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Publisher: American Geophysical Union (AGU)   (Total: 17 journals)

Geochemistry, Geophysics, Geosystems     Full-text available via subscription   (Followers: 21, SJR: 2.156, h-index: 61)
Geophysical Research Letters     Full-text available via subscription   (Followers: 46, SJR: 2.668, h-index: 142)
Global Biogeochemical Cycles     Full-text available via subscription   (Followers: 3, SJR: 2.4, h-index: 109)
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J. of Geophysical Research : Biogeosciences     Full-text available via subscription   (Followers: 5)
J. of Geophysical Research : Earth Surface     Partially Free   (Followers: 22)
J. of Geophysical Research : Oceans     Partially Free   (Followers: 15)
J. of Geophysical Research : Planets     Full-text available via subscription   (Followers: 13)
J. of Geophysical Research : Solid Earth     Full-text available via subscription   (Followers: 22)
J. of Geophysical Research : Space Physics     Full-text available via subscription   (Followers: 14)
Paleoceanography     Full-text available via subscription   (Followers: 4, SJR: 2.16, h-index: 82)
Radio Science     Full-text available via subscription   (Followers: 2, SJR: 0.527, h-index: 47)
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Water Resources Research     Full-text available via subscription   (Followers: 144, SJR: 1.769, h-index: 110)
Journal Cover Journal of Geophysical Research : Oceans
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     Published by American Geophysical Union (AGU) Homepage  [17 journals]
  • Confidence and sensitivity study of the OAFlux multisensor synthesis of
           the global ocean‐surface vector wind from 1987 onward
    • Authors: Lisan Yu; Xiangze Jin
      Pages: n/a - n/a
      Abstract: This study presents an uncertainty assessment of the high‐resolution global analysis of daily‐mean ocean‐surface vector winds (1987 onward) by the Objectively Analyzed air‐sea Fluxes (OAFlux) project. The time series was synthesized from multiple satellite sensors using a variational approach to find a best fit to input data in a weighted least‐squares cost function. This framework requires the a priori specification of the weights, or equivalently, the error covariances of input data, which are seldom known. Two issues are addressed. The first issue is the selection of the optimal weights used for the OAFlux synthesis. A set of weight‐varying experiments was conducted, from which the weights were selected by satisfying the condition that the chosen weights should make the best‐fit of the cost function be optimal with regard to both input satellite observations and the air‐sea buoy measurements at 126 locations. The buoy measurements were not part of the synthesis and served as an independent measure in the selection. The second issue is the quantification of the effect of the uncertainty in the resultant weight assignments on the synthesis. This was achieved by computing the ensemble statistics of weight perturbation experiments. The results show that high winds (>15 ms‐1) and rain conditions are the leading sources of uncertainty for the OAFlux vector wind analysis. High winds correlated with rain remain a major technical challenge for proper wind retrievals from satellites. Further improvement of the multi‐sensor synthesis in events of severe storms will rely on the future technical advancement in retrieval algorithms.
      PubDate: 2014-09-19T21:47:39.36176-05:0
      DOI: 10.1002/2014JC010194
  • Climatological monthly heat and freshwater flux estimates on a global
           scale from Argo
    • Authors: Donata Giglio; Dean Roemmich
      Pages: n/a - n/a
      Abstract: The global pattern of climatological monthly heat and freshwater fluxes at the ocean surface is estimated using Argo temperature and salinity profile data for the period 2004 to 2013. Temperature or salinity changes are calculated in a volume of water above an isopycnal that is below the mixed layer and not subject to mixed‐layer entrainment. Horizontal advection components from geostrophic velocity and from Ekman transport, based on wind stress, are also included. The climatological monthly heat or freshwater flux at the ocean surface is estimated as the sum of advective and time tendency contributions. The air‐sea flux estimates from Argo are described in global maps and basin‐wide integrals, in comparison to atmospheric reanalysis data and to air‐sea flux products based on observations. This ocean‐based estimate of surface fluxes is consistent with property variations in the subsurface ocean and indicates greater amplitude for the climatological monthly heat flux values in the subtropics compared to other products. Similarly, the combination of Argo freshwater flux and reanalysis evaporation, suggests greater amplitude for climatological monthly precipitation in the tropics.
      PubDate: 2014-09-19T05:12:45.396286-05:
      DOI: 10.1002/2014JC010083
  • Dust‐induced episodic phytoplankton blooms in the Arabian Sea during
           winter monsoon
    • Authors: Priyanka Banerjee; S. Prasanna Kumar
      Pages: n/a - n/a
      Abstract: Phytoplankton blooms mediated by the oceanic supply of nutrients is a well understood phenomenon in the Arabian Sea (AS), while the role of dust deposition in enhancing phytoplankton is less explored. In this paper we show that during winter monsoon the central Arabian Sea (CAS), away from the realm of active winter convection, supports episodic phytoplankton blooms. These blooms cannot be fully explained by the oceanic input of nutrients through processes such as advection and mixing in the upper ocean. Using satellite images we tracked about 45 dust storms over the AS during the winter monsoons of 2002‐2003 to 2010‐2011 of which only 8 were followed by chlorophyll enhancements. We used a regional climate model to get possible fluxes of dust and the amount of nutrients (nitrate, phosphate and iron) that can be derived from the dust depositions. Additionally, we used published in situ nutrients data in conjunction with carbon: nitrogen: phosphorus and iron: carbon molar ratios to compute the potential requirements of different nutrients for the 8 cases of chlorophyll enhancements. It is likely that the deepening of the mixed layer can incorporate nitrate and phosphate, but not enough iron from the subsurface waters leading to potential iron limitation. Although, all the phytoplankton blooms within CAS were observed following episodic dust events, only four blooms can be attributed to dust depositions. Our work shows that phytoplankton blooms fuelled by episodic dust storms are important in driving the inter‐annual variability in chlorophyll in a region away from active winter convection.
      PubDate: 2014-09-19T05:09:22.092955-05:
      DOI: 10.1002/2014JC010304
  • Mean sea level variability in the North Sea: Processes and implications
    • Authors: Sönke Dangendorf; Francisco M. Calafat, Arne Arns, Thomas Wahl, Ivan D. Haigh, Jürgen Jensen
      Pages: n/a - n/a
      Abstract: Mean sea level (MSL) variations across a range of timescales are examined for the North Sea under the consideration of different forcing factors since the late 19th century. We use multiple linear regression models, which are validated for the second half of the 20th century against the output of a tide+surge model, to determine the barotropic response of the ocean to fluctuations in atmospheric forcing. We find that local atmospheric forcing mainly initiates MSL variability on timescales up to a few years, with the inverted barometric effect dominating the variability along the UK and Norwegian coastlines and wind controlling the MSL variability in the south from Belgium up to Denmark. On decadal timescales, MSL variability mainly reflects steric changes, which are largely forced remotely. A spatial correlation analysis of altimetry observations and gridded steric heights suggests evidence for a coherent signal extending from the Norwegian shelf down to the Canary Islands. This fits with the theory of longshore wind forcing along the eastern boundary of the North Atlantic causing coastally trapped waves to propagate over thousands of kilometers along the continental slope. Implications of these findings are assessed with statistical Monte‐Carlo experiments. It is demonstrated that the removal of known variability increases the signal to noise ratio with the result that: (i) linear trends can be estimated more accurately; and (ii) possible accelerations (as expected e.g. due to anthropogenic climate change) can be detected much earlier. Such information is of crucial importance for anticipatory coastal management, engineering and planning.
      PubDate: 2014-09-19T05:08:15.632585-05:
      DOI: 10.1002/2014JC009901
  • Coastally trapped eddies in the north of the Gulf of Guinea
    • Authors: Sandrine Djakouré; Pierrick Penven, Bernard Bourlès, Jennifer Veitch, Vamara Koné
      Pages: n/a - n/a
      Abstract: Cyclonic eddies generated downstream of Cape Palmas and Cape Three points have been suggested to contribute to the coastal upwelling along the northern coast of the Gulf of Guinea. A numerical analysis using a high resolution model is used to investigate the mesoscale activity and the coastal upwelling generation processes. An eddy detection and tracking tool is applied to altimeter and model data, showing good agreement between these data sets. About 2 cyclonic eddies per year with an average radius of 60 km were identified downstream of both capes. These cyclonic eddies have an average lifetime of about 60 days during the major coastal upwelling period (boreal summer) and an eastward propagation. These cyclonic eddies are shallow, energetic (their relative vorticity can reach three times the earth's rotation) and dimensionless parameters show that they are in an eddy shedding regime. Mean flow interactions and barotropic instabilities associated with capes are their main generation processes. An idealized experiment is conducted in order to analyse the effect of capes on eddy generation and on coastal upwelling. It reveals that these cyclonic eddies generated downstream of capes are not the process responsible for this coastal upwelling. This experiment also suggests that the cyclonic eddies are the cause of the westward and coastal Guinea Counter Current that is associated with a transfer of energy from eddy kinetic to the mean flow.
      PubDate: 2014-09-19T05:04:25.010994-05:
      DOI: 10.1002/2014JC010243
  • A first estimation of SMOS‐based ocean surface T‐S diagrams
    • Authors: Roberto Sabia; Marlene Klockmann, Diego Fernández‐Prieto, Craig Donlon
      Pages: n/a - n/a
      Abstract: A first estimation of satellite‐based ocean surface T‐S diagrams is performed by using SMOS Sea Surface Salinity (SSS) and OSTIA Sea Surface Temperature (SST) and comparing them with in‐situ measurements interpolated fields obtained by the Argo‐buoys for the North Atlantic and over the entire year 2011. The key objectives at the base of this study are: 1) To demonstrate the feasibility of generating routinely satellite‐derived surface T‐S diagrams, obviating the lack of extensive sampling of the surface open ocean; 2) To display the T‐S diagrams variability and the distribution/dynamics of SSS, altogether with SST and the relative density with respect to in‐situ measurements, and 3) To assess the SMOS SSS data added value in detecting geophysical signals not sensed/resolved by the Argo measurements. To perform the latter analysis, the satellite‐Argo mismatches have been overlapped with geophysical parameters of precipitation rates, surface heat and freshwater fluxes and wind speed data. Ongoing and future efforts focus on enlarging the study area and the temporal frame of the analysis and aim at developing a method for the systematic identification of surface water masses formation areas by remotely‐sensed data.
      PubDate: 2014-09-19T04:59:30.519576-05:
      DOI: 10.1002/2014JC010120
  • Assimilating SMOS sea ice thickness into a coupled ice‐ocean model,
           using a local SEIK filter
    • Authors: Qinghua Yang; Svetlana N. Losa, Martin Losch, Xiangshan Tian‐Kunze, Lars Nerger, Jiping Liu, Lars Kaleschke, Zhanhai Zhang
      Pages: n/a - n/a
      Abstract: The impact of assimilating sea ice thickness data derived from ESA's Soil Moisture and Ocean Salinity (SMOS) satellite together with Special Sensor Microwave Imager/Sounder (SSMIS) sea ice concentration data of the National Snow and Ice Data Center (NSIDC) in a coupled sea ice‐ocean model is examined. A period of three months from November 1st, 2011 to January 31st, 2012 is selected to assess the forecast skill of the assimilation system. 24h‐forecasts and longer forecasts are based on the Massachusetts Institute of Technology general circulation model (MITgcm), and the assimilation is performed by a localized Singular Evolutive Interpolated Kalman (LSEIK) filter. For comparison, the assimilation is repeated only with the SSMIS sea ice concentrations. By running two different assimilation experiments, and comparing with the unassimilated model, independent satellite derived data, and in‐situ observation, it is shown that the SMOS ice thickness assimilation leads to improved thickness forecasts. With SMOS thickness data, the sea ice concentration forecasts also agree better with observations, although this improvement is smaller.
      PubDate: 2014-09-19T04:52:04.49611-05:0
      DOI: 10.1002/2014JC009963
  • Characteristics of surface signatures of Mediterranean water eddies
    • Authors: Bashmachnikov I; Carton X, Belonenko T.V.
      Pages: n/a - n/a
      Abstract: In this work we obtain new results on the manifestation of meddies (or of other deep eddies) at the sea‐surface, further developing the results by Bashmachnikov and Carton [2012]. The quasi‐geostrophic equations are used to describe a near‐axisymmetric vortex in the upper ocean, forced at its lower boundary by the isopycnal elevation of a moving meddy. The solution thus obtained provides a better approximation of the characteristics of meddy surface signals. The results show that in subtropics large meddies with dynamic radius Rm ≥ 30 km are always seen at the sea‐surface with AVISO altimetry, that medium‐size meddies with Rm = 20 km may be seen at the sea‐surface only if they are sufficiently shallow and strong, while small meddies with Rm = 10 km generally cannot be detected with the present accuracy of altimetry data. The intensity of meddy surface signals decreases to the south with the decrease of the f/N ratio. The seasonal variation in intensity of the surface signal for northern meddies (45° N) is on the order of 2‐3 cm, but for subtropical meddies (35° N) it can be on the order of 5‐10 cm. The radii of meddy surface signals range from 1 to 2 times the radii of the corresponding meddies. For most of the observed meddies the upper limit should be used. Numerical experiments show that surface signals of meddies translated with β‐ drift are efficiently dispersed by the radiation of Rossby waves. At the same time, for meddies translated by a background current, the surface signal does not show strong dissipation.
