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Publisher: American Geophysical Union (AGU)   (Total: 17 journals)

Geochemistry, Geophysics, Geosystems     Full-text available via subscription   (Followers: 25, SJR: 2.56, h-index: 69)
Geophysical Research Letters     Full-text available via subscription   (Followers: 53, SJR: 3.493, h-index: 157)
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J. of Geophysical Research : Oceans     Partially Free   (Followers: 15)
J. of Geophysical Research : Planets     Full-text available via subscription   (Followers: 13)
J. of Geophysical Research : Solid Earth     Full-text available via subscription   (Followers: 26)
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Paleoceanography     Full-text available via subscription   (Followers: 4, SJR: 3.22, h-index: 88)
Radio Science     Full-text available via subscription   (Followers: 3, SJR: 0.959, h-index: 51)
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Water Resources Research     Full-text available via subscription   (Followers: 207, SJR: 2.189, h-index: 121)
Journal Cover   Journal of Geophysical Research : Oceans
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   ISSN (Online) 2169-9291
   Published by American Geophysical Union (AGU) Homepage  [17 journals]
  • Vertical convergence of resuspended sediment and subducted phytoplankton
           to a persistent detached layer over the southern shelf of Monterey Bay,
    • Authors: Jeff C. Sevadjian; Erika E. McPhee‐Shaw, Ben Y. Raanan, Olivia M. Cheriton, Curt D. Storlazzi
      Abstract: Work done by the present authors and collaborators in the first year of a two‐year field study established the frequent presence of layers of suspended particulate matter detached from the sea floor over the southern shelf of Monterey Bay, California. In this contribution, we document similar findings over a one‐month period in Fall 2012, and investigate physical processes leading to vertical convergence of particles to the observed layers, both from re‐suspended sediment originating below the layer depth, and from phytoplankton originating above the layer depth. Physical and optical vertical structure was measured by an autonomous vertical profiler, thermistor chain, and acoustic Doppler current profiler, and optical sensors fixed to a bottom‐mounted frame measured beam attenuation and particle size distribution within the benthic boundary layer (BBL). These data support a conceptual model for layer formation in which (1) bottom material was mobilized into the BBL by semidiurnal internal tidal currents; (2) brief ‘updraft' events regularly injected particles into the interior water column during the down‐slope phase of the semidiurnal internal tide; and (3) particles converged at the detached layer due to a measured, but previously unreported, long‐time scale mean vertical convergence in flow. Subduction and vertical mixing of phytoplankton to the sub‐euphotic layer were also observed and are presented in two case studies in connection with horizontal convergences of surface water masses. This article is protected by copyright. All rights reserved.
      PubDate: 2015-04-21T00:39:34.756409-05:
      DOI: 10.1002/2015JC010785
  • Temporal and spatial variability of biological nitrogen fixation off the
           upwelling system of central Chile (35°S–38.5°S)
    • Authors: Camila Fernandez; Maria Lorena Gonzalez, Claudia Muñoz, Veronica Molina, Laura Farias
      Abstract: Although N2 fixation could represent a supplementary source of bioavailable nitrogen in coastal upwelling areas and underlying oxygen minimum zones (OMZs), the limited data available prevent assessing its variability and biogeochemical significance. Here we report the most extensive N2 fixation data set gathered to date in the upwelling area off central Chile (36ºS). It covers interannual to high frequency time scales in an area of about 82500 km2 in the Eastern South Pacific (ESP). Because heterotrophic N2 fixation may be regulated by DOM availability in the ESP, we conducted experiments at different oxygen conditions and included DOM amendments in order to test diazotrophic activity. Rates in the euphotic zone showed intense temporal variability which resulted in values reaching 0.5 nmol L−1 d−1 in 2006 (average 0.32 ± 0.17 nmol L−1 d−1) and up to 126.8 nmol L−1 d−1 (average 24.75 ± 37.9 nmol L−1 d−1) in 2011. N2 fixation in subsurface suboxic conditions (1.5 ± 1.16 nmol L−1 d−1) also occurred mainly during late summer and autumn while virtually absent in winter. The diversity of diazotrophs was dominated by heterotrophs, with higher richness in surface compared to OMZ waters. Rates in oxygen depleted conditions could exceed values obtained in the euphotic layer, but rates were not dependent on the availability of dissolved organic matter.N2 fixation also showed a positive correlation with total chlorophyll and the C:N ratio of phytoplankton, but not to the P excess compared to N. We conclude that the diazotrophic community responds to the composition of phytoplankton rather than the extent of N deficiency and the availability of bulk DOM in this system. This article is protected by copyright. All rights reserved.
      PubDate: 2015-04-17T16:17:05.445646-05:
      DOI: 10.1002/2014JC010410
  • Glider observations of the North Equatorial Current in the western
           tropical Pacific
    • Authors: Martha C. Schönau; Daniel L. Rudnick
      Abstract: The North Equatorial Current (NEC) of the Pacific Ocean advects subtropical, subpolar and tropical water masses. Repeat underwater glider observations of the NEC from June 2009 to January 2014 along 134.3°E provide absolute zonal geostrophic velocity, transport, and water mass structure at length scales of 10 km to 1000 km. The NEC is strongest near the surface and persistent eastward undercurrents are identified deeper than potential density surface 26 kg m−3 at 9.6°N and 13.1°N. Mean transport from the surface to 27.3 kg m−3 and 8.5°N to 16.5°N is 37.6 Sv (106 m3 s−1), with a standard deviation of 15.6 Sv. The transport variability is greatest deeper than 26 kg m−3 due to undercurrent variability. Wavelet analysis at scales of 10 km to 80 km reveals extrema of fine‐scale salinity variance along isopycnals (spice variance). High spice variance is found in the North Pacific Tropical Water (NPTW) and the North Pacific Intermediate Water (NPIW), with a spice variance minimum between water masses at 25.5 kg m−3. A horizontal Cox number, CH, relates salinity variance at fine‐scales (10 km to 80 km) to that at greater length scales (120 km to 200 km). As a function of density, CH is nearly vertically uniform, indicating that the stirring of mean salinity gradients enhances fine‐scale salinity variance. NPTW, with an estimated horizontal eddy diffusivity of order 104 m s−2, is a useful tracer for the region and may be used to relate the fine‐scale salinity variance to an eddy diffusivity. This article is protected by copyright. All rights reserved.
      PubDate: 2015-04-17T00:59:05.401393-05:
      DOI: 10.1002/2014JC010595
  • Activities of 50–80 day subthermocline eddies near the Philippine
    • Authors: Tzu‐Ling Chiang; Chau‐Ron Wu, Tangdong Qu, Yi‐Chia Hsin
      Abstract: Analyses of outputs from an eddy‐resolving ocean general circulation model show that there are at least two groups of subthermocline eddies near the Philippine coast: one originates from the southeast, and the other from the east. The dominant period and principal depth of the former (latter) group of eddies are about 55 days (67 days) and 600 m (350 m), respectively. The propagation speed (∼0.12 m s−1) and diameter (∼3°) of the two groups of eddies are similar. We suggest that the westward propagating eddies are generated by interactions between meridional movement of the westward‐flowing North Equatorial Current, the eastward‐flowing North Equatorial Undercurrent, and their interactions with topography. Besides, the analysis indicates that, in comparison with the northwestward propagating subthermocline eddies, the westward propagating ones play a more important role in modulating the subsurface circulation near the Philippine coast. This article is protected by copyright. All rights reserved.
      PubDate: 2015-04-17T00:40:44.571493-05:
      DOI: 10.1002/2013JC009626
  • Issue Information
    • PubDate: 2015-04-16T01:28:01.236367-05:
      DOI: 10.1002/jgrc.20859
  • Experimental investigation of sediment resuspension beneath internal
           solitary waves of depression
    • Authors: Payam Aghsaee; Leon Boegman
      Abstract: Internal solitary waves (ISWs) of depression are common features of coastal environments and believed to re‐suspend sediments where they shoal. In this study, the sediment re‐supension process associated with ISWs propagating over a flat bed was investigated in the laboratory. The first‐ever profile measurements of the three‐dimensional instantaneous velocity field beneath the ISWs revealed that re‐suspension occurs during burst like vertical velocity events, which lift sediments into the watercolumn, in the adverse pressure gradient region beneath the trailing part of the wave. Re‐suspension was not observed when the wave‐induced viscous bed‐stress was maximal directly beneath the ISW trough. Prediction of wave‐induced re‐suspension was, therefore, unsuccessful using a traditional viscous bed‐stress based Shields diagram. A parameterization for IWS‐induced re‐suspension is proposed as a function of the maximum instantaneous vertical velocity in the bursts wmax. Here, we have replaced the viscous bed stress with τISW = ρ2 wmax2, where τISW is the instantaneous re‐suspending bed‐stress and ρ2 is the near‐bed fluid density. From these results, it is possible for field‐oceanographers to predict the occurrence of ISW‐induced re‐suspension from the bulk wave and stratifications characteristics in a two‐layer stratification. Further research is required to extend the parameterization to larger Reynolds numbers at field‐scale. This article is protected by copyright. All rights reserved.
      PubDate: 2015-04-15T10:42:15.191375-05:
      DOI: 10.1002/2014JC010401
  • Ka‐band backscattering from water surface at small incidence: A
           wind‐wave tank study
    • Authors: Olivier Boisot; Sébastien Pioch, Christophe Fatras, Guillemette Caulliez, Alexandra Bringer, Pierre Borderies, Jean‐Claude Lalaurie, Charles‐Antoine Guérin
      Abstract: We report on an experiment conducted at the large Pytheas wind‐wave facility in Marseille to characterize the Ka‐band radar return from water surfaces when observed at small incidence. Simultaneous measurements of capillary‐gravity to gravity wave height and slopes and Normalized Radar Cross Section (NRCS) were carried out for various wind speeds and scattering angles. From this data set we construct an empirical two‐dimensional wave number spectrum accounting for the surface current to describe water surface motions from decimeter to millimeter scales. Some consistency tests are proposed to validate the surface wave spectrum, which is then incorporated into simple analytical scattering models. The resulting directional NRCS is found in overall good agreement with the experimental values. Comparisons are performed with oceanic models as well as in situ measurements over different types of natural surfaces. The applicability of the present findings to oceanic as well as continental surfaces is discussed. This article is protected by copyright. All rights reserved.
      PubDate: 2015-04-15T10:42:04.765584-05:
      DOI: 10.1002/2014JC010338
  • River‐tide dynamics: Exploration of non‐stationary and
           nonlinear tidal behavior in the Yangtze River estuary
    • Authors: Leicheng Guo; Mick van der Wegen, David A. Jay, Pascal Matte, Zheng Bing Wang, Dano J.A. Roelvink, Qing He
      Abstract: River‐tide dynamics remain poorly understood, in part because of conventional harmonic analysis (HA) does not cope effectively with non‐stationary signals. To explore non‐stationary behavior of river tides and the modulation effects of river discharge, this work analyzes tidal signals in the Yangtze River estuary using both HA in a non‐stationary mode and continuous wavelet transforms (CWT). The Yangtze is an excellent natural laboratory in which to analyze river tides, because of its high and variable flow, its length, and the fact that there are do dams or reflecting barriers within the tidal part of the system. Analysis of tidal frequencies by CWT and analysis of subtidal water level and tidal ranges reveal a broad range of subtidal variations over fortnightly, monthly, semi‐annual, and annual frequencies, driven by subtidal variations in friction and by variable river discharges. We employ HA in a non‐stationary mode (NSHA) by segregating data within defined flow ranges into separate analyses. NSHA quantifies the decay of the principal tides and the modulation of M4 tide with increasing river discharges. M4 amplitudes decrease far upriver (landward portion of the estuary) and conversely increase close to the ocean as river discharge increases. The fortnightly frequencies reach an amplitude maximum upriver of that for over tide frequencies, due to the longer wavelength of the fortnightly constituents. These methods and findings should be applicable to large tidal rivers globally, and have broad implications regarding management of navigation channels and ecosystems in tidal rivers. This article is protected by copyright. All rights reserved.
      PubDate: 2015-04-15T10:41:51.908837-05:
      DOI: 10.1002/2014JC010491
  • Waving in the rain
    • Authors: Cavaleri Luigi; Bertotti Luciana, Bidlot Jean‐Raymond
      Abstract: We consider the effect of rain on wind wave generation and dissipation. Rain falling on a wavy surface may have a marked tendency to dampen the shorter waves in the tail of the spectrum, the related range increasing with the rain rate. Historical and sailors' reports suggest this leads to calmer wave conditions, certainly so for the action of breakers. We have explored this situation using a fully coupled meteorological‐wave model system, adding an artificial rain rate dependent damping of the tail. Contrarily to direct marine experience, the experimental results show higher wind speeds and wave heights. A solid indication of the truth is achieved with the direct comparison between operational model (where rain effect is ignored) and measured data. These strongly support the sailors' claims of less severe wave conditions under heavy rain. This leads to a keen analysis of the overall process, in particular on the role of the tail of the spectrum in modulating the wind input and the white‐capping, and how this is presently modeled in operational activity. We suggest that some revision is due and that the relationship between white‐capping and generation by wind is deeper and more implicative than presently generally assumed. This article is protected by copyright. All rights reserved.
      PubDate: 2015-04-15T03:21:42.43977-05:0
      DOI: 10.1002/2014JC010348
  • Beam attenuation, scattering, and backscattering of marine particles in
           relation to particle size distribution and composition in Hudson Bay
    • Authors: Hongyan Xi; Pierre Larouche, Christine Michel, Shilin Tang
      Abstract: This study investigated the relationships between the concentration of biogeochemical parameters and particulate beam attenuation (cp), scattering (bp), and backscattering (bbp) in Hudson Bay. Results showed that most of the variability resulted from the presence of a deep chlorophyll maximum. cp, bp, and bbp were all adequate proxies to estimate total suspended matter (TSM) but were mostly sensitive to particulate inorganic matter (PIM) in the surface layer, and particulate organic matter (POM) at the chlorophyll maximum depth. The backscattering ratio varied in the range of [0.005 – 0.05] and was inversely related to the POM: TSM ratio. According to the Twardowski et al. [2001] model, the PSD slope ξ well represented and bulk refractive index in relation to particulate composition. For inorganic particulate dominated waters, both and had a larger range and a higher mean value than at organic particulate dominated waters. This knowledge on the optical properties related to the PSD and particulate composition provides valuable information for further investigation and broadens our understanding of ocean optics in high latitude waters leading to potential improvements of regional scale remote sensing algorithms. This article is protected by copyright. All rights reserved.
      PubDate: 2015-04-15T02:59:03.416315-05:
      DOI: 10.1002/2014JC010668
  • Transient River Flow into a Fjord and its Control of Plume Energy
    • Authors: J.M. O'Callaghan; C.L. Stevens
      Abstract: The influence of variable inflows on near‐field plume dynamics and energy partitioning was examined using observations of a controlled flow into Doubtful Sound, New Zealand. The high temporal changes in flows passing through the Manapouri hydroelectric power station mimic the magnitude and variability seen in small mountainous river systems (SMRS) globally. The variable flow, coupled with strong vertical density gradients akin to ambient conditions in coastal systems enabled plume behaviour to be characterized for differing flow, wind and tidal inputs in a quasi‐idealised ʻlaboratoryʼ system. Comparisons of the frequency distributions of energy for different forcing conditions showed that baroclinic and barotropic processes were closely intertwined for transient forcing. The periodicity of density due to tidal oscillations was initially absent; headwaters of the fjord absorbed the momentum when inflows were substantially increased from the mean of ∼420 m3s−1. From the buoyancy frequency squared N2 six events were identified when N2 was greater than 0.07 s−2. Seven occurrences of supercritical flow (Froude number, Fri˃1) and associated transitions to subcritical flow were observed over the duration of mooring deployment. Transient inflows induced internal hydraulic jumps in the near‐field region which lead to a rapid breakdown of vertical stratification. The horizontal length scale of an internal hydraulic jump is O(1 km). Not all transitions from Fri>1 to
      PubDate: 2015-04-15T02:10:34.580434-05:
      DOI: 10.1002/2015JC010721
  • Altimeter‐derived seasonal circulation on the Southwest Atlantic
           shelf: 27° – 43°S
    • Authors: P. Ted Strub; Corinne James, Vincent Combes, Ricardo Matano, Alberto Piola, Elbio Palma, Martin Saraceno, Raul Guerrero, Harold Fenco, Laura Ruiz Etcheverry
      Abstract: Altimeter sea surface height (SSH) fields are analyzed to define and discuss the seasonal circulation over the wide continental shelf in the SW Atlantic Ocean (27°‐43°S) during 2001‐2012. Seasonal variability is low south of the Rio de la Plata (RdlP), where winds and currents remain equatorward for most of the year. Winds and currents in the central and northern parts of our domain are also equatorward during autumn and winter but reverse to become poleward during spring and summer. Transports of shelf water to the deep ocean are strongest during summer offshore and to the southeast of the RdlP. Details of the flow are discussed using mean monthly seasonal cycles of winds, heights and currents, along with analyses of Empirical Orthogonal Functions. Principle Estimator Patterns bring out the patterns of wind forcing and ocean response. The largest part of the seasonal variability in SSH signals is due to changes in the wind forcing (described above) and changes in the strong boundary currents that flow along the eastern boundary of the shelf. The rest of the variability contains a smaller component due to heating and expansion of the water column, concentrated in the southern part of the region next to the coast. Our results compare well to previous studies using in situ data and to results from realistic numerical models of the regional circulation. This article is protected by copyright. All rights reserved.
      PubDate: 2015-04-14T14:11:07.458281-05:
      DOI: 10.1002/2015JC010769
  • Introduction to the Special Section on Western Pacific Ocean Circulation
           and Climate
    • Authors: Dunxin Hu; Wenju Cai, Alexandre Ganachaud, William Kessler, Janet Sprintall
      PubDate: 2015-04-10T02:34:34.314272-05:
      DOI: 10.1002/2015JC010856
  • In situ validation of sea surface temperatures from the GCOM‐W1
           AMSR2 RSS calibrated brightness temperatures
    • Authors: Chelle L. Gentemann; Kyle A. Hilburn
      Abstract: Remote Sensing Systems AMSR2 v7.2 data from 25 July 2012 – 9 October 2014 are collocated with in situ sea surface temperature (SST) data. The RSS SST algorithm uses AMSR2 brightness temperatures calibrated using a methodology developed at RSS rather than using the standard JAXA AMSR2 product, which includes the JAXA calibration. The new RFI exclusion methodology used for the AMSR2 v7.2 data is described. Buoy data are quality controlled using an internal quality indicator. Daytime collocations with wind speeds of less than 6 ms−1 are excluded to avoid diurnal contamination of the results. A mean bias (AMSR2 minus in situ) of ‐0.04 K and standard deviation 0.55 K with 109350 collocations is found. The geographical distribution of biases is investigated, with a small increase in biases found at higher latitudes. At lower SST the uncertainty increases and the bias. The dependencies of the bias and uncertainties on other geophysical variables are shown to be negligible. The time series of the bias and uncertainty show little variability, but a small seasonal dependence is determined to be related to a seasonal shift in wind speeds. Overall, the AMSR2 SSTs are of comparable quality to the AMSR‐E SSTs and continue the climate microwave SST record that started in 1997. This article is protected by copyright. All rights reserved.
      PubDate: 2015-04-10T01:36:45.44116-05:0
      DOI: 10.1002/2014JC010574
  • Drivers of decadal variability in the Tasman Sea
    • Authors: Bernadette M. Sloyan; Terence J. O'Kane
      Abstract: In this study we compare optimally interpolated monthly time‐series Tasman Sea XBT data and a comprehensive set of ocean data assimilation models forced by atmospheric reanalysis to investigate the stability of the Tasman Sea thermocline and the transport variability of the East Australian Current (EAC), the Tasman Front and EAC‐extension. We find that anomalously weaker EAC transport at 25S corresponds to an anomalously weaker Tasman Front and anomalously stronger EAC‐extension. We further show that, post about 1980 and relative to the previous 30 years, the anomalously weaker EAC transport at 25S is associated with large‐scale changes in the Tasman Sea; specifically stronger stratification above the thermocline, larger thermocline temperature gradients and enhanced energy conversion. Significant correlations are found between the Maria Island station Sea Surface Temperature (SST) variability and stratification, thermocline temperature gradient and baroclinic energy conversion suggesting that non‐linear dynamical responses to variability in the basin‐scale wind stress curl are important drivers of decadal varibility in the Tasman Sea. We further show that the stability of the EAC is linked, via the South Caledonian Jet, to the stability of the pan‐basin subtropical South Pacific Ocean ”storm track”. This article is protected by copyright. All rights reserved.
      PubDate: 2015-04-10T01:35:42.392109-05:
      DOI: 10.1002/2014JC010550
  • Surface roughness and breaking wave properties retrieved from polarimetric
           microwave radar backscattering
    • Authors: Paul A. Hwang; Franco Fois
      Abstract: Ocean surface roughness and wave breaking are the two main contributors of radar backscattering from the ocean surface. The relative weightings of the two contributions vary with the microwave polarization: the VV (vertical transmit vertical receive) is dominated by the Bragg resonance scattering mechanism, the HH (horizontal transmit horizontal receive) and VH (horizontal transmit vertical receive or vertical transmit horizontal receive) contain nontrivial non‐Bragg contributions mainly produced by breaking features. A method is developed to obtain the short scale properties of ocean surface roughness and wave breaking from Ku, C and L band polarimetric sea returns. The results are used for quantitative evaluation of the ocean surface roughness spectral models and for deriving understanding of the breaking contribution important to microwave ocean remote sensing, in particular its dependence on wind speed, microwave frequency and incidence angle. Implications of the results to air‐sea interaction applications are discussed. This article is protected by copyright. All rights reserved.
