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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)
Global Biogeochemical Cycles     Full-text available via subscription   (Followers: 5, SJR: 3.239, h-index: 119)
J. of Advances in Modeling Earth Systems     Open Access   (Followers: 2, SJR: 1.944, h-index: 7)
J. of Geophysical Research : Atmospheres     Partially Free   (Followers: 22)
J. of Geophysical Research : Biogeosciences     Full-text available via subscription   (Followers: 6)
J. of Geophysical Research : Earth Surface     Partially Free   (Followers: 24)
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)
J. of Geophysical Research : Space Physics     Full-text available via subscription   (Followers: 15)
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)
Reviews of Geophysics     Full-text available via subscription   (Followers: 20, SJR: 9.68, h-index: 94)
Space Weather     Full-text available via subscription   (Followers: 3, SJR: 1.319, h-index: 19)
Tectonics     Full-text available via subscription   (Followers: 9, SJR: 2.748, h-index: 85)
Water Resources Research     Full-text available via subscription   (Followers: 69, 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]
  • The Cape Ghir filament system in August 2009 (NW Africa)
    • Authors: Pablo Sangrà; Charles Troupin, Beatriz Barreiro‐González, Eric Desmond Barton, Abdellatif Orbi, Javier Arístegui
      Abstract: In the framework of the CAIBEX (Canaries‐Iberian marine Ecosystem Exchanges) experiment an interdisciplinary high‐resolution survey was conducted in the NW African region of Cape Ghir (30º38'N) during August 2009. The anatomy of a major filament is investigated on scales down to the submesoscale using in situ and remotely sensed data. The filament may be viewed as a system composed of three intimately connected structures: a small, shallow and cold filament embedded within a larger, deeper and cool filament and an intrathermocline anticyclonic eddy (ITE). The cold filament, which stretches 110 km offshore, is a shallow feature 60 m deep and 25 km wide, identified by minimal surface temperatures and rich in chlorophyll‐a. This structure comprises two asymmetrical submesoscale (∼18 km) fronts with jets flowing in opposite directions. The cold filament is embedded near the equatorward boundary of a much broader region of c.a. 120 km width and 150 m depth that forms the cool filament and stretches at least 200 km offshore. This cool region, partly resulting from the influence of cold filament, is limited by two asymmetrical mesoscale (∼50 km) frontal boundaries. At the ITE, located North of the cold filament, we observe evidence of downwelling as indicated by a relatively high concentration of particles extending from the surface to more than 200 m depth. We hypothesize that this ITE may act as a sink of carbon and thus the filament system may serve dual roles of offshore carbon export and carbon sink This article is protected by copyright. All rights reserved.
      PubDate: 2015-05-20T03:43:31.387664-05:
      DOI: 10.1002/2014JC010514
       
  • Satellite chlorophyll off the British Columbia coast, 1997–2010
    • Authors: Jennifer M. Jackson; Richard E. Thomson, Leslie N. Brown, Peter G. Willis, Gary A. Borstad
      Abstract: We examine the spatial and temporal variability of satellite‐sensed sea surface chlorophyll‐a off the west coast of North America from 1997 to 2010, with focus on coastal British Columbia. The variability in surface chlorophyll‐a is shown to be complex. Whereas the spring bloom generates the highest phytoplankton concentration for coastal Alaska, the north and east coasts of Haida Gwaii, Queen Charlotte Sound, the Strait of Georgia, and coastal Oregon and California, it is the fall bloom that normally generates the highest concentration for the west coast of Vancouver Island, Juan de Fuca Strait, and the west coast of Washington. The highest satellite‐sensed chlorophyll concentrations occur in the Strait of Georgia, where mean values are at least two times higher than elsewhere in the northeast Pacific. Moreover, the annual average surface chlorophyll concentration has increased significantly in the Strait of Georgia from 5.9 mg m−3 in 1998 (a major El Niño year) to 8.9 mg m−3 in 2010 (a moderate El Niño year), suggesting an enhancement of biological productivity. Similarly, surface chlorophyll in the waters north and east of Haida Gwaii increased from 2.0 mg m−3 in 1998 to 2.8 mg m−3 in 2010. In all British Columbia regions, except the Strait of Georgia, the annual average chlorophyll concentration was highest in 2008, a year with prolonged La Niña conditions in the North Pacific. In the Strait of Georgia, the highest chlorophyll concentration was observed during the near neutral ENSO conditions of the spring of 2007. This article is protected by copyright. All rights reserved.
      PubDate: 2015-05-20T03:31:37.348619-05:
      DOI: 10.1002/2014JC010496
       
  • On the seasonal variations of salinity of the tropical Atlantic mixed
           layer
    • Authors: I. Camara; Nicolas Kolodziejczyk, Juliette mignot, Alban Lazar, Amadou. T. Gaye
      Abstract: The physical processes controlling the mixed layer salinity (MLS) seasonal budget in the tropical Atlantic ocean are investigated using a regional configuration of a ocean general circulation model. The analysis reveals that the MLS cycle is generally weak in comparison of individual physical processes entering in the budget, because of strong compensation. In evaporative regions, around the surface salinity maxima, the ocean acts to freshen the mixed layer against the action of evaporation. Poleward of the southern SSS maxima, the freshening is ensured by geostrophic advection, the vertical salinity diffusion and, during winter, a dominant contribution of the convective entrainment. On the equatorward flanks of the SSS maxima, Ekman transport mainly contributes to supply freshwater from ITCZ regions while vertical salinity diffusion adds on the effect of evaporation. All these terms are phase‐locked through the effect of the wind. Under the seasonal march of the ITCZ and in coastal areas affected by river (7°S:15°N), the upper ocean freshening by precipitations and/or runoff is attenuated by vertical salinity diffusion. In the eastern equatorial regions seasonal cycle of wind forced surface currents advect freshwaters which are mixed with subsurface saline water because of the strong vertical turbulent diffusion. In all these regions, the vertical diffusion presents an important contribution to the MLS budget by providing, in general, an upwelling flux of salinity. It is generally due to vertical salinity gradient and mixing due to winds. Furthermore, in the equator where the vertical shear, associated to surface horizontal currents, is developed, the diffusion depends also on the sheared flow stability. This article is protected by copyright. All rights reserved.
      PubDate: 2015-05-18T04:04:32.59997-05:0
      DOI: 10.1002/2015JC010865
       
  • Decadal and seasonal changes in temperature, salinity, nitrate, and
           chlorophyll in inshore and offshore waters along southeast Australia
    • Authors: Paige Kelly; Lesley Clementson, Vincent Lyne
      Abstract: Sixty years of oceanographic in‐situ data at Port Hacking (34ºS) and Maria Island (42ºS), and 15 years of satellite‐derived chlorophyll‐a (chl‐a) in inshore and offshore waters of southeast Australia show changes in the seasonality and trend of water properties consistent with long‐term intensification and southerly extensions of East Australian Current (EAC) water. Decadal analyses reveal that the EAC extension water at Maria Island increased gradually from the 1940s‐1980s, followed by a rapid increase since the 1990s. This acceleration coincided with enhanced winter nitrate, implying increased injections of Sub‐Antarctic water at Maria Island. Satellite‐derived chl‐a at six coastal sites and offshore companion sites in the western Tasman Sea, showed significant inshore‐offshore variations in seasonal cycle and long‐term trend. After 2004/2005, the Maria Island seasonal cycle became increasingly similar to those of Bass Strait and St Helens, suggesting that the EAC extension water was extending further southward. Comparative analyses of inshore‐offshore sites showed that the presence of EAC extension water declined offshore. Seasonal cycles at Maria Island show a recent shift away from the traditional spring bloom, towards increased winter biomass, and enhanced primary productivity consistent with extensions of warm, energetic EAC extension water and more frequent injections of cooler, fresher nitrate‐replete waters. Overall, we find complex temporal, latitudinal and inshore‐offshore changes in multiple water masses, particularly at Maria Island, and changes in primary productivity that will profoundly impact fisheries and ecosystems. This article is protected by copyright. All rights reserved.
      PubDate: 2015-05-18T03:59:49.011985-05:
      DOI: 10.1002/2014JC010646
       
  • A global‐scale map of isoprene and volatile organic iodine in
           surface seawater of the Arctic, northwest Pacific, Indian, and Southern
           Oceans
    • Authors: Atsushi Ooki; Daiki Nomura, Shigeto Nishino, Takashi Kikuchi, Yoko Yokouchi
      Abstract: Isoprene (C5H8) and three volatile organic iodine compounds (VOIs: CH3I, C2H5I, and CH2ClI) in surface seawater were measured in the western Arctic, Northwest Pacific, Indian, and Southern oceans during the period 2008–2012. These compounds are believed to play an important role in the marine atmospheric chemistry after their emission. The measurements were performed with high time‐resolution (1–6 h intervals) using an online equilibrator gas‐chromatography mass‐spectrometer. C5H8 was most abundant in high‐productivity transitional waters and eutrophic tropical waters. The chlorophyll‐a normalized production rates of C5H8 were high in the warm sub‐tropical and tropical waters, suggesting the existence of a high emitter of C5H8 in the biological community of the warm waters. High concentrations of the three VOIs in highly productive transitional water were attributed to biological productions. For CH3I, the highest concentrations were widely distributed in the basin area of the oligotrophic subtropical NW Pacific, probably due to photochemical production and/or high emission rates from phytoplankton. In contrast, the lowest concentrations of C2H5I in subtropical waters were attributed to photochemical removal. Enhancement of CH2ClI concentrations in the shelf–slope areas of the Chukchi Sea and the transitional waters of the NW Pacific in winter suggested that vertical mixing with subsurface waters by regional upwelling or winter cooling act to increase the CH2ClI concentrations in surface layer. Sea–air flux calculations revealed that the fluxes of CH2ClI were the highest among the three VOIs in shelf–slope areas; the CH3I flux was highest in basin areas. This article is protected by copyright. All rights reserved.
      PubDate: 2015-05-15T20:56:21.885026-05:
      DOI: 10.1002/2014JC010519
       
