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

Geochemistry, Geophysics, Geosystems     Full-text available via subscription   (Followers: 27, SJR: 2.56, h-index: 69)
Geophysical Research Letters     Full-text available via subscription   (Followers: 60, SJR: 3.493, h-index: 157)
Global Biogeochemical Cycles     Full-text available via subscription   (Followers: 8, SJR: 3.239, h-index: 119)
J. of Advances in Modeling Earth Systems     Open Access   (Followers: 3, SJR: 1.944, h-index: 7)
J. of Geophysical Research : Atmospheres     Partially Free   (Followers: 28)
J. of Geophysical Research : Biogeosciences     Full-text available via subscription   (Followers: 9)
J. of Geophysical Research : Earth Surface     Partially Free   (Followers: 26)
J. of Geophysical Research : Oceans     Partially Free   (Followers: 15)
J. of Geophysical Research : Planets     Full-text available via subscription   (Followers: 19)
J. of Geophysical Research : Solid Earth     Full-text available via subscription   (Followers: 28)
J. of Geophysical Research : Space Physics     Full-text available via subscription   (Followers: 18)
Paleoceanography     Full-text available via subscription   (Followers: 3, SJR: 3.22, h-index: 88)
Radio Science     Full-text available via subscription   (Followers: 5, SJR: 0.959, h-index: 51)
Reviews of Geophysics     Full-text available via subscription   (Followers: 21, SJR: 9.68, h-index: 94)
Space Weather     Full-text available via subscription   (Followers: 4, SJR: 1.319, h-index: 19)
Tectonics     Full-text available via subscription   (Followers: 9, SJR: 2.748, h-index: 85)
Water Resources Research     Full-text available via subscription   (Followers: 79, SJR: 2.189, h-index: 121)
Journal Cover Journal of Geophysical Research : Oceans
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   Published by American Geophysical Union (AGU) Homepage  [17 journals]
  • Forum for Arctic Modeling and Observational Synthesis (FAMOS): Past,
           current, and future activities
    • Abstract: The overall goal of the Forum for Arctic Modeling and Observing Synthesis (FAMOS) community activities reported in this special issue is to enhance understanding of processes and mechanisms driving Arctic Ocean marine and sea ice changes, and the consequences of those changes especially in biogeochemical and ecosystem studies. Major 2013‐2015 FAMOS accomplishments to date are: identification of consistent errors across Arctic regional models; approaches to reduce these errors, and recommendations for the most effective coupled sea ice – ocean models for use in fully coupled regional and global climate models. 2013‐2015 FAMOS coordinated analyses include many process studies, using models together with observations to investigate: dynamics and mechanisms responsible for drift, deformation and thermodynamics of sea ice; pathways and mechanisms driving variability of the Atlantic, Pacific and river waters in the Arctic Ocean; processes of freshwater accumulation and release in the Beaufort Gyre; the fate of melt water from Greenland; characteristics of ocean eddies; biogeochemistry and ecosystem processes and change, climate variability and predictability. Future FAMOS collaborations will focus on employing models and conducting observations at high and very high spatial and temporal resolution to investigate the role of sub‐grid scale processes in regional Arctic Ocean and coupled ice‐ocean and atmosphere‐ice‐ocean models. This article is protected by copyright. All rights reserved.
      PubDate: 2016-04-26T10:16:21.930413-05:
      DOI: 10.1002/2016JC011898
       
  • Data and numerical analysis of astronomic tides, wind‐waves, and
           hurricane storm surge along the northern Gulf of Mexico
    • Authors: M.V. Bilskie; S.C. Hagen, S.C. Medeiros, A.T. Cox, M. Salisbury, D. Coggin
      Abstract: The northern Gulf of Mexico (NGOM) is a unique geophysical setting for complex tropical storm‐induced hydrodynamic processes that occur across a variety of spatial and temporal scales. Each hurricane includes its own distinctive characteristics and can cause unique and devastating storm surge when it strikes within the intricate geometric setting of the NGOM. While a number of studies have explored hurricane storm surge in the NGOM, few have attempted to describe storm surge and coastal inundation using observed data in conjunction with a single large‐domain high‐resolution numerical model. To better understand the oceanic and nearshore response to these tropical cyclones, we provide a detailed assessment, based on field measurements and numerical simulation, of the evolution of wind waves, water levels, and currents for Hurricanes Ivan (2004), Dennis (2005), Katrina (2005), and Isaac (2012), with focus on Mississippi, Alabama, and the Florida Panhandle coasts. The developed NGOM3 computational model describes the hydraulic connectivity among the various inlet and bay systems, Gulf Intracoastal Waterway, coastal rivers and adjacent marsh, and built infrastructure along the coastal floodplain. The outcome is a better understanding of the storm surge generating mechanisms and interactions among hurricane characteristics and the NGOM's geophysical configuration. The numerical analysis and observed data explain the ∼2 m/s hurricane‐induced geostrophic currents across the continental shelf, a 6 m/s outflow current during Ivan, the hurricane‐induced coastal Kelvin wave along the shelf, and for the first time a wealth of measured data and a detailed numerical simulation was performed and was presented for Isaac. This article is protected by copyright. All rights reserved.
      PubDate: 2016-04-26T10:15:50.672861-05:
      DOI: 10.1002/2015JC011400
       
  • Interannual modulation of eddy kinetic energy in the northeastern South
           China Sea as revealed by an eddy‐resolving OGCM
    • Authors: Zhongbin Sun; Zhiwei Zhang, Wei Zhao, Jiwei Tian
      Abstract: Interannual modulation of eddy kinetic energy (EKE) in the northeastern South China Sea (NE‐SCS) is investigated based on outputs of an eddy‐resolving oceanic general circulation model between 1980–2014. The EKE displays distinct interannual modulations with periods between 1.5 and 7 years. The maximum peak‐to‐trough amplitude of the interannual modulation occurred during period 2004–2005, which was about 1.5‐fold the time‐mean EKE level. Further analysis suggested that interannual variability of EKE in the NE‐SCS is primarily modulated by the Luzon Strait transport (LST). During high‐EKE years, the LST increases corresponding to a strengthened Kuroshio intrusion. The strengthened Kuroshio intrusion enhances the baroclinic instability of current in the NE‐SCS and thus leads to a strong EKE. The reverse is true during low‐EKE years when LST is smaller. Influences of ENSO and Pacific mesoscale eddies on the interannual modulation of LST are also discussed in this study. This article is protected by copyright. All rights reserved.
      PubDate: 2016-04-22T04:55:31.47693-05:0
      DOI: 10.1002/2015JC011497
       
  • Internal swells in the tropics: Near‐inertial wave energy fluxes and
           dissipation during CINDY
    • Authors: S. M. Soares; A. Natarov, K. J. Richards
      Abstract: A developing MJO event in the tropical Indian Ocean triggered wind disturbances that generated inertial oscillations in the surface mixed layer. Subsequent radiation of near‐inertial waves below the mixed layer produced strong turbulence in the pycnocline. Linear plane wave dynamics and spectral analysis are used to explain these observations, with the ultimate goal of estimating the wave energy flux in relation to both the energy input by the wind and the dissipation by turbulence. The results indicate that the wave packets carry approximately 30‐40% of the wind input of inertial kinetic energy, and propagate in an environment conducive to the occurrence of a critical level set up by a combination of vertical gradients in background relative vorticity and Doppler shifting of wave frequency. Turbulent kinetic energy dissipation measurements demonstrate that the waves lose energy as they propagate in the transition layer as well as in the pycnocline, where approaching this critical level may have dissipated approximately 20% of the wave packet energy in a single event. Our analysis, therefore, supports the notion that appreciable amounts of wind‐induced inertial kinetic energy escape the surface boundary layer into the interior. However, a large fraction of wave energy is dissipated within the pycnocline, limiting its full depth penetration and transfer to the abyssal ocean. This article is protected by copyright. All rights reserved.
      PubDate: 2016-04-21T19:29:18.876105-05:
      DOI: 10.1002/2015JC011600
       
  • The influence of environmental parameters on active and maturing oceanic
           whitecaps
    • Authors: B. Scanlon; B. Ward
      Abstract: High resolution images of the ocean surface are examined using digital processing, achieving quantifications of actively breaking (WA), maturing (WB) and total (WT=WA+WB) whitecaps. The images are selected from two datasets of the North Atlantic and Southern Ocean to sample a maximal range of environmental conditions. A total of 125,860 images were processed and averaged to establish 622 10‐minute periods. Parameterizing WA, WB and WT with wind speed achieved modest correlations while also displaying large variabilities. Parameterizing WT with wind speed and specific Reynolds numbers achieved correlation coefficients ranging from 0.76 to 0.79. The filtering of WT into its active stage of evolution WA and subsequent fittings with wind speed and specific Reynolds numbers achieved reduced correlation coefficients ranging from 0.62 to 0.66. Unresolved scatter was revealed for low and mid‐range Reynolds numbers. Thus Reynolds numbers may not suitable parameters for quantifying WA. We suggest that the contribution of WB serves to conceal and thus underestimate the variability of actively breaking waves. This article is protected by copyright. All rights reserved.
      PubDate: 2016-04-21T19:27:28.367005-05:
      DOI: 10.1002/2015JC011230
       
  • Vertical heat flux in the ocean: Estimates from observations and from a
           coupled general circulation model
    • Authors: Patrick F. Cummins; Diane Masson, Oleg A. Saenko
      Abstract: The net heat uptake by the ocean in a changing climate involves small imbalances between the advective and diffusive processes that transport heat vertically. Generally, it is necessary to rely on global climate models to study these processes in detail. In the present study, it is shown that a key component of the vertical heat flux, namely that associated with the large‐scale mean vertical circulation, can be diagnosed over extra‐tropical regions from global observational data sets. This component is estimated based on the vertical velocity obtained from the geostrophic vorticity balance, combined with estimates of absolute geostrophic flow. Results are compared with the output of a non‐eddy resolving, coupled atmosphere‐ocean general circulation model. Reasonable agreement is found in the latitudinal distribution of the vertical heat flux, as well as in the area‐integrated flux below about 250 meters depth. The correspondence with the coupled model deteriorates sharply at depths shallower than 250 m due to the omission of equatorial regions from the calculation. The vertical heat flux due to the mean circulation is found to be dominated globally by the downward contribution from the Southern Hemisphere, in particular the Southern Ocean. This is driven by the Ekman vertical velocity which induces an upward transport of seawater that is cold relative to the horizontal average at a given depth. The results indicate that the dominant characteristics of the vertical transport of heat due to the mean circulation can be inferred from simple linear vorticity dynamics over much of the ocean. This article is protected by copyright. All rights reserved.
      PubDate: 2016-04-21T19:21:11.212307-05:
      DOI: 10.1002/2016JC011647
       
  • Modeling surfzone to inner‐shelf tracer exchange
    • Abstract: A near‐shoreline, continuous dye release at an approximately alongshore‐uniform beach (IB09 experiment) is simulated with the wave‐resolving Boussinesq model funwaveC. The model generates surfzone eddies and transient rip currents but does not resolve inner‐shelf vertical variation or stratification. The funwaveC odel reproduces well the observed surfzone and inner‐shelf dye observations over roughly 350 m cross‐shore and 2000 m alongshore. Dye is advected alongshore by wave‐ and wind‐driven currents similarly in the observations and model. Near‐shoreline mean dye concentration decays downstream as a power law with similar observed (‐0.33) and modeled (‐0.38) exponents. Observed and modeled cross‐shore mean dye profiles are similar, though modeled inner‐shelf dye is somewhat elevated. Observed and modeled alongshore dye transports agree, though with compensating surfzone and inner‐shelf errors later in the release. For times
      PubDate: 2016-04-21T19:20:56.361575-05:
      DOI: 10.1002/2015JC011530
       
  • Effect of subseabed salt domes on tidal residual currents in the Persian
           Gulf
    • Authors: Hossein Mashayekh Poul; Jan Backhaus, Ali Dehghani, Udo Huebner
      Abstract: Geological studies in the Persian Gulf (PG) have revealed the existence of sub‐seabed salt‐domes. With suitable filtering of a high‐resolution PG seabed topography, it is seen that the domes leave their signature in the seabed, i.e. numerous hills and valleys with amplitudes of several tens of meters and radii from a few up to tens of kilometers. It was suspected that the 'shark skin' of the PG seabed may affect the tidal residual flow. The interaction of tidal dynamics and these obstacles was investigated in a non‐linear hydrodynamic numerical tidal model of the PG. The model was first used to characterize flow patterns of residual currents generated by a tidal wave passing over symmetric, elongated and tilted obstacles. Thereafter it was applied to the entire PG. The model was forced at its open boundary by the four dominant tidal constituents residing in the PG. Each tidal constituent was simulated separately. Results, i.e. tidal residual currents in the PG, as depicted by Lagrangian trajectories reveal a stationary flow that is very rich in eddies. Each eddy can be identified with a topographic obstacle. This confirms that the tidal residual flow field is strongly influenced by the nonlinear interaction of the tidal wave with the bottom relief which, in turn, is deformed by salt‐domes beneath the seabed. Different areas of maximum residual current velocities are identified for major tidal constituents. The pattern of trajectories indicates the presence of two main cyclonic gyres and several adjacent gyres rotating in opposite directions and a strong coastal current in the northern PG. This article is protected by copyright. All rights reserved.
      PubDate: 2016-04-21T19:20:36.78354-05:0
      DOI: 10.1002/2015JC011421
       
  • Eddy properties in the western Mediterranean Sea from satellite altimetry
           and a numerical simulation
    • Authors: Romain Escudier; Lionel Renault, Ananda Pascual, Pierre Brasseur, Dudley Chelton, Jonathan Beuvier
      Abstract: Three different eddy detection and tracking methods are applied to the outputs of a high‐resolution simulation in the Western Mediterranean Sea in order to extract mesoscale eddy characteristics. The results are compared with the same eddy statistics derived from satellite altimetry maps over the same period. Eddy radii are around 30 km in altimetry maps whereas, in the model, they are around 20 km. This is probably due to the inability of altimetry maps to resolve the smaller mesoscale in the region. About 30 eddies are detected per day in the basin with a very heterogeneous spatial distribution and relatively short lifespans (median life around 13 days). Unlike other areas of the open ocean, they do not have a preferred direction of propagation but appear to be advected by mean currents. The number of detected eddies seems to present an annual cycle when separated according to their lifespan. With the numerical simulation, we show that anticyclones extend deeper in the water column and have a more conic shape than cyclones. This article is protected by copyright. All rights reserved.
      PubDate: 2016-04-20T18:15:32.636372-05:
      DOI: 10.1002/2015JC011371
       
  • Sea ice circulation around the Beaufort Gyre: The changing role of wind
           forcing and the sea ice state
    • Abstract: Sea ice drift estimates from feature tracking of satellite passive microwave data are used to investigate seasonal trends and variability in the ice circulation around the Beaufort Gyre, over the multi‐decadal period 1980–2013. Our results suggest an amplified response of the Beaufort Gyre ice circulation to wind forcing, especially during the late 2000s. We find increasing anticyclonic ice drift across all seasons, with the strongest trend in autumn, associated with increased ice export out of the southern Beaufort Sea (into the Chukchi Sea). A flux gate analysis highlights consistency across a suite of drift products. Despite these seasonal anticyclonic ice drift trends, a significant anticyclonic wind trend occurs in summer only, driven, in‐part, by anomalously anticyclonic winds in 2007. Across all seasons, the ice drift curl is more anticyclonic than predicted from a linear relationship to the wind curl in the 2000s, compared to the 1980s/1990s. The strength of this anticyclonic ice drift curl amplification is strongest in autumn and appears to have increased since the 1980s (up to 2010). In spring and summer, the ice drift curl amplification occurs mainly between 2007 and 2010. These results suggest non‐linear ice interaction feedbacks (e.g. a weaker, more mobile sea ice pack), enhanced atmospheric drag, and/or an increased role of the ocean. The results also show a weakening of the anticyclonic wind and ice circulation since 2010. This article is protected by copyright. All rights reserved.
      PubDate: 2016-04-20T18:11:46.344743-05:
      DOI: 10.1002/2015JC010903
       
  • A tale of three islands: Downstream natural iron fertilization in the
           Southern Ocean
    • Authors: J. Robinson; E.E. Popova, M. Srokosz, A. Yool
      Abstract: Iron limitation of primary productivity prevails across much of the Southern Ocean but there are exceptions; in particular, the phytoplankton blooms associated with the Kerguelen Plateau, Crozet Islands and South Georgia. These blooms occur annually, fertilized by iron and nutrient‐rich shelf waters that are transported downstream from the islands. Here we use a highresolution (1/12°) ocean general circulation model and Lagrangian particle tracking to investigate whether inter‐annual variability in the potential lateral advection of iron, could explain the inter‐annual variability in the spatial extent of the blooms. Comparison with ocean color data, 1998 to 2007, suggests that iron fertilization via advection can explain the extent of each island's annual bloom, but only the inter‐annual variability of the Crozet bloom. The area that could potentially be fertilized by iron from Kerguelen was much larger than the bloom, suggesting that there is another primary limiting factor, potentially silicate, that controls the inter‐annual variability of bloom spatial extent. For South Georgia, there are differences in the year‐to‐year timing of advection and consequently fertilization, but no clear explanation of the inter‐annual variability observed in the bloom's spatial extent has been identified. The model results suggest that the Kerguelen and Crozet blooms are terminated by nutrient exhaustion, probably iron and or silicate, whereas the deepening of the mixed layer in winter terminates the South Georgia bloom. Therefore, iron fertilization via lateral advection alone can explain the annual variability of the Crozet bloom, but not fully that of the Kerguelen and South Georgia blooms. This article is protected by copyright. All rights reserved.
      PubDate: 2016-04-19T10:36:13.665908-05:
      DOI: 10.1002/2015JC011319
       
  • Observations of wave transformation over a fringing coral reef and the
           importance of low‐frequency waves and offshore water levels to
           runup, overwash, and coastal flooding
    • Authors: Olivia M. Cheriton; Curt D. Storlazzi, Kurt J. Rosenberger
      Abstract: Many low‐lying tropical islands are susceptible to sea‐level rise and often subjected to overwash and flooding during large wave events. To quantify wave dynamics and wave‐driven water levels on fringing coral reefs, a 5‐month deployment of wave gauges and a current meter was conducted across two shore‐normal transects on Roi‐Namur Island in the Republic of the Marshall Islands. These observations captured two large wave events that had waves with maximum heights greater than 6 m with peak periods of 16 s over the fore reef. The larger event coincided with a peak spring tide, leading to energetic, highly‐skewed infragravity (0.04‐0.004 Hz) and very low frequency (0.004‐0.001 Hz) waves at the shoreline, which reached heights of 1.0 and 0.7 m, respectively. Water surface elevations, combined with wave runup, reached 3.7 m above the reef bed at the innermost reef flat adjacent to the toe of the beach, resulting in flooding of inland areas. This overwash occurred during a 3‐hr time window that coincided with high tide and maximum low‐frequency reef flat wave heights. The relatively low‐relief characteristics of this narrow reef flat may further drive shoreline amplification of low‐frequency waves due to resonance modes. These results (1) demonstrate how the coupling of high offshore water levels with low‐frequency reef flat wave energetics can lead to large impacts along fringing reef‐lined shorelines, such as island overwash, and (2) lend support to the hypothesis that predicted higher sea levels will lead to more frequent occurrences of these extreme events, negatively impacting coastal resources and infrastructure. This article is protected by copyright. All rights reserved.
      PubDate: 2016-04-19T10:36:09.334382-05:
      DOI: 10.1002/2015JC011231
       
  • Decadal‐scale thermohaline variability in the Atlantic sector of the
           Southern Ocean
    • Authors: K. Hutchinson; S. Swart, A. Meijers, I. Ansorge, S. Speich
      Abstract: An enhanced Altimetry Gravest Empirical Mode (AGEM), including both adiabatic and diabatic trends, is developed for the Antarctic Circumpolar Current (ACC) south of Africa using updated hydrographic CTD sections, Argo data, and satellite altimetry. This AGEM has improved accuracy compared to traditional climatologies and other proxy methods. The AGEM for the Atlantic Southern Ocean offers an ideal technique to investigate the thermohaline variability over the past two decades in a key region for water mass exchanges and transformation. In order to assess and attribute changes in the hydrography of the region, we separate the changes into adiabatic and diabatic components. Integrated over the upper 2000 dbar of the ACC south of Africa, results show mean adiabatic changes of 0.16 ± 0.11°C.decade−1 and 0.006 ± 0.014 decade−1, and diabatic differences of ‐0.044 ± 0.13°C.decade−1 and ‐0.01 ± 0.017 .decade−1 for temperature and salinity, respectively. The trends of the resultant AGEM, that include both adiabatic and diabatic variability (termed AD‐AGEM), show a significant increase in the heat content of the upper 2000dbar of the ACC with a mean warming of 0.12 ± 0.087°C.decade−1. This study focuses on the Antarctic Intermediate Water (AAIW) mass where negative diabatic trends dominate positive adiabatic differences in the Subantarctic Zone (SAZ), with results indicating a cooling (‐0.17°C.decade−1) and freshening (‐0.032 decade−1) of AAIW in this area, whereas south of the SAZ positive adiabatic and diabatic trends together create a cumulative warming (0.31°C.decade−1) and salinification (0.014 decade−1) of AAIW. This article is protected by copyright. All rights reserved.
      PubDate: 2016-04-19T10:36:00.035638-05:
      DOI: 10.1002/2015JC011491
       
