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

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Geochemistry, Geophysics, Geosystems     Full-text available via subscription   (Followers: 25, SJR: 2.56, h-index: 69)
Geophysical Research Letters     Full-text available via subscription   (Followers: 80, SJR: 3.493, h-index: 157)
Global Biogeochemical Cycles     Full-text available via subscription   (Followers: 11, SJR: 3.239, h-index: 119)
J. of Advances in Modeling Earth Systems     Open Access   (Followers: 4, SJR: 1.944, h-index: 7)
J. of Geophysical Research : Atmospheres     Partially Free   (Followers: 89)
J. of Geophysical Research : Biogeosciences     Full-text available via subscription   (Followers: 13)
J. of Geophysical Research : Earth Surface     Partially Free   (Followers: 34)
J. of Geophysical Research : Oceans     Partially Free   (Followers: 29)
J. of Geophysical Research : Planets     Full-text available via subscription   (Followers: 27)
J. of Geophysical Research : Solid Earth     Full-text available via subscription   (Followers: 32)
J. of Geophysical Research : Space Physics     Full-text available via subscription   (Followers: 36)
Paleoceanography     Full-text available via subscription   (Followers: 4, SJR: 3.22, h-index: 88)
Radio Science     Full-text available via subscription   (Followers: 33, SJR: 0.959, h-index: 51)
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Water Resources Research     Full-text available via subscription   (Followers: 67, SJR: 2.189, h-index: 121)
Journal Cover Journal of Geophysical Research : Oceans
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   Published by AGU Homepage  [17 journals]
  • 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
           Ocean
    • 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 Sea
    • 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
           subtropics?
    • 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 modeling
    • 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 Grounds
    • 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 (ROMS)
    • 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
           Antarctica
    • 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
       
  • 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
      Abstract: Bottom sediment transport on the NW Iberian shelf was monitored during a downwelling storm in September 2014. Collected data was analyzed and fed into a 3D coastal ocean model to understand storm‐driven sediment transport on the shelf and its impact on mid‐shelf mud depocenters (MDCs). A significantly enhanced level of bottom sediment resuspension, nearly two orders of magnitude higher than that in the pre‐storm 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. This article is protected by copyright. All rights reserved.
      PubDate: 2016-07-20T10:26:02.115345-05:
      DOI: 10.1002/2015JC011526
       
  • 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
       
  • New perspectives for noble gases in oceanography
    • Authors: Werner Aeschbach
      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. This article is protected by copyright. All rights reserved.
      PubDate: 2016-07-15T04:16:47.656331-05:
      DOI: 10.1002/2016JC012133
       
  • 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 Carter, Emily Donham, Todd R. Martz, Nichole N. Price, Jennifer E. Smith
      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 timescales 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 two weeks in September, 2014. Measurements were made every ∼15 minutes. 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 timescales 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. This article is protected by copyright. All rights reserved.
      PubDate: 2016-07-15T04:16:39.299754-05:
      DOI: 10.1002/2016JC011886
       
  • 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
       
  • Effects of model physics on hypoxia simulations for the northern Gulf of
           Mexico: A model intercomparison
    • 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. This article is protected by copyright. All rights reserved.
      PubDate: 2016-07-14T09:15:42.233144-05:
      DOI: 10.1002/2015JC011577
       
  • Synoptic forcing of wind relaxations at Pt. Conception, California
    • Authors: Melanie R Fewings; Libe Washburn, Clive E. Dorman, Christopher Gotschalk, Kelly Lombardo
      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 dy 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 dy. 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. This article is protected by copyright. All rights reserved.
      PubDate: 2016-07-14T09:15:36.011722-05:
      DOI: 10.1002/2016JC011699
       
  • An observation and model‐based analysis of meridional transports in
           the South Atlantic
    • Authors: Sudip Majumder; Claudia Schmid, George Halliwell
      Abstract: A three dimensional velocity field constructed from Argo observations and sea surface heights (called Argo & 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 to 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 & 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 & 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 This article is protected by copyright. All rights reserved.
      PubDate: 2016-07-13T10:42:56.986488-05:
      DOI: 10.1002/2016JC011693
       
  • West Florida shelf upwelling: Origins and pathways
    • Authors: Robert H. Weisberg; Lianyuan Zheng, Yonggang Liu
      Abstract: Often described as oligotrophic, the west Florida continental shelf supports abundant fisheries, experiences blooms of the harmful algae, 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 inter‐annual renewal of west Florida shelf waters appears to have profound effects on west Florida shelf ecology. This article is protected by copyright. All rights reserved.
      PubDate: 2016-07-13T10:42:55.654529-05:
      DOI: 10.1002/2015JC011384
       
  • Stable reconstruction of Arctic sea level for the 1950–2010 period
    • Authors: Peter Limkilde Svendsen; Ole B. Andersen, Allan Aasbjerg Nielsen
      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 (68ºN and 82ºN). It is found that timestep cumulative reconstruction as suggested by Church and White (2000) 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/y for the period 1950 to 2010, between 68ºN and 82ºN. This value is in good agreement with the global mean trend of 1.8 +/‐ 0.3 mm/y over the same period as found by Church and White (2004). This article is protected by copyright. All rights reserved.
      PubDate: 2016-07-13T10:42:53.776801-05:
      DOI: 10.1002/2016JC011685
       
  • 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
       
  • Winter water properties and the Chukchi Polynya
    • Authors: C. Ladd; C. W. Mordy, S. A. Salo, P. J. Stabeno
      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 four 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. This article is protected by copyright. All rights reserved.
      PubDate: 2016-07-13T10:42:48.885569-05:
      DOI: 10.1002/2016JC011918
       
  • 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
       
  • Using multifractals to evaluate oceanographic model skill
    • 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, 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. This article is protected by copyright. All rights reserved.
      PubDate: 2016-07-13T10:42:30.280829-05:
      DOI: 10.1002/2016JC011741
       
  • 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
      Abstract: We reconstructed changes in biogenic opal export productivity (BOEP) in the southern Bering Sea (BS) over the last 4.3Ma, 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 ∼4.3Ma and ∼1.9Ma, extremely low and relatively stable from ∼1.9Ma to ∼1.1Ma, and then fluctuated frequently (generally high during interglacials and low during glacials) during the last ∼1.1Ma. One interval of enhanced BOEP from 4.3Ma∼3.2Ma is a response to the Late Miocene–Early Pliocene “Biogenic Bloom Event”. Another interval from 2.8Ma∼1.9Ma 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 1.1Ma 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 ∼4.3Ma. 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 ∼4.3Ma to ∼3.2Ma and the last ∼0.5Ma. This article is protected by copyright. All rights reserved.
      PubDate: 2016-07-13T10:42:25.363438-05:
      DOI: 10.1002/2016JC011750
       
  • Mixing of dissolved oxygen in Chesapeake Bay driven by the interaction
           between wind‐driven circulation and estuarine bathymetry
    • Authors: Malcolm Scully
      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. Secondly, 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. This article is protected by copyright. All rights reserved.
      PubDate: 2016-07-13T10:26:04.135407-05:
      DOI: 10.1002/2016JC011924
       
  • 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
      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. This article is protected by copyright. All rights reserved.
      PubDate: 2016-07-13T10:21:39.573799-05:
      DOI: 10.1002/2016JC011938
       
  • 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, Yan Jinpei, Yuhong Li, Man Wu
      Abstract: The 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. The 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. The N2O sink characteristics of this region are confirmed by the undersaturation of N2O in the water column. The 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 the dominant factors in the 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 has 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 whose area accounts for only 0.03% of the world ocean area. This article is protected by copyright. All rights reserved.
      PubDate: 2016-07-13T10:21:24.855803-05:
      DOI: 10.1002/2016JC011925
       
  • 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
       
  • Changes in summer sea ice, albedo, and portioning of surface solar
           radiation in the Pacific sector of Arctic Ocean during 1982‐2009
    • Abstract: SSM/I sea ice concentration and CLARA black‐sky composite albedo were used to estimate sea ice albedo in the region 70°–82°N, 130°–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. This article is protected by copyright. All rights reserved.
      PubDate: 2016-07-11T10:46:10.915699-05:
      DOI: 10.1002/2016JC011831
       
  • 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
       
  • High resolution modeling of dense water formation in the
           north‐western Mediterranean during winter 2012‐2013: Processes
           and budget
    • Abstract: The evolution of the stratification of the north‐western Mediterranean between summer 2012 and the end of winter 2013 was simulated and compared with different sets of observations. A summer cruise and profiler observations were used to improve the initial conditions of the simulation. This improvement was crucial to simulate winter convection. Variations of some parameters involved in air ‐ sea exchanges (wind, coefficient of transfer used in the latent heat flux formulation, and constant additive heat flux) showed that the characteristics of water masses and the volume of dense water formed during convection cannot be simply related to the time‐integrated buoyancy budget over the autumn ‐ winter period. The volume of dense water formed in winter was estimated to be about 50,000 km3 with a density anomaly larger than 29.113 kg m−3. The effect of advection and air/sea fluxes on the heat and salt budget of the convection zone was quantified during the preconditioning phase and the mixing period. Destratification of the surface layer in autumn occurs through an interaction of surface and Ekman buoyancy fluxes associated with displacements of the North Balearic front bounding the convection zone to the south. During winter convection, advection stratifies the convection zone: from December to March, the absolute value of advection represents 58% of the effect of surface buoyancy fluxes. This article is protected by copyright. All rights reserved.
      PubDate: 2016-07-04T10:40:28.024325-05:
      DOI: 10.1002/2016JC011935
       
  • 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
       
  • Evaluation of entrainment formulations for liquid/gas plumes from
           underwater blowouts
    • Authors: Luciana de Freitas Tessarolo; Valdir Innocentini
      Abstract: A numerical model using the Lagrangian approach developed to simulate the fate of liquid/gas blowouts in deepwater is presented, and three entrainment formulations are tested: HOULT, JETLAG and CORJET parameterizations, given by Hoult et al. (1969), Lee and Cheung (1990) and Jirka (2004), respectively. The results are discussed and compared with field and laboratory observations. These formulations differ both in shear and forced contributions to the entrainment. As expected, the qualitative analysis of the dynamics of a liquid plume shows that the entrainment of ambient water decreases the acceleration due to buoyancy, and the plume and ambient momentums become increasingly similar over time. However, simulations of field and laboratory cases, where different plumes (gas, liquid and gas/liquid) were discharged into environments with different ambient stratifications and cross‐flows, show that the JETLAG parameterization provides the best results, while HOULT (CORJET) overestimates (underestimates) the entrainment. Additional numerical experiments applying only the JETLAG formulation are performed, considering different plume composition, ambient condition, nozzle diameter and initial discharge. For all the studied cases, the simulated results are in good agreement with the observations. Especially noteworthy were field experiments with gas released at depth of 50‐60 m. The vertical plume velocity decreased during the ascending motion, but after a certain level, the velocity increased. This feature was simulated by the JETLAG parameterization, and a closer analysis reveals the increase of buoyancy due to gas expansion exceeding the decrease caused by the entrainment. These results encourage the use of this model in realistic and complex situations. This article is protected by copyright. All rights reserved.
      PubDate: 2016-07-04T10:35:22.690812-05:
      DOI: 10.1002/2016JC011735
       
