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Geophysical Research Letters     Full-text available via subscription   (Followers: 99, SJR: 3.323, h-index: 185)
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Journal Cover Journal of Geophysical Research : Oceans
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  • The study of droplet-laden turbulent airflow over waved water surface by
           direct numerical simulation
    • Authors: O.A. Druzhinin; Yu.I. Troitskaya, S.S. Zilitinkevich
      Abstract: The objective of the present paper is to elucidate possible effects of sea spray on the momentum transfer in marine boundary layer under strong wind-forcing conditions by performing direct numerical simulation (DNS) of turbulent, droplet-laden air-flow over a waved water surface. Three-dimensional, turbulent Couette air-flow is considered in DNS as a model of a constant-flux layer in the atmospheric surface layer. Two-dimensional stationary waves at the water surface are prescribed and assumed to be unaffected by the air-flow and/or droplets. Droplets are considered as non-deformable spheres and tracked in a Lagrangian framework, and their impact on the carrier flow is modeled with the use of a point-force approximation. The results show that drops dynamics and their impact on the carrier air-flow is controlled by the drops velocity at injection, the ratio of drops gravitational settling velocity versus the product of air friction velocity and Karman constant (Vg/κu∗), and the wave slope, ka. Drops injected into the flow with the surrounding air-flow velocity reduce the turbulent air-stress and increase mean air velocity as compared to the droplet-free case. On the other hand, the opposite effect is observed for drops injected with velocity equal to the water surface velocity, which increase the turbulent air-stress and reduce the mean wind velocity. This modification of the air-flow by drops is most pronounced for the ratio Vg/κu∗≈1, increases with drops mass loading and is reduced for steeper waves and smaller settling velocity. This article is protected by copyright. All rights reserved.
      PubDate: 2017-02-11T03:16:27.038481-05:
      DOI: 10.1002/2016JC012134
  • Ocean circulation and sea-ice thinning induced by melting ice shelves in
           the Amundsen Sea
    • Authors: Nicolas C. Jourdain; Pierre Mathiot, Nacho Merino, Gaël Durand, Julien Le Sommer, Paul Spence, Pierre Dutrieux, Gurvan Madec
      Abstract: A 1/12° ocean model configuration of the Amundsen Sea sector is developed to better understand the circulation induced by ice shelf melt and the impacts on the surrounding ocean and sea ice. Eighteen sensitivity experiments to drag and heat exchange coefficients at the ice shelf/ocean interface are performed. The total melt rate simulated in each cavity is function of the thermal Stanton number, and for a given thermal Stanton number, melt is slightly higher for lower values of the drag coefficient. Sub ice shelf melt induces a thermohaline circulation that pumps warm circumpolar deep water into the cavity. The related volume flux into a cavity is 100 to 500 times stronger than the melt volume flux itself. Ice-shelf melt also induces a coastal barotropic current that contributes 45±12% of the total simulated coastal transport. Due to the presence of warm circumpolar deep waters, the melt-induced inflow typically brings 4 to 20 times more heat into the cavities than the latent heat required for melt. For currently observed melt rates, approximately 6% to 31% of the heat that enters a cavity with melting potential is actually used to melt ice shelves. For increasing sub ice shelf melt rates, the transport in the cavity becomes stronger, and more heat is pumped from the deep layers to the upper part of the cavity then advected towards the ocean surface in front of the ice shelf. Therefore, more ice shelf melt induces less sea ice volume near the ice sheet margins. This article is protected by copyright. All rights reserved.
      PubDate: 2017-02-11T03:16:23.324722-05:
      DOI: 10.1002/2016JC012509
  • On the response of the Lorenz energy cycle for the Southern Ocean to
           intensified westerlies
    • Authors: Yang Wu; Zhaomin Wang, Chengyan Liu
      Abstract: The potential impact of intensified westerlies on the Lorenz Energy Cycle for the Southern Ocean is examined by employing a global eddy-permitting ocean-sea ice model. Two idealized sensitivity experiments are designed for this purpose: one is driven by 1992 forcing with weaker westerlies and the other driven by 1998 forcing with stronger westerlies. The intensified westerlies lead to the most significant increase of about 30% in the eddy kinetic energy (EKE) reservoir, followed by the mean kinetic energy (MKE) reservoir increase (17.9%), eddy available potential energy (EAPE) reservoir increase (8.6%), and mean available potential energy (MAPE) reservoir increase (6.5%). In contrast, the increases in the generations of kinetic energy and available potential energy are quite similar, ranging from 21% for EAPE generation to 26% for MKE generation. There are considerablly increased energy transfers from MKE to MAPE (about 75%) and from MAPE to EAPE (about 78%), reflecting greatly enhanced baroclinic instability pathway.The conversion rates are strongly influenced by large topography; in particular, a relatively large energy conversion from EKE to MKE exists in the regions associated with large topography, in contrast to the energy flow from MKE to EKE over the broad Southern Ocean. Under stronger wind forcing, all energy conversions are enhanced, and the increases in the conversion rates from EAPE to EKE and from EKE to MKE are more prominent than the increases from MKE to MAPE and from MAPE to EAPE near large topography. This article is protected by copyright. All rights reserved.
      PubDate: 2017-02-11T03:16:02.623644-05:
      DOI: 10.1002/2016JC012539
  • Estimating the distribution of colored dissolved organic matter during the
           Southern Ocean gas exchange experiment using four-dimensional variational
           data assimilation
    • Authors: C. E. Del Castillo; S. Dwivedi, T. W. N. Haine, D. T. Ho
      Abstract: We diagnosed the effect of various physical processes on the distribution of mixed-layer colored dissolved organic matter (CDOM) and a sulfur hexafluoride (SF6) tracer during the Southern Ocean Gas Exchange Experiment (SO GasEx). The biochemical upper ocean state estimate uses in situ and satellite biochemical and physical data in the study region, including CDOM (absorption coefficient and spectral slope), SF6, hydrography, and sea level anomaly. Modules for photo-bleaching of CDOM and surface transport of SF6 were coupled with an ocean circulation model for this purpose. The observed spatial and temporal variations in CDOM were captured by the state estimate without including any new biological source term for CDOM, assuming it to be negligible over the 26 days of the state estimate. Thermocline entrainment and photobleaching acted to diminish the mixed layer CDOM with time scales of 18 and 16 days, respectively. Lateral advection of CDOM played a dominant role and increased the mixed layer CDOM with a time scale of 12 days, whereas lateral diffusion of CDOM was negligible. A Lagrangian view on the CDOM variability was demonstrated by using the SF6 as a weighting function to integrate the CDOM fields. This and similar data assimilation methods can be used to provide reasonable estimates of optical properties, and other physical parameters over the short term duration of a research cruise, and help in the tracking of tracer releases in large scale oceanographic experiments, and in oceanographic process studies. This article is protected by copyright. All rights reserved.
      PubDate: 2017-02-11T03:15:57.046331-05:
      DOI: 10.1002/2016JC012406
  • Distribution of water masses and meltwater on the continental shelf near
           the Totten and Moscow University ice shelves
    • Authors: Alessandro Silvano; Stephen R. Rintoul, Beatriz Peña-Molino, Guy D. Williams
      Abstract: Warm waters flood the continental shelf of the Amundsen and Bellingshausen seas in West Antarctica, driving rapid basal melt of ice shelves. In contrast, waters on the continental shelf in East Antarctica are cooler and ice shelves experience relatively low rates of basal melt. An exception is provided by the Totten and Moscow University ice shelves on the Sabrina Coast, where satellite-derived basal melt rates are comparable to West Antarctica. Recent oceanographic observations have revealed that relatively warm (∼ -0.4°C) modified Circumpolar Deep Water (mCDW) enters the cavity beneath the Totten Ice Shelf through a 1100 m deep trough, delivering sufficient heat to drive rapid basal melt. Here we use observations from a recent summer survey to show that mCDW is widespread on the continental shelf of the Sabrina Coast, forming a warm (up to 0.3°C) and saline (34.5-34.6) bottom layer overlaid by cold (∼ freezing point) and fresh (salinity ∼ 34.3) Winter Water. Dense Shelf Water is not observed. A 1000 deep m trough allows water at -1.3°C to reach the Moscow University ice-shelf cavity to drive basal melt. Freshening by addition of glacial meltwater is widespread on the southern shelf at depths above 300-400 m, with maximum meltwater concentrations up to 4-5 ml l−1 observed in outflows from the ice-shelf cavities. Our observations indicate that the ocean properties on the Sabrina Coast more resemble those found on the continental shelf of the Amundsen and Bellingshausen seas than those typical of East Antarctica. This article is protected by copyright. All rights reserved.
      PubDate: 2017-02-11T03:15:51.487122-05:
      DOI: 10.1002/2016JC012115
  • An inverse method to derive surface fluxes from the closure of oceanic
           heat and water budgets: Application to the north-western Mediterranean Sea
    • Authors: G. Caniaux; L. Prieur, H. Giordani, J.-L. Redelsperger
      Abstract: The large amount of data collected during DeWEX, MOOSE and HyMeX campaigns in the north-western Mediterranean in 2012-2013 allowed to implement an inverse method to solve the difficult problem of heat and water budget closure. The inverse method is based on the simulation of the observed heat and water budgets, strongly constrained by observations collected during the campaigns and on the deduction of adjusted surface fluxes. The inverse method uses a genetic algorithm that generates 50.000 simulations of a single-column model and optimizes some adjustable coefficients introduced in the surface fluxes. Finally, the single-column model forced by the adjusted fluxes during one year and over a test area of about 300 x 300 km2 simulates the daily mean satellite bulk SST with an accuracy/uncertainty of 0.011±0.072°C, as well as daily mean SSS and residual buoyancy series deduced from wintertime analyses with an accuracy of 0.011±0.008 and 0.03±0.012 m2 s−2 respectively. The adjusted fluxes close the annual heat and rescaled water budgets by less than 5 W m−2. To our knowledge, this is the first time that such a flux dataset is produced. It can thus be considered as a reference for the north-western Mediterranean and be used for estimating other flux datasets, for forcing regional models and for process studies. Compared with the adjusted fluxes, some operational numerical weather prediction models (ARPEGE, NCEP, ERA-INTERIM, ECMWF and AROME), often used to force oceanic models, were evaluated: they are unable to retrieve the mean annual patterns and values. This article is protected by copyright. All rights reserved.
      PubDate: 2017-02-11T03:15:47.812012-05:
      DOI: 10.1002/2016JC012167
  • Seasonal and interannual variability of the eastern tropical Pacific fresh
    • Authors: S. Guimbard; N. Reul, B. Chapron, M. Umbert, C. Maes
      Abstract: The Eastern Pacific Fresh Pool (EPFP) is a large region of low Sea Surface Salinity (SSS) defined by values lower than 34 practical salinity scale within [5°S-30°N, 75°W-180°W]. The fresh pool dynamically responds to strong regional and seasonally varying ocean-atmosphere-land interactions (including monsoon rain, trade and gap winds and strong currents). Using more than five years of Soil Moisture and Ocean Salinity (SMOS) satellite sea surface salinity (SSS) and complementary satellite wind, rain, currents, and sea surface temperature data together with a historical ensemble of in situ products, the present study explores the seasonal and interannual dynamics of the fresh pool over the period 2004-2015. An important interannual variability of the maximal surface extension of the EPFP over the past decade is revealed with two extreme events (2012, 2015) occurring during the SMOS satellite period. These extremes are found to be related to the El Niño-Southern Oscillation (ENSO) phases and associated anomalies of precipitation, surface currents and trade wind in the central Pacific. In 2012 (La Niña), stronger trade winds coupled with a deficit of precipitation induced a minimum extension of the pool during the rainy season. Whereas, during the strong El Niño 2014-2015, the EPFP extension reached an unprecedented maximum value. A modification of the atmospheric freshwater fluxes and ocean surface currents during winter 2014 are found to have favored the onset of this abnormal fresh event. This article is protected by copyright. All rights reserved.
      PubDate: 2017-02-11T03:10:34.407101-05:
      DOI: 10.1002/2016JC012130
  • One-dimensional evolution of the upper water column in the Atlantic sector
           of the Arctic Ocean in winter
    • Authors: Ilker Fer; Algot K. Peterson, Achim Randelhoff, Amelie Meyer
      Abstract: A one-dimensional model is employed to reproduce the observed time evolution of hydrographic properties in the upper water column during winter, between 26 January and 11 March 2015, in a region north of Svalbard in the Nansen Basin of the Arctic Ocean. From an observed initial state, vertical diffusion equations for temperature and salinity give the hydrographic conditions at a later stage. Observations of microstructure are used to synthesize profiles of vertical diffusivity, K, representative of varying wind forcing conditions. The ice-ocean heat and salt fluxes at the ice-ocean interface are implemented as external source terms, estimated from the salt and enthalpy budgets, using friction velocity from the Rossby similarity drag relation, and the ice core temperature profiles. We are able to reproduce the temporal evolution of hydrography satisfactorily for two pairs of measured profiles, suggesting that the vertical processes dominated the observed changes. Sensitivity tests reveal a significant dependence on K. Variation in other variables, such as the temperature gradient of the sea ice, the fraction of heat going to ice melt, and the turbulent exchange coefficient for heat are relatively less important. The increase in salinity as a result of freezing and brine release is approximately 10%, significantly less than that due to entrainment (90%) from beneath the mixed layer. Entrainment was elevated during episodic storm events, leading to melting. The results highlight the contribution of storms to mixing in the upper Arctic Ocean and its impact on ice melt and mixed-layer salt and nutrient budgets. This article is protected by copyright. All rights reserved.
      PubDate: 2017-02-11T03:10:33.062768-05:
      DOI: 10.1002/2016JC012431
  • Understanding feedbacks between ocean acidification and coral reef
    • Authors: Yuichiro Takeshita
      Abstract: Biogeochemical feedbacks from benthic metabolism have been hypothesized as a potential mechanism to buffer some effects of ocean acidification on coral reefs. The article in JGR-Oceans by DeCarlo et al. demonstrates the importance of benthic community health on this feedback from Dongsha Atoll in the South China Sea. This article is protected by copyright. All rights reserved.
      PubDate: 2017-02-11T03:10:25.005992-05:
      DOI: 10.1002/2017JC012740
  • Physical forcing of late summer chlorophyll-a blooms in the oligotrophic
           eastern North Pacific
    • Authors: Takahiro Toyoda; Suguru Okamoto
      Abstract: We investigated physical forcing of late summer chlorophyll-a (chl-a) blooms in the oligotrophic eastern North Pacific Ocean by using ocean reanalysis and satellite data. Relatively large chl-a blooms as defined in this study occurred in August–October following sea surface temperature (SST) anomaly (SSTA) decreases, mixed layer deepening, and temperature and salinity increases at the bottom of the mixed layer. These physical conditions were apparently induced by the entrainment of subsurface water resulting from the destabilization of the surface layer caused by anomalous northward Ekman transport of subtropical waters of higher salinity. Salinity-normalized total alkalinity data provide supporting evidence for nutrient supply by the entrainment process. We next investigated the impact of including information about the entrainment on bloom identification. The results of analyses using reanalysis data and of those using only satellite data showed large SSTA decreases when the northward Ekman salinity transports were large, implying that the entrainment of subsurface water is well represented in both types of data. After surface-destabilizing conditions were established, relatively high surface chl-a concentrations were observed. The use of SST information can further improve the detection of high chl-a concentrations. Although the detection of high chl-a concentrations would be enhanced by finer data resolution and the inclusion of biogeochemical parameters in the ocean reanalysis, our results obtained by using existing reanalysis data as well as recent satellite data are valuable for better understanding and prediction of lower trophic ecosystem variability. This article is protected by copyright. All rights reserved.
      PubDate: 2017-02-09T10:56:13.757425-05:
      DOI: 10.1002/2016JC012423
  • From interannual to decadal—17 years of boundary current transports at
           the exit of the Labrador Sea
    • Authors: R. Zantopp; J. Fischer, M. Visbeck, J. Karstensen
      Abstract: Over the past 17 years, the western boundary current system of the Labrador Sea has been closely observed by maintaining the 53°N observatory (moorings and shipboard station data) measuring the top-to-bottom flow field offshore from the Labrador shelf break. Volume transports for the North Atlantic Deep Water (NADW) components were calculated using different methods, including gap filling procedures for deployment periods with suboptimal instrument coverage. On average the Deep Western Boundary Current (DWBC) carries 30.2 ± 6.6 Sv of NADW southward, which are almost equally partitioned between Labrador Sea Water (LSW, 14.9 Sv ± 3.9) and Lower North Atlantic Deep Water (LNADW, 15.3 Sv ± 3.8). The transport variability ranges from days to decades, with the most prominent multi-year fluctuations at interannual to near decadal time scales (± 5 Sv) in the LNADW overflow water mass. These long-term fluctuations appear to be in phase with the NAO-modulated wind fluctuations. The boundary current system off Labrador occurs as a conglomerate of nearly independent components, namely the shallow Labrador Current, the weakly sheared LSW range, and the deep baroclinic, bottom-intensified current core of the LNADW, all of which are part of the cyclonic Labrador Sea circulation. This structure is relatively stable over time, and the 120 km wide boundary current is constrained seaward by a weak counterflow which reduces the Deep Water export by 10 to 15%. This article is protected by copyright. All rights reserved.
      PubDate: 2017-02-09T10:51:26.67299-05:0
      DOI: 10.1002/2016JC012271
  • Eastward salinity anomaly propagation in the intermediate layer of the
           North Pacific
    • Authors: Shinya Kouketsu; Satoshi Osafune, Yuichiro Kumamoto, Hiroshi Uchida
      Abstract: An objective mapping with the data of profiling float array, maintained under the Argo project, revealed eastward propagation of long-term (>5 years) salinity anomalies in the subsurface and deep neutral density (γ) layers of 27.0–27.6 γ along the subarctic front in the North Pacific after 2000. Such propagation was previously inferred from water property variations along a few observation lines and from numerical simulations, mainly for shallow layers. In the western North Pacific, the signs of the anomalies were the same on and below the 27.0γ, whereas in the eastern North Pacific the sign on 27.0γ was opposite to those on 27.4γ. This difference was attributed mainly to slower advection in the deeper layers. These changes were larger than the standard errors inferred from the objective mapping at least. Furthermore, the variation revealed by the float array was similar to decadal changes observed along repeat ship-based observation lines, and they were also associated with changes in apparent oxygen utilization especially along 165°E. The small salinity changes in the deeper layers inferred from the float array were also detected as decadal differences in highly accurate trans-basin observations. Furthermore, because the extension of small changes into the subtropical gyre was also captured by the float and ship-based observations, the influence of the decadal changes on the isopycnal surfaces off the coast of Japan could appear relatively quickly, even in deeper layers (27.0–27.4 γ) in the North Pacific. This article is protected by copyright. All rights reserved.
      PubDate: 2017-02-09T10:51:20.162827-05:
      DOI: 10.1002/2016JC012118
  • A semianalytical MERIS green-red band algorithm for identifying
           phytoplankton bloom types in the East China Sea
    • Authors: Bangyi Tao; Zhihua Mao, Hui Lei, Delu Pan, Yan Bai, Qiankun Zhu, Zhenglong Zhang
      Abstract: A new bio-optical algorithm based on the green and red bands of the Medium Resolution Imaging Spectrometer (MERIS) is developed to differentiate the harmful algal blooms of Prorocentrum donghaiense Lu (P. donghaiense) from diatom blooms in the East China Sea (ECS). Specifically, a novel green-red index (GRI), actually an indicator for a(510) of bloom waters, is retrieved from a semi-analytical bio-optical model based on the green and red bands of phytoplankton-absorption and backscattering spectra. In addition, a MERIS-based diatom index (DIMERIS) is derived by adjusting a Moderate Resolution Imaging Spectroradiometer (MODIS) diatom index algorithm to the MERIS bands. Finally, bloom types are effectively differentiated in the feature spaces of the green-red index and DIMERIS. Compared with three previous MERIS-based quasi-analytical algorithm (QAA) algorithms and three existing classification methods, the proposed GRI and classification method have the best discrimination performance when using the MERIS data. Further validations of the algorithm by using several MERIS image series and near-concurrent in-situ observations indicate that our algorithm yields the best classification accuracy and thus can be used to reliably detect and classify P. donghaiense and diatom blooms in the ECS. This is the first time that the MERIS data have been used to identify bloom types in the ECS. Our algorithm can also be used for the successor of the MERIS, the Ocean and Land Color Instrument, which will aid the long-term observation of species succession in the ECS. This article is protected by copyright. All rights reserved.
      PubDate: 2017-02-09T10:51:05.256344-05:
      DOI: 10.1002/2016JC012368
  • The importance of lateral variability on exchange across the inner shelf
           south of Martha's Vineyard, MA
    • Authors: A. R. Kirincich; S. J. Lentz
      Abstract: This study examines the spatial variability of transport within the inner shelf south of Martha's Vineyard Massachusetts, its time and space dependence, and its importance to the total volume exchanged between the nearshore and the coastal ocean. The exchange of water across the inner shelf is often considered to be driven primarily by wind forcing, yet the effects of small-scale O(1-10 km) variability on the total exchange have not been well quantified. Using a combination of high-resolution HF radar-based surface currents and a dense array of moorings to document the lateral variability of across-shelf exchange, the cumulative wind-driven across-shelf transport over the summer stratified period was less than the volume of the inner-shelf onshore of the 25-m isobath. Along-shelf variations in the wind-driven exchange were as large as the spatial mean of the wind-driven exchange. A spatially varying time-averaged circulation caused by tidal rectification resulted in across-shelf exchange larger in magnitude than, and independent of, the integrated wind-forced exchange. Coherent submesoscale eddies also occurred frequently within the domain due to flow-topography effects onshore and horizontal density gradients offshore, generally with lifespans shorter than 10 hours, diameters smaller than 6 km, and vertical depths shallower than 10 meters. The across-shelf volume transport due to eddies, estimated by seeding particles within the surface current fields, was more than half the wind-driven depth-dependent exchange. Thus, accounting for the potential coherent along-shelf variability present over the inner-shelf can significantly increase estimates of the across-shelf transfer of water masses and particles. This article is protected by copyright. All rights reserved.
      PubDate: 2017-02-09T10:45:32.041551-05:
      DOI: 10.1002/2016JC012491
  • Zooplankton community response to the winter 2013 deep convection process
           in the NW Mediterranean Sea
    • Authors: Katty Donoso; François Carlotti, Marc Pagano, Brian P. V. Hunt, Rubén Escribano, Léo Berline
      Abstract: The Gulf of Lion is an important area of deep convection, where intense winter vertical mixing brings nutrients up from deeper layers, promoting the largest bloom in the Mediterranean at the end of winter/early spring. In DEWEX program conducted cruises in February and April 2013 to investigate the ecosystem level impacts of deep water convection. Zooplankton data were collected through net sampling and imaging with an Underwater Vision Profiler. In winter, low zooplankton abundance and biomass were observed in the Deep Convection Zone (DCZ) and higher values on its periphery. In spring, this pattern reversed with high biomass in the DCZ and lower values on the periphery. On average for the whole area, the potential grazing impact was estimated to increase by one order of magnitude from winter to spring. In April, all areas except the DCZ incurred top-down control by zooplankton on the phytoplankton stock. In the DCZ, the chlorophyll-a values remained high despite the high zooplankton biomass and carbon demand, indicating a sustained bottom-up control. The zooplankton community composition was comparable for both periods, typified by high copepod dominance, but with some differences between the DCZ and peripheral regions. In spring the DCZ was characterized by a strong increase in herbivorous species such as Centropages typicus and Calanus helgolandicus, and an increase in the number of large zooplankton individuals. Our study indicates that the DCZ is likely an area of both enhanced energy transfer to higher trophic levels and organic matter export in the North Western Mediterranean Sea. This article is protected by copyright. All rights reserved.
      PubDate: 2017-02-08T11:01:05.074732-05:
      DOI: 10.1002/2016JC012176
  • Semiannual variability of the California undercurrent along the Southern
           California current system: A tropical generated phenomenon
    • Authors: Felipe Gómez-Valdivia; Alejandro Parés-Sierra, Ana Laura Flores-Morales
      Abstract: We used a high-resolution numerical model implementation to analyze the California Undercurrent (CU) dynamics along the Southern California Current System. In agreement with reported observations, the modeled CU was stronger during June-July and December-January, when it flowed continuously along Baja California and Southern California reaching long-term averages up to 6 cms−1. Previous research has associated the biannual CU intensification to the local dynamics off Southern California. Our results evidenced, however, that the passage of remote Semiannual Coastally Trapped Waves (SCTW) primarily explained the semiannual CU variability. The CU was stronger 2-3 months after the passage of the upwelling SCTW phase, when the offshore propagation of Rossby waves, brought about by the SCTW transit, induced an energetic cross-shore pressure gradient that strengthened the subsurface poleward circulation along the continental slope. The SCTW were independent of the local wind; they corresponded to the northward extension of semiannual equatorial Kelvin waves that have been observed along the northeastern tropical Pacific. This article is protected by copyright. All rights reserved.
      PubDate: 2017-02-07T11:35:46.6912-05:00
      DOI: 10.1002/2016JC012350
  • Variability of upper ocean characteristics and tropical cyclones in the
           South West Indian Ocean
    • Authors: D. Mawren; C. J. C Reason
      Abstract: Track and intensity are key aspects of tropical cyclone behaviour. Intensity may be impacted by the upper-ocean heat content relevant for TC intensification (known as Tdy) and barrier layer thickness (BLT). Here, the variability of Tdy and BLT in the South West Indian Ocean and their relationships with tropical cyclones are investigated. It is shown that rapid cyclone intensification is influenced by large Tdy values, thick barrier layers and the presence of anticyclonic eddies. For TC generation in the South West Indian Ocean, the parameter Tdy was found to be important.Large BLT values overlay with large Tdy values during summer. Both fields are modulated by the westward propagation of Rossby waves, which are often associated with ENSO. For example, the 1997-1998 El Niño shows a strong signal in Tdy, SST and BLT over the South West Indian Ocean. After this event, an increasing trend in Tdy occurred over most of the basin which may be associated with changes in atmospheric circulation. Increasing SST, Power Dissipation Index and frequency of Category-5 tropical cyclones also occurred from 1980 to 2010.To further examine the links between tropical cyclones, Tdy and BLT, the ocean response to Category 5 Tropical Cyclone Bansi that developed near Madagascar during January 2015 was analysed. Its unusual track was found to be linked with the strengthening of the monsoonal north westerlies while its rapid intensification from Category-2 to Category-4 was linked to a high- Tdy region, associated with a warm core eddy and large BLT. This article is protected by copyright. All rights reserved.
      PubDate: 2017-02-07T11:35:41.486144-05:
      DOI: 10.1002/2016JC012028
  • Sensitivity of typhoon modeling to surface waves and rainfall
    • Authors: Biao Zhao; Fangli Qiao, Luigi Cavaleri, Guansuo Wang, Luciana Bertotti, Li Liu
      Abstract: Improving intensity simulation and forecast of tropical cyclones has always been a challenge, although in recent years the track forecasts have been remarkably improved. In this study, we explore the sensitivity of typhoon simulation to three physical processes using a fully coupled atmosphere-ocean-wave model. Two storms, a strong and a weak one, have been chosen. The effects of wave breaking induced sea spray, ocean vertical mixing associated with non-breaking surface waves, and sea surface cooling due to intense rainfall are assessed by means of a set of numerical experiments. The results show and confirm that sea spray leads to an increase of typhoon intensity by enhancing the air-sea heat flux, while non-breaking wave-induced vertical mixing and rainfall lead to a decrease. Each process can be relevant, depending on wind and wave conditions. These can vary dramatically when typhoons interact with not sufficiently well-defined coastal areas, typically an archipelago. Compared with the control runs, when all the three physical processes are considered, the (absolute) difference between the modeled sea level pressure and best track data is reduced from 26.05 to 0.70 hPa for typhoon Haiyan, and from -9.42 to -8.67 hPa for typhoon Jebi. We have found a steady overestimate of the dimensions of the typhoons. We have verified an extreme sensitivity to the initial conditions, especially when small differences in the typhoon track may imply different relevance of the physical processes, like the ones we have considered, governing the evolution of the storm. This article is protected by copyright. All rights reserved.