  • Sea ice production variability in Antarctic coastal polynyas
    • Authors: Takeshi Tamura; Kay I. Ohshima, Alexander D. Fraser, Guy D. Williams
      Abstract: Enhanced sea ice production (SIP) in Antarctic coastal polynyas forms dense shelf water (DSW), leading to Antarctic Bottom Water (AABW) formation that ultimately drives the lower limb of the meridional overturning circulation. Some studies suggest that the variability of SIP in Antarctic coastal polynyas is driven by the influence of atmospheric forcing, i.e., surface winds and air temperature. Our previous mapping of SIP in 13 major Antarctic coastal polynyas from 1992 to 2007, using a heat‐flux calculation with ice thickness data derived from satellite data, is extended here to examine the interannual and seasonal variability of SIP from 1992 to 2013. The interannual variability of total ice production correlates more strongly with polynya extent than with atmospheric forcing, with the exception of the Shackleton Polynya, which correlates well with wind. There is no coherent signal in the interannual variability between the major Antarctic coastal polynyas. We find that stochastic changes to the coastal ‘icescape', i.e., ice shelves, floating glaciers, fast ice, together with offshore first‐year ice, are also important factors driving SIP variability on multiyear timescales. Both the Ross Ice Shelf Polynya and Mertz Glacier Polynya experienced a significant reduction in SIP due to calving events and the re‐positioning of icebergs and fast‐ice. Our results also show opposing trends between polynya‐based SIP and sea ice extent in key regions of Antarctic sea ice change. Close monitoring of coastal icescape dynamics and change is essential to better understand the long‐term impact of coastal polynya variability and its influence on regional AABW production. This article is protected by copyright. All rights reserved.
      PubDate: 2016-04-15T18:56:17.784132-05:
      DOI: 10.1002/2015JC011537
       
  • Wind‐induced variability of estuarine circulation in a tidally
           energetic inlet with curvature
    • Authors: Kaveh Purkiani; Johannes Becherer, Knut Klingbeil, Hans Burchard
      Abstract: In numerous studies, the functioning of estuarine circulation has been investigated, under idealized conditions, by means of numerical models. This has led to a deep understanding of the theory of estuarine residual flows. However, the question as to how estuarine circulation is established in real estuaries, in response to their topographical and forcing characteristics, remains. The present study uses a highly accurate three‐dimensional numerical model simulation to calculate estuarine circulation in a curved, tidally‐energetic channel of the Wadden Sea in the southeastern North Sea.The specific momentum balance of this curved inlet shows an approximate pressure‐gradient ‐ frictional balance in the longitudinal direction and a pressure gradient ‐ centrifugal balance in the lateral direction. A local Wedderburn number is introduced to quantify the varying contributions of wind stress and gravitational forcing on estuarine circulation. A total exchange flow (TEF) analysis is combined with an analysis of the intensity of the vertical overturning circulation to understand the dynamics of estuarine exchange in this inlet. The results show how established forcing mechanisms of residual circulation, such as horizontal buoyancy gradients and wind stress, act in a combined way. In general, the strength of estuarine circulation is always positively correlated with wind stress, with frequent reversals of residual flow for wind stress directed towards higher buoyancy. Only during calm weather conditions are longitudinal and lateral estuarine circulation highly correlated with the respective buoyancy gradients. This article is protected by copyright. All rights reserved.
      PubDate: 2016-04-15T18:56:02.606185-05:
      DOI: 10.1002/2015JC010945
       
  • Cadmium in the waters off South Morocco: Nature of particles hosting Cd
           and insights into the mechanisms fractionating Cd from phosphate
    • Abstract: In this study we report the distributions of total dissolvable cadmium and particulate cadmium from 27 stations in southern Moroccan coastal waters (22‐30°N) which is part of the North‐West African upwelling system. These distributions were predominantly controlled by upwelling of the North Atlantic Central Waters (NACW) and uptake by primary production. Atmospheric inputs and phosphogypsum slurry inputs from the phosphate industry at Jorf Lasfar (33°N), recently estimated as an important source of dissolved cadmium (240 tCd year−1), are at best of minor importance for the studied waters. Our study provides new insights into the mechanisms fractionating cadmium from phosphate. In the upper 30 m, the anomalies observed in terms of Cd:P ratios in both the particulate and total dissolvable fractions were related to an overall preferential uptake of phosphate. We show that the type of phytoplanktonic assemblage (diatoms vs dinoflagellates) is also a determinant of the fractionation intensity. In sub‐surface waters (30‐60 m), a clear preferential release of P (vs Cd) was observed indicating that remineralization in oxygen minimum zones is a key process in sequestering Cd. This article is protected by copyright. All rights reserved.
      PubDate: 2016-04-15T18:55:46.225213-05:
      DOI: 10.1002/2016JC011688
       
  • A dynamical model of Kara Sea land‐fast ice
    • Authors: Einar Olason
      Abstract: This paper introduces modifications to the traditional viscous‐plastic sea‐ice dynamical model, which are necessary to model land‐fast ice in the Kara Sea in a realistic manner. The most important modifications are an increase in the maximum viscosity from the standard value of ζmax = (2.5 × 108 s)P to ζmax = (1013 s)P, and to use a solver for the momentum equation capable of correctly solving for small ice velocities (the limit here is set to $10−4 m/s). Given these modifications, a necessary condition for a realistic fast‐ice simulation is that the yield curve give sufficient uni‐axial compressive strength. This is consistent with the idea that land‐fast ice in the Kara Sea forms primarily via static arching. The modified model is tested and tuned using forcing data and observations from 1997 and 1998. The results show that it is possible to model land‐fast ice using this model with the modifications mentioned above. The model performs well in terms of modelled fast‐ice extent, but suffers from unrealistic break‐ups during the start and end of the fast‐ice season. The main results are that fast ice in the Kara Sea is supported by arching of the ice, the arches footers resting on a chain of islands off shore. This article is protected by copyright. All rights reserved.
      PubDate: 2016-04-15T18:55:34.37491-05:0
      DOI: 10.1002/2016JC011638
       
  • Understanding Stokes forces in the wave‐averaged equations
    • Abstract: The wave‐averaged, or Craik‐Leibovich, equations describe the dynamics of upper ocean flow interacting with non‐breaking, not steep, surface gravity waves. This paper formulates the wave effects in these equations in terms of three contributions to momentum: Stokes advection, Stokes Coriolis force, and Stokes shear force. Each contribution scales with a distinctive parameter. Moreover, these contributions affect the turbulence energetics differently from each other such that the classification of instabilities is possible accordingly. Stokes advection transfers energy between turbulence and Eulerian mean‐flow kinetic energy, and its form also parallels the advection of tracers such as salinity, buoyancy, and potential vorticity. Stokes shear force transfers energy between turbulence and surface waves. The Stokes Coriolis force can also transfer energy between turbulence and waves, but this occurs only if the Stokes drift fluctuates. Furthermore, this formulation elucidates the unique nature of Stokes shear force and also allows direct comparison of Stokes shear force with buoyancy. As a result, the classic Langmuir instabilities of Craik and Leibovich, wave‐balanced fronts and filaments, Stokes perturbations of symmetric and geostrophic instabilities, the wavy Ekman layer, and the wavy hydrostatic balance are framed in terms of intuitive physical balances. This article is protected by copyright. All rights reserved.
      PubDate: 2016-04-13T08:26:07.420753-05:
      DOI: 10.1002/2015JC011566
       
  • Seasonal variability of the meridional overturning circulation in the
           South China Sea and its connection with inter‐ocean transport based
           on SODA2.2.4
    • Authors: Yaohua Zhu; Guohong Fang, Zexun Wei, Yonggang Wang, Fei Teng, Tangdong Qu
      Abstract: We have proposed a five‐layer‐scheme to investigate the volume transport through the South China Sea (SCS) based on the updated Simple Ocean Data Assimilation (SODA2.2.4) product. By demonstrating horizontal transport in each layer, we have revealed different formation mechanisms for the meridional overturning circulation (MOC) in winter and summer in the SCS. Our analysis suggests three meridional circulation systems in the SCS: (1) the seasonal monsoon driven circulation in the surface layer, i.e., southward circulation in winter and northward in summer; (2) the compensatory transport induced seasonal intermediate MOC in the central SCS; (3) the persistent deep MOC in the southern SCS all year round. By examining vertical velocity distribution, we have identified that the major overturning process of the intermediate MOC is located along the continental slope east and southeast of Vietnam, while the major overturning process of the deep MOC is located along the continental slope northwest of Borneo. The downwelling in the intermediate MOC in winter and upwelling in the deep MOC all year round bring different water masses to the intermediate and sub‐intermediate layers to be mixed in the SCS. We found no evidence to suggest that the strength and extent of the MOC south of 18ºN are related to inter‐ocean volume transport. The surface layer transport in the Luzon Strait has been decreasing since 1960s. However, the causes of the meridionally staggered and interdecadal alternative acceleration/slowdown of the meridional stream function difference are unknown. This article is protected by copyright. All rights reserved.
      PubDate: 2016-04-13T08:25:54.200798-05:
      DOI: 10.1002/2015JC011443
       
  • The vertical structure of upper ocean variability at the Porcupine Abyssal
           Plain during 2012‐2013
    • Authors: Gillian M. Damerell; Karen J. Heywood, Andrew F. Thompson, Umberto Binetti, Jan Kaiser
      Abstract: This study presents the characterization of variability in temperature, salinity and oxygen concentration, including the vertical structure of the variability, in the upper 1000m of the ocean over a full year in the northeast Atlantic. Continuously profiling ocean gliders with vertical resolution between 0.5‐1m provide more information on temporal variability throughout the water column than time series from moorings with sensors at a limited number of fixed depths. The heat, salt and dissolved oxygen content are quantified at each depth. While the near surface heat content is consistent with the net surface heat flux, heat content of the deeper layers is driven by gyre‐scale water mass changes. Below ∼150m, heat and salt content display intraseasonal variability which has not been resolved by previous studies. A mode‐1 baroclinic internal tide is detected as a peak in the power spectra of water mass properties. The depth of minimum variability is at ∼415m for both temperature and salinity, but this is a depth of high variability for oxygen concentration. The deep variability is dominated by the intermittent appearance of Mediterranean Water, which shows evidence of filamentation. Susceptibility to salt fingering occurs throughout much of the water column for much of the year. Between about 700‐900m, the water column is susceptible to diffusive layering, particularly when Mediterranean Water is present. This unique ability to resolve both high vertical and temporal resolution highlights the importance of intraseasonal variability in upper ocean heat and salt content, variations that may be aliased by traditional observing techniques. This article is protected by copyright. All rights reserved.
      PubDate: 2016-04-13T08:25:39.265836-05:
      DOI: 10.1002/2015JC011423
       
  • Assessing climate impacts and risks of ocean albedo modification in the
           Arctic
    • Authors: N. Mengis; T. Martin, D.P. Keller, A. Oschlies
      Abstract: The ice albedo feedback is one of the key factors of accelerated temperature increase in the high northern latitudes under global warming. This study assesses climate impacts and risks of idealized Arctic Ocean albedo modification (AOAM), a proposed climate engineering method, during transient climate change simulations with varying representative concentration pathway (RCP) scenarios. We find no potential for reversing trends in all assessed Arctic climate metrics under increasing atmospheric CO2 concentrations. AOAM only yields an initial offset during the first years after implementation. Nevertheless, sea ice loss can be delayed by 25(60) years in the RCP8.5(RCP4.5) scenario and the delayed thawing of permafrost soils in the AOAM simulations prevents up to 40(32) Pg of carbon from being released by 2100. AOAM initially dampens the decline of the Atlantic Meridional Overturning and delays the onset of open ocean deep convection in the Nordic Seas under the RCP scenarios. Both these processes cause a sub‐surface warming signal in the AOAM simulations relative to the default RCP simulations with the potential to destabilize Arctic marine gas hydrates. Furthermore, in 2100, the RCP8.5 AOAM simulation diverts more from the 2005‐2015 reference state in many climate metrics than the RCP4.5 run without AOAM. Considering the demonstrated risks, we conclude that concerning longer time scales, reductions in emissions remain the safest and most effective way to prevent severe changes in the Arctic. This article is protected by copyright. All rights reserved.
      PubDate: 2016-04-12T20:31:46.651582-05:
      DOI: 10.1002/2015JC011433
       
  • On the nature of cross‐isobath energy fluxes in topographically
           modified barotropic semidiurnal Kelvin waves
    • Authors: Tianyi Zhang; Alexander E Yankovsky
      Abstract: Continental shelf topography modifies a Kelvin wave into a hybrid Kelvin‐edge wave with a non‐zero across‐isobath velocity and a phase speed that decreases with increasing wavenumber while the group velocity reaches a minimum at intermediate wavenumbers. We model the modified semidiurnal Kelvin wave adjustment to alongshore changes in the shelf width. The model domain consists of two alongshore‐uniform continental shelves of different widths adjoined through a 150 km‐long transition zone. The continental shelf and slope are adjacent to an ocean of a constant depth, allowing radiation of Poincaré waves. We consider three shelf widths of 150, 250, and 300 km, where properties of a zero mode at semidiurnal frequency change from Kelvin wave‐like to edge wave‐like. For each shelf width, a zero wave mode has its distinctive alongshore energy flux structure on the shelf. As the incident wave encounters a variable shelf width, the alongshore energy flux converges (diverges) on the shelf resulting in an offshore (onshore) energy flux over the continental slope. Furthermore, for a strongly convergent alongshore energy flux, the incident wave mode scatters into radiating Poincaré waves. On sufficiently wide shelves, a strong across‐isobath energy flux comparable with the incident wave energy flux can be triggered even by relatively modest changes of shelf width. An energy flux divergence parameter De is defined, which scales with magnitude and direction of the energy flux across the continental slope. More than 50% of the incident energy flux scatters into modes radiating offshore when De is ‐1 or less. This article is protected by copyright. All rights reserved.
      PubDate: 2016-04-12T20:31:26.914973-05:
      DOI: 10.1002/2015JC011617
       
  • Probabilistic assessment of erosion and flooding risk in the northern Gulf
           of Mexico
    • Authors: Thomas Wahl; Nathaniel G. Plant, Joseph W. Long
      Abstract: We assess erosion and flooding risk in the northern Gulf of Mexico by identifying interdependencies among oceanographic drivers and probabilistically modeling the resulting potential for coastal change. Wave and water level observations are used to determine relationships between six hydrodynamic parameters that influence total water level and therefore erosion and flooding, through consideration of a wide range of univariate distribution functions and multivariate elliptical copulas. Using these relationships, we explore how different our interpretation of the present‐day erosion/flooding risk could be if we had seen more or fewer extreme realizations of individual and combinations of parameters in the past by simulating 10,000 physically and statistically consistent sea‐storm time series. We find that seasonal total water levels associated with the 100‐year return period could be up to 3 m higher in summer and 0.6 m higher in winter relative to our best estimate based on the observational records. Impact hours of collision and overwash – where total water levels exceed the dune toe or dune crest elevations – could be on average 70% (collision) and 100% (overwash) larger than inferred from the observations. Our model accounts for non‐stationarity in a straightforward, non‐parametric way that can be applied (with little adjustments) to many other coastlines. The probabilistic model presented here, which accounts for observational uncertainty, can be applied to other coastlines where short record lengths limit the ability to identify the full range of possible wave and water level conditions that coastal mangers and planners must consider to develop sustainable management strategies. This article is protected by copyright. All rights reserved.
      PubDate: 2016-04-12T20:30:40.744773-05:
      DOI: 10.1002/2015JC011482
       
  • Collision of oil droplets with marine aggregates: Effect of droplet size
    • Authors: Ruth A. Lambert; Evan A. Variano
      Abstract: Interactions between oil droplets and marine particle aggregates, such as marine snow, may effect the behavior of oil spills. Marine snow is known to scavenge fine particles from the water column, and has the potential to scavenge oil droplets in the same manner. To determine the degree to which such a process is important in the evolution of oil spills, we quantify the collision of oil droplets and marine aggregates using existing collision rate equations. Results show that interaction of drops and aggregates can substantially influence the drop size distribution, but like all such processes this result is sensitive to the local concentration of oil and aggregates. The analysis also shows that as the size distribution of oil droplets shifts towards larger droplets, a greater fraction of the total oil volume collides with marine aggregates. This result is robust to a variety of different assumptions in the collision model. Results also show that there is a not always a dominant collision mechanism. For example, when droplets and aggregates are both close to 10 micrometers in radius, shear and differential settling contribute nearly equally to the collision rate. This overlap suggests that further research on the interaction of shear and differential settling could be useful. This article is protected by copyright. All rights reserved.
      PubDate: 2016-04-08T13:03:22.209515-05:
      DOI: 10.1002/2015JC011562
       
  • Major variations in subtropical North Atlantic heat transport at short (5
           day) time scales and their causes
    • Abstract: Variability in the North Atlantic ocean heat transport at 26.5°N on short (5‐day) timescales is identified and contrasted with different behaviour at monthly intervals using a combination of RAPID/MOCHA/WBTS measurements and the NEMO‐LIM2 1/12° ocean circulation/sea ice model. Wind forcing plays the leading role in establishing the heat transport variability through the Ekman transport response of the ocean and the associated driving atmospheric conditions vary significantly with timescale. We find that at 5‐day timescales the largest changes in the heat transport across 26.5°N coincide with north‐westerly airflows originating over the American land mass that drive strong southward anomalies in the Ekman flow. During these events the northward heat transport reduces by 0.5‐1.4 PW. In contrast, the Ekman transport response at longer monthly timescales is smaller in magnitude (up to 0.5 PW) and consistent with expected variations in the leading mode of North Atlantic atmospheric variability, the North Atlantic Oscillation. The north‐westerly airflow mechanism can have a prolonged influence beyond the central 5‐day timescale and on occasion can reduce the accumulated winter ocean heat transport into the North Atlantic by ∼40%. This article is protected by copyright. All rights reserved.
      PubDate: 2016-04-08T12:57:51.10661-05:0
      DOI: 10.1002/2016JC011660
       
  • Infragravity waves on fringing reefs in the tropical Pacific: Dynamic
           setup
    • Authors: J. M. Becker; M.A. Merrifield, H. Yoon
      Abstract: Cross‐shore pressure and current observations from four fringing reefs of lengths ranging from 135 to 420 m reveal energetic low frequency (∼ 0.001‐0.05 Hz) motions. The spatial structure and temporal amplitudes of an empirical orthogonal function analysis of the pressure measurements suggest the dominant low frequency variability is modal. Incoming and outgoing linear flux estimates also support partially standing modes on the reef flat during energetic events. A cross‐covariance analysis suggests that breakpoint forcing excites these partially standing modes, similar to previous findings at other steep reefs. The dynamics of Symonds et al. [1982] with damping are applied to a step reef, with forcing obtained by extending a point break model of Vetter et al. [2010] for breaking wave setup to the low frequency band using the shoaled envelope of the incident free surface elevation. A one parameter, linear analytical model for the reef flat free surface elevation is presented, which describes between 75% to 97% of the variance of the observed low frequency shoreline significant wave height for all reefs considered over a range of conditions. The linear model contains a single dimensionless parameter that is the ratio of the inertial to dissipative time scales, and the observations from this study exhibit more low frequency variability when the dissipative time scale is greater than the inertial time scale for the steep reefs considered. This article is protected by copyright. All rights reserved.
      PubDate: 2016-04-07T17:46:41.756588-05:
      DOI: 10.1002/2015JC011516
       
  • Characterizing storm water dispersion and dilution from small coastal
           streams
    • Authors: Leonel Romero; David A. Siegel, James C. McWilliams, Yusuke Uchiyama, Charles Jones
      Abstract: Characterizing the dispersion and dilution of stormwater from small coastal creeks is important for understanding the importance of land‐derived subsidies to nearby ecosystems and the management of anthropogenic pollutants. In Southern California, creek runoff is episodic, intense and short‐lived while the plumes are buoyant, all of which make the field sampling of freshwater plumes challenging. Numerical modeling offers a viable way to characterize these systems. The dilution and dispersion of freshwater from two creeks that discharge into the Santa Barbara Channel, California is investigated using Regional Ocean Modeling System (ROMS) simulations with a horizontal resolution of 100 m. Tight coupling is found among precipitation, hydrologic discharge, wind forcing and submesoscale flow structures which all contribute to plume evolution. During flooding, plumes are narrow and attached to the coast, due to downwelling/onshore wind forcing and intense vorticity filaments lying parallel to the shelf. As the storm passes, the winds typically shift to offshore/upwelling favorable conditions and the plume is advected offshore which enhances its dilution. Plumes reach the bottom nearshore while they form thin layers a few meters thick offshore. Dilution field of passive tracers released with the runoff is strongly anisotropic with stronger cross‐shelf gradients than along‐shelf. Dispersion analysis of statistical moments of the passive tracer distribution results in scale dependent diffusivities consistent with the particle‐pair analysis of Romero et al. (2013). Model validation, the roles of submesoscale processes and wind forcing on plume evolution and application to ecological issues and marine resource management are discussed. This article is protected by copyright. All rights reserved.
      PubDate: 2016-04-07T17:45:49.961528-05:
      DOI: 10.1002/2015JC011323
       
