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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: 25, SJR: 2.439, h-index: 91)
Geophysical Research Letters     Full-text available via subscription   (Followers: 101, SJR: 3.323, h-index: 185)
Global Biogeochemical Cycles     Full-text available via subscription   (Followers: 12, SJR: 3.22, h-index: 136)
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J. of Geophysical Research : Biogeosciences     Full-text available via subscription   (Followers: 24)
J. of Geophysical Research : Earth Surface     Partially Free   (Followers: 48)
J. of Geophysical Research : Oceans     Partially Free   (Followers: 45)
J. of Geophysical Research : Planets     Full-text available via subscription   (Followers: 96)
J. of Geophysical Research : Solid Earth     Full-text available via subscription   (Followers: 42)
J. of Geophysical Research : Space Physics     Full-text available via subscription   (Followers: 114)
Paleoceanography     Full-text available via subscription   (Followers: 7, SJR: 3.067, h-index: 100)
Radio Science     Full-text available via subscription   (Followers: 36, SJR: 1.072, h-index: 59)
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Water Resources Research     Full-text available via subscription   (Followers: 75, SJR: 2.661, h-index: 144)
Journal Cover Journal of Geophysical Research : Oceans
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   Published by AGU Homepage  [17 journals]
  • Evaluation of thermal and dynamic impacts of summer dust aerosols on the
           Red Sea
    • Authors: Bronwyn Cahill; Ralf Toumi, Georgiy Stenchikov, Sergey Osipov, Helen Brindley
      Abstract: The seasonal response of upper ocean processes in the Red Sea to summer-time dust aerosol perturbations is investigated using an uncoupled regional ocean model. We find that the upper limit response is highly sensitive to dust-induced reductions in radiative fluxes. Sea surface cooling of -1°C and -2°C is predicted in the northern and southern regions, respectively. This cooling is associated with a net radiation reduction of -40Wm−2 and-90Wm−2 over the northern and southern regions, respectively. Larger cooling occurs below the mixed layer at 75m in autumn, -1.2°C (north) and -1.9°C (south). SSTs adjust more rapidly (c. 30 days) than the subsurface temperatures (seasonal timescales), due to stronger stratification and increased mixed layer stability inhibiting the extent of vertical mixing. The basin average annual heat flux reverses sign and becomes positive, +4.2 Wm−2 (as compared to observed estimates -17.3Wm−2) indicating a small gain of heat from the atmosphere. When we consider missing feedbacks from atmospheric processes in our uncoupled experiment, we postulate that the magnitude of cooling and the timescales for adjustment will be much less, and that the annual heat flux will not reverse sign but nevertheless be reduced as a result of dust perturbations. While our study highlights the importance of considering coupled ocean-atmosphere processes on the net surface energy flux in dust perturbation studies, the results of our uncoupled dust experiment still provide an upper limit estimate of the response of the upper ocean to dust-induced radiative forcing perturbations. This article is protected by copyright. All rights reserved.
      PubDate: 2017-01-16T06:26:56.145914-05:
      DOI: 10.1002/2016JC011911
       
  • Percolation blockage: A process that enables melt pond formation on first
           year Arctic Sea ice
    • Authors: Chris Polashenski; Kenneth M. Golden, Donald K. Perovich, Eric Skyllingstad, Alexandra Arnsten, Carolyn Stwertka, Nicholas Wright
      Abstract: Melt pond formation atop Arctic sea ice is a primary control of shortwave energy balance in the Arctic Ocean. During late spring and summer, the ponds determine sea ice albedo and how much solar radiation is transmitted into the upper ocean through the sea ice. The initial formation of ponds requires that melt water be retained above sea level on the ice surface. Both theory and observations, however, show that first year sea ice is so highly porous prior to the formation of melt ponds that multi-day retention of water above hydraulic equilibrium should not be possible. Here we present results of percolation experiments that identify and directly demonstrate a mechanism allowing melt pond formation. The infiltration of fresh water into the pore structure of sea ice is responsible for blocking percolation pathways with ice, sealing the ice against water percolation, and allowing water to pool above sea level. We demonstrate that this mechanism is dependent on fresh water availability, known to be predominantly from snowmelt, and ice temperature at melt onset. We argue that the blockage process has the potential to exert significant control over inter-annual variability in ice albedo. Finally, we suggest that incorporating the mechanism into models would enhance their physical realism. Full treatment would be complex. We provide a simple temperature threshold-based scheme that maybe used to incorporate percolation blockage behavior into existing model frameworks. This article is protected by copyright. All rights reserved.
      PubDate: 2017-01-16T06:25:48.394936-05:
      DOI: 10.1002/2016JC011994
       
  • Local diurnal wind-driven variability and upwelling in a small coastal
           embayment
    • Authors: Ryan K. Walter; Emma C. Reid, Kristen A. Davis, Kevin J. Armenta, Kevin Merhoff, Nicholas J. Nidzieko
      Abstract: The oceanic response to high-frequency local diurnal wind forcing is examined in a small coastal embayment located along an understudied stretch of the central California coast. We show that local diurnal wind forcing is the dominant control on nearshore temperature variability and circulation patterns. A complex empirical orthogonal function (CEOF) analysis of velocities in San Luis Obispo Bay reveals that the first mode CEOF amplitude time series, which accounts for 47.9% of the variance, is significantly coherent with the local wind signal at the diurnal frequency and aligns with periods of weak and strong wind forcing. The diurnal evolution of the hydrographic structure and circulation in the bay is examined using both individual events and composite-day averages. During the late afternoon, the local wind strengthens and results in a sheared flow with near-surface warm waters directed out of the bay and a compensating flow of colder waters into the bay over the bottom portion of the water column. This cold water intrusion into the bay causes isotherms to shoal towards the surface and delivers subthermocline waters to shallow reaches of the bay, representing a mechanism for small-scale upwelling. When the local winds relax, the warm water mass advects back into the bay in the form of a buoyant plume front. Local diurnal winds are expected to play an important role in nearshore dynamics and local upwelling in other small coastal embayments with important implications for various biological and ecological processes. This article is protected by copyright. All rights reserved.
      PubDate: 2017-01-13T04:00:50.382849-05:
      DOI: 10.1002/2016JC012466
       
  • High-resolution observations of secondary circulation and tidally
           synchronized upwelling around a coastal headland
    • Authors: P. Russell; R. Vennell
      Abstract: The upwelling of nutrient rich bottom waters supports life at the oceans surface. Secondary circulation can produce localized upwelling at headlands. Secondary circulation develops in the curved flow around headlands resulting in a loose helical flow pattern within the curved flow. The magnitude of secondary flow can be up to 20% of the depth average current. Moving vessel ADCP measurements were taken at Cape Saunders, Otago Peninsula, New Zealand. New radial basis function interpolation techniques for smoothing noisy data allows the weak horizontal secondary flow to be extracted from the stronger along shore flows. During peak floods of 1 ms−1 the measured strength of secondary flow is 0.2 ms−1. A region of secondary flow approximately 1500 m long and 800 m wide is observed down-stream of the Cape. On the inshore edge this region, areas of vertical velocity inferred from horizontal ADCP measurements using mass continuity show a localized upwelling of up to 0.007 ms−1. Concurrent CTD measurements also show this tidally synchronized upwelling. Linear regression between upwelling from the CTD measurements and the inferred vertical velocity from the ADCP measurements show the data is well correlated, rp = 0.65. The upwelling is 4 m per hour so in this location with a mean depth of 25 m the entire water column is replacing itself with deep waters at least once during a tidal cycle. On a global scale the cumulative upwelling from headlands and islands due to secondary circulation could be a significant source of nutrient delivery to coastal surface waters. This article is protected by copyright. All rights reserved.
      PubDate: 2017-01-13T03:55:51.651267-05:
      DOI: 10.1002/2016JC012117
       
  • Middepth decadal warming and freshening in the South Atlantic
    • Authors: Donata Giglio; Gregory C. Johnson
      Abstract: South Atlantic Ocean mid-depth water property (temperature, salinity, oxygen, nutrients, etc.) distributions are set by salty, well-ventilated, and relatively nutrient-poor North Atlantic Deep Water (NADW) spreading southward towards the Southern Ocean (SO) underneath fresher, well-ventilated, and relatively nutrient-poor northward-spreading Antarctic Intermediate Water (AAIW). The layer between the NADW and AAIW is oxygen-poor and nutrient-rich, with small vertical temperature gradients. Salinity stratification dominates the vertical density gradient, hence the layer is referred to as Salinity Stratified Layer (SSL). Decadal warming (0.044°C decade−1) and freshening (0.006 g kg−1 decade−1) of this layer are analyzed using Argo data, a climatology, and repeat hydrographic sections. Warming within the SSL accumulates heat at a rate of ∼20 TW, is unlikely to be caused by vertical heave, and is consistent with anomalous southward advection of order 102 km decade−1 in the Atlantic Meridional Overturning Circulation (AMOC). Salinity changes within the SSL are consistent with a downward velocity anomaly of order 10 m decade−1. This article is protected by copyright. All rights reserved.
      PubDate: 2017-01-13T03:55:50.498364-05:
      DOI: 10.1002/2016JC012246
       
  • Development of a global gridded Argo data set with Barnes successive
           corrections
    • Authors: Hong Li; Fanghua Xu, Wei Zhou, Dongxiao Wang, Jonathon S. Wright, Zenghong Liu, Yanluan Lin
      Abstract: A new 11-year (2004-2014) monthly 1-degree gridded Argo temperature and salinity dataset with 49 vertical levels from the surface to 1950 m depth (named BOA-Argo) is generated for use in ocean research and modeling studies. The dataset is produced based on refined Barnes successive corrections by adopting flexible response functions based on a series of error analyses to minimize errors induced by non-uniform spatial distribution of Argo observations. These response functions allow BOA-Argo to capture a greater portion of mesoscale and large-scale signals while compressing small-sale and high frequency noise relative to the most recent version of the World Ocean Atlas (WOA). BOA-Argo dataset is evaluated against other gridded datasets, such as WOA13, Roemmich-Argo, Jamestec-Argo, EN4-Argo, and IPRC-Argo in terms of climatology, independent observations, mixed layer depth and so on. Generally, BOA-Argo compare well with other Argo gridded datasets. The RMSEs and correlation coefficients of compared variables from BOA-Argo agree most with those from the Roemmich-Argo. In particular, more mesoscale features are retained in BOA-Argo than others as compared to satellite sea surface heights. These results indicate that the BOA-Argo dataset is a useful and promising adding to the current Argo datasets. The proposed refined Barnes method is computationally simple and efficient, so that the BOA-Argo dataset can be easily updated to keep pace with tremendous daily increases in the volume of Argo temperature and salinity data.
      PubDate: 2017-01-10T19:15:56.735488-05:
      DOI: 10.1002/2016JC012285
       
  • Dissolved trace metal (Cu, Cd, Co, Ni, Ag) distribution and Cu speciation
           in the Southern Yellow Sea and Bohai Sea, China
    • Authors: Li Li; Xiaojing Wang, Jihua Liu, Xuefa Shi
      Abstract: Trace metals play an important role in biogeochemical cycling in ocean systems. However, because the use of trace-metal clean sampling and analytical techniques has been limited in coastal China, there are few accurate trace metal data for that region. This work studied spatial distribution of selected dissolved trace metals (Ag, Cu, Co, Cd, Ni) and Cu speciation in the southern Yellow Sea (SYS) and Bohai Sea (BS). In general, the average metal (Cu, Co, Cd, Ni) concentrations found in the SYS were lower by a factor of two than those in BS, and they are comparable to dissolved trace metal concentrations in coastal seawater of the United States and Europe.Possible sources and sinks, and physical and biological processes that influenced the distribution of these trace metals in the study region were further examined. Close relationships were found between the trace metal spatial distribution with local freshwater discharge, and processes such as sediment resuspension and biological uptake. Ag, owing to its extremely low concentrations, exhibited a unique distribution pattern that magnified the influences from the physical and biological processes. Cu speciation in the water column showed that, in the study region, Cu was strongly complexed with organic ligands and concentrations of free cupric ion were in the range of 10−12.6‒10−13.2 mol L−1. The distribution of Cu-complexing organic ligand, indicated by values of the side reaction coefficient α', were similar to the Chl a distribution, suggesting that in situ biota production may be one main source of Cu-complexing organic ligand.
      PubDate: 2017-01-10T17:40:22.767831-05:
      DOI: 10.1002/2016JC012500
       
  • Upper ocean observations in Eastern Caribbean Sea reveal barrier layer
           within a warm core eddy
    • Authors: J. E. Rudzin; L.K. Shay, B. Jaimes, J. K. Brewster
      Abstract: Three-dimensional measurements of a large warm core eddy (WCE) and the Caribbean Current are acquired using oceanic profilers deployed during a NOAA research aircraft study in September 2014 in the eastern Caribbean Sea. Measurements of the near-surface atmosphere are also collected to examine air-sea processes over the eddy. These novel measurements showcase temperature and salinity for the eddy and background flow, upper ocean stratification, a residing barrier layer (BL), velocity structure, and water mass characteristics. The eddy's thermal structure is alike that of WCEs in the Gulf of Mexico (GoM) whereas surrounding waters have relatively deeper isotherms compared to its GoM counterparts. Analyses suggest that upper ocean stratification within the study region is due to a BL. These are the first observations of a BL inside a WCE to the best of our knowledge. Reduced shear comparisons suggest that the upper ocean, especially within the WCE, would be more resistant to tropical cyclone (TC) induced mixing than the GoM because of the BL. The eddy is suspected to originate from North Brazil Current rings, given its fresh anomalies relative to climatology and surrounding waters and its trajectory prior to sampling. Atmospheric measurements suggest the WCE is influencing the lower atmosphere along its boundaries. These observations signify that not only does this WCE have deep thermal structure and modulate the near-surface atmosphere but it is unique because it has a BL. The findings and analyses suggest that a similar eddy could potentially influence air-sea processes, such as those during TC passage.
      PubDate: 2017-01-10T17:35:22.707102-05:
      DOI: 10.1002/2016JC012339
       
  • Observed cold filaments associated with mesoscale eddies in the South
           China Sea
    • Authors: Jiaxun Li; Guihua Wang, Xiaoming Zhai
      Abstract: Unusual cold filaments are uncovered during the spring intermonsoon season in the South China Sea (SCS) using a suite of satellite observations. They have a width of about 100 km on average and extend several hundreds of kilometers offshore on the sea surface, providing significant cross-shelf transport of heat and nutrients. The eastward current associated with mesoscale eddies in spring in the western SCS is found to play an important role in the filament formation by advecting coastal cold waters far offshore. The meridional location of the cold filament displays considerable interannual variability ranging between 9oN and 18oN, which can be attributed to the interannual south-north shift of the eastward current associated with eddies. It is also found that in the spring, cold filaments have profound effects on the chlorophyll a concentration in the upper ocean and the overlying atmosphere. These findings provide new insights into the role of eddies in cross-shelf exchange and mesoscale air-sea interaction in the marginal seas.
      PubDate: 2017-01-10T17:30:24.776939-05:
      DOI: 10.1002/2016JC012353
       
  • Variability of upper ocean thermohaline structure during a MJO event from
           DYNAMO aircraft observations
    • Authors: Denny P. Alappattu; Qing Wang, John Kalogiros, Nick Guy, David P. Jorgensen
      Abstract: This paper reports upper ocean thermohaline structure and variability observed during the life cycle of an intense Madden Julian Oscillation (MJO) event occurred in the southern tropical Indian Ocean (14°S-Eq, 70°E-81°E). Water column measurements for this study were collected using airborne expendable probes deployed from NOAA's WP-3D Orion aircraft operated as a part of Dynamics of MJO field experiment conducted during November-December, 2011. Purpose of the study is twofold; (1) to provide a statistical analysis of the upper ocean properties observed during different phases of MJO and, (2) to investigate how the upper ocean thermohaline structure evolved in the study region in response to the MJO induced perturbation.During the active phase of MJO, mixed layer depth (MLD) had a characteristic bimodal distribution. Primary and secondary modes were at ∼ 34 m and ∼ 65 m respectively. Spatial heterogeneity of the upper ocean response to the MJO forcing was the plausible reason for bimodal distribution. Thermocline and isothermal layer depth deepened respectively by 13 m and 19 m from the suppressed through the restoring phase of MJO. Thicker (> 30 m) barrier layers were found to occur more frequently in the active phase of MJO, associated with convective rainfalls. Additionally, the water mass analysis indicated that, in the active phase of this MJO event the sub-surface was dominated by Indonesian throughflow, nonetheless intrusion of Arabian Sea high saline water was also noted near the equator.
      PubDate: 2017-01-10T17:30:22.328789-05:
      DOI: 10.1002/2016JC012137
       
  • Seasonal warming of the Middle Atlantic Bight Cold Pool
    • Authors: S. J. Lentz
      Abstract: The Cold Pool is a 20 – 60 m thick band of cold, near-bottom water that persists from spring to fall over the mid and outer shelf of the Middle Atlantic Bight (MAB) and Southern Flank of Georges Bank. The Cold Pool is remnant winter water bounded above by the seasonal thermocline and offshore by warmer slope water. Historical temperature profiles are used to characterize the average annual evolution and spatial structure of the Cold Pool. The Cold Pool gradually warms from spring through summer at a rate of order 1°C per month. The warming rate is faster in shallower water where the Cold Pool is thinner, consistent with a vertical turbulent heat flux from the thermocline to the Cold Pool. The Cold Pool warming rate also varies along the shelf; it is larger over Georges Bank and smaller in the southern MAB. The mean turbulent diffusivities at the top of the Cold Pool, estimated from the spring to summer mean heat balance, are an order of magnitude larger over Georges Bank than in the southern MAB, consistent with much stronger tidal mixing over Georges Bank than in the southern MAB. The stronger tidal mixing causes the Cold Pool to warm more rapidly over Georges Bank and the eastern New England shelf than in the New York Bight or southern MAB. Consequently, the coldest Cold Pool water is located in the New York Bight from late spring through summer.
      PubDate: 2017-01-10T17:25:33.168175-05:
      DOI: 10.1002/2016JC012201
       
  • Community production modulates coral reef pH and the sensitivity of
           ecosystem calcification to ocean acidification
    • Authors: Thomas M. DeCarlo; Anne L. Cohen, George T.F. Wong, Fuh-Kwo Shiah, Steven J. Lentz, Kristen A. Davis, Kathryn E.F. Shamberger, Pat Lohmann
      Abstract: Coral reefs are built of calcium carbonate (CaCO3) produced biogenically by a diversity of calcifying plants, animals and microbes. As the ocean warms and acidifies, there is mounting concern that declining calcification rates could shift coral reef CaCO3 budgets from net accretion to net dissolution. We quantified net ecosystem calcification (NEC) and production (NEP) on Dongsha Atoll, northern South China Sea, over a two-week period that included a transient bleaching event. Peak daytime pH on the wide, shallow reef flat during the non-bleaching period was ∼8.5, significantly elevated above that of the surrounding open ocean (∼8.0-8.1) as a consequence of daytime NEP (up to 112 mmol C m−2 hr−1). Diurnal-averaged NEC was 390 ± 90 mmol CaCO3 m−2 day−1, higher than any other coral reef studied to date despite comparable calcifier cover (25%) and relatively high fleshy algal cover (19%). Coral bleaching linked to elevated temperatures significantly reduced daytime NEP by 29 mmol C m−2 hr−1. pH on the reef flat declined by 0.2 units, causing a 40% reduction in NEC in the absence of pH changes in the surrounding open ocean. Our findings highlight the interactive relationship between carbonate chemistry of coral reef ecosystems and ecosystem production and calcification rates, which are in turn impacted by ocean warming. As open-ocean waters bathing coral reefs warm and acidify over the 21st century, the health and composition of reef benthic communities will play a major role in determining on-reef conditions that will in turn dictate the ecosystem response to climate change.
      PubDate: 2017-01-10T17:25:31.868009-05:
      DOI: 10.1002/2016JC012326
       
  • Tidal-fluvial interaction in the Guadalquivir Estuary: Spatial and
           frequency-dependent response of currents and water levels
    • Authors: M.A. Losada; M. Díez-Minguito, M.Á. Reyes-Merlo
      Abstract: This paper presents a study on the tidal-fluvial interaction in the highly regulated Guadalquivir River Estuary (SW Spain), which is occasionally subjected to high discharge episodes that affect navigational conditions and increase flood risks. The study specifically focuses on the processes and controlling mechanisms of the non-stationary response of water levels and currents to high discharges. Measurements show a 60-day post-discharge amplification of tidal current and elevation amplitudes and a clockwise rotation of the tidal ellipse in the upper layers. A decrease of amplitudes and an anticlockwise rotation predominate near the bed. Such episodes significantly increase the tidal wave celerity, and especially at high and low water. These features are due to the suspended sediment stratification triggered by the discharge event. The increase in stratification restricts frictional influence to bottom layers, partially decoupling the overlying flow from the bottom. A non-stationary harmonic decomposition method, intended for identifying which non-linear terms in the governing hydrodynamic equations control overtide and compound tide generation, shows that quadratic bottom stress contributes the most during high discharge periods. The consequence in the subtidal balance is that, during peak discharge and in the upper stretches, friction is largely balanced by the water level gradient, although the density gradient term becomes comparable to the friction term soon after peak discharge. Advection is also important to the force balance in the lower estuary. For both parts, to correctly explain subtidal dynamics, it is necessary to account for the time variability of the friction coefficient due to flow-sediment feedback.
      PubDate: 2017-01-10T17:25:28.435786-05:
      DOI: 10.1002/2016JC011984
       
  • The Angola Current: Flow and hydrographic characteristics as observed at
           11°S
    • Authors: R. Kopte; P. Brandt, M. Dengler, P.C.M. Tchipalanga, M. Macuéria, M. Ostrowski
      Abstract: The eastern boundary circulation off the coast of Angola has been described only sparsely to date, although it is a key element in the understanding of the highly productive tropical marine ecosystem off Angola. Here, we report for the first time direct velocity observations of the Angola Current (AC) at ∼11°S between July 2013 and October 2015, covering the depth range from 45 to 450 m. The measurements reveal an alongshore flow that is dominated by intra-seasonal to seasonal variability with periodically alternating southward and northward velocities in the range of ±40 cm/s. During the observation period, a weak southward mean flow of 5-8 cm/s at 50 m depth is observed, with the southward current extending down to about 200 m depth. Corresponding mean southward transport of the AC is estimated to be 0.32 ± 0.046 Sv. An extensive set of hydrographic measurements is used to investigate the thermal structure and seasonality in the hydrography of the eastern boundary circulation. Within the depth range of the AC the superposition of annual and semi-annual harmonics explains a significant part of the total variability, although salinity in the near surface layer appears to be also impacted by year-to-year variability and/or short-term freshening events. In the central water layer, temperature and salinity on isopycnals vary only weakly on seasonal to annual time scales. The available dataset is further used to discuss biases in different reanalysis products particularly emphasizing the ocean's role in coupled climate model SST biases in the Eastern Tropical Atlantic. This article is protected by copyright. All rights reserved.
      PubDate: 2017-01-07T03:40:27.466608-05:
      DOI: 10.1002/2016JC012374
       
  • Relationship between optimal precursors for Indian Ocean Dipole events and
           optimally growing initial errors in its prediction
    • Authors: Mu Mu; Rong Feng, Wansuo Duan
      Abstract: Using the Geophysical Fluid Dynamics Laboratory Climate Model version 2p1, we explored the precursory disturbances that are most likely to develop into a positive Indian Ocean Dipole (IOD). The dominant spatial patterns of these precursors are defined as the optimal precursors (OPRs) of positive IOD as they are more inclined to cause a positive IOD than other superimposed initial perturbations in the experiments. Specifically, there are two types of OPRs with opposite patterns; the surface component of OPR-1 (OPR-2) is an indistinctive west–east dipole pattern, with a small area of negative (positive) perturbations to the coast of Sumatra and Java. Correspondingly, there is a significant west–east dipole pattern in the subsurface component of the OPRs, with the largest values located in the eastern equatorial Indian Ocean. The dominant mode of the time-dependent evolutions of the precursors features rapid development of positive IOD. Furthermore, the OPRs are similar to the optimally growing initial errors (OGEs) associated with IOD predictions that have been presented in previous studies. The shortwave radiation, latent heat flux and westward Rossby waves play an important role in the time-dependent evolution of OGEs. Moreover, the large values of the OPRs are located in the same areas as the sensitive areas of targeted observations identified by the OGEs. This infers that intensive observations over these areas would not only reduce initial errors, improve the accuracy of initial fields and decrease the prediction errors, but would also detect the precursory signals in advance, which substantially improves the forecast skill of IOD. This article is protected by copyright. All rights reserved.
      PubDate: 2017-01-07T03:40:24.841798-05:
      DOI: 10.1002/2016JC012527
       
  • Three-compartment structure of subsurface-intensified mesoscale eddies in
           the ocean
    • Authors: Zhengguang Zhang; Yu Zhang, Wei Wang
      Abstract: Mesoscale eddies are energetically dominant and pervasive over most of the world's oceans. Among them, many are subsurface intensified with strongest signals in the ocean interior such as mode water eddies, which trap water masses with distinctive properties and carry them over long distances. With both Argo profiling floats and atmospheric reanalysis data we showed that the structure of these eddies obeys a universal rule. Hence their three-dimensional hydrographic fields can be readily reconstructed from very limited information. More interestingly, the volume of water trapped and moved by a mode water eddy is much greater than previously thought; it has a three-compartment structure in the vertical with the mode water being sandwiched between two layers of notably different properties and accounting for only a portion of the total trapped volume. This article is protected by copyright. All rights reserved.
      PubDate: 2017-01-05T04:05:22.89151-05:0
      DOI: 10.1002/2016JC012376
       
