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Geophysical Research Letters     Full-text available via subscription   (Followers: 82, SJR: 3.323, h-index: 185)
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Journal Cover Journal of Geophysical Research : Oceans
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  • Laboratory experiments on diffusive convection layer thickness and its
           oceanographic implications
    • Abstract: We studied the thickness of diffusive convective layers that form when a linearly stratified fluid is subjected to heating from below in the laboratory. The thickness of the bottom convecting layer is much larger than subsequent layers. These thicknesses are systematically identified and used to examine the available convecting layer thickness parameterizations, which are consisted of the measured heat flux F (or thermal buoyancy flux qT), initial stratification N, density ratio Rρ, thermal diffusivity κT, etc. Parameterization with an intrinsic length scale (qT3κTN8)1/4 is shown to be superior. Including the present laboratory convecting layer thicknesses and those observed in oceans and lakes, where layer thickness ranges from 0.01 to 1000 m, the parameterization is updated as H=C(Rρ−1)2(qT3κTN8)1/4, where C=38.3 for the bottom convective layer and 10.8 for the subsequent layers. Different prefactors are proposed to be attributed to different convective instabilities induced by different boundary conditions. This article is protected by copyright. All rights reserved.
      PubDate: 2016-09-22T09:55:55.381865-05:
      DOI: 10.1002/2016JC012172
  • Teleconnection between the North Indian Ocean high swell events and
           meteorological conditions over the Southern Indian Ocean
    • Authors: P G Remya; S Vishnu, B Praveen Kumar, TM Balakrishnan Nair, B Rohith
      Abstract: The link between North Indian Ocean (NIO) high swell events and the meteorological conditions over the Southern Indian Ocean (SIO) is explored in this article, using a combination of in‐situ measurements and model simulations for the year 2005. High waves, without any sign in the local winds, sometimes cause severe flooding events along the south‐west coast of India, locally known as the Kallakkadal events and cause major societal problems along the coasts. In‐situ observations report ten high swell events in NIO during 2005. Our study confirm that these events are caused by the swells propagating from south of 30°S. In all cases, 3‐5 days prior to the high swell events in NIO, we observed a severe low pressure system, called the Cut‐Off Low (COL) in the Southern Ocean. These COLs are quasi‐stationary in nature, providing strong (∼25 ms−1) and long duration (∼3 days) surface winds over a large fetch; essential conditions for the generation of long period swells. The intense equator ward winds associated with COLs in the SIO trigger the generation of high waves, which propagate to NIO as swells. Furthermore, these swells cause high wave activity and sometimes Kallakkadal events along the NIO coastal regions, depending on the local topography, angle of incidence and tidal conditions. Our study shows that such natural hazards along the NIO coasts can be forecasted at least 2 days in advance if the meteorological conditions of the SIO are properly monitored. This article is protected by copyright. All rights reserved.
      PubDate: 2016-09-21T19:10:44.497883-05:
      DOI: 10.1002/2016JC011723
  • Measurement and modeling of oil slick transport
    • Abstract: Transport characteristics of oil slicks are reported from a controlled release experiment conducted in the North Sea in June 2015, during which mineral oil emulsions of different volumetric oil fractions and a look‐alike biogenic oil were released and allowed to develop naturally. The experiment used the Uninhabited Aerial Vehicle Synthetic Aperture Radar (UAVSAR) to track slick location, size, and shape for ∼8 hours following release. Wind conditions during the exercise were at the high end of the range considered suitable for radar‐based slick detection, but the slicks were easily detectable in all images acquired by the low noise, L‐band imaging radar. The measurements are used to constrain the entrainment length and representative droplet radii for oil elements in simulations generated using the OpenOil advanced oil drift model. Simultaneously released drifters provide near‐surface current estimates for the single biogenic release and one emulsion release, and are used to test model sensitivity to upper ocean currents and mixing. Results of the modeling reveal a distinct difference between the transport of the biogenic oil and the mineral oil emulsion, in particular in the vertical direction, with faster and deeper entrainment of significantly smaller droplets of the biogenic oil. The difference in depth profiles for the two types of oils is substantial, with most of the biogenic oil residing below depths of 10 m, compared to the majority of the emulsion remaining above 10 m depth. This difference was key to fitting the observed evolution of the two different types of slicks. This article is protected by copyright. All rights reserved.
      PubDate: 2016-09-21T18:35:26.123817-05:
      DOI: 10.1002/2016JC012113
  • Tsunami generation and associated waves in the water column and seabed due
           to an asymmetric earthquake motion within an anisotropic substratum
    • Authors: Amirhossein Bagheri; Stewart Greenhalgh, Ali Khojasteh, Mohammad Rahimian, Reza Attarnejad
      Abstract: In this paper, closed‐form integral expressions are derived to describe how surface gravity waves (tsunamis) are generated when general asymmetric ground displacement (due to earthquake rupturing), involving both horizontal and vertical components of motion, occurs at arbitrary depth within the interior of an anisotropic sub‐sea solid beneath the ocean. In addition, we compute the resultant hydrodynamic pressure within the seawater and the elastic wavefield within the seabed at any position. The method of potential functions and an integral transform approach, accompanied by a special contour integration scheme, are adopted to handle the equations of motion and produce the numerical results. The formulation accounts for any number of possible acoustic‐gravity modes and is valid for both shallow and deep water situations as well as for any focal depth of the earthquake source. Phase and group velocity dispersion curves are developed for surface gravity (tsunami mode), acoustic‐gravity, Rayleigh, and Scholte waves. Several asymptotic cases which arise from the general analysis are discussed and compared to existing solutions. The role of effective parameters such as hypocenter location and frequency of excitation are examined and illustrated through several figures which show the propagation pattern in the vertical and horizontal directions. Attention is directed to the unexpected contribution from the horizontal ground motion. The results have important application in several fields such as tsunami hazard prediction, marine seismology and offshore and coastal engineering. In a companion paper we examine the effect of ocean stratification on the appearance and character of internal and surface gravity waves. This article is protected by copyright. All rights reserved.
      PubDate: 2016-09-21T18:35:21.897982-05:
      DOI: 10.1002/2016JC011944
  • Enhanced turbulence driven by mesoscale motions and flow‐topography
           interaction in the Denmark Strait Overflow plume
    • Authors: Janin Schaffer; Torsten Kanzow, Kerstin Jochumsen, Klas Lackschewitz, Sandra Tippenhauer, Victor M. Zhurbas, Detlef Quadfasel
      Abstract: The Denmark Strait Overflow (DSO) contributes roughly half to the total volume transport of the Nordic overflows. The overflow increases its volume by entraining ambient water as it descends into the subpolar North Atlantic, feeding into the deep branch of the Atlantic Meridional Overturning Circulation. In June 2012 a multi‐platform experiment was carried out in the DSO plume on the continental slope off Greenland (180 km downstream of the sill in Denmark Strait), to observe the variability associated with the entrainment of ambient waters into the DSO plume. In this study we report on two high‐dissipation events captured by an autonomous underwater vehicle (AUV) by horizontal profiling in the interfacial layer between the DSO plume and the ambient water. Strong dissipation of turbulent kinetic energy of O(10‐6) W kg‐1 was associated with enhanced small‐scale temperature variance at wavelengths between 0.05 and 500 m as deduced from a fast‐response thermistor. Isotherm displacement slope spectra reveal a wavenumber‐dependence characteristic of turbulence in the inertial‐convective subrange (k1/3) at wavelengths between 0.14 and 100 m. The first event captured by the AUV was transient, and occurred near the edge of a bottom‐intensified energetic eddy. Our observations imply that both horizontal advection of warm water and vertical mixing of it into the plume are eddy‐driven and go hand in hand in entraining ambient water into the DSO plume. The second event was found to be a stationary feature on the upstream side of a topographic elevation located in the plume pathway. Flow‐topography interaction is suggested to drive the intense mixing at this site. This article is protected by copyright. All rights reserved.
      PubDate: 2016-09-21T11:10:38.629141-05:
      DOI: 10.1002/2016JC011653
  • Wind‐driven export of Weddell Sea slope water
    • Authors: A.J.S. Meijers; M.P. Meredith, E.P. Abrahamsen, M.A. Morales Maqueda, D.C. Jones, A.C. Naveira Garabato
      Abstract: The export of waters from the Weddell Gyre to lower latitudes is an integral component of the southern subpolar contribution to the three‐dimensional oceanic circulation. Here, we use more than 20 years of repeat hydrographic data on the continental slope on the northern tip of the Antarctic Peninsula and five years of bottom lander data on the slope at 1000 m to show the intermittent presence of a relatively cold, fresh westward‐flowing current. This is often bottom intensified between 600‐2000 dbar with velocities of over 20 cms−1, transporting an average of 1.5±1.5 Sv. By comparison with hydrography on the continental slope within the Weddell Sea and modelled tracer release experiments we show that this slope current is an extension of the Antarctic Slope Current that has crossed the South Scotia Ridge west of Orkney Plateau. On monthly to interannual timescales the density of the slope current is negatively correlated (r >0.6 with a significance of over 95%) with eastward wind stress over the northern Weddell Sea, but lagging it by 6‐13 months. This relationship holds in both the high temporal resolution bottom lander time series and the 20+ year annual hydrographic occupations and agrees with Weddell Sea export variability observed further east. We compare several alternative hypotheses for this wind‐stress/export relationship and find that it is most consistent with wind‐driven acceleration of the gyre boundary current, possibly modulated by eddy dynamics, and represents a mechanism by which climatic perturbations can be rapidly transmitted as fluctuations in the supply of intermediate‐level waters to lower latitudes. This article is protected by copyright. All rights reserved.
      PubDate: 2016-09-21T11:00:30.892787-05:
      DOI: 10.1002/2016JC011757
  • Direct observations of the Antarctic Slope Current transport at 113°E
    • Abstract: The Antarctic Slope Current (ASC), defined here as the region of westward flow along the continental slope off Antarctica, forms the southern limb of the subpolar gyres. It regulates the exchange of water across the shelf break, and provides a path for interbasin westward transport. Despite its significance, the ASC remains largely unobserved around most of the Antarctic continent. Here we present direct velocity observations from a 17‐month current meter moored array deployed across the continental slope between the 1000 and the 4200 m isobaths, in the southeastern Indian Ocean near 113°E. The observed time‐mean flow consists of a surface intensified jet associated with the Antarctic Slope Front (ASF) and a broader bottom intensified westward flow that extends out to approximately the 4000 m isobath and is strongest along the upper slope. The time‐mean transport of the ASC is ‐29.2 Sv. Fluctuations in the transport are large, typically exceeding the mean by a factor of 2. They are mainly due to changes in the northward extent of the current over the lower slope. However, seasonal changes in the wind also drive variations in the transport of the ASF and the flow in the upper slope. Both mean and variability are largely barotropic, thus invisible to traditional geostrophic methods. This article is protected by copyright. All rights reserved.
      PubDate: 2016-09-21T11:00:26.473997-05:
      DOI: 10.1002/2015JC011594
  • Wind relaxation and a coastal buoyant plume north of Pt. Conception, CA:
           Observations, simulations, and scalings
    • Authors: Sutara Suanda; Nirnimesh Kumar, Arthur J. Miller, Emanuele DiLorenzo, Kevin Haas, Donghua Cai, Christopher A. Edwards, Libe Washburn, Melanie Fewings, Rachel Torres, Falk Feddersen
      Abstract: In upwelling regions, wind relaxations lead to poleward propagating warm water plumes that are important to coastal ecosystems. The coastal ocean response to wind relaxation around Pt. Conception, CA is simulated with a Regional Ocean Model (ROMS) forced by realistic surface and lateral boundary conditions including tidal processes. The model reproduces well the statistics of observed subtidal water column temperature and velocity at both outer‐ and inner‐shelf mooring locations throughout the study. A poleward‐propagating plume of Southern California Bight water that increases shelf water temperatures by ≈ 5°C is also reproduced. Modeled plume propagation speed, spatial scales, and flow structure are consistent with a theoretical scaling for coastal buoyant plumes with both surface‐trapped and slope‐controlled dynamics. Plume momentum balances are distinct between the offshore (> 30‐m depth) region where the plume is surface‐trapped, and onshore of the 30‐m isobath (within 5 km from shore) where the plume water mass extends to the bottom and is slope‐controlled. In the onshore region, bottom stress is important in the alongshore momentum equation and generates vertical vorticity that is an order of magnitude larger than the vorticity in the plume core. Numerical experiments without tidal forcing show that modeled surface temperatures are biased 0.5°C high, potentially affecting plume propagation distance and persistence. This article is protected by copyright. All rights reserved.
      PubDate: 2016-09-20T03:45:32.764738-05:
      DOI: 10.1002/2016JC011919
  • Variability, trends, and predictability of seasonal sea ice retreat and
           advance in the Chukchi Sea
    • Authors: Mark C. Serreze; Alex D. Crawford, Julienne Stroeve, Andrew P. Barrett, Rebecca A. Woodgate
      Abstract: As assessed over the period 1979‐2014, the date that sea ice retreats to the shelf break (150m contour) of the Chukchi Sea has a linear trend of ‐0.7 days per year. The date of seasonal ice advance back to the shelf break has a steeper trend of about +1.5 days per year, together yielding an increase in the open water period of 80 days. Based on de‐trended time series, we ask how interannual variability in advance and retreat dates relate to various forcing parameters including radiation fluxes, temperature and wind (from numerical reanalyses), and the oceanic heat inflow through the Bering Strait (from in situ moorings). Of all variables considered, the retreat date is most strongly correlated (r∼0.8) with the April through June Bering Strait heat inflow. After testing a suite of statistical linear models using several potential predictors, the best model for predicting the date of retreat includes only the April through June Bering Strait heat inflow, which explains 68% of retreat date variance. The best model predicting the ice advance date includes the July through September inflow and the date of retreat, explaining 67% of advance date variance. We address these relationships by discussing heat balances within the Chukchi Sea, and the hypothesis of oceanic heat transport triggering ocean heat uptake and ice‐albedo feedback. Developing an operational prediction scheme for seasonal retreat and advance would require timely acquisition of Bering Strait heat inflow data. Predictability will likely always be limited by the chaotic nature of atmospheric circulation patterns. This article is protected by copyright. All rights reserved.
      PubDate: 2016-09-19T18:35:35.422629-05:
      DOI: 10.1002/2016JC011977
  • Remote sources for year‐to‐year changes in the seasonality of the
           Florida Current transport
    • Authors: Ricardo Domingues; Molly Baringer, Gustavo Goni
      Abstract: The seasonal variability of the Florida Current (FC) transport is often characterized by the presence of an average annual cycle (8% of the variance) of ∼3 Sv range peaking in boreal summer. However, the seasonality displayed by the FC transport in any individual year may have very distinct characteristics. In this study, the analysis focuses on seasonal changes (73—525 day frequency band) in the FC transport that are associated with a variable annual phase, which is defined as the transient seasonal component (FCt, 27% of the variance). It is shown that the FCt is largely modulated by westward propagating sea height anomaly (SHA) signals that are formed in the eastern North Atlantic 4 to 7 years earlier than observed at 27°N in the Florida Straits. These westward propagating SHA signals behave approximately like first baroclinic Rossby waves that can modulate changes in the FC seasonal variability upon arrival at the western boundary. The main finding from this study is that changes in coastal sea‐level between 25°N—42°N linked with westward propagating signals account for at least 50% of the FCt. The integrated changes in the coastal sea‐level between 25°N—42°N, in turn, drive adjustments in the geostrophic transport of the FC at 27°N. Results reported here provide an explanation for previously reported year‐to‐year changes in the FC seasonality, and suggest that large sea‐level variations along the coast of Florida may be partially predictable, given that these Rossby‐wave‐like signals propagate approximately at fixed rates in the open ocean along 27°N. This article is protected by copyright. All rights reserved.
      PubDate: 2016-09-19T18:35:31.731373-05:
      DOI: 10.1002/2016JC012070
  • Identification and classification of very‐low frequency waves on a
           coral reef flat
    • Authors: Matthijs Gawehn; Ap van Dongeren, Arnold van Rooijen, Curt Storlazzi, Olivia M. Cheriton, Ad Reniers
      Abstract: Very‐low frequency (VLF, 0.001‐0.005 Hz) waves are important drivers of flooding of low‐lying coral reef‐islands. In particular, VLF wave resonance is known to drive large wave runup and subsequent overwash. Using a five‐month dataset of water levels and waves collected along a cross‐reef transect on Roi‐Namur Island in the Republic of the Marshall Islands, the observed VLF motions were categorized into four different classes: (1) resonant, (2) (non‐resonant) standing, (3) progressive‐growing and (4) progressive‐dissipative waves. Each VLF class is set by the reef flat water depth and, in the case of resonance, the incident‐band offshore wave period. Using an improved method to identify VLF wave resonance, we find that VLF wave resonance caused prolonged (∼0.5 – 6.0 hr), large‐amplitude water surface oscillations at the inner reef flat ranging in wave height from 0.14 to 0.83 m. It was induced by relatively long‐period, grouped, incident‐band waves, and occurred under both storm and non‐storm conditions. Moreover, observed resonant VLF waves had non‐linear, bore‐like wave shapes, which likely have a larger impact on the shoreline than regular, sinusoidal waveforms. As an alternative technique to the commonly used Fast Fourier Transformation, we propose the Hilbert‐Huang Transformation that is more computationally expensive but can capture the wave shape more accurately. This research demonstrates that understanding VLF waves on reef flats is important for evaluating coastal flooding hazards. This article is protected by copyright. All rights reserved.
      PubDate: 2016-09-19T18:30:26.986267-05:
      DOI: 10.1002/2016JC011834
  • Glider observations of enhanced deep water upwelling at a shelf break
           canyon: A mechanism for cross‐slope carbon and nutrient exchange
    • Authors: M. Porter; M. E. Inall, J. Hopkins, M. R. Palmer, A. C. Dale, D. Aleynik, J. A. Barth, C. Mahaffey, D. A. Smeed
      Abstract: Using underwater gliders we have identified canyon driven upwelling across the Celtic Sea shelf‐break, in the vicinity of Whittard Canyon. The presence of this upwelling appears to be tied to the direction and strength of the local slope current, which is in itself highly variable. During typical summer time equatorward flow, an unbalanced pressure gradient force and the resulting disruption of geostrophic flow can lead to upwelling along the main axis of two small shelf break canyons. As the slope current reverts to poleward flow, the upwelling stops and the remnants of the upwelled features are mixed into the local shelf water or advected away from the region. The upwelled features are identified by the presence of sub‐pycnocline high salinity water on the shelf, and are upwelled from a depth of 300 m on the slope, thus providing a mechanism for the transport of nutrients across the shelf break onto the shelf. This article is protected by copyright. All rights reserved.
      PubDate: 2016-09-19T18:30:21.487541-05:
      DOI: 10.1002/2016JC012087
  • Surface shear stress dependence of gas transfer velocity parameterizations
           using DNS
    • Authors: S. T. Fredriksson; L. Arneborg, H. Nilsson, R. A. Handler
      Abstract: Air‐water gas‐exchange is studied in direct numerical simulations (DNS) of free‐surface flows driven by natural convection and weak winds. The wind is modeled as a constant surface‐shear‐stress and the gas‐transfer is modeled via a passive scalar. The simulations are characterized via a Richardson number Ri=Bν/u*4 where B, v, and u* are the buoyancy flux, kinematic viscosity, and friction velocity respectively. The simulations comprise 0 < Ri < ∞ ranging from convection‐dominated to shear‐dominated cases. The results are used to: (i) evaluate parameterizations of the air‐water gas‐exchange, (ii) determine, for a given buoyancy flux, the wind speed at which gas transfer becomes primarily shear driven, and (iii) find an expression for the gas‐transfer velocity for flows driven by both convection and shear. The evaluated gas transfer‐velocity parametrizations are based on either the rate of turbulent kinetic energy dissipation, the surface flow‐divergence, the surface heat‐flux, or the wind‐speed. The parametrizations using dissipation or divergence show an unfavorable Ri dependence for flows with combined forcing whereas the parametrization using heat‐flux only shows a limited Ri dependence. The two parametrizations using wind speed give reasonable estimates for the transfer‐velocity, depending however on the surface heat‐flux. The transition from convection‐ to shear‐dominated gas‐transfer‐velocity is shown to be at Ri ≈ 0.004. Furthermore, the gas‐transfer is shown to be well represented by two different approaches: (i) additive forcing expressed as kg,sum = AShear u* (Ri/Ric+1)1/4 Sc‐n where Ric = (AShear/ABuoy)4, and (ii) either buoyancy or shear dominated expressed as, kg = ABuoy (Bv)1/4Sc‐n, Ri>Ric or kg = Ashearu*Sc‐n, Ri
      PubDate: 2016-09-15T18:06:09.423857-05:
      DOI: 10.1002/2016JC011852
  • Trapping of the near‐inertial wave wakes of two consecutive
           hurricanes in the Loop Current
    • Abstract: The near‐inertial trapping of the wave wakes of two consecutive hurricanes, Gustav and Ike (2008), at the base of the Loop Current is described using observations of a triangular mooring array. Hurricane Gustav translates over the Loop Current twice as fast and leads to a sharper, but equally strong, wind intensity peak than Hurricane Ike. Following the passage of Gustav, near‐inertial oscillations in the main thermocline have large horizontal (∼340±45 km) and vertical (∼850 m) scales. The wave kinetic energy propagates down with a vertical group velocity of several tens of meters per day and leads to a downward energy flux of [1.82‐7.10]×10−3 W m−2.Observations suggest that the dispersion of the wave wakes produced by the hurricanes is driven by Doppler shifting in the upper ∼600 m and the vertical‐varying vorticity inside the Loop Current. Near‐inertial kinetic energy is enhanced and near‐inertial oscillations are circularly polarized in a critical layer at the base of the Loop Current where the group velocity vector is nearly horizontal. The trapping and amplification of the near‐inertial wave wakes of two consecutive hurricanes is addressed using a primitive equation numerical model. Numerical results suggest an increase of the near‐inertial kinetic energy and vertical shear by a factor of ∼2 near the critical layer due to wave‐mean flow and wave‐wave interactions. A K‐Profile parameterization of these interactions results in banded regions of elevated turbulent kinetic energy dissipation rates of ε ϵ [0.5‐6.6]×10−6 W m−3 for critical Richardson number Ric = 1, depending on the stratification of the water column. This article is protected by copyright. All rights reserved.
      PubDate: 2016-09-15T18:00:56.412806-05:
      DOI: 10.1002/2015JC011592
  • Cross‐shelf water exchange in the East China Sea as estimated by
           satellite altimetry and in situ hydrographic measurement
    • Authors: Ruibin Ding; Daji Huang, Jiliang Xuan, Bernhard Mayer, Feng Zhou, Thomas Pohlmann
      Abstract: Combining satellite altimetry and in situ hydrographic measurement, we estimated the cross‐shelf transport (CST) and its spatial and temporal variations across 200 m isobath in the East China Sea (ECS) from 1993 to 2014. The vertically‐integrated CST can be dynamically divided into three parts: surface Ekman transport, geostrophic transport, and bottom Ekman transport. The results show that the 22‐year‐mean, sectionally‐integrated CST to be 1.7±2.0 Sv (positive in the on‐shelf direction), comprised of bottom and surface Ekman transports of 2.7±1.0 Sv and 0.6±0.6 Sv, respectively, that are partially offset by a geostrophic transport of ‐1.5±1.7 Sv. The sectionally‐integrated CST shows significantly high power at roughly annual period from 1999‐2001, with lower power at intra‐annual period. The time‐averaged, vertically‐integrated CST to the northeast of Taiwan is the main source of sectionally‐integrated CST. The vertically‐integrated CST also shows significant variations in the 6–15 month period band to the northeast of Taiwan as well. The temporal variations of the sectionally‐integrated and vertically‐integrated CST are both controlled primarily by geostrophic transport and modulated by bottom Ekman transport. In the upper 50 m, the geostrophic current to the northeast of Taiwan exhibits large mean and significant variability. The empirical orthogonal function analysis of vertical structure of geostrophic current shows two significant modes with strong annual signal. The first mode is associated with the migration of Kuroshio axis near Taiwan, while the second mode is associated with the variation of the meander of the Kuroshio to the northeast of Taiwan. This article is protected by copyright. All rights reserved.