      PubDate: 2015-04-10T01:22:31.479545-05:
      DOI: 10.1002/2015JC010782
  • Estimation of the time series of the meridional heat transport across
           15°N in the Pacific Ocean from Argo and satellite data
    • Authors: Tingting Yang; Yongsheng Xu
      Abstract: The time series of the net meridional heat transport (MHT) at 15°N in the Pacific Ocean from 2003 to 2012 is estimated, by combining the Argo profiles with the satellite altimeter and scatterometer data. The estimate is validated against the climatological ocean data and the ECCO2 products, and is demonstrated to be reasonable. The MHT has a high degree of variability with a temporal mean of 0.70±0.31 PW which is concentrated in the upper 800 dbar. The time series of the MHT and Ekman temperature transport have a significant annual cycle which peaks near April and December, whereas the time series of the geostrophic temperature transport have a sub‐annual cycle. The results are consistent with previous estimates. The difference between the Argo/SSH estimate and ECCO2 estimate is further discussed, and the major difference is caused by the smoothing in the hydrographic data. The correlation between the air‐sea flux and MHT is 0.50 with a 3‐month delay. This report describes the first such attempt at a continuous transport of heat at 15°N in the Pacific Ocean from in situ observations. This article is protected by copyright. All rights reserved.
      PubDate: 2015-04-10T01:22:18.71921-05:0
      DOI: 10.1002/2015JC010752
  • Circulation of modified circumpolar deep water and basal melt beneath the
           Amery Ice Shelf, East Antarctica
    • Authors: Laura Herraiz–Borreguero; Richard Coleman, Ian Allison, Stephen R. Rintoul, Mike Craven, Guy Williams
      Abstract: Antarctic ice sheet mass loss has been linked to an increase in oceanic heat supply, which enhances basal melt and thinning of ice shelves. Here, we detail the interaction of modified Circumpolar Deep Water (mCDW) with the Amery Ice Shelf, the largest ice shelf in East Antarctica, and provide the first estimates of basal melting due to mCDW. We use sub–ice shelf ocean observations from a borehole site (AM02) situated ∼70 km inshore of the ice shelf front, together with open ocean observations in Prydz Bay. We find that mCDW transport into the cavity is about 0.22 ± 0.06 Sv (1Sv = 106 m3 s−1). The inflow of mCDW drives a net basal melt rate of up to 2 ± 0.5 m yr−1 during 2001 (23.9 ± 6.52 Gt yr−1 from under about 12,800 km2 of the north–eastern flank of the ice shelf). The heat content flux by mCDW at AM02 shows high intra–annual variability (up to 40%). Our results suggest two main modes of sub–ice shelf circulation and basal melt regimes: (1) the “ice pump”/high salinity shelf water circulation, on the western flank and (2) the mCDW meltwater–driven circulation in conjunction with the “ice pump”, on the eastern flank. These results highlight the sensitivity of the Amery's basal melting to changes in mCDW inflow. Improved understanding of such ice shelf–ocean interaction is crucial to refining projections of mass loss and associated sea‐level rise. This article is protected by copyright. All rights reserved.
      PubDate: 2015-04-08T10:22:58.490487-05:
      DOI: 10.1002/2015JC010697
  • Particle distributions and dynamics in the euphotic zone of the North
           Pacific Subtropical Gyre
    • Authors: Benedetto Barone; Robert R. Bidigare, Matthew J. Church, David M. Karl, Ricardo M. Letelier, Angelicque E. White
      Abstract: During the summer of 2012, we used laser diffractometry to investigate the temporal and vertical variability of the particle size spectrum (1.25‐100 µm in equivalent diameter) in the euphotic zone of the North Pacific Subtropical Gyre. Particles measured with this optical method (1.25‐100 µm) accounted for ∼40% of the particulate carbon stocks in the upper euphotic zone (25‐75 m), as estimated using an empirical formula to transform particle volume to carbon concentrations. Over the entire vertical layer considered (20‐180 m), the largest contribution to particle volume corresponded to particles between 3 and 10 µm in diameter. Although the exponent of a power‐law parameterization suggested that larger particles had a lower relative abundance than in other regions of the global ocean, this parameter, and hence conclusions about relative particle abundance, are sensitive to the shape of the size distribution and to the curve fitting method. Results on the vertical distribution of particles indicate that different size fractions varied independently with depth. Particles between 1.25 and 2 µm reached maximal abundances coincident with the depth of the chlorophyll a maximum (averaging 121 ± 10 m), where eukaryotic phytoplankton abundances increased. In contrast, particles between 2 and 20 µm tended to accumulate just below the base of the mixed layer (41 ± 14 m). Variability in particle size tracked changes in the abundance of specific photoautotrophic organisms (measured with flow cytometry and pigment concentration), suggesting that phytoplankton population dynamics are an important control of the spatiotemporal variability in particle concentration in this ecosystem. This article is protected by copyright. All rights reserved.
      PubDate: 2015-04-07T03:51:47.231564-05:
      DOI: 10.1002/2015JC010774
  • Variability in the origins and pathways of Pacific equatorial undercurrent
    • Authors: Xuerong Qin; Alex Sen Gupta, Erik van Sebille
      Abstract: The Pacific Equatorial Undercurrent (EUC) transports water, originating from a number of distinct source regions, eastward across the Pacific Ocean. It is responsible for supplying nutrients to the productive eastern Equatorial Pacific Ocean. Of particular importance is the transport of iron by the EUC; the limiting nutrient in that region. Although the mean circulation and sources of EUC water are reasonably well understood, it is unclear how the contribution of water from these sources to the EUC vary on seasonal to interannual timescales. Here, a Lagrangian analysis is applied to the eddy‐resolving OFAM3 ocean simulation, in order to identity variability in the make up of the EUC over an 18 year period (1993‐2010). While ENSO has an influence on the variability of source transport contributions to the EUC, the signal of increased (decreased) transport of water from the LLWBCs during El Niño (la Niña) periods does not translate to substantial changes in the make up of the EUC between 165oE and 140oW. It is hypothesized that this is due to the large spread in travel times of water parcels as they travel from the source regions into the EUC. The consequent erosion of transport anomalies generated at the different western boundary source regions associated with ENSO may help explain why previous studies found little relationship between variability in iron fluxes off Papua New Guinea shelves and the chlorophyll response in the eastern tropical Pacific. This article is protected by copyright. All rights reserved.
      PubDate: 2015-04-03T22:16:27.061884-05:
      DOI: 10.1002/2014JC010549
  • Internal tides and associated vertical mixing in the Indonesian
    • Authors: Taira Nagai; Toshiyuki Hibiya
      Abstract: Tidal mixing in the Indonesian Archipelago contributes to regulation of the tropical atmospheric circulation and water‐mass transformation in the Indonesian Throughflow. The present study quantifies the vertical diffusivity in the Indonesian Archipelago by driving a high resolution three‐dimensional numerical model and investigates the processes of internal tide generation, propagation and dissipation. The numerical experiment shows that M2 internal tides are effectively generated over prominent subsurface ridges. The conversion rate from M2 barotropic to baroclinic energy over the whole analyzed model domain is estimated to be 85.5 GW. The generated internal tides dissipate 50–100 % of their energy in close proximity to the generation sites (‘near‐field'), and the remaining baroclinic energy propagates away causing relatively large energy dissipation far from the generation sites (‘far‐field'). The local dissipation efficiency q, therefore, has an extremely non‐uniform spatial distribution, although it has been assumed to be constant in the existing tidal mixing parameterization for the Indonesian Archipelago. Compared with the model predicted values, the existing parameterization yields the same order of vertical diffusivity averaged within the Indonesian Archipelago, but significantly overestimated (or underestimated) vertical diffusivity in the near‐field (or the far‐field). This discrepancy is attributable to the fact that the effects of internal wave propagation are completely omitted in the existing parameterization, suggesting the potential danger of using such parameterized vertical mixing in predicting the distribution of SST as well as water‐mass transformation in the Indonesian Seas. This article is protected by copyright. All rights reserved.
      PubDate: 2015-04-03T22:16:03.061524-05:
      DOI: 10.1002/2014JC010592
  • Hydrographic variability in the St. Helena Bay region of the southern
           Benguela ecosystem
    • Authors: T. Lamont; L. Hutchings, M.A. van den Berg, W.S. Goschen, R.G. Barlow
      Abstract: Cross‐shelf distributions of temperature, salinity, water masses, and dissolved oxygen in St Helena Bay revealed substantial vertical and seasonal variations. In the surface layers, near‐shore and offshore temperature and salinity patterns differed, with bay‐scale variability linked to upwelling dynamics and coastal processes, while the offshore region was influenced by solar insolation. Spectral analysis revealed that an annual signal prevailed at most stations, and corroborated contrasting patterns between the offshore and near‐shore regions, with phase differences suggesting shoreward propagation of the offshore temperature signal. The shelf was dominated by Modified Upwelled Water (MUW) and Subantarctic Mode Water (SAMW), which comprised the primary source of upwelled water. Clear zonation of MUW was evident across the shelf, resulting from seasonal variations in locations of the oceanic and bifurcated shelf‐break fronts. Dynamics within St Helena Bay consistently differed from those further offshore, due to the influences of the shelf‐break front, Cape Columbine upwelling plume, and cyclonic recirculation, which appeared to be associated with an intra‐annual signal with a periodicity of 3‐4 months. Persistent hypoxia in the bottom waters suggested the occurrence of a permanent reservoir of Low Oxygen Water (LOW). Seasonal shoreward and offshore expansion of LOW occurred throughout the upwelling season, with maximum extent reached during summer and autumn, due to the coupled effects of advection and local phytoplankton decay. While wind mixing ventilated the water column at near‐shore stations in winter, and the onset of upwelling during spring introduced oxygen‐richer water from further offshore, hypoxia persisted in the centre of the Bay. This article is protected by copyright. All rights reserved.
      PubDate: 2015-04-03T11:27:47.245557-05:
      DOI: 10.1002/2014JC010619
  • Dynamics governing the response of tidal current along the mouth of
           Jiaozhou Bay to land reclamation
    • Authors: Lei Lin; Zhe Liu, Lian Xie, Huiwang Gao, Zhongya Cai, Ziyu Chen, Jianzhong Zhao
      Abstract: Data collected from previous studies show that the tidal current along the mouth of Jiaozhou Bay (JZB) appears to have weakened, whereas the spatial asymmetry (stronger flooding in the north region and stronger ebbing in the south region) has remained nearly unchanged during the past several decades of large‐scale land reclamation. This study is conducted to explain the underlying dynamics for this phenomenon. The analytic evaluation of the tidal motion indicates that the tidal current in a small coastal bay such as the JZB is linearly proportional to its length (L), if L is far less than one‐quarter of the tidal wave length. This relation suggests that the decrease in tidal current speed in JZB mouth results from the land reclamation within the Bay. Also, the relationship between bay areas and tidal current along the JZB mouth can be derived. The results of this simple theoretical method for predicting the change in mean tidal current amplitude after the land reclamation largely agree with previous three‐dimensional (3D) modeling studies. It is also found that the spatial asymmetry of the tidal current along the JZB mouth is controlled by the sharp headland (the local factor). The unchanged shoreline around the headland leads to the stable spatial asymmetry of the tidal current. The weaker tidal current can explain the weaker residual current, and the unchanged asymmetry of the tidal current explains the unaltered pattern of inflow over the north region and outflow over the south region for the tidally‐induced residual current along the JZB mouth. This article is protected by copyright. All rights reserved.
      PubDate: 2015-04-03T11:26:27.237851-05:
      DOI: 10.1002/2014JC010434
  • Surface wave effects in the NEMO ocean model: Forced and coupled
    • Authors: Øyvind Breivik; Kristian Mogensen, Jean‐Raymond Bidlot, Alonso Balmaseda Magdalena, Peter A.E.M. Janssen
      Abstract: The NEMO general circulation ocean model is extended to incorporate three physical processes related to ocean surface waves, namely the surface stress (modified by growth and dissipation of the oceanic wave field), the turbulent kinetic energy flux from breaking waves, and the Stokes‐Coriolis force. Experiments are done with NEMO in ocean‐only (forced) mode and coupled to the ECMWF atmospheric and wave models. Ocean‐only integrations are forced with fields from the ERA‐Interim reanalysis. All three effects are noticeable in the extra‐tropics, but the sea‐state dependent turbulent kinetic energy flux yields by far the largest difference. This is partly because the control run has too vigorous deep mixing due to an empirical mixing term in NEMO. We investigate the relation between this ad hoc mixing and Langmuir turbulence and find that it is much more effective than the Langmuir parameterization used in NEMO. The biases in sea surface temperature as well as subsurface temperature are reduced, and the total ocean heat content exhibits a trend closer to that observed in a recent ocean reanalysis (ORAS4) when wave effects are included. Seasonal integrations of the coupled atmosphere‐wave‐ocean model consisting of NEMO, the wave model ECWAM and the atmospheric model of ECMWF similarly show that the sea surface temperature biases are greatly reduced when the mixing is controlled by the sea state and properly weighted by the thickness of the uppermost level of the ocean model. These wave‐related physical processes were recently implemented in the operational coupled ensemble forecast system of ECMWF. This article is protected by copyright. All rights reserved.
      PubDate: 2015-04-03T11:17:23.977966-05:
      DOI: 10.1002/2014JC010565
  • Comparing in situ and satellite‐based parameterizations of oceanic
    • Authors: Aaron Paget; Mark A. Bourassa, Magdalena D. Anguelova
      Abstract: The majority of the parameterizations developed to estimate whitecap fraction use a stability‐dependent 10‐m wind (U10) measured in situ, but recent efforts to use satellite‐reported equivalent neutral winds (U10EN) to estimate whitecap fraction with the same parameterizations introduce additional error. This study identifies and quantifies the differences in whitecap parameterizations caused by U10 and U10EN for the active and total whitecap fractions. New power law coefficients are presented for both U10 and U10EN parameterizations based on available in situ whitecap observations. One‐way analysis of variance (ANOVA) tests are performed on the residuals of the whitecap parameterizations and the whitecap observations and identify that parameterizations in terms of U10 and U10EN perform similarly. The parameterizations are also tested against the satellite‐based WindSat Whitecap Database to assess differences. The improved understanding aids in estimating whitecap fraction globally using satellite products and in determining the global effects of whitecaps on air–sea processes and remote sensing of the surface. This article is protected by copyright. All rights reserved.
      PubDate: 2015-04-03T11:16:45.501284-05:
      DOI: 10.1002/2014JC010328
  • On Adler space‐time extremes during ocean storms
    • Authors: Alessandra Romolo; Felice Arena
      Abstract: The paper concerns the statistical properties of extreme ocean waves in the space‐time domain. In this regards, a solution for the exceedance probability of the maximum crest height during a sea state over a certain area is obtained. The approach is based on the Adler's solution for the extremal probability for Gaussian random processes in a multidimensional domain. The method is able to include effects of spatial variability of three‐dimensional sea waves on short‐term prediction, both over an assigned area XY and in a given direction. Next, the storm‐term predictions in the space‐time are investigated. For this purpose, the exceedance probability of during an ocean storm over an assigned area is derived. This solution gives a generalization to the space‐time of the Borgman's time‐based model for non‐stationary processes. The validity of the model is assessed from wave data of two buoys of the NOOA‐NDBC network located along the Pacific and the Atlantic US coasts. The results show that the size of the spatial domain A remarkably influences the expected maximum crest height during a sea storm. Indeed, the exceedance probabilities of the maximum crest height during an ocean storm over a certain area, significantly deviate from the classical Borgman's model in time, for increasing area. Then, for account to nonlinear contributions on crest height the proposed model is exploited jointly with the Forristall's distribution for nonlinear crest amplitudes in a given sea state. Finally, Monte Carlo simulations of a sea storm are performed showing a very good agreement with theoretical results. This article is protected by copyright. All rights reserved.
      PubDate: 2015-04-03T11:16:34.174065-05:
      DOI: 10.1002/2015JC010749
  • A basal stress parameterization for modeling landfast ice
    • Authors: Jean‐François Lemieux; Bruno Tremblay, Frédéric Dupont, Mathieu Plante, Gregory C. Smith, Dany Dumont
      Abstract: Current large‐scale sea ice models represent very crudely or are unable to simulate the formation, maintenance and decay of coastal landfast ice. We present a simple landfast ice parameterization representing the effect of grounded ice keels. This parameterization is based on bathymetry data and the mean ice thickness in a grid cell. It is easy to implement and can be used for two‐thickness and multi‐thickness category models. Two free parameters are used to determine the critical thickness required for large ice keels to reach the bottom and to calculate the basal stress associated with the weight of the ridge above hydrostatic balance. A sensitivity study was conducted and demonstrates that the parameter associated with the critical thickness has the largest influence on the simulated landfast ice area. A six year (2001‐2007) simulation with a 20‐km resolution sea ice model was performed. The simulated landfast ice areas for regions off the coast of Siberia and for the Beaufort Sea were calculated and compared with data from the National Ice Center. With optimal parameters, the basal stress parameterization leads to a slightly shorter landfast ice season but overall provides a realistic seasonal cycle of the landfast ice area in the East Siberian, Laptev and Beaufort Seas. However, in the Kara Sea, where ice arches between islands are key to the stability of the landfast ice, the parameterization consistently leads to an underestimation of the landfast area. This article is protected by copyright. All rights reserved.
      PubDate: 2015-04-03T03:21:56.458368-05:
      DOI: 10.1002/2014JC010678
  • Water mass pathways to the North Atlantic oxygen minimum zone
    • Authors: Jesús Peña‐Izquierdo; Erik van Sebille, Josep L. Pelegrí, Janet Sprintall, Evan Mason, Pedro J. Llanillo, Francisco Machín
      Abstract: The water mass pathways to the North Atlantic Oxygen Minimum Zone (naOMZ) are traditionally sketched within the cyclonic tropical circulation via the poleward branching from the eastward flowing jets that lie south of 10ºN. However, our water mass analysis of historic hydrographic observations together with numerical Lagrangian experiments consistently reveal that the potential density level of σθ = 26.8 kg m−3 (σ26.8, approximately 300 m depth) separates two distinct regimes of circulation within the Central Water (CW) stratum of the naOMZ. In the upper CW (above σ26.8), and in agreement with previous studies, the supply of water mainly comes from the south with a predominant contribution of South Atlantic CW. In the lower CW (below σ26.8), where minimal oxygen content is found, the tropical pathway is instead drastically weakened in favour of a subtropical pathway. More than two thirds of the total water supply to this lower layer takes place north of 10ºN, mainly via an eastward flow at 14ºN and northern recirculations from the northern subtropical gyre. The existence of these northern jets explains the greater contribution of North Atlantic CW observed in the lower CW, making up to 50% of the water mass at the naOMZ core. The equatorward transfer of mass from the well‐ventilated northern subtropical gyre emerges as an essential part of the ventilation of the naOMZ. This article is protected by copyright. All rights reserved.
      PubDate: 2015-04-03T03:15:51.877654-05:
      DOI: 10.1002/2014JC010557
  • An initial estimate of the global distribution of diurnal variation in sea
           surface salinity
    • Authors: E.C. Fine; F.O. Bryan, W.G. Large, D.A. Bailey
      Abstract: Diurnal variations in sea surface salinity (SSS) have been observed at a few select locations with adequate in situ instrumentation. Such variations result primarily from imbalances between surface freshwater fluxes and vertical mixing of deeper water to the surface. New observations becoming available from satellite salinity remote sensing missions could help to constrain estimates of diurnal variations in air‐sea exchange of freshwater, and provide insight into the processes governing diurnal variability of mixing processes in the upper ocean. Additionally, a better understanding of variation in near surface salinity is required to compare satellite measured SSS with in situ measurements at a few meters depth. The diurnal SSS variations should be reflected as differences between ascending and descending pass retrievals from the Aquarius and SMOS satellites; however, the diurnal signal can be masked by inadequacies of the geophysical corrections used in processing the satellite measurements. In this study we quantify the expected range of diurnal SSS variations using a model developed for predicting diurnal sea surface temperature variations. We present estimates for the mean and variance of the global diurnal SSS cycle, contrasting it with the diurnal cycle of sea surface temperature. We find the SSS diurnal cycle can be significant throughout the tropics, with mean amplitudes of up to 0.1 psu in areas with heavy precipitation. Predicted maximum diurnal ranges approach 2 psu in select regions. Surface freshening in Aquarius salinity retrievals is shown to be larger for ascending than descending passes, consistent with the expectations from the model simulation. This article is protected by copyright. All rights reserved.
      PubDate: 2015-04-03T02:44:06.281029-05:
      DOI: 10.1002/2014JC010483
  • Marine climate influences on interannual variability of tropical cyclones
           in the eastern Caribbean: 1979–2008
    • Authors: Mark R. Jury
      Abstract: Inter‐annual variability of tropical cyclones (TCs) in the eastern Caribbean is studied using MIT‐Hurdat fields during the July‐October season from 1979‐2008. TC intensity shows local climate sensitivity particularly for upper ocean currents, salinity and mixed layer depth, and 200‐850 mb wind shear. Remote influences from the Southern Oscillation, Saharan dust and the South American monsoon are also identified as important. Ocean currents diminish along the coast of South America, so inter‐basin transfer between the North Brazil and Caribbean Currents declines in seasons of frequent and intense TCs. This is related to a dipole pattern in the sea surface height formed mainly by reduced trade wind upwelling northeast of Venezuela. A low salinity plume from the Orinoco River spreads across the eastern Caribbean. It is the weaker currents and shallower mixed layer that conspire with surplus heat to build thermodynamic energy available for TC intensification. This article is protected by copyright. All rights reserved.
      PubDate: 2015-04-03T02:26:21.023441-05:
      DOI: 10.1002/2014JC010591
  • Northern Adriatic meteorological tsunamis: Assessment of their potential
           through ocean modeling experiments
    • Authors: Jadranka Šepić; Ivica Vilibić, Isaac Fine
      Abstract: Potential for generation of meteotsunami waves via open ocean resonance has been documented for the shallow northern Adriatic, based on a set of barotropic numerical modeling experiments. Model simulations were forced by a bell‐shaped traveling atmospheric (air pressure, wind) disturbance, with shape and propagation parameters chosen in accordance with measurements done during several observed northern Adriatic meteotsunamis. Air pressure disturbances were found to generate much larger meteotsunami waves than wind disturbances, with wind disturbances having a limited influence in the very coastal and shallow areas only. Numerical simulations reveal that the most important factor for generation of large meteotsunami waves is matching between the speed of the atmospheric disturbance and the speed of long‐ocean waves. Already a small (∼10%) deviation from resonant conditions stops the wave growth and dramatically decreases height of predicted waves. A train of atmospheric disturbances can significantly increase maximum wave heights at selected locations at which multiple reflections and superimpositions of meteotsunami waves occur. Sensitivity of model simulations to resonant conditions and limited cross‐propagation width of atmospheric disturbance explain the localization of destructive meteotsunami waves in a limited area during destructive historic events. Mapping of maximum predicted wave heights indicates places with large meteotsunami hazard potential, matching the locations where real events were observed, and may be a useful tool for assessing vulnerability and risks in coastal areas during extreme sea level events. This article is protected by copyright. All rights reserved.