  • Winds on the West Florida Shelf: Regional comparisons between observations
           and model estimates
    • Authors: Dennis A. Mayer; Robert H. Weisberg, Lianyuan Zheng, Yonggang Liu
      Abstract: Wind fields on the West Florida Continental Shelf are investigated using observations from five University of South Florida Coastal Ocean Monitoring and Prediction System buoys and seven of NOAA's National Ocean Service and National Weather Service, National Data Buoy Center stations or buoys spanning the 10 yr period, 2004 through 2013. These observations are compared with NOAA's National Center for Environmental Protection (NCEP) reanalysis wind fields (NCEP winds). The analyses consist of vector correlations in both the time and frequency domains. The primary findings are that winds observed on and near the coastline underestimate those observed offshore and that NCEP winds derived from assimilating mostly land-based observations also underestimate winds observed offshore. With regard to wind stress, and depending upon location, wind stress derived from NCEP winds are 6% to 49% lower than what is computed from observations over open water. A corollary is that wind forcing fields that are underestimated may result in coastal ocean model circulation fields that are also underestimated. These analyses stress the importance of having offshore wind observations, and suggest that adding more offshore wind observations will lead to improved coastal ocean wind fields and hence to improved model renditions of coastal ocean model circulation and related water property fields. This article is protected by copyright. All rights reserved.
      PubDate: 2017-01-05T04:00:26.775852-05:
      DOI: 10.1002/2016JC012112
       
  • Composition of freshwater in the spring of 2014 on the southern Labrador
           shelf and slope
    • Authors: M. Benetti; G. Reverdin, C. Lique, I. Yashayaev, N.P. Holliday, E. Tynan, S. Torres-Valdes, P. Lherminier, P. Tréguer, G. Sarthou
      Abstract: The Labrador Current is an important conduit of freshwater from the Arctic to the interior North Atlantic subpolar gyre. Here, we investigate the spatial variability of the freshwater sources over the southern Labrador shelf and slope during May-June 2014. Using measurements of seawater properties such as temperature, salinity, nutrients and oxygen isotopic composition, we estimate the respective contributions of saline water of Atlantic and Pacific origins, of brines released during sea ice formation, and of freshwater from sea ice melt and meteoric water origins. On the southern Labrador shelf, we find a large brine signal and Pacific Water influence indicating a large contribution of water from the Canadian Arctic. The brine signal implies that more than 4 m of sea ice formed upstream, either in the Arctic or in Baffin Bay and the northern Labrador Sea. Over the mid-shelf and slope at 52°N, we find a stronger influence of slope water from the West Greenland Current with a smaller contribution of Pacific water and no brine signal. Thus, there is advection of water from the slope region to the mid-shelf between 55°N and 52°N. Very fresh water with high meteoric content is found close to the coast in June 2014. Observations from 1995 and 2008 suggest a higher fraction of brine and Pacific water on the shelf compared to that observed in 2014. This article is protected by copyright. All rights reserved.
      PubDate: 2017-01-05T04:00:25.453902-05:
      DOI: 10.1002/2016JC012244
       
  • Water mass modification and mixing rates in a 1/12° simulation of the
           Canadian Arctic Archipelago
    • Authors: Kenneth G. Hughes; Jody M. Klymak, Xianmin Hu, Paul G. Myers
      Abstract: Strong spatial differences in diapycnal mixing across the Canadian Arctic Archipelago are diagnosed in a 1/12° basin-scale model. Changes in mass flux between water flowing into or out of several regions are analyzed using a volume-integrated advection–diffusion equation, and focus is given to denser water, the direct advective flux of which is mediated by sills. The unknown in the mass budget, mixing strength, is a quantity seldom explored in other studies of the Archipelago, which typically focus on fluxes. Regionally averaged diapycnal diffusivities and buoyancy fluxes are up to an order of magnitude larger in the eastern half of the Archipelago relative to those in the west. Much of the elevated mixing is concentrated near sills in Queens Channel and Barrow Strait, with stronger mixing particularly evident in the net shifts of the densest water to lower densities as it traverses these constrictions. Associated with these shifts are areally averaged buoyancy fluxes up to 10-8, m2, s-3 through the 1027 kg m-3 isopycnal surface, which lies at approximately 100 m depth. This value is similar in strength to the destabilizing buoyancy flux at the ocean surface during winter. Effective diffusivities estimated from the buoyancy fluxes can exceed 10-4 m2 s-1, but are often closer to 10-5 m2 s-1 across the Archipelago. Tidal forcing, known to modulate mixing in the Archipelago, is not included in the model. Nevertheless, mixing metrics derived from our simulation are of the same order of magnitude as the few comparable observations. This article is protected by copyright. All rights reserved.
      PubDate: 2017-01-04T03:25:21.88117-05:0
      DOI: 10.1002/2016JC012235
       
  • Satellite observation of particulate organic carbon dynamics on the
           Louisiana continental shelf
    • Authors: Chengfeng Le; John C. Lehrter, Chuanmin Hu, Hugh MacIntyre, Marcus W. Beck
      Abstract: Particulate organic carbon (POC) plays an important role in coastal carbon cycling and the formation of hypoxia. Yet, coastal POC dynamics are often poorly understood due to a lack of long term POC observations and the complexity of coastal hydrodynamic and biogeochemical processes that influence POC sources and sinks. Using field observations and satellite ocean color products, we developed a new multiple regression algorithm to estimate POC on the Louisiana Continental Shelf (LCS) from satellite observations. The algorithm had reliable performance with mean relative error (MRE) of ∼40% and root mean square error (RMSE) of ∼50% for MODIS and SeaWiFS images for POC ranging between ∼80 and ∼1200 mg m−3, and showed similar performance for a large estuary (Mobile Bay). Substantial spatio-temporal variability in the satellite-derived POC was observed on the LCS, with high POC found on the inner shelf (< 10 m depth) and lower POC on the middle (10-50 m depth) and outer shelf (50-200 m depth), and with high POC found in winter (January to March) and lower POC in summer to fall (August to October). Correlation analysis between long-term POC time series and several potential influencing factors indicated that river discharge played a dominant role in POC dynamics on the LCS, while wind and surface currents also affected POC spatial patterns on short time scales. This study adds another example where satellite data with carefully developed algorithms can greatly increase the spatial and temporal observations of important biogeochemical variables on continental shelf and estuaries. This article is protected by copyright. All rights reserved.
      PubDate: 2016-12-29T18:35:44.221314-05:
      DOI: 10.1002/2016JC012275
       
  • Importance of the Equatorial Undercurrent on the Sea Surface Salinity in
           the Eastern Equatorial Atlantic in boreal spring
    • Authors: C. Y. Da-Allada; J. Jouanno, F. Gaillard, N. Kolodziejczyk, C. Maes, N. Reul, B. Bourlès
      Abstract: The physical processes implied in the sea surface salinity (SSS) increase in the equatorial Atlantic Cold Tongue (ACT) region during boreal spring and the lag observed between boreal spring SSS maximum and sea surface temperature (SST) summer minimum are examined using mixed-layer salinity budgets computed from observations and model during the period 2010-2012. The boreal spring SSS maximum is mainly explained by an upward flux of high salinity originating from the core of the Equatorial Undercurrent (EUC) through vertical mixing and advection. The vertical mixing contribution to the mixed-layer salt budget peaks in April-May. It is controlled primarily by i) an increased zonal shear between the surface South Equatorial Current and the subsurface EUC and ii) the presence of a strong salinity stratification at the mixed-layer base from December to May. This haline stratification that is due to both high precipitations below the Inter Tropical Convergence Zone and zonal advection of low-salinity water from the Gulf of Guinea, explains largely the seasonal cycle of the vertical advection contribution to the mixed-layer salt budget. In the ACT region, the SST reaches its maximum in March/April and minimum in July/August. This SST minimum appears one month after the maximum of SSS. The 1-month lag observed between the maximum of SSS in June and the minimum of SST in July is explained by the shallowing of the EUC salinity core in June, then the weakening/erosion of the EUC in June-July which dramatically reduces the lateral subsurface supply of high saline waters. This article is protected by copyright. All rights reserved.
      PubDate: 2016-12-29T18:25:30.928408-05:
      DOI: 10.1002/2016JC012342
       
  • Multiscale Climate Emulator of Multimodal Wave Spectra: MUSCLE-spectra
    • Authors: Ana Rueda; Christie A. Hegermiller, Jose A.A. Antolinez, Paula Camus, Sean Vitousek, Peter Ruggiero, Patrick L. Barnard, Li H. Erikson, Antonio Tomás, Fernando J. Mendez
      Abstract: Characterization of multimodal directional wave spectra is important for many offshore and coastal applications, such as marine forecasting, coastal hazard assessment, and design of offshore wave energy farms and coastal structures. However, the multivariate and multiscale nature of wave climate variability makes this complex problem tractable using computationally-expensive numerical models. So far, the skill of statistical-downscaling model-based parametric (unimodal) wave conditions is limited in large ocean basins such as the Pacific. The recent availability of long-term directional spectral data from buoys and wave hindcast models allows for development of stochastic models that include multimodal sea-state parameters. This work introduces a statistical-downscaling framework based on weather types to predict multimodal wave spectra (e.g., significant wave height, mean wave period, and mean wave direction from different storm systems, including sea and swells) from large-scale atmospheric pressure fields. For each weather type, variables of interest are modeled using the categorical distribution for the sea-state type, the Generalized Extreme Value (GEV) distribution for wave height and wave period, a multivariate Gaussian copula for the interdependence between variables, and a Markov chain model for the chronology of daily weather types. We apply the model to the Southern California coast, where local seas and swells from both the Northern and Southern Hemispheres contribute to the multimodal wave spectrum. This work allows attribution of particular extreme multimodal wave events to specific atmospheric conditions, expanding knowledge of time-dependent, climate-driven offshore and coastal sea-state conditions that have a significant influence on local nearshore processes, coastal morphology, and flood hazards. This article is protected by copyright. All rights reserved.
      PubDate: 2016-12-29T18:20:35.14266-05:0
      DOI: 10.1002/2016JC011957
       
  • Reconstruction of Ocean's Interior from Observed Sea Surface Information
    • Authors: Lei Liu; Shiqiu Peng, Rui Xin Huang
      Abstract: Observational surface data are used to reconstruct the ocean's interior through the “interior + surface quasigeostrophic” (isQG) method. The input data include the satellite-derived sea surface height, satellite-derived sea surface temperature, satellite-derived or Argo-based sea surface salinity, and an estimated stratification of the region. The results show that the isQG retrieval of subsurface density anomalies is quite promising compared to Argo profile data. At ∼1000 m depth, the directions of retrieved velocity anomalies are comparable to those derived from Argo float trajectories. The reconstruction using surface density input field approximated only by SST (with constant SSS) performs less satisfactorily than that taking into account the contribution of SSS perturbations, suggesting that the observed SSS information is important for the application of the isQG method. Better reconstruction is obtained in the warm season than in the cold season, which is probably due to the stronger stratification in the warm season that confines the influence of the biases in the surface input data (especially SSS) in a shallow layer. The comparison between the performance of isQG with Argo-based SSS input and that with satellite-derived SSS input suggests that the biases in the SSS products could be a major factor that influences the isQG performance. With reduced biases in satellite-derived SSS in the future, the measurement-based isQG method is expected to achieve better reconstruction of ocean interior and thus is promising in practical application. This article is protected by copyright. All rights reserved.
      PubDate: 2016-12-29T18:20:31.47502-05:0
      DOI: 10.1002/2016JC011927
       
  • Mapping the Non-Stationary Internal Tide with Satellite Altimetry
    • Authors: Edward D. Zaron
      Abstract: Temporal variability of the internal tide has been inferred from the 23-year-long combined records of the TOPEX/Poseidon, Jason-1, and Jason-2 satellite altimeters by combining harmonic analysis with an analysis of along-track wavenumber spectra of sea-surface height (SSH). Conventional harmonic analysis is first applied to estimate and remove the stationary components of the tide at each point along the reference ground tracks. The wavenumber spectrum of the residual SSH is then computed, and the variance in a neighborhood around the wavenumber of the mode-1 baroclinic M2 tide is interpreted as the sum of noise, broadband non-tidal processes, and the non-stationary tide. At many sites a bump in the spectrum associated with the internal tide is noted, and an empirical model for the noise and non-tidal processes is used to estimate the non-stationary semidiurnal tidal variance. The results indicate a spatially inhomogeneous pattern of tidal variability. Non-stationary tides are larger than stationary tides throughout much of the Equatorial Pacific and Indian Oceans. This article is protected by copyright. All rights reserved.
      PubDate: 2016-12-29T18:20:26.400285-05:
      DOI: 10.1002/2016JC012487
       
  • Mixing efficiency in the thermocline of lakes observed from eddy
           correlation flux measurements
    • Authors: Julika Weck; Andreas Lorke
      Abstract: Vertical mixing in the thermocline of lakes is poorly understood and most of the current knowledge is based on ex situ methods like laboratory measurements and simulations. Here, we used the eddy correlation technique (EC) to directly measure oxygen and buoyancy fluxes in the thermocline of two lakes (Lake Scharmützelsee and Lake Arendsee in 2012 and 2013, respectively). Additionally, sets of temperature microstructure profiles (SCAMP) were measured during the EC deployments. We used these data to quantify the mixing efficiency as well as the turbulent diffusivity. The derived turbulent diffusivities from EC for the Prandtl number of DO were one order of magnitude higher than predicted by commonly applied parameterization, while the diffusivities for the Prandtl number of heat confirmed the parameterization. The results from EC and SCAMP showed strong differences which we attribute to the fact that SCAMP measurements reflect snapshots of the instantaneous turbulence field while EC provides a temporal average of the prevailing turbulence. Finally, we discuss problems of the EC and the inertial dissipation method in a strongly stratified environment and propose how they could be improved to resolve the full temporal variability of mixing in thermoclines. This article is protected by copyright. All rights reserved.
      PubDate: 2016-12-29T05:41:31.942552-05:
      DOI: 10.1002/2016JC012188
       
  • Tsunamis generated by long and thin granular landslides in a large flume
    • Authors: Garrett S. Miller; W. Andy Take, Ryan P. Mulligan, Scott McDougall
      Abstract: In this experimental study granular material is released down slope to investigate landslide-generated waves. Starting with a known volume and initial position of the landslide source, detailed data are obtained on the velocity and thickness of the granular flow, the shape and location of the submarine landslide deposit, the amplitude and shape of the near-field wave, the far-field wave evolution, and the wave runup elevation on a smooth impermeable slope. The experiments are performed on a 6.7 m long 30° slope on which gravity accelerates the landslides into a 2.1 m wide and 33.0 m long wave flume that terminates with a 27° runup ramp. For a fixed landslide volume of 0.34 m3, tests are conducted in a range of still water depths from 0.05-0.50 m. Observations from high-speed cameras and measurements from wave probes indicate that the granular landslide moves as a long and thin train of material, and that only a portion of the landslide (termed the ‘effective mass') is engaged in activating the leading wave. The wave behaviour is highly dependent on the water depth relative to the size of the landslide. In deeper water the near-field wave behaves as a stable solitary-like wave, while in shallower water the wave behaves as a breaking dissipative bore. Overall, the physical model observations are in good agreement with the results of existing empirical equations when the effective mass is used to predict the maximum near-field wave amplitude, the far-field amplitude and the runup of tsunamis generated by granular landslides. This article is protected by copyright. All rights reserved.
      PubDate: 2016-12-29T05:41:00.929071-05:
      DOI: 10.1002/2016JC012177
       
  • The influence of ENSO on an oceanic eddy pair in the South China Sea
    • Authors: Xiaoqing Chu; Changming Dong, Yiquan Qi
      Abstract: An eddy pair off the Vietnam coast is one of the most important features of the summertime South China Sea circulation. Its variability is of interest due to its profound impact on regional climate, ecosystems, biological processes, and fisheries. This study examines the influence of the El Niño–Southern Oscillation (ENSO), a basin-scale climatic mode, on the interannual variability of this regional eddy pair using satellite observational data and historical hydrographic measurements. Over the last three decades, the eddy pair strengthened in 1994 and 2002, and weakened in 2006, 2007, and 2008. It was absent in 1988, 1995, 1998, and 2010, coinciding with strong El Nino-to-La Nina transitions. Composite analyses showed that the strong transition events of ENSO led to radical changes in the summer monsoon, through the forcing of a unique sea surface temperature anomaly structure over the tropical Indo-Pacific basin. With weaker zonal wind, a more northward wind direction, and the disappearance of a pair of positive and negative wind stress curls, the eastward current jet turns northward along the Vietnam coast and the eddy pair disappears. This article is protected by copyright. All rights reserved.
      PubDate: 2016-12-29T05:40:26.947209-05:
      DOI: 10.1002/2016JC012642
       
  • A PV-approach for dense water formation along fronts: Application to the
           northwestern Mediterranean
    • Authors: Hervé Giordani; Cindy Lebeaupin-Brossier, Fabien Léger, Guy Caniaux
      Abstract: The mechanisms of dense water formation (σ >29.0 kg m−3) at work in the baroclinic cyclonic gyre of the North-Western Mediterranean basin are investigated through a PV-budget (PV: Potential Vorticity). The PV-budget is diagnosed from an eddy-resolving (1/36°) ocean simulation driven in surface by hourly air-sea fluxes provided by a non-hydrostatic atmospheric model at 2.5km-resolution. The PV-budget is controlled by the diabatic, frictional and advective PV-fluxes. Around the gyre the surface diabatic PV-flux dominates the PV-destruction, except along the northern branch of the North Current where the surface frictional PV-flux is strongly negative. In this region, the bathymetry stabilizes the front and maintains the current northerly in the same direction as the dominant northerly wind. This configuration leads to optimal wind-current interactions and explains the preponderance of frictional PV-destruction on diabatic PV-destruction. This mechanical forcing drives a cross-front ageostrophic circulation which subducts surface low-PV waters destroyed by wind on the dense side of the front and obducts high-PV waters from the pycnocline on the light side of the front. The horizontal PV-advections associated with the geostrophic cyclonic gyre and turbulent entrainment at the pycnocline also contribute to the PV-refueling in the frontal region. The surface non-advective PV-flux involves energy exchanges down to −1400 W m−2 in the frontal zone: this flux is 3.5 times stronger than atmospheric buoyancy flux. These energy exchanges quantify the coupling effects between the surface atmospheric forcing with the oceanic frontal structures at submesoscale. This article is protected by copyright. All rights reserved.
      PubDate: 2016-12-27T18:36:05.198883-05:
      DOI: 10.1002/2016JC012019
       
  • A new real-time tsunami detection algorithm
    • Authors: Francesco Chierici; Davide Embriaco, Luca Pignagnoli
      Abstract: Real-time tsunami detection algorithms play a key role in any Tsunami Early Warning System. We have developed a new algorithm for tsunami detection based on the real-time tide removal and real-time band-pass filtering of sea-bed pressure recordings. The algorithm greatly increases the tsunami detection probability, shortens the detection delay and enhances detection reliability with respect to the most widely used tsunami detection algorithm, while containing the computational cost. The algorithm is designed to be used also in autonomous early warning systems with a set of input parameters and procedures which can be reconfigured in real time. We have also developed a methodology based on Monte Carlo simulations to test the tsunami detection algorithms. The algorithm performance is estimated by defining and evaluating statistical parameters, namely the detection probability, the detection delay, which are functions of the tsunami amplitude and wavelength, and the occurring rate of false alarms. Pressure data sets acquired by Bottom Pressure Recorders in different locations and environmental conditions have been used in order to consider real working scenarios in the test. We also present an application of the algorithm to the tsunami event which occurred at Haida Gwaii on October 28th, 2012 using data recorded by the Bullseye underwater node of Ocean Networks Canada. The algorithm successfully ran for test purpose in year-long missions onboard abyssal observatories, deployed in the Gulf of Cadiz and in the Western Ionian Sea. This article is protected by copyright. All rights reserved.
      PubDate: 2016-12-27T05:25:26.25192-05:0
      DOI: 10.1002/2016JC012170
       
  • Significant wave heights from Sentinel-1 SAR: Validation and applications
    • Authors: J. E. Stopa; A. Mouche
      Abstract: Two empirical algorithms are developed for wave mode images measured from the synthetic aperture radar aboard Sentinel-1 A. The first method, called CWAVE_S1A, is an extension of previous efforts developed for ERS2 and the second method, called Fnn, uses the azimuth cutoff amongst other parameters to estimate significant wave heights and average wave periods without using a modulation transfer function. Neural networks are trained using co-located data generated from WAVEWATCH III and independently verified with data from altimeters and in-situ buoys. We use neural networks to relate the nonlinear relationships between the input SAR image parameters and output geophysical wave parameters. CWAVE_S1A performs well and has reduced precision compared to Fnn with Hs root mean square errors within 0.5 and 0.6 m respectively. The developed neural networks extend the SAR's ability to retrieve useful wave information under a large range of environmental conditions including extra-tropical and tropical cyclones. This article is protected by copyright. All rights reserved.
      PubDate: 2016-12-26T19:50:44.472802-05:
      DOI: 10.1002/2016JC012364
       
  • Methane and nitrous oxide distributions across the North American Arctic
           Ocean during summer, 2015
    • Authors: Lindsay Fenwick; David Capelle, Ellen Damm, Sarah Zimmermann, Bill Williams, Svein Vagle, Philippe D. Tortell
      Abstract: We collected Arctic Ocean water column samples for methane (CH4) and nitrous oxide (N2O) analysis on three separate cruises in the summer and fall of 2015, covering a ∼10,000 km transect from the Bering Sea to Baffin Bay. This provided a three-dimensional view of CH4 and N2O distributions across contrasting hydrographic environments, from the oligotrophic waters of the deep Canada Basin and Baffin Bay, to the productive shelves of the Bering and Chukchi Seas. Percent saturation relative to atmospheric equilibrium ranged from 30-800% for CH4 and 75-145% for N2O, with the highest concentrations of both gases occurring in the northern Chukchi Sea. Nitrogen cycling in the shelf sediments of the Bering and Chukchi Seas likely constituted the major source of N2O to the water column, and the resulting high N2O concentrations were transported across the Arctic Ocean in eastward-flowing water masses. Methane concentrations were more spatially heterogeneous, reflecting a variety of localized inputs, including likely sources from sedimentary methanogenesis and sea ice processes. Unlike N2O, CH4 was rapidly consumed through microbial oxidation in the water column, as shown by the 13C enrichment of CH4 with decreasing concentrations. For both CH4 and N2O, sea-air fluxes were close to neutral, indicating that our sampling region was neither a major source nor sink of these gases. Our results provide insight into the factors controlling the distribution of CH4 and N2O in the North American Arctic Ocean, and an important baseline data set against which future changes can be assessed. This article is protected by copyright. All rights reserved.
      PubDate: 2016-12-26T19:50:36.424758-05:
      DOI: 10.1002/2016JC012493
       
  • Modeling the intense 2012-2013 dense water formation event in the
           northwestern Mediterranean Sea: Evaluation with an ensemble simulation
           approach
    • Authors: Robin Waldman; Samuel Somot, Marine Herrmann, Anthony Bosse, Guy Caniaux, Claude Estournel, Loic Houpert, Louis Prieur, Florence Sevault, Pierre Testor
      Abstract: The northwestern Mediterranean Sea is a well-observed ocean deep convection site. Winter 2012-2013 was an intense and intensely documented dense water formation (DWF) event. We evaluate this DWF event in an ensemble configuration of the regional ocean model NEMOMED12. We then assess for the first time the impact of ocean intrinsic variability on DWF with a novel perturbed initial state ensemble method. Finally, we identify the main physical mechanisms driving water mass transformations.NEMOMED12 reproduces accurately the deep convection chronology between late January and March, its location off the Gulf of Lions although with a southward shift and its magnitude. It fails to reproduce the Western Mediterranean Deep Waters salinification and warming, consistently with too strong a surface heat loss.The Ocean Intrinsic Variability modulates half of the DWF area, especially in the open-sea where the bathymetry slope is low. It modulates marginally (3-5\%) the integrated DWF rate, but its increase with time suggests its impact could be larger at interannual timescales. We conclude that ensemble frameworks are necessary to evaluate accurately numerical simulations of DWF.Each phase of DWF has distinct diapycnal and thermohaline regimes: during preconditioning, the Mediterranean thermohaline circulation is driven by exchanges with the Algerian basin. During the intense mixing phase, surface heat fluxes trigger deep convection and internal mixing largely determines the resulting deep water properties. During restratification, lateral exchanges and internal mixing are enhanced. Finally, isopycnal mixing was shown to play a large role in water mass transformations during the preconditioning and restratification phases. This article is protected by copyright. All rights reserved.
      PubDate: 2016-12-26T19:50:27.646395-05:
      DOI: 10.1002/2016JC012437
       