  • Response time of mean square slope to wind forcing: An empirical
           investigation
    • Authors: David D. Chen; Christopher S. Ruf, Scott T. Gleason
      Abstract: We present an empirical study of the response time of surface wave mean square slope to local wind forcing using data collected over eleven years by forty‐six discus buoys moored at a wide variety of locations. The response time is defined as the time lag at which the time dependence of the waves exhibits the highest correlation with that of the local wind speed. The response time at each location is found to be fairly stable, with the time varying between 0.4 and 1.8 hours depending on the location. Examination of long‐term statistics reveals response time dependencies on wind speed magnitude, fetch, atmospheric stability, and wavelength. With the increasing reliance on satellite microwave remote sensing as a source of wind data, these results provide useful insights and bounds for their use. This article is protected by copyright. All rights reserved.
      PubDate: 2016-04-06T10:42:52.87033-05:0
      DOI: 10.1002/2016JC011661
       
  • Focusing of baroclinic tidal energy in a canyon
    • Authors: Vasiliy Vlasenko; Nataliya Stashchuk, Mark E. Inall, Marie Porter, Dmitry Aleynik
      Abstract: Strong three dimensional focusing of internal tidal energy in the Petite Sole Canyon in the Celtic Sea is analysed using observational data and numerical modelling. In a deep layer (500‐800 m) in the centre of the canyon shear variance was elevated by an order of magnitude. Corresponding large vertical oscillations of deep isotherms, and a local maximum of horizontal velocity were replicated numerically using the MITgcm. The elevated internal tidal activity in the deep part of the canyon is explained in terms of the downward propagation and focusing of multiple internal tidal beams generated at the shelf break. The near‐circular shape of the canyon head and steep bottom topography throughout the canyon (steeper than the tidal beam) create favourable conditions for the lens‐like focusing of tidal energy in the canyon's centre. Observations and modeling show that the energy focusing greatly intensifies local diapycnal mixing, that leads to local formation of a baroclinic eddy. This article is protected by copyright. All rights reserved.
      PubDate: 2016-04-06T10:42:51.253917-05:
      DOI: 10.1002/2015JC011314
       
  • Turbulent flow field and air entrainment in laboratory plunging breaking
           waves
    • Abstract: This paper presents laboratory measurements of turbulent flow fields and void fraction in deep‐water plunging breaking waves using imaging and optical fiber techniques. Bubble size distributions are also determined based on combined measurements of velocity and bubble residence time. The most excited mode of the local intermittency measure of the turbulent flow and its corresponding length scale are obtained using a wavelet‐based method and found to correlate with the swirling strength and vorticity. Concentrated vortical structures with high intermittency are observed near the lower boundaries of the aerated rollers where the velocity shear is high; the length scale of the deduced eddies ranges from 0.05 to 0.15 times the wave height. The number of bubbles with a chord length less than 2 mm demonstrates good correlation with the swirling strength. The power law scaling and the Hinze scale of the bubbles determined from the bubble chord length distribution compare favorably with existing measurements. The turbulent dissipation rate, accounting for void fraction, is estimated using mixture theory. When void fraction is not considered, the turbulent dissipation rate is underestimated by more than 70% in the initial impinging and the first splash‐up roller. A significant discrepancy of approximately 67% between the total energy dissipation rate and the turbulence dissipation rate is found. Of this uncounted dissipation, 23% is caused by bubble‐induced dissipation. This article is protected by copyright. All rights reserved.
      PubDate: 2016-04-06T10:30:59.176331-05:
      DOI: 10.1002/2015JC011377
       
  • An overview of uncertainty quantification techniques with application to
           oceanic and oil‐spill simulations
    • Authors: Mohamed Iskandarani; Shitao Wang, Ashwanth Srinivasan, W. Carlisle Thacker, Justin Winokur, Omar M. Knio
      Abstract: We give an overview of four different ensemble‐based techniques for uncertainty quantification and illustrate their application in the context of oil plume simulations. These techniques share the common paradigm of constructing a model proxy that efficiently captures the functional dependence of the model output on uncertain model inputs. This proxy is then used to explore the space of uncertain inputs using a large number of samples, so that reliable estimates of the model's output statistics can be calculated. Three of these techniques use Polynomial Chaos (PC) expansions to construct the model proxy, but they differ in their approach to determining the expansions' coefficients; the fourth technique uses Gaussian Process Regression (GPR). An integral plume model for simulating the Deepwater Horizon oil‐gas blowout provides examples for illustrating the different techniques. A Monte‐Carlo ensemble of 50,000 model simulations is used for gauging the performance of the different proxies. The examples illustrate how regression‐based techniques can outperform projection‐based techniques when the model output is noisy. They also demonstrate that robust uncertainty analysis can be performed at a fraction of the cost of the Monte‐Carlo calculation. This article is protected by copyright. All rights reserved.
      PubDate: 2016-04-04T06:13:14.523386-05:
      DOI: 10.1002/2015JC011366
       
  • Dissipation processes in the tongue of the ocean
    • Authors: James A. Hooper; Molly O. Baringer, Louis C. St. Laurent, William K. Dewar, Doug Nowacek
      Abstract: The Tongue of the Ocean (TOTO) region located within the Bahamas archipelago is a relatively under‐studied region in terms of both its biological and physical oceanographic characteristics. A prey‐field mapping cruise took place in the fall between 15 September 2008 and 01 October 2008, consisting of a series of transects and “clovers” to study the spatial and temporal variability. The region is characterized by a deep scattering layer (DSL), which is preyed on by nekton that serves as the food for beaked whale and other whale species. This study marks the first of its kind where concurrent measurements of acoustic backscatter and turbulence have been conducted for a nekton scattering layer well below the euphotic zone. Turbulence data collected from a deep microstructure profiler are compared to biological and shear data collected by a 38 kHz Simrad EK 60 echosounder and a hydrographic Doppler sonar system, respectively. From these measurements, the primary processes responsible for the turbulent production in the TOTO region are assessed. The DSL around 500 m and a surface scattering layer (SSL) are investigated for raised ε values. Strong correlation between turbulence levels and scattering intensity of prey is generally found in the SSL with dissipation levels as large as ∼10−7 W kg−1, three orders of magnitude above background levels. In the DSL and during the diel vertical migration dissipation levels ∼10−8 W kg−1 were observed. This article is protected by copyright. All rights reserved.
      PubDate: 2016-04-04T06:06:34.136921-05:
      DOI: 10.1002/2015JC011165
       
  • Contribution of the Yellow Sea Bottom Cold Water to the abnormal cooling
           of Sea Surface Temperature in the summer of 2011
    • Abstract: Satellite‐based sea surface temperature (SST) measurements revealed an abnormal cooling anomaly over the Yellow Sea (YS) in the summer of 2011. Using in situ hydrographic profiles, meteorological fields, and an ocean circulation model with a passive tracer experiment, we identified the cold SST anomaly and its connection with the YS Bottom Cold Water (YSBCW), which occupies the central part of the YS below the thermocline in the summer. The summer SST anomalies in the YS showed three cold peaks in 1993, 2003, and 2011 over the past 20 years, but the reasons for the cooling events were different, as one was due to weakened surface heating and the other was attributed to mixing with the YSBCW. In 1993 and 2003, relatively weak surface heating made the surface water cooler compared with that during the other years, whereas in 2011, a strong vertical mixing of water was induced by a typhoon that passed through the central YS, causing the surface water to cool by ∼8°C and the bottom water to warm up by ∼4°C. A tracer experiment further confirmed that the vertical heat transfers between the warm surface and the cold bottom water masses when the typhoon passed through the YS interior. This article is protected by copyright. All rights reserved.
      PubDate: 2016-04-04T03:47:40.247451-05:
      DOI: 10.1002/2016JC011658
       
  • Role of pulsed winds on detachment of low‐salinity water from the
           Pearl River plume: Upwelling and mixing processes
    • Authors: Zhaoyun Chen; Jiayi Pan, Yuwu Jiang
      Abstract: The detachment of low salinity water (LSW) from the Pearl River plume occurs frequently as revealed by in‐situ observations and satellite images, and plays an important role in cross‐shore transport of the nutrient‐rich plume water. In this study, the Regional Ocean Modeling System (ROMS) is used to simulate the LSW detachment process forced by real and idealized winds, and to explore its dynamical mechanisms. Modeling results show that the LSW detachment appears under a pulsed southwesterly wind, while tidal mixing modifies the size and salinity of the detached LSW. Strong pulsed wind causes the LSW to separate from the plume and move offshore quickly after the detachment. Under a pulsed northeasterly wind, however, the plume without separation of the LSW moves shoreward, indicating that the LSW detachment is sensitive to wind direction. In the plume region, upwelling develops under the forcing of the pulsed southwesterly wind, which transports high salinity bottom water to the surface layer, while the shear mixing in the upper layer further enhances the surface buoyancy flux, leading to appearance of high salinity water in the surface layer off the Pearl River estuary mouth, cutting off the eastward‐spreading plume water, and resulting in the plume LSW detachment. Further analysis shows that the pulsed southwesterly wind induces positive local salinity change rate in the LSW detachment area. The pulsed upwelling‐favorable wind with duration of 2–5 days is responsible for the detachment process. This article is protected by copyright. All rights reserved.
      PubDate: 2016-04-04T03:47:24.544802-05:
      DOI: 10.1002/2015JC011337
       
  • A global surface drifter data set at hourly resolution
    • Authors: S. Elipot; Rick Lumpkin, Renellys C. Perez, Jonathan M. Lilly, Jeffrey J. Early, Adam M. Sykulski
      Abstract: The surface drifting buoys, or drifters, of the Global Drifter Program (GDP) are predominantly tracked by the Argos positioning system, providing drifter locations with O(100 m)$errors at non‐uniform temporal intervals, with an average interval of 1.2 hours since January 2005. This dataset is thus a rich and global source of information on high‐frequency and small‐scale oceanic processes, yet is still relatively understudied because of the challenges associated with its large size and sampling characteristics. A methodology is described to produce a new high‐resolution global dataset since 2005, consisting of drifter locations and velocities estimated at hourly intervals, along with their respective errors. Locations and velocities are obtained by modeling locally in time trajectories as a first order polynomial with coefficients obtained by maximizing a likelihood function. This function is derived by modeling the Argos location errors with t location‐scale probability distribution functions. The methodology is motivated by analyzing 82 drifters tracked contemporaneously by Argos and by the Global Positioning System, where the latter is assumed to provide true locations. A global spectral analysis of the velocity variance from the new dataset reveals a sharply‐defined ridge of energy closely following the inertial frequency as a function of latitude, distinct energy peaks near diurnal and semi‐diurnal frequencies, as well as higher‐frequency peaks located near tidal harmonics as well as harmonics of the inertial frequency. Compared to the spectra that can be obtained using the standard 6‐hourly GDP product, the new dataset contains up to 100\% more spectral energy at some latitudes. This article is protected by copyright. All rights reserved.
      PubDate: 2016-04-04T03:47:07.845655-05:
      DOI: 10.1002/2016JC011716
       
  • A dynamic sediment model based on satellite‐measured concentration
           of the surface suspended matter in the East China Sea
    • Authors: Zhihua Mao; Delu Pan, C. L. Tang, Bangyi Tao, Jianyu Chen, Yan Bai, Peng Chen, Xianqiang He, Zengzhou Hao, Haiqing Huang, Qiankun Zhu
      Abstract: The concentration of total suspended matter (TSM) at the sea surface is derived from satellite data using a complex proxy TSM model in East China Sea from 1997 to 2008. The structure of the mean TSM image is similar to that of the topography, indicating that the distribution of the surface concentration is strongly related to the water depth. A dynamic sediment model (DSM) is constructed to relate the TSM concentration at the sea surface with suspended sediment at the benthic boundary layer, the Rouse number, and the water depth. The DSM model is improved through iteration with a convergence identified by the mean relative difference between two adjacent bottom TSM images which becomes smaller with the more iterations and the value is less than 1% after 50 iterations. The performance of the DSM model is validated by satellite‐measured concentration with a mean relative error of 5.2% for the monthly‐mean images. The DSM model is used to deduce the bottom TSM concentration at the benthic boundary layer and the distribution of the Rouse number. The spatial distribution of the sea surface TSM concentration is determined predominately by both the bottom suspended sediment concentration and water depth. The temporal variation of the sea surface concentration mainly depends upon the Rouse number in the water column. Our result shows that the discharge of the Changjiang River can change the distribution of the Rouse number to form a band‐shaped region in the Changjiang Estuary. The DSM model provides a framework for understanding some of the mechanisms of the formation and variation of the primary TSM plume and the secondary plume in the ECS. The primary TSM plume corresponds approximately to the region with depth shallower than 20 m and the secondary plume corresponds to the region with depths between 20‐50 m. This article is protected by copyright. All rights reserved.
      PubDate: 2016-04-04T03:34:13.402244-05:
      DOI: 10.1002/2015JC011466
       
  • Relationship between optimal precursory disturbances and optimally growing
           initial errors associated with ENSO events: Implications to target
           observations for ENSO prediction
    • Authors: Junya Hu; Wansuo Duan
      Abstract: By superimposing initial sea temperature disturbances in neutral years, we determine the precursory disturbances that are most likely to evolve into El Niño and La Niña events using an Earth System Model. These precursory disturbances for El Niño and La Niña events are deemed optimal precursory disturbances because they are more likely to trigger strong ENSO events. Specifically, the optimal precursory disturbance for El Niño exhibits negative sea surface temperature anomalies (SSTAs) in the central‐eastern equatorial Pacific. Additionally, the subsurface temperature component exhibits negative anomalies in the upper layers of the eastern equatorial Pacific and positive anomalies in the lower layers of the western equatorial Pacific. The optimal precursory disturbance for La Niña is almost opposite to that of El Niño. The optimal precursory disturbances show that both El Niño and La Niña originate from precursory signals in the subsurface layers of the western equatorial Pacific and in the surface layers of the eastern equatorial Pacific. We find that the optimal precursory disturbances for El Niño and La Niña are particularly similar to the optimally growing initial errors associated with El Niño prediction that have been presented in previous studies. The optimally growing initial errors show that the optimal precursor source areas represent the sensitive areas for target observations associated with ENSO prediction. Combining the optimal precursory disturbances and the optimally growing initial errors for ENSO, we infer that additional observations in these sensitive areas can reduce initial errors and be used to detect precursory signals, thereby improving ENSO predictions. This article is protected by copyright. All rights reserved.
      PubDate: 2016-03-31T03:27:03.172938-05:
      DOI: 10.1002/2015JC011386
       
  • Sources of dissolved inorganic carbon to the Canada Basin halocline: A
           multitracer study
    • Abstract: We examine the dissolved inorganic carbon maximum in the Canada Basin halocline using a suite of geochemical tracers to gain insight into the factors that contribute to the persistence of this feature. Hydrographic and geochemical samples were collected in the upper 500 m of the southwestern Canada Basin water column in the summer of 2008 and fall of 2009. These observations were used to identify conservative and non‐conservative processes that contribute dissolved inorganic carbon to halocline source waters, including shelf sediment organic matter remineralization, air‐sea gas exchange, and sea‐ice brine export. Our results indicate the remineralization of organic matter that occurs along the Bering and Chukchi Sea shelves is the overwhelming contributor of dissolved inorganic carbon to Pacific Winter Water that occupies the middle halocline in the southwestern Canada Basin. Non‐conservative contributions from air‐sea exchange and sea‐ice brine are not significant. The broad salinity range associated with the DIC maximum, compared to the narrow salinity range of the nutrient maximum, is due to mixing between Pacific and Atlantic water and not abiotic addition of DIC. This article is protected by copyright. All rights reserved.
      PubDate: 2016-03-31T03:23:02.995358-05:
      DOI: 10.1002/2015JC011535
       
  • The role of heating, winds, and topography on sea level changes in the
           North Atlantic
    • Authors: Jinting Zhang; Kathryn A. Kelly, LuAnne Thompson
      Abstract: Seasonal and interannual‐to‐decadal variations of large‐scale altimetric sea surface height (SSH) owing to surface heating and wind forcing in the presence of topography are investigated using simplified models. The dominant forcing mechanisms are timescale dependent. On the seasonal timescale locally forced thermosteric height explains most of the SSH variance north of 18°N. First mode linear long baroclinic Rossby waves forced by changes in the winds and eastern boundary conditions explain most of the variance between 10°N‐15°N and are also important east of Greenland. On interannual‐to‐decadal timescales, local thermosteric height remains important at several locations in the middle and high latitudes. A topographic Sverdrup response explains interannual‐to‐decadal SSH between 53°N and 63°N east of Greenland. Farther south, the linear Rossby wave model explains SSH variations on interannual‐to‐decadal timescales between 30°N and 50°N from mid‐basin to the eastern boundary. Propagation of the eastern boundary condition into the interior dominates the interannual‐to‐decadal SSH signals south of 30°N. The effect from NAO‐related heat flux on SSH is small, but forcing the topographic Sverdrup models with NAO‐regressed winds gives slightly better agreement with the observed SSH in the subpolar gyre on interannual‐to‐decadal timescales than using the full winds. This article is protected by copyright. All rights reserved.
      PubDate: 2016-03-31T03:22:10.030119-05:
      DOI: 10.1002/2015JC011492
       
  • Antarctic tides from GRACE satellite accelerations
    • Abstract: The extended length of the GRACE data timeseries (now 13.5 years) provides the unique opportunity to estimate global mass variations due to ocean tides at large (∼300 km) spatial scales. State of the art global tide models rely heavily on satellite altimetry data, which are sparse for latitudes higher than 66 degrees. Thus, the performance of the models is typically worse at higher latitudes. GRACE data, alternately, extend to polar latitudes and therefore provide information for both model validation and improvement at the higher latitudes. In this work, eleven years of GRACE inter‐satellite range‐acceleration measurements are inverted to solve for corrections to the amplitudes and phases of the major solar and lunar ocean tidal constituents (M2, K1, S2 and O1) from the GOT4.7 ocean tide model at latitudes south of 50°S. Two independent inversion and regularization methods are employed and compared against one another. Uncertainty estimates are derived by subtracting two independent solutions, each spanning a unique 5.5 years of data. Features above the noise floor in the derived solutions likely represent errors in GOT4.7. We find the GOT4.7 amplitudes to be generally too small for M2 and K1, and too large for S2 and O1, and to spatially correlate with geographic regions where GOT4.7 predicts the largest tidal amplitudes. In particular, we find GOT4.7 errors to be dominant over the Patagonia shelf (M2), the Filchner‐Ronne Ice Shelf (M2 and S2), the Ross Ice Shelf (S2), and the Weddell and Ross seas (K1 and O1). This article is protected by copyright. All rights reserved.
      PubDate: 2016-03-29T03:20:44.132605-05:
      DOI: 10.1002/2015JC011488
       
  • Sea level anomaly on the Patagonian continental shelf: Trends, annual
           patterns, and geostrophic flows
    • Authors: L.A. Ruiz Etcheverry; M. Saraceno, A.R. Piola, P.T. Strub
      Abstract: We study the annual patterns and linear trend of satellite sea level anomaly (SLA) over the southwest South Atlantic continental shelf (SWACS) between 54ºS and 36ºS. Results show that south of 42°S the thermal steric effect explains nearly 100% of the annual amplitude of the SLA, while north of 42°S it explains less than 60%. This difference is due to the halosteric contribution. The annual wind variability plays a minor role over the whole continental shelf. The temporal linear trend in SLA ranges between 1 and 5 mm/yr (95% confidence level). The largest linear trends are found north of 39°S, at 42°S and at 50°S. We propose that in the northern region the large positive linear trends are associated with local changes in the density field caused by advective effects in response to a southward displacement of the South Atlantic High. The causes of the relative large SLA trends in two southern coastal regions are discussed as a function meridional wind stress and river discharge. Finally, we combined the annual cycle of SLA with the mean dynamic topography to estimate the absolute geostrophic velocities. This approach provides the first comprehensive description of the seasonal component of SWACS circulation based on satellite observations. The general circulation of the SWACS is northeastward with stronger/weaker geostrophic currents in austral summer/winter. At all latitudes, geostrophic velocities are larger (up to 20 cm/s) close to the shelf‐break and decrease towards the coast. This spatio‐temporal pattern is more intense north of 45°S. This article is protected by copyright. All rights reserved.
      PubDate: 2016-03-24T18:00:37.623112-05:
      DOI: 10.1002/2015JC011265
       