      PubDate: 2016-09-15T08:35:25.891586-05:
      DOI: 10.1002/2016JC011972
  • Impacts of mesoscale activity on the water masses and circulation in the
           Coral Sea
    • Authors: L. Rousselet; A.M. Doglioli, C. Maes, B. Blanke, A. Petrenko
      Abstract: The climatological vision of the circulation within the Coral Sea is today well established with the westward circulation of two main jets, the North Caledonian Jet (NCJ) and the North Vanuatu Jet (NVJ) as a consequence of the separation of the South Equatorial Current (SEC) on the islands of New Caledonia, Vanuatu and Fiji. Each jet has its own dynamic and transports different water masses across the Coral Sea. The influence of mesoscale activity on mean flow and on water mass exchanges is not yet fully explored in this region of intense activity. Our study relies on the analysis of in situ, satellite and numerical data. Indeed we first use in situ data from the Bifurcation cruise and from an Argo float, jointly with satellite‐derived velocities, to study the eddy influence on the Coral Sea dynamics. We identify an anticyclonic eddy as participating in the transport of NVJ‐like water masses into the theoretical pathway of NCJ waters. This transfer from the NVJ to the NCJ is confirmed over the long‐term by a Lagrangian analysis. In particular, this numerical analysis shows that anticyclonic eddies can contribute up to 70% to 90% of the overall eddy transfer between those seemingly independent jets. Finally, transports calculated using S‐ADCP measurements (0‐500 m) show an eddy‐induced sensitivity that can reach up to 15 Sv, i.e, the order of the transport of the jets. This article is protected by copyright. All rights reserved.
      PubDate: 2016-09-15T08:30:53.907375-05:
      DOI: 10.1002/2016JC011861
  • Characterization and Assessment of the meteotsunami hazard in northern
           Lake Michigan
    • Abstract: The meteotsunami hazard is assessed in northern Lake Michigan from both short‐term and long‐term records of water level, wind speed, and air pressure. Cross‐wavelet analysis reveals that meteotsunamis can be caused by atmospheric disturbances that are pressure dominated, wind dominated, or both pressure and wind forced. In total, air pressure and wind stress are found to contribute similarly to meteotsunami initiation in northern Lake Michigan. The pressure‐driven meteotsunamis tend to be associated with convective storms, whereas meteotsunamis that are mainly wind‐driven are associated more with cyclonic‐type storms. The atmospheric disturbances responsible for largest meteotsunamis in northern Lake Michigan are found to have a propagation speed close to 32 m/s and from the south to north direction. A heuristic approach is developed to estimate the maximum meteotsunami height from the atmospheric disturbance strength and velocity. Overall, the heuristic approach is shown to be an effective methodology to assess the meteotsunami hazard over a wide range of potential atmospheric disturbance conditions. This article is protected by copyright. All rights reserved.
      PubDate: 2016-09-10T10:30:25.686312-05:
      DOI: 10.1002/2016JC011979
  • Improving the simulation of landfast ice by combining tensile strength and
           a parameterization for grounded ridges
    • Abstract: In some coastal regions of the Arctic Ocean, grounded ice ridges contribute to stabilizing and maintaining a landfast ice cover. Recently, a grounding scheme representing this effect on sea ice dynamics was introduced and tested in a viscous‐plastic sea ice model. This grounding scheme, based on a basal stress parameterization, improves the simulation of landfast ice in many regions such as in the East Siberian Sea, the Laptev Sea and along the coast of Alaska. Nevertheless, in some regions such as the Kara Sea, the area of landfast ice is systematically underestimated. This indicates that another mechanism such as ice arching is at play for maintaining the ice cover fast. To address this problem, the combination of the basal stress parameterization and tensile strength is investigated using a 0.25° pan‐Arctic CICE‐NEMO configuration. Both uniaxial and isotropic tensile strengths notably improve the simulation of landfast ice in the Kara Sea but also in the Laptev Sea. However, the simulated landfast ice season for the Kara Sea is too short compared to observations. This is especially obvious for the onset of the landfast ice season which systematically occurs later in the model and with a slower build up. This suggests that improvements to the sea ice thermodynamics could reduce these discrepancies with the data. This article is protected by copyright. All rights reserved.
      PubDate: 2016-09-10T10:30:22.49546-05:0
      DOI: 10.1002/2016JC012006
  • Remote Forcing of Subsurface Currents and Temperatures near the Northern
           Limit of the California Current System
    • Authors: Zelalem Engida; Adam Monahan, Debby Ianson, Richard E. Thomson
      Abstract: Local and remote wind forcing of upwelling along continental shelves of coastal upwelling regions play key roles in driving biogeochemical fluxes, including vertical net fluxes of carbon and nutrients. These fluxes are responsible for high primary productivity, which in turn supports a lucrative fishery in these regions. However, the relative contributions of local versus remote wind forcing is not well quantified or understood. We present results of coherence analyses between currents at a single mooring site (48.5° N, 126° W) in the northern portion of the California Current System (CalCS) from 1989–2008 and coincident time series of North America Regional Reanalysis (NARR) 10 m wind stress within the CalCS (36–54° N, 120–132° W). The two‐decade long current records from the three shallowest depths (35, 100 and 175 m) show a remote response to winds from as far south as 36° N. In contrast, only temperatures at the deepest depth (400 m) show strong coherences with remote winds. Weaker local wind influence is observed in both the currents and 400 m temperatures but is mostly due to the large spatial coherence within the wind field itself. Lack of coherence between distal winds and the 400 m currents suggests that the temperature variations at that depth are driven by vertical motion resulting from poleward travelling coastal trapped waves (CTWs). Understanding the effects of remote forcing in coastal upwelling regions is necessary for determining the occurrence and timing of extreme conditions in coastal oceans, and their subsequent impact on marine ecosystems. This article is protected by copyright. All rights reserved.
      PubDate: 2016-09-10T10:25:30.534334-05:
      DOI: 10.1002/2016JC011880
  • Physical drivers of chlorophyll variability in the open South China Sea
    • Abstract: The variability of chlorophyll a concentration (Chl a) in the open South China Sea (SCS) was examined using observations from two Bio‐Argo floats. During the period of September 2014 to August 2015, there was a permanent subsurface Chl a maximum (SCM) in the depth range of 48 to 96 m in the central basin of the SCS. In the northern basin, the SCM disappeared in winter, replaced by enhanced surface layer phytoplankton with high Chl a. The values of the SCM were influenced by the vertical displacement of isotherms. Strong wind forcing and surface cooling were the main physical drivers of high surface Chl a in winter. In the north, stronger wind than in the center, lower sea surface temperature (SST) than in the center, and Kuroshio water intrusion were more favorable for the upward transport of nutrient‐rich deep water. A large amount of nitrate could be advected from the Taiwan Strait and shallow continental shelf to the northern basin in winter. A combination of strong wind mixing, surface cooling, Kuroshio water intrusion, and horizontal advection caused the winter surface phytoplankton bloom in the north. This article is protected by copyright. All rights reserved.
      PubDate: 2016-09-10T10:25:22.807672-05:
      DOI: 10.1002/2016JC011983
  • Observed subseasonal variability of heat flux and the SST response of the
           tropical Indian Ocean
    • Authors: Sindu Raj Parampil; G. N. Bharathraj, Matthew Harrison, Debasis Sengupta
      Abstract: We develop an experimental daily surface heat flux dataset based on satellite observations to study subseasonal variability (periods shorter than 90 days) in the tropical Indian Ocean. We use incoming shortwave and longwave radiation from the International Satellite Cloud Climatology Project, and sea surface temperature (SST) from microwave sensors, to estimate net radiative flux. Latent and sensible heat fluxes are estimated from scatterometer winds and near‐surface air temperature and specific humidity from Atmospheric Infrared Sounder (AIRS) observations calibrated to buoy data. Seasonal biases in net heat flux are generally within 10 Wm−2 of estimates from moorings, and the phases and amplitudes of subseasonal variability of heat fluxes are realistic. We find that the contribution of subseasonal changes in air‐sea humidity gradients to latent heat flux equals or exceeds the contribution of subseasonal changes in wind speed in all seasons. SST responds coherently to subseasonal oscillations of net heat flux associated with active and suppressed phases of atmospheric convection in the summer hemisphere. Thus subseasonal SST changes are mainly forced by heat flux in the northeast Indian Ocean in northern summer, and in the 15oS‐5oN latitude belt in southern summer. In the winter hemisphere, subseasonal SST changes are not a one‐dimensional response to heat flux, implying that they are mainly due to oceanic advection, entrainment or vertical mixing. The coherent evolution of subseasonal SST variability and surface heat flux suggests active coupling between SST and large‐scale, organized tropical convection in the summer season. This article is protected by copyright. All rights reserved.
      PubDate: 2016-09-09T10:15:53.136856-05:
      DOI: 10.1002/2016JC011948
  • Temporal variability in the Antarctic Polar Front (2002‐2014)
    • Authors: Natalie M. Freeman; Nicole S. Lovenduski, Peter R. Gent
      Abstract: We investigate intra‐annual to interannual variability in the Antarctic Polar Front (PF) using weekly PF realizations spanning 2002 to 2014 (found at While several PF studies have used gradient maxima in sea surface temperature (SST) or height to define its location, results from this study are based on a PF defined using SST measurements that avoid cloud contamination and the influence of steric sea level change. With a few regional exceptions, we find that the latitudinal position of the PF does not vary seasonally, yet its temperature exhibits a clear seasonal cycle. Consistent with previous studies, the position and intensity of the PF is largely influenced by bathymetry; generally, over steep topography we find that the front intensifies and interannual variability in its position is low. We also investigate drivers of PF variability in the context of large‐scale climate variability on various spatial and temporal scales, but find that the major modes of Southern Hemisphere climate variability explain only a tiny fraction of the interannual PF variance. Over the study time period, the PF intensifies at nearly all longitudes while exhibiting no discernible meridional displacement in its zonal mean path. This article is protected by copyright. All rights reserved.
      PubDate: 2016-09-08T03:55:33.75221-05:0
      DOI: 10.1002/2016JC012145
  • Observations of the vertical and temporal evolution of a Natal Pulse along
           the Eastern Agulhas Bank
    • Authors: Xavier Pivan; Marjolaine Krug, Steven Herbette
      Abstract: This study re‐investigates the work of Lutjeharms et al. [2001,2003] who documented the properties of a Natal Pulse using isopycnal Lagrangian floats. We combined Lagrangian analyses and Eulerian maps derived from objective analysis to better describe the evolution of a Natal Pulse along three density surfaces referred to as the surface (satellite‐observed), shallow (isopycnal‐1026.8 kg.m−3) and deep (isopycnal‐1027.2 kg.m−3) layer. Our observations show that this Natal Pulse extended to a depth of 1000m and was associated with cyclonic relative vorticity values of about 6.5 to 8.5x10−5.s−1 in the surface and shallow layer and 4x10−5.s−1 in the deep layer. This Natal Pulse contributed to cross‐shelf exchange through the offshore advection of Eastern Agulhas Bank water near the surface, onshore advection of South Indian Central Water and/or Indian Equatorial Water in the shallow layer and Antarctic Intermediate Water in the deep layer. Sea Surface Temperature maps showed that the downstream progression of the Natal Pulse along the 3000m isobath was related to a readjustment of its rotation axis. This readjustment advected Eastern Agulhas Bank water into the Natal Pulse eddy and triggered a SST cooling of about 3°C in the cyclonic area. The importance of a warm recirculating Agulhas plume originating from the Natal Pulse was highlighted. This warm water plume extended to a depth of 700m and was associated with onshore velocities exceeding those experienced within the Natal Pulse eddy by a factor of two. Our observations indicate that the June/July 1998 Natal Pulse and its associated plumes enhanced cross‐shelf exchanges. This article is protected by copyright. All rights reserved.
      PubDate: 2016-09-08T03:55:32.652903-05:
      DOI: 10.1002/2015JC011582
  • Bayesian hindcast of acoustic transmission loss in the western Pacific
    • Authors: Margaret Palmsten; J. Paquin Fabre
      Abstract: A Bayesian network is developed to demonstrate the feasibility of using environmental acoustic feature vectors (EAFVs) to predict underwater acoustic transmission loss (TL) versus range at two locations for a single acoustic source depth and frequency. Features for the networks are chosen based on a sensitivity analysis. The final network design resulted in a well‐trained network, with high skill, little gain error, and low bias. The capability presented here shows promise for expansion to a more generalized approach, which could be applied at varying locations, depths and frequencies to estimate acoustic performance over a highly variable oceanographic area in real‐time or near‐real‐time. This article is protected by copyright. All rights reserved.
      PubDate: 2016-09-08T03:55:30.22015-05:0
      DOI: 10.1002/2016JC011982
  • A tropical cyclone genesis potential index over the western North Pacific
           by using oceanic parameters
    • Authors: Min Zhang; Lei Zhou, Dake Chen, Chunzai Wang
      Abstract: This study attempts to create a tropical cyclone (TC) genesis potential index (GPI) by considering oceanic parameters and necessary atmospheric parameters. Based on the general understanding of the oceanic impacts on the TC genesis, many candidate factors are evaluated and discriminated, resulting in a new GPI index, which is referred to as GPIocean. GPIocean includes the parameters of (1) absolute vorticity at 1000 hPa, (2) net sea surface longwave radiation, (3) mean ocean temperature in the upper mixed layer, and (4) depth of the 26°C isotherm. GPIocean is comparable to existing GPIs in representing the TC genesis over the western North Pacific on seasonal and interannual variations. The same procedure can be applied to create a similar GPI for the other ocean basins. In the context of climate change, this new index is expected to be useful for evaluating the oceanic influences on TC genesis by using ocean reanalysis products and climate model outputs. This article is protected by copyright. All rights reserved.
      PubDate: 2016-09-07T04:23:36.076128-05:
      DOI: 10.1002/2016JC011851
  • Surface water subduction during a downwelling event in a
           semi‐enclosed bay
    • Authors: Barton E.D; Torres R, Figueiras F.G, Gilcoto M, Largier J.
      Abstract: The Ría de Vigo is a bay strongly influenced by upwelling‐downwelling cycles along the adjacent coast of NW Iberia. Moored and ship‐board observations during September 2006 showed that subduction, initially associated with an estuarine circulation, strengthened when a strong downwelling circulation, resulting from northward wind over the coastal ocean, was generated in the outer ría causing ambient waters to be advected outward in the lower layer. Incoming surface waters confined the estuarine circulation to the shallow interior and displaced isopleths downward through the water column at ∼10 m d−1. As the estuarine circulation retreated inward, strong flow convergence developed between middle and inner ria in the layer above 15 m, while divergence developed beneath. The convergence increased through the period of downwelling‐favorable wind at a rate consistent with the observed isopleth displacement velocities. The coefficient of turbulent diffusion Kt, from a microstructure profiler, indicated that mixing was strong in the estuarine circulation and subsequently in the downwelling zone, where localized instabilities and temperature‐salinity inversions were observed. During the downwelling, concentrations of phytoplankton, including potentially harmful species, increased, especially in the middle and inner ria, as a result of inward advection, subduction and the ability of the dinoflagellates to maintain their position in the water column by swimming. In the course of the 5 day event, the water mass of all but the innermost ría was flushed completely and replaced by waters originating in the coastally‐trapped poleward flow along the Atlantic coastline. This article is protected by copyright. All rights reserved.
      PubDate: 2016-09-06T10:20:30.21031-05:0
      DOI: 10.1002/2016JC011950
  • Contribution of Asian dust and volcanic material to the western Philippine
           Sea over the last 220 kyr as inferred from grain size and Sr‐Nd isotopes
    • Authors: Fuqing Jiang; Ye Zhou, Qingyun Nan, Yu Zhou, Xufeng Zheng, Tiegang Li, Anchun Li, Hongli Wang
      Abstract: Asian dust and volcanogenic materials are two major components in the northwestern Pacific. Quantitatively distinguishing them and estimating their mass accumulation rates (MARs) are very important for understanding regional and global climate change. Here, we present the grain‐size composition of detrital sediments and the radiogenic strontium (Sr) and neodymium (Nd) isotopic compositions of different grain‐size fractions of detrital sediments that were recovered from the western Philippine Sea. These new records show that the different grain‐size distributions can be associated with 1) Asian dust from the western and central Chinese deserts and Chinese loess and 2) volcanogenic materials that were derived from the Luzon Islands. The MARs of this Asian dust and volcanic materials are obtained by using Weibull‐function fitting. The MARs of Asian dust and volcanic materials are coupled with the glacial‐interglacial cycle; these values are found to have been higher and more variable during the glacial period than during the interglacial period. We argue that the strengthening aridity of the Asian continent, which is connected to solar insolation and ice volume variations from orbital eccentricity, constitutes an important mechanism that drives the high MARs of glacial dust in the western Philippine Sea. The internal positive feedback of dust may be another important mechanism. The significant increase in volcanic material during the glacial period was caused by sea level changes, which were driven by the ice volume and solar insolation at high latitudes, and by strengthened precipitation from the El Niño/Southern Oscillation (ENSO), which is driven by orbital eccentricity and precession cycles on the Luzon Islands. This article is protected by copyright. All rights reserved.
      PubDate: 2016-09-06T10:20:24.872305-05:
      DOI: 10.1002/2016JC012000
  • Estimation of Ocean Surface Currents from Maximum Cross Correlation
           applied to GOCI geostationary satellite remote sensing data over the
           Tsushima (Korea) Straits
    • Authors: M. A. Warren; G. D. Quartly, J. D. Shutler, P. I. Miller, Y. Yoshikawa
      Abstract: Attempts to automatically estimate surface current velocities from satellite‐derived thermal or visible imagery face the limitations of data occlusion due to cloud cover, the complex evolution of features and the degradation of their surface signature. The Geostationary Ocean Color Imager (GOCI) provides a chance to reappraise such techniques due to its multi‐year record of hourly high‐resolution visible spectrum data. Here we present the results of applying a Maximum Cross Correlation (MCC) technique to GOCI data. Using a combination of simulated and real data we derive suitable processing parameters and examine the robustness of different satellite products, those being water‐leaving radiance and chlorophyll concentration. These estimates of surface currents are evaluated using High Frequency (HF) radar systems located in the Tsushima (Korea) Strait. We show the performance of the MCC approach varies depending on the amount of missing data and the presence of strong optical contrasts. Using simulated data it was found that patchy cloud cover occupying 25% of the image pair reduces the number of vectors by 20% compared to using perfect images. Root mean square errors between the MCC and HF radar velocities are of the order of 20 cm s−1. Performance varies depending on the wavelength of the data with the blue‐green products out‐performing the red and near infra‐red products. Application of MCC to GOCI chlorophyll data results in similar performance to radiances in the blue‐green bands. The technique has been demonstrated using specific examples of an eddy feature and tidal induced features in the region. This article is protected by copyright. All rights reserved.
      PubDate: 2016-09-06T04:15:26.973047-05:
      DOI: 10.1002/2016JC011814
  • Assessing surface heat fluxes in atmospheric reanalyses with a decade of
           data from the NOAA Kuroshio Extension Observatory
    • Authors: Dongxiao Zhang; Meghan F. Cronin, Caihong Wen, Yan Xue, Arun Kumar, Dai McClurg
      Abstract: Previous studies have found large biases and uncertainties in the air‐sea fluxes from Numerical Weather Prediction model reanalyses, which must be identified and reduced in order to make progress on weather and climate predictions. Here, air‐sea heat fluxes from NOAA Kuroshio Extension Observatory (KEO) measurements are used to assess two new reanalyses, NCEP's Climate Forecast System Reanalysis (CFSR) and ECMWF Reanalysis‐Interim (ERA‐I), suggesting that these two new generation reanalyses have significantly improved. In both reanalyses, all four flux components (sensible and latent heat flux and net longwave and shortwave radiation) are highly correlated with observation, with the correlation of total net surface heat fluxes above 0.96. Although errors of the net surface heat flux have significantly reduced from previous reanalyses, the Root Mean Square Errors (RMSEs) and biases remain high especially for CFSR: the RMSEs of CFSR and ERA‐I are reduced by 25‐30% to 64 and 61 W/m2 respectively, while biases are reduced by 40‐60% to 28 and 20 W/m2. But CFSR overestimates the winter heat release by 90 W/m2. The main cause of biases is the latent heat flux, while RMS errors are primarily due to latent heat flux and shortwave radiation errors. Both reanalyses overestimate the wind speed associated with winter storms and underestimate specific humidity in summer. The ERA‐I latent heat flux, and its total net surface heat flux, are however closer to observation. It is the bulk algorithm in CFSR that is found to be mainly responsible for overestimates of winter heat release in CFSR. This article is protected by copyright. All rights reserved.
      PubDate: 2016-09-02T14:20:23.681054-05:
      DOI: 10.1002/2016JC011905
  • Seasonal predictability of sea surface temperature anomalies over the
           Kuroshio‐Oyashio Extension: Low in summer and high in winter
    • Authors: Yujie Wu; Wansuo Duan, Xinyao Rong
      Abstract: The seasonal predictability of sea surface temperature anomalies (SSTA) in the Kuroshio–Oyashio Extension (KOE) is explored by performing perfect model predictability experiments from the viewpoint of initial error growth in a global coupled model. It is found that prediction errors of KOE–SSTA always increase in the boreal summer and decrease in the boreal winter. This leads to smaller (larger) prediction errors and higher (lower) prediction skills in boreal winter (summer). This seasonal characteristic of the KOE–SSTA error growth implies a season‐dependent predictability that is lower in summer and higher in winter. The mechanism responsible for error growth associated with seasonal predictability is also explored. The error increase in summer and error decrease in winter in the KOE–SSTA are both largely attributed to the seasonal evolution of latent heat flux error and mean temperature advection by vertical current error in the KOE region, both of which are forced by the prediction error of one‐month leading zonal wind stress per unit mass for the mixed layer over the KOE region. The shallowest (deepest) mixed layer in summer (winter) amplifies (reduces) the forcing of zonal wind stress errors on the error growth of KOE–SSTA, thereby causing the seasonal evolution of prediction errors of KOE–SSTA and ultimately resulting in the season‐dependent predictability of the KOE–SSTA, i.e., low in summer and high in winter. This article is protected by copyright. All rights reserved.
      PubDate: 2016-09-02T14:05:36.503-05:00
      DOI: 10.1002/2016JC011887
  • A modulating effect of tropical instability wave (TIW)‐induced surface
           wind feedback in a hybrid coupled model of the tropical Pacific
    • Abstract: Tropical Instability Waves (TIWs) and the El Niño‐Southern Oscillation (ENSO) are two air‐sea coupling phenomena that are prominent in the tropical Pacific, occurring at vastly different space‐time scales. It has been challenging to adequately represent both of these processes within a large‐scale coupled climate model, which has led to a poor understanding of the interactions between TIW‐induced feedback and ENSO. In this study, a novel modeling system was developed that allows representation of TIW‐scale air‐sea coupling and its interaction with ENSO. Satellite data were first used to derive an empirical model for TIW‐induced sea surface wind stress perturbations (τTIW). The model was then embedded in a basin‐wide hybrid‐coupled model (HCM) of the tropical Pacific. Because τTIW were internally determined from TIW‐scale sea surface temperatures (SSTTIW) simulated in the ocean model, the wind‐SST coupling at TIW scales was interactively represented within the large‐scale coupled model. Because the τTIW–SSTTIW coupling part of the model can be turned on or off in the HCM simulations, the related TIW wind feedback effects can be isolated and examined in a straightforward way. Then, the TIW‐scale wind feedback effects on the large‐scale mean ocean state and interannual variability in the tropical Pacific were investigated based on this embedded system. The interactively represented TIW‐scale wind forcing exerted an asymmetric influence on SSTs in the HCM, characterized by a mean‐state cooling and by a positive feedback on interannual variability, acting to enhance ENSO amplitude. Roughly speaking, the feedback tends to increase interannual SST variability by approximately 9%. Additionally, there is a tendency for TIW wind to have an effect on the phase transition during ENSO evolution, with slightly shortened interannual oscillation periods. Additional sensitivity experiments were performed to elucidate the details of TIW wind effects on SST evolution during ENSO cycles. This article is protected by copyright. All rights reserved.