      PubDate: 2017-02-07T11:35:34.512451-05:
      DOI: 10.1002/2016JC012262
  • A scale-aware subgrid model for quasigeostrophic turbulence
    • Authors: Scott D. Bachman; Baylor Fox-Kemper, Brodie Pearson
      Abstract: This paper introduces two methods for dynamically prescribing eddy-induced diffusivity, advection, and viscosity appropriate for primitive equation models with resolutions permitting the forward potential enstrophy cascade of quasigeostrophic dynamics, such as operational ocean models and high-resolution climate models with O(25)km horizontal resolution and finer. Where quasigeostrophic dynamics fail (e.g., the equator, boundary layers, deep convection), the method reverts to scalings based on a matched two-dimensional enstrophy cascade. A principle advantage is that these subgrid models are scale-aware, meaning that the model is suitable over a range of grid resolutions: from mesoscale grids that just permit baroclinic instabilities to grids below the submesoscale where ageostrophic effects dominate. Two approaches are presented here using Large Eddy Simulation (LES) techniques adapted for three-dimensional rotating, stratified turbulence. The simpler approach has one non-dimensional parameter, Λ, which has an optimal value near 1. The second approach dynamically optimizes Λ during simulation using a test filter. The new methods are tested in an idealized scenario by varying the grid resolution, and their use improves the spectra of potential enstrophy and energy in comparison to extant schemes. The new methods keep the gridscale Reynolds and Péclet numbers near one throughout the domain, which confers robust numerical stability and minimal spurious diapycnal mixing. Although there are no explicit parameters in the dynamic approach, there is strong sensitivity to the choice of test filter. Designing test filters for heterogeneous ocean turbulence adds cost and uncertainty, and we find the dynamic method does not noticeably improve over setting Λ = 1. This article is protected by copyright. All rights reserved.
      PubDate: 2017-02-07T03:20:43.590989-05:
      DOI: 10.1002/2016JC012265
  • Current-induced dissipation in spectral wave models
    • Authors: H. Rapizo; A. V. Babanin, D. Provis, W. E. Rogers
      Abstract: Despite many recent developments of the parameterization for wave dissipation in spectral models, it is evident that when waves propagate onto strong adverse currents the rate of energy dissipation is not properly estimated. The issue of current-induced dissipation is studied through a comprehensive data set in the tidal inlet of Port Phillip Heads, Australia. The wave parameters analyzed are significantly modulated by the tidal currents. Wave height in conditions of opposing currents (ebb tide) can reach twice the offshore value, whereas during co-flowing currents (flood) it can be reduced to half. The wind-wave model SWAN is able to reproduce the tide-induced modulation of waves and the results show that the variation of currents is the dominant factor in modifying the wave field. In stationary simulations the model provides an accurate representation of wave height for slack and flood tides. During ebb tides, wave energy is highly overestimated over the opposing current-jet. None of the four parameterizations for wave dissipation tested performs satisfactorily. A modification to enhance dissipation as a function of the local currents is proposed. It consists of the addition of a factor that represents current-induced wave steepening and it is scaled by the ratio of spectral energy to the threshold breaking level. The new term asymptotes to the original form as the current in the wave direction tends to zero. The proposed modification considerably improves wave height and mean period in conditions of adverse currents, whereas the good model performance in co-flowing currents is unaltered. This article is protected by copyright. All rights reserved.
      PubDate: 2017-02-07T03:15:33.779653-05:
      DOI: 10.1002/2016JC012367
  • Generation of internal solitary waves over a large sill: From Knight Inlet
           to Luzon Strait
    • Authors: Zhiwu Chen; Yuhua Nie, Jieshuo Xie, Jiexin Xu, Yinghui He, Shuqun Cai
      Abstract: A fully nonlinear, nonhydrostatic numerical model is utilized to investigate the generation of Internal Solitary Waves (ISWs) upstream of the Knight Inlet sill. While an upstream hydraulic jump initiates the ISW generation and both hydraulic jump and upstream influence contribute to the generation, it is found that upstream influence is dominant and the hydraulic jump is not necessary for the ultimate generation of ISWs. Decreasing the tidal forcing or upstream sill width may render the flow subcritical (i.e., the hydraulic jump disappears) and ISWs can be generated by nonlinear steepening of long wave disturbances induced by upstream influence. Increasing the tidal forcing or upstream sill width may generate a hydraulic jump blocking strong upstream propagating disturbances. The jump subsequently becomes a turbulent bore and later disperses into a train of ISWs as the tide relaxes. Further increase in the tidal forcing may sweep the turbulent bore downstream and a train of ISWs is emitted upstream towards the end of waning tide. By reducing the stratification strength by one order of magnitude, the near-sill flow is in the transcritical regime and ISWs are resonantly generated over the lee side slope. Connections to the internal tide release mechanism at Luzon Strait and to the unsteady lee wave model are also discussed. The present work provides some more insights into the ISW generation process at Knight Inlet and the connection between the generation mechanism at Knight Inlet and that at Luzon Strait is identified. This article is protected by copyright. All rights reserved.
      PubDate: 2017-02-07T03:15:31.652976-05:
      DOI: 10.1002/2016JC012206
  • How well does wind speed predict air-sea gas transfer in the sea ice zone?
           A synthesis of radon deficit profiles in the upper water column of the
           Arctic Ocean
    • Authors: B. Loose; R.P. Kelly, A. Bigdeli, W. Williams, R. Krishfield, M. Rutgers van der Loeff, S.B. Moran
      Abstract: We present 34 profiles of radon-deficit from the ice-ocean boundary layer of the Beaufort Sea. Including these 34, there are presently 58 published radon-deficit estimates of k, the air-sea gas transfer velocity in the Arctic Ocean; 52 of these estimates were derived from water covered by 10% sea ice or more. The average value of k collected since 2011 is 4.0 ± 1.2 m d−1 This exceeds the quadratic wind speed prediction of weighted kws = 2.85 m d−1 with mean weighted wind speed of 6.4 m s−1. We show how ice cover changes the mixed-layer radon budget, and yields an “effective gas transfer velocity”. We use these 58 estimates to statistically evaluate the suitability of a wind speed parameterization for k, when the ocean surface is ice covered. Whereas the six profiles taken from the open ocean indicate a statistically good fit to wind speed parameterizations, the same parameterizations could not reproduce k from the sea ice zone. We conclude that techniques for estimating k in the open ocean cannot be similarly applied to determine k in the presence of sea ice. The magnitude of k through gaps in the ice may reach high values as ice cover increases, possibly as a result of focused turbulence dissipation at openings in the free surface. These 58 profiles are presently the most complete set of estimates of k across seasons and variable ice cover; as dissolved tracer budgets they reflect air-sea gas exchange with no impact from air-ice gas exchange. This article is protected by copyright. All rights reserved.
      PubDate: 2017-02-07T03:15:27.304611-05:
      DOI: 10.1002/2016JC012460
  • Sea-ice thickness from field measurements in the northwestern Barents Sea
    • Authors: Jennifer King; Gunnar Spreen, Sebastian Gerland, Christian Haas, Stefan Hendricks, Lars Kaleschke, Caixin Wang
      Abstract: The Barents Sea is one of the fastest changing regions of the Arctic, and has experienced the strongest decline in winter-time sea-ice area in the Arctic, at −23±4% decade–1. Sea-ice thickness in the Barents Sea is not well studied. We present two previously unpublished helicopter-borne electromagnetic (HEM) ice thickness measurements from the north-western Barents Sea acquired in March 2003 and 2014. The HEM data are compared to ice thickness calculated from ice draft measured by ULS deployed between 1994-96. These data show that ice thickness varies greatly from year to year; influenced by the thermodynamic and dynamic processes that govern local formation vs long-range advection. In a year with a large inflow of sea-ice from the Arctic Basin the Barents Sea ice cover is dominated by thick multiyear ice; as was the case in 2003 and 1995. In a year with an ice cover that was mainly grown in-situ the ice will be thin and mechanically unstable; as was the case in 2014. The HEM data allow us to explore the spatial and temporal variability in ice thickness. In 2003 the dominant ice class was more than 2 years old; and modal sea-ice thickness varied regionally from 0.6 - 1.4 m, with the thinner ice being either first-year ice, or multiyear ice which had come into contact with warm Atlantic water. In 2014 the ice cover was predominantly locally-grown ice less than 1 month old (regional modes of 0.5 - 0.8 m). These two situations represent two extremes of a range of possible ice thickness distributions that can present very different conditions for shipping traffic; or have a different impact on heat transport from ocean to atmosphere. This article is protected by copyright. All rights reserved.
      PubDate: 2017-02-03T20:07:20.515122-05:
      DOI: 10.1002/2016JC012199
  • Snow contribution to first-year and second-year Arctic sea ice mass
           balance north of Svalbard
    • Authors: Mats A. Granskog; Anja Rösel, Paul A. Dodd, Dmitry Divine, Sebastian Gerland, Tõnu Martma, Melanie J. Leng
      Abstract: The salinity and water oxygen isotope composition (δ18O) of twenty-nine first-year (FYI) and second-year (SYI) Arctic sea ice cores (total length 32.0 m) from the drifting ice pack north of Svalbard were examined to quantify the contribution of snow to sea ice mass. Five cores (total length 6.4 m) were analyzed for their structural composition showing variable contribution of 10-30% by granular ice. In these cores snow had been entrained in 6 to 28% of the total ice thickness. We found evidence of snow contribution in about three quarter of the sea ice cores, when surface granular layers had very low δ18O values. Snow contributed 7.5-9.7% to sea ice mass balance on average (including also cores with no snow) using δ18O mass balance calculations. In SYI cores snow fraction by mass (12.7-16.3%) was much higher than in FYI cores (3.3-4.4%), while the bulk salinity of FYI (4.9) was distinctively higher than for SYI (2.7). We surmise that oxygen isotopes and salinity profiles can give information on the age of the ice and allows to distinguish between FYI and SYI (or older) ice in the area north of Svalbard. This article is protected by copyright. All rights reserved.
      PubDate: 2017-02-03T20:00:47.402082-05:
      DOI: 10.1002/2016JC012398
  • Variability of the Pacific North equatorial current from 1993 to 2012
           based on a 1/8˚ Pacific model simulation
    • Authors: Zhichun Zhang; Huijie Xue, Fei Chai, Yi Chao
      Abstract: Based on a multi-decadal Pacific basin model simulation, inter-annual variations of the North Equatorial Current (NEC) are investigated. The model reproduces well the characteristics of the NEC and its inter-annual variability. The NEC transport is calculated as an integral of the westward velocity from 6º N to 21º N and from the surface to the 1200 m depth. The magnitude and standard deviation of the NEC transport increase from 46.5 and 3.9 Sv at 175ºE to 66.7 and 6.5 Sv at 130º E, respectively, and both peak around 132º E prior to entering the bifurcation region. The NEC transport tends to be higher during positive Oceanic Niño Index (ONI) years but lower during negative ONI years with the maximum difference of more than 20 Sv. The inter-annual variability of the NEC transport is closely related to changes of the sea surface height in the tropical Pacific Ocean, and the increase of the NEC is mostly balanced by the increase in the North Equatorial Counter Current (NECC) on the tropical gyre side. The present study further suggests a long-term decline of the NEC transport from 1993 to 2012, which is consistent with the patterns in the trend of wind stress curl.Transport anomalies reconstructed from the normal modes of zonal velocity suggest that the 1st baroclinic mode captures about 95% of the variance in the NEC transport, while the 2nd mode adds only additional 3-4%. A 1.5-layer reduced gravity model further reveals that the 1st (2nd) baroclinic mode is driven primarily by the wind (thermal) forcing. The wind forcing plays a predominant role in determining the inter-annual variability in the NEC transport while the effect of the thermal forcing is rather limited. This article is protected by copyright. All rights reserved.
      PubDate: 2017-02-03T20:00:43.746693-05:
      DOI: 10.1002/2016JC012143
  • The coastal streamflow flux in the regional Arctic system model
    • Authors: Joseph Hamman; Bart Nijssen, Andrew Roberts, Anthony Craig, Wieslaw Maslowski, Robert Osinski
      Abstract: The coastal streamflow flux from the Arctic drainage basin is an important driver of dynamics in the coupled ice-ocean system. Comprising more than one-third of the total freshwater flux into the Arctic Ocean, streamflow is a key component of the regional and global freshwater cycle. To better represent the coupling of the streamflow flux to the ocean, we have developed and applied the RVIC streamflow routing model within the Regional Arctic System Model (RASM). The RASM is a high-resolution regional Earth System Model whose domain includes all of the Arctic drainage basin. In this paper, we introduce the RVIC streamflow routing model, detailing its application within RASM and its advancements in terms of representing high-resolution streamflow processes. We evaluate model simulated streamflow relative to in-situ observations and demonstrate a method for improving model performance using a simple optimization procedure. We also present a new, spatially and temporally consistent, high-resolution dataset of coastal freshwater fluxes for the Arctic drainage basin and surrounding areas that is based on a fully-coupled RASM simulation and intended for use in Arctic Ocean modeling applications. This dataset is evaluated relative to other coastal streamflow datasets commonly used by the ocean modeling community. We demonstrate that the RASM-simulated streamflow flux better represents the annual cycle than existing datasets, especially in ungauged areas. Finally, we assess the impact that streamflow has on the coupled ice-ocean system, finding that the presence of streamflow leads to reduced sea surface salinity, increased sea surface temperatures, and decreased sea ice thickness. This article is protected by copyright. All rights reserved.
      PubDate: 2017-02-03T20:00:38.778681-05:
      DOI: 10.1002/2016JC012323
  • Signature of mesoscale eddies in satellite sea surface salinity data
    • Authors: Oleg Melnichenko; Angel Amores, Nikolai Maximenko, Peter Hacker, James Potemra
      Abstract: A persistent signature of coherent mesoscale eddies in sea surface salinity (SSS) is revealed by analyzing the relationship between satellite SSS and sea surface height (SSH) variability in an eddy-following reference frame. Our analysis focuses on mid-ocean eddies in two representative regions, the southern Indian Ocean and the North Atlantic subtropical gyre. The resulting composite averages reveal a clear signature of mesoscale eddies in satellite SSS with typical SSS anomalies of 0.03-0.05 psu. The spatial structure of eddy-induced SSS perturbations can be characterized as a superposition of a dipole structure, arising from horizontal advection of the background SSS gradient by eddy velocity field, and a monopole structure related to the eddy core. The observed relationships between SSS and SSH anomalies are used to provide a regional assessment of the role of mesoscale eddies in the ocean freshwater transport in the North Atlantic subtropical gyre. This article is protected by copyright. All rights reserved.
      PubDate: 2017-02-03T20:00:35.711208-05:
      DOI: 10.1002/2016JC012420
  • Impact of wave breaking on upper ocean turbulence
    • Authors: Yongqing Cai; Yuanqiao Wen, Lichuan Wu, Chunhui Zhou, Fan Zhang
      Abstract: Previous studies have demonstrated that surface wave breaking can impact upper-ocean turbulence through wave-breaking-induced turbulence kinetic energy (TKE) flux and momentum flux. Wave-breaking-induced momentum flux decays approximately exponentially with depth, and the decay exponent depends on both the wind speed and wave age. With increasing wave age, the decay speed of wave-breaking-induced momentum flux first decreases, reaching a minimum around a wave age of 16, and then increases. In this study, a wave-breaking-induced momentum flux parameterization was proposed based on wave age and wind-speed dependence. The new proposed parameterization was introduced into a one-dimensional (1-D) ocean model along with a wave-age-dependent wave-breaking-induced TKE flux parameterization. The simulation results showed that the wave-breaking impact on the ocean mainly affected the upper-ocean layer. Adding the wave-age impact to the wave-breaking-induced TKE flux and momentum flux improved the 1-D model performance concerning the sea temperature. Moreover, the wave-breaking-induced momentum flux had a larger impact on the simulation results than the wave-breaking-induced TKE flux. This article is protected by copyright. All rights reserved.
      PubDate: 2017-02-03T20:00:31.973187-05:
      DOI: 10.1002/2016JC012654
  • Vertical structure anomalies of oceanic eddies in the Kuroshio Extension
    • Authors: Wenjin Sun; Changming Dong, Ruyun Wang, Yu Liu, Kai Yu
      Abstract: Using collocated altimetry sea surface height anomalies (SSHA) and Argo profiles within detected eddies, we investigated structures of temperature, salinity, potential density, geostrophic current, mixed layer depth (MLD), potential vorticity (PV), and buoyancy frequency (N) in the Kuroshio Extension (KE) region under the influences of oceanic eddies. We identified 54,302 oceanic eddies (snapshots) in the KE region during the period of 1999-2013. The composite analysis showed that changes in physical parameters modulated by the climatological composite eddies (hereinafter referred as composite eddies) were mainly confined in the upper 800 m. At the eddy core, the maximum cooling in the composite cyclonic eddy (CE) reaches 2.00°C at ∼360 m, with maximum salinity change of −0.13 psu at ∼260 m and maximum potential density change of +0.27 kg/m3 at ∼310 m. In contrast, the maximum warming in the composite anticyclonic eddy (AE) reaches 1.78°C at ∼410 m of the eddy core, with maximum salinity change of 0.12 psu at ∼260 m and maximum potential density change of −0.22 kg/m3 at ∼410 m. There were obvious anti-clockwise and clockwise geostrophic current anomalies associated with the composite CE and AE, respectively. The seasonal mean eddy-modulated MLD anomaly had significant seasonal variations. Besides, they could modulate opposite PV changes, the magnitude of which varied with depth. This article is protected by copyright. All rights reserved.
      PubDate: 2017-01-31T18:35:44.516564-05:
      DOI: 10.1002/2016JC012226
  • Investigation of turbulent momentum flux in the typhoon boundary layer
    • Authors: Ziqiang Duan; Xiaohong Yao, Yongping Li
      Abstract: Turbulent momentum flux in the typhoon boundary layer was measured at a turbulent flux tower in Southeast China using an eddy covariance technique during the passage of typhoons Fanapi and Megi in 2010. The anemometers were mounted on the cantilever bracket at the height of 26.6 m, 42.4 m, 60.4 m and 82.9 m. The momentum flux in the typhoon boundary layer decreased with increasing height during the two typhoon passages. The decrease was nearly linear during the passage of the Typhoon Fanapi. However, in the boundary layer of the Typhoon Megi, the momentum flux at 26.6 m height was much greater than that at 42.4 m height. The cospectra of wind components showed that the enhanced flux at the bottom of the boundary layer was caused by small scale turbulence with frequency in the range of 0.1–1 Hz and can be explained by the turbulent energy accumulation of small scale turbulence. This article is protected by copyright. All rights reserved.
      PubDate: 2017-01-31T18:35:40.826977-05:
      DOI: 10.1002/2016JC012169
  • Vertically varying Eulerian mean currents induced by internal coastal
           Kelvin waves
    • Authors: Jan Erik H. Weber
      Abstract: The lost momentum in spatially damped internal Kelvin waves reappears as Eulerian mean currents through the action of the nonlinear wave-wave interaction terms. A novel expression is derived for the steady balance between the frictional force on the coastally trapped horizontal Eulerian mean flow, and the forcing from the wave field in terms of the mean wave Reynolds stresses and the horizontal divergence of the Stokes drift. The forcing can be expressed in terms of orthogonal eigenfunctions for internal waves, yielding the vertical variation of the Eulerian mean flow. For arbitrary values of the Brunt-Väisälä frequency N, it is shown that the wave forcing on the Eulerian mean is always negative, yielding a Poiseuille type flow. Therefore, unlike the Stokes drift velocity in internal Kelvin waves which exhibits a backward drift for the first mode in the region of maximum N, the wave-induced horizontal Eulerian mean current is always in the direction of the waves. The results are illustrated by an example from Van Mijenfjorden in Svalbard, which is an arctic sill fjord where internal waves are generated by the action of the barotropic semi-diurnal tide. This article is protected by copyright. All rights reserved.
      PubDate: 2017-01-31T18:35:33.284288-05:
      DOI: 10.1002/2016JC012377
  • Long-term monitoring of ocean deep convection using multisensors altimetry
           and ocean color satellite data
    • Authors: Marine Herrmann; Pierre-Amael Auger, Caroline Ulses, Claude Estournel
      Abstract: Deep convection occurs in oceanic regions submitted to strong atmospheric buoyancy losses and results in the formation of deep water masses (DWF) of the ocean circulation. It shows a strong interannual variability, and could drastically weaken under the influence of climate change. In this study, a method is proposed to monitor quantitatively deep convection using multi-sensors altimetry and ocean color satellite data. It is applied and evaluated for the well observed Northwestern Mediterranean Sea (NWMS) case study. For that, a coupled hydrodynamical-biogeochemical numerical simulation is used to examine the signature of DWF on sea level anomaly (SLA) and surface chlorophyll concentration. Statistically significant correlations between DWF annual indicators and the areas of low surface chlorophyll concentration and low SLA in winter are obtained, and linear relationships between those indicators and areas are established. These relationships are applied to areas of low SLA and low chlorophyll concentration computed respectively from a 27-year altimetry dataset and a 19-year ocean color dataset. The first long time series (covering the last 2 decades) of DWF indicators obtained for the NWMS from satellite observations are produced. Model biases and smoothing effect induced by the low resolution of gridded altimetry data are partly taken into account by using corrective methods. Comparison with winter atmospheric heat flux and previous modeled and observed estimates of DWF indicators suggests that those DWF indicators time series capture realistically DWF interannual variability in the NWMS. The advantages as well as the weaknesses and uncertainties of the method are finally discussed. This article is protected by copyright. All rights reserved.
      PubDate: 2017-01-31T06:41:31.033425-05:
      DOI: 10.1002/2016JC011833
  • Turbulent heat and momentum fluxes in the upper ocean under Arctic Sea Ice
    • Authors: Algot K. Peterson; Ilker Fer, Miles G. McPhee, Achim Randelhoff
      Abstract: We report observations of heat and momentum fluxes measured in the ice-ocean boundary layer from four drift stations between January and June 2015, covering from the typical Arctic basin conditions in the Nansen Basin to energetic spots of interaction with the warm Atlantic Water branches near the Yermak Plateau and over the North Spitsbergen slope. A wide range of oceanic turbulent heat flux values are observed, reflecting the variations in space and time over the five month duration of the experiment. Oceanic heat flux is weakly positive in winter over the Nansen Basin during quiescent conditions, increasing by an order of magnitude during storm events. An event of local upwelling and mixing in the winter-time Nansen basin highlights the importance of individual events. Spring-time drift is confined to the Yermak Plateau and its slopes, where vertical mixing is enhanced. Wind events cause an approximate doubling of oceanic heat fluxes compared to calm periods. In June, melting conditions near the ice edge lead to heat fluxes of O(100 W m−2). The combination of wind forcing with shallow Atlantic Water layer and proximity to open waters leads to maximum heat fluxes reaching 367 W m−2, concurrent with rapid melting. Observed ocean-to-ice heat fluxes agree well with those estimated from a bulk parameterization except when accumulated freshwater from sea ice melt in spring probably causes the bulk formula to overestimate the oceanic heat flux. This article is protected by copyright. All rights reserved.
      PubDate: 2017-01-31T06:41:29.530446-05:
      DOI: 10.1002/2016JC012283
  • Sun glitter imagery of surface waves: 2. Waves Transformation on Ocean
    • Authors: Vladimir Kudryavtsev; Maria Yurovskaya, Bertrand Chapron, Fabrice Collard, Craig Donlon
      Abstract: Under favourable imaging conditions, the Sentinel-2 Multi-Spectral Instrument (MSI) can provide spectacular and novel quantitative ocean surface wave directional measurements in satellite Sun Glitter Imagery (SSGI). Owing to a relatively large-swath with high spatial resolution (10 m) ocean surface roughness mapping capabilities, changes in ocean wave energy and propagation direction can be precisely quantified at very high resolution, across spatial distances of 10 km and more. This provides unique opportunities to study ocean wave refraction induced by spatial varying surface currents. As expected and demonstrated over the Grand Agulhas current area, the mesoscale variability of near-surface currents, documented and reconstructed from satellite altimetry, can significantly deflect in-coming south-western swell systems. Based on ray-tracing calculations, and unambiguously revealed from the analysis of Sentinel-2 MSI SSGI measurements, the variability of the near-surface current explains significant wave-current refraction, leading to wave-trapping phenomenon and strong local enhancement of the total wave energy. In addition to its importance for wave modelling and hazard prediction, these results open new possibilities to combine different satellite measurements and greatly improve the determination of the upper ocean mesoscale vorticity motions. This article is protected by copyright. All rights reserved.
      PubDate: 2017-01-31T06:41:26.763297-05:
      DOI: 10.1002/2016JC012426
  • Winter to summer oceanographic observations in the Arctic Ocean north of
    • Authors: Amelie Meyer; Arild Sundfjord, Ilker Fer, Christine Provost, Nicolas Villacieros Robineau, Zoe Koenig, Ingrid H. Onarheim, Lars H. Smedsrud, Pedro Duarte, Paul A. Dodd, Robert M. Graham, Sunke Schmidtko, Hanna M. Kauko
      Abstract: Oceanographic observations from the Eurasian Basin north of Svalbard collected between January and June 2015 from the N-ICE2015 drifting expedition are presented. The unique winter observations are a key contribution to existing climatologies of the Arctic Ocean, and show a ∼100m deep winter mixed layer likely due to high sea ice growth rates in local leads. Current observations for the upper ∼200m show mostly a barotropic flow, enhanced over the shallow Yermak Plateau. The two branches of inflowing Atlantic Water are partly captured, confirming that the outer Yermak Branch follows the perimeter of the plateau, and the inner Svalbard Branch the coast. Atlantic Water observed to be warmer and shallower than in the climatology, is found directly below the mixed layer down to 800m depth, and is warmest along the slope, while properties inside the basin are quite homogeneous. From late May onwards, the drift was continually close to the ice edge and a thinner surface mixed layer and shallower Atlantic Water coincided with significant sea ice melt being observed. This article is protected by copyright. All rights reserved.
      PubDate: 2017-01-31T06:41:22.912925-05:
      DOI: 10.1002/2016JC012391
  • Sun glitter imagery of ocean surface waves: 1. Directional spectrum
           retrieval and validation
    • Authors: Vladimir Kudryavtsev; Maria Yurovskaya, Bertrand Chapron, Fabrice Collard, Craig Donlon
      Abstract: A practical method is suggested to quantitatively retrieve directional spectra of ocean surface waves from high-resolution satellite sun glitter imagery (SSGI). The method builds on direct determination of the imaging transfer function from the large-scale smoothed shape of sun glitter. Observed brightness modulations are then converted into sea surface elevations to perform directional spectral analysis. The method is applied to the Copernicus Sentinel-2 Multi-Spectral Instrument (MSI) measurements. Owing to the specific instrumental configuration of MSI (which has a primary mission dedicated to mapping land surfaces), a physical angular difference between channel detectors on the instrument focal plane array can be used to efficiently determine the surface brightness gradients in two directions, i.e. in sensor zenith and azimuthal directions. In addition, the detector configuration of MSI means that a small temporal lag between channel acquisitions exists. This feature can be exploited to detect surface waves and infer their space-time characteristics using cross-channel correlation. We demonstrate how this can be used to remove directional ambiguity in 2D detected wave spectra, and to obtain information describing local dispersion of surface waves. Directional spectra derived from Sentinel-2 MSI SSGI are compared with in situ buoy measurements. We report an encouraging agreement between SSGI-derived wave spectra and in situ measurements. This article is protected by copyright. All rights reserved.
      PubDate: 2017-01-31T06:41:15.983637-05:
      DOI: 10.1002/2016JC012425
  • Large eddy simulation of dam-break-driven swash on a rough-planar beach
    • Authors: Yeulwoo Kim; Zheyu Zhou, Tian-Jian Hsu, Jack A. Puleo
      Abstract: Turbulence characteristics in the swash zone are investigated using a 3D large eddy simulation model. The numerical model is implemented based on OpenFOAM which solves the filtered Navier-Stokes equations for two immiscible fluids with a standard Smagorinsky sub-grid-scale closure. The numerical model is validated with laboratory data for swash flow driven by a dam-break apparatus. The model results demonstrate that the main characteristics of turbulence in the swash zone are different from those in the surf zone, which are mainly induced by surface wave breaking. During uprush phase, bore-generated turbulence has 2D turbulence characteristics because of limited water depth. Near-bed generated turbulence is mainly observed during backwash. Turbulence production and turbulent dissipation rate estimated from the model results indicate an imbalance, possibly due to advection at swash front and large roughness used. Touching down of turbulent coherent structure (TCS) is observed during uprush, which drives intense bed shear stress. During the backwash, interaction between TCS and bed is less clear. However, finger-like patterns in the spatial extent of bed shear stress and vertical components of vorticity are predicted during the backwash. The location of the strongest finger patterns in the vertical direction is collocated with that of maximum turbulence production. These finger patterns are likely caused by boundary layer instabilities injected vertically from the bed. This article is protected by copyright. All rights reserved.