  • Sea‐surface salinity fronts and associated salinity minimum zones in
           the Tropical Ocean
    • Authors: Lisan Yu
      Abstract: The Intertropical Convergence Zone (ITCZ) is a major source of the surface freshwater input to the tropical open ocean. Under the ITCZ, sea‐surface salinity (SSS) fronts that extend zonally across the basins are observed by the Aquarius/SAC‐D mission and Argo floats. This study examined the evolution and forcing mechanisms of the SSS fronts. It is found that, although the SSS fronts are sourced from the ITCZ‐freshened surface waters, the formation, structure, and propagation of these fronts are governed by the trade‐wind driven Ekman processes. Three features characterize the governing role of Ekman forcing. First, the SSS fronts are associated with near‐surface salinity minimum zones (SMZs) of 50‐80 m deep. The SMZs are formed during December‐March when the near‐equatorial Ekman convergence zone concurs with an equatorward displaced ITCZ. Second, after the formation, the SMZs are carried poleward away at a speed of ∼3.5 km day−1 by Ekman transport. The monotonic poleward propagation is a sharp contrast to the seasonal north/south oscillation of the ITCZ. Lastly, each SMZ lasts about 12 – 15 months until dissipated at latitudes beyond 10°N/S. The persistence of more than one calendar year allows two SMZs to coexist during the formation season (December – March), with the newly formed SMZ located near the equator while the SMZ that is formed in the previous year located near the latitudes of 10‐15° poleward after one year's propagation. The contrast between the ITCZ and SMZ highlights the dominance of Ekman dynamics on the relationship between the SSS and the ocean water cycle. This article is protected by copyright. All rights reserved.
      PubDate: 2015-05-15T20:54:40.639214-05:
      DOI: 10.1002/2015JC010790
       
  • Photosynthetic parameters in the northern South China Sea in relation to
           phytoplankton community structure
    • Authors: Yuyuan Xie; Bangqin Huang, Lizhen Lin, Edward A. Laws, Lei Wang, Shaoling Shang, Tinglu Zhang, Minhan Dai
      Abstract: (Many recent models for retrieval of primary production in the sea from ocean‐colour data are temperature‐based. But previous studies in low latitudes have shown that models that include phytoplankton community structure can have improved predictive capability. In this study, we measured photosynthetic parameters from photosynthesis‐irrandiance (P‐E) experiments, phytoplankton absorption coefficients, and phytoplankton community structure derived from algal pigments during four cruises in the northern South China Sea. The maximum quantum yield of CO2 (ΦmC) and the chlorophyll a‐normalized P‐E curve light‐limited slope (αB) varied significantly with the blue‐to‐red ratio of phytoplankton absorption peaks (aph(435)/aph(676)) (p 
      PubDate: 2015-05-15T20:54:10.670397-05:
      DOI: 10.1002/2014JC010415
       
  • Local water mass modifications by a solitary meander in the Agulhas
           Current
    • Authors: Greta M. Leber; Lisa M. Beal
      Abstract: We present full‐depth hydrographic and velocity observations across a solitary meander within the Agulhas Current, and assess the degree to which the meander causes upwelling, cross‐frontal mixing, and diapycnal transport. These events can input nutrients onto the continental shelf as well as advect larvae offshore. We find that the meander drives inshore upwelling with vertical velocities of at least 13 m day– 1. The meander also causes diapycnal transport resulting in 1°C cooler and 0.25 fresher central waters and 1°C warmer, 0.25 saltier intermediate waters below the thermocline. We introduce a new coordinate system that separates these changes into kinematic changes due to meandering and property changes along transport streamlines. This reveals that most of the observed diapycnal transport below the thermocline is due to property changes. We find a small amount of enhanced cross‐frontal mixing associated with solitary meanders of the Agulhas Current, but it is statistically insignificant. We believe this is due to the strongly barotropic nature of the meandering Agulhas Current, which maintains a deep steering level that inhibits mixing. This article is protected by copyright. All rights reserved.
      PubDate: 2015-05-13T17:22:31.829362-05:
      DOI: 10.1002/2015JC010863
       
  • A review of trend models applied to sea level data with reference to the
           “acceleration‐deceleration debate”
    • Authors: Hans Visser; Sönke Dangendorf, Arthur C. Petersen
      Abstract: Global sea levels have been rising through the past century and are projected to rise at an accelerated rate throughout the 21st century. This has motivated a number of authors to search for already existing accelerations in observations, which would be, if present, vital for coastal protection planning purposes. No scientific consensus has been reached yet as to how a possible acceleration could be separated from intrinsic climate variability in sea level records. This has led to an intensive debate on its existence and, if absent, also on the general validity of current future projections. Here we shed light on the controversial discussion from a methodological point of view. To do so we provide a comprehensive review of trend methods used in the community so far. This resulted in an overview of 30 methods, each having its individual mathematical formulation, flexibilities and characteristics. We illustrate that varying trend approaches may lead to contradictory acceleration–deceleration inferences. As for statistics‐oriented trend methods we argue that checks on model assumptions and model selection techniques yield a way out. However, since these selection methods all have implicit assumptions, we show that good modeling practices are of importance too. We conclude at this point that (i) several differently characterized methods should be applied and discussed simultaneously, (ii) uncertainties should be taken into account to prevent biased or wrong conclusions, and (iii) removing internally generated climate variability by incorporating atmospheric or oceanographic information helps to uncover externally forced climate change signals. This article is protected by copyright. All rights reserved.
      PubDate: 2015-05-13T17:19:54.829059-05:
      DOI: 10.1002/2015JC010716
       
  • Formation of winter water on the Canadian Beaufort shelf: New insight from
           observations during 2009–2011
    • Authors: Jennifer M. Jackson; Humfrey Melling, Jennifer V. Lukovich, David Fissel, David G. Barber
      Abstract: The Arctic halocline forms a cold stratified barrier between the seasonally‐modified near‐surface layers and deeper Atlantic‐derived waters. Its low temperature is maintained by intrusions of cold water formed over Arctic shelves in winter. Surprisingly, cold salty (33) water capable of halocline ventilation (Beaufort Sea Winter Water: BSWW) has been observed in the Beaufort Sea during some winters despite the low salinity (20‐25) of shelf waters there in summer. This study uses year‐round data from moored instruments on the Beaufort shelf and slope during 2009‐2011 to investigate the mechanisms involved. Our analysis reveals that four air‐sea interaction processes contribute to the formation of BSWW – flushing of the low salinity surface water from the shelf via Ekman transport in late summer and early fall, compensatory upwelling of more saline halocline water onto the shelf, net seaward ice drift that promotes ice production by maintaining a flaw lead, and entrainment of dense upwelled water into the freezing surface layer on the inner shelf. This work moves beyond earlier studies in revealing that while weather conditions were more favourable to BSWW formation in the winter of 2010‐11 than in 2009‐10, the difference was more strongly influenced by Ekman transport (offshore at the surface, onshore at the seabed) than by differences in cumulative brine injection from ice growth. The strength of the Ekman circulation over the Canadian Beaufort shelf in winter, and its inter‐annual variation, have significance for surface nutrient renewal and for the cross‐shelf transport of pollutants at the surface and the seabed. This article is protected by copyright. All rights reserved.
      PubDate: 2015-05-13T17:19:29.242542-05:
      DOI: 10.1002/2015JC010812
       
  • A method for multiscale optimal analysis with application to Argo data
    • Authors: Alison R. Gray; Stephen C. Riser
      Abstract: This study presents an optimal analysis method for estimating from observations the large and small scale components of a field. This technique relies on an iterative generalized least squares procedure to determine the statistics of the small scale fluctuations directly from the data and is thus especially valuable when such information is not known a priori. The use of spherical radial basis functions in fitting the large scale signal is suggested, particularly when the domain is sufficiently large. Two test cases illustrate several of the properties of this procedure, demonstrate its utility, and provide practical guidelines for its use. This method is then applied to observations collected by the Argo array of profiling floats to produce global gridded absolute geostrophic velocity estimates. This article is protected by copyright. All rights reserved.
      PubDate: 2015-05-13T02:38:54.068474-05:
      DOI: 10.1002/2014JC010208
       