      PubDate: 2016-09-02T14:05:31.92588-05:0
      DOI: 10.1002/2015JC011567
  • Atmospheric forcing during active convection in the Labrador Sea and its
           impact on mixed‐layer depth
    • Authors: Lena M. Schulze; Robert S. Pickart, G.W.K. Moore
      Abstract: Hydrographic data from the Labrador Sea collected in February ‐ March 1997, together with atmospheric reanalysis fields, are used to explore relationships between the air‐sea fluxes and the observed mixed layer depths. The strongest winds and highest heat fluxes occurred in February, due to the nature and tracks of the storms. While greater numbers of storms occurred earlier and later in the winter, the storms in February followed a more organized track extending from the Gulf Stream region to the Irminger Sea where they slowed and deepened. The canonical low pressure system that drives convection is located east of the southern tip of Greenland, with strong westerly winds advecting cold air off the ice edge over the warm ocean. The deepest mixed layers were observed in the western interior basin, although the variability in mixed layer depth was greater in the eastern interior basin. The overall trend in mixed layer depth through the winter in both regions of the basin was consistent with that predicted by a 1‐D mixed layer model. We argue that the deeper mixed layers in the west were due to the enhanced heat fluxes on that side of the basin as opposed to oceanic preconditioning. This article is protected by copyright. All rights reserved.
      PubDate: 2016-09-01T10:05:21.616711-05:
      DOI: 10.1002/2015JC011607
  • A hybrid method to estimate suspended particle sizes from satellite
           measurements over Bohai Sea and Yellow Sea
    • Authors: Deyong Sun; Zhongfeng Qiu, Chuanmin Hu, Shengqiang Wang, Lin Wang, Lufei Zheng, Tian Peng, Yijun He
      Abstract: Particle size distribution (PSD), a measure of particle concentrations at different sizes, is of great importance to the understanding of many biogeochemical processes in coastal marine ecosystems. Here, a hybrid method, including analytical, semi‐analytical, and empirical steps, is developed to estimate PSD through the median diameter of suspended particles (Dv50). Four cruise surveys were conducted to measure optical scattering properties, particle concentrations, spectral reflectance, and particle size distributions (obtained with a LISST instrument covering a size range of 2.5‐500 μm) in coastal waters of Bohai Sea, Yellow Sea, and Jiangsu coastal region. Based on the Mie scattering theory, Dv50 is closely related to mass‐specific backscattering coefficient of suspended particles (bbp*), and their relationship is calibrated through a power model (R2=0.796, n=67, p
      PubDate: 2016-08-29T10:10:23.664014-05:
      DOI: 10.1002/2016JC011949
  • New insights on the upper layer circulation north of the Gulf of Guinea
    • Abstract: The oceanic circulation in the upper layers of the north of the Gulf of Guinea is analyzed, as inferred from in situ observations and numerical simulations. This particular region, in spite of the presence of a coastal upwelling and its impact on resources and regional climate, is still poorly documented. Cruises carried out in the framework of different international programs (e.g. EGEE/AMMA, PIRATA) allowed to show the existence of an eastward flowing undercurrent, found under the Guinea Current, named the Guinea UnderCurrent (GUC). Numerical results from high resolution simulation allowed the description of the seasonal variability of this current. It appears that the GUC is stronger in spring and reverses westward in August‐September. We also depict the fate and the sources of the GUC based on selected trajectories from numerical particle tracking. The simulated trajectories reveal: i) a preferred route of the GUC along 4°N from Cape Palmas to Cape Three Points and following the coast east of Cape Three Points; ii) strong recirculations in the most eastern part of the Gulf of Guinea and off Cape Palmas including warm and salty waters of the South Equatorial Current; iv) a weak inflow from northern latitudes through a subsurface current flowing southward along the West African coast. In addition, Lagrangian experiments show that the GUC is not an extension of the North Equatorial UnderCurrent and confirm that this current does not penetrate into the Gulf of Guinea. This article is protected by copyright. All rights reserved.
      PubDate: 2016-08-29T09:20:28.815618-05:
      DOI: 10.1002/2016JC011959
  • Diagnosing cross‐shelf transport along an ocean front: an observational
           case study in the Gulf of Lion
    • Authors: F. Nencioli; A. A. Petrenko, A. M. Doglioli
      Abstract: Exchanges between coastal regions and the open ocean are often associated with intermittent and localized processes such as eddies, fronts and filaments. Since these features are difficult to observe, their impact has been predominantly investigated using numerical models and remote sensing. In this study, satellite sea surface temperature maps, Lagrangian surface drifter trajectories, and ship‐based surveys of currents and hydrography from the Latex10 campaign are used to quantify cross‐shelf exchanges associated with a temperature front in the western Gulf of Lion. Satellite imagery and thermosalinograph sections provide the characterization of the various water masses associated with the front. Lagrangian drifter trajectories are used to identify the main transport structures and to quantify the velocity components associated with near‐inertial oscillations. These are removed from the instantaneous ADCP observations with which the cross‐shelf exchanges are then computed. The results indicate an average outflow of 0.074 ±0.013 Sv and an inflow of 0.021 ±0.006 Sv. Integrated over the two‐week lifetime of the front, such outflow induced a total export of ∼ 90 ± 14 km3 of water, indicating that 3 to 4 of such events are sufficient to completely renew the surface waters of the Gulf of Lion. The total import was ∼ 25 ± 7 km3, suggesting larger inflows at depth or in the eastern part of the gulf to maintain its volume balance. These in‐situ estimates represent a key term of comparison for the further development of numerical model‐ and satellite‐based studies of cross‐shelf exchanges associated with this type of processes. This article is protected by copyright. All rights reserved.
      PubDate: 2016-08-29T09:15:33.094553-05:
      DOI: 10.1002/2016JC011908
  • Seasonal evolution of the Yellow Sea Cold Water Mass and its interactions
           with ambient hydrodynamic system
    • Authors: Jianchao Li; Guangxue Li, Jishang Xu, Ping Dong, Lulu Qiao, Shidong Liu, Pingkuo Sun, Zhisong Fan
      Abstract: The Yellow Sea Cold Water Mass (YSCWM) is an important component of the hydrodynamic system in the South Yellow Sea (SYS). However, its intricate interactions with the ambient flows over long time scales are not fully understood. This paper presents the analysis of the dataset obtained from a seabed‐mounted Acoustic Doppler Current Profiler (ADCP) deployed for nearly one‐year in the western SYS. It allowed us to study the evolution of YSCWM, including the seasonal changes of tidal currents, near inertial oscillations (NIOs) and the wind‐driven currents due to typhoons and winter storms. Strong NIOs were found near the bottom of mixed layer and in the pycnocline with nearly opposite current directions, with maximum velocity of nearly 20cm·s−1 in summer. The YSCWM can also inhibit the direct downward energy transport in the water column due to typhoons. Conversely, the hydrodynamic system also feeds back to influence the change of YSCWM. A large current shear (S) of 20cm·s−1·m−1 is generated near the top of pycnocline. Generally the intensity and depth of the pycnocline determine S's magnitude and vertical location, respectively. Based on the monthly‐averaged density profile data, the Richardson number and wavelet analysis, the NIOs are considered to be capable of inducing predominant shear instability around the pycnocline. However the NIOs are not strong enough to influence the lower YSCWM. In addition, in autumn, each fortnightly spring tide corresponds with a bottom temperature increase of nearly 2°C, indicating that tidal currents are the leading hydrodynamic driving force to decline the YSCWM. This article is protected by copyright. All rights reserved.
      PubDate: 2016-08-29T09:15:27.859285-05:
      DOI: 10.1002/2016JC012186
  • Sea surface wind streaks in spaceborne synthetic aperture radar imagery
    • Abstract: Wind streaks are often observed in Synthetic Aperture Radar (SAR) images. They are used to determine the sea surface wind direction for sea surface wind field retrievals. It is generally understood that visible wind streaks are caused by roll vortices in the marine atmospheric boundary layer. In this study, 227 X‐band spaceborne SAR TerraSAR‐X and TanDEM‐X images acquired from the three FiNO platforms in the North Sea and Baltic Sea were thoroughly analyzed for a comprehensive understanding of the manifestation of wind streaks in SAR images. Approximately 48.0% of the 227 SAR images displayed wind streaks, among which 67.3%, 20.0% and 12.7% occurred under unstable, neutral, stable atmospheric conditions, respectively. The proportions indicate that wind streaks are more likely to be generated from thermal convection. Further investigations suggest that the inflection point and the wind shear may be essential for the appearance of wind streaks in SAR images under stable atmospheric conditions. This article is protected by copyright. All rights reserved.
      PubDate: 2016-08-29T04:10:24.087185-05:
      DOI: 10.1002/2016JC012040
  • The phenology of Arctic Ocean Surface warming
    • Authors: Michael Steele; Suzanne Dickinson
      Abstract: In this work, we explore the seasonal relationships (i.e., the phenology) between sea ice retreat, sea surface temperature (SST), and atmospheric heat fluxes in the Pacific Sector of the Arctic Ocean, using satellite and reanalysis data. We find that where ice retreats early in most years, maximum summertime SSTs are usually warmer, relative to areas with later retreat. For any particular year, we find that anomalously early ice retreat generally leads to anomalously warm SSTs. However, this relationship is weak in the Chukchi Sea, where ocean advection plays a large role. It is also weak where retreat in a particular year happens earlier than usual, but still relatively late in the season, primarily because atmospheric heat fluxes are weak at that time. This result helps to explain the very different ocean warming responses found in two recent years with extreme ice retreat, 2007 and 2012. We also find that the timing of ice retreat impacts the date of maximum SST, owing to a change in the ocean surface buoyancy and momentum forcing that occurs in early August that we term the Late Summer Transition (LST). After the LST, enhanced mixing of the upper ocean leads to cooling of the ocean surface even while atmospheric heat fluxes are still weakly downward. Our results indicate that in the near‐term, earlier ice retreat is likely to cause enhanced ocean surface warming in much of the Arctic Ocean, although not where ice retreat still occurs late in the season. This article is protected by copyright. All rights reserved.
      PubDate: 2016-08-29T04:05:32.287449-05:
      DOI: 10.1002/2016JC012089
  • Experiments with mixing in stratified flow over a topographic ridge
    • Authors: Yvan Dossmann; Madelaine G. Rosevear, Ross W. Griffiths, Andrew McC. Hogg, Graham O. Hughes, Michael Copeland
      Abstract: The interaction of quasi‐steady abyssal ocean flow with submarine topography is expected to generate turbulent mixing in the ocean. This mixing may occur locally, close to topography, or via breaking quasi‐steady lee waves that can carry energy into the ocean interior. There is currently no theoretical, or empirically‐derived, prediction for the relative amounts of local and interior mixing. We report measurements of the mixing rate in laboratory experiments with a topographic ridge towed through a density stratification. The experiments span three parameter regimes including linear lee waves, nonlinear flow and an evanescent regime in which wave radiation is weak. Full field density measurements provide the depth‐dependence of energy loss to turbulent mixing, allowing separation of the local mixing in the turbulent wake and remote mixing by wave radiation. Remote mixing is significant only for a narrow band of forcing parameters where the flow speed is resonant with internal waves; in all other parameter regimes local mixing close to the topography is dominant. The results suggest that mixing by local nonlinear mechanisms close to abyssal ocean topography may be much greater than the remote mixing by quasi‐steady lee waves. This article is protected by copyright. All rights reserved.
      PubDate: 2016-08-25T17:45:48.081132-05:
      DOI: 10.1002/2016JC011990
  • Remote sensing of normalized diffuse attenuation coefficient of
           downwelling irradiance
    • Authors: Junfang Lin; Zhongping Lee, Mike Ondrusek, Keping Du
      Abstract: The diffuse attenuation of downwelling irradiance, Kd (m−1), is an important property related to light penetration and availability in aquatic ecosystems. The standard Kd(490) product (the diffuse attenuation coefficient at 490 nm) of the global oceans from satellite remote sensing has been produced with an empirical algorithm, which limits its reliability and applicability in coastal regions. More importantly, as an apparent optical property (AOP), Kd is a function of the angular distribution of the light field (e.g., solar zenith angle). The empirically derived product thus contains ambiguities when compared with in situ measurements as there is no specification regarding the corresponding solar zenith angle associated with this Kd(490) product. To overcome these shortcomings, we refined the Kd product with a product termed as the normalized diffuse attenuation coefficient (nKd, m−1), which is equivalent to the Kd in the absence of the atmosphere and with the sun at zenith. Models were developed to get nKd from both in situ measurements and ocean color remote sensing. Evaluations using field measurements indicated that the semi‐analytically derived nKd product will not only remove the ambiguities when comparing Kd values of different light fields, but will also improve the quality of such a product, therefore maximizing the value offered by satellite ocean color remote sensing. This article is protected by copyright. All rights reserved.
      PubDate: 2016-08-25T05:10:33.4159-05:00
      DOI: 10.1002/2016JC011895
  • Seaglider surveys at Ocean Station Papa: Circulation and water mass
           properties in a meander of the North Pacific current
    • Authors: Noel A. Pelland; Charles C. Eriksen, Meghan F. Cronin
      Abstract: A Seaglider autonomous underwater vehicle augmented the Ocean Station Papa (OSP; 50°N, 145°W) surface mooring, measuring spatial structure on scales relevant to the monthly evolution of the moored time series. During each of three missions from June 2008‐January 2010, a Seaglider made biweekly 50 km × 50 km surveys in a bowtie‐shaped survey track. Horizontal temperature and salinity gradients measured by these surveys were an order of magnitude stronger than climatological values and sometimes of opposite sign. Geostrophically‐inferred circulation was corroborated by moored acoustic Doppler current profiler measurements and AVISO satellite altimetry estimates of surface currents, confirming that glider surveys accurately resolved monthly‐scale mesoscale spatial structure. In contrast to climatological North Pacific Current circulation, upper ocean flow was modestly northward during the first half of the 18 month survey period, and weakly westward during its latter half, with Rossby number (0.01). This change in circulation coincided with a shift from cool and fresh to warm, saline, oxygen‐rich water in the upper‐ocean halocline, and an increase in vertical finestructure there and in the lower pycnocline. The anomalous flow and abrupt water mass transition were due to the slow growth of an anticyclonic meander within the North Pacific Current with radius comparable to the scale of the survey pattern, originating to the southeast of OSP. This article is protected by copyright. All rights reserved.
      PubDate: 2016-08-24T05:50:31.454779-05:
      DOI: 10.1002/2016JC011920
  • The effect of Kuroshio current on nitrate dynamics in the southern East
           China Sea revealed by nitrate isotopic composition
    • Authors: Wentao Wang; Zhiming Yu, Xiuxian Song, Zaixing Wu, Yongquan Yuan, Peng Zhou, Xihua Cao
      Abstract: In spring 2014 (May‒June), in the southern East China Sea (ECS) and east of Taiwan, nutrient and isotope samples were collected and analyzed. Also, dissolved oxygen (DO), chlorophyll‐a, and physical parameters were determined. The Kuroshio subsurface water intruded into the ECS and separated into two branches on the continental shelf: the nearshore Kuroshio branch current (NKBC) and the offshore Kuroshio branch current (OKBC). Nitrate concentration in the Kuroshio subsurface water slightly decreased east of Taiwan by assimilation, and was supplied by upwelling currents northeast of Taiwan. The variations of DO, nitrate and nitrogen isotopes in the bottom water showed that continuous nitrification occurred in the NKBC after intrusion into the ECS. This process might contribute to the hypoxia zone near the coast of Zhejiang Province, China. Our results also indicated that internal nitrogen cycles appeared in NKBC since the isotope fractionation ratio of oxygen and nitrogen (18ɛ:15ɛ) in NO3‐ was 1.40. The NO3‐ flux and budget were calculated in the NKBC via numerical simulations. It demonstrated that at least ∼0.52 kmol NO3‐·s−1 was produced by nitrification from DH9 to DH5 transect, and ∼0.11 kmol NO3‐·s−1 was consumed in the DH4 transect. Moreover, according to the Rayleigh model, primary production in most of southern ECS was supported by the intrusion of the Kuroshio subsurface water, causing 5‰ isotope fractionation. In some of nearshore stations which located in the northern investigated area, the assimilated nitrate was contributed from both the NKBC and coastal currents originated from the Changjiang diluted water. This article is protected by copyright. All rights reserved.
      PubDate: 2016-08-24T05:40:24.59285-05:0
      DOI: 10.1002/2016JC011882
  • Mesoscale modulation of air‐sea CO2 flux in Drake Passage
    • Authors: Hajoon Song; John Marshall, David R. Munro, Stephanie Dutkiewicz, Colm Sweeney, D. J. McGillicuddy, Ute Hausmann
      Abstract: We investigate the role of mesoscale eddies in modulating air‐sea CO2 flux and associated biogeochemical fields in Drake Passage using in situ observations and an eddy‐resolving numerical model. Both observations and model show a negative correlation between temperature and partial pressure of CO2 (pCO2) anomalies at the sea surface in austral summer, indicating that warm/cold anticyclonic/cyclonic eddies take up more/less CO2. In austral winter, in contrast, relationships are reversed: warm/cold anticyclonic/cyclonic eddies are characterized by a positive/negative pCO2 anomaly and more/less CO2 outgassing. It is argued that DIC‐driven effects on pCO2 are greater than temperature effects in austral summer, leading to a negative correlation. In austral winter, however, the reverse is true. An eddy‐centric analysis of the model solution reveals that nitrate and iron respond differently to the same vertical mixing: vertical mixing has a greater impact on iron because its normalized vertical gradient at the base of the surface mixed layer is an order of magnitude greater than that of nitrate. This article is protected by copyright. All rights reserved.
      PubDate: 2016-08-24T05:20:27.601083-05:
      DOI: 10.1002/2016JC011714
  • Western boundary currents and climate change
    • Authors: Richard Seager; Isla R. Simpson
      Abstract: A recent paper in Journal of Geophysical Research‐Oceans connects recent changes in atmospheric circulation to poleward movement and intensification of western boundary currents. Causes and character of past and future trends in surface wind stress and western boundary currents are discussed here. This article is protected by copyright. All rights reserved.
      PubDate: 2016-08-24T05:20:21.93621-05:0
      DOI: 10.1002/2016JC012156
  • Forcing of recent decadal variability in the Equatorial and North Indian
    • Authors: P. R. Thompson; C. G. Piecuch, M. A. Merrifield, J. P. McCreary, E. Firing
      Abstract: Recent decadal sea surface height (SSH) variability across the Equatorial and North Indian Ocean (ENIO, north of 5°S) is spatially coherent and related to a reversal in basin‐scale, upper‐ocean‐temperature trends. Analysis of ocean and forcing fields from a data‐assimilating ocean synthesis (ECCOv4) suggests that two equally‐important mechanisms of wind‐driven heat redistribution within the Indian Ocean account for a majority of the decadal variability. The first is the Cross‐Equatorial Cell (CEC) forced by zonal‐wind‐stress curl at the equator. The wind‐stress curl variability relates to the strength and position of the Mascarene High, which is influenced by the phase of the Indian Ocean Subtropical Dipole. The second mechanism is deep (700 m) upwelling related to zonal wind stress at the equator that causes deep, cross‐equatorial overturning due to the unique geometry of the basin. The CEC acts to cool the upper ocean throughout most of the first decade of satellite altimetry, while the deep upwelling delays and then amplifies the effect of the CEC on SSH. During the subsequent decade, reversals in the forcing anomalies drive warming of the upper ocean and increasing SSH, with the effect of the deep upwelling leading the CEC. This article is protected by copyright. All rights reserved.
      PubDate: 2016-08-24T03:56:09.328435-05:
      DOI: 10.1002/2016JC012132
  • Alongcoast structure and interannual variability of seasonal midshelf
           water properties and velocity in the Northern California Current System
    • Authors: B. Hickey; S. Geier, N. Kachel, S. Ramp, P. M. Kosro, T. Connolly
      Abstract: Moored sensors were maintained for ∼5 years on the northern California Current System (CCS) midshelf. The alongcoast sensor array spanned the area of influence of the plume from the Columbia River, several submarine canyons, as well as a coastal promontory where the equatorward coastal jet frequently separates from the shelf. Upwelling‐favorable wind stress magnitude decreases poleward by more than a factor of three over the latitudinal range and shelf width varies by a factor of two. In spite of the alongcoast structure in setting, both seasonal and interannual patterns in subsurface layer water properties were remarkably similar at all sites. Higher in the water column, freshwater forcing was substantial. Because of the near surface freshwater input, seasonal sea surface and subsurface temperatures were almost perfectly out of phase in the northernmost CCS (WA and OR), with a mid water column inversion in winter. Year to year differences in subsurface layer wintertime water properties were similar to spatial and temporal patterns of wind stress variability: little alongcoast structure except in salinity, but pronounced interannual differences. Summertime wind and property patterns were the opposite of those in winter: pronounced alongcoast wind stress structure, but little or no alongcoast or interannual variability in water property extremes, and only a weak relationship to local wind stress. Summertime interannual water property variability, including source waters, was shown to be more consistent with “remote forcing” via larger scale wind stress rather than with local wind stress, particularly in the northernmost CCS. This article is protected by copyright. All rights reserved.
      PubDate: 2016-08-24T03:56:02.562191-05:
      DOI: 10.1002/2015JC011424
  • The Atlantic Water boundary current in the Nansen Basin: Transport and
           mechanisms of lateral exchange
    • Abstract: Data from a shipboard hydrographic survey near 30°E in the Nansen Basin of the Arctic Ocean are used to investigate the structure and transport of the Atlantic Water boundary current. Two high‐resolution synoptic crossings of the current indicate that it is roughly 30 km wide and weakly mid‐depth intensified. Using a previously‐determined definition of Atlantic Water, the transport of this water mass is calculated to be 1.6 ± 0.3 Sv, which is similar to the transport of Atlantic Water in the inner branch of the West Spitsbergen Current. At the time of the survey a small anti‐cyclonic eddy of Atlantic Water was situated just offshore of the boundary current. The data suggest that the feature was recently detached from the boundary current, and, due to compensating effects of temperature and salinity on the thermal wind shear, the maximum swirl speed was situated below the hydrographic property core. Two other similar features were detected within our study domain, suggesting that these eddies are common and represent an effective means of fluxing warm and salty water from the boundary current into the interior. An atmospheric low pressure system transiting south of our study area resulted in southeasterly winds prior to and during the field measurements. A comparison to hydrographic data from the Pacific Water boundary current in the Canada Basin under similar atmospheric forcing suggests that upwelling was taking place during the survey. This provides a second mechanism related to cross‐stream exchange of heat and salt in this region of the Nansen Basin. This article is protected by copyright. All rights reserved.
      PubDate: 2016-08-24T03:50:58.865812-05:
      DOI: 10.1002/2016JC011715
  • Annual cycle and destruction of Eighteen Degree Water
    • Authors: Sam Billheimer; Lynne D. Talley
      Abstract: Eighteen Degree Water (EDW), the subtropical mode water of the western North Atlantic, is a voluminous, weakly‐stratified upper ocean water mass that acts as a subsurface reservoir of heat, nutrients, and CO2. This thick layer persists throughout the year, but nearly half of its volume is dispersed or mixed away, diffusing its properties into the thermocline, from the time it outcrops in winter until it is renewed the following year. CTD observations from Argo profiling floats and acoustically‐tracked, isothermally‐bound profiling floats are used to quantify EDW destruction rates and investigate the relevant processes responsible for the large annual cycle of EDW. EDW destruction occurs primarily at the top of the EDW layer, with the highest EDW destruction rates occurring during early summer. Slower, steadier EDW destruction is observed in early winter. EDW destruction is dominated by 1‐D vertical diffusion, while mesoscale, along‐isopycnal stirring is also significant, explaining approximately 1/3 of the total annual EDW destruction. Destruction via along‐isopycnal processes is more prevalent near the Gulf Stream than in the southern Sargasso Sea, due to higher potential vorticity gradients and enhanced mesoscale activity. This article is protected by copyright. All rights reserved.
      PubDate: 2016-08-20T04:06:02.409243-05:
      DOI: 10.1002/2016JC011799
  • High export via small particles before the onset of the North Atlantic
           spring bloom
    • Abstract: Sinking organic matter in the North Atlantic Ocean transfers 1‐3 Gt carbon year−1 from the surface ocean to the interior. The majority of this exported material is thought to be in form of large, rapidly sinking particles that aggregate during or after the spring phytoplankton bloom. However, recent work has suggested that intermittent water column stratification resulting in the termination of deep convection can isolate phytoplankton from the euphotic zone, leading to export of small particles. We present depth profiles of large (>0.1mm equivalent spherical diameter, ESD) and small (300m depth, leading to deep mixing of particles as deep as 600m. Subsequent re‐stratification could trap these particles at depth and lead to high particle fluxes at depth without the need for aggregation (‘mixed layer pump'). Overall we suggest that pre‐bloom fluxes to the mesopelagic are significant, and the role of small sinking particles requires careful consideration. This article is protected by copyright. All rights reserved.