      PubDate: 2014-09-19T04:45:27.208541-05:
      DOI: 10.1002/2014JC010244
  • Influence of underwater barriers on the distribution of tsunami waves
    • Authors: V. Chugunov; S. Fomin, R. Shankar
      Pages: n/a - n/a
      Abstract: Solitary wave propagation over underwater shelves and bumps is examined using straightforward analytical methods. Explicit solutions for wave propagation are obtained. Using the nonlinear shallow‐water equations, it was found that propagation of small amplitude long waves can be well described by a linear approximation. The effects of topographical variety and proportion of underwater barriers (steps, bumps, multiple bumps) on the incident wave are demonstrated using linear wave theory. At a step, the incident wave is shown to be more strongly reflected for increased barrier size. The incident wave also transmits an amplified wave with smaller wavelength onto the obstacle. After propagating off of a bump, the wave experiences an amplitude decay. The decay rate is shown to be exponential with a variable number of bumps. Accounting for the presence of the small parameter, which represents the wave amplitude/water depth ratio, the non‐linear shallow water equations were solved by the method of asymptotic expansions. Using the method of renormalization, a uniformly valid solution was obtained accounting for nonlinear effects in the vicinity of the sharp depth change. Far‐field comparisons of the constructed solutions with the associated Riemann waves show good accuracy of the obtained solutions. Over an infinitely long shelf, the amplified transmitted wave breaks.
      PubDate: 2014-09-19T04:25:44.371039-05:
      DOI: 10.1002/2014JC010296
  • Global and regional sea level change during the 20th century
    • Authors: Manfred Wenzel; Jens Schröter
      Pages: n/a - n/a
      Abstract: Sea level variations prior to the launch of satellite altimeters are estimated by analysing historic tide gauge records. Recently, a number of groups have reconstructed sea level by applying EOF techniques to fill missing observations. We complement this study with alternative methods. In a first step gaps in 178 records of sea level change are filled using the pattern recognition capabilities of artificial neural networks. Afterwards satellite altimetry is used to extrapolate local sea level change to global fields. Patterns of sea level change are compared to prior studies. Global mean sea level change since 1900 is found to be 1.77 ± 0.38 mm year−1 on average. Local trends are essentially positive with the highest values found in the western tropical Pacific and in the Indian Ocean east of Madagascar where it reaches about +6 mm year−1. Regions with negative trends are spotty with a minimum value of about −2 mm year−1 south of the Aleutian Islands. Although the acceleration found for the global mean, +0.0042 ± 0.0092 mm year−2, is not significant, local values range from −0.1 mm year−2 in the central Indian Ocean to +0.1 mm year−2 in the western tropical Pacific and east of Japan. These extrema are associated with patterns of sea level change that differ significantly from the first half of the analyzed period (i.e. 1900 to 1950) to the second half (1950 to 2000). We take this as an indication of long period oceanic processes that are superimposed to the general sea level rise.
      PubDate: 2014-09-19T00:14:31.502269-05:
      DOI: 10.1002/2014JC009900
  • Mesoscale eddy variability in the southern extension of the East
           Madagascar Current: Seasonal cycle, energy conversion terms, and eddy mean
    • Authors: Issufo Halo; Pierrick Penven, Björn Backeberg, Isabelle Ansorge, Frank Shillington, Raymond Roman
      Pages: n/a - n/a
      Abstract: In this study we used more than seventeen years of satellite altimetry observations and output from an ocean model to investigate the mesoscale eddy variability and forcing mechanisms to the south of Madagascar. Analysis of energy conversion terms in the model has shown seasonality on eddy formation, both by barotropic and baroclinic instabilities: maximum in winter (JJA) and minimum in summer (DJF). The eddies were mainly formed in the upper ocean (0 ‐ 300 m) and at intermediate depths (800 ‐ 2000 m) by barotropic and baroclinic instabilities respectively. The former dominated in the southeastern margin of Madagascar, and the latter to the southwest, where the South East Madagascar Current (SEMC) separates from the continental shelf. Seasonality of the eddy formation appeared linked with the seasonal intensification of the SEMC. The energy conversion terms indicated that the eddies have a significant contribution to the large‐scale circulation, but not being persistent throughout the year, occurring mainly during the fall‐season (MAM). Eddy demography from altimetry and model provided information on eddy preferential sites for birth, annual occurrence (6 ‐ 13 per year), eddy mean diameter (124 ‐ 178 km), mean amplitude (9 ‐ 28 cm), life‐time (90 ‐ 183 days) and maximum traveling distances (325 ‐ 1052 km). Eddies formed to the southwest of Madagascar exhibited distinct characteristics from those formed in the southeast. Nevertheless, all eddies were highly nonlinear, suggesting that they are potential vectors of connectivity between Madagascar and Africa. This may have a significant impact on the ecology of this region.
      PubDate: 2014-09-18T23:59:42.138228-05:
      DOI: 10.1002/2014JC009820
  • Formation and distribution of sea ice in the Gulf of St. Lawrence: A
           process‐oriented study using a coupled ocean‐ice model
    • Authors: Jorge Urrego‐Blanco; Jinyu Sheng
      Pages: n/a - n/a
      Abstract: A coupled ocean‐ice model for the eastern Canadian shelf is used to examine main physical processes affecting sea ice conditions in the Gulf of St. Lawrence (GSL) and adjacent waters. The coupled model is based on NEMO and uses OPA9 as the ocean circulation component and the 2‐category dynamic‐thermodynamic LIM2 as the ice model. The coupled model is forced by atmospheric reanalysis fields produced by Large and Yeager [2004]. The model results are used to examine the roles of thermodynamics and dynamics on sea ice distributions and patterns of ice production and melting, and the influence of ice capping on the circulation in the study region. Analysis of model results indicates that local production of sea ice is important in shallower areas over the northern and western GSL. Equatorward advection of sea ice from the St. Lawrence Estuary is affected significantly by the Gaspé Current. An index is used to quantify the relative importance of thermodynamic and dynamics of sea ice in the GSL. It is found that both thermodynamics and dynamics are important over most of the GSL, except for waters around Anticosti Island, the southeastern Gulf, and the eastern Scotian Shelf, where dynamics (or sea ice movements) are the most important mechanism for the presence of sea ice. The study also demonstrates that ice capping significantly reduces the strength of the winter circulation in the GSL.
      PubDate: 2014-09-18T07:28:17.329459-05:
      DOI: 10.1002/2014JC010185
  • Local feedback mechanisms of the shallow water region around the Maritime
    • Authors: Pengfei Xue; Elfatih A. B. Eltahir, Paola Malanotte‐Rizzoli, Jun Wei
      Pages: n/a - n/a
      Abstract: The focus of this study is the local‐scale air‐sea feedback mechanisms over the shallow shelf water region (water depth < 200m) of the Maritime Continent (MC). MC was selected as a pilot study site for its extensive shallow water coverage, geographic complexity and importance in the global climate system. To identify the local‐scale air‐sea feedback processes, we ran numerical experiments with perturbed surface layer water temperature using a coupled ocean‐atmosphere model and an uncoupled ocean model. By examining the responses of the coupled and uncoupled models to the water temperature perturbation, we identify that, at a local‐scale, a negative feedback process through the coupled dynamics that tends to restore the SST from its perturbation could dominate the shallow water region of the MC at a short time scale of several days. The energy budget shows that 38% of initial perturbation‐induced heat energy was adjusted through the air‐sea feedback mechanisms within 2 weeks, of which 58% is directly transferred into the atmosphere by the adjustment of latent heat flux due to the evaporative cooling mechanism. The increased inputs of heat and moisture into the lower atmosphere then modifies its thermal structure and increases the formation of low‐level clouds, which act as a shield preventing incoming solar radiation from reaching the sea surface, accounts for 38% of the total adjustment of surface heat fluxes, serving as the second mechanism for the negative feedback process. The adjustment of sensible heat flux and net long‐wave radiation play a secondary role. The response of the coupled system to the SST perturbation suggests a response time scale of the coupled feedback process of about 3‐5 days. The two‐way air‐sea feedback tightly links the surface heat fluxes, clouds and SST, and can play an important role in regulating the short‐term variability of the SST over the shallow shelf water regions.
      PubDate: 2014-09-18T05:20:59.142648-05:
      DOI: 10.1002/2013JC009700
  • Sea surface salinity variability in the East China Sea observed by the
           Aquarius instrument
    • Authors: Seung‐bum Kim; Jae Hak Lee, Paolo de Matthaeis, Simon Yueh, Chang‐Su Hong, Joon‐Ho Lee, Gary Lagerloef
      Pages: n/a - n/a
      Abstract: This study demonstrates that the spaceborne Aquarius instrument is able to monitor the sea surface salinity (SSS) variations in the East China Sea (ECS) with the spatial resolution of about 150 km at 7‐day interval, where routine observations are difficult. The two geophysical contaminants enter the sidelobes of the Aquarius antenna and bias the coastal SSS low: the emission from the land surface and the radiofrequency interference (RFI). Away from about one Aquarius pixel (150 km) from the coastline, the Aquarius SSS is fairly insensitive (less than about 0.2 psu) to the radiometric details of the method to correct for the land emission. The ascending orbits appear to be affected by unfiltered RFI much less than the descending tracks. The Aquarius SSS along the ascending tracks is low over the ECS by 0.40 to 0.93 psu (with respect to the in situ data during the two separate 7‐day periods) and is biased low by 0.41 to 1.07 psu (accuracy, or the time‐mean of difference from the regional model along three Aquarius tracks over a 18‐month period). The presence of the ascending and descending differences in the Aquarius SSS, and the spatially widespread bias suggest that the bias is attributed to the unfiltered RFI originating from strong point sources (rather than to the land contamination from weak distributed sources, or to other seasonally‐varying geophysical contaminants). Despite the bias, the Aquarius data describe well the temporal and spatial variability of the ECS SSS. The temporal trend and magnitude of salinity changes agree remarkably between Aquarius and a regional numerical model, during both the freshwater discharge season from the Yangtze river and the rest of the year. The precision of the Aquarius observation in the ECS is comparable with the Aquarius mission requirement (0.2 psu one‐sigma for a monthly average over the open ocean). The river discharge rate correlates with the Aquarius SSS with the coefficient of 0.71 on a seasonal scale with the discharge leading the SSS changes. The Aquarius SSS increases away from the coast, in response to the river outflow. The interannual changes in the Aquarius SSS capture the effect of the regional drought in summer 2013.
      PubDate: 2014-09-18T05:09:39.316904-05:
      DOI: 10.1002/2014JC009983
  • Laboratory study on coprecipitation of phosphate with ikaite in sea ice
    • Authors: Yu‐Bin Hu; Gerhard S. Dieckmann, Dieter A. Wolf‐Gladrow, Gernot Nehrke
      Pages: n/a - n/a
      Abstract: Ikaite (CaCO3·6H2O) has recently been discovered in sea ice, providing first direct evidence of CaCO3 precipitation in sea ice. However, the impact of ikaite precipitation on phosphate (PO4) concentration has not been considered so far. Experiments were set up at pH from 8.5 to 10.0, salinities from 0 to 105, temperatures from −4 to 0°C and PO4 concentrations from 5 to 50 µmol kg−1 in artificial sea ice brine so as to understand how ikaite precipitation affects the PO4 concentration in sea ice under different conditions. Our results show that PO4 is coprecipitated with ikaite under all experimental conditions. The amount of PO4 removed by ikaite precipitation increases with increasing pH. Changes in salinity (S ≥ 35) as well as temperature have little impact on PO4 removal by ikaite precipitation. The initial PO4 concentration affects the PO4 coprecipitation. These findings may shed some light on the observed variability of PO4 concentration in sea ice.
      PubDate: 2014-09-18T05:04:25.347027-05:
      DOI: 10.1002/2014JC010079
  • Enhanced sea‐air CO2 exchange influenced by a tropical depression in
           the South China Sea
    • Authors: Qingyang Sun; Danling Tang, Louis Legendre, Ping Shi
      Pages: n/a - n/a
      Abstract: Ship measurements made two days after the passage of a tropical depression (TD) in the South China Sea (SCS, April 2011) showed two contrasted responses of the partial pressure of CO2 at sea surface (pCO2,sw). In low sea‐surface salinity (SSS) waters, pCO2,sw was low (349 ± 7 µatm), and the area was a carbon sink (‐4.7 ± 1.8 mmol CO2 m‐2 d‐1), whereas in water with high SSS and chlorophyll a and low dissolved oxygen and sea surface temperature, pCO2,sw was higher than normal SCS water (376 ± 8 vs. 362 ± 4 µatm) and the area was a carbon source (1.2 ± 3.1 mmol CO2 m‐2 d‐1). Satellite data showed two large areas of low SSS before the TD, which were likely influenced by rainfall, and these areas were considered to have low pCO2,sw because of their low SSS. The high pCO2,sw after the TD is explained by the uplifting to the surface of deeper and CO2‐rich water, due to winds accompanied by the TD. The difference in sea‐air CO2 flux between the TD‐affected area and the lower‐SSS water was (1.99 + 4.70 = 6.7 mmol CO2 m‐2 d‐1), indicating a 100% change caused by the TD compared to the average seasonal value in spring in southern SCS (3.3 ± 0.3 mmol CO2 m‐2 d‐1). Undersaturation of CO2 prior to the TD due to dilution by freshwater and the pre‐existing cold eddy, and slow translation speed of the TD, are considered to be accounted for the CO2 flux change.