  • Uncertainty quantification and global sensitivity analysis of the Los
           Alamos Sea Ice Model
    • Abstract: Changes in the high latitude climate system have the potential to affect global climate through feedbacks with the atmosphere and connections with mid latitudes. Sea ice and climate models used to understand these changes have uncertainties that need to be characterized and quantified. We present a quantitative way to assess uncertainty in complex computer models, which is a new approach in the analysis of sea ice models. We characterize parametric uncertainty in the Los Alamos sea ice model (CICE) in a standalone configuration and quantify the sensitivity of sea ice area, extent and volume with respect to uncertainty in 39 individual model parameters. Unlike common sensitivity analyses conducted in previous studies where parameters are varied one at a time, this study uses a global variance‐based approach in which Sobol' sequences are used to efficiently sample the full 39‐dimensional parameter space. We implement a fast emulator of the sea ice model whose predictions of sea ice extent, area, and volume are used to compute the Sobol' sensitivity indices of the 39 parameters. Main effects and interactions among the most influential parameters are also estimated by a non‐parametric regression technique based on generalized additive models. A ranking based on the sensitivity indices indicates that model predictions are most sensitive to snow parameters such as conductivity and grain size, and the drainage of melt ponds. It is recommended that research be prioritized towards more accurately determining these most influential parameter values by observational studies or by improving parameterizations in the sea ice model. This article is protected by copyright. All rights reserved.
      PubDate: 2016-03-24T03:31:17.898207-05:
      DOI: 10.1002/2015JC011558
       
  • Attenuation coefficient of usable solar radiation of the global oceans
    • Authors: Junfang Lin; Zhongping Lee, Michael Ondrusek, Mati Kahru
      Abstract: Usable solar radiation (USR) represents spectrally integrated solar energy in the spectral range of 400‐560 nm, a domain where photons penetrate the most in oceanic waters and thus contribute to photosynthesis and heating at deeper depths. Through purely numerical simulations [Lee et al., 2014], it was found that the diffuse attenuation coefficient of downwelling USR (Kd(USR), m−1) is nearly a constant vertically in the upper water column for clear waters and most turbid waters. Subsequently an empirical model was developed to estimate Kd(USR) based on the diffuse attenuation coefficient at 490 nm (Kd(490), m−1). We here evaluate this relationship using data collected from a wide range of oceanic and coastal environments and found that the relationship between Kd(490) and Kd(USR) developed via the numerical simulation is quite robust. We further refined this relationship to extend the applicability to “clearest” natural waters. This refined relationship was then used to produce sample distribution of Kd(USR) of global oceans. As expected, extremely low Kd(USR) (∼ 0.02 m−1) was observed in ocean gyres, while significantly higher Kd(USR) (∼ 5.2 m−1) was found in very turbid coastal regions. A useful application of Kd(USR) is to easily and accurately propagate surface USR to deeper depths, potentially to significantly improve the estimation of basin scale primary production and heat fluxes in the upper water column. This article is protected by copyright. All rights reserved.
      PubDate: 2016-03-24T03:31:04.148584-05:
      DOI: 10.1002/2015JC011528
       
  • An oceanic heat transport pathway to the Amundsen Sea Embayment
    • Authors: Angelica R. Rodriguez; Matthew R. Mazloff, Sarah T. Gille
      Abstract: The Amundsen Sea Embayment (ASE) on the West Antarctic coastline has been identified as a region of accelerated glacial melting. A Southern Ocean State Estimate (SOSE) is analyzed over the 2005–2010 time period in the Amundsen Sea region. The SOSE oceanic heat budget reveals that the contribution of parameterized small‐scale mixing to the heat content of the ASE waters is small compared to advection and local air‐sea heat flux, both of which contribute significantly to the heat content of the ASE waters. Above the permanent pycnocline the local air‐sea flux dominates the heat budget and is controlled by seasonal changes in sea ice coverage. Overall, between 2005 and 2010, the model shows a net heating in the surface above the pycnocline within the ASE. Sea water below the permanent pycnocline is isolated from the influence of air‐sea heat fluxes, and thus, the divergence of heat advection is the major contributor to increased oceanic heat content of these waters. Oceanic transport of mass and heat into the ASE is dominated by the cross‐shelf input and is primarily geostrophic below the permanent pycnocline. Diagnosis of the time‐mean SOSE vorticity budget along the continental shelf slope indicates that the cross‐shelf transport is sustained by vorticity input from the localized wind‐stress curl over the shelf break. This article is protected by copyright. All rights reserved.
      PubDate: 2016-03-23T17:51:06.851762-05:
      DOI: 10.1002/2015JC011402
       
  • Potential and timescales for oxygen depletion in coastal upwelling
           systems: A box‐model analysis
    • Authors: C. S. Harrison; B. Hales, S. Siedlecki, R. M. Samelson
      Abstract: A simple box model is used to examine oxygen depletion in an idealized ocean‐margin upwelling system. Near‐bottom oxygen depletion is controlled by a competition between flushing with oxygenated offshore source waters and respiration of particulate organic matter produced near the surface and retained near the bottom. Upwelling‐supplied nutrients are consumed in the surface box, and some surface particles sink to the bottom where they respire, consuming oxygen. Steady states characterize the potential for hypoxic near‐bottom oxygen depletion; this potential is greatest for faster sinking rates, and largely independent of production timescales except in that faster production allows faster sinking. Timescales for oxygen depletion depend on upwelling and productivity differently, however, as oxygen depletion can only be reached in meaningfully short times when productivity is rapid. Hypoxia thus requires fast production, to capture upwelled nutrients, and fast sinking, to deliver the respiration potential to model bottom waters. Combining timescales allows generalizations about tendencies toward hypoxia. If timescales of sinking are comparable to or smaller than the sum of those for respiration and flushing, the steady state will generally be hypoxic, and results indicate optimal timescales and conditions exist to generate hypoxia. For example, the timescale for approach to hypoxia lengthens with stronger upwelling, since surface particle and nutrient are shunted off‐shelf, in turn reducing subsurface respiration and oxygen depletion. This suggests that if upwelling winds intensify with climate change the increased forcing could offer mitigation of coastal hypoxia, even as the oxygen levels in upwelled source waters decline. This article is protected by copyright. All rights reserved.
      PubDate: 2016-03-23T17:50:52.097507-05:
      DOI: 10.1002/2015JC011328
       
  • Turbulent boundary layers under irregular waves and currents: Experiments
           and the equivalent‐wave concept
    • Authors: Jing Yuan
      Abstract: A full‐scale experimental study of turbulent boundary layer flows under irregular waves and currents is conducted with the primary objective to investigate the equivalent‐wave concept by Madsen [1994]. Irregular oscillatory flows following the bottom‐velocity spectrum under realistic surface irregular waves are produced over two fixed rough bottoms in an oscillatory water tunnel, and flow velocities are measured using a Particle Image Velocimetry. The root‐mean‐square (RMS) value and representative phase lead of wave velocities have vertical variations very similar to those of the first‐harmonic velocity of periodic wave boundary layers, e.g. the RMS wave velocity follows a logarithmic distribution controlled by the physical bottom roughness in the very near‐bottom region. The RMS wave bottom shear stress and the associated representative phase lead can be accurately predicted using the equivalent‐wave approach. The spectra of wave bottom shear stress and boundary layer velocity are found to be proportional to the spectrum of free‐stream velocity. Currents in the presence of irregular waves exhibit the classic two‐log‐profile structure with the lower log‐profile controlled by the physical bottom roughness and the upper log‐profile controlled by a much larger apparent roughness. Replacing the irregular waves by their equivalent sinusoidal waves virtually makes no difference for the co‐existing currents. These observations, together with the excellent agreement between measurements and model predictions, suggest that the equivalent‐wave representation adequately characterizes the basic wave‐current interaction under irregular waves. This article is protected by copyright. All rights reserved.
      PubDate: 2016-03-23T17:50:34.663287-05:
      DOI: 10.1002/2015JC011551
       
  • Intraseasonal coastal upwelling signal along the southern coast of Java
           observed using Indonesian tidal station data
    • Authors: Takanori Horii; Iwao Ueki, Fadli Syamsudin, Ibnu Sofian, Kentaro Ando
      Abstract: Sea level variations along the coasts of Sumatra and Java were investigated to determine the coastal upwelling signal that is linked to local sea surface temperature (SST) variability. We used Indonesian tidal station data together with satellite SST data and atmospheric reanalysis data. The sea level variations along the southern coast of Java have a significant coherence with remote wind, local wind, and local SST variations, with an intraseasonal time scale of 20–50 days. Assuming that a coastal upwelling signal would appear as a sea level drop (SLD), we focused on intraseasonal‐scale SLD events in the data. Significant upwelling signals are frequently observed during both the boreal summer and winter. To evaluate the impact of the coastal upwelling on local SST, we examined statistical relationships between sea level and SST variations. The results demonstrated that events that occurred during April‐August were associated with local SST cooling. The horizontal distribution of the SST cooling was analogous with annual‐mean SST, suggesting the importance of intraseasonal‐scale coastal upwelling in forming the climatic conditions of the southeastern tropical Indian Ocean. This article is protected by copyright. All rights reserved.
      PubDate: 2016-03-22T01:25:51.543252-05:
      DOI: 10.1002/2015JC010886
       
  • Recent changes in sea ice area flux through the Beaufort Sea during the
           summer
    • Authors: Stephen E.L. Howell; Michael Brady, Chris Derksen, Richard E.J. Kelly
      Abstract: Over the annual cycle, sea ice is sequestered from the Canadian Basin and transported through the Beaufort Sea toward the Chukchi Sea. In recent years, the Beaufort Sea has experienced considerable ice loss during the summer, which may be indicative of recent changes to this process. In order to investigate this, we quantify the sea ice area flux using RADARSAT from 1997‐2014 at three gates in Beaufort Sea: the Canadian Basin (entrance), Mid‐Beaufort (mid‐point) and Chukchi (exit). There was a mean annual flux of 42±56x103 km2 at the Canadian Basin gate, 94±92x103 km2 at the Mid‐Beaufort gate and ‐83±68x103 km2 at the Chukchi gate (positive and negative flux signs correspond to ice inflow and outflow, respectively). The majority of ice transport in Beaufort Sea was found to occur from October to May providing replenishment for ice lost during the summer months. The cross‐strait gradient in sea level pressure explains ∼40% of the variance in the ice area flux at the gates. Remarkably, the mean July to October net sea ice area flux at the Chukchi gate decreased by ∼80% from 2008‐2014 relative to 1997‐2007 and became virtually ice‐free every year since 2008. This reduction was associated with younger (thinner) ice that was unable to survive the summer melt season when either being sequestered from the Canadian Basin and transported through Beaufort Sea during the melt season (2008‐2011) or remaining immobile and present in the vicinity of the Chukchi gate during the melt season (2012‐2014). This article is protected by copyright. All rights reserved.
      PubDate: 2016-03-22T01:25:37.446149-05:
      DOI: 10.1002/2015JC011464
       
  • Understanding the formation and evolution of rain‐formed fresh
           lenses at the ocean surface
    • Authors: Kyla Drushka; William E. Asher, Brian Ward, Kieran Walesby
      Abstract: Rain falling on the ocean produces a layer of buoyant fresher surface water, or “fresh lens”. Fresh lenses can have significant impacts on satellite‐in situ salinity comparisons and on exchanges between the surface and the bulk mixed layer. However, because these are small, transient features, relatively few observations of fresh lenses have been made. Here, the Generalized Ocean Turbulence Model (GOTM) is used to explore the response of the upper few meters of the ocean to rain events. Comparisons with observations from several platforms demonstrate that GOTM can reproduce the main characteristics of rain‐formed fresh lenses. Idealized sensitivity tests show that the near‐surface vertical salinity gradient within fresh lenses has a linear dependence on rain rate, and an inverse dependence on wind speed. Year‐long simulations forced with satellite rainfall and reanalysis atmospheric parameters demonstrate that the mean salinity difference between 0.01 and 5 m, equivalent to the measurement depths of satellite radiometers and Argo floats, is ‐0.04 psu when averaged over the 20°S‐20°N tropical band. However, when averaged regionally, the mean vertical salinity difference exceeds ‐0.15 psu in the Indo‐Pacific warm pool, in the Pacific and Atlantic intertropical convergence zone, and in the South Pacific convergence zone. In most of these regions, salinities measured by the Aquarius satellite instrument have a fresh bias relative to Argo measurements at 5 m depth. These results demonstrate that the fresh bias in Aquarius salinities in rainy, low‐wind regions may be caused by the presence of rain‐produced fresh lenses. This article is protected by copyright. All rights reserved.
      PubDate: 2016-03-22T01:20:34.441731-05:
      DOI: 10.1002/2015JC011527
       
  • A neural network‐based method for merging ocean color and Argo data
           to extend surface bio‐optical properties to depth: Retrieval of the
           particulate backscattering coefficient
    • Abstract: The present study proposes a novel method that merges satellite ocean‐color bio‐optical products with Argo temperature‐salinity profiles to infer the vertical distribution of the particulate backscattering coefficient (bbp). This neural network‐based method (SOCA‐BBP for Satellite Ocean‐Color merged with Argo data to infer the vertical distribution of the Particulate Backscattering coefficient) uses three main input components: (1) satellite‐based surface estimates of bbp and chlorophyll a concentration matched‐up in space and time with (2) depth‐resolved physical properties derived from temperature‐salinity profiles measured by Argo profiling floats and (3) the day of the year of the considered satellite‐Argo matchup. The neural network is trained and validated using a database including 4725 simultaneous profiles of temperature‐salinity and bio‐optical properties collected by Bio‐Argo floats, with concomitant satellite‐derived products. The Bio‐Argo profiles are representative of the global open‐ocean in terms of oceanographic conditions, making the proposed method applicable to most open‐ocean environments. SOCA‐BBP is validated using 20% of the entire database (global error of 21%). We present additional validation results based on two other independent datasets acquired (1) by four Bio‐Argo floats deployed in major oceanic basins, not represented in the database used to train the method; and (2) during an AMT (Atlantic Meridional Transect) field cruise in 2009. These validation tests based on two fully independent datasets indicate the robustness of the predicted vertical distribution of bbp. To illustrate the potential of the method, we merged monthly climatological Argo profiles with ocean color products to produce a depth‐resolved climatology of bbp for the global ocean. This article is protected by copyright. All rights reserved.
      PubDate: 2016-03-21T10:37:02.965107-05:
      DOI: 10.1002/2015JC011408
       
  • Shelfbreak current over the Canadian Beaufort Sea continental slope:
           Wind‐driven events in January 2005
    • Authors: Igor A. Dmitrenko; Sergei A. Kirillov, Alexandre Forest, Yves Gratton, Denis L. Volkov, William J. Williams, Jennifer V. Lukovich, Claude Belanger, David G. Barber
      Abstract: The shelfbreak current over the Beaufort Sea continental slope is known to be one of the most energetic features of the Beaufort Sea hydrography. In January 2005, three oceanographic moorings deployed over the Canadian (eastern) Beaufort Sea continental slope simultaneously recorded two consecutive shelfbreak current events with along‐slope eastward bottom‐intensified flow up to 120 cm s−1. Both events were generated by the local wind forcing associated with two Pacific‐born cyclones passing north of the Beaufort Sea continental slope towards the Canadian Archipelago. Over the mooring array the associated westerly wind exceeded 15 m s−1. These two cyclones generated storm surges along the Beaufort Sea coast with sea surface height (SSH) rising up to 1.4 m following the two westerly wind maxima. We suggest that the westerly along‐slope wind generated a surface Ekman on‐shore transport. The associated SSH increase over the shelf produced a cross‐slope pressure gradient that drove an along‐slope eastward geostrophic current, in the same direction as the wind. This wind‐driven barotropic flow was superimposed on the background baroclinic bottom‐intensified shelfbreak current that consequently amplified. Summer‐fall satellite altimetry data for 1992‐2013 show that the SSH gradient in the southeastern Beaufort Sea is enhanced over the upper continental slope in response to frequent storm surge events. Because the local wind forcing and/or sea‐ice drift could not explain the reduction of sea‐ice concentration over the Beaufort Sea continental slope in January 2005, we speculate that wind‐driven sea level fluctuations may impact the sea‐ice cover in winter. This article is protected by copyright. All rights reserved.
      PubDate: 2016-03-18T18:06:05.737806-05:
      DOI: 10.1002/2015JC011514
       
  • A high‐resolution salinity time series 1993–2012 in the North
           Atlantic from Argo and altimeter data
    • Authors: I. Stendardo; M. Rhein, R. Hollmann
      Abstract: The study of salinity changes has been hampered by the lack of temporal and spatial resolution of the observations. In order to improve the spatial and temporal distribution of salinity observations, we used the Gravest Empirical Mode (GEM) technique to calculate high‐resolution salinity distributions as a function of dynamic height for the period 1993‐2012. This technique combined Argo and altimeter data to exploit the relationship between T/S profiles and dynamic height in the North Atlantic. The method was valid in the upper 700 m mainly at and near the pathways of the North Atlantic Current (NAC), but failed in regions with weak stratification or with ambiguities in the T/S relationships. Coherent, multiannual large‐scale variability was observed, with many features present in all regions, albeit with weaker amplitudes in the eastern basins. Some of the interannual features in the northeastern Atlantic basins were unrelated to the variability further south and west, pointing to an occasional advection of subtropical water in the eastern Atlantic. Origin and advection of salinity anomalies with the NAC from the North American Basin into the western subpolar North Atlantic are correlated with the state of the North Atlantic Oscillation (NAO) and dampened by the surface freshwater fluxes. Other mechanisms influencing the salinity pattern are the changing location of the subpolar front, also related to the NAO. The large multiyear variability in the 20‐years time series obscured any potential trends caused by global warming. Only the Rockall Trough showed a salinity increase of 0.03 per decade. This article is protected by copyright. All rights reserved.
      PubDate: 2016-03-18T18:05:48.424849-05:
      DOI: 10.1002/2015JC011439
       
  • Interannual variability of the Indonesian throughflow: The salinity effect
    • Authors: Shijian Hu; Janet Sprintall
      Abstract: The Indonesian Throughflow (ITF) region possesses strong mixing and experiences significant freshwater input, but the role of salinity variability in the Indonesian Seas remains unclear. The goal of this study is to understand how salinity variability influences the ITF transport on interannual time scales. The ITF transport is calculated using observations and assimilation datasets and verified using direct ITF transport estimates. We find that the halosteric component of the ITF transport contributes (36±7)% of the total ITF variability, in contrast to (63±6)% by the thermosteric component. Thus, while not dominant, this result nonetheless implies that the salinity variability in the Indonesian Seas is of remarkable importance in determining the interannual variability of ITF transport. Correlation analysis indicates that the interannual variability of the total ITF transport is mainly influenced by the El Niño‐Southern Oscillation (ENSO) rather than the Indian Ocean Dipole. Under the ENSO cycle, the Walker Circulation shifts longitudinally resulting in fluctuations in precipitation over the Indonesian Seas that modulates salinity and subsequently influences the interannual variability of ITF transport. This result signals the importance of precipitation and the subsequent salinity effect in determining the interannual variability of the ITF transport. The role of wind forcing and oceanic planetary waves is also revisited using this newly calculated ITF transport series. ENSO‐related wind forcing is found to modulate the ITF transport via Rossby waves through the wave guide in the Indonesian Seas, which is in agreement with previous studies. This article is protected by copyright. All rights reserved.
      PubDate: 2016-03-18T17:55:53.950329-05:
      DOI: 10.1002/2015JC011495
       
  • Resonant near‐surface inertial oscillations in the northeastern Gulf
           of Mexico
    • Authors: Matt K. Gough; A. J. H. M. Reniers, Jamie H. MacMahan, Stephan D. Howden
      Abstract: The inertial frequency is nearly diurnal at 30°N latitude which transects the northeastern Gulf of Mexico (NeGoM). At this latitude near‐surface inertial oscillations can amplify due to resonance with diurnal wind forcing. Diurnal oscillations have also been attributed to diurnal tidal forcing in this region. Because tidal forcing, wind forcing, and inertial oscillations are nearly diurnal, a unique series of comparative analyses are required to determine their relative influence on surface circulation. By comparing surface currents obtained by HF radar to predictions of the inertial response to wind forcing and barotropic tidal currents it is found that diurnal oscillations in the NeGoM were predominantly due to wind‐forced inertial oscillations in June 2010. The analyses provide a unique spatiotemporal perspective of inertial oscillations in the NeGoM where there is evidence of propagation, frequency and phase shifts, and amplitude variability. Because inertial oscillations mix the ocean differently than the tides, these results provide insight into how inertial oscillations potentially mixed oil from the Deepwater Horizon spill in June 2010. Near‐diurnal oscillations during the winter were found to be predominantly due to tidal forcing when wind‐driven inertial oscillations were diminished due to a presumably deeper mixed layer. This article is protected by copyright. All rights reserved.
      PubDate: 2016-03-18T17:55:35.793248-05:
      DOI: 10.1002/2015JC011372
       