      PubDate: 2015-04-01T22:21:52.860949-05:
      DOI: 10.1002/2015JC010795
  • High turnover rates indicated by changes in the fixed N forms and their
           stable isotopes in Antarctic landfast sea ice
    • Authors: François Fripiat; Daniel M. Sigman, Guillaume Massé, Jean‐Louis Tison
      Abstract: We report concentration and nitrogen and oxygen isotopic measurements of nitrate, total dissolved nitrogen, and particulate nitrogen from Antarctic landfast sea ice, covering almost the complete seasonal cycle of sea ice growth and decay (from April to November). When sea ice forms in autumn, ice algae growth depletes nitrate and accumulates organic N within the ice. Subsequent low biological activity in winter imposes minor variations in the partitioning of fixed N. In early spring, the coupling between nitrate assimilation and brine convection at the sea ice bottom traps a large amount of fixed N within sea ice, up to 20 times higher than in the underlying seawater. At this time, remineralization and nitrification also accelerate, yielding nitrate concentrations up to 5 times higher than in seawater. Nitrate δ15N and δ18O are both elevated, indicating a near‐balance between nitrification and nitrate assimilation. These findings require high microbially mediated turnover rates for the large fixed N pools, including nitrate. When sea ice warms in the spring, ice algae grow through the full thickness of the ice. The warming stratifies the brine network, which limits the exchange with seawater, causing the once‐elevated nitrate pool to be nearly completely depleted. The nitrate isotope data point to light limitation at the base of landfast ice as a central characteristic of the environment, affecting its N cycling (e.g., allowing for nitrification) and impacting algal physiology (e.g., as reflected in the N and O isotope effects of nitrate assimilation). This article is protected by copyright. All rights reserved.
      PubDate: 2015-03-31T18:00:03.351134-05:
      DOI: 10.1002/2014JC010583
  • Anisotropic response of surface circulation to wind forcing, as inferred
           from high‐frequency radar currents in the southeastern Bay of Biscay
    • Authors: Almudena Fontán; Bruce Cornuelle
      Abstract: The short‐term (less than 20 days) response of surface circulation to wind has been determined in waters of the southeastern Bay of Biscay, using wind impulse response (time domain) and transfer (frequency domain) functions relating high‐frequency radar currents and reanalysis winds. The response of surface currents is amplified at the near‐inertial frequency and the low‐frequency and it varies spatially. The analysis indicates that the response of the ocean to the wind is slightly anisotropic, likely due to pressure gradients and friction induced by the bottom and coastline boundaries in this region. Thus, the transfer function at the near‐inertial frequency decreases onshore due to the coastline inhibition of circularly‐polarized near‐inertial motion. In contrast, the low‐frequency transfer function is enhanced towards the coast as a result of the geostrophic balance between the cross‐shore pressure gradient and the Coriolis forces. The transfer functions also vary with season. In summer, the current response to wind is expected to be stronger but shallower due to stratification; in winter, the larger mixed layer depth results in a weaker but deeper response. The results obtained are consistent with the theoretical description of wind‐driven circulation and can be used to develop a statistical model with a broad range of applications including accurate oceanic forecasting and understanding of the coupled atmosphere‐ocean influence on marine ecosystems. This article is protected by copyright. All rights reserved.
      PubDate: 2015-03-31T17:59:52.194323-05:
      DOI: 10.1002/2014JC010671
  • Indonesian throughflow proxy from satellite altimeters and gravimeters
    • Authors: R. Dwi Susanto; Y. Tony Song
      Abstract: The Indonesian Throughflow (ITF) from the Pacific to the Indian Ocean plays an important role in global ocean circulation and climate. Yet, continuous ITF measurement is difficult and expensive. The longest time series of in situ measurements of the ITF to date were taken in the Makassar Strait, the main pathway of the ITF. Here we have demonstrated a plausible approach to derive the ITF transport proxy using satellite altimetry sea surface height (SSH), gravimetry ocean bottom pressure (OBP) data, in situ measurements from the Makassar Strait from 1996‐1998 and 2004‐2011, and a theoretical formulation. We first identified the optimal locations of the correlation between the observed ITF transport through the Makassar Strait and the pressure gradients, represented by the SSH and OBP differences between the Pacific and Indian Oceans at a 1°x1° horizontal resolution. The optimal locations were found centred at 162°E and 11°N in the Pacific Ocean and 80°E and 0° in the Indian Ocean, then were used in the theoretical formulation to estimate the throughflow. The proxy time series follow the observation time series quite well, with the 1993‐2011 mean proxy transport of 11.6±3.2Sv southward, varying from 5.6Sv during the strong 1997 El Niño to 16.9Sv during the 2007 La Nina period, which are consistent with previous estimates. The observed Makassar mean transport is 13.0±3.8Sv southward over 2004‐2011, while the SSH proxy (for the same period) gives an ITF mean transport of 13.9±4.1Sv and the SSH+OBP proxy (for 2004‐2010) is 15.8±3.2Sv. This article is protected by copyright. All rights reserved.
      PubDate: 2015-03-25T08:26:53.25965-05:0
      DOI: 10.1002/2014JC010382
  • Processes controlling mid‐water column oxygen minima over the
           Texas‐Louisiana shelf
    • Authors: Wenxia Zhang; Robert D. Hetland, Steven F. DiMarco, Katja Fennel
      Abstract: We investigate distributions of dissolved oxygen over the Texas‐Louisiana shelf using spatially highly resolved observations in combination with a regional circulation model with simple oxygen dynamics. The observations were collected using a towed, undulating CTD during the Mechanisms Controlling Hypoxia (MCH) program. Mid‐water oxygen minimum layers (dissolved oxygen lower than 3.2 mL L– 1) were detected in many transects. These oxygen minimum layers are connected with the bottom boundary layer and follow the pycnocline seaward as a tongue of low oxygen into the mid‐water column. T‐S diagrams highlighting the low oxygen minima in both observations and simulations imply direct connections between low‐oxygen bottom water and the oxygen minimum layer. The dynamics of these oxygen minimum layers in the mid‐water column are examined using a three‐dimensional hydrodynamic model, based on the Regional Ocean Modeling System (ROMS). Convergence within the bottom boundary layer relative to density surfaces is calculated, results show that there is a convergence in the bottom boundary layer at the location where the pycnocline intersects the bottom. Buoyancy advection forced by bottom Ekman transport creates this convergent flow, and the corresponding low‐oxygen intrusion. Similar intrusions of near‐bottom water into the pycnocline are observed in other regions. The presence of hypoxia within the bottom boundary layer in the northern Gulf of Mexico creates a unique situation in which these intrusions are also associated with low dissolved oxygen. This article is protected by copyright. All rights reserved.
      PubDate: 2015-03-24T14:56:22.716125-05:
      DOI: 10.1002/2014JC010568
  • Seasonal differences in intraseasonal and interannual variability of
           Mediterranean sea surface temperature
    • Authors: Igor I. Zveryaev
      Abstract: Sea surface temperature (SST) data from the NOAA OI SST dataset for 1982‐2011 are used to investigate intraseasonal and interannual variability of Mediterranean SST during winter and summer seasons. It is shown that during winter the intraseasonal SST fluctuations are larger than the interannual SST variations in the western Mediterranean (e.g., the Tyrrhenian Sea), but smaller in the central and eastern Mediterranean Sea. In summer, the intraseasonal SST fluctuations are larger in almost the entire Mediterranean basin. Also summertime intraseasonal SST fluctuations are larger (up to three times near the Gulf of Lions) than their wintertime counterparts in the entire Mediterranean basin. The interannual SST variations are larger during summer in the western and central Mediterranean Sea and during winter in its eastern part. The leading empirical orthogonal functions (EOFs) of the Mediterranean SST and of the intensities of its intraseasonal fluctuations are characterized by the differing spatial‐temporal structures both during winter and summer implying that their interannual variability is driven by different physical mechanisms. During winter the EOF‐1 of SST is associated with the East Atlantic teleconnection, whereas EOF‐1 of the intensity of intraseasonal fluctuations is not linked significantly to regional atmospheric dynamics. The second EOFs of these variables are associated, respectively, with the East Atlantic/West Russia and the North Atlantic teleconnections. While during summer the atmospheric influence on Mediterranean SST is generally weaker, it is revealed that the EOF‐1 of the intensity of intraseasonal SST fluctuations is linked to the Polar teleconnection. This article is protected by copyright. All rights reserved.
      PubDate: 2015-03-24T14:55:55.333727-05:
      DOI: 10.1002/2014JC010387
  • Temporal and spatial variability of particle transport in the deep Arctic
           Canada Basin
    • Authors: Jeomshik Hwang; Minkyoung Kim, Steven J. Manganini, Cameron P. McIntyre, Negar Haghipour, JongJin Park, Richard A. Krishfield, Robie W. Macdonald, Fiona A. McLaughlin, Timothy I. Eglinton
      Abstract: To better understand the current carbon cycle and potentially detect its change in the rapidly changing Arctic Ocean, we examined sinking particles collected quasi‐continuously over a period of 7 years (2004‐2011) by bottom‐tethered sediment trap moorings in the central Canada Basin. Total mass flux was very low (< 100 mgm−2d−1) at all sites and was temporally decoupled from the cycle of primary production in surface waters. Extremely low radiocarbon contents of particulate organic carbon and high aluminum contents in sinking particles reveal high contributions of resuspended sediment to total sinking particle flux in the deep Canada Basin. Station A (75 ºN, 150 ºW) in the southwest quadrant of the Canada Basin is most strongly influenced while Station C (77 ºN, 140 ºW) in the northeast quadrant is least influenced by lateral particle supply based on radiocarbon content and Al concentration. The results at Station A, where three sediment traps were deployed at different depths, imply that the most likely mode of lateral particle transport was as thick clouds of enhanced particle concentration extending well above the seafloor. At present, only 1‐2% of the low levels of new production in Canada Basin surface waters reaches the interior basin. Lateral POC supply therefore appears to be the major source of organic matter to the interior basin. However, ongoing changes to surface ocean boundary conditions may influence both lateral and vertical supply of particulate material to the deep Canada Basin. This article is protected by copyright. All rights reserved.
      PubDate: 2015-03-24T14:53:12.753706-05:
      DOI: 10.1002/2014JC010643
  • Baroclinic instability and the mesoscale eddy field around the Lofoten
    • Authors: P. E. Isachsen
      Abstract: The vigorous mesoscale eddy field around the Lofoten Basin west of northern Norway is thought to be related to eddy shedding from the Norwegian Atlantic Current flowing along the Norwegian coast. Here we study baroclinic instability in the current with a particular focus on the influence of topography. The flow over the steepest part of the continental slope is found to be the most unstable. The growth characteristics can not be understood from Eady theory alone but require the consideration of interior potential vorticity gradients. A study of the fully‐developed macroturbulent field shows that eddy kinetic energy is advected away from the generation regions and that non‐linear effects likely draw the eddy statistics away from the linear growth regime. This article is protected by copyright. All rights reserved.
      PubDate: 2015-03-23T10:06:23.110691-05:
      DOI: 10.1002/2014JC010448
  • Mesoscale variability in the habitat of the Humboldt Current krill, spring
    • Authors: Ramiro Riquelme‐Bugueño; Marco Correa‐Ramírez, Rubén Escribano, Sergio Núñez, Samuel Hormazábal
      Abstract: Mesoscale eddies are prominent structures in the world's oceans generating a high degree of spatial and temporal heterogeneity that influences zooplankton distribution. Euphausiids (krill) are a key zooplankton group mainly inhabiting coastal upwelling areas where high productivity, advection and eddy kinetic energy (EKE) play pivotal roles in the distribution and structure of krill habitats. We analyzed the spatial distribution of the Humboldt Current krill, Euphausia mucronata, in relation to environmental variability and mesoscale circulation during the 2007 austral spring. Using net‐based zooplankton samples, remotely sensed environmental conditions, multivariate analysis and generalized additive models, we described and tested the effect of oceanographic variability and mesoscale eddies on E. mucronata abundance and biomass. E. mucronata was significantly more abundant in coastal (97%) than oceanic habitats, and more abundant in cyclonic cores (mean: 76 indiv. m−2) than in surrounding waters (mean: 13‐29 indiv. m−2). Abundance correlated to current and EKE fields at >10‐20 cm s−1 and >50‐200 cm2 s−2, respectively, and biomass correlated negatively to sea level anomaly and positively to alongshore winds. Krill abundance and biomass were also strongly coupled to both eddy dynamics and the coastal upwelling regime in spring 2007. Mesoscale eddies may doubly influence the E. mucronata population dynamic by retaining krill within them and, by advection from coastal to oligotrophic regions. This article is protected by copyright. All rights reserved.
      PubDate: 2015-03-23T09:56:39.461323-05:
      DOI: 10.1002/2014JC010460
  • Shift of anammox bacterial community structure along the Pearl Estuary and
           the impact of environmental factors
    • Authors: Bingbing Fu; Jiwen Liu, Hongmei Yang, Ting Chang Hsu, Biyan He, Minhan Dai, Shuh Ji Kao, Meixun Zhao, Xiao‐Hua Zhang
      Abstract: Anaerobic ammonium oxidation (anammox) plays an important role in the marine nitrogen cycle. The Pearl Estuary, a typical subtropical estuary characterized by hypoxia upstream and high loads of organic matter and inorganic nutrients caused by anthropogenic activities, has received extensive attention. In this study, anammox bacterial community structures in surface sediments along the Pearl Estuary were investigated using 16S rRNA and hydrazine oxidoreductase (HZO) genes. In addition, abundance of anammox bacteria in both water and surface sediments was investigated by quantitative PCR. Obvious anammox bacterial community structure shift was observed in surface sediments, in which the dominant genus changed from “Candidatus Brocadia” or “Candidatus Anammoxoglobus” to “Candidatus Scalindua” along the salinity gradient from freshwater to the open ocean based on 16S rRNA gene and HZO amino acid phylotypes. This distribution pattern was associated with salinity, temperature, pH of overlying water and particularly C/N ratio. Phylogenetic analysis unraveled a rich diversity of anammox bacteria including four novel clusters provisionally named “Candidatus Jugangensis”, “Candidatus Oceanicum”, “Candidatus Anammoxidans” and “Candidatus Aestuarianus”. The abundance of anammox bacteria in surface sediments, bottom and surface waters ranged from 4.22 × 105 to 2.55 × 106 copies g−1, 1.24 × 104 to 1.01 × 105 copies L−1 and 8.07 × 103 to 8.86 × 105 copies L−1, respectively. The abundance of anammox bacteria in the water column was positively correlated with NO2‐ and NO3‐, and negatively correlated with dissolved oxygen, although an autochthonous source might contribute to the observed abundance of anammox bacteria. This article is protected by copyright. All rights reserved.
      PubDate: 2015-03-23T09:29:40.761143-05:
      DOI: 10.1002/2014JC010554
  • Low salinity water off West Luzon Island in summer
    • Authors: Yunwei Yan; Guihua Wang, Chunzai Wang, Jilan Su
      Abstract: Low salinity water with two cores is found off West Luzon Island in the South China Sea (SCS) during summer. A series of salinity observations and model results show that the low salinity water begins to appear in June, reaches its lowest salinity in September, and disappears after October. Rainfall associated with the summer monsoon impinging on the Philippine mountain ranges plays an important role in the formation of the low salinity water, while upward Ekman pumping of high salinity subsurface water caused by the strong winter monsoon is important for its disappearance. Variation in mixed layer depth is responsible for the formation of the two cores of the low salinity water, while advection also contributes. The study further demonstrates that the low salinity water has considerable interannual variability associated with El Niño‐Southern Oscillation (ENSO): During the summer of the decaying year of an El Niño, an anticyclonic wind anomaly occurs in the SCS. The anticyclonic wind anomaly is associated with a northeasterly anomaly south of 18°N, reducing precipitation and causing salting of the low salinity water off West Luzon Island. The situation is reversed during the summer of the decaying year of a La Niña. This article is protected by copyright. All rights reserved.
      PubDate: 2015-03-23T09:29:13.457927-05:
      DOI: 10.1002/2014JC010465
  • An observational study of salt fluxes in Delaware Bay
    • Authors: María. F. Aristizábal; Robert J. Chant
      Abstract: An observational study was conducted in Delaware Bay during the summer of 2011 aiming to quantify different mechanisms driving the salt flux in this system. Seven moorings, equipped with bottom mounted ADCPs and CT sensors at difference depths, were deployed across a section of the estuary. The total area‐averaged and tidal‐averaged salt flux was decomposed in three different contributions: the advective salt flux that represents the flux caused by river input and meteorological‐induced flows, the steady shear dispersion that is the salt flux driven by the estuarine exchange flow, and the tidal oscillatory salt flux that is induced by the tidal currents. The advective salt flux dominated over the steady shear dispersion and tidal oscillatory salt flux because it was driven mainly by changes in sea surface height associated with wind driven set‐up and set‐down. The steady shear dispersion was always positive and presented a spring/neap variability that was consistent with a two layer exchange flow. On the other hand, the tidal oscillatory salt flux fluctuated between positive and negative values, but increased around a strong neap tide and decreased on the following spring tide. This variability is contrary to previous parameterizations whereby the tidal salt flux is proportional to the amplitude of the tidal currents. The observational estimate was compared to a parameterization that relates tidal salt flux as proportional to tidal current amplitude and stratification. The observational estimate agreed with this new parameterization when the river discharge was relatively constant. This article is protected by copyright. All rights reserved.
      PubDate: 2015-03-23T08:53:12.429884-05:
      DOI: 10.1002/2014JC010680
  • Observations of hysteresis in the annual exchange circulation of a large
           microtidal estuary
    • Authors: Steven D. Meyers; Monica Wilson, Mark. E. Luther
      Abstract: A nonlinear relation between the salinity field and the subtidal exchange circulation in the Tampa Bay estuary is demonstrated using observational data from 1999‐2011. The data are averaged to form mean monthly climatological values of total freshwater discharge (Q), axial and vertical salinity gradients, and subtidal vertical shear. Well‐known steady‐state solutions indicate the exchange circulation is linearly proportional to the horizontal salinity gradient, assuming a constant vertical eddy viscosity (Aeff). The exchange flow is found to be multi‐valued with respect to the horizontal salinity gradient, forming a hysteresis loop in parameter space that passes through three dynamical regimes. Regime I is relatively dry with weak salinity gradients and exchange circulation. Regime II is the wet season (June‐September) in which all quantities rapidly increase. In regime III the exchange flow persists even though Q and the axial salinity gradient are again low. Gradient Richardson numbers and Simpson numbers also form a loop in parameter space with Ri remaining subcritical (turbulent) until the wet season when Ri rises above criticality (weak vertical mixing) where it remains through the end of regime III. The Simpson number is in a narrow range around 0.2, indicating that the horizontal salinity gradient is always a driver of the exchange circulation. The Aeff, estimated from a parameterization of the Richardson number, decreases by almost an order of magnitude from regime I to II. It remains low during III, indicating the persistent stratification is insulating the exchange flow from destruction by tidal mixing during this time period. This article is protected by copyright. All rights reserved.
      PubDate: 2015-03-23T08:38:25.42237-05:0
      DOI: 10.1002/2014JC010342
  • Salt intrusion at a submarine spring in a fringing reef lagoon
    • Authors: Sabrina M. Parra; Arnoldo Valle‐Levinson, Ismael Mariño‐Tapia, Cecilia Enriquez
      Abstract: Variations in discharge and turbulent kinetic energy (TKE) were studied at a point‐source submarine groundwater discharge (SGD), within a fringing reef lagoon, from quadrature (neap) to syzygy (spring) tides. The principal factors affecting discharge and TKE variations were tides and waves. Field data indicated discharge and TKE varied with high and low tides, and with quadrature and syzygy. Maximum discharge and TKE values were observed during low tides when the hydrostatic pressure over the jet was minimal, while the lowest discharge and TKE values were found at high tides. Syzygy tides produced consistent saltwater intrusion during high tides, while quadrature tides produced the greatest TKE values. In general as the discharge intensified during low tides, jet temperatures decreased suggesting that waters originated further within the aquifer. At the same time jet salinities increased, suggesting a mixing of aquifer and seawater. To reconcile these two seemingly opposing views, it is proposed that the jet conduit is connected to a stratified chamber with seawater below brackish water. The greatest subtidal discharge occurred during quadrature tides. Syzygy produced low subtidal discharge driven by flow reversals (flow into the aquifer) observed throughout syzygy high tides in conjunction with the peak wave setup (>5cm) observed during a storm. While tides were the primary driving force of the discharge, waves played a non‐negligible role. Wave effects on the discharge were most evident during syzygy high tides combined with a storm, when the subtidal spring discharge was weakest and salt intrusion developed. This article is protected by copyright. All rights reserved.
      PubDate: 2015-03-23T08:09:23.073638-05:
      DOI: 10.1002/2014JC010459
  • Annual sea level variability of the coastal ocean: The Baltic
           Sea‐North Sea transition zone
    • Authors: M. Passaro; P. Cipollini, J. Benveniste
      Abstract: The annual cycle is a major contribution to the non‐tidal variability in sea level. Its characteristics can vary substantially even at a regional scale, particularly in an area of high variability such as the coastal ocean. This study uses previously validated coastal altimetry solutions (from ALES dataset) and the reference ESA Sea Level Climate Change Initiative dataset to improve the understanding of the annual cycle during the Envisat years (2002‐2010) in the North Sea ‐ Baltic Sea transition area. This area of study is chosen because of the complex coastal morphology and the availability of in‐situ measurements. To our knowledge, this is the first time that the improvements brought by coastal satellite altimetry to the description of the annual variability of the sea level have been evaluated and discussed. The findings are interpreted with the help of a local climatology and wind stress from a reanalysis model. The coastal amplitude of the annual cycle estimated from ALES altimetry data is in better agreement with estimations derived from in‐situ data than the one from the reference dataset. Wind stress is found to be the main driver of annual cycle variability throughout the domain, while different steric contributions are responsible for the differences within and among the sub‐basins. We conclude that the ALES coastal altimetry product is a reliable dataset to study the annual cycle of the sea level at a regional scale and the strategy described in this research can be applied to other areas of the coastal ocean where the coverage from the tide gauges is not sufficient. This article is protected by copyright. All rights reserved.