  • Zonal evolution of Alaskan stream structure and transport quantified with
           Argo data
    • Authors: Paige D. Logan; Gregory C. Johnson
      Abstract: The Alaskan Stream (AS) flows west-southwestward along the south side of Alaska and the Aleutian Island Arc; a western boundary current at the northern edge of the North Pacific subpolar gyre. The Argo float array has improved sampling of the Gulf of Alaska, allowing quantification of the AS's zonal evolution from 140°W to 175°W. Geostrophic along-shore transport of the AS in the upper 1000 dbar referenced to an assumed level of no motion at 1000 dbar shows little change from east to west. However, along-shore absolute geostrophic transports in the top 2000 dbar (obtained by combining mean absolute 1000-dbar velocities from float displacements with the geostrophic velocity fields) generally increase to the west. We estimate full-depth transports by fitting a barotropic and the first two baroclinic modes calculated from a climatology to the absolute geostrophic velocities in the upper 2000 dbar and applying the velocities from these fits from 2000 dbar to the seafloor. Flowing west from its formation region at 140°W–145°W the full-depth AS becomes stronger, more barotropic, and also narrower once it reaches ∼160°W, with along-shore transports increasing from -16.4 ± 4.9 Sv (1 Sv = 106 m3 s−1) at 140°W to -32.6 ± 5.2 Sv at 175°W. Mean concentrations of relatively warm, salty, oxygen-poor, and nutrient-rich Pacific Equatorial Water (PEW) in the AS decrease from 17.8 ± 0.3% to 8.5 ± 0.5% between 140°W and 175°W. However, the volume transport of PEW by the AS exhibits little change over the PEW density range between these longitudes. This article is protected by copyright. All rights reserved.
      PubDate: 2016-12-26T19:45:52.931213-05:
      DOI: 10.1002/2016JC012302
       
  • Bioavailable dissolved organic matter and biological hot spots during
           austral winter in Antarctic waters
    • Authors: Yuan Shen; Ronald Benner, Alison E. Murray, Carla Gimpel, B. Greg Mitchell, Elliot L. Weiss, Christian Reiss
      Abstract: Primary production and heterotrophic bacterial activity in the Antarctic Ocean are generally low during the austral winter. Organic carbon is considered to be a major factor limiting bacterial metabolism, but few studies have investigated the bioavailability of organic matter during winter. Herein, the chemical composition and bioavailability of dissolved organic matter (DOM) were investigated in surface (5-100 m) and mesopelagic (200-750 m) waters off the northwestern Antarctic Peninsula during August 2012. Concentrations of dissolved organic carbon (DOC) were low (42±4 µmol L−1) and showed no apparent spatial patterns. By contrast, the composition of DOM exhibited significant spatial trends that reflected the varying ecosystem productivity and water masses. Surface distributions of chlorophyll-a and particulate organic carbon depicted a southward decline in primary productivity from open waters (60.0˚S-61.5˚S) to ice-covered regions (61.5˚S-62.5˚S). This trend was evident from concentrations and DOC-normalized yields of dissolved amino acids in the surface waters, indicating decreasing DOM bioavailability with increasing latitude. A different pattern of DOM bioavailability was observed in the mesopelagic water masses, where amino acids indicated highly altered DOM in the Circumpolar Deep Water and bioavailable DOM in the Transitional Weddell Water. Depth distributions of amino acid yields and compositions revealed hot spots of elevated bioavailable DOM at ∼75 m relative to surrounding waters at most ice-free stations. Relatively low mole percentages of bacterially-derived d-amino acids in hot spots were consistent with an algal source of bioavailable DOM. Overall, these results reveal spatial heterogeneity of bioavailable substrates in Antarctic waters during winter. This article is protected by copyright. All rights reserved.
      PubDate: 2016-12-26T19:45:50.326231-05:
      DOI: 10.1002/2016JC012301
       
  • Low- and high-frequency oscillatory winds synergistically enhance nutrient
           entrainment and phytoplankton at fronts
    • Authors: D. B. Whitt; M. Lévy, J. R. Taylor
      Abstract: When phytoplankton growth is limited by low nutrient concentrations, full-depth-integrated phytoplankton biomass increases in response to intermittent mixing events that bring nutrient-rich waters into the sunlit surface layer. Here, it is shown how oscillatory winds can induce intermittent nutrient entrainment events and thereby sustain more phytoplankton at fronts in nutrient-limited oceans. Low frequency (i.e. synoptic to planetary scale) along-front wind drives oscillatory cross-front Ekman transport, which induces intermittent deeper mixing layers on the less dense side of fronts. High-frequency wind with variance near the Coriolis frequency resonantly excites inertial oscillations, which also induce deeper mixing layers on the less dense side of fronts. Moreover, we show that low and high frequency winds have a synergistic effect and larger impact on the deepest mixing layers, nutrient entrainment, and phytoplankton growth on the less dense side of fronts than either high-frequency winds or low frequency winds acting alone. These theoretical results are supported by two-dimensional numerical simulations of fronts in an idealized nutrient-limited open-ocean region forced by low and high frequency along-front winds. In these model experiments, higher-amplitude low-frequency wind strongly modulates and enhances the impact of the lower-amplitude high-frequency wind on phytoplankton at a front. Moreover, sensitivity studies emphasize that the synergistic phytoplankton response to low and high frequency wind relies on the high-frequency wind just below the Coriolis frequency. This article is protected by copyright. All rights reserved.
      PubDate: 2016-12-26T19:45:47.05623-05:0
      DOI: 10.1002/2016JC012400
       
  • Factors influencing the skill of synthesized satellite wind products in
           the tropical Pacific
    • Authors: Shayne McGregor; Alex Sen Gupta, Dietmar Dommenget, Tony Lee, Michael J. McPhaden, William S. Kessler
      Abstract: Given the importance of tropical Pacific winds to global climate, it is interesting to examine differences in the mean and trend among various wind products, and their implications for ocean circulation. Past analysis has revealed that despite the assimilation of observational data, there remain large differences among reanalysis products. Thus, here we examine if satellite-based synthesis products may provide more consistent estimate than reanalysis. Reanalysis product winds are, however, typically used as a background constraint in constructing the synthesis products to fill spatiotemporal gaps and to deal with satellite wind direction ambiguity. Our study identified two important factors that influence both the mean and trends from synthesized wind products. Firstly, the choice of background wind product in synthesised satellite wind products affects the mean and long-term trends, which has implications for simulations of ocean circulation, sea level, and presumably SST. Secondly, we identify a clear need for developing a better understanding of, and correcting differences between in-situ observations of absolute winds with the satellite-derived relative winds prior to synthesizing. This correction requires careful analysis of satellite surface winds with existing co-located in-situ measurements of surface winds and currents, and will benefit from near surface current observations of the proposed Tropical Pacific Observing System. These results also illustrate the difficulty in independently evaluating the synthesis wind products because the in-situ data have been utilised at numerous steps during their development. Addressing these identified issues effectively, will require enhanced collaborations among the wind observation (both satellite and in-situ), reanalysis, and synthesis communities. This article is protected by copyright. All rights reserved.
      PubDate: 2016-12-26T19:45:43.492037-05:
      DOI: 10.1002/2016JC012340
       
  • Surface flux and ocean heat transport convergence contributions to
           seasonal and interannual variations of ocean heat content
    • Authors: C. D. Roberts; M. D. Palmer, R. P. Allan, D.G.D. Desbruyeres, P. Hyder, C. Liu, D. Smith
      Abstract: We present an observation-based heat budget analysis for seasonal and interannual variations of ocean heat content (H) in the mixed layer (Hmld) and full depth ocean (Htot). Surface heat flux and ocean heat content estimates are combined using a novel Kalman smoother-based method. Regional contributions from ocean heat transport convergences are inferred as a residual and the dominant drivers of Hmld and Htot are quantified for seasonal and interannual time scales. We find that non-Ekman ocean heat transport processes dominate Hmld variations in the equatorial oceans and regions of strong ocean currents and substantial eddy activity. In these locations, surface temperature anomalies generated by ocean dynamics result in turbulent flux anomalies that drive the overlying atmosphere. In addition, we find large regions of the Atlantic and Pacific oceans where heat transports combine with local air-sea fluxes to generate mixed layer temperature anomalies. In all locations except regions of deep convection and water mass transformation, interannual variations in Htot are dominated by the internal rearrangement of heat by ocean dynamics rather than the loss or addition of heat at the surface. Our analysis suggests that, even in extra-tropical latitudes, initialization of ocean dynamical processes could be an important source of skill for interannual predictability of Hmld and Htot. Furthermore, we expect variations in Htot (and thus thermosteric sea level) to be more predictable than near surface temperature anomalies due to the increased importance of ocean heat transport processes for full-depth heat budgets. This article is protected by copyright. All rights reserved.
      PubDate: 2016-12-26T19:45:38.7027-05:00
      DOI: 10.1002/2016JC012278
       
  • Upper-ocean thermal variability controlled by ocean dynamics in the
           Kuroshio-Oyashio Extension region
    • Authors: Gyundo Pak; Young-Hyang Park, Frederic Vivier, Romain Bourdalle-Badie, Gilles Garric, Kyung-Il Chang
      Abstract: To understand the upper-ocean thermal variability in the Kuroshio−Oyashio Extension (KOE) region, the upper 400 m heat budget in the western North Pacific is analyzed for the 1981 − 2013 period using outputs from a high-resolution (1/12°) ocean general circulation model. Winter heat storage rate on interannual to decadal time scales is mainly determined by oceanic heat advection rather than by net air-sea heat flux. The role of heat advection becomes particularly prominent and widely spread over the entire western North Pacific after the 1990 regime shift in association with the reduced variability of surface heat flux caused by weakened SST variability. The net heat flux acts to dampen temperature anomalies caused by the ocean dynamics. The ocean dynamics causing the upper-ocean heat storage rate is principally associated with the meridional shift of the Oyashio Extension front, which is significantly correlated with both the West Pacific and Pacific-North America teleconnection patterns. This article is protected by copyright. All rights reserved.
      PubDate: 2016-12-26T19:45:36.944535-05:
      DOI: 10.1002/2016JC012076
       
  • Observed mesoscale eddy signatures in Southern Ocean surface mixed-layer
           depth
    • Authors: U. Hausmann; Dennis J. McGillicuddy, John Marshall
      Abstract: Combining satellite altimetry with Argo profile data a systematic observational estimate of mesoscale eddy signatures in surface mixed-layer depth (MLD) is provided across the Southern Ocean (SO). Eddy composite MLD anomalies are shallow in cyclones, deep in anticyclones, and increase in magnitude with eddy amplitude. Their magnitudes show a pronounced seasonal modulation roughly following the depth of the climatological mixed layer. Weak eddies of the relatively quiescent SO subtropics feature peak late-winter perturbations of ±10 m. Much larger MLD perturbations occur over the vigorous eddies originating along the Antarctic Circumpolar Current (ACC) and SO western boundary current systems, with late-winter peaks of −30 m and +60 m in the average over cyclonic and anticyclonic eddy cores (a difference of ≈ 100 m). The asymmetry between modest shallow cyclonic and pronounced deep anticyclonic anomalies is systematic and not accompanied by corresponding asymmetries in eddy amplitude. Nonetheless the net deepening of the climatological SO mixed layer by this asymmetry in eddy MLD perturbations is estimated to be small (few meters). Eddies are shown to enhance SO MLD variability with peaks in late winter and eddy-intense regions. Anomalously deep late-winter mixed layers occur disproportionately within the cores of anticyclonic eddies, suggesting the mesoscale heightens the frequency of deep winter surface-mixing events along the eddy-intense regions of the SO. The eddy modulation in MLD reported here provides a pathway via which the oceanic mesoscale can impact air-sea fluxes of heat and carbon, the ventilation of water masses, and biological productivity across the SO. This article is protected by copyright. All rights reserved.
      PubDate: 2016-12-26T19:45:31.602686-05:
      DOI: 10.1002/2016JC012225
       
  • Decadal changes in salinity in the oceanic subtropical gyres
    • Authors: Bryce A. Melzer; Bulusu Subrahmanyam
      Abstract: We analyzed spatial and temporal salinity trends in five subtropical gyre regions over the past six decades using Simple Ocean Data Assimilation (SODA) reanalysis with a focus on the subsurface salinity of the upper 1000 m of the ocean. Our results indicate an overall salinity increase within the mixed layer, and a salinity decrease at depths greater than 200m in the global subtropical gyres over 61 years, of which each individual gyre was analyzed in further detail. We determine that freshwater fluxes at the air-sea interface are the primary drivers of the sea surface salinity (SSS) signature over these open ocean regions by quantifying the advective contribution within the surface layer. This was demonstrated through a mixed layer salinity budget in each subtropical gyre based on the vertically integrated advection and entrainment of salt. Our analysis of decadal variability of fluxes into and out of the gyres reveals little change in the strength of the mean currents through this region despite an increase in the annual export of salt in all subtropical gyres, with the meridional component dominating the zonal. This study reveals that the salt content of E-P maximum waters advected into the subtropical gyres is increasing over time. A combination of increasing direct evaporation over the regions with increasing remote evaporation over nearby E-P maxima is believed to be the main driver in increasing salinity of the subtropical oceans, suggesting an intensification of the global water cycle over decadal timescales. This article is protected by copyright. All rights reserved.
      PubDate: 2016-12-26T19:45:26.679908-05:
      DOI: 10.1002/2016JC012243
       
  • Regional modeling of the water masses and circulation annual variability
           at the Southern Brazilian Continental Shelf
    • Authors: L. F. Mendonça; R. B. Souza, C. R. C. Aseff, L. P. Pezzi, O. O. Möller, R. C. M. Alves
      Abstract: The Southern Brazilian Continental Shelf (SBCS) is one of the more productive areas for fisheries in Brazilian waters. The water masses and the dynamical processes of the region present a very seasonal behavior that imprint strong effects in the ecosystem and the weather of the area and its vicinity. This paper makes use of the Regional Ocean Modeling System (ROMS) for studying the water mass distribution and circulation variability in the SBCS during the year of 2012. Model outputs were compared to in situ, historical observations and to satellite data. The model was able to reproduce the main thermohaline characteristics of the waters dominating the SBCS and the adjacent region. The mixing between the Subantarctic Shelf Water and the Subtropical Shelf Water, known as the Subtropical Shelf Front (STSF), presented a clear seasonal change in volume. As a consequence of the mixing and of the seasonal oscillation of the STSF position, the stability of the water column inside the SBCS also changes seasonally. Current velocities and associated transports estimated for the Brazil Current (BC) and for the Brazilian Coastal Current (BCC) agree with previous measurements and estimates, stressing the fact that the opposite flow of the BCC occurring during winter in the study region is about 2 orders of magnitude smaller than that of the BC. Seasonal maps of simulated Mean Kinetic Energy and Eddy Kinetic Energy demonstrate the known behavior of the BC and stressed the importance of the mean coastal flow off Argentina throughout the year. This article is protected by copyright. All rights reserved.
      PubDate: 2016-12-26T19:40:25.384959-05:
      DOI: 10.1002/2016JC011780
       
  • Water Mass Modification and Mixing Rates in a 1/12° Simulation of the
           Canadian Arctic Archipelago
    • Authors: Kenneth G. Hughes; Jody M. Klymak, Xianmin Hu, Paul G. Myers
      Abstract: Strong spatial differences in diapycnal mixing across the Canadian Arctic Archipelago are diagnosed in a 1/12° basin-scale model. Changes in mass flux between water flowing into or out of several regions are analyzed using a volume-integrated advection–diffusion equation, and focus is given to denser water, the direct advective flux of which is mediated by sills. The unknown in the mass budget, mixing strength, is a quantity seldom explored in other studies of the Archipelago, which typically focus on fluxes. Regionally averaged diapycnal diffusivities and buoyancy fluxes are up to an order of magnitude larger in the eastern half of the Archipelago relative to those in the west. Much of the elevated mixing is concentrated near sills in Queens Channel and Barrow Strait, with stronger mixing particularly evident in the net shifts of the densest water to lower densities as it traverses these constrictions. Associated with these shifts are areally averaged buoyancy fluxes up to 10−8 m2 s−3 through the 1027 kg m−3 isopycnal surface, which lies at approximately 100m depth. This value is similar in strength to the destabilizing buoyancy flux at the ocean surface during winter. Effective diffusivities estimated from the buoyancy fluxes can exceed 10−4 m2 s−1, but are often closer to 10−5 m2 s−1 across the Archipelago. Tidal forcing, known to modulate mixing in the Archipelago, is not included in the model. Nevertheless, mixing metrics derived from our simulation are of the same order of magnitude as the few comparable observations. This article is protected by copyright. All rights reserved.
      PubDate: 2016-12-23T06:56:23.849476-05:
      DOI: 10.1002/2016JC012200
       
  • Nitrogen fixation in the eastern Atlantic reaches similar levels in the
           southern and northern hemisphere
    • Authors: Debany Fonseca-Batista; Frank Dehairs, Virginie Riou, François Fripiat, Marc Elskens, Florian Deman, Natacha Brion, Fabien Quéroué, Maya Bode, Holger Auel
      Abstract: Euphotic layer dinitrogen (N2) fixation and primary production (PP) were measured in the eastern Atlantic Ocean (38°N–21°S) using 15N2 and 13C bicarbonate tracer incubations. This region is influenced by Saharan dust deposition and waters with low nitrogen to phosphorus (N/P) ratios originating from the Subantarctic and the Benguela upwelling system. Depth-integrated rates of N2 fixation in the north (0–38°N) ranged from 59 to 370 µmol N m−2 d−1, with the maximal value at 19°N under the influence of the northwest African upwelling. Diazotrophic activity in the south (0–21°S), though slightly lower, was surprisingly close to observations in the north, with values ranging from 47 to 119 µmol N m−2 d−1. Our North Atlantic N2 fixation rates correlate well with dust deposition, while those in the South Atlantic correlate strongly with excess phosphate relative to nitrate. There the necessary iron is assumed to be supplied from the Benguela upwelling system. When converting N2 fixation to carbon uptake using a Redfield ratio (6.6), we find that N2 fixation may support up to 9% of PP in the subtropical North Atlantic (20–38°N), 5% in the tropical North Atlantic (0–20°N) and 1% of PP in the South Atlantic (0–21°S). Combining our data with published datasets, we estimate an annual N input of 27.6 ± 10 Tg N yr−1 over the open Atlantic Ocean, 11% of which enters the region between 20° to 50°N, 71% between 20°N and 10°S and 18% between 10 and 45°S. This article is protected by copyright. All rights reserved.
      PubDate: 2016-12-22T04:50:34.570514-05:
      DOI: 10.1002/2016JC012335
       
  • Turbulent and numerical mixing in a salt wedge estuary: Dependence on grid
           resolution, bottom roughness, and turbulence closure
    • Authors: David K. Ralston; Geoffrey W. Cowles, W. Rockwell Geyer, Rusty C. Holleman
      Abstract: The Connecticut River is a tidal salt wedge estuary, where advection of sharp salinity gradients through channel constrictions and over steeply sloping bathymetry leads to spatially heterogeneous stratification and mixing. A 3-d unstructured-grid finite-volume hydrodynamic model (FVCOM) was evaluated against shipboard and moored observations, and mixing by both the turbulent closure and numerical diffusion were calculated. Excessive numerical mixing in regions with strong velocities, sharp salinity gradients, and steep bathymetry reduced model skill for salinity. Model calibration was improved by optimizing both the bottom roughness (z0), based on comparison with the barotropic tidal propagation, and the mixing threshold in the turbulence closure (steady-state Richardson number, Rist), based on comparison with salinity. Whereas a large body of evidence supports a value of Rist ∼ 0.25, model skill for salinity improved with Rist ∼ 0.1. With Rist = 0.25, numerical mixing contributed about 1/2 the total mixing, while with Rist = 0.10 it accounted for ∼2/3, but salinity structure was more accurately reproduced. The combined contributions of numerical and turbulent mixing were quantitatively consistent with high-resolution measurements of turbulent mixing. A coarser grid had increased numerical mixing, requiring further reductions in turbulent mixing and greater bed friction to optimize skill. The optimal Rist for the fine grid case was closer to 0.25 than for the coarse grid, suggesting that additional grid refinement might correspond with Rist approaching the theoretical limit. Numerical mixing is rarely assessed in realistic models, but comparisons with high-resolution observations in this study suggest it is an important factor. This article is protected by copyright. All rights reserved.
      PubDate: 2016-12-22T04:46:07.28881-05:0
      DOI: 10.1002/2016JC011738
       
  • Extension of the prognostic model of sea surface temperature to
           rain-induced cool and fresh lenses
    • Authors: H. Bellenger; K. Drushka, W. Asher, G. Reverdin, M. Katsumata, M. Watanabe
      Abstract: The Zeng and Beljaars (2005) sea surface temperature prognostic scheme, developed to represent diurnal warming, is extended to represent rain-induced freshening and cooling. Effects of rain on salinity and temperature in the molecular skin layer (first few hundred micrometers) and the near-surface turbulent layer (first few meters) are separately parameterized by taking into account rain-induced fluxes of sensible heat and freshwater, surface stress, and mixing induced by droplets penetrating the water surface. Numerical results from this scheme are compared to observational data from two field studies of near-surface ocean stratifications caused by rain, to surface drifter observations and to previous computations with an idealized ocean mixed layer model, demonstrating that the scheme produces temperature variations consistent with in situ observations and model results. It reproduces the dependency of salinity on wind and rainfall rate and the lifetime of fresh lenses. In addition, the scheme reproduces the observed lag between temperature and salinity minimum at low wind speed and is sensitive to the peak rain rate for a given amount of rain. Finally, a first assessment of the impact of these fresh lenses on ocean surface variability is given for the near-equatorial western Pacific. In particular, the variability due to the mean rain-induced cooling is comparable to the variability due to the diurnal warming so that they both impact large-scale horizontal surface temperature gradients. The present parameterization can be used in a variety of models to study the impact of rain-induced fresh and cool lenses at different spatial and temporal scales. This article is protected by copyright. All rights reserved.
      PubDate: 2016-12-22T04:46:01.203328-05:
      DOI: 10.1002/2016JC012429
       
  • Hydrodynamic influences on acoustical and optical backscatter in a
           fringing reef environment
    • Authors: G. Pawlak; M. A. Moline, E. J. Terrill, P. L. Colin
      Abstract: Observations of hydrodynamics along with optical and acoustical water characteristics in a tropical fringing reef environment reveal a distinct signature associated with flow characteristics and tidal conditions. Flow conditions are dominated by tidal forcing with an offshore component from the reef flat during ebb. Measurements span variable wave conditions enabling identification of wave effects on optical and acoustical water properties.High frequency acoustic backscatter (6 MHz) is strongly correlated with tidal forcing increasing with offshore directed flow and modulated by wave height, indicating dominant hydrodynamic influence. Backscatter at 300 kHz and 1200 kHz is predominantly diurnal suggesting a biological component. Optical backscatter is closely correlated with high frequency acoustic backscatter across the range of study conditions. Acoustic backscatter frequency dependence is used along with changes in optical properties to interpret particle size variations. Changes across wave heights suggest shifts in particle size distributions with increases in relative concentrations of smaller particles for larger wave conditions. Establishing a connection between the physical processes of a fringing tropical reef and the resulting acoustical and optical signals allows for interpretation and forecasting of the remote sensing response of these phenomena over larger scales. This article is protected by copyright. All rights reserved.
      PubDate: 2016-12-22T04:45:56.961567-05:
      DOI: 10.1002/2016JC012497
       
  • Sea surface pCO2 and O2 dynamics in the partially ice-covered Arctic Ocean
    • Authors: Fakhrul Islam; Michael D. DeGrandpre, Cory M. Beatty, Mary-Louise Timmermans, Richard A. Krishfield, John M. Toole, Samuel R. Laney
      Abstract: Understanding the physical and biogeochemical processes that control CO2 and dissolved oxygen (DO) dynamics in the Arctic Ocean (AO) is crucial for predicting future air-sea CO2 fluxes and ocean acidification. Past studies have primarily been conducted on the AO continental shelves during low-ice periods and we lack information on gas dynamics in the deep AO basins where ice typically inhibits contact with the atmosphere. To study these gas dynamics, in situ time-series data have been collected in the Canada Basin during late summer to autumn of 2012. Partial pressure of CO2 (pCO2), DO concentration, temperature, salinity, and chlorophyll-a fluorescence (Chl-a) were measured in the upper ocean in a range of sea ice states by two drifting instrument systems. Although the two systems were on average only 222 km apart, they experienced considerably different ice cover and external forcings during the 40-50 d periods when data were collected. The pCO2 levels at both locations were well below atmospheric saturation whereas DO was almost always slightly supersaturated. Modeling results suggest that air-sea gas exchange, net community production (NCP) and horizontal gradients were the main sources of pCO2 and DO variability in the sparsely ice-covered AO. In areas more densely covered by sea ice, horizontal gradients were the dominant source of variability, with no significant NCP in the surface mixed layer. If the AO reaches equilibrium with atmospheric CO2 as ice cover continues to decrease, aragonite saturation will drop from a present mean of 1.00 ± 0.02 to 0.86 ± 0.01. This article is protected by copyright. All rights reserved.
      PubDate: 2016-12-22T04:45:47.136148-05:
      DOI: 10.1002/2016JC012162
       
