−1 and the maximum transport rates are >5 kg m−1 s−1. The direction of sediment transport rotates clockwise progressively through nearly 360° during the passage of a storm. Although peak transport typically occurs on central and southeastern Grand Bank with a southeastward direction, the magnitude, direction and timing of peak transport show strong spatial and temporal variability. The variability of the peak transport largely depends on the timing and relative intensity of the waves and the total bottom currents which in turn depends on the addition of the storm-induced and tidal currents. The calculation of the maximum transport potential suggests that sediments as coarse as small pebbles are mobile in water depths 
      PubDate: 2017-03-02T10:45:30.440671-05:
      DOI: 10.1002/2016JC012215
       
  • Concentrations and cycling of DMS, DMSP, and DMSO in coastal and offshore
           waters of the Subarctic Pacific during summer, 2010–2011
    • Authors: Elizabeth Asher; John W. Dacey, Debby Ianson, Angelica Peña, Philippe D. Tortell
      Abstract: Concentrations of dimethylsulfide (DMS), measured in the Subarctic Pacific during summer 2010 and 2011, ranged from ∼ 1 – 40 nM, while dissolved DMSO concentrations (range 13 - 23 nM) exceeded those of dissolved DMSP (range 1.3 – 8.8 nM). Particulate DMSP dominated the reduced sulfur pool, reaching maximum concentrations of 100 nM. Coastal and off shore waters exhibited similar overall DMS concentration ranges, but sea-air DMS fluxes were lower in the oceanic waters due to lower wind speeds. Surface DMS concentrations showed statistically significant correlations with various hydrographic variables including the upwelling intensity (r2 = 0.52, p 
      PubDate: 2017-02-28T03:35:34.563188-05:
      DOI: 10.1002/2016JC012465
       
  • Subtidal circulation in a deep-silled fjord: Douglas Channel, British
           Columbia
    • Authors: Di Wan; Charles G. Hannah, Michael G.G. Foreman, Stan Dosso
      Abstract: Douglas Channel, a deep fjord on the west coast of British Columbia, Canada, is the main waterway in the fjord system that connects the town of Kitimat to Queen Charlotte Sound and Hecate Strait. A 200 m depth sill divides Douglas Channel into an outer and an inner basin. This study examines the low-frequency (from seasonal to meteorological bands) circulation in Douglas Channel from data collected at three moorings deployed during 2013 – 2015. The deep flows are dominated by a yearly renewal that takes place from May/June to early September. A dense bottom layer with a thickness of 100 m that cascades through the system at the speed of 0.1 – 0.2 m s−1, which is consistent with gravity currents. Estuarine flow dominates the circulation above the sill-depth, and the observed landward net volume flux suggests that it is necessary to include the entire complex channel network to fully understand the estuarine circulation in the system. The influence of the wind forcing on the sub-tidal circulation is not only at the surface, but also at mid-depth. The along-channel wind dominates the surface current velocity fluctuations and the sealevel response to the wind produces a velocity signal at 100 – 120 m in the counter-wind direction. Overall, the circulation in the seasonal and the meteorological bands is a mix of estuarine flow, direct wind driven flow, and the barotropic and baroclinic responses to changes to the surface pressure gradient caused by the wind stress. This article is protected by copyright. All rights reserved.
      PubDate: 2017-02-15T10:35:29.040964-05:
      DOI: 10.1002/2016JC012022
       
  • Observations of flooding and snow-ice formation in a thinner Arctic sea
           ice regime during the N-ICE2015 campaign: Influence of basal ice melt and
           storms
    • Authors: Christine Provost; Nathalie Sennéchael, Jonas Miguet, Polona Itkin, Anja Rösel, Zoé Koenig, Nicolas Villacieros-Robineau, Mats A. Granskog
      Abstract: Seven ice mass balance instruments deployed near 83°N on different first-year and second-year ice floes, representing variable snow and ice conditions, documented the evolution of snow and ice conditions in the Arctic Ocean north of Svalbard in Jan-Mar 2015. Frequent profiles of temperature and thermal diffusivity proxy were recorded to distinguish changes in snow depth and ice thickness with 2 cm vertical resolution. Four instruments documented flooding and snow-ice formation. Flooding was clearly detectable in the simultaneous changes in thermal diffusivity proxy, increased temperature and heat propagation through the underlying ice. Slush then progressively transformed into snow-ice. Flooding resulted from two different processes; i) after storm-induced break-up of snow-loaded floes and ii) after loss of buoyancy due to basal ice melt. In the case of break-up, when the ice was cold and not permeable, rapid flooding, probably due to lateral intrusion of seawater, led to slush and snow-ice layers at the ocean freezing temperature (-1.88°C). After the storm the instruments documented basal sea-ice melt over warm Atlantic waters and ocean-to-ice heat flux peaked at up to 400 Wm−2. The warm ice was then permeable and flooding was more gradual probably involving vertical intrusion of brines and led to colder slush and snow-ice (-3°C). The N-ICE2015 campaign provided the first documentation of significant flooding and snow-ice formation in the Arctic ice pack as the slush partially refroze. Snow-ice formation may become a more-frequently observed process in a thinner-ice Arctic. This article is protected by copyright. All rights reserved.
      PubDate: 2017-02-14T03:35:35.750566-05:
      DOI: 10.1002/2016JC012011
       