      PubDate: 2015-03-23T07:42:55.069082-05:
      DOI: 10.1002/2014JC010510
  • Ice thickness effects on Aquarius brightness temperatures over Antarctica
    • Authors: Miriam Pablos; María Piles, Verónica González‐Gambau, Adriano Camps, Mercè Vall‐llossera
      Abstract: The Dome‐C region, in the East Antarctic Plateau, is regarded as an ideal natural laboratory for calibration/validation of space‐borne microwave radiometers. At L‐band, the thermal stability of this region has been confirmed by several experimental campaigns. However, its use as an independent external calibration target has recently been questioned due to some spatial inhomogeneities and seasonal effects revealed in the brightness temperatures (TB) acquired in this area. This paper shows the observed relationship, from exploratory research, between the Antarctic ice thickness spatial variations and the measured Aquarius TB changes. A three‐months no‐daylight period during the Austral winter has been analyzed. Four transects have been defined over East Antarctica covering areas with different ice thickness variations and ranges. The theoretical L‐band penetration depth has been estimated to understand the possible contributions to the measured signal. A good agreement has been observed between Aquarius TB and ice thickness variations over the whole Antarctica, with correlations of ∼0.6–0.7. The two variables show a linear trend with slopes of ∼8.3–9.5 K/km. No correlation has been observed with the subglacial bedrock. The maximum L‐band penetration depth has been estimated to be ∼1–1.5 km. Results are therefore consistent: the spatial variations found on Aquarius TB are not related to the emissivity of the bedrock, which lies deeper. This study provides evidence that new L‐band satellite observations could contribute to further our understanding of Antarctic geophysical processes. This article is protected by copyright. All rights reserved.
      PubDate: 2015-03-19T21:20:39.671117-05:
      DOI: 10.1002/2014JC010151
  • Whales and waves: Humpback whale foraging response and the shoaling of
           internal waves at Stellwagen Bank
    • Authors: Jesús Pineda; Victoria Starczak, José C.B. da Silva, Karl Helfrich, Michael Thompson, David Wiley
      Abstract: We tested the hypothesis that humpback whales aggregate at the southern flank of Stellwagen Bank (SB) in response to internal waves (IWs) generated semi‐diurnally at Race Point (RP) channel because of the presence of their preferred prey, planktivorous fish, which in turn respond to zooplankton concentrated by the predictable IWs. Analysis of synthetic aperture radar (SAR) images indicates that RP IWs approach the southern flank of SB frequently (∼62% of the images). Published reports of whale sighting data and archived SAR images point to a coarse spatial coincidence between whales and Race Point IWs at SB's southern flank. The responses of whales to IWs were evaluated via sightings and behavior of humpback whales, and IWs were observed in situ by acoustic backscatter and temperature measurements. Modeling of IWs complemented the observations, and results indicate a change of ∼0.4m/s in current velocity, and ∼1.5 Pa in dynamic pressure near the bottom, which may be sufficient for bottom fish to detect the IWs. However, fish were rare in our acoustic observations, and fish response to the IWs could not be evaluated. RP IWs do not represent the leading edge of the internal tide, and they may have less mass‐transport potential than typical coastal IWs. There was large interannual variability in whale sightings at SB's southern flank, with decreases in both numbers of sightings and proportion of sightings where feeding was observed from 2008 to 2013. Coincidence of whales and IWs was inconsistent, and results do not support the hypothesis. This article is protected by copyright. All rights reserved.
      PubDate: 2015-03-18T10:47:18.811386-05:
      DOI: 10.1002/2014JC010564
  • Persistent organic pollutants in ocean sediments from the North Pacific to
           the Arctic Ocean
    • Authors: Yuxin Ma; Crispin J. Halsall, John D. Crosse, Carola Graf, Minghong Cai, Jianfeng He, Guoping Gao, Kevin Jones
      Abstract: Concentrations of polychlorinated biphenyls (PCBs), organochlorine pesticides (OC pesticides) and polybrominated diphenyl ethers (PBDEs) are reported in surficial sediments sampled along cruise transects from the Bering Sea to the central Arctic Ocean. OCs and PCBs all had significantly higher concentrations in the relatively shallow water (500 m) of the Bering Sea and Arctic Ocean (e.g. Canada Basin ΣPCB 149±102 pg g−1 dw). Concentrations were similar to, or slightly lower than, studies from the 1990s, indicating a lack of a declining trend. PBDEs (excluding BDE‐209) displayed very low concentrations (e.g. range of median values, 3.5‐6.6 pg/g dw). In the shelf areas, the sediments comprised similar proportions of silt and clay, whereas the deep basin sediments were dominated by clay, with a lower total organic carbon (TOC) content. While significant positive correlations were observed between persistent organic pollutant (POP) concentrations and TOC (Pearson correlation, r=0.66‐0.75, p
      PubDate: 2015-03-18T08:15:38.604053-05:
      DOI: 10.1002/2014JC010651
  • Observation of the surface horizontal thermohaline variability at meso to
           submesoscales in the north‐eastern Subtropical Atlantic Ocean
    • Authors: Nicolas Kolodziejczyk; Gilles Reverdin, Jacqueline Boutin, Olga Hernandez
      Abstract: The seasonal variability of the surface horizontal thermohaline structure is investigated in the north‐eastern Atlantic Surface Salinity Maximum (SSM) at length scales from five to hundreds of kilometers, i.e. at submeso‐ to meso‐scales. The near‐surface temperature and salinity data from merchant ship thermosalinograph (TSG) transects across the Atlantic are used to compute the horizontal temperature, salinity and density fluctuations, and the density ratio. During late winter in the north‐eastern SSM, thermohaline compensation is observed for wavelengths from 5 km to more than 200 km; in spite of large and sharp surface thermohaline fronts, a weak density surface horizontal gradient is observed. Temperature and salinity exhibit an energetic spectra in k−2 slope scale between 20‐100 km scale and a steeper slope at shorter wavelengths, while density spectra exhibit lower energy level with a clear k−3 slope below 20 km, consistent with interior quasi‐geostrophic turbulence. During summer in the north‐eastern SSM, salinity and temperature gradients are no longer compensated due to strong atmospheric heating of the upper ocean. In comparison with winter, the surface density spectrum is enhanced with a flatter slope between k−2‐k−1 between 5 and 20 km. This article is protected by copyright. All rights reserved.
      PubDate: 2015-03-18T04:54:41.155929-05:
      DOI: 10.1002/2014JC010455
  • Estimates of Lagrangian transport by surface gravity wave groups: The
           effects of finite depth and directionality
    • Authors: T.S. van den Bremer; P.H. Taylor
      Abstract: Two physical phenomena drive the Lagrangian trajectories of neutrally buoyant particles underneath surface gravity wave groups: the Stokes drift results in a net displacement of particles in the direction of propagation of the group, whereas the Eulerian return flow transports such particles in the opposite direction. Generally, the Stokes drift is the larger of the two near the surface, whereas the effects of the return flow dominate at depth. A transition depth can be defined that separates the two regimes. Using a multiple‐scales expansion we provide leading‐order estimates of the forward transport, the backward transport and the transition depth for realistic sea states. We consider the effects of both finite depth and the directionally spread nature of the waves on our estimates. We show that from the perspective of the return flow, almost all seas are of finite depth. In fact, many seas can be shown to be “shallow” from the perspective of the return flow with little variation of this flow with depth. Furthermore, even small degrees of directional spreading can considerably reduce the magnitude of the return flow and its transport. This article is protected by copyright. All rights reserved.
      PubDate: 2015-03-18T03:06:51.466182-05:
      DOI: 10.1002/2015JC010712
  • Comparison of spaceborne measurements of sea surface salinity and colored
           detrital matter in the Amazon plume
    • Authors: S. Fournier; B. Chapron, J. Salisbury, D. Vandemark, N. Reul
      Abstract: Large rivers are key hydrologic components in oceanography, particularly regarding air‐sea and land‐sea exchanges and biogeochemistry. We enter now in a new era of Sea Surface Salinity (SSS) observing system from Space with the recent launches of the ESA Soil Moisture and Ocean Salinity (SMOS) and the NASA Aquarius/Sac‐D missions. With these new sensors, we are now in an excellent position to revisit SSS and ocean color investigations in the tropical northwest Atlantic using multi‐year remote sensing time series and concurrent in situ observations. The Amazon is the world's largest river in terms of discharge. In its plume, SSS and upper water column optical properties such as the absorption coefficient of colored detrital matter (acdm) are strongly negatively correlated (
      PubDate: 2015-03-16T00:27:47.44327-05:0
      DOI: 10.1002/2014JC010109
  • Upwelling induced by the frictional stress curl and vertical squeezing of
           the vortex tube over a submerged valley in the East China Sea
    • Authors: Zhiqiang Liu; Jianping Gan
      Abstract: We conducted a process‐oriented modeling study to investigate the characteristics and dynamics of the prominent upwelling over a vast submerged valley in the East China Sea (ECS). The valley is inversely funnel‐shaped with the west bank and the east bank oriented in the north‐south direction. A cross‐bank upward transport occurred along the west bank. It intensified northward and peaked around the head of the valley. An along‐bank southward pressure gradient force (PGF) formed the cross‐bank geostrophic transport for the upwelling over the valley. The PGF reached its maxima at the head of the valley. Our momentum and vorticity dynamic analyses revealed that a bottom stress curl mainly contributed the PGF along the west bank. At the same time, both the bottom stress curl and the nonlinear vorticity advection contributed to the PGF around the head. The bottom stress curl was due to the bottom shear vorticity of the coastal current and the curvature vorticity around the head. The nonlinear vorticity advection formed because of the vertical squeezing of vortex tube as the current flowed over the valley. The nonlinearity mainly affected the PGF around the head, whereas the bottom stress curl contributed to the PGF over the entire valley. The ratio of the nonlinear to frictional contributions to the PGF increased as the coastal current intensified. Our study demonstrates that the PGF that drives the upwelling over the valley is the combined result of the nonlinearity due to vertical squeezing of vortex tube and bottom frictional effects. This article is protected by copyright. All rights reserved.
      PubDate: 2015-03-16T00:27:25.020841-05:
      DOI: 10.1002/2015JC010715
  • Spatial and temporal variability of zooplankton off New Caledonia
           (Southwestern Pacific) from acoustics and net measurements
    • Authors: Houssem Smeti; Marc Pagano, Christophe Menkes, Florian de Boissieu, Anne Lebourges‐Dhaussy, Brian P. V. Hunt, Valerie Allain, Martine Rodier, Elodie Kestenare, Cherif Sammari
      Abstract: Spatial and temporal distribution of zooplankton off New Caledonia in the eastern Coral Sea was studied during two multidisciplinary cruises in 2011, during the cool and the hot seasons. Acoustic measurements of zooplankton were made using a shipborne acoustic Doppler current profiler (S‐ADCP), a scientific echosounder and a Tracor acoustic profiling system (TAPS). Relative backscatter from ADCP was converted to biomass estimates using zooplankton weights from net‐samples collected during the cruises. Zooplankton biomass was estimated using four methods: weighing, digital imaging (ZooScan), ADCP and TAPS. Significant correlations were found between the different biomass estimators and between the backscatters of the ADCP and the echosounder. There was a consistent diel pattern in ADCP derived biomass and echosounder backscatter resulting from the diel vertical migration (DVM) of zooplankton. Higher DVM amplitudes were associated with higher abundance of small zooplankton and cold waters to the south of the study area, while lower DVM amplitudes in the north were associated with warmer waters and higher abundance of large organisms. Zooplankton was largely dominated by copepods (71‐73%) among which calanoids prevailed (40‐42%), with Paracalanus spp. as the dominant species (16‐17%). Overall, zooplankton exhibited low abundance and biomass (mean night dry biomass of 4.7 ± 2.2mg m3 during the cool season and 2.4 ± 0.4mg m3 during the hot season) but high richness and diversity (Shannon index ∼4). Substantially enhanced biomass and abundance appeared to be episodically associated with mesoscale features contributing to shape a rather patchy zooplankton distribution. This article is protected by copyright. All rights reserved.
      PubDate: 2015-03-13T23:41:17.248975-05:
      DOI: 10.1002/2014JC010441
  • Ocean variability east of Mindanao: Mooring observations at 7°N,
    • Authors: Yuji Kashino; Iwao Ueki, Hedeharu Sasaki
      Abstract: Two subsurface moorings were deployed east of Mindanao Island, the Philippines, at 7°01'N, 126°55'E and 7°01'N, 127°46'E, at the location of the inshore and offshore cores of the Mindanao Undercurrent (MUC) suggested by past studies, from September 2011 to October 2012 and March 2013. A steady northward undercurrent, the MUC, was not confirmed by these observations, not only at the location of its inshore core but also of the offshore core. The observed mean flow at the mooring sites seems to be part of an anti‐cyclonic eddy rather than the MUC. A Particle‐tracking experiment using a high‐resolution general circulation model output showed that the northward mean flow, called the MUC by past studies, was too weak to advect water to the north. The Mindanao Current during 2011–2012 was weaker than during 1999–2002 because the sea surface height in the Philippine Sea during 2011–2012 was lower than that during 1999–2002. Intraseasonal variability with periods of 50–100 days was observed at the mooring sites, comparable to previous observations during 1999–2002. Westward signal propagations were observed with periods and speeds of 50 days and 0.20m s−1 at 300m depth, and of 60–72 days and 0.11–0.14m s−1 at 960m depth. This article is protected by copyright. All rights reserved.
      PubDate: 2015-03-13T21:10:27.457726-05:
      DOI: 10.1002/2015JC010703
  • Interdecadal decrease of the Oyashio transport on the continental slope
           off the southeastern coast of Hokkaido, Japan
    • Authors: Hiroshi Kuroda; Taku Wagawa, Yugo Shimizu, Shin‐ichi Ito, Shigeho Kakehi, Takashi Okunishi, Sosuke Ohno, Akira Kusaka
      Abstract: We investigated the interdecadal trend of Oyashio velocity and transport during 1993–2011 based mainly on linear trend analysis of altimetry and in situ temperature‐salinity data from a monitoring line (“A‐line”) off the southeastern Hokkaido coast. Significant trends of increasing sea level were detected on the continental slope, north of the Kuril‐Kamchatka Trench. Sea level anomaly data revealed a localized clockwise circulation centered near the trench, the suggestion being that the strength of the Oyashio on the slope and the offshore return flow had decreased. The Oyashio main stream seemed to have shifted from a nearshore to an offshore path. Steric heights estimated from the A‐line data exhibited an increasing trend north of the trench, where 50–80% of the increase was determined by halosteric components attributable to a trend of decreasing salinity in the subsurface. The trend of decreasing salinity was related to downward displacement of isohaline/isopycnal surfaces. The largest displacement was above the trench. Horizontal pressure gradients associated with southwestward flows on the slope were weakened. The Oyashio transport decreased by 8.9 Sv (106 m3 s−1) in 19 years. A mesoscale eddy analysis revealed that clockwise eddies appeared more frequently in recent years near the trench around the A‐line and could decrease the Oyashio transport. A baroclinic, long Rossby‐wave model also predicted that a large‐scale baroclinic response to the wind stress could weaken the Oyashio velocity in the upper layer. Dynamical linkage between the localized eddies and large‐scale response remains to be clarified in future work. This article is protected by copyright. All rights reserved.
      PubDate: 2015-03-13T10:49:40.330281-05:
      DOI: 10.1002/2014JC010402
  • Strengthening Kuroshio observed at its origin during November 2010 to
           October 2012
    • Authors: Zhaohui Chen; Lixin Wu, Bo Qiu, Lei Li, Dunxin Hu, Chengyan Liu, Fan Jia, Xi Liang
      Abstract: Direct measurements of Kuroshio at its origin (18°N, east of the Luzon Island) are conducted from November 2010 to October 2012. It is found that the depth‐averaged Kuroshio between 200–700 m has increased over 15cm s−1 during the two‐year observational period and it is accompanied by the pronounced southward shift of the North Equatorial Current (NEC) bifurcation. Further analysis indicates that the Kuroshio's strengthening is confined to the upstream segment east of the Luzon Island while the Kuroshio decreased as it passed the Luzon Strait due to a dipole‐like sea surface height (SSH) trend between 15°N and 23°N. It is demonstrated that the two‐year strengthening of the Kuroshio, as well as the dipole‐like SSH trend can be adequately reproduced by a 1.5‐layer non‐linear reduced gravity model, suggesting an important role of upper ocean response to low‐frequency wind forcing in the western Pacific. Salinity at 500‐m depth is also found to increase during the concurrent 2 years. This subthermocline salinity increase is a combined outcome of vertical (basin‐scale isopycnal surface movement) and horizontal advections (i.e., strengthened Kuroshio) due to changes in the large‐scale wind‐driven ocean circulation. This article is protected by copyright. All rights reserved.
      PubDate: 2015-03-13T10:27:03.128493-05:
      DOI: 10.1002/2014JC010590
  • Frontal structures associated with coastal upwelling and ice‐edge
           subduction events in southern Beaufort Sea during the Canadian Arctic
           Shelf Exchange Study
    • Authors: Caroline Sévigny; Yves Gratton, Peter S. Galbraith
      Abstract: The near‐surface temperature structure in the southeastern Beaufort Sea is shown to have been largely dependent on frontal dynamics in spring 2004, which may be typical for the region. Easterly wind events generated coastal upwelling along the Cap Bathurst peninsula; a recurring event in that area. Further west, a large mesoscale anticyclone simultaneously developed and subsequently controlled the sea surface circulation in the central Amundsen Gulf. Sharp temperature and density fronts were created at the surface at both eastern and western ends of the domain. Sampling north of Cape Bathurst and Cape Parry showed evidence of frontal intensification. Frontal features were detected near the 50–200‐m isobaths, at the mouth of the gulf, where density‐compensated near‐surface intrusions driven by agesotrophic vertical circulation were identified. These warm water tongues intruded into the outcropping isopycnal layers, which dipped down between 5 and 25 m over the Mackenzie Shelf. They then crossed the density surfaces with an inverse slope consistent with N/f as predicted for quasi‐geostrophic flows. The front event ended prior to the breakup of the landfast‐ice bridge in late June with sea‐surface temperature undergoing quick and widespread changes throughout the Amundsen Gulf. This article is protected by copyright. All rights reserved.
      PubDate: 2015-03-13T10:12:33.273992-05:
      DOI: 10.1002/2014JC010641
  • Advection of North Atlantic deep water from the Labrador Sea to the
           Southern Hemisphere
    • Authors: Monika Rhein; Dagmar Kieke, Reiner Steinfeldt
      Abstract: Recently formed Labrador Sea Water (LSW) and overflow water from Denmark Strait (DSOW) are main components of the Atlantic Meridional Overturning Circulation. Both exhibit a distinct chlorofluorocarbon (CFC) maximum. Here, we use 25 years of CFC observations in the Atlantic to study the main features of the circulation of LSW and DSOW. From the CFC data, the age and fraction of young deep water are inferred. Due to the superior spatial data resolution compared to former attempts, regional differences in the spreading velocity and pathways of young deep water become evident, dependent on the regional circulation. The observed distributions of young LSW and DSOW showed that the DWBC is the fastest pathway to reach the southern hemisphere. The downstream decrease of the fractions of young LSW in the DWBC is slower compared to model studies. From 47°N to 42°N, DWBC transports of young LSW and DSOW decrease by 44% and 49%, respectively. At 26°N, the DWBC transport of young water is still 39% of the LSW formation rate and 44% of the DSOW overflow transport. Interior pathways also exist, especially in the subpolar North Atlantic and in the transition zone between the subpolar and subtropical gyre. Compared to DSOW, the distributions indicate a higher tendency for LSW to follow additional interior pathways. North of 45°N the major part of LSW is younger than 20 years. The general weakening of new LSW formation since the 1990s worked towards a homogenization between the LSW in the western and the eastern subpolar North Atlantic. This article is protected by copyright. All rights reserved.
      PubDate: 2015-03-11T01:49:38.477365-05:
      DOI: 10.1002/2014JC010605
  • Seasonal variability of alongshore winds and sea surface temperature
           fronts in Eastern Boundary Current Systems
    • Authors: Yuntao Wang; Renato M. Castelao, Yeping Yuan
      Abstract: Seven years of satellite observations (2003–2009) are used to describe variability in sea surface temperature (SST) fronts and in the alongshore component of ocean winds, and to investigate their relations in four Eastern Boundary Current Systems (EBCS). The general patterns of SST frontal activity are remarkably similar in all EBCS, with high frontal probabilities along the coast decreasing with distance from the coastline. Results from empirical orthogonal function decompositions reveal that the seasonal evolution of SST fronts and wind stress are significantly correlated, with intensified upwelling favorable winds associated with an increase in frontal probabilities. An offshore migration of the region of high frontal activity is observed during the period of upwelling favorable alongshore wind stress in EBCS. In all regions, the seasonal variability of frontal activity and wind stress is stronger at mid than at low latitudes. The width of the region of high frontal activity is relatively broader in the California and Benguela Current Systems, and narrower in the Canary and Humboldt Current Systems. The width of the band of high frontal activity may be influenced by multiple factors, including wind forcing, flow topography interactions and mesoscale dynamics. While seasonal variability in frontal activity in the California Current System acts to reinforce or weaken the average pattern, they are substantially different in the Canary Current System, where there is little overlap in the areas characterized by persistent and seasonally varying front activity. This article is protected by copyright. All rights reserved.