  • Monitoring remote ocean waves using P-wave microseisms
    • Authors: Jennifer Neale; Nicholas Harmon, Meric Srokosz
      Abstract: Oceanic microseisms are generated by the interaction of opposing ocean waves and subsequent coupling with the seabed, so microseisms should contain information on the ocean conditions that generated them. This leads to the possibility of using seismic records as a proxy for the ocean gravity wavefield. Here we investigate the P-wave component of microseisms, which has previously been linked to areas of high wave interaction intensity in mid-ocean regions. We compare modeled P-wave microseismic sources with those observed at an array in California, and also investigate the relationship between observed sources and significant wave height. We found that the time-varying location of microseism sources in the North Pacific, mapped from beamforming and backprojection of seismic data, was accurate to ≤10° in 90% of cases. The modeled sources were found to dominate at ∼0.2 Hz which was also reflected in the seismic observations. An empirical relationship between observed beampower and modeled source power allowed sources during an independent data period to be estimated with a correlation coefficient of 0.63. Likewise, significant wave height was also estimated with a correlation coefficient of 0.63. Our findings suggest that with improvements in resolution and amplitude retrieval from beamforming, correlations up to 0.78 should be possible between observed P-wave microseisms and significant wave height in remote ocean regions. This article is protected by copyright. All rights reserved.
      PubDate: 2016-12-22T04:45:34.545346-05:
      DOI: 10.1002/2016JC012183
       
  • Interannual variability of tropical Pacific Sea level from 1993 to 2014
    • Authors: Xiaoting Zhu; Richard J. Greatbatch, Martin Claus
      Abstract: A multi-mode, linear reduced-gravity model, driven by ERA-Interim monthly mean wind stress anomalies, is used to investigate interannual variability in tropical Pacific sea level as seen in satellite altimeter data. The model output is fitted to the altimeter data along the equator, in order to derive the vertical profile for the model forcing, showing that a signature from modes higher than mode six cannot be extracted from the altimeter data. It is shown that the model has considerable skill at capturing interannual sea level variability both on and off the equator. The correlation between modelled and satellite-derived sea level data exceeds 0.8 over a wide range of longitudes along the equator and readily captures the observed ENSO events. Overall, the combination of the first, second, third and fifth modes can provide a robust estimate of the interannual sea level variability, the second mode being dominant. A remarkable feature of both the model and the altimeter data is the presence of a pivot point in the western Pacific on the equator. We show that the westward displacement of the pivot point from the centre of the basin is strongly influenced by the fact that most of the wind stress variance is found in the western part of the basin. We also show that the Sverdrup transport is not fundamental to the dynamics of the recharge/discharge mechanism in our model, although the spatial structure of the wind forcing does play a role in setting the amplitude of the “warm water volume”. This article is protected by copyright. All rights reserved.
      PubDate: 2016-12-22T04:45:27.105334-05:
      DOI: 10.1002/2016JC012347
       
  • Atlantic water in the Nordic Seas: Locally eddy-permitting ocean
           simulation in a global setup
    • Authors: Claudia Wekerle; Qiang Wang, Sergey Danilov, Vibe Schourup-Kristensen, Wilken-Jon von Appen, Thomas Jung
      Abstract: Warm and salty Atlantic Water is transported by the Norwegian Atlantic Current through the Nordic Seas. A fraction of it enters the Arctic Ocean where it contributes significantly to its heat budget. Resolving the complex circulations structure in the Nordic Seas, in particular eddies, presents a numerical challenge in ocean models. Here, we present a hindcast experiment for the years 1969–2009 with a global configuration of the Finite Element Sea-ice Ocean Model, employing high resolution in the Nordic Seas and Arctic Ocean (4.5 km). We show that substantial improvements can be achieved in the circulation structure, hydrography and eddy kinetic energy in the Nordic Seas compared with a coarse-resolution reference run. A better represented Norwegian Atlantic Front Current (NwAFC) in the high-resolution setup leads to a reduction of a strong negative temperature bias in the eastern Nordic Seas. The Atlantic Water inflow through the Iceland-Faroe Ridge is found to be very sensitive to mesh resolution, and high resolution is required to adequately represent this inflow and the downstream NwAFC. With increased mesh resolution, the simulated ocean temperature is significantly improved at Barents Sea Opening (BSO), and the Atlantic Water volume transport in Fram Strait becomes much closer to observations in terms of both magnitude and variability. By using passive tracers the origins of water masses at BSO and Fram Strait are identified. Our study also indicates that eddy-resolving meshes are required to further improve the representation of dynamical processes in the region, in particular at Fram Strait. This article is protected by copyright. All rights reserved.
      PubDate: 2016-12-21T03:30:51.601123-05:
      DOI: 10.1002/2016JC012121
       
  • The fate of terrigenous dissolved organic carbon on the Eurasian shelves
           and export to the North Atlantic
    • Authors: K. Kaiser; R. Benner, R.M.W. Amon
      Abstract: Dissolved lignin phenols, colored dissolved organic matter (CDOM) absorption and CDOM fluorescence were analyzed along cross-slope mooring locations in the Barents, Laptev and East Siberian Seas to gain a better understanding of terrigenous dissolved organic carbon (tDOC) dynamics in Arctic Shelf seas and the Arctic Ocean. A gradient of river water and tDOC was observed along the continental shelf eastward into the East Siberian Sea. Correlations of carbon-normalized yields of lignin-derived phenols supplied by Siberian rivers with river water fractions and known water residence times yielded in-situ decay constants of 0.18-0.58 yr−1. Calculations showed ∼50% of annual tDOC discharged by Siberian rivers was mineralized in rivers estuaries and on the Eurasian shelves per year indicating extensive removal of tDOC. Bioassay experiments and in-situ decay constants indicated a reactivity continuum for tDOC. CDOM parameters and acid/aldehyde ratios of vanillyl (V) and syringyl (S) lignin phenols showed biomineralization was the dominant mechanism for the removal of tDOC. Characteristic ratios of p-hydroxy (P), S, and V phenols (P/V, S/V) also identified shelf regions in the Kara Sea and regions along the Western Laptev Sea shelf where formation of Low Salinity Halocline Waters (LSHW) and Lower Halocline Water (LHW) occurred. The efficient removal of tDOC demonstrates the importance of Eurasian margins as sinks of tDOC derived from the large Siberian Rivers and confirms tDOC mineralization has a major impact on nutrients budgets, air-sea CO2 exchange and acidification in the Siberian Shelf Seas. This article is protected by copyright. All rights reserved.
      PubDate: 2016-12-21T03:16:11.328582-05:
      DOI: 10.1002/2016JC012380
       
  • A comparison of the climates of the medieval climate anomaly, little ice
           age, and current warm period reconstructed using coral records from the
           northern South China Sea
    • Authors: Wenfeng Deng; Xi Liu, Xuefei Chen, Gangjian Wei, Ti Zeng, Luhua Xie, Jian-xin Zhao
      Abstract: For the global oceans, the characteristics of high-resolution climate changes during the last millennium remain uncertain because of the limited availability of proxy data. This study reconstructs climate conditions using annually resolved coral records from the South China Sea (SCS) to provide new insights into climate change over the last millennium. The results indicate that the climate of the Medieval Climate Anomaly (MCA, AD 900–1300) was similar to that of the Current Warm Period (CWP, AD 1850–present), which contradicts previous studies. The similar warmth levels for the MCA and CWP have also been recorded in the Makassar Strait of Indonesia, which suggests that the MCA was not warmer than the CWP in the western Pacific and that this may not have been a globally uniform change. Hydrological conditions were drier/saltier during the MCA and similar to those of the CWP. The drier/saltier MCA and CWP in the western Pacific may be associated with the reduced precipitation caused by variations in the Pacific Walker Circulation. As for the Little Ice Age (LIA, AD 1550–1850), the results from this study, together with previous data from the Makassar Strait, indicate a cold and wet period compared with the CWP and the MCA in the western Pacific. The cold LIA period agrees with the timing of the Maunder sunspot minimum and is therefore associated with low solar activity. The fresher/wetter LIA in the western Pacific may have been caused by the synchronized retreat of both the East Asian Summer Monsoon and the Australian Monsoon. This article is protected by copyright. All rights reserved.
      PubDate: 2016-12-21T03:15:59.551029-05:
      DOI: 10.1002/2016JC012458
       
  • Seasonal controls of aragonite saturation states in the Gulf of Maine
    • Authors: Zhaohui Aleck Wang; Gareth L. Lawson, Cynthia H. Pilskaln, Amy E. Maas
      Abstract: The Gulf of Maine (GoME) is a shelf region especially vulnerable to ocean acidification (OA) due to natural conditions of low pH and aragonite saturation states (Ω-Ar). This study is the first to assess the major oceanic processes controlling seasonal variability of the carbonate system and its linkages with pteropod abundance in Wilkinson Basin in the GoME. Two years of seasonal sampling cruises suggest that water-column carbonate chemistry in the region undergoes a seasonal cycle, wherein the annual cycle of stratification-overturn, primary production, respiration-remineralization and mixing all play important roles, at distinct spatiotemporal scales. Surface production was tightly coupled with remineralization in the benthic nepheloid layer during high production seasons, which results in occasional aragonite undersaturation. From spring to summer, carbonate chemistry in the surface across Wilkinson Basin reflects a transition from a production-respiration balanced system to a net autotropic system. Mean water-column Ω-Ar and abundance of large thecosomatous pteropods show some correlation, although patchiness and discrete cohort reproductive success likely also influence their abundance. Overall, photosynthesis-respiration is the primary driving force controlling Ω-Ar variability during the spring-to-summer transition as well as over the seasonal cycle. However, calcium carbonate (CaCO3) dissolution appears to occur near bottom in fall and winter when bottom water Ω-Ar is generally low but slightly above 1. This is accompanied by a decrease in pteropod abundance that is consistent with previous CaCO3 flux trap measurements. The region might experience persistent subsurface aragonite undersaturation in 30-40 years under continued ocean acidification. This article is protected by copyright. All rights reserved.
      PubDate: 2016-12-20T18:15:29.583734-05:
      DOI: 10.1002/2016JC012373
       
  • Using Landsat 8 data to estimate suspended particulate matter in the
           Yellow River estuary
    • Authors: Zhongfeng Qiu; Cong Xiao, William Perrie, Deyong Sun, Shengqiang Wang, Hui Shen, Dezhou Yang, Yijun He
      Abstract: The distribution of suspended particulate matter (SPM) and its variations in estuary regions are key to promoting carbon, oxygen and nutrient cycling in coastal regions and nearby seas. This study presents SPM estimations for the Yellow River estuary from Landsat 8 Operational Land Imager (L8/OLI) data from 2013 to 2016. L8/OLI-measured remote sensing reflectance (Rrs) was cross-validated with Moderate Resolution Imaging Spectroradiometer (MODIS) measurements, and SPM concentrations calculated from the tuned retrieval model, were validated with in situ observations. The validation shows that L8/OLI can provide reasonably Rrs, which can be used to quantify SPM distributions and variations in the Yellow River estuary. Three-year averaged SPM maps show that highly turbid waters are mostly found in an ovate area surrounding the mouth of the Yellow River. The corresponding area proportion is less than 30%, with SPM concentrations greater than 100 g m−3. High variations of SPM distributions are consistent with high SPM concentrations, and vice versa. Significant difference is observed between dry and wet seasons. Higher SPM in the dry season are observed both in range and intensity compared to those of the wet season. Furthermore, multiyear averaged SPM distributions with high concentrations are mainly attributable to currents. Significant seasonal variations are mainly controlled by sediment re-suspension processes driven by wind-wave forces. Due to human interventions, seasonal variability in river runoff and sediment discharge from the Yellow River has decreased in recent years. Accordingly, seasonal variability in SPM distributions in the Yellow River estuary due to sediment discharge has decreased. This article is protected by copyright. All rights reserved.
      PubDate: 2016-12-20T18:15:27.342577-05:
      DOI: 10.1002/2016JC012412
       
  • Satellite assessment of particulate matter and phytoplankton variations in
           the Santa Barbara Channel and its surrounding waters: Role of surface
           waves
    • Authors: Fernanda Henderikx Freitas; David A. Siegel, Stéphane Maritorena, Erik Fields
      Abstract: Satellite observations of chlorophyll in coastal waters are often described in terms of changes in productivity in response to regional upwelling processes while optical backscattering coefficients are more often linked to episodic inputs of suspended sediments from storm runoff. Here we show that the surface gravity wave resuspension of sediments has a larger role in controlling backscatter than previously considered. Almost 18 years of SeaWiFS, MODIS, MERIS and VIIRS satellite imagery of the Santa Barbara Channel, California and its surrounding waters spectrally-merged with the Garver-Siegel-Maritorena bio-optical model were used to assess the controls on suspended particle distributions. Analysis revealed that chlorophyll blooms in the warmer portions of the domain occur in phase with SST minima, usually in early spring, while blooms in the cooler regions lag SST minima and occur simultaneously to the strongest equatorward winds every year, often in the summer. Tight coupling between the optical variables was seen in offshore areas, as expected for productive waters. However, values of backscatter near the coast were primarily modulated by surface waves. This relationship holds throughout all seasons and is stronger within the 100-m isobath, but often extends tens of kilometers offshore. This forcing of particle resuspension by surface waves is likely a feature ubiquitous in all coastal oceans characterized by fine sediments. The implication of surface wave processes determining suspended particle loads far beyond the surf zone has large consequences for the interpretation of satellite ocean color signals in coastal waters and potentially redefines the extent of the littoral zone. This article is protected by copyright. All rights reserved.
      PubDate: 2016-12-20T18:15:24.861734-05:
      DOI: 10.1002/2016JC012152
       
  • Observations of frazil ice formation and upward sediment transport in the
           Sea of Okhotsk: A possible mechanism of iron supply to sea ice–
    • Authors: Masato Ito; Kay I. Ohshima, Yasushi Fukamachi, Genta Mizuta, Yoshimu Kusumoto, Jun Nishioka
      Abstract: In the Sea of Okhotsk, sediment incorporation, transport and release by sea ice potentially plays important roles in the bio-related material (such as iron) cycle and ecosystem. The backscatter strength data of bottom-mounted Acoustic Doppler Current Profilers have suggested signals of frazil ice down to 30 m depth, and signals of upward sediment transport throughout the water column simultaneously in the region northeast of Sakhalin, with a water depth of ∼100 m. Such events occurred under turbulent conditions with strong winds of 10 – 20 m s−1. During such events, newly formed ice was present near the observational sites, shown by satellite microwave imagery. Sediment dispersion from the bottom occurred in association with strong currents of 1.0 – 1.5 m s−1. During these events, the mixed layer reaches near the bottom due to wind-induced stirring, inferred from the high frequency component of vertical velocity. Thus the winter time turbulent mixing brings re-suspended sediment up to near the ocean surface. This study provides the first observational evidence of a series of processes on the incorporation of sedimentary materials into sea ice: sedimentary particles are dispersed by the strong bottom current, subsequently brought up to near the surface by winter time mixing, and finally incorporated into sea ice through underwater interaction with frazil ice and/or flooding of sea ice floes. This wintertime incorporation of bottom sediment into sea ice is a possible mechanism of iron supply to sea ice which melts in spring, and releases bio-reactive iron into the ocean. This article is protected by copyright. All rights reserved.
      PubDate: 2016-12-20T18:15:21.338966-05:
      DOI: 10.1002/2016JC012198
       
  • Vertical structure of pore pressure under surface gravity waves on a
           steep, megatidal, mixed sand-gravel-cobble beach
    • Authors: Tristan B. Guest; Alex E. Hay
      Abstract: The vertical structure of surface gravity wave-induced pore pressure is investigated within the intertidal zone of a natural, steeply sloping, megatidal, mixed sand-gravel-cobble beach. Results from a coherent vertical array of buried pore pressure sensors are presented in terms of signal phase lag and attenuation as functions of oscillatory forcing frequency and burial depth. Comparison of the observations with the predictions of a theoretical poro-elastic bed response model indicates that the large observed phase lags and attenuation are attributable to interstitial trapped air. In addition to the dependence on entrapped air volume, the pore pressure phase and attenuation are shown to be sensitive to the hydraulic conductivity of the sediment, to the changing mean water depth during the tidal cycle, and to the redistribution/rearrangement of beach face material by energetic wave action during storm events. The latter result indicates that the effects on pore pressure of sediment column disturbance during instrument burial can persist for days to weeks, depending upon wave forcing conditions. Taken together, these results raise serious questions as to the practicality of using pore pressure measurements to estimate the kinematic properties of surface gravity waves on steep, mixed sand-gravel beaches. This article is protected by copyright. All rights reserved.
      PubDate: 2016-12-20T18:10:28.49583-05:0
      DOI: 10.1002/2016JC012257
       
  • The use of a wave boundary layer model in SWAN
    • Authors: Jianting Du; Rodolfo. Bolaños, Xiaoli Guo Larsén
      Abstract: A Wave Boundary Layer Model (WBLM) is implemented in the third-generation ocean wave model SWAN to improve the wind-input source function under idealized, fetch-limited condition. Accordingly, the white capping dissipation parameters are re-calibrated to fit the new wind-input source function to parametric growth curves. The performance of the new pair of wind-input and dissipation source functions is validated by numerical simulations of fetch-limited evolution of wind-driven waves. As a result, fetch-limited growth curves of significant wave height and peak frequency show close agreement with benchmark studies at all wind speeds (5 ∼ 60 ms−1) and fetches (1 ∼ 3000 km). The WBLM wind-input source function explicitly calculates the drag coefficient based on the momentum and kinetic energy conservation. The modeled drag coefficient using WBLM wind-input source function is in rather good agreement with field measurements. Thus, the new pair of wind-input and dissipation source functions not only improve the wave simulation but also have the potential of improving air-sea coupling systems by providing reliable momentum flux estimation at the air-sea interface. This article is protected by copyright. All rights reserved.
      PubDate: 2016-12-20T13:20:54.251237-05:
      DOI: 10.1002/2016JC012104
       
  • On the hydrography of Denmark Strait
    • Authors: Dana Mastropole; Robert S. Pickart, Héðinn Valdimarsson, Kjetil Våge, Kerstin Jochumsen, James Girton
      Abstract: Using 111 shipboard hydrographic sections across Denmark Strait occupied between 1990-2012, we characterize the mean conditions at the sill, quantify the water mass constituents, and describe the dominant features of the Denmark Strait Overflow Water (DSOW). The mean vertical sections of temperature, salinity, and density reveal the presence of circulation components found upstream of the sill, in particular the shelfbreak East Greenland Current (EGC) and the separated EGC. These correspond to hydrographic fronts consistent with surface-intensified southward flow. Deeper in the water column the isopycnals slope oppositely, indicative of bottom-intensified flow of DSOW. An end-member analysis indicates that the deepest part of Denmark Strait is dominated by Arctic-Origin Water with only small amounts of Atlantic-Origin Water. On the western side of the strait the overflow water is a mixture of both constituents, with a contribution from Polar Surface Water. Weakly stratified “boluses” of dense water are present in 41% of the occupations, revealing that this is a common configuration of DSOW. The bolus water is primarily Arctic-Origin Water and constitutes the densest portion of the overflow. The boluses have become warmer and saltier over the 22-year record, which can be explained by changes in end member properties and their relative contributions to bolus composition. This article is protected by copyright. All rights reserved.
      PubDate: 2016-12-20T13:20:44.897778-05:
      DOI: 10.1002/2016JC012007
       
  • Tidal flow separation at protruding beach nourishments
    • Authors: Max Radermacher; Matthieu A. de Schipper, Cilia Swinkels, Jamie H. MacMahan, Ad J.H.M. Reniers
      Abstract: In recent years, the application of large-scale beach nourishments has been discussed, with the Sand Motor in the Netherlands as the first real-world example. Such protruding beach nourishments have an impact on tidal currents, potentially leading to tidal flow separation and the generation of tidal eddies of length scales larger than the nourishment itself. The present study examines the characteristics of the tidal flow field around protruding beach nourishments under varying nourishment geometry and tidal conditions, based on extensive field observations and numerical flow simulations.Observations of the flow field around the Sand Motor, obtained with a ship-mounted current profiler and a set of fixed current profilers, show that a tidal eddy develops along the northern edge of the mega-nourishment every flood period. The eddy is generated around peak tidal flow and gradually gains size and strength, growing much larger than the cross-shore dimension of the coastline perturbation. Based on a three-week measurement period, it is shown that the intensity of the eddy modulates with the spring-neap tidal cycle.Depth-averaged tidal currents around coastline perturbations are simulated and compared to the field observations. The occurrence and behavior of tidal eddies is derived for a large set of simulations with varying nourishment size and shape. Results show that several different types of behavior exist, characterized by different combinations of the nourishment aspect ratio, the size of the nourishment relative to the tidal excursion length and the influence of bed friction. This article is protected by copyright. All rights reserved.
      PubDate: 2016-12-20T13:20:41.21706-05:0
      DOI: 10.1002/2016JC011942
       
  • A periodic freshwater patch detachment process from the block Island sound
           estuarine plume
    • Authors: Qianqian Liu; Lewis M. Rothstein, Yiyong Luo
      Abstract: Previous observations suggest periodic freshwater patches separating from the Block Island Sound (BIS) estuarine plume. In this study, the dynamics of the separation process is investigated through a series of numerical experiments using the Regional Ocean Modeling System (ROMS). In addition, we explore the seasonal variability of the freshwater patches and their response to river discharge and ambient current. The model results indicate that episodic freshwater patches are triggered by small changes in tidal currents over the spring-neap tidal cycle. The spring-neap variation in tidal currents causes significant, monthly fluctuations in turbulent mixing and vertical stratification in BIS, modulating the freshwater discharge thereby generating episodic freshwater patches that move both downstream along the southern shore of Long Island and toward Rhode Island Sound (RIS). The realistically configured model shows that the freshwater patches experience strong seasonal variability. They are largest in spring when the river discharge peaks, and smallest in summer due to the weak river discharge and a robust upstream ambient current from RIS. According to the analysis of the freshwater transport out of BIS, we conclude that such detachment occurs at tidal mixing fronts. This article is protected by copyright. All rights reserved.
      PubDate: 2016-12-20T13:20:37.919238-05:
      DOI: 10.1002/2015JC011546
       
  • Coherent mesoscale eddies in the North Atlantic subtropical gyre: 3D
           structure and transport with application to the salinity maximum
    • Authors: Angel Amores; Oleg Melnichenko, Nikolai Maximenko
      Abstract: The mean vertical structure and transport properties of mesoscale eddies are investigated in the North Atlantic subtropical gyre by combining historical records of Argo temperature/salinity profiles and satellite sea level anomaly data in the framework of the eddy tracking technique. The study area is characterized by a low eddy kinetic energy and sea surface salinity maximum. Although eddies have a relatively weak signal at surface (amplitudes around 3-7 cm), the eddy composites reveal a clear deep signal that penetrates down to at least 1200 m depth. The analysis also reveals that the vertical structure of the eddy composites is strongly affected by the background stratification. The horizontal patterns of temperature/salinity anomalies can be reconstructed by a linear combination of a monopole, related to the elevation/depression of the isopycnals in the eddy core, and a dipole, associated with the horizontal advection of the background gradient by the eddy rotation. A common feature of all the eddy composites reconstructed is the phase coherence between the eddy temperature/salinity and velocity anomalies in the upper ∼ 300 m layer, resulting in the transient eddy transports of heat and salt. As an application, a box model of the near-surface layer is used to estimate the role of mesoscale eddies in maintaining a quasi-steady-state distribution of salinity in the North Atlantic subtropical salinity maximum. The results show that mesoscale eddies are able to provide between 4 and 21% of the salt flux out of the area required to compensate for the local excess of evaporation over precipitation. This article is protected by copyright. All rights reserved.
      PubDate: 2016-12-20T13:20:36.063113-05:
      DOI: 10.1002/2016JC012256
       
  • Deep temperature variability in Drake passage
    • Authors: Yvonne L. Firing; Elaine L. McDonagh, Brian A. King, Damien G. Desbruyères
      Abstract: Observations made on 21 occupations between 1993 and 2016 of GO-SHIP line SR1b in eastern Drake Passage show an average temperature of 0.53°C deeper than 2000 dbar, with no significant trend, but substantial year-to-year variability (standard deviation 0.08°C). Using a neutral density framework to decompose the temperature variability into isopycnal displacement (heave) and isopycnal property change components shows that approximately 95% of the year-to-year variance in deep temperature is due to heave. Changes on isopycnals make a small contribution to year-to-year variability but contribute a significant trend of -1.4±0.6 m°C per year, largest for density (γn) > 28.1, south of the Polar Front (PF). The heave component is depth-coherent and results from either vertical or horizontal motions of neutral density surfaces, which trend upward and northward around the PF, downward for the densest levels in the southern section, and downward and southward in the Subantarctic Front and Southern Antarctic Circumpolar Current Front (SACCF). A proxy for the locations of the Antarctic Circumpolar Current (ACC) fronts is constructed from the repeat hydrographic data and has a strong relationship with deep ocean heat content, explaining 76% of deep temperature variance. The same frontal position proxy based on satellite altimeter-derived surface velocities explains 73% of deep temperature variance. The position of the PF plays the strongest role in this relationship between ACC fronts and deep temperature variability in Drake Passage, although much of the temperature variability in the southern half of the section can be explained by the position of the SACCF. This article is protected by copyright. All rights reserved.
      PubDate: 2016-12-20T13:20:32.809602-05:
      DOI: 10.1002/2016JC012452
       