  • Modeling long‐term change of planktonic ecosystems in the northern
           South China Sea and the upstream Kuroshio Current
    • Authors: Qian P. Li; Yanjun Wang, Yuan Dong, Jianping Gan
      Abstract: Field studies suggested that the biogeochemical settings and community structures are substantial different between the central Northern South China Sea (NSCS) and the upstream Kuroshio Current (KC). In particular, the water column of KC is characterized by substantially lower nutrients and productivity but higher Trichodesmium abundance and nitrogen fixation compared to the NSCS. The mechanism driving the difference of the two marine ecosystems, however, remains inadequately understood. Here, a one‐dimensional biogeochemical model was developed to simulate the long‐term variability of lower‐trophic planktonic ecosystem for two pelagic stations in the NSCS and the KC near the Luzon Strait. The physical model included the vertical mixing driven by air‐sea interaction and the Ekman pumping induced by wind stress curl. The biological model was constructed by modifying a nitrogen‐based NPZD model with the incorporation of phosphorus cycle and diazotroph nitrogen fixation. After validation by several field datasets, the model was used to study the impact of long‐term physical forcing on ecosystem variability in the two distinct stations. Our results suggested that nutrient transport above nitracline during summer was largely controlled by vertical turbulent mixing, while Ekman pumping was important for nutrient transport below the nitracline. Our results also indicated that diazotroph community structure and N2 fixation in the NSCS and the KC could be strongly influenced by physical processes through the impacts on vertical nutrient fluxes. The disadvantage of diazotroph in the NSCS in compared to the KC during the summer could be attributed to its high nitrate fluxes from subsurface leading to outcompete of diazotrophs by faster growing non‐diazotroph phytoplankton. This article is protected by copyright. All rights reserved.
      PubDate: 2015-05-13T02:36:54.82823-05:0
      DOI: 10.1002/2014JC010609
       
  • Dynamics of ocean surface mixed layer variability in the Indian Ocean
    • Authors: Andreas Schiller; Peter R. Oke
      Abstract: We present a new methodology that allows quantifying the impact of individual terms of the temperature and salinity conservation within the mixed layer on mixed layer depth (MLD). The method is applied to output from an ocean general circulation model in the Indian Ocean to investigate variability and changes in MLD. On seasonal timescales and for most areas of the Indian Ocean variability of MLD is tightly linked to all thermohaline budget terms. In the Indian Ocean at approximately 20oS the MLD co‐varies with surface heat and freshwater fluxes on intraseasonal and interannual timescales. The geography of the region includes the Leeuwin Current, plus the tropical eastern Indian Ocean for interannual surface freshwater fluxes. The range of seasonal amplitudes of MLD variability varies with individual budget terms but is typically within 1 m/month to 100 m/month. The ocean footprints of an intraseasonal tropical cyclone, tropical and mid‐latitude seasonal temperature and salinity budgets and interannual variability associated with the Indian Ocean Dipole Mode are analyzed. The results reveal close relationships of the thermohaline budgets within the mixed layer with the variability of the MLD. The associated tendencies of changes in MLD are consistent with Argo and satellite‐based observations of tendencies within the mixed layer and sea‐surface temperature and salinity. This article is protected by copyright. All rights reserved.
      PubDate: 2015-05-12T18:19:03.238594-05:
      DOI: 10.1002/2014JC010538
       
  • On the wave and current interaction with a rippled seabed in the coastal
           ocean bottom boundary layer
    • Authors: Aditya R. Nayak; Cheng Li, Bobak T. Kiani, Joseph Katz
      Abstract: Interactions of currents and waves with a rippled seabed in the inner part of the coastal ocean bottom boundary layer are studied using particle image velocimetry, ADV and bottom roughness measurements. Mean velocity profiles collapse with appropriate scaling in the log layer, but vary substantially in the roughness sublayer. When wave induced motions are similar or greater than the mean current, the hydrodynamic roughness (z0) determined from velocity profiles is substantially larger than directly measured values. The roughness signature in turbulent energy spectra persists with elevation when its scale falls in the dissipation range, but decays in the log layer for larger roughness elements. Reynolds shear stress profiles peak in the lower parts of the log layer, diminishing below it, and gradually decaying at higher elevations. In contrast, wave shear stresses are negligible within the log layer, but become significant within the roughness sublayer. This phenomenon is caused by an increase in the magnitude and phase lag of the vertical component of wave‐induced motion. No single boundary layer length scale collapses the Reynolds stresses, but both the Prandtl mixing length and eddy viscosity profiles agree well with the classical model of linear increase with elevation, especially near the seabed. Within the log region, profiles of shear production and dissipation rates of turbulence converge. Below it, dissipation rapidly increases, peaking near the seabed. Conversely, the shear production decays near the seabed, in agreement with the eddy viscosity model, but in contrast to both laboratory and computational rough wall studies. This article is protected by copyright. All rights reserved.
      PubDate: 2015-05-12T17:56:47.341721-05:
      DOI: 10.1002/2014JC010606
       
  • Estimating wave energy dissipation in the surf zone using thermal infrared
           imagery
    • Authors: Roxanne J. Carini; C. Chris Chickadel, Andrew T. Jessup, Jim Thomson
      Abstract: Thermal infrared (IR) imagery is used to quantify the high spatial and temporal variability of dissipation due to wave breaking in the surf zone. The foam produced in an actively breaking crest, or wave roller, has a distinct signature in IR imagery. A retrieval algorithm is developed to detect breaking waves and extract wave roller length using measurements taken during the Surf Zone Optics 2010 experiment at Duck, NC. The remotely‐derived roller length and an in situ estimate of wave slope are used to estimate dissipation due to wave breaking by means of the wave‐resolving model by Duncan [1981]. The wave energy dissipation rate estimates show a pattern of increased breaking during low tide over a sand bar, consistent with in situ turbulent kinetic energy dissipation rate estimates from fixed and drifting instruments over the bar. When integrated over the surf zone width, these dissipation rate estimates account for 40‐69% of the incoming wave energy flux. The Duncan [1981] estimates agree with those from a dissipation parameterization by Janssen and Battjes [2007], a wave energy dissipation model commonly applied within nearshore circulation models. This article is protected by copyright. All rights reserved.
      PubDate: 2015-05-12T17:56:36.569721-05:
      DOI: 10.1002/2014JC010561
       
  • A breaking internal wave in the surface ocean boundary layer
    • Authors: Danielle J. Wain; Jonathan M. Lilly, Adrian H. Callaghan, Igor Yashayaev, Brian Ward
      Abstract: High‐temporal resolution measurements in the Labrador Sea surface layer are presented using an upwardly‐profiling autonomous microstructure instrument, which captures an internal wave in the act of breaking at the base of the surface mixed layer, driving turbulence levels two to three orders of magnitude above the background. While lower‐frequency (near‐inertial) internal waves are known to be important sources of turbulence, we report here a higher frequency internal wave breaking near the ocean surface. Due to observational limitations, the exact nature of the instability cannot be conclusively identified, but the interaction of wave‐induced velocity with unresolved background shear appears to be the most likely candidate. These observations add a new process to the list of those currently being considered as potentially important for near‐surface mixing. The geographical distribution and global significance of such features is unknown, and underscores the need for more extensive small‐scale, rapid observations of the ocean surface layer. This article is protected by copyright. All rights reserved.
      PubDate: 2015-05-12T17:56:26.541053-05:
      DOI: 10.1002/2014JC010416
       
  • Infragravity‐wave dynamics in a barred coastal region, a numerical
           study
    • Authors: Dirk P. Rijnsdorp; Gerben Ruessink, Marcel Zijlema
      Abstract: This paper presents a comprehensive numerical study into the infragravity‐wave dynamics at a field site, characterised by a gently‐sloping barred beach. The non‐hydrostatic wave‐flow model SWASH was used to simulate the local wave field for a range of wave conditions (including mild and storm conditions). The extensive spatial coverage of the model allowed us to analyse the infragravity‐wave dynamics at spatial scales not often covered before. Overall, the model predicted a wave field that was representative of the natural conditions, supporting the model application to analyse the wave dynamics. The infragravity‐wave field was typically dominated by leaky waves, except near the outer bar where bar‐trapped edge waves were observed. Relative contributions of bar‐trapped waves peaked during mild conditions, when they explained up to 50% of the infragravity variance. Near the outer bar, the infragravity wave growth was partly explained by nonlinear energy transfers from short‐waves. This growth was strongest for mild conditions, and decreased for more energetic conditions when short‐waves were breaking at the outer bar. Further shoreward, infragravity waves lost most of their energy, due to a combination of nonlinear transfers, bottom friction, and infragravity‐wave breaking. Nonlinear transfers were only effective near the inner bar, whereas near the shoreline (where losses were strongest) the dissipation was caused by the combined effect of bottom friction and breaking. This study demonstrated the model's potential to study wave dynamics at field scales not easily covered by in‐situ observations. This article is protected by copyright. All rights reserved.
      PubDate: 2015-05-12T17:56:18.574539-05:
      DOI: 10.1002/2014JC010450
       
  • The effect of the El Niño‐Southern Oscillation on U.S. regional
           and coastal sea level
    • Authors: B.D. Hamlington; R.R. Leben, K.‐Y. Kim, R.S. Nerem, L.P. Atkinson, P.R. Thompson
      Abstract: Although much of the focus on future sea level rise concerns the long‐term trend associated with anthropogenic warming, on shorter timescales, internal climate variability can contribute significantly to regional sea level. Such sea level variability should be taken into consideration when planning efforts to mitigate the effects of future sea level change. In this study, we quantify the contribution to regional sea level of the El Niño‐Southern Oscillation (ENSO). Through cyclostationary empirical orthogonal function analysis (CSEOF) of the long reconstructed sea level dataset and of a set of United States tide gauges, two global modes dominated by Pacific Ocean variability are identified and related to ENSO and, by extension, the Pacific Decadal Oscillation. By estimating the combined contribution of these two modes to regional sea level, we find that ENSO can contribute significantly on short time scales, with contributions of up to 20 cm along the west coast of the U.S. The CSEOF decomposition of the long tide gauge records around the U.S. highlights the influence of ENSO on the U.S. east coast. Tandem analyses of both the reconstructed and tide gauge records also examine the utility of the sea level reconstructions for near‐coast studies. This article is protected by copyright. All rights reserved.
      PubDate: 2015-05-12T17:53:25.419219-05:
      DOI: 10.1002/2014JC010602
       