      PubDate: 2015-03-11T01:48:50.221928-05:
      DOI: 10.1002/2014JC010379
  • Local and remote wind stress forcing of the seasonal variability of the
           Atlantic Meridional Overturning Circulation (AMOC) transport at
    • Authors: Jiayan Yang
      Abstract: The transport of the Atlantic Meridional Overturning Circulation (AMOC) varies considerably on the seasonal time scale at 26.5˚N according to observations made at the RAPID‐MOCHA array. Previous studies indicate that the local wind stress at 26.5˚N, especially a large wind‐stress curl at the African coast, is the leading contributor to this seasonal variability. The purpose of the present study is to examine whether non‐local wind stress forcing, i.e., remote forcing from latitudes away from 26.5˚N, affects the seasonal AMOC variability at the RAPID‐MOCHA array. Our tool is a two‐layer and wind‐driven model with a realistic topography and an observation‐derived wind stress. The seasonal cycle of the modeled AMOC transport agrees well with RAPID‐MOCHA observations while the amplitude is in the lower end of the observational range. In contrast to previous studies, the seasonal AMOC variability at 26.5˚N is not primarily forced by the wind‐stress curl at the eastern boundary, but is a result of a basin‐wide adjustment of ocean circulation to seasonal changes in wind stress. Both the amplitude and phase of the seasonal cycle at 26.5˚N are strongly influenced by wind‐stress forcing from other latitudes, especially from the subpolar North Atlantic. The seasonal variability of the AMOC transport at 26.5˚N is due to the seasonal re‐distribution of the water‐mass volume and is driven by both local and remote wind stress. Barotropic processes make significant contributions to the seasonal AMOC variability through topography‐gyre interactions. This article is protected by copyright. All rights reserved.
      PubDate: 2015-03-11T01:40:20.201194-05:
      DOI: 10.1002/2014JC010317
  • A mechanistic semi‐analytical method for remotely sensing sea
           surface pCO2 in river‐dominated coastal oceans: A case study from
           the East China Sea
    • Authors: Yan Bai; Wei‐Jun Cai, Xianqiang He, Weidong Zhai, Delu Pan, Minhan Dai, Peisong Yu
      Abstract: While satellite remote sensing has become a very useful tool contributing to assessments of sea surface partial pressure of carbon dioxide (pCO2) that subsequently allow quantification of air‐sea CO2 flux, the application of empirical approaches in coastal oceans has proven challenging owing to the interaction of multiple controlling factors. We propose a “mechanistic semi‐analytic algorithm” (MeSAA) to estimate sea surface pCO2 in river‐dominated coastal oceans using satellite data. Observed pCO2 can be analytically expressed as the sum of individual components controlled by major factors such as thermodynamics (or temperature), mixing, and biology. With marine carbonate system calculations, temperature and mixing effects can be predicted using thermodynamic principles and by assuming conservative two end‐member mixing of total dissolved inorganic carbon and total alkalinity (e.g. the Changjiang River and Kuroshio water in the East China Sea, ECS). Next, an integral expression for pCO2 drawdown due to biological effects can be parameterized using the chlorophyll a concentration (chla). We demonstrate the validity and applicability of the algorithm in the ECS during summertime. Sensitivity analysis shows that errors in empirical coefficients and three input satellite parameters (salinity, SST, chla) have limited influence on the algorithm, and satellite‐derived pCO2 is consistent with underway data, even though no in situ pCO2 data from the ECS shelves was used to train the algorithm. Our algorithm has more physical and biogeochemical mechanistic meaning than empirical methods, and should be applicable to other similar systems. This article is protected by copyright. All rights reserved.
      PubDate: 2015-03-02T14:16:29.790285-05:
      DOI: 10.1002/2014JC010632
  • Assessment of the upper‐ocean mixed layer parameterizations using a
           large eddy simulation model
    • Authors: Naoki Furuichi; Toshiyuki Hibiya
      Abstract: A large eddy simulation (LES) of the upper ocean mixed layer processes in the winter northwestern Pacific is carried out and compared with concurrent microstructure measurements. We find that dissipation rates of turbulent kinetic energy (ε) and temperature variance from LES agree well with field observations in the areas where the Ozmidov length scale, calculated from the modeled ε and buoyancy frequency, exceeds the grid size of LES. It is also found that, even though the Ozmidov length scale is less than the grid size of LES, model estimates of the vertical turbulent heat flux near the base of the mixed layer are very similar to observed values, suggesting that this LES model reproduces the intensity of entrainment well. This enables us to use the results from LES to assess the turbulence closure models of Mellor and Yamada [1982, MY] and Nakanishi and Niino [2009, NN] for several forcing scenarios consisting of strong winds, surface heating, and surface cooling. It is found that NN performs better than MY in reproducing the results from LES for each forcing scenario, particularly when the turbulent length scale is adjusted to be more restricted by density stratification. This article is protected by copyright. All rights reserved.
      PubDate: 2015-03-02T14:16:19.774068-05:
      DOI: 10.1002/2014JC010665
  • Response of Arctic Ocean stratification to changing river runoff in a
           column model
    • Authors: Aleksi Nummelin; Camille Li, Lars H. Smedsrud
      Abstract: A 1‐dimensional model of the atmosphere‐ice‐ocean column is used to study the effects of changing river runoff to the Arctic Ocean. River runoff is the largest contributor of freshwater to the Arctic and is expected to increase as the hydrological cycle accelerates due to global warming. The column model simulates the stratification of the Arctic Ocean reasonably well, capturing important features such as the fresh surface layer, the salty cold halocline, and the temperature maximum within the Atlantic Water layer. The model is run for 500 years with prescribed boundary conditions to reach steady‐state solutions. Increasing river runoff is found to strengthen the stratification and to produce a fresher and shallower surface mixed layer with warming (up to ∼1C for a doubling of present day runoff) in the Atlantic Water layer below. An important consequence is that the effect of the larger vertical temperature gradient is able to balance that of the stronger stratification and yield a close to constant vertical heat flux towards the surface. As a result the sea ice response is small, showing only slight increase (up to ∼15cm for a doubling of present day runoff) in annual mean ice thickness. Limitations of the study include the idealized nature of the column model and uncertainties in the background vertical mixing within the Arctic Ocean. This article is protected by copyright. All rights reserved.
      PubDate: 2015-03-02T14:15:32.858082-05:
      DOI: 10.1002/2014JC010571
  • Argo data assimilation and its effect on climate state estimation and
           forecasting in the western North Pacific using a coupled model
    • Authors: Shiro Nishikawa; Yoichi Ishikawa, Shuhei Masuda, Yoshihisa Hiyoshi, Yuji Sasaki, Hiromichi Igarashi
      Abstract: In this study, we investigated the effects of ocean subsurface data (Argo data) on climate state estimation and forecasting, focusing on the reproduction of North Pacific subtropical mode water (STMW) using a four‐dimensional variational data assimilation system with a coupled model. We produced two reanalysis plus forecast datasets for the ocean and atmosphere in 2010 using a 3‐month assimilation period: the first including Argo data (Argo case) and the second did not include Argo data (control case). In the control case, the Kuroshio, Kuroshio Extension front, and recirculation gyres along the front were not adequately reproduced. Consequently, there were large biases in temperature and salinity in the western North Pacific. The assimilation of Argo data effectively corrected these biases and significantly improved reproduction of the Kuroshio fronts and recirculation gyres, resulting in a more realistic reproduction of the winter mixed layer and STMW. The correction of these biases is critical to the 1–3‐year predictions of the STMW core properties, and the assimilation of Argo data enables prediction of these properties for more than a year. We showed that assimilation of Argo data affects the surface atmospheric temperature above the STMW formation region. This article is protected by copyright. All rights reserved.
      PubDate: 2015-03-02T14:11:47.290814-05:
      DOI: 10.1002/2014JC010095
  • Recent accelerated warming of the continental shelf off New Jersey:
           Observations from the CMV Oleander XBT line
    • Authors: J. Forsyth; M. Andres, G.G. Gawarkiewicz
      Abstract: Expendable bathythermographs (XBTs) have been launched along a repeat track from New Jersey to Bermuda from the CMV Oleander through the NOAA/NEFSC Ship of Opportunity Program about 14 times per year since 1977. The XBT temperatures on the Middle Atlantic Bight (MAB) shelf are binned with 10km horizontal and 5m vertical resolution to produce monthly, seasonally and annually‐averaged cross‐shelf temperature sections. The depth‐averaged shelf temperature, Ts, calculated from annually‐averaged sections that are spatially‐averaged across the shelf, increases at 0.026 ± 0.001 ˚C yr−1 from 1977‐2013, with the recent trend substantially larger than the overall 37‐year trend (0.11 ± 0.02 ˚C yr−1 since 2002). The Oleander temperature sections suggest that the recent acceleration in warming on the shelf is not confined to the surface, but occurs throughout the water column with some contribution from interactions between the shelf and the adjacent Slope Sea reflected in cross‐shelf motions of the shelfbreak front. The local warming on the shelf cannot explain the region's amplified rate of sea level rise relative to the global mean. Additionally, Ts exhibits significant interannual variability with the warmest anomalies increasing in intensity over the 37‐year record even as the cold anomalies remain relatively uniform throughout the record. Ts anomalies are not correlated with annually‐averaged coastal sea level anomalies at zero lag. However, positive correlation is found between 2‐year lagged Ts anomalies and coastal sea level anomalies, suggesting that the region's sea level anomalies may serve as a predictor of shelf temperature. This article is protected by copyright. All rights reserved.
      PubDate: 2015-03-02T14:11:36.77134-05:0
      DOI: 10.1002/2014JC010516
  • Bottom‐pressure signature of annual baroclinic Rossby waves in the
           northeast tropical Pacific Ocean
    • Authors: Christopher G. Piecuch
      Abstract: The annual cycle in bottom pressure (pb) in the northeast tropical Pacific Ocean (NTPO) is studied. Focus is on a zonal section along 12∘N between 105–145∘W that is characterized by a strong annual cycle in sea level from satellite altimetry. Estimates of pb from the Gravity Recovery and Climate Experiment (GRACE), a state estimate produced by the Estimating the Circulation and Climate of the Ocean (ECCO) consortium, and a linear Rossby wave model (LRWM) are used. The GRACE NTPO pb annual cycle shows amplitudes as large as 1cm water equivalent. The GRACE data also evidence westward propagation that is consistent with the behavior of long mode‐1 Rossby waves at this latitude, with phase increasing from east to west at a rate of ∼0.34m s– 1. The ECCO and LRWM pb estimates corroborate the notion that GRACE reveals the pb signature of annual Rossby waves driven by interior wind stress curl and possibly damped by frictional processes. Results have implications for attempts to constrain global ocean mass using a single point mooring. This article is protected by copyright. All rights reserved.
      PubDate: 2015-03-02T14:11:04.102414-05:
      DOI: 10.1002/2014JC010667
  • Turbulent nitrate fluxes in the Lower St. Lawrence Estuary (Canada)
    • Authors: Frédéric Cyr; Daniel Bourgault, Peter S. Galbraith, Michel Gosselin
      Abstract: Turbulent vertical nitrate fluxes were calculated using new turbulent microstructure observations in the Lower St. Lawrence Estuary (LSLE), Canada. Two stations were compared: the head of the Laurentian Channel (HLC), where intense mixing occurs on the shallow sill that marks the upstream limit of the LSLE, and another station located about 100 km downstream (St. 23), more representative of the LSLE mean mixing conditions. Mean turbulent diffusivities and nitrate fluxes at the base of the surface layer for both stations were respectively (with 95% confidence intervals): ,, , and . Observations suggest that the interplay between large isopleth heaving near the sill and strong turbulence is the key mechanism to sustain such high turbulent nitrate fluxes at the HLC (two to three orders of magnitude higher than those at Station 23). Calculations also suggest that nitrate fluxes at the HLC alone can sustain primary production rates of 3.4(0.6, 11) g Cm−2 mo−1 over the whole LSLE, approximately enough to account for a large part of the phytoplankton bloom and for most of the post‐bloom production. Surfacing nitrates are also believed to be consumed within the LSLE, not leaving much to be exported to the rest of the Gulf of St. Lawrence. This article is protected by copyright. All rights reserved.
      PubDate: 2015-02-26T10:46:32.186764-05:
      DOI: 10.1002/2014JC010272
  • Along‐isopycnal variability of spice in the North Pacific
    • Authors: Jody M. Klymak; William Crawford, Matthew H. Alford, Jennifer A. MacKinnon, Robert Pinkel
      Abstract: Two hydrographic surveys in the Gulf of Alaska and the North Pacific subtropical gyre are presented. Both surveys are roughly perpendicular to lateral temperature gradients, and were collected in the summer when there was a shallow mixed layer and a seasonal thermocline. Isopycnal displacements and horizontal velocities are dominated by internal waves. Spice anomalies along isopycnals are examined to diagnose lateral stirring mechanisms. The spectra of spice anomaly gradients along near‐surface isopycnals roughly follow power laws of (variance spectra power laws of ), and in most cases the spectra become redder at depth. The near‐surface spectra are possibly consistent with the predictions of quasigeostrophic turbulence theory (when surface buoyancy effects are accounted for), but the spectra at depth are inconsistent with any quasi‐geostrophic theory. Probability distributions of spice gradients exhibit a large peak at low gradients and long tails for large gradients, symptomatic of fronts. Vertical coherence of the spice signal falls off with a decorrelation depth scale that has a maximum of about 80 m at 100‐ km wavelengths and depends on horizontal wavelength with a power law of approximately . Lateral decorrelation length scales are 20‐40 km, close to the baroclinic Rossby radius. Lateral stirring occurs over large scales, with average lateral displacements of about 200 km in the upper 75 m, decreasing to 100 km at greater depths. The depth variation of the statistics indicates that time history of tracer stirring on each isopycnal is important, or that there are unconsidered depth‐dependent stirring mechanisms. This article is protected by copyright. All rights reserved.
      PubDate: 2015-02-25T07:49:55.731256-05:
      DOI: 10.1002/2013JC009421
  • Characteristics of the meltwater field from a large Antarctic iceberg
           using δ18O
    • Authors: John J. Helly; Maria Vernet, Alison Murray, Gordon R. Stephenson
      Abstract: Large tabular icebergs represent a disruptive influence on a stable water column when drifting in the open ocean. This is a study of one iceberg, C18A, encountered in the Powell Basin in the Weddell Sea in March, 2009, formed from iceberg C18 (76 × 7 km) originating from the Ross Ice Shelf in May, 2002. C18A was lunate in shape with longest dimensions of 31 km × 7 km × 184 m. The meltwater field from C18A was characterized using δ18O from water samples collected near C18A (Near‐field, 0.4‐2 km) and contrasted with a Far‐field comprised of samples from an Away site (19 km from C18A), a Control site (70 km away) and a region populated with small icebergs (Iceberg Alley, 175 km away). The in‐sample fractions of meteoric water were calculated relative δ18O in iceberg ice and Weddell Deep Water and converted to meteoric water height (m) and a percentage within 100 m depth‐bins. The the Near‐field and Far‐fielddifference from surface to 200 m was 0.51±0.28%. The concentration of meteoric water dropped to approximately half that value below 200 m, approximate keel depth of the iceberg, although detectable to 600 m. From surface to 600 m, the overall difference was statistically significant (P  
      PubDate: 2015-02-25T06:24:19.651993-05:
      DOI: 10.1002/2015JC010772
  • Intraseasonal sea surface salinity variability in the equatorial
           Indo‐Pacific Ocean induced by Madden‐Julian oscillations
    • Authors: Yuanlong Li; Weiqing Han, Tong Lee
      Abstract: Intraseasonal sea surface salinity (SSS) variability in the equatorial Indo‐Pacific Ocean is investigated using the Aquarius/SAC‐D satellite measurements and Hybrid Coordinate Ocean Model (HYCOM). Large‐scale SSS variations at 20‐90 day timescales induced by Madden‐Julian oscillations (MJOs) are prominent in the central‐to‐eastern Indian Ocean (IO) and western Pacific Ocean (PO) with a standard deviation of ∼0.15 psu. The relationship between SSS anomaly and freshwater flux is nearly in phase in the central‐to‐eastern IO and out of phase in the western PO during a MJO cycle. A series of HYCOM experiments are performed to explore the causes for SSS variability. In most areas of the equatorial Indo‐Pacific Ocean, wind stress‐forced ocean dynamical processes act as the main driver of intraseasonal SSS, while precipitation plays a secondary role. In some areas of the western PO and western IO, however, precipitation effect is the leading contributor. In comparison, evaporation effect induced by radiation and wind speed changes is generally much smaller. Besides the external forcing by MJOs, ocean internal variability can also cause considerable intraseasonal SSS changes, explaining 10%‐20% of the total variance in some regions. Composite analysis for MJO events reveals that the effects of wind stress, precipitation, and evaporation vary at different phases of a MJO cycle. The MJO‐induced SSS signature is the result of complicated superimposition and interaction of these effects. The effect of wind stress also varies significantly from event to event. It affects SSS variability primarily through horizontal ocean current advection and to a lesser extent through vertical entrainment. This article is protected by copyright. All rights reserved.
      PubDate: 2015-02-25T06:07:09.427211-05:
      DOI: 10.1002/2014JC010647
  • Thermodynamic sea ice growth in the central Weddell Sea, observed in
           upward‐looking sonar data
    • Authors: A. Behrendt; W. Dierking, H. Witte
      Abstract: Upward‐looking sonar (ULS) data were used to analyse thermodynamic sea ice growth. The study was carried out for an ocean region in the central Weddell Sea, for which data of sea ice thickness variability and of the oceanic heat flux through the ice are rare. In the study area the contribution of sea ice deformation to vertical ice growth is relatively small in some years. This provides the opportunity to simulate thermodynamic sea ice growth considering the influence of a snow cover and of the oceanic heat flux. To this end, a modified version of Stefan's Law was used. The resulting ice thickness variations were then compared with the ULS measurements. For the investigated cases, the best consistency between data and model results was obtained assuming a snow layer of less than 5cm thickness and average oceanic heat fluxes between 6 and 14 W m−2. It is demonstrated that in conjunction with ice drift data and analytical models for thermal sea ice growth, ULS ice thickness measurements are useful for studying the seasonal cycle of growth and decay, and for inferring the magnitude of the average oceanic heat flux under sea ice. This article is protected by copyright. All rights reserved.
      PubDate: 2015-02-25T03:47:43.076434-05:
      DOI: 10.1002/2014JC010408
  • Investigation of interbasin exchange and interannual variability in Lake
           Erie using an unstructured‐grid hydrodynamic model
    • Authors: Qianru Niu; Meng Xia, Edwards Rutherford, Doran Mason, Eric J. Anderson, David J. Schwab
      Abstract: Inter‐basin exchange and interannual variability in Lake Erie's three basins are investigated with the help of a three‐dimensional unstructured‐grid based Finite Volume Coastal Ocean Model (FVCOM). Experiments were carried out to investigate the influence of grid resolutions and different sources of wind forcing on the lake dynamics. Based on the calibrated model, we investigated the sensitivity of lake dynamics to major external forcing, and seasonal climatological circulation patterns are presented and compared with the observational data and existing model results. It was found that water exchange between the western basin (WB) and the central basin (CB) was mainly driven by hydraulic and density‐driven flows, while density‐driven flows dominate the interaction between the CB and the eastern basin (EB). River‐induced hydraulic flows magnify the eastward water exchange and impede the westward one. Surface wind forcing shifts the pathway of hydraulic flows in the WB, determines the gyre pattern in the CB, contributes to thermal mixing, and magnifies inter‐basin water exchange during winter. Interannual variability is mainly driven by the differences in atmospheric forcing, and is most prominent in the CB. This article is protected by copyright. All rights reserved.
      PubDate: 2015-02-20T16:08:22.332516-05:
      DOI: 10.1002/2014JC010457
  • On the eastward shift of winter surface chlorophyll‐a bloom peak in
           the Bay of Bengal
    • Authors: M. V. Martin; C. Shaji
      Abstract: Analysis of satellite‐derived surface layer phytoplankton chlorophyll‐a concentration (Chl‐a) in the Bay of Bengal revealed an eastward shift of peak of Chl‐a bloom during northeast monsoon period (November‐March) from western to the eastern Bay. The winter Chl‐a bloom in the western Bay is shorter duration (∼1‐2 months) while that in the eastern Bay is longer duration (∼3‐4 months). Unlike other oceans, the eastward bloom shift found in the Bay of Bengal is opposite to the direction of propagation of Rossby wave and seasonal mean zonal currents. During winter, sufficient light is available in the Bay of Bengal for phytoplankton growth. Herein, we studied the processes controlling the eastward winter bloom shift and the zonal depiction of bloom duration in the Bay of Bengal. There is no single dominant process that drives the eastward bloom shift everywhere in the Bay of Bengal. Rather, different physical processes operate in different regions. The physical mechanisms governing the eastward winter Chl‐a bloom shift reported here include open ocean Ekman pumping, coastal upwelling, upwelling driven by Rossby wave, wind induced vertical mixing, nutrients supply from river input, and westward advection through prevailing zonal currents. This article is protected by copyright. All rights reserved.
      PubDate: 2015-02-20T16:08:11.760274-05:
      DOI: 10.1002/2014JC010162
  • Shoreline variability from days to decades: Results of long‐term
           video imaging
    • Authors: C. Pianca; R. Holman, E. Siegle
      Abstract: The present work characterizes the time‐space scales of variability and forcing dependencies of a unique 26‐year record of daily to hourly shoreline data from a steep beach at Duck, North Carolina. Shoreline positions over a 1500m alongshore span were estimated using a new algorithm called ASLIM based on fitting the band of high light intensity in time exposure images to a local Gaussian fit, with a subsequent Kalman filter to reduce noise and uncertainty. Our findings revealed that the shoreline change at long times scales dominates seasonal variability, despite that wave forcing had only 2\% variance at inter‐annual frequencies. The shoreline response presented 66\% of the variance at inter‐annual scales. These results were not expected since from wave forcing it would have expected that the shoreline response should similarly lack inter‐annual variability, but we found it to be dominated by this scale. The alongshore‐mean shoreline time series revealed no significant annual cycle. However, there are annual oscillations in the shoreline response that are coherent with wave forcing and deserves further explanations. The pier was found to have a significant influence on shoreline behavior since restricts the seasonal longshore transport between the sides, resulting in a seasonally‐reversing sediment accumulation. Thus, there is a significant annual peak in shoreline variability that is coherent with the annual forcing but becomes insignificant in the longshore‐average. This article is protected by copyright. All rights reserved.