      PubDate: 2014-09-18T04:48:52.290777-05:
      DOI: 10.1002/2014JC010131
  • Storms modify baroclinic energy fluxes in a seasonally stratified shelf
           sea: Inertial‐tidal interaction
    • Authors: Joanne. E. Hopkins; Gordon R. Stephenson, J. Mattias Green, Mark E. Inall, Matthew R. Palmer
      Pages: n/a - n/a
      Abstract: Observations made near the Celtic Sea shelf edge are used to investigate the interaction between wind generated near‐inertial oscillations and the semi‐diurnal internal tide. Linear, baroclinic energy fluxes within the near‐inertial (f) and semi‐diurnal (M2) wave bands are calculated from measurements of velocity and density structure at two moorings located 40 km from the internal tidal generation zone. Over the two week deployment period the semi‐diurnal tide drove 28‐48 W m‐1 of energy directly on‐shelf. Little spring‐neap variability could be detected. Horizontal near‐inertial energy fluxes were an order of magnitude weaker, but non‐linear interaction between the vertical shear of inertial oscillations and the vertical velocity associated with the semi‐diurnal internal tide led to a 25‐43% increase in positive on‐shelf energy flux. The phase relationship between f and M2 determines whether this non‐linear interaction enhances or dampens the linear tidal component of the flux, and introduces a 2‐day counter clockwise beating to the energy transport. Two very clear contrasting regimes of (a) tidally and (b) inertially driven shear and energy flux are captured in the observations.
      PubDate: 2014-09-18T04:48:50.374667-05:
      DOI: 10.1002/2014JC010011
  • A numerical investigation of wave‐breaking‐induced turbulent
           coherent structure under a solitary wave
    • Authors: Zheyu Zhou; Jacob Sangermano, Tian‐Jian Hsu, Francis C. K. Ting
      Pages: n/a - n/a
      Abstract: To better understand the effect of wave‐breaking‐induced turbulence on the bed, we report a 3D Large‐Eddy Simulation (LES) study of a breaking solitary wave in spilling condition. Using a turbulence‐resolving approach, we study the generation and the fate of wave‐breaking‐induced turbulent coherent structures, commonly known as obliquely descending eddies (ODEs). Specifically, we focus on how these eddies may impinge onto bed. The numerical model is implemented using an open‐source CFD library of solvers, called OpenFOAM, where the incompressible 3D filtered Navier‐Stokes equations for the water and the air phases are solved with a finite volume scheme. The evolution of the water‐air interfaces are approximated with a volume of fluid method. Using the dynamic Smagorinsky closure, the numerical model has been validated with wave flume experiments of solitary wave breaking over a 1/50 sloping beach. Simulation results show that during the initial overturning of the breaking wave, 2D horizontal rollers are generated, accelerated and further evolve into a couple of 3D hairpin vortices. Some of these vortices are sufficiently intense to impinge onto the bed. These hairpin vortices possess counter‐rotating and downburst features, which are key characteristics of ODEs observed by earlier laboratory studies using Particle Image Velocimetry. Model results also suggest that those ODEs that impinge onto bed can induce strong near‐bed turbulence and bottom stress. The intensity and locations of these near‐bed turbulent events could not be parameterized by near‐surface (or depth integrated) turbulence unless in very shallow depth.
      PubDate: 2014-09-18T04:47:15.170217-05:
      DOI: 10.1002/2014JC009854
  • Issue Information
    • Pages: i - vi
      PubDate: 2014-09-16T16:16:11.100361-05:
      DOI: 10.1002/jgrc.20353
  • Energy transfer between wind waves and low‐frequency oscillations on
           a fringing reef, Ipan Guam
    • Authors: Anne‐Christine N. Péquignet; Janet M. Becker, Mark A. Merrifield
      Pages: n/a - n/a
      Abstract: Field observations from a Guam fringing reef are used to examine the cross‐reef energy exchange between high frequency sea and swell (SS) and low frequency infragravity (IG) and far infragravity (fIG) waves. Energetic SS waves (significant wave heights 2‐4 m) break at the outer reef, leading to weak (
      PubDate: 2014-09-15T21:57:36.268634-05:
      DOI: 10.1002/2014JC010179
  • Swell dissipation by induced atmospheric shear stress
    • Authors: Y. Perignon; F. Ardhuin, M. Cathelain, M. Robert
      Pages: n/a - n/a
      Abstract: Observations of swell dissipation across oceans reveal a significant loss of energy that can be the result of many of processes. Among these candidate mechanisms, this paper examines the properties of the viscous air‐sea boundary layer driven by swells in order to characterize the induced atmospheric flow regime and its associated viscous dissipation over swells. A series of 3D numerical experiments is carried out with a RANS model and appropriate turbulence closure. These experiments reveal a laminar to turbulent transition in the near free‐surface region for a common range of characteristic amplitudes and periods of swells under stationary conditions. At low Reynolds number, laminar conditions prevail and computed decay rates conform to the analytical formulation μv of the Stokes interfacial boundary layer for this problem. The turbulent regimes are characterized as well and the new decay rates follow a non‐dimensional relation μ=1.42μv(Re⁡1.5×105)0.41 above Re = 1.5 × 105 (e.g. amplitude larger than 1.1m for a 14s monochromatic wave period). Typical decay rates are up to 4 times above the laminar values, which is a factor 10 less than the largest rates estimated for oceanic conditions. A sensitivity analysis is finally conducted to evaluate the influence of the stationary hypothesis. It demonstrates a short setup‐length and low relative variations of the unsteady decay rates for laminar, transitioning and developed turbulent conditions, which confirms the evaluation of steady decay rates.
      PubDate: 2014-09-15T07:11:31.561552-05:
      DOI: 10.1002/2014JC009896
  • South Indian Countercurrent and associated fronts
    • Authors: Viviane V. Menezes; Helen E. Phillips, Andreas Schiller, Nathaniel L. Bindoff, Catia M. Domingues, Marcio L. Vianna
      Pages: n/a - n/a
      Abstract: A striking feature of the South Indian Ocean circulation is the presence of the eastward South Indian Countercurrent (SICC) that flows in a direction opposite to that predicted by the classical theories of wind‐driven circulation. Several authors suggest that the SICC resembles the subtropical countercurrents (STCCs) observed in other oceans, which are defined as narrow eastward jets on the equatorward side of subtropical gyres, where the depth‐integrated flow is westward. These jets are associated with subsurface thermal fronts at thermocline depths by the thermal wind relation. However, the subsurface thermal front associated with the SICC has not been described to date. Other studies conjecture an important role for salinity in controlling the SICC. In the present work, we analyse three Argo‐based atlases and data from six hydrographic cruises to investigate whether the SICC is accompanied by permanent thermal and density fronts including salinity effects. The seasonal cycle of these fronts in relation to the SICC strength are also investigated. We find that the SICC is better described as composed of three distinct jets, which we name the northern, central and southern SICC. We find that the southern SICC around 26° S has an associated thermal front at subsurface depths around 100‐200 m with salinity being of secondary importance. The southern branch strength is related to mode waters poleward of the front, similar to a STCC‐like current. However, the SICC multiple jet structure seems to be better described as resulting from PV staircases.
      PubDate: 2014-09-15T05:42:52.254511-05:
      DOI: 10.1002/2014JC010076
  • Impact of periodic intermediary flows on submarine melting of a Greenland
    • Authors: R. Sciascia; C. Cenedese, D. Nicolì, P. Heimbach, F. Straneo
      Pages: n/a - n/a
      Abstract: The submarine melting of a vertical glacier front, induced by an intermediary circulation forced by periodic density variations at the mouth of a fjord, is investigated using a non–hydrostatic ocean general circulation model and idealized laboratory experiments. The idealized configurations broadly match that of Sermilik Fjord, southeast Greenland, a largely two‐layers system characterized by strong seasonal variability of subglacial discharge. Consistent with observations, the numerical results suggest that the intermediary circulation is an effective mechanism for the advection of shelf anomalies inside the fjord. In the numerical simulations, the advection mechanism is a density intrusion with a velocity which is an order of magnitude larger than the velocities associated with a glacier–driven circulation. In summer, submarine melting is mostly influenced by the discharge of surface runoff at the base of the glacier and the intermediary circulation induces small changes in submarine melting. In winter, on the other hand, submarine melting depends only on the water properties and velocity distribution at the glacier front. Hence, the properties of the waters advected by the intermediary circulation to the glacier front are found to be the primary control of the submarine melting. When the density of the intrusion is intermediate between those found in the fjord's two layers there is a significant reduction in submarine melting. On the other hand, when the density is close to that of the bottom layer, only a slight reduction in submarine melting is observed. The numerical results compare favorably to idealized laboratory experiments with a similar setup.
      PubDate: 2014-09-12T00:01:13.838272-05:
      DOI: 10.1002/2014JC009953
  • First “in situ” determination of gas transport coefficients
           (DO2, DAr, and DN2) from bulk gas concentration measurements (O2, N2, Ar)
           in natural sea ice
    • Authors: Odile Crabeck; B Delille, S Rysgaard, D.N. Thomas, N.X. Geilfus, B Else, J.L. Tison
      Pages: n/a - n/a
      Abstract: We report bulk gas concentrations of O2, N2 and Ar, as well as their transport coefficients, in natural landfast subarctic sea ice in southwest Greenland. The observed bulk ice gas composition was 27.5% O2, 71.4% N2 and 1.09% Ar. Most previous studies suggest that convective transport is the main driver of gas displacement in sea ice and have neglected diffusion processes. According to our data, brines were stratified within the ice, so that no convective transport could occur within the brine system. Therefore, diffusive transport was the main driver of gas migration. By analysing the temporal evolution of an internal gas peak within the ice, we deduced the bulk gas transport coefficients for oxygen (DO2), argon (DAr) and nitrogen (DN2). The values fit to the few existing estimates from experimental work, and are close to the diffusivity values in water (10‐5 cm2 s‐1). We suggest that gas bubbles escaping from the brine to the atmosphere ‐ as the ice gets more permeable during melt ‐ could be responsible for the previously reported high transport coefficients. These results underline that when there is no convective transport within the sea ice, the transport of gas by diffusion through the brines, either in the liquid or gaseous phases, is a major factor in controlling the ocean–atmosphere exchange.
      PubDate: 2014-09-10T01:58:08.847943-05:
      DOI: 10.1002/2014JC009849
  • Distinguishing meanders of the Kuroshio using machine learning
    • Authors: David A. Plotkin; Jonathan Weare, Dorian S. Abbot
      Pages: n/a - n/a
      Abstract: The Kuroshio south of Japan is often described as being bimodal, with abrupt transitions between a straight path state that stays near the coast (small meander) and a meandering state the deviates from the coast (large meander). Despite evidence of the existence of two or more states of the Kuroshio, previous data‐driven studies have shown only high variability of the current; they have not, however, demonstrated bimodality in the sense of two states of relatively high probability separated by a region of relatively low probability. We use singular value decomposition (SVD), a standard time series analysis method for characterizing variability, and diffusion maps and spectral clustering (DMSC), a machine learning algorithm that seeks multimodality, to investigate Kuroshio reanalysis output. By applying these methods to a time series of velocity fields, we find that 1) the Kuroshio is bimodal, with high inflow and low path variability in the small meander and low inflow and high path variability in the large meander, 2) the state of the system correlates highly with the location of the recirculation gyre south of Japan, and 3) the meanders are better characterized by path variability than by mean path. Because these results are consistent with satellite sea surface height data, they are not an artifact of the model used for reanalysis. Further, our results provide evidence for a previously proposed transition mechanism based on the strengthening, migration, and weakening of the recirculation gyre south of Japan and can therefore help direct future modeling studies.