  • An OGCM study of the impact of Rain and River Water Forcing on the Bay of
           Bengal
    • Authors: Ambica Behara; P. N. Vinayachandran
      Abstract: Individual and combined effects of rainfall and river discharge on the Bay of Bengal (BoB) is investigated using an Ocean General Circulation Model. A set of four sensitivity experiments, forced with same air‐sea heat flux, but retaining either river runoff or rainfall or both is carried out. These experiments show that the river water is exported out of the bay along the western boundary during winter and rain water along the eastern boundary during summer. Runoff leads to a large (>3 psu) decrease in salinity in the northern bay during summer and along the western boundary during winter, with a weaker contribution from rainfall. The sea surface temperature response to freshwater forcing shows large spatial variations with eastern bay showing higher differences. The northwestern bay warms by ∼1.5°C in the presence of freshwater during summer, due to greater heat absorption within a shallow mixed layer (ML). This warming is caused in nearly equal proportions by rain and river water in early summer, but the contribution by river water dominates during peak and withdrawal phases of the summer monsoon. Northeastern bay, in contrast, is cooler by 1.5–3°C in the presence of freshwater, caused primarily by river runoff, owing to the winter cooling over a thin ML. Temperature inversions form due to surface cooling of a river stratified layer during winter in the northwestern bay and due to radiation penetrating below the ML during summer in the northeastern bay. This article is protected by copyright. All rights reserved.
      PubDate: 2016-03-18T10:57:11.757005-05:
      DOI: 10.1002/2015JC011325
       
  • Intensification of the subpolar front in the Sea of Japan during winter
           cyclones
    • Abstract: The response of the subpolar front in the Sea of Japan (also known as the East Sea) to winter cyclones is investigated based on quantitative analyses of gridded and satellite datasets. Cyclone passages affecting the sea are detected using time series of spatially averaged surface turbulent heat fluxes. As the cyclones develop, there are strong cold‐air outbreaks that produce twice the normal heat loss over the sea. After removal of sea surface temperature (SST) seasonal trends, we found that cyclone passage (hence, cooling) mainly occurred over 3 days, with maximum SST reduction of −0.4°C. The greatest reduction was found along the subpolar front, where frontal sharpness (i.e., SST gradient) increased by 0.1°C(100 km)−1. Results of a mixed layer model were consistent with both temperature and frontal sharpness, and localized surface cooling along the subpolar front resulted from both horizontal heat advection and turbulent heat fluxes at the sea surface. Further analyses show that this localized cooling from horizontal heat advection is caused by the cross‐frontal Ekman flow (vertically averaged over the mixed layer) and strong northwesterly winds associated with the cold‐air outbreak during cyclone passage. This article is protected by copyright. All rights reserved.
      PubDate: 2016-03-15T11:02:01.130754-05:
      DOI: 10.1002/2015JC011565
       
  • Ocean bottom pressure records from the Cascadia array and short surface
           gravity waves
    • Authors: Charles Peureux; Fabrice Ardhuin
      Abstract: The ocean bottom pressure records from eight stations of the Cascadia array are used to investigate the properties of short surface gravity waves with frequencies ranging from 0.2 to 5 Hz. It is found that the pressure spectrum at all sites is a well‐defined function of the wind speed U10 and frequency f, with only a minor shift of a few dB from one site to another that can be attributed to variations in bottom properties. This observation can be combined with the theoretical prediction that the ocean bottom pressure spectrum is proportional to the surface gravity wave spectrum E(f) squared, times the overlap integral I(f) which is given by the directional wave spectrum at each frequency. This combination, using E(f) estimated from modeled spectra [e.g. Rascle and Ardhuin, 2013] or parametric spectra [e.g. Elfouhaily et al 1997], yields an overlap integral I(f) that is a function of the local wave age f/fPM = fU10/0.13g. This function is maximum for f/fPM = 8 and decreases by 10, dB for f/fPM = 2 and f/fPM = 30. This shape of I(f) can be interpreted as a maximum width of the directional wave spectrum at f/fPM = 8, possibly equivalent to an isotropic directional spectrum, and a narrower directional distribution towards both the dominant low frequencies and the higher capillary‐gravity wave frequencies. This article is protected by copyright. All rights reserved.
      PubDate: 2016-03-15T11:01:58.97326-05:0
      DOI: 10.1002/2015JC011580
       
  • Sea‐ice melt onset associated with lead opening during the
           spring/summer transition near the North Pole
    • Abstract: In the central Arctic Ocean, autonomous observations of the ocean mixed layer and ice documented the transition from cold spring to early summer in 2011. Ice‐motion measurements using GPS drifters captured three events of lead opening and ice ridge formation in May and June. Satellite sea‐ice concentration observations suggest that locally observed lead openings were part of a larger scale pattern. We clarify how these ice deformation events are linked with the onset of basal sea ice melt, which preceded surface melt by 20 days. Observed basal melt and ocean warming are consistent with the available input of solar radiation into leads, once the advent of mild atmospheric conditions prevents lead refreezing. We use a one‐dimensional numerical simulation incorporating a Local Turbulence Closure scheme to investigate the mechanisms controlling basal melt and upper ocean warming. According to the simulation, a combination of rapid ice motion and increased solar energy input at leads promotes basal ice melt, through enhanced mixing in the upper mixed layer, while slow ice motion during a large lead opening in mid June produced a thin, low density surface layer. This enhanced stratification near the surface facilitates storage of solar radiation within the thin layer, instead of exchange with deeper layers, leading to further basal ice melt preceding the upper surface melt. This article is protected by copyright. All rights reserved.
      PubDate: 2016-03-15T11:01:53.752126-05:
      DOI: 10.1002/2015JC011588
       
  • Variability of sea ice melt and meteoric water input in the surface
           Labrador Current off Newfoundland
    • Authors: M. Benetti; G. Reverdin, C. Pierre, S. Khatiwala, B. Tournadre, S. Olafsdottir, A. Naamar
      Abstract: The respective contributions of saline (Atlantic and Pacific water) and freshwater (sea ice melt, meteoric water) components in the surface Labrador Current are quantified using salinity, δ18O and nutrient data collected between 2012 and 2015 east of Newfoundland to investigate the seasonal variability of salinity in relation with the different freshwater contributions. Nutrient data indicate that the surface saline water is composed on average over 2012‐2015 of roughly 62% Atlantic Water and 38% Pacific Water. A large salinity seasonal cycle of ≈ 1.5 peak‐to‐peak amplitude is found over the middle continental shelf, which is explained by the freshwater input seasonal variability: 2/3 of the amplitude of the salinity seasonal cycle can be explained by meteoric water input and 1/3 by the sea ice melt. A smaller seasonal salinity cycle (≈1.3) is observed over the inner shelf compared to the middle shelf, because of smaller variability in the large meteoric water inputs. Furthermore, the data reveal that sea ice melt (SIM) input was particularly important during July 2014, following a larger extension of sea ice over the Labrador shelf during the 2013/2014 winter season, compared to both previous winter seasons. Some patches of large SIM contribution observed during July 2014 and April 2015 were located on the continental slope or further offshore. The comparison of 2012‐2015 data with data collected in 1994‐1995 shows that the surface water over the Newfoundland shelf and slope is strongly affected by sea ice processes in both periods and suggests a larger contribution of brines over the slope during 1994‐1995. This article is protected by copyright. All rights reserved.
      PubDate: 2016-03-14T10:38:39.585291-05:
      DOI: 10.1002/2015JC011302
       
  • Flocculation in a Decaying Shear Field and Its Implications for Mud
           Removal In Near‐Field River Mouth Discharges
    • Authors: Kyle Strom; Ali Keyvani
      Abstract: We measure the change in floc size in a decaying shear field that mimics the decay found in a river mouth discharge. The primary questions explored are: (1) how far from equilibrium are the flocs during the decay? And, (2) how significant are the changes in size, and hence settling velocity, brought on by the changes in shear when it comes to predicting settling flux? These questions are examined in the laboratory using a camera system that allows for flocs to be sized within a turbulent suspension. We also examine how inclusion of various approaches to account for flocculation impact the plume concentration and deposition rates in a simple river‐mouth discharge model. In the experiments, flocs grew from their initial size of 20 to 50 μm up to 100 to 200 μm due only to changes in shear over time scales of decay similar to those in a small river mouth discharge. It is estimated that such growth would lead to 4 or greater fold increase in the mud settling velocity within the first few km from the river mouth, even though floc sizes were only 0.5 to 0.8 of their equilibrium values. The plume modeling highlights the strong dependence of the mud deposition rate on the method chosen to account for flocs, and the importance of the settling to entrainment velocity ratio in dictating plume concentrations. This article is protected by copyright. All rights reserved.
      PubDate: 2016-03-14T10:38:38.455822-05:
      DOI: 10.1002/2015JC011169
       
  • Patterns of the Loop Current System and Regions of Sea Surface Height
           Variability in the Eastern Gulf of Mexico Revealed by the
           Self‐Organizing Maps
    • Authors: Yonggang Liu; Robert H. Weisberg, Stefano Vignudelli, Gary T. Mitchum
      Abstract: The Self‐Organizing Map (SOM), an unsupervised learning neural network, is employed to extract patterns evinced by the Loop Current (LC) system and to identify regions of sea surface height (SSH) variability in the eastern Gulf of Mexico (GoM) from 23 years (1993 – 2015) of altimetry data. Spatial patterns are characterized as different LC extensions and different stages in the process of LC eddy shedding. The temporal evolutions and the frequency of occurrences of these patterns are obtained, and the typical trajectories of the LC system progression on the SOM grid are investigated. For an elongated, northwest‐extended, or west‐positioned LC, it is common for the LC anticyclonic eddy (LCE) to separate and propagate into the western GoM, while an initially separated LCE in close proximity to the west Florida continental slope often reattaches to the LC and develops into an elongated LC, or reduces intensity locally before moving westward as a smaller eddy. Regions of differing SSH variations are also identified using the joint SOM – wavelet analysis. Along the general axis of the LC, SSH exhibits strong variability on time scales of 3 months to 2 years, also with energetic intraseasonal variations, which is consistent with the joint Empirical Orthogonal Function (EOF) – wavelet analysis. In the more peripheral regions, the SSH has a dominant seasonal variation that also projects across the coastal ocean. The SOM, when applied to both space and time domains of the same data, provides a powerful tool for diagnosing ocean processes from such different perspectives. This article is protected by copyright. All rights reserved.
      PubDate: 2016-03-14T10:38:36.169831-05:
      DOI: 10.1002/2015JC011493
       
  • Annual cycles of deep‐ocean biogeochemical export fluxes in
           subtropical and subantarctic waters, Southwest Pacific Ocean
    • Authors: Scott D. Nodder; Stephen M. Chiswell, Lisa C. Northcote
      Abstract: The annual cycles of particle fluxes derived from moored sediment trap data collected during 2000‐2012 in subtropical (STW) and subantarctic waters (SAW) east of New Zealand are presented. These observations are the most comprehensive export flux time‐series from temperate Southern Hemisphere latitudes to date. With high levels of variability, fluxes in SAW were markedly lower than in STW, reflecting the picophytoplankton‐dominated communities in the iron‐limited, high nutrient‐low chlorophyll SAW. Austral spring chlorophyll blooms in surface STW were near‐synchronous with elevated fluxes of bio‐siliceous, carbonate and organic carbon‐rich materials to the deep ocean, probably facilitated by diatom and/or coccolithophorid sedimentation. Lithogenic fluxes were also high in STW, compared to SAW, reflecting proximity to the New Zealand landmass. In contrast, the highest biogenic fluxes in SAW occurred in spring when surface chlorophyll concentrations were low, while highest annual chlorophyll concentrations were in summer with no associated flux increase. We hypothesize that the high spring export in SAW results from subsurface chlorophyll accumulation that is not evident from remote‐sensing satellites. This material was also rich in biogenic silica, perhaps related to the preferential export of diatoms and other silica‐producing organisms, such as silicoflagellates and radiolarians. Organic carbon fluxes in STW are similar to that of other mesotrophic to oligotrophic waters (∼6‐7 mgC m−2 d−1), whereas export from SAW is below the global average (∼3 mgC m−2 d−1). Regional differences in flux across the SW Pacific and Tasman region reflect variations in physical processes and ecosystem structure and function. This article is protected by copyright. All rights reserved.
      PubDate: 2016-03-14T10:38:33.912168-05:
      DOI: 10.1002/2015JC011243
       
  • Wind‐driven changes of surface current, temperature, and chlorophyll
           observed by satellites north of New Guinea
    • Abstract: Satellite observations of wind, sea level and derived currents, sea surface temperature (SST) and chlorophyll are used to expand our understanding of the physical and biological variability of the ocean surface north of New Guinea. Based on scarce cruise and mooring data, previous studies differentiated a trade wind situation (austral winter) when the New Guinea Coastal Current (NGCC) flows northwestward and a northwest monsoon situation (austral summer) when a coastal upwelling develops and the NGCC reverses. This circulation pattern is confirmed by satellite observations, except in Vitiaz Strait where the surface northwestward flow persists. We find that intraseasonal and seasonal time scale variations explain most of the variance north of New Guinea. SST and chlorophyll variabilities are mainly driven by two processes: penetration of Solomon Sea waters and coastal upwelling. In the trade wind situation, the NGCC transports cold Solomon Sea waters through Vitiaz Strait in a narrow vein hugging the coast. Coastal upwelling is generated in westerly wind situations (westerly wind event, northwest monsoon). Highly productive coastal waters are advected towards the equator and, during some westerly wind events, towards the eastern part of the warm pool. During El Niño, coastal upwelling events and northward penetration of Solomon Sea waters combine to influence SST and chlorophyll anomalies. This article is protected by copyright. All rights reserved.
      PubDate: 2016-03-14T10:38:29.553462-05:
      DOI: 10.1002/2015JC011438
       
  • Observations of bubbles in natural seep flares at MC 118 and GC 600 using
           in situ quantitative imaging
    • Authors: Binbin Wang; Scott A. Socolofsky, John A. Breier, Jeff Seewald
      Abstract: This paper reports the results of quantitative imaging using a stereoscopic, high‐speed camera system at two natural gas seep sites in the northern Gulf of Mexico during the Gulf Integrated Spill Research G07 cruise in July 2014. The cruise was conducted on the E/V Nautilus using the ROV Hercules for in situ observation of the seeps as surrogates for the behavior of hydrocarbon bubbles in subsea blowouts. The seeps originated between 890 m and 1190 m depth in Mississippi Canyon block 118 and Green Canyon block 600. The imaging system provided qualitative assessment of bubble behavior (e.g., break‐up, coalescence, etc.) and verified the formation of clathrate hydrate skins on all bubbles above 1.3 m altitude. Quantitative image analysis yielded the bubble size distributions, rise velocity, total gas flux, and void fraction, with most measurements conducted from the seafloor to an altitude of 200 m. Bubble size distributions fit well to log‐normal distributions, with median bubble sizes between 3 mm and 4.5 mm. Measurements of rise velocity fluctuated between two ranges: fast‐rising bubbles following helical‐type trajectories and bubbles rising about 40% slower following a zig‐zag pattern. Rise speed was uncorrelated with hydrate formation, and bubbles following both speeds were observed at both sites. Ship‐mounted multi‐beam sonar provided the flare rise heights, which corresponded closely with the boundary of the hydrate stability zone for the measured gas compositions. The evolution of bubble size with height agreed well with mass transfer rates predicted by equations for dirty bubbles. This article is protected by copyright. All rights reserved.
      PubDate: 2016-03-11T01:03:31.490894-05:
      DOI: 10.1002/2015JC011452
       
  • Fram Strait and Greenland Sea transports, water masses, and water mass
           transformations 1999–2010 (and beyond)
    • Abstract: Transports between the Nordic Seas and the Arctic Ocean are estimated using hydrographic data from the Greenland Sea and the Fram Strait. Geostrophic transports are computed from hydrographic sections at 75°N in the Greenland Sea and at about 79°N in the Fram Strait. Geostrophic velocities are adjusted with velocities derived from Argo floats, and 4 conservation constraints are applied to a box closed by the two sections. The estimated net volume transports are 0.8±1.5 Sv southward. Net freshwater transports through the Greenland Sea section are estimated at 54±20 mSv and through the Fram Strait section at 66±9 mSv. Heat loss in the area between the two sections is estimated at 9±12 TW. Convection depths in the Greenland Sea are estimated from observations. Water mass properties in the Greenland Sea are affected both by convection and lateral mixing. The properties of Atlantic, intermediate, and deep waters are studied. Deep water properties are defined using water mass triangles. This article is protected by copyright. All rights reserved.
      PubDate: 2016-03-11T01:02:21.478518-05:
      DOI: 10.1002/2015JC011312
       
  • Do the Amazon and Orinoco freshwater plumes really matter for
           hurricane‐induced ocean surface cooling?
    • Authors: O. Hernandez; J. Jouanno, F. Durand
      Abstract: Recent studies suggested that the plume of low‐saline waters formed by the discharge of the Amazon and Orinoco rivers could favor Atlantic Tropical Cyclone (TC) intensification by weakening the cool wake and its impact on the hurricane growth potential. The main objective of this study is to quantify the effects of the Amazon‐Orinoco river discharges in modulating the amplitude of TC‐induced cooling in the western Tropical Atlantic. Our approach is based on the analysis of TC cool wake statistics obtained from an ocean regional numerical simulation with ¼º horizontal resolution over the 1998‐2012 period, forced with realistic TC winds. In both model and observations, the amplitude of TC‐induced cooling in plume waters (0.3‐0.4ºC) is reduced significantly by around 50‐60% compared to the cooling in open ocean waters out of the plume (0.6‐0.7ºC). A twin simulation without river runoff shows that TC‐induced cooling over the plume region (defined from the reference experiment) is almost unchanged (∼0.03ºC) despite strong differences in salinity stratification and the absence of barrier layers. This argues for a weaker than thought cooling inhibition effect of salinity stratification and barrier layers in this region. Indeed, results suggest that haline stratification and barrier layers caused by the river runoff may explain only ∼10% of the cooling difference between plume waters and open ocean waters. Instead, the analysis of the background oceanic conditions suggests that the regional distribution of the thermal stratification is the main factor controlling the amplitude of cooling in the plume region. This article is protected by copyright. All rights reserved.
      PubDate: 2016-03-11T01:02:17.719502-05:
      DOI: 10.1002/2015JC011021
       
  • Turbulent production in an internal wave bottom boundary layer maintained
           by a vertically propagating seiche
    • Authors: Stephen M. Henderson
      Abstract: Internal seiches, which supply the energy responsible for mixing many lakes, are often modeled as vertically standing waves. However, recent observations of vertical seiche propagation in a small lake are inconsistent with the standard, vertically standing model. To examine the processes responsible for such propagation, drag and turbulent production in the bottom boundary layer of a small lake are related to the energy supplied by a propagating seiche (period 10–24 hours). Despite complex and fluctuating stratification, which often inhibited mixing within 0.4m of the bed, bottom stress was well represented by a simple drag coefficient model (drag coefficient 1.5 × 10−3). The net supply of seiche energy to the boundary layer was estimated by fitting a model for internal wave vertical propagation to velocity profiles measured above the boundary layer (1–4.5m above lakebed). Fitted reflection coefficients ranged from 0.3 at 1 cycle/day frequency to 0.7 at 2.4 cycles/day (c.f. near‐unity coefficients of classical seiche theories). The net supply of seiche energy approximately balanced boundary layer turbulent production. Three of four peaks in production and energy flux occurred 0.8–2.2 days after strong oscillating winds, a delay comparable to the time required for seiche energy to propagate to the lakebed. A model based on the estimated drag coefficient predicted the observed frequency‐dependence of the seiche reflection coefficient. For flat‐bed regions in narrow lakes, the model predicts that reflection is controlled by the ratio of water velocity to vertical wave propagation speed, with sufficiently large ratios leading to weak reflection, and clear vertical seiche propagation. This article is protected by copyright. All rights reserved.
      PubDate: 2016-03-11T00:57:18.307026-05:
      DOI: 10.1002/2015JC011071
       