  • Small meanders of the Kuroshio Extension and associated northward
           spreading of warm water: Three‐vessel simultaneous observations and
           an eddy‐resolving ocean model simulation
    • Abstract: Small meanders of the Kuroshio Extension (KE) were examined in the three vessel simultaneous observations carried out across the KE along 143°E between 2 and 7 July 2012 and the hindcast simulation by a high‐resolution ocean model for the Earth Simulator (OFES). The observations captured the crest of the small meander that passed 143°E. Behind the meander crest, the meander showed the northward separation of the front from the meandering jet. It was suggested that the meander spread the low‐density KE water to the north above the main pycnocline there. The OFES showed similar characteristics of small meanders. We focused on three major small meanders. In the upstream KE region west of 143°E, these meanders were subject to northeastward ageostrophic advection, by which the low‐density KE water was carried to the north and their fronts moved away from the KE jet above the main pycnocline. This resulted in the KE water and the front extending to the west behind the crest of the downstream‐propagating meanders. This feature was particularly obvious for the two cases of the meander and was consistent with the observations. The displaced front formed a shallow geostrophic jet to the north of the main KE jet, which induced geostrophic advection along the jet and advected the front further northward and downstream near the crest of the meandering front. The KE water spread by the small meanders then mostly evolved into warm core rings or warm streamers. This article is protected by copyright. All rights reserved.
      PubDate: 2016-06-30T03:45:34.726128-05:
      DOI: 10.1002/2016JC011969
       
  • An abrupt shift in the Labrador Current system in relation to winter NAO
           events
    • Authors: Zeliang Wang; David Brickman, Blair J.W. Greenan, Igor Yashayaev
      Abstract: The behavior of the Labrador Current during the period from 1990 to 2007 is investigated with an eddy‐resolving circulation model for the North Atlantic Ocean. An EOF analysis of the model output suggests that the variability in the Labrador Current can be partitioned into a western Labrador Current (WLC; from the 300‐2500 m isobaths), and an eastern Labrador Current (ELC; from the 2500‐3300 m isobaths). The model results demonstrate that the WLC transport experienced an abrupt increase during 2000‐2002, consistent with data. This differed significantly from the ELC transport which was strong during the high winter NAO (North Atlantic Oscillation) years (1990‐95) and then steadily declined. This ELC trend is consistent with changes in the modelled Atlantic Meridional Overturning Circulation and convection depth. Our study proposes that the change in the WLC is due to a southwestward shift of the atmospheric circulation pattern starting in 2001, coincident with a change in the 2001 NAO index, and also in a westward shift of the action centers of the winter NAO events. This article is protected by copyright. All rights reserved.
      PubDate: 2016-06-30T03:45:22.931321-05:
      DOI: 10.1002/2016JC011721
       
  • 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)
    • Abstract: This work demonstrates the feasibility of 2D 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 pre‐stack multi‐channel 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 pre‐conditioning 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 ∼ 70m, 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. This article is protected by copyright. All rights reserved.
      PubDate: 2016-06-29T10:55:50.123671-05:
      DOI: 10.1002/2016JC011844
       
  • 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
      Abstract: A two‐dimensional array of fourteen 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. This article is protected by copyright. All rights reserved.
      PubDate: 2016-06-28T03:26:18.771975-05:
      DOI: 10.1002/2016JC011866
       
  • Predictability of wind‐induced sea surface transport in coastal
           areas
    • 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
       
  • Vertical distribution of buoyant Microcystis blooms in a Lagrangian
           particle tracking model for short‐term forecasts in Lake Erie
    • Authors: M. D. Rowe; E. J. Anderson, T. T. Wynne, R. P. Stumpf, D. L. Fanslow, K. Kijanka, H. A. Vanderploeg, J. R. Strickler, T. W. Davis
      Abstract: Cyanobacterial harmful algal blooms (CHABs) are a problem in western Lake Erie, and in eutrophic fresh waters worldwide. Western Lake Erie is a large (3000 km2), shallow (8 m mean depth), freshwater system. CHABs occur from July to October, when stratification is intermittent in response to wind and surface heating or cooling (polymictic). Existing forecast models give the present location and extent of CHABs from satellite imagery, then predict two‐dimensional (surface) CHAB movement in response to meteorology. In this study, we simulated vertical distribution of buoyant Microcystis colonies, and 3D advection, using a Lagrangian particle model forced by currents and turbulent diffusivity from the Finite Volume Community Ocean Model (FVCOM). We estimated the frequency distribution of Microcystis colony buoyant velocity from measured size distributions and buoyant velocities. We evaluated several random‐walk numerical schemes to efficiently minimize particle accumulation artifacts. We selected the Milstein scheme, with linear interpolation of the diffusivity profile in place of cubic splines, and varied the time step at each particle and step based on the curvature of the local diffusivity profile to ensure that the Visser time step criterion was satisfied. Inclusion of vertical mixing with buoyancy significantly improved model skill statistics compared to an advection‐only model, and showed greater skill than a persistence forecast through simulation day 6, in a series of 26 hindcast simulations from 2011. The simulations and in‐situ observations show the importance of subtle thermal structure, typical of a polymictic lake, along with buoyancy in determining vertical and horizontal distribution of Microcystis. This article is protected by copyright. All rights reserved.
      PubDate: 2016-06-27T03:40:47.944411-05:
      DOI: 10.1002/2016JC011720
       
  • Horizontal mixing in the Southern Ocean from Argo float trajectories
    • Authors: Christopher J. Roach; Dhruv Balwada, Kevin Speer
      Abstract: We provide the first observational estimate of the circumpolar distribution of cross‐stream eddy diffusivity at 1000m 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 behaviour 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 1000m, calculated from Argo float trajectories, ranged between 300 and 2500 m2s−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 modelling 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. This article is protected by copyright. All rights reserved.
      PubDate: 2016-06-27T03:30:40.87102-05:0
      DOI: 10.1002/2015JC011440
       
  • 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
       
  • 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
      Abstract: Surface Layer Temperature Inversion (SLTI), a warm layer sandwiched between surface and subsurface colder waters, have been reported to frequently occur in conjunction with barrier layers in the Bay of Bengal (BoB), with potentially commensurable impacts on climate and post‐monsoon tropical cyclones. Lack of systematic measurements from the BoB in the past prevented a thorough investigation of the SLTI spatio‐temporal 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 15oN 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 ∼ 0.7oC) 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 ∼ 0.44oC) than the interannual amplitude (∼ 0.26oC). 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. This article is protected by copyright. All rights reserved.
      PubDate: 2016-06-27T03:30:35.650938-05:
      DOI: 10.1002/2016JC011674
       
  • Dense water formation in the north‐western mediterranean area during
           HyMeX‐SOP2 in 1/36° ocean simulations: Sensitivity to initial
           conditions
    • 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 (2.5km‐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 datasets 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. This article is protected by copyright. All rights reserved.
      PubDate: 2016-06-27T03:30:31.358546-05:
      DOI: 10.1002/2015JC011542
       
  • 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
      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 idealised 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. This article is protected by copyright. All rights reserved.
      PubDate: 2016-06-24T03:27:19.676648-05:
      DOI: 10.1002/2015JC011597
       
  • Vertical fluxes of nitrate in the seasonal nitracline of the Atlantic
           sector of the Arctic Ocean
    • Abstract: This study compiles co‐located oceanic observations of high‐resolution vertical profiles of nitrate concentration and turbulent microstructure around the Svalbard shelf slope, covering both the permanently ice‐free Fram Strait and the pack ice north of Svalbard. The authors present an overview over the seasonal evolution of the distribution of nitrate and its relation to upper ocean stratification. The average upward turbulent diffusive nitrate flux across the seasonal nitracline during the Arctic summer season is derived, with average values of 0.3 and 0.7 mmolm−2,d−1 for stations with and without ice cover, respectively. The increase under ice‐free conditions is attributed to different patterns of stratification under sea ice versus open water. The nitrate flux obtained from microstructure measurements lacked a seasonal signal. However, bottle incubations indicate that August nitrate uptake was reduced by more than an order of magnitude relative to the May values. It remains inconclusive whether the new production was limited by an unidentified factor other than NO3− supply in late summer, or the uptake was underestimated by the incubation method. This article is protected by copyright. All rights reserved.
      PubDate: 2016-06-23T03:50:29.871292-05:
      DOI: 10.1002/2016JC011779
       
  • 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
      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 such as 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 oxidized 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 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–‐enriched gases and non‐sea‐salt aerosols. This article is protected by copyright. All rights reserved.
      PubDate: 2016-06-22T18:55:42.980452-05:
      DOI: 10.1002/2015JC011409
       
  • A comparative study of wave‐current interactions over the eastern
           Canadian shelf under severe weather conditions using a coupled
           wave‐circulation model
    • Authors: Pengcheng Wang; Jinyu Sheng
      Abstract: A coupled wave‐circulation model is used to examine interactions between surface gravity waves and ocean currents over the eastern Canadian shelf and adjacent deep waters during three severe weather events. The simulated significant wave heights (SWHs) and peak wave periods reveal the importance of wave‐current interactions (WCI) during and after the storm. In two fast‐moving hurricane cases, the maximum SWHs are reduced by more than 11% on the right‐hand side of the storm track and increased by about 5% on the left‐hand side due to different WCI mechanisms on waves on two sides of the track. The dominate mechanisms of the WCI on waves include the current‐induced modification of wind energy input to the wave generation, and current‐induced wave advection and refraction. In the slow‐moving winter storm case, the effect of WCI decreases the maximum SWHs on both sides of the storm track due to different results of the current‐induced wave advection, which is affected greatly by the storm translation speed. The simulated sea surface temperature (SST) cooling induced by hurricanes and SST warming induced by the winter storm are also enhanced (up to 1.2oC) by the WCI mechanisms on circulation and hydrography. The 3D wave forces can affect water columns up to 200 m in all three storm cases. By comparison, the effect of breaking wave‐induced mixing in the ocean upper layer is more important under strong stratification conditions in two hurricane cases than under weak stratification conditions in the winter storm case. This article is protected by copyright. All rights reserved.
      PubDate: 2016-06-20T11:00:36.990861-05:
      DOI: 10.1002/2016JC011758
       
  • Relevance of infragravity waves in a wave‐dominated inlet
    • Authors: Xavier Bertin; Maitane Olabarrieta
      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 to 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 two hours 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. This article is protected by copyright. All rights reserved.
      PubDate: 2016-06-20T10:55:28.529948-05:
      DOI: 10.1002/2015JC011444
       
  • A numerical model for the entire Wadden Sea: skill assessment and analysis
           of hydrodynamics
    • Abstract: A baroclinic three‐dimensional numerical model for the entire Wadden Sea of the German Bight in the southern North Sea is first assessed by comparison to field data for surface elevation, current velocity, temperature and salinity at selected stations and then used to calculate fluxes of volume and salt inside the Wadden Sea and the exchange between the Wadden Sea and the adjacent North Sea through the major tidal inlets. The model is simulating the reference years 2009‐2011. An overview of tidal prisms and residual volume fluxes of the main inlets and their variability is given. In addition, data from an intensive observational campaign in a tidal channel south of the island of Spiekeroog as well as satellite images and observations of sea surface properties from a ship of opportunity are used for the skill assessment. Finally, the intensity of estuarine overturning circulation and its variability in the tidal gullies are quantified and analyzed as function of gravitational and wind straining using various estimates including Total Exchange Flow (TEF). Regional differences between the gullies are assessed and drivers of the estuarine circulation are identified. For some inlets, the longitudinal buoyancy gradient dominates the exchange flow, for some others wind straining is more important. Also the intensity of tidal straining (scaled covariance of eddy viscosity and vertical shear) depends on buoyancy gradient and wind forcing in different ways, depending on local topography, orientation towards the main wind direction and influence by freshwater run‐off inside or outside the tidal basin. This article is protected by copyright. All rights reserved.
      PubDate: 2016-06-16T03:30:34.650685-05:
      DOI: 10.1002/2016JC011655
       