      PubDate: 2017-01-25T20:25:41.032087-05:
      DOI: 10.1002/2016JC012366
  • On the importance of Major Baltic Inflows for oxygenation of the central
           Baltic Sea
    • Authors: Thomas Neumann; Hagen Radtke, Torsten Seifert
      Abstract: In December 2014, the third strongest salt water inflow into the Baltic Sea occurred since 1880. It was assumed that the inflow would turn the entire bottom water of the Baltic Sea from anoxic into oxic conditions for an extended period. However, already in late 2015, the central Eastern Baltic Sea had turned back into anoxic conditions. This rapid oxygen decline was in fact surprising since a weaker inflow in 2003 ventilated the Baltic Sea for a longer period of time.With the aid of an ecosystem model of the Baltic Sea, the two inflows in 2003 and 2014 were analyzed in detail. Although the 2014 inflow event was twice as strong as the 2003 inflow event, oxygen transport continued after the latter one, supplying about the same amount of oxygen again. In addition to the major inflow event, a series of smaller inflows in 2003 supplied the extra oxygen transport. Therefore, the strength of a major inflow event alone cannot be used to predict the oxygenation impact. Instead, it is necessary to consider smaller events, in particular those occurring just before and after a major inflow event, as well.An element tagging method showed that the share of oxygen imported across the Danish Straits on the total oxygen arriving at the central Eastern Baltic Sea is between 10% and 20%. Therefore, the oxygen concentration of the inflowing water seems to be of less importance for the oxygenation effect on the central Baltic Sea due to the strong dilution effect. This article is protected by copyright. All rights reserved.
      PubDate: 2017-01-25T20:25:39.833069-05:
      DOI: 10.1002/2016JC012525
  • Spatial variability of the Arctic Ocean's double-diffusive staircase
    • Authors: N. Shibley; M.-L. Timmermans, J.R. Carpenter, J. Toole
      Abstract: The Arctic Ocean thermohaline stratification frequently exhibits a staircase structure overlying the Atlantic Water Layer that can be attributed to the diffusive form of double-diffusive convection. The staircase consists of multiple layers of O(1) m in thickness separated by sharp interfaces, across which temperature and salinity change abruptly. Through a detailed analysis of Ice-Tethered Profiler measurements from 2004-2013, the double-diffusive staircase structure is characterized across the entire Arctic Ocean. We demonstrate how the large-scale Arctic Ocean circulation influences the small-scale staircase properties. These staircase properties (layer thicknesses and temperature and salinity jumps across interfaces) are examined in relation to a bulk vertical density ratio spanning the staircase stratification. We show that the Lomonosov Ridge serves as an approximate boundary between regions of low density ratio (approximately 3 to 4) on the Eurasian side and higher density ratio (approximately 6 to 7) on the Canadian side. We find that the Eurasian Basin staircase is characterized by fewer, thinner layers than that in the Canadian Basin, although the margins of all basins are characterized by relatively thin layers and the absence of a well-defined staircase. A double-diffusive 4/3-flux law parametrization is used to estimate vertical heat fluxes in the Canadian Basin to be O(0.1) Wm-2. It is shown that the 4/3-flux law may not be an appropriate representation of heat fluxes through the Eurasian Basin staircase. Here, molecular heat fluxes are estimated to be between O(0.01) Wm-2 and O(0.1) Wm-2. However, many uncertainties remain about the exact nature of these fluxes. This article is protected by copyright. All rights reserved.
      PubDate: 2017-01-25T20:25:35.387478-05:
      DOI: 10.1002/2016JC012419
  • Deep sediment resuspension and thick nepheloid layer generation by
           open-ocean convection
    • Authors: X. Durrieu de Madron; S. Ramondenc, L. Berline, L. Houpert, A. Bosse, S. Martini, L. Guidi, P. Conan, C. Curtil, N. Delsaut, S. Kunesch, J.F. Ghiglione, P. Marsaleix, M. Pujo-Pay, T. Séverin, P. Testor, C. Tamburini,
      Abstract: The Gulf of Lions in the northwestern Mediterranean is one of the few sites around the world ocean exhibiting deep open-ocean convection. Based on 6-year long (2009-2015) time series from a mooring in the convection region, shipborne measurements from repeated cruises, from 2012 to 2015, and glider measurements, we report evidence of bottom thick nepheloid layer formation, which is coincident with deep sediment resuspension induced by bottom-reaching convection events. This bottom nepheloid layer, which presents a maximum thickness of around 2000 m in the center of the convection region, probably results from the action of cyclonic eddies that are formed during the convection period and can persist within their core while they travel through the basin. The residence time of this bottom nepheloid layer appears to be less than a year. In-situ measurements of suspended particle size further indicate that the bottom nepheloid layer is primarily composed of aggregates between 100 and 1000 µm in diameter, probably constituted of fine silts. Bottom-reaching open ocean convection, as well as deep dense shelf water cascading that occurred concurrently some years, lead to recurring deep sediments resuspension episodes. They are key mechanisms that control the concentration and characteristics of the suspended particulate matter in the basin, and in turn affect the bathypelagic biological activity. This article is protected by copyright. All rights reserved.
      PubDate: 2017-01-21T04:35:31.68472-05:0
      DOI: 10.1002/2016JC012062
  • Atlantic Waters inflow north of Svalbard: Insights from IAOOS observations
           and Mercator Ocean global operational system during N-ICE2015
    • Authors: Zoé Koenig; Christine Provost, Nicolas Villacieros-Robineau, Nathalie Sennéchael, Amelie Meyer, Jean-Michel Lellouche, Gilles Garric
      Abstract: As part of the N-ICE2015 campaign, IAOOS (Ice Atmosphere Ocean Observing System) platforms gathered intensive winter data at the entrance of Atlantic Water (AW) inflow to the Arctic Ocean north of Svalbard. These data are used to examine the performance of the 1/12° resolution Mercator Ocean global operational ice/ocean model in the marginal ice zone north of Svalbard. Modeled sea-ice extent, ocean heat fluxes, mixed layer depths, and AW mass characteristics are in good agreement with observations. Model outputs are then used to put the observations in a larger spatial and temporal context. Model outputs show that AW pathways over and around the Yermak Plateau differ in winter from summer. In winter, the large AW volume transport of the West Spitsbergen Current (WSC) (∼4 Sv) proceeds to the North East through 3 branches: the Svalbard Branch (∼0.5 Sv) along the northern shelf break of Svalbard, the Yermak Branch (∼1.1 Sv) along the western slope of the Yermak Plateau and the Yermak Pass Branch (∼2.0 Sv) through a pass in the Yermak Plateau at 80.8°N. In summer, the AW transport in the WSC is smaller (∼2 Sv) and there is no transport through the Yermak Pass. Although only eddy-permitting in the area, the model suggests an important mesoscale activity throughout the AW flow. The large differences in ice extent between winters 2015 and 2016 follow very distinct atmospheric and oceanic conditions in the preceding summer and autumn seasons. Convection-induced upward heat fluxes maintained the area free of ice in winter 2016. This article is protected by copyright. All rights reserved.
      PubDate: 2017-01-21T04:30:36.903107-05:
      DOI: 10.1002/2016JC012424
  • Three-dimensional simulation of high-frequency nonlinear internal wave
           dynamics in Cayuga Lake
    • Authors: Abbas Dorostkar; Leon Boegman, Andrew Pollard
      Abstract: Three-dimensional (3D) hydrostatic and nonhydrostatic versions of the MITgcm were applied to simulate the dynamics of the internal wave field (basin-scale seiches, nonlinear surges and high-frequency nonlinear internal waves, NLIWs) in Cayuga Lake, NY. The simulations were performed using up to 226 million computational cells with several horizontal grid resolutions, varying from 450 × 450 m to 22 × 22 m. Vertical grid spacing was not varied and ranged from 0.5 m to 2.95 m. The 22 × 22 m nonhydrostatic grid reproduced qualitatively the formation, propagation and shoaling of observed NLIWs using >10 grid points along the wavelength and a grid lepticity λ of O(1). This ensured, respectively, that the waves were not aliased and physical dispersion predominated over numerical dispersion. Using a sensitivity analysis, we generalize that correctly simulating NLIWs in real domains, using second-order discretization, requires grid resolutions that are an order of magnitude smaller than the wavelength and amplitude with λ ∼ 2; consistent with published work on idealized domains. Local gyre-like circulation was simulated, near mid-basin headlands, and transverse shoaling of NLIW packets on lateral boundaries was associated with topographic reflection and refraction, in agreement with published field observations from estuaries, which show NLIW propagation in long narrow quasi-2D systems (e.g., Finger Lakes, lochs, fjords, estuaries and straits) is fundamentally 3D. These results, therefore, help fill the gap in understanding and correctly modelling the multiscale 3D dynamics of NLIWs in complex natural systems. This article is protected by copyright. All rights reserved.
      PubDate: 2017-01-21T04:30:33.627065-05:
      DOI: 10.1002/2016JC011862
  • Impact of recently upwelled water on productivity investigated using in
           situ and incubation-based methods in Monterey Bay
    • Authors: Cara C. Manning; Rachel H.R. Stanley, David P. Nicholson, Jason M. Smith, J. Timothy Pennington, Melanie R. Fewings, Michael E. Squibb, Francisco P. Chavez
      Abstract: Photosynthetic conversion of CO2 to organic carbon and the transport of this carbon from the surface to the deep ocean is an important regulator of atmospheric CO2. To understand the controls on carbon fluxes in a productive region impacted by upwelling, we measured biological productivity via multiple methods during a cruise in Monterey Bay, California. We quantified net community production and gross primary production from measurements of O2/Ar and O2 triple isotopes (17Δ), respectively. We simultaneously conducted incubations measuring the uptake of 14C, 15NO3- and 15NH4+, and nitrification, and deployed sediment traps. At the start of the cruise (Phase 1) the carbon cycle was at steady state and the estimated net community production was 35(10) and 35(8) mmol C m−2 d−1 from O2/Ar and 15N incubations respectively, a remarkably good agreement. During Phase 1, net primary production was 96(27) mmol C m−2 d−1 from C uptake, and gross primary production was 209(17) mmol C m−2 d−1 from 17Δ. Later in the cruise (Phase 2), recently upwelled water with higher nutrient concentrations entered the study area, causing 14C and 15NO3- uptake to increase substantially. Continuous O2/Ar measurements revealed submesoscale variability in water mass structure and likely productivity in Phase 2 that was not evident from the incubations. These data demonstrate that O2/Ar and 15N incubation-based NCP estimates can give equivalent results in an N-limited, coastal system, when the non-steady state O2 fluxes are negligible or can be quantified. This article is protected by copyright. All rights reserved.
      PubDate: 2017-01-21T04:25:35.999603-05:
      DOI: 10.1002/2016JC012306
  • Air-Sea heat flux climatologies in the Mediterranean Sea: Surface energy
           balance and its consistency with ocean heat storage
    • Authors: Xiangzhou Song; Lisan Yu
      Abstract: This study provides an analysis of the Mediterranean Sea surface energy budget using nine surface heat flux climatologies. The ensemble mean estimation shows that the net downward shortwave radiation (192±19 Wm−2) is balanced by latent heat flux (-98±10 Wm−2), followed by net longwave radiation (-78±13 Wm−2) and sensible heat flux (-13±4 Wm−2). The resulting net heat budget (Qnet) is 2±12 Wm−2 into the ocean, which appears to be warm biased. The annual-mean Qnet should be -5.6±1.6 Wm−2 when estimated from the observed net transport through the Strait of Gibraltar. To diagnose the uncertainty in nine Qnet climatologies, we constructed Qnet from the heat budget equation by using historic hydrological observations to determine the heat content changes and advective heat flux. We also used the Qnet from a data-assimilated global ocean state estimation as an additional reference. By comparing with the two reference Qnet estimates, we found that seven products (NCEP 1, NCEP 2, CFSR, ERA-Interim, MERRA, NOCSv2.0 and OAFlux+ISCCP) overestimate Qnet, with magnitude ranging from 6 to 27 Wm−2, while two products underestimate Qnet by -6 Wm−2 (JRA55) and -14 Wm−2 (CORE.2). Together with the previous warm pool work of Song and Yu, [2013], we show that CFSR, MERRA, NOCSv2.0, and OAFlux+ISCCP are warm biased not only in the western Pacific warm pool but also in the Mediterranean Sea, whilst CORE.2 is cold biased in both regions. The NCEP 1, 2 and ERA-Interim are cold biased over the warm pool but warm biased in the Mediterranean Sea. This article is protected by copyright. All rights reserved.
      PubDate: 2017-01-21T04:25:27.820129-05:
      DOI: 10.1002/2016JC012254
  • Sediment transport in the presence of large reef bottom roughness
    • Authors: Andrew W. M. Pomeroy; Ryan J. Lowe, Marco Ghisalberti, Curt Storlazzi, Graham Symonds, Dano Roelvink
      Abstract: The presence of large bottom roughness, such as that formed by benthic organisms on coral reef flats, has important implications for the size, concentration, and transport of suspended sediment in coastal environments. A three-week field study was conducted in approximately 1.5 m water depth on the reef flat at Ningaloo Reef, Western Australia, to quantify the cross-reef hydrodynamics and suspended sediment dynamics over the large bottom roughness (∼20–40 cm) at the site. A logarithmic mean current profile consistently developed above the height of the roughness; however, the flow was substantially reduced below the height of the roughness (canopy region). Shear velocities inferred from the logarithmic profile and Reynolds stresses measured at the top of the roughness, which are traditionally used in predictive sediment transport formulations, were similar but much larger than that required to suspend the relatively coarse sediment present at the bed. Importantly, these stresses did not represent the stresses imparted on the sediment measured in suspension and are therefore not relevant to the description of suspended sediment transport in systems with large bottom roughness. Estimates of the bed shear stresses that accounted for the reduced near-bed flow in the presence of large roughness vastly improved the relationship between the predicted and observed grain sizes that were in suspension. Thus the impact of roughness, not only on the overlying flow but also on bed stresses, must be accounted for to accurately estimate suspended sediment transport in regions with large bottom roughness, a common feature of many shallow coastal ecosystems. This article is protected by copyright. All rights reserved.
      PubDate: 2017-01-20T05:01:14.167143-05:
      DOI: 10.1002/2016JC011755
  • Evaluation and reformulation of the maximum peak height algorithm (MPH)
           and application in a hypertrophic lagoon
    • Authors: Jaime Pitarch; Antonio Ruiz-Verdú, María D. Sendra, Rosalia Santoleri
      Abstract: We studied the performance of the MERIS maximum peak height (MPH) algorithm in the retrieval of chlorophyll-a concentration (CHL), using a matchup dataset of Bottom-of-Rayleigh Reflectances (BRR) and CHL from a hypertrophic lake (Albufera de Valencia). The MPH algorithm produced a slight underestimation of CHL in the pixels classified as cyanobacteria (83% of the total) and a strong overestimation in those classified as eukaryotic phytoplankton (17%). In situ biomass data showed that the binary classification of MPH was not appropriate for mixed phytoplankton populations, producing also unrealistic discontinuities in the CHL maps. We recalibrated MPH using our matchup dataset and found that a single calibration curve of third degree fitted equally well to all match ups regardless of how they were classified. As a modification to the former approach, we incorporated the Phycocyanin Index (PCI) in the formula, thus taking into account the gradient of phytoplankton composition, which reduced the CHL retrieval errors. By using in-situ biomass data, we also proved that PCI was indeed an indicator of cyanobacterial dominance. We applied our recalibration of the MPH algorithm to the whole MERIS dataset (2002-2012). Results highlight the usefulness of the MPH algorithm as a tool to monitor eutrophication. The relevance of this fact is higher since MPH does not require a complete atmospheric correction, which often fails over such waters. An adequate flagging or correction of sun glint is advisable though, since the MPH algorithm was sensitive to sun glint. This article is protected by copyright. All rights reserved.
      PubDate: 2017-01-20T04:55:46.219961-05:
      DOI: 10.1002/2016JC012174
  • Evaluation of thermal and dynamic impacts of summer dust aerosols on the
           Red Sea
    • Authors: Bronwyn Cahill; Ralf Toumi, Georgiy Stenchikov, Sergey Osipov, Helen Brindley
      Abstract: The seasonal response of upper ocean processes in the Red Sea to summer-time dust aerosol perturbations is investigated using an uncoupled regional ocean model. We find that the upper limit response is highly sensitive to dust-induced reductions in radiative fluxes. Sea surface cooling of -1°C and -2°C is predicted in the northern and southern regions, respectively. This cooling is associated with a net radiation reduction of -40Wm−2 and-90Wm−2 over the northern and southern regions, respectively. Larger cooling occurs below the mixed layer at 75m in autumn, -1.2°C (north) and -1.9°C (south). SSTs adjust more rapidly (c. 30 days) than the subsurface temperatures (seasonal timescales), due to stronger stratification and increased mixed layer stability inhibiting the extent of vertical mixing. The basin average annual heat flux reverses sign and becomes positive, +4.2 Wm−2 (as compared to observed estimates -17.3Wm−2) indicating a small gain of heat from the atmosphere. When we consider missing feedbacks from atmospheric processes in our uncoupled experiment, we postulate that the magnitude of cooling and the timescales for adjustment will be much less, and that the annual heat flux will not reverse sign but nevertheless be reduced as a result of dust perturbations. While our study highlights the importance of considering coupled ocean-atmosphere processes on the net surface energy flux in dust perturbation studies, the results of our uncoupled dust experiment still provide an upper limit estimate of the response of the upper ocean to dust-induced radiative forcing perturbations. This article is protected by copyright. All rights reserved.
      PubDate: 2017-01-16T06:26:56.145914-05:
      DOI: 10.1002/2016JC011911
  • Percolation blockage: A process that enables melt pond formation on first
           year Arctic Sea ice
    • Authors: Chris Polashenski; Kenneth M. Golden, Donald K. Perovich, Eric Skyllingstad, Alexandra Arnsten, Carolyn Stwertka, Nicholas Wright
      Abstract: Melt pond formation atop Arctic sea ice is a primary control of shortwave energy balance in the Arctic Ocean. During late spring and summer, the ponds determine sea ice albedo and how much solar radiation is transmitted into the upper ocean through the sea ice. The initial formation of ponds requires that melt water be retained above sea level on the ice surface. Both theory and observations, however, show that first year sea ice is so highly porous prior to the formation of melt ponds that multi-day retention of water above hydraulic equilibrium should not be possible. Here we present results of percolation experiments that identify and directly demonstrate a mechanism allowing melt pond formation. The infiltration of fresh water into the pore structure of sea ice is responsible for blocking percolation pathways with ice, sealing the ice against water percolation, and allowing water to pool above sea level. We demonstrate that this mechanism is dependent on fresh water availability, known to be predominantly from snowmelt, and ice temperature at melt onset. We argue that the blockage process has the potential to exert significant control over inter-annual variability in ice albedo. Finally, we suggest that incorporating the mechanism into models would enhance their physical realism. Full treatment would be complex. We provide a simple temperature threshold-based scheme that maybe used to incorporate percolation blockage behavior into existing model frameworks. This article is protected by copyright. All rights reserved.
      PubDate: 2017-01-16T06:25:48.394936-05:
      DOI: 10.1002/2016JC011994
  • Local diurnal wind-driven variability and upwelling in a small coastal
    • Authors: Ryan K. Walter; Emma C. Reid, Kristen A. Davis, Kevin J. Armenta, Kevin Merhoff, Nicholas J. Nidzieko
      Abstract: The oceanic response to high-frequency local diurnal wind forcing is examined in a small coastal embayment located along an understudied stretch of the central California coast. We show that local diurnal wind forcing is the dominant control on nearshore temperature variability and circulation patterns. A complex empirical orthogonal function (CEOF) analysis of velocities in San Luis Obispo Bay reveals that the first mode CEOF amplitude time series, which accounts for 47.9% of the variance, is significantly coherent with the local wind signal at the diurnal frequency and aligns with periods of weak and strong wind forcing. The diurnal evolution of the hydrographic structure and circulation in the bay is examined using both individual events and composite-day averages. During the late afternoon, the local wind strengthens and results in a sheared flow with near-surface warm waters directed out of the bay and a compensating flow of colder waters into the bay over the bottom portion of the water column. This cold water intrusion into the bay causes isotherms to shoal towards the surface and delivers subthermocline waters to shallow reaches of the bay, representing a mechanism for small-scale upwelling. When the local winds relax, the warm water mass advects back into the bay in the form of a buoyant plume front. Local diurnal winds are expected to play an important role in nearshore dynamics and local upwelling in other small coastal embayments with important implications for various biological and ecological processes. This article is protected by copyright. All rights reserved.
      PubDate: 2017-01-13T04:00:50.382849-05:
      DOI: 10.1002/2016JC012466
  • High-resolution observations of secondary circulation and tidally
           synchronized upwelling around a coastal headland
    • Authors: P. Russell; R. Vennell
      Abstract: The upwelling of nutrient rich bottom waters supports life at the oceans surface. Secondary circulation can produce localized upwelling at headlands. Secondary circulation develops in the curved flow around headlands resulting in a loose helical flow pattern within the curved flow. The magnitude of secondary flow can be up to 20% of the depth average current. Moving vessel ADCP measurements were taken at Cape Saunders, Otago Peninsula, New Zealand. New radial basis function interpolation techniques for smoothing noisy data allows the weak horizontal secondary flow to be extracted from the stronger along shore flows. During peak floods of 1 ms−1 the measured strength of secondary flow is 0.2 ms−1. A region of secondary flow approximately 1500 m long and 800 m wide is observed down-stream of the Cape. On the inshore edge this region, areas of vertical velocity inferred from horizontal ADCP measurements using mass continuity show a localized upwelling of up to 0.007 ms−1. Concurrent CTD measurements also show this tidally synchronized upwelling. Linear regression between upwelling from the CTD measurements and the inferred vertical velocity from the ADCP measurements show the data is well correlated, rp = 0.65. The upwelling is 4 m per hour so in this location with a mean depth of 25 m the entire water column is replacing itself with deep waters at least once during a tidal cycle. On a global scale the cumulative upwelling from headlands and islands due to secondary circulation could be a significant source of nutrient delivery to coastal surface waters. This article is protected by copyright. All rights reserved.
      PubDate: 2017-01-13T03:55:51.651267-05:
      DOI: 10.1002/2016JC012117
  • Middepth decadal warming and freshening in the South Atlantic
    • Authors: Donata Giglio; Gregory C. Johnson
      Abstract: South Atlantic Ocean mid-depth water property (temperature, salinity, oxygen, nutrients, etc.) distributions are set by salty, well-ventilated, and relatively nutrient-poor North Atlantic Deep Water (NADW) spreading southward towards the Southern Ocean (SO) underneath fresher, well-ventilated, and relatively nutrient-poor northward-spreading Antarctic Intermediate Water (AAIW). The layer between the NADW and AAIW is oxygen-poor and nutrient-rich, with small vertical temperature gradients. Salinity stratification dominates the vertical density gradient, hence the layer is referred to as Salinity Stratified Layer (SSL). Decadal warming (0.044°C decade−1) and freshening (0.006 g kg−1 decade−1) of this layer are analyzed using Argo data, a climatology, and repeat hydrographic sections. Warming within the SSL accumulates heat at a rate of ∼20 TW, is unlikely to be caused by vertical heave, and is consistent with anomalous southward advection of order 102 km decade−1 in the Atlantic Meridional Overturning Circulation (AMOC). Salinity changes within the SSL are consistent with a downward velocity anomaly of order 10 m decade−1. This article is protected by copyright. All rights reserved.
      PubDate: 2017-01-13T03:55:50.498364-05:
      DOI: 10.1002/2016JC012246
  • Development of a global gridded Argo data set with Barnes successive
    • Authors: Hong Li; Fanghua Xu, Wei Zhou, Dongxiao Wang, Jonathon S. Wright, Zenghong Liu, Yanluan Lin
      Abstract: A new 11-year (2004-2014) monthly 1-degree gridded Argo temperature and salinity dataset with 49 vertical levels from the surface to 1950 m depth (named BOA-Argo) is generated for use in ocean research and modeling studies. The dataset is produced based on refined Barnes successive corrections by adopting flexible response functions based on a series of error analyses to minimize errors induced by non-uniform spatial distribution of Argo observations. These response functions allow BOA-Argo to capture a greater portion of mesoscale and large-scale signals while compressing small-sale and high frequency noise relative to the most recent version of the World Ocean Atlas (WOA). BOA-Argo dataset is evaluated against other gridded datasets, such as WOA13, Roemmich-Argo, Jamestec-Argo, EN4-Argo, and IPRC-Argo in terms of climatology, independent observations, mixed layer depth and so on. Generally, BOA-Argo compare well with other Argo gridded datasets. The RMSEs and correlation coefficients of compared variables from BOA-Argo agree most with those from the Roemmich-Argo. In particular, more mesoscale features are retained in BOA-Argo than others as compared to satellite sea surface heights. These results indicate that the BOA-Argo dataset is a useful and promising adding to the current Argo datasets. The proposed refined Barnes method is computationally simple and efficient, so that the BOA-Argo dataset can be easily updated to keep pace with tremendous daily increases in the volume of Argo temperature and salinity data.
      PubDate: 2017-01-10T19:15:56.735488-05:
      DOI: 10.1002/2016JC012285
  • Dissolved trace metal (Cu, Cd, Co, Ni, Ag) distribution and Cu speciation
           in the Southern Yellow Sea and Bohai Sea, China
    • Authors: Li Li; Xiaojing Wang, Jihua Liu, Xuefa Shi
      Abstract: Trace metals play an important role in biogeochemical cycling in ocean systems. However, because the use of trace-metal clean sampling and analytical techniques has been limited in coastal China, there are few accurate trace metal data for that region. This work studied spatial distribution of selected dissolved trace metals (Ag, Cu, Co, Cd, Ni) and Cu speciation in the southern Yellow Sea (SYS) and Bohai Sea (BS). In general, the average metal (Cu, Co, Cd, Ni) concentrations found in the SYS were lower by a factor of two than those in BS, and they are comparable to dissolved trace metal concentrations in coastal seawater of the United States and Europe.Possible sources and sinks, and physical and biological processes that influenced the distribution of these trace metals in the study region were further examined. Close relationships were found between the trace metal spatial distribution with local freshwater discharge, and processes such as sediment resuspension and biological uptake. Ag, owing to its extremely low concentrations, exhibited a unique distribution pattern that magnified the influences from the physical and biological processes. Cu speciation in the water column showed that, in the study region, Cu was strongly complexed with organic ligands and concentrations of free cupric ion were in the range of 10−12.6‒10−13.2 mol L−1. The distribution of Cu-complexing organic ligand, indicated by values of the side reaction coefficient α', were similar to the Chl a distribution, suggesting that in situ biota production may be one main source of Cu-complexing organic ligand.
      PubDate: 2017-01-10T17:40:22.767831-05:
      DOI: 10.1002/2016JC012500
  • Upper ocean observations in Eastern Caribbean Sea reveal barrier layer
           within a warm core eddy
    • Authors: J. E. Rudzin; L.K. Shay, B. Jaimes, J. K. Brewster
      Abstract: Three-dimensional measurements of a large warm core eddy (WCE) and the Caribbean Current are acquired using oceanic profilers deployed during a NOAA research aircraft study in September 2014 in the eastern Caribbean Sea. Measurements of the near-surface atmosphere are also collected to examine air-sea processes over the eddy. These novel measurements showcase temperature and salinity for the eddy and background flow, upper ocean stratification, a residing barrier layer (BL), velocity structure, and water mass characteristics. The eddy's thermal structure is alike that of WCEs in the Gulf of Mexico (GoM) whereas surrounding waters have relatively deeper isotherms compared to its GoM counterparts. Analyses suggest that upper ocean stratification within the study region is due to a BL. These are the first observations of a BL inside a WCE to the best of our knowledge. Reduced shear comparisons suggest that the upper ocean, especially within the WCE, would be more resistant to tropical cyclone (TC) induced mixing than the GoM because of the BL. The eddy is suspected to originate from North Brazil Current rings, given its fresh anomalies relative to climatology and surrounding waters and its trajectory prior to sampling. Atmospheric measurements suggest the WCE is influencing the lower atmosphere along its boundaries. These observations signify that not only does this WCE have deep thermal structure and modulate the near-surface atmosphere but it is unique because it has a BL. The findings and analyses suggest that a similar eddy could potentially influence air-sea processes, such as those during TC passage.