      PubDate: 2015-02-20T16:05:30.520333-05:
      DOI: 10.1002/2014JC010329
  • A vertical gradient of nitrous oxide below the subsurface of the Canada
           Basin and its formation mechanisms
    • Authors: Liyang Zhan; Liqi Chen, Jiexia Zhang, Yuhong Li
      Abstract: The ocean is regarded as a significant source of N2O, which is an ozone depleting greenhouse gas. However, the contribution of the Arctic Ocean to the global N2O budget is not yet known. Herein, the first observations of N2O concentrations in the Canada Basin (CB) and Greenland Sea Basin (GSB) are presented. A correlation between the historic atmospheric N2O record and N2O concentrations at the corresponding depth in the GSB suggests that the N2O distribution pattern is dominated by air‐sea exchanges and hydrographic processes in this region. The consistency between the observed N2O concentrations in the CB and calculated results based on the above correlation suggest that the N2O concentrations in the CB are most likely dominated by N2O dynamics and subsequent hydrographic processes in the sea adjacent to the GSB. The N2O concentration in the Canada Basin Intermediate Water (CBIW) reflects anthropogenic influences, whereas the N2O concentration in the Canada Basin Deep Water (CBDW) suggests that the CBDW may be a preindustrial “relict”. This article is protected by copyright. All rights reserved.
      PubDate: 2015-02-20T16:05:14.978946-05:
      DOI: 10.1002/2014JC010337
  • Spectral characterization of fine‐scale wind waves using shipboard
           optical polarimetry
    • Authors: Nathan J. M. Laxague; Brian K. Haus, Darek Bogucki, Tamay Özgökmen
      Abstract: Fine‐scale sea surface waves are of profound importance to a number of air‐sea interaction processes. Due to a number of reasons, there exists a great degree of difficulty in obtaining quality in situ observations of these waves. This paper presents the application of a shipboard wave‐sensing method towards the following quantifications: regime‐specific contribution to sea surface slope and sensitivity to wind speed increases. Measurements were made via polarimetric camera, resolving waves with wavelengths ranging from 0.21 m to 0.003 m (30 rad/m 
      PubDate: 2015-02-20T03:54:30.756186-05:
      DOI: 10.1002/2014JC010403
  • Ocean salinity changes in the northwest Pacific subtropical gyre: The
           quasi‐decadal oscillation and the freshening trend
    • Authors: Feng Nan; Fei Yu, Huijie Xue, Ran Wang, Guangcheng Si
      Abstract: Ocean salinity changes play an important role in modulating ocean and climate variability. Analyses of the repeating observations along PN, TK, and 137º E sections reveal that both surface and subsurface salinity in the Northwest Pacific Subtropical Gyre (NPSG) had clear quasi‐decadal oscillation (QDO) of ∼10‐year and a sustained freshening trend during 1987‐2012. Surface salinity in the NPSG troughed in 1989, 1999, and 2008, and peaked in 1993, and 2003. The peak‐to‐tough range of surface salinity oscillation can reach 0.3 psu. Meanwhile, surface salinity decreased about 0.10 psu from 1987 to 2012 with a freshening trend of ‐0.0042 psu yr−1. These surface salinity anomalies were subducted into the subsurface layer mainly in the ventilated zone along the Kuroshio Extension, and advected over the NPSG. The QDO of the subsurface salinity maximum (Smax) lagged that of the surface salinity by about 1∼2 years. Both the peak‐to‐tough range of the Smax oscillation (0.15 psu) and its freshening trend (‐0.0036 psu yr−1) are smaller than those of the surface salinity. Salinity changes in the NPSG likely began in the mid‐1970s associated with the North Pacific regime shift during 1976/77. Analyses of mixed layer salinity budget indicated that air‐sea freshwater flux change in the NPSG is the leading factor controlling the surface salinity anomalies, while change of large‐scale ocean circulation (Geostrophic advection) also plays a minor role. Salinity and air‐sea freshwater flux changes in the NPSG are all closely related to the Pacific Decadal Oscillation, indicating the large‐scale ocean‐atmosphere interaction. This article is protected by copyright. All rights reserved.
      PubDate: 2015-02-20T03:41:34.342954-05:
      DOI: 10.1002/2014JC010536
  • Short wind waves on the ocean: Wave number‐frequency spectra
    • Authors: William J. Plant
      Abstract: Dominant surface waves on the ocean exhibit a dispersion relation that confines their energy to a curve in a wavenumber‐frequency spectrum. Short wind waves on the ocean, on the other hand, are advected by these dominant waves so that they do not exhibit a well defined dispersion relation over many realizations of the surface. Here we show that the short‐wave analog to the dispersion relation is a distributed spectrum in the wavenumber‐frequency plane that collapses to the standard dispersion relation in the absence of long waves. We compute probability distributions of short‐wave wavenumber given a [frequency, direction] pair and of short‐wave frequency given a [wavenumber, direction] pair. These two probability distributions must yield a single spectrum of surface displacements as a function of wavenumber and frequency, F(k,f). We show that the folded, azimuthally averaged version of this spectrum has a “butterfly” pattern in the wavenumber‐frequency plane if significant long waves are present. Integration of this spectrum over frequency yields the well‐known k−3 wavenumber spectrum. When integrated over wavenumber, the spectrum yields an f−4 form that agrees with measurement. We also show that a cut through the unfolded F(k,f) at constant k produces the well‐known form of moderate‐incidence‐angle Doppler spectra for electromagnetic scattering from the sea. This development points out the dependence of the short‐wave spectrum on the amplitude of the long waves. This article is protected by copyright. All rights reserved.
      PubDate: 2015-02-20T00:05:28.484172-05:
      DOI: 10.1002/2014JC010586
  • Summer upwelling and thermal fronts in the northwestern South China Sea:
           Observational analysis of two mesoscale mapping surveys
    • Authors: Zhiyou Jing; Yiquan Qi, Yan Du, Shuwen Zhang, Lingling Xie
      Abstract: Persistent coastal upwelling and upwelling‐induced thermal fronts in the northwestern South China Sea are investigated using satellite measurements, two intensive mesoscale mapping surveys and three bottom‐mounted ADCPs. The results indicate that pronounced surface cooling and upwelling‐related fronts with a width of 20‐50 km occur around Hainan Island and persist through the summer upwelling season. Driven by the prevailing southwesterly monsoon, the subsurface cooling band is ~6°C colder than the water offshore of the East Coast, where the thermal gradients are generally more than 0.1°C/km. The cold and nutrient‐rich coastal water is identified to be derived primarily from the deep water of the outer shelf. At the same time, the spatial structure of the upwelling and thermal front, as well as the upwelling‐related coastal currents, are significantly regulated by wind forcing. A prominent lagged correlation between the moored temperature records and alongshore wind stress is detected in the East Coast. The correlation coefficient is ‐0.8 with the temperature lagging behind wind stress by 2.2 days, indicating that the cooling band off the East Coast is dominated mostly by the alongshore southwesterly monsoon during the upwelling season. This article is protected by copyright. All rights reserved.
      PubDate: 2015-02-19T10:47:56.138641-05:
      DOI: 10.1002/2014JC010601
  • On the coefficients of small eddy and surface divergence models for the
           air‐water gas transfer velocity
    • Authors: Binbin Wang; Qian Liao, Joseph H. Fillingham, Harvey A. Bootsma
      Abstract: Recent studies suggested that under low to moderate wind conditions without bubble entraining wave breaking, the air‐water gas transfer velocity k+ can be mechanistically parameterized by the near‐surface turbulence, following the small eddy model (SEM). Field measurements have supported this model in a variety of environmental forcing systems. Alternatively, surface divergence model (SDM) has also been shown to predict the gas transfer velocity across the air‐water interface in laboratory settings. However, the empirically determined model coefficients (α in SEM and c1 in SDM) scattered over a wide range. Here we present the first field measurement of the near‐surface turbulence with a novel floating PIV system on Lake Michigan, which allows us to evaluate the SEM and SDM in situ in the natural environment. k+ was derived from the CO2 flux that was measured simultaneously with a floating gas chamber. Measured results indicate that α and c1 are not universal constants. Regression analysis showed that α ~ log(ε) while the near surface turbulence dissipation rate ε is approximately greater than 10–6 m2s–3 according to data measured for this study as well as from other published results measured in similar environments or in laboratory settings. It also showed that α scales linearly with the turbulent Reynolds number. Similarly, coefficient c1 in the SDM was found to linearly scale with the Reynolds number. These findings suggest that larger eddies are also important parameters, and the dissipation rate in the SEM or the surface divergence β’ in the SDM alone may not be adequate to determine k+ completely. This article is protected by copyright. All rights reserved.
      PubDate: 2015-02-19T10:47:12.311951-05:
      DOI: 10.1002/2014JC010253
  • Physical and biogeochemical mechanisms of internal carbon cycling in Lake
    • Authors: Darren J. Pilcher; Galen A. McKinley, Harvey A. Bootsma, Val Bennington
      Abstract: The lakewide seasonal carbon cycle of Lake Michigan is poorly quantified and lacks the mechanistic links necessary to determine impacts upon it from eutrophication, invasive species, and climate change. A first step towards a full appreciation of Lake Michigan's carbon cycle is to quantify the dominant mechanisms of its internal carbon cycle. To achieve this, we use the MIT general circulation model configured to the bathymetry of Lake Michigan and coupled to an ecosystem model to simulate the seasonal cycle of productivity, temperature, circulation, and the partial pressure of CO2 in water (pCO2). This biogeochemistry is designed to be appropriate for the pre‐quagga mussel state of the lake. The primary mechanism behind the seasonal cycle of primary productivity is lake physics. The offshore spring phytoplankton bloom begins following a reduction in deep vertical mixing and ends with the depletion of nutrients via thermal stratification. The exception is the western shoreline, where summer winds drive coastal upwelling, providing hypolimnetic nutrients and generating significant productivity. Surface pCO2 is controlled by the net effect from temperature on solubility, and is modulated by biological uptake of dissolved inorganic carbon (DIC) and isothermal mixing of DIC‐rich water in winter. Temperature tends to have the greatest seasonal impact in nearshore regions, while local DIC has the greatest impact in offshore regions. Lakewide, the model suggests that carbon is absorbed from the atmosphere during the spring bloom and released to the atmosphere during winter mixing and when summer surface temperatures are at their maximum. This article is protected by copyright. All rights reserved.
      PubDate: 2015-02-18T11:28:17.460134-05:
      DOI: 10.1002/2014JC010594
  • Structure of turbulence and sediment stratification in
           wave‐supported mud layers
    • Authors: A. Hooshmand; A. R. Horner‐Devine, M. P. Lamb
      Abstract: We present results from laboratory experiments in a wave flume with and without a sediment bed to investigate the turbulent structure and sediment dynamics of wave‐supported mud layers. The presence of sediment on the bed significantly alters the structure of the wave boundary layer relative to that observed in the absence of sediment, increasing the TKE by more than a factor of three at low wave orbital velocities and suppressing it at the highest velocities. The transition between the low and high velocity regimes occurs when Re∆ ≃ 450, where Re∆ is the Stokes Reynolds number. In the low velocity regime (Re∆ < 450) the flow is significantly influenced by the formation of ripples, which enhances the TKE and Reynolds stress, and increases the wave boundary layer thickness. In the high velocity regime (Re∆ > 450) the ripples are significantly smaller, the near‐bed sediment concentrations are significantly higher and density stratification due to sediment becomes important. In this regime the TKE and Reynolds stress are lower in the sediment bed runs than in comparable runs with no sediment. The regime transition at Re∆ = 450 appears to result from washout of the ripples and increased concentrations of fine sand suspended in the boundary layer, which increases the settling flux and the stratification near the bed. The increased stratification damps turbulence, especially near the top of the high concentration layer, reducing the layer thickness. We anticipate that these effects will influence the transport capacity of wave‐supported gravity currents on the continental shelf. This article is protected by copyright. All rights reserved.
      PubDate: 2015-02-18T11:27:54.955232-05:
      DOI: 10.1002/2014JC010231
  • Nutrient supply and biological response to wind‐induced mixing,
    • Authors: Shigeto Nishino; Yusuke Kawaguchi, Jun Inoue, Toru Hirawake, Amane Fujiwara, Ryosuke Futsuki, Jonaotraro Onodera, Michio Aoyama
      Abstract: A fixed‐point observation station was set up in the northern Chukchi Sea during autumn 2013, and for about 2 weeks conductivity‐temperature‐depth (CTD)/water samplings (6‐hourly) and microstructure turbulence measurements (two to three times a day) were performed. This enabled us to estimate vertical nutrient fluxes and the impact of different types of turbulent mixing on biological activity. There have been no such fixed‐point observations in this region, where incoming low‐salinity water from the Pacific Ocean, river water, and sea‐ice meltwater promote a strong pycnocline (halocline) that stabilizes the water column. Previous studies have suggested that because of the strong pycnocline wind‐induced ocean mixing could not change the stratification to impact biological activity. However, the present study indicates that a combined effect of an uplifted pycnocline accompanied by wind‐induced inertial motion and turbulent mixing caused by intense gale‐force winds (>10 m s‐1) did result in increases in upward nutrient fluxes, primary productivity, and phytoplankton biomass, particularly large phytoplankton such as diatoms. Convective mixing associated with internal waves around the pycnocline also increased the upward nutrient fluxes and might have an impact on biological activity there. For diatom production at the fixed‐point observation station, it was essential that silicate was supplied from a subsurface silicate maximum, a new feature that we identified during autumn in the northern Chukchi Sea. Water mass distributions obtained from wide‐area observations suggest that the subsurface silicate maximum water was possibly derived from the ventilated halocline in the Canada Basin. This article is protected by copyright. All rights reserved.
      PubDate: 2015-02-18T11:22:47.687857-05:
      DOI: 10.1002/2014JC010407
  • Large icebergs characteristics from altimeter waveforms analysis
    • Authors: J. Tournadre; N. Bouhier, F. Girard‐Ardhuin, F. Remy
      Abstract: Large uncertainties exist on the volume of ice transported by the Southern Ocean large icebergs, a key parameter for climate studies, because of the paucity of information, especially on iceberg thickness. Using icebergs tracks from the National Ice Center (NIC) and Brigham Young University (BYU) databases to select altimeter data over icebergs and a method of analysis of altimeter waveforms, a database of 5366 icebergs freeboard elevation, length and backscatter covering the 2002‐2012 period has been created. The database is analyzed in terms of distributions of freeboard, length and backscatter showing differences as a function of the iceberg's quadrant of origin. The database allows to analyze the temporal evolution of icebergs and to estimate a melt rate of 35 to 39 m/yr– 1 (neglecting the firn compaction). The total daily volume of ice, estimated by combining the NIC and altimeter sizes and the altimeter freeboards, regularly decreases from 2.2 104km3 in 2002 to 0.9 104km3 in 2012. During this decade, the total loss of ice (~ 1,800km3) is twice as large as than the input (~ 960km3) showing that the system is out of equilibrium after a very large input of ice between 1997 and 2002. Breaking into small icebergs represents 80% (~ 1,500km3) of the total ice loss while basal melting is only 18% (~ 320km3). Small icebergs are thus the major vector of freshwater input in the Southern Ocean. This article is protected by copyright. All rights reserved.
      PubDate: 2015-02-18T11:22:37.459056-05:
      DOI: 10.1002/2014JC010502
  • Eddy‐driven sediment transport in the Argentine Basin: Is the height
           of the Zapiola Rise hydrodynamically controlled?
    • Authors: Wilbert Weijer; Mathew E. Maltrud, William B. Homoky, Kurt L. Polzin, Leo R. M. Maas
      Abstract: In this study we address the question whether eddy‐driven transports in the Argentine Basin can be held responsible for enhanced sediment accumulation over the Zapiola Rise, hence accounting for the existence and growth of this sediment drift. To address this question, we perform a 6‐year simulation with a strongly eddying ocean model. We release two passive tracers, with settling velocities that are consistent with silt and clay size particles. Our experiments show contrasting behavior between the silt fraction and the lighter clay. Due to its larger settling velocity, the silt fraction reaches a quasi‐steady state within a few years, with abyssal sedimentation rates that match net input. In contrast, clay settles only slowly, and its distribution is heavily stratified, being transported mainly along isopycnals. Yet, both size classes display a significant and persistent concentration minimum over the Zapiola Rise. We show that the Zapiola Anticyclone, a strong eddy‐driven vortex that circulates around the Zapiola Rise, is a barrier to sediment transport, and hence prevents significant accumulation of sediments on the Rise. We conclude that sediment transport by the turbulent circulation in the Argentine Basin alone cannot account for the preferred sediment accumulation over the Rise. We speculate that resuspension is a critical process in the formation and maintenance of the Zapiola Rise. This article is protected by copyright. All rights reserved.
      PubDate: 2015-02-18T11:22:01.540707-05:
      DOI: 10.1002/2014JC010573
  • Marine microbes rapidly adapt to consume ethane, propane, and butane
           within the dissolved hydrocarbon plume of a natural seep
    • Authors: Stephanie D. Mendes; Molly C. Redmond, Karl Voigritter, Christian Perez, Rachel Scarlett, David L. Valentine
      Abstract: Simple hydrocarbon gases containing two to four carbons (ethane, propane, and butane) are among the most abundant compounds present in petroleum reservoirs, and are introduced into the ocean through natural seepage and industrial discharge. Yet little is known about the bacterial consumption of these compounds in ocean waters. To assess the timing by which microbes metabolize these gases, we conducted a 3‐phase study that tested and applied a radiotracer‐based method to quantify the oxidation rates of ethane, propane and butane in fresh seawater samples. Phase 1 involved the synthesis of tritiated ethane, propane, and butane using Grignard reagents and tritiated water. Phase 2 was a systematic assessment of experimental conditions, wherein the indigenous microbial community was found to rapidly oxidize ethane, propane, and butane. Phase 3 was the application of this tritium method near the Coal Oil Point seeps, offshore California. Spatial and temporal patterns of ethane, propane, and butane oxidation down current from the hydrocarbon seeps demonstrated that >99% of these gases are metabolized within 1.3 days following initial exposure. The oxidation of ethane outpaced oxidation of propane and butane with patterns indicating the microbial community responded to these gases by rapid adaptation or growth. Methane oxidation responded the slowest in plume waters. Estimates based on the observed metabolic rates and carbon mass balance suggest that ethane, propane, and butane consuming microorganisms may transiently account for a majority of the total microbial community in these impacted waters. This article is protected by copyright. All rights reserved.
      PubDate: 2015-02-18T11:16:13.489761-05:
      DOI: 10.1002/2014JC010362
  • Multiscale seismic reflectivity of shallow thermoclines
    • Authors: S. Ker; Y. Le Gonidec, L. Marié, Y. Thomas, D. Gibert
      Abstract: Seismic Oceanography is coming of age as an established technique of observation of the thermohaline structure of the ocean. The present paper deals with the seismic reflectivity of the Armorican Shelf seasonal thermocline, west of France, based on two seismic experiments performed with a sparker source. The peak frequency was 500 Hz for the ASPEX experiment, where the thermocline was located at 27 m water depth, and reduced to 400 Hz associated to a higher source level for the IFOSISMO experiment, where the thermocline was 12 m deeper. Despite this settings, only the first experiment could clearly highlight the thermocline reflector, providing the first seismic observation of a shallow oceanic structure. To better understand the limitation of high‐resolution seismic devices in detecting weak oceanic features, we develop a wavelet‐based seismic analysis and consider, as a first approximation, a simple thermocline modelled by a Gauss Error function, allowing an analytical expression for the associated seismic reflectivity. We show that the acoustic impedance profile of the thermocline is mainly controlled by a sound velocity proportional to the temperature. We show that the seismic reflectivity is controlled by the reflection coefficient of the large‐scale structure of the thermocline and by an attenuation factor which depends on the ratio between the seismic wavelength and the characteristic size of the thermocline. Depending on this ratio, the strength of the thermocline‐related reflection may be too weak to be detected by seismic measurement. This article is protected by copyright. All rights reserved.
      PubDate: 2015-02-14T09:52:45.205229-05:
      DOI: 10.1002/2014JC010478
  • Role of particle stock and phytoplankton community structure in regulating
           particulate organic carbon export in a large marginal sea
    • Authors: Pinghe Cai; Daochen Zhao, Lei Wang, Bangqin Huang, Minhan Dai
      Abstract: In this study, we utilize 234Th/238U disequilibrium to determine particulate organic carbon (POC) export from the euphotic zone in the South China Sea. Depth profiles of 234Th, total chlorophyll, pigments, and POC were collected during four cruises from August 2009 to May 2011, covering an entire seasonal cycle of spring, summer, autumn, and winter. The extensive data set that was acquired allows for an evaluation of the seasonal variability of upper ocean POC export and its controls in a large marginal sea. The results show that 234Th fluxes from the euphotic zone fall in the range of 528 − 1550, 340 − 2694, and 302–2647 dpm m−2 d−1 for the coastal, shelf, and basin regimes, respectively. In these regimes, POC/234Th ratios at the base of the euphotic zone fall in the range of 5.7‐58.2, 4.6‐44.0, and 2.5‐15.5 μmol dpm−1, respectively. Accordingly, for the coastal, shelf, and basin regimes, the mean POC export fluxes from the euphotic zone are 24.3, 18.3, and 6.3 mmolC m−2 d−1, respectively. Seasonal variations in POC export flux are remarkable in the study area, and POC export peaks were generally observed in autumn. We use a simple linear regression (LLS) method to examine the correlation of POC export vs. POC stock and vs. plankton community structure. We found a strong correlation (R2=0.73, p
      PubDate: 2015-02-14T09:24:29.148865-05:
      DOI: 10.1002/2014JC010432
  • On the transport modes of fine sediment in the wave boundary layer due to
           resuspension/deposition—A turbulence‐resolving numerical
    • Authors: Zhen Cheng; Xiao Yu, Tian‐Jian Hsu, Celalettin E. Ozdemir, S. Balachandar
      Abstract: Previous field observations revealed that the wave boundary layer is one of the main conduits delivering fine sediments from the nearshore to continental shelves. Recently, a series of turbulence‐resolving simulations further demonstrated the existence of a range of flow regimes due to different degrees of sediment‐induced density stratification controlled by the sediment availability. In this study, we investigate the scenario in which sediment availability is governed by the resuspension/deposition from/to the bed. Specifically, we focus on how the critical shear stress of erosion and the settling velocity, can determine the modes of transport. Simulations reveal that at a given wave intensity, which is associated with more energetic muddy shelves and a settling velocity of about 0.5 mm/s, three transport modes, ranging from the well‐mixed transport (mode I), two‐layer like transport with the formation of lutocline (mode II) and laminarized transport (mode III) are obtained as the critical shear stress of erosion reduces. Moreover, reductions in the settling velocity also yield similar transitions of transport modes. We also demonstrate that the onset of laminarization can be well‐explained by the reduction of wave‐averaged bottom stress to about 0.39 Pa due to attenuated turbulence by sediments. A 2D parametric map is proposed to characterize the transition from one transport mode to another as a function of the critical shear stress and the settling velocity at a fixed wave intensity. This article is protected by copyright. All rights reserved.