  • Simulation of oil bioremediation in a tidally influenced beach:
           Spatiotemporal evolution of nutrient and dissolved oxygen
    • Authors: Xiaolong Geng; Zhong Pan, Michel C. Boufadel, Tamay Ozgokmen, Kenneth Lee, Lin Zhao
      Abstract: Numerical experiments of oil bioremediation of tidally‐influenced beach were simulated using the model BIOMARUN. Nutrient and dissolved oxygen were assumed present in a solution applied on the exposed beach face, and the concentration of these amendments was tracked throughout the beach for up to 6 months. It was found that, in comparison to natural attenuation, bioremediation increased the removal efficiency by 76% and 65% for alkanes and aromatics, respectively. Increasing the nutrient concentration in the applied solution did not always enhance biodegradation as oxygen became limiting even when the beach was originally oxygen‐rich. Therefore, replenishment of oxygen to oil‐contaminated zone was also essential. Stimulation of oil biodegradation was more evident in the upper and mid‐ intertidal zone of the beach, and less in the lower intertidal zone. This was due to reduced nutrient and oxygen replenishment, as very little of the amendment solution reached that zone. It was found that under continual application, most of the oil biodegraded within two months, while it persisted for six months under natural conditions. While the difference in duration suggests minimal long‐term effects, there are situations where the beach would need to be cleaned for major ecological functions, such as temporary nesting or feeding for migratory birds. Biochemical retention time map (BRTM) showed that the duration of solution application was dependent upon the stimulated oil biodegradation rate. By contrast, the application rate of the amendment solution was dependent upon the surface extent of the oil‐contaminated zone. Delivery of nutrient and oxygen into coastal beach involved complex interaction among amendment solution, groundwater, and seawater. Therefore, approaches that ignore the hydrodynamics due to tide are unlikely to provide the optimal solutions for shoreline bioremediation. This article is protected by copyright. All rights reserved.
      PubDate: 2016-03-11T00:57:17.347389-05:
      DOI: 10.1002/2015JC011221
       
  • Anatomy of the Antarctic Circumpolar Current VOLUME TRANSPORTS through
           Drake Passage
    • Abstract: The 20‐year (October 1992‐ August 2013) observation‐based volume transport time series of the Antarctic Circumpolar Current (ACC) through Drake Passage (DP) across the Jason altimeter track #104 is analyzed to better understand the ACC transport variability and its potential causes. The time series of three transport components (total (TT), barotropic (BT), and baroclinic (BC)) referenced to 3000 m present energetic intraseasonal fluctuations, with a salient spectral peak at 50 and 36 days, with the largest (least) variance being associated with the BT (BC) component. Low‐frequency variations are much less energetic with a significant variance limited to the annual and biannual timescales and show a non‐stationary intermittent link with the Southern Annular Mode and the Nino 3.4 index for interannual timescales. The region around 57°S in the Yaghan Basin appears to be a strategic point for a practical monitoring of the ACC transport, as the whole‐track TT is significantly correlated with the local TT (r = 0.53) and BT (r = 0.69) around 57°S. These local BT (and TT) variations are associated with a well‐defined tripole pattern in altimetric sea level anomaly (SLA). There is evidence that the tripole pattern associated with BT is locally generated when the BC‐associated meso‐scale SLAs, which have propagated eastward from an upstream area of DP, cross the Shackleton Fracture Zone to penetrate into the Yaghan Basin. Barotropic basin modes excited within the Yaghan Basin are discussed as a plausible mechanism for the observed energy‐containing intraseasonal spectral peaks found in the transport variability. This article is protected by copyright. All rights reserved.
      PubDate: 2016-03-11T00:54:04.888996-05:
      DOI: 10.1002/2015JC011436
       
  • Influence of upwelling on SST trends in La Guajira system
    • Abstract: La Guajira upwelling system has shown a moderate cooling (∼ ‐0.05 ºC dec−1) over the period 1982 ‐ 2014 for the months with strong upwelling (JFM). This contrasts with the general warming observed for most of the Caribbean region (∼0.25 ºC dec−1) over the same period. Cooling is in good agreement with the upwelling increase (∼0.04 m2s−1dec−1) observed in the region during winter months, although the position of the peaks in upwelling and SST trends does not coincide exactly due to the presence of the Caribbean Coastal Undercurrent. This article is protected by copyright. All rights reserved.
      PubDate: 2016-03-07T04:47:29.330441-05:
      DOI: 10.1002/2015JC011420
       
  • Stochastic secular trends in sea level rise
    • Abstract: Global mean sea level (GMSL) has been rising since (at least) the 19th century and the rate of rise may be increasing. Several studies that attempt to explain the long term trend of GMSL during the instrumental record share the common assumption that this trend is deterministic in nature and different from natural variations. Here we show that the trend can alternatively be explained, at least in part, as being caused by random variations within the coupled ocean‐atmosphere‐cryosphere system, and hence not having a deterministic origin. These random trends, which add to externally forced changes (e.g. through anthropogenic climate change), are a consequence of the integrated character of GMSL, which is the cumulative addition of temporal contributions that exhibit random character, and whose integration results in GMSL variations with persistence on decadal‐centennial time scales. The generation of trends by integration of random stationary noise (i.e. even in a constant climate) is a robust and fundamental feature of stochastically forced systems with memory. The integrated character of GMSL results in an intrinsic difficulty in distinguishing internal from externally forced trends. This article is protected by copyright. All rights reserved.
      PubDate: 2016-03-07T04:44:34.95683-05:0
      DOI: 10.1002/2015JC011301
       
  • Observations of wave dispersion and attenuation in landfast ice
    • Authors: Graig Sutherland; Jean Rabault
      Abstract: Observations of wave propagation in landfast ice were obtained in Tempelfjorden, Svalbard during March 2015. Wave motion was measured near the ice edge using inertial motion units and consisted of a combination of swell from the North Atlantic and wind‐generated waves. The waves were observed to be unidirectional in the ice with comparable magnitudes in the vertical and horizontal displacements. The dispersion relation was calculated from the measured phase difference between two adjacent sensors separated by a distance of approximately 60 m. Deviations from the gravity wave dispersion relation were observed during the growth phase of the waves and were consistent with the presence of flexural waves. This period of wave growth was accompanied by significant wave attenuation in the high frequency portion of the wave spectrum which persisted for 3 to 5 hours. This article is protected by copyright. All rights reserved.
      PubDate: 2016-03-07T04:43:45.366894-05:
      DOI: 10.1002/2015JC011446
       
  • Evolution of wind‐driven flows in the Yellow Sea during winter
    • Abstract: To examine the evolution of the wind‐driven flows in the Yellow Sea (YS) during winter, ocean circulation was simulated using a three‐dimensional ocean model with realistic topography and atmospheric forcing. The simulated sea surface temperature, ocean currents, and path of the Yellow Sea Warm Current (YSWC) agreed with observations. Southward currents along the Korean coast and the Chinese coast in winter were also effectively identified. Spectra of the daily mean winds and the YSWC velocities in the subsurface layer had dominant peaks at 12‐ and 20‐day periods. Time‐lagged correlation analysis suggested that the downwind flow in the surface layer reacts concurrently to the northwesterly wind in winter whereas the subsurface layer responds with a delay. One day after the wind burst, an upwind current in the subsurface layer appeared in the center of the trough, whereas the downwind flow in the surface layer decreased significantly. Two days later, the upwind flow in the subsurface layer shifted to the west of the trough while the downwind flow along the Korean coast strengthened. These flow responses to the wind variations resulted in a clockwise circulation in the YS during winter. This article is protected by copyright. All rights reserved.
      PubDate: 2016-03-07T04:43:32.369813-05:
      DOI: 10.1002/2016JC011622
       
  • Seasonal thermal fronts on the northern South China Sea shelf: Satellite
           measurements and three repeated field surveys
    • Abstract: Seasonal thermal fronts associated with wind‐driven coastal downwelling/upwelling in the northern South China Sea are investigated using satellite measurements and three repeated fine‐resolution mapping surveys in winter, spring and summer. The results show that vigorous thermal fronts develop over the broad shelf with variable widths and intensities in different seasons, which tend to be approximately aligned with the 20‐100 m isobaths. Driven by the prevailing winter/summer monsoon, the band‐shaped fronts were observed with a magnitude exceeding 0.1°C/km in the subsurface, and accompanied by energetic coastal downwelling/upwelling due to shoreward/offshore Ekman transport. The downward/upward tilting of seasonal thermoclines across the shelf exceeds 20 m, significantly contributing to the development of thermal fronts over the shelf. In addition, the diagnostic analysis of Potential Vorticity (PV) suggests that the summer frontal activities induced by the coastal upwelling are more stable to convection and symmetric instabilities in comparison to the winter fronts associated with downwelling‐favorable monsoon forcing. This is primarily due to their essential differences in the upper ocean stratification and horizontal buoyancy gradients arising from wind forcing. At the same time, the coastal currents are substantially regulated by the seasonal winds. An expected lag correlation between the velocity from mooring measurements and alongshore wind stress is detected near the frontal region. These results indicate that seasonal wind forcing plays an important role in the frontal activities and coastal water transport over the shelf. This article is protected by copyright. All rights reserved.
      PubDate: 2016-03-07T04:43:12.328203-05:
      DOI: 10.1002/2015JC011222
       
  • Spectral slopes of the absorption coefficient of colored dissolved and
           detrital material inverted from UV‐visible remote sensing
           reflectance
    • Authors: Jianwei Wei; Zhongping Lee, Michael Ondrusek, Antonio Mannino, Maria Tzortziou, Roy Armstrong
      Abstract: The spectral slope of the absorption coefficient of colored dissolved and detrital material (CDM), Scdm (units: nm−1), is an important optical parameter for characterizing the absorption spectral shape of CDM. Although highly variable in natural waters, in most remote sensing algorithms, this slope is either kept as a constant or empirically modeled with multi‐band ocean color in the visible domain. In this study, we explore the potential of semi‐analytically retrieving Scdm with added ocean color information in the ultraviolet (UV) range between 360‐400 nm. Unique features of hyperspectral remote sensing reflectance in the UV‐visible wavelengths (360‐500 nm) have been observed in various waters across a range of coastal and open ocean environments. Our data and analyses indicate that ocean color in the UV domain is particularly sensitive to the variation of the CDM spectral slope. Here, we used a synthesized dataset to show that adding UV wavelengths to the ocean color measurements will improve the retrieval of Scdm from remote sensing reflectance considerably, while the spectral band settings of past and current satellite ocean color sensors cannot fully account for the spectral variation of remote sensing reflectance. Results of this effort support the concept to include UV wavelengths in the next generation of satellite ocean color sensors. This article is protected by copyright. All rights reserved.
      PubDate: 2016-03-04T13:51:54.903234-05:
      DOI: 10.1002/2015JC011415
       
  • Scales and properties of cold filaments in the Benguela upwelling system
           off Lüderitz
    • Abstract: The Benguela upwelling system is one of the four strongest upwelling systems in the world. Meso‐scale and submeso‐scale structures like eddies and filaments build up at the front between the cold upwelled water and the warm surface water offshore. From three and a half years of satellite data (MODIS) of sea surface temperature (SST) more than 450 filaments were identified and used for extensive statistics of the scales, the occurrence frequency and the location of filaments in the Lüderitz upwelling cell and its vicinity. In‐situ data of SST, sea surface salinity and profiles of temperature, salinity and velocity collected during six cruises between September 2011 and February 2014 provide information on the properties of filaments and on their vertical structure. The occurrence of the filaments shows a maximum in austral summer and a minimum in September similar to the seasonal intensity of the upwelling. Within the Lüderitz cell, filaments have a meridional extent between 5 km and 80 km, an offshore extent between 20 km and more than 500 km and a vertical extent between 70 m and 180 m. The temperature within the filaments is 0.25 – 2.5°C lower than the ambient temperature and their salinity is up to 0.6 lower than the ambient salinity. Within the filaments westerly currents with velocities up to 30 cm/s were observed. Mean filament properties and occurrence result in an offshore volume transport of 3 Sv and a heat transport anomaly of ‐3*1013W. This article is protected by copyright. All rights reserved.
      PubDate: 2016-03-03T20:20:52.003859-05:
      DOI: 10.1002/2015JC011411
       
  • Variability of nonlinear internal waves in the South China Sea affected by
           the Kuroshio and mesoscale eddies
    • Abstract: Long‐term observations of nonlinear internal waves in the South China Sea reveal seasonal to interannual variability. During two selected segments of inverted echo sounder observations, tidal forcing in Luzon Strait is almost identical, but the observed amplitudes of nonlinear internal waves in the South China Sea are very different. The effects of the Kuroshio and mesoscale eddies, reproduced by HYbrid Cooridnate Ocean Model (HYCOM) reanalysis simulation, are then investigated. The Kuroshio can enhance the zonal tilt of the thermocline and induce intruding flow in Luzon Strait. During the two selected segments, different thermocline slopes did not significantly change the internal tide generation, but the intruding flow may result in a 11% difference in the amplitude of generated M2 internal tides. During the two selected segments, mesoscale eddies appeared on the path of internal wave propagation, a cold eddy in one case and a warm one in the other. The eddies changed local stratification and induced additional background currents, thus affecting the nonlinear evolution of internal tides. In addition, wave front steering due to the mesoscale eddies dramatically affected the observed amplitude changes of the nonlinear internal waves: the edge, rather than the center, of the nonlinear internal wave front passed through the observational stations, resulting in reduced amplitude in the observations. This article is protected by copyright. All rights reserved.
      PubDate: 2016-03-03T20:19:53.398856-05:
      DOI: 10.1002/2015JC011134
       
  • Time scales in Galveston Bay: An unsteady estuary
    • Authors: Matthew D. Rayson; Edward S. Gross, Robert D. Hetland, Oliver B. Fringer
      Abstract: Estuarine time scales including the turnover, particle e‐folding time, the age (calculated with a passive tracer) and residence time (calculated with Lagrangian particles), were computed using a three‐dimensional hydrodynamic model of Galveston Bay, a low‐flow, partially‐stratified estuary. Time scales were computed during a time period when river flow varied by several orders of magnitude and all time scales therefore exhibited significant temporal variability because of the unsteadiness of the system. The spatial distributions of age and residence time were qualitatively similar and increased from 15 d in a shipping channel to > 45 d in the upper estuary. Volume‐averaged age and residence time decreased during high‐flow conditions. Bulk time scales, including the freshwater and salinity turnover times, were far more variable due to the changing river discharge and salt flux through the estuary mouth. A criteria for calculating a suitable averaging time is discussed to satisfy a steady‐state assumption and to estimate a more representative bulk time scale. When scaled with a freshwater advective time, all time scales were approximately equal to the advective time scale during high‐flow conditions and many times higher during low flow conditions. The mean age, Lagrangian residence and flushing times exhibited a relationship that was weakly dependent on the freshwater advective time scale demonstrating predictability even in an unsteady, realistic estuary. This article is protected by copyright. All rights reserved.
      PubDate: 2016-03-03T20:16:25.659903-05:
      DOI: 10.1002/2015JC011181
       
  • A framework to quantify uncertainty in simulations of oil transport in the
           ocean
    • Abstract: An uncertainty quantification framework is developed for the Deep‐C Oil Model based on a non‐intrusive polynomial chaos method. This allows the model's output to be presented in a probabilistic framework so that the model's predictions reflect the uncertainty in the model's input data. The new capability is illustrated by simulating the far field dispersal of oil in a Deep Water Horizon blowout scenario. The uncertain input consisted of ocean current and oil droplet size data and the main model output analyzed is the ensuing oil concentration in the Gulf of Mexico. A 1,331 member ensemble was used to construct a surrogate for the model which was then mined for statistical information. The mean and standard deviations in the oil concentration were calculated for up to 30 days, and the total contribution of each input parameter to the model's uncertainty was quantified at different depths. Also, probability density functions of oil concentration were constructed by sampling the surrogate and used to elaborate probabilistic hazard maps of oil impact. The performance of the surrogate was constantly monitored in order to demarcate the space‐time zones where its estimates are reliable. This article is protected by copyright. All rights reserved.
      PubDate: 2016-03-01T19:23:33.371816-05:
      DOI: 10.1002/2015JC011311
       
  • Optics of the offshore Columbia River plume from glider observations and
           satellite imagery
    • Abstract: The Columbia River (CR) is the largest source of freshwater along the U.S. Pacific coast. The resultant plume is often transported southward and offshore forming a large buoyant feature off Oregon and northern California in spring‐summer ‐ the offshore CR plume. Observations from autonomous underwater gliders and Moderate Resolution Imaging Spectroradiometer (MODIS) satellite imagery are used to characterize the optics of the offshore CR plume off Newport, Oregon. Vertical sections, under contrasting river flow conditions, reveal a low‐salinity and warm surface layer of ∼20‐25 m (fresher in spring and warmer in summer), high Colored Dissolved Organic Matter (CDOM) concentration and backscatter, and associated with the base of the plume high chlorophyll fluorescence. Plume characteristics vary in the offshore direction as the warm and fresh surface layer thickens progressively to an average 30‐40 m of depth 270‐310 km offshore; CDOM, backscatter, and chlorophyll fluorescence decrease in the upper 20 m and increase at subsurface levels (30‐50 m depth). MODIS normalized water‐leaving radiance (nLw(λ)) spectra for CR plume cases show enhanced water‐leaving radiance at green bands (as compared to no‐CR plume cases) up to ∼154 km from shore. Farther offshore, the spectral shapes for both cases are very similar, and consequently, a contrasting color signature of low‐salinity plume water is practically imperceptible from ocean color remote sensing. Empirical algorithms based on multivariate regression analyses of nLw(λ) plus SST data produce more accurate results detecting offshore plume waters than previous studies using single visible bands (e.g. adg(412) or nLw(555)). This article is protected by copyright. All rights reserved.
      PubDate: 2016-03-01T19:22:55.747442-05:
      DOI: 10.1002/2015JC011431
       
  • Surface circulation at the Strait of Gibraltar: A combined HF radar and
           high‐resolution model study
    • Abstract: Observations from a high frequency radar system and outputs from a high resolution operational ocean model working at the Strait of Gibraltar have been analyzed and compared during the period February 2013 – September 2014 in order to evaluate their capability to resolve the surface circulation of the region. The description of the mean circulation patterns has been statistically assessed, showing good agreement, particularly in the central region of the strait corresponding with the Atlantic Jet (AJ) stream, although some short scale features are not reproduced by the model. In the frequency domain very high concordance is observed. Tidal maps of diurnal and semi‐diurnal constituents are in good agreement with previous observations. The analysis of the model and radar response to the wind forcing reveals that the low resolution of the model wind – forcing field and its deeper superficial level smoothes the wind effect on the simulated currents. The first three EOF modes account for the 86% of model and radar variances. The coincidence between the observed and simulated patterns is very significant for the first two modes, which account for the mean velocity field and the latitudinal shifting of the AJ consequence of the flow‐topography interaction. The third mode captures the wind – induced circulation, and greater discrepancies are found in this case. Results underline the complementary character of both systems: radar observations improve the model description, resolving short scale processes, while the model completes the radar information when the time or spatial coverage is poorer. This article is protected by copyright. All rights reserved.
      PubDate: 2016-03-01T19:21:23.761462-05:
      DOI: 10.1002/2015JC011354
       
  • Estimating river discharge using multiple‐tide gauges distributed
           along a channel
    • Authors: H. R. Moftakhari; D. A. Jay, S. A. Talke
      Abstract: Reliable estimation of freshwater inflow to the ocean from large tidal rivers is vital for water resources management and climate analyses. Discharge gauging stations are typically located beyond the tidal intrusion reach, such that inputs and losses occurring closer to the ocean are often not included. Here, we develop a method of estimating river discharge using multiple gauges and time‐dependent tidal statistics determined via wavelet analysis. The Multiple‐gauge Tidal Discharge Estimate (MTDE) method is developed using data from the Columbia River and Fraser River estuaries and calibrated against river discharge. Next, we evaluate the general applicability of MTDE by testing an idealized two‐dimensional numerical model, with a convergent cross‐sectional profile, for eighty‐one cases in which non‐dimensional numbers for friction, river flow, and convergence length scale are varied.The simulations suggest that MTDE is applicable to a variety of tidal systems. Model results and data analyses together suggest that MTDE works best with at least three gauges: a reference station near the river mouth, and two upstream gauges that respond strongly to distinct portions of the observed range of flow. The balance between tidal damping and amplifying factors determines the favorable location of the gauges. Compared to previous studies, the MTDE method improves the time resolution of estimates (from 2.5 to
      PubDate: 2016-03-01T19:21:13.280365-05:
      DOI: 10.1002/2015JC010983
       
  • Poleward propagation of parametric subharmonic instability‐induced
           inertial waves
    • Authors: Xiaohui Xie; Qian Liu, Xiaodong Shang, Guiying Chen, Dongxiao Wang
      Abstract: This study presents two sets of current records obtained from the South China Sea and satellite altimeter data, and it suggests that near‐inertial waves induced by parametric subharmonic instability (PSI) associated with internal tides can be transported poleward beyond their critical latitude φc by background geostrophic flow (BGF). The two mooring locations were poleward of φc (≈14°N) for diurnal subharmonics (0.5D1; half diurnal frequency D1); however, both of the current records revealed clear signals at 0.5D1. The enhanced sub‐inertial motion at 0.5D1 exhibited a fortnightly spring‐neap cycle but did not agree with that of D1, indicating that it may not be generated via PSI associated with the local D1. Observations from the altimeter data and a ray‐tracing simulation suggested that these non‐locally generated 0.5D1 waves may be excited near their φc, after which they propagated poleward under the role of the BGF to the observation site with a latitude higher than φc. The poleward propagation of near‐inertial waves can produce elevated vertical shears; thus, it may play an important role in enhancing the local turbulent mixing. This article is protected by copyright. All rights reserved.
      PubDate: 2016-02-26T04:33:57.189184-05:
      DOI: 10.1002/2015JC011194
       