  • Winter to summer oceanographic observations in the Arctic Ocean north of
           Svalbard
    • Authors: Amelie Meyer; Arild Sundfjord, Ilker Fer, Christine Provost, Nicolas Villacieros Robineau, Zoe Koenig, Ingrid H. Onarheim, Lars H. Smedsrud, Pedro Duarte, Paul A. Dodd, Robert M. Graham, Sunke Schmidtko, Hanna M. Kauko
      Abstract: Oceanographic observations from the Eurasian Basin north of Svalbard collected between January and June 2015 from the N-ICE2015 drifting expedition are presented. The unique winter observations are a key contribution to existing climatologies of the Arctic Ocean, and show a ∼100m deep winter mixed layer likely due to high sea ice growth rates in local leads. Current observations for the upper ∼200m show mostly a barotropic flow, enhanced over the shallow Yermak Plateau. The two branches of inflowing Atlantic Water are partly captured, confirming that the outer Yermak Branch follows the perimeter of the plateau, and the inner Svalbard Branch the coast. Atlantic Water observed to be warmer and shallower than in the climatology, is found directly below the mixed layer down to 800m depth, and is warmest along the slope, while properties inside the basin are quite homogeneous. From late May onwards, the drift was continually close to the ice edge and a thinner surface mixed layer and shallower Atlantic Water coincided with significant sea ice melt being observed. This article is protected by copyright. All rights reserved.
      PubDate: 2017-01-31T06:41:22.912925-05:
      DOI: 10.1002/2016JC012391
       
  • Issue Information
    • Pages: 2643 - 2646
      PubDate: 2017-05-26T02:13:30.355625-05:
      DOI: 10.1002/jgrc.21927
       
  • Sedimentation from buoyant muddy plumes in the presence of interface
           mixing: An experimental study
    • Authors: Mohamad Rouhnia; Kyle Strom
      Pages: 2652 - 2670
      Abstract: A series of flume experiments were conducted to study the sediment removal rates, characterized by an effective settling velocity, from buoyant muddy plumes overriding clear saltwater at steady state conditions under different Richardson numbers and initial suspended sediment concentrations. The experiments were all carried out in the subcritical regime, leading to cusps style instabilities at the interface of the two fluids. Flocculation in the experiments was not suppressed, yet flocs remained small while in the plume layer. Data from the experiments allowed for calculation of the individual floc settling velocity, Ws,f, and the overall effective settling velocity, Ws,eff, i.e., that velocity needed to predict the downward flux with CWs,eff. Results showed that Ws,eff was greater than the floc settling velocity in all runs. Ws,eff was found to increase with plume velocity and interface mixing but not with plume concentration. The difference between the effective and floc settling velocities was therefore attributed to turbulent diffusion brought on by mixing at the interface and not to convective sedimentation or leaking. The turbulence enhanced settling velocity, Ws,t, was found to be a strong function of the Richardson number, and conceptual and empirical equations are presented to predict Ws,t as a function of the plume surface velocity, Ups, and Richardson number, Ri; this analysis showed that Ws,t∝UpsRi−2. In the runs with low Richardson number, Ws,t was approximately equal to the floc settling velocity, leading to an overall doubling of the effective settling velocity relative to that of the individual particles.
      PubDate: 2017-04-01T10:40:47.213905-05:
      DOI: 10.1002/2016JC012053
       
  • On the role of sea-state in bubble-mediated air-sea gas flux during a
           winter storm
    • Authors: Jun-Hong Liang; Steven R. Emerson, Eric A. D'Asaro, Craig L. McNeil, Ramsey R. Harcourt, Peter P. Sullivan, Bo Yang, Meghan F. Cronin
      Pages: 2671 - 2685
      Abstract: Oceanic bubbles play an important role in the air-sea exchange of weakly soluble gases at moderate to high wind speeds. A Lagrangian bubble model embedded in a large eddy simulation model is developed to study bubbles and their influence on dissolved gases in the upper ocean. The transient evolution of mixed-layer dissolved oxygen and nitrogen gases at Ocean Station Papa (50°N, 145°W) during a winter storm is reproduced with the model. Among different physical processes, gas bubbles are the most important in elevating dissolved gas concentrations during the storm, while atmospheric pressure governs the variability of gas saturation anomaly (the relative departure of dissolved gas concentration from the saturation concentration). For the same wind speed, bubble-mediated gas fluxes are larger during rising wind with smaller wave age than during falling wind with larger wave age. Wave conditions are the primary cause for the bubble gas flux difference: when wind strengthens, waves are less-developed with respect to wind, resulting in more frequent large breaking waves. Bubble generation in large breaking waves is favorable for a large bubble-mediated gas flux. The wave-age dependence is not included in any existing bubble-mediated gas flux parameterizations.
      PubDate: 2017-04-01T10:40:29.374808-05:
      DOI: 10.1002/2016JC012408
       