      PubDate: 2014-09-10T01:51:57.218342-05:
      DOI: 10.1002/2014JC010128
  • Variability of Arctic sea ice topography and its impact on the atmospheric
           surface drag
    • Authors: G. Castellani; C. Lüpkes, R. Gerdes, S. Hendricks
      Pages: n/a - n/a
      Abstract: Over the polar oceans, near‐surface atmospheric transport of momentum is strongly influenced by sea‐ice surface topography. The latter is analyzed on the basis of laser altimeter data obtained during airborne campaigns between 1995 and 2011 over more than 10000 km of flight distance in different regions of the Arctic Ocean. Spectra of height and spacing between topographic features averaged over 10 km flight sections show that typical values are 0.45 m for the mean height and about 20 m for the mean spacing. Nevertheless the variability is high and the spatial variability is stronger than the temporal one. The total topography spectrum is divided into a range with small obstacles (between 0.2 m and 0.8 m height) and large obstacles (≥0.8 m). Results show that large pressure ridges represent the dominant topographic feature only along the coast of Greenland. In the Central Arctic the concentration of large ridges decreased over the years, accompanied by an increase of small obstacles concentration and this might be related to decreasing multi‐year ice. The application of a topography dependent parameterization of neutral atmospheric drag coefficients reflects the large variability in the sea ice topography and reveals characteristic differences between the regions. Based on the analysis of the two spectral ranges we find that the consideration of only large pressure ridges is not enough to characterize the roughness degree of an ice field, and the values of drag coefficients are in most regions strongly influenced by small obstacles.
      PubDate: 2014-09-10T01:49:56.147693-05:
      DOI: 10.1002/2013JC009712
  • Mesoscale dynamics in the Arabian Sea and a focus on the Great Whirl life
           cycle: A numerical investigation using ROMS
    • Authors: C. Vic; G. Roullet, X. Carton, X. Capet
      Pages: n/a - n/a
      Abstract: The Great Whirl (GW) is a persistent anticyclonic mesoscale eddy that is observed seasonally in the Arabian Sea during a period embedding the three months of the southwest monsoon (June‐July‐August) at a quasi‐steady location. Its dynamics remain unclear despite it being one of the largest coherent vortices in the world ocean. Realistic regional numerical experiments using ROMS are performed to investigate the life cyle of the GW, which is not well resolved by sparse available in situ measurements in the region. Using a set of sensitivity experiments and an accurate temporal characterization of the eddy properties (including position, radius, depth and vorticity) we (i) confirm the role of basin‐scale downwelling Rossby waves in the GW generation (ii) clarify the role of the monsoonal strong anticyclonic wind in its maintenance and barotropization (iii) suggest a connection between basin‐scale Rossby wave dynamics and GW collapse.
      PubDate: 2014-09-09T23:51:31.112037-05:
      DOI: 10.1002/2014JC009857
  • Quantifying tidally driven benthic oxygen exchange across permeable
           sediments: An aquatic eddy correlation study
    • Authors: Daniel F. McGinnis; Stefan Sommer, Andreas Lorke, Ronnie N. Glud, Peter Linke
      Pages: n/a - n/a
      Abstract: Continental shelves are predominately (~70%) covered with permeable, sandy sediments. While identified as critical sites for intense oxygen, carbon and nutrient turnover, constituent exchange across permeable sediments remains poorly quantified. The central North Sea largely consists of permeable sediments and has been identified as increasingly at risk for developing hypoxia. Therefore, we investigate the benthic O2 exchange across the permeable North Sea sediments using a combination of in situ microprofiles, a benthic chamber and aquatic eddy correlation. Tidal bottom currents drive the variable sediment O2 penetration depth (from ~3 to 8 mm) and the concurrent turbulence‐driven 25‐fold variation in the benthic sediment O2 uptake. The O2 flux and variability were reproduced using a simple 1‐D model linking the benthic turbulence to the sediment porewater exchange. The high O2 flux variability results from deeper sediment O2 penetration depths and increased O2 storage during high velocities, which is then utilized during low flow periods. The study reveals that the benthic hydrodynamics, sediment permeability and porewater redox oscillations are all intimately linked and crucial parameters determining the oxygen availability in permeable sediments. These parameters must all be considered when evaluating mineralization pathways of organic matter and nutrients in permeable sediments.
      PubDate: 2014-09-09T19:25:33.632231-05:
      DOI: 10.1002/2014JC010303
  • Intrusion of Rhone River diluted water into the Bay of Marseille:
           Generation processes and impacts on ecosystem functioning
    • Authors: Marion Fraysse; Ivane Pairaud, Oliver N. Ross, Vincent M. Faure, Christel Pinazo
      Pages: n/a - n/a
      Abstract: The Rhone River provides the largest inputs of terrestrial freshwater and nutrients into the Mediterranean Sea. The Rhone River diluted water intrusions into the Bay of Marseille were investigated, examining their physical generation processes and associated biogeochemical impact by using in situ observations, remote sensing data, and a three‐dimensional physical/biogeochemical coupled model. During our study period from 2007 to 2011, Rhone River intrusions occurred on average 7.6 times per year and affected more frequently the northern part of the bay. A classification of intrusion events in three categories is proposed (short‐lived, big and small) as a function of their duration and spatial extent. The intrusions appeared to be driven by: (i) wind forcing, (ii) the presence of a mesoscale eddy, (iii) the Rhone River discharge volume, and (iv) the variation in thermocline depth. Typically, a combination of these favorable factors was necessary to induce an intrusion. An intrusion strongly impacts the biogeochemical functioning of the Bay of Marseille by bringing large quantities of nutrients into the bay. Mass balances were computed allowing us to quantify this impact on the Bay of Marseille. The results show that the ecological impact depends very much on the type of intrusion, with big intrusions having the highest impact.
      PubDate: 2014-09-05T04:21:53.953366-05:
      DOI: 10.1002/2014JC010022
  • Modulation of Rossby waves on the Pacific North Equatorial Current
           bifurcation associated with the 1976 climate regime shift
    • Authors: Li‐Chiao Wang; Chau‐Ron Wu, Bo Qiu
      Pages: n/a - n/a
      Abstract: Simulated current velocity and long‐term reanalysis wind data are used to investigate interannual variations in the bifurcation of the Pacific North Equatorial Current (NEC) after the 1976 climate regime shift. Wind stress curl anomaly (WSCA) in the region of 10°N–15°N and 160°E–170°E generates Rossby waves and affects the NEC bifurcation along the Philippine coast. From 1976 to 1992, following a regime shift to the positive Pacific Decadal Oscillation (PDO) phase, PDO and El Niño‐Southern Oscillation (ENSO) match each other in strength and have a neutralized effect on the WSCA. From 1993 to 2009, WSCA matches PDO well, and its correlation with ENSO is lower. Using a linear regression model, we show that the influence of PDO has nearly thirteen times weight over that of ENSO. Prior to the 1976 regime shift, WSCA is closely related to ENSO from 1961 to 1975, and it does not correlate significantly with PDO. Our analysis results show that Rossby waves are preferentially generated in either the negative PDO phase when the ENSO signal dominates, or in the positive PDO phase when the ENSO signal is overshadowed. In the phase when the positive PDO counteracts with the ENSO signal, neither ENSO nor PDO has a significant influence on Rossby wave generations through the WSCA.
      PubDate: 2014-09-05T04:20:49.538107-05:
      DOI: 10.1002/2014JC010233
  • Heat fluxes across the Antarctic Circumpolar Current in Drake Passage:
           Mean flow and eddy contributions
    • Authors: Ramiro Ferrari; Christine Provost, Young‐Hyang Park, Nathalie Sennéchael, Zoé Koenig, Hela Sekma, Gilles Garric, Romain Bourdallé‐Badie
      Pages: n/a - n/a
      Abstract: In contrast to a long‐standing belief, observations in the Antarctic Circumpolar Current (ACC) show that mean velocity vectors rotate with depth, thus suggesting a possible importance of the time‐mean flow for the local poleward heat transport. The respective contributions of the eddy and mean flows to the heat flux across the ACC in Drake Passage (DP) are investigated using recently acquired and historical time series of velocity and temperature from a total of 24 current meter moorings and outputs of a high‐resolution (1/12 degree) model with realistic topography. Only 11 out of the 24 depth‐integrated eddy heat flux estimates are found to be significant, and they are poleward. Model depth‐integrated eddy heat fluxes have similar signs and amplitudes as the in situ estimates at the mooring sites. They are mostly poleward or non‐significant, with amplitude decreasing to the south. The cross‐stream temperature fluxes caused by the mean flow at the moorings have a sign that varies with location and corresponds to the opposite of the vertical velocity estimates. The depth‐integrated temperature fluxes due to the mean flow in the model exhibit small spatial scales and are of opposite sign to the bottom vertical velocities. This suggests that the rotation of the mean velocity vectors with depth is mainly due to bottom topography. The rough hilly topography in DP likely promotes the small‐scale vertical velocities and temperature fluxes. Eddy heat fluxes and cross‐stream temperature fluxes are integrated over mass balanced regions defined by the model transport streamlines. The contribution of the mean flow to the ocean heat fluxes across the Southern ACC Front in DP (covering about 4 % of the circumpolar longitudes) is about four times as large as the eddy heat flux contribution and the sum of the two represent on the order of 10% of the heat loss to the atmosphere south of 60°S.
      PubDate: 2014-09-05T03:38:20.554388-05:
      DOI: 10.1002/2014JC010201
  • Observations on stratified flow over a bank at low Froude numbers
    • Authors: Ewa Jarosz; Hemantha W. Wijesekera, William J. Teague, Diane B. Fribance, Mark A. Moline
      Pages: n/a - n/a
      Abstract: In June 2011, a nine‐day oceanographic survey was conducted over the East Flower Garden Bank, a coral reef, located on the outer shelf in the northwestern Gulf of Mexico. Current, temperature, conductivity, and microstructure measurements were collected to characterize flow evolution, turbulence, and mixing over the bank. During the experiment, the flow was highly stratified, subcritical (Froude number below 0.4), hydrostatic, and nonlinear with rotational effects being important. Observations showed that flow structure, turbulence, and mixing were highly dependent on the direction and strength of the current; thus, they varied spatially and temporarily. Responses resulting from interactions between the free‐stream flow and the obstacle were significantly different on the upstream and downstream sides of the bank. Blocking and diverging of the flow just below the bank height was observed on the upstream side. On the downstream side, a wake with imbedded vortices developed. Moreover, turbulence was amplified over the bank top and on its downstream side. Turbulent dissipation rates were as high as 10‐6 W kg‐1 and resulted in measured rates of energy dissipation and mixing by turbulence per unit width as high as 40 W m‐1. Mixing on the downstream side was elevated with eddy diffusivities reaching 10‐3 m2 s‐1, well above a typical value of 10‐5 m2 s‐1 commonly found in the ocean thermocline and over shelves with flat topography. On the upstream side, estimated eddy diffusivities were close to that for the ocean thermocline, i.e., they were generally less than 0.5·10‐4 m2 s‐1.
      PubDate: 2014-09-05T03:37:59.808359-05:
      DOI: 10.1002/2014JC009934
  • Net community production and export from seaglider measurements in the
           North Atlantic after the spring bloom
    • Authors: Matthew B. Alkire; Craig Lee, Eric D’Asaro, Mary Jane Perry, Nathan Briggs, Ivona Cetinić, Amanda Gray
      Pages: n/a - n/a
      Abstract: Mean rates of net community production (NCP) and particulate organic carbon (POC) export were estimated from sensor measurements of dissolved oxygen (O2), chlorophyll fluorescence (chl F), and particulate backscatter (bbp700) collected from three Seagliders that surveyed a 20 x 20 km area in the North Atlantic subsequent to a large diatom bloom. Since the Seagliders sampled geographically fixed patterns, care was taken in the calculation of all terms applicable to the Eulerian reference frame, including local rate of change, vertical mixing, air‐sea exchange, and horizontal advection. Although similar studies of NCP in the open ocean have generally assumed advection to be insignificant we have found that this term cannot be ignored when dealing with temporal scales of ≤ 1 month and/or spatial scales ≤ 20 km. The overlapping sampling pattern of the Seagliders was sufficiently rapid such that 4‐5 day timescales observed in the O2 and POC data were adequately resolved and variations were not a consequence of aliasing spatial variability. During the study period, ratios of chlorophyll fluorescence‐to‐particulate backscatter (chl:bbp700) were lower than values encountered during the spring diatom bloom, suggesting the phytoplankton community was predominantly composed of smaller cells (pico‐ and nanoplankton) and/or coccolithophorids. Coupled budgets of oxygen and POC indicated a net community production of 1.0 mol C m‐2 and carbon export of 0.6 mol C m‐2, respectively over a period of 23 days. Thus, the production and export of carbon that occurred over the month‐long experiment period was comparable to that encountered during the spring bloom.