  • 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
       
  • The spectral albedo of sea ice and salt crusts on the tropical ocean of
           Snowball Earth: II. Optical modeling
    • Authors: Regina C. Carns; Bonnie Light, Stephen G. Warren
      Abstract: During the Snowball Earth events of the Neoproterozoic, tropical regions of the ocean could have developed a precipitated salt lag deposit left behind by sublimating sea ice. The major salt would have been hydrohalite, NaCl•2H2O. The crystals in such a deposit can be small and highly scattering, resulting in an allwave albedo similar to that of snow. The snow‐free sea ice from which such a crust could develop has a lower albedo, around 0.5, so the development of a crust would substantially increase the albedo of tropical regions on Snowball Earth. Hydrohalite crystals are much less absorptive than ice in the near‐infrared part of the solar spectrum, so their presence at the surface would increase the overall albedo as well as altering its spectral distribution. In this paper, we use laboratory measurements of the spectral albedo of a hydrohalite lag deposit, in combination with a radiative transfer model, to infer the inherent optical properties of hydrohalite as functions of wavelength. Using this result, we model mixtures of hydrohalite and ice representing both artificially created surfaces in the laboratory and surfaces relevant to Snowball Earth. The model is tested against sequences of laboratory measurements taken during the formation and the dissolution of a lag deposit of hydrohalite. We present a parameterization for the broadband albedo of cold, sublimating sea ice as it forms and evolves a hydrohalite crust, for use in climate models of Snowball Earth. This article is protected by copyright. All rights reserved.
      PubDate: 2016-06-16T03:25:59.963095-05:
      DOI: 10.1002/2016JC011804
       
  • 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
       
  • Effects of rotation on turbulent buoyant plumes in stratified environments
    • 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 non‐rotating 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 non‐rotating 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 crossflow speeds. This article is protected by copyright. All rights reserved.
      PubDate: 2016-06-16T03:25:40.110743-05:
      DOI: 10.1002/2016JC011737
       
  • Pathways and mechanisms of offshore water intrusions on the Espírito
           Santo Basin shelf (18°‐22°S, Brazil)
    • Abstract: The pathways and physical mechanisms associated with intrusions of cold, nutrient–rich South Atlantic Central Water (SACW) on the continental shelf of the Espírito Santo Basin (ESB), off southeast Brazil (18°S‐22°S), are investigated. To this end, a set of process–oriented, Primitive–Equation (PE) numerical models are used, together with an independent and more complete PE model, available observations and simple theoretical ideas. SACW enters the model ESB shelf mostly through two preferential pathways along the Tubarão Bight (TB, 19.5°S‐22°S). These pathways are found to be locations where an equatorward along–isobath pressure gradient force (PGFy*) of O(10−6m s−2) develops in response to steady wind forcing. This equatorward PGFy* is essentially in geostrophic balance, inducing onshore flow across the shelf edge and most of the shelf proper. The Brazil Current (BC) imparts an additional periodic (in the along–shelf direction) PGFy* on the shelf. The intrinsic pycnocline uplifting effect of the BC in making colder water available at the shelf edge is quantified. The BC also induces local intrusions by inertially overshooting the shelf edge, consistent with estimated Rossby numbers of ≈0.3–0.5. In addition, the planetary β–effect is related to a background equatorward PGFy*. A modified Arrested Topographic Wave model is shown to be a plausible rationalization for the shelf–wide spreading of the pressure field imparted by the BC at the shelf edge. The deep–ocean processes examined here are found to enhance the onshore transport of SACW, while wind forcing is found to dominate it at leading order. This article is protected by copyright. All rights reserved.
      PubDate: 2016-06-06T10:25:42.082206-05:
      DOI: 10.1002/2015JC011468
       
  • Examining features of enhanced phytoplankton biomass in the Bay of Bengal
           using a coupled physical‐biological model
    • Abstract: A coupled bio‐physical ocean model is used to describe areas of enhanced phytoplankton biomass, seen remotely sensed observations, in the otherwise oligotrophic environment of the Bay of Bengal. The model is based on the Naval Coastal Ocean Model (NCOM), which is one‐way coupled to the 13‐component Carbon, Silicate, and Nitrogen Ecosystem (CoSiNE) model and configured for the Indian Ocean. Model results are compared and evaluated against a set of in situ shipboard observations as well as ocean color data acquired from several remote sensing platforms. The model is shown to successfully simulate the seasonal cycle of phytoplankton, the markedly contrasting scenarios of phytoplankton distribution in the north versus the south Bay of Bengal, and the biological impact from the 1997/1998 Indian Ocean Dipole (IOD) event. The model simulation provides us with vertical cross sections of phytoplankton biomass from summer and winter blooms in the southwest of the bay; information not found in remotely sensed data. It also successfully reproduces the timing of the onset of the blooms and their spatial extent, thereby providing a measure of its potential for augmenting in situ and remotely sensed observations to improve understanding of the dynamics of primary producers and carbon cycling in one of the most poorly sampled regions of the world's oceans. This article is protected by copyright. All rights reserved.
      PubDate: 2016-05-23T11:14:46.032803-05:
      DOI: 10.1002/2015JC011508
       
  • Issue Information
    • Pages: 4435 - 4437
      PubDate: 2016-08-23T01:32:24.413393-05:
      DOI: 10.1002/jgrc.21407
       
  • On the long‐term stability of the Lofoten Basin Eddy
    • Pages: 4438 - 4449
      Abstract: In recent years, several studies have identified an area of intense anticyclonic activity about 500 km straight west of the Lofoten Islands at 70°N in the northern Norwegian Sea. Now recognized as the coherent Lofoten Basin Eddy (LBE), it is maintained by a supply of anticyclonic eddies that break away from the Norwegian Atlantic Current. Here we show from ship‐based surveys of its velocity field that it is quite stable with a central core in solid body rotation ∼1000 m deep, ∼8 km radius, and a relative vorticity close to its theoretical limit –f. The surveys also show the LBE typically has a >60 km radius with maximum swirl velocities at 17–20 km radius. From the velocity field, we estimate the dynamic height amplitude at the surface to be about ∼0.21 ± 0.03 dyn. m. Second, altimetry from the last 20 years shows the extremum in sea surface height relative to the surrounding waters to be about the same, 0.2 dyn. m. Third, a float trapped in the LBE for many months reveals a clear cyclonic wandering of the eddy over the deepest parts of the basin. Last, three hydrographic sections from the 1960s show the dynamic height signal to be virtually the same then as it is now. From these observations, we conclude that the LBE is a permanent feature of the Nordic Seas and plays a central role in maintaining the pool of warm water in the western Lofoten Basin.
      PubDate: 2016-07-01T10:41:21.264338-05:
      DOI: 10.1002/2016JC011726
       
  • Arctic Ocean stability: The effects of local cooling, oceanic heat
           transport, freshwater input, and sea ice melt with special emphasis on the
           Nansen Basin
    • Authors: Bert Rudels
      Pages: 4450 - 4473
      Abstract: The Arctic loses energy to space and heat is transported northward in the atmosphere and ocean. The largest transport occurs in the atmosphere. The oceanic heat flux is significantly smaller, and the warm water that enters the Arctic Ocean becomes covered by a low‐salinity surface layer, which reduces the heat transfer to the sea surface. This upper layer has two distinct regimes. In most of the deep basins it is due to the input of low‐salinity shelf water, ultimately conditioned by net precipitation and river runoff. The Nansen Basin is different. Here warm Atlantic water is initially in direct contact with and melts sea ice, its upper part being transformed into less dense surface water. The characteristics and depth of this layer are determined as functions of the temperature of the Atlantic water and for different energy losses using a one‐dimensional energy balance model. The amount of transformed Atlantic water is estimated for two different sea ice melt rates and the assumption of a buoyant boundary outflow. To create the upper layer sea ice formed elsewhere has to drift to the Nansen Basin. With reduced ice cover, this ice drift might weaken and the ice could disappear by the end of winter. The surface buoyancy input would disappear, and the upper layer might eventually convect back into the Atlantic water, reducing the formation of less dense Polar water. The created ice‐free areas would release more heat to the atmosphere and affect the atmospheric circulation.
      PubDate: 2016-07-01T10:41:05.523105-05:
      DOI: 10.1002/2015JC011045
       
  • On theories dealing with the interaction of surface waves and ocean
           circulation
    • Authors: George Mellor
      Pages: 4474 - 4486
      Abstract: The classic theory for the interaction of surface gravity waves and the general ocean circulation entails the so‐called wave radiation stress terms in the phase‐averaged momentum equation. The equations of motion are for the combined Eulerian current and Stokes drift. On the other hand, a more recent approach includes the so‐called vortex force term in the momentum equation wherein the only wave property is Stokes drift. The equations of motion are for the Eulerian current. The idea has gained traction in the ocean science community, a fact that motivates this paper. A question is: can both theories be correct? This paper answers the question in the negative and presents arguments in favor of the wave radiation theory. The vortex force approach stems from an interesting mathematical construct, but it does stand up to physical or mathematical scrutiny as described in this paper. Although not the primary focus of the paper, some discussion of Langmuir circulation is included since the vortex force was first introduced as the basis of this oceanic cellular phenomenon. Finaly the paper explains the difference in the derivation of the radiation stress theory and the vortex force theory: the later theory entails errors related to its use of curl and reverse‐curl [or uncurl] processes.
      PubDate: 2016-07-01T10:41:15.017843-05:
      DOI: 10.1002/2016JC011768
       
  • Potential new production in two upwelling regions of the western Arabian
           Sea: Estimation and comparison
    • Authors: Xiaomei Liao; Haigang Zhan, Yan Du
      Pages: 4487 - 4502
      Abstract: Using satellite‐derived and in situ data, the wind‐driven potential new production (nitrate supply) for the 300 km wide coastal band in two upwelling regions of the western Arabian Sea (AS) during the southwest monsoon is estimated. The upward nitrate flux to the euphotic zone is generally based on the physical processes of coastal transport (Ekman transport and geostrophic transport) and offshore Ekman pumping. The coastal geostrophic current in the western AS influences the upwelling intensity and latitudinal distributions of nitrate supply. The Oman and Somalia upwelling regions have similar level of potential new production (nitrate supply) during the summer monsoon, while the satellite estimates of primary production off Oman are 2 times greater than those off Somalia. The much higher potential f‐ratio in the Somalia upwelling region indicates that the primary production could be limited by availability of other macronutrients (e.g., silicate). The correlation analysis of the primary production and the aerosol optical thickness shows that the Oman upwelling region displays a stronger coupling between the atmospheric deposition and the phytoplankton abundance. The high summertime dust levels in the atmosphere are suggested to contribute to the high primary production in the Oman upwelling region.
      PubDate: 2016-07-01T10:41:30.091341-05:
      DOI: 10.1002/2016JC011707
       
  • Quantifying mesoscale eddies in the Lofoten Basin
    • Pages: 4503 - 4521
      Abstract: The Lofoten Basin is the most eddy rich region in the Norwegian Sea. In this paper, the characteristics of these eddies are investigated from a comprehensive database of nearly two decades of satellite altimeter data (1995–2013) together with Argo profiling floats and surface drifter data. An automated method identified 1695/1666 individual anticyclonic/cyclonic eddies in the Lofoten Basin from more than 10,000 altimeter‐based eddy observations. The eddies are found to be predominantly generated and residing locally. The spatial distributions of lifetime, occurrence, generation sites, size, intensity, and drift of the eddies are studied in detail. The anticyclonic eddies in the Lofoten Basin are the most long‐lived eddies (>60 days), especially in the western part of the basin. We reveal two hotspots of eddy occurrence on either side of the Lofoten Basin. Furthermore, we infer a cyclonic drift of eddies in the western Lofoten Basin. Barotropic energy conversion rates reveals energy transfer from the slope current to the eddies during winter. An automated colocation of surface drifters trapped inside the altimeter‐based eddies are used to corroborate the orbital speed of the anticyclonic and cyclonic eddies. Moreover, the vertical structure of the altimeter‐based eddies is examined using colocated Argo profiling float profiles. Combination of altimetry, Argo floats, and surface drifter data is therefore considered to be a promising observation‐based approach for further studies of the role of eddies in transport of heat and biomass from the slope current to the Lofoten Basin.
      PubDate: 2016-07-02T02:30:35.394935-05:
      DOI: 10.1002/2016JC011637
       
