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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: 2.439, h-index: 91)
Geophysical Research Letters     Full-text available via subscription   (Followers: 101, SJR: 3.323, h-index: 185)
Global Biogeochemical Cycles     Full-text available via subscription   (Followers: 12, SJR: 3.22, h-index: 136)
J. of Advances in Modeling Earth Systems     Open Access   (Followers: 4, SJR: 4.444, h-index: 18)
J. of Geophysical Research : Atmospheres     Partially Free   (Followers: 105)
J. of Geophysical Research : Biogeosciences     Full-text available via subscription   (Followers: 24)
J. of Geophysical Research : Earth Surface     Partially Free   (Followers: 48)
J. of Geophysical Research : Oceans     Partially Free   (Followers: 45)
J. of Geophysical Research : Planets     Full-text available via subscription   (Followers: 101)
J. of Geophysical Research : Solid Earth     Full-text available via subscription   (Followers: 42)
J. of Geophysical Research : Space Physics     Full-text available via subscription   (Followers: 112)
Paleoceanography     Full-text available via subscription   (Followers: 6, SJR: 3.067, h-index: 100)
Radio Science     Full-text available via subscription   (Followers: 36, SJR: 1.072, h-index: 59)
Reviews of Geophysics     Full-text available via subscription   (Followers: 33, SJR: 8.833, h-index: 107)
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Tectonics     Full-text available via subscription   (Followers: 12, SJR: 2.628, h-index: 96)
Water Resources Research     Full-text available via subscription   (Followers: 74, SJR: 2.661, h-index: 144)
Journal Cover Journal of Geophysical Research : Oceans
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  • Quantifying the residual volume transport through a multiple-inlet system
           in response to wind forcing: The case of the western Dutch Wadden Sea
    • Authors: Matias Duran-Matute; Theo Gerkema, Maximiliano G. Sassi
      Abstract: In multiple-inlet coastal systems like the western Dutch Wadden Sea, the tides (and their interaction with the bathymetry), the fresh water discharge, and the wind drive a residual flow through the system. In the current paper, we study the effect of the wind on the residual volume transport through the inlets and the system as a whole on both the short (one tidal period) and long (seasonal or yearly) time scales. The results are based on realistic three-dimensional baroclinic numerical simulations for the years 2009-2011. The length of the simulations (over 2000 tidal periods) allowed us to analyze a large variety of conditions and quantify the effect of wind on the residual volume transport. We found that each inlet has an anisotropic response to wind; i.e. the residual volume transport is much more sensitive to the wind from two inherent preferential directions than from any other directions. We quantify the effects of wind on the residual volume transport through the system and introduce the concept of the system's conductance for such wind driven residual transport. For the western Dutch Wadden Sea, the dominant wind direction in the region is close to the direction with the highest conductance and opposes the tidally driven residual volume transport. This translates a large variability of the residual volume transport and a dominance of the wind in its long-term characteristics in spite of the episodic nature of storms. This article is protected by copyright. All rights reserved.
      PubDate: 2016-11-28T14:36:06.232328-05:
      DOI: 10.1002/2016JC011807
       
  • Tropical Cyclone asymmetry - development and evaluation of a new
           parametric model
    • Authors: M. Olfateh; David P. Callaghan, Peter Nielsen, Tom E. Baldock
      Abstract: A new parametric model is developed to describe the asymmetry commonly observed in Tropical Cyclones or Hurricanes. Observations from 21 Hurricanes from the Gulf of Mexico basin and TC Roger in the Coral Sea are analysed to determine the azimuthal and radial asymmetry typical in these mesoscale systems. On the basis of the observations a new asymmetric directional wind model is proposed which adjusts the widely used Holland (1980) axisymmetric wind model to account for the action of blocking high pressure systems, boundary layer friction and forward speed. The model is tested against the observations and demonstrated to capture the physical features of asymmetric cyclones and provides a better fit to observed winds than the Holland model. Optimum values and distributions of the model parameters are derived for use in statistical modelling. Finally, the model is used to investigation of the asymmetric character of TC systems, including the azimuth of the maximum wind speed, the degree of asymmetry and the re-lationship between asymmetry and forward speed. This article is protected by copyright. All rights reserved.
      PubDate: 2016-11-28T14:36:02.040704-05:
      DOI: 10.1002/2016JC012237
       
  • Large-eddy simulation of wave-breaking induced turbulent coherent
           structures and suspended sediment transport on a barred beach
    • Authors: Zheyu Zhou; Tian-Jian Hsu, Daniel Cox, Xiaofeng Liu
      Abstract: To understand the interaction between wave-breaking induced turbulent coherent structures and suspended sediment transport, we report a Large-Eddy Simulation (LES) study of wave breaking processes over a near-prototype scale barred beach. The numerical model is implemented using the open-source CFD toolbox, OpenFOAM®, in which the incompressible three-dimensional filtered Navier-Stokes equations for the water and air phases are solved with a finite volume scheme. A Volume of Fluid (VOF) method is used to capture the evolution of the water-air interface. The numerical model is validated with measured free surface elevation, turbulence averaged flow velocity, turbulent intensity, and for the first time, the intermittency of breaking wave turbulence. Simulation results confirm that as the obliquely descending eddies (ODEs) approach the bottom, significant bottom shear stress is generated. Remarkably, the collapse of ODEs onto the bed can also cause drastic spatial and temporal changes of dynamic pressure on the bottom. By allowing sediment to be suspended from the bar crest, intermittently high sediment suspension events and their correlation with high turbulence and/or high bottom shear stress events are investigated. The simulated intermittency of sediment suspension is similar to previous field and large wave flume observations. Coherent suspension events account for only 10% of the record but account for about 50% of the sediment load. Model results suggest that about 60∼70% of coherent bottom stress events are associated with surface-generated turbulence. Nearly all the coherent sand suspension events are associated with coherent turbulence events due to wave-breaking turbulence approaching the bed. This article is protected by copyright. All rights reserved.
      PubDate: 2016-11-28T14:35:58.920394-05:
      DOI: 10.1002/2016JC011884
       
  • The ocean mixed-layer under Southern Ocean sea-ice: Seasonal cycle and
           forcing
    • Authors: Violaine Pellichero; Jean-Baptiste Sallée, Sunke Schmidtko, Fabien Roquet, Jean-Benoît Charrassin
      Abstract: The oceanic mixed-layer is the gateway for the exchanges between the atmosphere and the ocean; in this layer all hydrographic ocean properties are set for months to millennia. A vast area of the Southern Ocean is seasonally capped by sea-ice, which alters the characteristics of the ocean mixed-layer. The interaction between the ocean mixed-layer and sea-ice plays a key role for water-mass transformation, the carbon cycle, sea-ice dynamics, and ultimately for the climate as a whole. However, the structure and characteristics of the under-ice mixed-layer are poorly understood due to the sparseness of in-situ observations and measurements. In this study, we combine distinct sources of observations to overcome this lack in our understanding of the Polar Regions. Working with Elephant Seal-derived observations, ship-based and Argo float observations, we describe the seasonal cycle of the ocean mixed-layer characteristics and stability of the ocean mixed-layer over the Southern Ocean and specifically under sea-ice. Mixed-layer heat and freshwater budgets are used to investigate the main forcing mechanisms of the mixed-layer seasonal cycle. The seasonal variability of sea surface salinity and temperature are primarily driven by surface processes, dominated by sea-ice freshwater flux for the salt budget, and by air-sea flux for the heat budget. Ekman advection, vertical diffusivity and vertical entrainment play only secondary roles.Our results suggest that changes in regional sea-ice distribution and annual duration, as currently observed, widely affect the buoyancy budget of the underlying mixed-layer, and impact large-scale water-mass formation and transformation with far reaching consequences for ocean ventilation. This article is protected by copyright. All rights reserved.
      PubDate: 2016-11-28T14:35:55.289042-05:
      DOI: 10.1002/2016JC011970
       
  • Bottom-slope-induced net sheet-flow sediment transport rate under
           sinusoidal oscillatory flows
    • Authors: Jing Yuan; Zhiwei Li, O. S. Madsen
      Abstract: It is generally believed that the slope of beaches can lead to a net downslope (usually offshore) sediment transport rate under shoaling waves, but very few high-quality measurements have been reported for a quantitative understanding of this phenomenon. In this study, full-scale (1:1) experiments of bottom-slope-induced net sheet-flow sediment transport rate under sinusoidal oscillatory flows are conducted using a tilting oscillatory water tunnel. The tests cover a variety of flow-sediment conditions on bottom slopes up to 2.6°. A laser-based bottom profiler system is developed for measuring net transport rate based on the principle of mass conservation. Experimental results suggest that for a given flow-sediment condition the net transport rate is in the downslope direction and increases linearly with bottom slope. A conceptual model is presented based on the idea that gravity helps bottom shear stress drive bedload transport and consequently enhances (reduces) bedload transport and suspension when the flow is in the downslope (up-slope) direction. The model predicts both the measured net sediment transport rates and the experimental linear relationship between net transport rates and bottom slope with an accuracy generally better than a factor of 2. Some measured net transport rates in this study are comparable to those due to flow skewness obtained in similar sheet-flow studies, despite that our maximum slope could be milder than the actual bottom slope in surf zones, where sheet-flow conditions usually occur. This shows that the slope effect may be as important as wave nonlinearity in producing net cross-shore sheet-flow sediment transport. This article is protected by copyright. All rights reserved.
      PubDate: 2016-11-28T14:35:52.278979-05:
      DOI: 10.1002/2016JC011996
       
  • Gulf of Maine salinity variation and its correlation with upstream Scotian
           Shelf currents at seasonal and interannual time scales
    • Authors: Hui Feng; Doug Vandemark, John Wilkin
      Abstract: In the Gulf of Maine (GoM), a network of buoy hydrography measurements collected since 2001 provide a subsurface salinity time series showing a strong seasonal cycle and interannual variations that are both consistent with remote forcing of Gulf hydrography by upstream advection. These long-term mooring data are combined with satellite altimeter estimates of upper ocean current anomaly on the adjoining Scotian Shelf (SS) in a new attempt to use disparate regional observations as proxies to detect and evaluate remote forcing of water mass change inside the Gulf from 2002-2015. Focusing on buoys moored along the Maine coastal current (MCC), lagged cross correlations with upstream altimeter-derived SS current anomalies are found to be as high as 0.84 and explain 50-70% of variance in the MCC subsurface salinity data at both seasonal and interannual time scales. Significant MCC freshening in 2004-2005 and 2010-2011 follow SS velocity strengthening, while salting events in 2002-2004 and 2012-2015 are associated with relaxation of SS currents. Estimated time lags translate to advective SS inflow velocity estimates of 6±2 cm/s that are consistent with past modeling and observational work. Investigation of wind stress control on SS velocity anomalies indicates that wind directions away from the along-shore can factor into flow modulation. Overall, the study findings are consistent with past freshwater flux observations and modeling examining southwest SS inflow to the GoM, provide a new empirical means to diagnose GoM hydrographic change, and point to one potential application of an altimeter measurement record that extends from 1992 into the future. This article is protected by copyright. All rights reserved.
      PubDate: 2016-11-28T14:35:50.974821-05:
      DOI: 10.1002/2016JC012337
       
  • On the generation and evolution of internal solitary waves in the southern
           Red Sea
    • Authors: Daquan Guo; T. R. Akylas, Peng Zhan, Aditya Kartadikaria, Ibrahim Hoteit
      Abstract: Satellite observations recently revealed trains of internal solitary waves (ISWs) in the off-shelf region between 16.0°N and 16.5°N in the southern Red Sea. The generation mechanism of these waves is not entirely clear, though, as the observed generation sites are far away (50 km) from the shelf break and tidal currents are considered relatively weak in the Red Sea. Upon closer examination of the tide properties in the Red Sea and the unique geometry of the basin, it is argued that the steep bathymetry and a relatively strong tidal current in the southern Red Sea provide favorable conditions for the generation of ISWs. To test this hypothesis and further explore the evolution of ISWs in the basin, 2D numerical simulations with the non-hydrostatic MIT general circulation model (MITgcm) were conducted. The results are consistent with the satellite observations in regard to the generation sites, peak amplitudes and the speeds of first-mode ISWs. Moreover, our simulations suggest that the generation process of ISWs in the southern Red Sea is similar to the tide-topography interaction mechanism seen in the South China Sea. Specifically, instead of ISWs arising in the immediate vicinity of the shelf break via a hydraulic lee wave mechanism, a broad, energetic internal tide is first generated, which subsequently travels away from the shelf break and eventually breaks down into ISWs. Sensitivity runs suggest that ISW generation may also be possible under summer stratification conditions, characterized by an intermediate water intrusion from the strait of Bab el Mandeb. This article is protected by copyright. All rights reserved.
      PubDate: 2016-11-28T14:35:49.785171-05:
      DOI: 10.1002/2016JC012221
       
  • Role of wind in erosion–accretion cycles on an estuarine mudflat
    • Authors: B.W. Shi; S.L. Yang, Y.P. Wang, G.C. Li, M.L. Li, P. Li, C. Li
      Abstract: Wind is an important regulator of coastal erosion and accretion processes that have significant ecological and engineering implications. Nevertheless, previous studies have mainly focused on storm−generated changes in the bed level. This paper aims to improve the understanding of wind−induced erosion–accretion cycles on intertidal flats under normal (non−stormy) weather conditions using data that relates to the wave climate, near−bed 3D flow velocity, suspended sediment concentration, and bed−level changes on a mudflat at the Yangtze Delta front. The following parameters were calculated at 10−minute intervals over 10 days: the wind wave orbital velocity (Ûδ), bed shear stress from combined current–wave action, erosion flux, deposition flux, and predicted bed−level change. The time series of measured and predicted bed−level changes both show tidal cycles and a 10−day cycle. We attribute the tidal cycles of bed−level changes to tidal dynamics, but we attribute the 10−day cycle of bed−level changes to the interaction between wind speed/direction and neap−spring cyclicity. We conclude that winds can significantly affect bed−level changes in mudflats even during non−stormy weather and under macro−mesotidal conditions and that the bed−level changes can be predicted well using current–wave–sediment combined models. This article is protected by copyright. All rights reserved.
      PubDate: 2016-11-28T14:35:48.454254-05:
      DOI: 10.1002/2016JC011902
       
  • Effect of tides and source location on nearshore tsunami-induced currents
    • Authors: Aykut Ayca; Patrick J. Lynett
      Abstract: Here, we present the results of a numerical modeling study that investigates how event-maximum tsunami-induced currents vary due to the dynamic effects of tides and wave directivity. First, analyses of tide-tsunami interaction are presented in three harbors by coupling the tsunami with the tide, and allowing the initial tsunami wave to arrive at various tidal phases. We find that tsunami-tide interaction can change the event-maximum current speed experienced in a harbor by up to 25% for the events and harbors studied, and we note that this effect is highly site-specific. Second, to evaluate the effect of wave directionality on event-maximum currents, earthquakes sources were placed throughout the Pacific, with magnitudes tuned to create the same maximum near-coast amplitude at the harbor of study. Our analysis also shows that, for the harbor and sources examined, the effect of offshore directionality and tsunami frequency content has a weak effect on the event-maximum currents experienced in the harbor. The more important dependency of event-maximum currents is the near-harbor amplitude of the wave, indicating that event-maximum currents in a harbor from a tsunami generated by a large far-field earthquake may be reasonably well predicted with only information about the predicted local maximum tsunami amplitude. This study was motivated by the hope of constructing a basis for understanding the dynamic effects of tides and wave directivity on current-based tsunami hazards in a coastal zone. The consideration of these aspects is crucial and yet challenging in the modeling of tsunami currents. This article is protected by copyright. All rights reserved.
      PubDate: 2016-11-28T14:35:43.818112-05:
      DOI: 10.1002/2016JC012435
       
  • The impact of glacier meltwater on the underwater noise field in a glacial
           bay
    • Authors: Oskar Glowacki; Mateusz Moskalik, Grant B. Deane
      Abstract: Ambient noise oceanography is proving to be an efficient and effective tool for the study of ice-ocean interactions in the bays of marine-terminating glaciers. However, obtaining quantitative estimates of ice melting or calving processes from ambient noise requires an understanding of how sound propagation through the bay attenuates and filters the noise spectrum. Measurements of the vertical structure in sound speed in the vicinity of the Hans Glacier in Hornsund Fjord, Spitsbergen, made with O(130) CTD casts between May and November 2015, reveal high-gradient, upward-refracting sound speed profiles created by cold, fresh meltwater during summer months. Simultaneous recordings of underwater ambient noise made at depths of 1, 10 and 20 meters in combination with propagation model calculations using the model Bellhop illustrate the dominant role these surface ducts play in shaping the underwater soundscape. The surface ducts lead to a higher intensity and greater variability of acoustic energy in the near-surface layer covered by glacially-modified waters relative to deeper waters, indicating deeper zones as most appropriate for inter-seasonal acoustic monitoring of the glacial melt. Surface waveguides in Hornsund are relatively shallow and trap sound above O(1 kHz). Deeper waveguides observed elsewhere will also trap low-frequency sounds, such as those generated by calving events for example. Finally, the ambient noise field in Hornsund is shown to be strongly dependent on the distribution of ice throughout the bay, stressing the importance of performing complementary environmental measurements when interpreting the results of acoustic surveys. This article is protected by copyright. All rights reserved.
      PubDate: 2016-11-28T14:35:40.161873-05:
      DOI: 10.1002/2016JC012355
       
  • Factors controlling enhanced N2O concentrations over the southwestern
           Indian shelf
    • Authors: V. Sudheesh; G.V.M. Gupta, K.V. Sudharma, H. Naik, D.M. Shenoy, M. Sudhakar, S.W.A. Naqvi
      Abstract: Repeat measurements of dissolved nitrous oxide (N2O) along two transects of the western continental shelf of India in 2012 revealed high concentrations of 45±32 nM (off Kochi) and 73±63 nM (off Mangalore) during the summer monsoon (SM). N2O concentrations increased non-linearly during the peak of the SM upwelling, when low O2 (
      PubDate: 2016-11-28T14:35:38.617189-05:
      DOI: 10.1002/2016JC012166
       
  • Temporal variability of diapycnal mixing in the northern South China Sea
    • Authors: Hui Sun; Qingxuan Yang, Wei Zhao, Xinfeng Liang, Jiwei Tian
      Abstract: Temporal variability of diapycnal mixing over seven months in the northern South China Sea was examined based on McLane Moored Profiler observations from 850 to 2,200 m by employing a finescale parameterization. Intensified diffusivity exceeding the order of 10−3 m2/s in magnitude was found over the first half of Oct 2014, and from 2 Dec 2014 to 21 Jan 2015 (a typical wintertime). Strong internal tides and winds in winter were the likely candidates for the high-level diapycnal mixing in winter. As for the enhanced mixing during Oct 2014, we suspect the generation of near-bottom near-inertial waves through the interaction of mesoscale eddies and unique bottom topography was the cause. This article is protected by copyright. All rights reserved.
      PubDate: 2016-11-28T14:35:34.842422-05:
      DOI: 10.1002/2016JC012044
       