      PubDate: 2017-01-10T17:35:22.707102-05:
      DOI: 10.1002/2016JC012339
  • Observed cold filaments associated with mesoscale eddies in the South
           China Sea
    • Authors: Jiaxun Li; Guihua Wang, Xiaoming Zhai
      Abstract: Unusual cold filaments are uncovered during the spring intermonsoon season in the South China Sea (SCS) using a suite of satellite observations. They have a width of about 100 km on average and extend several hundreds of kilometers offshore on the sea surface, providing significant cross-shelf transport of heat and nutrients. The eastward current associated with mesoscale eddies in spring in the western SCS is found to play an important role in the filament formation by advecting coastal cold waters far offshore. The meridional location of the cold filament displays considerable interannual variability ranging between 9oN and 18oN, which can be attributed to the interannual south-north shift of the eastward current associated with eddies. It is also found that in the spring, cold filaments have profound effects on the chlorophyll a concentration in the upper ocean and the overlying atmosphere. These findings provide new insights into the role of eddies in cross-shelf exchange and mesoscale air-sea interaction in the marginal seas.
      PubDate: 2017-01-10T17:30:24.776939-05:
      DOI: 10.1002/2016JC012353
  • Variability of upper ocean thermohaline structure during a MJO event from
           DYNAMO aircraft observations
    • Authors: Denny P. Alappattu; Qing Wang, John Kalogiros, Nick Guy, David P. Jorgensen
      Abstract: This paper reports upper ocean thermohaline structure and variability observed during the life cycle of an intense Madden Julian Oscillation (MJO) event occurred in the southern tropical Indian Ocean (14°S-Eq, 70°E-81°E). Water column measurements for this study were collected using airborne expendable probes deployed from NOAA's WP-3D Orion aircraft operated as a part of Dynamics of MJO field experiment conducted during November-December, 2011. Purpose of the study is twofold; (1) to provide a statistical analysis of the upper ocean properties observed during different phases of MJO and, (2) to investigate how the upper ocean thermohaline structure evolved in the study region in response to the MJO induced perturbation.During the active phase of MJO, mixed layer depth (MLD) had a characteristic bimodal distribution. Primary and secondary modes were at ∼ 34 m and ∼ 65 m respectively. Spatial heterogeneity of the upper ocean response to the MJO forcing was the plausible reason for bimodal distribution. Thermocline and isothermal layer depth deepened respectively by 13 m and 19 m from the suppressed through the restoring phase of MJO. Thicker (> 30 m) barrier layers were found to occur more frequently in the active phase of MJO, associated with convective rainfalls. Additionally, the water mass analysis indicated that, in the active phase of this MJO event the sub-surface was dominated by Indonesian throughflow, nonetheless intrusion of Arabian Sea high saline water was also noted near the equator.
      PubDate: 2017-01-10T17:30:22.328789-05:
      DOI: 10.1002/2016JC012137
  • Seasonal warming of the Middle Atlantic Bight Cold Pool
    • Authors: S. J. Lentz
      Abstract: The Cold Pool is a 20 – 60 m thick band of cold, near-bottom water that persists from spring to fall over the mid and outer shelf of the Middle Atlantic Bight (MAB) and Southern Flank of Georges Bank. The Cold Pool is remnant winter water bounded above by the seasonal thermocline and offshore by warmer slope water. Historical temperature profiles are used to characterize the average annual evolution and spatial structure of the Cold Pool. The Cold Pool gradually warms from spring through summer at a rate of order 1°C per month. The warming rate is faster in shallower water where the Cold Pool is thinner, consistent with a vertical turbulent heat flux from the thermocline to the Cold Pool. The Cold Pool warming rate also varies along the shelf; it is larger over Georges Bank and smaller in the southern MAB. The mean turbulent diffusivities at the top of the Cold Pool, estimated from the spring to summer mean heat balance, are an order of magnitude larger over Georges Bank than in the southern MAB, consistent with much stronger tidal mixing over Georges Bank than in the southern MAB. The stronger tidal mixing causes the Cold Pool to warm more rapidly over Georges Bank and the eastern New England shelf than in the New York Bight or southern MAB. Consequently, the coldest Cold Pool water is located in the New York Bight from late spring through summer.
      PubDate: 2017-01-10T17:25:33.168175-05:
      DOI: 10.1002/2016JC012201
  • Community production modulates coral reef pH and the sensitivity of
           ecosystem calcification to ocean acidification
    • Authors: Thomas M. DeCarlo; Anne L. Cohen, George T.F. Wong, Fuh-Kwo Shiah, Steven J. Lentz, Kristen A. Davis, Kathryn E.F. Shamberger, Pat Lohmann
      Abstract: Coral reefs are built of calcium carbonate (CaCO3) produced biogenically by a diversity of calcifying plants, animals and microbes. As the ocean warms and acidifies, there is mounting concern that declining calcification rates could shift coral reef CaCO3 budgets from net accretion to net dissolution. We quantified net ecosystem calcification (NEC) and production (NEP) on Dongsha Atoll, northern South China Sea, over a two-week period that included a transient bleaching event. Peak daytime pH on the wide, shallow reef flat during the non-bleaching period was ∼8.5, significantly elevated above that of the surrounding open ocean (∼8.0-8.1) as a consequence of daytime NEP (up to 112 mmol C m−2 hr−1). Diurnal-averaged NEC was 390 ± 90 mmol CaCO3 m−2 day−1, higher than any other coral reef studied to date despite comparable calcifier cover (25%) and relatively high fleshy algal cover (19%). Coral bleaching linked to elevated temperatures significantly reduced daytime NEP by 29 mmol C m−2 hr−1. pH on the reef flat declined by 0.2 units, causing a 40% reduction in NEC in the absence of pH changes in the surrounding open ocean. Our findings highlight the interactive relationship between carbonate chemistry of coral reef ecosystems and ecosystem production and calcification rates, which are in turn impacted by ocean warming. As open-ocean waters bathing coral reefs warm and acidify over the 21st century, the health and composition of reef benthic communities will play a major role in determining on-reef conditions that will in turn dictate the ecosystem response to climate change.
      PubDate: 2017-01-10T17:25:31.868009-05:
      DOI: 10.1002/2016JC012326
  • Tidal-fluvial interaction in the Guadalquivir Estuary: Spatial and
           frequency-dependent response of currents and water levels
    • Authors: M.A. Losada; M. Díez-Minguito, M.Á. Reyes-Merlo
      Abstract: This paper presents a study on the tidal-fluvial interaction in the highly regulated Guadalquivir River Estuary (SW Spain), which is occasionally subjected to high discharge episodes that affect navigational conditions and increase flood risks. The study specifically focuses on the processes and controlling mechanisms of the non-stationary response of water levels and currents to high discharges. Measurements show a 60-day post-discharge amplification of tidal current and elevation amplitudes and a clockwise rotation of the tidal ellipse in the upper layers. A decrease of amplitudes and an anticlockwise rotation predominate near the bed. Such episodes significantly increase the tidal wave celerity, and especially at high and low water. These features are due to the suspended sediment stratification triggered by the discharge event. The increase in stratification restricts frictional influence to bottom layers, partially decoupling the overlying flow from the bottom. A non-stationary harmonic decomposition method, intended for identifying which non-linear terms in the governing hydrodynamic equations control overtide and compound tide generation, shows that quadratic bottom stress contributes the most during high discharge periods. The consequence in the subtidal balance is that, during peak discharge and in the upper stretches, friction is largely balanced by the water level gradient, although the density gradient term becomes comparable to the friction term soon after peak discharge. Advection is also important to the force balance in the lower estuary. For both parts, to correctly explain subtidal dynamics, it is necessary to account for the time variability of the friction coefficient due to flow-sediment feedback.
      PubDate: 2017-01-10T17:25:28.435786-05:
      DOI: 10.1002/2016JC011984
  • The Angola Current: Flow and hydrographic characteristics as observed at
    • Authors: R. Kopte; P. Brandt, M. Dengler, P.C.M. Tchipalanga, M. Macuéria, M. Ostrowski
      Abstract: The eastern boundary circulation off the coast of Angola has been described only sparsely to date, although it is a key element in the understanding of the highly productive tropical marine ecosystem off Angola. Here, we report for the first time direct velocity observations of the Angola Current (AC) at ∼11°S between July 2013 and October 2015, covering the depth range from 45 to 450 m. The measurements reveal an alongshore flow that is dominated by intra-seasonal to seasonal variability with periodically alternating southward and northward velocities in the range of ±40 cm/s. During the observation period, a weak southward mean flow of 5-8 cm/s at 50 m depth is observed, with the southward current extending down to about 200 m depth. Corresponding mean southward transport of the AC is estimated to be 0.32 ± 0.046 Sv. An extensive set of hydrographic measurements is used to investigate the thermal structure and seasonality in the hydrography of the eastern boundary circulation. Within the depth range of the AC the superposition of annual and semi-annual harmonics explains a significant part of the total variability, although salinity in the near surface layer appears to be also impacted by year-to-year variability and/or short-term freshening events. In the central water layer, temperature and salinity on isopycnals vary only weakly on seasonal to annual time scales. The available dataset is further used to discuss biases in different reanalysis products particularly emphasizing the ocean's role in coupled climate model SST biases in the Eastern Tropical Atlantic. This article is protected by copyright. All rights reserved.
      PubDate: 2017-01-07T03:40:27.466608-05:
      DOI: 10.1002/2016JC012374
  • Relationship between optimal precursors for Indian Ocean Dipole events and
           optimally growing initial errors in its prediction
    • Authors: Mu Mu; Rong Feng, Wansuo Duan
      Abstract: Using the Geophysical Fluid Dynamics Laboratory Climate Model version 2p1, we explored the precursory disturbances that are most likely to develop into a positive Indian Ocean Dipole (IOD). The dominant spatial patterns of these precursors are defined as the optimal precursors (OPRs) of positive IOD as they are more inclined to cause a positive IOD than other superimposed initial perturbations in the experiments. Specifically, there are two types of OPRs with opposite patterns; the surface component of OPR-1 (OPR-2) is an indistinctive west–east dipole pattern, with a small area of negative (positive) perturbations to the coast of Sumatra and Java. Correspondingly, there is a significant west–east dipole pattern in the subsurface component of the OPRs, with the largest values located in the eastern equatorial Indian Ocean. The dominant mode of the time-dependent evolutions of the precursors features rapid development of positive IOD. Furthermore, the OPRs are similar to the optimally growing initial errors (OGEs) associated with IOD predictions that have been presented in previous studies. The shortwave radiation, latent heat flux and westward Rossby waves play an important role in the time-dependent evolution of OGEs. Moreover, the large values of the OPRs are located in the same areas as the sensitive areas of targeted observations identified by the OGEs. This infers that intensive observations over these areas would not only reduce initial errors, improve the accuracy of initial fields and decrease the prediction errors, but would also detect the precursory signals in advance, which substantially improves the forecast skill of IOD. This article is protected by copyright. All rights reserved.
      PubDate: 2017-01-07T03:40:24.841798-05:
      DOI: 10.1002/2016JC012527
  • Three-compartment structure of subsurface-intensified mesoscale eddies in
           the ocean
    • Authors: Zhengguang Zhang; Yu Zhang, Wei Wang
      Abstract: Mesoscale eddies are energetically dominant and pervasive over most of the world's oceans. Among them, many are subsurface intensified with strongest signals in the ocean interior such as mode water eddies, which trap water masses with distinctive properties and carry them over long distances. With both Argo profiling floats and atmospheric reanalysis data we showed that the structure of these eddies obeys a universal rule. Hence their three-dimensional hydrographic fields can be readily reconstructed from very limited information. More interestingly, the volume of water trapped and moved by a mode water eddy is much greater than previously thought; it has a three-compartment structure in the vertical with the mode water being sandwiched between two layers of notably different properties and accounting for only a portion of the total trapped volume. This article is protected by copyright. All rights reserved.
      PubDate: 2017-01-05T04:05:22.89151-05:0
      DOI: 10.1002/2016JC012376
  • Winds on the West Florida Shelf: Regional comparisons between observations
           and model estimates
    • Authors: Dennis A. Mayer; Robert H. Weisberg, Lianyuan Zheng, Yonggang Liu
      Abstract: Wind fields on the West Florida Continental Shelf are investigated using observations from five University of South Florida Coastal Ocean Monitoring and Prediction System buoys and seven of NOAA's National Ocean Service and National Weather Service, National Data Buoy Center stations or buoys spanning the 10 yr period, 2004 through 2013. These observations are compared with NOAA's National Center for Environmental Protection (NCEP) reanalysis wind fields (NCEP winds). The analyses consist of vector correlations in both the time and frequency domains. The primary findings are that winds observed on and near the coastline underestimate those observed offshore and that NCEP winds derived from assimilating mostly land-based observations also underestimate winds observed offshore. With regard to wind stress, and depending upon location, wind stress derived from NCEP winds are 6% to 49% lower than what is computed from observations over open water. A corollary is that wind forcing fields that are underestimated may result in coastal ocean model circulation fields that are also underestimated. These analyses stress the importance of having offshore wind observations, and suggest that adding more offshore wind observations will lead to improved coastal ocean wind fields and hence to improved model renditions of coastal ocean model circulation and related water property fields. This article is protected by copyright. All rights reserved.
      PubDate: 2017-01-05T04:00:26.775852-05:
      DOI: 10.1002/2016JC012112
  • Composition of freshwater in the spring of 2014 on the southern Labrador
           shelf and slope
    • Authors: M. Benetti; G. Reverdin, C. Lique, I. Yashayaev, N.P. Holliday, E. Tynan, S. Torres-Valdes, P. Lherminier, P. Tréguer, G. Sarthou
      Abstract: The Labrador Current is an important conduit of freshwater from the Arctic to the interior North Atlantic subpolar gyre. Here, we investigate the spatial variability of the freshwater sources over the southern Labrador shelf and slope during May-June 2014. Using measurements of seawater properties such as temperature, salinity, nutrients and oxygen isotopic composition, we estimate the respective contributions of saline water of Atlantic and Pacific origins, of brines released during sea ice formation, and of freshwater from sea ice melt and meteoric water origins. On the southern Labrador shelf, we find a large brine signal and Pacific Water influence indicating a large contribution of water from the Canadian Arctic. The brine signal implies that more than 4 m of sea ice formed upstream, either in the Arctic or in Baffin Bay and the northern Labrador Sea. Over the mid-shelf and slope at 52°N, we find a stronger influence of slope water from the West Greenland Current with a smaller contribution of Pacific water and no brine signal. Thus, there is advection of water from the slope region to the mid-shelf between 55°N and 52°N. Very fresh water with high meteoric content is found close to the coast in June 2014. Observations from 1995 and 2008 suggest a higher fraction of brine and Pacific water on the shelf compared to that observed in 2014. This article is protected by copyright. All rights reserved.
      PubDate: 2017-01-05T04:00:25.453902-05:
      DOI: 10.1002/2016JC012244
  • Water mass modification and mixing rates in a 1/12° simulation of the
           Canadian Arctic Archipelago
    • Authors: Kenneth G. Hughes; Jody M. Klymak, Xianmin Hu, Paul G. Myers
      Abstract: Strong spatial differences in diapycnal mixing across the Canadian Arctic Archipelago are diagnosed in a 1/12° basin-scale model. Changes in mass flux between water flowing into or out of several regions are analyzed using a volume-integrated advection–diffusion equation, and focus is given to denser water, the direct advective flux of which is mediated by sills. The unknown in the mass budget, mixing strength, is a quantity seldom explored in other studies of the Archipelago, which typically focus on fluxes. Regionally averaged diapycnal diffusivities and buoyancy fluxes are up to an order of magnitude larger in the eastern half of the Archipelago relative to those in the west. Much of the elevated mixing is concentrated near sills in Queens Channel and Barrow Strait, with stronger mixing particularly evident in the net shifts of the densest water to lower densities as it traverses these constrictions. Associated with these shifts are areally averaged buoyancy fluxes up to 10-8, m2, s-3 through the 1027 kg m-3 isopycnal surface, which lies at approximately 100 m depth. This value is similar in strength to the destabilizing buoyancy flux at the ocean surface during winter. Effective diffusivities estimated from the buoyancy fluxes can exceed 10-4 m2 s-1, but are often closer to 10-5 m2 s-1 across the Archipelago. Tidal forcing, known to modulate mixing in the Archipelago, is not included in the model. Nevertheless, mixing metrics derived from our simulation are of the same order of magnitude as the few comparable observations. This article is protected by copyright. All rights reserved.
      PubDate: 2017-01-04T03:25:21.88117-05:0
      DOI: 10.1002/2016JC012235
  • Issue Information
    • Pages: 8417 - 8418
      PubDate: 2017-01-24T23:23:11.721358-05:
      DOI: 10.1002/jgrc.21412
  • Satellite observation of particulate organic carbon dynamics on the
           Louisiana continental shelf
    • Authors: Chengfeng Le; John C. Lehrter, Chuanmin Hu, Hugh MacIntyre, Marcus W. Beck
      Abstract: Particulate organic carbon (POC) plays an important role in coastal carbon cycling and the formation of hypoxia. Yet, coastal POC dynamics are often poorly understood due to a lack of long term POC observations and the complexity of coastal hydrodynamic and biogeochemical processes that influence POC sources and sinks. Using field observations and satellite ocean color products, we developed a new multiple regression algorithm to estimate POC on the Louisiana Continental Shelf (LCS) from satellite observations. The algorithm had reliable performance with mean relative error (MRE) of ∼40% and root mean square error (RMSE) of ∼50% for MODIS and SeaWiFS images for POC ranging between ∼80 and ∼1200 mg m−3, and showed similar performance for a large estuary (Mobile Bay). Substantial spatio-temporal variability in the satellite-derived POC was observed on the LCS, with high POC found on the inner shelf (< 10 m depth) and lower POC on the middle (10-50 m depth) and outer shelf (50-200 m depth), and with high POC found in winter (January to March) and lower POC in summer to fall (August to October). Correlation analysis between long-term POC time series and several potential influencing factors indicated that river discharge played a dominant role in POC dynamics on the LCS, while wind and surface currents also affected POC spatial patterns on short time scales. This study adds another example where satellite data with carefully developed algorithms can greatly increase the spatial and temporal observations of important biogeochemical variables on continental shelf and estuaries. This article is protected by copyright. All rights reserved.
      PubDate: 2016-12-29T18:35:44.221314-05:
      DOI: 10.1002/2016JC012275
  • Importance of the Equatorial Undercurrent on the Sea Surface Salinity in
           the Eastern Equatorial Atlantic in boreal spring
    • Authors: C. Y. Da-Allada; J. Jouanno, F. Gaillard, N. Kolodziejczyk, C. Maes, N. Reul, B. Bourlès
      Abstract: The physical processes implied in the sea surface salinity (SSS) increase in the equatorial Atlantic Cold Tongue (ACT) region during boreal spring and the lag observed between boreal spring SSS maximum and sea surface temperature (SST) summer minimum are examined using mixed-layer salinity budgets computed from observations and model during the period 2010-2012. The boreal spring SSS maximum is mainly explained by an upward flux of high salinity originating from the core of the Equatorial Undercurrent (EUC) through vertical mixing and advection. The vertical mixing contribution to the mixed-layer salt budget peaks in April-May. It is controlled primarily by i) an increased zonal shear between the surface South Equatorial Current and the subsurface EUC and ii) the presence of a strong salinity stratification at the mixed-layer base from December to May. This haline stratification that is due to both high precipitations below the Inter Tropical Convergence Zone and zonal advection of low-salinity water from the Gulf of Guinea, explains largely the seasonal cycle of the vertical advection contribution to the mixed-layer salt budget. In the ACT region, the SST reaches its maximum in March/April and minimum in July/August. This SST minimum appears one month after the maximum of SSS. The 1-month lag observed between the maximum of SSS in June and the minimum of SST in July is explained by the shallowing of the EUC salinity core in June, then the weakening/erosion of the EUC in June-July which dramatically reduces the lateral subsurface supply of high saline waters. This article is protected by copyright. All rights reserved.
      PubDate: 2016-12-29T18:25:30.928408-05:
      DOI: 10.1002/2016JC012342
  • Multiscale Climate Emulator of Multimodal Wave Spectra: MUSCLE-spectra
    • Authors: Ana Rueda; Christie A. Hegermiller, Jose A.A. Antolinez, Paula Camus, Sean Vitousek, Peter Ruggiero, Patrick L. Barnard, Li H. Erikson, Antonio Tomás, Fernando J. Mendez
      Abstract: Characterization of multimodal directional wave spectra is important for many offshore and coastal applications, such as marine forecasting, coastal hazard assessment, and design of offshore wave energy farms and coastal structures. However, the multivariate and multiscale nature of wave climate variability makes this complex problem tractable using computationally-expensive numerical models. So far, the skill of statistical-downscaling model-based parametric (unimodal) wave conditions is limited in large ocean basins such as the Pacific. The recent availability of long-term directional spectral data from buoys and wave hindcast models allows for development of stochastic models that include multimodal sea-state parameters. This work introduces a statistical-downscaling framework based on weather types to predict multimodal wave spectra (e.g., significant wave height, mean wave period, and mean wave direction from different storm systems, including sea and swells) from large-scale atmospheric pressure fields. For each weather type, variables of interest are modeled using the categorical distribution for the sea-state type, the Generalized Extreme Value (GEV) distribution for wave height and wave period, a multivariate Gaussian copula for the interdependence between variables, and a Markov chain model for the chronology of daily weather types. We apply the model to the Southern California coast, where local seas and swells from both the Northern and Southern Hemispheres contribute to the multimodal wave spectrum. This work allows attribution of particular extreme multimodal wave events to specific atmospheric conditions, expanding knowledge of time-dependent, climate-driven offshore and coastal sea-state conditions that have a significant influence on local nearshore processes, coastal morphology, and flood hazards. This article is protected by copyright. All rights reserved.
      PubDate: 2016-12-29T18:20:35.14266-05:0
      DOI: 10.1002/2016JC011957
  • Reconstruction of Ocean's Interior from Observed Sea Surface Information
    • Authors: Lei Liu; Shiqiu Peng, Rui Xin Huang
      Abstract: Observational surface data are used to reconstruct the ocean's interior through the “interior + surface quasigeostrophic” (isQG) method. The input data include the satellite-derived sea surface height, satellite-derived sea surface temperature, satellite-derived or Argo-based sea surface salinity, and an estimated stratification of the region. The results show that the isQG retrieval of subsurface density anomalies is quite promising compared to Argo profile data. At ∼1000 m depth, the directions of retrieved velocity anomalies are comparable to those derived from Argo float trajectories. The reconstruction using surface density input field approximated only by SST (with constant SSS) performs less satisfactorily than that taking into account the contribution of SSS perturbations, suggesting that the observed SSS information is important for the application of the isQG method. Better reconstruction is obtained in the warm season than in the cold season, which is probably due to the stronger stratification in the warm season that confines the influence of the biases in the surface input data (especially SSS) in a shallow layer. The comparison between the performance of isQG with Argo-based SSS input and that with satellite-derived SSS input suggests that the biases in the SSS products could be a major factor that influences the isQG performance. With reduced biases in satellite-derived SSS in the future, the measurement-based isQG method is expected to achieve better reconstruction of ocean interior and thus is promising in practical application. This article is protected by copyright. All rights reserved.
      PubDate: 2016-12-29T18:20:31.47502-05:0
      DOI: 10.1002/2016JC011927
  • Mapping the Non-Stationary Internal Tide with Satellite Altimetry
    • Authors: Edward D. Zaron
      Abstract: Temporal variability of the internal tide has been inferred from the 23-year-long combined records of the TOPEX/Poseidon, Jason-1, and Jason-2 satellite altimeters by combining harmonic analysis with an analysis of along-track wavenumber spectra of sea-surface height (SSH). Conventional harmonic analysis is first applied to estimate and remove the stationary components of the tide at each point along the reference ground tracks. The wavenumber spectrum of the residual SSH is then computed, and the variance in a neighborhood around the wavenumber of the mode-1 baroclinic M2 tide is interpreted as the sum of noise, broadband non-tidal processes, and the non-stationary tide. At many sites a bump in the spectrum associated with the internal tide is noted, and an empirical model for the noise and non-tidal processes is used to estimate the non-stationary semidiurnal tidal variance. The results indicate a spatially inhomogeneous pattern of tidal variability. Non-stationary tides are larger than stationary tides throughout much of the Equatorial Pacific and Indian Oceans. This article is protected by copyright. All rights reserved.
      PubDate: 2016-12-29T18:20:26.400285-05:
      DOI: 10.1002/2016JC012487
  • Mixing efficiency in the thermocline of lakes observed from eddy
           correlation flux measurements
    • Authors: Julika Weck; Andreas Lorke
      Abstract: Vertical mixing in the thermocline of lakes is poorly understood and most of the current knowledge is based on ex situ methods like laboratory measurements and simulations. Here, we used the eddy correlation technique (EC) to directly measure oxygen and buoyancy fluxes in the thermocline of two lakes (Lake Scharmützelsee and Lake Arendsee in 2012 and 2013, respectively). Additionally, sets of temperature microstructure profiles (SCAMP) were measured during the EC deployments. We used these data to quantify the mixing efficiency as well as the turbulent diffusivity. The derived turbulent diffusivities from EC for the Prandtl number of DO were one order of magnitude higher than predicted by commonly applied parameterization, while the diffusivities for the Prandtl number of heat confirmed the parameterization. The results from EC and SCAMP showed strong differences which we attribute to the fact that SCAMP measurements reflect snapshots of the instantaneous turbulence field while EC provides a temporal average of the prevailing turbulence. Finally, we discuss problems of the EC and the inertial dissipation method in a strongly stratified environment and propose how they could be improved to resolve the full temporal variability of mixing in thermoclines. This article is protected by copyright. All rights reserved.
      PubDate: 2016-12-29T05:41:31.942552-05:
      DOI: 10.1002/2016JC012188
  • Tsunamis generated by long and thin granular landslides in a large flume
    • Authors: Garrett S. Miller; W. Andy Take, Ryan P. Mulligan, Scott McDougall
      Abstract: In this experimental study granular material is released down slope to investigate landslide-generated waves. Starting with a known volume and initial position of the landslide source, detailed data are obtained on the velocity and thickness of the granular flow, the shape and location of the submarine landslide deposit, the amplitude and shape of the near-field wave, the far-field wave evolution, and the wave runup elevation on a smooth impermeable slope. The experiments are performed on a 6.7 m long 30° slope on which gravity accelerates the landslides into a 2.1 m wide and 33.0 m long wave flume that terminates with a 27° runup ramp. For a fixed landslide volume of 0.34 m3, tests are conducted in a range of still water depths from 0.05-0.50 m. Observations from high-speed cameras and measurements from wave probes indicate that the granular landslide moves as a long and thin train of material, and that only a portion of the landslide (termed the ‘effective mass') is engaged in activating the leading wave. The wave behaviour is highly dependent on the water depth relative to the size of the landslide. In deeper water the near-field wave behaves as a stable solitary-like wave, while in shallower water the wave behaves as a breaking dissipative bore. Overall, the physical model observations are in good agreement with the results of existing empirical equations when the effective mass is used to predict the maximum near-field wave amplitude, the far-field amplitude and the runup of tsunamis generated by granular landslides. This article is protected by copyright. All rights reserved.
      PubDate: 2016-12-29T05:41:00.929071-05:
      DOI: 10.1002/2016JC012177
  • The influence of ENSO on an oceanic eddy pair in the South China Sea
    • Authors: Xiaoqing Chu; Changming Dong, Yiquan Qi
      Abstract: An eddy pair off the Vietnam coast is one of the most important features of the summertime South China Sea circulation. Its variability is of interest due to its profound impact on regional climate, ecosystems, biological processes, and fisheries. This study examines the influence of the El Niño–Southern Oscillation (ENSO), a basin-scale climatic mode, on the interannual variability of this regional eddy pair using satellite observational data and historical hydrographic measurements. Over the last three decades, the eddy pair strengthened in 1994 and 2002, and weakened in 2006, 2007, and 2008. It was absent in 1988, 1995, 1998, and 2010, coinciding with strong El Nino-to-La Nina transitions. Composite analyses showed that the strong transition events of ENSO led to radical changes in the summer monsoon, through the forcing of a unique sea surface temperature anomaly structure over the tropical Indo-Pacific basin. With weaker zonal wind, a more northward wind direction, and the disappearance of a pair of positive and negative wind stress curls, the eastward current jet turns northward along the Vietnam coast and the eddy pair disappears. This article is protected by copyright. All rights reserved.