  • Internal wave generation from tidal flow exiting a constricted opening
    • Authors: Caixia Wang; Richard Pawlowicz
      Abstract: The southern Strait of Georgia, British Columbia, often contains packets of large, near-surface internal waves. Wave crests at the leading edge of the packet, spaced a few hundred meters apart, can have a longitudinal extent of more than 10∼km. It has long been assumed that these waves are generated by tidal flow through narrow passages and channels at the Strait's southern boundaries, but no actual link has ever been made between these waves and a specific passage or generation mechanism. Here we identify the location and extent of a number of these large packets at specific times using mosaics of photogrammetrically rectified oblique air photos. Wave speeds are determined by analyzing a time-sequence of images, with water column measurements used to subtract effects of tidal advection. The location and extent of these internal waves are then compared with the predicted location and extent of hypothetical waves generated in different passages, at different stages of the tide, which are then propagated through a predicted time-varying barotropic flow field. It is found that the observed waves are most likely generated near or after the time of the peak flood tide, or peak inflow into the Strait. They are therefore inconsistent with generation mechanisms involving the release and upstream propagation of waves by the relaxation of an ebb tide. Instead they are probably associated with the nonlinear adjustment of conditions at the edge of an inflowing injection of relatively weakly stratified water. This article is protected by copyright. All rights reserved.
      PubDate: 2016-12-20T13:20:25.800621-05:
      DOI: 10.1002/2016JC011765
       
  • Submarine groundwater discharge and associated nutrient fluxes into the
           Southern Yellow Sea: A case study for semienclosed and oligotrophic
           seas-implication for green tide bloom
    • Authors: Jian'an Liu; Ni Su, Xilong Wang, Jinzhou Du
      Abstract: The biogenic elements concentrations in a coastal bay/estuary are strongly influenced not only by riverine input but also by submarine groundwater discharge (SGD) which has been identified as a typical process of land/ocean interactions in coastal zones. To assess the role of SGD in nutrient fluxes in the Southern Yellow Sea (SYS), 228Ra activities were measured in seawater collected in May 2015. Analyzing the sources and sinks of 228Ra, the flux of excess 228Ra through SGD was estimated to be (2.2 ± 1.0) ×1015 dpm yr−1. Based on the 228Ra mass balance model, we estimated the average SGD flux to be approximately (1.3 ± 0.87) ×1012 m3 yr−1 over the entire SYS, which is at least 3.3 times the estimated annual delivery from the Changjiang River into the SYS (∼1.3 × 1011 m3 yr−1). The SGD-derived biogenic elements loads (dissolved inorganic nitrogen [DIN], phosphorus [DIP] and silicon [DSi]) were estimated as (487 ± 384) × 109 mol yr−1, (2.8 ± 2.2) × 109 mol yr−1, and (313 ± 259) × 109 mol yr−1, respectively, which are approximately 18 times, 7 times and 13 times the riverine input from both mainland China and Korea. The accumulation nutrient fluxes derived by SGD may play one of the most important roles in the green tide bloom that originated from the Subei Shoal zone in the SYS. Additionally, DIN and DIP via SGD can provide the necessary amounts of nutrient for recovering nutrient concentrations to normal levels after the green tide bloom is terminated. This article is protected by copyright. All rights reserved.
      PubDate: 2016-12-17T10:23:18.341302-05:
      DOI: 10.1002/2016JC012282
       
  • On the observed synoptic signal in the Mississippi-Alabama slope flow
    • Authors: E. V. Maksimova
      Abstract: This communication discusses a strong near-barotropic current signal and its dynamics observed on the Mississippi-Alabama upper slope in the northeastern Gulf of Mexico. When the variability related to mesoscale eddies is not present or removed, the subinertial current is found to be controlled by the synoptic-scale wind fluctuations, qualitatively in agreement with coastally trapped wave theory. Specifically, the along-isobath synoptic velocity component on the Mississippi-Alabama slope is correlated with the wind stress component in the direction along the Florida peninsula. Moreover, the observed along-isobath flows on the Mississippi-Alabama slope and on the west Florida shelf are highly coherent. These relations are pronounced in wintertime but less obvious during summertime, less energetic, wind forcing. This article is protected by copyright. All rights reserved.
      PubDate: 2016-12-17T10:22:47.806588-05:
      DOI: 10.1002/2016JC012320
       
  • Improved forecasts of winter weather extremes over midlatitudes with extra
           Arctic observations
    • Authors: Kazutoshi Sato; Jun Inoue, Akira Yamazaki, Joo-Hong Kim, Marion Maturilli, Klaus Dethloff, Stephen R. Hudson, Mats A. Granskog
      Abstract: Recent cold winter extremes over Eurasia and North America have been considered to be a consequence of a warming Arctic. More accurate weather forecasts are required to reduce human and socioeconomic damages associated with severe winters. However, the sparse observing network over the Arctic brings errors in initializing a weather prediction model, which might impact accuracy of prediction results at midlatitudes. Here we show that additional Arctic radiosonde observations from the Norwegian young sea ICE cruise project 2015 drifting ice camps and existing land stations during winter improved forecast skill and reduced uncertainties of weather extremes at midlatitudes of the Northern Hemisphere. For two winter storms over East Asia and North America in February 2015, ensemble forecast experiments were performed with initial conditions taken from an ensemble atmospheric reanalysis in which the observation data were assimilated. The observations reduced errors in initial conditions in the upper troposphere over the Arctic region, yielding more precise prediction of the locations and strengths of upper troughs and surface synoptic disturbances. Errors and uncertainties of predicted upper troughs at midlatitudes would be brought with upper-level high potential vorticity (PV) intruding southward from the observed Arctic region. This is because the PV contained a “signal” of the additional Arctic observations as it moved along an isentropic surface. This suggests that a coordinated sustainable Arctic observing network would be effective not only for regional weather services but also for reducing weather risks in locations distant from the Arctic. This article is protected by copyright. All rights reserved.
      PubDate: 2016-12-13T12:45:03.629595-05:
      DOI: 10.1002/2016JC012197
       
  • Combined observations of Arctic sea ice with near-coincident colocated X,
           C, and L-band SAR satellite remote sensing and helicopter-borne
           measurements
    • Authors: A.M. Johansson; J.A. King, A.P. Doulgeris, S. Gerland, S. Singha, G. Spreen, T. Busche
      Abstract: In this study we compare co-located near-coincident X-, C- and L-band fully polarimetry SAR satellite images with helicopter-borne ice thickness measurements acquired during the Norwegian Young sea ICE 2015 (N-ICE2015) expedition in the region of the Arctic Ocean north of Svalbard in April 2015. The air-borne surveys provide near-coincident snow plus ice thickness, surface roughness data and photographs. This unique dataset allows us to investigate how the different frequencies can complement one another for sea ice studies; but also to raise awareness of limitations. X- and L-band satellite scenes were shown to be a useful complement to the standard SAR frequency for sea ice monitoring (C-band) for lead ice and young ice identification. This may be in part be due to the frequency but also the high spatial resolution of these sensors. Importantly this is true even when the scenes are outside their nominal performance range. We found a relatively low correlation between snow plus ice thickness and surface roughness. Therefore, ice thickness cannot directly be observed by SAR which has important implications for operational ice charting based on automatic segmentation. This article is protected by copyright. All rights reserved.
      PubDate: 2016-12-13T12:45:02.140353-05:
      DOI: 10.1002/2016JC012273
       
  • Liquid freshwater transport estimates from the East Greenland Current
           based on continuous measurements north of Denmark Strait
    • Authors: L. de Steur; R. S. Pickart, A. Macrander, K. Våge, B. Harden, S. Jónsson, S. Østerhus, H. Valdimarsson
      Abstract: Liquid freshwater transports of the shelfbreak East Greenland Current (EGC) and the separated EGC are determined from mooring records from the Kögur section north of Denmark Strait between August 2011 and July 2012. The 11-month mean freshwater transport (FWT), relative to a salinity of 34.8, was 65 ± 11 mSv to the south. Approximately 70% of this was associated with the shelfbreak EGC and the remaining 30% with the separated EGC. Very large southward FWT ranging from 160 mSv to 120 mSv was observed from September to mid-October 2011 and was foremost due to anomalously low upper-layer salinities. The FWT may, however, be underestimated by approximately 5 mSv due to sampling biases in the upper ocean. The FWT on the Greenland shelf was estimated using additional inshore moorings deployed from 2012-14. While the annual mean ranged from nearly zero during the first year to 18 mSv to the south during the second year, synoptically the FWT on the shelf can be significant. Furthermore, an anomalous event in autumn 2011 caused the shelfbreak EGC to reverse, leading to a large reduction in FWT. This reversed circulation was due to the passage of a large, 100 km wide anticyclone originating upstream from the shelfbreak. The late summer FWT of -131 mSv is 150% larger than earlier estimates based on sections in the late-1990s and early-2000s. This increase is likely the result of enhanced freshwater flux from the Arctic Ocean to the Nordic Seas during the early 2010s. This article is protected by copyright. All rights reserved.
      PubDate: 2016-12-01T11:35:36.520632-05:
      DOI: 10.1002/2016JC012106
       
  • Quantifying the residual volume transport through a multiple-inlet system
           in response to wind forcing: The case of the western Dutch Wadden Sea
    • Authors: Matias Duran-Matute; Theo Gerkema, Maximiliano G. Sassi
      Abstract: In multiple-inlet coastal systems like the western Dutch Wadden Sea, the tides (and their interaction with the bathymetry), the fresh water discharge, and the wind drive a residual flow through the system. In the current paper, we study the effect of the wind on the residual volume transport through the inlets and the system as a whole on both the short (one tidal period) and long (seasonal or yearly) time scales. The results are based on realistic three-dimensional baroclinic numerical simulations for the years 2009-2011. The length of the simulations (over 2000 tidal periods) allowed us to analyze a large variety of conditions and quantify the effect of wind on the residual volume transport. We found that each inlet has an anisotropic response to wind; i.e. the residual volume transport is much more sensitive to the wind from two inherent preferential directions than from any other directions. We quantify the effects of wind on the residual volume transport through the system and introduce the concept of the system's conductance for such wind driven residual transport. For the western Dutch Wadden Sea, the dominant wind direction in the region is close to the direction with the highest conductance and opposes the tidally driven residual volume transport. This translates a large variability of the residual volume transport and a dominance of the wind in its long-term characteristics in spite of the episodic nature of storms. This article is protected by copyright. All rights reserved.
      PubDate: 2016-11-28T14:36:06.232328-05:
      DOI: 10.1002/2016JC011807
       
  • Tropical Cyclone asymmetry - development and evaluation of a new
           parametric model
    • Authors: M. Olfateh; David P. Callaghan, Peter Nielsen, Tom E. Baldock
      Abstract: A new parametric model is developed to describe the asymmetry commonly observed in Tropical Cyclones or Hurricanes. Observations from 21 Hurricanes from the Gulf of Mexico basin and TC Roger in the Coral Sea are analysed to determine the azimuthal and radial asymmetry typical in these mesoscale systems. On the basis of the observations a new asymmetric directional wind model is proposed which adjusts the widely used Holland (1980) axisymmetric wind model to account for the action of blocking high pressure systems, boundary layer friction and forward speed. The model is tested against the observations and demonstrated to capture the physical features of asymmetric cyclones and provides a better fit to observed winds than the Holland model. Optimum values and distributions of the model parameters are derived for use in statistical modelling. Finally, the model is used to investigation of the asymmetric character of TC systems, including the azimuth of the maximum wind speed, the degree of asymmetry and the re-lationship between asymmetry and forward speed. This article is protected by copyright. All rights reserved.
      PubDate: 2016-11-28T14:36:02.040704-05:
      DOI: 10.1002/2016JC012237
       
  • Large-eddy simulation of wave-breaking induced turbulent coherent
           structures and suspended sediment transport on a barred beach
    • Authors: Zheyu Zhou; Tian-Jian Hsu, Daniel Cox, Xiaofeng Liu
      Abstract: To understand the interaction between wave-breaking induced turbulent coherent structures and suspended sediment transport, we report a Large-Eddy Simulation (LES) study of wave breaking processes over a near-prototype scale barred beach. The numerical model is implemented using the open-source CFD toolbox, OpenFOAM®, in which the incompressible three-dimensional filtered Navier-Stokes equations for the water and air phases are solved with a finite volume scheme. A Volume of Fluid (VOF) method is used to capture the evolution of the water-air interface. The numerical model is validated with measured free surface elevation, turbulence averaged flow velocity, turbulent intensity, and for the first time, the intermittency of breaking wave turbulence. Simulation results confirm that as the obliquely descending eddies (ODEs) approach the bottom, significant bottom shear stress is generated. Remarkably, the collapse of ODEs onto the bed can also cause drastic spatial and temporal changes of dynamic pressure on the bottom. By allowing sediment to be suspended from the bar crest, intermittently high sediment suspension events and their correlation with high turbulence and/or high bottom shear stress events are investigated. The simulated intermittency of sediment suspension is similar to previous field and large wave flume observations. Coherent suspension events account for only 10% of the record but account for about 50% of the sediment load. Model results suggest that about 60∼70% of coherent bottom stress events are associated with surface-generated turbulence. Nearly all the coherent sand suspension events are associated with coherent turbulence events due to wave-breaking turbulence approaching the bed. This article is protected by copyright. All rights reserved.
      PubDate: 2016-11-28T14:35:58.920394-05:
      DOI: 10.1002/2016JC011884
       
  • The ocean mixed-layer under Southern Ocean sea-ice: Seasonal cycle and
           forcing
    • Authors: Violaine Pellichero; Jean-Baptiste Sallée, Sunke Schmidtko, Fabien Roquet, Jean-Benoît Charrassin
      Abstract: The oceanic mixed-layer is the gateway for the exchanges between the atmosphere and the ocean; in this layer all hydrographic ocean properties are set for months to millennia. A vast area of the Southern Ocean is seasonally capped by sea-ice, which alters the characteristics of the ocean mixed-layer. The interaction between the ocean mixed-layer and sea-ice plays a key role for water-mass transformation, the carbon cycle, sea-ice dynamics, and ultimately for the climate as a whole. However, the structure and characteristics of the under-ice mixed-layer are poorly understood due to the sparseness of in-situ observations and measurements. In this study, we combine distinct sources of observations to overcome this lack in our understanding of the Polar Regions. Working with Elephant Seal-derived observations, ship-based and Argo float observations, we describe the seasonal cycle of the ocean mixed-layer characteristics and stability of the ocean mixed-layer over the Southern Ocean and specifically under sea-ice. Mixed-layer heat and freshwater budgets are used to investigate the main forcing mechanisms of the mixed-layer seasonal cycle. The seasonal variability of sea surface salinity and temperature are primarily driven by surface processes, dominated by sea-ice freshwater flux for the salt budget, and by air-sea flux for the heat budget. Ekman advection, vertical diffusivity and vertical entrainment play only secondary roles.Our results suggest that changes in regional sea-ice distribution and annual duration, as currently observed, widely affect the buoyancy budget of the underlying mixed-layer, and impact large-scale water-mass formation and transformation with far reaching consequences for ocean ventilation. This article is protected by copyright. All rights reserved.
      PubDate: 2016-11-28T14:35:55.289042-05:
      DOI: 10.1002/2016JC011970
       
  • Bottom-slope-induced net sheet-flow sediment transport rate under
           sinusoidal oscillatory flows
    • Authors: Jing Yuan; Zhiwei Li, O. S. Madsen
      Abstract: It is generally believed that the slope of beaches can lead to a net downslope (usually offshore) sediment transport rate under shoaling waves, but very few high-quality measurements have been reported for a quantitative understanding of this phenomenon. In this study, full-scale (1:1) experiments of bottom-slope-induced net sheet-flow sediment transport rate under sinusoidal oscillatory flows are conducted using a tilting oscillatory water tunnel. The tests cover a variety of flow-sediment conditions on bottom slopes up to 2.6°. A laser-based bottom profiler system is developed for measuring net transport rate based on the principle of mass conservation. Experimental results suggest that for a given flow-sediment condition the net transport rate is in the downslope direction and increases linearly with bottom slope. A conceptual model is presented based on the idea that gravity helps bottom shear stress drive bedload transport and consequently enhances (reduces) bedload transport and suspension when the flow is in the downslope (up-slope) direction. The model predicts both the measured net sediment transport rates and the experimental linear relationship between net transport rates and bottom slope with an accuracy generally better than a factor of 2. Some measured net transport rates in this study are comparable to those due to flow skewness obtained in similar sheet-flow studies, despite that our maximum slope could be milder than the actual bottom slope in surf zones, where sheet-flow conditions usually occur. This shows that the slope effect may be as important as wave nonlinearity in producing net cross-shore sheet-flow sediment transport. This article is protected by copyright. All rights reserved.
      PubDate: 2016-11-28T14:35:52.278979-05:
      DOI: 10.1002/2016JC011996
       
  • Gulf of Maine salinity variation and its correlation with upstream Scotian
           Shelf currents at seasonal and interannual time scales
    • Authors: Hui Feng; Doug Vandemark, John Wilkin
      Abstract: In the Gulf of Maine (GoM), a network of buoy hydrography measurements collected since 2001 provide a subsurface salinity time series showing a strong seasonal cycle and interannual variations that are both consistent with remote forcing of Gulf hydrography by upstream advection. These long-term mooring data are combined with satellite altimeter estimates of upper ocean current anomaly on the adjoining Scotian Shelf (SS) in a new attempt to use disparate regional observations as proxies to detect and evaluate remote forcing of water mass change inside the Gulf from 2002-2015. Focusing on buoys moored along the Maine coastal current (MCC), lagged cross correlations with upstream altimeter-derived SS current anomalies are found to be as high as 0.84 and explain 50-70% of variance in the MCC subsurface salinity data at both seasonal and interannual time scales. Significant MCC freshening in 2004-2005 and 2010-2011 follow SS velocity strengthening, while salting events in 2002-2004 and 2012-2015 are associated with relaxation of SS currents. Estimated time lags translate to advective SS inflow velocity estimates of 6±2 cm/s that are consistent with past modeling and observational work. Investigation of wind stress control on SS velocity anomalies indicates that wind directions away from the along-shore can factor into flow modulation. Overall, the study findings are consistent with past freshwater flux observations and modeling examining southwest SS inflow to the GoM, provide a new empirical means to diagnose GoM hydrographic change, and point to one potential application of an altimeter measurement record that extends from 1992 into the future. This article is protected by copyright. All rights reserved.
      PubDate: 2016-11-28T14:35:50.974821-05:
      DOI: 10.1002/2016JC012337
       
  • On the generation and evolution of internal solitary waves in the southern
           Red Sea
    • Authors: Daquan Guo; T. R. Akylas, Peng Zhan, Aditya Kartadikaria, Ibrahim Hoteit
      Abstract: Satellite observations recently revealed trains of internal solitary waves (ISWs) in the off-shelf region between 16.0°N and 16.5°N in the southern Red Sea. The generation mechanism of these waves is not entirely clear, though, as the observed generation sites are far away (50 km) from the shelf break and tidal currents are considered relatively weak in the Red Sea. Upon closer examination of the tide properties in the Red Sea and the unique geometry of the basin, it is argued that the steep bathymetry and a relatively strong tidal current in the southern Red Sea provide favorable conditions for the generation of ISWs. To test this hypothesis and further explore the evolution of ISWs in the basin, 2D numerical simulations with the non-hydrostatic MIT general circulation model (MITgcm) were conducted. The results are consistent with the satellite observations in regard to the generation sites, peak amplitudes and the speeds of first-mode ISWs. Moreover, our simulations suggest that the generation process of ISWs in the southern Red Sea is similar to the tide-topography interaction mechanism seen in the South China Sea. Specifically, instead of ISWs arising in the immediate vicinity of the shelf break via a hydraulic lee wave mechanism, a broad, energetic internal tide is first generated, which subsequently travels away from the shelf break and eventually breaks down into ISWs. Sensitivity runs suggest that ISW generation may also be possible under summer stratification conditions, characterized by an intermediate water intrusion from the strait of Bab el Mandeb. This article is protected by copyright. All rights reserved.
      PubDate: 2016-11-28T14:35:49.785171-05:
      DOI: 10.1002/2016JC012221
       
  • Role of wind in erosion–accretion cycles on an estuarine mudflat
    • Authors: B.W. Shi; S.L. Yang, Y.P. Wang, G.C. Li, M.L. Li, P. Li, C. Li
      Abstract: Wind is an important regulator of coastal erosion and accretion processes that have significant ecological and engineering implications. Nevertheless, previous studies have mainly focused on storm−generated changes in the bed level. This paper aims to improve the understanding of wind−induced erosion–accretion cycles on intertidal flats under normal (non−stormy) weather conditions using data that relates to the wave climate, near−bed 3D flow velocity, suspended sediment concentration, and bed−level changes on a mudflat at the Yangtze Delta front. The following parameters were calculated at 10−minute intervals over 10 days: the wind wave orbital velocity (Ûδ), bed shear stress from combined current–wave action, erosion flux, deposition flux, and predicted bed−level change. The time series of measured and predicted bed−level changes both show tidal cycles and a 10−day cycle. We attribute the tidal cycles of bed−level changes to tidal dynamics, but we attribute the 10−day cycle of bed−level changes to the interaction between wind speed/direction and neap−spring cyclicity. We conclude that winds can significantly affect bed−level changes in mudflats even during non−stormy weather and under macro−mesotidal conditions and that the bed−level changes can be predicted well using current–wave–sediment combined models. This article is protected by copyright. All rights reserved.
      PubDate: 2016-11-28T14:35:48.454254-05:
      DOI: 10.1002/2016JC011902
       
  • Effect of tides and source location on nearshore tsunami-induced currents
    • Authors: Aykut Ayca; Patrick J. Lynett
      Abstract: Here, we present the results of a numerical modeling study that investigates how event-maximum tsunami-induced currents vary due to the dynamic effects of tides and wave directivity. First, analyses of tide-tsunami interaction are presented in three harbors by coupling the tsunami with the tide, and allowing the initial tsunami wave to arrive at various tidal phases. We find that tsunami-tide interaction can change the event-maximum current speed experienced in a harbor by up to 25% for the events and harbors studied, and we note that this effect is highly site-specific. Second, to evaluate the effect of wave directionality on event-maximum currents, earthquakes sources were placed throughout the Pacific, with magnitudes tuned to create the same maximum near-coast amplitude at the harbor of study. Our analysis also shows that, for the harbor and sources examined, the effect of offshore directionality and tsunami frequency content has a weak effect on the event-maximum currents experienced in the harbor. The more important dependency of event-maximum currents is the near-harbor amplitude of the wave, indicating that event-maximum currents in a harbor from a tsunami generated by a large far-field earthquake may be reasonably well predicted with only information about the predicted local maximum tsunami amplitude. This study was motivated by the hope of constructing a basis for understanding the dynamic effects of tides and wave directivity on current-based tsunami hazards in a coastal zone. The consideration of these aspects is crucial and yet challenging in the modeling of tsunami currents. This article is protected by copyright. All rights reserved.
      PubDate: 2016-11-28T14:35:43.818112-05:
      DOI: 10.1002/2016JC012435
       
  • The impact of glacier meltwater on the underwater noise field in a glacial
           bay
    • Authors: Oskar Glowacki; Mateusz Moskalik, Grant B. Deane
      Abstract: Ambient noise oceanography is proving to be an efficient and effective tool for the study of ice-ocean interactions in the bays of marine-terminating glaciers. However, obtaining quantitative estimates of ice melting or calving processes from ambient noise requires an understanding of how sound propagation through the bay attenuates and filters the noise spectrum. Measurements of the vertical structure in sound speed in the vicinity of the Hans Glacier in Hornsund Fjord, Spitsbergen, made with O(130) CTD casts between May and November 2015, reveal high-gradient, upward-refracting sound speed profiles created by cold, fresh meltwater during summer months. Simultaneous recordings of underwater ambient noise made at depths of 1, 10 and 20 meters in combination with propagation model calculations using the model Bellhop illustrate the dominant role these surface ducts play in shaping the underwater soundscape. The surface ducts lead to a higher intensity and greater variability of acoustic energy in the near-surface layer covered by glacially-modified waters relative to deeper waters, indicating deeper zones as most appropriate for inter-seasonal acoustic monitoring of the glacial melt. Surface waveguides in Hornsund are relatively shallow and trap sound above O(1 kHz). Deeper waveguides observed elsewhere will also trap low-frequency sounds, such as those generated by calving events for example. Finally, the ambient noise field in Hornsund is shown to be strongly dependent on the distribution of ice throughout the bay, stressing the importance of performing complementary environmental measurements when interpreting the results of acoustic surveys. This article is protected by copyright. All rights reserved.
      PubDate: 2016-11-28T14:35:40.161873-05:
      DOI: 10.1002/2016JC012355
       
  • Factors controlling enhanced N2O concentrations over the southwestern
           Indian shelf
    • Authors: V. Sudheesh; G.V.M. Gupta, K.V. Sudharma, H. Naik, D.M. Shenoy, M. Sudhakar, S.W.A. Naqvi
      Abstract: Repeat measurements of dissolved nitrous oxide (N2O) along two transects of the western continental shelf of India in 2012 revealed high concentrations of 45±32 nM (off Kochi) and 73±63 nM (off Mangalore) during the summer monsoon (SM). N2O concentrations increased non-linearly during the peak of the SM upwelling, when low O2 (
      PubDate: 2016-11-28T14:35:38.617189-05:
      DOI: 10.1002/2016JC012166
       
  • Temporal variability of diapycnal mixing in the northern South China Sea
    • Authors: Hui Sun; Qingxuan Yang, Wei Zhao, Xinfeng Liang, Jiwei Tian
      Abstract: Temporal variability of diapycnal mixing over seven months in the northern South China Sea was examined based on McLane Moored Profiler observations from 850 to 2,200 m by employing a finescale parameterization. Intensified diffusivity exceeding the order of 10−3 m2/s in magnitude was found over the first half of Oct 2014, and from 2 Dec 2014 to 21 Jan 2015 (a typical wintertime). Strong internal tides and winds in winter were the likely candidates for the high-level diapycnal mixing in winter. As for the enhanced mixing during Oct 2014, we suspect the generation of near-bottom near-inertial waves through the interaction of mesoscale eddies and unique bottom topography was the cause. This article is protected by copyright. All rights reserved.
      PubDate: 2016-11-28T14:35:34.842422-05:
      DOI: 10.1002/2016JC012044
       