  • Hydrographic responses to regional covariates across the Kara Sea
    • Authors: Jussi Mäkinen; Jarno Vanhatalo
      Abstract: The Kara Sea is a shelf sea in the Arctic Ocean which has a strong spatiotemporal hydrographic variation driven by river discharge, air pressure and sea ice. There is a lack of information about the effects of environmental variables on surface hydrography in different regions of the Kara Sea. We use a hierarchical spatially varying coefficient model to study the variation of sea surface temperature (SST) and salinity (SSS) in the Kara Sea between years 1980 and 2000. The model allows us to study the effects of climatic (Arctic oscillation index, AO) and seasonal (river discharge and ice concentration) environmental covariates on hydrography. The hydrographic responses to covariates vary considerably between different regions of the Kara Sea. River discharge decreases SSS in the shallow shelf area and has a neutral effect in the northern Kara Sea. The responses of SST and SSS to AO show the effects of different wind and air pressure conditions on water circulation and hence on hydrography. Ice concentration has a constant effect across the Kara Sea. We estimated the average SST and SSS in the Kara Sea in 1980-2000. The average August SST over the Kara Sea in 1995-2000 was higher than the respective average in 1980-1984 with 99.9% probability and August SSS decreased with 77% probability between these time periods. We found a support that the winter season AO has an impact on the summer season hydrography, and temporal trends may be related to the varying level of winter season AO index. This article is protected by copyright. All rights reserved.
      PubDate: 2016-11-28T14:35:33.545373-05:
      DOI: 10.1002/2016JC011981
       
  • Turbulence-resolving, two-phase flow simulations of wave-supported gravity
           flows: A conceptual study
    • Authors: Celalettin Emre Ozdemir
      Abstract: Discoveries over the last three decades have shown that wave-supported gravity flows (WSGFs) are among the participating physical processes that carry substantial amount of fine sediments across low-gradient shelves. Therefore, understanding the full range of mechanisms responsible for such gravity flows is likely to shed light on the dynamics of subaqueous delta and clinoform development. As wave-induced boundary layer turbulence is the major agent to suspend sediments in WSGFs, the scale of WSGFs in the water column is also bounded by the wave-induced boundary layer thickness which is on the order of decimeters. Therefore, in order to explore the details of participating physical mechanisms, especially that due to turbulence-sediment interaction, highly resolved and accurate numerical models or measurements in the laboratory and the field are required. In this study, the dynamics of WSGFs is investigated by using turbulence-resolving two-phase flow simulations that utilize Direct Numerical Simulations (DNS). The effect of variable sediment loading, slope, and wave orbital velocity is investigated via 21 simulations. This article is protected by copyright. All rights reserved.
      PubDate: 2016-11-28T14:35:27.589828-05:
      DOI: 10.1002/2016JC012061
       
  • Formation of summer phytoplankton bloom in the northwestern Bay of Bengal
           in a coupled physical-ecosystem model
    • Authors: V. Thushara; P. N. Vinayachandran
      Abstract: The Bay of Bengal (BoB) is considered to be a region of low biological productivity, owing to nutrient limitation, caused by strong salinity stratification induced by the freshwater influx from rivers and precipitation. Satellite and in situ observations, however, reveal the presence of prominent regional blooms in the bay in response to monsoonal forcings. Bloom dynamics of the BoB are presumably determined by freshwater as well as the local and remote effect of winds and remain to be explored in detail. Using a coupled physical-ecosystem model, we have examined the oceanic processes controlling productivity in the northwestern BoB during the summer monsoon. The region exhibits a prominent bloom lasting for a period of about two months, supporting major fishing zones along the northeast coast of India. The ecosystem model simulates the spatial and temporal evolution of the surface bloom in good agreement with Sea-Viewing Wide Field-of-View Sensor (SeaWiFS) observations. Vertical distribution of upper ocean physical and biological tracers and a nitrate budget analysis reveal the dominant role of coastal upwelling induced by alongshore winds in triggering the bloom. Horizontal advection plays a secondary role by supplying nutrients from coastal to offshore regions. The bloom decays with the weakening of winds and upwelling by the end of summer monsoon. The simulated bloom in the northwestern bay remains largely unaffected by the freshwater effects, since the peak bloom occur before the arrival of river plumes. This article is protected by copyright. All rights reserved.
      PubDate: 2016-11-28T14:35:23.407116-05:
      DOI: 10.1002/2016JC011987
       
  • Seasonal variation of the Beaufort shelfbreak jet and its relationship to
           Arctic cetacean occurrence
    • Authors: Peigen Lin; Robert S. Pickart, Kathleen M. Stafford, G.W.K. Moore, Daniel J. Torres, Frank Bahr, Jianyu Hu
      Abstract: Using mooring timeseries from September 2008 to August 2012, together with ancillary atmospheric and satellite data sets, we quantify the seasonal variations of the shelfbreak jet in the Alaskan Beaufort Sea and explore connections to the occurrences of bowhead and beluga whales. Wind patterns during the four-year study period are different than the long-term climatological conditions in that the springtime peak in easterly winds shifted from May to June, and the autumn peak was limited to October instead of extending farther into the fall. These changes were primarily due to the behavior of the two regional atmospheric centers of action, the Aleutian Low and Beaufort High. The volume transport of the shelfbreak jet, which peaks in the summer, was decomposed into a background (weak wind) component and a wind-driven component. The wind-driven component is correlated to the Pt. Barrow, AK alongcoast windspeed record, although a more accurate prediction is obtained when considering the ice thickness at the mooring site. An upwelling index reveals that wind-driven upwelling is enhanced in June and October when storms are stronger and longer-lasting. The seasonal variation of Arctic cetacean occurrence is dominated by the eastward migration in spring, dictated by pack-ice patterns, and westward migration in fall, coincident with the autumn peak in shelfbreak upwelling intensity. This article is protected by copyright. All rights reserved.
      PubDate: 2016-11-16T16:06:01.205411-05:
      DOI: 10.1002/2016JC011890
       
  • Sea ice algae chlorophyll a concentrations derived from under-ice spectral
           radiation profiling platforms
    • Authors: Benjamin A. Lange; Christian Katlein, Marcel Nicolaus, Ilka Peeken, Hauke Flores
      Abstract: Multi-scale sea ice algae observations are fundamentally important for projecting changes to sea ice ecosystems, as the physical environment continues to change. In this study we developed upon previously established methodologies for deriving sea ice-algal chlorophyll a concentrations (chl a) from spectral radiation measurements, and applied these to larger-scale spectral surveys. We conducted four different under-ice spectral measurements: irradiance; radiance; transmittance; and transflectance, and applied three statistical approaches: Empirical Orthogonal Functions (EOF); Normalized Difference Indices (NDI); and multi-NDI. We developed models based on ice core chl a and coincident spectral irradiance/transmittance (N=49) and radiance/transflectance (N=50) measurements conducted during two cruises to the central Arctic Ocean in 2011 and 2012. These reference models were ranked based on two criteria: mean robustness R2; and true prediction error estimates. For estimating the biomass of a large scale dataset, the EOF approach performed better than the NDI, due to its ability to account for the high variability of environmental properties experienced over large areas. Based on robustness and true prediction error, the three most reliable models, EOF-transmittance, EOF-transflectance and NDI-transmittance, were applied to two remotely operated vehicle (ROV) and two Surface and Under-Ice Trawl (SUIT) spectral radiation surveys. In these larger-scale chl a estimates, EOF-transmittance showed the best fit to ice core chl a. Application of our most reliable model, EOF-transmittance, to an 85 m horizontal ROV transect revealed large differences compared to published biomass estimates from the same site with important implications for projections of Arctic-wide ice-algal biomass and primary production. This article is protected by copyright. All rights reserved.
      PubDate: 2016-11-16T16:05:54.138874-05:
      DOI: 10.1002/2016JC011991
       
  • Net primary productivity estimates and environmental variables in the
           Arctic Ocean: An assessment of coupled physical-biogeochemical models
    • Authors: Younjoo J. Lee; Patricia A. Matrai, Marjorie A. M. Friedrichs, Vincent S. Saba, Olivier Aumont, Marcel Babin, Erik T. Buitenhuis, Matthieu Chevallier, Lee de Mora, Morgane Dessert, John P. Dunne, Ingrid Ellingsen, Doron Feldman, Robert Frouin, Marion Gehlen, Thomas Gorgues, Tatiana Ilyina, Meibing Jin, Jasmin G. John, Jonathan Lawrence, Manfredi Manizza, Christophe Eugène Menkes, Coralie Perruche, Vincent Le Fouest, Ekaterina Popova, Anastasia Romanou, Annette Samuelsen, Jörg Schwinger, Roland Séférian, Charles A. Stock, Jerry Tjiputra, L. Bruno Tremblay, Kyozo Ueyoshi, Marcello Vichi, Andrew Yool, Jinlun Zhang
      Abstract: The relative skill of 21 regional and global biogeochemical models was assessed in terms of how well the models reproduced observed net primary productivity (NPP) and environmental variables such as nitrate concentration (NO3), mixed layer depth (MLD), euphotic layer depth (Zeu), and sea ice concentration, by comparing results against a newly updated, quality-controlled in situ NPP database for the Arctic Ocean (1959-2011). The models broadly captured the spatial features of integrated NPP (iNPP) on a pan-Arctic scale. Most models underestimated iNPP by varying degrees in spite of overestimating surface NO3, MLD, and Zeu throughout the regions. Among the models, iNPP exhibited little difference over sea ice condition (ice-free vs. ice-influenced) and bottom depth (shelf vs. deep ocean). The models performed relatively well for the most recent decade and towards the end of Arctic summer. In the Barents and Greenland Seas, regional model skill of surface NO3 was best associated with how well MLD was reproduced. . Regionally, iNPP was relatively well simulated in the Beaufort Sea and the central Arctic Basin, where in situ NPP is low and nutrients are mostly depleted. Models performed less well at simulating iNPP in the Greenland and Chukchi Seas, despite the higher model skill in MLD and sea ice concentration, respectively. iNPP model skill was constrained by different factors in different Arctic Ocean regions. Our study suggests that better parameterization of biological and ecological microbial rates (phytoplankton growth and zooplankton grazing) are needed for improved Arctic Ocean biogeochemical modeling. This article is protected by copyright. All rights reserved.
      PubDate: 2016-11-14T14:18:16.980562-05:
      DOI: 10.1002/2016JC011993
       
  • Seasonal and interannual variability of the Arctic sea ice: A comparison
           between AO-FVCOM and observations
    • Authors: Yu Zhang; Changsheng Chen, Robert C. Beardsley, Guoping Gao, Jianhua Qi, Huichan Lin
      Abstract: A high-resolution (up to 2 km), unstructured-grid, fully ice-sea coupled Arctic Ocean Finite-Volume Community Ocean Model (AO-FVCOM) was used to simulate the sea ice in the Arctic over the period 1978-2014. The spatial-varying horizontal model resolution was designed to better resolve both topographic and baroclinic dynamics scales over the Arctic slope and narrow straits. The model-simulated sea ice was in good agreement with available observed sea ice extent, concentration, drift velocity and thickness, not only in seasonal and interannual variability but also in spatial distribution. Compared with six other Arctic Ocean models (ECCO2, GSFC, INMOM, ORCA, NAME, and UW), the AO-FVCOM-simulated ice thickness showed a higher mean correlation coefficient of ∼0.63 and a smaller residual with observations. Model-produced ice drift speed and direction errors varied with wind speed: the speed and direction errors increased and decreased as the wind speed increased, respectively. Efforts were made to examine the influences of parameterizations of air-ice external and ice-water interfacial stresses on the model-produced bias. The ice drift direction was more sensitive to air-ice drag coefficients and turning angles than the ice drift speed. Increasing or decreasing either 10% in water-ice drag coefficient or 10° in water-ice turning angle did not show a significant influence on the ice drift velocity simulation results although the sea ice drift speed was more sensitive to these two parameters than the sea ice drift direction. Using the COARE 4.0 derived parameterization of air-water drag coefficient for wind stress did not significantly influence the ice drift velocity simulation. This article is protected by copyright. All rights reserved.
      PubDate: 2016-11-11T17:04:26.786954-05:
      DOI: 10.1002/2016JC011841
       
  • Seasonal cycle of near-bottom transport and currents in the northern Gulf
           of California
    • Authors: R. Navarro; M. López, J. Candela
      Abstract: Seasonal cycles of near-bottom transport and temperature over the sills of the Northern Gulf of California, as well as surface geostrophic velocity anomalies, are presented. Transport at the sills, where overflows occur, is towards the head of the gulf all year round with maximum in October and minimum in June. Furthermore, transport is 180° out of phase with the surface geostrophic velocity across the northern gulf, consistent with the exchange being strongest in October. Seasonal cycles of near-bottom temperature and transport are also 180° out of phase, indicating that maximum water inflow is associated with the coolest water entering from the Pacific Ocean. Near-bottom temperature over the northern Ballenas Channel sill has a maximum in early August, which is more in phase with the surface temperature and consistent with intense mixing in the channel. Geostrophic velocity at the northern gulf is in phase with that near the mouth of the gulf, and approximately in phase with the seasonal heat input through the mouth, calculated previously by Beron-Vera and Ripa [2000]. Moreover, the maximum lower-layer, horizontal heat output of the Ballenas Channel occurs in November, approximately one month after the maximum transport through the San Lorenzo and Delfín sills. Therefore, heat loss results from the continuous near-bottom inflow of relatively cold water at both sills which bound the deepest basins of the northern gulf. Moreover, the mean and seasonal cycles of heat and mass fluxes in the deepest basins of the northern gulf are almost everywhere in opposite directions. This article is protected by copyright. All rights reserved.
      PubDate: 2016-11-11T17:04:12.638414-05:
      DOI: 10.1002/2016JC012063
       
  • GNSS-R nonlocal sea state dependencies: Model and empirical verification
    • Authors: David D. Chen-Zhang; Christopher S. Ruf, Fabrice Ardhuin, Jeonghwan Park
      Abstract: Global Navigation Satellite System Reflectometry (GNSS-R) is an active, bistatic remote sensing technique operating at L-band frequencies. GNSS-R signals scattered from a rough ocean surface are known to interact with longer surface waves than traditional scatterometery and altimetry signals. A revised forward model for GNSS-R measurements is presented which assumes an ocean surface wave spectrum that is forced by other sources than just the local near-surface winds. The model is motivated by recent spaceborne GNSS-R observations that indicate a strong scattering dependence on significant wave height, even after controlling for local wind speed. This behavior is not well represented by the most commonly used GNSS-R scattering model, which features a one-to-one relationship between wind speed and the mean-square-slope of the ocean surface. The revised forward model incorporates a third generation wave model that is skillful at representing long waves, an anchored spectral tail model, and a GNSS-R electromagnetic scattering model. In comparisons with the spaceborne measurements, the new model is much better able to reproduce the empirical behavior. This article is protected by copyright. All rights reserved.
      PubDate: 2016-11-11T17:04:08.99925-05:0
      DOI: 10.1002/2016JC012308
       
  • Intrusive upwelling in the Central Great Barrier Reef
    • Authors: Jessica A. Benthuysen; Hemerson Tonin, Richard Brinkman, Michael Herzfeld, Craig Steinberg
      Abstract: In the Central Great Barrier Reef, the outer continental shelf has an open reef matrix that facilitates the exchange of waters with the Coral Sea. During austral summer, cool water intrudes onto the shelf along the seafloor. Temperature observations reveal cool, bottom intrusions during a six-year period from the Queensland Integrated Mooring Observational System's Palm Passage mooring. A metric is used to identify 64 intrusion events. These intrusions predominantly occur from October through March including the wet season. During an event, the outer-shelf's near-bottom temperature decreases by 1 to 3ºC typically over one week. The near-bottom salinity tends to increase, while near-surface changes do not reflect these tendencies. Intrusion events occur predominantly with either weakening equatorward winds or poleward wind bursts. A regional hydrodynamic model for the Great Barrier Reef captures the timing and amplitude of these intrusions. During intrusion events, isotherms tend to uplift over the continental slope and onto the shelf and the East Australian Current intensifies poleward. Over the shelf, a bottom-intensified onshore current coincides with bottom cooling. For numerous events, the model diagnostics reveal that the cross-shelf flow is dominated by the geostrophic contribution. A vertical circulation tilts the isopycnals upward on the southern side of the passage, causing an along-shelf density gradient and geostrophic onshore flow with depth. While wind fluctuations play a major role in controlling the along-shelf currents, model results indicate that a concurrent topographically induced circulation can assist the onshore spread of cool water. This article is protected by copyright. All rights reserved.
      PubDate: 2016-11-11T17:04:04.859169-05:
      DOI: 10.1002/2016JC012294
       
  • Ocean mixing beneath Pine Island Glacier ice shelf, West Antarctica
    • Authors: Satoshi Kimura; Adrian Jenkins, Pierre Dutrieux, Alexander Forryan, Alberto C. Naveira Garabato, Yvonne Firing
      Abstract: Ice shelves around Antarctica are vulnerable to an increase in ocean-driven melting, with the melt rate depending on ocean temperature and the strength of circulations inside the ice-shelf cavities. We present measurements of velocity, temperature, salinity, turbulent kinetic energy dissipation rate and thermal variance dissipation rate beneath Pine Island Glacier ice shelf, West Antarctica. These measurements were obtained by CTD, ADCP and turbulence sensors mounted on an Autonomous Underwater Vehicle (AUV). The highest turbulent kinetic energy dissipation rate is found near the grounding line. The thermal variance dissipation rate increases closer to the ice-shelf base, with a maximum value found ∼0.5 m away from the ice. The observed dissipation rates near the ice are used to estimate basal melting of the ice shelf. We argue that our estimates of basal melting from dissipation rates are within a range of previous estimates of basal melting. This article is protected by copyright. All rights reserved.
      PubDate: 2016-11-11T17:04:03.134314-05:
      DOI: 10.1002/2016JC012149
       
  • Reconciling estimates of the ratio of heat and salt fluxes at the
           ice-ocean interface
    • Authors: T. Keitzl; J.-P. Mellado, D. Notz
      Abstract: The heat exchange between floating ice and the underlying ocean is determined by the interplay of diffusive fluxes directly at the ice-ocean interface and turbulent fluxes away from it. In this study, we examine this interplay through direct numerical simulations of free convection. Our results show that an estimation of the interface flux ratio based on direct measurements of the turbulent fluxes can be difficult because the flux ratio varies with depth. As an alternative, we present a consistent evaluation of the flux ratio based on the total heat and salt fluxes across the boundary layer. This approach allows us to reconcile previous estimates of the ice–ocean interface conditions. We find that the ratio of heat and salt fluxes directly at the interface is 83 to 100 rather than 33 as determined by previous turbulence measurements in the outer layer. This can cause errors in the estimated ice-ablation rate from field measurements of up to 40% if they are based on the three-equation formulation. This article is protected by copyright. All rights reserved.
      PubDate: 2016-11-11T17:03:08.327637-05:
      DOI: 10.1002/2016JC012018
       
  • The diurnal cycle of sea-surface temperature and estimation of the heat
           budget of the Mediterranean Sea
    • Authors: S. Marullo; P. J. Minnett, R. Santoleri, M. Tonani
      Abstract: The diurnal cycle in sea-surface temperature (SST) is reconstructed for the year 2013 by combining numerical model analyses and satellite measurements using Optimal Interpolation (OI). The method is applied to derive hourly Mediterranean SST fields using Spinning Enhanced Visible and Infrared Imager (SEVIRI) data and Mediterranean Forecasting System analyses (Copernicus Marine Environment Monitoring Service - Analysis and Forecast product). The evaluation of the Diurnal OI SST (
      DOI SST) values against drifter measurements results in a mean bias of −0.1°C and a RMS of 0.4°C. The
      DOI SST fields reproduce well the diurnal cycle in SST including extreme Diurnal Warming events as measured by drifting buoys. We evaluate the impact of resolving the SST diurnal cycle, including extreme events, on estimates of the heat budget of the Mediterranean Sea over an entire annual cycle. It results in the mean annual difference in the heat loss derived using SST's with and without diurnal variations of 4 Wm−2 with a peak of 9 Wm−2 in July. This value is comparable to several other sources of uncertainty in the calculation of the heat and water budgets of the Mediterranean Sea. The results are an important step towards reducing uncertainties in the “Mediterranean Sea Heat Budget Closure Problem. This article is protected by copyright. All rights reserved.
      PubDate: 2016-11-11T17:03:06.997545-05:
       