  • Carbon exchange between a shelf sea and the ocean: The Hebrides Shelf,
           west of Scotland
    • Pages: 4522 - 4544
      Abstract: Global mass balance calculations indicate the majority of particulate organic carbon (POC) exported from shelf seas is transferred via downslope exchange processes. Here we demonstrate the downslope flux of POC from the Hebrides Shelf is approximately 3‐ to 5‐fold larger per unit length/area than the global mean. To reach this conclusion, we quantified the offshore transport of particulate and dissolved carbon fractions via the “Ekman Drain,” a strong downwelling feature of the NW European Shelf circulation, and subsequently compared these fluxes to simultaneous regional air‐sea CO2 fluxes and onshore wind‐driven Ekman fluxes to constrain the carbon dynamics of this shelf. Along the shelf break, we estimate a mean offshelf total carbon (dissolved + particulate) flux of 4.2 tonnes C m−1 d−1 compared to an onshelf flux of 4.5 tonnes C m−1 d−1. Organic carbon represented 3.3% of the onshelf carbon flux but 6.4% of the offshelf flux indicating net organic carbon export. Dissolved organic carbon represented 95% and POC 5% of the exported organic carbon pool. When scaled along the shelf break the total offshelf POC flux (0.007 Tg C d−1) was found to be 3 times larger than the regional air‐sea CO2 ingassing flux (0.0021 Tg C d−1), an order of magnitude larger than the particulate inorganic carbon flux (0.0003 Tg C d−1) but far smaller than the DIC (2.03 Tg C d−1) or DOC (0.13 Tg C d−1) fluxes. Significant spatial heterogeneity in the Ekman drain transport confirms that offshelf carbon fluxes via this mechanism are also spatially heterogeneous.
      PubDate: 2016-07-02T07:41:19.505372-05:
      DOI: 10.1002/2015JC011599
       
  • The Sicily Channel surface circulation revisited using a neural clustering
           analysis of a high‐resolution simulation
    • Pages: 4545 - 4567
      Abstract: The Sicily Channel surface circulation is investigated by analyzing the outputs of a high‐resolution ocean model MED12 forced during 46 years by the ARPERA atmospheric fields. Applying a neural network classifier, we show that the surface circulation in the Sicily Channel can be decomposed into 8 modes characterizing the major patterns of that circulation, particularly the Algerian Current separation at the entrance to the Sicily Channel, the features of the Atlantic Tunisian Current and of the Atlantic Ionian Stream. These modes reflect the variability of the circulation in space and time at seasonal and inter‐annual scales. Some modes preferably occur in winter whilst others are characteristic of summer. The mode sequence presents an inter‐annual variability in good agreement with observations. The topography of the Sicily Channel sill plays a major role in steering the circulation. In particular the summer upwelling along the southern coast of Sicily, which is present in several modes, could be explained by a large‐scale density forcing. A combination of barotropic/baroclinic double Kelvin waves generated on both sides of the sill provides a mechanism for explaining the complexity of the surface circulation advecting the surface waters from the Western Mediterranean toward the Eastern Mediterranean, the most salient features of which are the Atlantic Tunisian Current, the Atlantic Ionian Stream and the Tyrrhenian Sicilian Current which is a new feature highlighted by the present study.
      PubDate: 2016-07-02T07:41:15.520383-05:
      DOI: 10.1002/2015JC011472
       
  • On subsurface cooling associated with the Biobio River Canyon (Chile)
    • Pages: 4568 - 4584
      Abstract: Submarine canyons cutting across the continental shelf can modulate the cross‐shelf circulation being effective pathways to bring water from the deep ocean onto the shelf. Here, we use 69 days of moored array observations of temperature and ocean currents collected during the spring of 2013 and winter‐spring 2014, as well as shipboard hydrographic surveys and sea‐level observations to characterize cold, oxygen poor, and nutrient‐rich upwelling events along the Biobio Submarine Canyon (BbC). The BbC is located within the Gulf of Arauco at 36° 50'S in the Central Chilean Coast. The majority of subtidal temperature at 150 m depth is explained by subtidal variability in alongshore currents on the canyon with a lag of less than a day (r2 = 0.65). Using the vertical displacement of the 10° and 10.5°C isotherms, we identified nine upwelling events, lasting between 20 h to 4.5 days, that resulted in vertical isothermal displacements ranging from 29 to 137 m. The upwelled water likely originated below 200 m. Majority of the cooling events were related with strong northward (opposite Kelvin wave propagation) flow and low pressure at the coast. Most of these low pressure events occur during relatively weak local wind forcing conditions, and were instead related with Coastal Trapped Waves (CTWs) propagating southwards from lower latitudes. These cold, high‐nutrient, low‐oxygen waters may be further upwelled and advected into the Gulf of Arauco by wind forcing. Thus, canyon upwelling may be a key driver of biological productivity and oxygen conditions in this Gulf.
      PubDate: 2016-07-02T05:59:20.889345-05:
      DOI: 10.1002/2016JC011796
       
  • Saw‐tooth modulation of the deep‐water thermohaline properties
           in the southern Adriatic Sea
    • Pages: 4585 - 4600
      Abstract: In this study, we investigate the dynamics of the bottom layer of the southern Adriatic Sea (eastern Mediterranean basin) by merging experimental measurements and numerical simulations. We hypothesize that the recently observed continuous density decrease over time, which was basically related to a temperature increase, and the following sudden density rise, which was caused by the intrusion of very dense water masses (cold but relatively fresh), constitute one cycle of a general “saw‐tooth” pattern: the alternation of long‐lasting and almost linear density decreases (mixing phases) and sudden density increases (dense water intrusion phases). The model results, which provide a basin‐scale view of the process, corroborate this theory because they satisfactorily reproduced the observed oceanographic features. We describe the almost linear density decrease in terms of local mixing fostered by the advection of flow instabilities that originate from the large‐scale quasi‐permanent cyclonic circulation. Conversely, diffusive processes play a minor role in determining the bottom layer thermohaline variability. The interpretation of the experimental findings, supported by the numerical simulations, suggests that similar dynamics might be observed in other basins characterized by similar bathymetric and hydrodynamic features.
      PubDate: 2016-07-02T06:00:13.003097-05:
      DOI: 10.1002/2015JC011522
       
  • Time series of temperature in Fram Strait determined from the
           2008–2009 DAMOCLES acoustic tomography measurements and an ocean
           model
    • Pages: 4601 - 4617
      Abstract: A pilot acoustic tomography program in Fram Strait during 2008–2009 measured a year‐long record of acoustic travel times along a 130 km range acoustic path crossing the West Spitsbergen Current. Individual ray arrivals were not observed. Rather, the arrival patterns consisted of a single, stable, broad arrival pulse of about 100 ms duration. Travel time variations of ±0.15 s recorded the vigorous mesoscale environment of the region and the seasonal cycle. To estimate ocean temperature from the tomography data an inverse scheme employed a high‐resolution ocean model for Fram Strait as the reference ocean. The information from the tomographic measurements is primarily average temperature. Estimated temperatures, averaged over 0–1000 m depth and over range, had a mean of 1.11°C and variations of ±0.33°C; the uncertainty of the tomography estimates was about 60m°C. Agreement with an alternate inverse approach based on EOFs and a Markov Chain Monte Carlo inversion scheme relying on a matched‐peak approach was excellent, indicating a robust estimate for ocean temperature. The inverse estimates for average temperature agreed with the equivalent estimates from hydrographic sections obtained along the acoustic path at the start and end of the program. Among other deficiencies, the ocean model greatly underestimated the intensity of the mesoscale fluctuations and exhibited a warm bias of about 0.38°C in section‐averaged temperature. Tomographic measurements in Fram Strait offer unique large‐scale temperature constraints for ocean models through data assimilation. It is anticipated that these constraints will lead to more accurate estimates of the circulation and transports in Fram Strait.
      PubDate: 2016-07-02T07:41:09.430205-05:
      DOI: 10.1002/2015JC011591
       
  • Changes in anthropogenic carbon storage in the Northeast Pacific in the
           last decade
    • Authors: Sophie N. Chu; Zhaohui Aleck Wang, Scott C. Doney, Gareth L. Lawson, Katherine A. Hoering
      Pages: 4618 - 4632
      Abstract: In order to understand the ocean's role as a sink for anthropogenic carbon dioxide (CO2), it is important to quantify changes in the amount of anthropogenic CO2 stored in the ocean interior over time. From August to September 2012, an ocean acidification cruise was conducted along a portion of the P17N transect (50°N 150°W to 33.5°N 135°W) in the Northeast Pacific. These measurements are compared with data from the previous occupation of this transect in 2001 to estimate the change in the anthropogenic CO2 inventory in the Northeast Pacific using an extended multiple linear regression (eMLR) approach. Maximum increases in the surface waters were 11 µmol kg−1 over 11 years near 50°N. Here, the penetration depth of anthropogenic CO2 only reached ∼300 m depth, whereas at 33.5°N, penetration depth reached ∼600 m. The average increase of the depth‐integrated anthropogenic carbon inventory was 0.41 ± 0.12 mol m−2 yr−1 across the transect. Lower values down to 0.20 mol m−2 yr−1 were observed in the northern part of the transect near 50°N and increased up to 0.55 mol m−2 yr−1 toward 33.5°N. This increase in anthropogenic carbon in the upper ocean resulted in an average pH decrease of 0.002 ± 0.0003 pH units yr−1 and a 1.8 ± 0.4 m yr−1 shoaling rate of the aragonite saturation horizon. An average increase in apparent oxygen utilization of 13.4 ± 15.5 µmol kg−1 centered on isopycnal surface 26.6 kg m−3 from 2001 to 2012 was also observed.
      PubDate: 2016-07-02T07:40:47.469193-05:
      DOI: 10.1002/2016JC011775
       
  • Swell impact on wind stress and atmospheric mixing in a regional coupled
           atmosphere‐wave model
    • Pages: 4633 - 4648
      Abstract: Over the ocean, the atmospheric turbulence can be significantly affected by swell waves. Change in the atmospheric turbulence affects the wind stress and atmospheric mixing over swell waves. In this study, the influence of swell on atmospheric mixing and wind stress is introduced into an atmosphere‐wave‐coupled regional climate model, separately and combined. The swell influence on atmospheric mixing is introduced into the atmospheric mixing length formula by adding a swell‐induced contribution to the mixing. The swell influence on the wind stress under wind‐following swell, moderate‐range wind, and near‐neutral and unstable stratification conditions is introduced by changing the roughness length. Five year simulation results indicate that adding the swell influence on atmospheric mixing has limited influence, only slightly increasing the near‐surface wind speed; in contrast, adding the swell influence on wind stress reduces the near‐surface wind speed. Introducing the wave influence roughness length has a larger influence than does adding the swell influence on mixing. Compared with measurements, adding the swell influence on both atmospheric mixing and wind stress gives the best model performance for the wind speed. The influence varies with wave characteristics for different sea basins. Swell occurs infrequently in the studied area, and one could expect more influence in high‐swell‐frequency areas (i.e., low‐latitude ocean). We conclude that the influence of swell on atmospheric mixing and wind stress should be considered when developing climate models.
      PubDate: 2016-07-02T05:58:40.499775-05:
      DOI: 10.1002/2015JC011576
       