      PubDate: 2016-08-19T09:45:29.206809-05:
      DOI: 10.1002/2016JC012048
  • Consequences of inhibition of mixed‐layer deepening by the West India
           Coastal Current for winter phytoplankton bloom in the northeastern Arabian
    • Authors: V. Vijith; P. N. Vinayachandran, V. Thushara, P. Amol, D. Shankar, A. C. Anil
      Abstract: The intense winter phytoplankton bloom during November – February in the northeastern Arabian Sea (NEAS) was thought, until recently, to be controlled only by a convective deepening of the mixed layer (ML) owing to cool and dry northeasterlies. But, a recent study has shown that the deepening of the ML in the southern NEAS is inhibited by the poleward advection of low‐salinity water from the south by the West India Coastal Current (WICC). Using an Ocean General Circulation Model coupled with an ecosystem model, we investigate the consequences of the inhibition of mixed‐layer deepening for winter phytoplankton bloom in the NEAS. We show that, during the winter monsoon, the shallow ML inhibits the entrainment of nutrients in the southern NEAS. Strong (weak) positive nitrate tendency in the northern (southern) NEAS seen in the model during the winter monsoon is maintained by strong (weak) entrainment. As a result, the chlorophyll integrated to 200 m depth from the surface is lower in the southern NEAS than in the northern NEAS. The inhibition of mixed‐layer deepening in the south affects the size‐based distribution of small and large phytoplankton, nutrient limitation terms and growth rate, and their elemental composition. The WICC, which inhibits the deepening of the ML and affects the winter bloom in the NEAS, is driven by coastal Kelvin waves generated by remote winds. This paper demonstrates a mechanism by which remotely forced coastal Kelvin waves impact the biology in the north Indian Ocean. This article is protected by copyright. All rights reserved.
      PubDate: 2016-08-19T09:35:30.038659-05:
      DOI: 10.1002/2016JC012004
  • Estimating dense water volume and its evolution for the year 2012‐2013
           in the North‐western Mediterranean Sea: An observing system simulation
           experiment approach
    • Authors: Robin Waldman; Samuel Somot, Marine Herrmann, Pierre Testor, Claude Estournel, Florence Sevault, Louis Prieur, Laurent Mortier, Laurent Coppola, Vincent Taillandier, Pascal Conan, Denis Dausse
      Abstract: The northwestern Mediterranean (NWMed) sea includes one of the best observed ocean deep convection sites in the World. An Observing System Simulation Experiment (OSSE) is developed to provide a methodology for estimating observing network errors. It is applied to quantify dense water volumes in the NWMed during 2012‐2013 with their observation error from MOOSE network. Results from the OSSE show low spatio‐temporal sampling errors, which confirms MOOSE network ability to measure dense waters. However, results are highly sensitive to instrumental stability. The dense water volume is then estimated in observations from four ship cruises between summers 2012 and 2013. A large seasonal cycle is found, maximal in spring 2013 and dominated by the area west of 6.5°E. The dense water volume (σ0>29.11kg/m3) is stable between summer 2012 (13.3±0.6 1013m3) and winter 2013 (13.7±1.3 1013m3). It increases dramatically in spring 2013 (17.7±0.9 1013m3) due to an intense convective event, and it finally decreases rapidly in summer 2013 (15.1±0.6 1013m3) due to restratification and spreading. We estimate an open‐sea dense water formation (DWF) rate of 1.4±0.3Sυ between summer 2012 and spring 2013 over the studied area, extrapolated to 2.3±0.5Sυ over the whole NWMed sea and for the optimal timing. This is to our knowledge the highest measured DWF rate, suggesting winter 2013 was exceptionally convective. The observed restratification rate between spring and summer 2013 is ‐0.8±0.4S4Sυ. This study provides robust quantifications of deep convection during an exceptional event that will allow to evaluate numerical simulations. This article is protected by copyright. All rights reserved.
      PubDate: 2016-08-19T09:35:26.150189-05:
      DOI: 10.1002/2016JC011694
  • An ensemble of ocean reanalyses for 1815–2013 with sparse
           observational input
    • Authors: Benjamin S. Giese; Howard F. Seidel, Gilbert P. Compo, Prashant D. Sardeshmukh
      Abstract: This paper describes a new 8‐member ensemble of oceanic reanalyses spanning nearly 200 years from 1815 to 2013 generated using the Simple Ocean Data Assimilation system with sparse observational input (SODAsi) to explore long‐term changes in the oceans. The 8 ensemble members assimilate surface temperature observations and use surface boundary conditions from an atmospheric reanalysis that is loosely coupled to the ocean reanalysis. Both surface and subsurface quantities, such as dynamic height and heat content, show a broad spectrum of variability. Surface temperature trends from 1815 to 2013 are positive in most regions, with some important exceptions; the central Tropical Pacific, around Antarctica, and in the Gulf Stream and Kuroshio extension regions all show cooling trends. A near‐global average shows warming of about 0.8°C over the full period, with most of the warming occurring after 1920. There is pronounced multi‐decadal variability in both the mid‐latitude and tropical oceans. In the North Atlantic Ocean, temperature variability is highly correlated with the meridional overturning streamfunction, with the largest correlation occurring when the streamfunction is advanced by 9 years. Trends of upper ocean heat content and dynamic height from the 1950s onward compare well with previously published values. Globally averaged heat content of the upper 700 m shows a nearly linear rise after the 1920s, requiring a net downward surface heat flux increase of 0.47 W m−2 into the ocean. This is close to published estimates of the increased flux required to explain the heat content increase from 1971 to 2010. This article is protected by copyright. All rights reserved.
      PubDate: 2016-08-18T06:20:49.310455-05:
      DOI: 10.1002/2016JC012079
  • Geochemical observations within the water column at the CO2‐rich
           hydrothermal systems Hatoma Knoll and Yonaguni Knoll IV, in the southern
           Okinawa Trough
    • Abstract: The Okinawa Trough is one of three known hydrothermal sites worldwide where liquid carbon dioxide is emitted from the seafloor into the water column. In March 2008 investigations were performed at two active areas, Yonaguni Knoll IV and Hatoma Knoll, in order to identify impacts of hydrothermal venting on the water column chemistry. Vertical profiles of pH and redox potential (Eh) were recorded and discrete water samples were taken for the analysis of total carbon dioxide (CT) and helium (3He, 4He). Anomalies with respect to reference stations (ΔCT, ΔpH) and 3He with respect to saturation with the atmosphere (3Heexcess) were used to characterize the impact of hydrothermal vents. These data indicate that the flux of CO2 into the water column is dominated by hot hydrothermal CO2‐rich vents located in close proximity to the liquid CO2 emission sites. Bubbles and droplets sampled at the cold gas outlets at Hatoma Knoll differed considerably from the water column regarding CO2/3He ratios, and thus, provide additional evidence that cold liquid phase CO2 is of minor importance for the total CO2 flux at both hydrothermal systems. Although hydrothermal vents at back‐arc basins are known to emit large amounts of acids other than CO2, the correlation between ΔpH and ΔCT at both research areas clearly suggests that the observed pH reduction is mainly caused by the addition of CO2. Deviating ΔCT/3He and ΔCT/ΔpH ratios and the prevailing water currents indicate a yet undiscovered vent site at the flank of a seamount in the northeast. This article is protected by copyright. All rights reserved.
      PubDate: 2016-08-18T06:16:35.563099-05:
      DOI: 10.1002/2016JC012003
  • Upstream control of the frontal jet regulating plankton production in the
           Alboran Sea (Western Mediterranean)
    • Abstract: Using a coupled physical‐biological model, we document that a ∼30 km wide meandering jet constitutes a major source of biological enrichment in the Alboran Sea (Western Mediterranean) even in the absence of wind forcing and tidal dynamics. The level of enrichment is shown to vary markedly during the year depending on the upstream characteristics of the jet as it exits from the Gibraltar Strait. When its intensity is sufficiently low and characterized by weaker cross‐frontal density gradients during winter‐spring, the jet is weakly nonlinear and may not fulfill the necessary conditions for frontogenesis. It then remains weakly productive. In the case of stronger jet intensity (>1.1 Sv) accompanied by stronger cross‐frontal density and velocity gradients within the Alboran Sea during summer‐autumn, the frontal jet becomes strongly nonlinear and ageostrophic with large cross‐frontal vorticity changes on the order of planetary vorticity. Under these conditions, upward vertical velocities in the range 10‐50 m d−1 supply nutrients into the euphotic layer more effectively and support high level frontogenesis‐induced phytoplankton production on the anticyclonic side of the main jet axis. The strong eddy pumping mechanism also provides a comparable level of plankton production within strongly nonlinear elongated cyclonic eddies along the outer periphery of the frontal jet. The plankton biomass is advected partially by the jet along its trajectory and dispersed within the basin by mesoscale eddies and meanders. This article is protected by copyright. All rights reserved.
      PubDate: 2016-08-18T06:16:23.855833-05:
      DOI: 10.1002/2016JC011667
  • Upper ocean response to Typhoon Kalmaegi (2014)
    • Authors: Han Zhang; Dake Chen, Lei Zhou, Xiaohui Liu, Tao Ding, Beifeng Zhou
      Abstract: Typhoon Kalmaegi passed over an array of buoys and moorings in the northern South China Sea in September 2014, leaving a rare set of observations on typhoon‐induced dynamical and thermohaline responses in the upper‐ocean. The dynamical response was characterized by strong near‐inertial currents with opposite phases in the surface mixed layer and in the thermocline, indicating the dominance of the response by the excitation of the first baroclinic mode. The thermohaline response showed considerable changes in the mean fields in addition to a near‐inertial oscillation. In particular, temperature and salinity anomalies generally exhibited a three‐layer vertical structure, with the surface layer becoming cooler and saltier, the subsurface layer warmer and fresher, and the lower layer cooler and saltier again. The response in the surface and subsurface layers was much stronger to the right of the typhoon track, while that in the lower layer was stronger along the track and to the left. These features of the upper ocean response were grossly reproduced by a three‐dimensional numerical model. A model‐based heat budget analysis suggests that vertical mixing was mainly responsible for the surface cooling and subsurface warming, while upwelling was the cause of cooling from below. Both observations and model results indicate that the whole upper ocean experienced an overall cooling in the wake of typhoon Kalmaegi. This article is protected by copyright. All rights reserved.
      PubDate: 2016-08-18T06:16:20.004006-05:
      DOI: 10.1002/2016JC012064
  • How does Subantarctic Mode Water ventilate the Southern Hemisphere
    • Abstract: In several regions north of the Antarctic Circumpolar Current (ACC), deep wintertime convection refreshes pools of weakly stratified subsurface water collectively referred to as Subantarctic Mode Water (SAMW). SAMW ventilates the subtropical thermocline on decadal timescales, providing nutrients for low‐latitude productivity and potentially trapping anthropogenic carbon in the deep ocean interior for centuries. In this work, we investigate the spatial structure and timescales of mode water export and associated thermocline ventilation. We use passive tracers in an eddy‐permitting, observationally‐informed Southern Ocean model to identify the pathways followed by mode waters between their formation regions and the areas where they first enter the subtropics. We find that the pathways followed by the mode water tracers are largely set by the mean geostrophic circulation. Export from the Indian and Central Pacific mode water pools is primarily driven by large‐scale gyre circulation, whereas export from the Australian and Atlantic pools is heavily influenced by the ACC. Export from the Eastern Pacific mode water pool is driven by a combination of deep boundary currents and subtropical gyre circulation. More than 50% of each mode water tracer reaches the subtropical thermocline within 50 years, with significant variability between pools. The Eastern Pacific pathway is especially efficient, with roughly 80% entering the subtropical thermocline within 50 years. The time required for 50% of the mode water tracers to leave the Southern Ocean domain varies significantly between mode water pools, from 9 years for the Indian mode water pool to roughly 40 years for the Central Pacific mode water pool. This article is protected by copyright. All rights reserved.
      PubDate: 2016-08-18T06:16:17.051279-05:
      DOI: 10.1002/2016JC011680
  • Air‐sea interaction at the Southern Brazilian continental shelf: In
           situ observations
    • Authors: L. P. Pezzi; R. B. Souza, P. C. Farias, O. Acevedo, A. J. Miller
      Abstract: The influence of the cross‐shelf oceanographic front occurring between the Brazil Current (BC) and the Brazilian Coastal Current (BCC) on the local Marine Atmospheric Boundary Layer (MABL) is investigated here. This front is typical of wintertime in the Southern Brazilian Continental Shelf (SBCS) and this is the first time that its effects are investigated over the above MABL. Here we analyze variability, vertical structure and stability of MABL as well as heat fluxes at air‐sea interface, across five oceanographic transects in the SBCS made during a winter 2012 cruise. Local thermal gradients associated with mixing between distinct water masses, play an essential role on MABL modulation and stability. Although weaker when compared with other frontal regions, the cross‐shelf thermal gradients reproduce exactly what is expected for open ocean regions: Stronger (weaker) winds, lower (higher) sea level pressure and a more unstable (stable) MABL are found over the warm (cold) side of the oceanographic front between the BC (warm) and coastal (cold) waters. Our findings strongly support the coexistence of both known MABL modulation mechanisms: the static and hydrostatic MABL stability. This is the first time that these modulation mechanisms are documented for this region. Turbulent fluxes were found to be markedly dependent on the cross‐shelf SST gradients resulting in differences of up to 100 W.m−2 especially in the southernmost region where the gradients were more intense. This article is protected by copyright. All rights reserved.
      PubDate: 2016-08-18T06:15:59.672622-05:
      DOI: 10.1002/2016JC011774
  • Budget of organic carbon in the North‐Western Mediterranean Open Sea
           over the period 2004–2008 using 3‐D coupled physical‐biogeochemical
    • Abstract: A 3D hydrodynamic‐biogeochemical coupled model has been used to estimate a budget of organic carbon and its interannual variability over the 5‐year period 2004‐2008 in the North‐Western Mediterranean Open Sea (NWMOS). The comparison of its results with in situ and satellite observations reveals that the timing and the magnitude of the convection and bloom processes during the study period, marked by contrasted atmospheric conditions, are reasonably well reproduced by the model. Model outputs show that the amount of nutrients annually injected into the surface layer is clearly linked to the intensity of the events of winter convection. During cold winters, primary production is reduced by intense mixing events but then spectacularly increases when the water column restratifies. In contrast, during mild winters, the primary production progressively and continuously increases, sustained by moderate new production followed by regenerated production. Overall, interannual variability in the annual primary production is low. The export in sub‐surface and at mid‐depth is however affected by the intensity of the convection process, with annual values twice as high during cold winters than during mild winters. Finally, the estimation of a global budget of organic carbon reveals that the NWMOS acts as a sink for the shallower areas and as a source for the Algerian and Balearic sub‐basins. This article is protected by copyright. All rights reserved.
      PubDate: 2016-08-18T06:10:54.493874-05:
      DOI: 10.1002/2016JC011818
  • Autumn CO2 chemistry in the Japan Sea and the impact of discharges from
           the Changjiang River
    • Authors: Naohiro Kosugi; Daisuke Sasano, Masao Ishii, Kazutaka Enyo, Shu Saito
      Abstract: We made comprehensive surface water CO2 chemistry observations in the Japan Sea during each autumn from 2010 to 2014. The partial pressure of CO2 (pCO2) in surface water, 312–329 μatm, was 10–30 μatm lower in the Japan Sea than in the same latitude range of the western North Pacific adjacent to Japan. According to the sensitivity analysis of pCO2, the lower pCO2 in the Japan Sea was primarily attributable to a large seasonal decrease of pCO2 associated with strong cooling in autumn, particularly in the northern Japan Sea. In contrast, the lower pCO2 in relatively warm, fresh water in the southern Japan Sea was attributable to not only the thermodynamic effect of the temperature changes but also high total alkalinity. This alkalinity had its origin in Changjiang River and was transported by Changjiang diluted water (CDW) which seasonally runs into the Japan Sea from the East China Sea. The input of total alkalinity through CDW also elevated the saturation state of calcium carbonate minerals and mitigated the effects of anthropogenic ocean acidification, at least during autumn. These biogeochemical impacts of CDW in the Japan Sea last until November, although the inflow from the East China Sea to the Japan Sea almost ceases by the end of September. The long duration of the high saturation state of calcium carbonate benefits calcareous marine organisms. This article is protected by copyright. All rights reserved.
      PubDate: 2016-08-18T06:10:53.067154-05:
      DOI: 10.1002/2016JC011838
  • Estimation of melt pond fraction over high‐concentration Arctic sea ice
           using AMSR‐E passive microwave data
    • Authors: Yasuhiro Tanaka; Kazutaka Tateyama, Takao Kameda, Jennifer K. Hutchings
      Abstract: Melt pond fraction (MPF) on sea ice is an important factor for ice‐albedo feedback throughout the Arctic Ocean. We propose an algorithm to estimate MPF using satellite passive microwave data in this study. The brightness temperature (TB) data obtained from the Advanced Microwave Scanning Radiometer‐Earth observing system (AMSR‐E) were compared to the ship‐based MPF in the Beaufort Sea and Canadian Arctic Archipelago. The difference between the TB at horizontal and vertical polarizations of 6.9‐ and 89.0‐GHz (MP06H–89V), respectively, depend on the MPF. The correlation between MP06H–89V and ship‐based MPF was higher than that between ship‐based MPF and two individual channels (6.9‐ and 89.0‐GHz of horizontal and vertical polarizations, respectively). The MPF determined with the highest resolution channel, 89.0‐GHz (5 km × 5 km) provides spatial information with more detail than the 6.9‐GHz channel. The algorithm estimates the relative fraction of ice covered by water (1) over areas where sea ice concentration is higher than 95%, (2) during late summer, and (3) in areas with low atmospheric humidity. The MPF estimated from AMSR‐E data (AMSR‐E MPF) in early summer was underestimated at lower latitudes and overestimated at higher latitudes, compared to the MPF obtained from the Moderate Resolution Image Spectrometer (MODIS MPF). The differences between AMSR‐E MPF and MODIS MPF were less than 5% in most the regions and the periods. Our results suggest that the proposal algorithm serves as a basis for building time series of MPF in regions of consolidated ice pack. This article is protected by copyright. All rights reserved.
      PubDate: 2016-08-18T06:10:29.516362-05:
      DOI: 10.1002/2016JC011876
  • Bottom pressure variability in the Kuroshio Extension driven by the
           atmosphere and ocean instabilities
    • Abstract: The relative importance of atmospheric forcing on oceanic intraseasonal (7–60 days) barotropic variability is investigated in the Kuroshio Extension region by comparing in‐situ measurements with two models: a wind‐forced barotropic model and an ocean general circulation model. Large‐scale wind‐stress curl forcing (an atmospheric mode) becomes successively more influential with decreasing periods (at 7–15‐day band, compared to 15–30‐day and 30–60‐day bands). On the other hand, oceanic instabilities (an oceanic mode) become more important with increasing period (at 30–60‐day band, compared to 15–30‐day and 7–15‐day bands). Comparison between the barotropic model and the ocean general circulation model results reveals differences on the broader gyre scale: the atmospheric mode exhibits basin‐mode‐like spatial patterns with faster phase propagation from east to west, whereas the oceanic mode shows eddy‐like spatial patterns with slower westward propagation. The atmospheric mode, which has received less attention on the intraseasonal time scale, contributes an important fraction to the barotropic variability in the Kuroshio Extension region. This article is protected by copyright. All rights reserved.
      PubDate: 2016-08-17T10:30:30.934255-05:
      DOI: 10.1002/2016JC012097
  • Waves in polar lows
    • Authors: A. P. Orimolade; B. R. Furevik, G. Noer, O. T. Gudmestad, R.M. Samelson
      Abstract: In a rather stationary fetch, one would not expect large waves in polar low situations. However, the picture changes when one considers a moving fetch. The significant wave heights that may be associated with the recorded polar lows on the Norwegian continental shelf from December 1999 to October 2015 are estimated using a one‐dimensional parametric wave model. A comparison of the measured and the forecasted significant wave heights in two recent polar low cases in the Barents Sea is presented. The estimated significant wave heights show that the values could have been up to and above 9 m. The forecasted significant wave heights considerably underestimated the measured significant wave heights in the two recent polar low cases that are considered. Furthermore, a generalization of the fetch‐limited wave equation in polar lows is proposed, which allows the wind field to vary in space and time, and is shown to give results that are consistent with the one‐dimensional parametric model. This article is protected by copyright. All rights reserved.
      PubDate: 2016-08-13T08:55:46.684655-05:
      DOI: 10.1002/2016JC012086
  • Simulated response of the mid‐Holocene Atlantic Meridional Overturning
           Circulation in ECHAM6‐FESOM/MPIOM
    • Authors: Xiaoxu Shi; Gerrit Lohmann
      Abstract: Changes of the Atlantic meridional overturning circulation (AMOC) in the mid‐Holocene compared to the pre‐industrial state are explored in different coupled climate models. Using time‐slice integrations by a newly developed global finite‐element model ECHAM6‐FESOM with unstructured mesh and high resolution, our simulations show an enhanced mid‐Holocene AMOC, accompanied by an increase in the ocean salinity over regions of deep water formation. We identify two different processes affecting the AMOC: 1) a more positive phase of North Atlantic Oscillation (NAO) increased water density over the Labrador Sea through anomalous net evaporation and surface heat loss; 2) a decreased import of sea ice from the Arctic causes a freshwater reduction in the northern North Atlantic Ocean. Using the coupled model ECHAM6‐MPIOM in T63GR15 and T31GR30 grids, we find that the simulated AMOC has significant discrepancy with different model resolutions. In detail, stronger‐than‐present mid‐Holocene AMOC is revealed by simulations with the T63GR15 grid, which resembles the result of ECHAM6‐FESOM, while a decline of the mid‐Holocene AMOC is simulated by the low resulution model with the T31GR30 grid. Such discrepancy can be attributed to different changes in Labrador Sea density which is mainly affected by 1) NAO‐induced net precipitation and deep water convection, 2) freshwater transport from the Arctic Ocean, and 3) the strength of AMOC itself. Finally, we analyzed available coupled climate models showing a diversity of responses of AMOC to mid‐Holocene forcings, most of which reveal positive AMOC changes related to northern high latitudes salinification. This article is protected by copyright. All rights reserved.
      PubDate: 2016-08-12T10:10:59.918352-05:
      DOI: 10.1002/2015JC011584
  • Saltier sea surface water conditions recorded by multiple mid‐Holocene
           corals in the northern South China Sea
    • Abstract: The typical features of the mid‐Holocene can be used to better understand present‐day climate conditions and the potential trends of future climate change. The surface conditions, including sea surface temperature (SST) and sea surface salinity (SSS), of the South China Sea (SCS) are largely controlled by the East Asian monsoon system. Surface water conditions reconstructed from coral proxies can be used to study the evolution of the East Asian monsoon during the mid‐Holocene. However, there are some discrepancies among existing coral‐based studies regarding whether the mid‐Holocene sea surface water was much saltier than the present day surface waters. Based on paired Sr/Ca and δ18O of modern and three fossil corals, this paper reconstructs the patterns of seasonal variation in SSS during the mid‐Holocene in the northern SCS. The Δδ18O records (a proxy for SSS) derived from the three fossil corals were all heavier than that from the modern coral, which suggests the presence of more saline surface waters during the mid‐Holocene in the northern SCS. These results are consistent with previous studies based on records reconstructed from coral and foraminifera, as well as from numerical simulations. Reduced rainfall caused by the strengthened Asian Monsoon and/or the northward shift of the intertropical convergence zone during the mid‐Holocene would explain the increased salinity of the surface waters of the northern SCS. The findings presented here clarify the discrepancies among previous studies and confirm the existence of saltier surface waters in the northern SCS during the mid‐Holocene. This article is protected by copyright. All rights reserved.