  • Climate change signal and uncertainty in CMIP5‐based projections of
           global ocean surface wave heights
    • Authors: Xiaolan L. Wang; Yang Feng, Val R. Swail
      Abstract: This study uses the analysis of variance approaches to quantify the climate change signal and uncertainty in multi‐model ensembles of statistical simulations of significant wave height (Hs), which are based on the CMIP5 historical, RCP4.5 and RCP8.5 forcing scenario simulations of sea level pressure. Here, the signal of climate change refers to the temporal variations caused by the prescribed forcing. “Significant” means “significantly different from zero at 5% level”. In a 4‐model ensemble of Hs simulations, the common signal ‐ the signal that is simulated in all the 4 models ‐ is found to strengthen over time. For the historical followed by RCP8.5 scenario, the common signal in annual mean Hs is found to be significant in 16.6% and 82.2% of the area by year 2005 and 2099, respectively. The global average of the variance proportion of the common signal increases from 0.75% in year 2005 to 12.0% by year 2099. The signal is strongest in the eastern tropical Pacific (ETP), featuring significant increases in both the annual mean and maximum of Hs in this region. The climate model uncertainty (i.e., inter‐model variability) is significant nearly globally; its magnitude is comparable to or greater than that of the common signal in most areas, except in the ETP where the signal is much larger. In a 20‐model ensemble of Hs simulations for the period 2006‐2099, the model uncertainty is found to be significant globally; it is about 10 times as large as the variability between the RCP4.5 and 8.5 scenarios. This article is protected by copyright. All rights reserved.
      PubDate: 2015-05-12T17:53:12.469206-05:
      DOI: 10.1002/2015JC010699
       
  • Linkage between lateral circulation and near‐surface vertical mixing
           in a coastal plain estuary
    • Authors: K. D. Huguenard; A. Valle‐Levinson, M. Li, R. J. Chant, A. J. Souza
      Abstract: Microstructure and current velocity measurements were collected at a cross‐channel transect in the James River under spring and neap tidal conditions in May 2010 to study cross‐estuary variations in vertical mixing. Results showed that near‐surface mixing was related to lateral circulation during the ebb phase of a tidal cycle, and that the linkage was somewhat similar from neap to spring tides. During neap tides, near‐surface mixing was generated by the straining of lateral density gradients influenced by the advection of fresh, riverine water on the right side (looking seaward) of the transect. Spring tide results revealed similar findings on the right side of the cross‐section. However on the left side, the straining by velocity shears acted in concert with density straining. Weak along‐estuary velocities over the left shoal were connected to faster velocities in the channel via a clockwise lateral circulation (looking seaward). These results provided evidence that in the absence of direct wind forcing, near‐surface vertical mixing can occur from mechanisms uncoupled from bottom friction. This article is protected by copyright. All rights reserved.
      PubDate: 2015-05-12T17:51:30.257075-05:
      DOI: 10.1002/2014JC010679
       
  • Issue Information
    • PubDate: 2015-05-12T07:01:38.587409-05:
      DOI: 10.1002/jgrc.20860
       
  • Passive buoyant tracers in the ocean surface boundary layer: 1. Influence
           of equilibrium wind‐waves on vertical distributions
    • Authors: T. Kukulka; K. Brunner
      Abstract: This paper is the first of a two part series that investigates passive buoyant tracers in the ocean surface boundary layer. The first part examines the influence of equilibrium wind‐waves on vertical tracer distributions, based on large eddy simulations (LES) of the wave‐averaged Navier‐Stokes equation. The second part applies the model to investigate observations of buoyant microplastic marine debris, which has emerged as a major ocean pollutant. The LES model captures both Langmuir turbulence (LT) and enhanced turbulent kinetic energy input due to breaking waves (BW) by imposing equilibrium wind‐wave statistics for a range of wind and wave conditions. Concentration profiles of LES agree well with analytic solutions obtained for an eddy diffusivity profile that is constant near the surface and transitions into the K‐Profile Parameterization (KPP) profile shape at greater depth. For a range of wind and wave conditions, the eddy diffusivity normalized by the product of water‐side friction velocity and mixed layer depth, $h$, mainly depends on a single non‐dimensional parameter, the peak wavelength (which is related to Stokes drift decay depth) normalized by $h$. For smaller wave ages, BW critically enhances near surface mixing, while LT effects are relatively small. For greater wave ages, both BW and LT contribute to elevated near surface mixing and LT significantly increases turbulent transport at greater depth. We identify a range of realistic wind and wave conditions for which only Langmuir (and not BW or shear‐driven) turbulence is capable of deeply submerging buoyant tracers. This article is protected by copyright. All rights reserved.
      PubDate: 2015-05-09T03:10:53.598473-05:
      DOI: 10.1002/2014JC010487
       
  • Arctic sea ice and freshwater sensitivity to the treatment of the
           atmosphere‐ice‐ocean surface layer
    • Authors: François Roy; Matthieu Chevallier, Greg Smith, Frédéric Dupont, Gilles Garric, Jean‐François Lemieux, Youyu Lu, Fraser Davidson
      Abstract: Global simulations are presented focusing on the atmosphere‐ice‐ocean (AIO) surface layer (SL) in the Arctic. Results are produced using an ocean model (NEMO) coupled to two different sea ice models: the Louvain‐La‐Neuve single‐category model (LIM2) and the Los Alamos multi‐category model (CICE4). A more objective way to adjust the sea ice‐ocean drag is proposed compared to a coefficient tuning approach. The air‐ice drag is also adjusted to be more consistent with the atmospheric forcing dataset. Improving the AIO SL treatment leads to more realistic results, having a significant impact on the sea ice volume trend, sea ice thickness and the Arctic freshwater (FW) budget. The physical mechanisms explaining this sensitivity are studied. Improved sea ice drift speeds result in less sea ice accumulation in the Beaufort Sea, correcting a typical ice thickness bias. Sea ice thickness and drag parameters affect how atmospheric stress is transferred to the ocean, thereby influencing Ekman transport and FW retention in the Beaufort Gyre (BG). Increasing sea ice‐ocean roughness reduces sea ice growth in winter by reducing ice deformation and lead fractions in the BG. It also increases the total Arctic FW content by reducing sea ice export through Fram Strait. Similarly, increasing air‐ice roughness increases the total Arctic FW content by increasing FW retention in the BG. This article is protected by copyright. All rights reserved.
      PubDate: 2015-05-09T03:05:33.62801-05:0
      DOI: 10.1002/2014JC010677
       
  • Shallow ocean response to tropical cyclones observed on the continental
           shelf of the northwestern South China Sea
    • Authors: Bing Yang; Yijun Hou, Po Hu, Ze Liu, Yahao Liu
      Abstract: Based on observed temperature and velocity in 2005 in northwestern South China Sea, the shallow ocean responses to three tropical cyclones were examined. The oceanic response to Washi was similar to common observations with 2°C cooling of the ocean surface and slight warming of the thermocline resulted from vertical entrainment. Moreover, the wave field was dominated by first mode near‐inertial oscillations which were red‐shifted and trapped by negative background vorticity leading to an e‐folding time scale of 12 days. The repeated reflections by the surface and bottom boundaries were thought to yield the successive emergence of higher modes. The oceanic responses to Vicente appeared to be insignificant with cooling of the ocean surface by only 0.5°C and near‐inertial currents no larger than 0.10 m/s as a result of a deepened surface mixed layer. However, the oceanic responses to Typhoon Damrey were drastic with cooling of 4.5°C near the surface and successive barotropic‐like near‐inertial oscillations. During the forced stage, the upper ocean heat content decreased conspicuously by 11.65% and the stratification was thoroughly destroyed by vertical mixing. In the relaxation stage, the water particle had vertical displacement of 20 to 30 m generated by inertial pumping. The current response to Damrey was weaker than Washi due to the deepened mixed layer and the destroyed stratification. Our results suggested that the shallow water oceanic responses to tropical cyclones varied significantly with the intensity of tropical cyclones, and was affected by local stratification and background vorticity. This article is protected by copyright. All rights reserved.
      PubDate: 2015-05-09T03:05:24.031859-05:
      DOI: 10.1002/2015JC010783
       
  • Shear‐generated turbulence in the equatorial Pacific produced by
           small vertical‐scale flow features
    • Authors: K. J. Richards; A. Natarov, E. Firing, Y. Kashino, S. M. Soares, M. Ishizu, G. S. Carter, J. H. Lee, K. I. Chang
      Abstract: We investigate the characteristics of shear–generated turbulence in the natural environment by considering data from a number of cruises in the western equatorial Pacific. In this region the vertical shear of the flow is dominated by flow structures that have a relatively small vertical scale of O(10m). Combining data from all cruises we find a strong relationship between the turbulent dissipation rate, ϵ, vertical shear, S, and buoyancy frequency, N. Examination of ϵ at a fixed value of Richardson number, Ri = N2∕S2, shows that ∈∝ut2N for a wide range of values of N, where ut is an appropriate velocity scale which we assume to be the horizontal velocity scale of the turbulence. The implied vertical length scale, ℓv = ut∕N, is consistent with theoretical and numerical studies of stratified turbulence. Such behavior is found for Ri 
      PubDate: 2015-05-09T02:47:40.231859-05:
      DOI: 10.1002/2014JC010673
       