  • Storm surge along the Pacific Coast of North America
    • Authors: Peter D. Bromirski; Reinhard E. Flick, Arthur J. Miller
      Abstract: Storm surge is an important factor that contributes to coastal flooding and erosion. Storm surge magnitude along eastern North Pacific coasts results primarily from low sea level pressure (SLP). Thus coastal regions where high surge occurs identify the dominant locations where intense storms make landfall, controlled by storm track across the North Pacific. Here storm surge variability along the Pacific coast of North America is characterized by positive non-tide residuals at a network of tide gauge stations from southern California to Alaska. The magnitudes of mean and extreme storm surge generally increase from south to north, with typically high amplitude surge north of Cape Mendocino and lower surge to the south. Correlation of mode 1 non-tide principal component (PC1) during winter months (Dec.-Feb.) with anomalous SLP over the Northeast Pacific indicates that the dominant storm landfall region is along the Cascadia/British Columbia coast. Although empirical orthogonal function spatial patterns show substantial interannual variability, similar correlation patterns of non-tide PC1 over the 1948-1975 and 1983-2014 epochs with anomalous SLP suggest that, when considering decadal-scale time periods, storm surge and associated tracks have generally not changed appreciably since 1948. Non-tide PC1 is well-correlated with PC1 of both anomalous SLP and modeled wave height near the tide gauge stations, reflecting the inter-relationship between storms, surge, and waves. Weaker surge south of Cape Mendocino during the 2015-16 Niño compared with 1982-83 may result from changes in Hadley circulation. Importantly from a coastal impacts perspective, extreme storm surge events are often accompanied by high waves. This article is protected by copyright. All rights reserved.
      PubDate: 2016-11-11T17:03:03.014741-05:
      DOI: 10.1002/2016JC012178
       
  • Nitrous oxide during the onset of the Atlantic Cold Tongue
    • Authors: D. L. Arévalo-Martínez; A. Kock, T. Steinhoff, P. Brandt, M. Dengler, T. Fischer, A. Körtzinger, H. W. Bange
      Abstract: The tropical Atlantic exerts a major influence in climate variability through strong air-sea interactions. Within this region, the eastern side of the equatorial band is characterized by strong seasonality, whereby the most prominent feature is the annual development of the Atlantic Cold Tongue (ACT). This band of low sea surface temperatures (∼22-23°C) is typically associated with upwelling-driven enhancement of surface nutrient concentrations and primary production. Based on a detailed investigation of the distribution and sea-to-air fluxes of N2O in the eastern equatorial Atlantic (EEA), we show that the onset and seasonal development of the ACT can be clearly observed in surface N2O concentrations, which increase progressively as the cooling in the equatorial region proceeds during spring-summer. We observed a strong influence of the surface currents of the EEA on the N2O distribution, which allowed identifying “high” and “low” concentration regimes that were, in turn, spatially delimited by the extent of the warm eastward-flowing North Equatorial Countercurrent and the cold westward-flowing South Equatorial Current. Estimated sea-to-air fluxes of N2O from the ACT (mean 5.18±2.59 µmol m−2 d−1) suggests that in May-July 2011 this cold-water band doubled the N2O efflux to the atmosphere with respect to the adjacent regions, highlighting its relevance for marine tropical emissions of N2O. This article is protected by copyright. All rights reserved.
      PubDate: 2016-11-11T17:02:57.869086-05:
      DOI: 10.1002/2016JC012238
       
  • Assessing recent air-sea freshwater flux changes using a surface
           temperature-salinity space framework
    • Authors: Jeremy P. Grist; Simon A. Josey, Jan D. Zika, Dafydd Gwyn Evans, Nikolaos Skliris
      Abstract: A novel assessment of recent changes in air-sea freshwater fluxes has been conducted using a surface temperature-salinity framework applied to four atmospheric reanalyses. Viewed in the T-S space of the ocean surface, the complex pattern of the longitude-latitude space mean global Precipitation minus Evaporation (PME) reduces to three distinct regions. The analysis is conducted for the period 1979-2007 for which there is most evidence for a broadening of the (atmospheric) tropical belt. All four of the reanalyses display an increase in strength of the water cycle. The range of increase is between 2%-30% over the period analysed, with an average of 14%. Considering the average across the reanalyses, the water cycle changes are dominated by changes in tropical as opposed to mid-high latitude precipitation. The increases in the water cycle strength, are consistent in sign, but larger than in a 1% greenhouse gas run of the HadGEM3 climate model. In the model a shift of the precipitation/evaporation cells to higher temperatures is more evident, due to the much stronger global warming signal. The observed changes in freshwater fluxes appear to be reflected in changes in the T-S distribution of the Global Ocean. Specifically, across the diverse range of atmospheric reanalyses considered here, there was an acceleration of the hydrological cycle during 1979-2007 which led to a broadening of the ocean's salinity distribution. Finally, although the reanalyses indicate that the warm temperature tropical precipitation dominated water cycle change, ocean observations suggest that ocean processes redistributed the freshening to lower ocean temperatures. This article is protected by copyright. All rights reserved.
      PubDate: 2016-11-11T17:01:55.921755-05:
      DOI: 10.1002/2016JC012091
       
  • Barrier island breach evolution: Alongshore transport and bay-ocean
           pressure gradient interactions
    • Authors: Ilgar Safak; John C. Warner, Jeffrey H. List
      Abstract: Physical processes controlling repeated openings and closures of a barrier island breach between a bay and the open ocean are studied using aerial photographs and atmospheric and hydrodynamic observations. The breach site is located on Pea Island along the Outer Banks, separating Pamlico Sound from the Atlantic Ocean. Wind direction was a major control on the pressure gradients between the bay and the ocean to drive flows that initiate or maintain the breach opening. Alongshore sediment flux was found to be a major contributor to breach closure. During the analysis period from 2011 to 2016, three hurricanes had major impacts on the breach. First, Hurricane Irene opened the breach with wind-driven flow from bay to ocean in August 2011. Hurricane Sandy in October 2012 quadrupled the channel width from pressure gradient flows due to water levels that were first higher on the ocean side and then higher on the bay side. The breach closed sometime in Spring 2013, most likely due to an event associated with strong alongshore sediment flux but minimal ocean-bay pressure gradients. Then, in July 2014, Hurricane Arthur briefly opened the breach again from the bay side, in a similar fashion to Irene. In summary, opening and closure of breaches are shown to follow a dynamic and episodic balance between along-channel pressure gradient driven flows and alongshore sediment fluxes. This article is protected by copyright. All rights reserved.
      PubDate: 2016-11-11T17:01:17.6961-05:00
      DOI: 10.1002/2016JC012029
       
  • Subduction of North Pacific Tropical Water and Its equatorward pathways as
           shown by a simulated passive tracer
    • Authors: Xunwei Nie; Shan Gao, Fan Wang, Tangdong Qu
      Abstract: The subduction and equatorward pathways of North Pacific Tropical Water (NPTW) are investigated using a simulated passive tracer of the consortium Estimating the Circulation and Climate of the Ocean (ECCO). The results demonstrate that the subduction of NPTW occurs in a large area that extends from about 150ºE to 130ºW between 20ºN and 30ºN, but the main subduction region lies in its eastern part. After subduction, the main body of NPTW first spreads westward in the North Equatorial Current. Then it splits into two branches. One flows northward in the Kuroshio upon reaching the western boundary, and the other enters the tropical Pacific either via its western boundary pathway (WBP) or interior pathway (IP). Less than half of the transport through the WBP can eventually reach the central and eastern Pacific by the Equatorial Undercurrent, while the rest is seen to flow into the Indian Ocean by the Indonesian Throughflow. The IP is found to play a significant role in equatorward transport of the NPTW. About 30% of the NPTW that reached the equatorial Pacific is transported through the IP. This article is protected by copyright. All rights reserved.
      PubDate: 2016-11-11T17:00:57.829169-05:
      DOI: 10.1002/2016JC012305
       
  • Radar imaging of shallow water bathymetry: A case study in the Yangtze
           Estuary
    • Authors: Peng Yu; Johnny A. Johannessen, Vladimir Kudryavtsev, Xiaojing Zhong, Yunxuan Zhou
      Abstract: This study focuses on 2-dimensional (2-D) radar imaging of bathymetric features in the shallow water of the Yangtze Estuary using synthetic aperture radar (SAR) observations and model simulations. A validated 2-D shallow water numerical model simulates the barotropic current velocity, and the simulated current fields together with the relevant parameters of radar observations are then invoked in the radar imaging model as the input. The results show that variations in the simulated image intensity are mainly dominated by distinct radar backscatter anomalies caused by wave-current interactions in the vicinity of rapidly changing underwater topographies. The comparison between the simulated and observed SAR images shows a reasonable agreement, demonstrating that our approach may be implemented to monitor changes in the shallow water bathymetry of the Yangtze Estuary in the future. This article is protected by copyright. All rights reserved.
      PubDate: 2016-11-11T17:00:46.082791-05:
      DOI: 10.1002/2016JC011973
       
  • Carbon cycling dynamics in the seasonal sea-ice zone of East Antarctica
    • Authors: Nicholas P. Roden; Bronte Tilbrook, Thomas W. Trull, Patti Virtue, Guy D. Williams
      Abstract: The carbon cycle of the seasonally ice covered region of the southwest Indian Ocean sector of East Antarctica (30°-80°E, 60°-69°S) was investigated during austral summer (January – March 2006). Large variability in the drivers and timing of carbon cycling dynamics were observed and indicated that the study site was a weak net source of carbon dioxide (CO2) to the atmosphere of 0.8 ± 1.6 grams C m−2 during the ice-free period, with narrow bands of CO2 uptake observed near the continental margin and north of the Southern Antarctic Circumpolar Current Front. Continuous surface measurements of dissolved oxygen and the fugacity of CO2 were combined with net community production estimates from oxygen/argon ratios to show that surface heat gain and photosynthesis were responsible for the majority of observed surface water variability. On seasonal timescales, winter sea-ice cover reduced the flux of CO2 to the atmosphere in the study area, followed by biologically driven drawdown of CO2 as the ice retreated in spring-summer highlighting the important role that sea-ice formation and retreat has on the biogeochemical cycling of the region. This article is protected by copyright. All rights reserved.
      PubDate: 2016-11-11T17:00:41.888061-05:
      DOI: 10.1002/2016JC012008
       
  • Efficient boundary mixing due to near-inertial waves in a non-tidal basin:
           Observations from the Baltic Sea
    • Authors: C. Lappe; L. Umlauf
      Abstract: Diapycnal mixing in large non-tidal basins is often assumed to be related to the effect of near-inertial waves. While the role of near-inertial shear for the generation of shear instabilities in the stratified interior is relatively well understood, much less is known about the impact of near-inertial motions on boundary mixing processes in non-tidal systems, mainly owing to the lack of appropriate observations. Here, an extensive data set is discussed, describing the variability of boundary mixing induced by near-inertial motions near the sloping topography of one of the main basins of the Baltic Sea. These data reveal the existence of a vigorously turbulent bottom boundary layer of a few meters thickness covering the entire slope region above and below the permanent halocline that constitutes the main obstacle for vertical transport in the Baltic Sea. Near-bottom turbulence was driven by the near-inertial shear in the frictional boundary layer rather than by breaking of near-inertial waves near critical slopes. Large regions of the bottom boundary layer remained strongly stratified, and therefore, different from the traditional view of inefficient boundary mixing, mixing efficiencies reached values typical for the ocean's interior. This article is protected by copyright. All rights reserved.
      PubDate: 2016-11-11T17:00:38.033652-05:
      DOI: 10.1002/2016JC011985
       
  • Footprints of obliquely incident internal solitary waves and internal
           tides near the shelf break in the northern South China Sea
    • Authors: Xiaochuan Ma; Jun Yan, Yijun Hou, Feilong Lin, Xufeng Zheng
      Abstract: A mooring system and two sites of bottom currents were deployed over the slope and near the shelf break on the propagating paths of internal solitary waves (ISWs), west off Dongsha Atoll in the northern South China Sea. Data indicated that energetic ISWs obliquely shoaled onto the shelf west off Dongsha Atoll in an approximately 290° direction, causing strong reversing currents (some exceeding 80 cm/s) near the bottom. Two types of sandwaves and short scour channels are discernible on the seafloor near the shelf break, which have reasonable correlations with the obliquely incident ISWs and internal tides. Type 1 sandwaves, featured by ISWs at the depths of 130-150 m, have flat crests interacting with the isobaths at a angle of nearly 45° which slightly incline and migrate upslope. Type 2 sandwaves are associated with internal tides, which have crests parallel to the isobaths and distinctly incline and migrate downslope. Short channels are parallel to the depth contours and truncate the strata, which could be formed and maintained by along-slope currents that are probably produced by the obliquely ISWs on a large gradient (γ>0.8°). The ISWs can move coarse grains or suspend fine grains but do not change the long-term trend of sediment transport on the seabed with larger gradients (γ/c>1), which is dominated by internal tides. These features are likely widespread near the shelf break in the northern South China Sea and other seas but are limited on mild slopes where ISWs do not break. This article is protected by copyright. All rights reserved.
      PubDate: 2016-11-11T17:00:24.762668-05:
      DOI: 10.1002/2016JC012009
       
  • Effects of ocean grid resolution on tropical cyclone-induced upper ocean
           responses using a global ocean general circulation model
    • Authors: Hui Li; Ryan L. Sriver
      Abstract: Tropical cyclones (TCs) have the potential to influence regional and global climate through interactions with the upper ocean. Here we present results from a suite of ocean-only model experiments featuring the Community Earth System Model, in which we analyze the effect of tropical cyclone wind forcing on the global ocean using three different horizontal ocean grid resolutions (3˚, 1˚, and 0.1˚). The ocean simulations are forced with identical atmospheric inputs from the Coordinated Ocean-Ice Reference Experiments version 2 (COREv2) normal year forcing conditions, featuring global blended TC winds from a fully-coupled CESM simulation with a 25 km atmosphere [Small et al., 2014]. The simulated TC climatology shows good agreement with observational estimates of annual TC statistics, including annual frequency, intensity distributions, and geographic distributions. Each ocean simulation is comprised of a 5-year spin up with COREv2 normal year forcing, followed by 18 months with blended TC winds. In addition, we conduct corresponding control simulations for each grid resolution configuration without blended TC winds. We find that ocean horizontal and vertical grid resolutions affect TC-induced heat and momentum fluxes, post-storm cold wake features, and ocean subsurface temperature profiles. The responses are amplified for smaller grid spacing. Moreover, analyses show that the annually accumulated TC-induced ocean heat uptake is also sensitive to ocean grid resolution, which may have important implications for modeled ocean heat budgets and variability. This article is protected by copyright. All rights reserved.
      PubDate: 2016-11-07T09:19:03.335803-05:
      DOI: 10.1002/2016JC011951
       
  • Equatorial Pacific thermostad response to El Niño
    • Authors: Gregory C. Johnson; Abigail N. Birnbaum
      Abstract: El Niños are characterized by a shift of warm surface water from the western to eastern equatorial Pacific due to weakening of easterly trade winds. This shift is associated with the pycnocline (or thermocline), the large vertical density gradient beneath the surface mixed layer, shoaling in the west and deepening in the east, inducing a redistribution of ocean heat with global impacts. Here the response of the Pacific Equatorial Thermostad, a layer of low vertical stratification below the pycnocline, to El Niño is investigated using a monthly Argo float climatology and Argo float deep velocity data. A mean, seasonal cycle, trend, and time-lagged linear response to the Niño3.4 index are fit by least squares to temperature and salinity at each gridpoint as well as to deep float velocities (omitting the trend). The results of these fits are used to characterize the response of physical properties in the Thermostad, including layer thickness and velocity, to El Niño by comparing the mean properties following neutral conditions (Niño3.4 = 0°C) versus those following a moderate El Niño (Niño3.4 = 1°C). Following an El Niño, a strengthening of the westward-flowing Equatorial Intermediate Current of about 2.7 × 106 m3 s−1 shifts about 97 × 1012 m3 of thermostad water from the east to the west, allowing conservation of volume within the Thermostad as the pycnocline above deepens in the east and shoals in the west. This transport and volume change imply a 14-month time scale, consistent with El Niño. This article is protected by copyright. All rights reserved.
      PubDate: 2016-11-07T09:18:26.783043-05:
      DOI: 10.1002/2016JC012304
       
  • Effects of an Arctic under-ice bloom on solar radiant heating of the water
           column
    • Authors: Torbjørn Taskjelle; Mats A. Granskog, Alexey K. Pavlov, Stephen R. Hudson, Børge Hamre
      Abstract: The deposition of solar energy in the upper Arctic Ocean depends, among other things, on the composition of the water column. During the N-ICE2015 expedition, a drift in the Arctic pack ice north of Svalbard, an under-ice phytoplankton bloom was encountered in May 2015. This bloom led to significant changes in the inherent optical properties (IOPs) of the upper ocean. Mean values of total water absorption in the upper 20 m of the water column were up to 4 times higher during the bloom than prior to it. The total water attenuation coefficient increased by a factor of up to around 7. Radiative transfer modeling, with measured IOPs as input, has been performed with a coupled atmosphere-ice-ocean model. Simulations are used to investigate the change in depth dependent solar heating of the ocean after the onset of the bloom, for wavelengths in the region 350–700 nm. Effects of clouds, sea ice cover, solar zenith angle, as well as the average cosine for scattering of the ocean inclusions are evaluated. An increase in energy absorption in the upper 10 m of about 36% is found under 25 cm ice with 2 cm snow, for bloom conditions relative to pre-bloom conditions, which may have implications for ice melt and growth in spring. Thicker clouds and lower sun reduce the irradiance available, but lead to an increase in relative absorption. This article is protected by copyright. All rights reserved.
      PubDate: 2016-11-07T09:18:00.909254-05:
      DOI: 10.1002/2016JC012187
       
  • Long-range propagation and associated variability of internal tides in the
           South China Sea
    • Authors: Zhenhua Xu; Kun Liu, Baoshu Yin, Zhongxiang Zhao, Yang Wang, Qun Li
      Abstract: The variability of internal tides during their generation and long-range propagation in the South China Sea (SCS) is investigated by driving a high-resolution numerical model. The present study clarifies the notably different processes of generation, propagation and dissipation between diurnal and semidiurnal internal tides. Internal tides in the SCS originate from multiple source sites, among which the Luzon Strait is dominant, and contributes approximately 90% and 74% of the baroclinic energy for M2 and K1, respectively. To the west of the Luzon Strait, local generation of K1 internal tides inside the SCS is more energetic than the M2 tides. Diurnal and semidiurnal internal tides from the Luzon Strait radiate into the SCS in a north-south asymmetry but with different patterns because of the complex two-ridge system. The tidal beams can travel across the deep basin and finally arrive at the Vietnam coast and Nansha Island more than 1000-1500 km away. During propagation, M2 internal tides maintain a southwestward direction, whereas K1 exhibit complicated wave fields because of the superposition of waves from local sources and island scattering effects. After significant dissipation within the Luzon Strait, the remaining energy travels into the SCS and reduces by more than 90% over a distance of ∼1000 km. Inside the SCS, the K1 internal tides with long crests and flat beam angles are more influenced by seafloor topographical features and thus undergo apparent dissipation along the entire path, whereas the prominent dissipation of M2 internal tides only occurs after their arrival at Zhongsha Island. This article is protected by copyright. All rights reserved.
      PubDate: 2016-11-07T09:17:43.023495-05:
      DOI: 10.1002/2016JC012105
       