      PubDate: 2016-12-29T05:40:26.947209-05:
      DOI: 10.1002/2016JC012642
  • A PV-approach for dense water formation along fronts: Application to the
           northwestern Mediterranean
    • Authors: Hervé Giordani; Cindy Lebeaupin-Brossier, Fabien Léger, Guy Caniaux
      Abstract: The mechanisms of dense water formation (σ >29.0 kg m−3) at work in the baroclinic cyclonic gyre of the North-Western Mediterranean basin are investigated through a PV-budget (PV: Potential Vorticity). The PV-budget is diagnosed from an eddy-resolving (1/36°) ocean simulation driven in surface by hourly air-sea fluxes provided by a non-hydrostatic atmospheric model at 2.5km-resolution. The PV-budget is controlled by the diabatic, frictional and advective PV-fluxes. Around the gyre the surface diabatic PV-flux dominates the PV-destruction, except along the northern branch of the North Current where the surface frictional PV-flux is strongly negative. In this region, the bathymetry stabilizes the front and maintains the current northerly in the same direction as the dominant northerly wind. This configuration leads to optimal wind-current interactions and explains the preponderance of frictional PV-destruction on diabatic PV-destruction. This mechanical forcing drives a cross-front ageostrophic circulation which subducts surface low-PV waters destroyed by wind on the dense side of the front and obducts high-PV waters from the pycnocline on the light side of the front. The horizontal PV-advections associated with the geostrophic cyclonic gyre and turbulent entrainment at the pycnocline also contribute to the PV-refueling in the frontal region. The surface non-advective PV-flux involves energy exchanges down to −1400 W m−2 in the frontal zone: this flux is 3.5 times stronger than atmospheric buoyancy flux. These energy exchanges quantify the coupling effects between the surface atmospheric forcing with the oceanic frontal structures at submesoscale. This article is protected by copyright. All rights reserved.
      PubDate: 2016-12-27T18:36:05.198883-05:
      DOI: 10.1002/2016JC012019
  • A new real-time tsunami detection algorithm
    • Authors: Francesco Chierici; Davide Embriaco, Luca Pignagnoli
      Abstract: Real-time tsunami detection algorithms play a key role in any Tsunami Early Warning System. We have developed a new algorithm for tsunami detection based on the real-time tide removal and real-time band-pass filtering of sea-bed pressure recordings. The algorithm greatly increases the tsunami detection probability, shortens the detection delay and enhances detection reliability with respect to the most widely used tsunami detection algorithm, while containing the computational cost. The algorithm is designed to be used also in autonomous early warning systems with a set of input parameters and procedures which can be reconfigured in real time. We have also developed a methodology based on Monte Carlo simulations to test the tsunami detection algorithms. The algorithm performance is estimated by defining and evaluating statistical parameters, namely the detection probability, the detection delay, which are functions of the tsunami amplitude and wavelength, and the occurring rate of false alarms. Pressure data sets acquired by Bottom Pressure Recorders in different locations and environmental conditions have been used in order to consider real working scenarios in the test. We also present an application of the algorithm to the tsunami event which occurred at Haida Gwaii on October 28th, 2012 using data recorded by the Bullseye underwater node of Ocean Networks Canada. The algorithm successfully ran for test purpose in year-long missions onboard abyssal observatories, deployed in the Gulf of Cadiz and in the Western Ionian Sea. This article is protected by copyright. All rights reserved.
      PubDate: 2016-12-27T05:25:26.25192-05:0
      DOI: 10.1002/2016JC012170
  • Significant wave heights from Sentinel-1 SAR: Validation and applications
    • Authors: J. E. Stopa; A. Mouche
      Abstract: Two empirical algorithms are developed for wave mode images measured from the synthetic aperture radar aboard Sentinel-1 A. The first method, called CWAVE_S1A, is an extension of previous efforts developed for ERS2 and the second method, called Fnn, uses the azimuth cutoff amongst other parameters to estimate significant wave heights and average wave periods without using a modulation transfer function. Neural networks are trained using co-located data generated from WAVEWATCH III and independently verified with data from altimeters and in-situ buoys. We use neural networks to relate the nonlinear relationships between the input SAR image parameters and output geophysical wave parameters. CWAVE_S1A performs well and has reduced precision compared to Fnn with Hs root mean square errors within 0.5 and 0.6 m respectively. The developed neural networks extend the SAR's ability to retrieve useful wave information under a large range of environmental conditions including extra-tropical and tropical cyclones. This article is protected by copyright. All rights reserved.
      PubDate: 2016-12-26T19:50:44.472802-05:
      DOI: 10.1002/2016JC012364
  • Methane and nitrous oxide distributions across the North American Arctic
           Ocean during summer, 2015
    • Authors: Lindsay Fenwick; David Capelle, Ellen Damm, Sarah Zimmermann, Bill Williams, Svein Vagle, Philippe D. Tortell
      Abstract: We collected Arctic Ocean water column samples for methane (CH4) and nitrous oxide (N2O) analysis on three separate cruises in the summer and fall of 2015, covering a ∼10,000 km transect from the Bering Sea to Baffin Bay. This provided a three-dimensional view of CH4 and N2O distributions across contrasting hydrographic environments, from the oligotrophic waters of the deep Canada Basin and Baffin Bay, to the productive shelves of the Bering and Chukchi Seas. Percent saturation relative to atmospheric equilibrium ranged from 30-800% for CH4 and 75-145% for N2O, with the highest concentrations of both gases occurring in the northern Chukchi Sea. Nitrogen cycling in the shelf sediments of the Bering and Chukchi Seas likely constituted the major source of N2O to the water column, and the resulting high N2O concentrations were transported across the Arctic Ocean in eastward-flowing water masses. Methane concentrations were more spatially heterogeneous, reflecting a variety of localized inputs, including likely sources from sedimentary methanogenesis and sea ice processes. Unlike N2O, CH4 was rapidly consumed through microbial oxidation in the water column, as shown by the 13C enrichment of CH4 with decreasing concentrations. For both CH4 and N2O, sea-air fluxes were close to neutral, indicating that our sampling region was neither a major source nor sink of these gases. Our results provide insight into the factors controlling the distribution of CH4 and N2O in the North American Arctic Ocean, and an important baseline data set against which future changes can be assessed. This article is protected by copyright. All rights reserved.
      PubDate: 2016-12-26T19:50:36.424758-05:
      DOI: 10.1002/2016JC012493
  • Modeling the intense 2012-2013 dense water formation event in the
           northwestern Mediterranean Sea: Evaluation with an ensemble simulation
    • Authors: Robin Waldman; Samuel Somot, Marine Herrmann, Anthony Bosse, Guy Caniaux, Claude Estournel, Loic Houpert, Louis Prieur, Florence Sevault, Pierre Testor
      Abstract: The northwestern Mediterranean Sea is a well-observed ocean deep convection site. Winter 2012-2013 was an intense and intensely documented dense water formation (DWF) event. We evaluate this DWF event in an ensemble configuration of the regional ocean model NEMOMED12. We then assess for the first time the impact of ocean intrinsic variability on DWF with a novel perturbed initial state ensemble method. Finally, we identify the main physical mechanisms driving water mass transformations.NEMOMED12 reproduces accurately the deep convection chronology between late January and March, its location off the Gulf of Lions although with a southward shift and its magnitude. It fails to reproduce the Western Mediterranean Deep Waters salinification and warming, consistently with too strong a surface heat loss.The Ocean Intrinsic Variability modulates half of the DWF area, especially in the open-sea where the bathymetry slope is low. It modulates marginally (3-5\%) the integrated DWF rate, but its increase with time suggests its impact could be larger at interannual timescales. We conclude that ensemble frameworks are necessary to evaluate accurately numerical simulations of DWF.Each phase of DWF has distinct diapycnal and thermohaline regimes: during preconditioning, the Mediterranean thermohaline circulation is driven by exchanges with the Algerian basin. During the intense mixing phase, surface heat fluxes trigger deep convection and internal mixing largely determines the resulting deep water properties. During restratification, lateral exchanges and internal mixing are enhanced. Finally, isopycnal mixing was shown to play a large role in water mass transformations during the preconditioning and restratification phases. This article is protected by copyright. All rights reserved.
      PubDate: 2016-12-26T19:50:27.646395-05:
      DOI: 10.1002/2016JC012437
  • Zonal evolution of Alaskan stream structure and transport quantified with
           Argo data
    • Authors: Paige D. Logan; Gregory C. Johnson
      Abstract: The Alaskan Stream (AS) flows west-southwestward along the south side of Alaska and the Aleutian Island Arc; a western boundary current at the northern edge of the North Pacific subpolar gyre. The Argo float array has improved sampling of the Gulf of Alaska, allowing quantification of the AS's zonal evolution from 140°W to 175°W. Geostrophic along-shore transport of the AS in the upper 1000 dbar referenced to an assumed level of no motion at 1000 dbar shows little change from east to west. However, along-shore absolute geostrophic transports in the top 2000 dbar (obtained by combining mean absolute 1000-dbar velocities from float displacements with the geostrophic velocity fields) generally increase to the west. We estimate full-depth transports by fitting a barotropic and the first two baroclinic modes calculated from a climatology to the absolute geostrophic velocities in the upper 2000 dbar and applying the velocities from these fits from 2000 dbar to the seafloor. Flowing west from its formation region at 140°W–145°W the full-depth AS becomes stronger, more barotropic, and also narrower once it reaches ∼160°W, with along-shore transports increasing from -16.4 ± 4.9 Sv (1 Sv = 106 m3 s−1) at 140°W to -32.6 ± 5.2 Sv at 175°W. Mean concentrations of relatively warm, salty, oxygen-poor, and nutrient-rich Pacific Equatorial Water (PEW) in the AS decrease from 17.8 ± 0.3% to 8.5 ± 0.5% between 140°W and 175°W. However, the volume transport of PEW by the AS exhibits little change over the PEW density range between these longitudes. This article is protected by copyright. All rights reserved.
      PubDate: 2016-12-26T19:45:52.931213-05:
      DOI: 10.1002/2016JC012302
  • Bioavailable dissolved organic matter and biological hot spots during
           austral winter in Antarctic waters
    • Authors: Yuan Shen; Ronald Benner, Alison E. Murray, Carla Gimpel, B. Greg Mitchell, Elliot L. Weiss, Christian Reiss
      Abstract: Primary production and heterotrophic bacterial activity in the Antarctic Ocean are generally low during the austral winter. Organic carbon is considered to be a major factor limiting bacterial metabolism, but few studies have investigated the bioavailability of organic matter during winter. Herein, the chemical composition and bioavailability of dissolved organic matter (DOM) were investigated in surface (5-100 m) and mesopelagic (200-750 m) waters off the northwestern Antarctic Peninsula during August 2012. Concentrations of dissolved organic carbon (DOC) were low (42±4 µmol L−1) and showed no apparent spatial patterns. By contrast, the composition of DOM exhibited significant spatial trends that reflected the varying ecosystem productivity and water masses. Surface distributions of chlorophyll-a and particulate organic carbon depicted a southward decline in primary productivity from open waters (60.0˚S-61.5˚S) to ice-covered regions (61.5˚S-62.5˚S). This trend was evident from concentrations and DOC-normalized yields of dissolved amino acids in the surface waters, indicating decreasing DOM bioavailability with increasing latitude. A different pattern of DOM bioavailability was observed in the mesopelagic water masses, where amino acids indicated highly altered DOM in the Circumpolar Deep Water and bioavailable DOM in the Transitional Weddell Water. Depth distributions of amino acid yields and compositions revealed hot spots of elevated bioavailable DOM at ∼75 m relative to surrounding waters at most ice-free stations. Relatively low mole percentages of bacterially-derived d-amino acids in hot spots were consistent with an algal source of bioavailable DOM. Overall, these results reveal spatial heterogeneity of bioavailable substrates in Antarctic waters during winter. This article is protected by copyright. All rights reserved.
      PubDate: 2016-12-26T19:45:50.326231-05:
      DOI: 10.1002/2016JC012301
  • Low- and high-frequency oscillatory winds synergistically enhance nutrient
           entrainment and phytoplankton at fronts
    • Authors: D. B. Whitt; M. Lévy, J. R. Taylor
      Abstract: When phytoplankton growth is limited by low nutrient concentrations, full-depth-integrated phytoplankton biomass increases in response to intermittent mixing events that bring nutrient-rich waters into the sunlit surface layer. Here, it is shown how oscillatory winds can induce intermittent nutrient entrainment events and thereby sustain more phytoplankton at fronts in nutrient-limited oceans. Low frequency (i.e. synoptic to planetary scale) along-front wind drives oscillatory cross-front Ekman transport, which induces intermittent deeper mixing layers on the less dense side of fronts. High-frequency wind with variance near the Coriolis frequency resonantly excites inertial oscillations, which also induce deeper mixing layers on the less dense side of fronts. Moreover, we show that low and high frequency winds have a synergistic effect and larger impact on the deepest mixing layers, nutrient entrainment, and phytoplankton growth on the less dense side of fronts than either high-frequency winds or low frequency winds acting alone. These theoretical results are supported by two-dimensional numerical simulations of fronts in an idealized nutrient-limited open-ocean region forced by low and high frequency along-front winds. In these model experiments, higher-amplitude low-frequency wind strongly modulates and enhances the impact of the lower-amplitude high-frequency wind on phytoplankton at a front. Moreover, sensitivity studies emphasize that the synergistic phytoplankton response to low and high frequency wind relies on the high-frequency wind just below the Coriolis frequency. This article is protected by copyright. All rights reserved.
      PubDate: 2016-12-26T19:45:47.05623-05:0
      DOI: 10.1002/2016JC012400
  • Factors influencing the skill of synthesized satellite wind products in
           the tropical Pacific
    • Authors: Shayne McGregor; Alex Sen Gupta, Dietmar Dommenget, Tony Lee, Michael J. McPhaden, William S. Kessler
      Abstract: Given the importance of tropical Pacific winds to global climate, it is interesting to examine differences in the mean and trend among various wind products, and their implications for ocean circulation. Past analysis has revealed that despite the assimilation of observational data, there remain large differences among reanalysis products. Thus, here we examine if satellite-based synthesis products may provide more consistent estimate than reanalysis. Reanalysis product winds are, however, typically used as a background constraint in constructing the synthesis products to fill spatiotemporal gaps and to deal with satellite wind direction ambiguity. Our study identified two important factors that influence both the mean and trends from synthesized wind products. Firstly, the choice of background wind product in synthesised satellite wind products affects the mean and long-term trends, which has implications for simulations of ocean circulation, sea level, and presumably SST. Secondly, we identify a clear need for developing a better understanding of, and correcting differences between in-situ observations of absolute winds with the satellite-derived relative winds prior to synthesizing. This correction requires careful analysis of satellite surface winds with existing co-located in-situ measurements of surface winds and currents, and will benefit from near surface current observations of the proposed Tropical Pacific Observing System. These results also illustrate the difficulty in independently evaluating the synthesis wind products because the in-situ data have been utilised at numerous steps during their development. Addressing these identified issues effectively, will require enhanced collaborations among the wind observation (both satellite and in-situ), reanalysis, and synthesis communities. This article is protected by copyright. All rights reserved.
      PubDate: 2016-12-26T19:45:43.492037-05:
      DOI: 10.1002/2016JC012340
  • Surface flux and ocean heat transport convergence contributions to
           seasonal and interannual variations of ocean heat content
    • Authors: C. D. Roberts; M. D. Palmer, R. P. Allan, D.G.D. Desbruyeres, P. Hyder, C. Liu, D. Smith
      Abstract: We present an observation-based heat budget analysis for seasonal and interannual variations of ocean heat content (H) in the mixed layer (Hmld) and full depth ocean (Htot). Surface heat flux and ocean heat content estimates are combined using a novel Kalman smoother-based method. Regional contributions from ocean heat transport convergences are inferred as a residual and the dominant drivers of Hmld and Htot are quantified for seasonal and interannual time scales. We find that non-Ekman ocean heat transport processes dominate Hmld variations in the equatorial oceans and regions of strong ocean currents and substantial eddy activity. In these locations, surface temperature anomalies generated by ocean dynamics result in turbulent flux anomalies that drive the overlying atmosphere. In addition, we find large regions of the Atlantic and Pacific oceans where heat transports combine with local air-sea fluxes to generate mixed layer temperature anomalies. In all locations except regions of deep convection and water mass transformation, interannual variations in Htot are dominated by the internal rearrangement of heat by ocean dynamics rather than the loss or addition of heat at the surface. Our analysis suggests that, even in extra-tropical latitudes, initialization of ocean dynamical processes could be an important source of skill for interannual predictability of Hmld and Htot. Furthermore, we expect variations in Htot (and thus thermosteric sea level) to be more predictable than near surface temperature anomalies due to the increased importance of ocean heat transport processes for full-depth heat budgets. This article is protected by copyright. All rights reserved.
      PubDate: 2016-12-26T19:45:38.7027-05:00
      DOI: 10.1002/2016JC012278
  • Upper-ocean thermal variability controlled by ocean dynamics in the
           Kuroshio-Oyashio Extension region
    • Authors: Gyundo Pak; Young-Hyang Park, Frederic Vivier, Romain Bourdalle-Badie, Gilles Garric, Kyung-Il Chang
      Abstract: To understand the upper-ocean thermal variability in the Kuroshio−Oyashio Extension (KOE) region, the upper 400 m heat budget in the western North Pacific is analyzed for the 1981 − 2013 period using outputs from a high-resolution (1/12°) ocean general circulation model. Winter heat storage rate on interannual to decadal time scales is mainly determined by oceanic heat advection rather than by net air-sea heat flux. The role of heat advection becomes particularly prominent and widely spread over the entire western North Pacific after the 1990 regime shift in association with the reduced variability of surface heat flux caused by weakened SST variability. The net heat flux acts to dampen temperature anomalies caused by the ocean dynamics. The ocean dynamics causing the upper-ocean heat storage rate is principally associated with the meridional shift of the Oyashio Extension front, which is significantly correlated with both the West Pacific and Pacific-North America teleconnection patterns. This article is protected by copyright. All rights reserved.
      PubDate: 2016-12-26T19:45:36.944535-05:
      DOI: 10.1002/2016JC012076
  • Observed mesoscale eddy signatures in Southern Ocean surface mixed-layer
    • Authors: U. Hausmann; Dennis J. McGillicuddy, John Marshall
      Abstract: Combining satellite altimetry with Argo profile data a systematic observational estimate of mesoscale eddy signatures in surface mixed-layer depth (MLD) is provided across the Southern Ocean (SO). Eddy composite MLD anomalies are shallow in cyclones, deep in anticyclones, and increase in magnitude with eddy amplitude. Their magnitudes show a pronounced seasonal modulation roughly following the depth of the climatological mixed layer. Weak eddies of the relatively quiescent SO subtropics feature peak late-winter perturbations of ±10 m. Much larger MLD perturbations occur over the vigorous eddies originating along the Antarctic Circumpolar Current (ACC) and SO western boundary current systems, with late-winter peaks of −30 m and +60 m in the average over cyclonic and anticyclonic eddy cores (a difference of ≈ 100 m). The asymmetry between modest shallow cyclonic and pronounced deep anticyclonic anomalies is systematic and not accompanied by corresponding asymmetries in eddy amplitude. Nonetheless the net deepening of the climatological SO mixed layer by this asymmetry in eddy MLD perturbations is estimated to be small (few meters). Eddies are shown to enhance SO MLD variability with peaks in late winter and eddy-intense regions. Anomalously deep late-winter mixed layers occur disproportionately within the cores of anticyclonic eddies, suggesting the mesoscale heightens the frequency of deep winter surface-mixing events along the eddy-intense regions of the SO. The eddy modulation in MLD reported here provides a pathway via which the oceanic mesoscale can impact air-sea fluxes of heat and carbon, the ventilation of water masses, and biological productivity across the SO. This article is protected by copyright. All rights reserved.
      PubDate: 2016-12-26T19:45:31.602686-05:
      DOI: 10.1002/2016JC012225
  • Decadal changes in salinity in the oceanic subtropical gyres
    • Authors: Bryce A. Melzer; Bulusu Subrahmanyam
      Abstract: We analyzed spatial and temporal salinity trends in five subtropical gyre regions over the past six decades using Simple Ocean Data Assimilation (SODA) reanalysis with a focus on the subsurface salinity of the upper 1000 m of the ocean. Our results indicate an overall salinity increase within the mixed layer, and a salinity decrease at depths greater than 200m in the global subtropical gyres over 61 years, of which each individual gyre was analyzed in further detail. We determine that freshwater fluxes at the air-sea interface are the primary drivers of the sea surface salinity (SSS) signature over these open ocean regions by quantifying the advective contribution within the surface layer. This was demonstrated through a mixed layer salinity budget in each subtropical gyre based on the vertically integrated advection and entrainment of salt. Our analysis of decadal variability of fluxes into and out of the gyres reveals little change in the strength of the mean currents through this region despite an increase in the annual export of salt in all subtropical gyres, with the meridional component dominating the zonal. This study reveals that the salt content of E-P maximum waters advected into the subtropical gyres is increasing over time. A combination of increasing direct evaporation over the regions with increasing remote evaporation over nearby E-P maxima is believed to be the main driver in increasing salinity of the subtropical oceans, suggesting an intensification of the global water cycle over decadal timescales. This article is protected by copyright. All rights reserved.
      PubDate: 2016-12-26T19:45:26.679908-05:
      DOI: 10.1002/2016JC012243
  • Regional modeling of the water masses and circulation annual variability
           at the Southern Brazilian Continental Shelf
    • Authors: L. F. Mendonça; R. B. Souza, C. R. C. Aseff, L. P. Pezzi, O. O. Möller, R. C. M. Alves
      Abstract: The Southern Brazilian Continental Shelf (SBCS) is one of the more productive areas for fisheries in Brazilian waters. The water masses and the dynamical processes of the region present a very seasonal behavior that imprint strong effects in the ecosystem and the weather of the area and its vicinity. This paper makes use of the Regional Ocean Modeling System (ROMS) for studying the water mass distribution and circulation variability in the SBCS during the year of 2012. Model outputs were compared to in situ, historical observations and to satellite data. The model was able to reproduce the main thermohaline characteristics of the waters dominating the SBCS and the adjacent region. The mixing between the Subantarctic Shelf Water and the Subtropical Shelf Water, known as the Subtropical Shelf Front (STSF), presented a clear seasonal change in volume. As a consequence of the mixing and of the seasonal oscillation of the STSF position, the stability of the water column inside the SBCS also changes seasonally. Current velocities and associated transports estimated for the Brazil Current (BC) and for the Brazilian Coastal Current (BCC) agree with previous measurements and estimates, stressing the fact that the opposite flow of the BCC occurring during winter in the study region is about 2 orders of magnitude smaller than that of the BC. Seasonal maps of simulated Mean Kinetic Energy and Eddy Kinetic Energy demonstrate the known behavior of the BC and stressed the importance of the mean coastal flow off Argentina throughout the year. This article is protected by copyright. All rights reserved.
      PubDate: 2016-12-26T19:40:25.384959-05:
      DOI: 10.1002/2016JC011780
  • Water Mass Modification and Mixing Rates in a 1/12° Simulation of the
           Canadian Arctic Archipelago
    • Authors: Kenneth G. Hughes; Jody M. Klymak, Xianmin Hu, Paul G. Myers
      Abstract: Strong spatial differences in diapycnal mixing across the Canadian Arctic Archipelago are diagnosed in a 1/12° basin-scale model. Changes in mass flux between water flowing into or out of several regions are analyzed using a volume-integrated advection–diffusion equation, and focus is given to denser water, the direct advective flux of which is mediated by sills. The unknown in the mass budget, mixing strength, is a quantity seldom explored in other studies of the Archipelago, which typically focus on fluxes. Regionally averaged diapycnal diffusivities and buoyancy fluxes are up to an order of magnitude larger in the eastern half of the Archipelago relative to those in the west. Much of the elevated mixing is concentrated near sills in Queens Channel and Barrow Strait, with stronger mixing particularly evident in the net shifts of the densest water to lower densities as it traverses these constrictions. Associated with these shifts are areally averaged buoyancy fluxes up to 10−8 m2 s−3 through the 1027 kg m−3 isopycnal surface, which lies at approximately 100m depth. This value is similar in strength to the destabilizing buoyancy flux at the ocean surface during winter. Effective diffusivities estimated from the buoyancy fluxes can exceed 10−4 m2 s−1, but are often closer to 10−5 m2 s−1 across the Archipelago. Tidal forcing, known to modulate mixing in the Archipelago, is not included in the model. Nevertheless, mixing metrics derived from our simulation are of the same order of magnitude as the few comparable observations. This article is protected by copyright. All rights reserved.
      PubDate: 2016-12-23T06:56:23.849476-05:
      DOI: 10.1002/2016JC012200
  • Nitrogen fixation in the eastern Atlantic reaches similar levels in the
           southern and northern hemisphere
    • Authors: Debany Fonseca-Batista; Frank Dehairs, Virginie Riou, François Fripiat, Marc Elskens, Florian Deman, Natacha Brion, Fabien Quéroué, Maya Bode, Holger Auel
      Abstract: Euphotic layer dinitrogen (N2) fixation and primary production (PP) were measured in the eastern Atlantic Ocean (38°N–21°S) using 15N2 and 13C bicarbonate tracer incubations. This region is influenced by Saharan dust deposition and waters with low nitrogen to phosphorus (N/P) ratios originating from the Subantarctic and the Benguela upwelling system. Depth-integrated rates of N2 fixation in the north (0–38°N) ranged from 59 to 370 µmol N m−2 d−1, with the maximal value at 19°N under the influence of the northwest African upwelling. Diazotrophic activity in the south (0–21°S), though slightly lower, was surprisingly close to observations in the north, with values ranging from 47 to 119 µmol N m−2 d−1. Our North Atlantic N2 fixation rates correlate well with dust deposition, while those in the South Atlantic correlate strongly with excess phosphate relative to nitrate. There the necessary iron is assumed to be supplied from the Benguela upwelling system. When converting N2 fixation to carbon uptake using a Redfield ratio (6.6), we find that N2 fixation may support up to 9% of PP in the subtropical North Atlantic (20–38°N), 5% in the tropical North Atlantic (0–20°N) and 1% of PP in the South Atlantic (0–21°S). Combining our data with published datasets, we estimate an annual N input of 27.6 ± 10 Tg N yr−1 over the open Atlantic Ocean, 11% of which enters the region between 20° to 50°N, 71% between 20°N and 10°S and 18% between 10 and 45°S. This article is protected by copyright. All rights reserved.
      PubDate: 2016-12-22T04:50:34.570514-05:
      DOI: 10.1002/2016JC012335
  • Turbulent and numerical mixing in a salt wedge estuary: Dependence on grid
           resolution, bottom roughness, and turbulence closure
    • Authors: David K. Ralston; Geoffrey W. Cowles, W. Rockwell Geyer, Rusty C. Holleman
      Abstract: The Connecticut River is a tidal salt wedge estuary, where advection of sharp salinity gradients through channel constrictions and over steeply sloping bathymetry leads to spatially heterogeneous stratification and mixing. A 3-d unstructured-grid finite-volume hydrodynamic model (FVCOM) was evaluated against shipboard and moored observations, and mixing by both the turbulent closure and numerical diffusion were calculated. Excessive numerical mixing in regions with strong velocities, sharp salinity gradients, and steep bathymetry reduced model skill for salinity. Model calibration was improved by optimizing both the bottom roughness (z0), based on comparison with the barotropic tidal propagation, and the mixing threshold in the turbulence closure (steady-state Richardson number, Rist), based on comparison with salinity. Whereas a large body of evidence supports a value of Rist ∼ 0.25, model skill for salinity improved with Rist ∼ 0.1. With Rist = 0.25, numerical mixing contributed about 1/2 the total mixing, while with Rist = 0.10 it accounted for ∼2/3, but salinity structure was more accurately reproduced. The combined contributions of numerical and turbulent mixing were quantitatively consistent with high-resolution measurements of turbulent mixing. A coarser grid had increased numerical mixing, requiring further reductions in turbulent mixing and greater bed friction to optimize skill. The optimal Rist for the fine grid case was closer to 0.25 than for the coarse grid, suggesting that additional grid refinement might correspond with Rist approaching the theoretical limit. Numerical mixing is rarely assessed in realistic models, but comparisons with high-resolution observations in this study suggest it is an important factor. This article is protected by copyright. All rights reserved.