      PubDate: 2015-02-14T09:24:15.917949-05:
      DOI: 10.1002/2014JC010623
  • An analysis of the evolution of Meddies in the North Atlantic using floats
           and multisensor satellite data
    • Authors: Young‐Heon Jo; Federico Ienna, Xiao‐Hai Yan
      Abstract: Previous studies focusing on the remote detection of Mediterranean Eddies (Meddies) have reported that the isopycnal surface changes derived from satellite multi‐sensor measurements at the approximate depth of 400m can be used to sense the presence of underlying Meddies. While the isopycnal surface near that depth does indeed reveal the locations of Meddies, an analysis of isopycnal surface changes in response to the evolution of Meddies has yet to be made. Accordingly, this research focuses on analyzing the relationship between isopycnal surface changes and the evolution of Meddies. The vertical isopycnal surface variability of Meddies, which is directly related to contributions from rotational velocity, interior thermal variation, and vertical displacement of Meddies, is observed and studied using float observations from A Mediterranean Undercurrent Seeding Experiment (AMUSE). The contributions of each of the three aforementioned parameters are estimated, enabling us to understand their relative role in changing the isopycnal surface above Meddies. Furthermore, in order to further understand Meddies' evolution and their associated forcing, the dominant frequencies of their horizontal and vertical displacements, as well as the sea surface height variability above the Meddy, are analyzed using the Hilbert – Huang Transform. Finally, the horizontal and the vertical eddy viscosity dissipation of Meddies is computed and compared with a theoretical model. The empirical horizontal and the vertical eddy viscosities are found to be 7x106cm2sec−1 and 200cm2sec−1, respectively. This study will therefore contribute to understanding how the isopycnal surface is related to the presence of Meddies, what frequencies are dominate its variability, and the values of eddy viscosity which can be used for a numerical model. This article is protected by copyright. All rights reserved.
      PubDate: 2015-02-12T08:46:28.843521-05:
      DOI: 10.1002/2014JC010495
  • Impact of submesoscale processes on dynamics of phytoplankton filaments
    • Authors: Igor Shulman; Bradley Penta, Jim Richman, Gregg Jacobs, Stephanie Anderson, Peter Sakalaukus
      Abstract: In Monterey Bay, CA, during northwesterly, upwelling favorable winds, the development of a southward flowing cold jet along the entrance to the Bay is often observed. This dense cold jet separates warm waters of the anticyclonic circulation offshore from the water masses inside the Bay. Interactions between the cold jet and the offshore anticyclonic circulation generate ageostrophic secondary circulation (ASC) cells due to submesoscale processes as, for example, flow interaction with the development of surface frontogenesis and nonlinear Ekman pumping. Based on observations and modeling studies we evaluate the impact of these submesoscale processes on the formation of chlorophyll a filaments during late spring‐earlier summer and late summer time frames. We show that during the late summer time frame, ASC leads to the development of filaments with high values of chlorophyll a concentration along the edge of the cold jet–in contrast to the earlier summer time, when the ASC mixes phytoplankton much deeper to the area below of the euphotic depth, and chlorophyll a filaments are 3‐4 times weaker. This article is protected by copyright. All rights reserved.
      PubDate: 2015-02-12T04:28:14.461296-05:
      DOI: 10.1002/2014JC010326
  • Modeled alongshore circulation and force balances onshore of a submarine
    • Authors: Jeff E. Hansen; Britt Raubenheimer, Jeffrey H. List, Steve Elgar
      Abstract: Alongshore force balances, including the role of nonlinear advection, in the shoaling and surf zones onshore of a submarine canyon are investigated using a numerical modeling system (Delft3D/SWAN). The model is calibrated with waves and alongshore flows recorded over a period of 1.5 months at 26 sites along the 1.0‐, 2.5‐, and 5.0‐m depth contours spanning about 2km of coast. Field‐observation‐based estimates of the alongshore pressure and radiation‐stress gradients are reproduced well by the model. Model simulations suggest that the alongshore momentum balance is between the sum of the pressure and radiation‐stress gradients and the sum of the nonlinear advective terms and bottom stress, with the remaining terms (e.g., wind stress and turbulent mixing) being negligible. The simulations also indicate that unexplained residuals in previous field‐based estimates of the momentum balance may be owing to the neglect of the nonlinear advective terms, which are similar in magnitude to the sum of the forcing (pressure and radiations stress gradients) and to the bottom stress. This article is protected by copyright. All rights reserved.
      PubDate: 2015-02-06T17:41:31.262621-05:
      DOI: 10.1002/2014JC010555
  • Turbulence in the East China Sea: The summertime stratification
    • Authors: Iossif Lozovatsky; Jae‐Hak Lee, H.J.S. Fernando, Sok Kuh Kang, S.U.P. Jinadasa
      Abstract: A series of 134 microstructure profiles were taken in the central East China Sea from a drifting ship, covering more than 12 miles over the sloping bottom. The water depth z varied in the range 52 ‐ 62m; the tidal elevation of semidiurnal component was ± 1m. Prior to the onset of 25‐hour drift, a hydrographic section consisting of 9 stations, 15 miles apart, was taken in the area. A shallow mixed surface layer (SL) underlain by a diurnal pycnocline (z 
      PubDate: 2015-02-06T17:25:53.906176-05:
      DOI: 10.1002/2014JC010596
  • Experimental study on plunging breaking waves in deep water
    • Authors: Ho‐Joon Lim; Kuang‐An Chang, Zhi‐Cheng Huang, Byoungjoon Na
      Abstract: This study presents a unique data set that combines measurements of velocities and void fraction under an unsteady deep‐water plunging breaker in a laboratory. Flow properties in the aerated crest region of the breaking wave were measured using modified particle image velocimetry (PIV) and bubble image velocimetry (BIV). Results show that the maximum velocity in the plunging breaker reached 1.68C at the first impingement of the overturning water jet with C being the phase speed of the primary breaking wave, while the maximum velocity reached 2.14C at the beginning of the first splash‐up. A similarity profile of void fraction was found in the successive impinging and splash‐up rollers. In the highly foamy splashing roller, the increase of turbulent level and vorticity level were strongly correlated with the increase of void fraction when the range of void fraction was between 0 and 0.4 (from the trough level to approximately the center of the roller). The levels became constant when void fraction was greater than 0.5. The mass flux, momentum flux, kinetic energy, potential energy, and total energy were computed and compared with and without the void fraction being accounted for. The results show that all the mean and turbulence properties related to the air‐water mixture are considerably overestimated unless void fraction is considered. When including the density variation due to the air bubbles, the wave energy dissipated exponentially a short distance after breaking about 54% and 85% of the total energy dissipated within one and two wavelengths beyond the breaking wave impingement point, respectively. This article is protected by copyright. All rights reserved.
      PubDate: 2015-02-06T17:25:44.270641-05:
      DOI: 10.1002/2014JC010269
  • Effect of glacial drainage water on the CO2 system and ocean acidification
           state in an Arctic tidewater‐glacier fjord during two contrasting
    • Authors: Agneta Fransson; Melissa Chierici, Daiki Nomura, Mats A. Granskog, Svein Kristiansen, Tõnu Martma, Gernot Nehrke
      Abstract: In order to investigate the effect of glacial water on the CO2 system in the fjord, we studied the variability of the total alkalinity (AT), total dissolved inorganic carbon (CT), dissolved inorganic nutrients, oxygen isotopic ratio (δ18O), and freshwater fractions from the glacier front to the outer Tempelfjorden on Spitsbergen in winter 2012 (January, March and April) and 2013 (April) and summer/fall 2013 (September). The two contrasting years clearly showed that the influence of freshwater, mixing and haline convection affected the chemical and physical characteristics of the fjord. The seasonal variability showed the lowest calcium carbonate saturation state (Ω) and pH values in March 2012 coinciding with the highest freshwater fractions. The highest Ω and pH were found in September 2013, mostly due to CO2 uptake during primary production. Overall, we found that increased freshwater supply decreased Ω, pH and AT. On the other hand, we observed higher AT relative to salinity in the freshwater end‐member in the mild and rainy winter of 2012 (1142 µmol kg−1) compared to AT in 2013 (526 µmol kg−1). Observations of calcite and dolomite crystals in the glacial ice suggested supply of carbonate‐rich glacial drainage water to the fjord. This implies that winters with a large amount of glacial drainage water partly provide a lessening of further ocean acidification, which will also affect the air‐sea CO2 exchange. This article is protected by copyright. All rights reserved.
      PubDate: 2015-02-03T22:16:13.843762-05:
      DOI: 10.1002/2014JC010320
  • A model for partitioning the light absorption coefficient of natural
           waters into phytoplankton, nonalgal particulate, and colored dissolved
           organic components: A case study for the Chesapeake Bay
    • Authors: Guangming Zheng; Dariusz Stramski, Paul M. DiGiacomo
      Abstract: We present a model, referred to as Generalized Stacked Constraints Model (G‐SCM), for partitioning the total light absorption coefficient of water (with pure‐water contribution subtracted), anw(λ), into phytoplankton, aph(λ), non‐algal particulate, ad(λ), and CDOM, ag(λ), components. The formulation of the model is based on the so‐called stacked‐constraints approach which utilizes a number of inequality constraints that must be satisfied simultaneously by the model outputs of component absorption coefficients. A major advancement is that G‐SCM provides a capability to separate the ad(λ) and ag(λ) coefficients from each other using only weakly restrictive assumptions about the component absorption coefficients. In contrast to the common assumption of exponential spectral shape of ad(λ) and ag(λ) in previous models, in our model these two coefficients are parameterized in terms of several distinct spectral shapes. These shapes are determined from field data collected in the Chesapeake Bay with an ultimate goal to adequately account for the actual variability in spectral shapes of ad(λ) and ag(λ) in the study area. Another advancement of this model lies in its capability to account for potentially non‐negligible magnitude of ad(λ) in the near‐infrared spectral region. Evaluation of model performance demonstrates good agreement with measurements in the Chesapeake Bay. For example, the median ratio of the model‐derived to measured ad(λ), ag(λ), and aph(λ) at 443 nm is 0.913, 1.064, and 1.056, respectively. Whereas our model in its present form can be a powerful tool for regional studies in the Chesapeake Bay, the overall approach is readily adaptable to other regions or bio‐optical water types. This article is protected by copyright. All rights reserved.
      PubDate: 2015-01-27T09:33:06.104342-05:
      DOI: 10.1002/2014JC010604
  • Landfast ice affects the stability of the Arctic halocline: Evidence from
           a numerical model
    • Authors: Polona Itkin; Martin Losch, Rüdiger Gerdes
      Abstract: Landfast ice covers large surface areas of the winter Siberian Seas. The immobile landfast ice cover inhibits divergent and convergent motion, hence dynamical sea ice growth and re‐distribution, decouples winter river plumes in coastal seas from the atmosphere and positions polynyas at the landfast ice edge offshore. In spite of the potentially large effects, state‐of‐the‐art numerical models usually do not represent landfast ice in its correct extent. A simple parametrization of landfast ice based on bathymetry and internal sea ice strength is introduced and its effects on the simulated Arctic Ocean are demonstrated. The simulations suggest that the Siberian landfast ice impacts the Arctic halocline stability through enhanced brine production in polynyas located closer to the shelf break and by re‐directing river water to the Canadian Basin. These processes strengthen the halocline in the Canadian Basin, but erode its stability in the Makarov and Eurasian Basins. This article is protected by copyright. All rights reserved.
      PubDate: 2015-01-09T23:06:50.341598-05:
      DOI: 10.1002/2014JC010353
  • The carbon dioxide system on the Mississippi River‐dominated
           continental shelf in the northern Gulf of Mexico: 1. Distribution and
           air‐sea CO2 flux
    • Authors: Wei‐Jen Huang; Wei‐Jun Cai, Yongchen Wang, Steven E. Lohrenz, Michael C. Murrell
      Pages: 1429 - 1445
      Abstract: River‐dominated continental shelf environments are active sites of air‐sea CO2 exchange. We conducted 13 cruises in the northern Gulf of Mexico, a region strongly influenced by fresh water and nutrients delivered from the Mississippi and Atchafalaya River system. The sea surface partial pressure of carbon dioxide (pCO2) was measured, and the air‐sea CO2 flux was calculated. Results show that CO2 exchange exhibited a distinct seasonality: the study area was a net sink of atmospheric CO2 during spring and early summer, and it was neutral or a weak source of CO2 to the atmosphere during midsummer, fall, and winter. Along the salinity gradient, across the shelf, the sea surface shifted from a source of CO2 in low‐salinity zones (0≤S
      PubDate: 2015-03-03T03:09:56.896693-05:
      DOI: 10.1002/2014JC010498
  • Failure to bloom: Intense upwelling results in negligible phytoplankton
           response and prolonged CO2 outgassing over the Oregon shelf
    • Authors: Wiley Evans; Burke Hales, Peter G. Strutton, R. Kipp Shearman, John A. Barth
      Pages: 1446 - 1461
      Abstract: During summer, upwelled water with elevated CO2 partial pressure (pCO2) and nutrients outcrops over the Oregon (OR) inner shelf. As this water transits across the shelf, high rates of primary production fueled by the upwelled nutrients results in net atmospheric CO2 drawdown. Upwelled source‐waters typically have pCO2 approaching 1000 µatm that is then reduced to ∼200 µatm. For almost the entire month of July 2008, strong and persistent upwelling brought cold (∼8°C), saline (∼33.5), high‐pCO2 (>600 µatm) water to our midshelf buoy site, and high‐pCO2 water was broadly distributed over the shelf. Chlorophyll levels, as a proxy for phytoplankton biomass, were low (< 2 mg m−3) on the shelf during the period of most intense upwelling, and satellite data showed no evidence of a downstream phytoplankton bloom. A small chlorophyll increase to ∼4 mg m−3 was observed at our buoy site following a decrease in the strength of southward wind stress 10 days after upwelling initiated. Chlorophyll levels further increased to ∼10 mg m−3 only after a cease in upwelling. These higher levels were coincident with the appearance of water masses having temperature and salinity properties distinct from recently upwelled water. We suggest that rapid offshore transport and subsequent subduction before phytoplankton populations could respond is the most likely explanation for the persistent low chlorophyll and elevated surface‐water pCO2 throughout the July upwelling event. This mechanism likely dominates under conditions of strong and persistent upwelling‐favorable winds that coincide with close proximity of low‐density offshore waters, which may have implications for the biogeochemical functioning of this system under future climate scenarios.
      PubDate: 2015-03-05T00:25:44.891743-05:
      DOI: 10.1002/2014JC010580
  • Overturning circulation that ventilates the intermediate layer of the Sea
           of Okhotsk and the North Pacific: The role of salinity advection
    • Authors: Junji Matsuda; Humio Mitsudera, Tomohiro Nakamura, Yuichiro Sasajima, Hiroyasu Hasumi, Masaaki Wakatsuchi
      Pages: 1462 - 1489
      Abstract: Dense Shelf Water (DSW) formation in the northwestern continental shelf of the Sea of Okhotsk is the beginning of the lower limb of the overturning circulation that ventilates the intermediate layer of the North Pacific Ocean. The upper limb consisting of surface currents in the Okhotsk Sea and the subarctic gyre has not been clarified. Using a high‐resolution North Pacific Ocean model with a curvilinear grid as fine as 3 km × 3 km in the Sea of Okhotsk, we succeeded in representing the three‐dimensional structure of the overturning circulation including the narrow boundary currents and flows through straits that constitute the upper limb, as well as the lower limb consisting of DSW formation and ventilation. In particular, pathways and time scales from the Bering Sea to the intermediate layer via the ventilation in the Sea of Okhotsk were examined in detail using tracer experiments. Further, we found that the overturning circulation that connects the surface and intermediate layer is sensitive to wind stress. In the case of strong winds, the coastal current under polynyas where DSW forms is intensified, and consequently diapycnal transport from the surface layer to the intermediate layer increases. Strong winds also induce a positive sea surface salinity anomaly in the subarctic region, causing a significant decrease in the density stratification and increase in the DSW salinity (i.e., density). These processes act together to produce intense overturning circulation and deep ventilation, which may subduct even to the bottom of the Sea of Okhotsk if the wind is strong.
      PubDate: 2015-03-06T09:00:03.423744-05:
      DOI: 10.1002/2014JC009995
  • Analysis of ageostrophy in strong surface eddies in the Atlantic Ocean
    • Authors: E. M. Douglass; J. G. Richman
      Pages: 1490 - 1507
      Abstract: Strongly nonlinear surface eddies are identified and analyzed in a general circulation model. Agulhas rings and Gulf Stream cold‐core eddies are examples of eddies that cannot be properly characterized using linear geostrophic dynamics. These eddies are compact, highly circular, persistent in time, and travel long distances while maintaining their characteristics. The nonlinear eddies can be identified by a large Rossby number and high circularity. The majority of the anomalous eddies are anticyclones. Calculation of the balance of forces on these eddies demonstrates that the centrifugal force associated with strong curvature is significant, and the force balance shifts from geostrophy toward a gradient wind balance. Using geostrophy instead of the gradient wind balance produces large errors in estimates of rotational velocity of these eddies. The gradient wind velocity can be calculated from geostrophic velocity and eddy radius. Comparison between the results demonstrates that even when only sea surface height and associated geostrophic velocities are available, strongly nonlinear eddies can be identified and properly characterized. This analysis is then applied to altimetric maps of sea surface height. Nonlinear eddies are present in the altimetric maps, but are less common and not as strongly nonlinear. This analysis demonstrates that by properly accounting for the dynamics of the eddy field, a more complete statistical description including nonlinear terms can be obtained from readily available observations.
      PubDate: 2015-03-06T09:02:23.501407-05:
      DOI: 10.1002/2014JC010350
  • Drivers of deep‐water renewal events observed over 13 years in the
           South Basin of Lake Baikal
    • Authors: Chrysanthi Tsimitri; Burkhardt Rockel, Alfred Wüest, Nikolay M. Budnev, Michael Sturm, Martin Schmid
      Pages: 1508 - 1526
      Abstract: Lake Baikal, with a depth of 1637 m, is characterized by deep‐water intrusions that bridge the near‐surface layer to the hypolimnion. These episodic events transfer heat and oxygen over large vertical scales and maintain the permanent temperature stratified deep‐water status of the lake. Here we evaluate a series of intrusion events that reached the bottom of the lake in terms of the stratification and the wind conditions under which they occurred and provide a new insight into the triggering mechanisms. We make use of long‐term temperature and current meter data (2000–2013) recorded in the South Basin of the lake combined with wind data produced with a regional downscaling of the global NCEP‐RA1 reanalysis product. A total of 13 events were observed during which near‐surface cold water reached the bottom of the South Basin at 1350 m depth. We found that the triggering mechanism of the events is related to the time of the year that they take place. We categorized the events in three groups: (1) winter events, observed shortly before the complete ice cover of the lake that are triggered by Ekman coastal downwelling, (2) under‐ice events, and (3) spring events, that show no correlation to the wind conditions and are possibly connected to the increased spring outflow of the Selenga River.
      PubDate: 2015-03-12T03:24:04.585904-05:
      DOI: 10.1002/2014JC010449
  • Evidence for multidecadal variability in US extreme sea level records
    • Authors: Thomas Wahl; Don P. Chambers
      Pages: 1527 - 1544
      Abstract: We analyze a set of 20 tide gauge records covering the contiguous United States (US) coastline and the period from 1929 to 2013 to identify long‐term trends and multidecadal variations in extreme sea levels (ESLs) relative to changes in mean sea level (MSL). Different data sampling and analysis techniques are applied to test the robustness of the results against the selected methodology. Significant but small long‐term trends in ESLs above/below MSL are found at individual sites along most coastline stretches, but are mostly confined to the southeast coast and the winter season when storm surges are primarily driven by extratropical cyclones. We identify six regions with broadly coherent and considerable multidecadal ESL variations unrelated to MSL changes. Using a quasi‐nonstationary extreme value analysis, we show that the latter would have caused variations in design relevant return water levels (50–200 year return periods) ranging from ∼10 cm to as much as 110 cm across the six regions. The results raise questions as to the applicability of the “MSL offset method,” assuming that ESL changes are primarily driven by changes in MSL without allowing for distinct long‐term trends or low‐frequency variations. Identifying the coherent multidecadal ESL variability is crucial in order to understand the physical driving factors. Ultimately, this information must be included into coastal design and adaptation processes.
      PubDate: 2015-03-12T03:34:44.184227-05:
      DOI: 10.1002/2014JC010443
  • Cross‐shelf exchange in the shelf of the East China Sea
    • Authors: Feng Zhou; Huijie Xue, Daji Huang, Jiliang Xuan, Xiaobo Ni, Peng Xiu, Qiang Hao
      Pages: 1545 - 1572
      Abstract: A high‐resolution, 3‐dimensional primitive equation model is used to investigate the cross‐shelf exchange in the East China Sea (ECS). Favorable comparisons between field data and model simulations from both climatological run and hindcast run for 2006 indicate that the model has essential skills in capturing the key physics of the ECS. Temporal and spatial variations of the cross‐shelf exchanges are further analyzed. It was demonstrated from both observations and simulations that in 2006 high saline water could be delivered to the north of the Changjiang River mouth (near 32°N) as a result of stronger than typical cross‐shelf exchanges at the shelf break and flows through the Taiwan Strait with an annual mean rate of 2.59 and 1.83 Sv, respectively. A few new places at the shelf break were also identified where persistent and vigorous onshore or offshore exchanges occur throughout the year. Cross‐shelf exchange is largely determined by the along‐shelf geostrophic balance with weak seasonality, which is modulated in upper layers by northeasterly monsoon from early‐fall to late‐spring and at seabed by bottom friction during December–January, May, and August–September. Nonlinear effect, with strong spatial variations and intraseasonal variability, is a secondary but persistent contributor to the net seaward transport, except for northeast of Taiwan where the nonlinear effect becomes significant but more varied.