  • A modeling study of processes controlling the Bay of Bengal sea surface
           salinity interannual variability
    • Authors: V.P. Akhil; M. Lengaigne, J. Vialard, F. Durand, M. G. Keerthi, A.V.S. Chaitanya, F. Papa, V. V. Gopalakrishna, Clément de Boyer Montégut
      Abstract: Recent observational studies provided preliminary insights on the interannual variability of Bay of Bengal (BoB) Sea Surface Salinity (SSS), but are limited by the poor data coverage. Here, we describe the BoB interannual SSS variability and its driving processes from a regional eddy-permitting ocean general circulation model forced by interannually varying air-sea fluxes and altimeter-derived discharges of major rivers over the past two decades. Simulated interannual SSS variations compare favourably with both in-situ and satellite data and are largest in boreal fall in three regions: the northern BoB, the coastal region off east India and the Andaman Sea. In the northern BoB these variations are independent from those in other regions and mostly driven by summer-fall Ganga-Brahmaputra runoff interannual variations. In fall, remote forcing from the Indian Ocean Dipole results in anti-clockwise anomalous horizontal currents that drive interannual SSS variations of opposite polarity along the east coast of India and in the Southern Andaman Sea. From winter onward, these anomalies are damped by vertical mixing in the northern BoB and along the east coast of India and by horizontal advection in the Southern Andaman Sea. While river runoff fluctuations locally play a strong role near the Ganga-Brahmaputra river mouth, wind-driven interannual current anomalies are responsible for a large fraction of SSS interannual variability in most of the basin. This article is protected by copyright. All rights reserved.
      PubDate: 2016-11-07T09:11:06.488671-05:
      DOI: 10.1002/2016JC011662
       
  • Direct estimates of friction factors for a mobile rippled bed
    • Authors: S. Rodríguez-Abudo; D. L. Foster
      Abstract: New friction factor estimates are computed from the total momentum transfer applied to a rippled sediment bed. The total time-dependent momentum flux is achieved by implementing the double-averaged horizontal momentum equation on the nearbed flow field collected with PIV. Time-independent friction factors are obtained by regressing the total momentum flux to the common quadratic stress law given by . The resulting friction factors compare favorably with available analysis techniques including energy dissipation, vertical turbulence intensity, and maximum shear stress, but can be 2-6 times smaller than estimates determined with the model by [1994] and the formula of Swart [1974] using the ripple roughness. This article is protected by copyright. All rights reserved.
      PubDate: 2016-11-07T09:10:54.266417-05:
      DOI: 10.1002/2016JC012055
       
  • Isotope constraints on seasonal dynamics of dissolved and particulate N in
           the Pearl River Estuary, South China
    • Authors: Feng Ye; Guodong Jia, Luhua Xie, Gangjian Wei, Jie Xu
      Abstract: Isotope measurements were performed on dissolved NO3−, NH4+ and suspended particulate total N along a salinity gradient in the Pearl River Estuary (PRE) to investigate seasonal changes in main N sources and its biogeochemical processing under the influence of monsoon climate. Our data revealed that municipal sewage and re-mineralized soil organic N were the major sources of DIN (NO3− and/or NH4+) in freshwater during winter and summer, respectively, whereas phytoplankton biomass was a major component of PN in both seasons. In low salinity waters (
      PubDate: 2016-11-02T09:10:54.169304-05:
      DOI: 10.1002/2016JC012066
       
  • Long-range sediment transport in the world's oceans by stably stratified
           turbidity currents
    • Authors: Benjamin Kneller; Mohamad M. Nasr-Azadani, Senthil Radhakrishnan, Eckart Meiburg
      Abstract: Submarine fans, supplied primarily by turbidity currents, constitute the largest sediment accumulations on Earth. Generally accepted models of turbidity current behavior imply they should dissipate rapidly on the very small gradients of submarine fans, thus their persistence over long distances is enigmatic. We present numerical evidence, constrained by published field data, suggesting that turbidity currents traveling on low slopes and carrying fine particles have a stably stratified shear layer along their upper interface, which dramatically reduces dissipation and entrainment of ambient fluid, allowing the current to propagate over long distances. We propose gradient Richardson number as a useful criterion to discriminate between the different behaviors exhibited by turbidity currents on high and low slopes. This article is protected by copyright. All rights reserved.
      PubDate: 2016-10-27T03:40:35.040523-05:
      DOI: 10.1002/2016JC011978
       
  • A validated tropical-extratropical flood hazard assessment for New York
           Harbor
    • Authors: P.M. Orton; T.M. Hall, S.A. Talke, A.F. Blumberg, N. Georgas, S. Vinogradov
      Abstract: Recent studies of flood risk at New York Harbor (NYH) have shown disparate results for the 100-year storm tide, providing an uncertain foundation for the flood mitigation response after Hurricane Sandy. Here, we present a flood hazard assessment that improves confidence in our understanding of the region's present-day potential for flooding, by separately including the contribution of tropical cyclones (TCs) and extratropical cyclones (ETCs), and validating our modeling study at multiple stages against historical observations. The TC assessment is based on a climatology of 606 synthetic storms developed from a statistical-stochastic model of North Atlantic TCs. The ETC assessment is based on simulations of historical storms with many random tide scenarios. Synthetic TC landfall rates and the final TC and ETC flood exceedance curves are all shown to be consistent with curves computed using historical data, within 95% confidence ranges. Combining the ETC and TC results together, the 100-year return period storm tide at NYH is 2.70 m (2.51-2.92 at 95% confidence), and Hurricane Sandy's storm tide of 3.38 m was a 260-year (170-420) storm tide. Deeper analyses of historical flood reports from estimated Category-3 hurricanes in 1788 and 1821 lead to new estimates and reduced uncertainties for their floods, and show that Sandy's storm tide was the largest at NYH back to at least 1700. The flood exceedance curves for ETCs and TCs have sharply different slopes due to their differing meteorology and frequency, warranting separate treatment in hazard assessments. This article is protected by copyright. All rights reserved.
      PubDate: 2016-10-25T08:05:34.956258-05:
      DOI: 10.1002/2016JC011679
       
  • Circulation and oxygen cycling in the Mediterranean Sea: Sensitivity to
           future climate change
    • Authors: Helen Powley; Michael Krom, Philippe Van Cappellen
      Abstract: Climate change is expected to increase temperatures and decrease precipitation in the Mediterranean Sea (MS) basin, causing substantial changes in the thermohaline circulation (THC) of both the Western Mediterranean Sea (WMS) and Eastern Mediterranean Sea (EMS). The exact nature of future circulation changes remains highly uncertain, however, with forecasts varying from a weakening to a strengthening of the THC. Here, we assess the sensitivity of dissolved oxygen (O2) distributions in the WMS and EMS to THC changes using a mass balance model, which represents the exchanges of O2 between surface, intermediate and deep water reservoirs, and through the Straits of Sicily and Gibraltar. Perturbations spanning the ranges in O2 solubility, aerobic respiration kinetics and THC changes projected for the year 2100 are imposed to the O2 model. In all scenarios tested, the entire MS remains fully oxygenated after 100 years; depending on the THC regime, average deep-water O2 concentrations fall in the ranges 151-205 and 160-219 µM in the WMS and EMS, respectively. On longer timescales (>1000 years), the scenario with the largest (> 74%) decline in deep-water formation rate leads to deep-water hypoxia in the EMS but, even then, the WMS deep-water remains oxygenated. In addition, a weakening of THC may result in a negative feedback on O2 consumption as supply of labile dissolved organic carbon to deep-water decreases. Thus, it appears unlikely that climate-driven changes in THC will cause severe O2 depletion of the deep-water masses of the MS in the foreseeable future. This article is protected by copyright. All rights reserved.
      PubDate: 2016-10-25T08:05:33.37871-05:0
      DOI: 10.1002/2016JC012224
       
  • Nonlinear multiscale interactions and internal dynamics underlying a
           typical eddy-shedding event at Luzon Strait
    • Authors: Yuan-Bing Zhao; X. San Liang, Jianping Gan
      Abstract: Eddy-shedding is a highly nonlinear process that presents a major challenge in geophysical fluid dynamics. Using the newly developed localized multiscale energy and vorticity analysis (MS-EVA), this study investigates an observed typical warm eddy-shedding event as the Kuroshio passes the Luzon Strait, in order to gain insight into the underlying internal dynamics. Through multiscale window transform (MWT), it is found that the loop-form Kuroshio intrusion into the South China Sea (SCS) is not a transient feature, but a quasi-equilibrium state of the system. A mesoscale reconstruction reveals that the eddy does not have its origin at the intrusion path, but comes from the Northwest Pacific. It propagates westward, preceded by a cyclonic (cold) eddy, through the Kuroshio into the SCS. As the eddy pair runs across the main current, the cold one weakens and the warm one intensifies through a mixed instability. In its development, another cold eddy is generated to its southeast, which also experiences a mixed instability. It develops rapidly and cuts the warm eddy off the stream. Both the warm and cold eddies then propagate westward in the form of a Rossby wave (first baroclinic mode). As the eddies approach the Dongsha Islands, they experience another baroclinic instability, accompanied by a sudden accumulation of eddy available potential energy. This part of potential energy is converted to eddy kinetic energy through buoyancy conversion, and is afterwards transferred back to the large-scale field through inverse cascading, greatly reducing the intensity of the eddy and eventually leading to its demise. This article is protected by copyright. All rights reserved.
      PubDate: 2016-10-25T08:05:28.945498-05:
      DOI: 10.1002/2016JC012483
       
  • A system to measure the data quality of spectral remote sensing
           reflectance of aquatic environments
    • Authors: Jianwei Wei; Zhongping Lee, Shaoling Shang
      Abstract: Spectral remote sensing reflectance (Rrs, sr−1) is the key for ocean color retrieval of water bio-optical properties. Since Rrs from in-situ and satellite systems are subject to errors or artifacts, assessment of the quality of Rrs data is critical. From a large collection of high quality in situ hyperspectral Rrs datasets, we developed a novel quality assurance (QA) system that can be used to objectively evaluate the quality of an individual Rrs spectrum. This QA scheme consists of a unique Rrs spectral reference and a score metric. The reference system includes Rrs spectra of 23 optical water types ranging from purple blue to yellow waters, with an upper and a lower bound defined for each water type. The scoring system is to compare any target Rrs spectrum with the reference and a score between 0 and 1 will be assigned to the target spectrum, with 1 for perfect Rrs spectrum and 0 for unusable Rrs spectrum. The effectiveness of this QA system is evaluated with both synthetic and in situ Rrs spectra and it is found to be robust. Further testing is performed with the NOMAD dataset as well as with satellite Rrs over coastal and oceanic waters, where questionable or likely erroneous Rrs spectra are shown to be well identifiable with this QA system. Our results suggest that applications of this QA system to in situ datasets can improve the development and validation of bio-optical algorithms and its application to ocean color satellite data can improve the short- and long-term products by objectively excluding questionable Rrs data. This article is protected by copyright. All rights reserved.
      PubDate: 2016-10-25T08:05:23.160646-05:
      DOI: 10.1002/2016JC012126
       
  • Intraseasonal sea level variability along the West Coast of India
    • Authors: Laxmikant Dhage; P. Ted Strub
      Abstract: The importance of local versus distant forcing is studied for the wind driven intra-seasonal (30-120 day) sea level anomaly (SLA) variations along the west coast of India. Significant correlations of altimeter derived SLA on the west coast are found with the mid-basin SLA east of Sri-Lanka and SLA as far as Sumatra and the Equator, with increased lags, connecting with the remote forcing from the Equator in the form of reflected Rossby waves. The highest correlations between SLA on the west coast and winds are found with the winds at the southern tip of India. Coherence calculations help to identify the importance of a narrow band (40-60 day) for the interactions of winds with the intra-seasonal SLA variations. A multivariate regression model, along with the coherences within this narrower band, suggest the lags of SLA on the west coast with winds to range from 0-2 days with the local forcing to 11-13 days with the forcing along south east coast of India. Hovmöller diagrams illustrate the propagation of signals by estimating phase speed for Rossby waves (57 cm/s) across the Indian Ocean from Sumatra and Coastal trapped Waves (CTWs) along the west coast of India (178 cm/s). Propagation from the south-east coast of India is not as robust as Rossby waves from Sumatra. This article is protected by copyright. All rights reserved.
      PubDate: 2016-10-20T21:00:27.430364-05:
      DOI: 10.1002/2016JC011904
       
  • Coherent motions and time scales that control heat and mass transfer at
           wind-swept water surfaces
    • Authors: D. E. Turney
      Abstract: Forecast of the heat and chemical budgets of lakes, rivers and oceans requires improved predictive understanding of air-water interfacial transfer coefficients. Here we present laboratory observations of the coherent motions that occupy the air-water interface at wind speeds (U10) 1.1 to 8.9 m/s. Spatio-temporal near-surface velocity data and interfacial renewal data are made available by a novel flow tracer method. The relative activity, velocity scales and time scales of the various coherent interfacial motions are measured, namely for Langmuir circulations, streamwise streaks, non-breaking wind waves, parasitic capillary waves, non-turbulent breaking wind waves, and turbulence-generating breaking wind waves. Breaking waves exhibit a sudden jump in streamwise interfacial velocity wherein the velocity jumps up to exceed the wave celerity and destroys nearby parasitic capillary waves. Four distinct hydrodynamic regimes are found to exist between U10 = 0 and 8.9 m/s, each with a unique population balance of the various coherent motions. The velocity scales, time scales and population balance of the different coherent motions are input to a first-principles gas transfer model to explain the waterside transfer coefficient (kw) as well as experimental patterns of temperature and gas concentration. The model mixes concepts from surface renewal and divergence theories, and requires surface divergence strength (β), the Lagrangian residence time inside the upwelling zone (tLu), and the total lifetime of new interface before it is downwelled (tLT). The model's output agrees with time-averaged measurements kw, patterns of temperature in infrared photographs, and spatial patterns of gas concentration and kw from direct numerical simulations. Several non-dimensional parameters, e.g. βtLu and τstLT where τs is the interfacial shear rate, determine the effectiveness of a particular type of coherent motion for affecting kw. This article is protected by copyright. All rights reserved.
      PubDate: 2016-10-19T10:26:45.800745-05:
      DOI: 10.1002/2016JC012139
       
  • Seasonal patterns of SST diurnal variation over the Tropical Warm Pool
           region
    • Authors: Haifeng Zhang; Helen Beggs, Xiao Hua Wang, Andrew E. Kiss, Christopher Griffin
      Abstract: Five-year (2010 through 2014) Advanced Very High Resolution Radiometer (AVHRR) sea surface temperature (SST) data produced by the Australian Bureau of Meteorology have been validated against drifting buoy data and then used to study the seasonal patterns of the SST diurnal variation (DV) events over the Tropical Warm Pool region (TWP, 25°S–15°N, 90°E–170°E). The in-situ validation results illustrate the overall good quality of the AVHRR SST data set, although an average 0.19 K underestimation of the daytime measurements has been observed. The night-time observations are in good agreement with in-situ buoys with an average bias of 0.03 K and a 0.30 K standard deviation of the biases. This SST data set is then used to characterise the SST DV seasonal patterns, together with wind speeds, daily maximum solar shortwave insolation (SSImax) and latent heat flux (LHF). A double-peak seasonal pattern of SST DV is observed over the study region: the strongest DVs are found in March and October and the weakest in June. Sensitivity tests of DV to wind, SSImax, and LHF are conducted. The results indicate: (1) different morning and early afternoon winds (7 am to 2 pm local time, LT) affect DV by as much as 0.73 K when the half-daily (defined as 2 am to 2 pm LT in this study) average winds are fixed between 2-3 ms−1; (2) SSImax levels regulate DV less significantly (< 0.68 K) under fixed winds; and (3) LHF effects on DV are relatively weak (< 0.35 K). This article is protected by copyright. All rights reserved.
      PubDate: 2016-10-19T10:26:34.038591-05:
      DOI: 10.1002/2016JC012210
       
  • Structure and dynamics of a subglacial discharge plume in a Greenlandic
           Fjord
    • Authors: Kenneth D. Mankoff; Fiammetta Straneo, Claudia Cenedese, Sarah B. Das, Clark G. Richards, Hanumant Singh
      Abstract: Discharge of surface-derived meltwater at the submerged base of Greenland's marine-terminating glaciers creates subglacial discharge plumes that rise along the glacier/ocean interface. These plumes impact submarine melting, calving and fjord circulation. Observations of plume properties and dynamics are challenging due to their proximity to the calving edge of glaciers. Therefore to date information on these plumes has been largely derived from models. Here we present temperature, salinity, and velocity data collected in a plume that surfaced at the edge of Saqqarliup Sermia, a mid-sized Greenlandic glacier. The plume is associated with a narrow core of rising waters approximately 20 m in diameter at the ice edge that spreads to a 200 m by 300 m plume pool as it reaches the surface, before descending to its equilibrium depth. Volume flux estimates indicate that the plume is primarily driven by subglacial discharge and that this has been diluted in a ratio of 1:10 by the time the plume reaches the surface. While highly uncertain, meltwater fluxes are likely two orders of magnitude smaller than the subglacial discharge flux. The overall plume characteristics agree with those predicted by theoretical plume models for a convection driven plume with limited influence from submarine melting. This article is protected by copyright. All rights reserved.
      PubDate: 2016-10-19T10:20:27.03883-05:0
      DOI: 10.1002/2016JC011764
       
  • Air pressure effects on sea level changes during the Twentieth Century
    • Authors: Christopher G. Piecuch; Philip R. Thompson, Kathleen A. Donohue
      Abstract: Interpretation of tide gauge data in terms sea level (η) and ocean dynamics requires estimates of air pressure (pa) to determine the ocean's isostatic response—the inverted barometer effect (ηib). Three gridded pa products (HadSLP2, NOAA-20CRv2, ERA-20C) are used alongside meteorological station pa and tide gauge η records to evaluate the contribution of ηib to η changes over the Twentieth Century. Agreement between gridded products is better during more recent periods and over regions with good historical data coverage, whereas it is worse for earlier time periods or in ocean areas with poor observational data coverage. Comparison against station data reveals the presence of systematic errors in the gridded products, for example, such that uncertainties estimated through differencing the gridded products underestimate the true errors by roughly $40\%$on interannual and decadal time scales. Notwithstanding such correlated errors, gridded products are still {useful} for interpretation of tide gauge data. Removing gridded estimates of ηib from η records reduces spatial variance in centennial trends across tide gauges by 10–30%, formal errors in centennial trends from individual gauges by ∼ 5%, and the temporal variance in detrended records by 10–15% on average (depending on choice of gridded product). Results here advocate for making the ηib correction to tide gauge records in studies of ocean circulation and global η over long, multidecadal and centennial time scales using an ensemble mean taken across several gridded ηib products. This article is protected by copyright. All rights reserved.
      PubDate: 2016-10-18T02:50:53.148746-05:
      DOI: 10.1002/2016JC012131
       