      PubDate: 2016-12-22T04:46:07.28881-05:0
      DOI: 10.1002/2016JC011738
  • Extension of the prognostic model of sea surface temperature to
           rain-induced cool and fresh lenses
    • Authors: H. Bellenger; K. Drushka, W. Asher, G. Reverdin, M. Katsumata, M. Watanabe
      Abstract: The Zeng and Beljaars (2005) sea surface temperature prognostic scheme, developed to represent diurnal warming, is extended to represent rain-induced freshening and cooling. Effects of rain on salinity and temperature in the molecular skin layer (first few hundred micrometers) and the near-surface turbulent layer (first few meters) are separately parameterized by taking into account rain-induced fluxes of sensible heat and freshwater, surface stress, and mixing induced by droplets penetrating the water surface. Numerical results from this scheme are compared to observational data from two field studies of near-surface ocean stratifications caused by rain, to surface drifter observations and to previous computations with an idealized ocean mixed layer model, demonstrating that the scheme produces temperature variations consistent with in situ observations and model results. It reproduces the dependency of salinity on wind and rainfall rate and the lifetime of fresh lenses. In addition, the scheme reproduces the observed lag between temperature and salinity minimum at low wind speed and is sensitive to the peak rain rate for a given amount of rain. Finally, a first assessment of the impact of these fresh lenses on ocean surface variability is given for the near-equatorial western Pacific. In particular, the variability due to the mean rain-induced cooling is comparable to the variability due to the diurnal warming so that they both impact large-scale horizontal surface temperature gradients. The present parameterization can be used in a variety of models to study the impact of rain-induced fresh and cool lenses at different spatial and temporal scales. This article is protected by copyright. All rights reserved.
      PubDate: 2016-12-22T04:46:01.203328-05:
      DOI: 10.1002/2016JC012429
  • Hydrodynamic influences on acoustical and optical backscatter in a
           fringing reef environment
    • Authors: G. Pawlak; M. A. Moline, E. J. Terrill, P. L. Colin
      Abstract: Observations of hydrodynamics along with optical and acoustical water characteristics in a tropical fringing reef environment reveal a distinct signature associated with flow characteristics and tidal conditions. Flow conditions are dominated by tidal forcing with an offshore component from the reef flat during ebb. Measurements span variable wave conditions enabling identification of wave effects on optical and acoustical water properties.High frequency acoustic backscatter (6 MHz) is strongly correlated with tidal forcing increasing with offshore directed flow and modulated by wave height, indicating dominant hydrodynamic influence. Backscatter at 300 kHz and 1200 kHz is predominantly diurnal suggesting a biological component. Optical backscatter is closely correlated with high frequency acoustic backscatter across the range of study conditions. Acoustic backscatter frequency dependence is used along with changes in optical properties to interpret particle size variations. Changes across wave heights suggest shifts in particle size distributions with increases in relative concentrations of smaller particles for larger wave conditions. Establishing a connection between the physical processes of a fringing tropical reef and the resulting acoustical and optical signals allows for interpretation and forecasting of the remote sensing response of these phenomena over larger scales. This article is protected by copyright. All rights reserved.
      PubDate: 2016-12-22T04:45:56.961567-05:
      DOI: 10.1002/2016JC012497
  • Sea surface pCO2 and O2 dynamics in the partially ice-covered Arctic Ocean
    • Authors: Fakhrul Islam; Michael D. DeGrandpre, Cory M. Beatty, Mary-Louise Timmermans, Richard A. Krishfield, John M. Toole, Samuel R. Laney
      Abstract: Understanding the physical and biogeochemical processes that control CO2 and dissolved oxygen (DO) dynamics in the Arctic Ocean (AO) is crucial for predicting future air-sea CO2 fluxes and ocean acidification. Past studies have primarily been conducted on the AO continental shelves during low-ice periods and we lack information on gas dynamics in the deep AO basins where ice typically inhibits contact with the atmosphere. To study these gas dynamics, in situ time-series data have been collected in the Canada Basin during late summer to autumn of 2012. Partial pressure of CO2 (pCO2), DO concentration, temperature, salinity, and chlorophyll-a fluorescence (Chl-a) were measured in the upper ocean in a range of sea ice states by two drifting instrument systems. Although the two systems were on average only 222 km apart, they experienced considerably different ice cover and external forcings during the 40-50 d periods when data were collected. The pCO2 levels at both locations were well below atmospheric saturation whereas DO was almost always slightly supersaturated. Modeling results suggest that air-sea gas exchange, net community production (NCP) and horizontal gradients were the main sources of pCO2 and DO variability in the sparsely ice-covered AO. In areas more densely covered by sea ice, horizontal gradients were the dominant source of variability, with no significant NCP in the surface mixed layer. If the AO reaches equilibrium with atmospheric CO2 as ice cover continues to decrease, aragonite saturation will drop from a present mean of 1.00 ± 0.02 to 0.86 ± 0.01. This article is protected by copyright. All rights reserved.
      PubDate: 2016-12-22T04:45:47.136148-05:
      DOI: 10.1002/2016JC012162
  • Monitoring remote ocean waves using P-wave microseisms
    • Authors: Jennifer Neale; Nicholas Harmon, Meric Srokosz
      Abstract: Oceanic microseisms are generated by the interaction of opposing ocean waves and subsequent coupling with the seabed, so microseisms should contain information on the ocean conditions that generated them. This leads to the possibility of using seismic records as a proxy for the ocean gravity wavefield. Here we investigate the P-wave component of microseisms, which has previously been linked to areas of high wave interaction intensity in mid-ocean regions. We compare modeled P-wave microseismic sources with those observed at an array in California, and also investigate the relationship between observed sources and significant wave height. We found that the time-varying location of microseism sources in the North Pacific, mapped from beamforming and backprojection of seismic data, was accurate to ≤10° in 90% of cases. The modeled sources were found to dominate at ∼0.2 Hz which was also reflected in the seismic observations. An empirical relationship between observed beampower and modeled source power allowed sources during an independent data period to be estimated with a correlation coefficient of 0.63. Likewise, significant wave height was also estimated with a correlation coefficient of 0.63. Our findings suggest that with improvements in resolution and amplitude retrieval from beamforming, correlations up to 0.78 should be possible between observed P-wave microseisms and significant wave height in remote ocean regions. This article is protected by copyright. All rights reserved.
      PubDate: 2016-12-22T04:45:34.545346-05:
      DOI: 10.1002/2016JC012183
  • Interannual variability of tropical Pacific Sea level from 1993 to 2014
    • Authors: Xiaoting Zhu; Richard J. Greatbatch, Martin Claus
      Abstract: A multi-mode, linear reduced-gravity model, driven by ERA-Interim monthly mean wind stress anomalies, is used to investigate interannual variability in tropical Pacific sea level as seen in satellite altimeter data. The model output is fitted to the altimeter data along the equator, in order to derive the vertical profile for the model forcing, showing that a signature from modes higher than mode six cannot be extracted from the altimeter data. It is shown that the model has considerable skill at capturing interannual sea level variability both on and off the equator. The correlation between modelled and satellite-derived sea level data exceeds 0.8 over a wide range of longitudes along the equator and readily captures the observed ENSO events. Overall, the combination of the first, second, third and fifth modes can provide a robust estimate of the interannual sea level variability, the second mode being dominant. A remarkable feature of both the model and the altimeter data is the presence of a pivot point in the western Pacific on the equator. We show that the westward displacement of the pivot point from the centre of the basin is strongly influenced by the fact that most of the wind stress variance is found in the western part of the basin. We also show that the Sverdrup transport is not fundamental to the dynamics of the recharge/discharge mechanism in our model, although the spatial structure of the wind forcing does play a role in setting the amplitude of the “warm water volume”. This article is protected by copyright. All rights reserved.
      PubDate: 2016-12-22T04:45:27.105334-05:
      DOI: 10.1002/2016JC012347
  • Atlantic water in the Nordic Seas: Locally eddy-permitting ocean
           simulation in a global setup
    • Authors: Claudia Wekerle; Qiang Wang, Sergey Danilov, Vibe Schourup-Kristensen, Wilken-Jon von Appen, Thomas Jung
      Abstract: Warm and salty Atlantic Water is transported by the Norwegian Atlantic Current through the Nordic Seas. A fraction of it enters the Arctic Ocean where it contributes significantly to its heat budget. Resolving the complex circulations structure in the Nordic Seas, in particular eddies, presents a numerical challenge in ocean models. Here, we present a hindcast experiment for the years 1969–2009 with a global configuration of the Finite Element Sea-ice Ocean Model, employing high resolution in the Nordic Seas and Arctic Ocean (4.5 km). We show that substantial improvements can be achieved in the circulation structure, hydrography and eddy kinetic energy in the Nordic Seas compared with a coarse-resolution reference run. A better represented Norwegian Atlantic Front Current (NwAFC) in the high-resolution setup leads to a reduction of a strong negative temperature bias in the eastern Nordic Seas. The Atlantic Water inflow through the Iceland-Faroe Ridge is found to be very sensitive to mesh resolution, and high resolution is required to adequately represent this inflow and the downstream NwAFC. With increased mesh resolution, the simulated ocean temperature is significantly improved at Barents Sea Opening (BSO), and the Atlantic Water volume transport in Fram Strait becomes much closer to observations in terms of both magnitude and variability. By using passive tracers the origins of water masses at BSO and Fram Strait are identified. Our study also indicates that eddy-resolving meshes are required to further improve the representation of dynamical processes in the region, in particular at Fram Strait. This article is protected by copyright. All rights reserved.
      PubDate: 2016-12-21T03:30:51.601123-05:
      DOI: 10.1002/2016JC012121
  • A comparison of the climates of the medieval climate anomaly, little ice
           age, and current warm period reconstructed using coral records from the
           northern South China Sea
    • Authors: Wenfeng Deng; Xi Liu, Xuefei Chen, Gangjian Wei, Ti Zeng, Luhua Xie, Jian-xin Zhao
      Abstract: For the global oceans, the characteristics of high-resolution climate changes during the last millennium remain uncertain because of the limited availability of proxy data. This study reconstructs climate conditions using annually resolved coral records from the South China Sea (SCS) to provide new insights into climate change over the last millennium. The results indicate that the climate of the Medieval Climate Anomaly (MCA, AD 900–1300) was similar to that of the Current Warm Period (CWP, AD 1850–present), which contradicts previous studies. The similar warmth levels for the MCA and CWP have also been recorded in the Makassar Strait of Indonesia, which suggests that the MCA was not warmer than the CWP in the western Pacific and that this may not have been a globally uniform change. Hydrological conditions were drier/saltier during the MCA and similar to those of the CWP. The drier/saltier MCA and CWP in the western Pacific may be associated with the reduced precipitation caused by variations in the Pacific Walker Circulation. As for the Little Ice Age (LIA, AD 1550–1850), the results from this study, together with previous data from the Makassar Strait, indicate a cold and wet period compared with the CWP and the MCA in the western Pacific. The cold LIA period agrees with the timing of the Maunder sunspot minimum and is therefore associated with low solar activity. The fresher/wetter LIA in the western Pacific may have been caused by the synchronized retreat of both the East Asian Summer Monsoon and the Australian Monsoon. This article is protected by copyright. All rights reserved.
      PubDate: 2016-12-21T03:15:59.551029-05:
      DOI: 10.1002/2016JC012458
  • Seasonal controls of aragonite saturation states in the Gulf of Maine
    • Authors: Zhaohui Aleck Wang; Gareth L. Lawson, Cynthia H. Pilskaln, Amy E. Maas
      Abstract: The Gulf of Maine (GoME) is a shelf region especially vulnerable to ocean acidification (OA) due to natural conditions of low pH and aragonite saturation states (Ω-Ar). This study is the first to assess the major oceanic processes controlling seasonal variability of the carbonate system and its linkages with pteropod abundance in Wilkinson Basin in the GoME. Two years of seasonal sampling cruises suggest that water-column carbonate chemistry in the region undergoes a seasonal cycle, wherein the annual cycle of stratification-overturn, primary production, respiration-remineralization and mixing all play important roles, at distinct spatiotemporal scales. Surface production was tightly coupled with remineralization in the benthic nepheloid layer during high production seasons, which results in occasional aragonite undersaturation. From spring to summer, carbonate chemistry in the surface across Wilkinson Basin reflects a transition from a production-respiration balanced system to a net autotropic system. Mean water-column Ω-Ar and abundance of large thecosomatous pteropods show some correlation, although patchiness and discrete cohort reproductive success likely also influence their abundance. Overall, photosynthesis-respiration is the primary driving force controlling Ω-Ar variability during the spring-to-summer transition as well as over the seasonal cycle. However, calcium carbonate (CaCO3) dissolution appears to occur near bottom in fall and winter when bottom water Ω-Ar is generally low but slightly above 1. This is accompanied by a decrease in pteropod abundance that is consistent with previous CaCO3 flux trap measurements. The region might experience persistent subsurface aragonite undersaturation in 30-40 years under continued ocean acidification. This article is protected by copyright. All rights reserved.
      PubDate: 2016-12-20T18:15:29.583734-05:
      DOI: 10.1002/2016JC012373
  • Using Landsat 8 data to estimate suspended particulate matter in the
           Yellow River estuary
    • Authors: Zhongfeng Qiu; Cong Xiao, William Perrie, Deyong Sun, Shengqiang Wang, Hui Shen, Dezhou Yang, Yijun He
      Abstract: The distribution of suspended particulate matter (SPM) and its variations in estuary regions are key to promoting carbon, oxygen and nutrient cycling in coastal regions and nearby seas. This study presents SPM estimations for the Yellow River estuary from Landsat 8 Operational Land Imager (L8/OLI) data from 2013 to 2016. L8/OLI-measured remote sensing reflectance (Rrs) was cross-validated with Moderate Resolution Imaging Spectroradiometer (MODIS) measurements, and SPM concentrations calculated from the tuned retrieval model, were validated with in situ observations. The validation shows that L8/OLI can provide reasonably Rrs, which can be used to quantify SPM distributions and variations in the Yellow River estuary. Three-year averaged SPM maps show that highly turbid waters are mostly found in an ovate area surrounding the mouth of the Yellow River. The corresponding area proportion is less than 30%, with SPM concentrations greater than 100 g m−3. High variations of SPM distributions are consistent with high SPM concentrations, and vice versa. Significant difference is observed between dry and wet seasons. Higher SPM in the dry season are observed both in range and intensity compared to those of the wet season. Furthermore, multiyear averaged SPM distributions with high concentrations are mainly attributable to currents. Significant seasonal variations are mainly controlled by sediment re-suspension processes driven by wind-wave forces. Due to human interventions, seasonal variability in river runoff and sediment discharge from the Yellow River has decreased in recent years. Accordingly, seasonal variability in SPM distributions in the Yellow River estuary due to sediment discharge has decreased. This article is protected by copyright. All rights reserved.
      PubDate: 2016-12-20T18:15:27.342577-05:
      DOI: 10.1002/2016JC012412
  • Satellite assessment of particulate matter and phytoplankton variations in
           the Santa Barbara Channel and its surrounding waters: Role of surface
    • Authors: Fernanda Henderikx Freitas; David A. Siegel, Stéphane Maritorena, Erik Fields
      Abstract: Satellite observations of chlorophyll in coastal waters are often described in terms of changes in productivity in response to regional upwelling processes while optical backscattering coefficients are more often linked to episodic inputs of suspended sediments from storm runoff. Here we show that the surface gravity wave resuspension of sediments has a larger role in controlling backscatter than previously considered. Almost 18 years of SeaWiFS, MODIS, MERIS and VIIRS satellite imagery of the Santa Barbara Channel, California and its surrounding waters spectrally-merged with the Garver-Siegel-Maritorena bio-optical model were used to assess the controls on suspended particle distributions. Analysis revealed that chlorophyll blooms in the warmer portions of the domain occur in phase with SST minima, usually in early spring, while blooms in the cooler regions lag SST minima and occur simultaneously to the strongest equatorward winds every year, often in the summer. Tight coupling between the optical variables was seen in offshore areas, as expected for productive waters. However, values of backscatter near the coast were primarily modulated by surface waves. This relationship holds throughout all seasons and is stronger within the 100-m isobath, but often extends tens of kilometers offshore. This forcing of particle resuspension by surface waves is likely a feature ubiquitous in all coastal oceans characterized by fine sediments. The implication of surface wave processes determining suspended particle loads far beyond the surf zone has large consequences for the interpretation of satellite ocean color signals in coastal waters and potentially redefines the extent of the littoral zone. This article is protected by copyright. All rights reserved.
      PubDate: 2016-12-20T18:15:24.861734-05:
      DOI: 10.1002/2016JC012152
  • Observations of frazil ice formation and upward sediment transport in the
           Sea of Okhotsk: A possible mechanism of iron supply to sea ice–
    • Authors: Masato Ito; Kay I. Ohshima, Yasushi Fukamachi, Genta Mizuta, Yoshimu Kusumoto, Jun Nishioka
      Abstract: In the Sea of Okhotsk, sediment incorporation, transport and release by sea ice potentially plays important roles in the bio-related material (such as iron) cycle and ecosystem. The backscatter strength data of bottom-mounted Acoustic Doppler Current Profilers have suggested signals of frazil ice down to 30 m depth, and signals of upward sediment transport throughout the water column simultaneously in the region northeast of Sakhalin, with a water depth of ∼100 m. Such events occurred under turbulent conditions with strong winds of 10 – 20 m s−1. During such events, newly formed ice was present near the observational sites, shown by satellite microwave imagery. Sediment dispersion from the bottom occurred in association with strong currents of 1.0 – 1.5 m s−1. During these events, the mixed layer reaches near the bottom due to wind-induced stirring, inferred from the high frequency component of vertical velocity. Thus the winter time turbulent mixing brings re-suspended sediment up to near the ocean surface. This study provides the first observational evidence of a series of processes on the incorporation of sedimentary materials into sea ice: sedimentary particles are dispersed by the strong bottom current, subsequently brought up to near the surface by winter time mixing, and finally incorporated into sea ice through underwater interaction with frazil ice and/or flooding of sea ice floes. This wintertime incorporation of bottom sediment into sea ice is a possible mechanism of iron supply to sea ice which melts in spring, and releases bio-reactive iron into the ocean. This article is protected by copyright. All rights reserved.
      PubDate: 2016-12-20T18:15:21.338966-05:
      DOI: 10.1002/2016JC012198
  • Vertical structure of pore pressure under surface gravity waves on a
           steep, megatidal, mixed sand-gravel-cobble beach
    • Authors: Tristan B. Guest; Alex E. Hay
      Abstract: The vertical structure of surface gravity wave-induced pore pressure is investigated within the intertidal zone of a natural, steeply sloping, megatidal, mixed sand-gravel-cobble beach. Results from a coherent vertical array of buried pore pressure sensors are presented in terms of signal phase lag and attenuation as functions of oscillatory forcing frequency and burial depth. Comparison of the observations with the predictions of a theoretical poro-elastic bed response model indicates that the large observed phase lags and attenuation are attributable to interstitial trapped air. In addition to the dependence on entrapped air volume, the pore pressure phase and attenuation are shown to be sensitive to the hydraulic conductivity of the sediment, to the changing mean water depth during the tidal cycle, and to the redistribution/rearrangement of beach face material by energetic wave action during storm events. The latter result indicates that the effects on pore pressure of sediment column disturbance during instrument burial can persist for days to weeks, depending upon wave forcing conditions. Taken together, these results raise serious questions as to the practicality of using pore pressure measurements to estimate the kinematic properties of surface gravity waves on steep, mixed sand-gravel beaches. This article is protected by copyright. All rights reserved.
      PubDate: 2016-12-20T18:10:28.49583-05:0
      DOI: 10.1002/2016JC012257
  • On the hydrography of Denmark Strait
    • Authors: Dana Mastropole; Robert S. Pickart, Héðinn Valdimarsson, Kjetil Våge, Kerstin Jochumsen, James Girton
      Abstract: Using 111 shipboard hydrographic sections across Denmark Strait occupied between 1990-2012, we characterize the mean conditions at the sill, quantify the water mass constituents, and describe the dominant features of the Denmark Strait Overflow Water (DSOW). The mean vertical sections of temperature, salinity, and density reveal the presence of circulation components found upstream of the sill, in particular the shelfbreak East Greenland Current (EGC) and the separated EGC. These correspond to hydrographic fronts consistent with surface-intensified southward flow. Deeper in the water column the isopycnals slope oppositely, indicative of bottom-intensified flow of DSOW. An end-member analysis indicates that the deepest part of Denmark Strait is dominated by Arctic-Origin Water with only small amounts of Atlantic-Origin Water. On the western side of the strait the overflow water is a mixture of both constituents, with a contribution from Polar Surface Water. Weakly stratified “boluses” of dense water are present in 41% of the occupations, revealing that this is a common configuration of DSOW. The bolus water is primarily Arctic-Origin Water and constitutes the densest portion of the overflow. The boluses have become warmer and saltier over the 22-year record, which can be explained by changes in end member properties and their relative contributions to bolus composition. This article is protected by copyright. All rights reserved.
      PubDate: 2016-12-20T13:20:44.897778-05:
      DOI: 10.1002/2016JC012007
  • A periodic freshwater patch detachment process from the block Island sound
           estuarine plume
    • Authors: Qianqian Liu; Lewis M. Rothstein, Yiyong Luo
      Abstract: Previous observations suggest periodic freshwater patches separating from the Block Island Sound (BIS) estuarine plume. In this study, the dynamics of the separation process is investigated through a series of numerical experiments using the Regional Ocean Modeling System (ROMS). In addition, we explore the seasonal variability of the freshwater patches and their response to river discharge and ambient current. The model results indicate that episodic freshwater patches are triggered by small changes in tidal currents over the spring-neap tidal cycle. The spring-neap variation in tidal currents causes significant, monthly fluctuations in turbulent mixing and vertical stratification in BIS, modulating the freshwater discharge thereby generating episodic freshwater patches that move both downstream along the southern shore of Long Island and toward Rhode Island Sound (RIS). The realistically configured model shows that the freshwater patches experience strong seasonal variability. They are largest in spring when the river discharge peaks, and smallest in summer due to the weak river discharge and a robust upstream ambient current from RIS. According to the analysis of the freshwater transport out of BIS, we conclude that such detachment occurs at tidal mixing fronts. This article is protected by copyright. All rights reserved.
      PubDate: 2016-12-20T13:20:37.919238-05:
      DOI: 10.1002/2015JC011546
  • Deep temperature variability in Drake passage
    • Authors: Yvonne L. Firing; Elaine L. McDonagh, Brian A. King, Damien G. Desbruyères
      Abstract: Observations made on 21 occupations between 1993 and 2016 of GO-SHIP line SR1b in eastern Drake Passage show an average temperature of 0.53°C deeper than 2000 dbar, with no significant trend, but substantial year-to-year variability (standard deviation 0.08°C). Using a neutral density framework to decompose the temperature variability into isopycnal displacement (heave) and isopycnal property change components shows that approximately 95% of the year-to-year variance in deep temperature is due to heave. Changes on isopycnals make a small contribution to year-to-year variability but contribute a significant trend of -1.4±0.6 m°C per year, largest for density (γn) > 28.1, south of the Polar Front (PF). The heave component is depth-coherent and results from either vertical or horizontal motions of neutral density surfaces, which trend upward and northward around the PF, downward for the densest levels in the southern section, and downward and southward in the Subantarctic Front and Southern Antarctic Circumpolar Current Front (SACCF). A proxy for the locations of the Antarctic Circumpolar Current (ACC) fronts is constructed from the repeat hydrographic data and has a strong relationship with deep ocean heat content, explaining 76% of deep temperature variance. The same frontal position proxy based on satellite altimeter-derived surface velocities explains 73% of deep temperature variance. The position of the PF plays the strongest role in this relationship between ACC fronts and deep temperature variability in Drake Passage, although much of the temperature variability in the southern half of the section can be explained by the position of the SACCF. This article is protected by copyright. All rights reserved.
      PubDate: 2016-12-20T13:20:32.809602-05:
      DOI: 10.1002/2016JC012452
  • Submarine groundwater discharge and associated nutrient fluxes into the
           Southern Yellow Sea: A case study for semienclosed and oligotrophic
           seas-implication for green tide bloom
    • Authors: Jian'an Liu; Ni Su, Xilong Wang, Jinzhou Du
      Abstract: The biogenic elements concentrations in a coastal bay/estuary are strongly influenced not only by riverine input but also by submarine groundwater discharge (SGD) which has been identified as a typical process of land/ocean interactions in coastal zones. To assess the role of SGD in nutrient fluxes in the Southern Yellow Sea (SYS), 228Ra activities were measured in seawater collected in May 2015. Analyzing the sources and sinks of 228Ra, the flux of excess 228Ra through SGD was estimated to be (2.2 ± 1.0) ×1015 dpm yr−1. Based on the 228Ra mass balance model, we estimated the average SGD flux to be approximately (1.3 ± 0.87) ×1012 m3 yr−1 over the entire SYS, which is at least 3.3 times the estimated annual delivery from the Changjiang River into the SYS (∼1.3 × 1011 m3 yr−1). The SGD-derived biogenic elements loads (dissolved inorganic nitrogen [DIN], phosphorus [DIP] and silicon [DSi]) were estimated as (487 ± 384) × 109 mol yr−1, (2.8 ± 2.2) × 109 mol yr−1, and (313 ± 259) × 109 mol yr−1, respectively, which are approximately 18 times, 7 times and 13 times the riverine input from both mainland China and Korea. The accumulation nutrient fluxes derived by SGD may play one of the most important roles in the green tide bloom that originated from the Subei Shoal zone in the SYS. Additionally, DIN and DIP via SGD can provide the necessary amounts of nutrient for recovering nutrient concentrations to normal levels after the green tide bloom is terminated. This article is protected by copyright. All rights reserved.
      PubDate: 2016-12-17T10:23:18.341302-05:
      DOI: 10.1002/2016JC012282
  • On the observed synoptic signal in the Mississippi-Alabama slope flow
    • Authors: E. V. Maksimova
      Abstract: This communication discusses a strong near-barotropic current signal and its dynamics observed on the Mississippi-Alabama upper slope in the northeastern Gulf of Mexico. When the variability related to mesoscale eddies is not present or removed, the subinertial current is found to be controlled by the synoptic-scale wind fluctuations, qualitatively in agreement with coastally trapped wave theory. Specifically, the along-isobath synoptic velocity component on the Mississippi-Alabama slope is correlated with the wind stress component in the direction along the Florida peninsula. Moreover, the observed along-isobath flows on the Mississippi-Alabama slope and on the west Florida shelf are highly coherent. These relations are pronounced in wintertime but less obvious during summertime, less energetic, wind forcing. This article is protected by copyright. All rights reserved.
      PubDate: 2016-12-17T10:22:47.806588-05:
      DOI: 10.1002/2016JC012320
  • Improved forecasts of winter weather extremes over midlatitudes with extra
           Arctic observations
    • Authors: Kazutoshi Sato; Jun Inoue, Akira Yamazaki, Joo-Hong Kim, Marion Maturilli, Klaus Dethloff, Stephen R. Hudson, Mats A. Granskog
      Abstract: Recent cold winter extremes over Eurasia and North America have been considered to be a consequence of a warming Arctic. More accurate weather forecasts are required to reduce human and socioeconomic damages associated with severe winters. However, the sparse observing network over the Arctic brings errors in initializing a weather prediction model, which might impact accuracy of prediction results at midlatitudes. Here we show that additional Arctic radiosonde observations from the Norwegian young sea ICE cruise project 2015 drifting ice camps and existing land stations during winter improved forecast skill and reduced uncertainties of weather extremes at midlatitudes of the Northern Hemisphere. For two winter storms over East Asia and North America in February 2015, ensemble forecast experiments were performed with initial conditions taken from an ensemble atmospheric reanalysis in which the observation data were assimilated. The observations reduced errors in initial conditions in the upper troposphere over the Arctic region, yielding more precise prediction of the locations and strengths of upper troughs and surface synoptic disturbances. Errors and uncertainties of predicted upper troughs at midlatitudes would be brought with upper-level high potential vorticity (PV) intruding southward from the observed Arctic region. This is because the PV contained a “signal” of the additional Arctic observations as it moved along an isentropic surface. This suggests that a coordinated sustainable Arctic observing network would be effective not only for regional weather services but also for reducing weather risks in locations distant from the Arctic. This article is protected by copyright. All rights reserved.