  • Climatology and linear trends of seasonal water temperature and heat
           budget in a semienclosed sea connected to the Kuroshio region
    • Authors: Eisuke Tsutsumi; Xinyu Guo
      Pages: 4649 - 4669
      Abstract: The climatology and linear trend of seasonal water temperature and heat budget in a semienclosed sea connected to the Kuroshio region (Seto Inland Sea: SIS) are investigated by constructing and analyzing a gridded data set of water temperature, salinity, and air‐sea heat fluxes. In the SIS, winter‐time water temperature and ocean heat content (OHC) showed a significant increasing trend while those in summer‐time exhibited no significant change. By analyzing the heat budget between the tendency of ocean heat content ∂OHC/∂t and air‐sea net heat flux Qnet, the driving factor of the winter‐time warming trend in the SIS was identified as autumn Qnet, while the unchanged summer‐time OHC is mainly due to decreased spring‐time oceanic heat transport to the SIS from the Kuroshio region. In addition, we showed that the decreased spring‐time heat transport could have been induced by upwelling due to wind curl and wind speed in the shelf‐slope region, heat transport by the Kuroshio south of Shikoku Island, and freshwater input to the SIS from rivers. The importance of both oceanic and atmospheric forcing mechanisms for explaining variation in the water temperature in coastal areas is demonstrated.
      PubDate: 2016-07-02T07:40:59.7108-05:00
      DOI: 10.1002/2016JC011748
       
  • Turbulent plumes from a glacier terminus melting in a stratified ocean
    • Authors: Samuel J. Magorrian; Andrew J. Wells
      Pages: 4670 - 4696
      Abstract: The melting of submerged faces of marine‐terminating glaciers is a key contributor to the glacial mass budget via direct thermodynamic ablation and the impact of ablation on calving. This study considers the behavior of turbulent plumes of buoyant meltwater in a stratified ocean, generated by melting of either near‐vertical calving faces or sloping ice shelves. We build insight by applying a turbulent plume model to describe melting of a locally planar region of ice face in a linearly stratified ocean, in a regime where subglacial discharge is insignificant. The plumes rise until becoming neutrally buoyant, before intruding into the ocean background. For strong stratifications, we obtain leading‐order scaling laws for the flow including the height reached by the plume before intrusion, and the melt rate, expressed in terms of the background ocean temperature and salinity stratifications. These scaling laws provide a new perspective for parameterizing glacial melting in response to a piecewise‐linear discretization of the ocean stratification.
      PubDate: 2016-07-10T03:21:30.778657-05:
      DOI: 10.1002/2015JC011160
       
  • Numerical modeling of intrinsically and extrinsically forced seasonal
           circulation in the China Seas: A kinematic study
    • Authors: Jianping Gan; Zhiqiang Liu, Linlin Liang
      Pages: 4697 - 4715
      Abstract: We developed a new three‐dimensional, high‐resolution ocean circulation model for the entire China Seas (CS) region. The model considered the linked physics associated with the western boundary current, monsoonal wind, and tidal forcings, and topography in both the CS and the adjacent oceans. From this well‐validated model, we derived new insights into the three‐dimensional seasonal circulation of the CS in response to the intrinsic forcing of monsoonal winds and extrinsic forcing of flow exchange with adjacent oceans through the straits and over the slope around the periphery of the CS. Besides the East Asian monsoon forcing, we found that the extrinsic forcings interact coherently with each other and with the interior circulation to jointly shape the CS circulation. Specifically, we revealed rotating layered circulation in the CS. The circulation in the South China Sea has a vertical cyclonic‐anticyclonic‐cyclonic pattern in the upper‐middle‐lower layers, which we relate to the inflow‐outflow‐inflow transport in those layers in the Luzon Strait. The circulation in the East China Sea (ECS) is characterized by a vertically variable cyclonically rotating flow, and the circulation in the Yellow Sea (YS) is represented by a cyclonic movement in the upper layer and an anticyclonic movement in the lower layer. We attribute the cross‐shelf variation of the along‐shelf current to the ECS circulation pattern, while the vertically variable intrusive current at the central trough, together with the seasonally varied west and east coastal currents, shape the two‐layer circulation in the YS.
      PubDate: 2016-07-10T03:21:12.981071-05:
      DOI: 10.1002/2016JC011800
       
  • Seasonal and interannual variations of mixed layer salinity in the
           southeast tropical Indian Ocean
    • Authors: Ningning Zhang; Ming Feng, Yan Du, Jian Lan, Susan E. Wijffels
      Pages: 4716 - 4731
      Abstract: In this study, seasonal and interannual variations of the mixed layer salinity (MLS) in the southeast tropical Indian Ocean (SETIO) are analyzed using satellite observations, historical data sets, and data‐assimilating ocean model outputs. On the seasonal cycle, the MLS in the SETIO becomes fresher in austral winter and saltier in austral summer: between the Java‐Lesser Sunda coast and the South Equatorial Current (SEC, 12°S), where positive entrainment and fresh advections counterbalance each other, the annual cycle of the MLS closely follows the variation of the air‐sea freshwater forcing; off the northwest and west Australian coasts, the MLS variations are influenced by the annual cycles of the Indonesian Throughflow (ITF) and Leeuwin Current (LC) transports as well as the air‐sea freshwater forcing, with eddy fluxes acting to freshen the MLS along the SEC, the Eastern Gyral Current, and the LC. On the interannual‐scale, El Niño (La Niña) events are typically associated with saltier (fresher) MLS in the SETIO. Composite and budget analyses reveal that interannual variations in precipitations drive the MLS anomalies off the Java‐Lesser Sunda coast; between 12°S and the northwest Australian coast, the MLS variations are influenced by both advection anomalies and local precipitation anomalies; whereas anomalous meridional currents contribute to the MLS variations off the west Australian coast. Both enhanced local precipitations and the ITF transport anomalies have substantial contributions to the drastic freshening of the Indonesian‐Australian Basin between the Java‐Lesser Sunda coast and the northwest Australian coast during the extended La Niña events in 1999–2001 and 2010–2012.
      PubDate: 2016-07-10T03:21:20.606067-05:
      DOI: 10.1002/2016JC011854
       
  • The eddy kinetic energy budget in the Red Sea
    • Authors: Peng Zhan; Aneesh C. Subramanian, Fengchao Yao, Aditya R. Kartadikaria, Daquan Guo, Ibrahim Hoteit
      Pages: 4732 - 4747
      Abstract: The budget of eddy kinetic energy (EKE) in the Red Sea, including the sources, redistributions, and sink, is examined using a high'resolution eddy‐resolving ocean circulation model. A pronounced seasonally varying EKE is identified, with its maximum intensity occurring in winter, and the strongest EKE is captured mainly in the central and northern basins within the upper 200 m. Eddies acquire kinetic energy from conversion of eddy available potential energy (EPE), from transfer of mean kinetic energy (MKE), and from direct generation due to time‐varying (turbulent) wind stress, the first of which contributes predominantly to the majority of the EKE. The EPE‐to‐EKE conversion occurs almost in the entire basin, while the MKE‐to‐EKE transfer appears mainly along the shelf boundary of the basin (200 m isobath) where high horizontal shear interacts with topography. The EKE generated by the turbulent wind stress is relatively small and limited to the southern basin. All these processes are intensified during winter, when the rate of energy conversion is about 4–5 times larger than that in summer. The EKE is redistributed by the vertical and horizontal divergence of energy flux and the advection of the mean flow. As a main sink of EKE, dissipation processes is ubiquitously found in the basin. The seasonal variability of these energy conversion terms can explain the significant seasonality of eddy activities in the Red Sea.
      PubDate: 2016-07-10T03:21:05.833984-05:
      DOI: 10.1002/2015JC011589
       
  • Interannual variability in net community production at the Western
           Antarctic Peninsula region (1997–2014)
    • Authors: Zuchuan Li; Nicolas Cassar, Kuan Huang, Hugh Ducklow, Oscar Schofield
      Pages: 4748 - 4762
      Abstract: In this study, we examined the interannual variability of net community production (NCP) in the Western Antarctic Peninsula (WAP) using in situ O2/Ar‐NCP estimates (2008–2014) and satellite data (SeaWiFS and MODIS‐Aqua) from 1997 to 2014. We found that NCP generally first peaks offshore and follows sea‐ice retreat from offshore to inshore. Annually integrated NCP (ANCP) displays an onshore‐to‐offshore gradient, with coastal and shelf regions up to 8 times more productive than offshore regions. We examined potential drivers of interannual variability in the ANCP using an Empirical Orthogonal Function (EOF) analysis. The EOF's first mode explains ∼50% of the variance, with high interannual variability observed seaward of the shelf break. The first principal component is significantly correlated with the day of sea‐ice retreat (R = −0.58, p 
      PubDate: 2016-07-10T03:21:00.78141-05:0
      DOI: 10.1002/2015JC011378
       
  • Impact of current speed on mass flux to a model flexible seagrass blade
    • Authors: Jiarui Lei; Heidi Nepf
      Pages: 4763 - 4776
      Abstract: Seagrass and other freshwater macrophytes can acquire nutrients from surrounding water through their blades. This flux may depend on the current speed (U), which can influence both the posture of flexible blades (reconfiguration) and the thickness of the flux‐limiting diffusive layer. The impact of current speed (U) on mass flux to flexible blades of model seagrass was studied through a combination of laboratory flume experiments, numerical modeling and theory. Model seagrass blades were constructed from low‐density polyethylene (LDPE), and 1, 2‐dichlorobenzene was used as a tracer chemical. The tracer mass accumulation in the blades was measured at different unidirectional current speeds. A numerical model was used to estimate the transfer velocity (K) by fitting the measured mass uptake to a one‐dimensional diffusion model. The measured transfer velocity was compared to predictions based on laminar and turbulent boundary layers developing over a flat plate parallel to flow, for which K∝U0.5 and ∝U, respectively. The degree of blade reconfiguration depended on the dimensionless Cauchy number, Ca, which is a function of both the blade stiffness and flow velocity. For large Ca, the majority of the blade was parallel to the flow, and the measured transfer velocity agreed with laminar boundary layer theory,  K∝U0.5. For small Ca, the model blades remained upright, and the flux to the blade was diminished relative to the flat‐plate model. A meadow‐scale analysis suggests that the mass exchange at the blade scale may control the uptake at the meadow scale.
      PubDate: 2016-07-10T03:20:56.979029-05:
      DOI: 10.1002/2016JC011826
       
  • Revisiting the cause of the eastern equatorial Atlantic cold event in 2009
    • Pages: 4777 - 4789
      Abstract: An extreme cold sea surface temperature event occurred in the Atlantic cold tongue region in boreal summer 2009. It was preceded by a strong negative Atlantic meridional mode event associated with north‐westerly wind anomalies along the equator from March to May. Although classical equatorial wave dynamics suggest that westerly wind anomalies should be followed by a warming in the eastern equatorial Atlantic, an abrupt cooling took place. In the literature two mechanisms—meridional advection of subsurface temperature anomalies and planetary wave reflection—are discussed as potential causes of such an event. Here, for the first time we use in situ measurements in addition to satellite and reanalysis products to investigate the contribution of both mechanisms to the 2009 cold event. Our results suggest that meridional advection is less important in cold events than in corresponding warm events, and, in particular, did not cause the 2009 cold event. Argo float data confirm previous findings that planetary wave reflection contributed to the onset of the 2009 cold event. Additionally, our analysis suggests that higher baroclinic modes were involved.
      PubDate: 2016-07-10T03:20:52.493957-05:
      DOI: 10.1002/2016JC011719
       