  • Flow paths and variability of the North Atlantic Current: A comparison of
           observations and a high-resolution model
    • Authors: Tilia Breckenfelder; Monika Rhein, Achim Roessler, Claus W. Böning, Arne Biastoch, Erik Behrens, Christian Mertens
      Pages: 2686 - 2708
      Abstract: The North Atlantic Current (NAC) is subject to variability on multiannual to decadal time scales, influencing the transport of volume, heat, and freshwater from the subtropical to the eastern subpolar North Atlantic (NA). Current observational time series are either too short or too episodic to study the processes involved. Here we compare the observed continuous NAC transport time series at the western flank of the Mid-Atlantic Ridge (MAR) and repeat hydrographic measurements at the OVIDE line in the eastern Atlantic with the NAC transport and circulation in the high-resolution (1/20°) ocean model configuration VIKING20 (1960–2008). The modeled baroclinic NAC transport relative to 3400 m (24.5 ± 7.1 Sv) at the MAR is only slightly lower than the observed baroclinic mean of 27.4 ± 4.7 Sv from 1993 to 2008, and extends further north by about 0.5°. In the eastern Atlantic, the western NAC (WNAC) carries the bulk of the transport in the model, while transport estimates based on hydrographic measurements from five repeated sections point to a preference for the eastern NAC (ENAC). The model is able to simulate the main features of the subpolar NA, providing confidence to use the model output to analyze the influence of the North Atlantic Oscillation (NAO). Model based velocity composites reveal an enhanced NAC transport across the MAR of up to 6.7 Sv during positive NAO phases. Most of that signal (5.4 Sv) is added to the ENAC transport, while the transport of the WNAC was independent of the NAO.
      PubDate: 2017-04-01T10:45:32.748392-05:
      DOI: 10.1002/2016JC012444
       
  • Detailed spatiotemporal impacts of El Niño on phytoplankton biomass
           in the South China Sea
    • Authors: Eko Siswanto; Haijun Ye, Dai Yamazaki, DanLing Tang
      Pages: 2709 - 2723
      Abstract: The lagging and leading correlations among satellite observations, reanalyzed biogeophysical data, and the Nino3.4 El Niño index were investigated to reveal the impacts of El Niño on the phytoplankton biomass (chlorophyll a [Chl a]) in the South China Sea (SCS), in an attempt to identify the probable responsible factors in greater spatiotemporal detail. A basin-scale high Chl a concentration during the developing phase of El Niño changed to basin-scale low Chl a during the weakening phase. Cyclonic wind circulation in the northern basin, increased wind speed in the southern basin, and strengthened upwelling off the Vietnamese coast likely caused a basin-scale nutrient increase during the developing phase of an El Niño event; the opposite conditions led to low nutrient levels during the weakening phase. Decreases in Chl a east of the Vietnamese coast and northwest of Borneo Island were due to decreases in nutrients supplied by rivers. These spatiotemporal changes are considered biogeophysical responses to a variety of types of El Niño. Regardless of the El Niño type, reanalyzing biogeophysical data sets during central Pacific warming separately from those during eastern Pacific warming is recommended for a more robust understanding of the detailed spatiotemporal impacts of different El Niño types on the biogeophysical environment of the SCS.
      PubDate: 2017-04-03T23:50:34.960032-05:
      DOI: 10.1002/2016JC012276
       
  • Importance of mesoscale eddies and mean circulation in ventilation of the
           Southern Ocean
    • Authors: Igor Kamenkovich; Zulema Garraffo, Romain Pennel, Rana A. Fine
      Pages: 2724 - 2741
      Abstract: This study examines the relative importance of the mean advection and mesoscale currents in the property exchange between the Southern Ocean mixed layer and downstream in the upper 2000 m; this exchange is referred to as ventilation. A new, highly efficient off-line tracer model employed here uses precalculated velocities to advect dynamically passive tracers. Two idealized tracers are considered: the Boundary Impulse Response (BIR) tracer, which helps to determine the ventilation pathways and time scales, and the Transient Surface Tracer (TST), which is relevant to transient atmospheric tracers. The importance of eddies is isolated by contrasting the control simulation with a simulation without mesoscale currents. The analysis reveals complex three-dimensional ventilation pathways, controlled by the interplay between the mean advection and eddy-induced spreading. The mean currents carry the tracers eastward within ACC and contribute to the formation of the Antarctic Intermediate Water (AAIW) in the South Pacific and South Atlantic. The main effect of eddies is to disperse tracers away from the mean pathways, and this dispersion acts to retain the BIR tracer in the Atlantic and Indian sectors and reduce the upstream influence of these regions on the South Pacific. In addition, the eddy-induced along-isopycnal spreading within ACC increases the ventilated depth and the inventory of TST. The results can be used to interpret distribution of tracers in the ocean in numerical simulations and observations.
      PubDate: 2017-04-03T23:55:47.596375-05:
      DOI: 10.1002/2016JC012292
       
  • Sea-level rise impacts on the temporal and spatial variability of extreme
           water levels: A case study for St. Peter-Ording, Germany
    • Authors: S. Santamaria-Aguilar; A. Arns, A. T. Vafeidis
      Pages: 2742 - 2759
      Abstract: Both the temporal and spatial variability of storm surge water level (WL) curves are usually not taken into account in flood risk assessments as observational data are often scarce. In addition, sea-level rise (SLR) can further affect the variability of WLs. We analyze the temporal and spatial variability of the WL curve of 75 historical storm surge events that have been numerically simulated for St. Peter-Ording at the German North Sea coast, considering the effects induced by three SLR scenarios (RCP 4.5, RCP 8.5, and a RCP 8.5 high end scenario). We assess potential impacts of these scenarios on two parameters related to flooding: overflow volumes and fullness. Our results indicate that due to both the temporal and spatial variability of those events the resulting overflow volume can be two or even three times greater. We observe a steepening of the WL curve with an increase of the tidal range under the three SLR scenarios, although SLR induced effects are relatively higher for the RCP 4.5. The steepening of the WL curve with SLR produces a reduction of the fullness, but the changes in overflow volumes also depend on the magnitude of the storm surge event.
      PubDate: 2017-04-03T23:55:43.281832-05:
      DOI: 10.1002/2016JC012579
       