  • Mapping error in Southern Ocean transport computed from satellite
           altimetry and Argo
    • Authors: Michael Kosempa; Don P. Chambers
      Abstract: In an effort to better estimate transport dynamics in response to wind forcing (primarily the Southern Annual Mode), this study quantifies the uncertainty in mapping zonal geostrophic transport of the Antarctic Circumpolar Current from sparse temperature, salinity and sea surface height observations. To do this, we sampled an ocean state estimate at the locations of both Argo floats and the Jason-1 altimeter groundtrack. These sampled values were then optimally interpolated to create SSH and temperature/salinity grids with 1° resolution. The temperature, salinity and SSH grids were then combined to compute the zonal geostrophic transport and compared to that estimated from the full state estimate. There are significant correlations between the baroclinic and barotropic error contributions to the total transport error. The increase in Argo floats in the Southern Ocean is effective in reducing mapping error. However, that error improvement is not uniform. By analyzing systematic errors in transport time series, we find the transects that are most appropriate for analyzing the dynamics of ACC transport using Argo and altimetric gridded fields. Based on our analysis, we conclude region south of Tasmania is most appropriate, with lowest uncertainty. Using real-world data, we calculated zonal transport variability at a transect south of Tasmania. There is an insignificant trend (0.3 ± 0.4 Sv yr−1, 90% confidence) but significant low-frequency variability correlated with the Southern Annular Mode (0.53, p 
      PubDate: 2016-10-18T02:50:46.211771-05:
      DOI: 10.1002/2016JC011956
       
  • Projecting nuisance flooding in a warming climate using generalized linear
           models and Gaussian processes
    • Authors: Alexander Vandenberg-Rodes; Hamed R. Moftakhari, Amir AghaKouchak, Babak Shahbaba, Brett F. Sanders, Richard A. Matthew
      Abstract: Nuisance flooding corresponds to minor and frequent flood events that have significant socio-economic and public health impacts on coastal communities. Yearly-averaged local mean sea level can be used as proxy to statistically predict the impacts of sea level rise (SLR) on the frequency of nuisance floods (NF). In this study, we use Generalized Linear Models (GLM) and Gaussian Process (GP) models combined to (i) estimate the frequency of NF associated with the change in mean sea level, and (ii) quantify the associated uncertainties via a novel and statistically robust approach. We calibrate our models to the water level data from eighteen tide gauges along the coasts of United States, and after validation, we estimate the frequency of NF associated with the SLR projections in year 2030 (under RCPs 2.6 and 8.5), along with their 90% bands, at each gauge. The historical NF-SLR data is very noisy, and shows large changes in variability (heteroscedasticity) with SLR. Prior models in the literature do not properly account for the observed heteroscedasticity, and thus their projected uncertainties are highly suspect. Among the models used in this study the Negative Binomial Distribution GLM with GP best characterizes the uncertainties associated with NF estimates; on validation data ≈ 93% of the points fall within the 90% credible limit, showing our approach to be a robust model for uncertainty quantification. This article is protected by copyright. All rights reserved.
      PubDate: 2016-10-18T02:50:23.747946-05:
      DOI: 10.1002/2016JC012084
       
  • Processes of multibathyal aragonite undersaturation in the Arctic Ocean
    • Authors: J.G. Wynn; L.L. Robbins, L.G. Anderson
      Abstract: During three years of study (2010-2012), the western Arctic Ocean was found to have unique aragonite saturation profiles with up to three distinct aragonite undersaturation zones. This complexity is produced as inflow of Atlantic- and Pacific-derived water masses mix with Arctic-derived waters, which are further modified by physiochemical and biological processes. The shallowest aragonite undersaturation zone, from the surface to ∼ 30 m depth is characterized by relatively low alkalinity and other dissolved ions. Besides local influence of biological processes on aragonite undersaturation of shallow coastal waters, the nature of this zone is consistent with dilution by sea-ice melt and invasion of anthropogenic CO2 from the atmosphere. A second undersaturated zone at ∼ 90-220 m depth (salinity ∼31.8–35.4) occurs within the Arctic Halocline and is characterized by elevated pCO2 and nutrients. The nature of this horizon is consistent with remineralization of organic matter on shallow continental shelves bordering the Canada Basin and the input of the nutrients and CO2 entrained by currents from the Pacific Inlet. Finally, the deepest aragonite undersaturation zone is at greater than 2000 m depth and is controlled by similar processes as deep aragonite saturation horizons in the Atlantic and Pacific Oceans. The comparatively shallow depth of this deepest aragonite saturation horizon in the Arctic is maintained by relatively low temperatures, and stable chemical composition. Understanding the mechanisms controlling the distribution of these aragonite undersaturation zones, and the timescales over which they operate will be crucial to refine predictive models. This article is protected by copyright. All rights reserved.
      PubDate: 2016-10-13T03:46:18.997671-05:
      DOI: 10.1002/2016JC011696
       
  • Gulf Stream variability and a triggering mechanism of its large meander in
           the South Atlantic Bight
    • Authors: Xiangming Zeng; Ruoying He
      Abstract: The Gulf Stream (GS) variability has an important impact on coastal circulation, shelf ecosystem, and regional weather and climate systems. Here we focus on the variability of the GS south of Cape Hatteras in the South Atlantic Bight (SAB). Statistical analysis on the 21-year satellite altimetry data reveals that the GS path in the SAB has two patterns: weakly and strongly deflected. The strongly deflected pattern is more likely to occur in winter. Over the last two decades, the largest GS offshore meander occurred in November 2009-April 2010. Realistic ocean hindcast simulation and adjoint sensitivity analysis are used to investigate the triggering mechanisms for this extreme event. Our analyses show that a net increase of relative vorticity near the Charleston Bump was generated by strong interaction between increased GS velocity and local bathymetry, pushing the GS further offshore by virtue of conserving the potential vorticity. Quantitative vorticity analysis confirms this finding. This article is protected by copyright. All rights reserved.
      PubDate: 2016-10-13T03:46:11.526959-05:
      DOI: 10.1002/2016JC012077
       
  • Impact of an upgraded model in the NCEP Global Ocean Data Assimilation
           System: The tropical Indian Ocean
    • Authors: Hasibur Rahaman; David Behringer, Stephen G. Penny, M Ravichandran
      Abstract: The National Centers for Environmental Prediction (NCEP) and the Indian National Centre for Ocean Information Services (INCOIS) produces global ocean analysis based on the Global Ocean Data Assimilation System (GODAS). This study shows how upgrades to the forward model simulations from MOM4p0d to MOM4p1 impact ocean analyses over the tropical Indian Ocean in GODAS. Three experiments were performed with same atmospheric forcing fields:(i) using MOM4p0d (GODAS_p0), (ii) using MOM4p1 (GODAS_p1), both using observed temperature and synthetic salinity, and (iii) using MOM4p1 (GODAS_p1S) assimilating both observed temperature and observed salinity. Validation with independent observations show significant improvement of sub-surface temperature and salinity in the new analysis using MOM4p1 versus MOM4p0d. There is also improvement in the upper ocean current of the equatorial Indian Ocean. The impact of observed salinity on the upper ocean surface current is marginal, but there is significant improvement in the sub-surface current. The seasonal and inter-annual variability of the Wyrtki jet and the equatorial undercurrent is improved in GODAS_p1 versus GODAS_p0. All analyses reproduced the Indian Ocean dipole, with the GODAS_p1S simulated sea surface temperature (SST) the most accurate. The temperature inversion over the north Bay of Bengal (BoB) is reproduced only in GODAS_p1S. The mean sea level over BoB and equatorial Indian Ocean improved in GODAS_p1S as compared with AVISO observation. The combined model upgrade and assimilation of observed salinity led to reduced root mean square deviation and higher correlation coefficient values in the sea level anomaly (SLA) when compared with satellite observations. This article is protected by copyright. All rights reserved.
      PubDate: 2016-10-13T03:45:59.559059-05:
      DOI: 10.1002/2016JC012056
       
  • Observations of open-ocean deep convection in the northwestern
           Mediterranean Sea: Seasonal and interannual variability of mixing and deep
           water masses for the 2007–2013 period
    • Authors: L. Houpert; X. Durrieu de Madron, P. Testor, A. Bosse, F. D'Ortenzio, M.N. Bouin, D. Dausse, H. Le Goff, S. Kunesch, M. Labaste, L. Coppola, L. Mortier, P. Raimbault
      Abstract: We present here a unique oceanographic and meteorological dataset focus on the deep convection processes. Our results are essentially based on in situ data (mooring, research vessel, glider, and profiling float) collected from a multi-platform and integrated monitoring system (MOOSE: Mediterranean Ocean Observing System on Environment), which monitored continuously the northwestern Mediterranean Sea since 2007, and in particular high-frequency potential temperature, salinity and current measurements from the mooring LION located within the convection region.From 2009 to 2013, the mixed layer depth reaches the seabed, at a depth of 2330m, in February. Then, the violent vertical mixing of the whole water column lasts between 9 and 12 days setting up the characteristics of the newly-formed deep water. Each deep convection winter formed a new warmer and saltier '“vintage” of deep water. These sudden inputs of salt and heat in the deep ocean are responsible for trends in salinity (3.3+/-0.2 *10−3/yr) and potential temperature (3.2+/-0.5 *10−3°C/yr) observed from 2009 to 2013 for the 600-2300m layer.For the first time, the overlapping of the 3 “phases” of deep convection can be observed with secondary vertical mixing events (2-4 days) after the beginning of the restratification phase, and the restratification/spreading phase still active at the beginning of the following deep convection event. This article is protected by copyright. All rights reserved.
      PubDate: 2016-10-13T03:43:19.995887-05:
      DOI: 10.1002/2016JC011857
       
  • Dissipation of wind waves by pancake and frazil ice in the autumn Beaufort
           Sea
    • Authors: W. Erick Rogers; Jim Thomson, Hayley H. Shen, Martin J. Doble, Peter Wadhams, Sukun Cheng
      Abstract: A model for wind-generated surface gravity waves, WAVEWATCH III®, is used to analyze and interpret buoy measurements of wave spectra. The model is applied to a hindcast of a wave event in sea ice in the western Arctic, October 11-14 2015, for which extensive buoy and ship-borne measurements were made during a research cruise. The model, which uses a viscoelastic parameterization to represent the impact of sea ice on the waves, is found to have good skill—after calibration of the effective viscosity—for prediction of total energy, but over-predicts dissipation of high frequency energy by the sea ice. This shortcoming motivates detailed analysis of the apparent dissipation rate. A new inversion method is applied to yield, for each buoy spectrum, the inferred dissipation rate as a function of wave frequency. For 102 of the measured wave spectra, visual observations of the sea ice were available from buoy-mounted cameras, and ice categories (primarily for varying forms of pancake and frazil ice) are assigned to each based on the photographs. When comparing the inversion-derived dissipation profiles against the independently derived ice categories, there is remarkable correspondence, with clear sorting of dissipation profiles into groups of similar ice type. These profiles are largely monotonic: they do not exhibit the “roll-over” that has been found at high frequencies in some previous observational studies. This article is protected by copyright. All rights reserved.
      PubDate: 2016-10-13T03:35:53.941718-05:
      DOI: 10.1002/2016JC012251
       
  • Chromophoric and fluorescent dissolved organic matter in and above the
           oxygen minimum zone off Peru
    • Authors: A.N. Loginova; S. Thomsen, A. Engel
      Abstract: As a result of nutrient upwelling, the Peruvian coastal system is one of the most productive regions in the ocean. Sluggish ventilation of intermediate waters, characteristic for the Eastern Tropical South Pacific (ETSP) and microbial degradation of a high organic matter load promotes deoxygenation at depth. Dissolved organic matter (DOM) plays a key role in microbial respiration and carbon cycling, but little is known on DOM distribution and cycling in the ETSP. DOM optical properties give important insights on DOM sources, structure and biogeochemical reactivity. Here, we present data and a conceptual view on distribution and cycling of chromophoric (CDOM) and fluorescent (FDOM) DOM in and above the oxygen minimum zone (OMZ) off Peru. Five fluorescent components were identified during PARAFAC analysis. Highest intensities of CDOM and of the amino acid-like fluorescent component (C3) occurred above the OMZ and coincided with maximum chl a concentrations, suggesting phytoplankton productivity as major source. High intensities of a marine humic-like fluorescent component (C1), observed in subsurface waters, indicated in situ microbial reworking of DOM. FDOM release from inner shelf sediment was determined by seawater analysis and continuous glider sensor measurement and included a humic-like component (C2) with a signature typical for terrestrially derived humic acids. Upwelling supplied humic-like substances to the euphotic zone. Photo-reactions were likely involved in the production of a humic-like fluorescent component (C5). Our data show that variable biological and physical processes need to be considered for understanding DOM cycling in a highly dynamic coastal upwelling system like the ETSP off Peru. This article is protected by copyright. All rights reserved.
      PubDate: 2016-10-13T03:30:53.767001-05:
      DOI: 10.1002/2016JC011906
       
  • Decomposition of thermal and dynamic changes in the South China Sea
           induced by boundary forcing and surface fluxes during 1970‒2000
    • Authors: Jun Wei; Paola Malanotte-Rizzoli, Ming-Ting Li, Hao Wang
      Abstract: Based on a fully-coupled, high-resolution regional climate model, this study analyzed three-dimensional temperature and momentum changes in the South China Sea (SCS) from 1970 to 2000, during which period the climate shifts from a decadal La Niña-like condition (before 1976/77) to a decadal El Niño-like condition afterward. With a set of partially-coupled experiments, sea surface temperature (SST) and kinetic energy (KE) changes during this period are first decomposed into two components: those induced by lateral boundary forcing and those induced by atmospheric surface fluxes. The results showed that the total SST and KE changes show an increasing trend from 1970 to 2000. The two decomposed components together determined 96% and 89% of the SST and KE changes respectively, implying their dominant roles on the SCS's surface variability. Spatially, a sandwich pattern of air-sea forcing relationship is revealed in the SCS basin. The increased KE, represented by a cyclonic flow anomaly in the northern SCS, was induced by enhanced cold water intrusion from Pacific into the SCS via the Luzon Strait (boundary forcing). This cold-water inflow, however, resulted in SST cooling along the northern shelf of the SCS. The maximal SST warming occurred in the central SCS and was attributed to the wind-evaporation-SST (WES) positive feedback (surface forcing), in which a southwestward wind anomaly is initialized by SST gradients between the northern and southern SCS. This wind anomaly decelerates the southwestly summer monsoons and in turn increases the SST gradients. Over the shallow Sunda shelf, which is far from the Luzon Strait, the SST/KE variability appeared to be determined primarily by local air–sea interactions. Furthermore, analyses on subsurface components indicated that the subsurface temperature changes are primarily induced by internal ocean mixing, which becomes significantly important below the thermocline. The enhanced subsurface flow is driven by the Luzon Strait inflow as well, and exits the SCS via the Mindoro-Sibutu passage. This article is protected by copyright. All rights reserved.
      PubDate: 2016-10-07T10:30:51.901249-05:
      DOI: 10.1002/2016JC012078
       
  • Interannual and interdecadal variability of the North Equatorial
           Countercurrent in the Western Pacific
    • Authors: Xiao Chen; Bo Qiu, Yan Du, Shuiming Chen, Yiquan Qi
      Abstract: Interannual and longer timescale variations of the North Equatorial Countercurrent (NECC) in the western Pacific are investigated using the multi-decade (1960-2014) hindcast by the Ocean general circulation model for the Earth Simulator (OFES). The OFES-simulated sea level and upper ocean circulation changes show favorable comparisons with available tide gauge data and repeat hydrographic surveys along the 137ºE meridian. An empirical orthogonal function (EOF) analysis reveals that the low-frequency NECC variability is dominated by two distinct modes. The first mode fluctuates interannually and shows strengthening and southward migration of the NECC concurrent with the development of El Niño events. Unlike the extra-tropical western Pacific Ocean circulation variability controlled by wind forcing west of the dateline, the interannual NECC variations are forced by equatorial wind forcing cumulative across the entire Pacific basin. The second mode of the NECC variability has a inter-decadal timescale and is characterized by NECC's progressive weakening in strength, migrating poleward, and broadening in width over the past 50 years. These long-term changes in NECC are caused by the corresponding changes in the trade wind system that weakened and expanded poleward in the past half a century across the Pacific basin. This article is protected by copyright. All rights reserved.
      PubDate: 2016-10-06T10:15:26.106931-05:
      DOI: 10.1002/2016JC012190
       
  • Long-term decrease in phosphate concentrations in the surface layer of the
           southern Japan Sea
    • Authors: Taketoshi Kodama; Yosuke Igeta, Mizuki Kuga, Shoko Abe
      Abstract: To identify possible causes for the long-term trends in nutrient concentrations in the southern Japan Sea (JS), we studied nutrient concentrations that were obtained by the Japan Meteorological Agency. Our evaluation shows that phosphate concentrations declined in the surface layers in summer (0–20 m and 21–50 m depth) and winter (0–20 m, 21–50 m, and 51–100 m depth) over the last 40 years, while no significant linear trend was observed for nitrate concentrations. The declining trend in the phosphate concentration was quantified as 1.8–3.3 nM year−1. The increase in atmospheric nutrient deposition to the JS could not explain the decline in phosphate concentration. In addition, the mixed layer depth during winter did not demonstrate any significant trend, and an increase in phosphate concentrations was not observed in any layers; therefore, the decrease in nutrient supply from deep JS water was not considered a major possible cause for the decline in the phosphate concentration. In contrast, the phosphate concentration in the surface of the southern JS during winter showed a significant positive correlation with the concentration in the 21–50 m depth layer of the saline East China Sea (ECS) water in the preceding summer, and the surface water of the southern JS was almost entirely replaced by water originating from the ECS during May–October. Therefore, it is concluded that the declining trend in the phosphate concentrations in the southern JS is caused by horizontal advection of ECS water. This article is protected by copyright. All rights reserved.
      PubDate: 2016-10-05T10:26:00.060089-05:
      DOI: 10.1002/2016JC012168
       
  • Winter ocean-ice interactions under thin sea ice observed by IAOOS
           platforms during N-ICE2015: Salty surface mixed layer and active basal
           melt
    • Authors: Zoé Koenig; Christine Provost, Nicolas Villacieros-Robineau, Nathalie Sennéchael, Amélie Meyer
      Abstract: IAOOS (Ice Atmosphere Arctic Ocean Observing System) platforms, measuring physical parameters at the atmosphere-snow-ice-ocean interface deployed as part of the N-ICE2015 campaign, provide new insights on winter conditions North of Svalbard. The three regions crossed during the drifts, the Nansen Basin, the Sofia Deep and the Svalbard northern continental slope featured distinct hydrographic properties and ice-ocean exchanges. In the Nansen Basin the quiescent warm layer was capped by a stepped halocline (60 and 110 m) and a deep thermocline (110 m). Ice was forming and the winter mixed layer salinity was larger by ∼0.1 g/kg than previously observed. Over the Svalbard continental slope, the Atlantic Water (AW) was very shallow (20 m from the surface) and extended offshore from the 500 m isobath by a distance of about 70 km, sank along the slope (40 m from the surface) and probably shed eddies into the Sofia Deep. In the Sofia Deep, relatively warm waters of Atlantic origin extended from 90 m downward. Resulting from different pathways, these waters had a wide range of hydrographic characteristics. Sea-ice melt was widespread over the Svalbard continental slope and ocean-to-ice heat fluxes reached values of 400 Wm−2 (mean of ∼150 Wm−2 over the continental slope). Sea-ice melt events were associated with near 12-hour fluctuations in the mixed-layer temperature and salinity corresponding to the periodicity of tides and near-inertial waves potentially generated by winter storms, large barotropic tides over steep topography and/or geostrophic adjustments. This article is protected by copyright. All rights reserved.
      PubDate: 2016-10-05T10:25:57.729296-05:
      DOI: 10.1002/2016JC012195
       