      PubDate: 2014-09-04T07:36:41.236157-05:
      DOI: 10.1002/2014JC010105
  • Estimating dissolved organic carbon inventories in the East China Sea
           using remote sensing data
    • Authors: Qiong Liu; Delu Pan, Yan Bai, Kai Wu, Chen‐Tung Authur Chen, Zhiliang Liu, Lin Zhang
      Pages: n/a - n/a
      Abstract: The coastal ocean is characterized by a high dissolved organic carbon (DOC) concentration due to large terrestrial inputs and high primary production. Monitoring the dynamic variation of DOC inventories in coastal oceans provide more information on carbon flux, but is very challenging in practice. We propose a method of estimating DOC inventories in the East China Sea (ECS) by integrating the surface DOC distribution with a vertical model of the DOC profile via satellite data. Surface DOC concentration was retrieved from satellite‐derived chromophoric dissolved organic matter and chlorophyll concentration. Two vertical DOC profile models (uniform model and stratified model) were established based on water‐density profiles, and the usage of these two models was corresponded to water mass classification determined by a water mass index. The average bias of the satellite‐derived DOC inventory, in the euphotic layer was 23.8% in spring, 24.8% in autumn and 14.7% in winter, and in the water column was 15.6% in spring, 12.3% in autumn and 10.2% in winter. The total satellite‐derived DOC inventory integrated in the water column was about 31.84 Tg, 31.96 Tg, 28.59 Tg and 31.18 Tg in four seasons in the ECS (5.84×105 km2). Sensitivity analysis indicated that the remote‐sensing method of estimating DOC inventory is stable and reliable. The available of long‐term and large‐scale satellite‐derived DOC inventories in marginal sea would provide us basic information on carbon cycle, and the difference between the seasonal DOC inventories would help to understand the DOC export and relative biogeochemical processes in the ECS.
      PubDate: 2014-09-03T12:36:29.559443-05:
      DOI: 10.1002/2014JC009868
  • Vertical structure of eddies and Rossby waves and their effect on the
           Atlantic meridional overturning circulation at 26.5°N
    • Authors: L. Clément; E. Frajka‐Williams, Z. B. Szuts, S. A. Cunningham
      Pages: n/a - n/a
      Abstract: The meridional overturning circulation (MOC) at 26.5°N in the Atlantic has a standard deviation of 4.9 Sv and contains large fluctuations at subannual periods. The geostrophic component of the MOC is believed to be influenced on subannual timescales by eddies and Rossby waves. To quantify this effect, the vertical structure and surface characteristics of westward propagating signals are studied using altimetric data and full‐depth mooring measurements from the RAPID array at 26.5°N. Westward propagating features are observed in the western North Atlantic in both datasets and have periods of 80‐250 days in the first baroclinic mode. These features are still observed by the RAPID moorings 20 km offshore of the western boundary. The western boundary also exhibits deep variability characterized by enhanced energy in higher baroclinic modes. The effect of eddies and Rossby waves on the geostrophic transport is quantified by representing their vertical structure with the first baroclinic mode. In total, 42% of the variance of the transbasin thermocline transport inferred from geostrophic calculations at 26.5°N can be attributed to first mode variability, which is associated with eddies and Rossby waves at periods of 80‐250 days. The standard deviation of the transbasin thermocline transport due to eddies and Rossby waves is estimated to be 2.6 Sv.
      PubDate: 2014-09-03T03:09:55.031686-05:
      DOI: 10.1002/2014JC010146
  • Differential distribution of diatoms and dinoflagellates in a cyclonic
           eddy confined in the Bay of La Paz, Gulf of California
    • Authors: Erik Coria‐Monter; María Adela Monreal‐Gómez, David Alberto Salas‐de‐León, Javier Aldeco‐Ramírez, Martín Merino‐Ibarra
      Pages: n/a - n/a
      Abstract: The differential distribution of diatoms and dinoflagellates in the Bay of La Paz, Gulf of California, Mexico, was analyzed in summer of 2009, when a cyclonic eddy confined in the bay dominated the circulation. An uplift of the nutricline in the eddy drove high concentrations of nutrients to the euphotic layer. A differential phytoplankton distribution was observed to be associated with the eddy: there was an abundance of dinoflagellates close to the center of the cyclonic eddy, whereas diatoms were more abundant at the periphery. A significant inverse correlation (R=‐0.62, p
      PubDate: 2014-09-02T03:50:54.679207-05:
      DOI: 10.1002/2014JC009916
  • Diurnal cross‐shore thermal exchange on a tropical fore reef
    • Authors: L. Molina; G. Pawlak, J. R. Wells, S. G. Monismith, M. A. Merrifield
      Pages: n/a - n/a
      Abstract: Observations of the velocity structure at the Kilo Nalu Observatory on the south shore of Oahu, Hawaii show that thermally driven baroclinic exchange is a dominant mechanism for cross‐shore transport for this tropical forereef environment. Estimates of the exchange and net volume fluxes are comparable and show that the average residence time for the zone shoreward of the 12 m isobath is generally much less than one day. Although cross‐shore wind stress influences the diurnal cross‐shore exchange, surface heat flux is identified as the primary forcing mechanism from the phase relationships and from analysis of momentum and buoyancy balances for the record‐averaged diurnal structure. Dynamic flow regimes are characterized based on a two‐dimensional theoretical framework and the observations of the thermal structure at Kilo Nalu are shown to be in the unsteady temperature regime. Diurnal phasing and the cross‐shore momentum balance suggest that turbulent stress divergence is an important driver of the baroclinic exchange. While the thermally driven exchange has a robust diurnal profile in the long‐term, there is high temporal variability on shorter timescales. Ensemble averaged diurnal profiles indicate that the exchange is strongly modulated by surface heat flux, wind speed/direction and along‐shore velocity direction. The latter highlights the role of along‐shore variability in the thermally driven exchange. Analysis of the thermal balance in the nearshore region indicates that the cross‐shore exchange accounts for roughly 38 of the advective heat transport on a daily basis.
      PubDate: 2014-09-02T03:50:44.508846-05:
      DOI: 10.1002/2013JC009621
  • Annual sea‐air CO2 fluxes in the Bering Sea: Insights from new
           autumn and winter observations of a seasonally ice‐covered
           continental shelf
    • Authors: Jessica N. Cross; Jeremy T. Mathis, Karen E. Frey, Catherine E. Cosca, Seth L. Danielson, Nicholas R. Bates, Richard A. Feely, Taro Takahashi, Wiley Evans
      Pages: n/a - n/a
      Abstract: High‐resolution data collected from several programs has greatly increased the spatiotemporal resolution of pCO2 data in the Bering Sea, and provided the first autumn and winter observations. Using data from 2008 – 2012, monthly climatologies of sea‐air CO2 fluxes for the Bering Sea shelf area from April through December were calculated, and contributions of physical and biological processes to observed monthly sea‐air pCO2 gradients (ΔpCO2) were investigated. Net efflux of CO2 was observed during November, December, and April, despite the impact of sea surface cooling on ΔpCO2. Although the Bering Sea was believed to be a moderate to strong atmospheric CO2 sink, we found that autumn and winter CO2 effluxes balanced 65% of spring and summer CO2 uptake. Ice cover reduced sea‐air CO2 fluxes in December, April, and May. Our estimate for ice‐cover corrected fluxes suggests the mechanical inhibition of CO2 flux by sea‐ice cover has only a small impact on the annual scale (< 2%). An important data gap still exists for January through March, the period of peak ice cover and the highest expected retardation of the fluxes. By interpolating between December and April using assumptions of the described autumn and winter conditions, we estimate the Bering Sea shelf area is an annual CO2 sink of ~6.8 Tg C yr‐1. With changing climate, we expect warming sea surface temperatures, reduced ice cover, and greater wind speeds with enhanced gas exchange to decrease the size of this CO2 sink by augmenting conditions favorable for greater wintertime outgassing.
      PubDate: 2014-09-02T03:50:34.239473-05:
      DOI: 10.1002/2013JC009579
  • Geometry of tidal inlet systems: A key factor for the net sediment
           transport in tidal inlets
    • Authors: W. Ridderinkhof; H. E. de Swart, M. van der Vegt, N. C. Alebregtse, P. Hoekstra
      Pages: n/a - n/a
      Abstract: The net transport of sediment between the back‐barrier basin and the sea is an important process for determining the stability of tidal inlet systems. Earlier studies showed that in a short basin, tidal flats favor peak ebb‐currents stronger than peak flood currents, implying export of coarse sediment, while shallow basins favor stronger flood currents. The new elements considered in this study are 1) arbitrary basin lengths, 2) a narrow inlet that connects the basin to the sea, 3) an asymmetric tidal forcing, and 4) radiation damping. The objective is to gain fundamental insight in how the geometry of a tidal inlet system affects the net sand transport in a tidal inlet. For this purpose, a width‐ and depth averaged analytical model was constructed. It is found that the length of a back‐barrier basin controls the effect that nonlinear hydrodynamic processes have on the tidal asymmetry, and consequently controls whether the currents in the inlet are flood‐ or ebb‐dominant. Furthermore, the cross‐sectional area of the inlet controls the ratio between the net sediment transport that results from tidal asymmetry and that caused by the interaction of the principal tide with the residual current. Finally, it is shown that the effect of an asymmetric tidal forcing on the net sand transport depends on the length of the back‐barrier basin with respect to the tidal wavelength in that basin.
      PubDate: 2014-09-02T03:50:12.081442-05:
      DOI: 10.1002/2014JC010226
  • The central Pacific El Niño intraseasonal Kelvin wave
    • Authors: K. Mosquera‐Vásquez; B. Dewitte, Serena Illig
      Pages: n/a - n/a
      Abstract: In this study we document and interpret the characteristics of the Intraseasonal Kelvin wave (ISKw) in the Pacific over the 1989‐2011 period, based on observations, a linear model and the outputs of an Ocean General Circulation Model (OGCM). We focus on the wave activity during the Central Pacific (CP) El Niño events contrasting with the extraordinary El Niño of 1997/1998. We find that ISKw activity is enhanced in Austral Summer (Spring) in the central Pacific (west of ~120°W) during CP El Niño events. The linear model experiment indicates that the Austral Summer peak is wind‐forced while the Austral Spring peak is not and consequently results from non‐linear processes. In addition, a strong dissipation of the ISKws is observed east of 120°W which cannot be accounted for by a linear model using a Rayleigh friction. A vertical and horizontal mode decomposition of the OGCM simulation further confirms the sharp changes in characteristics of the ISKws as well as the reflection of the latter into first‐meridional Rossby wave at the longitude where the maximum zonal gradient of the thermocline is found (~ 120°W). Our analysis suggests that the confinement of CP El Niño warming in the central Pacific may result from the reinforcement of the zonal gradient in stratification associated with the La Niña‐like conditions since the late of the 1990s, leading to scattering of the energy of the ISKws in the eastern Pacific.
      PubDate: 2014-09-02T03:27:05.514268-05:
      DOI: 10.1002/2014JC010044
  • Seasonal variability of primary production and phytoplankton biomass in
           the western Pacific subarctic gyre: Control by light availability within
           the mixed layer
    • Authors: Kazuhiko Matsumoto; Makio C. Honda, Kosei Sasaoka, Masahide Wakita, Hajime Kawakami, Shuichi Watanabe
      Pages: n/a - n/a
      Abstract: A distinct seasonal variation of primary production was revealed from shipboard observations conducted from 2005 to 2013 at time‐series station K2 in the western Pacific subarctic gyre (WSG). The mean depth‐integrated primary production was highest (569 ± 162 mg C m–2 d–1) in summer and lowest (101 ± 16 mg C m–2 d–1) in winter. Strong winter mixing enriched the mixed layer (ML) with nutrients that were not fully consumed during the remainder of the year, the result being that the WSG was a high‐nutrient, low‐chlorophyll (HNLC) region. The deep ML reduced primary production by reducing light availability in winter, whereas primary production was enhanced by strong light availability in the shallower ML as summer progressed. However, primary production was often attenuated by a reduction of light availability attributable to dense sea fog in summer. We found a significant relationship between primary production and light availability in this HNLC region. However, chlorophyll a was less variable seasonally than primary production. The highest depth‐integrated chlorophyll a was observed in summer (54.6 ± 13.4 mg m–2), but chlorophyll a remained high in winter (45.3 ± 7.7 mg m–2). Reduced light availability depressed primary production, but a reduction of the chlorophyll a concentration was prevented by a relaxation of grazing in the deep ML during winter. We found that light availability exerted an important control on the seasonal variability of primary production and phytoplankton biomass in the WSG.
      PubDate: 2014-09-02T03:26:09.433622-05:
      DOI: 10.1002/2014JC009982
  • Meteotsunami‐tide interactions and high‐frequency sea level
           oscillations in the eastern Yellow Sea
    • Authors: Byoung‐Ju Choi; Chorong Hwang, Sang‐Ho Lee
      Pages: n/a - n/a
      Abstract: While an air pressure jump was moving southeastward over the shallow water region of the eastern Yellow Sea in March 2007, a long ocean wave (meteotsunami) was generated and amplified due to the Proudman resonance. The long wave arrived at the coast during high tide with wave amplitude of 1.4 m and seawater overflew seawalls and inundated the land. High‐frequency sea level oscillations continued for 8‐9 hours after the long wave hit a local coast. The Moon's age was 12 days, and the tidal range was about 4 m between neap and spring tides. Two‐dimensional numerical simulations were performed, to reproduce amplification of the long ocean wave in offshore and oscillations of sea level at the coast. Both tidal elevation and tidal currents were found to affect the growth of the long wave amplitude by the interactions between tides and the long wave. Long wave‐tides interactions are important processes for the accurate prediction of long wave arrival time and maximum height and for the reduction of coastal hazards in the macro‐tidal region. After the long wave hit the coast of remote regions, reflected waves propagated radially from remote regions to a local coast. The high‐frequency sea level oscillations at a local observation station continued, until all of the reflected waves at remote regions had passed by. It was concluded that high‐frequency oscillations of sea level are generated not only by local reflection of the long wave, but also by propagation of the reflected waves from remote regions.