  • Observations and a linear model of water level in an interconnected
           inlet-bay system
    • Authors: Alfredo L. Aretxabaleta; Neil K. Ganju, Bradford Butman, Richard P. Signell
      Pages: 2760 - 2780
      Abstract: A system of barrier islands and back-barrier bays occurs along southern Long Island, New York, and in many coastal areas worldwide. Characterizing the bay physical response to water level fluctuations is needed to understand flooding during extreme events and evaluate their relation to geomorphological changes. Offshore sea level is one of the main drivers of water level fluctuations in semienclosed back-barrier bays. We analyzed observed water levels (October 2007 to November 2015) and developed analytical models to better understand bay water level along southern Long Island. An increase (∼0.02 m change in 0.17 m amplitude) in the dominant M2 tidal amplitude (containing the largest fraction of the variability) was observed in Great South Bay during mid-2014. The observed changes in both tidal amplitude and bay water level transfer from offshore were related to the dredging of nearby inlets and possibly the changing size of a breach across Fire Island caused by Hurricane Sandy (after December 2012). The bay response was independent of the magnitude of the fluctuations (e.g., storms) at a specific frequency. An analytical model that incorporates bay and inlet dimensions reproduced the observed transfer function in Great South Bay and surrounding areas. The model predicts the transfer function in Moriches and Shinnecock bays where long-term observations were not available. The model is a simplified tool to investigate changes in bay water level and enables the evaluation of future conditions and alternative geomorphological settings.
      PubDate: 2017-04-04T00:05:33.984932-05:
      DOI: 10.1002/2016JC012318
       
  • Boundary layers at a dynamic interface: Air-sea exchange of heat and mass
    • Authors: Andrew J. Szeri
      Pages: 2781 - 2794
      Abstract: Exchange of mass or heat across a turbulent liquid-gas interface is a problem of critical interest, especially in air-sea transfer of natural and anthropogenic gases involved in the study of climate. The goal in this research area is to determine the gas flux from air to sea or vice versa. For sparingly soluble nonreactive gases, this is controlled by liquid phase turbulent velocity fluctuations that act on the thin species concentration boundary layer on the liquid side of the interface. If the fluctuations in surface-normal velocity w′ and gas concentration c′ are known, then it is possible to determine the turbulent contribution to the gas flux. However, there is no suitable fundamental direct approach in the general case where neither w′ nor c′ can be easily measured. A new approach is presented to deduce key aspects about the near-surface turbulent motions from measurements that can be taken by an infrared (IR) camera. An equation is derived with inputs being the surface temperature and heat flux, and a solution method developed for the surface-normal strain experienced over time by boundary layers at the interface. Because the thermal and concentration boundary layers experience the same near-surface fluid motions, the solution for the surface-normal strain determines the gas flux or gas transfer velocity. Examples illustrate the approach in the cases of complete surface renewal, partial surface renewal, and insolation. The prospects for use of the approach in flows characterized by sheared interfaces or rapid boundary layer straining are explored.
      PubDate: 2017-04-04T00:00:41.156877-05:
      DOI: 10.1002/2016JC012312
       
  • Factors responsible for the limited inland extent of sand deposits on
           Leyte Island during 2013 Typhoon Haiyan
    • Authors: M. Watanabe; J. D. Bricker, K. Goto, F. Imamura
      Pages: 2795 - 2812
      Abstract: Previous geological studies suggest that the maximum inland extent of storm-induced sand deposits is shorter, but their thickness is larger, than those of tsunami-induced sand deposits. However, factors that determine the maximum extent and thickness of storm deposits are still uncertain. We conducted numerical simulations of storm surge, waves, and sediment transport during Typhoon Haiyan in order to understand the distribution and sedimentary processes responsible for storm deposits. Numerical results showed that wave-induced currents slightly offshore were strong, but attenuated rapidly in the inland direction after wave breaking. Therefore, sediments were not transported far inland by waves and storm surge. Consequently, the maximum inland extent of storm deposits was remarkably shorter than the inland extent of inundation. We also revealed that vegetation (roughness coefficient) and typhoon intensity greatly affect the calculation of maximum extent and thickness distribution of storm deposits. As the duration of wave impact on a coast is relatively long during a storm (hours, compared to minutes for a tsunami), sediments are repeatedly supplied by multiple waves. Therefore, storm deposits tend to be thicker than tsunami deposits, and multiple layers can form in the internal sedimentary structure of the deposits. We infer that limitation of the sand deposit to within only a short distance inland from the shoreline and multiple layers found in a deposit can be used as appropriate identification proxies for storm deposits.
      PubDate: 2017-04-04T00:10:37.105146-05:
      DOI: 10.1002/2016JC012023
       