      PubDate: 2016-08-12T10:10:22.536148-05:
      DOI: 10.1002/2016JC012034
  • An east‐west contrast of upper ocean heat content variation south of the
           subpolar front in the East/Japan Sea
    • Abstract: Long‐term variability of non‐seasonal ocean heat content (OHC) in the upper 500 m in the East/Japan Sea (EJS) exhibits a distinct east‐west contrast during the recent 30 years. The contrasting OHC variations are revisited and investigated more in detail by analyzing two observational datasets, the gridded data from 1976 to 2007 and in situ data from 1976 to 2011 in the southwestern EJS that covers the zone of western boundary current. The OHC variability shows in‐phase and predominant decadal variation in both east and west regions before 1995, but uncorrelated and predominant interannual variations after 1995. Heaving effects due to major branches of warm currents in the EJS, the East Korea Warm Current (EKWC) in the western part and other two branches in the eastern part, mainly contribute to the OHC variations. The heaving effect in the western EJS is shown to be associated with changes in winter wind‐stress curl field in the northern EJS. Weakening of the subpolar gyre due to weakening of positive wind‐stress curl in the Japan Basin related with wintertime Western Pacific teleconnection pattern and Siberian High appears to enhance the northward penetration of the EKWC resulting in an increase of OHC in the western EJS. The heaving effect in the eastern EJS is significantly correlated with the Siberian High, but the causative mechanism is inconclusive. This study also demonstrates the importance of using highly‐resolved datasets for areas affected by strong and narrow boundary currents in computing and understanding the OHC variability. This article is protected by copyright. All rights reserved.
      PubDate: 2016-08-11T03:50:36.151912-05:
      DOI: 10.1002/2016JC011891
  • Size characteristics of chromophoric dissolved organic matter in the
           Chukchi Sea
    • Authors: Hui Lin; Min Chen, Jian Zeng, Qi Li, Renming Jia, Xiuwu Sun, Minfang Zheng, Yusheng Qiu
      Abstract: With the Arctic warming, terrestrial input plays a more important role in carbon cycle in the Arctic Ocean than before. Chromophoric dissolved organic matter (CDOM) as a tracer of terrestrial dissolved organic matter (tDOM) becomes more valuable in elucidating the source and compositions of DOM. Although measurements of DOM in the Arctic Ocean have been widely reported, characteristics of high molecular weight colloids are still poorly understood. In this study, the bulk absorbance and size fractograms of CDOM were measured in the Chukchi Seas using an asymmetrical flow field‐flow fractionation (AF4) coupled online with UV‐vis detectors. Both CDOM a254, absorption coefficient at 254 nm, and the integrated UV254 (from AF4 UV‐Vis detector) of three colloidal fractions (1‐10 kDa, 10‐100 kDa, >100 kDa) significantly correlated with the fraction of meteoric water (fmw) calculated from δ18O in seawater, which indicates that the CDOM was mainly derived from terrestrial input and a254 is a potential tracer of tDOM in the Chukchi Sea. Compared with the larger colloidal fractions (10‐100 kDa and >100 kDa), the smaller colloidal fraction (1‐10 kDa) showed a stronger correlation with the fmw, suggesting the smaller colloids were of mostly terrigenous origin. Values of field measured spectral slope at 275‐295 nm (s275‐295), a tDOM proxy, were significantly lower than the model‐estimated s275‐295 calculated from the MODIS Aqu satellite remote sensing data, which indicated that terrestrial input of CDOM derived from model calculation was likely underestimated in the Chukchi Sea. This article is protected by copyright. All rights reserved.
      PubDate: 2016-08-08T10:13:29.896907-05:
      DOI: 10.1002/2016JC011771
  • Lagrangian pathways of upwelling in the Southern Ocean
    • Authors: Giuliana A. Viglione; Andrew F. Thompson
      Abstract: The spatial and temporal variability of upwelling into the mixed layer in the Southern Ocean is studied using a 1/10° ocean general circulation model. Virtual drifters are released in a regularly‐spaced pattern across the Southern Ocean at depths of 250, 500, and 1000 m during both summer and winter months. The drifters are advected along isopycnals for a period of four years, unless they outcrop into the mixed layer, where lateral advection and a parameterization of vertical mixing is applied. The focus of this study is on the discrete exchange between the model mixed layer and the interior. Localization of interior‐mixed layer exchange occurs downstream of major topographic features across the Indian and Pacific basins, creating “hotspots” of outcropping. Minimal outcropping occurs in the Atlantic basin, while 59% of drifters outcrop in the Pacific sector and in Drake Passage (the region from 140° W to 40° W), a disproportionately large amount even when considering the relative basin sizes. Due to spatial and temporal variations in mixed layer depth, the Lagrangian trajectories provide a statistical measure of mixed layer residence times. For each exchange into the mixed layer, the residence time has a Rayleigh distribution with a mean of 30 days; the cumulative residence time of the drifters is 261 ± 194 days, over a period of four years. These results suggest that certain oceanic gas concentrations, such as CO2 and 14C, will likely not reach equilibrium with the atmosphere before being re‐subducted. This article is protected by copyright. All rights reserved.
      PubDate: 2016-08-08T10:12:06.235901-05:
      DOI: 10.1002/2016JC011773
  • Tropical Cyclone Activity over the Southwest Tropical Indian Ocean
    • Authors: Jessica M. Burns; Bulusu Subrahmanyam, Ebenezer S. Nyadjro, V. S. N. Murty
      Abstract: The Southwest Tropical Indian Ocean (SWTIO) is a key region for air‐sea interaction. Tropical cyclones (TCs) regularly form over the SWTIO and subsurface ocean variability influences the cyclogenesis of this region. Tropical cyclone days for this region span from November through April, and peak in January and February during austral summer. Past research provides evidence for more tropical cyclone days over the SWTIO during austral summer (December to June) with a deep thermocline ridge than in austral summer with a shallow thermocline ridge. We have analyzed the Argo temperature data and HYbrid Coordinate Ocean Model (HYCOM) outputs while focusing on the austral summer of 2012/2013 (a positive Indian Ocean Dipole (IOD) year and neutral El Niño Southern Oscillation (ENSO) year) when seven named tropical cyclones developed over the SWTIO region. This study reveals that the climatic events like the IOD and ENSO influence the cyclonic activity and number of TC days over the SWTIO. We ascertain that the IOD events have linkages with the Barrier Layer Thickness (BLT) in the SWTIO region through propagating Rossby waves, and further show that the BLT variability influences the cyclonic activity in this region. This article is protected by copyright. All rights reserved.
      PubDate: 2016-08-08T10:12:01.648396-05:
      DOI: 10.1002/2016JC011992
  • Circulation and haline structure of a microtidal bay in the Sea of Japan
           influenced by the winter monsoon and the Tsushima Warm Current
    • Authors: Sachihiko Itoh; Akihide Kasai, Aigo Takeshige, Kei Zenimoto, Shingo Kimura, Keita W. Suzuki, Yoichi Miyake, Tatsuhiro Funahashi, Yoh Yamashita, Yoshiro Watanabe
      Abstract: Mooring and hydrographic surveys were conducted in Tango Bay, a microtidal region of freshwater influence (ROFI) in the Sea of Japan, in order to clarify the circulation pattern in the bay and its driving forces. Monthly mean velocity records at four stations revealed an inflow and outflow at the eastern and northern openings of the bay, respectively, which indicates an anticyclonic circulation across the bay mouth. The circulation was significantly intensified in winter, in accordance with the prevailing NW wind component of the winter monsoon. The anticyclonic circulation at the bay mouth was connected to an estuarine circulation that was evident near the mouth of the Yura River at the bay head. Surface salinity just offshore of the river mouth was closely related to the Yura River discharge, whereas in lower layers the offshore water had a stronger influence on salinity. Prior to a seasonal increase in the Yura River discharge, summer salinity decreased markedly through the water column in Tango Bay, possibly reflecting intrusion of the Changjiang Diluted Water transported by the Tsushima Warm Current. In contrast with the traditional assumption that estuarine circulation is controlled mainly by river discharge and tidal forcing, the circulation in Tango Bay is strongly influenced by seasonal wind and the Tsushima Warm Current. The narrow shelf may be responsible for the strong influence of the Tsushima Warm Current on circulation and water exchange processes in Tango Bay. This article is protected by copyright. All rights reserved.
      PubDate: 2016-08-08T10:11:48.529013-05:
      DOI: 10.1002/2015JC011441
  • Sub‐surface circulation and mesoscale variability in the Algerian
           sub‐basin from altimeter‐derived eddy trajectories
    • Abstract: Algerian eddies are the strongest and largest propagating mesoscale structures in the Western Mediterranean Sea. They have a large influence on the mean circulation, water masses and biological processes. Over 20 years of satellite altimeter data have been analyzed to characterize the propagation of these eddies using automatic detection methods and cross‐correlation analysis. We found that, on average, Algerian eddy trajectories form two sub‐basin scale anticlockwise gyres that coincide with the two Algerian gyres which were described in the literature as the barotropic circulation in the area. This result suggests that altimetry sea surface observations can provide information on sub‐surface currents and their variability through the study of the propagation of deep mesoscale eddies in semi‐enclosed seas. The analysis of eddy sea level anomalies along the mean pathways reveals three preferred areas of formation. Eddies are usually formed at a specific time of the year in these areas, with a strong interannual variability over the last 20 years. This article is protected by copyright. All rights reserved.
      PubDate: 2016-08-08T10:11:29.07074-05:0
      DOI: 10.1002/2016JC011760
  • Hydrodynamic condition and suspended sediment diffusion in the Yellow Sea
           and East China Sea
    • Authors: Guangxue Li, Lulu Qiao, Ping Dong, Yanyan Ma, Jishang Xu, Shidong Liu, Yong Liu, Jianchao Li, Pin Li, Dong Ding, Nan Wang, Dada Olusegun A; Ling Liu
      Abstract: Based on monthly‐averaged current, temperature and salinity, we analyzed the changes of suspended sediment concentration (SSC) and the relationship with the warm current, coastal current and cold water mass (CWM) in the East China Seas (ECSs). The result shows that the coastal current and surface diluted water are the route for transporting suspended sediment. The Kuroshio and its derived warm current branches play the important role of the continental shelf circulation system and control the diffusion of suspended sediment. High SSC has been mainly concentrated in coastal current and CWM. Two sedimentary dynamic patterns have been identified. The winter‐half‐year pattern lasts almost seven months. The coastal currents off the Shandong Peninsula, northern Jiangsu, Zhejiang‐Fujian coast are the main routes for diffusion and deposition of the suspended sediment from the Yellow River and Changjiang River. The summer‐half‐year pattern is characterized by the well‐developed CWM. All CWMs have a unique function to trap suspended sediment under the thermocline due to weakening tidal current and residual current there. These CWMs in the Yellow Sea (YS) and north ECS are connected together. The layer above the thermocline is characterized by diluted water with low salinity, high temperature. Suspended sediment can be transported into the Okinawa Trough and the South Korea coast during this period. A strong eddy always occur nearby the Kuroshio bend at northeast Taiwan, which has promoted the exchange between the ECS shelf and Okinawa Trough, and the development of the shelf edge current and Taiwan warm current (TWC). This article is protected by copyright. All rights reserved.
      PubDate: 2016-08-06T03:20:37.024816-05:
      DOI: 10.1002/2015JC011442
  • The plumbing of the global biological pump: Efficiency control through
           leaks, pathways, and timescales
    • Abstract: We systematically quantify the pathways and timescales that set the efficiency, Ebio, of the global biological pump by applying Green‐function‐based diagnostics to a data‐assimilated phosphorus cycle embedded in a jointly assimilated ocean circulation. We consider “bio pipes” that consist of phosphorus paths that connect specified regions of last biological utilization with regions where regenerated phosphate first reemerges into the euphotic zone. The dominant bio pipes connect the Eastern Equatorial Pacific (EEqP) and Equatorial Atlantic to the Southern Ocean ((21±3)% of Ebio), as well as the Southern Ocean to itself ((15±3)% of Ebio). The bio pipes with the largest phosphorus flow rates connect the EEqP to itself and the subantarctic Southern Ocean to itself. The global mean sequestration time of the biological pump is 130±$70yr, while the sequestration time of the bio pipe from anywhere to the Antarctic region of the Southern Ocean is 430±30yr. The distribution of the phosphorus flowing within a given bio pipe is quantified by its transit‐time partitioned path density. For the largest bio pipes, ∼1/7 of their phosphorus is carried by thermocline paths with transit times less than ∼300‐400yr, while ∼4/7 of their phosphorus is carried by abyssal paths with transit times exceeding ∼700yr. The path density reveals that Antarctic Intermediate Water carries about a third of the regenerated phosphate last utilized in the EEqP that is destined for the Southern Ocean euphotic zone. The Southern Ocean is where (62±2)% of the regenerated inventory and (69±1)% of the preformed inventory first reemerge into the euphotic zone. This article is protected by copyright. All rights reserved.
      PubDate: 2016-08-06T03:17:09.575556-05:
      DOI: 10.1002/2016JC011821
  • Evaluating the use of 1d transit time distributions to infer the mean
           state and variability of oceanic ventilation
    • Authors: Andrew E. Shao; Sabine Mecking, LuAnne Thompson, Rolf E. Sonnerup
      Abstract: An offline tracer transport model transport is used to simulate chlorofluorocarbon (CFCs), sulfur hexafluoride (SF6), oxygen, ideal age, and model transit time distributions (TTDs) to evaluate how well tracers can be used to constrain both the mean state and variability of oceanic ventilation. Using climatological transports, the two‐parameter 1d inverse Gaussian approximation of the model TTD is found to be an adequate representation of ventilation pathways within {the parts of the subtropical gyres with simple ventilation dynamics}, but a poor approximation for regions with large gradients in ideal age (i.e. near the base of the thermocline and the continental boundaries). TTDs inferred from CFC‐12 and SF6 using a Peclet number based lookup table approach yield poor representations of the model TTD with a consistent bias towards ventilation being strongly dominated by along‐isopycnal diffusion. In a run with variable circulation, ideal age is used to track changes in thermocline ventilation. Variability in both apparent oxygen utilization (AOU) and tracer‐inferred TTD mean ages inferred using CFC‐12 (assuming fixed Peclet number) and dual tracers (SF6 and CFC‐12) are well‐correlated to ideal age variability in most of the thermocline. Changes in AOU are correlated with ideal age variability in even more regions compared to the TTD ages both horizontally and vertically down to intermediate depths. Generally when changes in TTD mean age and AOU agreed in sign, correlations of both with ideal age changes were positive indicating the usefulness of tracers in diagnosing ventilation changes. This article is protected by copyright. All rights reserved.
      PubDate: 2016-08-06T03:14:14.535536-05:
      DOI: 10.1002/2016JC011900
  • Bomb‐produced radiocarbon in the western tropical Pacific Ocean‐Guam
           coral reveals operation‐specific signals from the Pacific Proving
    • Authors: Allen H. Andrews; Ryuji Asami, Yasufumi Iryu, Donald R. Kobayashi, Frank Camacho
      Abstract: High‐resolution radiocarbon (14C) analyses on a coral core extracted from Guam, a western tropical Pacific island, revealed a series of early bomb‐produced 14C spikes. The typical marine bomb 14C signal—phase lagged and attenuated relative to atmospheric records—is present in the coral and is consistent with other regional coral records. However, 14C levels well above what can be attributed to air‐sea diffusion alone punctuate this pattern. This anomaly was observed in other Indo‐Pacific coral records, but the Guam record is unmatched in magnitude and temporal resolution. The Guam coral Δ14C record provided three spikes in 1954–55, 1956–57, and 1958–59 that are superimposed on a normal 14C record. Relative to mean pre‐bomb levels, the first peak rises an incredible ∼700‰ and remained elevated for ∼1.2 years. A follow up assay with finer resolution increased the peak by ∼300‰. Subsequent spikes were less intense with a rise of ∼35‰ and ∼70‰. Each can be linked to thermonuclear testing in the Pacific Proving Grounds at Bikini and Enewetak atolls in Operations Castle (1954), Redwing (1956), and Hardtack I (1958). These 14C signals can be explained by vaporization of coral reef material in the nuclear fireball, coupled with neutron activation of atmospheric nitrogen (14C production), and subsequent absorption of 14CO2 to form particulate carbonates of close‐in fallout. The lag time in reaching Guam and other coral records abroad was tied to ocean surface currents and modeling provided validation of 14C arrival observations. This article is protected by copyright. All rights reserved.
      PubDate: 2016-08-06T03:10:35.620395-05:
      DOI: 10.1002/2016JC012043
  • Mixing and dissipation in a geostrophic buoyancy‐driven circulation
    • Authors: Catherine A. Vreugdenhil; Bishakhdatta Gayen, Ross W. Griffiths
      Abstract: Turbulent mixing and energy dissipation have important roles in the global circulation but are not resolved by ocean models. We use direct numerical simulations of a geostrophic circulation, resolving turbulence and convection, to examine the rates of dissipation and mixing. As a starting point, we focus on circulation in a rotating rectangular basin forced by a surface temperature difference but no wind stress. Emphasis is on the geostrophic regime for the horizontal circulation, but also on the case of strong buoyancy forcing (large Rayleigh number), which implies a turbulent convective boundary layer. The computed results are consistent with existing scaling theory that predicts dynamics and heat transport dependent on the relative thicknesses of thermal and Ekman boundary layers, hence on the relative roles of buoyancy and rotation. Scaling theory is extended to describe the volume‐integrated rate of mixing, which is proportional to heat transport and decreases with increasing rotation rate or decreasing temperature difference. In contrast, viscous dissipation depends crucially on whether the thermal boundary layer is laminar or turbulent, with no direct Coriolis effect on the turbulence unless rotation is extremely strong. For strong forcing, in the geostrophic regime, the mechanical energy input from buoyancy goes primarily into mixing rather than dissipation. For a buoyancy‐driven circulation in a basin comparable to the North Atlantic we estimate that the total rate of mixing accounts for over $95$\% of the mechanical energy supply, implying that buoyancy is an efficient driver of mixing in the oceans. This article is protected by copyright. All rights reserved.
      PubDate: 2016-08-03T03:07:57.310009-05:
      DOI: 10.1002/2016JC011691
  • Near‐bed hydrodynamics and turbulence below a large‐scale plunging
           breaking wave over a mobile barred bed profile
    • Abstract: Detailed measurements are presented of velocities and turbulence under a large‐scale regular plunging breaking wave in a wave flume. Measurements were obtained at 12 cross‐shore locations around a mobile medium‐sand breaker bar. They focused particularly on the dynamics of the wave bottom boundary layer (WBL) and near‐bed turbulent kinetic energy (TKE), measured with an Acoustic Concentration and Velocity Profiler (ACVP). The breaking process and outer‐flow hydrodynamics are in agreement with previous laboratory and field observations of plunging waves, including a strong undertow in the bar trough region. The WBL thickness matches with previous studies at locations offshore from the bar crest, but it increases near the breaking‐wave plunge point. This relates possibly to breaking‐induced TKE or to the diverging flow at the shoreward slope of the bar. Outer‐flow TKE is dominated by wave breaking and exhibits strong spatial variation with largest TKE above the breaker bar crest. Below the plunge point, breaking‐induced turbulence invades the WBL during both crest and trough half‐cycle. This results in an increase in the time‐averaged TKE in the WBL (with a factor 3) and an increase in peak onshore and offshore near‐bed Reynolds stresses (with a factor 2) from shoaling to breaking region. A fraction of locally‐produced TKE is advected offshore over a distance of a few meters to shoaling locations during the wave trough phase, and travels back onshore during the crest half‐cycle. The results imply that breaking‐induced turbulence, for large‐scale conditions, may significantly affect near‐bed sediment transport processes. This article is protected by copyright. All rights reserved.
      PubDate: 2016-08-03T03:07:53.580155-05:
      DOI: 10.1002/2016JC011909
  • Internal‐tide Interactions with the Gulf Stream and Middle Atlantic
           Bight Shelfbreak Front
    • Authors: Samuel M. Kelly; Pierre F. J. Lermusiaux
      Abstract: Internal tides in the Middle Atlantic Bight region are found to be noticeably influenced by the presence of the shelfbreak front and the Gulf Stream, using a combination of observations, equations, and data‐driven model simulations. To identify the dominant interactions of these waves with subtidal flows, vertical‐mode momentum and energy partial differential equations are derived for small‐amplitude waves in a horizontally‐ and vertically‐sheared mean flow and in a horizontally‐ and vertically‐variable density field. First, the energy balances are examined in idealized simulations with mode‐1 internal tides propagating across and along the Gulf Stream. Next, the fully‐nonlinear dynamics of regional tide‐mean‐flow interactions are simulated with a primitive‐equation model, which incorporates realistic summer‐mesoscale features and atmospheric forcing. The shelfbreak front, which has horizontally‐variable stratification, decreases topographic internal‐tide generation by about 10% and alters the wavelengths and arrival times of locally‐generated mode‐1 internal tides on the shelf and in the abyss. The (sub)‐mesoscale variability at the front and on the shelf, as well as the summer stratification itself, also alter internal‐tide propagation. The Gulf Stream produces anomalous regions of (20 mW m−2) mode‐1 internal‐tide energy‐flux divergence, which are explained by tide‐mean‐flow terms in the mode‐1 energy balance. Advection explains most tide‐mean‐flow interaction, suggesting that geometric wave theory explains mode‐1 reflection and refraction at the Gulf Stream. Geometric theory predicts that offshore‐propagating mode‐1 internal tides that strike the Gulf Stream at oblique angles (more than thirty degrees from normal) are reflected back to the coastal ocean, preventing their radiation into the central North Atlantic. This article is protected by copyright. All rights reserved.
      PubDate: 2016-08-03T03:07:48.494852-05:
      DOI: 10.1002/2016JC011639
  • Seasonal variation of speed and width from kinematic parameters of
           mode‐1 nonlinear internal waves in the northeastern East China Sea
    • Authors: Chomgun Cho; SungHyun Nam, H.C. Song
      Abstract: To better understand the statistical and theoretical characteristics of nonlinear internal waves (NLIWs) in the broad continental shelf of the northeastern East China Sea (ECS), historical hydrographic data collected over 50 years between 1962 and 2011 are analyzed to calculate monthly climatology. Based on KdV and extended KdV models under the two‐layer approximation (i.e., mode‐1 NLIWs), the monthly climatology for propagating speed and characteristic width is constructed, ranging from 0.8 to 1.2 m s−1 and from O(102) to O(103) m, respectively. The result is consistent with a few previous in situ observations in the region. When NLIWs originating in the southeastern slope area approach the shallower regime (northwestward propagation), they propagate more slowly with neither break nor extinction, but with a shorter width, since both the Iribarren and Ostrovsky numbers are small (Ir ≪ 0.45 and Os ≪ 1, respectively). Limitations of the two‐layered KdV‐type models are discussed (e.g., an importance of mode‐2 waves) in the context of occasional extension of the low‐salinity Changjiang Discharged Water onto the area, which implies distinct effects on the kinematic parameters of NLIWs in the ECS. This article is protected by copyright. All rights reserved.
      PubDate: 2016-08-02T10:20:52.388852-05:
      DOI: 10.1002/2016JC012035
  • Prediction of M2 tidal surface currents by a global baroclinic ocean model
           and evaluation using observed drifter trajectories
    • Authors: Tsubasa Kodaira; Keith R. Thompson, Natacha B. Bernier
      Abstract: Global M2 tidal surface currents are predicted using a global baroclinic ocean model with horizontal grid spacing of 1/12° and 19 z‐levels in the vertical. After first showing the predicted tidal elevations are in reasonable agreement with observations made by bottom pressure recorders and altimeters, the predicted tidal surface currents are evaluated by comparing them with independent estimates based on observed drifter trajectories. Both predicted and observed tidal surface currents can exceed 0.1 m s– 1 in the deep ocean. Internal tides are shown to make a significant contribution to the predicted tidal surface currents. Phase locking of the surface and internal tides causes spatial changes in the predicted tidal surface currents that vary with approximately the same wavenumber as that of the lowest mode internal tide. Qualitatively similar, small‐scale variations are also detected in the observed estimates but the variations do not line up exactly with the predictions. Possible explanations for the mismatch are given. The seasonal variation of M2 tidal surface currents, and the energy conversion rate from surface to internal tides, is also predicted by initializing, and restoring, the model to an observed seasonal climatology of temperature and salinity. Compared to tidal elevation, the seasonal change of tidal surface current can be large (order 10% for each hemisphere). It is caused by seasonal variations in the vertical structure of the baroclinic modes and the energy conversion rate. In the vicinity of major bathymetric features, the seasonal variation of second and higher order modes can be much larger (up to 50%). This article is protected by copyright. All rights reserved.