      PubDate: 2014-09-02T03:21:22.846231-05:
      DOI: 10.1002/2013JC009788
  • The impact of the assimilation of Aquarius sea surface salinity data in
           the GEOS Ocean Data Assimilation System
    • Authors: G. Vernieres; R. Kovach, C. Keppenne, S. Akella, L. Brucker, E. Dinnat
      Pages: n/a - n/a
      Abstract: Ocean salinity and temperature differences drive thermohaline circulations. These properties also play a key role in the ocean‐atmosphere coupling. With the availability of L‐band space‐borne observations, it becomes possible to provide global scale sea surface salinity (SSS) distribution. This study analyzes globally the along‐track (Level 2) Aquarius SSS retrievals obtained using both passive and active L‐band observations. Aquarius along‐track retrieved SSS are assimilated into the ocean data assimilation component of Version 5 of the Goddard Earth Observing System (GEOS‐5) assimilation and forecast model. We present a methodology to correct the large biases and errors apparent in Version 2.0 of the Aquarius SSS retrieval algorithm and map the observed Aquarius SSS retrieval into the ocean model’s bulk salinity in the topmost layer. The impact of the assimilation of the corrected SSS on the salinity analysis is evaluated by comparisons with in‐situ salinity measurements from Argo. The results show a significant reduction of the global biases and RMS of observations‐minus‐forecast differences at in‐situ locations. The most striking results are found in the tropics and southern latitudes. Our results highlight the complementary role and problems that arise during the assimilation of salinity information from in‐situ (Argo) and space‐borne SSS retrievals.
      PubDate: 2014-08-30T11:44:40.548264-05:
      DOI: 10.1002/2014JC010006
  • Polar Front around the Kerguelen Islands: An up‐to‐date
           determination and associated circulation of surface/subsurface waters
    • Authors: Young‐Hyang Park; Isabelle Durand, Elodie Kestenare, Gilles Rougier, Meng Zhou, Francesco d’Ovidio, Cédric Cotté, Jae‐Hak Lee
      Pages: n/a - n/a
      Abstract: The circulation of iron‐rich shelf waters around the Kerguelen Islands plays a crucial role for a climatically important, annually‐recurrent phytoplankton spring bloom over the sluggish shelf region and its downstream plume area along the Antarctic circumpolar flow. However, there is a long‐standing confusion about the Polar Front (PF) in the Kerguelen region due to diverse suggestions in the literature for its geographical location with an extreme difference over 10° of latitude. Based on abundant historical hydrographic data, the in situ hydrographic and current measurements during the 2011 KEOPS2 cruise, satellite chlorophyll images, and altimetry‐derived surface velocity fields, we determine and validate an up‐to‐date location of the PF around the Kerguelen Islands. Artificial Lagrangian particle trajectories computed from altimetric velocity time series are analysed for the possible pathways and sources of different surface/subsurface waters advected into the chlorophyll bloom area east off the islands studied during the KEOPS2 cruise. The PF location determined as the northernmost boundary of the Winter Water colder than 2°C, which is also associated with a band of strong currents, appears to be primarily controlled by topography. The PF rounds the Kerguelen Islands from the south to deflect northward along the eastern escarpment up to the northeastern corner of the Kerguelen Plateau before making its southward retroflection. It is shown that the major surface/subsurface waters found within the deep basin east of the Kerguelen Islands originate from the shelf around the Heard Island, rather than from the shallow shelf north of the Kerguelen Islands.
      PubDate: 2014-08-30T11:20:29.471501-05:
      DOI: 10.1002/2014JC010061
  • On the temporal memory of coastal upwelling off NW Africa
    • Authors: Aïssa Benazzouz; Josep L. Pelegrí, Herve Demarcq, Francisco Machín, Evan Mason, Abdellatif Orbi, Jesus Peña‐Izquierdo, Mordane Soumia
      Pages: n/a - n/a
      Abstract: We use a combination of satellite, in situ and numerical data to provide a comprehensive view of the seasonal coastal upwelling cycle off NW Africa in terms of both wind forcing and sea surface temperature (SST) response. Wind forcing is expressed in terms of both instantaneous (local) and time‐integrated (non‐local) indices, and the ocean response is expressed as the SST difference between coastal and offshore waters. The classical local index, the cross‐shore Ekman transport, reproduces reasonably well the time‐latitude distribution of SST differences but with significant time lags at latitudes higher than Cape Blanc. Two non‐local indices are examined. One of them, a cumulative index calculated as the backward averaged Ekman transport that provides the highest correlation with SST differences, reproduces well the timing of the SST differences at all latitudes (except near Cape Blanc). The corresponding time lags are close to zero south of Cape Blanc and range between two and four months at latitudes between Cape Blanc and the southern Gulf of Cadiz. The results are interpreted based on calculations of spatial and temporal auto‐ and cross‐correlations for wind forcing and SST differences. At temporal scales of 2‐3 weeks the along‐shore advection of along‐shore momentum compensates for interfacial friction, allowing the upwelling jet and associated frontal system to remain active. We conclude that the coastal jet plays a key role in maintaining the structure of coastal upwelling, even at times of relaxed winds, by introducing a seasonal memory to the system in accordance with the atmospheric‐forcing annual cycle.
      PubDate: 2014-08-30T11:17:16.417032-05:
      DOI: 10.1002/2013JC009559
  • Interannual variability of wintertime temperature on the inner continental
           shelf of the Middle Atlantic Bight
    • Authors: Thomas P. Connolly; Steven J. Lentz
      Pages: n/a - n/a
      Abstract: The shallow depth of the inner continental shelf allows for rapid adjustment of the ocean to air‐sea exchange of heat and momentum compared with offshore locations. Observations during 2001‐2013 are used to evaluate the contributions of air‐sea heat flux and oceanic advection to interannual variability of inner‐shelf temperature in the Middle Atlantic Bight. Wintertime processes are important for interpreting regional interannual variability at nearshore locations since winter anomalies account for 69–77% of the variance of the annual anomalies and are correlated over broad alongshelf scales, from New England to North Carolina. At the Martha's Vineyard Coastal Observatory on the 12‐m isobath, a heat budget is used to test the hypothesis that interannual differences in winter temperatures are due solely to air‐sea heat flux. Bimonthly averages of air‐sea heat flux are correlated with temporal changes in temperature, but overestimate the observed wintertime cooling. Velocity and satellite‐derived temperature data show that interannual variability in wintertime surface cooling is partially compensated for by alongshore advection of warmer water from the west at this particular location. It is also shown that surface heat flux is a strong function of air‐sea temperature difference. Because of this coupling between ocean and air temperatures in shallow water, along‐shelf advection can significantly modify the surface heat flux at seasonal and interannual time scales. While alongshelf advection at relatively small (˜100‐km) scales can be an important component of the heat budget over the inner shelf, interannual temperature variability is still largely determined by adjustment to large‐scale air‐temperature anomalies.
      PubDate: 2014-08-30T04:28:38.691208-05:
      DOI: 10.1002/2014JC010153
  • Sea surface salinity under rain cells: SMOS satellite and in situ drifters
    • Authors: J. Boutin; N. Martin, G. Reverdin, S. Morisset, X. Yin, L. Centurioni, N. Reul
      Pages: n/a - n/a
      Abstract: We study the signature of rainfall on S1cm, the sea surface salinity retrieved from the Soil Moisture and Ocean Salinity (SMOS) satellite mission first by comparing SMOS S1cm with ARGO sea surface salinity measured at about 5 m depth in the Intertropical Convergence Zone (ITCZ) and in the Southern Pacific Convergence Zone; second by investigating spatial variability of SMOS S1cm related to rainfall. The resulting estimated S1cm decrease associated with rainfall occurring within less than 1 h from the salinity measurement is close to −0.2 pss (mm h−1) −1. We estimate that rain induced roughness and atmospheric effects are responsible for no more than 20% of this value. We also study the signature of rainfall on sea surface salinity measured by surface drifters at 45 cm depth and find a decrease associated with rainfall of −0.21 (±0.14) pss (mm h−1) −1, consistent with SMOS observations. When averaged over one month, this rain associated salinity decrease is at most −0.2 in monthly 100 × 100 km2 pixels, and at most 40% of the difference between SMOS S1cm and interpolated in situ bulk salinity in pixels near the ITCZ. This suggests that more than half of this difference is related to the in situ products obtained from optimal interpolation and therefore influenced by smoothing and relaxation to climatology. Finally, further studies on the satellite‐derived salinities should pay attention to that as well as to other sources of uncertainties in satellite measurements and not interpret fully the observed differences between in situ and satellite mapped products, as rain induced SSS variability.
      PubDate: 2014-08-28T10:37:00.094574-05:
      DOI: 10.1002/2014JC010070
  • High‐resolution modeling of the Eastern Tropical Pacific oxygen
           minimum zone: Sensitivity to the tropical oceanic circulation
    • Authors: Ivonne Montes; Boris Dewitte, Elodie Gutknecht, Aurélien Paulmier, Isabelle Dadou, Andreas Oschlies, Véronique Garçon
      Pages: n/a - n/a
      Abstract: The connection between the equatorial mean circulation and the oxygen minimum zone (OMZ) in the Eastern Tropical Pacific is investigated through sensitivity experiments with a high‐resolution coupled physical‐biogeochemical model. A validation against in situ observations indicates a realistic simulation of the vertical and horizontal oxygen distribution by the model. Two sets of climatological open‐boundary conditions for the physical variables, which differ slightly with respect to the intensity and vertical structure of the Equatorial Current System, are shown to lead to contrasting characteristics of the simulated OMZ dynamics. From a Lagrangian perspective, the mean differences near the coast originate to a large extent from the different transport of deoxygenated waters by the secondary Tsuchiya Jet (secondary Southern Subsurface Countercurrent, sSSCC). The O2 budget further indicates a large difference in the balance between tendency terms, with advection exhibiting the largest difference between both simulations, which is shown to result from both linear and nonlinear advection. At regional scale, we also find that the variability of the physical contribution to the rate of O2 change is one order of magnitude larger than the variability associated with the biogeochemical contribution, which originates from internal high‐frequency variability. Overall our study illustrates the large sensitivity of the OMZ dynamics to the equatorial circulation.
      PubDate: 2014-08-27T12:57:35.821886-05:
      DOI: 10.1002/2014JC009858
  • Impact of assimilating surface salinity from SMOS on ocean circulation
    • Authors: A. Köhl; M. Sena Martins, D. Stammer
      Pages: n/a - n/a
      Abstract: In a pilot attempt, the GECCO2 synthesis system is being used to investigate the impact of SMOS sea surface salinity (SSS) observations on estimates of SSS and freshwater fluxes. The paper focuses on the period 2010–2011, during which, in addition to traditional in situ and satellite observations, SMOS SSS is assimilated. A prior SMOS SSS error field is inferred through a comparison of the satellite data with in situ salinity data and reveals large biases (>1 g/kg) in the SMOS product near continents and in the Southern Ocean. Employing this error estimate in the assimilation procedure leads only to an insignificant impact of SMOS SSS on the estimated ocean state. However, when reducing the error artificially by a factor of 10, the SMOS data can be reproduced well in the interior ocean. In this case, the previously remaining positive model bias with respect to in situ salinity is changed to a negative bias while the misfit slightly increased. The clear freshening can be attributed to the SMOS bias with respect to in situ data. The associated increase in freshwater input in the tropical oceans enhances slightly the correspondence of the estimated fluxes to the independent satellite‐based estimate from HOAPS except for the South Pacific and South Atlantic. On short‐timescales, changes in the estimated surface salinity result primarily from changes in surface freshwater fluxes, while over longer periods ocean dynamics become increasingly more important for changing the near‐surface salinity.