  • The role of the Indian Ocean sector for prediction of the coupled
           Indo-Pacific system: Impact of atmospheric coupling
    • Authors: E. C. Hackert; A. J. Busalacchi, J. Carton, R. Murtugudde, P. Arkin, M. N. Evans
      Pages: 2813 - 2829
      Abstract: Indian Ocean (IO) dynamics impact ENSO predictability by influencing wind and precipitation anomalies in the Pacific. To test if the upstream influence of the IO improves ENSO validation statistics, a combination of forced ocean, atmosphere, and coupled models are utilized. In one experiment, the full tropical Indo-Pacific region atmosphere is forced by observed interannual SST anomalies. In the other, the IO is forced by climatological SST. Differences between these two forced atmospheric model experiments spotlight a much richer wind response pattern in the Pacific than previous studies that used idealized forcing and simple linear atmospheric models. Weak westerlies are found near the equator similar to earlier literature. However, at initialization strong easterlies between 30°S–10°S and 0°N–25°N and equatorial convergence of the meridional winds across the entire Pacific are unique findings from this paper. The large-scale equatorial divergence west of the dateline and northeasterly-to-northwesterly cross-equatorial flow converging on the equator east of the dateline in the Pacific are generated from interannual IO SST coupling. In addition, off-equatorial downwelling curl impacts large-scale oceanic waves (i.e., Rossby waves reflect as western boundary Kelvin waves). After 3 months, these downwelling equatorial Kelvin waves propagate across the Pacific and strengthen the NINO3 SST. Eventually Bjerknes feedbacks take hold in the eastern Pacific which allows this warm anomaly to grow. Coupled forecasts for NINO3 SST anomalies for 1993–2014 demonstrate that including interannual IO forcing significantly improves predictions for 3–9 month lead times.
      PubDate: 2017-04-04T23:30:47.559027-05:
      DOI: 10.1002/2016JC012632
       
  • Composition and variability of the Denmark Strait Overflow Water in a
           high-resolution numerical model hindcast simulation
    • Authors: Erik Behrens; Kjetil Våge, Benjamin Harden, Arne Biastoch, Claus W. Böning
      Pages: 2830 - 2846
      Abstract: The upstream sources and pathways of the Denmark Strait Overflow Water and their variability have been investigated using a high-resolution model hindcast. This global simulation covers the period from 1948 to 2009 and uses a fine model mesh (1/20°) to resolve mesoscale features and the complex current structure north of Iceland explicitly. The three sources of the Denmark Strait Overflow, the shelfbreak East Greenland Current (EGC), the separated EGC, and the North Icelandic Jet, have been analyzed using Eulerian and Lagrangian diagnostics. The shelfbreak EGC contributes the largest fraction in terms of volume and freshwater transport to the Denmark Strait Overflow and is the main driver of the overflow variability. The North Icelandic Jet contributes the densest water to the Denmark Strait Overflow and shows only small temporal transport variations. During summer, the net volume and freshwater transports to the south are reduced. On interannual time scales, these transports are highly correlated with the large-scale wind stress curl around Iceland and, to some extent, influenced by the North Atlantic Oscillation, with enhanced southward transports during positive phases. The Lagrangian trajectories support the existence of a hypothesized overturning loop along the shelfbreak north of Iceland, where water carried by the North Icelandic Irminger Current is transformed and feeds the North Icelandic Jet. Monitoring these two currents and the region north of the Iceland shelfbreak could provide the potential to track long-term changes in the Denmark Strait Overflow and thus also the AMOC.
      PubDate: 2017-04-04T00:10:32.013965-05:
      DOI: 10.1002/2016JC012158
       
  • Striations and preferred eddy tracks triggered by topographic steering of
           the background flow in the eastern South Pacific
    • Authors: Ali Belmadani; Emilio Concha, David Donoso, Alexis Chaigneau, François Colas, Nikolai Maximenko, Emanuele Di Lorenzo
      Pages: 2847 - 2870
      Abstract: In recent years, persistent quasi-zonal jets or striations have been ubiquitously detected in the world ocean using satellite and in situ data as well as numerical models. This study aims at determining the role of mesoscale eddies in the generation and persistence of striations off Chile in the eastern South Pacific. A 50 year climatological integration of an eddy-resolving numerical ocean model is used to assess the long-term persistence of striations. Automated eddy tracking algorithms are applied to the model outputs and altimetry data. Results reveal that striations coincide with both polarized eddy tracks and the offshore formation of new eddies in the subtropical front and coastal transition zone, without any significant decay over time that discards random eddies as a primary driver of the striations. Localized patches of vortex stretching and relative vorticity advection, alternating meridionally near the eastern edge of the subtropical front, are associated with topographic steering of the background flow in the presence of steep topography, and with baroclinically and barotropically unstable meridional flow. These sinks and sources of vorticity are suggested to generate the banded structure further west, consistently with a β-plume mechanism. On the other hand, zonal/meridional eddy advection of relative vorticity and the associated Reynolds stress covariance are consistent with eddy deformation over rough topography and participate to sustain the striations in the far field. Shear instability of mean striations is proposed to feedback onto the eddy field, acting to maintain the subtropical front eddy streets and thus the striations.
      PubDate: 2017-04-04T00:10:48.104401-05:
      DOI: 10.1002/2016JC012348
       