  • Time series measurements of transient tracers and tracer derived transport
           in the deep western boundary current between the Labrador Sea and the
           subtropical Atlantic Ocean at Line W
    • Authors: John N. Smith; William M. Smethie, Igor Yashayev, Ruth Curry, Kumiko Azetsu-Scott
      Abstract: Time series measurements of the nuclear fuel reprocessing tracer, 129I and the gas ventilation tracer, CFC-11 were undertaken on the AR7W section in the Labrador Sea (1997-2014) and on Line W (2004-2014), located over the US continental slope off Cape Cod, to determine advection and mixing time scales for the transport of Denmark Strait Overflow Water (DSOW) within the Deep Western Boundary Current (DWBC). Tracer measurements were also conducted in 2010 over the continental rise southeast of Bermuda to intercept the equator-ward flow of DSOW by interior pathways. The Labrador Sea tracer and hydrographic time series data were used as input functions in a boundary current model that employs transit time distributions to simulate the effects of mixing and advection on downstream tracer distributions. Model simulations of tracer levels in the boundary current core and adjacent interior (shoulder) region with which mixing occurs were compared with the Line W time series measurements to determine boundary current model parameters. These results indicate that DSOW is transported from the Labrador Sea to Line W via the DWBC on a time scale of 5-6 y corresponding to a mean flow velocity of 2.7 cm/s while mixing between the core and interior regions occurs with a time constant of 2.6 y. A tracer section over the southern flank of the Bermuda rise indicates that the flow of DSOW that separated from the DWBC had undergone transport through interior pathways on a time scale of 9 y with a mixing time constant of 4 y. This article is protected by copyright. All rights reserved.
      PubDate: 2016-10-05T10:25:55.624726-05:
      DOI: 10.1002/2016JC011759
       
  • CMIP5 earth system models with biogeochemistry: An assessment for the
           southwest Pacific Ocean
    • Authors: Graham J. Rickard; Erik Behrens, Stephen M. Chiswell
      Abstract: An assessment is made of the ability of CMIP5 models to represent the seasonal biogeochemical cycles over the late twentieth century in the southwest Pacific Ocean. In particular, sea surface temperature (SST), surface chlorophyll a, nitrate, phosphate, silicate, and the depth of the seasonal thermocline, are examined to quantify the physical-biogeochemical capabilities of each model; the result is a “ranking” estimate enabling model ensemble generation. The better/less ranked ensembles we refer to as inner/outer, respectively. The ensembles then allow less well observed variables such as iron and vertically integrated primary production to be assessed. The assessment establishes model output confidence limits for setting bounds on future model scenario ecosystem change projections. By the end of the twenty first century under Representative Concentration Pathways (RCP) RCP4.5 and/or RCP8.5 our best estimates suggest that there will be average domain wide increases in SST and surface iron, but average decreases in surface chlorophyll a, nitrate, and phosphate, accompanied by relatively large decreases in the depth of the seasonal thermocline (all changes realised by both ensembles). On the other hand, for surface silicate the inner ensemble suggests general declines, and vice versa for the outer ensemble. For integrated primary production the ensembles predict declines in subtropical water, but elsewhere generally less significant changes. This article is protected by copyright. All rights reserved.
      PubDate: 2016-10-05T10:25:53.513578-05:
      DOI: 10.1002/2016JC011736
       
  • Observations and Modeling of a Tidal Inlet Dye Tracer Plume
    • Authors: Falk Feddersen; Maitane Olabarrieta, R. T. Guza, D. Winters, Britt Raubenheimer, Steve Elgar
      Abstract: A 9 km-long tracer plume was created by continuously releasing Rhodamine WT dye for 2.2 h during ebb tide within the southern edge of the main tidal channel at New River Inlet, NC on May 7, 2012, with highly obliquely incident waves and alongshore winds. Over 6 h from release, COAWST (coupled ROMS and SWAN, including wave, wind, and tidal forcing) modeled dye compares well with (aerial hyperspectral and in situ) observed dye concentration. Dye first was transported rapidly seaward along the main channel and partially advected across the ebb-tidal shoal until reaching the offshore edge of the shoal. Dye did not eject offshore in an ebb-tidal jet because the obliquely incident breaking waves retarded the inlet-mouth ebb-tidal flow and forced currents along the ebb shoal. The dye plume largely was confined to 
      PubDate: 2016-10-04T10:25:55.161221-05:
      DOI: 10.1002/2016JC011922
       
  • The Beaufort Gyre intensification and stabilization: A model-observation
           synthesis
    • Authors: Jinlun Zhang; Michael Steele, Kay Runciman, Sarah Dewey, James Morison, Craig Lee, Luc Rainville, Sylvia Cole, Richard Krishfield, Mary-Louise Timmermans, John Toole
      Abstract: A model–observation synthesis is conducted to investigate changes in the upper ocean circulation and stratification in the Canada Basin (CB) of the Arctic Ocean. Results show that the Beaufort Gyre (BG) has been generally intensifying during 1992–2015 in conjunction with changes in sea ice and the upper ocean including increasing sea surface height (SSH), sea ice and ocean speed, Ekman transport convergence and downwelling, and freshwater content, decreasing ice thickness and upper ocean salinity, shoaling summer halocline and mixed layer, and deepening winter halocline and mixed layer. Increasing Ekman transport convergence draws more water from surrounding areas into the CB, thus lowering SSH in those areas and raising SSH in the CB. The rate of change in the CB began to decrease in 2008 and the BG circulation appears to be stabilizing, if not relaxing slightly. This is reflected in the general plateauing of SSH, the intensity of the sea ice and ocean circulation, and various measures of the CB thermohaline stratification. The BG intensification and subsequent stabilization appear to have been strongly controlled by atmospheric changes in the CB characterized by generally increasing anticyclonic wind circulation and sea level pressure (SLP) before 2008 and falling wind strength and SLP to below-average levels in some years after 2008. Changes in SLP are highly correlated with changes in ocean surface stress curl and downwelling. Since 2008, the magnitude of the stress curl and downwelling in much of the CB has declined, contributing to BG stabilization. The general leveling-off of sea ice thickness also contributes to the stabilization by limiting melt water input to the CB that increases freshwater content. Temperatures in the Near Surface Temperature Maximum layer trended upward slightly over 1992–2015, which is closely correlated with decreasing sea ice thickness. Upper ocean heat content increased over the study period mainly due to strong temperature increases in the summer Pacific Water layer. This article is protected by copyright. All rights reserved.
      PubDate: 2016-10-04T10:25:53.440564-05:
      DOI: 10.1002/2016JC012196
       
  • Impact of model resolution for on-shelf heat transport along the West
           Antarctic Peninsula
    • Authors: Jennifer A Graham; Michael S Dinniman, John M Klinck
      Abstract: The flux of warm deep water onto Antarctic continental shelves plays a vital role in determining water mass properties adjacent to the continent. A regional model, with two different grid resolutions, has been used to simulate ocean processes along the West Antarctic Peninsula. At both 4 km and 1.5 km resolution, the model reproduces the locations of warm intrusions, as shown through comparison with observations from instrumented seals. However, the 1.5 km simulation shows greater on-shelf heat transport, leading to improved representation of heat content on the shelf. This increased heat transport is associated with increased eddy activity, both at the shelf-break and in the deep ocean off-shore. Cross-shelf troughs are key locations of on-shelf heat transport. Comparison of two troughs, Belgica and Marguerite, shows differing responses to increased resolution. At higher resolution, there is an increased on-shelf volume transport at Belgica Trough, but not at Marguerite Trough. This is likely related to the differing structure of the shelf-break jet between these two locations. The increased heat flux at Marguerite Trough is attributed to increased heat content in the on-shelf transport. Increased eddy activity off-shelf may lead to greater cross-front heat transport, and therefore increased heat available above the continental slope. While these simulations differ in their magnitude of heat transport, both show similar patterns of variability. Variations in wind stress lead to variations in speed of the shelf-break jet, and therefore on-shelf heat transport. These results demonstrate the importance of model resolution for understanding cross-shelf transport around Antarctica. This article is protected by copyright. All rights reserved.
      PubDate: 2016-10-04T10:20:53.977797-05:
      DOI: 10.1002/2016JC011875
       
  • Recurrent Replenishment of Labrador Sea Water and Associated Decadal-Scale
           Variability
    • Authors: Igor Yashayaev; John W. Loder
      Abstract: Winter convective overturning in the Labrador Sea reached an “aggregate” maximum depth of 1700m in 2015 – the deepest since 1994 – with the resulting Labrador Sea Water (LSW) “year class” being one of the deepest and thickest observed outside of the early 1990s. Argo float, annual survey and moored measurements in recent decades provide an unprecedented view of important seasonal, interannual and longer-term LSW variability in the Labrador Sea region. During the 2002-2015 “Argo” era, the average winter LSW pycnostad volume was about 70% larger in relatively strong convection years than in relatively weak ones. However, the winter-to-fall LSW disappearance volume was 180% larger, pointing to a factor of 2.8 difference in the potential LSW export rates from the region between relatively strong and weak convection years. Intermittently recurrent deep convection is contributing to predominant decadal-scale variations in intermediate-depth temperature, salinity and density in the LS, with implications for decadal-scale variability across the subpolar North Atlantic and potentially in the Atlantic Meridional Overturning Circulation. Comparison of the LS ocean heat content changes and cumulative surface heat losses during the fall-winter cooling seasons indicates that anomalously strong winter atmospheric cooling, associated at least in part with the North Atlantic Oscillation, is continuing to be a major forcing of the recurrent convection. This article is protected by copyright. All rights reserved.
      PubDate: 2016-09-30T03:45:42.867176-05:
      DOI: 10.1002/2016JC012046
       
  • Tidal distortion caused by the resonance of sexta-diurnal tides in a
           micro-mesotidal embayment
    • Authors: Dehai Song; Yuhan Yan, Wen Wu, Xiliang Diao, Yang Ding, Xianwen Bao
      Abstract: Double high water and double-peak flood current were observed in Daya Bay (DYB), China, which is a shallow, mixed, mainly semidiurnal-tide dominated bay with a micro- to meso-tidal range. Harmonic analysis reveals that the quarter- and especially the sexta-diurnal constituents are getting much stronger as tides propagating into the bay. The astronomical tides induced tidal asymmetry is yet dominant at the bay entrance, but overtaken by the sexta-diurnal tides at the end of the bay. Both the M4 and M6 tide meet the requirement proposed in previous studies, but still unable to produce a double high water alone. Therefore the conditions to produce a double high water between a fundamental tide and its higher harmonics need to be revisited. Analytical solutions were obtained in this paper, which fit the numerical solutions very well. Modelling result indicates M6 alone with M2 can produce the double high water in DYB, but limited in some regions; while the combination of M2, M4 and M6 tides would enhance the capability. The amplification of sexta-diurnal tides in DYB is dominated by resonance, and followed by shoaling effect. Bottom friction damped M6 a lot, and largely confined its amplification. However, the quadratic friction and other nonlinear processes are just responsible for about 10% of the total M6 increase. This article is protected by copyright. All rights reserved.
      PubDate: 2016-09-30T03:40:43.680092-05:
      DOI: 10.1002/2016JC012039
       
  • Century-scale perspectives on observed and simulated Southern Ocean sea
           ice trends from proxy reconstructions
    • Authors: Will Hobbs; Mark Curran, Nerilie Abram, Elizabeth R. Thomas
      Abstract: Observations show that Southern Ocean sea ice extent has increased since 1979, whereas global coupled climate models simulate a decrease over the same period. It is uncertain whether the observed trends are anthropogenically forced or due to internal variability, and whether the discrepancy between models and observations is also due to internal variability or indicative of a significant deficiency in the models. The length of the passive microwave satellite record hampers research into this question, which is too short to resolve multidecadal variability. Here, we use a number of proxies for regional seasonal sea ice extent around the Antarctic region to reconstruct sea ice extent for the full 20th century, which we compare with CMIP5 model simulations. We find that for the East Antarctic, Amundsen and Bellingshausen Seas, models and reconstructions agree that there has been a decrease in sea ice extent since the early 1970s, but this decrease is small compared to the simulated internal variability of the system. The Ross Sea is a confounding factor, with a significant increase in sea ice since 1979 that is not captured by climate models; however, existing proxy reconstructions of this region are not yet sufficiently reliable for formal change detection. This article is protected by copyright. All rights reserved.
      PubDate: 2016-09-30T03:36:08.469928-05:
      DOI: 10.1002/2016JC012111
       
  • Laboratory simulation of the geothermal heating effects on ocean
           overturning circulation
    • Authors: Fei Wang; Shi-Di Huang, Sheng-Qi Zhou, Ke-Qing Xia
      Abstract: Motivated by a desire to understand the geothermal heating effects on ocean circulation, a large-scale circulation generated and sustained by thermal forcing at the surface subject to a small amount of heating from the bottom boundary is investigated through laboratory experiments, motivated by understanding the geothermal heating effects on ocean circulation. Despite its idealization, our experiments demonstrate that the leading order effect of geothermal heating is to significantly enhance the abyssal overturning, in agreement with the findings in ocean circulation models. Our experiments also demonstrate that geothermal heating cannot influence the poleward heat transport due to the strong stratification in the thermocline. Our study further reveals that the ratio of geothermal-flux-induced turbulent dissipation to the dissipation due to other energies is the key parameter determining the dynamical importance of geothermal heating. This quantity explains why the impact of geothermal heating is sensitive to the deep stratification, the diapycnal mixing and the amount of geothermal flux. Moreover, it is found that this dissipation ratio may be used to understand results from different studies in a consistent way. This article is protected by copyright. All rights reserved.
      PubDate: 2016-09-30T03:36:06.617896-05:
      DOI: 10.1002/2016JC012068
       
  • Effects of mesoscale eddies on the volume transport and branch pattern of
           the Kuroshio east of Taiwan
    • Authors: Xiaomei Yan; Xiao‐Hua Zhu, Chongguang Pang, Linlin Zhang
      Abstract: Using satellite altimeter data and a long‐term altimetric transport index for the Kuroshio inflow northeast of Taiwan, the effects of mesoscale eddies on the Kuroshio volume transport (KVT) at the East Taiwan Channel and the branching pattern of the Kuroshio east of Taiwan are investigated at scales from those of individual events to interannual timescales. Both anticyclonic and cyclonic eddies are found to be able either to strengthen or weaken the KVT, depending on the relative strength of mass convergence and divergence produced upstream and downstream of the eddies. The major factor influencing the intensity of the Kuroshio inflow is the meridional location of the eddies. For single eddy events, the KVT is significantly correlated with the latitude of the eddy's center, the correlations being 0.44 and ‐0.48 for anticyclonic and cyclonic eddies, respectively. For dipole eddy events, when the direction angle falls in the interval 40°‐150° (240°‐300°), the KVT anomaly tends to be positive (negative). Furthermore, low KVT events generally correspond to the formation of the Ryukyu Current branch, which is also generated from strong mass divergence produced by the eddies. In addition, on interannual time scales, the variation of KVT is closely related to the relative number of anticyclonic to cyclonic eddies west of 125°E, with a correlation of 0.5. This article is protected by copyright. All rights reserved.
      PubDate: 2016-09-28T03:32:07.999989-05:
      DOI: 10.1002/2016JC012038
       
  • Weakest winter South China Sea western boundary current caused by the
           2015–2016 El Niño event
    • Authors: Ruixiang Zhao; Xiao‐Hua Zhu
      Abstract: During the winter of 2015–2016, the strongest El Niño event of the 21st century occurred. At the same time, volume transport (VT) time series of the South China Sea western boundary current (SCSWBC) exhibited a minimum value of 3.7 Sv (1 Sv = 1 × 106 m3 s−1) toward the southwest, indicating the weakest strength ever recorded in boreal winter (from November to February). The South China Sea (SCS) cyclonic gyre, inferred from the satellite‐derived surface absolute geostrophic current, was significantly reduced. It was considered that the weakened wind stress curl (negative anomaly) over the SCS resulting from an anticyclone over the Philippine Sea played an essential role. The anticyclone arose from a Rossby‐wave response to a negative sea surface temperature anomaly in the northwest Pacific. This idea is further supported by composite analysis, which shows that during El Niño (La Niña) winter, negative (positive) wind stress curl anomalies prevail in the Philippines Sea and the SCS; thus, the wind stress curl over the SCS is reduced (strengthened), leading to a weaker (stronger) SCS cyclonic gyre and SCSWBC. The mean VT of SCSWBC is 4.7 Sv (5.6 Sv), which is smaller (larger) than 5.2 Sv in normal years. This study provides robust observational evidence from long‐term in situ volume transport monitoring that El Niño can have a significant impact on the SCSWBC through an atmosphere‐bridged teleconnection. This article is protected by copyright. All rights reserved.
      PubDate: 2016-09-28T03:31:49.945392-05:
      DOI: 10.1002/2016JC012252
       
  • Pressure field induced in the water column by acoustic‐gravity waves
           generated from sea bottom motion
    • Authors: Tiago C. A. Oliveira; Usama Kadri
      Abstract: An uplift of the ocean bottom caused by a submarine earthquake can trigger acoustic‐gravity waves that travel at near the speed of sound in water and thus may act as early tsunami precursors. We study the spatio‐temporal evolution of the pressure field induced by acoustic‐gravity modes during submarine earthquakes, analytically. We show that these modes may all induce comparable temporal variations in pressure at different water depths in regions far from the epicenter, though the pressure field depends on the presence of a leading acoustic‐gravity wave mode. Practically, this can assist in the implementation of an early tsunami detection system by identifying the pressure and frequency ranges of measurement equipment and appropriate installation locations. This article is protected by copyright. All rights reserved.
      PubDate: 2016-09-28T03:27:02.815803-05:
      DOI: 10.1002/2016JC011742
       
  • Vernal distribution and turnover of dimethylsulfide (DMS) in the surface
           water of the Yellow Sea
    • Authors: Cheng‐Xuan Li; Gui‐Peng Yang, Bao‐Dong Wang, Zong‐Jun Xu
      Abstract: The spatial and interannual variations of dimethylsulfide (DMS) and its precursors, dissolved and particulate dimethylsulfoniopropionate (DMSP), were discussed on the basis of field observations in the surface waters of the Yellow Sea during spring 2007. Maxima of dimethylated sulfur compounds and low chlorophyll a concentrations were found in the central southern Yellow Sea, whereas low concentrations of DMS and DMSP were detected at the boundary between the northern and southern parts of the Yellow Sea. This frontal region is influenced by active water currents, air–sea interface exchanges, and biological turnover. The horizontal variations in DMS production and consumption rates showed a decreasing tendency from the coastal to offshore areas mainly due to the complicated biological features. DMS positively correlated with dissolved CH4 and CO2 but negatively correlated with nutrients (nitrite and phosphate). Particulate DMSP concentrations and DMS production rates positively correlated with dinoflagellate abundances but negatively correlated with diatom cell densities. DMS and DMSP concentrations, as well as DMS production and consumption rates, exhibited approximately 2.0‐2.8 fold increases from 2005 to 2012. This finding was likely caused by shifts in the phytoplankton communities from diatoms to dinoflagellates and the increases in abundances of zooplankton and bacteria. Average sea‐to‐air DMS fluxes were estimated to be 8.12 ± 1.24 µmol·(m−2·d−1), and DMS microbial consumption was approximately 1.68 times faster than the DMS sea–air exchange. These findings imply that biological consumption, relative to ventilation, is a predominant mechanism in DMS removal from the surface water. This article is protected by copyright. All rights reserved.
      PubDate: 2016-09-28T03:21:26.848701-05:
      DOI: 10.1002/2016JC011901
       