      PubDate: 2016-08-02T10:20:50.714952-05:
      DOI: 10.1002/2015JC011549
  • Open boundary conditions for tidally and subtidally forced circulation in
           a limited‐area coastal model using the regional ocean modeling system
    • Authors: Zhiqiang Liu; Jianping Gan
      Abstract: In limited‐area ocean models, open boundary conditions (OBCs) often create dynamic inconsistencies and perform poorly in resolving tidal or subtidal flow when both forces exist. Orlanski‐type radiation OBCs are reasonably efficient at treating the subtidally forced flow, and Flather‐type OBCs are commonly adapted for the tidally forced flow. However, neither of them performs well when tidal and subtidal forces simultaneously drive the flows. We have developed a novel OBC that integrates the active OBC in Gan and Allen [2005] and a Flather‐type OBC. This new OBC accommodates the concurrent Tidal and Subtidal (TST) forcing, and the respective tidal or subtidal forcing, at the open boundary of a limited‐area model. This new TST‐OBC treats the tidal component with a Flather‐type OBC, and it separates subtidal barotropic and baroclinic components into local (forced) and global (unforced) components. Then an unforced Orlanski‐type OBC can be applied to the global part. We applied the TST‐OBC to all model variables to reduce dynamic inconsistence. Using the Regional Ocean Modeling System, we applied the TST‐OBC to the shallow East China Sea shelf where strong tidal and subtidal forces over complex topography govern the circulation. Our numerical experiments and analyses suggest that the TST‐OBC was robust for both concurrent tidal‐subtidal forcing and solely tidal or subtidal forcing at the open boundary. It reduced spurious energy reflection, and, overall, it performed better than an Orlanski‐type or Flather‐type OBC in reproducing realistic tidal and subtidal shelf circulation. This article is protected by copyright. All rights reserved.
      PubDate: 2016-08-02T10:20:46.776689-05:
      DOI: 10.1002/2016JC011975
  • The Draupner wave: A fresh look and the emerging view
    • Abstract: Using the new high‐resolution operational model of ECMWF, we revisit the storm during which the Draupner freak wave of January 1, 1995 was recorded. The modeling system gives a realistic evolution of the storm highlighting the crucial role played by the southward propagating polar low in creating the extreme wave conditions present at the time the freak wave was recorded. We also discuss the predictability of the meteorological event. The hindcast wave spectra allow a new analysis of the probability of occurrence of the Draupner wave that we analyze not only in time at a specific position, but also in space. This leads us to discuss how exceptional the so‐called freak waves really are. For a given sea state, as characterized by the significant wave height, they are namely part of the reality of the ocean, the key point being the probability of encountering them. In this respect, the often considered record at a specific location can be misleading because the probability of detecting a freak wave must be considered both in space and time. This article is protected by copyright. All rights reserved.
      PubDate: 2016-08-02T10:15:25.930224-05:
      DOI: 10.1002/2016JC011649
  • Multifrequency seismic detectability of seasonal thermoclines assessed
           from ARGO data
    • Abstract: Seismic Oceanography is a developing research topic where new acoustic methods allow high‐resolution teledetection of the thermohaline structure of the ocean. First implementations to study the Ocean Surface Boundary Layer have recently been achieved but remain very challenging due to the weakness and shallowness of such seismic reflectors. In this article, we develop a multifrequency seismic analysis of hydrographic datasets collected in a seasonally stratified midlatitude shelf by ARGO network floats to assess the detectability issue of shallow thermoclines. This analysis, for which sensitivity to the data reduction scheme used by ARGO floats for the transmission of the profiles is discussed, allows characterizing both the depth location and the frequency dependency of the dominant reflective feature of such complex structures. This approach provides the first statistical distribution of the range of variability of the frequency‐dependent seismic reflection amplitude of the midlatitude seasonal thermoclines. We introduce a new parameter to quantify the overall capability of a multichannel seismic setup, including the source strength, the fold and the ambient noise level, to detect shallow thermoclines. Seismic source signals are approximated by Ricker wavelets, providing quantitative guidelines to help in the design of seismic experiments targeting such oceanic reflectors. For shallow midlatitude seasonal thermoclines, we show that the detectability is optimal for seismic peak frequencies between 200 and 400 Hz: this means that airgun and Sparker sources are not well suited and that significant improvements of source devices will be necessary before seismic imaging of OSBL structures can be reliably attempted. This article is protected by copyright. All rights reserved.
      PubDate: 2016-08-02T10:10:39.556956-05:
      DOI: 10.1002/2016JC011793
  • Surface mixed layer deepening through wind shear alignment in a seasonally
           stratified shallow sea
    • Authors: B.J. Lincoln; T.P. Rippeth, J.H. Simpson
      Abstract: Inertial Oscillations are a ubiquitous feature of the surface ocean. Here we combine new observations with a numerical model to investigate the role of inertial oscillations in driving deepening of the surface mixed layer in a seasonally stratified sea. Observations of temperature and current structure, from a mooring in the Western Irish Sea, reveal episodes of strong currents (>0.3ms−1) lasting several days, resulting in enhanced shear across the thermocline. Whilst the episodes of strong currents are coincident with windy periods, the variance in the shear is not directly related to the wind stress. The shear varies on a sub‐inertial timescale with the formation of shear maxima lasting several hours occurring at the local inertial period of 14.85h. These shear maxima coincide with the orientation of the surface current being at an angle of approximately 90° to the right of the wind direction. Observations of the water column structure during windy periods reveal deepening of the surface mixed layer in a series of steps which coincide with a period of enhanced shear. During the periods of enhanced shear gradient Richardson number estimates indicate Ri−1≥4 at the base of the surface mixed layer, implying the deepening as a result of shear instability. A one‐dimensional vertical exchange model successfully reproduces the magnitude and phase of the shear spikes as well as the step like deepening. The observations and model results therefore identify the role of wind‐shear alignment as a key entrainment mechanism driving surface mixed layer deepening in a shallow, seasonally stratified sea. This article is protected by copyright. All rights reserved.
      PubDate: 2016-07-25T18:05:37.654515-05:
      DOI: 10.1002/2015JC011382
  • Early ice retreat and ocean warming may induce copepod biogeographic
           boundary shifts in the Arctic Ocean
    • Authors: Zhixuan Feng; Rubao Ji, Robert G. Campbell, Carin J. Ashjian, Jinlun Zhang
      Abstract: Early ice retreat and ocean warming are changing various facets of the Arctic marine ecosystem, including the biogeographic distribution of marine organisms. Here, an endemic copepod species, Calanus glacialis, was used as a model organism, to understand how and why Arctic marine environmental changes may induce biogeographic boundary shifts. A copepod individual‐based model was coupled to an ice‐ocean‐ecosystem model to simulate temperature‐ and food‐dependent copepod life history development. Numerical experiments were conducted for two contrasting years: a relatively cold and normal sea ice year (2001) and a well‐known warm year with early ice retreat (2007). Model results agreed with commonly known biogeographic distributions of C. glacialis, which is a shelf/slope species and cannot colonize the vast majority of the central Arctic basins. Individuals along the northern boundaries of this species' distribution were most susceptible to reproduction timing and early food availability (released sea ice algae). In the Beaufort, Chukchi, East Siberian, and Laptev Seas where severe ocean warming and loss of sea ice occurred in summer 2007, relatively early ice retreat, elevated ocean temperature (about 1‐2°C higher than 2001), increased phytoplankton food, and prolonged growth season created favorable conditions for C. glacialis development and caused a remarkable poleward expansion of its distribution. From a pan‐Arctic perspective, despite the great heterogeneity in the temperature and food regimes, common biogeographic zones were identified from model simulations, thus allowing a better characterization of habitats and prediction of potential future biogeographic boundary shifts. This article is protected by copyright. All rights reserved.
      PubDate: 2016-07-25T04:19:49.632712-05:
      DOI: 10.1002/2016JC011784
  • Large flux of iron from the Amery Ice Shelf marine ice to Prydz Bay, East
    • Abstract: The Antarctic continental shelf supports a high level of marine primary productivity and is a globally important carbon dioxide (CO2) sink through the photosynthetic fixation of CO2 via the biological pump. Sustaining such high productivity requires a large supply of the essential micronutrient iron (Fe); however, the pathways for Fe delivery to these zones vary spatially and temporally. Our study is the first to report a previously unquantified source of concentrated bio‐available Fe to Antarctic surface waters. We hypothesize that Fe derived from subglacial processes is delivered to euphotic waters through the accretion (Fe storage) and subsequent melting (Fe release) of a marine‐accreted layer of ice at the base of the Amery Ice Shelf (AIS). Using satellite‐derived Chlorophyll‐a data, we show that the soluble Fe supplied by the melting of the marine ice layer is an order of magnitude larger than the required Fe necessary to sustain the large annual phytoplankton bloom in Prydz Bay. Our finding of high concentrations of Fe in AIS marine ice and recent data on increasing rates of ice shelf basal melt in many of Antarctica's ice shelves [Paolo et al., 2015] should encourage further research into glacial and marine sediment transport beneath ice shelves and their sensitivity to current changes in basal melt. Currently, the distribution, volume and Fe concentration of Antarctic marine ice is poorly constrained. This uncertainty, combined with variable forecasts of increased rates of ice shelf basal melt, limits our ability to predict future Fe supply to Antarctic coastal waters. This article is protected by copyright. All rights reserved.
      PubDate: 2016-07-20T10:26:03.355568-05:
      DOI: 10.1002/2016JC011687
  • The influence of Pacific equatorial water on fish diversity in the
           southern California Current System
    • Authors: Sam McClatchie; Andrew R. Thompson, Simone R. Alin, Samantha Siedlecki, William Watson, Steven J. Bograd
      Abstract: The California Undercurrent transports Pacific Equatorial Water (PEW) into the Southern California Bight from the eastern tropical Pacific Ocean. PEW is characterized by higher temperatures and salinities, with lower pH, representing a source of potentially corrosive (aragonite, Ω
      PubDate: 2016-07-20T10:25:55.525557-05:
      DOI: 10.1002/2016JC011672
  • Asymmetric oceanic response to a hurricane: Deepwater observations during
           Hurricane Isaac
    • Authors: Laura J. Spencer; Steven F. DiMarco, Zhankun Wang, Joseph J. Kuehl, David A. Brooks
      Abstract: The eye of Hurricane Isaac passed through the center of an array of six deepwater water‐column current meter moorings deployed in the northern Gulf of Mexico. The trajectory of the hurricane provided for a unique opportunity to quantify differences in the full water‐column oceanic response to a hurricane to the left and right of the hurricane trajectory. Prior to the storm passage, relative vorticity on the right side of the hurricane was strongly negative; while on the left, relative vorticity was positive. This resulted in an asymmetry in the near inertial frequencies oceanic response at depth and horizontally. A shift in the response to a slightly larger inertial frequencies ∼1.11f was observed and verified by theory. Additionally, the storm passage coincided with an asymmetric change in relative vorticity in the upper 1000 m, which persisted for ∼15 inertial periods. Vertical propagation of inertial energy was estimated at 29 m/day, while horizontal propagation at this frequency was approximately 5.7 km/day. Wavelet analysis showed two distinct sub‐inertial responses, one with a period of 2‐5 days and another with a period of 5‐12 days. Analysis of the sub‐inertial bands reveals that the spatial and temporal scales are shorter and less persistent than the near‐inertial variance. As the array is geographically located near the site of the Deepwater Horizon oil spill, the spatial and temporal scales of response have significant implications for the fate, transport, and distribution of hydrocarbons following a deepwater spill event. This article is protected by copyright. All rights reserved.
      PubDate: 2016-07-18T03:57:13.921395-05:
      DOI: 10.1002/2015JC011560
  • Impact of climate variability on the tidal oceanic magnetic signal ‐ a
           model based sensitivity study
    • Authors: J. Saynisch; J. Petereit, C. Irrgang, A. Kuvshinov, M. Thomas
      Abstract: ESA's satellite magnetometer mission Swarm is supposed to lower the limit of observability for oceanic processes. While periodic magnetic signals from ocean tides are already detectable in satellite magnetometer observations, changes in the general ocean circulation are yet too small or irregular for a successful separation. An approach is presented that utilizes the good detectability of tidal magnetic signals to detect changes in the oceanic electric conductivity distribution. Ocean circulation, tides and the resultant magnetic fields are calculated with a global general ocean circulation model coupled to a 3D electromagnetic induction model. For the decay of the meridional overturning circulation, as an example, the impact of climate variability on tidal oceanic magnetic signals is demonstrated. Total overturning decay results in anomalies of up to 0.7 nT in the radial magnetic M2 signal at sea level. The anomalies are spatially heterogeneous and reach in extended areas 30% or more of the unperturbed tidal magnetic signal. The anomalies should be detectable in long time series from magnetometers on land or at the ocean bottom. The anomalies at satellite height (430 km) reach 0.1 nT and pose a challenge for the precision of the Swarm mission. Climate variability induced deviations in the tide system (e.g., tidal velocities and phases) are negligible. Changes in tidal magnetic fields are dominated by changes in sea water salinity and temperature. Therefore, it is concluded that observations of tidal magnetic signals could be used as a tool to detect respective state changes in the ocean. This article is protected by copyright. All rights reserved.
      PubDate: 2016-07-18T03:40:30.059138-05:
      DOI: 10.1002/2016JC012027
  • Impact of the Antarctic bottom water formation on the Weddell G/yre and
           its northward propagation characteristics in GFDL model
    • Authors: Liping Zhang; Thomas L. Delworth
      Abstract: The impact of Antarctic bottom water (AABW) formation on the Weddell Gyre and its northward propagation characteristics are studied using a 4000‐yr long control run of the GFDL CM2.1 model as well as sensitivity experiments. In the control run, the AABW cell and Weddell Gyre are highly correlated when the AABW cell leads the Weddell Gyre by several years, with an enhanced AABW cell corresponding to a strengthened Weddell Gyre and vice versa. An additional sensitivity experiment shows that the response of the Weddell Gyre to AABW cell changes is primarily attributed to interactions between the AABW outflow and ocean topography, instead of the surface wind stress curl and freshwater anomalies. As the AABW flows northward, it encounters topography with steep slopes that induce strong downwelling and negative bottom vortex stretching. The anomalous negative bottom vortex stretching induces a cyclonic barotropic streamfunction over the Weddell Sea, thus leading to an enhanced Weddell Gyre. The AABW cell variations in the control run have significant meridional coherence in density space. Using passive dye tracers, it is found that the slow propagation of AABW cell anomalies south of 35oS corresponds to the slow tracer advection time scale. The dye tracers escape the Weddell Sea through the western limb of the Weddell Gyre and then go northwestward to the Argentine Basin through South Sandwich Trench and Georgia Basin. This slow advection by deep ocean currents determines the AABW cell propagation speed south of 35oS. North of 35oS the propagation speed is determined both by advection in the deep western boundary current and through Kelvin waves. This article is protected by copyright. All rights reserved.
      PubDate: 2016-07-15T04:16:19.028248-05:
      DOI: 10.1002/2016JC011790
  • A snapshot of internal waves and hydrodynamic instabilities in the
           Southern Bay of Bengal
    • Abstract: Measurements conducted in the southern Bay of Bengal (BoB) as a part of the ASIRI‐EBoB Program portray the characteristics of high‐frequency internal waves in the upper pycnocline as well as the velocity structure with episodic events of shear instability. A 20‐hour time series of CTD, ADCP and acoustic backscatter profiles down to 150 m as well as temporal CTD measurements in the pycnocline at z = 54 m were taken to the east of Sri Lanka. Internal waves of periods ∼ 10 to 40 min were recorded at all depths below a shallow (∼ 20 – 30 m) surface mixed layer in the background of an 8‐m amplitude internal tide. The absolute values of vertical displacements associated with high‐frequency waves followed the Nakagami distribution with a median value of 2.1 m and a 95% quintile 6.5 m. The internal wave amplitudes are normally distributed. The tails of the distribution deviate from normality due to episodic high‐amplitude displacements. The sporadic appearance of internal waves with amplitudes exceeding ∼ 5 m usually coincided with patches of low Richardson numbers, pointing to local shear instability as a possible mechanism of internal‐wave induced turbulence. The probability of shear instability in the summer BoB pycnocline based on an exponential distribution of the inverse Richardson number, however, appears to be relatively low, not exceeding 4% for Ri 
      PubDate: 2016-07-15T04:16:15.146988-05:
      DOI: 10.1002/2016JC011697
  • Timing and regional patterns of snowmelt on Antarctic sea ice from passive
           microwave satellite observations
    • Authors: Stefanie Arndt; Sascha Willmes, Wolfgang Dierking, Marcel Nicolaus
      Abstract: An improved understanding of the temporal variability and the spatial distribution of snowmelt on Antarctic sea ice is crucial to better quantify atmosphere‐ice‐ocean interactions, in particular sea‐ice mass and energy budgets. It is therefore important to understand the mechanisms that drive snowmelt, both at different times of the year and in different regions around Antarctica. In this study, we combine diurnal brightness temperature differences (dTB(37GHz)) and ratios (TB(19GHz)/TB(37GHz)) to detect and classify snowmelt processes. We distinguish temporary snowmelt from continuous snowmelt to characterize dominant melt patterns for different Antarctic sea ice regions from 1988/89 to 2014/15. Our results indicate four characteristic melt types. On average, 38.9±6.0% of all detected melt events are diurnal freeze‐thaw cycles in the surface snow layer, characteristic of temporary melt (Type A). Less than 2% reveal immediate continuous snowmelt throughout the snowpack, i.e. strong melt over a period of several days (Type B). In 11.7±4.0%, Type A and B take place consecutively (Type C), and for 47.8±6.8% no surface melt is observed at all (Type D). Continuous snowmelt is primarily observed in the outflow of the Weddell Gyre and in the northern Ross Sea, usually 17 days after the onset of temporary melt. Comparisons with Snow Buoy data suggest that also the onset of continuous snowmelt does not translate into changes in snow depth for a longer period but might rather affect the internal stratigraphy and density structure of the snowpack. Considering the entire data set, the timing of snowmelt processes does not show significant temporal trends. This article is protected by copyright. All rights reserved.
      PubDate: 2016-07-14T09:35:28.432404-05:
      DOI: 10.1002/2015JC011504
  • Anomalous Java cooling at the initiation of positive IOD events
    • Authors: Andrew S. Delman; Janet Sprintall, Julie L. McClean, Lynne D. Talley
      Abstract: Anomalous sea surface temperature (SST) cooling south of Java, initiated during May‐July, is an important precursor to positive Indian Ocean Dipole (pIOD) events. As shown previously, the Java SST anomalies are spatially and temporally coincident with seasonal upwelling induced locally by southeasterly trade winds. However, we confirm earlier findings that interannual variability of the Java cooling is primarily driven by remote wind forcing from coastal Sumatra and the equatorial Indian Ocean (EqIO); we also find an inuence from winds along the Indonesian Throughow. The wind forcing in the EqIO and along coastal Sumatra does not initiate SST cooling locally due to a deep thermocline and thick barrier layer, but can force upwelling Kelvin waves that induce substantial surface cooling once they reach the seasonally shallower thermocline near the coast of Java. Satellite altimetry is used to obtain a Kelvin wave coefficient that approximates Kelvin wave amplitude variations along the equator. All pIOD years in the satellite record have anomalous levels of upwelling Kelvin wave activity along the equator during April‐June, suggesting that upwelling waves during this season are necessary for pIOD event development. However, a change to wind‐forced downwelling Kelvin waves during July‐August can abruptly terminate cool Java SST anomalies and weaken the pIOD event. Upwelling Kelvin wave activity along the equator and wind stress anomalies west of Sumatra are both robust predictors of the IOD index later in the calendar year, while values of the Kelvin wave coefficient are the most reliable predictor of pIOD events specifically. This article is protected by copyright. All rights reserved.
      PubDate: 2016-07-13T10:42:51.050031-05:
      DOI: 10.1002/2016JC011635
  • Float observations of an anticyclonic eddy off Hokkaido
    • Authors: Ryuichiro Inoue; Vincent Faure, Shinya Kouketsu
      Abstract: To clarify the formation process of the salinity minimum in the Kuroshio–Oyashio mixed water region and understand the mechanism of meridional heat transport between the subtropical and subpolar gyres, 16 profiling floats were deployed within a warm‐core anticyclonic eddy off Hokkaido from June 2012 to December 2013. Then, the evolution of an anticyclonic eddy was examined using time series of the water properties. The largest fluctuations in water properties were observed in April and May 2013, when the anticyclonic eddy first moved south to interact with a warm front, then back north. Salinity in the salinity minimum layer increased during the interaction. After the eddy detached from the frontal structure, low‐salinity water was again observed with small intrusive structures, which eventually converged to a smooth zigzag structure in the potential temperature‐salinity diagram, suggesting that both vertical mixing and vertical heaving played a role in the temporal changes observed after the eddy detached from the front. Since the salinity variation during the interaction event was about half the total salinity change during the whole experimental period, the interaction of an eddy with a front might be important for modifying the water properties of the eddy, and, therefore, for the meridional transport of heat and fresh water. This article is protected by copyright. All rights reserved.
      PubDate: 2016-07-13T10:42:36.911384-05:
      DOI: 10.1002/2016JC011698
  • Evolution of a Canada Basin ice‐ocean boundary layer and mixed layer
           across a developing thermodynamically forced marginal ice zone
    • Authors: Shawn G. Gallaher; Timothy P. Stanton, William J. Shaw, Sylvia T. Cole, John M. Toole, Jeremy P. Wilkinson, Ted Maksym, Byongjun Hwang
      Abstract: A comprehensive set of autonomous, ice‐ocean measurements were collected across the Canada Basin to study the summer evolution of the ice‐ocean boundary layer (IOBL) and ocean mixed layer (OML). Evaluation of local heat and freshwater balances and associated turbulent forcing reveals that melt ponds (MP's) strongly influence the summer IOBL‐OML evolution. Areal expansion of MP's in mid‐June start the upper ocean evolution resulting in significant increases to ocean absorbed radiative flux (19 Wm−2 in this study). Buoyancy provided by MP drainage shoals and freshens the IOBL resulting in a 39 MJm−2 increase in heat storage in just 19 days (52% of the summer total). Following MP drainage, a near‐surface fresh layer deepens through shear‐forced mixing to form the summer mixed layer (sML). In late summer, basal melt increases due to stronger turbulent mixing in the thin sML and the expansion of open water areas due in part to wind forced divergence of the sea ice. Thermal heterogeneities in the marginal ice zone (MIZ) upper ocean led to large ocean‐to‐ice heat fluxes (100‐200 Wm−2) and enhanced basal ice melt (3‐6 cm‐day−1), well away from the ice edge. Calculation of the upper ocean heat budget show that local radiative heat input accounted for at least 89% of the observed latent heat losses and heat storage (partitioned 0.77/0.23). These results suggest that the extensive area of deteriorating sea ice observed away from the ice edge during the 2014 season, termed the “thermodynamically forced MIZ,” was driven primarily by local radiative forcing. This article is protected by copyright. All rights reserved.
      PubDate: 2016-07-13T10:18:02.46016-05:0
      DOI: 10.1002/2016JC011778
  • The effects of Antarctic iceberg calving‐size distribution in a
           global climate model
    • Authors: A.A. Stern; A. Adcroft, O. Sergienko
      Abstract: Icebergs calved from the Antarctic continent act as moving sources of freshwater while drifting in the Southern Ocean. The lifespan of these icebergs strongly depends on their original size during calving. In order to investigate the effects (if any) of the calving size of icebergs on the Southern Ocean, we use a coupled general circulation model with an iceberg component. Iceberg calving length is varied from 62 m up to 2.3 km, which is the typical range used in climate models. Results show that increasing the size of calving icebergs leads to an increase in the westward iceberg freshwater transport around Antarctica. In simulations using larger icebergs, the reduced availability of meltwater in the Amundsen and Bellingshausen Seas suppresses the sea‐ice growth in the region. In contrast, the increased iceberg freshwater transport leads to increased sea‐ice growth around much of the East Antarctic coastline. These results suggest that the absence of large tabular icebergs with horizontal extent of tens of kilometers in climate models may introduces systematic biases in sea‐ice formation, ocean temperatures and salinities around Antarctica. This article is protected by copyright. All rights reserved.