      PubDate: 2016-12-13T12:45:03.629595-05:
      DOI: 10.1002/2016JC012197
  • Combined observations of Arctic sea ice with near-coincident colocated X,
           C, and L-band SAR satellite remote sensing and helicopter-borne
    • Authors: A.M. Johansson; J.A. King, A.P. Doulgeris, S. Gerland, S. Singha, G. Spreen, T. Busche
      Abstract: In this study we compare co-located near-coincident X-, C- and L-band fully polarimetry SAR satellite images with helicopter-borne ice thickness measurements acquired during the Norwegian Young sea ICE 2015 (N-ICE2015) expedition in the region of the Arctic Ocean north of Svalbard in April 2015. The air-borne surveys provide near-coincident snow plus ice thickness, surface roughness data and photographs. This unique dataset allows us to investigate how the different frequencies can complement one another for sea ice studies; but also to raise awareness of limitations. X- and L-band satellite scenes were shown to be a useful complement to the standard SAR frequency for sea ice monitoring (C-band) for lead ice and young ice identification. This may be in part be due to the frequency but also the high spatial resolution of these sensors. Importantly this is true even when the scenes are outside their nominal performance range. We found a relatively low correlation between snow plus ice thickness and surface roughness. Therefore, ice thickness cannot directly be observed by SAR which has important implications for operational ice charting based on automatic segmentation. This article is protected by copyright. All rights reserved.
      PubDate: 2016-12-13T12:45:02.140353-05:
      DOI: 10.1002/2016JC012273
  • Liquid freshwater transport estimates from the East Greenland Current
           based on continuous measurements north of Denmark Strait
    • Authors: L. de Steur; R. S. Pickart, A. Macrander, K. Våge, B. Harden, S. Jónsson, S. Østerhus, H. Valdimarsson
      Abstract: Liquid freshwater transports of the shelfbreak East Greenland Current (EGC) and the separated EGC are determined from mooring records from the Kögur section north of Denmark Strait between August 2011 and July 2012. The 11-month mean freshwater transport (FWT), relative to a salinity of 34.8, was 65 ± 11 mSv to the south. Approximately 70% of this was associated with the shelfbreak EGC and the remaining 30% with the separated EGC. Very large southward FWT ranging from 160 mSv to 120 mSv was observed from September to mid-October 2011 and was foremost due to anomalously low upper-layer salinities. The FWT may, however, be underestimated by approximately 5 mSv due to sampling biases in the upper ocean. The FWT on the Greenland shelf was estimated using additional inshore moorings deployed from 2012-14. While the annual mean ranged from nearly zero during the first year to 18 mSv to the south during the second year, synoptically the FWT on the shelf can be significant. Furthermore, an anomalous event in autumn 2011 caused the shelfbreak EGC to reverse, leading to a large reduction in FWT. This reversed circulation was due to the passage of a large, 100 km wide anticyclone originating upstream from the shelfbreak. The late summer FWT of -131 mSv is 150% larger than earlier estimates based on sections in the late-1990s and early-2000s. This increase is likely the result of enhanced freshwater flux from the Arctic Ocean to the Nordic Seas during the early 2010s. This article is protected by copyright. All rights reserved.
      PubDate: 2016-12-01T11:35:36.520632-05:
      DOI: 10.1002/2016JC012106
  • Quantifying the residual volume transport through a multiple-inlet system
           in response to wind forcing: The case of the western Dutch Wadden Sea
    • Authors: Matias Duran-Matute; Theo Gerkema, Maximiliano G. Sassi
      Abstract: In multiple-inlet coastal systems like the western Dutch Wadden Sea, the tides (and their interaction with the bathymetry), the fresh water discharge, and the wind drive a residual flow through the system. In the current paper, we study the effect of the wind on the residual volume transport through the inlets and the system as a whole on both the short (one tidal period) and long (seasonal or yearly) time scales. The results are based on realistic three-dimensional baroclinic numerical simulations for the years 2009-2011. The length of the simulations (over 2000 tidal periods) allowed us to analyze a large variety of conditions and quantify the effect of wind on the residual volume transport. We found that each inlet has an anisotropic response to wind; i.e. the residual volume transport is much more sensitive to the wind from two inherent preferential directions than from any other directions. We quantify the effects of wind on the residual volume transport through the system and introduce the concept of the system's conductance for such wind driven residual transport. For the western Dutch Wadden Sea, the dominant wind direction in the region is close to the direction with the highest conductance and opposes the tidally driven residual volume transport. This translates a large variability of the residual volume transport and a dominance of the wind in its long-term characteristics in spite of the episodic nature of storms. This article is protected by copyright. All rights reserved.
      PubDate: 2016-11-28T14:36:06.232328-05:
      DOI: 10.1002/2016JC011807
  • Tropical Cyclone asymmetry - development and evaluation of a new
           parametric model
    • Authors: M. Olfateh; David P. Callaghan, Peter Nielsen, Tom E. Baldock
      Abstract: A new parametric model is developed to describe the asymmetry commonly observed in Tropical Cyclones or Hurricanes. Observations from 21 Hurricanes from the Gulf of Mexico basin and TC Roger in the Coral Sea are analysed to determine the azimuthal and radial asymmetry typical in these mesoscale systems. On the basis of the observations a new asymmetric directional wind model is proposed which adjusts the widely used Holland (1980) axisymmetric wind model to account for the action of blocking high pressure systems, boundary layer friction and forward speed. The model is tested against the observations and demonstrated to capture the physical features of asymmetric cyclones and provides a better fit to observed winds than the Holland model. Optimum values and distributions of the model parameters are derived for use in statistical modelling. Finally, the model is used to investigation of the asymmetric character of TC systems, including the azimuth of the maximum wind speed, the degree of asymmetry and the re-lationship between asymmetry and forward speed. This article is protected by copyright. All rights reserved.
      PubDate: 2016-11-28T14:36:02.040704-05:
      DOI: 10.1002/2016JC012237
  • Large-eddy simulation of wave-breaking induced turbulent coherent
           structures and suspended sediment transport on a barred beach
    • Authors: Zheyu Zhou; Tian-Jian Hsu, Daniel Cox, Xiaofeng Liu
      Abstract: To understand the interaction between wave-breaking induced turbulent coherent structures and suspended sediment transport, we report a Large-Eddy Simulation (LES) study of wave breaking processes over a near-prototype scale barred beach. The numerical model is implemented using the open-source CFD toolbox, OpenFOAM®, in which the incompressible three-dimensional filtered Navier-Stokes equations for the water and air phases are solved with a finite volume scheme. A Volume of Fluid (VOF) method is used to capture the evolution of the water-air interface. The numerical model is validated with measured free surface elevation, turbulence averaged flow velocity, turbulent intensity, and for the first time, the intermittency of breaking wave turbulence. Simulation results confirm that as the obliquely descending eddies (ODEs) approach the bottom, significant bottom shear stress is generated. Remarkably, the collapse of ODEs onto the bed can also cause drastic spatial and temporal changes of dynamic pressure on the bottom. By allowing sediment to be suspended from the bar crest, intermittently high sediment suspension events and their correlation with high turbulence and/or high bottom shear stress events are investigated. The simulated intermittency of sediment suspension is similar to previous field and large wave flume observations. Coherent suspension events account for only 10% of the record but account for about 50% of the sediment load. Model results suggest that about 60∼70% of coherent bottom stress events are associated with surface-generated turbulence. Nearly all the coherent sand suspension events are associated with coherent turbulence events due to wave-breaking turbulence approaching the bed. This article is protected by copyright. All rights reserved.
      PubDate: 2016-11-28T14:35:58.920394-05:
      DOI: 10.1002/2016JC011884
  • The ocean mixed-layer under Southern Ocean sea-ice: Seasonal cycle and
    • Authors: Violaine Pellichero; Jean-Baptiste Sallée, Sunke Schmidtko, Fabien Roquet, Jean-Benoît Charrassin
      Abstract: The oceanic mixed-layer is the gateway for the exchanges between the atmosphere and the ocean; in this layer all hydrographic ocean properties are set for months to millennia. A vast area of the Southern Ocean is seasonally capped by sea-ice, which alters the characteristics of the ocean mixed-layer. The interaction between the ocean mixed-layer and sea-ice plays a key role for water-mass transformation, the carbon cycle, sea-ice dynamics, and ultimately for the climate as a whole. However, the structure and characteristics of the under-ice mixed-layer are poorly understood due to the sparseness of in-situ observations and measurements. In this study, we combine distinct sources of observations to overcome this lack in our understanding of the Polar Regions. Working with Elephant Seal-derived observations, ship-based and Argo float observations, we describe the seasonal cycle of the ocean mixed-layer characteristics and stability of the ocean mixed-layer over the Southern Ocean and specifically under sea-ice. Mixed-layer heat and freshwater budgets are used to investigate the main forcing mechanisms of the mixed-layer seasonal cycle. The seasonal variability of sea surface salinity and temperature are primarily driven by surface processes, dominated by sea-ice freshwater flux for the salt budget, and by air-sea flux for the heat budget. Ekman advection, vertical diffusivity and vertical entrainment play only secondary roles.Our results suggest that changes in regional sea-ice distribution and annual duration, as currently observed, widely affect the buoyancy budget of the underlying mixed-layer, and impact large-scale water-mass formation and transformation with far reaching consequences for ocean ventilation. This article is protected by copyright. All rights reserved.
      PubDate: 2016-11-28T14:35:55.289042-05:
      DOI: 10.1002/2016JC011970
  • Bottom-slope-induced net sheet-flow sediment transport rate under
           sinusoidal oscillatory flows
    • Authors: Jing Yuan; Zhiwei Li, O. S. Madsen
      Abstract: It is generally believed that the slope of beaches can lead to a net downslope (usually offshore) sediment transport rate under shoaling waves, but very few high-quality measurements have been reported for a quantitative understanding of this phenomenon. In this study, full-scale (1:1) experiments of bottom-slope-induced net sheet-flow sediment transport rate under sinusoidal oscillatory flows are conducted using a tilting oscillatory water tunnel. The tests cover a variety of flow-sediment conditions on bottom slopes up to 2.6°. A laser-based bottom profiler system is developed for measuring net transport rate based on the principle of mass conservation. Experimental results suggest that for a given flow-sediment condition the net transport rate is in the downslope direction and increases linearly with bottom slope. A conceptual model is presented based on the idea that gravity helps bottom shear stress drive bedload transport and consequently enhances (reduces) bedload transport and suspension when the flow is in the downslope (up-slope) direction. The model predicts both the measured net sediment transport rates and the experimental linear relationship between net transport rates and bottom slope with an accuracy generally better than a factor of 2. Some measured net transport rates in this study are comparable to those due to flow skewness obtained in similar sheet-flow studies, despite that our maximum slope could be milder than the actual bottom slope in surf zones, where sheet-flow conditions usually occur. This shows that the slope effect may be as important as wave nonlinearity in producing net cross-shore sheet-flow sediment transport. This article is protected by copyright. All rights reserved.
      PubDate: 2016-11-28T14:35:52.278979-05:
      DOI: 10.1002/2016JC011996
  • Gulf of Maine salinity variation and its correlation with upstream Scotian
           Shelf currents at seasonal and interannual time scales
    • Authors: Hui Feng; Doug Vandemark, John Wilkin
      Abstract: In the Gulf of Maine (GoM), a network of buoy hydrography measurements collected since 2001 provide a subsurface salinity time series showing a strong seasonal cycle and interannual variations that are both consistent with remote forcing of Gulf hydrography by upstream advection. These long-term mooring data are combined with satellite altimeter estimates of upper ocean current anomaly on the adjoining Scotian Shelf (SS) in a new attempt to use disparate regional observations as proxies to detect and evaluate remote forcing of water mass change inside the Gulf from 2002-2015. Focusing on buoys moored along the Maine coastal current (MCC), lagged cross correlations with upstream altimeter-derived SS current anomalies are found to be as high as 0.84 and explain 50-70% of variance in the MCC subsurface salinity data at both seasonal and interannual time scales. Significant MCC freshening in 2004-2005 and 2010-2011 follow SS velocity strengthening, while salting events in 2002-2004 and 2012-2015 are associated with relaxation of SS currents. Estimated time lags translate to advective SS inflow velocity estimates of 6±2 cm/s that are consistent with past modeling and observational work. Investigation of wind stress control on SS velocity anomalies indicates that wind directions away from the along-shore can factor into flow modulation. Overall, the study findings are consistent with past freshwater flux observations and modeling examining southwest SS inflow to the GoM, provide a new empirical means to diagnose GoM hydrographic change, and point to one potential application of an altimeter measurement record that extends from 1992 into the future. This article is protected by copyright. All rights reserved.
      PubDate: 2016-11-28T14:35:50.974821-05:
      DOI: 10.1002/2016JC012337
  • On the generation and evolution of internal solitary waves in the southern
           Red Sea
    • Authors: Daquan Guo; T. R. Akylas, Peng Zhan, Aditya Kartadikaria, Ibrahim Hoteit
      Abstract: Satellite observations recently revealed trains of internal solitary waves (ISWs) in the off-shelf region between 16.0°N and 16.5°N in the southern Red Sea. The generation mechanism of these waves is not entirely clear, though, as the observed generation sites are far away (50 km) from the shelf break and tidal currents are considered relatively weak in the Red Sea. Upon closer examination of the tide properties in the Red Sea and the unique geometry of the basin, it is argued that the steep bathymetry and a relatively strong tidal current in the southern Red Sea provide favorable conditions for the generation of ISWs. To test this hypothesis and further explore the evolution of ISWs in the basin, 2D numerical simulations with the non-hydrostatic MIT general circulation model (MITgcm) were conducted. The results are consistent with the satellite observations in regard to the generation sites, peak amplitudes and the speeds of first-mode ISWs. Moreover, our simulations suggest that the generation process of ISWs in the southern Red Sea is similar to the tide-topography interaction mechanism seen in the South China Sea. Specifically, instead of ISWs arising in the immediate vicinity of the shelf break via a hydraulic lee wave mechanism, a broad, energetic internal tide is first generated, which subsequently travels away from the shelf break and eventually breaks down into ISWs. Sensitivity runs suggest that ISW generation may also be possible under summer stratification conditions, characterized by an intermediate water intrusion from the strait of Bab el Mandeb. This article is protected by copyright. All rights reserved.
      PubDate: 2016-11-28T14:35:49.785171-05:
      DOI: 10.1002/2016JC012221
  • Role of wind in erosion–accretion cycles on an estuarine mudflat
    • Authors: B.W. Shi; S.L. Yang, Y.P. Wang, G.C. Li, M.L. Li, P. Li, C. Li
      Abstract: Wind is an important regulator of coastal erosion and accretion processes that have significant ecological and engineering implications. Nevertheless, previous studies have mainly focused on storm−generated changes in the bed level. This paper aims to improve the understanding of wind−induced erosion–accretion cycles on intertidal flats under normal (non−stormy) weather conditions using data that relates to the wave climate, near−bed 3D flow velocity, suspended sediment concentration, and bed−level changes on a mudflat at the Yangtze Delta front. The following parameters were calculated at 10−minute intervals over 10 days: the wind wave orbital velocity (Ûδ), bed shear stress from combined current–wave action, erosion flux, deposition flux, and predicted bed−level change. The time series of measured and predicted bed−level changes both show tidal cycles and a 10−day cycle. We attribute the tidal cycles of bed−level changes to tidal dynamics, but we attribute the 10−day cycle of bed−level changes to the interaction between wind speed/direction and neap−spring cyclicity. We conclude that winds can significantly affect bed−level changes in mudflats even during non−stormy weather and under macro−mesotidal conditions and that the bed−level changes can be predicted well using current–wave–sediment combined models. This article is protected by copyright. All rights reserved.
      PubDate: 2016-11-28T14:35:48.454254-05:
      DOI: 10.1002/2016JC011902
  • Effect of tides and source location on nearshore tsunami-induced currents
    • Authors: Aykut Ayca; Patrick J. Lynett
      Abstract: Here, we present the results of a numerical modeling study that investigates how event-maximum tsunami-induced currents vary due to the dynamic effects of tides and wave directivity. First, analyses of tide-tsunami interaction are presented in three harbors by coupling the tsunami with the tide, and allowing the initial tsunami wave to arrive at various tidal phases. We find that tsunami-tide interaction can change the event-maximum current speed experienced in a harbor by up to 25% for the events and harbors studied, and we note that this effect is highly site-specific. Second, to evaluate the effect of wave directionality on event-maximum currents, earthquakes sources were placed throughout the Pacific, with magnitudes tuned to create the same maximum near-coast amplitude at the harbor of study. Our analysis also shows that, for the harbor and sources examined, the effect of offshore directionality and tsunami frequency content has a weak effect on the event-maximum currents experienced in the harbor. The more important dependency of event-maximum currents is the near-harbor amplitude of the wave, indicating that event-maximum currents in a harbor from a tsunami generated by a large far-field earthquake may be reasonably well predicted with only information about the predicted local maximum tsunami amplitude. This study was motivated by the hope of constructing a basis for understanding the dynamic effects of tides and wave directivity on current-based tsunami hazards in a coastal zone. The consideration of these aspects is crucial and yet challenging in the modeling of tsunami currents. This article is protected by copyright. All rights reserved.
      PubDate: 2016-11-28T14:35:43.818112-05:
      DOI: 10.1002/2016JC012435
  • The impact of glacier meltwater on the underwater noise field in a glacial
    • Authors: Oskar Glowacki; Mateusz Moskalik, Grant B. Deane
      Abstract: Ambient noise oceanography is proving to be an efficient and effective tool for the study of ice-ocean interactions in the bays of marine-terminating glaciers. However, obtaining quantitative estimates of ice melting or calving processes from ambient noise requires an understanding of how sound propagation through the bay attenuates and filters the noise spectrum. Measurements of the vertical structure in sound speed in the vicinity of the Hans Glacier in Hornsund Fjord, Spitsbergen, made with O(130) CTD casts between May and November 2015, reveal high-gradient, upward-refracting sound speed profiles created by cold, fresh meltwater during summer months. Simultaneous recordings of underwater ambient noise made at depths of 1, 10 and 20 meters in combination with propagation model calculations using the model Bellhop illustrate the dominant role these surface ducts play in shaping the underwater soundscape. The surface ducts lead to a higher intensity and greater variability of acoustic energy in the near-surface layer covered by glacially-modified waters relative to deeper waters, indicating deeper zones as most appropriate for inter-seasonal acoustic monitoring of the glacial melt. Surface waveguides in Hornsund are relatively shallow and trap sound above O(1 kHz). Deeper waveguides observed elsewhere will also trap low-frequency sounds, such as those generated by calving events for example. Finally, the ambient noise field in Hornsund is shown to be strongly dependent on the distribution of ice throughout the bay, stressing the importance of performing complementary environmental measurements when interpreting the results of acoustic surveys. This article is protected by copyright. All rights reserved.
      PubDate: 2016-11-28T14:35:40.161873-05:
      DOI: 10.1002/2016JC012355
  • Factors controlling enhanced N2O concentrations over the southwestern
           Indian shelf
    • Authors: V. Sudheesh; G.V.M. Gupta, K.V. Sudharma, H. Naik, D.M. Shenoy, M. Sudhakar, S.W.A. Naqvi
      Abstract: Repeat measurements of dissolved nitrous oxide (N2O) along two transects of the western continental shelf of India in 2012 revealed high concentrations of 45±32 nM (off Kochi) and 73±63 nM (off Mangalore) during the summer monsoon (SM). N2O concentrations increased non-linearly during the peak of the SM upwelling, when low O2 (
      PubDate: 2016-11-28T14:35:38.617189-05:
      DOI: 10.1002/2016JC012166
  • Temporal variability of diapycnal mixing in the northern South China Sea
    • Authors: Hui Sun; Qingxuan Yang, Wei Zhao, Xinfeng Liang, Jiwei Tian
      Abstract: Temporal variability of diapycnal mixing over seven months in the northern South China Sea was examined based on McLane Moored Profiler observations from 850 to 2,200 m by employing a finescale parameterization. Intensified diffusivity exceeding the order of 10−3 m2/s in magnitude was found over the first half of Oct 2014, and from 2 Dec 2014 to 21 Jan 2015 (a typical wintertime). Strong internal tides and winds in winter were the likely candidates for the high-level diapycnal mixing in winter. As for the enhanced mixing during Oct 2014, we suspect the generation of near-bottom near-inertial waves through the interaction of mesoscale eddies and unique bottom topography was the cause. This article is protected by copyright. All rights reserved.
      PubDate: 2016-11-28T14:35:34.842422-05:
      DOI: 10.1002/2016JC012044
  • Hydrographic responses to regional covariates across the Kara Sea
    • Authors: Jussi Mäkinen; Jarno Vanhatalo
      Abstract: The Kara Sea is a shelf sea in the Arctic Ocean which has a strong spatiotemporal hydrographic variation driven by river discharge, air pressure and sea ice. There is a lack of information about the effects of environmental variables on surface hydrography in different regions of the Kara Sea. We use a hierarchical spatially varying coefficient model to study the variation of sea surface temperature (SST) and salinity (SSS) in the Kara Sea between years 1980 and 2000. The model allows us to study the effects of climatic (Arctic oscillation index, AO) and seasonal (river discharge and ice concentration) environmental covariates on hydrography. The hydrographic responses to covariates vary considerably between different regions of the Kara Sea. River discharge decreases SSS in the shallow shelf area and has a neutral effect in the northern Kara Sea. The responses of SST and SSS to AO show the effects of different wind and air pressure conditions on water circulation and hence on hydrography. Ice concentration has a constant effect across the Kara Sea. We estimated the average SST and SSS in the Kara Sea in 1980-2000. The average August SST over the Kara Sea in 1995-2000 was higher than the respective average in 1980-1984 with 99.9% probability and August SSS decreased with 77% probability between these time periods. We found a support that the winter season AO has an impact on the summer season hydrography, and temporal trends may be related to the varying level of winter season AO index. This article is protected by copyright. All rights reserved.
      PubDate: 2016-11-28T14:35:33.545373-05:
      DOI: 10.1002/2016JC011981
  • Turbulence-resolving, two-phase flow simulations of wave-supported gravity
           flows: A conceptual study
    • Authors: Celalettin Emre Ozdemir
      Abstract: Discoveries over the last three decades have shown that wave-supported gravity flows (WSGFs) are among the participating physical processes that carry substantial amount of fine sediments across low-gradient shelves. Therefore, understanding the full range of mechanisms responsible for such gravity flows is likely to shed light on the dynamics of subaqueous delta and clinoform development. As wave-induced boundary layer turbulence is the major agent to suspend sediments in WSGFs, the scale of WSGFs in the water column is also bounded by the wave-induced boundary layer thickness which is on the order of decimeters. Therefore, in order to explore the details of participating physical mechanisms, especially that due to turbulence-sediment interaction, highly resolved and accurate numerical models or measurements in the laboratory and the field are required. In this study, the dynamics of WSGFs is investigated by using turbulence-resolving two-phase flow simulations that utilize Direct Numerical Simulations (DNS). The effect of variable sediment loading, slope, and wave orbital velocity is investigated via 21 simulations. This article is protected by copyright. All rights reserved.
      PubDate: 2016-11-28T14:35:27.589828-05:
      DOI: 10.1002/2016JC012061
  • Formation of summer phytoplankton bloom in the northwestern Bay of Bengal
           in a coupled physical-ecosystem model
    • Authors: V. Thushara; P. N. Vinayachandran
      Abstract: The Bay of Bengal (BoB) is considered to be a region of low biological productivity, owing to nutrient limitation, caused by strong salinity stratification induced by the freshwater influx from rivers and precipitation. Satellite and in situ observations, however, reveal the presence of prominent regional blooms in the bay in response to monsoonal forcings. Bloom dynamics of the BoB are presumably determined by freshwater as well as the local and remote effect of winds and remain to be explored in detail. Using a coupled physical-ecosystem model, we have examined the oceanic processes controlling productivity in the northwestern BoB during the summer monsoon. The region exhibits a prominent bloom lasting for a period of about two months, supporting major fishing zones along the northeast coast of India. The ecosystem model simulates the spatial and temporal evolution of the surface bloom in good agreement with Sea-Viewing Wide Field-of-View Sensor (SeaWiFS) observations. Vertical distribution of upper ocean physical and biological tracers and a nitrate budget analysis reveal the dominant role of coastal upwelling induced by alongshore winds in triggering the bloom. Horizontal advection plays a secondary role by supplying nutrients from coastal to offshore regions. The bloom decays with the weakening of winds and upwelling by the end of summer monsoon. The simulated bloom in the northwestern bay remains largely unaffected by the freshwater effects, since the peak bloom occur before the arrival of river plumes. This article is protected by copyright. All rights reserved.
      PubDate: 2016-11-28T14:35:23.407116-05:
      DOI: 10.1002/2016JC011987
  • Seasonal variation of the Beaufort shelfbreak jet and its relationship to
           Arctic cetacean occurrence
    • Authors: Peigen Lin; Robert S. Pickart, Kathleen M. Stafford, G.W.K. Moore, Daniel J. Torres, Frank Bahr, Jianyu Hu
      Abstract: Using mooring timeseries from September 2008 to August 2012, together with ancillary atmospheric and satellite data sets, we quantify the seasonal variations of the shelfbreak jet in the Alaskan Beaufort Sea and explore connections to the occurrences of bowhead and beluga whales. Wind patterns during the four-year study period are different than the long-term climatological conditions in that the springtime peak in easterly winds shifted from May to June, and the autumn peak was limited to October instead of extending farther into the fall. These changes were primarily due to the behavior of the two regional atmospheric centers of action, the Aleutian Low and Beaufort High. The volume transport of the shelfbreak jet, which peaks in the summer, was decomposed into a background (weak wind) component and a wind-driven component. The wind-driven component is correlated to the Pt. Barrow, AK alongcoast windspeed record, although a more accurate prediction is obtained when considering the ice thickness at the mooring site. An upwelling index reveals that wind-driven upwelling is enhanced in June and October when storms are stronger and longer-lasting. The seasonal variation of Arctic cetacean occurrence is dominated by the eastward migration in spring, dictated by pack-ice patterns, and westward migration in fall, coincident with the autumn peak in shelfbreak upwelling intensity. This article is protected by copyright. All rights reserved.
      PubDate: 2016-11-16T16:06:01.205411-05:
      DOI: 10.1002/2016JC011890
  • Sea ice algae chlorophyll a concentrations derived from under-ice spectral
           radiation profiling platforms
    • Authors: Benjamin A. Lange; Christian Katlein, Marcel Nicolaus, Ilka Peeken, Hauke Flores
      Abstract: Multi-scale sea ice algae observations are fundamentally important for projecting changes to sea ice ecosystems, as the physical environment continues to change. In this study we developed upon previously established methodologies for deriving sea ice-algal chlorophyll a concentrations (chl a) from spectral radiation measurements, and applied these to larger-scale spectral surveys. We conducted four different under-ice spectral measurements: irradiance; radiance; transmittance; and transflectance, and applied three statistical approaches: Empirical Orthogonal Functions (EOF); Normalized Difference Indices (NDI); and multi-NDI. We developed models based on ice core chl a and coincident spectral irradiance/transmittance (N=49) and radiance/transflectance (N=50) measurements conducted during two cruises to the central Arctic Ocean in 2011 and 2012. These reference models were ranked based on two criteria: mean robustness R2; and true prediction error estimates. For estimating the biomass of a large scale dataset, the EOF approach performed better than the NDI, due to its ability to account for the high variability of environmental properties experienced over large areas. Based on robustness and true prediction error, the three most reliable models, EOF-transmittance, EOF-transflectance and NDI-transmittance, were applied to two remotely operated vehicle (ROV) and two Surface and Under-Ice Trawl (SUIT) spectral radiation surveys. In these larger-scale chl a estimates, EOF-transmittance showed the best fit to ice core chl a. Application of our most reliable model, EOF-transmittance, to an 85 m horizontal ROV transect revealed large differences compared to published biomass estimates from the same site with important implications for projections of Arctic-wide ice-algal biomass and primary production. This article is protected by copyright. All rights reserved.