  • Hydrographic responses to regional covariates across the Kara Sea
    • Authors: Jussi Mäkinen; Jarno Vanhatalo
      Abstract: The Kara Sea is a shelf sea in the Arctic Ocean which has a strong spatiotemporal hydrographic variation driven by river discharge, air pressure and sea ice. There is a lack of information about the effects of environmental variables on surface hydrography in different regions of the Kara Sea. We use a hierarchical spatially varying coefficient model to study the variation of sea surface temperature (SST) and salinity (SSS) in the Kara Sea between years 1980 and 2000. The model allows us to study the effects of climatic (Arctic oscillation index, AO) and seasonal (river discharge and ice concentration) environmental covariates on hydrography. The hydrographic responses to covariates vary considerably between different regions of the Kara Sea. River discharge decreases SSS in the shallow shelf area and has a neutral effect in the northern Kara Sea. The responses of SST and SSS to AO show the effects of different wind and air pressure conditions on water circulation and hence on hydrography. Ice concentration has a constant effect across the Kara Sea. We estimated the average SST and SSS in the Kara Sea in 1980-2000. The average August SST over the Kara Sea in 1995-2000 was higher than the respective average in 1980-1984 with 99.9% probability and August SSS decreased with 77% probability between these time periods. We found a support that the winter season AO has an impact on the summer season hydrography, and temporal trends may be related to the varying level of winter season AO index. This article is protected by copyright. All rights reserved.
      PubDate: 2016-11-28T14:35:33.545373-05:
      DOI: 10.1002/2016JC011981
       
  • Turbulence-resolving, two-phase flow simulations of wave-supported gravity
           flows: A conceptual study
    • Authors: Celalettin Emre Ozdemir
      Abstract: Discoveries over the last three decades have shown that wave-supported gravity flows (WSGFs) are among the participating physical processes that carry substantial amount of fine sediments across low-gradient shelves. Therefore, understanding the full range of mechanisms responsible for such gravity flows is likely to shed light on the dynamics of subaqueous delta and clinoform development. As wave-induced boundary layer turbulence is the major agent to suspend sediments in WSGFs, the scale of WSGFs in the water column is also bounded by the wave-induced boundary layer thickness which is on the order of decimeters. Therefore, in order to explore the details of participating physical mechanisms, especially that due to turbulence-sediment interaction, highly resolved and accurate numerical models or measurements in the laboratory and the field are required. In this study, the dynamics of WSGFs is investigated by using turbulence-resolving two-phase flow simulations that utilize Direct Numerical Simulations (DNS). The effect of variable sediment loading, slope, and wave orbital velocity is investigated via 21 simulations. This article is protected by copyright. All rights reserved.
      PubDate: 2016-11-28T14:35:27.589828-05:
      DOI: 10.1002/2016JC012061
       
  • Formation of summer phytoplankton bloom in the northwestern Bay of Bengal
           in a coupled physical-ecosystem model
    • Authors: V. Thushara; P. N. Vinayachandran
      Abstract: The Bay of Bengal (BoB) is considered to be a region of low biological productivity, owing to nutrient limitation, caused by strong salinity stratification induced by the freshwater influx from rivers and precipitation. Satellite and in situ observations, however, reveal the presence of prominent regional blooms in the bay in response to monsoonal forcings. Bloom dynamics of the BoB are presumably determined by freshwater as well as the local and remote effect of winds and remain to be explored in detail. Using a coupled physical-ecosystem model, we have examined the oceanic processes controlling productivity in the northwestern BoB during the summer monsoon. The region exhibits a prominent bloom lasting for a period of about two months, supporting major fishing zones along the northeast coast of India. The ecosystem model simulates the spatial and temporal evolution of the surface bloom in good agreement with Sea-Viewing Wide Field-of-View Sensor (SeaWiFS) observations. Vertical distribution of upper ocean physical and biological tracers and a nitrate budget analysis reveal the dominant role of coastal upwelling induced by alongshore winds in triggering the bloom. Horizontal advection plays a secondary role by supplying nutrients from coastal to offshore regions. The bloom decays with the weakening of winds and upwelling by the end of summer monsoon. The simulated bloom in the northwestern bay remains largely unaffected by the freshwater effects, since the peak bloom occur before the arrival of river plumes. This article is protected by copyright. All rights reserved.
      PubDate: 2016-11-28T14:35:23.407116-05:
      DOI: 10.1002/2016JC011987
       
  • Seasonal variation of the Beaufort shelfbreak jet and its relationship to
           Arctic cetacean occurrence
    • Authors: Peigen Lin; Robert S. Pickart, Kathleen M. Stafford, G.W.K. Moore, Daniel J. Torres, Frank Bahr, Jianyu Hu
      Abstract: Using mooring timeseries from September 2008 to August 2012, together with ancillary atmospheric and satellite data sets, we quantify the seasonal variations of the shelfbreak jet in the Alaskan Beaufort Sea and explore connections to the occurrences of bowhead and beluga whales. Wind patterns during the four-year study period are different than the long-term climatological conditions in that the springtime peak in easterly winds shifted from May to June, and the autumn peak was limited to October instead of extending farther into the fall. These changes were primarily due to the behavior of the two regional atmospheric centers of action, the Aleutian Low and Beaufort High. The volume transport of the shelfbreak jet, which peaks in the summer, was decomposed into a background (weak wind) component and a wind-driven component. The wind-driven component is correlated to the Pt. Barrow, AK alongcoast windspeed record, although a more accurate prediction is obtained when considering the ice thickness at the mooring site. An upwelling index reveals that wind-driven upwelling is enhanced in June and October when storms are stronger and longer-lasting. The seasonal variation of Arctic cetacean occurrence is dominated by the eastward migration in spring, dictated by pack-ice patterns, and westward migration in fall, coincident with the autumn peak in shelfbreak upwelling intensity. This article is protected by copyright. All rights reserved.
      PubDate: 2016-11-16T16:06:01.205411-05:
      DOI: 10.1002/2016JC011890
       
  • Sea ice algae chlorophyll a concentrations derived from under-ice spectral
           radiation profiling platforms
    • Authors: Benjamin A. Lange; Christian Katlein, Marcel Nicolaus, Ilka Peeken, Hauke Flores
      Abstract: Multi-scale sea ice algae observations are fundamentally important for projecting changes to sea ice ecosystems, as the physical environment continues to change. In this study we developed upon previously established methodologies for deriving sea ice-algal chlorophyll a concentrations (chl a) from spectral radiation measurements, and applied these to larger-scale spectral surveys. We conducted four different under-ice spectral measurements: irradiance; radiance; transmittance; and transflectance, and applied three statistical approaches: Empirical Orthogonal Functions (EOF); Normalized Difference Indices (NDI); and multi-NDI. We developed models based on ice core chl a and coincident spectral irradiance/transmittance (N=49) and radiance/transflectance (N=50) measurements conducted during two cruises to the central Arctic Ocean in 2011 and 2012. These reference models were ranked based on two criteria: mean robustness R2; and true prediction error estimates. For estimating the biomass of a large scale dataset, the EOF approach performed better than the NDI, due to its ability to account for the high variability of environmental properties experienced over large areas. Based on robustness and true prediction error, the three most reliable models, EOF-transmittance, EOF-transflectance and NDI-transmittance, were applied to two remotely operated vehicle (ROV) and two Surface and Under-Ice Trawl (SUIT) spectral radiation surveys. In these larger-scale chl a estimates, EOF-transmittance showed the best fit to ice core chl a. Application of our most reliable model, EOF-transmittance, to an 85 m horizontal ROV transect revealed large differences compared to published biomass estimates from the same site with important implications for projections of Arctic-wide ice-algal biomass and primary production. This article is protected by copyright. All rights reserved.
      PubDate: 2016-11-16T16:05:54.138874-05:
      DOI: 10.1002/2016JC011991
       
  • Net primary productivity estimates and environmental variables in the
           Arctic Ocean: An assessment of coupled physical-biogeochemical models
    • Authors: Younjoo J. Lee; Patricia A. Matrai, Marjorie A. M. Friedrichs, Vincent S. Saba, Olivier Aumont, Marcel Babin, Erik T. Buitenhuis, Matthieu Chevallier, Lee de Mora, Morgane Dessert, John P. Dunne, Ingrid Ellingsen, Doron Feldman, Robert Frouin, Marion Gehlen, Thomas Gorgues, Tatiana Ilyina, Meibing Jin, Jasmin G. John, Jonathan Lawrence, Manfredi Manizza, Christophe Eugène Menkes, Coralie Perruche, Vincent Le Fouest, Ekaterina Popova, Anastasia Romanou, Annette Samuelsen, Jörg Schwinger, Roland Séférian, Charles A. Stock, Jerry Tjiputra, L. Bruno Tremblay, Kyozo Ueyoshi, Marcello Vichi, Andrew Yool, Jinlun Zhang
      Abstract: The relative skill of 21 regional and global biogeochemical models was assessed in terms of how well the models reproduced observed net primary productivity (NPP) and environmental variables such as nitrate concentration (NO3), mixed layer depth (MLD), euphotic layer depth (Zeu), and sea ice concentration, by comparing results against a newly updated, quality-controlled in situ NPP database for the Arctic Ocean (1959-2011). The models broadly captured the spatial features of integrated NPP (iNPP) on a pan-Arctic scale. Most models underestimated iNPP by varying degrees in spite of overestimating surface NO3, MLD, and Zeu throughout the regions. Among the models, iNPP exhibited little difference over sea ice condition (ice-free vs. ice-influenced) and bottom depth (shelf vs. deep ocean). The models performed relatively well for the most recent decade and towards the end of Arctic summer. In the Barents and Greenland Seas, regional model skill of surface NO3 was best associated with how well MLD was reproduced. . Regionally, iNPP was relatively well simulated in the Beaufort Sea and the central Arctic Basin, where in situ NPP is low and nutrients are mostly depleted. Models performed less well at simulating iNPP in the Greenland and Chukchi Seas, despite the higher model skill in MLD and sea ice concentration, respectively. iNPP model skill was constrained by different factors in different Arctic Ocean regions. Our study suggests that better parameterization of biological and ecological microbial rates (phytoplankton growth and zooplankton grazing) are needed for improved Arctic Ocean biogeochemical modeling. This article is protected by copyright. All rights reserved.
      PubDate: 2016-11-14T14:18:16.980562-05:
      DOI: 10.1002/2016JC011993
       
  • Seasonal and interannual variability of the Arctic sea ice: A comparison
           between AO-FVCOM and observations
    • Authors: Yu Zhang; Changsheng Chen, Robert C. Beardsley, Guoping Gao, Jianhua Qi, Huichan Lin
      Abstract: A high-resolution (up to 2 km), unstructured-grid, fully ice-sea coupled Arctic Ocean Finite-Volume Community Ocean Model (AO-FVCOM) was used to simulate the sea ice in the Arctic over the period 1978-2014. The spatial-varying horizontal model resolution was designed to better resolve both topographic and baroclinic dynamics scales over the Arctic slope and narrow straits. The model-simulated sea ice was in good agreement with available observed sea ice extent, concentration, drift velocity and thickness, not only in seasonal and interannual variability but also in spatial distribution. Compared with six other Arctic Ocean models (ECCO2, GSFC, INMOM, ORCA, NAME, and UW), the AO-FVCOM-simulated ice thickness showed a higher mean correlation coefficient of ∼0.63 and a smaller residual with observations. Model-produced ice drift speed and direction errors varied with wind speed: the speed and direction errors increased and decreased as the wind speed increased, respectively. Efforts were made to examine the influences of parameterizations of air-ice external and ice-water interfacial stresses on the model-produced bias. The ice drift direction was more sensitive to air-ice drag coefficients and turning angles than the ice drift speed. Increasing or decreasing either 10% in water-ice drag coefficient or 10° in water-ice turning angle did not show a significant influence on the ice drift velocity simulation results although the sea ice drift speed was more sensitive to these two parameters than the sea ice drift direction. Using the COARE 4.0 derived parameterization of air-water drag coefficient for wind stress did not significantly influence the ice drift velocity simulation. This article is protected by copyright. All rights reserved.
      PubDate: 2016-11-11T17:04:26.786954-05:
      DOI: 10.1002/2016JC011841
       
  • Seasonal cycle of near-bottom transport and currents in the northern Gulf
           of California
    • Authors: R. Navarro; M. López, J. Candela
      Abstract: Seasonal cycles of near-bottom transport and temperature over the sills of the Northern Gulf of California, as well as surface geostrophic velocity anomalies, are presented. Transport at the sills, where overflows occur, is towards the head of the gulf all year round with maximum in October and minimum in June. Furthermore, transport is 180° out of phase with the surface geostrophic velocity across the northern gulf, consistent with the exchange being strongest in October. Seasonal cycles of near-bottom temperature and transport are also 180° out of phase, indicating that maximum water inflow is associated with the coolest water entering from the Pacific Ocean. Near-bottom temperature over the northern Ballenas Channel sill has a maximum in early August, which is more in phase with the surface temperature and consistent with intense mixing in the channel. Geostrophic velocity at the northern gulf is in phase with that near the mouth of the gulf, and approximately in phase with the seasonal heat input through the mouth, calculated previously by Beron-Vera and Ripa [2000]. Moreover, the maximum lower-layer, horizontal heat output of the Ballenas Channel occurs in November, approximately one month after the maximum transport through the San Lorenzo and Delfín sills. Therefore, heat loss results from the continuous near-bottom inflow of relatively cold water at both sills which bound the deepest basins of the northern gulf. Moreover, the mean and seasonal cycles of heat and mass fluxes in the deepest basins of the northern gulf are almost everywhere in opposite directions. This article is protected by copyright. All rights reserved.
      PubDate: 2016-11-11T17:04:12.638414-05:
      DOI: 10.1002/2016JC012063
       
  • GNSS-R nonlocal sea state dependencies: Model and empirical verification
    • Authors: David D. Chen-Zhang; Christopher S. Ruf, Fabrice Ardhuin, Jeonghwan Park
      Abstract: Global Navigation Satellite System Reflectometry (GNSS-R) is an active, bistatic remote sensing technique operating at L-band frequencies. GNSS-R signals scattered from a rough ocean surface are known to interact with longer surface waves than traditional scatterometery and altimetry signals. A revised forward model for GNSS-R measurements is presented which assumes an ocean surface wave spectrum that is forced by other sources than just the local near-surface winds. The model is motivated by recent spaceborne GNSS-R observations that indicate a strong scattering dependence on significant wave height, even after controlling for local wind speed. This behavior is not well represented by the most commonly used GNSS-R scattering model, which features a one-to-one relationship between wind speed and the mean-square-slope of the ocean surface. The revised forward model incorporates a third generation wave model that is skillful at representing long waves, an anchored spectral tail model, and a GNSS-R electromagnetic scattering model. In comparisons with the spaceborne measurements, the new model is much better able to reproduce the empirical behavior. This article is protected by copyright. All rights reserved.
      PubDate: 2016-11-11T17:04:08.99925-05:0
      DOI: 10.1002/2016JC012308
       
  • Intrusive upwelling in the Central Great Barrier Reef
    • Authors: Jessica A. Benthuysen; Hemerson Tonin, Richard Brinkman, Michael Herzfeld, Craig Steinberg
      Abstract: In the Central Great Barrier Reef, the outer continental shelf has an open reef matrix that facilitates the exchange of waters with the Coral Sea. During austral summer, cool water intrudes onto the shelf along the seafloor. Temperature observations reveal cool, bottom intrusions during a six-year period from the Queensland Integrated Mooring Observational System's Palm Passage mooring. A metric is used to identify 64 intrusion events. These intrusions predominantly occur from October through March including the wet season. During an event, the outer-shelf's near-bottom temperature decreases by 1 to 3ºC typically over one week. The near-bottom salinity tends to increase, while near-surface changes do not reflect these tendencies. Intrusion events occur predominantly with either weakening equatorward winds or poleward wind bursts. A regional hydrodynamic model for the Great Barrier Reef captures the timing and amplitude of these intrusions. During intrusion events, isotherms tend to uplift over the continental slope and onto the shelf and the East Australian Current intensifies poleward. Over the shelf, a bottom-intensified onshore current coincides with bottom cooling. For numerous events, the model diagnostics reveal that the cross-shelf flow is dominated by the geostrophic contribution. A vertical circulation tilts the isopycnals upward on the southern side of the passage, causing an along-shelf density gradient and geostrophic onshore flow with depth. While wind fluctuations play a major role in controlling the along-shelf currents, model results indicate that a concurrent topographically induced circulation can assist the onshore spread of cool water. This article is protected by copyright. All rights reserved.
      PubDate: 2016-11-11T17:04:04.859169-05:
      DOI: 10.1002/2016JC012294
       
  • Ocean mixing beneath Pine Island Glacier ice shelf, West Antarctica
    • Authors: Satoshi Kimura; Adrian Jenkins, Pierre Dutrieux, Alexander Forryan, Alberto C. Naveira Garabato, Yvonne Firing
      Abstract: Ice shelves around Antarctica are vulnerable to an increase in ocean-driven melting, with the melt rate depending on ocean temperature and the strength of circulations inside the ice-shelf cavities. We present measurements of velocity, temperature, salinity, turbulent kinetic energy dissipation rate and thermal variance dissipation rate beneath Pine Island Glacier ice shelf, West Antarctica. These measurements were obtained by CTD, ADCP and turbulence sensors mounted on an Autonomous Underwater Vehicle (AUV). The highest turbulent kinetic energy dissipation rate is found near the grounding line. The thermal variance dissipation rate increases closer to the ice-shelf base, with a maximum value found ∼0.5 m away from the ice. The observed dissipation rates near the ice are used to estimate basal melting of the ice shelf. We argue that our estimates of basal melting from dissipation rates are within a range of previous estimates of basal melting. This article is protected by copyright. All rights reserved.
      PubDate: 2016-11-11T17:04:03.134314-05:
      DOI: 10.1002/2016JC012149
       
  • Reconciling estimates of the ratio of heat and salt fluxes at the
           ice-ocean interface
    • Authors: T. Keitzl; J.-P. Mellado, D. Notz
      Abstract: The heat exchange between floating ice and the underlying ocean is determined by the interplay of diffusive fluxes directly at the ice-ocean interface and turbulent fluxes away from it. In this study, we examine this interplay through direct numerical simulations of free convection. Our results show that an estimation of the interface flux ratio based on direct measurements of the turbulent fluxes can be difficult because the flux ratio varies with depth. As an alternative, we present a consistent evaluation of the flux ratio based on the total heat and salt fluxes across the boundary layer. This approach allows us to reconcile previous estimates of the ice–ocean interface conditions. We find that the ratio of heat and salt fluxes directly at the interface is 83 to 100 rather than 33 as determined by previous turbulence measurements in the outer layer. This can cause errors in the estimated ice-ablation rate from field measurements of up to 40% if they are based on the three-equation formulation. This article is protected by copyright. All rights reserved.
      PubDate: 2016-11-11T17:03:08.327637-05:
      DOI: 10.1002/2016JC012018
       
  • The diurnal cycle of sea-surface temperature and estimation of the heat
           budget of the Mediterranean Sea
    • Authors: S. Marullo; P. J. Minnett, R. Santoleri, M. Tonani
      Abstract: The diurnal cycle in sea-surface temperature (SST) is reconstructed for the year 2013 by combining numerical model analyses and satellite measurements using Optimal Interpolation (OI). The method is applied to derive hourly Mediterranean SST fields using Spinning Enhanced Visible and Infrared Imager (SEVIRI) data and Mediterranean Forecasting System analyses (Copernicus Marine Environment Monitoring Service - Analysis and Forecast product). The evaluation of the Diurnal OI SST (
      DOI SST) values against drifter measurements results in a mean bias of −0.1°C and a RMS of 0.4°C. The
      DOI SST fields reproduce well the diurnal cycle in SST including extreme Diurnal Warming events as measured by drifting buoys. We evaluate the impact of resolving the SST diurnal cycle, including extreme events, on estimates of the heat budget of the Mediterranean Sea over an entire annual cycle. It results in the mean annual difference in the heat loss derived using SST's with and without diurnal variations of 4 Wm−2 with a peak of 9 Wm−2 in July. This value is comparable to several other sources of uncertainty in the calculation of the heat and water budgets of the Mediterranean Sea. The results are an important step towards reducing uncertainties in the “Mediterranean Sea Heat Budget Closure Problem. This article is protected by copyright. All rights reserved.
      PubDate: 2016-11-11T17:03:06.997545-05:
       
  • Nitrous oxide during the onset of the Atlantic Cold Tongue
    • Authors: D. L. Arévalo-Martínez; A. Kock, T. Steinhoff, P. Brandt, M. Dengler, T. Fischer, A. Körtzinger, H. W. Bange
      Abstract: The tropical Atlantic exerts a major influence in climate variability through strong air-sea interactions. Within this region, the eastern side of the equatorial band is characterized by strong seasonality, whereby the most prominent feature is the annual development of the Atlantic Cold Tongue (ACT). This band of low sea surface temperatures (∼22-23°C) is typically associated with upwelling-driven enhancement of surface nutrient concentrations and primary production. Based on a detailed investigation of the distribution and sea-to-air fluxes of N2O in the eastern equatorial Atlantic (EEA), we show that the onset and seasonal development of the ACT can be clearly observed in surface N2O concentrations, which increase progressively as the cooling in the equatorial region proceeds during spring-summer. We observed a strong influence of the surface currents of the EEA on the N2O distribution, which allowed identifying “high” and “low” concentration regimes that were, in turn, spatially delimited by the extent of the warm eastward-flowing North Equatorial Countercurrent and the cold westward-flowing South Equatorial Current. Estimated sea-to-air fluxes of N2O from the ACT (mean 5.18±2.59 µmol m−2 d−1) suggests that in May-July 2011 this cold-water band doubled the N2O efflux to the atmosphere with respect to the adjacent regions, highlighting its relevance for marine tropical emissions of N2O. This article is protected by copyright. All rights reserved.
      PubDate: 2016-11-11T17:02:57.869086-05:
      DOI: 10.1002/2016JC012238
       
  • Assessing recent air-sea freshwater flux changes using a surface
           temperature-salinity space framework
    • Authors: Jeremy P. Grist; Simon A. Josey, Jan D. Zika, Dafydd Gwyn Evans, Nikolaos Skliris
      Abstract: A novel assessment of recent changes in air-sea freshwater fluxes has been conducted using a surface temperature-salinity framework applied to four atmospheric reanalyses. Viewed in the T-S space of the ocean surface, the complex pattern of the longitude-latitude space mean global Precipitation minus Evaporation (PME) reduces to three distinct regions. The analysis is conducted for the period 1979-2007 for which there is most evidence for a broadening of the (atmospheric) tropical belt. All four of the reanalyses display an increase in strength of the water cycle. The range of increase is between 2%-30% over the period analysed, with an average of 14%. Considering the average across the reanalyses, the water cycle changes are dominated by changes in tropical as opposed to mid-high latitude precipitation. The increases in the water cycle strength, are consistent in sign, but larger than in a 1% greenhouse gas run of the HadGEM3 climate model. In the model a shift of the precipitation/evaporation cells to higher temperatures is more evident, due to the much stronger global warming signal. The observed changes in freshwater fluxes appear to be reflected in changes in the T-S distribution of the Global Ocean. Specifically, across the diverse range of atmospheric reanalyses considered here, there was an acceleration of the hydrological cycle during 1979-2007 which led to a broadening of the ocean's salinity distribution. Finally, although the reanalyses indicate that the warm temperature tropical precipitation dominated water cycle change, ocean observations suggest that ocean processes redistributed the freshening to lower ocean temperatures. This article is protected by copyright. All rights reserved.
      PubDate: 2016-11-11T17:01:55.921755-05:
      DOI: 10.1002/2016JC012091
       
  • Barrier island breach evolution: Alongshore transport and bay-ocean
           pressure gradient interactions
    • Authors: Ilgar Safak; John C. Warner, Jeffrey H. List
      Abstract: Physical processes controlling repeated openings and closures of a barrier island breach between a bay and the open ocean are studied using aerial photographs and atmospheric and hydrodynamic observations. The breach site is located on Pea Island along the Outer Banks, separating Pamlico Sound from the Atlantic Ocean. Wind direction was a major control on the pressure gradients between the bay and the ocean to drive flows that initiate or maintain the breach opening. Alongshore sediment flux was found to be a major contributor to breach closure. During the analysis period from 2011 to 2016, three hurricanes had major impacts on the breach. First, Hurricane Irene opened the breach with wind-driven flow from bay to ocean in August 2011. Hurricane Sandy in October 2012 quadrupled the channel width from pressure gradient flows due to water levels that were first higher on the ocean side and then higher on the bay side. The breach closed sometime in Spring 2013, most likely due to an event associated with strong alongshore sediment flux but minimal ocean-bay pressure gradients. Then, in July 2014, Hurricane Arthur briefly opened the breach again from the bay side, in a similar fashion to Irene. In summary, opening and closure of breaches are shown to follow a dynamic and episodic balance between along-channel pressure gradient driven flows and alongshore sediment fluxes. This article is protected by copyright. All rights reserved.
      PubDate: 2016-11-11T17:01:17.6961-05:00
      DOI: 10.1002/2016JC012029
       