      PubDate: 2016-07-11T10:46:05.674207-05:
      DOI: 10.1002/2016JC011835
  • Modeling the influence of deep water application of dispersants on the
           surface expression of oil: A sensitivity study
    • Authors: Jeremy Testa; E. Eric Adams, Elizabeth W. North, Ruoying He
      Abstract: Although the effects of chemical dispersants on oil droplet sizes and ascent speeds are well‐known, the fate and transport of dispersed oil droplets of different sizes under varying hydrodynamic conditions can be difficult to assess with observations alone. We used a particle tracking model to evaluate the effect of changes in droplet sizes due to dispersant application on the short‐term transport and surface expression of oil released under conditions similar to those following the June 3, 2010 riser cutting during the Deepwater Horizon event. We used simulated injections of oil droplets of varying size and number under conditions associated with no dispersant application and with dispersant application at 50% and 100% efficiency. Due to larger droplet sizes in the no‐dispersant scenario, all of the simulated oil reached the surface within 7 hrs, while only 61% and 28% of the oil reached the surface after 12 hs in the 50% and 100% dispersant efficiency cases, respectively. The length of the surface slick after 6 hrs was ∼2 km in the no‐dispersant case whereas there was no surface slick after 6 hrs in the 100% dispersant case, because the smaller oil droplets which resulted from dispersant application had not yet reached the surface. Model results suggest that the application of dispersants at the well head had the following effects: (1) less oil reached the surface in the 6‐12 hrs after application, (2) oil had a longer residence time in the water‐column, and (3) oil was more highly influenced by sub‐surface transport. This article is protected by copyright. All rights reserved.
      PubDate: 2016-07-11T10:40:32.871846-05:
      DOI: 10.1002/2015JC011571
  • Deep ventilation in the Okinawa Trough induced by Kerama Gap overflow
    • Abstract: Near‐bottom water flowing over the Kerama Gap's sills is thought to ventilate the deep water below ∼1100 m depth in the Okinawa Trough and then upwell with 5‒10 years residence time. The present study follows up on this phenomenon, using comprehensive profile data of temperature, salinity, dissolved oxygen, currents and turbulence obtained by intensive shipboard observations performed in June 2013 and June 2014 in the region. Strong near‐bottom sub‐tidal flow with speeds exceeding 0.5 m s−1 was observed within a layer of about 100 m thickness over the western side of the peak of the main sill. Temperature and salinity sections along the Kerama Gap indicated some depressions and overturns of the deep water downstream of the strong overflow, suggesting the existence of breaking internal gravity waves and hydraulic jumps. Associated vertical diffusivities, estimated using the Thorpe scale and the buoyancy frequency, were three to four orders of magnitude larger than typical values observed in the thermocline of the open ocean (∼10−5 m2 s−1). The dissolved oxygen section also indicated strong vertical mixing and associated upwelling with the entrainment of the near‐bottom overflow water into the lower thermocline beneath the Kuroshio in the Okinawa Trough. The present study not only supports the previous conceptual model but also provides new evidence that the Okinawa Trough is an upwelling location where nutrient rich Philippine Sea intermediate water is sucked up into the lower thermocline below the Kuroshio. This article is protected by copyright. All rights reserved.
      PubDate: 2016-07-11T10:40:31.449209-05:
      DOI: 10.1002/2016JC011822
  • Evaluation of the structure function method to compute turbulent
           dissipation within boundary layers using numerical simulations
    • Authors: Aidin Jabbari; Amirreza Rouhi, Leon Boegman
      Abstract: Well‐resolved numerical simulations of turbulent open channel flows are analyzed to evaluate the accuracy of the 2nd order structure function method (SFM) in estimating the rate of dissipation of turbulent kinetic energy within boundary layers. The objective is to assess the variation in the 2/3 Kolmogorov constants due to flow anisotropy with distance from the wall. Comparison of the dissipation calculated directly from the numerical data, with that from the SFM shows that usage of the canonical constants, based on the assumption of local isotropy, can result in considerable error (>50%) in the prediction of dissipation when using the vertical or spanwise velocity components. From the numerically calculated dissipation, optimal Kolmogorov 2/3 constants were obtained and empirical relations, which account for near‐wall effects, were proposed. Usage of the optimal constants will improve estimation of the dissipation rate when the SFM is applied to compute dissipation in geophysical boundary‐layer flows. This article is protected by copyright. All rights reserved.
      PubDate: 2016-07-04T10:36:04.749997-05:
      DOI: 10.1002/2015JC011608
  • A process study of the Adriatic‐Ionian System baroclinic dynamics
    • Authors: M. Reale; A. Crise, R. Farneti, R. Mosetti
      Abstract: The driving mechanisms behind the decadal reversal of the Ionian Sea upper layer circulation recently sparked a considerable discussion in the Mediterranean scientific community. It has been suggested that the reversal can be driven by variations in wind stress curl over the basin, baroclinic dynamics acting within the Adriatic‐Ionian System (AISys) or baroclinic dynamics driven by thermohaline properties at the AISys eastern boundary. Here, we perform numerical simulations in order to assess the relative importance of remote forcings (wind stress, thermohaline fluxes, thermohaline open boundary conditions) on the vorticity and energy budget of the Ionian Sea. A mechanistic understanding of the AISys dynamics is achieved with an approach based on an increasing complexity in the model forcings and domain. Our experiments suggest that wind stress does not play a leading role in the vorticity and energy budgets of the Ionian Sea. Wind stress can reinforce or weaken the circulation but it is not able to reverse its sign. Its role becomes dominant only in the absence of inflows through the Antikythira Strait and Cretan Passage. Instead, reversals in the upper layer circulation of the Ionian Sea take place only in the presence of an active boundary on the Aegean Sea/Levantine Basin side and appear to be correlated with substantial exchanges of Availalble Potential Energy between the two basins (as observed at the end of the Eastern Mediterranean Transient). From an energetic point of view, AISys can be explained therefore only if the role of the Aegean Sea is explicitly condidered. This article is protected by copyright. All rights reserved.
      PubDate: 2016-07-04T10:35:58.603421-05:
      DOI: 10.1002/2016JC011763
  • Predictability of wind‐induced sea surface transport in coastal
    • Authors: A. Cucco; G. Quattrocchi, A. Satta, F. Antognarelli, F. de Biasio, E. Cadau, G. Umgiesser, S. Zecchetto
      Abstract: In this work we investigated the predictability of the wind induced sea surface transport in coastal areas. The wind fields predicted by two state‐of‐the‐art meteorological models, namely ECMWF and SKIRON, were used as forcing for a hydrodynamic and particles tracking model applied to reproduce a set of observed drifters trajectories in a coastal area of the Mediterranean Sea. A set of anemometric data derived by in situ measurements was also adopted as model forcing to reproduce the observed drifter paths. This approach provided a baseline that was used as a reference for evaluating the effects of the predicted wind accuracy on the numerical model solution. The accuracy of the simulation results obtained using, as model forcing, the observed wind data was fair and suitable for most of the operational oceanographic purposes. It decreased when using the wind data predicted by the two meteorological models. In particular, the results obtained using ECMWF data were about 3 times more accurate than the ones obtained using SKIRON ones. The uncertainties were strongly dependent on the range of observed wind speed classes with a different behavior depending on the type of adopted wind data. Finally the amplification of the errors in predicting the sea surface transport generated by the inaccuracies of the predicted wind fields was quantified. This article is protected by copyright. All rights reserved.
      PubDate: 2016-06-27T03:40:58.367251-05:
      DOI: 10.1002/2016JC011643
  • Contributions of different tidal interactions to fortnightly variation in
           tidal duration asymmetry
    • Authors: Wenyun Guo; Dehai Song, Xiao Hua Wang, Pingxing Ding, Jianzhong Ge
      Abstract: The general framework for identifying tidal duration asymmetry proposed by Song et al. (2011) is extended to express fortnightly variability in duration asymmetry. The extended metrics are verified and studied using observed sea‐level data at 481 stations worldwide. The results reveal that fortnightly variability is universal and that duration asymmetry can be stronger during neap tide than during spring tide. The fortnightly variability in duration asymmetry is primarily induced by three types of tidal interactions: interactions within the principal tidal constituents, interactions between high‐frequency and principal tidal constituents, and interactions between long‐period and principal tidal constituents. Among these interactions, the first type is most important at most of the stations and is related to the form number F. The contributions of different interactions can be quantified using their frequencies, amplitudes and phases. Global patterns of the fortnightly variation are illustrated using TOPEX/Poseidon altimetry data. The findings show that remarkable fortnightly variation in the tidal duration asymmetry occurs in most open oceans and is significant around an amphidromic point. The metrics derived in this study can be used to examine any time‐varying characteristics in tidal asymmetry (not limited to duration asymmetry) by selecting a suitable frequency threshold. This article is protected by copyright. All rights reserved.
      PubDate: 2016-06-27T03:30:38.876762-05:
      DOI: 10.1002/2016JC011689
  • Estuarine Circulation vs Tidal Pumping: Sediment Transport in a
           Well‐Mixed Tidal Inlet
    • Abstract: High‐resolution water column observations have been carried out in the Wadden Sea to understand suspended particulate mater (SPM) transport in well‐mixed tidal channels . These observations include more than 4000 consecutive CTD‐, micro‐structure shear and turbidity profiles from a free‐falling micro‐structure probe, as well as velocity data from an ADCP and SPM samples for calibration. A horizontal density gradient was established by a landward temperature gradient built up during an extraordinarily warm and calm spring season. Tidal averaging along σ‐layers (relative depth) provides the first direct observations of along‐channel estuarine circulation in the Wadden Sea, with net inflow near the bottom and outflow near the surface. Increased westerly (up‐estuary) winds during the second part of the campaign weakened and eventually even reversed estuarine circulation and yielded a net barotropic eastward transport. SPM concentrations showed a strong quarter‐diurnal signal with maxima near full flood and full ebb and were generally lower during the calm period and increased during the windy period, mainly due to wave‐related resuspension over nearby inter‐tidal flats. The sediment flux analysis was based on a decomposition of the vertically integrated SPM flux into a barotropic advective component, an estuarine circulation component and a tidal pumping component. As a result, tidal pumping (due to ebb‐dominance weakly seaward) dominated the SPM flux during calm conditions, whereas barotropic advection dominated the strong landward SPM flux during the windy period. Along‐channel estuarine circulation is found to be of minor importance for the net SPM‐transport in such well‐mixed systems. This article is protected by copyright. All rights reserved.
      PubDate: 2016-06-16T03:26:15.426205-05:
      DOI: 10.1002/2016JC011640
  • Estimating the recharge properties of the deep ocean using noble gases and
           helium isotopes
    • Authors: Brice Loose; William J. Jenkins, Roisin Moriarty, Peter Brown, Loic Jullion, Alberto C. Naveira Garabato, Sinhue Torres Valdes, Mario Hoppema, Chris Ballentine, Michael P. Meredith
      Abstract: The distribution of noble gases and helium isotopes in the dense shelf waters of Antarctica reflect the boundary conditions near the ocean surface: air‐sea exchange, sea ice formation and subsurface ice melt. We use a non‐linear least‐squares solution to determine the value of the recharge temperature and salinity, as well as the excess air injection and glacial meltwater content throughout the water column and in the precursor to Antarctic Bottom Water. The noble gas‐derived recharge temperature and salinity in the Weddell Gyre are ‐1.95 °C and 34.95 psu near 5500 m; these cold, salty recharge values are a result of surface cooling as well as brine rejection during sea ice formation in Antarctic polynyas. In comparison, the global value for deep water recharge temperature is ‐0.44 °C at 5500 m, which is 1.5 °C warmer than the southern hemisphere deep water recharge temperature, reflecting the contribution from the north Atlantic. The contrast between northern and southern hemisphere recharge properties highlight the impact of sea ice formation on setting the gas properties in southern sourced deep water. Below 1000 m, glacial meltwater averages 3.5 ‰ by volume and represents greater than 50% of the excess neon and argon found in the water column. These results indicate glacial melt has a non‐negligible impact on the atmospheric gas content of Antarctic Bottom Water. This article is protected by copyright. All rights reserved.
      PubDate: 2016-06-16T03:25:53.371533-05:
      DOI: 10.1002/2016JC011809
  • Issue Information
    • Pages: 5393 - 5396
      PubDate: 2016-09-21T03:53:00.834955-05:
      DOI: 10.1002/jgrc.21408
  • Effects of rotation on turbulent buoyant plumes in stratified environments
    • Pages: 5397 - 5417
      Abstract: We numerically investigate the effects of rotation on the turbulent dynamics of thermally driven buoyant plumes in stratified environments at the large Rossby numbers characteristic of deep oceanic releases. When compared to nonrotating environments, rotating plumes are distinguished by a significant decrease in vertical buoyancy and momentum fluxes leading to lower and thicker neutrally buoyant intrusion layers. The primary dynamic effect of background rotation is the concentration of entraining fluid into a strong cyclonic flow at the base of the plume resulting in cyclogeostrophic balance in the radial momentum equation. The structure of this cyclogeostrophic balance moving upward from the well head is associated with a net adverse vertical pressure gradient producing an inverted hydrostatic balance in the mean vertical momentum budgets. The present simulations reveal that the primary response to the adverse pressure gradient is an off‐axis deflection of the plume that evolves into a robust, organized anticyclonic radial precession about the buoyancy source. The off‐axis evolution is responsible for the weaker inertial overshoots, the increased thickness of lateral intrusion layers, and the overall decrease in the vertical extent of rotating plumes at intermediate Rossby numbers compared to the nonrotating case. For inlet buoyancy forcings and environmental Rossby numbers consistent with those expected in deepwater blowout plumes, the speed of the organized precession is found to be as large as typical oceanic cross‐flow speeds.
      PubDate: 2016-08-05T10:04:25.777717-05:
      DOI: 10.1002/2016JC011737
  • Relevance of infragravity waves in a wave‐dominated inlet
    • Authors: Xavier Bertin; Maitane Olabarrieta
      Pages: 5418 - 5435
      Abstract: This study investigates the relevance of infragravity (IG) waves at Albufeira Lagoon Inlet, a shallow wave‐dominated inlet located on the western coast of Portugal. A field experiment carried out in September 2010 revealed the occurrence of low‐frequency oscillations (i.e., 25–300 s) in water levels and current velocities. While these fluctuations were present over the ebb‐tidal delta along the whole tidal cycle, they only appeared between the beginning of the flood and up to 2 h after high tide inside the lagoon. The XBeach modeling system was applied to Albufeira Lagoon Inlet and reproduced the generation and propagation of IG waves and their blocking during the ebb. This behavior was explained by blocking due to opposing tidal currents reaching 2.5 m s−1 in shallow water depths. Numerical results suggest that the breakpoint mechanism and the long bound wave shoaling mechanisms contributed significantly to the generation of IG waves in the inlet. IG waves induced fluctuations in flood currents inside the lagoon reaching temporarily 100% of their magnitude. The fact that these fluctuations occur mostly at flood and not at ebb could promote flood dominance in the lagoon. This hypothesis will have to be verified, namely under storm wave conditions.
      PubDate: 2016-08-05T09:59:15.065048-05:
      DOI: 10.1002/2015JC011444
  • Forcing of the overturning circulation across a circumpolar channel by
           internal wave breaking
    • Authors: Maria B. Broadbridge; Alberto C. Naveira Garabato, A. J. George Nurser
      Pages: 5436 - 5451
      Abstract: The hypothesis that the impingement of mesoscale eddy flows on small‐scale topography regulates diapycnal mixing and meridional overturning across the deep Southern Ocean is assessed in an idealized model. The model simulates an eddying circumpolar current coupled to a double‐celled meridional overturning with properties broadly resembling those of the Southern Ocean circulation and represents lee wave‐induced diapycnal mixing using an online formulation grounded on wave radiation theory. The diapycnal mixing generated by the simulated eddy field is found to play a major role in sustaining the lower overturning cell in the model, and to underpin a significant sensitivity of this cell to wind forcing. The vertical structure of lower overturning is set by mesoscale eddies, which propagate the effects of near‐bottom diapycnal mixing by displacing isopycnals vertically.
      PubDate: 2016-08-05T09:54:00.511105-05:
      DOI: 10.1002/2015JC011597
  • Fine‐scale thermohaline ocean structure retrieved with 2‐D prestack
           full‐waveform inversion of multichannel seismic data: Application to the
           Gulf of Cadiz (SW Iberia)
    • Pages: 5452 - 5469
      Abstract: This work demonstrates the feasibility of 2‐D time‐domain, adjoint‐state acoustic full‐waveform inversion (FWI) to retrieve high‐resolution models of ocean physical parameters such as sound speed, temperature and salinity. The proposed method is first described and then applied to prestack multichannel seismic (MCS) data acquired in the Gulf of Cadiz (SW Iberia) in 2007 in the framework of the Geophysical Oceanography project. The inversion strategy flow includes specifically designed data preconditioning for acoustic noise reduction, followed by the inversion of sound speed in the shotgather domain. We show that the final sound speed model has a horizontal resolution of ∼ 70 m, which is two orders of magnitude better than that of the initial model constructed with coincident eXpendable Bathy Thermograph (XBT) data, and close to the theoretical resolution of O(λ). Temperature (T) and salinity (S) are retrieved with the same lateral resolution as sound speed by combining the inverted sound speed model with the thermodynamic equation of seawater and a local, depth‐dependent T‐S relation derived from regional conductivity‐temperature‐depth (CTD) measurements of the National Oceanic and Atmospheric Administration (NOAA) database. The comparison of the inverted T and S models with XBT and CTD casts deployed simultaneously to the MCS acquisition shows that the thermohaline contrasts are resolved with an accuracy of 0.18oC for temperature and 0.08 PSU for salinity. The combination of oceanographic and MCS data into a common, pseudo‐automatic inversion scheme allows to quantitatively resolve submeso‐scale features that ought to be incorporated into larger‐scale ocean models of oceans structure and circulation.
      PubDate: 2016-08-05T09:47:03.510192-05:
      DOI: 10.1002/2016JC011844
  • Changes in summer sea ice, albedo, and portioning of surface solar
           radiation in the Pacific sector of Arctic Ocean during 1982–2009
    • Pages: 5470 - 5486
      Abstract: SSM/I sea ice concentration and CLARA black‐sky composite albedo were used to estimate sea ice albedo in the region 70°N–82°N, 130°W–180°W. The long‐term trends and seasonal evolutions of ice concentration, composite albedo, and ice albedo were then obtained. In July–August 1982–2009, the linear trend of the composite albedo and the ice albedo was −0.069 and −0.046 units per decade, respectively. During 1 June to 19 August, melting of sea ice resulted in an increase of solar heat input to the ice‐ocean system by 282 MJ·m−2 from 1982 to 2009. However, because of the counter‐balancing effects of the loss of sea ice area and the enhanced ice surface melting, the trend of solar heat input to the ice was insignificant. The summer evolution of ice albedo matched the ice surface melting and ponding well at basin scale. The ice albedo showed a large difference between the multiyear and first‐year ice because the latter melted completely by the end of a melt season. At the SHEBA geolocations, a distinct change in the ice albedo has occurred since 2007, because most of the multiyear ice has been replaced by first‐year ice. A positive polarity in the Arctic Dipole Anomaly could be partly responsible for the rapid loss of summer ice within the study region in the recent years by bringing warmer air masses from the south and advecting more ice toward the north. Both these effects would enhance ice‐albedo feedback.
      PubDate: 2016-08-05T09:40:42.739367-05:
      DOI: 10.1002/2016JC011831
  • Using multifractals to evaluate oceanographic model skill
    • Pages: 5487 - 5500
      Abstract: We are in an era of unprecedented data volumes generated from observations and model simulations. This is particularly true from satellite Earth Observations (EO) and global scale oceanographic models. This presents us with an opportunity to evaluate large‐scale oceanographic model outputs using EO data. Previous work on model skill evaluation has led to a plethora of metrics. The paper defines two new model skill evaluation metrics. The metrics are based on the theory of universal multifractals and their purpose is to measure the structural similarity between the model predictions and the EO data. The two metrics have the following advantages over the standard techniques: (a) they are scale‐free and (b) they carry important part of information about how model represents different oceanographic drivers. Those two metrics are then used in the paper to evaluate the performance of the FVCOM model in the shelf seas around the south‐west coast of the UK.
      PubDate: 2016-08-05T09:24:58.186182-05:
      DOI: 10.1002/2016JC011741
  • Offshore forcing on the “pressure point” of the West Florida Shelf:
           Anomalous upwelling and its influence on harmful algal blooms
    • Authors: Yonggang Liu; Robert H. Weisberg, Jason M. Lenes, Lianyuan Zheng, Katherine Hubbard, John J. Walsh
      Pages: 5501 - 5515
      Abstract: Gulf of Mexico Loop Current (LC) interactions with the West Florida Shelf (WFS) slope play an important role in shelf ecology through the upwelling of new inorganic nutrients across the shelf break. This is particularly the case when the LC impinges upon the shelf slope in the southwest portion of the WFS near the Dry Tortugas. By contacting shallow water isobaths at this “pressure point” the LC forcing sets the entire shelf into motion. Characteristic patterns of LC interactions with the WFS and their occurrences are identified using unsupervised neural network, self‐organizing map, from 23 years (1993–2015) of altimetry data. The duration of the occurrences of such LC patterns is used as an indicator of offshore forcing of anomalous upwelling. Consistency is found between the altimetry‐derived offshore forcing and the occurrence and severity of WFS coastal blooms of the toxic dinoflagellate, Karenia brevis: years without major blooms tend to have prolonged LC contact at the “pressure point,” whereas years with major blooms tend not to have prolonged offshore forcing. Resetting the nutrient state of the shelf by the coastal ocean circulation in response to deep‐ocean forcing demonstrates the importance of physical oceanography in shelf ecology. A satellite altimetry‐derived seasonal predictor for major K. brevis blooms is also proposed.
      PubDate: 2016-08-05T09:35:21.8832-05:00
      DOI: 10.1002/2016JC011938
  • Winter Water Properties and the Chukchi Polynya
    • Authors: C. Ladd; C. W. Mordy, S. A. Salo, P. J. Stabeno
      Pages: 5516 - 5534
      Abstract: Water properties from moored measurements (2010–2015) near Icy Cape on the eastern Chukchi shelf have been examined in relation to satellite observations of ice cover. Atlantic Water (AW), with temperature >−1°C and salinity >33.6, has been observed to upwell from deeper than 200 m in the Arctic Basin onto the Chukchi Shelf via Barrow Canyon. Most previous observations of AW on the Chukchi shelf have been in or near Barrow Canyon; observations of AW farther onto the shelf are rare. Despite mooring location on the shelf ∼225 km from the head of Barrow Canyon, five AW events have been observed at mooring C1 (70.8°N, 163.2°W) in 4 years of data. All but one of the events occurred under openings in the sea‐ice cover (either a polynya or the ice edge). No events were observed during the winter of 2011/2012, a year with little polynya activity in the region. In addition to changes in temperature and salinity, the AW events are typically associated with southwestward winds and currents, changes in sea‐ice cover, and increased nutrient concentrations in the bottom water. Estimates of heat content associated with the AW events suggest that the Chukchi Polynya can often be classified as a hybrid sensible heat/wind‐driven polynya.
      PubDate: 2016-08-05T09:29:58.234443-05:
      DOI: 10.1002/2016JC011918
  • Bromide and chloride distribution across the snow‐sea ice‐ocean
           interface: A comparative study between an Arctic coastal marine site and
           an experimental sea ice mesocosm
    • Authors: Wen Xu; Mario Tenuta, Feiyue Wang
      Pages: 5535 - 5548
      Abstract: During springtime in the Arctic, bromine explosion events occur when high concentrations of reactive bromine species are observed in the boundary layer with the concurrence of ozone depletion events and mercury depletion events. While a variety of substrates including snow, sea ice, frost flowers, and aerosols have been proposed to be the substrate and/or source of bromine activation in the Arctic, recent studies have highlighted the role of snow. Here we report concentration profiles of halides (Br− and Cl−), Na+, and mercury across the snow‐sea ice‐seawater interface at a coastal marine site in the Canadian Arctic Archipelago in March and June 2014, as well as in an experimental sea ice mesocosm in Winnipeg in January and February 2014. The occurrence of bromine activation at the Arctic site in March was indicated by the high mercury concentrations in snowpack. At both the Arctic and mesocosm sites, the molar ratios of Br−/Na+ were nearly constant throughout the sea ice depth, but highly variable in the upper layer of the overlying snowpack, revealing that bromine activation takes place in the sunlit snow instead of sea ice. This is supported by calculations showing that the loss of Br– from the upper layer of the snowpack is large enough to produce the observed concentrations of reactive bromine in the atmospheric boundary layer. However, the upper layer of the Arctic snowpack tends to be generally enriched in Br– due to the net addition of Br–‐containing gases and nonsea‐salt aerosols.