  • Replacement of Multiyear sea ice and Changes in the Open Water Season
           Duration in the Beaufort Sea Since 2004
    • Abstract: The last decade has witnessed the nine lowest Arctic September sea ice extents in the observational record. It also forms the most recent third of the long‐term trend in that record, which reached ‐13.4% decade−1 in 2015. While hemispheric analyses paint a compelling picture of sea ice loss across the Arctic, the situation with multiyear ice in the Beaufort Sea is particularly dire. This study was undertaken in light of substantial changes that have occurred in the extent of summer multiyear sea ice in the Arctic inferred from the passive microwave record. To better elucidate these changes at a sub‐regional scale, we use data from the Canadian Ice Service archive, the most direct observations of sea ice stage‐of‐development available. We also build upon the only previous sea ice climatological analysis for Canada's western Arctic region by sea ice stage‐of‐development that ended in 2004. The annual evolution of sea ice by stage of development in Canada's western Arctic changed dramatically between 1983 and 2014. The rate of these changes and their spatial prevalence were most prominent in the last decade. In summer, total sea ice loss occurred via reductions in old and first year sea ice over increasingly large areas and over more months per year. Resultant delay of thermodynamic freeze up has increased the annual open water duration in the study region. The winter sea ice cover was increasingly composed of first‐year sea ice at the expense of old ice. Breakup timing has not significantly changed in the region. This article is protected by copyright. All rights reserved.
      PubDate: 2016-02-26T04:32:24.569051-05:
      DOI: 10.1002/2015JC011583
       
  • Ensemble simulations of the magnetic field induced by global ocean
           circulation: Estimating the uncertainty
    • Authors: C. Irrgang; Jan Saynisch, M. Thomas
      Abstract: The modelling of the ocean global circulation induced magnetic field is affected by various uncertainties that originate from errors in the input data and from the model itself. The amount of aggregated uncertainties and their effect on the modelling of electromagnetic induction in the ocean is unknown. For many applications, however, the knowledge of uncertainties in the modelling is essential. To investigate the uncertainty in the modelling of motional induction at the sea surface, simulation experiments are performed on the basis of different error scenarios and error covariance matrices. For these error scenarios, ensembles of an ocean general circulation model and an electromagnetic induction model are generated. This ensemble‐based approach allows to estimate both the spatial distribution and temporal variation of the uncertainty in the ocean induced magnetic field. The largest uncertainty in the ocean induced magnetic field occurs in the area of the Antarctic Circumpolar Current. Local maxima reach values of up to 0.7 nano Tesla. The estimated global annual mean uncertainty in the ocean induced magnetic field ranges from 0.1 to 0.4 nT. The relative amount of uncertainty reaches up to 30 % of the signal strength with largest values in regions in the northern hemisphere. The major source of uncertainty is found to be introduced by wind stress from the atmospheric forcing of the ocean model. In addition, the temporal evolution of the uncertainty in the induced magnetic field shows distinct seasonal variations. Specific regions are identified which are robust with respect to the introduced uncertainties. This article is protected by copyright. All rights reserved.
      PubDate: 2016-02-25T18:09:39.324092-05:
      DOI: 10.1002/2016JC011633
       
  • A laboratory study of spilling breakers in the presence of
           light‐wind and surfactants
    • Authors: Xinan Liu
      Abstract: Spilling breaking waves in the presence of light‐wind and surfactants are studied experimentally in a wind‐wave tank. The breaking waves are mechanically generated with a single wave maker motion that produces a weak spilling breaker in clean water without wind. Separate experiments are performed with the same wave maker motion at different low wind speeds in clean water and in water with various concentrations of Triton X‐100 (soluble surfactant). The crest‐profiles of the waves along the center plane of the tank are measured with a laser induced fluorescence (LIF) technique that utilizes a high‐speed camera. In clean water with wind speeds lower than 2.3∼m/s (the minimum wind speed of wind‐generated waves for clean water in our tank), the breaking of the waves is initiated with a similar bulge‐capillary‐waves pattern on the forward face of the wave crest as reported in Duncan et al (1999). When the wind speed is above 3 m/s, wind waves are generated. The wind waves strongly affect the breaking process of the mechanically generated waves. It is found that the bulge‐capillary‐waves pattern is independent of the wind, but is dramatically affected by surfactants. The slope of the back face of the wave crest decreases with increasing wind speed. At the moment of incipient breaking, the distances between the leading edge of the bulge (called the toe) and the highest point of the wave crest in all cases are linearly proportional to the surface wind drift. After the fluid in the bulge slides down the front face of the wave, the maximum horizontal distance of the toe away from the crest increases as the wind speed increases. This article is protected by copyright. All rights reserved.
      PubDate: 2016-02-24T18:22:20.677446-05:
      DOI: 10.1002/2015JC011200
       
  • Simulated effect of deep‐sea sedimentation and terrestrial
           weathering on projections of ocean acidification
    • Authors: Long Cao; Meidi Zheng, Ken Caldeira
      Abstract: Projections of ocean acidification have often been based on ocean carbon cycle models that do not represent deep‐sea sedimentation and terrestrial weathering. Here we use an Earth system model of intermediate complexity to quantify the effect of sedimentation and weathering on projections of ocean acidification under an intensive CO2 emission scenario that releases 5000 PgC after year 2000. In our simulations, atmospheric CO2 reaches a peak concentration of 2123 ppm near year 2300 with a maximum reduction in surface pH of 0.8. Consideration of deep‐sea sedimentation and terrestrial weathering has negligible effect on these peak changes. Only after several millenniums, sedimentation and weathering feedbacks substantially affect projected ocean acidification. Ten thousand years from today, in the constant‐alkalinity simulation, surface pH is reduced by ∼0.7 with 95% of the polar oceans undersaturated with respect to calcite, and no ocean has a calcite saturation horizon (CSH) that is deeper than 1000 m. With the consideration of sediment feedback alone, surface pH is reduced by ∼0.5 with 35% of the polar oceans experiencing calcite undersaturation, and 8% global ocean has a CSH deeper than 1000 m. With the addition of weathering feedback, depending on the weathering parameterizations, surface pH is reduced by 0.2‐0.4 with no polar oceans experiencing calcite undersaturation, and 30‐80% ocean has a CSH that is deeper than 1000 m. Our results indicate that deep‐sea sedimentation and terrestrial weathering play an important role in long‐term ocean acidification, but have little effect on mitigating ocean acidification in the coming centuries. This article is protected by copyright. All rights reserved.
      PubDate: 2016-02-22T18:03:46.425582-05:
      DOI: 10.1002/2015JC011364
       
  • The Polar Front in Drake Passage: A composite‐mean
           stream‐coordinate view
    • Authors: Annie Foppert; Kathleen A. Donohue, D. Randolph Watts
      Abstract: The Polar Front (PF) is studied using 4 years of data collected by a line of current‐ and pressure‐recording inverted echo sounders in Drake Passage complemented with satellite altimetry. The location of the PF is bimodal in latitude. A northern and southern PF exist at separate times, separated geographically by a seafloor ridge ‐ the Shackleton Fracture Zone ‐ and hydrographically by 17 cm of geopotential height. Expressed in stream coordinates, vertical structures of buoyancy are determined with a gravest empirical mode analysis. Baroclinic velocity referenced to zero at 3500 dbar, width, and full transport (about 70 Sv) of the jets are statistically indistinguishable; the two jets alternate carrying the baroclinic transport rather than coexisting. Influences of local bathymetry and deep cyclogenesis manifest as differences in deep reference velocity structures. Downstream reference velocities of the PF‐N and PF‐S reach maximum speeds of 0.09 and 0.06 m s‐1, respectively. Buoyancy fields are indicative of upwelling and poleward residual circulation at the PF. Based on potential vorticity and mixing lengths, the northern and southern PF both act as a barrier to cross‐frontal exchange while remaining susceptible to baroclinic instability. This article is protected by copyright. All rights reserved.
      PubDate: 2016-02-22T18:03:21.738028-05:
      DOI: 10.1002/2015JC011333
       
  • An inverse modeling study of circulation in the Eastern Bering Sea during
           2007‐2010
    • Authors: Gleb Panteleev; Max Yaremchuk, Oceana Francis, Philis J. Stabeno, T. Weingartner, J. Zhang
      Abstract: A two‐way nested 4d‐variational data assimilation system is implemented in the Eastern Bering Sea (EBS) to investigate changes in circulation and thermodynamic state for a 3.8‐year period. Assimilated observations include data from 19 moorings deployed on the shelf and in the Bering Strait, 1705 hydrographic stations occupied during eight surveys, and remotely sensed sea surface temperature and sea surface height (SSH) data. Validation of the presented 4dVar reanalysis against the output of two sequential data‐assimilative systems (the Bering Ecosystem Study ice‐ocean Modeling and Assimilation System (BESTMAS) and the Arctic Cap Nowcast‐Forecast System (ACNFS)) has shown that the product is more consistent with the observed transports in the Bering Strait and in the EBS interior both in terms of their magnitude and time variability. Analysis of the data‐optimized solution quantifies a sequence of wind‐forced events that resulted in the anomalous heat and freshwater transports through the Bering Strait, including a 28‐day long flow reversal that occurred in November of 2009 and carried Siberian Coastal Current water down to the Gulf of Anadyr. Lagrangian study of the Arctic‐bound Pacific waters indicates the extreme importance of the cross‐shelf exchange along the path of the Bering Slope Current and quantifies the spectrum of residence times for the waters entering EBS through Unimak Pass and through Aleutian passages. Residence times in the EBS cold pool are diagnosed to be 2‐3 times longer than those in the surrounding waters. This article is protected by copyright. All rights reserved.
      PubDate: 2016-02-22T18:03:09.359725-05:
      DOI: 10.1002/2015JC011287
       
  • Paralarvae of the complex Sthenoteuthis oualaniensis‐Dosidicus gigas
           (Cephalopoda: Ommastrephidae) in the northern limit of the shallow oxygen
           minimum zone of the Eastern Tropical Pacific Ocean (April 2012)
    • Abstract: The three‐dimensional distribution of the paralarvae of the complex Sthenoteuthis oualaniensis ‐ Dosidicus gigas (Cephalopoda: Ommastrephidae) was analyzed at the northern limit of the shallow oxygen minimum zone in the Eastern Tropical Pacific in April, 2012. The hypoxic water (∼ 44 μmol/kg or 1 mL/L) rises from ∼ 100 m depth in the entrance of the Gulf of California to ∼ 20 m depth off Cabo Corrientes. Most of the paralarvae of this complex, dominated by D. gigas, were concentrated in the Gulf entrance, between the thermocline (∼20 to ∼ 50 m depth) and the sea surface, in the warmest (> 19°C) oxygenated (> 176 μmol/kg) layer. The highest abundance of paralarvae was detected in an anticyclonic eddy (∼120 km diameter and > 500 m deep), which contained lower salinity water (< 35 g/kg), consistent with formation in the California Current. Lower paralarvae abundance was recorded further south off Cabo Corrientes, where hypoxic layers were elevated as water shoaled near shore. Almost no paralarvae were found in the north of the study area beyond the strong salinity front (∼ 34.8 ‐ 35.4 g/kg) that bounded the anticyclone. These results showed an affinity of the paralarvae for lower salinity, oxygenated water, illustrated by the influence of the mesoscale anticyclonic eddy and the salinity front in their distribution. Based on this study, it can be concluded that the expansion of the depth range of hypoxic water observed in the Eastern Tropical Pacific may be increasing environmental stress on the paralarvae by vertically restricting their habitat, and so affecting their survival. This article is protected by copyright. All rights reserved.
      PubDate: 2016-02-22T18:02:50.697695-05:
      DOI: 10.1002/2015JC011534
       
  • Modeling of circulation in the Arabian Gulf and the Sea of Oman: Skill
           assessment and seasonal thermohaline structure
    • Authors: Muchamad Al Azhar; Marouane Temimi, Jun Zhao, Hosni Ghedira
      Abstract: Hindcast simulations of the Arabian Gulf and the Sea of Oman using the Regional Ocean Modeling System (ROMS) are quantitatively evaluated with basin‐wide hydrographic data and time series measurements. The model shows comparable skill in reproducing moored observations of current velocities structure in upper and bottom depths. The skill in simulating observed temperature is higher of 0.93 (scale 0 – 1) in upper depths compared to 0.52 in bottom depths. Model results are sensitive to parameterization of water clarity. A lower sensitivity was noticed to KPP, GLS, and MY2.5 turbulence closures. When coastal turbid water parameterization is used, accuracy of the model in reproducing seasonal and spatial variations of temperature and salinity increased by 25% compared to the clear water case whereas only 10% increase was noticed when applying KPP turbulent closure. The model reproduces well anti‐clockwise circulation in the Gulf. A stronger surface inflow of fresher water to the Arabian Gulf through the Strait of Hormuz is simulated in summer compared to winter conditions, mainly due to upper‐layer horizontal gradient of density between the Arabian Gulf and the Sea of Oman. Less seasonal variability of outflow between 0.15 to 0.20 m s−1 at 50 m to bottom depth around the Strait of Hormuz was noticed in the model results. Modeled surface layer stratification is stronger in summer than winter and varies spatially in the Arabian Gulf with highest stratification near the Strait of Hormuz. Overall, the stratification in shallow water area of the Arabian Gulf remains low throughout the year. This article is protected by copyright. All rights reserved.
      PubDate: 2016-02-17T12:26:00.975698-05:
      DOI: 10.1002/2015JC011038
       
  • Wintertime variability of the Beaufort Gyre in the Arctic Ocean derived
           from CryoSat‐2/SIRAL observations
    • Authors: Kohei Mizobata; Eiji Watanabe, Noriaki Kimura
      Abstract: We processed the sea surface height measured by the SAR (Synthetic Aperture Radar)/Interferometric Radar Altimeter (SIRAL) on board CryoSat‐2 (CS‐2) and successfully estimated the monthly dynamic ocean topography (DOT) of the Arctic Ocean. The CS‐2 monthly DOT showed the interannual and monthly variability of the Beaufort Gyre (BG) during winter between 2010/2011 and 2014/2015. The northward flow at the western edge of the BG was primarily estimated over the Chukchi Borderland (CBL). However, the BG extended across the CBL, and the northward flow was estimated over the Mendeleev Ridge in the winter of 2012/2013. Our analyses revealed a significantly variable BG in response to changes in the sea surface stress field. Our analysis indicated that 1) sea ice motion, driven by wind fields, acts as a driving force for the BG when sea ice motion was intensified during winter and 2) sea ice motion can also act as an inhibiting force for the BG when sea ice motion is weakened during winter. In addition, the relationship between the DOT, steric height and ocean bottom pressure implied that the DOT during winter responded to varying wind stresses through baroclinic and barotropic adjustments. According to a tracer experiment, we inferred that in the winter of 2012/2013, the Pacific‐origin water carried into the BG through the Barrow Canyon was transported to the northern shelf and shelf break of the Chukchi Sea rather than the CBL, which is where the Pacific‐origin water had been transported in the other years of the observation period. This article is protected by copyright. All rights reserved.
      PubDate: 2016-02-17T12:24:37.897127-05:
      DOI: 10.1002/2015JC011218
       
  • The tug‐of‐war between the West Philippine Sea and South China
           Sea Tropical Waters and Intermediate Waters in the Okinawa Trough
    • Abstract: Kuroshio subsurface waters are the major source of nutrients to the East China Sea continental shelf, a major fishing ground. These subsurface waters are heavily affected by the South China Sea (SCS) Tropical and Intermediate Waters which contain more nutrients than the tropical (Smax) and intermediate (Smin) waters from the West Philippine Sea (WPS). A front was found to separate these waters from the SCS and WPS. The reported front in the Okinawa Trough, however, was identified based only on one‐time data from a single cross‐section. Here historical hydrographic data between Mar. 1950 and Dec. 2011 are analyzed. A vertical front tilted toward the west is found in all seasons in all years in the Okinawa Trough. In winter the presence of the SCS Tropical Water is the most prominent. It is the weakest in autumn. As for intermediate waters (Smin) the WPS and SCS Intermediate Waters show much weaker seasonality compared with tropical waters although in summer the WPS Intermediate Water contribution is slightly larger than during other times. This article is protected by copyright. All rights reserved.
      PubDate: 2016-02-17T12:23:26.420875-05:
      DOI: 10.1002/2015JC011274
       
  • Seasonal heat and freshwater cycles in the Arctic Ocean in CMIP5 coupled
           models
    • Authors: Yanni Ding; James A. Carton, Gennady A. Chepurin, Michael Steele, Sirpa Hakkinen
      Abstract: This study examines the processes governing the seasonal response of the Arctic Ocean and sea ice to surface forcings as they appear in historical simulations of 14 Coupled Model Intercomparison Project Phase 5 coupled climate models. In both models and observations the seasonal heat budget is dominated by a local balance between net surface heating and storage in the heat content of the ocean and in melting/freezing of sea ice. Observations suggest ocean heat storage is more important than sea ice melt, while in most of these models sea ice melt dominates. Seasonal horizontal heat flux divergence driven by the seasonal cycle of volume transport is only important locally. In models and observations the dominant terms in the basin‐average seasonal freshwater budget are the storages of freshwater between the ocean and sea ice, and the exchange between the two. The largest external source term is continental discharge in early summer, which is an order of magnitude smaller. The appearance of sea ice (extent and volume), and also ocean stratification in both the heat and freshwater budgets provides two links between the budgets and provides two mechanisms for feedback. One consequence of such an interaction is the fact that models with strong/weak seasonal surface heating also have strong/weak seasonal haline and temperature stratification. This article is protected by copyright. All rights reserved.
      PubDate: 2016-02-15T22:52:33.741438-05:
      DOI: 10.1002/2015JC011124
       
  • Revisiting nitrification in the eastern tropical South Pacific: A focus on
           controls
    • Authors: Xuefeng Peng; Clara A. Fuchsman, Amal Jayakumar, Mark J. Warner, Allan H. Devol, Bess B. Ward
      Abstract: Nitrification, the oxidation of ammonium (NH4+) to nitrite (NO2‐) and to nitrate (NO3‐), is a component of the nitrogen (N) cycle internal to the fixed N pool. In oxygen minimum zones (OMZs), which are hotspots for oceanic fixed N loss, nitrification plays a key role because it directly supplies substrates for denitrification and anaerobic ammonia oxidation (anammox), and may compete for substrates with these same processes. However, the control of oxygen and substrate concentrations on nitrification are not well understood. We performed onboard incubations with 15N‐labeled substrates to measure rates of NH4+ and NO2‐ oxidation in the eastern tropical South Pacific (ETSP). The spatial and depth distributions of NH4+ and NO2‐ oxidation rates were primarily controlled by NH4+ and NO2‐ availability, oxygen concentration, and light. In the euphotic zone, nitrification was partially photoinhibited. In the anoxic layer, NH4+ oxidation was negligible or below detection, but high rates of NO2‐ oxidation were observed. NH4+ oxidation displayed extremely high affinity for both NH4+ and oxygen. The positive linear correlations between NH4+ oxidation rates and in situ NH4+ concentrations and ammonia monooxygenase subunit A (amoA) gene abundances in the upper oxycline indicate that the natural assemblage of ammonia oxidizers responds to in situ NH4+ concentrations or supply by adjusting their population size, which determines the NH4+ oxidation potential. The depth distribution of archaeal and bacterial amoA gene abundances and N2O concentration, along with independently reported simultaneous direct N2O production rate measurements, suggests that AOA were predominantly responsible for NH4+ oxidation, which was a major source of N2O production at oxygen concentrations > 5 µM. This article is protected by copyright. All rights reserved.
      PubDate: 2016-02-15T22:52:03.757503-05:
      DOI: 10.1002/2015JC011455
       
  • Riverine skin temperature response to subsurface processes in low wind
           speeds
    • Authors: Sophia E. Brumer; Christopher J. Zappa, Steven P. Anderson, John P. Dugan
      Abstract: Both surface and subsurface processes modulate the surface thermal skin and as such the skin temperature may serve as an indicator for coastal, estuarine and alluvial processes. Infrared (IR) imagery offers the unique tool to survey such systems, allowing not only to assess temperature variability of the thermal boundary layer, but also to derive surface flow fields through digital particle image velocimetry, optical flow techniques, or spectral methods. In this study, IR time‐series imagery taken from a boat moored in the Hudson River estuary is used to determine surface flow, turbulent kinetic energy dissipation rate, and characteristic temperature and velocity length scales. These are linked to subsurface measurements provided by in situ instruments. Under the low wind conditions and weak stratification, surface currents and dissipation rate are found to reflect subsurface mean flow (r2=0.89) and turbulence (r2=0.75). For relatively low dissipation rates, better correlations are obtained by computing dissipation rates directly from wavenumber spectra rather than when having to assume the validity of the Taylor hypothesis. Furthermore, the subsurface dissipation rate scales with the surface length scales (L) and mean flow (U) using . The surface length scale derived from the thermal fields, is found to have a strong linear relationship (r2=0.88) to water depth (D) with D/L ∼ 13. Such a relation may prove useful for remote bathymetric surveys when no waves are present. This article is protected by copyright. All rights reserved.
      PubDate: 2016-02-12T07:39:27.704062-05:
      DOI: 10.1002/2015JC010746
       