      PubDate: 2014-08-26T13:27:10.618481-05:
      DOI: 10.1002/2014JC010040
  • Diel variability of the beam attenuation and backscattering coefficients
           in the northwestern Mediterranean Sea (BOUSSOLE site)
    • Authors: Malika Kheireddine; David Antoine
      Pages: n/a - n/a
      Abstract: The diel variability of the particulate beam attenuation coefficient, cp, and of the particulate backscattering coefficient, bbp, were investigated during five seasonal cycles at an oceanic site in the northwestern Mediterranean Sea, covering contrasting physical and trophic situations. We observed a diel cycle in cp and bbp, related to changes in phytoplankton properties (i.e., size and refractive index) induced by the accumulation of carbon within phytoplankton cells associated with photosynthetic processes, during the winter mixing of the water column, the development of the spring phytoplankton bloom, its decline, and during the summer oligotrophy. The relative amplitude of the cp diel variability was much larger during the spring bloom (20–50%) than during other seasons (10–20%), whereas that of bbp is steadily around 20% and does not show significant seasonal variability. The minimal cp and bbp occurred at sunrise and are synchronized, whereas maximum bbp values are often reached 3–6 h before those for cp (except during bloom conditions), which occur near sunset. These different amplitudes and timing are tentatively explained using Mie computations, which allow discerning the respective roles of changes in the particle size distribution and refractive index. The differences observed here in the diel cycles of cp and bbp show that they cannot be used interchangeably to determine the daily increase of the particle pool. This result has implications on the feasibility to determine net community production from the bbp diel changes, when only bbp is measured in situ or available from ocean color observations.
      PubDate: 2014-08-26T13:25:10.453849-05:
      DOI: 10.1002/2014JC010007
  • Seasonal salinity stratifications in the near‐surface layer from
           Aquarius, Argo, and an ocean model: Focusing on the tropical
           Atlantic/Indian Oceans
    • Authors: Jae‐Hong Moon; Y. Tony Song
      Pages: n/a - n/a
      Abstract: A newly available sea surface salinity (SSS) measurement from Aquarius/SAC‐D satellite reveals strong seasonal variability in the tropical Atlantic and Indian Oceans. The seasonal SSS variability at skin layer differs/agrees regionally in their amplitude from/with Argo‐measured salinity at 5 m depth and model salinity at the top layer, indicating various characteristics of near‐surface salinity stratifications. By comparing the three different salinity products, we have examined the near‐surface salinity stratifications with emphasis on the dynamical processes that differ from one region to another. Our analysis shows that for the western part of tropical Atlantic and southern tropical Indian, a large amount of river runoff and/or surface freshwater significantly stratifies the surface layer above 5m depth, resulting in the differences among the Aquarius, Argo and model. Differently for the southern Arabian Sea, the surface water can be mixed down to the depth of 5 m due to seasonally reversing currents driven by monsoons, resulting in an agreement among the datasets. The comparison suggests that dynamical differences can lead to different vertical salinity stratifications locally, which explain the differences between the Aquarius observations in the first cm of the sea surface, the Argo measurements at the 5m depth, and model’s representation of the surface‐layer averaged salinity.
      PubDate: 2014-08-26T11:12:06.537099-05:
      DOI: 10.1002/2014JC009969
  • ENSO‐driven interhemispheric Pacific mass transports
    • Authors: Shayne McGregor; Paul Spence, Franziska U. Schwarzkopf, Matthew H. England, Agus Santoso, William S. Kessler, Axel Timmermann, Claus W. Böning
      Pages: n/a - n/a
      Abstract: Previous studies have shown that ENSO’s anomalous equatorial winds, including the observed southward shift of zonal winds that occurs around the event peak, can be reconstructed with the first two Empirical Orthogonal Functions (EOFs) of equatorial region wind stresses. Using a high resolution Ocean General Circulation Model we investigate the effect of these two EOFs on changes in warm water volume (WWV), interhemispheric mass transports and Indonesian Throughflow (ITF). Wind stress anomalies associated with the first EOF produce changes in WWV that are dynamically consistent with the conceptual recharge oscillator paradigm. The ITF is found to heavily damp these WWV changes, reducing their variance by half. Wind stress anomalies associated with the second EOF, which depicts the southward wind shift, are responsible for WWV changes that are of comparable magnitude to those driven by the first mode. The southward wind shift is also responsible for the majority of the observed interhemispheric upper ocean mass exchanges. These winds transfer mass between the Northern and the Southern Hemisphere during El Niño events. Whilst water is transferred in the opposite direction during La Niña events, the magnitude of this exchange is roughly half of that seen during El Niño events. Thus, the discharging of WWV during El Niño events is meridionally asymmetric, while the WWV recharging during a La Niña event is largely symmetric. The inclusion of the southward wind shift is also shown to allow ENSO to exchange mass with much higher latitudes than that allowed by the first EOF alone.
      PubDate: 2014-08-26T11:12:00.726183-05:
      DOI: 10.1002/2014JC010286
  • Summertime Changjiang River plume variation during 1998–2010
    • Authors: Yan Bai; Xianqiang He, Delu Pan, Chen‐Tung Arthur Chen, Yan Kang, Xiaoyan Chen, Wei‐Jun Cai
      Pages: n/a - n/a
      Abstract: Using an improved satellite‐derived salinity algorithm in the East China Sea (ECS), we presented and examined a general view on summertime Changjiang River plume variation during 1998–2010. Three types of plume shapes were identified: 1) the commonly known northeastward transportation, 2) a case in which most of the plume water crossed the Cheju Strait into the Tsushima‐Korea Straits with only a small fraction staying on the shelf of the ECS, and 3) a rare case in which the plume front moved southeastward. Satellite time‐series data suggested that, during the peak river discharge time in July with favorable southwest monsoon, the plume area was highly correlated with the river discharge of the same month. Interestingly, the plume area in August was also dominated by the discharge in July. In August, as the direct effect of freshwater discharge weakening, the plume area also became positively correlated with wind speed in the 45o and 60odirection, suggesting that the plume extension was more influenced by the southwesterly wind during periods of smaller discharge. Furthermore, a few special cases with unique plume extensions were found under extreme weather conditions. Finally, we found no significant long‐term trend of plume area change over 1998–2010 in summertime and concluded that the interannual variation was probably regulated by natural variation rather than anthropogenic effects, such as construction of the Three Gorges Dam. This study will have implications for biogeochemical and modeling studies in large river plume areas.
      PubDate: 2014-08-26T11:08:08.287214-05:
      DOI: 10.1002/2014JC009866
  • Evaluation of directly wind‐coherent near‐inertial surface
           currents off Oregon using a statistical parameterization and analytical
           and numerical models
    • Authors: Sung Yong Kim; P. Michael Kosro, Alexander L. Kurapov
      Pages: n/a - n/a
      Abstract: Directly wind‐coherent near‐inertial surface currents off the Oregon coast are investigated with a statistical parameterization of observations and outputs of a regional numerical ocean model and three one‐dimensional analytical models including the slab layer, Ekman, and near‐surface averaged Ekman models. The transfer functions and response functions, statistically estimated from observed wind stress at NDBC buoys and surface currents derived from shoredbased high‐frequency radars, enable us to isolate the directly wind‐forced near‐inertial surface currents. Concurrent observations of the wind and currents are crucial to evaluate the directly wind‐forced currents. Thus, the wind stress and surface current fields obtained from a regional ocean model, which simulates variability of the wind and surface currents on scales comparable to those in observations, are analyzed with the same statistical parameterization to derive the point‐by‐point transfer functions and response functions. Model and data comparisons suggest that the regional ocean model describe near‐inertial variability of surface currents in quality and quantity correctly. The estimated response functions exhibit decay time scales in a range of 3 to 5 days, and about 40% of near‐inertial motions are explained by local wind stress. Among the one‐dimensional analytical models, the near‐surface averaged Ekman model explains the statistically derived wind‐current relationship better than other analytical models.
      PubDate: 2014-08-26T10:55:23.696434-05:
      DOI: 10.1002/2014JC010115
  • Distribution of vertical velocity inferred from secondary flow in a curved
           tidal channel
    • Authors: P. Russell; R. Vennell
      Pages: n/a - n/a
      Abstract: High resolution observations from the curved tidal channel of the Otago Harbour show secondary flows up to 20 % of the primary flow and vertical velocity inferred from secondary flow up to 1% of the primary flow. This vertical velocity is inferred on a much finer scale than previous works. The spatial pattern of this vertical velocity is upwards on the inside and downwards on the outside of the bend, consistent with previous laboratory flume measurements. Linear regression, rp = 0.95, shows the cross‐channel distribution of the observed secondary flow can be resolved from the horizontal ADCP measurements well enough to be consistent with the cross‐channel distribution of secondary flow derived from the observed primary flow using the model of Kalkwijk and Booij [1986]. Linear regression, rp = 0.80, shows the vertical velocity inferred from observed secondary flow is consistent with vertical velocity derived from the observed primary flow using the model of Kalkwijk and Booij [1986]. This also shows the cross‐channel distribution of the observed secondary flow is resolved well enough from the horizontal ADCP measurements to be able to infer the vertical velocity from mass continuity. The required horizontal resolution is made possible by Radial Basis Function (RBF) smoothing and spatial interpolation that allows for continuity of the spatial derivatives. Modeling trajectories using channel dimensions and velocity field values equivalent to the observations show that 3‐dimensional secondary circulation forms a loose helical flow pattern.
      PubDate: 2014-08-26T10:53:10.901529-05:
      DOI: 10.1002/2014JC010003
  • Direct velocity observations of volume flux between Iceland and the
           Shetland Islands
    • Authors: Katelin H. Childers; Charles N. Flagg, Thomas Rossby
      Pages: n/a - n/a
      Abstract: Atlantic Waters flowing northward into the Nordic Seas are important for their role as an early indicator of changes to deepwater formation. As such this requires a fundamental understanding of the pathways and volume fluxes through the primary passageways from the Atlantic into the Nordic Seas. A mean annual volume transport of 6.1±0.3 Sv was observed flowing in above the σt =27.8 isopycnal (a proxy for the lower limit of Atlantic Water depth), through the Faroe Shetland Channel (FSC) and over the Iceland Faroes Ridge (IFR) from March 2008 through June 2012, using repeat velocity sections obtained from a vessel mounted Acoustic Doppler Current Profiler (ADCP). A new vessel route has expanded the spatial coverage of FSC observations and reveals a difference in average inflow transport, which most likely results from an interannual variation in the total transport through the FSC, which in turn is tied to a weakening of the southerly flow over the western slope of the channel. This interannual variability has increased the mean transport through the FSC from 0.9 Sv observed over the first two years of this program by Rossby and Flagg [2012] to a 4.5 year mean of 1.7±0.2 Sv, which emphasizes the importance of knowing the flow along the Faroese shelf. Interannual fluctuations in transport observed over the IFR are related to the width of the inflow over the Faroese half of the ridge.
      PubDate: 2014-08-25T10:12:37.453855-05:
      DOI: 10.1002/2014JC009946
  • Internal tide radiation from the Luzon Strait
    • Authors: Zhongxiang Zhao
      Pages: n/a - n/a
      Abstract: The M2, K1, and O1 internal tides originating in the Luzon Strait are investigated using the sea surface height measurements by multiple satellites ERS‐2, Envisat, TOPEX/Poseidon, Jason‐1/2, and Geosat Follow‐On. A plane wave fit method is used to resolve multiple internal tides in arbitrary horizontal directions. The Luzon Strait is an energetic internal tide generation site, and radiates internal tides both westward into the South China Sea (SCS) and eastward into the western Pacific (WP). In the SCS, the K1 and O1 internal tides propagate over 1600 km, reaching the Vietnam coast; in the WP, they propagate over 2500 km and arrive to the Mariana Ridge and Guam. The K1 and O1 internal tides refract toward the Equator during propagation. The M2 internal tides in the SCS bifurcate into two beams. The northwestward beam is coincident with the frequent occurrence of internal solitary waves in this region, implying their causative relation. The phase speeds inferred from the altimetric along‐beam propagation agree with the theoretical values. Due to the influence of the Earth's rotation, the K1 and O1 phase speeds decrease remarkably from high to low latitudes. For the diurnal internal tides, the eastward radiation is about 50% greater than the westward radiation. For M2, the westward radiation is about two times the eastward radiation. The altimetric energy fluxes are about 50% of those in numerical model simulations.
      PubDate: 2014-08-25T09:41:28.290948-05:
      DOI: 10.1002/2014JC010014
  • Volume transport of the Antarctic Circumpolar Current: Production and
           validation of a 20 year long time series obtained from in situ and
           satellite observations
    • Authors: Zoé Koenig; Christine Provost, Ramiro Ferrari, Nathalie Sennéchael, Marie‐Hélène Rio
      Pages: n/a - n/a
      Abstract: A 20 year long volume transport time series of the Antarctic Circumpolar Current across the Drake Passage is estimated from the combination of information from in situ current meter data (2006–2009) and satellite altimetry data (1992–2012). A new method for transport estimates had to be designed. It accounts for the dependence of the vertical velocity structure on surface velocity and latitude. Yet unpublished velocity profile time series from Acoustic Doppler Current Profilers are used to provide accurate vertical structure estimates in the upper 350 m. The mean cross‐track surface geostrophic velocities are estimated using an iterative error/correction scheme to the mean velocities deduced from two recent mean dynamic topographies. The internal consistency and the robustness of the method are carefully assessed. Comparisons with independent data demonstrate the accuracy of the method. The full‐depth volume transport has a mean of 141 Sv (standard error of the mean 2.7 Sv), a standard deviation (std) of 13 Sv, and a range of 110 Sv. Yearly means vary from 133.6 Sv in 2011 to 150 Sv in 1993 and standard deviations from 8.8 Sv in 2009 to 17.9 Sv in 1995. The canonical ISOS values (mean 133.8 Sv, std 11.2 Sv) obtained from a year‐long record in 1979 are very similar to those found here for year 2011 (133.6 Sv and 12 Sv). Full‐depth transports and transports over 3000 m barely differ as in that particular region of Drake Passage the deep recirculations in two semiclosed basins have a close to zero net transport.