      PubDate: 2016-08-06T09:09:24.048169-05:
      DOI: 10.1002/2015JC011409
  • Dense water formation in the north‐western Mediterranean area during
           HyMeX‐SOP2 in 1/36° ocean simulations: Sensitivity to initial
    • Pages: 5549 - 5569
      Abstract: The north‐western Mediterranean Sea is a key location where intense air‐sea exchanges occur in autumn and winter. The succession of strong mistral and tramontane situations, leading to significant evaporation and ocean heat loss, is well known as the controlling factor in the dense water formation (DWF) with deep convection episodes. During HyMeX‐SOP2 (1 February to 15 March 2013), several platforms sampled the area in order to document DWF and air‐sea exchanges. This study investigates the ability of the NEMO‐WMED36 ocean model (1/36°‐resolution), driven in surface by the hourly air‐sea fluxes from the AROME‐WMED forecasts (2.5 km resolution), to represent DWF during HyMeX‐SOP2 and focuses on the sensitivity to initial conditions. After a short evaluation of the atmospheric forcing, the high‐resolution oceanic simulations using three different data sets as initial and boundary conditions are compared to observations collected during the field campaign. It evidences that using regional model outputs may lead to unrealistic thermohaline characteristics for the intermediate and deep waters, which degrade the simulated new dense water formed. Using ocean analyses built from observations, permits to obtain more realistic characteristics of the Western Mediterranean dense water. However, a low stratification favors an overestimation of the convective area and of the DWF rate. The DWF chronology is also impacted. Nevertheless, in every run, SOP2 is characterized by the production of water denser than 29.11 kg m−3 with a peak during the strong mistral event of 23–25 February followed by a period of restratification, before a last event of bottom convection on 13–15 March.
      PubDate: 2016-08-06T03:16:36.074311-05:
      DOI: 10.1002/2015JC011542
  • Horizontal mixing in the Southern Ocean from Argo float trajectories
    • Authors: Christopher J. Roach; Dhruv Balwada, Kevin Speer
      Pages: 5570 - 5586
      Abstract: We provide the first observational estimate of the circumpolar distribution of cross‐stream eddy diffusivity at 1000 m in the Southern Ocean using Argo float trajectories. We show that Argo float trajectories, from the float surfacing positions, can be used to estimate lateral eddy diffusivities in the ocean and that these estimates are comparable to those obtained from RAFOS floats, where they overlap. Using the Southern Ocean State Estimate (SOSE) velocity fields to advect synthetic particles with imposed behavior that is “Argo‐like” and “RAFOS‐like” diffusivity estimates from both sets of synthetic particles agreed closely at the three dynamically very different test sites, the Kerguelen Island region, the Southeast Pacific Ocean, and the Scotia Sea, and support our approach. Observed cross‐stream diffusivities at 1000 m, calculated from Argo float trajectories, ranged between 300 and 2500 m2 s−1, with peaks corresponding to topographic features associated with the Scotia Sea, the Kerguelen Plateau, the Campbell Plateau, and the Southeast Pacific Ridge. These observational estimates agree with previous regional estimates from the Diapycnal and Isopycnal Mixing Experiment in the Southern Ocean (DIMES) near the Drake Passage, and other estimates from natural tracers (helium), inverse modeling studies, and current meter measurements. These estimates are also compared to the suppressed eddy diffusivity in the presence of mean flows. The comparison suggests that away from regions of strong topographic steering suppression explains both the structure and magnitude of eddy diffusivity but that eddy diffusivities in the regions of topographic steering are greater than what would be theoretically expected and the ACC experiences localized enhanced cross‐stream mixing in these regions.
      PubDate: 2016-08-06T09:08:38.985138-05:
      DOI: 10.1002/2015JC011440
  • Horizontal variability of high‐frequency nonlinear internal waves in
           Massachusetts Bay detected by an array of seafloor pressure sensors
    • Authors: J. A. Thomas; J. A. Lerczak, J. N. Moum
      Pages: 5587 - 5607
      Abstract: A two‐dimensional array of 14 seafloor pressure sensors was deployed to measure properties of tidally generated, nonlinear, high‐frequency internal waves over a 14 km by 12 km area west of Stellwagen Bank in Massachusetts Bay during summer 2009. Thirteen high‐frequency internal wave packets propagated through the region over 6.5 days (one packet every semidiurnal cycle). Propagation speed and direction of wave packets were determined by triangulation, using arrival times and distances between triads of sensor locations. Wavefront curvature ranged from straight to radially spreading, with wave speeds generally faster to the south. Waves propagated to the southwest, rotating to more westward with shoreward propagation. Linear theory predicts a relationship between kinetic energy and bottom pressure variance of internal waves that is sensitive to sheared background currents, water depth, and stratification. By comparison to seafloor acoustic Doppler current profiler measurements, observations nonetheless show a strong relationship between kinetic energy and bottom pressure variance. This is presumably due to phase‐locking of the wave packets to the internal tide that dominates background currents and to horizontally uniform and relatively constant stratification throughout the study. This relationship was used to qualitatively describe variations in kinetic energy of the high‐frequency wave packets. In general, high‐frequency internal wave kinetic energy was greater near the southern extent of wavefronts and greatly decreased upon propagating shoreward of the 40 m isobath.
      PubDate: 2016-08-06T09:11:04.090429-05:
      DOI: 10.1002/2016JC011866
  • A permanent N2O sink in the Nordic Seas and its strength and possible
           variability over the past four decades
    • Authors: Liyang Zhan; Liqi Chen, Jiexia Zhang, Jinpei Yan, Yuhong Li, Man Wu
      Pages: 5608 - 5621
      Abstract: Nordic Seas have been assumed to be a net sink of the ozone‐depleting greenhouse gas N2O. However, few studies have been conducted in this region. N2O profile data obtained during the 5th Chinese National Arctic Research Expedition demonstrate that the N2O distribution pattern in the Nordic Seas differs from that of most other oceans. N2O sink characteristics of this region are confirmed by the undersaturation of N2O in the water column. Distributions of N2O in three subbasins of the Nordic Seas vary in the upper 1000 m but are homogenous below 1000 m due to a shared origin in the Greenland Basin (GB). Air‐sea exchange and vertical convection are thought to be dominant factors in N2O distribution in the GB, resulting in a distribution pattern that correlates significantly with the atmospheric mixing ratio variation over the past 40 years. Although recent studies have shown that weakened convection and/or enhanced Arctic outflow below the mid‐depth have occurred, our results show that these variations have yet to significantly affect the above relationship. The distribution could be considered a “historical record” that can be used to evaluate the air‐to‐sea flux over the past 40 years in the GB. The annual amount of N2O absorbed by the GB is ∼0.016–0.029 Tg N, which is equal to 0.4–0.8% of the world ocean emissions. This amount should not be simply neglected because it is absorbed by a region that accounts for only 0.03% of the world ocean area.
      PubDate: 2016-08-06T03:11:11.742197-05:
      DOI: 10.1002/2016JC011925
  • An observations and model‐based analysis of meridional transports in
           the South Atlantic
    • Authors: Sudip Majumder; Claudia Schmid, George Halliwell
      Pages: 5622 - 5638
      Abstract: A three‐dimensional velocity field constructed from Argo observations and sea surface heights (called Argo and SSH, hereinafter) is used to estimate meridional overturning volume transport and meridional heat transport (MHT) across 20°S, 25°S, 30°S, and 35°S for the years 2000–2014 in the South Atlantic. Volume transport in the upper branch of Meridional Overturning Circulation (MOC) and MHT from the observations are consistent with the previous observations, but are higher than the estimates derived from three data assimilative ocean models, at some of the latitudes. Both the observations and models show strong correlations between the strength of MOC and MHT at all the latitudes. The corresponding change in MHT for 1 Sv change of MOC strength, in the observations, increases from 0.046 PW in 25°S, 30°S, and 35°S to 0.056 PW across 20°S. A comparison of model‐based transports at 35°S at the boundaries and in the interior with those from Argo and SSH shows significant differences between them with respect to the contributions in the three segments of the section. In addition, the contributions also vary greatly between the different models. An analysis of the seasonality of MOC in the models and in the observations reveals that MOC anomalies in the models mostly show strong annual cycles at all the latitudes, whereas those derived from Argo and SSH exhibit annual cycles at three latitudes (35°S, 30°S, and to a lesser extent at 25°S) and a semiannual cycle at 20°S.
      PubDate: 2016-08-06T09:07:14.113919-05:
      DOI: 10.1002/2016JC011693
  • Mixing of dissolved oxygen in Chesapeake Bay driven by the interaction
           between wind‐driven circulation and estuarine bathymetry
    • Authors: Malcolm E. Scully
      Pages: 5639 - 5654
      Abstract: Field observations collected in Chesapeake Bay demonstrate how wind‐driven circulation interacts with estuarine bathymetry to control when and where the vertical mixing of dissolved oxygen occurs. In the across‐Bay direction, the lateral Ekman response to along‐Bay wind forcing contributes to the vertical mixing of dissolved oxygen in two ways. First, the lateral tilting of the pycnocline/oxycline, consistent with the thermal wind relationship, advects the region of high vertical gradient into the surface and bottom boundary layers where mixing can occur. Second, upwelling of low‐oxygen water to the surface enhances the atmospheric influx. In the along‐Bay direction, the abrupt change in bottom depth associated with Rappahannock Shoal results in surface convergence and downwelling, leading to localized vertical mixing. Water that is mixed on the shoal is entrained into the up‐Bay residual bottom flow resulting in increases in bottom dissolved oxygen that propagate up the system. The increases in dissolved oxygen are often associated with increases in temperature and decreases in salinity, consistent with vertical mixing. However, the lagged arrival moving northward suggests that the propagation of this signal up the Bay is due to advection.
      PubDate: 2016-08-08T08:05:09.177925-05:
      DOI: 10.1002/2016JC011924
  • Assessment of net community production and calcification of a coral reef
           using a boundary layer approach
    • Authors: Yuichiro Takeshita; Wade McGillis, Ellen M. Briggs, Amanda L. Carter, Emily M. Donham, Todd R. Martz, Nichole N. Price, Jennifer E. Smith
      Pages: 5655 - 5671
      Abstract: Coral reefs are threatened worldwide, and there is a need to develop new approaches to monitor reef health under natural conditions. Because simultaneous measurements of net community production (NCP) and net community calcification (NCC) are used as important indicators of reef health, tools are needed to assess them in situ. Here we present the Benthic Ecosystem and Acidification Measurement System (BEAMS) to provide the first fully autonomous approach capable of sustained, simultaneous measurements of reef NCP and NCC under undisturbed, natural conditions on time scales ranging from tens of minutes to weeks. BEAMS combines the chemical and velocity gradient in the benthic boundary layer to quantify flux from the benthos for a variety of parameters to measure NCP and NCC. Here BEAMS was used to measure these rates from two different sites with different benthic communities on the western reef terrace at Palmyra Atoll for 2 weeks in September 2014. Measurements were made every ∼15 min. The trends in metabolic rates were consistent with the benthic communities between the two sites with one dominated by fleshy organisms and the other dominated by calcifiers (degraded and healthy reefs, respectively). This demonstrates the potential utility of BEAMS as a reef health monitoring tool. NCP and NCC were tightly coupled on time scales of minutes to days, and light was the primary driver for the variability of daily integrated metabolic rates. No correlation between CO2 levels and daily integrated NCC was observed, indicating that NCC at these sites were not significantly affected by CO2.
      PubDate: 2016-08-10T08:02:20.243475-05:
      DOI: 10.1002/2016JC011886
  • West Florida shelf upwelling: Origins and pathways
    • Authors: Robert H. Weisberg; Lianyuan Zheng, Yonggang Liu
      Pages: 5672 - 5681
      Abstract: Often described as oligotrophic, the west Florida continental shelf supports abundant fisheries, experiences blooms of the harmful alga, Karenia brevis, and exhibits subsurface chlorophyll maxima evident in shipboard and glider surveys. Renewal of inorganic nutrients by the upwelling of deeper ocean water onto the shelf may account for this, but what are the origins and pathways by which such new water may broach the shelf break and advance toward the shoreline? We address these questions via numerical model simulations of pseudo‐Lagrangian, isopycnic water parcel trajectories. Focus is on 2010, when the west Florida shelf was subjected to an anomalously protracted period of upwelling caused by Gulf of Mexico Loop Current interactions with the shelf slope. Origins and pathways are determined by integrating trajectories over successive 45 day intervals, beginning from different locations along the shelf break and at various locations and depths along the shelf slope. Waters upwelling across the shelf break are found to originate from relatively shallow depths along the shelf slope. Even for the anomalous 2010 year, much of this upwelling occurs from about 150 m and above, although waters may broach the shelf break from 300 m depth, particularly in the Florida Panhandle. Such interannual renewal of west Florida shelf waters appears to have profound effects on west Florida shelf ecology.
      PubDate: 2016-08-10T08:02:28.947884-05:
      DOI: 10.1002/2015JC011384
  • Surface layer temperature inversion in the Bay of Bengal: Main
           characteristics and related mechanisms
    • Authors: Pankajakshan Thadathil; I. Suresh, S. Gautham, S. Prasanna Kumar, Matthieu Lengaigne, R. R. Rao, S. Neetu, Akshay Hegde
      Pages: 5682 - 5696
      Abstract: Surface layer temperature inversion (SLTI), a warm layer sandwiched between surface and subsurface colder waters, has been reported to frequently occur in conjunction with barrier layers in the Bay of Bengal (BoB), with potentially commensurable impacts on climate and postmonsoon tropical cyclones. Lack of systematic measurements from the BoB in the past prevented a thorough investigation of the SLTI spatiotemporal variability, their formation mechanisms, and their contribution to the surface temperature variations. The present study benefits from the recent Research Moored Array for African‐Asian‐Australian Monsoon Analysis and Prediction (RAMA) buoys located in BoB along 90°E at 4°N, 8°N, 12°N, and 15°N over the 2006–2014 period. Analysis of data from these RAMA buoys indicates that SLTI forms after the summer monsoon and becomes fully developed during winter (December–February). SLTI exhibits a strong geographical dependency, with more frequent (80% times during winter) and intense inversions (amplitude, ΔT ∼ 0.7°C) occurring only in the northern BoB compared to central and southern Bay. SLTI also exhibits large interannual and intraseasonal variations, with intraseasonal amplitude significantly larger (ΔT ∼ 0.44°C) than the interannual amplitude (∼0.26°C). Heat budget analysis of the mixed layer reveals that the net surface heat loss is the most dominant process controlling the formation and maintenance of SLTI. However, there are instances of episodic advection of cold, low‐saline waters over warm‐saline waters leading to the formation of SLTI as in 2012–2013. Vertical processes contribute significantly to the mixed layer heat budget during winter, by warming the surface layer through entrainment and vertical diffusion.
      PubDate: 2016-08-11T10:05:32.117479-05:
      DOI: 10.1002/2016JC011674
  • Stable reconstruction of Arctic sea level for the 1950–2010 period
    • Authors: Peter Limkilde Svendsen; Ole B. Andersen, Allan Aasbjerg Nielsen
      Pages: 5697 - 5710
      Abstract: Reconstruction of historical Arctic sea level is generally difficult due to the limited coverage and quality of both tide gauge and altimetry data in the area. Here a strategy to achieve a stable and plausible reconstruction of Arctic sea level from 1950 to today is presented. This work is based on the combination of tide gauge records and a new 20 year reprocessed satellite altimetry‐derived sea level pattern. Hence, the study is limited to the area covered by satellite altimetry (68°N and 82°N). It is found that time step cumulative reconstruction as suggested by Church and White (2011) may yield widely variable results and is difficult to stabilize due to the many gaps in both tide gauge and satellite data. A more robust sea level reconstruction approach is to use datum adjustment of the tide gauges in combination with satellite altimetry, as described by Ray and Douglas (2011). In this approach, a datum‐fit of each tide gauges is used and the method takes into account the entirety of each tide gauge record. This makes the Arctic sea level reconstruction much less prone to drifting. From our reconstruction, we found that the Arctic mean sea level trend is around 1.5 mm ± 0.3 mm/yr for the period 1950–2010, between 68°N and 82°N. This value is in good agreement with the global mean trend of 1.8 ± 0.3 mm/yr over the same period as found by Church and White (2004).
      PubDate: 2016-08-11T01:45:59.727325-05:
      DOI: 10.1002/2016JC011685
  • Synoptic forcing of wind relaxations at Pt. Conception, California
    • Authors: Melanie R. Fewings; Libe Washburn, Clive E. Dorman, Christopher Gotschalk, Kelly Lombardo
      Pages: 5711 - 5730
      Abstract: Over the California Current upwelling system in summer, the prevailing upwelling‐favorable winds episodically weaken (relax) or reverse direction for a few days. Near Pt. Conception, California, the wind usually does not reverse, but wind relaxation allows poleward oceanic coastal flow with ecological consequences. To determine the offshore extent and synoptic forcing of these wind relaxations, we formed composite averages of wind stress from the QuikSCAT satellite and atmospheric pressure from the North American Regional Reanalysis (NARR) using 67 wind relaxations during summer 2000–2009. Wind relaxations at Pt. Conception are the third stage of an event sequence that repeatedly affects the west coast of North America in summer. First, 5–7 days before the wind weakens near Pt. Conception, the wind weakens or reverses off Oregon and northern California. Second, the upwelling‐favorable wind intensifies along central California. Third, the wind relaxes at Pt. Conception, and the area of weakened winds extends poleward to northern California over 3–5 days. The NARR underestimates the wind stress within ∼200 km of coastal capes by a factor of 2. Wind relaxations at Pt. Conception are caused by offshore extension of the desert heat low. This synoptic forcing is related to event cycles that cause wind reversal as in Halliwell and Allen (1987) and Mass and Bond (1996), but includes weaker events. The wind relaxations extend ∼600 km offshore, similarly to the California‐scale hydraulic expansion fan shaping the prevailing winds, and ∼1000 km alongshore, limited by an opposing pressure gradient force at Cape Mendocino.
      PubDate: 2016-08-11T01:45:47.737178-05:
      DOI: 10.1002/2016JC011699
  • Effects of model physics on hypoxia simulations for the northern Gulf of
           Mexico: A model intercomparison
    • Pages: 5731 - 5750
      Abstract: A large hypoxic zone forms every summer on the Texas‐Louisiana Shelf in the northern Gulf of Mexico due to nutrient and freshwater inputs from the Mississippi/Atchafalaya River System. Efforts are underway to reduce the extent of hypoxic conditions through reductions in river nutrient inputs, but the response of hypoxia to such nutrient load reductions is difficult to predict because biological responses are confounded by variability in physical processes. The objective of this study is to identify the major physical model aspects that matter for hypoxia simulation and prediction. In order to do so, we compare three different circulation models (ROMS, FVCOM, and NCOM) implemented for the northern Gulf of Mexico, all coupled to the same simple oxygen model, with observations and against each other. By using a highly simplified oxygen model, we eliminate the potentially confounding effects of a full biogeochemical model and can isolate the effects of physical features. In a systematic assessment, we found that (1) model‐to‐model differences in bottom water temperatures result in differences in simulated hypoxia because temperature influences the uptake rate of oxygen by the sediments (an important oxygen sink in this system), (2) vertical stratification does not explain model‐to‐model differences in hypoxic conditions in a straightforward way, and (3) the thickness of the bottom boundary layer, which sets the thickness of the hypoxic layer in all three models, is key to determining the likelihood of a model to generate hypoxic conditions. These results imply that hypoxic area, the commonly used metric in the northern Gulf which ignores hypoxic layer thickness, is insufficient for assessing a model's ability to accurately simulate hypoxia, and that hypoxic volume needs to be considered as well.
      PubDate: 2016-08-11T01:45:56.247404-05:
      DOI: 10.1002/2015JC011577
  • Storm‐driven bottom sediment transport on a high‐energy narrow shelf
           (NW Iberia) and development of mud depocenters
    • Authors: Wenyan Zhang; Yongsheng Cui, Ana I. Santos, Till J. J. Hanebuth
      Pages: 5751 - 5772
      Abstract: Bottom sediment transport on the NW Iberian shelf was monitored during a downwelling storm in September 2014. Collected data were analyzed and fed into a 3‐D coastal ocean model to understand storm‐driven sediment transport on the shelf and its impact on midshelf mud depocenters (MDCs). A significantly enhanced level of bottom sediment resuspension, nearly two orders of magnitude higher than that in the prestorm period, was recorded at the mooring site. Field data analysis reveals that it was induced by a short‐lasting strong bottom current in combination with enhanced wave‐current interaction. Simulation results indicate that this strong current was part of a coastal jet resulted from downwelling. An across‐shelf horizontal density gradient as high as 0.32 g/m4 occurred at the interface between the downwelling and the bottom waters, forming a remarkable front. Due to buoyancy effect, the downwelling water was mostly confined to the coast with a depth limit of 80 m in the south and 120 m in the north of the region, resulting in a northward‐directed coastal jet. Simulation results suggest that during the storm, local near‐bottom sediment suspensions with concentrations on the order of 10 kg/m3 would be triggered by wave‐current interaction and flow convergence associated with the front. Direct impact on the development of MDCs by transport and deposition of concentrated sediment suspensions is indicated by model results. The seaward limit of the front coincided with the shoreward edge of the MDC nucleus, suggesting the front as a primary control on the deposition of fine‐grained sediment.
      PubDate: 2016-08-11T01:30:48.958898-05:
      DOI: 10.1002/2015JC011526
  • Changes and influencing factors in biogenic opal export productivity in
           the Bering Sea over the last 4.3 Ma: Evidence from the records at IODP
           Site U1340
    • Authors: Qiang Zhang; Muhong Chen, Lanlan Zhang, Xiang Su, Rong Xiang
      Pages: 5789 - 5804
      Abstract: We reconstructed changes in biogenic opal export productivity (BOEP) in the southern Bering Sea (BS) over the last ∼4.3 Ma, based on mass accumulation rate (MAR) of biogenic opal from Integrated Ocean Drilling Program (IODP) Site U1340. The results show that the BOEP in the BS was high and variable between ∼4.3 and ∼1.9 Ma, extremely low and relatively stable from ∼1.9 to ∼1.1 Ma, and then fluctuated frequently (generally high during interglacials and low during glacials) during the last ∼1.1 Ma. One interval of enhanced BOEP from ∼4.3 to ∼3.2 Ma is a response to the Late Miocene‐Early Pliocene “Biogenic Bloom Event.” Another interval from ∼2.8 to ∼1.9 Ma correlates with global opal burial shifting from high‐latitude oceans to upwelling‐influenced regions following the intensification of the Northern Hemisphere Glaciation (NHG). Whereas, the increase in BS opal export productivity during the last ∼1.1 Ma tends to be a “local” phenomenon. Overall, the BOEP shows a similar trend and good correspondence to the input of the Alaskan Stream (AS), which can be traced using the Na2O/K2O ratio. We thus conclude that the AS may be the direct, and primary factor on BOEP variability in the BS during the last ∼4.3 Ma. In addition, although the poor correlation between opal MAR and volcanic glass suggests that BOEP variability was not controlled by long‐term variations in the volcanism or ash abundance, increased ash abundance indicated by high contents of volcanic glasses was also a possible reason for enhanced BOEP during the period from ∼4.3 to ∼3.2 Ma and the last ∼0.5 Ma.
      PubDate: 2016-08-12T01:20:34.389944-05:
      DOI: 10.1002/2016JC011750
  • New perspectives for noble gases in oceanography
    • Authors: Werner Aeschbach
      Pages: 6550 - 6554
      Abstract: Conditions prevailing in regions of deep water formation imprint their signature in the concentrations of dissolved noble gases, which are conserved in the deep ocean. Such “recharge conditions” including temperature, salinity, and interactions with sea ice are important in view of ocean‐atmosphere CO2 partitioning. Noble gases, especially the temperature sensitive Kr and Xe, are well‐established tracers to reconstruct groundwater recharge conditions. In contrast, tracer oceanography has traditionally focused on He isotopes and the light noble gases Ne and Ar, which could be analyzed at the required high precision. Recent developments of analytical and data interpretation methods now provide fresh perspectives for noble gases in oceanography.
      PubDate: 2016-08-11T01:45:41.066436-05:
      DOI: 10.1002/2016JC012133
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