  • Kuroshio subsurface water feeds the wintertime Taiwan Warm Current on the
           inner East China Sea shelf
    • Authors: Ergang Lian; Shouye Yang, Hui Wu, Chengfan Yang, Chao Li, James T. Liu
      Pages: 4790 - 4803
      Abstract: The Taiwan Warm Current (TWC) has an overwhelming influence on the heat, salt, and nutrients balance on one of the broadest shelf in the world, the East China Sea shelf. In winter, the TWC flows in an unusual upwind direction and reaches the Changjiang (Yangtze River) Estuary, but its origin and pathway are intensely debated. Here combined evidences from current measurement, hydrographic, and stable isotopic data all suggest that the wintertime TWC intrusion off the Changjiang Estuary mainly originates from the Kuroshio subsurface water northeast of Taiwan, rather than from the Taiwan Strait warm water. The Kuroshio‐branched water northeast of Taiwan can intrude into the inner shelf near the Zhe‐Min Coast via bottom layer, manifesting by a pronounced boundary at 50 m isobath around 28°N, and thereby feeds the TWC intrusion into the Changjiang Estuary. The intrusion complicates the hydrological process in the estuary and shelf sea, and its impact on marine environment deserves more research attentions.
      PubDate: 2016-07-10T03:20:47.82007-05:0
      DOI: 10.1002/2016JC011869
       
  • Wintertime water dynamics and moonlight disruption of the acoustic
           backscatter diurnal signal in an ice‐covered Northeast Greenland
           fjord
    • Pages: 4804 - 4818
      Abstract: Six and a half month records from three ice‐tethered Acoustic Doppler Current Profilers deployed in October 2013 in Young Sound fjord in Northeast Greenland are used to analyze the acoustic backscatter signal. The acoustic data suggest a systematic diel vertical migration (DVM) of scatters below the land‐fast ice during polar night. The scatters were likely composed of zooplankton. The acoustic signal pattern typical to DVM persisted in Young Sound throughout the entire winter including the period of civil polar night. However, polynya‐enhanced estuarine‐like cell circulation that occurred during winter disrupted the DVM signal favoring zooplankton to occupy the near‐surface water layer. This suggests that zooplankton avoided spending additional energy crossing the interface with a relatively strong velocity gradient comprised by fjord inflow in the intermediate layer and outflow in the subsurface layer. Instead, the zooplankton tended to remain in the upper 40 m layer where relatively warmer water temperatures associated with upward heat flux during enhanced estuarine‐like circulation could be energetically favorable. Furthermore, our data show moonlight disruption of DVM in the subsurface layer and weaker intensity of vertical migration beneath snow covered land‐fast ice during polar night. Finally, by using existing models for lunar illuminance and light transmission through sea ice and snow cover, we estimated under ice illuminance and compared it with known light sensitivity of Arctic zooplankton species.
      PubDate: 2016-07-16T10:10:51.408066-05:
      DOI: 10.1002/2016JC011703
       
  • Quantifying uncertainty in Gulf of Mexico forecasts stemming from
           uncertain initial conditions
    • Pages: 4819 - 4832
      Abstract: Polynomial Chaos (PC) methods are used to quantify the impacts of initial conditions uncertainties on oceanic forecasts of the Gulf of Mexico circulation. Empirical Orthogonal Functions are used as initial conditions perturbations with their modal amplitudes considered as uniformly distributed uncertain random variables. These perturbations impact primarily the Loop Current system and several frontal eddies located in its vicinity. A small ensemble is used to sample the space of the modal amplitudes and to construct a surrogate for the evolution of the model predictions via a nonintrusive Galerkin projection. The analysis of the surrogate yields verification measures for the surrogate's reliability and statistical information for the model output. A variance analysis indicates that the sea surface height predictability in the vicinity of the Loop Current is limited to about 20 days.
      PubDate: 2016-07-16T10:11:08.296055-05:
      DOI: 10.1002/2015JC011573
       
  • Tropical and extratropical‐origin storm wave types and their
           influence on the East Australian longshore sand transport system under a
           changing climate
    • Authors: Ian D. Goodwin; Thomas R. Mortlock, Stuart Browning
      Pages: 4833 - 4853
      Abstract: Tropical expansion is potentially an amplifier of coastal change in the subtropics, through directional wave climate shifts. The storm wave climate and directional wave power distribution along the Southeast Australian Shelf (SEAS) is investigated with respect to tropical extent. Forty years of storm wave observations from nine midshelf wave buoys are evaluated using synoptic storm wave typing. A robust latitudinal and along‐shelf gradient in storm wave types and wave propagation patterns exists. The tropical origin storms produce a shore‐normal propagation pattern along the SEAS, reduce the connectivity of coastal compartments through minor headland bypassing events or episodically reversing the net northward transport. In contrast, the extratropical origin storms produce a shore‐oblique propagation pattern from the Southern Tasman to the Coral Sea, and are an important control on the connectivity of regional longshore sand transport through episodic major headland bypassing events between compartments, and the maintenance of down‐drift coastlines in dynamic equilibrium. Future climate change projections indicate that the recent trend in the expansion of the latitudinal extent of the tropics in the south‐west Pacific region will continue throughout this century. The combined impacts of a projected 2.5° poleward shift on the storm wave climate is a significant reduction in net northward longshore sand transport and the efficiency of headland bypassing events. On the North and Central Coasts of New South Wales we project a ∼30% reduction in longshore sand transport for the dominant extratropical‐origin storm events, together with a ∼5% increase in reversed (net southward) longshore sand transport for tropical‐origin storm events.
      PubDate: 2016-07-16T10:11:04.036531-05:
      DOI: 10.1002/2016JC011769
       
  • Malvinas Current variability from Argo floats and satellite altimetry
    • Pages: 4854 - 4872
      Abstract: The Malvinas Current (MC) is an offshoot of the Antarctic Circumpolar Current (ACC). Downstream of Drake Passage, the northern fronts of the ACC veer northward, cross over the North Scotia Ridge (NSR) and the Malvinas Plateau, and enter the Argentine Basin. We investigate the variations of the MC circulation between the NSR and 41°S and their possible relations with the ACC circulation using data from Argo floats and satellite altimetry. The data depict meandering and eddy shedding of the northern ACC jets as they cross the NSR. The altimetry fields show that these eddies are trapped, break down, and dissipate over the Malvinas Plateau, suggesting that this region is a hot spot for dissipation of mesoscale variability. Variations of sea level anomalies (SLA) across the NSR do not impact the MC further north, except for intra‐seasonal variability associated with coastal trapped waves. Altimetry and float trajectories show events during which a large fraction of the MC is cut off from the ACC. Blocking events at around 48.5°S are a recurrent feature of the MC circulation. Over the 23 year altimetry record, we detected 26 events during which the MC surface transport at 48.5°S was reduced to less than half its long‐term mean. Blocking events last from 10 to 35 days and do not present any significant trend. These events were tracked back to positive SLA that built up over the Argentine Abyssal Plain. Future work is needed to understand the processes responsible for these blocking events.
      PubDate: 2016-07-16T10:10:35.266508-05:
      DOI: 10.1002/2016JC011889
       
  • Nonlinear and minor ocean tides in the Bay of Biscay from the strain tides
           observed by two geodetic laser strainmeters at Canfranc (Spain)
    • Authors: Antonella Amoruso; Luca Crescentini
      Pages: 4873 - 4887
      Abstract: This work presents and discusses the tidal analysis of about 2.5 years of strain data recorded by two 70 m‐long high‐resolution laser strainmeters, which are operating in the Canfranc underground laboratory (Central Pyrenees, Spain). Spectra show clear tidal peaks whose frequencies range from the diurnal band to at least 8 cycles per day; strain amplitudes (relative change in length for each strainmeter) range from few 10−12 to 10−8. The reliability of observations and corrections for local distortions are testified by the excellent agreement between measurements and computations for all the diurnal and semidiurnal tides included in the TPXO8 and FES2012 tidal atlases, with the exception of L2. Observed higher‐frequency strain tides are mostly attributed to loading by nonlinear shallow‐water constituents in the Bay of Biscay, more than 120 km from the measurement site. The signals are quantitatively compared with computations using TPXO8 (MN4, M4, and MS4) and FES2012 (M3, N4, MN4, M4, MS4, and M6). Computations fully agree with M4 observations for one strainmeter, overestimate M4 by about 30% for the other strainmeter and M6 by about a factor of two, and underestimate the other tides. This work shows that data from high‐sensitivity strainmeters installed in high‐quality sites may provide valuable additional information for studying the nonlinear tidal dynamics and energetics of coastal waters and minor ocean tides, at spatial resolutions of tens to thousands square kilometers, depending on the strainmeter location with respect to the coastline.
      PubDate: 2016-07-16T10:10:39.625724-05:
      DOI: 10.1002/2016JC011733
       
  • Exploiting coastal altimetry to improve the surface circulation scheme
           over the central Mediterranean Sea
    • Pages: 4888 - 4909
      Abstract: This work is the first study exploiting along track altimetry data to observe and monitor coastal ocean features over the transition area between the western and eastern Mediterranean Basins. The relative performances of both the AVISO and the X‐TRACK research regional altimetric data sets are compared using in situ observations. Both products are cross validated with tide gauge records. The altimeter‐derived geostrophic velocities are also compared with observations from a moored Acoustic Doppler Current Profiler. Results indicate the good potential of satellite altimetry to retrieve dynamic features over the area. However, X‐TRACK shows a more homogenous data coverage than AVISO, with longer time series in the 50 km coastal band. The seasonal evolution of the surface circulation is therefore analyzed by conjointly using X‐TRACK data and remotely sensed sea surface temperature observations. This combined data set clearly depicts different current regimes and bifurcations, which allows us to propose a new seasonal circulation scheme for the central Mediterranean. The analysis shows variations of the path and temporal behavior of the main circulation features: the Atlantic Tunisian Current, the Atlantic Ionian Stream, the Atlantic Libyan Current, and the Sidra Gyre. The resulting bifurcating veins of these currents are also discussed, and a new current branch is observed for the first time.
      PubDate: 2016-07-16T10:10:47.151286-05:
      DOI: 10.1002/2016JC011961
       
  • Mesoscale resolution capability of altimetry: Present and future
    • Pages: 4910 - 4927
      Abstract: Wavenumber spectra of along‐track Sea Surface Height from the most recent satellite radar altimetry missions [Jason‐2, Cryosat‐2, and SARAL/Altika) are used to determine the size of ocean dynamical features observable with the present altimetry constellation. A global analysis of the along‐track 1‐D mesoscale resolution capability of the present‐day altimeter missions is proposed, based on a joint analysis of the spectral slopes in the mesoscale band and the error levels observed for horizontal wavelengths lower than 20km. The global sea level spectral slope distribution provided by Xu and Fu () with Jason‐1 data is revisited with more recent altimeter missions, and maps of altimeter error levels are provided and discussed for each mission. Seasonal variations of both spectral slopes and altimeter error levels are also analyzed for Jason‐2. SARAL/Altika, with its lower error levels, is shown to detect smaller structures everywhere. All missions show substantial geographical and temporal variations in their mesoscale resolution capabilities, with variations depending mostly on the error level change but also on slight regional changes in the spectral slopes. In western boundary currents where the signal to noise ratio is favorable, the along‐track mesoscale resolution is approximately 40 km for SARAL/AltiKa, 45 km for Cryosat‐2, and 50 km for Jason‐2. Finally, a prediction of the future 2‐D mesoscale sea level resolution capability of the Surface Water and Ocean Topography (SWOT) mission is given using a simulated error level.
      PubDate: 2016-07-17T04:05:43.652884-05:
      DOI: 10.1002/2015JC010904
       