      PubDate: 2016-11-16T16:05:54.138874-05:
      DOI: 10.1002/2016JC011991
  • Net primary productivity estimates and environmental variables in the
           Arctic Ocean: An assessment of coupled physical-biogeochemical models
    • Authors: Younjoo J. Lee; Patricia A. Matrai, Marjorie A. M. Friedrichs, Vincent S. Saba, Olivier Aumont, Marcel Babin, Erik T. Buitenhuis, Matthieu Chevallier, Lee de Mora, Morgane Dessert, John P. Dunne, Ingrid Ellingsen, Doron Feldman, Robert Frouin, Marion Gehlen, Thomas Gorgues, Tatiana Ilyina, Meibing Jin, Jasmin G. John, Jonathan Lawrence, Manfredi Manizza, Christophe Eugène Menkes, Coralie Perruche, Vincent Le Fouest, Ekaterina Popova, Anastasia Romanou, Annette Samuelsen, Jörg Schwinger, Roland Séférian, Charles A. Stock, Jerry Tjiputra, L. Bruno Tremblay, Kyozo Ueyoshi, Marcello Vichi, Andrew Yool, Jinlun Zhang
      Abstract: The relative skill of 21 regional and global biogeochemical models was assessed in terms of how well the models reproduced observed net primary productivity (NPP) and environmental variables such as nitrate concentration (NO3), mixed layer depth (MLD), euphotic layer depth (Zeu), and sea ice concentration, by comparing results against a newly updated, quality-controlled in situ NPP database for the Arctic Ocean (1959-2011). The models broadly captured the spatial features of integrated NPP (iNPP) on a pan-Arctic scale. Most models underestimated iNPP by varying degrees in spite of overestimating surface NO3, MLD, and Zeu throughout the regions. Among the models, iNPP exhibited little difference over sea ice condition (ice-free vs. ice-influenced) and bottom depth (shelf vs. deep ocean). The models performed relatively well for the most recent decade and towards the end of Arctic summer. In the Barents and Greenland Seas, regional model skill of surface NO3 was best associated with how well MLD was reproduced. . Regionally, iNPP was relatively well simulated in the Beaufort Sea and the central Arctic Basin, where in situ NPP is low and nutrients are mostly depleted. Models performed less well at simulating iNPP in the Greenland and Chukchi Seas, despite the higher model skill in MLD and sea ice concentration, respectively. iNPP model skill was constrained by different factors in different Arctic Ocean regions. Our study suggests that better parameterization of biological and ecological microbial rates (phytoplankton growth and zooplankton grazing) are needed for improved Arctic Ocean biogeochemical modeling. This article is protected by copyright. All rights reserved.
      PubDate: 2016-11-14T14:18:16.980562-05:
      DOI: 10.1002/2016JC011993
  • Seasonal cycle of near-bottom transport and currents in the northern Gulf
           of California
    • Authors: R. Navarro; M. López, J. Candela
      Abstract: Seasonal cycles of near-bottom transport and temperature over the sills of the Northern Gulf of California, as well as surface geostrophic velocity anomalies, are presented. Transport at the sills, where overflows occur, is towards the head of the gulf all year round with maximum in October and minimum in June. Furthermore, transport is 180° out of phase with the surface geostrophic velocity across the northern gulf, consistent with the exchange being strongest in October. Seasonal cycles of near-bottom temperature and transport are also 180° out of phase, indicating that maximum water inflow is associated with the coolest water entering from the Pacific Ocean. Near-bottom temperature over the northern Ballenas Channel sill has a maximum in early August, which is more in phase with the surface temperature and consistent with intense mixing in the channel. Geostrophic velocity at the northern gulf is in phase with that near the mouth of the gulf, and approximately in phase with the seasonal heat input through the mouth, calculated previously by Beron-Vera and Ripa [2000]. Moreover, the maximum lower-layer, horizontal heat output of the Ballenas Channel occurs in November, approximately one month after the maximum transport through the San Lorenzo and Delfín sills. Therefore, heat loss results from the continuous near-bottom inflow of relatively cold water at both sills which bound the deepest basins of the northern gulf. Moreover, the mean and seasonal cycles of heat and mass fluxes in the deepest basins of the northern gulf are almost everywhere in opposite directions. This article is protected by copyright. All rights reserved.
      PubDate: 2016-11-11T17:04:12.638414-05:
      DOI: 10.1002/2016JC012063
  • Ocean mixing beneath Pine Island Glacier ice shelf, West Antarctica
    • Authors: Satoshi Kimura; Adrian Jenkins, Pierre Dutrieux, Alexander Forryan, Alberto C. Naveira Garabato, Yvonne Firing
      Abstract: Ice shelves around Antarctica are vulnerable to an increase in ocean-driven melting, with the melt rate depending on ocean temperature and the strength of circulations inside the ice-shelf cavities. We present measurements of velocity, temperature, salinity, turbulent kinetic energy dissipation rate and thermal variance dissipation rate beneath Pine Island Glacier ice shelf, West Antarctica. These measurements were obtained by CTD, ADCP and turbulence sensors mounted on an Autonomous Underwater Vehicle (AUV). The highest turbulent kinetic energy dissipation rate is found near the grounding line. The thermal variance dissipation rate increases closer to the ice-shelf base, with a maximum value found ∼0.5 m away from the ice. The observed dissipation rates near the ice are used to estimate basal melting of the ice shelf. We argue that our estimates of basal melting from dissipation rates are within a range of previous estimates of basal melting. This article is protected by copyright. All rights reserved.
      PubDate: 2016-11-11T17:04:03.134314-05:
      DOI: 10.1002/2016JC012149
  • Reconciling estimates of the ratio of heat and salt fluxes at the
           ice-ocean interface
    • Authors: T. Keitzl; J.-P. Mellado, D. Notz
      Abstract: The heat exchange between floating ice and the underlying ocean is determined by the interplay of diffusive fluxes directly at the ice-ocean interface and turbulent fluxes away from it. In this study, we examine this interplay through direct numerical simulations of free convection. Our results show that an estimation of the interface flux ratio based on direct measurements of the turbulent fluxes can be difficult because the flux ratio varies with depth. As an alternative, we present a consistent evaluation of the flux ratio based on the total heat and salt fluxes across the boundary layer. This approach allows us to reconcile previous estimates of the ice–ocean interface conditions. We find that the ratio of heat and salt fluxes directly at the interface is 83 to 100 rather than 33 as determined by previous turbulence measurements in the outer layer. This can cause errors in the estimated ice-ablation rate from field measurements of up to 40% if they are based on the three-equation formulation. This article is protected by copyright. All rights reserved.
      PubDate: 2016-11-11T17:03:08.327637-05:
      DOI: 10.1002/2016JC012018
  • Storm surge along the Pacific Coast of North America
    • Authors: Peter D. Bromirski; Reinhard E. Flick, Arthur J. Miller
      Abstract: Storm surge is an important factor that contributes to coastal flooding and erosion. Storm surge magnitude along eastern North Pacific coasts results primarily from low sea level pressure (SLP). Thus coastal regions where high surge occurs identify the dominant locations where intense storms make landfall, controlled by storm track across the North Pacific. Here storm surge variability along the Pacific coast of North America is characterized by positive non-tide residuals at a network of tide gauge stations from southern California to Alaska. The magnitudes of mean and extreme storm surge generally increase from south to north, with typically high amplitude surge north of Cape Mendocino and lower surge to the south. Correlation of mode 1 non-tide principal component (PC1) during winter months (Dec.-Feb.) with anomalous SLP over the Northeast Pacific indicates that the dominant storm landfall region is along the Cascadia/British Columbia coast. Although empirical orthogonal function spatial patterns show substantial interannual variability, similar correlation patterns of non-tide PC1 over the 1948-1975 and 1983-2014 epochs with anomalous SLP suggest that, when considering decadal-scale time periods, storm surge and associated tracks have generally not changed appreciably since 1948. Non-tide PC1 is well-correlated with PC1 of both anomalous SLP and modeled wave height near the tide gauge stations, reflecting the inter-relationship between storms, surge, and waves. Weaker surge south of Cape Mendocino during the 2015-16 Niño compared with 1982-83 may result from changes in Hadley circulation. Importantly from a coastal impacts perspective, extreme storm surge events are often accompanied by high waves. This article is protected by copyright. All rights reserved.
      PubDate: 2016-11-11T17:03:03.014741-05:
      DOI: 10.1002/2016JC012178
  • Nitrous oxide during the onset of the Atlantic Cold Tongue
    • Authors: D. L. Arévalo-Martínez; A. Kock, T. Steinhoff, P. Brandt, M. Dengler, T. Fischer, A. Körtzinger, H. W. Bange
      Abstract: The tropical Atlantic exerts a major influence in climate variability through strong air-sea interactions. Within this region, the eastern side of the equatorial band is characterized by strong seasonality, whereby the most prominent feature is the annual development of the Atlantic Cold Tongue (ACT). This band of low sea surface temperatures (∼22-23°C) is typically associated with upwelling-driven enhancement of surface nutrient concentrations and primary production. Based on a detailed investigation of the distribution and sea-to-air fluxes of N2O in the eastern equatorial Atlantic (EEA), we show that the onset and seasonal development of the ACT can be clearly observed in surface N2O concentrations, which increase progressively as the cooling in the equatorial region proceeds during spring-summer. We observed a strong influence of the surface currents of the EEA on the N2O distribution, which allowed identifying “high” and “low” concentration regimes that were, in turn, spatially delimited by the extent of the warm eastward-flowing North Equatorial Countercurrent and the cold westward-flowing South Equatorial Current. Estimated sea-to-air fluxes of N2O from the ACT (mean 5.18±2.59 µmol m−2 d−1) suggests that in May-July 2011 this cold-water band doubled the N2O efflux to the atmosphere with respect to the adjacent regions, highlighting its relevance for marine tropical emissions of N2O. This article is protected by copyright. All rights reserved.
      PubDate: 2016-11-11T17:02:57.869086-05:
      DOI: 10.1002/2016JC012238
  • Assessing recent air-sea freshwater flux changes using a surface
           temperature-salinity space framework
    • Authors: Jeremy P. Grist; Simon A. Josey, Jan D. Zika, Dafydd Gwyn Evans, Nikolaos Skliris
      Abstract: A novel assessment of recent changes in air-sea freshwater fluxes has been conducted using a surface temperature-salinity framework applied to four atmospheric reanalyses. Viewed in the T-S space of the ocean surface, the complex pattern of the longitude-latitude space mean global Precipitation minus Evaporation (PME) reduces to three distinct regions. The analysis is conducted for the period 1979-2007 for which there is most evidence for a broadening of the (atmospheric) tropical belt. All four of the reanalyses display an increase in strength of the water cycle. The range of increase is between 2%-30% over the period analysed, with an average of 14%. Considering the average across the reanalyses, the water cycle changes are dominated by changes in tropical as opposed to mid-high latitude precipitation. The increases in the water cycle strength, are consistent in sign, but larger than in a 1% greenhouse gas run of the HadGEM3 climate model. In the model a shift of the precipitation/evaporation cells to higher temperatures is more evident, due to the much stronger global warming signal. The observed changes in freshwater fluxes appear to be reflected in changes in the T-S distribution of the Global Ocean. Specifically, across the diverse range of atmospheric reanalyses considered here, there was an acceleration of the hydrological cycle during 1979-2007 which led to a broadening of the ocean's salinity distribution. Finally, although the reanalyses indicate that the warm temperature tropical precipitation dominated water cycle change, ocean observations suggest that ocean processes redistributed the freshening to lower ocean temperatures. This article is protected by copyright. All rights reserved.
      PubDate: 2016-11-11T17:01:55.921755-05:
      DOI: 10.1002/2016JC012091
  • Barrier island breach evolution: Alongshore transport and bay-ocean
           pressure gradient interactions
    • Authors: Ilgar Safak; John C. Warner, Jeffrey H. List
      Abstract: Physical processes controlling repeated openings and closures of a barrier island breach between a bay and the open ocean are studied using aerial photographs and atmospheric and hydrodynamic observations. The breach site is located on Pea Island along the Outer Banks, separating Pamlico Sound from the Atlantic Ocean. Wind direction was a major control on the pressure gradients between the bay and the ocean to drive flows that initiate or maintain the breach opening. Alongshore sediment flux was found to be a major contributor to breach closure. During the analysis period from 2011 to 2016, three hurricanes had major impacts on the breach. First, Hurricane Irene opened the breach with wind-driven flow from bay to ocean in August 2011. Hurricane Sandy in October 2012 quadrupled the channel width from pressure gradient flows due to water levels that were first higher on the ocean side and then higher on the bay side. The breach closed sometime in Spring 2013, most likely due to an event associated with strong alongshore sediment flux but minimal ocean-bay pressure gradients. Then, in July 2014, Hurricane Arthur briefly opened the breach again from the bay side, in a similar fashion to Irene. In summary, opening and closure of breaches are shown to follow a dynamic and episodic balance between along-channel pressure gradient driven flows and alongshore sediment fluxes. This article is protected by copyright. All rights reserved.
      PubDate: 2016-11-11T17:01:17.6961-05:00
      DOI: 10.1002/2016JC012029
  • Subduction of North Pacific Tropical Water and Its equatorward pathways as
           shown by a simulated passive tracer
    • Authors: Xunwei Nie; Shan Gao, Fan Wang, Tangdong Qu
      Abstract: The subduction and equatorward pathways of North Pacific Tropical Water (NPTW) are investigated using a simulated passive tracer of the consortium Estimating the Circulation and Climate of the Ocean (ECCO). The results demonstrate that the subduction of NPTW occurs in a large area that extends from about 150ºE to 130ºW between 20ºN and 30ºN, but the main subduction region lies in its eastern part. After subduction, the main body of NPTW first spreads westward in the North Equatorial Current. Then it splits into two branches. One flows northward in the Kuroshio upon reaching the western boundary, and the other enters the tropical Pacific either via its western boundary pathway (WBP) or interior pathway (IP). Less than half of the transport through the WBP can eventually reach the central and eastern Pacific by the Equatorial Undercurrent, while the rest is seen to flow into the Indian Ocean by the Indonesian Throughflow. The IP is found to play a significant role in equatorward transport of the NPTW. About 30% of the NPTW that reached the equatorial Pacific is transported through the IP. This article is protected by copyright. All rights reserved.
      PubDate: 2016-11-11T17:00:57.829169-05:
      DOI: 10.1002/2016JC012305
  • Radar imaging of shallow water bathymetry: A case study in the Yangtze
    • Authors: Peng Yu; Johnny A. Johannessen, Vladimir Kudryavtsev, Xiaojing Zhong, Yunxuan Zhou
      Abstract: This study focuses on 2-dimensional (2-D) radar imaging of bathymetric features in the shallow water of the Yangtze Estuary using synthetic aperture radar (SAR) observations and model simulations. A validated 2-D shallow water numerical model simulates the barotropic current velocity, and the simulated current fields together with the relevant parameters of radar observations are then invoked in the radar imaging model as the input. The results show that variations in the simulated image intensity are mainly dominated by distinct radar backscatter anomalies caused by wave-current interactions in the vicinity of rapidly changing underwater topographies. The comparison between the simulated and observed SAR images shows a reasonable agreement, demonstrating that our approach may be implemented to monitor changes in the shallow water bathymetry of the Yangtze Estuary in the future. This article is protected by copyright. All rights reserved.
      PubDate: 2016-11-11T17:00:46.082791-05:
      DOI: 10.1002/2016JC011973
  • Carbon cycling dynamics in the seasonal sea-ice zone of East Antarctica
    • Authors: Nicholas P. Roden; Bronte Tilbrook, Thomas W. Trull, Patti Virtue, Guy D. Williams
      Abstract: The carbon cycle of the seasonally ice covered region of the southwest Indian Ocean sector of East Antarctica (30°-80°E, 60°-69°S) was investigated during austral summer (January – March 2006). Large variability in the drivers and timing of carbon cycling dynamics were observed and indicated that the study site was a weak net source of carbon dioxide (CO2) to the atmosphere of 0.8 ± 1.6 grams C m−2 during the ice-free period, with narrow bands of CO2 uptake observed near the continental margin and north of the Southern Antarctic Circumpolar Current Front. Continuous surface measurements of dissolved oxygen and the fugacity of CO2 were combined with net community production estimates from oxygen/argon ratios to show that surface heat gain and photosynthesis were responsible for the majority of observed surface water variability. On seasonal timescales, winter sea-ice cover reduced the flux of CO2 to the atmosphere in the study area, followed by biologically driven drawdown of CO2 as the ice retreated in spring-summer highlighting the important role that sea-ice formation and retreat has on the biogeochemical cycling of the region. This article is protected by copyright. All rights reserved.
      PubDate: 2016-11-11T17:00:41.888061-05:
      DOI: 10.1002/2016JC012008
  • Footprints of obliquely incident internal solitary waves and internal
           tides near the shelf break in the northern South China Sea
    • Authors: Xiaochuan Ma; Jun Yan, Yijun Hou, Feilong Lin, Xufeng Zheng
      Abstract: A mooring system and two sites of bottom currents were deployed over the slope and near the shelf break on the propagating paths of internal solitary waves (ISWs), west off Dongsha Atoll in the northern South China Sea. Data indicated that energetic ISWs obliquely shoaled onto the shelf west off Dongsha Atoll in an approximately 290° direction, causing strong reversing currents (some exceeding 80 cm/s) near the bottom. Two types of sandwaves and short scour channels are discernible on the seafloor near the shelf break, which have reasonable correlations with the obliquely incident ISWs and internal tides. Type 1 sandwaves, featured by ISWs at the depths of 130-150 m, have flat crests interacting with the isobaths at a angle of nearly 45° which slightly incline and migrate upslope. Type 2 sandwaves are associated with internal tides, which have crests parallel to the isobaths and distinctly incline and migrate downslope. Short channels are parallel to the depth contours and truncate the strata, which could be formed and maintained by along-slope currents that are probably produced by the obliquely ISWs on a large gradient (γ>0.8°). The ISWs can move coarse grains or suspend fine grains but do not change the long-term trend of sediment transport on the seabed with larger gradients (γ/c>1), which is dominated by internal tides. These features are likely widespread near the shelf break in the northern South China Sea and other seas but are limited on mild slopes where ISWs do not break. This article is protected by copyright. All rights reserved.
      PubDate: 2016-11-11T17:00:24.762668-05:
      DOI: 10.1002/2016JC012009
  • Effects of an Arctic under-ice bloom on solar radiant heating of the water
    • Authors: Torbjørn Taskjelle; Mats A. Granskog, Alexey K. Pavlov, Stephen R. Hudson, Børge Hamre
      Abstract: The deposition of solar energy in the upper Arctic Ocean depends, among other things, on the composition of the water column. During the N-ICE2015 expedition, a drift in the Arctic pack ice north of Svalbard, an under-ice phytoplankton bloom was encountered in May 2015. This bloom led to significant changes in the inherent optical properties (IOPs) of the upper ocean. Mean values of total water absorption in the upper 20 m of the water column were up to 4 times higher during the bloom than prior to it. The total water attenuation coefficient increased by a factor of up to around 7. Radiative transfer modeling, with measured IOPs as input, has been performed with a coupled atmosphere-ice-ocean model. Simulations are used to investigate the change in depth dependent solar heating of the ocean after the onset of the bloom, for wavelengths in the region 350–700 nm. Effects of clouds, sea ice cover, solar zenith angle, as well as the average cosine for scattering of the ocean inclusions are evaluated. An increase in energy absorption in the upper 10 m of about 36% is found under 25 cm ice with 2 cm snow, for bloom conditions relative to pre-bloom conditions, which may have implications for ice melt and growth in spring. Thicker clouds and lower sun reduce the irradiance available, but lead to an increase in relative absorption. This article is protected by copyright. All rights reserved.
      PubDate: 2016-11-07T09:18:00.909254-05:
      DOI: 10.1002/2016JC012187
  • A modeling study of processes controlling the Bay of Bengal sea surface
           salinity interannual variability
    • Authors: V.P. Akhil; M. Lengaigne, J. Vialard, F. Durand, M. G. Keerthi, A.V.S. Chaitanya, F. Papa, V. V. Gopalakrishna, Clément de Boyer Montégut
      Abstract: Recent observational studies provided preliminary insights on the interannual variability of Bay of Bengal (BoB) Sea Surface Salinity (SSS), but are limited by the poor data coverage. Here, we describe the BoB interannual SSS variability and its driving processes from a regional eddy-permitting ocean general circulation model forced by interannually varying air-sea fluxes and altimeter-derived discharges of major rivers over the past two decades. Simulated interannual SSS variations compare favourably with both in-situ and satellite data and are largest in boreal fall in three regions: the northern BoB, the coastal region off east India and the Andaman Sea. In the northern BoB these variations are independent from those in other regions and mostly driven by summer-fall Ganga-Brahmaputra runoff interannual variations. In fall, remote forcing from the Indian Ocean Dipole results in anti-clockwise anomalous horizontal currents that drive interannual SSS variations of opposite polarity along the east coast of India and in the Southern Andaman Sea. From winter onward, these anomalies are damped by vertical mixing in the northern BoB and along the east coast of India and by horizontal advection in the Southern Andaman Sea. While river runoff fluctuations locally play a strong role near the Ganga-Brahmaputra river mouth, wind-driven interannual current anomalies are responsible for a large fraction of SSS interannual variability in most of the basin. This article is protected by copyright. All rights reserved.
      PubDate: 2016-11-07T09:11:06.488671-05:
      DOI: 10.1002/2016JC011662
  • Isotope constraints on seasonal dynamics of dissolved and particulate N in
           the Pearl River Estuary, South China
    • Authors: Feng Ye; Guodong Jia, Luhua Xie, Gangjian Wei, Jie Xu
      Abstract: Isotope measurements were performed on dissolved NO3−, NH4+ and suspended particulate total N along a salinity gradient in the Pearl River Estuary (PRE) to investigate seasonal changes in main N sources and its biogeochemical processing under the influence of monsoon climate. Our data revealed that municipal sewage and re-mineralized soil organic N were the major sources of DIN (NO3− and/or NH4+) in freshwater during winter and summer, respectively, whereas phytoplankton biomass was a major component of PN in both seasons. In low salinity waters (
      PubDate: 2016-11-02T09:10:54.169304-05:
      DOI: 10.1002/2016JC012066
  • Long-range sediment transport in the world's oceans by stably stratified
           turbidity currents
    • Authors: Benjamin Kneller; Mohamad M. Nasr-Azadani, Senthil Radhakrishnan, Eckart Meiburg
      Abstract: Submarine fans, supplied primarily by turbidity currents, constitute the largest sediment accumulations on Earth. Generally accepted models of turbidity current behavior imply they should dissipate rapidly on the very small gradients of submarine fans, thus their persistence over long distances is enigmatic. We present numerical evidence, constrained by published field data, suggesting that turbidity currents traveling on low slopes and carrying fine particles have a stably stratified shear layer along their upper interface, which dramatically reduces dissipation and entrainment of ambient fluid, allowing the current to propagate over long distances. We propose gradient Richardson number as a useful criterion to discriminate between the different behaviors exhibited by turbidity currents on high and low slopes. This article is protected by copyright. All rights reserved.
      PubDate: 2016-10-27T03:40:35.040523-05:
      DOI: 10.1002/2016JC011978
  • A validated tropical-extratropical flood hazard assessment for New York
    • Authors: P.M. Orton; T.M. Hall, S.A. Talke, A.F. Blumberg, N. Georgas, S. Vinogradov
      Abstract: Recent studies of flood risk at New York Harbor (NYH) have shown disparate results for the 100-year storm tide, providing an uncertain foundation for the flood mitigation response after Hurricane Sandy. Here, we present a flood hazard assessment that improves confidence in our understanding of the region's present-day potential for flooding, by separately including the contribution of tropical cyclones (TCs) and extratropical cyclones (ETCs), and validating our modeling study at multiple stages against historical observations. The TC assessment is based on a climatology of 606 synthetic storms developed from a statistical-stochastic model of North Atlantic TCs. The ETC assessment is based on simulations of historical storms with many random tide scenarios. Synthetic TC landfall rates and the final TC and ETC flood exceedance curves are all shown to be consistent with curves computed using historical data, within 95% confidence ranges. Combining the ETC and TC results together, the 100-year return period storm tide at NYH is 2.70 m (2.51-2.92 at 95% confidence), and Hurricane Sandy's storm tide of 3.38 m was a 260-year (170-420) storm tide. Deeper analyses of historical flood reports from estimated Category-3 hurricanes in 1788 and 1821 lead to new estimates and reduced uncertainties for their floods, and show that Sandy's storm tide was the largest at NYH back to at least 1700. The flood exceedance curves for ETCs and TCs have sharply different slopes due to their differing meteorology and frequency, warranting separate treatment in hazard assessments. This article is protected by copyright. All rights reserved.
      PubDate: 2016-10-25T08:05:34.956258-05:
      DOI: 10.1002/2016JC011679
  • Coherent motions and time scales that control heat and mass transfer at
           wind-swept water surfaces
    • Authors: D. E. Turney
      Abstract: Forecast of the heat and chemical budgets of lakes, rivers and oceans requires improved predictive understanding of air-water interfacial transfer coefficients. Here we present laboratory observations of the coherent motions that occupy the air-water interface at wind speeds (U10) 1.1 to 8.9 m/s. Spatio-temporal near-surface velocity data and interfacial renewal data are made available by a novel flow tracer method. The relative activity, velocity scales and time scales of the various coherent interfacial motions are measured, namely for Langmuir circulations, streamwise streaks, non-breaking wind waves, parasitic capillary waves, non-turbulent breaking wind waves, and turbulence-generating breaking wind waves. Breaking waves exhibit a sudden jump in streamwise interfacial velocity wherein the velocity jumps up to exceed the wave celerity and destroys nearby parasitic capillary waves. Four distinct hydrodynamic regimes are found to exist between U10 = 0 and 8.9 m/s, each with a unique population balance of the various coherent motions. The velocity scales, time scales and population balance of the different coherent motions are input to a first-principles gas transfer model to explain the waterside transfer coefficient (kw) as well as experimental patterns of temperature and gas concentration. The model mixes concepts from surface renewal and divergence theories, and requires surface divergence strength (β), the Lagrangian residence time inside the upwelling zone (tLu), and the total lifetime of new interface before it is downwelled (tLT). The model's output agrees with time-averaged measurements kw, patterns of temperature in infrared photographs, and spatial patterns of gas concentration and kw from direct numerical simulations. Several non-dimensional parameters, e.g. βtLu and τstLT where τs is the interfacial shear rate, determine the effectiveness of a particular type of coherent motion for affecting kw. This article is protected by copyright. All rights reserved.
      PubDate: 2016-10-19T10:26:45.800745-05:
      DOI: 10.1002/2016JC012139
  • Structure and dynamics of a subglacial discharge plume in a Greenlandic
    • Authors: Kenneth D. Mankoff; Fiammetta Straneo, Claudia Cenedese, Sarah B. Das, Clark G. Richards, Hanumant Singh
      Abstract: Discharge of surface-derived meltwater at the submerged base of Greenland's marine-terminating glaciers creates subglacial discharge plumes that rise along the glacier/ocean interface. These plumes impact submarine melting, calving and fjord circulation. Observations of plume properties and dynamics are challenging due to their proximity to the calving edge of glaciers. Therefore to date information on these plumes has been largely derived from models. Here we present temperature, salinity, and velocity data collected in a plume that surfaced at the edge of Saqqarliup Sermia, a mid-sized Greenlandic glacier. The plume is associated with a narrow core of rising waters approximately 20 m in diameter at the ice edge that spreads to a 200 m by 300 m plume pool as it reaches the surface, before descending to its equilibrium depth. Volume flux estimates indicate that the plume is primarily driven by subglacial discharge and that this has been diluted in a ratio of 1:10 by the time the plume reaches the surface. While highly uncertain, meltwater fluxes are likely two orders of magnitude smaller than the subglacial discharge flux. The overall plume characteristics agree with those predicted by theoretical plume models for a convection driven plume with limited influence from submarine melting. This article is protected by copyright. All rights reserved.
      PubDate: 2016-10-19T10:20:27.03883-05:0
      DOI: 10.1002/2016JC011764
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