  • Subduction of North Pacific Tropical Water and Its equatorward pathways as
           shown by a simulated passive tracer
    • Authors: Xunwei Nie; Shan Gao, Fan Wang, Tangdong Qu
      Abstract: The subduction and equatorward pathways of North Pacific Tropical Water (NPTW) are investigated using a simulated passive tracer of the consortium Estimating the Circulation and Climate of the Ocean (ECCO). The results demonstrate that the subduction of NPTW occurs in a large area that extends from about 150ºE to 130ºW between 20ºN and 30ºN, but the main subduction region lies in its eastern part. After subduction, the main body of NPTW first spreads westward in the North Equatorial Current. Then it splits into two branches. One flows northward in the Kuroshio upon reaching the western boundary, and the other enters the tropical Pacific either via its western boundary pathway (WBP) or interior pathway (IP). Less than half of the transport through the WBP can eventually reach the central and eastern Pacific by the Equatorial Undercurrent, while the rest is seen to flow into the Indian Ocean by the Indonesian Throughflow. The IP is found to play a significant role in equatorward transport of the NPTW. About 30% of the NPTW that reached the equatorial Pacific is transported through the IP. This article is protected by copyright. All rights reserved.
      PubDate: 2016-11-11T17:00:57.829169-05:
      DOI: 10.1002/2016JC012305
       
  • Radar imaging of shallow water bathymetry: A case study in the Yangtze
           Estuary
    • Authors: Peng Yu; Johnny A. Johannessen, Vladimir Kudryavtsev, Xiaojing Zhong, Yunxuan Zhou
      Abstract: This study focuses on 2-dimensional (2-D) radar imaging of bathymetric features in the shallow water of the Yangtze Estuary using synthetic aperture radar (SAR) observations and model simulations. A validated 2-D shallow water numerical model simulates the barotropic current velocity, and the simulated current fields together with the relevant parameters of radar observations are then invoked in the radar imaging model as the input. The results show that variations in the simulated image intensity are mainly dominated by distinct radar backscatter anomalies caused by wave-current interactions in the vicinity of rapidly changing underwater topographies. The comparison between the simulated and observed SAR images shows a reasonable agreement, demonstrating that our approach may be implemented to monitor changes in the shallow water bathymetry of the Yangtze Estuary in the future. This article is protected by copyright. All rights reserved.
      PubDate: 2016-11-11T17:00:46.082791-05:
      DOI: 10.1002/2016JC011973
       
  • Carbon cycling dynamics in the seasonal sea-ice zone of East Antarctica
    • Authors: Nicholas P. Roden; Bronte Tilbrook, Thomas W. Trull, Patti Virtue, Guy D. Williams
      Abstract: The carbon cycle of the seasonally ice covered region of the southwest Indian Ocean sector of East Antarctica (30°-80°E, 60°-69°S) was investigated during austral summer (January – March 2006). Large variability in the drivers and timing of carbon cycling dynamics were observed and indicated that the study site was a weak net source of carbon dioxide (CO2) to the atmosphere of 0.8 ± 1.6 grams C m−2 during the ice-free period, with narrow bands of CO2 uptake observed near the continental margin and north of the Southern Antarctic Circumpolar Current Front. Continuous surface measurements of dissolved oxygen and the fugacity of CO2 were combined with net community production estimates from oxygen/argon ratios to show that surface heat gain and photosynthesis were responsible for the majority of observed surface water variability. On seasonal timescales, winter sea-ice cover reduced the flux of CO2 to the atmosphere in the study area, followed by biologically driven drawdown of CO2 as the ice retreated in spring-summer highlighting the important role that sea-ice formation and retreat has on the biogeochemical cycling of the region. This article is protected by copyright. All rights reserved.
      PubDate: 2016-11-11T17:00:41.888061-05:
      DOI: 10.1002/2016JC012008
       
  • Efficient boundary mixing due to near-inertial waves in a non-tidal basin:
           Observations from the Baltic Sea
    • Authors: C. Lappe; L. Umlauf
      Abstract: Diapycnal mixing in large non-tidal basins is often assumed to be related to the effect of near-inertial waves. While the role of near-inertial shear for the generation of shear instabilities in the stratified interior is relatively well understood, much less is known about the impact of near-inertial motions on boundary mixing processes in non-tidal systems, mainly owing to the lack of appropriate observations. Here, an extensive data set is discussed, describing the variability of boundary mixing induced by near-inertial motions near the sloping topography of one of the main basins of the Baltic Sea. These data reveal the existence of a vigorously turbulent bottom boundary layer of a few meters thickness covering the entire slope region above and below the permanent halocline that constitutes the main obstacle for vertical transport in the Baltic Sea. Near-bottom turbulence was driven by the near-inertial shear in the frictional boundary layer rather than by breaking of near-inertial waves near critical slopes. Large regions of the bottom boundary layer remained strongly stratified, and therefore, different from the traditional view of inefficient boundary mixing, mixing efficiencies reached values typical for the ocean's interior. This article is protected by copyright. All rights reserved.
      PubDate: 2016-11-11T17:00:38.033652-05:
      DOI: 10.1002/2016JC011985
       
  • Footprints of obliquely incident internal solitary waves and internal
           tides near the shelf break in the northern South China Sea
    • Authors: Xiaochuan Ma; Jun Yan, Yijun Hou, Feilong Lin, Xufeng Zheng
      Abstract: A mooring system and two sites of bottom currents were deployed over the slope and near the shelf break on the propagating paths of internal solitary waves (ISWs), west off Dongsha Atoll in the northern South China Sea. Data indicated that energetic ISWs obliquely shoaled onto the shelf west off Dongsha Atoll in an approximately 290° direction, causing strong reversing currents (some exceeding 80 cm/s) near the bottom. Two types of sandwaves and short scour channels are discernible on the seafloor near the shelf break, which have reasonable correlations with the obliquely incident ISWs and internal tides. Type 1 sandwaves, featured by ISWs at the depths of 130-150 m, have flat crests interacting with the isobaths at a angle of nearly 45° which slightly incline and migrate upslope. Type 2 sandwaves are associated with internal tides, which have crests parallel to the isobaths and distinctly incline and migrate downslope. Short channels are parallel to the depth contours and truncate the strata, which could be formed and maintained by along-slope currents that are probably produced by the obliquely ISWs on a large gradient (γ>0.8°). The ISWs can move coarse grains or suspend fine grains but do not change the long-term trend of sediment transport on the seabed with larger gradients (γ/c>1), which is dominated by internal tides. These features are likely widespread near the shelf break in the northern South China Sea and other seas but are limited on mild slopes where ISWs do not break. This article is protected by copyright. All rights reserved.
      PubDate: 2016-11-11T17:00:24.762668-05:
      DOI: 10.1002/2016JC012009
       
  • Effects of ocean grid resolution on tropical cyclone-induced upper ocean
           responses using a global ocean general circulation model
    • Authors: Hui Li; Ryan L. Sriver
      Abstract: Tropical cyclones (TCs) have the potential to influence regional and global climate through interactions with the upper ocean. Here we present results from a suite of ocean-only model experiments featuring the Community Earth System Model, in which we analyze the effect of tropical cyclone wind forcing on the global ocean using three different horizontal ocean grid resolutions (3˚, 1˚, and 0.1˚). The ocean simulations are forced with identical atmospheric inputs from the Coordinated Ocean-Ice Reference Experiments version 2 (COREv2) normal year forcing conditions, featuring global blended TC winds from a fully-coupled CESM simulation with a 25 km atmosphere [Small et al., 2014]. The simulated TC climatology shows good agreement with observational estimates of annual TC statistics, including annual frequency, intensity distributions, and geographic distributions. Each ocean simulation is comprised of a 5-year spin up with COREv2 normal year forcing, followed by 18 months with blended TC winds. In addition, we conduct corresponding control simulations for each grid resolution configuration without blended TC winds. We find that ocean horizontal and vertical grid resolutions affect TC-induced heat and momentum fluxes, post-storm cold wake features, and ocean subsurface temperature profiles. The responses are amplified for smaller grid spacing. Moreover, analyses show that the annually accumulated TC-induced ocean heat uptake is also sensitive to ocean grid resolution, which may have important implications for modeled ocean heat budgets and variability. This article is protected by copyright. All rights reserved.
      PubDate: 2016-11-07T09:19:03.335803-05:
      DOI: 10.1002/2016JC011951
       
  • Equatorial Pacific thermostad response to El Niño
    • Authors: Gregory C. Johnson; Abigail N. Birnbaum
      Abstract: El Niños are characterized by a shift of warm surface water from the western to eastern equatorial Pacific due to weakening of easterly trade winds. This shift is associated with the pycnocline (or thermocline), the large vertical density gradient beneath the surface mixed layer, shoaling in the west and deepening in the east, inducing a redistribution of ocean heat with global impacts. Here the response of the Pacific Equatorial Thermostad, a layer of low vertical stratification below the pycnocline, to El Niño is investigated using a monthly Argo float climatology and Argo float deep velocity data. A mean, seasonal cycle, trend, and time-lagged linear response to the Niño3.4 index are fit by least squares to temperature and salinity at each gridpoint as well as to deep float velocities (omitting the trend). The results of these fits are used to characterize the response of physical properties in the Thermostad, including layer thickness and velocity, to El Niño by comparing the mean properties following neutral conditions (Niño3.4 = 0°C) versus those following a moderate El Niño (Niño3.4 = 1°C). Following an El Niño, a strengthening of the westward-flowing Equatorial Intermediate Current of about 2.7 × 106 m3 s−1 shifts about 97 × 1012 m3 of thermostad water from the east to the west, allowing conservation of volume within the Thermostad as the pycnocline above deepens in the east and shoals in the west. This transport and volume change imply a 14-month time scale, consistent with El Niño. This article is protected by copyright. All rights reserved.
      PubDate: 2016-11-07T09:18:26.783043-05:
      DOI: 10.1002/2016JC012304
       
  • Effects of an Arctic under-ice bloom on solar radiant heating of the water
           column
    • Authors: Torbjørn Taskjelle; Mats A. Granskog, Alexey K. Pavlov, Stephen R. Hudson, Børge Hamre
      Abstract: The deposition of solar energy in the upper Arctic Ocean depends, among other things, on the composition of the water column. During the N-ICE2015 expedition, a drift in the Arctic pack ice north of Svalbard, an under-ice phytoplankton bloom was encountered in May 2015. This bloom led to significant changes in the inherent optical properties (IOPs) of the upper ocean. Mean values of total water absorption in the upper 20 m of the water column were up to 4 times higher during the bloom than prior to it. The total water attenuation coefficient increased by a factor of up to around 7. Radiative transfer modeling, with measured IOPs as input, has been performed with a coupled atmosphere-ice-ocean model. Simulations are used to investigate the change in depth dependent solar heating of the ocean after the onset of the bloom, for wavelengths in the region 350–700 nm. Effects of clouds, sea ice cover, solar zenith angle, as well as the average cosine for scattering of the ocean inclusions are evaluated. An increase in energy absorption in the upper 10 m of about 36% is found under 25 cm ice with 2 cm snow, for bloom conditions relative to pre-bloom conditions, which may have implications for ice melt and growth in spring. Thicker clouds and lower sun reduce the irradiance available, but lead to an increase in relative absorption. This article is protected by copyright. All rights reserved.
      PubDate: 2016-11-07T09:18:00.909254-05:
      DOI: 10.1002/2016JC012187
       
  • Long-range propagation and associated variability of internal tides in the
           South China Sea
    • Authors: Zhenhua Xu; Kun Liu, Baoshu Yin, Zhongxiang Zhao, Yang Wang, Qun Li
      Abstract: The variability of internal tides during their generation and long-range propagation in the South China Sea (SCS) is investigated by driving a high-resolution numerical model. The present study clarifies the notably different processes of generation, propagation and dissipation between diurnal and semidiurnal internal tides. Internal tides in the SCS originate from multiple source sites, among which the Luzon Strait is dominant, and contributes approximately 90% and 74% of the baroclinic energy for M2 and K1, respectively. To the west of the Luzon Strait, local generation of K1 internal tides inside the SCS is more energetic than the M2 tides. Diurnal and semidiurnal internal tides from the Luzon Strait radiate into the SCS in a north-south asymmetry but with different patterns because of the complex two-ridge system. The tidal beams can travel across the deep basin and finally arrive at the Vietnam coast and Nansha Island more than 1000-1500 km away. During propagation, M2 internal tides maintain a southwestward direction, whereas K1 exhibit complicated wave fields because of the superposition of waves from local sources and island scattering effects. After significant dissipation within the Luzon Strait, the remaining energy travels into the SCS and reduces by more than 90% over a distance of ∼1000 km. Inside the SCS, the K1 internal tides with long crests and flat beam angles are more influenced by seafloor topographical features and thus undergo apparent dissipation along the entire path, whereas the prominent dissipation of M2 internal tides only occurs after their arrival at Zhongsha Island. This article is protected by copyright. All rights reserved.
      PubDate: 2016-11-07T09:17:43.023495-05:
      DOI: 10.1002/2016JC012105
       
  • A modeling study of processes controlling the Bay of Bengal sea surface
           salinity interannual variability
    • Authors: V.P. Akhil; M. Lengaigne, J. Vialard, F. Durand, M. G. Keerthi, A.V.S. Chaitanya, F. Papa, V. V. Gopalakrishna, Clément de Boyer Montégut
      Abstract: Recent observational studies provided preliminary insights on the interannual variability of Bay of Bengal (BoB) Sea Surface Salinity (SSS), but are limited by the poor data coverage. Here, we describe the BoB interannual SSS variability and its driving processes from a regional eddy-permitting ocean general circulation model forced by interannually varying air-sea fluxes and altimeter-derived discharges of major rivers over the past two decades. Simulated interannual SSS variations compare favourably with both in-situ and satellite data and are largest in boreal fall in three regions: the northern BoB, the coastal region off east India and the Andaman Sea. In the northern BoB these variations are independent from those in other regions and mostly driven by summer-fall Ganga-Brahmaputra runoff interannual variations. In fall, remote forcing from the Indian Ocean Dipole results in anti-clockwise anomalous horizontal currents that drive interannual SSS variations of opposite polarity along the east coast of India and in the Southern Andaman Sea. From winter onward, these anomalies are damped by vertical mixing in the northern BoB and along the east coast of India and by horizontal advection in the Southern Andaman Sea. While river runoff fluctuations locally play a strong role near the Ganga-Brahmaputra river mouth, wind-driven interannual current anomalies are responsible for a large fraction of SSS interannual variability in most of the basin. This article is protected by copyright. All rights reserved.
      PubDate: 2016-11-07T09:11:06.488671-05:
      DOI: 10.1002/2016JC011662
       
  • Direct estimates of friction factors for a mobile rippled bed
    • Authors: S. Rodríguez-Abudo; D. L. Foster
      Abstract: New friction factor estimates are computed from the total momentum transfer applied to a rippled sediment bed. The total time-dependent momentum flux is achieved by implementing the double-averaged horizontal momentum equation on the nearbed flow field collected with PIV. Time-independent friction factors are obtained by regressing the total momentum flux to the common quadratic stress law given by . The resulting friction factors compare favorably with available analysis techniques including energy dissipation, vertical turbulence intensity, and maximum shear stress, but can be 2-6 times smaller than estimates determined with the model by [1994] and the formula of Swart [1974] using the ripple roughness. This article is protected by copyright. All rights reserved.
      PubDate: 2016-11-07T09:10:54.266417-05:
      DOI: 10.1002/2016JC012055
       
  • Isotope constraints on seasonal dynamics of dissolved and particulate N in
           the Pearl River Estuary, South China
    • Authors: Feng Ye; Guodong Jia, Luhua Xie, Gangjian Wei, Jie Xu
      Abstract: Isotope measurements were performed on dissolved NO3−, NH4+ and suspended particulate total N along a salinity gradient in the Pearl River Estuary (PRE) to investigate seasonal changes in main N sources and its biogeochemical processing under the influence of monsoon climate. Our data revealed that municipal sewage and re-mineralized soil organic N were the major sources of DIN (NO3− and/or NH4+) in freshwater during winter and summer, respectively, whereas phytoplankton biomass was a major component of PN in both seasons. In low salinity waters (
      PubDate: 2016-11-02T09:10:54.169304-05:
      DOI: 10.1002/2016JC012066
       
  • Long-range sediment transport in the world's oceans by stably stratified
           turbidity currents
    • Authors: Benjamin Kneller; Mohamad M. Nasr-Azadani, Senthil Radhakrishnan, Eckart Meiburg
      Abstract: Submarine fans, supplied primarily by turbidity currents, constitute the largest sediment accumulations on Earth. Generally accepted models of turbidity current behavior imply they should dissipate rapidly on the very small gradients of submarine fans, thus their persistence over long distances is enigmatic. We present numerical evidence, constrained by published field data, suggesting that turbidity currents traveling on low slopes and carrying fine particles have a stably stratified shear layer along their upper interface, which dramatically reduces dissipation and entrainment of ambient fluid, allowing the current to propagate over long distances. We propose gradient Richardson number as a useful criterion to discriminate between the different behaviors exhibited by turbidity currents on high and low slopes. This article is protected by copyright. All rights reserved.
      PubDate: 2016-10-27T03:40:35.040523-05:
      DOI: 10.1002/2016JC011978
       
  • A validated tropical-extratropical flood hazard assessment for New York
           Harbor
    • Authors: P.M. Orton; T.M. Hall, S.A. Talke, A.F. Blumberg, N. Georgas, S. Vinogradov
      Abstract: Recent studies of flood risk at New York Harbor (NYH) have shown disparate results for the 100-year storm tide, providing an uncertain foundation for the flood mitigation response after Hurricane Sandy. Here, we present a flood hazard assessment that improves confidence in our understanding of the region's present-day potential for flooding, by separately including the contribution of tropical cyclones (TCs) and extratropical cyclones (ETCs), and validating our modeling study at multiple stages against historical observations. The TC assessment is based on a climatology of 606 synthetic storms developed from a statistical-stochastic model of North Atlantic TCs. The ETC assessment is based on simulations of historical storms with many random tide scenarios. Synthetic TC landfall rates and the final TC and ETC flood exceedance curves are all shown to be consistent with curves computed using historical data, within 95% confidence ranges. Combining the ETC and TC results together, the 100-year return period storm tide at NYH is 2.70 m (2.51-2.92 at 95% confidence), and Hurricane Sandy's storm tide of 3.38 m was a 260-year (170-420) storm tide. Deeper analyses of historical flood reports from estimated Category-3 hurricanes in 1788 and 1821 lead to new estimates and reduced uncertainties for their floods, and show that Sandy's storm tide was the largest at NYH back to at least 1700. The flood exceedance curves for ETCs and TCs have sharply different slopes due to their differing meteorology and frequency, warranting separate treatment in hazard assessments. This article is protected by copyright. All rights reserved.
      PubDate: 2016-10-25T08:05:34.956258-05:
      DOI: 10.1002/2016JC011679
       
  • Circulation and oxygen cycling in the Mediterranean Sea: Sensitivity to
           future climate change
    • Authors: Helen Powley; Michael Krom, Philippe Van Cappellen
      Abstract: Climate change is expected to increase temperatures and decrease precipitation in the Mediterranean Sea (MS) basin, causing substantial changes in the thermohaline circulation (THC) of both the Western Mediterranean Sea (WMS) and Eastern Mediterranean Sea (EMS). The exact nature of future circulation changes remains highly uncertain, however, with forecasts varying from a weakening to a strengthening of the THC. Here, we assess the sensitivity of dissolved oxygen (O2) distributions in the WMS and EMS to THC changes using a mass balance model, which represents the exchanges of O2 between surface, intermediate and deep water reservoirs, and through the Straits of Sicily and Gibraltar. Perturbations spanning the ranges in O2 solubility, aerobic respiration kinetics and THC changes projected for the year 2100 are imposed to the O2 model. In all scenarios tested, the entire MS remains fully oxygenated after 100 years; depending on the THC regime, average deep-water O2 concentrations fall in the ranges 151-205 and 160-219 µM in the WMS and EMS, respectively. On longer timescales (>1000 years), the scenario with the largest (> 74%) decline in deep-water formation rate leads to deep-water hypoxia in the EMS but, even then, the WMS deep-water remains oxygenated. In addition, a weakening of THC may result in a negative feedback on O2 consumption as supply of labile dissolved organic carbon to deep-water decreases. Thus, it appears unlikely that climate-driven changes in THC will cause severe O2 depletion of the deep-water masses of the MS in the foreseeable future. This article is protected by copyright. All rights reserved.
      PubDate: 2016-10-25T08:05:33.37871-05:0
      DOI: 10.1002/2016JC012224
       
  • Nonlinear multiscale interactions and internal dynamics underlying a
           typical eddy-shedding event at Luzon Strait
    • Authors: Yuan-Bing Zhao; X. San Liang, Jianping Gan
      Abstract: Eddy-shedding is a highly nonlinear process that presents a major challenge in geophysical fluid dynamics. Using the newly developed localized multiscale energy and vorticity analysis (MS-EVA), this study investigates an observed typical warm eddy-shedding event as the Kuroshio passes the Luzon Strait, in order to gain insight into the underlying internal dynamics. Through multiscale window transform (MWT), it is found that the loop-form Kuroshio intrusion into the South China Sea (SCS) is not a transient feature, but a quasi-equilibrium state of the system. A mesoscale reconstruction reveals that the eddy does not have its origin at the intrusion path, but comes from the Northwest Pacific. It propagates westward, preceded by a cyclonic (cold) eddy, through the Kuroshio into the SCS. As the eddy pair runs across the main current, the cold one weakens and the warm one intensifies through a mixed instability. In its development, another cold eddy is generated to its southeast, which also experiences a mixed instability. It develops rapidly and cuts the warm eddy off the stream. Both the warm and cold eddies then propagate westward in the form of a Rossby wave (first baroclinic mode). As the eddies approach the Dongsha Islands, they experience another baroclinic instability, accompanied by a sudden accumulation of eddy available potential energy. This part of potential energy is converted to eddy kinetic energy through buoyancy conversion, and is afterwards transferred back to the large-scale field through inverse cascading, greatly reducing the intensity of the eddy and eventually leading to its demise. This article is protected by copyright. All rights reserved.
      PubDate: 2016-10-25T08:05:28.945498-05:
      DOI: 10.1002/2016JC012483
       
  • A system to measure the data quality of spectral remote sensing
           reflectance of aquatic environments
    • Authors: Jianwei Wei; Zhongping Lee, Shaoling Shang
      Abstract: Spectral remote sensing reflectance (Rrs, sr−1) is the key for ocean color retrieval of water bio-optical properties. Since Rrs from in-situ and satellite systems are subject to errors or artifacts, assessment of the quality of Rrs data is critical. From a large collection of high quality in situ hyperspectral Rrs datasets, we developed a novel quality assurance (QA) system that can be used to objectively evaluate the quality of an individual Rrs spectrum. This QA scheme consists of a unique Rrs spectral reference and a score metric. The reference system includes Rrs spectra of 23 optical water types ranging from purple blue to yellow waters, with an upper and a lower bound defined for each water type. The scoring system is to compare any target Rrs spectrum with the reference and a score between 0 and 1 will be assigned to the target spectrum, with 1 for perfect Rrs spectrum and 0 for unusable Rrs spectrum. The effectiveness of this QA system is evaluated with both synthetic and in situ Rrs spectra and it is found to be robust. Further testing is performed with the NOMAD dataset as well as with satellite Rrs over coastal and oceanic waters, where questionable or likely erroneous Rrs spectra are shown to be well identifiable with this QA system. Our results suggest that applications of this QA system to in situ datasets can improve the development and validation of bio-optical algorithms and its application to ocean color satellite data can improve the short- and long-term products by objectively excluding questionable Rrs data. This article is protected by copyright. All rights reserved.
      PubDate: 2016-10-25T08:05:23.160646-05:
      DOI: 10.1002/2016JC012126
       
  • Intraseasonal sea level variability along the West Coast of India
    • Authors: Laxmikant Dhage; P. Ted Strub
      Abstract: The importance of local versus distant forcing is studied for the wind driven intra-seasonal (30-120 day) sea level anomaly (SLA) variations along the west coast of India. Significant correlations of altimeter derived SLA on the west coast are found with the mid-basin SLA east of Sri-Lanka and SLA as far as Sumatra and the Equator, with increased lags, connecting with the remote forcing from the Equator in the form of reflected Rossby waves. The highest correlations between SLA on the west coast and winds are found with the winds at the southern tip of India. Coherence calculations help to identify the importance of a narrow band (40-60 day) for the interactions of winds with the intra-seasonal SLA variations. A multivariate regression model, along with the coherences within this narrower band, suggest the lags of SLA on the west coast with winds to range from 0-2 days with the local forcing to 11-13 days with the forcing along south east coast of India. Hovmöller diagrams illustrate the propagation of signals by estimating phase speed for Rossby waves (57 cm/s) across the Indian Ocean from Sumatra and Coastal trapped Waves (CTWs) along the west coast of India (178 cm/s). Propagation from the south-east coast of India is not as robust as Rossby waves from Sumatra. This article is protected by copyright. All rights reserved.
      PubDate: 2016-10-20T21:00:27.430364-05:
      DOI: 10.1002/2016JC011904
       