  • Dependence of subsurface chlorophyll on seasonal water masses in the
           Chukchi Sea
    • Authors: Kim I. Martini; Phyllis J. Stabeno, Carol Ladd, Peter Winsor, Tom J. Weingartner, Calvin W. Mordy, Lisa B. Eisner
      Abstract: During the late summer, phytoplankton in the northeastern Chukchi Sea are typically found in subsurface layers. These layers and their sensitivity to local changes in hydrography and nutrient concentrations are characterized by combining data from a high‐resolution towed sampling platform with traditional shipboard observations. The replacement of surface meltwater and deeper nutrient‐rich Chukchi Winter Water by northward flowing nutrient‐poor Chukchi Summer Water and Remnant Winter water leads to a net decrease in biomass and smaller phytoplankton. Between 17‐67% of phytoplankton biomass is contained within the subsurface layers. This estimate is nearly twice as high as previous estimates from sparser shipboard data and suggests subsurface phytoplankton contribute significantly to the net biomass in the Chukchi Sea in late summer. This article is protected by copyright. All rights reserved.
      PubDate: 2016-02-12T07:10:15.848421-05:
      DOI: 10.1002/2015JC011359
       
  • Decadal reanalysis of biogeochemical indicators and fluxes in the North
           West European shelf‐sea ecosystem
    • Abstract: In this paper we present the first decadal reanalysis simulation of the biogeochemistry of the North West European shelf, along with a full evaluation of its skill and value. An error‐characterized satellite product for chlorophyll was assimilated into a physical‐biogeochemical model of the North East Atlantic, applying a localized Ensemble Kalman filter. The results showed that the reanalysis improved the model predictions of assimilated chlorophyll in 60% of the study region. Model validation metrics showed that the reanalysis had skill in matching a large dataset of in situ observations for ten ecosystem variables. Spearman rank correlations were significant and higher than 0.7 for physical‐chemical variables (temperature, salinity, oxygen), ∼0.6 for chlorophyll and nutrients (phosphate, nitrate, silicate), and significant, though lower in value, for partial pressure of dissolved carbon dioxide (∼0.4). The reanalysis captured the magnitude of pH and ammonia observations, but not their variability. The value of the reanalysis for assessing environmental status and variability has been exemplified in two case studies. The first shows that between 340,000‐380,000 km2 of shelf bottom waters were oxygen deficient potentially threatening bottom fishes and benthos. The second application confirmed that the shelf is a net sink of atmospheric carbon dioxide, but the total amount of uptake varies between 36‐46 Tg C yr−1 at a 90% confidence level. These results indicate that the reanalysis output dataset can inform the management of the North West European shelf ecosystem, in relation to eutrophication, fishery, and variability of the carbon cycle. This article is protected by copyright. All rights reserved.
      PubDate: 2016-02-12T07:09:18.65337-05:0
      DOI: 10.1002/2015JC011496
       
  • The impact of variable sea ice roughness on changes in Arctic Ocean
           surface stress: A model study
    • Authors: Torge Martin; Michel Tsamados, David Schroeder, Daniel L. Feltham
      Abstract: The Arctic sea ice cover is thinning and retreating, causing changes in surface roughness that in turn modify the momentum flux from the atmosphere through the ice into the ocean. New model simulations comprising variable sea ice drag coefficients for both the air and water interface demonstrate that the heterogeneity in sea ice surface roughness significantly impacts the spatial distribution and trends of ocean surface stress during the last decades. Simulations with constant sea ice drag coefficients as used in most climate models show an increase in annual mean ocean surface stress (0.003 N/m2 per decade, 4.6%) due to the reduction of ice thickness leading to a weakening of the ice and accelerated ice drift. In contrast, with variable drag coefficients our simulations show annual mean ocean surface stress is declining at a rate of ‐0.002 N/m2 per decade (3.1%) over the period 1980‐2013 because of a significant reduction in surface roughness associated with an increasingly thinner and younger sea ice cover. The effectiveness of sea ice in transferring momentum does not only depend on its resistive strength against the wind forcing but is also set by its top and bottom surface roughness varying with ice types and ice conditions. This reveals the need to account for sea ice surface roughness variations in climate simulations in order to correctly represent the implications of sea ice loss under global warming. This article is protected by copyright. All rights reserved.
      PubDate: 2016-02-12T03:11:22.496068-05:
      DOI: 10.1002/2015JC011186
       
  • The distribution of glacial meltwater in the Amundsen Sea, Antarctica,
           revealed by dissolved helium and neon
    • Abstract: The light noble gases, helium (He) and neon (Ne), dissolved in seawater, can be useful tracers of freshwater input from glacial melting because the dissolution of air bubbles trapped in glacial ice results in an approximately ten‐fold supersaturation. Using He and Ne measurements, we determined, for the first time, the distribution of glacial meltwater (GMW) within the water columns of the Dotson Trough (DT) and in front of the Dotson and Getz Ice Shelves (DIS and GIS, respectively) in the western Amundsen Sea, Antarctica, in the austral summers of 2011 and 2012. The measured saturation anomalies of He and Ne (ΔHe and ΔNe) were in the range of 3 – 35% and 2 – 12%, respectively, indicating a significant presence of GMW. Throughout the DT, the highest values of ΔHe (21%) were observed at depths of 400 – 500 m, corresponding to the layer between the incoming warm Circumpolar Deep Water and the overlying Winter Water. The high ΔHe (and ΔNe) area extended outside of the shelf break, suggesting that GMW is transported more than 300 km offshore. The ΔHe was substantially higher in front of the DIS than the GIS, and the highest ΔHe (31%) was observed in the western part of the DIS, where concentrated outflow from the shelf to the offshore was observed. In 2012, the calculated GMW fraction in seawater based on excess He and Ne, decreased by 30 – 40% compared with that in 2011 in both ice shelves, indicating strong temporal variability in glacial melting. This article is protected by copyright. All rights reserved.
      PubDate: 2016-02-06T11:18:34.362884-05:
      DOI: 10.1002/2015JC011211
       
  • Drivers of spring and summer variability in the coastal ocean offshore of
           Cape Cod, MA
    • Authors: Anthony R. Kirincich; Glen G. Gawarkiewicz
      Abstract: The drivers of spring and summer variability within the coastal ocean east of Cape Cod, Massachusetts, a critical link between the Gulf of Maine and Mid‐Atlantic Bight, are investigated using two years of shipboard and moored hydrographic and velocity observations from 2010 and 2011. The observations reveal sharp differences in the spring transition and along‐shelf circulation due to variable freshwater and meteorological forcing, along with along‐shelf pressure gradients. The role of the along‐shelf pressure gradient is inferred using in situ observations of turbulent momentum flux, or Reynolds stresses, estimated from the ADCP‐based velocities using recently developed methods and an inversion of the along‐shelf momentum balance. During spring, the locally relevant along‐shelf pressure gradient contains a sizable component that is not coupled to the along‐shelf winds and often opposes the regional sea level gradient. Together with the winds, local pressure gradients dominate along‐shelf transport variability during spring, while density‐driven geostrophic flows appear to match the contribution of the local winds during summer. These results suggest that local effects along the Outer Cape have the potential to cause significant changes in exchange between the basins. This article is protected by copyright. All rights reserved.
      PubDate: 2016-02-02T17:43:06.949544-05:
      DOI: 10.1002/2015JC011252
       
  • Nearshore sandbar rotation at single‐barred embayed beaches
    • Authors: B. Blossier; K.R. Bryan, C.J. Daly, C. Winter
      Abstract: The location of a shore‐parallel nearshore sandbar derived from seven years of video imagery data at the single‐barred embayed Tairua Beach (NZ) is investigated to assess the contribution of barline rotation to the overall morphodynamics of sandbars in embayed environments and to characterise the process of rotation in relation to external conditions. Rotation induces cross‐shore barline variations at the embayment extremities on the order of magnitude of those induced by alongshore uniform cross‐shore migration of the bar. Two semi‐empirical models have been developed to relate the barline cross‐shore migration and rotation to external wave forcing conditions. The rotation model is directly derived from the cross‐shore migration model. Therefore its formulation advocates for a primary role of cross‐shore processes in the rotation of sandbars at embayed beaches. The orientation evolves towards an equilibrium angle directly related to the alongshore wave energy gradient due to two different mechanisms. Either the bar extremities migrate in opposite directions with no overall cross‐shore bar migration (pivotal rotation) or the rotation relates to an overall migration of the barline which is not uniform along the beach (migration‐driven rotation). Migration and rotation characteristic response times are similar, ranging from 10 to 30 days for mild and energetic wave conditions and above 200 days during very calm conditions or when the bar is located far offshore. This article is protected by copyright. All rights reserved.
      PubDate: 2016-01-18T10:31:47.259714-05:
      DOI: 10.1002/2015JC011031
       
  • Issue Information
    • Pages: 1527 - 1528
      PubDate: 2016-04-26T02:49:19.394785-05:
      DOI: 10.1002/jgrc.21403
       
  • Estimating decadal variability in sea level from tide gauge records: An
           application to the North Sea
    • Authors: Thomas Frederikse; Riccardo Riva, Cornelis Slobbe, Taco Broerse, Martin Verlaan
      Pages: 1529 - 1545
      Abstract: One of the primary observational data sets of sea level is represented by the tide gauge record. We propose a new method to estimate variability on decadal time scales from tide gauge data by using a state space formulation, which couples the direct observations to a predefined state space model by using a Kalman filter. The model consists of a time‐varying trend and seasonal cycle, and variability induced by several physical processes, such as wind, atmospheric pressure changes and teleconnection patterns. This model has two advantages over the classical least‐squares method that uses regression to explain variations due to known processes: a seasonal cycle with time‐varying phase and amplitude can be estimated, and the trend is allowed to vary over time. This time‐varying trend consists of a secular trend and low‐frequency variability that is not explained by any other term in the model. As a test case, we have used tide gauge data from stations around the North Sea over the period 1980–2013. We compare a model that only estimates a trend with two models that also remove intra‐annual variability: one by means of time series of wind stress and sea level pressure, and one by using a two‐dimensional hydrodynamic model. The last two models explain a large part of the variability, which significantly improves the accuracy of the estimated time‐varying trend. The best results are obtained with the hydrodynamic model. We find a consistent low‐frequency sea level signal in the North Sea, which can be linked to a steric signal over the northeastern part of the Atlantic.
      PubDate: 2016-03-04T05:00:26.978848-05:
      DOI: 10.1002/2015JC011174
       
  • A study of radar backscattering from water surface in response to rainfall
    • Pages: 1546 - 1562
      Abstract: In this paper, radar backscattering from a water surface in response to rainfall was studied. The paper consists of two parts. First, the spatial characteristics of raindrops in rain fields were analyzed based on published data and the response of a water surface to rainfall was experimentally studied in the laboratory. Rain‐generated surface features including stalks, crowns, ring waves, and secondary drops were measured. It was found that stalks and crowns are dominant in terms of their height and energy. Second, the radar signatures of a rainfall event simultaneously observed by C band ENVISAT (European satellite), ASAR (Advanced Synthetic Aperture Radar), and ground‐based weather radar in the Northwest Pacific were investigated. The relationship between the radar return intensity extracted from the C band ASAR image and the reflectivity factor (rain rate) obtained from ground‐based weather radar was analyzed. For light/moderate rain (with low reflectivity factors), the radar backscattering intensity increases as the reflectivity factor increases. For heavy rain (with high reflectivity factors), the radar backscattering intensity decreases as the reflectivity factor increases. The maximum radar backscattering intensity occurs at a reflectivity factor of 45 dBZ (with rain rate of 24 mm/h). It was found that the spaceborne radar backscattering intensity strongly correlates with the average distance between the stalks on the water surface in the rain field in a nonlinear manner. The physics of the radar signatures of the rain event are explored.
      PubDate: 2016-03-04T05:03:59.530814-05:
      DOI: 10.1002/2015JC010975
       
  • Mapping surface tidal currents and Changjiang plume in the East China Sea
           from Geostationary Ocean Color Imager
    • Pages: 1563 - 1572
      Abstract: The spatial pattern of the semidiurnal M2 tidal currents in the East China Sea (ECS) is mapped from the Geostationary Ocean Color Imager (GOCI), taking advantage of the satellite's unique 8 hourly local daytime sequential images. The GOCI‐derived surface M2 tidal currents are validated with a comprehensive set of twenty‐eight surface drifters and four mooring observations. The agreement is outstanding with the error variance less than 10% of the total variance. The gridded GOCI‐derived tidal currents are also in good agreement with the Oregon State University (OSU) high‐resolution regional tidal model of the China Seas. The detided mean flow shows a strong Changjiang plume extending hundreds of kilometers offshore, in agreement with the concurrent satellite sea surface temperature (SST) and sea surface salinity (SSS) distributions. The observed surface currents are compared with the daily mean flows derived from the Japan Coastal Ocean Predictability Experiment (JCOPE2). The model results are consistent with the observations, showing the sensitivity of Changjiang plume to wind forcing. The study clearly demonstrates the utility of geostationary satellite in mapping the surface currents over a wide (∼400 km), tidally dominated continental shelf.
      PubDate: 2016-03-04T05:01:23.500009-05:
      DOI: 10.1002/2015JC011469
       
  • Evolution of bubble size distribution from gas blowout in shallow water
    • Authors: Lin Zhao; Michel C. Boufadel, Kenneth Lee, Thomas King, Norman Loney, Xiaolong Geng
      Pages: 1573 - 1599
      Abstract: Gas is often emanated from the sea bed during a subsea oil and gas blowout. The size of a gas bubble changes due to gas dissolution in the ambient water and expansion as a result of a decrease in water pressure during the rise. It is important to understand the fate and transport of gas bubbles for the purpose of environmental and safety concerns. In this paper, we used the numerical model, VDROP‐J to simulate gas formation in jet/plume upon release, and dissolution and expansion while bubble rising during a relatively shallow subsea gas blowout. The model predictions were an excellent match to the experimental data. Then a gas dissolution and expansion module was included in the VDROP‐J model to predict the fate and transport of methane bubbles rising due to a blowout through a 0.10 m vertical orifice. The numerical results indicated that gas bubbles would increase the mixing energy in released jets, especially at small distances and large distances from the orifice. This means that models that predict the bubble size distribution (BSD) should account for this additional mixing energy. It was also found that only bubbles of certain sizes would reach the water surfaces; small bubbles dissolve fast in the water column, while the size of the large bubbles decreases. This resulted in a BSD that was bimodal near the orifice, and then became unimodal.
      PubDate: 2016-03-06T07:51:14.472881-05:
      DOI: 10.1002/2015JC011403
       
  • Locally driven interannual variability of near‐surface pH and
           ΩA in the Strait of Georgia
    • Pages: 1600 - 1625
      Abstract: Declines in mean ocean pH and aragonite saturation state (ΩA) driven by anthropogenic CO2 emissions have raised concerns regarding the trends of pH and ΩA in estuaries. Low pH and ΩA can be harmful to a variety of marine organisms, especially those with calcium carbonate shells, and so may threaten the productive ecosystems and commercial fisheries found in many estuarine environments. The Strait of Georgia is a large, temperate, productive estuarine system with numerous wild and aquaculture shellfish and finfish populations. We determine the seasonality and variability of near‐surface pH and ΩA in the Strait using a one‐dimensional, biophysical, mixing layer model. We further evaluate the sensitivity of these quantities to local wind, freshwater, and cloud forcing by running the model over a wide range of scenarios using 12 years of observations. Near‐surface pH and ΩA demonstrate strong seasonal cycles characterized by low pH, aragonite‐undersaturated waters in winter and high pH, aragonite‐supersaturated waters in summer. The aragonite saturation horizon generally lies at ∼20 m depth except in winter and during strong Fraser River freshets when it shoals to the surface. Periods of strong interannual variability in pH and aragonite saturation horizon depth arise in spring and summer. We determine that at different times of year, each of wind speed, freshwater flux, and cloud fraction are the dominant drivers of this variability. These results establish the mechanisms behind the emerging observations of highly variable near‐surface carbonate chemistry in the Strait.
      PubDate: 2016-03-06T06:27:38.880906-05:
      DOI: 10.1002/2015JC011118
       
  • Sources, fate, and pathways of Leeuwin Current water in the Indian Ocean
           and Great Australian Bight: A Lagrangian study in an eddy‐resolving
           ocean model
    • Authors: Christopher Yit Sen Bull; Erik van Sebille
      Pages: 1626 - 1639
      Abstract: The Leeuwin Current is the dominant circulation feature in the eastern Indian Ocean, transporting tropical and subtropical water southward. While it is known that the Leeuwin Current draws its water from a multitude of sources, existing Indian Ocean circulation schematics have never quantified the fluxes of tropical and subtropical source water flowing into the Leeuwin Current. This paper uses virtual Lagrangian particles to quantify the transport of these sources along the Leeuwin Current's mean pathway. Here the pathways and exchange of Leeuwin Current source waters across six coastally bound sectors on the south‐west Australian coast are analyzed. This constitutes the first quantitative assessment of Leeuwin Current pathways within an offline, 50 year integration time, eddy‐resolving global ocean model simulation. Along the Leeuwin Current's pathway, we find a mean poleward transport of 3.7 Sv in which the tropical sources account for 60–78% of the transport. While the net transport is small, we see large transports flowing in and out of all the offshore boundaries of the Leeuwin Current sectors. Along the Leeuwin Current's pathway, we find that water from the Indonesian Throughflow contributes 50–66% of the seasonal signal. By applying conditions on the routes particles take entering the Leeuwin Current, we find particles are more likely to travel offshore north of 30°S, while south of 30°S, particles are more likely to continue downstream. We find a 0.2 Sv pathway of water from the Leeuwin Current's source regions, flowing through the entire Leeuwin Current pathway into the Great Australian Bight.
      PubDate: 2016-03-06T06:27:57.117342-05:
      DOI: 10.1002/2015JC011486
       
  • Wind‐wave‐induced velocity in ATI SAR ocean surface currents:
           First experimental evidence from an airborne campaign
    • Authors: Adrien C. H. Martin; Christine Gommenginger, Jose Marquez, Sam Doody, Victor Navarro, Christopher Buck
      Pages: 1640 - 1653
      Abstract: Conventional and along‐track interferometric (ATI) Synthetic Aperture Radar (SAR) senses the motion of the ocean surface by measuring the Doppler shift of reflected signals. Measurements are affected by a Wind‐wave‐induced Artifact Surface Velocity (WASV) which was modeled theoretically in past studies and has been estimated empirically only once before with Envisat ASAR by Mouche et al. (2012). An airborne campaign in the tidally dominated Irish Sea served to evaluate this effect and the current retrieval capabilities of a dual‐beam SAR interferometer known as Wavemill. A comprehensive collection of Wavemill airborne data acquired in a star pattern over a well‐instrumented validation site made it possible for the first time to estimate the magnitude of the WASV, and its dependence on azimuth and incidence angle from data alone. In light wind (5.5 m/s) and moderate current (0.7 m/s) conditions, the wind‐wave‐induced contribution to the measured ocean surface motion reaches up to 1.6 m/s upwind, with a well‐defined second‐order harmonic dependence on direction to the wind. The magnitude of the WASV is found to be larger at lower incidence angles. The airborne WASV results show excellent consistency with the empirical WASV estimated from Envisat ASAR. These results confirm that SAR and ATI surface velocity estimates are strongly affected by WASV and that the WASV can be well characterized with knowledge of the wind knowledge and of the geometry. These airborne results provide the first independent validation of Mouche et al. (2012) and confirm that the empirical model they propose provides the means to correct airborne and spaceborne SAR and ATI SAR data for WASV to obtain accurate ocean surface current measurements. After removing the WASV, the airborne Wavemill‐retrieved currents show very good agreement against ADCP measurements with a root‐mean‐square error (RMSE) typically around 0.1 m/s in velocity and 10° in direction.
      PubDate: 2016-03-07T07:58:52.523299-05:
      DOI: 10.1002/2015JC011459
       
  • Comment on “Influence of sea level rise on the dynamics of salt
           inflows in the Baltic Sea” by R. Hordoir, L. Axell, U. Löptien,
           H. Dietze, and I. Kuznetsov
    • Authors: Lars Arneborg
      Pages: 2035 - 2040
      PubDate: 2016-03-09T12:39:30.308169-05:
      DOI: 10.1002/2015JC011451
       
 
 
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