  • Assessing controls on cross‐shelf phytoplankton and suspended particle
           distributions using repeated bio‐optical glider surveys
    • Authors: Fernanda Henderikx Freitas; David A. Siegel, Libe Washburn, Stuart Halewood, Erik Stassinos
      Abstract: Characterizing the space/time variability of bio‐optical properties is essential to understanding the mechanisms that control cross‐shelf phytoplankton and suspended particle distributions in coastal waters. Approximately 400 high‐resolution cross‐shelf sections of bio‐optical properties collected with an oceanographic glider in the coastal Santa Barbara Channel, California, revealed complex relationships among optical properties and environmental conditions. Surface waves were found as a proximate control on suspended sediment variability for both inner and mid‐shelves. Increases in phytoplankton abundances attributed to horizontal advection and upwelling events were observed only on episodic time scales. The lack of all‐encompassing linear relationships between environmental forcings and changes in cross‐shelf phytoplankton highlight the challenge of decoupling bio‐optical signals from their controlling processes in coastal zones where phytoplankton distributions are patchy, and where nearshore and offshore phytoplankton populations and suspended sediments often occupy the same portion of the water column. Clear relationships between runoff and productivity were not observed. Temporal variability of suspended particles and phytoplankton distributions were roughly independent from each other during stratified conditions. Synchronous increases in phytoplankton and suspended sediments were observed when associated with strong upwelling events that may induce mixing and promote productivity. The repeated glider sections illustrated many processes regulating phytoplankton and particle transport in the innershelf and showed the difficulty in establishing general connections between high‐frequency changes in optical properties and potential environmental forcings in a complex coastal environment. This article is protected by copyright. All rights reserved.
      PubDate: 2016-09-26T18:15:30.835054-05:
      DOI: 10.1002/2016JC011781
       
  • Scales and dynamics of submesoscale coherent vortices formed by deep
           convection in the northwestern Mediterranean Sea
    • Authors: Anthony Bosse; Pierre Testor, Loïc Houpert, Pierre Damien, Louis Prieur, Daniel Hayes, Vincent Taillandier, Xavier Durrieu de Madron, Fabrizio d'Ortenzio, Laurent Coppola, Johannes Karstensen, Laurent Mortier
      Abstract: Since 2010, an intense effort in the collection of\textit{in situ} observations has been carried out in the northwestern Mediterranean Sea thanks to gliders, profiling floats, regular cruises and mooring lines. This integrated observing system enabled a year‐to‐year monitoring of the deep waters formation that occurred in the Gulf of Lions area during 4 consecutive winters (2010‐2013). Vortical structures remnant of wintertime deep vertical mixing events were regularly sampled by the different observing platforms. These are Submesoscale Coherent Vortices (SCVs) characterized by a small radius (∼5‐8 km), strong depth‐intensified orbital velocities (∼10‐20 cm s−1) with often a weak surface signature, high Rossby (∼0.5) and Burger numbers O(0.5‐1). Anticyclones transport convected waters resulting from intermediate (∼300 m) to deep (∼2000 m) vertical mixing. Cyclones are characterized by a 500‐1000 m thick layer of weakly stratified deep waters (or bottom waters that cascaded from the shelf of the Gulf of Lions in 2012) extending down to the bottom of the ocean at ∼2500 m. The formation of cyclonic eddies seems to be favored by bottom‐reaching convection occurring during the study period or cascading events reaching the abyssal plain. We confirm the prominent role of anticyclonic SCVs and shed light on the important role of cyclonic SCVs in the spreading of a significant amount (30%) of the newly‐formed deep waters away from the winter mixing areas. Since they can survive until the following winter, they can potentially have a great impact on the mixed layer deepening through a local preconditioning effect. This article is protected by copyright. All rights reserved.
      PubDate: 2016-09-26T02:57:39.972444-05:
      DOI: 10.1002/2016JC012144
       
  • Laboratory experiments on diffusive convection layer thickness and its
           oceanographic implications
    • Authors: Shuang‐Xi Guo; Sheng‐Qi Zhou, Ling Qu, Yuan‐Zheng Lu
      Abstract: We studied the thickness of diffusive convective layers that form when a linearly stratified fluid is subjected to heating from below in the laboratory. The thickness of the bottom convecting layer is much larger than subsequent layers. These thicknesses are systematically identified and used to examine the available convecting layer thickness parameterizations, which are consisted of the measured heat flux F (or thermal buoyancy flux qT), initial stratification N, density ratio Rρ, thermal diffusivity κT, etc. Parameterization with an intrinsic length scale (qT3κTN8)1/4 is shown to be superior. Including the present laboratory convecting layer thicknesses and those observed in oceans and lakes, where layer thickness ranges from 0.01 to 1000 m, the parameterization is updated as H=C(Rρ−1)2(qT3κTN8)1/4, where C=38.3 for the bottom convective layer and 10.8 for the subsequent layers. Different prefactors are proposed to be attributed to different convective instabilities induced by different boundary conditions. This article is protected by copyright. All rights reserved.
      PubDate: 2016-09-22T09:55:55.381865-05:
      DOI: 10.1002/2016JC012172
       
  • Teleconnection between the North Indian Ocean high swell events and
           meteorological conditions over the Southern Indian Ocean
    • Authors: P G Remya; S Vishnu, B Praveen Kumar, TM Balakrishnan Nair, B Rohith
      Abstract: The link between North Indian Ocean (NIO) high swell events and the meteorological conditions over the Southern Indian Ocean (SIO) is explored in this article, using a combination of in‐situ measurements and model simulations for the year 2005. High waves, without any sign in the local winds, sometimes cause severe flooding events along the south‐west coast of India, locally known as the Kallakkadal events and cause major societal problems along the coasts. In‐situ observations report ten high swell events in NIO during 2005. Our study confirm that these events are caused by the swells propagating from south of 30°S. In all cases, 3‐5 days prior to the high swell events in NIO, we observed a severe low pressure system, called the Cut‐Off Low (COL) in the Southern Ocean. These COLs are quasi‐stationary in nature, providing strong (∼25 ms−1) and long duration (∼3 days) surface winds over a large fetch; essential conditions for the generation of long period swells. The intense equator ward winds associated with COLs in the SIO trigger the generation of high waves, which propagate to NIO as swells. Furthermore, these swells cause high wave activity and sometimes Kallakkadal events along the NIO coastal regions, depending on the local topography, angle of incidence and tidal conditions. Our study shows that such natural hazards along the NIO coasts can be forecasted at least 2 days in advance if the meteorological conditions of the SIO are properly monitored. This article is protected by copyright. All rights reserved.
      PubDate: 2016-09-21T19:10:44.497883-05:
      DOI: 10.1002/2016JC011723
       
  • Measurement and modeling of oil slick transport
    • Authors: Cathleen E. Jones; Knut‐Frode Dagestad, Øyvind Breivik, Benjamin Holt, Johannes Röhrs, Kai Håkon Christensen, Martine Espeseth, Camilla Brekke, Stine Skrunes
      Abstract: Transport characteristics of oil slicks are reported from a controlled release experiment conducted in the North Sea in June 2015, during which mineral oil emulsions of different volumetric oil fractions and a look‐alike biogenic oil were released and allowed to develop naturally. The experiment used the Uninhabited Aerial Vehicle Synthetic Aperture Radar (UAVSAR) to track slick location, size, and shape for ∼8 hours following release. Wind conditions during the exercise were at the high end of the range considered suitable for radar‐based slick detection, but the slicks were easily detectable in all images acquired by the low noise, L‐band imaging radar. The measurements are used to constrain the entrainment length and representative droplet radii for oil elements in simulations generated using the OpenOil advanced oil drift model. Simultaneously released drifters provide near‐surface current estimates for the single biogenic release and one emulsion release, and are used to test model sensitivity to upper ocean currents and mixing. Results of the modeling reveal a distinct difference between the transport of the biogenic oil and the mineral oil emulsion, in particular in the vertical direction, with faster and deeper entrainment of significantly smaller droplets of the biogenic oil. The difference in depth profiles for the two types of oils is substantial, with most of the biogenic oil residing below depths of 10 m, compared to the majority of the emulsion remaining above 10 m depth. This difference was key to fitting the observed evolution of the two different types of slicks. This article is protected by copyright. All rights reserved.
      PubDate: 2016-09-21T18:35:26.123817-05:
      DOI: 10.1002/2016JC012113
       
  • Tsunami generation and associated waves in the water column and seabed due
           to an asymmetric earthquake motion within an anisotropic substratum
    • Authors: Amirhossein Bagheri; Stewart Greenhalgh, Ali Khojasteh, Mohammad Rahimian, Reza Attarnejad
      Abstract: In this paper, closed‐form integral expressions are derived to describe how surface gravity waves (tsunamis) are generated when general asymmetric ground displacement (due to earthquake rupturing), involving both horizontal and vertical components of motion, occurs at arbitrary depth within the interior of an anisotropic sub‐sea solid beneath the ocean. In addition, we compute the resultant hydrodynamic pressure within the seawater and the elastic wavefield within the seabed at any position. The method of potential functions and an integral transform approach, accompanied by a special contour integration scheme, are adopted to handle the equations of motion and produce the numerical results. The formulation accounts for any number of possible acoustic‐gravity modes and is valid for both shallow and deep water situations as well as for any focal depth of the earthquake source. Phase and group velocity dispersion curves are developed for surface gravity (tsunami mode), acoustic‐gravity, Rayleigh, and Scholte waves. Several asymptotic cases which arise from the general analysis are discussed and compared to existing solutions. The role of effective parameters such as hypocenter location and frequency of excitation are examined and illustrated through several figures which show the propagation pattern in the vertical and horizontal directions. Attention is directed to the unexpected contribution from the horizontal ground motion. The results have important application in several fields such as tsunami hazard prediction, marine seismology and offshore and coastal engineering. In a companion paper we examine the effect of ocean stratification on the appearance and character of internal and surface gravity waves. This article is protected by copyright. All rights reserved.
      PubDate: 2016-09-21T18:35:21.897982-05:
      DOI: 10.1002/2016JC011944
       
  • Enhanced turbulence driven by mesoscale motions and flow‐topography
           interaction in the Denmark Strait Overflow plume
    • Authors: Janin Schaffer; Torsten Kanzow, Kerstin Jochumsen, Klas Lackschewitz, Sandra Tippenhauer, Victor M. Zhurbas, Detlef Quadfasel
      Abstract: The Denmark Strait Overflow (DSO) contributes roughly half to the total volume transport of the Nordic overflows. The overflow increases its volume by entraining ambient water as it descends into the subpolar North Atlantic, feeding into the deep branch of the Atlantic Meridional Overturning Circulation. In June 2012 a multi‐platform experiment was carried out in the DSO plume on the continental slope off Greenland (180 km downstream of the sill in Denmark Strait), to observe the variability associated with the entrainment of ambient waters into the DSO plume. In this study we report on two high‐dissipation events captured by an autonomous underwater vehicle (AUV) by horizontal profiling in the interfacial layer between the DSO plume and the ambient water. Strong dissipation of turbulent kinetic energy of O(10‐6) W kg‐1 was associated with enhanced small‐scale temperature variance at wavelengths between 0.05 and 500 m as deduced from a fast‐response thermistor. Isotherm displacement slope spectra reveal a wavenumber‐dependence characteristic of turbulence in the inertial‐convective subrange (k1/3) at wavelengths between 0.14 and 100 m. The first event captured by the AUV was transient, and occurred near the edge of a bottom‐intensified energetic eddy. Our observations imply that both horizontal advection of warm water and vertical mixing of it into the plume are eddy‐driven and go hand in hand in entraining ambient water into the DSO plume. The second event was found to be a stationary feature on the upstream side of a topographic elevation located in the plume pathway. Flow‐topography interaction is suggested to drive the intense mixing at this site. This article is protected by copyright. All rights reserved.
      PubDate: 2016-09-21T11:10:38.629141-05:
      DOI: 10.1002/2016JC011653
       
  • Wind‐driven export of Weddell Sea slope water
    • Authors: A.J.S. Meijers; M.P. Meredith, E.P. Abrahamsen, M.A. Morales Maqueda, D.C. Jones, A.C. Naveira Garabato
      Abstract: The export of waters from the Weddell Gyre to lower latitudes is an integral component of the southern subpolar contribution to the three‐dimensional oceanic circulation. Here, we use more than 20 years of repeat hydrographic data on the continental slope on the northern tip of the Antarctic Peninsula and five years of bottom lander data on the slope at 1000 m to show the intermittent presence of a relatively cold, fresh westward‐flowing current. This is often bottom intensified between 600‐2000 dbar with velocities of over 20 cms−1, transporting an average of 1.5±1.5 Sv. By comparison with hydrography on the continental slope within the Weddell Sea and modelled tracer release experiments we show that this slope current is an extension of the Antarctic Slope Current that has crossed the South Scotia Ridge west of Orkney Plateau. On monthly to interannual timescales the density of the slope current is negatively correlated (r >0.6 with a significance of over 95%) with eastward wind stress over the northern Weddell Sea, but lagging it by 6‐13 months. This relationship holds in both the high temporal resolution bottom lander time series and the 20+ year annual hydrographic occupations and agrees with Weddell Sea export variability observed further east. We compare several alternative hypotheses for this wind‐stress/export relationship and find that it is most consistent with wind‐driven acceleration of the gyre boundary current, possibly modulated by eddy dynamics, and represents a mechanism by which climatic perturbations can be rapidly transmitted as fluctuations in the supply of intermediate‐level waters to lower latitudes. This article is protected by copyright. All rights reserved.
      PubDate: 2016-09-21T11:00:30.892787-05:
      DOI: 10.1002/2016JC011757
       
  • Direct observations of the Antarctic Slope Current transport at 113°E
    • Authors: B. Peña‐Molino; M. S. McCartney, S. R. Rintoul
      Abstract: The Antarctic Slope Current (ASC), defined here as the region of westward flow along the continental slope off Antarctica, forms the southern limb of the subpolar gyres. It regulates the exchange of water across the shelf break, and provides a path for interbasin westward transport. Despite its significance, the ASC remains largely unobserved around most of the Antarctic continent. Here we present direct velocity observations from a 17‐month current meter moored array deployed across the continental slope between the 1000 and the 4200 m isobaths, in the southeastern Indian Ocean near 113°E. The observed time‐mean flow consists of a surface intensified jet associated with the Antarctic Slope Front (ASF) and a broader bottom intensified westward flow that extends out to approximately the 4000 m isobath and is strongest along the upper slope. The time‐mean transport of the ASC is ‐29.2 Sv. Fluctuations in the transport are large, typically exceeding the mean by a factor of 2. They are mainly due to changes in the northward extent of the current over the lower slope. However, seasonal changes in the wind also drive variations in the transport of the ASF and the flow in the upper slope. Both mean and variability are largely barotropic, thus invisible to traditional geostrophic methods. This article is protected by copyright. All rights reserved.
      PubDate: 2016-09-21T11:00:26.473997-05:
      DOI: 10.1002/2015JC011594
       
  • Wind relaxation and a coastal buoyant plume north of Pt. Conception, CA:
           Observations, simulations, and scalings
    • Authors: Sutara Suanda; Nirnimesh Kumar, Arthur J. Miller, Emanuele DiLorenzo, Kevin Haas, Donghua Cai, Christopher A. Edwards, Libe Washburn, Melanie Fewings, Rachel Torres, Falk Feddersen
      Abstract: In upwelling regions, wind relaxations lead to poleward propagating warm water plumes that are important to coastal ecosystems. The coastal ocean response to wind relaxation around Pt. Conception, CA is simulated with a Regional Ocean Model (ROMS) forced by realistic surface and lateral boundary conditions including tidal processes. The model reproduces well the statistics of observed subtidal water column temperature and velocity at both outer‐ and inner‐shelf mooring locations throughout the study. A poleward‐propagating plume of Southern California Bight water that increases shelf water temperatures by ≈ 5°C is also reproduced. Modeled plume propagation speed, spatial scales, and flow structure are consistent with a theoretical scaling for coastal buoyant plumes with both surface‐trapped and slope‐controlled dynamics. Plume momentum balances are distinct between the offshore (> 30‐m depth) region where the plume is surface‐trapped, and onshore of the 30‐m isobath (within 5 km from shore) where the plume water mass extends to the bottom and is slope‐controlled. In the onshore region, bottom stress is important in the alongshore momentum equation and generates vertical vorticity that is an order of magnitude larger than the vorticity in the plume core. Numerical experiments without tidal forcing show that modeled surface temperatures are biased 0.5°C high, potentially affecting plume propagation distance and persistence. This article is protected by copyright. All rights reserved.
      PubDate: 2016-09-20T03:45:32.764738-05:
      DOI: 10.1002/2016JC011919
       
  • Variability, trends, and predictability of seasonal sea ice retreat and
           advance in the Chukchi Sea
    • Authors: Mark C. Serreze; Alex D. Crawford, Julienne Stroeve, Andrew P. Barrett, Rebecca A. Woodgate
      Abstract: As assessed over the period 1979‐2014, the date that sea ice retreats to the shelf break (150m contour) of the Chukchi Sea has a linear trend of ‐0.7 days per year. The date of seasonal ice advance back to the shelf break has a steeper trend of about +1.5 days per year, together yielding an increase in the open water period of 80 days. Based on de‐trended time series, we ask how interannual variability in advance and retreat dates relate to various forcing parameters including radiation fluxes, temperature and wind (from numerical reanalyses), and the oceanic heat inflow through the Bering Strait (from in situ moorings). Of all variables considered, the retreat date is most strongly correlated (r∼0.8) with the April through June Bering Strait heat inflow. After testing a suite of statistical linear models using several potential predictors, the best model for predicting the date of retreat includes only the April through June Bering Strait heat inflow, which explains 68% of retreat date variance. The best model predicting the ice advance date includes the July through September inflow and the date of retreat, explaining 67% of advance date variance. We address these relationships by discussing heat balances within the Chukchi Sea, and the hypothesis of oceanic heat transport triggering ocean heat uptake and ice‐albedo feedback. Developing an operational prediction scheme for seasonal retreat and advance would require timely acquisition of Bering Strait heat inflow data. Predictability will likely always be limited by the chaotic nature of atmospheric circulation patterns. This article is protected by copyright. All rights reserved.
      PubDate: 2016-09-19T18:35:35.422629-05:
      DOI: 10.1002/2016JC011977
       