  • Intensification and poleward shift of subtropical western boundary
           currents in a warming climate
    • Authors: Hu Yang; Gerrit Lohmann, Wei Wei, Mihai Dima, Monica Ionita, Jiping Liu
      Pages: 4928 - 4945
      Abstract: A significant increase in sea surface temperature (SST) is observed over the midlatitude western boundary currents (WBCs) during the past century. However, the mechanism for this phenomenon remains poorly understood due to limited observations. In the present paper, several coupled parameters (i.e., sea surface temperature (SST), ocean surface heat fluxes, ocean water velocity, ocean surface winds and sea level pressure (SLP)) are analyzed to identify the dynamic changes of the WBCs. Three types of independent data sets are used, including reanalysis products, satellite‐blended observations. and climate model outputs from the fifth phase of the Climate Model Intercomparison Project (CMIP5). Based on these broad ranges of data, we find that the WBCs (except the Gulf Stream) are intensifying and shifting toward the poles as long‐term effects of global warming. An intensification and poleward shift of near‐surface ocean winds, attributed to positive annular mode‐like trends, are proposed to be the forcing of such dynamic changes. In contrast to the other WBCs, the Gulf Stream is expected to be weaker under global warming, which is most likely related to a weakening of the Atlantic Meridional Overturning Circulation (AMOC). However, we also notice that the natural variations of WBCs might conceal the long‐term effect of global warming in the available observational data sets, especially over the Northern Hemisphere. Therefore, long‐term observations or proxy data are necessary to further evaluate the dynamics of the WBCs.
      PubDate: 2016-07-17T04:05:50.82148-05:0
      DOI: 10.1002/2015JC011513
       
  • Basal melt, seasonal water mass transformation, ocean current variability,
           and deep convection processes along the Amery Ice Shelf calving front,
           East Antarctica
    • Pages: 4946 - 4965
      Abstract: Despite the Amery Ice Shelf (AIS) being the third largest ice shelf in Antarctica, the seasonal variability of the physical processes involved in the AIS‐ocean interaction remains undocumented and a robust observational, oceanographic‐based basal melt rate estimate has been lacking. Here we use year‐long time series of water column temperature, salinity, and horizontal velocities measured along the ice shelf front from 2001 to 2002. Our results show strong zonal variations in the distribution of water masses along the ice shelf front: modified Circumpolar Deep Water (mCDW) arrives in the east, while in the west, Ice Shelf Water (ISW) and Dense Shelf Water (DSW) formed in the Mackenzie polynya dominate the water column. Baroclinic eddies, formed during winter deep convection (down to 1100 m), drive the inflow of DSW into the ice shelf cavity. Our net basal melt rate estimate is 57.4 ± 25.3 Gt yr−1 (1 ± 0.4 m yr−1), larger than previous modeling‐based and glaciological‐based estimates, and results from the inflow of DSW (0.52 ± 0.38 Sv; 1 Sv = 106 m3 s−1) and mCDW (0.22 ± 0.06 Sv) into the cavity. Our results highlight the role of the Mackenzie polynya in the seasonal exchange of water masses across the ice shelf front, and the role of the ISW in controlling the formation rate and thermohaline properties of DSW. These two processes directly impact on the ice shelf mass balance, and on the contribution of DSW/ISW to the formation of Antarctic Bottom Water.
      PubDate: 2016-07-17T04:05:35.525352-05:
      DOI: 10.1002/2016JC011858
       
  • The spectral albedo of sea ice and salt crusts on the tropical ocean of
           Snowball Earth: 1. Laboratory measurements
    • Authors: Bonnie Light; Regina C. Carns, Stephen G. Warren
      Pages: 4966 - 4979
      Abstract: The ice‐albedo feedback mechanism likely contributed to global glaciation during the Snowball Earth events of the Neoproterozoic era (1 Ga to 544 Ma). This feedback results from the albedo contrast between sea ice and open ocean. Little is known about the optical properties of some of the possible surface types that may have been present, including sea ice that is both snow‐free and cold enough for salts to precipitate within brine inclusions. A proxy surface for such ice was grown in a freezer laboratory using the single salt NaCl and kept below the eutectic temperature (−21.2°C) of the NaCl‐H2O binary system. The resulting ice cover was composed of ice and precipitated hydrohalite crystals (NaCl · 2H2O). As the cold ice sublimated, a thin lag‐deposit of salt formed on the surface. To hasten its growth in the laboratory, the deposit was augmented by addition of a salt‐enriched surface crust. Measurements of the spectral albedo of this surface were carried out over 90 days as the hydrohalite crust thickened due to sublimation of ice, and subsequently over several hours as the crust warmed and dissolved, finally resulting in a surface with puddled liquid brine. The all‐wave solar albedo of the subeutectic crust is 0.93 (in contrast to 0.83 for fresh snow and 0.67 for melting bare sea ice). Incorporation of these processes into a climate model of Snowball Earth will result in a positive salt‐albedo feedback operating between −21°C and −36°C.
      PubDate: 2016-07-17T04:05:26.501227-05:
      DOI: 10.1002/2016JC011803
       
  • Assessing the impact of vertical land motion on twentieth century global
           mean sea level estimates
    • Authors: B. D. Hamlington; P. Thompson, W. C. Hammond, G. Blewitt, R. D. Ray
      Pages: 4980 - 4993
      Abstract: Near‐global and continuous measurements from satellite altimetry have provided accurate estimates of global mean sea level in the past two decades. Extending these estimates further into the past is a challenge using the historical tide gauge records. Not only is sampling nonuniform in both space and time, but tide gauges are also affected by vertical land motion (VLM) that creates a relative sea level change not representative of ocean variability. To allow for comparisons to the satellite altimetry estimated global mean sea level (GMSL), typically the tide gauges are corrected using glacial isostatic adjustment (GIA) models. This approach, however, does not correct other sources of VLM that remain in the tide gauge record. Here we compare Global Positioning System (GPS) VLM estimates at the tide gauge locations to VLM estimates from GIA models, and assess the influence of non‐GIA‐related VLM on GMSL estimates. We find that the tide gauges, on average, are experiencing positive VLM (i.e., uplift) after removing the known effect of GIA, resulting in an increase of 0.24 ± 0.08 mm yr−1 in GMSL trend estimates from 1900 to present when using GPS‐based corrections. While this result is likely dependent on the subset of tide gauges used and the actual corrections used, it does suggest that non‐GIA VLM plays a significant role in twentieth century estimates of GMSL. Given the relatively short GPS records used to obtain these VLM estimates, we also estimate the uncertainty in the GMSL trend that results from limited knowledge of non‐GIA‐related VLM.
      PubDate: 2016-07-22T09:56:10.237053-05:
      DOI: 10.1002/2016JC011747
       
  • The combined effect of transient wind‐driven upwelling and eddies on
           vertical nutrient fluxes and phytoplankton dynamics along Ningaloo Reef,
           Western Australia
    • Authors: Zhenlin Zhang; Ryan Lowe, Greg Ivey, Jiangtao Xu, James Falter
      Pages: 4994 - 5016
      Abstract: We investigate the influence of wind stresses, stratification, and coastal mesoscale eddies on upwelling intensity, vertical nutrient fluxes, and phytoplankton biomass on the continental shelf off Ningaloo Reef in northwestern Australia during an austral spring‐summer period. A three‐dimensional (3‐D) hydrodynamic model, ROMS (Regional Ocean Modeling System), was coupled with a four‐component nitrogen‐based biogeochemical NPZD model (Nitrogen Phytoplankton Zooplankton Detritus) to resolve the shelf circulation as well as the coupled nutrient and phytoplankton dynamics within a broad shelf region surrounding Ningaloo Reef. The simulated currents, temperatures, and chlorophyll a concentrations generally agreed well with both the remotely sensed satellite images and observational data collected during a field experiment from September to November 2010. Scenario tests for an individual wind‐driven upwelling event under a variety of hypothetical physical forcing conditions showed that shelf currents and biogeochemical variables were largely a function of wind stress and stratification. However, the functional relationships derived from this single wind event could not be extrapolated to other periods of the upwelling season, due to the additional influence of 3‐D mesoscale processes on the shelf. The presence, intensification, and propagation of a coastal anticyclonic eddy during the study period strongly influenced the spatial and temporal variations in nutrient profiles, which in turn caused fluctuations in vertical nutrient fluxes that were largely independent of wind stress. These results emphasize that it is necessary to fully capture the 3‐D details of the mesoscale and submesoscale coastal dynamics to properly predict upwelling‐induced coastal phytoplankton dynamics in eddy‐intensive shelf regions such as Ningaloo Reef.
      PubDate: 2016-07-22T09:56:07.273557-05:
      DOI: 10.1002/2016JC011791
       
  • Sunlight induced chlorophyll fluorescence in the near‐infrared
           spectral region in natural waters: Interpretation of the narrow
           reflectance peak around 761 nm
    • Authors: Yingcheng Lu; Linhai Li, Chuanmin Hu, Lin Li, Minwei Zhang, Shaojie Sun, Chunguang Lv
      Pages: 5017 - 5029
      Abstract: Sunlight induced chlorophyll a fluorescence (SICF) can be used as a probe to estimate chlorophyll a concentrations (Chl) and infer phytoplankton physiology. SICF at ∼685 nm has been widely applied to studies of natural waters. SICF around 740 nm has been demonstrated to cause a narrow reflectance peak at ∼761 nm in the reflectance spectra of terrestrial vegetation. This narrow peak has also been observed in the reflectance spectra of natural waters, but its mechanism and applications have not yet been investigated and it has often been treated as measurement artifacts. In this study, we aimed to interpret this reflectance peak at ∼761 nm and discuss its potential applications for remote monitoring of natural waters. A derivative analysis of the spectral reflectance suggests that the 761 nm peak is due to SICF. It was also found that the fluorescence line height (FLH) at 761 nm significantly and linearly correlates with Chl. FLH(761 nm) showed a tighter relationship with Chl than the relationship between FLH(∼685 nm) and Chl mainly due to weaker perturbations by nonalgal materials around 761 nm. While it is not conclusive, a combination of FLH(761 nm) and FLH(∼685 nm) might have some potentials to discriminate cyanobacteria from other phytoplankton due to their different fluorescence responses at the two wavelengths. It was further found that reflectance spectra with a 5 nm spectral resolution are adequate to capture the spectral SICF feature at ∼761 nm. These preliminary results suggest that FLH(761 nm) need to be explored more for future applications in optically complex coastal and inland waters.
      PubDate: 2016-07-24T05:25:45.508657-05:
      DOI: 10.1002/2016JC011797
       