  • Long‐term observations of North Atlantic Current transport at the
           gateway between western and eastern Atlantic
    • Authors: Achim Roessler; Monika Rhein, Dagmar Kieke, Christian Mertens
      Abstract: In the western North Atlantic, warm and saline water is brought by the North Atlantic Current (NAC) from the subtropics into the subpolar gyre. Four Inverted Echo Sounders with high precision pressure sensors (PIES) were moored between 47°40'N and 52°30'N to study the main pathways of the NAC from the western into the eastern basin. The array configuration that forms three segments (northern, central and southern) allows partitioning of the NAC and some assessment of NAC flow paths through the different Mid‐Atlantic Ridge fracture zones. We exploit the correlation between the NAC transport measured between 2006 and 2010 and the geostrophic velocity from altimeter data to extend the time series of NAC transports to the period from 1992 to 2013. The mean NAC transport over the entire 21‐years is 27±5 Sv, consisting of 60% warm water of subtropical origin, and 40% subpolar water. We did not find a significant trend in the total transport time series, but individual segments had opposing trends, leading to a more focused NAC in the central subsection and decreasing transports in the southern and northern segments. The spectral analysis exhibits several significant peaks. The two most prominent are around 120 days, identified as the time scale of meanders and eddies, and at 4‐9 years, most likely related to the NAO. Transport composites for the years of highest and lowest NAO indices showed a significantly higher transport (+2.9 Sv) during strong NAO years, mainly in the southern segment. This article is protected by copyright. All rights reserved.
      PubDate: 2015-05-08T13:53:40.115138-05:
      DOI: 10.1002/2014JC010662
       
  • Sea surface temperature as a tracer to estimate cross‐shelf
           turbulent diffusivity
    • Authors: Yadan Mao; Peter V. Ridd
      Abstract: Accurate parameterization of spatially‐variable diffusivity in complex shelf regions such as the Great Barrier Reef (GBR) lagoon is an unresolved issue for hydrodynamic models. This leads to large uncertainties to the flushing time derived from them and to the evaluation of ecosystem resilience to terrestrially‐derived pollution. In fact, numerical hydrodynamic models and analytical cross‐shore diffusion models have predicted very different flushing times for the GBR lagoon. Nevertheless, scarcity of in‐situ measurements used previously in the latter method prevents derivation of detailed diffusivity profiles. Here, detailed cross‐shore profiles of diffusivity were calculated explicitly in a closed form for the first time from the steady state transects of sea surface temperature for different sections of the GBR lagoon. We find that diffusivity remains relatively constant within the inner lagoon (< ∼20km) where tidal current is weak, and increases linearly with sufficiently large tidal amplitude in reef‐devoid regions, but increases dramatically where the reef matrixes start and fluctuates with reef size and density. The cross‐shelf profile of steady‐state salinity calculated using the derived diffusivity values agrees well with field measurements. The calculated diffusivity values are also consistent with values derived from satellite‐tracked drifters. Flushing time by offshore diffusion is of the order of 1 month, suggesting the important role of turbulent diffusion in flushing the lagoon, especially in reef‐distributed regions. The results imply that previous very large residence times predicted by numerical hydrodynamic models may result from underestimation of diffusivity. Our findings can guide parameterization of diffusivity in hydrodynamic modelling. This article is protected by copyright. All rights reserved.
      PubDate: 2015-05-08T13:53:02.635678-05:
      DOI: 10.1002/2015JC010745
       
  • Role of circulation scales and water mass distributions on larval fish
           habitats in the eastern tropical Pacific off Mexico
    • Authors: Cristina A. León‐Chávez; Emilio Beier, Laura Sánchez‐Velasco, Eric Desmond Barton, Victor M. Godínez
      Abstract: On the basis of five oceanographic cruises carried out in the Eastern Tropical Pacific off Mexico, relationships between the larval fish habitats (areas inhabited by larval fish assemblages) and the environmental circulation scales (mesoscale, seasonal and interannual) were examined. Analysis of in situ data over a grid of hydrographic stations and oblique zooplankton hauls with bongo net (505 µm) was combined with orthogonal robust functions decomposition applied to altimetry anomalies obtained from satellite. During both cool (March and June) and warm (August and November) periods, Bray‐Curtis dissimilarity Index defined three recurrent larval fish habitats which varied in species composition and extent as a function of the environmental scales. The variability of the Tropical larval fish habitat (characterized by high species richness, and dominated by Vinciguerria lucetia, Diogenichthys laternatus and Diaphus pacificus) was associated with the seasonal changes. The Transitional‐California Current larval fish habitat (dominated by V. lucetia and D. laternatus, with lower mean abundance and lower species richness than in the Tropical habitat) and Coastal‐and‐Upwelling larval fish habitat (dominated by Bregmaceros bathymaster) was associated mainly with mesoscale activity induced by eddies and with coastal upwelling. During February 2010, the Tropical larval fish habitat predominated offshore and the Transitional‐California Current larval fish habitat was not present, which we attribute to the effect of El Niño conditions. Thus the mesoscale, seasonal and interannual environmental scales affect the composition and extension of larval fish habitats. This article is protected by copyright. All rights reserved.
      PubDate: 2015-05-08T13:47:43.646649-05:
      DOI: 10.1002/2014JC010289
       
  • Mesoscale eddies and Trichodesmium spp. distributions in the southwestern
           North Atlantic
    • Authors: Elise M. Olson; Dennis J. McGillicuddy, Glenn R. Flierl, Cabell S. Davis, Sonya T. Dyhrman, John B. Waterbury
      Abstract: Correlations of Trichodesmium colony abundance with the eddy field emerged in two segments of Video Plankton Recorder observations made in the southwestern North Atlantic during fall 2010 and spring 2011. In fall 2010, local maxima in abundance were observed in cyclones. We hypothesized surface Ekman transport convergence as a mechanism for trapping buoyant colonies in cyclones. Idealized models supported the potential of this process to influence the distribution of buoyant colonies over timescales of several months. In spring 2011, the highest vertically integrated colony abundances were observed in anticyclones. These peaks in abundance correlated with anomalously fresh water, suggesting riverine input as a driver of the relationship. These contrasting results in cyclones and anticyclones highlight distinct mechanisms by which mesoscale eddies can influence the abundance and distribution of Trichodesmium populations of the southwestern North Atlantic. This article is protected by copyright. All rights reserved.
      PubDate: 2015-05-05T09:26:44.865176-05:
      DOI: 10.1002/2015JC010728
       
  • Using ocean bottom pressure from the Gravity Recovery and Climate
           Experiment (GRACE) to estimate transport variability in the southern
           Indian Ocean
    • Authors: Jessica K. Makowski; Don P. Chambers, Jennifer A. Bonin
      Abstract: Previous studies have suggested that ocean bottom pressure (OBP) from the Gravity Recovery and Climate Experiment (GRACE) can be used to measure the depth‐averaged, or barotropic, transport variability of the Antarctic Circumpolar Current (ACC). Here, we use GRACE OBP observations to calculate transport variability in a region of the Southern Indian Ocean encompassing the major fronts of the ACC. We use a statistical analysis of a simulated GRACE‐like data set to determine the uncertainty of the estimated transport for the 2003.0‐2013.0 time period. We find that when the transport is averaged over 60° of longitude, the uncertainty (one standard error) is close to 1 Sv (1 Sv = 106 m3 s−1) for low‐pass filtered transport, which is significantly smaller than the signal and lower than previous studies have found. The interannual variability is correlated with the Southern Annual Mode (SAM) (0.61), but more highly correlated with circumpolar zonally averaged winds between 45°S – 65°S (0.88). GRACE transport reflects significant changes in transport between 2007 and 2009 that is observed in the zonal wind variations but not in the SAM index. We also find a statistically significant trend in transport (‐1.0 ± 0.4 Sv yr−1, 90% confidence) that is correlated with a local deceleration in zonal winds related to an asymmetry in the SAM on multi‐decadal periods. This article is protected by copyright. All rights reserved.
      PubDate: 2015-05-05T09:22:34.951344-05:
      DOI: 10.1002/2014JC010575
       
  • Subannual variability of total alkalinity distributions in the
           northeastern Gulf of Mexico
    • Authors: Bo Yang; Robert H. Byrne, Rik Wanninkhof
      Abstract: The subannual variability of total alkalinity (TA) distributions in the northeastern Gulf of Mexico was examined through the use of TA data from ship‐based water sampling, historical records of riverine TA, and contemporaneous model output of surface currents and salinity. TA variability was restricted to the upper 150 m of the water column, where relationships between salinity and TA were controlled primarily by subannual variations in the extent of mixing between seawater and river water. A transition in TA distribution patterns between the river‐dominated northern margin (near the Mississippi–Atchafalaya River System) and the ocean current‐dominated eastern margin (West Florida Shelf) was observed. An index for riverine alkalinity input was formulated to provide insights about riverine alkalinity contributions in the upper water column. Spatial and temporal variations of total alkalinity in the northeastern Gulf of Mexico are primarily controlled by riverine TA inputs and ocean currents. This article is protected by copyright. All rights reserved.
      PubDate: 2015-05-04T03:29:30.526174-05:
      DOI: 10.1002/2015JC010780
       