  • Coherent motions and time scales that control heat and mass transfer at
           wind-swept water surfaces
    • Authors: D. E. Turney
      Abstract: Forecast of the heat and chemical budgets of lakes, rivers and oceans requires improved predictive understanding of air-water interfacial transfer coefficients. Here we present laboratory observations of the coherent motions that occupy the air-water interface at wind speeds (U10) 1.1 to 8.9 m/s. Spatio-temporal near-surface velocity data and interfacial renewal data are made available by a novel flow tracer method. The relative activity, velocity scales and time scales of the various coherent interfacial motions are measured, namely for Langmuir circulations, streamwise streaks, non-breaking wind waves, parasitic capillary waves, non-turbulent breaking wind waves, and turbulence-generating breaking wind waves. Breaking waves exhibit a sudden jump in streamwise interfacial velocity wherein the velocity jumps up to exceed the wave celerity and destroys nearby parasitic capillary waves. Four distinct hydrodynamic regimes are found to exist between U10 = 0 and 8.9 m/s, each with a unique population balance of the various coherent motions. The velocity scales, time scales and population balance of the different coherent motions are input to a first-principles gas transfer model to explain the waterside transfer coefficient (kw) as well as experimental patterns of temperature and gas concentration. The model mixes concepts from surface renewal and divergence theories, and requires surface divergence strength (β), the Lagrangian residence time inside the upwelling zone (tLu), and the total lifetime of new interface before it is downwelled (tLT). The model's output agrees with time-averaged measurements kw, patterns of temperature in infrared photographs, and spatial patterns of gas concentration and kw from direct numerical simulations. Several non-dimensional parameters, e.g. βtLu and τstLT where τs is the interfacial shear rate, determine the effectiveness of a particular type of coherent motion for affecting kw. This article is protected by copyright. All rights reserved.
      PubDate: 2016-10-19T10:26:45.800745-05:
      DOI: 10.1002/2016JC012139
       
  • Seasonal patterns of SST diurnal variation over the Tropical Warm Pool
           region
    • Authors: Haifeng Zhang; Helen Beggs, Xiao Hua Wang, Andrew E. Kiss, Christopher Griffin
      Abstract: Five-year (2010 through 2014) Advanced Very High Resolution Radiometer (AVHRR) sea surface temperature (SST) data produced by the Australian Bureau of Meteorology have been validated against drifting buoy data and then used to study the seasonal patterns of the SST diurnal variation (DV) events over the Tropical Warm Pool region (TWP, 25°S–15°N, 90°E–170°E). The in-situ validation results illustrate the overall good quality of the AVHRR SST data set, although an average 0.19 K underestimation of the daytime measurements has been observed. The night-time observations are in good agreement with in-situ buoys with an average bias of 0.03 K and a 0.30 K standard deviation of the biases. This SST data set is then used to characterise the SST DV seasonal patterns, together with wind speeds, daily maximum solar shortwave insolation (SSImax) and latent heat flux (LHF). A double-peak seasonal pattern of SST DV is observed over the study region: the strongest DVs are found in March and October and the weakest in June. Sensitivity tests of DV to wind, SSImax, and LHF are conducted. The results indicate: (1) different morning and early afternoon winds (7 am to 2 pm local time, LT) affect DV by as much as 0.73 K when the half-daily (defined as 2 am to 2 pm LT in this study) average winds are fixed between 2-3 ms−1; (2) SSImax levels regulate DV less significantly (< 0.68 K) under fixed winds; and (3) LHF effects on DV are relatively weak (< 0.35 K). This article is protected by copyright. All rights reserved.
      PubDate: 2016-10-19T10:26:34.038591-05:
      DOI: 10.1002/2016JC012210
       
  • Structure and dynamics of a subglacial discharge plume in a Greenlandic
           Fjord
    • Authors: Kenneth D. Mankoff; Fiammetta Straneo, Claudia Cenedese, Sarah B. Das, Clark G. Richards, Hanumant Singh
      Abstract: Discharge of surface-derived meltwater at the submerged base of Greenland's marine-terminating glaciers creates subglacial discharge plumes that rise along the glacier/ocean interface. These plumes impact submarine melting, calving and fjord circulation. Observations of plume properties and dynamics are challenging due to their proximity to the calving edge of glaciers. Therefore to date information on these plumes has been largely derived from models. Here we present temperature, salinity, and velocity data collected in a plume that surfaced at the edge of Saqqarliup Sermia, a mid-sized Greenlandic glacier. The plume is associated with a narrow core of rising waters approximately 20 m in diameter at the ice edge that spreads to a 200 m by 300 m plume pool as it reaches the surface, before descending to its equilibrium depth. Volume flux estimates indicate that the plume is primarily driven by subglacial discharge and that this has been diluted in a ratio of 1:10 by the time the plume reaches the surface. While highly uncertain, meltwater fluxes are likely two orders of magnitude smaller than the subglacial discharge flux. The overall plume characteristics agree with those predicted by theoretical plume models for a convection driven plume with limited influence from submarine melting. This article is protected by copyright. All rights reserved.
      PubDate: 2016-10-19T10:20:27.03883-05:0
      DOI: 10.1002/2016JC011764
       
  • Mapping error in Southern Ocean transport computed from satellite
           altimetry and Argo
    • Authors: Michael Kosempa; Don P. Chambers
      Abstract: In an effort to better estimate transport dynamics in response to wind forcing (primarily the Southern Annual Mode), this study quantifies the uncertainty in mapping zonal geostrophic transport of the Antarctic Circumpolar Current from sparse temperature, salinity and sea surface height observations. To do this, we sampled an ocean state estimate at the locations of both Argo floats and the Jason-1 altimeter groundtrack. These sampled values were then optimally interpolated to create SSH and temperature/salinity grids with 1° resolution. The temperature, salinity and SSH grids were then combined to compute the zonal geostrophic transport and compared to that estimated from the full state estimate. There are significant correlations between the baroclinic and barotropic error contributions to the total transport error. The increase in Argo floats in the Southern Ocean is effective in reducing mapping error. However, that error improvement is not uniform. By analyzing systematic errors in transport time series, we find the transects that are most appropriate for analyzing the dynamics of ACC transport using Argo and altimetric gridded fields. Based on our analysis, we conclude region south of Tasmania is most appropriate, with lowest uncertainty. Using real-world data, we calculated zonal transport variability at a transect south of Tasmania. There is an insignificant trend (0.3 ± 0.4 Sv yr−1, 90% confidence) but significant low-frequency variability correlated with the Southern Annular Mode (0.53, p 
      PubDate: 2016-10-18T02:50:46.211771-05:
      DOI: 10.1002/2016JC011956
       
  • Projecting nuisance flooding in a warming climate using generalized linear
           models and Gaussian processes
    • Authors: Alexander Vandenberg-Rodes; Hamed R. Moftakhari, Amir AghaKouchak, Babak Shahbaba, Brett F. Sanders, Richard A. Matthew
      Abstract: Nuisance flooding corresponds to minor and frequent flood events that have significant socio-economic and public health impacts on coastal communities. Yearly-averaged local mean sea level can be used as proxy to statistically predict the impacts of sea level rise (SLR) on the frequency of nuisance floods (NF). In this study, we use Generalized Linear Models (GLM) and Gaussian Process (GP) models combined to (i) estimate the frequency of NF associated with the change in mean sea level, and (ii) quantify the associated uncertainties via a novel and statistically robust approach. We calibrate our models to the water level data from eighteen tide gauges along the coasts of United States, and after validation, we estimate the frequency of NF associated with the SLR projections in year 2030 (under RCPs 2.6 and 8.5), along with their 90% bands, at each gauge. The historical NF-SLR data is very noisy, and shows large changes in variability (heteroscedasticity) with SLR. Prior models in the literature do not properly account for the observed heteroscedasticity, and thus their projected uncertainties are highly suspect. Among the models used in this study the Negative Binomial Distribution GLM with GP best characterizes the uncertainties associated with NF estimates; on validation data ≈ 93% of the points fall within the 90% credible limit, showing our approach to be a robust model for uncertainty quantification. This article is protected by copyright. All rights reserved.
      PubDate: 2016-10-18T02:50:23.747946-05:
      DOI: 10.1002/2016JC012084
       
  • Processes of multibathyal aragonite undersaturation in the Arctic Ocean
    • Authors: J.G. Wynn; L.L. Robbins, L.G. Anderson
      Abstract: During three years of study (2010-2012), the western Arctic Ocean was found to have unique aragonite saturation profiles with up to three distinct aragonite undersaturation zones. This complexity is produced as inflow of Atlantic- and Pacific-derived water masses mix with Arctic-derived waters, which are further modified by physiochemical and biological processes. The shallowest aragonite undersaturation zone, from the surface to ∼ 30 m depth is characterized by relatively low alkalinity and other dissolved ions. Besides local influence of biological processes on aragonite undersaturation of shallow coastal waters, the nature of this zone is consistent with dilution by sea-ice melt and invasion of anthropogenic CO2 from the atmosphere. A second undersaturated zone at ∼ 90-220 m depth (salinity ∼31.8–35.4) occurs within the Arctic Halocline and is characterized by elevated pCO2 and nutrients. The nature of this horizon is consistent with remineralization of organic matter on shallow continental shelves bordering the Canada Basin and the input of the nutrients and CO2 entrained by currents from the Pacific Inlet. Finally, the deepest aragonite undersaturation zone is at greater than 2000 m depth and is controlled by similar processes as deep aragonite saturation horizons in the Atlantic and Pacific Oceans. The comparatively shallow depth of this deepest aragonite saturation horizon in the Arctic is maintained by relatively low temperatures, and stable chemical composition. Understanding the mechanisms controlling the distribution of these aragonite undersaturation zones, and the timescales over which they operate will be crucial to refine predictive models. This article is protected by copyright. All rights reserved.
      PubDate: 2016-10-13T03:46:18.997671-05:
      DOI: 10.1002/2016JC011696
       
  • Gulf Stream variability and a triggering mechanism of its large meander in
           the South Atlantic Bight
    • Authors: Xiangming Zeng; Ruoying He
      Abstract: The Gulf Stream (GS) variability has an important impact on coastal circulation, shelf ecosystem, and regional weather and climate systems. Here we focus on the variability of the GS south of Cape Hatteras in the South Atlantic Bight (SAB). Statistical analysis on the 21-year satellite altimetry data reveals that the GS path in the SAB has two patterns: weakly and strongly deflected. The strongly deflected pattern is more likely to occur in winter. Over the last two decades, the largest GS offshore meander occurred in November 2009-April 2010. Realistic ocean hindcast simulation and adjoint sensitivity analysis are used to investigate the triggering mechanisms for this extreme event. Our analyses show that a net increase of relative vorticity near the Charleston Bump was generated by strong interaction between increased GS velocity and local bathymetry, pushing the GS further offshore by virtue of conserving the potential vorticity. Quantitative vorticity analysis confirms this finding. This article is protected by copyright. All rights reserved.
      PubDate: 2016-10-13T03:46:11.526959-05:
      DOI: 10.1002/2016JC012077
       
  • Impact of an upgraded model in the NCEP Global Ocean Data Assimilation
           System: The tropical Indian Ocean
    • Authors: Hasibur Rahaman; David Behringer, Stephen G. Penny, M Ravichandran
      Abstract: The National Centers for Environmental Prediction (NCEP) and the Indian National Centre for Ocean Information Services (INCOIS) produces global ocean analysis based on the Global Ocean Data Assimilation System (GODAS). This study shows how upgrades to the forward model simulations from MOM4p0d to MOM4p1 impact ocean analyses over the tropical Indian Ocean in GODAS. Three experiments were performed with same atmospheric forcing fields:(i) using MOM4p0d (GODAS_p0), (ii) using MOM4p1 (GODAS_p1), both using observed temperature and synthetic salinity, and (iii) using MOM4p1 (GODAS_p1S) assimilating both observed temperature and observed salinity. Validation with independent observations show significant improvement of sub-surface temperature and salinity in the new analysis using MOM4p1 versus MOM4p0d. There is also improvement in the upper ocean current of the equatorial Indian Ocean. The impact of observed salinity on the upper ocean surface current is marginal, but there is significant improvement in the sub-surface current. The seasonal and inter-annual variability of the Wyrtki jet and the equatorial undercurrent is improved in GODAS_p1 versus GODAS_p0. All analyses reproduced the Indian Ocean dipole, with the GODAS_p1S simulated sea surface temperature (SST) the most accurate. The temperature inversion over the north Bay of Bengal (BoB) is reproduced only in GODAS_p1S. The mean sea level over BoB and equatorial Indian Ocean improved in GODAS_p1S as compared with AVISO observation. The combined model upgrade and assimilation of observed salinity led to reduced root mean square deviation and higher correlation coefficient values in the sea level anomaly (SLA) when compared with satellite observations. This article is protected by copyright. All rights reserved.
      PubDate: 2016-10-13T03:45:59.559059-05:
      DOI: 10.1002/2016JC012056
       
  • Observations of open-ocean deep convection in the northwestern
           Mediterranean Sea: Seasonal and interannual variability of mixing and deep
           water masses for the 2007–2013 period
    • Authors: L. Houpert; X. Durrieu de Madron, P. Testor, A. Bosse, F. D'Ortenzio, M.N. Bouin, D. Dausse, H. Le Goff, S. Kunesch, M. Labaste, L. Coppola, L. Mortier, P. Raimbault
      Abstract: We present here a unique oceanographic and meteorological dataset focus on the deep convection processes. Our results are essentially based on in situ data (mooring, research vessel, glider, and profiling float) collected from a multi-platform and integrated monitoring system (MOOSE: Mediterranean Ocean Observing System on Environment), which monitored continuously the northwestern Mediterranean Sea since 2007, and in particular high-frequency potential temperature, salinity and current measurements from the mooring LION located within the convection region.From 2009 to 2013, the mixed layer depth reaches the seabed, at a depth of 2330m, in February. Then, the violent vertical mixing of the whole water column lasts between 9 and 12 days setting up the characteristics of the newly-formed deep water. Each deep convection winter formed a new warmer and saltier '“vintage” of deep water. These sudden inputs of salt and heat in the deep ocean are responsible for trends in salinity (3.3+/-0.2 *10−3/yr) and potential temperature (3.2+/-0.5 *10−3°C/yr) observed from 2009 to 2013 for the 600-2300m layer.For the first time, the overlapping of the 3 “phases” of deep convection can be observed with secondary vertical mixing events (2-4 days) after the beginning of the restratification phase, and the restratification/spreading phase still active at the beginning of the following deep convection event. This article is protected by copyright. All rights reserved.
      PubDate: 2016-10-13T03:43:19.995887-05:
      DOI: 10.1002/2016JC011857
       
  • Dissipation of wind waves by pancake and frazil ice in the autumn Beaufort
           Sea
    • Authors: W. Erick Rogers; Jim Thomson, Hayley H. Shen, Martin J. Doble, Peter Wadhams, Sukun Cheng
      Abstract: A model for wind-generated surface gravity waves, WAVEWATCH III®, is used to analyze and interpret buoy measurements of wave spectra. The model is applied to a hindcast of a wave event in sea ice in the western Arctic, October 11-14 2015, for which extensive buoy and ship-borne measurements were made during a research cruise. The model, which uses a viscoelastic parameterization to represent the impact of sea ice on the waves, is found to have good skill—after calibration of the effective viscosity—for prediction of total energy, but over-predicts dissipation of high frequency energy by the sea ice. This shortcoming motivates detailed analysis of the apparent dissipation rate. A new inversion method is applied to yield, for each buoy spectrum, the inferred dissipation rate as a function of wave frequency. For 102 of the measured wave spectra, visual observations of the sea ice were available from buoy-mounted cameras, and ice categories (primarily for varying forms of pancake and frazil ice) are assigned to each based on the photographs. When comparing the inversion-derived dissipation profiles against the independently derived ice categories, there is remarkable correspondence, with clear sorting of dissipation profiles into groups of similar ice type. These profiles are largely monotonic: they do not exhibit the “roll-over” that has been found at high frequencies in some previous observational studies. This article is protected by copyright. All rights reserved.
      PubDate: 2016-10-13T03:35:53.941718-05:
      DOI: 10.1002/2016JC012251
       
  • Chromophoric and fluorescent dissolved organic matter in and above the
           oxygen minimum zone off Peru
    • Authors: A.N. Loginova; S. Thomsen, A. Engel
      Abstract: As a result of nutrient upwelling, the Peruvian coastal system is one of the most productive regions in the ocean. Sluggish ventilation of intermediate waters, characteristic for the Eastern Tropical South Pacific (ETSP) and microbial degradation of a high organic matter load promotes deoxygenation at depth. Dissolved organic matter (DOM) plays a key role in microbial respiration and carbon cycling, but little is known on DOM distribution and cycling in the ETSP. DOM optical properties give important insights on DOM sources, structure and biogeochemical reactivity. Here, we present data and a conceptual view on distribution and cycling of chromophoric (CDOM) and fluorescent (FDOM) DOM in and above the oxygen minimum zone (OMZ) off Peru. Five fluorescent components were identified during PARAFAC analysis. Highest intensities of CDOM and of the amino acid-like fluorescent component (C3) occurred above the OMZ and coincided with maximum chl a concentrations, suggesting phytoplankton productivity as major source. High intensities of a marine humic-like fluorescent component (C1), observed in subsurface waters, indicated in situ microbial reworking of DOM. FDOM release from inner shelf sediment was determined by seawater analysis and continuous glider sensor measurement and included a humic-like component (C2) with a signature typical for terrestrially derived humic acids. Upwelling supplied humic-like substances to the euphotic zone. Photo-reactions were likely involved in the production of a humic-like fluorescent component (C5). Our data show that variable biological and physical processes need to be considered for understanding DOM cycling in a highly dynamic coastal upwelling system like the ETSP off Peru. This article is protected by copyright. All rights reserved.
      PubDate: 2016-10-13T03:30:53.767001-05:
      DOI: 10.1002/2016JC011906
       
  • Decomposition of thermal and dynamic changes in the South China Sea
           induced by boundary forcing and surface fluxes during 1970‒2000
    • Authors: Jun Wei; Paola Malanotte-Rizzoli, Ming-Ting Li, Hao Wang
      Abstract: Based on a fully-coupled, high-resolution regional climate model, this study analyzed three-dimensional temperature and momentum changes in the South China Sea (SCS) from 1970 to 2000, during which period the climate shifts from a decadal La Niña-like condition (before 1976/77) to a decadal El Niño-like condition afterward. With a set of partially-coupled experiments, sea surface temperature (SST) and kinetic energy (KE) changes during this period are first decomposed into two components: those induced by lateral boundary forcing and those induced by atmospheric surface fluxes. The results showed that the total SST and KE changes show an increasing trend from 1970 to 2000. The two decomposed components together determined 96% and 89% of the SST and KE changes respectively, implying their dominant roles on the SCS's surface variability. Spatially, a sandwich pattern of air-sea forcing relationship is revealed in the SCS basin. The increased KE, represented by a cyclonic flow anomaly in the northern SCS, was induced by enhanced cold water intrusion from Pacific into the SCS via the Luzon Strait (boundary forcing). This cold-water inflow, however, resulted in SST cooling along the northern shelf of the SCS. The maximal SST warming occurred in the central SCS and was attributed to the wind-evaporation-SST (WES) positive feedback (surface forcing), in which a southwestward wind anomaly is initialized by SST gradients between the northern and southern SCS. This wind anomaly decelerates the southwestly summer monsoons and in turn increases the SST gradients. Over the shallow Sunda shelf, which is far from the Luzon Strait, the SST/KE variability appeared to be determined primarily by local air–sea interactions. Furthermore, analyses on subsurface components indicated that the subsurface temperature changes are primarily induced by internal ocean mixing, which becomes significantly important below the thermocline. The enhanced subsurface flow is driven by the Luzon Strait inflow as well, and exits the SCS via the Mindoro-Sibutu passage. This article is protected by copyright. All rights reserved.
      PubDate: 2016-10-07T10:30:51.901249-05:
      DOI: 10.1002/2016JC012078
       
  • Time series measurements of transient tracers and tracer derived transport
           in the deep western boundary current between the Labrador Sea and the
           subtropical Atlantic Ocean at Line W
    • Authors: John N. Smith; William M. Smethie, Igor Yashayev, Ruth Curry, Kumiko Azetsu-Scott
      Abstract: Time series measurements of the nuclear fuel reprocessing tracer, 129I and the gas ventilation tracer, CFC-11 were undertaken on the AR7W section in the Labrador Sea (1997-2014) and on Line W (2004-2014), located over the US continental slope off Cape Cod, to determine advection and mixing time scales for the transport of Denmark Strait Overflow Water (DSOW) within the Deep Western Boundary Current (DWBC). Tracer measurements were also conducted in 2010 over the continental rise southeast of Bermuda to intercept the equator-ward flow of DSOW by interior pathways. The Labrador Sea tracer and hydrographic time series data were used as input functions in a boundary current model that employs transit time distributions to simulate the effects of mixing and advection on downstream tracer distributions. Model simulations of tracer levels in the boundary current core and adjacent interior (shoulder) region with which mixing occurs were compared with the Line W time series measurements to determine boundary current model parameters. These results indicate that DSOW is transported from the Labrador Sea to Line W via the DWBC on a time scale of 5-6 y corresponding to a mean flow velocity of 2.7 cm/s while mixing between the core and interior regions occurs with a time constant of 2.6 y. A tracer section over the southern flank of the Bermuda rise indicates that the flow of DSOW that separated from the DWBC had undergone transport through interior pathways on a time scale of 9 y with a mixing time constant of 4 y. This article is protected by copyright. All rights reserved.
      PubDate: 2016-10-05T10:25:55.624726-05:
      DOI: 10.1002/2016JC011759
       
  • The Beaufort Gyre intensification and stabilization: A model-observation
           synthesis
    • Authors: Jinlun Zhang; Michael Steele, Kay Runciman, Sarah Dewey, James Morison, Craig Lee, Luc Rainville, Sylvia Cole, Richard Krishfield, Mary-Louise Timmermans, John Toole
      Abstract: A model–observation synthesis is conducted to investigate changes in the upper ocean circulation and stratification in the Canada Basin (CB) of the Arctic Ocean. Results show that the Beaufort Gyre (BG) has been generally intensifying during 1992–2015 in conjunction with changes in sea ice and the upper ocean including increasing sea surface height (SSH), sea ice and ocean speed, Ekman transport convergence and downwelling, and freshwater content, decreasing ice thickness and upper ocean salinity, shoaling summer halocline and mixed layer, and deepening winter halocline and mixed layer. Increasing Ekman transport convergence draws more water from surrounding areas into the CB, thus lowering SSH in those areas and raising SSH in the CB. The rate of change in the CB began to decrease in 2008 and the BG circulation appears to be stabilizing, if not relaxing slightly. This is reflected in the general plateauing of SSH, the intensity of the sea ice and ocean circulation, and various measures of the CB thermohaline stratification. The BG intensification and subsequent stabilization appear to have been strongly controlled by atmospheric changes in the CB characterized by generally increasing anticyclonic wind circulation and sea level pressure (SLP) before 2008 and falling wind strength and SLP to below-average levels in some years after 2008. Changes in SLP are highly correlated with changes in ocean surface stress curl and downwelling. Since 2008, the magnitude of the stress curl and downwelling in much of the CB has declined, contributing to BG stabilization. The general leveling-off of sea ice thickness also contributes to the stabilization by limiting melt water input to the CB that increases freshwater content. Temperatures in the Near Surface Temperature Maximum layer trended upward slightly over 1992–2015, which is closely correlated with decreasing sea ice thickness. Upper ocean heat content increased over the study period mainly due to strong temperature increases in the summer Pacific Water layer. This article is protected by copyright. All rights reserved.
      PubDate: 2016-10-04T10:25:53.440564-05:
      DOI: 10.1002/2016JC012196
       
  • Recurrent Replenishment of Labrador Sea Water and Associated Decadal-Scale
           Variability
    • Authors: Igor Yashayaev; John W. Loder
      Abstract: Winter convective overturning in the Labrador Sea reached an “aggregate” maximum depth of 1700m in 2015 – the deepest since 1994 – with the resulting Labrador Sea Water (LSW) “year class” being one of the deepest and thickest observed outside of the early 1990s. Argo float, annual survey and moored measurements in recent decades provide an unprecedented view of important seasonal, interannual and longer-term LSW variability in the Labrador Sea region. During the 2002-2015 “Argo” era, the average winter LSW pycnostad volume was about 70% larger in relatively strong convection years than in relatively weak ones. However, the winter-to-fall LSW disappearance volume was 180% larger, pointing to a factor of 2.8 difference in the potential LSW export rates from the region between relatively strong and weak convection years. Intermittently recurrent deep convection is contributing to predominant decadal-scale variations in intermediate-depth temperature, salinity and density in the LS, with implications for decadal-scale variability across the subpolar North Atlantic and potentially in the Atlantic Meridional Overturning Circulation. Comparison of the LS ocean heat content changes and cumulative surface heat losses during the fall-winter cooling seasons indicates that anomalously strong winter atmospheric cooling, associated at least in part with the North Atlantic Oscillation, is continuing to be a major forcing of the recurrent convection. This article is protected by copyright. All rights reserved.
      PubDate: 2016-09-30T03:45:42.867176-05:
      DOI: 10.1002/2016JC012046
       
  • Issue Information
    • Pages: 7931 - 7932
      PubDate: 2016-12-25T12:58:02.210013-05:
      DOI: 10.1002/jgrc.21411
       
 
 
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