  • Response of the Kuroshio Extension path state to near-term global warming
           in CMIP5 experiments with MIROC4h
    • Authors: Rui Li; Zhao Jing, Zhaohui Chen, Lixin Wu
      Pages: 2871 - 2883
      Abstract: In this study, responses of the Kuroshio Extension (KE) path state to near-term (2006–2035) global warming are investigated using a Kuroshio-resolving atmosphere-ocean coupled model. Under the representative concentration pathway 4.5 (RCP4.5) forcing, the KE system is intensified and its path state tends to move northward and becomes more stable. It is suggested that the local anticyclonic wind stress anomalies in the KE region favor the spin-up of the southern recirculation gyre, and the remote effect induced by the anticyclonic wind stress anomalies over the central and eastern midlatitude North Pacific also contributes to the stabilization of the KE system substantially. The dominant role of wind stress forcing on KE variability under near-term global warming is further confirmed by adopting a linear 1.5 layer reduced-gravity model forced by wind stress curl field from the present climate model. It is also found that the main contributing longitudinal band for KE index (KEI) moves westward in response to the warmed climate. This results from the northwestward expansion of the large-scale sea level pressure (SLP) field.
      PubDate: 2017-04-04T00:00:50.464006-05:
      DOI: 10.1002/2016JC012468
       
  • An inverse method to derive surface fluxes from the closure of oceanic
           heat and water budgets: Application to the north-western Mediterranean Sea
           
    • Authors: G. Caniaux; L. Prieur, H. Giordani, J.-L. Redelsperger
      Pages: 2884 - 2908
      Abstract: The large amount of data collected during DeWEX, MOOSE, and HyMeX campaigns in the north-western Mediterranean in 2012–2013 allowed to implement an inverse method to solve the difficult problem of heat and water budget closure. The inverse method is based on the simulation of the observed heat and water budgets, strongly constrained by observations collected during the campaigns and on the deduction of adjusted surface fluxes. The inverse method uses a genetic algorithm that generates 50,000 simulations of a single-column model and optimizes some adjustable coefficients introduced in the surface fluxes. Finally, the single-column model forced by the adjusted fluxes during 1 year and over a test area of about 300 × 300 km2 simulates the daily mean satellite bulk SST with an accuracy/uncertainty of 0.011 ± 0.072°C, as well as daily mean SSS and residual buoyancy series deduced from wintertime analyses with an accuracy of 0.011 ± 0.008 and 0.03 ± 0.012 m2 s−2, respectively. The adjusted fluxes close the annual heat and rescaled water budgets by less than 5 W m−2. To our knowledge, this is the first time that such a flux data set is produced. It can thus be considered as a reference for the north-western Mediterranean and be used for estimating other flux data sets, for forcing regional models and for process studies. Compared with the adjusted fluxes, some operational numerical weather prediction models (ARPEGE, NCEP, ERA-INTERIM, ECMWF, and AROME), often used to force oceanic models, were evaluated: they are unable to retrieve the mean annual patterns and values.
      PubDate: 2017-04-06T04:30:00.496148-05:
      DOI: 10.1002/2016JC012167
       
  • Space and time scales of sea surface salinity and freshwater forcing
           variability in the global ocean (60°S–60°N)
    • Authors: Frederick M. Bingham; Tong Lee
      Pages: 2909 - 2922
      Abstract: Using Aquarius Version-4.0 data, we have investigated the time and space scales of sea surface salinity (SSS) over the global ocean between 60°S and 60°N. Decorrelation time scales of SSS were found to be divided among less than 80 days (covering 1/2 of ocean area), 80–100 days (1/3), and greater than 100 days (remainder). Once the seasonal cycle is removed, shorter time scales (less than 80 days) dominate. Spatial scales are largest in the tropics along the intertropical convergence zones of all oceans and the South Pacific convergence zone in the South Pacific. Time scales were also calculated for time-integrated (cumulative) surface freshwater forcing (CFWF) using precipitation from Tropical Rainfall Measurement Mission and evaporation from OAFlux data. These showed little spatial pattern, but a dominance of the seasonal and longer time scales over the globe. The lack of correspondence between dominant temporal and spatial scales of SSS and CFWF highlights the importance of ocean processes in regulating SSS variability.
      PubDate: 2017-04-07T01:57:10.422958-05:
      DOI: 10.1002/2016JC012216
       
  • Validation of sea ice models using an uncertainty-based distance metric
           for multiple model variables
    • Authors: Jorge R. Urrego-Blanco; Elizabeth C. Hunke, Nathan M. Urban, Nicole Jeffery, Adrian K. Turner, James R. Langenbrunner, Jane M. Booker
      Pages: 2923 - 2944
      Abstract: We implement a variance-based distance metric (Dn) to objectively assess skill of sea ice models when multiple output variables or uncertainties in both model predictions and observations need to be considered. The metric compares observations and model data pairs on common spatial and temporal grids improving upon highly aggregated metrics (e.g., total sea ice extent or volume) by capturing the spatial character of model skill. The Dn metric is a gamma-distributed statistic that is more general than the χ2 statistic commonly used to assess model fit, which requires the assumption that the model is unbiased and can only incorporate observational error in the analysis. The Dn statistic does not assume that the model is unbiased, and allows the incorporation of multiple observational data sets for the same variable and simultaneously for different variables, along with different types of variances that can characterize uncertainties in both observations and the model. This approach represents a step to establish a systematic framework for probabilistic validation of sea ice models. The methodology is also useful for model tuning by using the Dn metric as a cost function and incorporating model parametric uncertainty as part of a scheme to optimize model functionality. We apply this approach to evaluate different configurations of the standalone Los Alamos sea ice model (CICE) encompassing the parametric uncertainty in the model, and to find new sets of model configurations that produce better agreement than previous configurations between model and observational estimates of sea ice concentration and thickness.
      PubDate: 2017-04-08T05:37:17.713928-05:
      DOI: 10.1002/2016JC012602
       