  • Carbon export efficiency and phytoplankton community composition in the
           Atlantic sector of the Arctic Ocean
    • Authors: Frédéric A.C. Le Moigne; Alex J. Poulton, Stephanie A. Henson, Chris J. Daniels, Glaucia M. Fragoso, Elaine Mitchell, Sophie Richier, Benjamin C. Russell, Helen E. K. Smith, Geraint A. Tarling, Jeremy R. Young, Mike Zubkov
      Abstract: Arctic primary production is sensitive to reductions in sea ice cover, and will likely increase into the future. Whether this increased primary production (PP) will translate into increased export of particulate organic carbon (POC) is currently unclear. Here we report on the POC export efficiency during summer 2012 in the Atlantic sector of the Arctic Ocean. We coupled 234‐Thorium based estimates of the export flux of POC to onboard incubation based estimates of PP. Export efficiency (defined as the fraction of PP that is exported below 100 m depth: ThE‐ratio) showed large variability (0.09 ± 0.19 to 1.3 ± 0.3). The highest ThE‐ratio (1.3 ± 0.3) was recorded in a mono‐specific bloom of Phaeocystis pouchetii located in the ice edge. Blooming diatom dominated areas also had high ThE‐ratios (0.1 ± 0.1 to 0.5 ± 0.2), while mixed and/or pre‐bloom communities showed lower ThE‐ratios (0.10 ± 0.03 to 0.19 ± 0.05). Furthermore, using oxygen saturation, bacterial abundance, bacterial production, and zooplankton oxygen demand, we also investigated spatial variability in the degree to which this sinking material may be remineralised in the upper mesopelagic (< 300 m). Our results suggest that blooming diatoms and P. pouchetii can export a significant fraction of their biomass below the surface layer (100 m) in the open Arctic Ocean. Also, we show evidence that the material sinking from a P. pouchetii bloom may be remineralised (>100m) at a similar rate as the material sinking from diatom blooms in the upper mesopelagic, contrary to previous findings. This article is protected by copyright. All rights reserved.
      PubDate: 2015-05-04T03:29:21.091699-05:
      DOI: 10.1002/2015JC010700
       
  • Hydrodynamic and sediment transport modeling of New River Inlet (NC) under
           the interaction of tides and waves
    • Authors: Jia‐Lin Chen; Tian‐Jian Hsu, Fengyan Shi, Britt Raubenheimer, Steve Elgar
      Abstract: The interactions between waves, tidal currents, and bathymetry near New River Inlet, NC, USA are investigated to understand the effects on the resulting hydrodynamics and sediment transport. A quasi‐3D nearshore community model, NearCoM‐TVD, is used in this integrated observational and modeling study. The model is validated with observations of waves and currents at 30 locations, including in a recently dredged navigation channel and a shallower channel, and on the ebb tidal delta, for a range of flow and offshore wave conditions during May 2012. In the channels, model skills for flow velocity and wave height are high. Near the ebb tidal delta, the model reproduces the observed rapid onshore (offshore) decay of wave heights (current velocities). Model results reveal that this sharp transition coincides with the location of the breaker zone over the ebb tidal delta, which is modulated by semi‐diurnal tides and by wave intensity. The modulation of wave heights is primarily owing to depth changes rather than direct wave‐current interaction. The modeled tidally averaged residual flow patterns show that waves play an important role in generating vortices and landward‐directed currents near the inlet entrance. Numerical experiments suggest that these flow patterns are associated with the channel‐shoal bathymetry near the inlet, similar to the generation of rip currents. Consistent with other inlet studies, model results suggest that tidal currents drive sediment fluxes in the channels, but that sediment fluxes on the ebb tidal delta are driven primarily by waves. This article is protected by copyright. All rights reserved.
      PubDate: 2015-05-04T03:29:12.719333-05:
      DOI: 10.1002/2014JC010425
       
  • Transit time distributions and oxygen utilization rates from
           chlorofluorocarbons and sulfur hexafluoride in the Southeast Pacific Ocean
           
    • Authors: Rolf E. Sonnerup; Sabine Mecking, John L. Bullister, Mark J. Warner
      Abstract: Chlorofluorocarbons‐11 (CFC‐11), CFC‐12, and sulfur hexafluoride (SF6) were measured during the December 2007 – February 2008 CLIVAR/Repeat Hydrography (RH) P18 section along ∼103ºW in the Southeast Pacific Ocean. Transit‐time distributions (TTDs) of 1‐D transport that matched all three tracers were consistent with high Peclet number flow ventilating the subtropical mode water and the main subtropical thermocline (30ºS‐42ºS, 200‐800 m). In the subtropics, TTDs with predominantly advective transport predicted decadal increases in CFC‐12 and CFC‐11 consistent with those observed comparing 1994 WOCE with 2007/8 CLIVAR/RH data, indicating steady ventilation in this region, and consistent with the near‐zero changes observed in dissolved oxygen. The mean transport timescales from the tracer‐tuned TTDs were used to estimate apparent oxygen utilization rates (OURs) on the order of 8‐20 μmol kg−1 yr−1 at ∼200 m depth, attenuating to ∼2 μmol kg−1 yr−1 typically by 500 m depth in this region. Depth‐integrated over the thermocline, these OURs implied carbon export rates from the overlying sea surface on the order of ∼1.8 moles C m−2 yr−1 from 30°S to 45°S, 2 – 2.5 moles C m−2 yr−1 from 45°S to 52°S, and 2.5 – 3.5 moles C m−2 yr−1 from 52°S to 60°S. This article is protected by copyright. All rights reserved.
      PubDate: 2015-04-29T10:41:34.419839-05:
      DOI: 10.1002/2015JC010781
       
  • Precessional forced evolution of the Indian Ocean Dipole
    • Authors: Yue Wang; ZhiMin Jian, Ping Zhao, JunMing Chen, Dong Xiao
      Abstract: In a transient accelerated simulation of a coupled climate model, we identified a zonal dipole‐like pattern of sea surface temperature (SST) anomalies in the tropical Indian Ocean, which is forced by precessional insolation changes since 300 ka and named as the paleo‐IOD. A positive paleo‐IOD mean state at 23‐kyr's precessional band exhibits warmer and wetter conditions over the western Indian Ocean and cooler and drier conditions over the eastern tropical Indian Ocean from August to October. The zonal thermal seesaw at the sea surface can extend downward to the subsurface ocean between 60 m and 80 m and accompanies stronger oceanic upwelling in the eastern tropical Indian Ocean. The associated boreal summer‐autumn tropospheric circulation anomalies are characterized by anomalous ascent over the western Indian Ocean and anomalous descent over the southeastern tropical Indian Ocean, with anomalous easterlies at the surface along the equatorial Indian Ocean. This positive paleo‐IOD largely originates from local air‐sea interactions that are induced by the increased summer insolation, and is also contributed by the reduced boreal winter insolation through oceanic “heat memory effect”. Our simulated dipole mode index (DMI) of SST is qualitatively consistent with the paleoceanographic reconstructed DMI based on the UK37 proxy of SST at precessional band. This paleo‐IOD provides a possible explanation for the in‐phase precessional variation between boreal winter insolation and the UK37 proxy of SST in the eastern tropical Indian Ocean. This article is protected by copyright. All rights reserved.
      PubDate: 2015-04-28T15:27:42.448712-05:
      DOI: 10.1002/2015JC010713
       
  • Characterizing the semidiurnal internal tide off Tasmania using glider
           data
    • Authors: Daniel Boettger; Robin Robertson, Luc Rainville
      Abstract: The spatial structure of the semidiurnal internal tide in the vicinity of Tasmania is characterized using temperature and salinity data from Seaglider and Slocum glider deployments. Wavelet analysis of isopycnal displacements measured by the gliders was used to isolate the semidiurnal internal tide, with a solid signal observed both to the east and to the south of Tasmania. The signal south of Tasmania was attributed to local forcing, whilst that to the east of Tasmania was found to have propagated from the south east to the north west – a result which supports previous studies indicating the presence of an internal tidal beam originating over the Macquarie Ridge, south of New Zealand. Displacement amplitudes were observed to be amplified in the vicinity of the continental slope, with the incoming tidal beam shown to be both reflected and scattered on the continental slope and shelf, and energy transferred to higher modes. This article is protected by copyright. All rights reserved.
      PubDate: 2015-04-28T15:26:58.344949-05:
      DOI: 10.1002/2015JC010711
       
  • M2 baroclinic tide variability modulated by the ocean circulation south of
           Japan
    • Authors: Sergey M. Varlamov; Xinyu Guo, Toru Miyama, Kaoru Ichikawa, Takuji Waseda, Yasumasa Miyazawa
      Abstract: We analyze a concurrent simulation result of the ocean circulation and tidal currents using a data assimilative ocean general circulation model covering the Western North Pacific with horizontal resolution of 1/36to investigate possible interactions between them. Four sites of active M2 internal tide variability in open ocean (hot spots): Tokara Strait, Izu Ridge, Luzon Strait, and Ogasawara Ridge, are detected from both the satellite observation and the simulation. Energy cycle analysis of the simulated M2 baroclinic tide indicates two types of the hot spots: dissipation (Tokara Strait and Izu Ridge) and radiation (Luzon Strait and Ogasawara Ridge) dominant sites. Energy conversion from barotropic to baroclinic M2 tides at the hot spots is modulated considerably by the lower‐frequency changes in the density field. Modulation at the two spots (Tokara Strait and Izu Ridge) is affected by the Kuroshio path variation together with the seasonal variation of the shallow thermocline. At the other two sites, influence from changes in the relatively deep stratification through the Kuroshio intrusion into South China Sea (Luzon Strat) and mesoscale eddy activity (Ogasawara Ridge) is dominant in the modulation. This article is protected by copyright. All rights reserved.
      PubDate: 2015-04-25T02:52:13.345462-05:
      DOI: 10.1002/2015JC010739
       