  • Remote sources for year‐to‐year changes in the seasonality of the
           Florida Current transport
    • Authors: Ricardo Domingues; Molly Baringer, Gustavo Goni
      Abstract: The seasonal variability of the Florida Current (FC) transport is often characterized by the presence of an average annual cycle (8% of the variance) of ∼3 Sv range peaking in boreal summer. However, the seasonality displayed by the FC transport in any individual year may have very distinct characteristics. In this study, the analysis focuses on seasonal changes (73—525 day frequency band) in the FC transport that are associated with a variable annual phase, which is defined as the transient seasonal component (FCt, 27% of the variance). It is shown that the FCt is largely modulated by westward propagating sea height anomaly (SHA) signals that are formed in the eastern North Atlantic 4 to 7 years earlier than observed at 27°N in the Florida Straits. These westward propagating SHA signals behave approximately like first baroclinic Rossby waves that can modulate changes in the FC seasonal variability upon arrival at the western boundary. The main finding from this study is that changes in coastal sea‐level between 25°N—42°N linked with westward propagating signals account for at least 50% of the FCt. The integrated changes in the coastal sea‐level between 25°N—42°N, in turn, drive adjustments in the geostrophic transport of the FC at 27°N. Results reported here provide an explanation for previously reported year‐to‐year changes in the FC seasonality, and suggest that large sea‐level variations along the coast of Florida may be partially predictable, given that these Rossby‐wave‐like signals propagate approximately at fixed rates in the open ocean along 27°N. This article is protected by copyright. All rights reserved.
      PubDate: 2016-09-19T18:35:31.731373-05:
      DOI: 10.1002/2016JC012070
       
  • Identification and classification of very‐low frequency waves on a
           coral reef flat
    • Authors: Matthijs Gawehn; Ap van Dongeren, Arnold van Rooijen, Curt Storlazzi, Olivia M. Cheriton, Ad Reniers
      Abstract: Very‐low frequency (VLF, 0.001‐0.005 Hz) waves are important drivers of flooding of low‐lying coral reef‐islands. In particular, VLF wave resonance is known to drive large wave runup and subsequent overwash. Using a five‐month dataset of water levels and waves collected along a cross‐reef transect on Roi‐Namur Island in the Republic of the Marshall Islands, the observed VLF motions were categorized into four different classes: (1) resonant, (2) (non‐resonant) standing, (3) progressive‐growing and (4) progressive‐dissipative waves. Each VLF class is set by the reef flat water depth and, in the case of resonance, the incident‐band offshore wave period. Using an improved method to identify VLF wave resonance, we find that VLF wave resonance caused prolonged (∼0.5 – 6.0 hr), large‐amplitude water surface oscillations at the inner reef flat ranging in wave height from 0.14 to 0.83 m. It was induced by relatively long‐period, grouped, incident‐band waves, and occurred under both storm and non‐storm conditions. Moreover, observed resonant VLF waves had non‐linear, bore‐like wave shapes, which likely have a larger impact on the shoreline than regular, sinusoidal waveforms. As an alternative technique to the commonly used Fast Fourier Transformation, we propose the Hilbert‐Huang Transformation that is more computationally expensive but can capture the wave shape more accurately. This research demonstrates that understanding VLF waves on reef flats is important for evaluating coastal flooding hazards. This article is protected by copyright. All rights reserved.
      PubDate: 2016-09-19T18:30:26.986267-05:
      DOI: 10.1002/2016JC011834
       
  • Glider observations of enhanced deep water upwelling at a shelf break
           canyon: A mechanism for cross‐slope carbon and nutrient exchange
    • Authors: M. Porter; M. E. Inall, J. Hopkins, M. R. Palmer, A. C. Dale, D. Aleynik, J. A. Barth, C. Mahaffey, D. A. Smeed
      Abstract: Using underwater gliders we have identified canyon driven upwelling across the Celtic Sea shelf‐break, in the vicinity of Whittard Canyon. The presence of this upwelling appears to be tied to the direction and strength of the local slope current, which is in itself highly variable. During typical summer time equatorward flow, an unbalanced pressure gradient force and the resulting disruption of geostrophic flow can lead to upwelling along the main axis of two small shelf break canyons. As the slope current reverts to poleward flow, the upwelling stops and the remnants of the upwelled features are mixed into the local shelf water or advected away from the region. The upwelled features are identified by the presence of sub‐pycnocline high salinity water on the shelf, and are upwelled from a depth of 300 m on the slope, thus providing a mechanism for the transport of nutrients across the shelf break onto the shelf. This article is protected by copyright. All rights reserved.
      PubDate: 2016-09-19T18:30:21.487541-05:
      DOI: 10.1002/2016JC012087
       
  • Surface shear stress dependence of gas transfer velocity parameterizations
           using DNS
    • Authors: S. T. Fredriksson; L. Arneborg, H. Nilsson, R. A. Handler
      Abstract: Air‐water gas‐exchange is studied in direct numerical simulations (DNS) of free‐surface flows driven by natural convection and weak winds. The wind is modeled as a constant surface‐shear‐stress and the gas‐transfer is modeled via a passive scalar. The simulations are characterized via a Richardson number Ri=Bν/u*4 where B, v, and u* are the buoyancy flux, kinematic viscosity, and friction velocity respectively. The simulations comprise 0 < Ri < ∞ ranging from convection‐dominated to shear‐dominated cases. The results are used to: (i) evaluate parameterizations of the air‐water gas‐exchange, (ii) determine, for a given buoyancy flux, the wind speed at which gas transfer becomes primarily shear driven, and (iii) find an expression for the gas‐transfer velocity for flows driven by both convection and shear. The evaluated gas transfer‐velocity parametrizations are based on either the rate of turbulent kinetic energy dissipation, the surface flow‐divergence, the surface heat‐flux, or the wind‐speed. The parametrizations using dissipation or divergence show an unfavorable Ri dependence for flows with combined forcing whereas the parametrization using heat‐flux only shows a limited Ri dependence. The two parametrizations using wind speed give reasonable estimates for the transfer‐velocity, depending however on the surface heat‐flux. The transition from convection‐ to shear‐dominated gas‐transfer‐velocity is shown to be at Ri ≈ 0.004. Furthermore, the gas‐transfer is shown to be well represented by two different approaches: (i) additive forcing expressed as kg,sum = AShear u* (Ri/Ric+1)1/4 Sc‐n where Ric = (AShear/ABuoy)4, and (ii) either buoyancy or shear dominated expressed as, kg = ABuoy (Bv)1/4Sc‐n, Ri>Ric or kg = Ashearu*Sc‐n, Ri
      PubDate: 2016-09-15T18:06:09.423857-05:
      DOI: 10.1002/2016JC011852
       
  • Trapping of the near‐inertial wave wakes of two consecutive
           hurricanes in the Loop Current
    • Authors: E. Pallàs‐Sanz; J. Candela, J. Sheinbaum, J. Ochoa, J. Jouanno
      Abstract: The near‐inertial trapping of the wave wakes of two consecutive hurricanes, Gustav and Ike (2008), at the base of the Loop Current is described using observations of a triangular mooring array. Hurricane Gustav translates over the Loop Current twice as fast and leads to a sharper, but equally strong, wind intensity peak than Hurricane Ike. Following the passage of Gustav, near‐inertial oscillations in the main thermocline have large horizontal (∼340±45 km) and vertical (∼850 m) scales. The wave kinetic energy propagates down with a vertical group velocity of several tens of meters per day and leads to a downward energy flux of [1.82‐7.10]×10−3 W m−2.Observations suggest that the dispersion of the wave wakes produced by the hurricanes is driven by Doppler shifting in the upper ∼600 m and the vertical‐varying vorticity inside the Loop Current. Near‐inertial kinetic energy is enhanced and near‐inertial oscillations are circularly polarized in a critical layer at the base of the Loop Current where the group velocity vector is nearly horizontal. The trapping and amplification of the near‐inertial wave wakes of two consecutive hurricanes is addressed using a primitive equation numerical model. Numerical results suggest an increase of the near‐inertial kinetic energy and vertical shear by a factor of ∼2 near the critical layer due to wave‐mean flow and wave‐wave interactions. A K‐Profile parameterization of these interactions results in banded regions of elevated turbulent kinetic energy dissipation rates of ε ϵ [0.5‐6.6]×10−6 W m−3 for critical Richardson number Ric = 1, depending on the stratification of the water column. This article is protected by copyright. All rights reserved.
      PubDate: 2016-09-15T18:00:56.412806-05:
      DOI: 10.1002/2015JC011592
       
  • Impacts of mesoscale activity on the water masses and circulation in the
           Coral Sea
    • Authors: L. Rousselet; A.M. Doglioli, C. Maes, B. Blanke, A. Petrenko
      Abstract: The climatological vision of the circulation within the Coral Sea is today well established with the westward circulation of two main jets, the North Caledonian Jet (NCJ) and the North Vanuatu Jet (NVJ) as a consequence of the separation of the South Equatorial Current (SEC) on the islands of New Caledonia, Vanuatu and Fiji. Each jet has its own dynamic and transports different water masses across the Coral Sea. The influence of mesoscale activity on mean flow and on water mass exchanges is not yet fully explored in this region of intense activity. Our study relies on the analysis of in situ, satellite and numerical data. Indeed we first use in situ data from the Bifurcation cruise and from an Argo float, jointly with satellite‐derived velocities, to study the eddy influence on the Coral Sea dynamics. We identify an anticyclonic eddy as participating in the transport of NVJ‐like water masses into the theoretical pathway of NCJ waters. This transfer from the NVJ to the NCJ is confirmed over the long‐term by a Lagrangian analysis. In particular, this numerical analysis shows that anticyclonic eddies can contribute up to 70% to 90% of the overall eddy transfer between those seemingly independent jets. Finally, transports calculated using S‐ADCP measurements (0‐500 m) show an eddy‐induced sensitivity that can reach up to 15 Sv, i.e, the order of the transport of the jets. This article is protected by copyright. All rights reserved.
      PubDate: 2016-09-15T08:30:53.907375-05:
      DOI: 10.1002/2016JC011861
       
  • Improving the simulation of landfast ice by combining tensile strength and
           a parameterization for grounded ridges
    • Authors: Jean‐François Lemieux; Frédéric Dupont, Philippe Blain, François Roy, Gregory C. Smith, Gregory M. Flato
      Abstract: In some coastal regions of the Arctic Ocean, grounded ice ridges contribute to stabilizing and maintaining a landfast ice cover. Recently, a grounding scheme representing this effect on sea ice dynamics was introduced and tested in a viscous‐plastic sea ice model. This grounding scheme, based on a basal stress parameterization, improves the simulation of landfast ice in many regions such as in the East Siberian Sea, the Laptev Sea and along the coast of Alaska. Nevertheless, in some regions such as the Kara Sea, the area of landfast ice is systematically underestimated. This indicates that another mechanism such as ice arching is at play for maintaining the ice cover fast. To address this problem, the combination of the basal stress parameterization and tensile strength is investigated using a 0.25° pan‐Arctic CICE‐NEMO configuration. Both uniaxial and isotropic tensile strengths notably improve the simulation of landfast ice in the Kara Sea but also in the Laptev Sea. However, the simulated landfast ice season for the Kara Sea is too short compared to observations. This is especially obvious for the onset of the landfast ice season which systematically occurs later in the model and with a slower build up. This suggests that improvements to the sea ice thermodynamics could reduce these discrepancies with the data. This article is protected by copyright. All rights reserved.
      PubDate: 2016-09-10T10:30:22.49546-05:0
      DOI: 10.1002/2016JC012006
       
  • Remote Forcing of Subsurface Currents and Temperatures near the Northern
           Limit of the California Current System
    • Authors: Zelalem Engida; Adam Monahan, Debby Ianson, Richard E. Thomson
      Abstract: Local and remote wind forcing of upwelling along continental shelves of coastal upwelling regions play key roles in driving biogeochemical fluxes, including vertical net fluxes of carbon and nutrients. These fluxes are responsible for high primary productivity, which in turn supports a lucrative fishery in these regions. However, the relative contributions of local versus remote wind forcing is not well quantified or understood. We present results of coherence analyses between currents at a single mooring site (48.5° N, 126° W) in the northern portion of the California Current System (CalCS) from 1989–2008 and coincident time series of North America Regional Reanalysis (NARR) 10 m wind stress within the CalCS (36–54° N, 120–132° W). The two‐decade long current records from the three shallowest depths (35, 100 and 175 m) show a remote response to winds from as far south as 36° N. In contrast, only temperatures at the deepest depth (400 m) show strong coherences with remote winds. Weaker local wind influence is observed in both the currents and 400 m temperatures but is mostly due to the large spatial coherence within the wind field itself. Lack of coherence between distal winds and the 400 m currents suggests that the temperature variations at that depth are driven by vertical motion resulting from poleward travelling coastal trapped waves (CTWs). Understanding the effects of remote forcing in coastal upwelling regions is necessary for determining the occurrence and timing of extreme conditions in coastal oceans, and their subsequent impact on marine ecosystems. This article is protected by copyright. All rights reserved.
      PubDate: 2016-09-10T10:25:30.534334-05:
      DOI: 10.1002/2016JC011880
       
  • Observed subseasonal variability of heat flux and the SST response of the
           tropical Indian Ocean
    • Authors: Sindu Raj Parampil; G. N. Bharathraj, Matthew Harrison, Debasis Sengupta
      Abstract: We develop an experimental daily surface heat flux dataset based on satellite observations to study subseasonal variability (periods shorter than 90 days) in the tropical Indian Ocean. We use incoming shortwave and longwave radiation from the International Satellite Cloud Climatology Project, and sea surface temperature (SST) from microwave sensors, to estimate net radiative flux. Latent and sensible heat fluxes are estimated from scatterometer winds and near‐surface air temperature and specific humidity from Atmospheric Infrared Sounder (AIRS) observations calibrated to buoy data. Seasonal biases in net heat flux are generally within 10 Wm−2 of estimates from moorings, and the phases and amplitudes of subseasonal variability of heat fluxes are realistic. We find that the contribution of subseasonal changes in air‐sea humidity gradients to latent heat flux equals or exceeds the contribution of subseasonal changes in wind speed in all seasons. SST responds coherently to subseasonal oscillations of net heat flux associated with active and suppressed phases of atmospheric convection in the summer hemisphere. Thus subseasonal SST changes are mainly forced by heat flux in the northeast Indian Ocean in northern summer, and in the 15oS‐5oN latitude belt in southern summer. In the winter hemisphere, subseasonal SST changes are not a one‐dimensional response to heat flux, implying that they are mainly due to oceanic advection, entrainment or vertical mixing. The coherent evolution of subseasonal SST variability and surface heat flux suggests active coupling between SST and large‐scale, organized tropical convection in the summer season. This article is protected by copyright. All rights reserved.
      PubDate: 2016-09-09T10:15:53.136856-05:
      DOI: 10.1002/2016JC011948
       
  • Temporal variability in the Antarctic Polar Front (2002‐2014)
    • Authors: Natalie M. Freeman; Nicole S. Lovenduski, Peter R. Gent
      Abstract: We investigate intra‐annual to interannual variability in the Antarctic Polar Front (PF) using weekly PF realizations spanning 2002 to 2014 (found at doi.pangaea.de/10.1594/PANGAEA.855640). While several PF studies have used gradient maxima in sea surface temperature (SST) or height to define its location, results from this study are based on a PF defined using SST measurements that avoid cloud contamination and the influence of steric sea level change. With a few regional exceptions, we find that the latitudinal position of the PF does not vary seasonally, yet its temperature exhibits a clear seasonal cycle. Consistent with previous studies, the position and intensity of the PF is largely influenced by bathymetry; generally, over steep topography we find that the front intensifies and interannual variability in its position is low. We also investigate drivers of PF variability in the context of large‐scale climate variability on various spatial and temporal scales, but find that the major modes of Southern Hemisphere climate variability explain only a tiny fraction of the interannual PF variance. Over the study time period, the PF intensifies at nearly all longitudes while exhibiting no discernible meridional displacement in its zonal mean path. This article is protected by copyright. All rights reserved.
      PubDate: 2016-09-08T03:55:33.75221-05:0
      DOI: 10.1002/2016JC012145
       
  • A modulating effect of tropical instability wave (TIW)‐induced surface
           wind feedback in a hybrid coupled model of the tropical Pacific
    • Authors: Rong‐Hua Zhang
      Abstract: Tropical Instability Waves (TIWs) and the El Niño‐Southern Oscillation (ENSO) are two air‐sea coupling phenomena that are prominent in the tropical Pacific, occurring at vastly different space‐time scales. It has been challenging to adequately represent both of these processes within a large‐scale coupled climate model, which has led to a poor understanding of the interactions between TIW‐induced feedback and ENSO. In this study, a novel modeling system was developed that allows representation of TIW‐scale air‐sea coupling and its interaction with ENSO. Satellite data were first used to derive an empirical model for TIW‐induced sea surface wind stress perturbations (τTIW). The model was then embedded in a basin‐wide hybrid‐coupled model (HCM) of the tropical Pacific. Because τTIW were internally determined from TIW‐scale sea surface temperatures (SSTTIW) simulated in the ocean model, the wind‐SST coupling at TIW scales was interactively represented within the large‐scale coupled model. Because the τTIW–SSTTIW coupling part of the model can be turned on or off in the HCM simulations, the related TIW wind feedback effects can be isolated and examined in a straightforward way. Then, the TIW‐scale wind feedback effects on the large‐scale mean ocean state and interannual variability in the tropical Pacific were investigated based on this embedded system. The interactively represented TIW‐scale wind forcing exerted an asymmetric influence on SSTs in the HCM, characterized by a mean‐state cooling and by a positive feedback on interannual variability, acting to enhance ENSO amplitude. Roughly speaking, the feedback tends to increase interannual SST variability by approximately 9%. Additionally, there is a tendency for TIW wind to have an effect on the phase transition during ENSO evolution, with slightly shortened interannual oscillation periods. Additional sensitivity experiments were performed to elucidate the details of TIW wind effects on SST evolution during ENSO cycles. This article is protected by copyright. All rights reserved.
      PubDate: 2016-09-02T14:05:31.92588-05:0
      DOI: 10.1002/2015JC011567
       
  • Diagnosing cross‐shelf transport along an ocean front: an observational
           case study in the Gulf of Lion
    • Authors: F. Nencioli; A. A. Petrenko, A. M. Doglioli
      Abstract: Exchanges between coastal regions and the open ocean are often associated with intermittent and localized processes such as eddies, fronts and filaments. Since these features are difficult to observe, their impact has been predominantly investigated using numerical models and remote sensing. In this study, satellite sea surface temperature maps, Lagrangian surface drifter trajectories, and ship‐based surveys of currents and hydrography from the Latex10 campaign are used to quantify cross‐shelf exchanges associated with a temperature front in the western Gulf of Lion. Satellite imagery and thermosalinograph sections provide the characterization of the various water masses associated with the front. Lagrangian drifter trajectories are used to identify the main transport structures and to quantify the velocity components associated with near‐inertial oscillations. These are removed from the instantaneous ADCP observations with which the cross‐shelf exchanges are then computed. The results indicate an average outflow of 0.074 ±0.013 Sv and an inflow of 0.021 ±0.006 Sv. Integrated over the two‐week lifetime of the front, such outflow induced a total export of ∼ 90 ± 14 km3 of water, indicating that 3 to 4 of such events are sufficient to completely renew the surface waters of the Gulf of Lion. The total import was ∼ 25 ± 7 km3, suggesting larger inflows at depth or in the eastern part of the gulf to maintain its volume balance. These in‐situ estimates represent a key term of comparison for the further development of numerical model‐ and satellite‐based studies of cross‐shelf exchanges associated with this type of processes. This article is protected by copyright. All rights reserved.
      PubDate: 2016-08-29T09:15:33.094553-05:
      DOI: 10.1002/2016JC011908
       
  • 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
       
  • 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
       
  • Issue Information
    • Pages: 7215 - 7217
      PubDate: 2016-11-22T01:31:29.876017-05:
      DOI: 10.1002/jgrc.21410
       
 
 
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