      PubDate: 2015-03-12T10:59:31.421758-05:
      DOI: 10.1002/2014JC010567
  • Variability in the meteoric water, sea‐ice melt, and Pacific water
           contributions to the central Arctic Ocean, 2000–2014
    • Authors: Matthew B. Alkire; James Morison, Roger Andersen
      Pages: 1573 - 1598
      Abstract: Fourteen years (2000–2014) of bottle chemistry data collected during the North Pole Environmental Observatory were compiled to examine variations in the composition of freshwater (meteoric water, net sea‐ice meltwater, and Pacific water) over mixed layer of the Central Arctic Ocean. In addition to significant spatial and interannual variability, there was a general decrease in meteoric water (MW) fractions at the majority of stations reoccupied over the duration of the program that was approximately balanced by a concomitant increase in freshwater from sea‐ice melt (SIM FW) between 2000 and 2012. Inventories (0–120 m) of MW and SIM FW computed using available data between 2005 and 2012 exhibited similar variations over the study area, allowing for first‐order estimates of the mean annual changes in MW (−389 ± 194 km3 yr−1) and SIM FW (292 ± 97 km3 yr−1) for the Central Arctic region. These mean annual changes were attributed to the diversion of Siberian river runoff to the Beaufort Gyre and the overall reduction of sea ice volume across the Arctic, respectively. In addition to this lower‐frequency variability, spatial gradients and interannual variations in MW, SIM FW, and Pacific water contributions to specific locations were attributed to shifts in the Transpolar Drift that advects waters of eastern and western Arctic origin through the study area.
      PubDate: 2015-03-12T03:40:36.686758-05:
      DOI: 10.1002/2014JC010023
  • Spreading of Levantine Intermediate Waters by submesoscale coherent
           vortices in the northwestern Mediterranean Sea as observed with gliders
    • Authors: Anthony Bosse; Pierre Testor, Laurent Mortier, Louis Prieur, Vincent Taillandier, Fabrizio d'Ortenzio, Laurent Coppola
      Pages: 1599 - 1622
      Abstract: Since 2007, gliders have been regularly deployed in the northwestern Mediterranean Sea, a crucial region regarding the thermohaline circulation of the Mediterranean Sea. It revealed for the first time very warm (+0.4∘C) and saline (+0.1) submesoscale anticyclones at intermediate depth characterized by a small radius (∼5 km), high Rossby (∼0.3), and Burger (∼0.7) numbers. They are likely order of 10 to be formed each year, have a life time order a year and certainly contribute significantly to the spreading of the Levantine Intermediate Waters (LIW) toward the whole subbasin, thus potentially impacting wintertime vertical mixing through hydrographical and dynamical preconditioning. They could be mainly formed by the combined action of turbulent mixing and flow detachment of the northward flow of LIW at the northwestern headland of Sardinia. Upwelling conditions along the western coast of Sardinia associated with a southward geostrophic flow within the upper layers seem to play a key role in their formation process.
      PubDate: 2015-03-12T03:41:19.27895-05:0
      DOI: 10.1002/2014JC010263
  • Multiyear ice replenishment in the Canadian Arctic Archipelago:
    • Authors: Stephen E. L. Howell; Chris Derksen, Larissa Pizzolato, Michael Brady
      Pages: 1623 - 1637
      Abstract: In the Canadian Arctic Archipelago (CAA), multiyear ice (MYI) replenishment from first‐year ice aging (CAAMYI‐Oct‐1) and Arctic Ocean MYI exchange (CAAMYI‐exchange) contribute to the CAA's relatively heavy sea ice conditions at the end of the summer melt season. We estimate these components using RADARSAT and the Canadian Ice Service Digital Archive and explore processes responsible for interannual variability from 1997 to 2013. CAAMYI‐Oct‐1 (52 ± 36 × 103 km2) provides a larger contribution than CAAMYI‐exchange (13 ± 11 × 103 km2). CAAMYI‐Oct‐1 represents ∼10% of the amount that occurs in the Arctic Ocean. CAAMYI‐exchange represents ∼50% of Nares Strait MYI export to Baffin Bay and ∼12% of Fram Strait MYI export to the Greenland Sea. CAAMYI‐Oct‐1 exhibits dependence on warmer (cooler) summers that increase (decrease) melt evident from strong relationships to surface air temperature (SAT), albedo and total absorbed solar radiation (Qtotal). CAAMYI‐exchange is influenced by summer sea level pressure (SLP) anomalies over the Beaufort Sea and Canadian Basin which shifts the primary source of CAAMYI‐exchange between less obstructed M'Clure Strait (low SLP anomalies) and the more obstructed Queen Elizabeth Islands (high SLP anomalies). Over the 17‐record, appreciable replenishment occurred for most years from 1997 to 2004, reduced replenishment from 2005 to 2012, and large replenishment in 2013. The reduced replenishment period was associated with positive SAT, negative albedo, and positive Qtotal anomalies that facilitated more melt and less CAAMYI‐Oct‐1, together with high SLP anomalies that facilitated less CAAMYI‐exchange. Large replenishment in 2013 was primarily from CAAMYI‐Oct‐1 attributed to strongly negative SAT and Qtotal anomalies and strongly positive albedo that impeded melt.
      PubDate: 2015-03-14T06:33:11.619468-05:
      DOI: 10.1002/2015JC010696
  • Mechanisms of surface wave energy dissipation over a
           high‐concentration sediment suspension
    • Authors: Peter Traykovski; John Trowbridge, Gail Kineke
      Pages: 1638 - 1681
      Abstract: Field observations from the spring of 2008 on the Louisiana shelf were used to elucidate the mechanisms of wave energy dissipation over a muddy seafloor. After a period of high discharge from the Atchafalaya River, acoustic measurements showed the presence of 20 cm thick mobile fluid‐mud layers during and after wave events. While total wave energy dissipation (D) was greatest during the high energy periods, these periods had relatively low normalized attenuation rates (κ = Dissipation/Energy Flux). During declining wave‐energy conditions, as the fluid‐mud layer settled, the attenuation process became more efficient with high κ and low D. The transition from high D and low κ to high κ and low D was caused by a transition from turbulent to laminar flow in the fluid‐mud layer as measured by a Pulse‐coherent Doppler profiler. Measurements of the oscillatory boundary layer velocity profile in the fluid‐mud layer during laminar flow reveal a very thick wave boundary layer with curvature filling the entire fluid‐mud layer, suggesting a kinematic viscosity 2–3 orders of magnitude greater than that of clear water. This high viscosity is also consistent with a high wave‐attenuation rates measured by across‐shelf energy flux differences. The transition to turbulence was forced by instabilities on the lutocline, with wavelengths consistent with the dispersion relation for this two‐layer system. The measurements also provide new insight into the dynamics of wave‐supported turbidity flows during the transition from a laminar to turbulent fluid‐mud layer.
      PubDate: 2015-03-19T05:58:30.696542-05:
      DOI: 10.1002/2014JC010245
  • Seasonal variation in the South China Sea deep circulation
    • Authors: Jian Lan; Yu Wang, Fengjuan Cui, Ningning Zhang
      Pages: 1682 - 1690
      Abstract: The previous studies show that the SCS deep circulation is featured by a basin‐scale cyclonic gyre. On the basis of the Hybrid Coordinate Ocean Model (HYCOM) and the Simple Ocean Data Assimilation (SODA), this study attempts to examine its seasonal variability and to investigate the driving mechanism. During summer season, the basin‐scale cyclonic gyre is dominant and strong, corresponding to higher value of the deepwater overflow transport. During winter season, the basin‐scale cyclonic gyre can hardly be identified, corresponding to lower value of the deepwater overflow transport. The control run and the SODA show the similar results. Two sensitivity experiments are designed to investigate what could be possible responsible for the seasonal variation in the SCS deep circulation. The results reveal that the deepwater overflow through the Luzon Strait contributes to the seasonal variability of the SCS deep circulation, and the seasonal variation of the surface forcings have less influence on that. The mechanism is related to the potential vorticity flux by the deepwater overflow.
      PubDate: 2015-03-19T05:51:11.436578-05:
      DOI: 10.1002/2014JC010413
  • ENSO and the California Current coastal upwelling response
    • Authors: Michael G. Jacox; Jerome Fiechter, Andrew M. Moore, Christopher A. Edwards
      Pages: 1691 - 1702
      Abstract: A 31 year (1980–2010) sequence of historical analyses of the California Current System (CCS) is used to describe the central CCS (35–43˚N) coastal upwelling response to El Niño‐Southern Oscillation (ENSO) variability. The analysis period captures 10 El Niño and 10 La Niña events, including the extreme El Niños of 1982–1983 and 1997–1998. Data‐assimilative model runs and backward trajectory calculations of passive tracers are used to elucidate physical conditions and source water characteristics during the upwelling season of each year. In general, El Niño events produce anomalously weak upwelling and source waters that are unusually shallow, warm, and fresh, while La Niña conditions produce the opposite. Maximum vertical transport anomalies in the CCS occur ∼1 month after El Niño peaks in midwinter, and before the onset of the upwelling season. Source density anomalies peak later than transport anomalies and persist more strongly through the spring and summer, causing the former to impact the upwelling season more directly. As nitrate concentration covaries with density in the central CCS, El Niño may exert more influence over the nitrate concentration of upwelled waters than it does over vertical transport, although both factors are expected to reduce nitrate supply during El Niño events. Interannual comparison of individual diagnostics highlights their relative impacts during different ENSO events, as well as years deviating from the canonical response to ENSO variability. The net impact of ENSO on upwelling is explored through an “Upwelling Efficacy Index”, which may be a useful indicator of bottom‐up control on primary productivity.
      PubDate: 2015-03-19T05:54:00.798366-05:
      DOI: 10.1002/2014JC010650
  • Seasonal evolution of an ice‐shelf influenced fast‐ice regime,
           derived from an autonomous thermistor chain
    • Authors: M. Hoppmann; M. Nicolaus, P. A. Hunkeler, P. Heil, L.‐K. Behrens, G. König‐Langlo, R. Gerdes
      Pages: 1703 - 1724
      Abstract: Ice shelves strongly interact with coastal Antarctic sea ice and the associated ecosystem by creating conditions favorable to the formation of a sub‐ice platelet layer. The close investigation of this phenomenon and its seasonal evolution remains a challenge due to logistical constraints and a lack of suitable methodology. In this study, we characterize the seasonal cycle of Antarctic fast ice adjacent to the Ekström Ice Shelf in the eastern Weddell Sea. We used a thermistor chain with the additional ability to record the temperature response induced by cyclic heating of resistors embedded in the chain. Vertical sea‐ice temperature and heating profiles obtained daily between November 2012 and February 2014 were analyzed to determine sea‐ice and snow evolution, and to calculate the basal energy budget. The residual heat flux translated into an ice‐volume fraction in the platelet layer of 0.18 ± 0.09, which we reproduced by a independent model simulation and agrees with earlier results. Manual drillings revealed an average annual platelet‐layer thickness increase of at least 4 m, and an annual maximum thickness of 10 m beneath second‐year sea ice. The oceanic contribution dominated the total sea‐ice production during the study, effectively accounting for up to 70% of second‐year sea‐ice growth. In summer, an oceanic heat flux of 21 W m−2 led to a partial thinning of the platelet layer. Our results further show that the active heating method, in contrast to the acoustic sounding approach, is well suited to derive the fast‐ice mass balance in regions influenced by ocean/ice‐shelf interaction, as it allows subdiurnal monitoring of the platelet‐layer thickness.
      PubDate: 2015-03-19T10:26:18.582573-05:
      DOI: 10.1002/2014JC010327
  • Development of HF radar inversion algorithm for spectrum estimation (HIAS)
    • Authors: Yukiharu Hisaki
      Pages: 1725 - 1740
      Abstract: A method for estimating ocean wave directional spectra using an HF (high‐frequency) ocean radar was developed. This method represents the development of work conducted in previous studies. In the present method, ocean wave directional spectra are estimated on polar coordinates whose center is the radar position, while spectra are estimated on regular grids. This method can be applied to both single and multiple radar cases. The area for wave estimation is more flexible than that of the previous method. As the signal to noise (SN) ratios of Doppler spectra are critical for wave estimation, we develop a method to exclude low SN ratio Doppler spectra. The validity of the method is demonstrated by comparing results with in situ observed wave data that it would be impossible to estimate by the methods of other groups.
      PubDate: 2015-03-19T05:53:47.989358-05:
      DOI: 10.1002/2014JC010548
  • A semianalytical ocean color inversion algorithm with explicit water
           column depth and substrate reflectance parameterization
    • Authors: Lachlan I. W. McKinna; Peter R. C. Fearns, Scarla J. Weeks, P. Jeremy Werdell, Martina Reichstetter, Bryan A. Franz, Donald M. Shea, Gene C. Feldman
      Pages: 1741 - 1770
      Abstract: A semianalytical ocean color inversion algorithm was developed for improving retrievals of inherent optical properties (IOPs) in optically shallow waters. In clear, geometrically shallow waters, light reflected off the seafloor can contribute to the water‐leaving radiance signal. This can have a confounding effect on ocean color algorithms developed for optically deep waters, leading to an overestimation of IOPs. The algorithm described here, the Shallow Water Inversion Model (SWIM), uses pre‐existing knowledge of bathymetry and benthic substrate brightness to account for optically shallow effects. SWIM was incorporated into the NASA Ocean Biology Processing Group's L2GEN code and tested in waters of the Great Barrier Reef, Australia, using the Moderate Resolution Imaging Spectroradiometer (MODIS) Aqua time series (2002–2013). SWIM‐derived values of the total non‐water absorption coefficient at 443 nm, at(443), the particulate backscattering coefficient at 443 nm, bbp(443), and the diffuse attenuation coefficient at 488 nm, Kd(488), were compared with values derived using the Generalized Inherent Optical Properties algorithm (GIOP) and the Quasi‐Analytical Algorithm (QAA). The results indicated that in clear, optically shallow waters SWIM‐derived values of at(443), bbp(443), and Kd(443) were realistically lower than values derived using GIOP and QAA, in agreement with radiative transfer modeling. This signified that the benthic reflectance correction was performing as expected. However, in more optically complex waters, SWIM had difficulty converging to a solution, a likely consequence of internal IOP parameterizations. Whilst a comprehensive study of the SWIM algorithm's behavior was conducted, further work is needed to validate the algorithm using in situ data.
      PubDate: 2015-03-19T05:54:28.876531-05:
      DOI: 10.1002/2014JC010224
  • Glider observations and modeling of sediment transport in Hurricane Sandy
    • Authors: Travis Miles; Greg Seroka, Josh Kohut, Oscar Schofield, Scott Glenn
      Pages: 1771 - 1791
      Abstract: Regional sediment resuspension and transport are examined as Hurricane Sandy made landfall on the Mid‐Atlantic Bight (MAB) in October 2012. A Teledyne‐Webb Slocum glider, equipped with a Nortek Aquadopp current profiler, was deployed on the continental shelf ahead of the storm, and is used to validate sediment transport routines coupled to the Regional Ocean Modeling System (ROMS). The glider was deployed on 25 October, 5 days before Sandy made landfall in southern New Jersey (NJ) and flew along the 40 m isobath south of the Hudson Shelf Valley. We used optical and acoustic backscatter to compare with two modeled size classes along the glider track, 0.1 and 0.4 mm sand, respectively. Observations and modeling revealed full water column resuspension for both size classes for over 24 h during peak waves and currents, with transport oriented along‐shelf toward the southwest. Regional model predictions showed over 3 cm of sediment eroded on the northern portion of the NJ shelf where waves and currents were the highest. As the storm passed and winds reversed from onshore to offshore on the southern portion of the domain waves and subsequently orbital velocities necessary for resuspension were reduced leading to over 3 cm of deposition across the entire shelf, just north of Delaware Bay. This study highlights the utility of gliders as a new asset in support of the development and verification of regional sediment resuspension and transport models, particularly during large tropical and extratropical cyclones when in situ data sets are not readily available.
      PubDate: 2015-03-19T10:26:40.941709-05:
      DOI: 10.1002/2014JC010474
  • Multidecadal variations of the surface Kuroshio between 1950s and 2000s
           and its impacts on surrounding waters
    • Authors: Yi‐Chia Hsin
      Pages: 1792 - 1808
      Abstract: Based on the analyses of 59 year (1950–2008) surface geostrophic velocities, the multidecadal changes of Kuroshio from the eastern Luzon to the southern Japan are investigated. Result shows that the upstream Kuroshio from the east of Luzon to southern East China Sea suffers much more obvious multidecadal changes. Except for the decade of 1980, the Kuroshio east of Luzon possesses a multidecadal tendency opposite to that east of Taiwan. Besides 1980s, the multidecadal change of Kuroshio bordering Taiwan is mainly governed by the eddy activity off the eastern Taiwan, while the wind stress curl plays a major role in the Kuroshio in the east of Luzon and in the Luzon Strait. The wind‐stress‐induced Ekman transport plays a secondary role in regulating the Kuroshio east of Luzon. In addition, the multidecadal fluctuation of Kuroshio east of Luzon also modulates the westward intrusion in the Luzon Strait. Instead, the Ekman transport dominated the whole upstream Kuroshio area from the eastern Luzon to the vicinity of Taiwan during the exceptional decade of 1980. Associated changes of water properties in the northern South China Sea and southern East China Sea are also ascribed to the multidecadal changes of surface Kuroshio in the upstream area.
      PubDate: 2015-03-19T10:24:51.882032-05:
      DOI: 10.1002/2014JC010582
  • Anomalous dispersion of sea ice in the Fram Strait region
    • Authors: A. Gabrielski; G. Badin, L. Kaleschke
      Pages: 1809 - 1824
      Abstract: The single‐particle dispersion of sea ice in the Fram Strait region is investigated using ice drift buoys deployed from 2002 to 2009 within the Fram Strait Cyclones and the Arctic Climate System Study campaigns. A new method to estimate the direction of the mean flow, based on a satellite drift product, is introduced. As a result, the bias in the dispersion introduced by the mean flow is eliminated considering only the displacements of the buoys in the cross‐stream direction. Results show an absolute dispersion growing quadratically in time for the first 3 days and an anomalous dispersion regime exhibiting a strongly self‐similar scaling following a 5/4 power law for time scales larger than 6 days persisting over the whole time series of length 32 days. The non‐Gaussian distribution of the velocity fluctuations with a skewness of −0.15 and a kurtosis of 7.33 as well as the slope of the Lagrangian frequency spectrum between −2 and −1 are in agreement with the anomalous diffusion regime. Comparison with data from the International Arctic Buoy Program yields similar results with an anomalous dispersion starting after 10 days and persisting over the whole time series of length 50 days. The results suggest the presence of deformation and shear acting on the sea ice dispersion. The high correlation between the cross‐stream displacements and the cross‐stream wind velocities shows the important role of the wind as a source for the anomalous dispersion.
      PubDate: 2015-03-19T06:13:39.045878-05:
      DOI: 10.1002/2014JC010359
  • Large variability of the Kuroshio at 23.75°N east of Taiwan
    • Authors: Sen Jan; Yiing Jang Yang, Joe Wang, Vigan Mensah, Tien‐Hsia Kuo, Ming‐Da Chiou, Ching‐Sheng Chern, Ming‐Huei Chang, Hwa Chien
      Pages: 1825 - 1840
      Abstract: Synoptic features of the Kuroshio at 23.75°N were quantified using nine ship‐based surveys between September 2012 and September 2014. The new ship‐based data set provides an unprecedented view of the Kuroshio east of Taiwan and suggest tremendous variability in its velocity, hydrography, volume, heat, and salt transports, and water masses. The Kuroshio maximum velocity varied in 0.7–1.4 m s−1; the core current width, delineated with the limit v ≥ 0.2 m s−1, ranged from 85 to 135 km, and the thickness varied from 400 to 600 m. A dual velocity maximum in the Kuroshio core current, though unexpected, was observed in three of nine cruises. The Kuroshio core transport, integrated from the directly measured velocity, varied between 10.46 and 22.92 Sv (1 Sv=106 m3 s−1). The corresponding heat transport referenced to 0°C was 0.838–1.793 × 1015 W, and the salt transport was 345.0–775.9 × 106 kg s−1. The geostrophic transport estimated using the thermal wind relation with the observed hydrographic data and reference velocity at 900 m is comparable to the directly measured Kuroshio transport during most of the surveys, suggesting the directly measured velocity is mostly in geostrophic balance.
      PubDate: 2015-03-19T06:14:13.6806-05:00
      DOI: 10.1002/2014JC010614
  • On the origin and propagation of Denmark Strait overflow water anomalies
           in the Irminger Basin
    • Authors: Kerstin Jochumsen; Manuela Köllner, Detlef Quadfasel, Stephen Dye, Bert Rudels, Heðinn Valdimarsson
      Pages: 1841 - 1855
      Abstract: Denmark Strait Overflow Water (DSOW) supplies the densest contribution to North Atlantic Deep Water and is monitored at several locations in the subpolar North Atlantic. Hydrographic (temperature and salinity) and velocity time series from three multiple‐mooring arrays at the Denmark Strait sill, at 180 km downstream (south of Dohrn Bank) and at a further 320 km downstream on the east Greenland continental slope near Tasiilaq (formerly Angmagssalik), were analyzed to quantify the variability and track anomalies in DSOW in the period 2007–2012. No long‐term trends were detected in the time series, while variability on time scales from interannual to weekly was present at all moorings. The hydrographic time series from different moorings within each mooring array showed coherent signals, while the velocity fluctuations were only weakly correlated. Lagged correlations of anomalies between the arrays revealed a propagation from the sill of Denmark Strait to the Angmagssalik array in potential temperature with an average propagation time of 13 days, while the correlations in salinity were low. Entrainment of warm and saline Atlantic Water and fresher water from the East Greenland Current (via the East Greenland Spill Jet) can explain the whole range of hydrographic changes in the DSOW measured downstream of the sill. Changes in the entrained water masses and in the mixing ratio can thus strongly influence the salinity variability of DSOW. Fresh anomalies found in downstream measurements of DSOW within the Deep Western Boundary Current can therefore not be attributed to Arctic climate variability in a straightforward way.
      PubDate: 2015-03-19T06:13:14.595269-05:
      DOI: 10.1002/2014JC010397
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