  • Using a 1-D model to reproduce the diurnal variability of SST
    • Authors: Ioanna Karagali; Jacob L. Høyer, Craig J. Donlon
      Pages: 2945 - 2959
      Abstract: A wide range of applications, from air-sea interaction studies to fisheries and biological modeling, need accurate, high resolution SST which requires that the diurnal signal is known; for many applications, diurnal estimates are necessary and should be included in blended SST products. A widely preferred approach to bridge the gap between in situ and remotely sensed measurements and obtain diurnal warming estimates at large spatial scales is modeling of the upper ocean temperature. This study uses the one-dimensional General Ocean Turbulence Model (GOTM) to resolve diurnal signals identified from satellite SSTs and in situ measurements. Focus is given on testing and validation of different parameterizations of the basic physical processes known to influence the generation of a warm surface layer. GOTM is tested and validated using in situ measurements obtained at three locations, two in the Atlantic Ocean and one in the Baltic Sea, where different oceanographic and atmospheric conditions occur, in order to obtain an insight into its general performance. It is found that the model, with a 9 band solar absorption model rather than the standard 2 band scheme, performs well when using 3 hourly NWP forcing fields and is able to resolve daily SST variability seen both from satellite and in situ measurements. As such, and due to its low computational cost, it is proposed as a candidate model for diurnal variability estimates.
      PubDate: 2017-04-08T02:51:10.047093-05:
      DOI: 10.1002/2016JC012542
       
  • Submesoscale processes promote seasonal restratification in the
           Subantarctic Ocean
    • Authors: M. du Plessis; S. Swart, I. J. Ansorge, A. Mahadevan
      Pages: 2960 - 2975
      Abstract: Traditionally, the mechanism driving the seasonal restratification of the Southern Ocean mixed layer (ML) is thought to be the onset of springtime warming. Recent developments in numerical modeling and North Atlantic observations have shown that submesoscale ML eddies (MLE) can drive a restratifying flux to shoal the deep winter ML prior to solar heating at high latitudes. The impact of submesoscale processes on the intraseasonal variability of the Subantarctic ML is still relatively unknown. We compare 5 months of glider data in the Subantarctic Zone to simulations of a 1-D mixing model to show that the magnitude of restratification of the ML cannot be explained by heat, freshwater, and momentum fluxes alone. During early spring, we estimate that periodic increases in the vertical buoyancy flux by MLEs caused small increases in stratification, despite predominantly down-front winds that promote the destruction of stratification. The timing of seasonal restratification was consistent between 1-D model estimates and the observations. However, during up-front winds, the strength of springtime stratification increased over twofold compared to the 1-D model, with a rapid shoaling of the MLD from >200 m to
      PubDate: 2017-04-08T02:46:22.429012-05:
      DOI: 10.1002/2016JC012494
       
  • Buoyancy-driven coastal current blocks ventilation of an anoxic fjord on
           the Pacific coast of Canada
    • Authors: Richard E. Thomson; David J. Spear, Maxim V. Krassovski, Roy A. S. Hourston, Tamás A. Juhász, Steven F. Mihály
      Pages: 2976 - 2998
      Abstract: Shallow sills restrict the ventilation of deep coastal fjords. Dense oceanic water seaward of the sill and lower density water within the receiving basin are generally required for oxygenated water to cross the sill and descend deep into the fjord. Here we use concurrent 10 year time series from current meters in the fjord and on the continental shelf to examine ventilation of the 120 m deep, anoxic inner basin of Effingham Inlet on the west coast of Vancouver Island. Whereas density currents traverse the 40 m deep sill and flow into the inner basin at mid-depth at quasi-fortnightly tidal intervals, only five current intrusions descended to the bottom of the basin over the decade-long measurement period. The deep intrusions had a mean (±SD) return interval of 1.75 (±1.33) years and induced bottom-water changes that persisted for 1–2 months. We show that, in addition to conditions within the inlet, deep ventilation of the inner basin is dependent on a coordinated sequence of external processes: (1) the onset of upwelling winds along the outer coast; (2) reversal of the buoyancy-driven coastal current that normally flows poleward over the inner shelf off Vancouver Island; and (3) reversal of the estuarine circulation in the channel connecting the inlet to the ocean. As the observed ventilation intervals are short compared to the decadal intervals derived from the spacing of “homogenites” (sedimentation sequences disrupted by intruding bottom currents), the use of homogenites as proxies for past upwelling conditions in the northeast Pacific may need to be reexamined.
      PubDate: 2017-04-08T02:46:16.919412-05:
      DOI: 10.1002/2016JC012512
       