  • Seagrass metabolism across a productivity gradient using the eddy
           covariance, Eulerian control volume, and biomass addition techniques
    • Authors: Matthew H. Long; Peter Berg, James L. Falter
      Abstract: The net ecosystem metabolism of the seagrass Thalassia testudinum was studied across a nutrient and productivity gradient in Florida Bay, Florida, using the Eulerian control volume, eddy covariance, and biomass addition techniques. In situ oxygen fluxes were determined by a triangular Eulerian control volume with sides 250m long and by eddy covariance instrumentation at its center. The biomass addition technique evaluated the aboveground seagrass productivity through the net biomass added. The spatial and temporal resolutions, accuracies, and applicability of each method were compared. The eddy covariance technique better resolved the short‐term flux rates and the productivity gradient across the bay, which was consistent with the long‐term measurements from the biomass addition technique. The net primary production rates from the biomass addition technique, which were expected to show greater autotrophy due to the exclusion of sediment metabolism and belowground production, were 71, 53, and 30 mmol carbon m−2 d−1 at 3 sites across the bay. The net ecosystem metabolism was 35, 25, and 11 mmol oxygen m−2 d−1 from the eddy covariance technique and 10, ‐103, and 14 mmol oxygen m−2 d−1 from the Eulerian control volume across the same sites, respectively. The low‐flow conditions in the shallow bays allowed for periodic stratification and long residence times within the Eulerian control volume that likely limited its precision. Overall, the eddy covariance technique had the highest temporal resolution while producing accurate long‐term flux rates that surpassed the capabilities of the biomass addition and Eulerian control volume techniques in these shallow coastal bays. This article is protected by copyright. All rights reserved.
      PubDate: 2015-04-25T02:16:01.954549-05:
      DOI: 10.1002/2014JC010352
       
  • Distinguishing ichthyogenic turbulence from geophysical turbulence
    • Authors: Kandaga Pujiana; James N. Moum, William D. Smyth, Sally J. Warner
      Abstract: Measurements of currents and turbulence beneath a geostationary ship in the equatorial Indian Ocean during a period of weak surface forcing revealed unexpectely strong turbulence beneath the surface mixed layer. Coincident with the turbulence was a marked reduction of the current speeds registered by shipboard Doppler current profilers, and an increase in their variability. At a mooring 1 km away, measurements of turbulence and currents showed no such anomalies. Correlation with the shipboard echosounder measurements indicate that these nighttime anomalies were associated with fish aggregations beneath the ship. The fish created turbulence by swimming against the strong zonal current in order to remain beneath the ship, and their presence affected the Doppler speed measurements. The principal characteristics of the resultant ichthyogenic turbulence are i) low wavenumber rolloff of shear spectra in the inertial subrange relative to geophysical turbulence, ii) Thorpe overturning scales that are small compared with the Ozmidov scale, and iii) low mixing efficiency. These factors extend previous findings by Gregg and Horne [2009] to a very different biophysical regime, and support the general conclusion that the biological contribution to mixing the ocean via turbulence is negligible. This article is protected by copyright. All rights reserved.
      PubDate: 2015-04-25T02:01:20.640456-05:
      DOI: 10.1002/2014JC010659
       
  • Inorganic carbon system dynamics in landfast Arctic sea ice during the
           early‐melt period
    • Authors: Kristina A. Brown; Lisa A. Miller, C.J. Mundy, Tim Papakyriakou, Roger Francois, Michel Gosselin, Gauthier Carnat, Kyle Swystun, Philippe D. Tortell
      Abstract: We present results of a six‐week time series of carbonate system and stable isotope measurements investigating the effects of sea ice on air‐sea CO2 exchange during the early melt period in the Canadian Arctic Archipelago. Our observations revealed significant changes in sea ice and sackhole brine carbonate system parameters that were associated with increasing temperatures and the buildup of chlorophyll a in bottom ice. The warming sea‐ice column could be separated into distinct geochemical zones where biotic and abiotic processes exerted different influences on inorganic carbon and pCO2 distributions. In the bottom ice, biological carbon uptake maintained undersaturated pCO2 conditions throughout the time series, while pCO2 was supersaturated in the upper ice. Low CO2 permeability of the sea ice matrix and snow cover effectively impeded CO2 efflux to the atmosphere, despite a strong pCO2 gradient. Throughout the middle of the ice column, brine pCO2 decreased significantly with time and was tightly controlled by solubility, as sea ice temperature and in situ melt dilution increased. Once the influence of melt dilution was accounted for, both CaCO3 dissolution and seawater mixing were found to contribute alkalinity and dissolved inorganic carbon to brines, with the CaCO3 contribution driving brine pCO2 to values lower than predicted from melt‐water dilution alone. This field study reveals a dynamic carbon system within the rapidly warming sea ice, prior to snow melt. We suggest that the early spring period drives the ice column towards pCO2 undersaturation, contributing to a weak atmospheric CO2 sink as the melt period advances. This article is protected by copyright. All rights reserved.
      PubDate: 2015-04-25T02:01:08.403843-05:
      DOI: 10.1002/2014JC010620
       
  • Role of tides on the formation of the Antarctic Slope Front at the
           Weddell‐Scotia Confluence
    • Authors: M. M. Flexas; M. P. Schodlok, L. Padman, D. Menemenlis, A. H. Orsi
      Abstract: The structure of the Antarctic Slope Front (ASF) and the associated Antarctic Slope Current (ASC) on the Scotia Sea side of the Weddell‐Scotia Confluence (WSC) is described using data from a hydrographic survey and three 1‐year long moorings across the continental slope. The ASC in this region flows westwards along isobaths with an annual mean speed of ∼0.2 m s– 1, with time variability dominated by the K1 and O1 tidal diurnal constituents, a narrowband oscillation with ∼2‐week period attributable to the spring/neap tidal cycle, and seasonal variability. Realistic and idealized high‐resolution numerical simulations are used to determine the contribution of tides to the structure of the ASF and the speed of the ASC. Two simulations forced by realistic atmospheric forcing and boundary conditions integrated with and without tidal forcing show that tidal forcing is essential to reproduce the measured ASF/ASC cross‐slope structure, the time variability at our moorings, and the reduced stratification within the WSC. Two idealized simulations run with tide‐only forcing, one with a homogeneous ocean and the other with initial vertical stratification that is laterally homogeneous, show that tides can generate the ASC and ASF through volume flux convergence along the slope initiated by effects including the Lagrangian component of tidal rectification and mixing at the seabed and in the stratified ocean interior. Climate models that exclude the effects of tides will not correctly represent the ASF and ASC or their influence on the injection of intermediate and dense waters from the WSC to the deep ocean. This article is protected by copyright. All rights reserved.
      PubDate: 2015-04-24T15:16:37.868895-05:
      DOI: 10.1002/2014JC010372
       
  • What drives seasonal change in oligotrophic area in the subtropical North
           Atlantic?
    • Authors: Apurva C. Dave; Andrew D. Barton, M. Susan Lozier, Galen A. McKinley
      Abstract: The oligotrophic regions of the subtropical gyres cover a significant portion of the global ocean, and exhibit considerable but poorly understood intraseasonal, interannual, and longer‐term variations in spatial extent. Here, using historical observations of surface ocean nitrate, wind, and currents, we have investigated how horizontal and vertical supplies of nitrate control seasonal changes in the size and shape of oligotrophic regions of the subtropical North Atlantic. In general, the oligotrophic region of the subtropical North Atlantic is associated with the region of weak vertical supply of nitrate. Though the total vertical supply of nitrate here is generally greater than the total horizontal supply, we find that seasonal expansion and contraction of the oligotrophic region is consistent with changes in horizontal supply of nitrate. In this dynamic periphery of the subtropical gyre, the seasonal variations in chlorophyll are linked to variations in horizontal nitrate supply that facilitate changes in intracellular pigment concentrations, and to a lesser extent, phytoplankton biomass. Our results suggest that horizontal transports of nutrient are crucial in setting seasonal cycles of chlorophyll in large expanses of the subtropical North Atlantic, and may play a key and underappreciated role in regulating interannual variations in these globally important marine ecosystems. This article is protected by copyright. All rights reserved.
      PubDate: 2015-04-24T15:11:01.182347-05:
      DOI: 10.1002/2015JC010787
       
  • Sea ice floe size distribution in the marginal ice zone: Theory and
           numerical experiments
    • Authors: Jinlun Zhang; Axel Schweiger, Michael Steele, Harry Stern
      Abstract: To better describe the state of sea ice in the marginal ice zone (MIZ) with floes of varying thicknesses and sizes, both an ice thickness distribution (ITD) and a floe size distribution (FSD) are needed. In this work, we have developed a FSD theory that is coupled to the ITD theory of Thorndike et al. [1975] in order to explicitly simulate the evolution of FSD and ITD jointly. The FSD theory includes a FSD function and a FSD conservation equation in parallel with the ITD equation. The FSD equation takes into account changes in FSD due to ice advection, thermodynamic growth, and lateral melting. It also includes changes in FSD because of mechanical redistribution of floe size due to ice ridging and, particularly, ice fragmentation induced by stochastic ocean surface waves. The floe size redistribution due to ice fragmentation is based on the assumption that wave‐induced breakup is a random process such that when an ice floe is broken, floes of any smaller sizes have an equal opportunity to form, without being either favored or excluded. To focus only on the properties of mechanical floe size redistribution, the FSD theory is implemented in a simplified ITD and FSD sea ice model for idealized numerical experiments. Model results show that the simulated cumulative floe number distribution (CFND) follows a power law as observed by satellites and airborne surveys. The simulated values of the exponent of the power law, with varying levels of ice breakups, are also in the range of the observations. It is found that floe size redistribution and the resulting FSD and mean floe size do not depend on how floe size categories are partitioned over a given floe size range. The ability to explicitly simulate multicategory FSD and ITD together may help to incorporate additional model physics, such as FSD‐dependent ice mechanics, surface exchange of heat, mass, and momentum, and wave‐ice interactions. This article is protected by copyright. All rights reserved.
      PubDate: 2015-04-24T15:06:27.024355-05:
      DOI: 10.1002/2015JC010770
       