  • Carbon export fluxes and export efficiency in the central Arctic during
           the record sea‐ice minimum in 2012: a joint 234Th/238U and
           210Po/210Pb study
    • Pages: 5030 - 5049
      Abstract: The Arctic sea‐ice extent reached a record minimum in September 2012. Sea‐ice decline increases the absorption of solar energy in the Arctic Ocean, affecting primary production and the plankton community. How this will modulate the sinking of particulate organic carbon (POC) from the ocean surface remains a key question. We use the 234Th/238U and 210Po/210Pb radionuclide pairs to estimate the magnitude of the POC export fluxes in the upper ocean of the central Arctic in summer 2012, covering time scales from weeks to months. The 234Th/238U proxy reveals that POC fluxes at the base of the euphotic zone were very low (2 ± 2 mmol C m−2 d−1) in late summer. Relationships obtained between the 234Th export fluxes and the phytoplankton community suggest that prasinophytes contributed significantly to the downward fluxes, likely via incorporation into sea‐ice algal aggregates and zooplankton‐derived material. The magnitude of the depletion of 210Po in the upper water column over the entire study area indicates that particle export fluxes were higher before July/August than later in the season. 210Po fluxes and 210Po‐derived POC fluxes correlated positively with sea‐ice concentration, showing that particle sinking was greater under heavy sea‐ice conditions than under partially ice‐covered regions. Although the POC fluxes were low, a large fraction of primary production (>30%) was exported at the base of the euphotic zone in most of the study area during summer 2012, indicating a high export efficiency of the biological pump in the central Arctic.
      PubDate: 2016-07-24T05:25:55.845579-05:
      DOI: 10.1002/2016JC011816
       
  • Distinct characteristics of the intermediate water observed off the east
           coast of Korea during two contrasting years
    • Pages: 5050 - 5068
      Abstract: The intermediate water known as “East Sea Intermediate Water” found south of the Subpolar Front (SF) is known to be formed in the northern East/Japan Sea (EJS), and its physical properties are determined by wintertime air‐sea interaction north of the SF. Hydrographic data collected off the Korean coast show significant decadal oscillations in spiciness (π) following isopycnals of intermediate layer, which are explained by the Arctic Oscillation (AO) and consequent cold‐air outbreaks. During positive AO phases, the cold‐air outbreak and water formation are more active and the intermediate water having the same π reaches higher density (higher π following the same isopycnals). At interannual time scale, however, the π variability is well beyond the relationship with the AO. Especially, significantly lower π (both fresher and colder) intermediate water was observed in spring of 2010 than 2001 under the same surface net‐heat flux peaks in the northern EJS in the two winters. Such contrasting characteristics of intermediate water between 2001 and 2010 are consistent with the HYCOM reanalysis results which indicate widespread extension of low‐ (high‐) π intermediate water in the southwestern EJS in 2010 (2001). A clear contrast in circulation pattern is suggested to derive the distinctly different characteristics of the intermediate water. Northward penetration of the East Korea Warm Current (EKWC) inhibited the southward extension of the intermediate water in 2001 off the east coast of Korea. On the other hand, the EKWC that poorly developed in 2010 allowed low‐π intermediate water to prevail in the southwestern EJS.
      PubDate: 2016-07-24T05:26:10.672859-05:
      DOI: 10.1002/2015JC011593
       
  • Mixing and phytoplankton dynamics in a submarine canyon in the West
           Antarctic Peninsula
    • Authors: Filipa Carvalho; Josh Kohut, Matthew J. Oliver, Robert M. Sherrell, Oscar Schofield
      Pages: 5069 - 5083
      Abstract: Bathymetric depressions (canyons) exist along the West Antarctic Peninsula shelf and have been linked with increased phytoplankton biomass and sustained penguin colonies. However, the physical mechanisms driving this enhanced biomass are not well understood. Using a Slocum glider data set with over 25,000 water column profiles, we evaluate the relationship between mixed layer depth (MLD, estimated using the depth of maximum buoyancy frequency) and phytoplankton vertical distribution. We use the glider deployments in the Palmer Deep region to examine seasonal and across canyon variability. Throughout the season, the ML becomes warmer and saltier, as a result of vertical mixing and advection. Shallow ML and increased stratification due to sea ice melt are linked to higher chlorophyll concentrations. Deeper mixed layers, resulting from increased wind forcing, show decreased chlorophyll, suggesting the importance of light in regulating phytoplankton productivity. Spatial variations were found in the canyon head region where local physical water column properties were associated with different biological responses, reinforcing the importance of local canyon circulation in regulating phytoplankton distribution in the region. While the mechanism initially hypothesized to produce the observed increases in phytoplankton over the canyons was the intrusion of warm, nutrient enriched modified Upper Circumpolar Deep Water (mUCDW), our analysis suggests that ML dynamics are key to increased primary production over submarine canyons in the WAP.
      PubDate: 2016-07-24T05:25:50.63314-05:0
      DOI: 10.1002/2016JC011650
       
  • An ongoing shift in Pacific Ocean sea level
    • Pages: 5084 - 5097
      Abstract: Based on the satellite altimeter data, sea level off the west coast of the United States has increased over the past 5 years, while sea level in the western tropical Pacific has declined. Understanding whether this is a short‐term shift or the beginning of a longer‐term change in sea level has important implications for coastal planning efforts in the coming decades. Here, we identify and quantify the recent shift in Pacific Ocean sea level, and also seek to describe the variability in a manner consistent with recent descriptions of El Nino‐Southern Oscillation (ENSO) and particularly the Pacific Decadal Oscillation (PDO). More specifically, we extract two dominant modes of sea level variability, one related to the biennial oscillation associated with ENSO and the other representative of lower‐frequency variability with a strong signal in the northern Pacific. We rely on cyclostationary empirical orthogonal function (CSEOF) analysis along with sea level reconstructions to describe these modes and provide historical context for the recent sea level changes observed in the Pacific. As a result, we find that a shift in sea level has occurred in the Pacific Ocean over the past few years that will likely persist in the coming years, leading to substantially higher sea level off the west coast of the United States and lower sea level in the western tropical Pacific.
      PubDate: 2016-07-30T00:15:48.272694-05:
      DOI: 10.1002/2016JC011815
       
  • Satellite estimate of freshwater exchange between the Indonesian Seas and
           the Indian Ocean via the Sunda Strait
    • Authors: James T. Potemra; Peter W. Hacker, Oleg Melnichenko, Nikolai Maximenko
      Pages: 5098 - 5111
      Abstract: The straits in Indonesia allow for low‐latitude exchange of water between the Pacific and Indian Oceans. Collectively known as the Indonesian Throughflow (ITF), this exchange is thought to occur primarily via the Makassar Strait and downstream via Lombok Strait, Ombai Strait, and Timor Passage. The Sunda Strait, between the islands of Sumatra and Java, is a very narrow ( ≈10 km) and shallow ( ≈20 m) gap, but it connects the Java Sea directly to the Indian Ocean. Flow through this strait is presumed to be small, given the size of the passage; however, recent observations from the Aquarius satellite indicate periods of significant freshwater transport, suggesting the Sunda Strait may play a more important role in Pacific to Indian Ocean exchange. The nature of this exchange is short‐duration (several days) bursts of freshwater injected into the eastern Indian Ocean superimposed on a mean seasonal cycle. The mean volume transport is small averaging about 0.1 Sv toward the Indian Ocean, but the freshwater transport is nonnegligible (estimated at 5.8 mSv). Transport through the strait is hydraulically controlled and directly correlates to the along‐strait pressure difference. The episodic low‐salinity plumes observed by Aquarius do not, however, appear to be forced by this same mechanism but are instead controlled by convergence of flow at the exit of the Strait in the Indian Ocean. Numerical model results show the fate of this freshwater plume varies with season and is either advected to the northwest along the coast of Sumatra or southerly into the ITF pathway.
      PubDate: 2016-07-30T00:15:36.219878-05:
      DOI: 10.1002/2015JC011618
       
  • Using CryoSat‐2 altimeter data to evaluate M2 internal tides
           observed from multisatellite altimetry
    • Authors: Zhongxiang Zhao
      Pages: 5164 - 5180
      Abstract: This paper evaluates M2 internal tides observed from multisatellite altimetry (MultiSat20yr) using CryoSat‐2 altimeter data. MultiSat20yr is constructed using 20 years of sea surface height measurements made by multiple satellite altimeters from 1992 to 2012. Here it is demonstrated that M2 internal tides can also be extracted using 4 years of CryoSat‐2 data from 2011 to 2014 (CryoSat4yr) by the same plane wave fit method. MultiSat20yr and CryoSat4yr are in good agreement in the central North Pacific, although they are from satellite data of different sampling patterns (1998 versus 10,688 tracks) and different observational periods (20 versus 4 years). Further comparisons are carried out for three isolated wave components. MultiSat20yr and CryoSat4yr agree very well for both Hawaiian components, suggesting that the Hawaiian Ridge is a relatively stable generation site. In contrast, the Aleutian Ridge is a relatively unstable source in that the M2 amplitudes in MultiSat20yr and CryoSat4yr are very different. With respect to MultiSat20yr, the M2 internal tide in 2011–2014 propagates slower (faster) to the south (north) of Hawaii, respectively, suggesting that the internal tide's propagation speed is subject to significant interannual variability. This feature is supported by M2 internal tides observed using multisatellite altimeter data in 2005 (MultiSat2005) and Argo measured upper ocean temperature profiles. MultiSat20yr is used to correct M2 internal tides in the CryoSat‐2 data. Significant and efficient variance reduction suggests that MultiSat20yr is a reliable internal tide model. A phase‐adjusted MultiSat20yr is built to account for the interannual variations, and it works better in internal tide correction.
      PubDate: 2016-07-30T00:20:44.669949-05:
      DOI: 10.1002/2016JC011805
       
  • Evidence of rising and poleward shift of storm surge in western North
           Pacific in recent decades
    • Pages: 5181 - 5192
      Abstract: Recently, there has been considerable interest in examining how sea‐level extremes due to storm surge may be related to climate change. Evidence of how storm‐surge extremes have evolved since the start of the most recent warming of mid‐1970s and early 1980s has not been firmly established however. Here we use 64 years (1950–2013) of observations and model simulations, and find evidence of a significant rise in the intensity as well as poleward‐shifting of location of typhoon surges in the western North Pacific after 1980s. The rising and poleward‐shifting trends are caused by the weakening of the steering flow in the tropics, which is related to climate warming, resulting in slower‐moving and longer‐lasting typhoons which had shifted northward.
      PubDate: 2016-07-30T00:20:39.974394-05:
      DOI: 10.1002/2016JC011777
       
  • Statistical properties of the surface velocity field in the northern Gulf
           of Mexico sampled by GLAD drifters
    • Pages: 5193 - 5216
      Abstract: The Grand LAgrangian Deployment (GLAD) used multiscale sampling and GPS technology to observe time series of drifter positions with initial drifter separation of O(100 m) to O(10 km), and nominal 5 min sampling, during the summer and fall of 2012 in the northern Gulf of Mexico. Histograms of the velocity field and its statistical parameters are non‐Gaussian; most are multimodal. The dominant periods for the surface velocity field are 1–2 days due to inertial oscillations, tides, and the sea breeze; 5–6 days due to wind forcing and submesoscale eddies; 9–10 days and two weeks or longer periods due to wind forcing and mesoscale variability, including the period of eddy rotation. The temporal e‐folding scales of a fitted drifter velocity autocorrelation function are bimodal with time scales, 0.25–0.50 days and 0.9–1.4 days, and are the same order as the temporal e‐folding scales of observed winds from nearby moored National Data Buoy Center stations. The Lagrangian integral time scales increase from coastal values of 8 h to offshore values of approximately 2 days with peak values of 3–4 days. The velocity variance is large, O(1) m2/s2, the surface velocity statistics are more anisotropic, and increased dispersion is observed at flow bifurcations. Horizontal diffusivity estimates are O(103) m2/s in coastal regions with weaker flow to O(105) m2/s in flow bifurcations, a strong jet, and during the passage of Hurricane Isaac. The Gulf of Mexico surface velocity statistics sampled by the GLAD drifters are a strong function of the feature sampled, topography, and wind forcing.
      PubDate: 2016-07-30T00:20:49.123326-05:
      DOI: 10.1002/2015JC011569
       
 
 
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