  • The variations of sediment transport patterns in the outer Changjiang
           Estuary and Hangzhou Bay over the last 30 years
    • Authors: Dongfeng Xie; Cunhong Pan, Xiuguang Wu, Shu Gao, Zhengbing Wang
      Pages: 2999 - 3020
      Abstract: The research objective is to investigate the variations of sediment transport in outer Changjiang Estuary and adjacent Hangzhou Bay, induced by the decline of Changjiang River sediment discharge and massive land reclamation in the last three decades. A synchronous hydrographic survey was conducted along two transects (at the bay-mouth and outer Changjiang Estuary, respectively) during the spring-neap tides of January and July 2014. The results show that the suspended sediment grain size, current velocity, suspended sediment concentration (SSC), and the water and sediment fluxes varied with the tidal cycles. Quantitative correlations with the tidal range were found for SSC and fluxes. These data have been compared with those at the same hydrographic stations in the summer and winter of the early 1980s. Along the outer Changjiang Estuary transect, the SSCs and sediment fluxes decreased in the winter, but no apparent changes occurred in the summer. The SSCs in the northern Hangzhou Bay decreased in both summer and winter, while the southern bay mouth has evolved from a low SSC region to a high SSC region. The findings clarify that the SSC and sediment flux changes in this area have only an indirect connection to the dramatic riverine sediment decline, because the sediment resuspension by the strong tidal currents provided a major source. At the present stage, the impact of the riverine sediment decline is insignificant for the SSC variation off the Changjiang River mouth. Finally, a sediment flux model is proposed to explain and predict the morphological evolution trends.
      PubDate: 2017-04-08T02:41:36.381133-05:
      DOI: 10.1002/2016JC012264
       
  • Impact of mesoscale eddies on Kuroshio intrusion variability northeast of
           Taiwan
    • Authors: Yuqi Yin; Xiaopei Lin, Ruoying He, Yijun Hou
      Pages: 3021 - 3040
      Abstract: Abundant mesoscale eddies propagate to the east of Taiwan and affect the Kuroshio Current and its onshore intrusion northeast of Taiwan. But the dynamical process of how eddy activities modulate the Kuroshio intrusion is still unclear. In this study, in situ ocean velocity observations, drifter trajectories, tide gauge measurements, satellite sea surface height data, and assimilative Hybrid Coordinate Ocean Model (HYCOM) reanalysis outputs were used to analyze a cyclonic eddy-induced large Kuroshio onshore intrusion during late autumn 2008. Along with nine anticyclonic eddy events and seven cyclonic eddy events extracted from long-term HYCOM reanalysis outputs, the variability of the eddy-induced intrusion was quantified. The impacts of the cyclonic eddy and the anticyclonic eddy were opposite. Under the impact of cyclonic (anticyclonic) eddy, the total onshore intrusion between 122°E and 124°E across the 200 m isobaths was decreased (increased) by 15.7% (7.3%), while the onshore intrusion west of 122.6°E and the northeastward current on the outer shelf were increased (decreased) by 30.6% (4.7%) and 31% (10.1%), respectively. The arrival of cyclonic (anticyclonic) eddy brought positive (negative) potential vorticity (PV) flux and modulated the local ocean vertical stratification, which weakened (enhanced) the cross-slope PV gradient and the shelf slope constraint on the upper layer current and, therefore, favored (inhibited) the Kuroshio onshore intrusion.
      PubDate: 2017-04-10T05:08:22.685235-05:
      DOI: 10.1002/2016JC012263
       
  • Parameterizing air-sea gas transfer velocity with dissipation
    • Authors: L. Esters; S. Landwehr, G. Sutherland, T. G. Bell, K. H. Christensen, E. S. Saltzman, S. D. Miller, B. Ward
      Pages: 3041 - 3056
      Abstract: The air-sea gas transfer velocity k is frequently estimated as an empirical function of wind speed. However, it is widely recognized that k depends on processes other than wind speed alone. The small-eddy model, which describes periodic events of small eddies disturbing the sea surface with water from below, suggests a direct relation between k and the dissipation rate of turbulent kinetic energy ϵ at the air-sea interface. This relation has been proven both in laboratories and in the field in various freshwater and coastal environments, but to date has not been verified in open ocean conditions. Here, concurrent North Atlantic field observations of ϵ and eddy covariance measurements of DMS and CO2 air-sea gas flux are presented. Using ϵ measurements, we compare the small-eddy model at various depths to previously published observations. Extrapolating the measured ϵ profiles to the thickness of the viscous sublayer allows us to formulate a function of k that depends solely on the water side friction velocity u∗w, which can be inferred from direct eddy covariance measurements of the air-side friction velocity u∗a. These field observations are generally consistent with the theoretical small-eddy model. Utilizing a variable Schmidt number exponent in the model, rather than a constant value of 12 yields improved agreement between model and observations.
      PubDate: 2017-04-10T05:06:56.487817-05:
      DOI: 10.1002/2016JC012088
       
  • Remote sensing of the correlation between breakpoint oscillations and
           infragravity waves in the surf and swash zone
    • Authors: T. Moura; T. E. Baldock
      Pages: 3106 - 3122
      Abstract: A novel remote sensing methodology to determine the dominant infragravity mechanism in the inner surf and swash zone in the field is presented. Video observations of the breakpoint motion are correlated with the shoreline motion and inner surf zone water levels to determine the relationship between the time-varying breakpoint oscillations and the shoreline motion. The results of 13 field data sets collected from three different beaches indicate that, inside the surf zone, the dominance of bound wave or breakpoint forcing is strongly dependent on the surf zone width and the type of short wave breaking. Infragravity generation by bound wave release was stronger for conditions with relatively narrow surf zones and plunging waves; breakpoint forcing was dominant for wider surf zones and spilling breaker conditions.
      PubDate: 2017-04-11T02:30:50.783793-05:
      DOI: 10.1002/2016JC012233
       
 
 
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