  • Observed interannual variability of near‐surface salinity in the Bay
           of Bengal
    • Authors: Vimlesh Pant; M. S. Girishkumar, T.V.S. Udaya Bhaskar, M. Ravichandran, Fabrice Papa, V. P. Thangaprakash
      Abstract: An in‐situ gridded data of salinity, comprising Argo and CTD profiles, has been used to study the interannual variability of near‐surface salinity (within 30 m from sea surface) in the Bay of Bengal (BoB) during the years 2005‐2013. In addition to the broad agreement with earlier studies on the north‐to‐south gradient of surface salinity and general features of seasonal variability of salinity, the data also revealed few episodes of enhanced freshening in the BoB. The observations show distinct anomalous low salinity (< 2 psu) waters in the northern BoB during June‐February of the years 2006‐07 (Y67), 2011‐12 (Y12), and 2012‐13 (Y23). The anomalous freshening during these years show similar life cycle, such as, it starts in the northern BoB during July‐September of current summer and extends up to February‐March of next winter with a southward propagation. Analysis showed that the oceanic and atmospheric conditions associated with positive Indian Ocean Dipole (pIOD) lead to these freshening events, and IOD rather than El Niño/Southern Oscillation (ENSO) controls the interannual variability of salinity in the BoB. The mixed layer salt budget analysis indicates the dominant role of local fresh water flux (horizontal advection) on the observed salinity tendency during summer (winter) monsoon season. Enhanced precipitation associated with pIOD lead to enhanced freshening in northern BoB during June‐September, which remains to this region with prevailing summer monsoon circulation. The weakening or absence of southward east India coastal current (EICC) during October‐December of these freshening years trapped anomalous freshwater in the northern BoB. This article is protected by copyright. All rights reserved.
      PubDate: 2015-04-24T05:42:42.137606-05:
      DOI: 10.1002/2014JC010340
       
  • Diurnal cycling of sea surface temperature, salinity, and current in the
           CESM coupled climate model
    • Authors: W. G. Large; J. M. Caron
      Abstract: A simple scheme is developed to represent Sea Surface Diurnal Cycling (SSDC) in Coupled General Circulation Models (CGCM). It follows Zeng and Beljaars [2005], but in addition to a night‐time deep well mixed ocean boundary layer and a deep day‐time stable layer, a shallow sub‐grid‐scale stable diurnal boundary layer is allowed to develop during the day, followed by a deepening convective layer. These four regimes have empirical property profiles and their governing parameters are determined by comparison of idealized experiments with published in situ and satellite observations. Mixing across the base of the shallow stable layer is governed by a gradient Richardson number, so prognostic equations are solved for salinity and current, as well as temperature. A conclusion is that the timing of peak warming depends on diurnal shear. The SSDC is implemented in the Community Earth System Model (CESM) for multiple purposes: the maximum diurnal amplitude of warming is found to exceed 5∘C and to be more than 2∘C over most of the ocean; the global distribution of average daytime minus night‐time SST is used to validate the SSDC against a satellite SST product; and the mean seasonal surface heat flux and precipitation from an uncoupled CESM atmosphere are used to show the climate impacts that might be expected in a CGCM. Two major conclusions are that these impacts are not negligible and that much of the observed signals of diurnal cycling are captured by SSDC without the computational expense of resolving the relevant ocean processes. This article is protected by copyright. All rights reserved.
      PubDate: 2015-04-24T03:27:39.8201-05:00
      DOI: 10.1002/2014JC010691
       
  • Wave power variability and trends across the North Atlantic influenced by
           decadal climate patterns
    • Authors: Peter D. Bromirski; Daniel R. Cayan
      Abstract: Climate variations influence North Atlantic winter storm intensity and resultant variations in wave energy levels. A 60‐year hindcast allows investigation of the influence of decadal climate variability on long‐term trends of North Atlantic wave power, PW, spanning the 1948‐2008 epoch. PW variations over much of the eastern North Atlantic are strongly influenced by the fluctuating North Atlantic Oscillation (NAO) atmospheric circulation pattern, consistent with previous studies of significant wave height, Hs. Wave activity in the western Atlantic also responds to fluctuations in Pacific climate modes, including the Pacific North American (PNA) pattern and the El Niño/Southern Oscillation. The magnitude of upward long‐term trends during winter over the northeast Atlantic is strongly influcenced by heightened storm activity under the extreme positive phase of winter NAO in the early 1990's. In contrast, PW along the United States East Coast shows no increasing trend, with wave activity there most closely associated with the PNA. Strong wave power “events” exhibit significant upward trends along the Atlantic coasts of Iceland and Europe during winter months. Importantly, in opposition to the long‐term increase of PW, a recent general decrease in PW across the North Atlantic from 2000 to 2008 occurred. The 2000‐2008 decrease was associated with a general shift of winter NAO to its negative phase, underscoring the control exerted by fluctuating North Atlantic atmospheric circulation on PW trends. This article is protected by copyright. All rights reserved.
      PubDate: 2015-04-24T02:29:48.725286-05:
      DOI: 10.1002/2014JC010440
       
  • Variational data assimilative modeling of the Gulf of Maine in spring and
           summer 2010
    • Authors: Yizhen Li; Ruoying He, Ke Chen, Dennis J. McGillicuddy
      Abstract: A data assimilative ocean circulation model is used to hindcast the Gulf of Maine (GOM) circulation in spring and summer 2010. Using the recently developed incremental strong constraint 4D Variational data assimilation algorithm, the model assimilates satellite sea surface temperature and in situ temperature and salinity profiles measured by expendable bathythermograph, Argo floats, and shipboard CTD casts. Validation against independent observations shows that the model skill is significantly improved after data assimilation. The data‐assimilative model hindcast reproduces the temporal and spatial evolution of the ocean state, showing that a sea level depression southwest of the Scotian Shelf played a critical role in shaping the gulf‐wide circulation. Heat budget analysis further demonstrates that both advection and surface heat flux contribute to temperature variability. The estimated time scale for coastal water to travel from the Scotian Shelf to the Jordan Basin is around 60 days, which is consistent with previous estimates based on in situ observations. Our study highlights the importance of resolving upstream and offshore forcing conditions in predicting the coastal circulation in the GOM. This article is protected by copyright. All rights reserved.
      PubDate: 2015-04-21T18:42:11.126721-05:
      DOI: 10.1002/2014JC010492
       
  • Vertical convergence of resuspended sediment and subducted phytoplankton
           to a persistent detached layer over the southern shelf of Monterey Bay,
           California
    • 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
           coast
    • 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
       
  • 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
           (Canada)
    • 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
           Partitioning
    • 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
           water
    • 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
           Archipelago
    • 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
           experiments
    • 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
           whitecaps
    • 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
           Basin
    • 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
           2007
    • 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,
           revisited
    • 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
       
  • 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
       
  • 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
       
  • 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
       
  • 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
       
  • Effect of glacial drainage water on the CO2 system and ocean acidification
           state in an Arctic tidewater‐glacier fjord during two contrasting
           years
    • 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
       
  • Bottom‐pressure signature of annual baroclinic Rossby waves in the
           northeast tropical Pacific Ocean
    • Authors: Christopher G. Piecuch
      Pages: 2449 - 2459
      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°W and 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 1 cm 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 ∼0.34 m 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.
      PubDate: 2015-04-02T09:49:19.861535-05:
      DOI: 10.1002/2014JC010667
       
  • 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
      Pages: 2460 - 2470
      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 and 700 m has increased over 15 cm s−1 during the 2 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 2 year strengthening of the Kuroshio, as well as the dipole‐like SSH trend can be adequately reproduced by a 1.5 layer nonlinear 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.
      PubDate: 2015-04-02T09:49:02.571312-05:
      DOI: 10.1002/2014JC010590
       
  • Advection of North Atlantic Deep Water from the Labrador Sea to the
           southern hemisphere
    • Authors: Monika Rhein; Dagmar Kieke, Reiner Steinfeldt
      Pages: 2471 - 2487
      Abstract: Recently formed Labrador Seawater (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 toward a homogenization between the LSW in the western and the eastern subpolar North Atlantic.
      PubDate: 2015-04-02T09:48:48.108009-05:
      DOI: 10.1002/2014JC010605
       
  • Local and remote wind stress forcing of the seasonal variability of the
           Atlantic Meridional Overturning Circulation (AMOC) transport at
           26.5°N
    • Authors: Jiayan Yang
      Pages: 2488 - 2503
      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 nonlocal 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 redistribution 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.
      PubDate: 2015-04-02T09:48:12.754271-05:
      DOI: 10.1002/2014JC010317
       
 
 
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