      PubDate: 2014-08-25T09:40:52.408652-05:
      DOI: 10.1002/2014JC009966
  • Storm tracks in the Southern Hemisphere subtropical oceans
    • Authors: T. J. O’Kane; R. J. Matear, M. A. Chamberlain, E. C. J. Oliver, N. J. Holbrook
      Pages: n/a - n/a
      Abstract: Ocean storm tracks have previously been associated with the mid‐latitude western boundary currents (WBCs) and the Antarctic Circumpolar Current (ACC). Here we identify and examine large‐scale baroclinically unstable waves occurring within waveguides associated with potential density gradients in the subtropical regions of the Southern Hemisphere (SH) oceans where the trade winds and westerlies meet and at depths associated with mode water formation. In contrast to the Northern Hemisphere subtropics, the SH pathways are more extensive allowing large scale coherent disturbances to communicate information westward from the mid‐latitudes to the subtropics (South Pacific Ocean) and from the subtropics to the tropics (Indian Ocean). Particular consideration is given to the subtropical South Pacific Ocean as this is a region where resonant interactions between large‐scale Rossby waves and significant topographic features have been reported to occur. Using an ocean general circulation model and a simple potential energy transfer diagnostic we identify the relevant nonlinearly modified structures comparing their propagation characteristics to planetary Rossby waves calculated using a shallow water model. Although at first appearance baroclinic disturbances resemble planetary Rossby waves, we show they are inherently nonlinear, multi‐scale and are amplified where topography occurs. The location of the disturbances coincides with regions of high variability in sea surface height observed in satellite altimetry and their speeds closely match the large‐scale coherent westward propagating structures described in the observational literature. Our study provides evidence that, in addition to the mid‐latitude WBCs and the ACC, significant ocean storm tracks are also manifest in the SH subtropics.
      PubDate: 2014-08-23T04:43:21.763814-05:
      DOI: 10.1002/2014JC009990
  • Interannual variability of the eastward current in the western South China
           Sea associated with the summer Asian monsoon
    • Authors: Changlin Chen; Guihua Wang
      Pages: n/a - n/a
      Abstract: The interannual variability of the eastward current in the western South China Sea (SCS) during the summer of 1993‐2012 is examined with satellite altimeter data and Regional Ocean Modeling System (ROMS) model output. It is found that the meridional location of the eastward current displays apparent interannual variability. The core of the eastward currentshifts between 10.7°N and 17.6°N with a standard deviation of 1.6°. Results from Sverdrup theory and ROMS experiments demonstrate a close dynamic linkage between the north‐south migration of the eastward current and the SCS summer monsoon anomaly on the interannual time scale. When the summer monsoon has southwesterly (northeasterly) anomaly, the eastward current moves southward (northward). With the southward (northward) shift of the eastward current, the summer cold filament in the SCS moves southward (northward) as well.
      PubDate: 2014-08-23T04:40:28.57138-05:0
      DOI: 10.1002/2014JC010309
  • The diurnal salinity cycle in the tropics
    • Authors: Kyla Drushka; Sarah T. Gille, Janet Sprintall
      Pages: n/a - n/a
      Abstract: Observations from 35 tropical moorings are used to characterize the diurnal cycle in salinity at 1‐m depth. The amplitude of diurnal salinity anomalies is up to 0.01 psu and more typically ~0.005 psu. Diurnal variations in precipitation and vertical entrainment appear to be the dominant drivers of diurnal salinity variability, with evaporation also contributing. Areas where these processes are strong are expected to have relatively strong salinity cycles: the eastern Atlantic and Pacific equatorial regions, the southwestern Bay of Bengal, the Amazon outflow region, and the Indo‐Pacific warm pool. We hypothesize that salinity anomalies resulting from precipitation and evaporation are initially trapped very near the surface and may not be observed at the 1‐m instrument depths until they are mixed downward. As a result, the pattern of diurnal salinity variations is not only dependent on the strength of the forcing terms, but also on the phasing of winds and convective overturning. A comparison of mixed‐layer depth computed with hourly and with daily‐averaged salinity reveals that diurnal salinity variability can have a significant effect on upper‐ocean stratification, suggesting that representing diurnal salinity variability could potentially improve air‐sea interaction in climate models. Comparisons between salinity observations from moorings and from the Aquarius satellite (level 2 version 2.5.1 data) reveal that the typical difference between ascending‐node and descending‐node Aquarius salinity is an order of magnitude greater than the observed diurnal salinity anomalies at 1‐m depth.
      PubDate: 2014-08-23T04:38:16.525451-05:
      DOI: 10.1002/2014JC009924
  • The emission and scattering of L‐band microwave radiation from rough
           ocean surfaces and wind speed measurements from the Aquarius sensor
    • Authors: Thomas Meissner; Frank J. Wentz, Lucrezia Ricciardulli
      Pages: n/a - n/a
      Abstract: In order to achieve the required accuracy in sea surface salinity (SSS) measurements from L‐band radiometers such as the Aquarius/SAC‐D or SMOS (Soil Moisture and Ocean Salinity) mission, it is crucial to accurately correct the radiation that is emitted from the ocean surface for roughness effects. We derive a geophysical model function (GMF) for the emission and backscatter of L‐band microwave radiation from rough ocean surfaces. The analysis is based on radiometer brightness temperature and scatterometer backscatter observations both taken on board Aquarius. The data are temporally and spatially collocated with wind speeds from WindSat and F17 SSMIS (Special Sensor Microwave Imager Sounder) and wind directions from NCEP (National Center for Environmental Prediction) GDAS (Global Data Assimilation System). This GMF is the basis for retrieval of ocean surface wind speed combining L‐band H‐pol radiometer and HH‐pol scatterometer observations. The accuracy of theses combined passive/active L‐band wind speeds matches those of many other satellite microwave sensors. The L‐band GMF together with the combined passive/active L‐band wind speeds are utilized in the Aquarius SSS retrieval algorithm for the surface roughness correction. We demonstrate that using these L‐band wind speeds instead of NCEP wind speeds leads to a significant improvement in the SSS accuracy. Further improvements in the roughness correction algorithm can be obtained by adding VV‐pol scatterometer measurements and wave height (WH) data into the GMF.
      PubDate: 2014-08-23T04:05:33.650966-05:
      DOI: 10.1002/2014JC009837
  • Effect of mesoscale eddies and streamers on sardine spawning habitat and
           recruitment success off southern and central California
    • Authors: Karen Nieto; Sam McClatchie, Edward D. Weber, Cleridy E. Lennert‐Cody
      Pages: n/a - n/a
      Abstract: We quantified the effect of mesoscale eddies and streamers on the spatial distribution of Pacific sardine spawning habitat using a merged altimetry dataset and a statistical spawning habitat model. The distribution of eggs could be predicted using sea‐surface temperature, chlorophyll concentration, and eddy kinetic energy (EKE) similarly to previous studies. Eddies alone did not have a significant additional or emergent effect on the probability of capturing eggs beyond these predictors. Rather, mesoscale features (eddies and streamers) entrained water with the appropriate conditions in terms of temperature, chlorophyll, and EKE. These dynamic features moved appropriate spawning habitat for sardine offshore to areas where appropriate habitat otherwise would not exist. Using centroids of predicted sardine habitat, we showed that sardine recruitment success was inversely correlated with distance from shore of predicted sardine habitat centroids. This indicates that offshore transport has a negative effect on sardine recruitment, despite expanding favorable spawning habitat further offshore.
      PubDate: 2014-08-22T04:19:02.575041-05:
      DOI: 10.1002/2014JC010251
  • Aquarius geophysical model function and combined active passive algorithm
           for ocean surface salinity and wind retrieval
    • Authors: Simon Yueh; Wenqing Tang, Alexander Fore, Akiko Hayashi, Yuhe T. Song, Gary Lagerloef
      Pages: n/a - n/a
      Abstract: This paper describes the updated Combined Active‐Passive (CAP) retrieval algorithm for simultaneous retrieval of surface salinity and wind from Aquarius' brightness temperature and radar backscatter. Unlike the algorithm developed by Remote Sensing Systems (RSS), implemented in the Aquarius Data Processing System (ADPS) to produce Aquarius standard products, the Jet Propulsion Laboratory's CAP algorithm does not require monthly climatology SSS maps for the salinity retrieval. Furthermore, the ADPS‐RSS algorithm fully uses the National Center for Environmental Predictions (NCEP) wind for data correction, while the CAP algorithm uses the NCEP wind only as a constraint. The major updates to the CAP algorithm include the galactic reflection correction, Faraday rotation, Antenna Pattern Correction, and geophysical model functions of wind or wave impacts. Recognizing the limitation of geometric optics scattering, we improve the modeling of the reflection of galactic radiation; the results are better salinity accuracy and significantly reduced ascending‐descending bias. We assess the accuracy of CAP's salinity by comparison with ARGO monthly gridded salinity products provided by the Asia‐Pacific Data‐Research Center (APDRC) and Japan Agency for Marine‐Earth Science and Technology (JAMSTEC). The RMS differences between Aquarius CAP and APDRC's or JAMSTEC's ARGO salinities are less than 0.2 psu for most parts of the ocean, except for the regions in the Intertropical Convergence Zone, near the outflow of major rivers and at high latitudes.
      PubDate: 2014-08-21T14:49:18.765692-05:
      DOI: 10.1002/2014JC009939
  • Evidence of Mediterranean Water dipole collision in the Gulf of Cadiz
    • Authors: Pierre L'Hégaret; Xavier Carton, Isabel Ambar, Claire Ménesguen, Bach Lien Hua, Laurent Chérubin, Ana Aguiar, Bernard Cann, Nathalie Daniault, Nuno Serra
      Pages: n/a - n/a
      Abstract: A collision of Mediterranean Water dipoles in the Gulf of Cadiz is studied here, using data from the MedTop and Semane experiments. First, a Mediterranean Water eddy (meddy) was surveyed hydrologically in November 2000 southwest of Cape Saint Vincent. Then, this meddy drifted northeastward from this position, accompanied by a cyclone (detected only via altimetry), thus forming a first dipole. In February 2001, a dipole of Mediterranean Water was measured hydrologically just after its formation near Portimão Canyon. This second dipole drifted southwestward. The western and eastern meddies had hydrological radii of about 22 and 25 km, respectively, with corresponding temperature and salinity maxima of (13.45°C, 36.78) and (11.40°C, 36.40). Rafos float trajectories and satellite altimetry indicate that these two dipoles collided early April 2001, south of Cape Saint Vincent, near 35°30′N, 10°15′W. More precisely, the eastern meddy wrapped around the western one. This merger resulted in an anticyclone (a meddy) which drifted southeastward, coupled with the eastern cyclone. Hydrological sections across this final third resulting dipole, performed in July 2001 in the southern Gulf of Cadiz, confirm this interaction: the thermohaline characteristics of the final meddy can be tracked back to the original structures. The subsequent evolution of this dipole was analyzed with Rafos float trajectories. A numerical simulation of the interaction between the two earlier dipoles is also presented. We suggest that these dipole collisions at the Mediterranean Water level may represent a mechanism of generation of the larger meddies that finally leave the Gulf of Cadiz.
      PubDate: 2014-08-21T14:11:49.502179-05:
      DOI: 10.1002/2014JC009972
  • Quad‐polarization SAR features of ocean currents
    • Authors: V. Kudryavtsev; I. Kozlov, B. Chapron, J. A. Johannessen
      Pages: n/a - n/a
      Abstract: A methodology is demonstrated to exploit the polarization sensitivity of high‐resolution radar measurements to interpret and quantify upper ocean dynamics. This study particularly illustrates the potential of quad‐polarization synthetic aperture radar (SAR) measurements. The analysis relies on essential characteristics of the electromagnetic scattering mechanisms and hydrodynamical principles. As the relaxation scale of centimeter‐scale ocean surface scatters is typically small, radar signal anomalies associated with surface manifestations of the upper ocean dynamics on spatial scales exceeding 100 m are mostly dominated by non‐resonant and non‐polarized scatters. These “scalar” contributions can thus efficiently trace local breaking and near‐breaking areas, caused by surface current variations. Using dual co‐polarized measurements, the polarized Bragg‐type radar scattering is isolated by considering the difference (PD) between vertically and horizontally polarized radar signals. The non‐polarized (NP) contribution associated with wave breaking is then deduced, using the measured polarization ratio (PR) between polarized signals. Considering SAR scenes depicting various surface manifestations of the upper ocean dynamics (internal waves, meso‐scale surface current features, SST front), the proposed methodology and set of decompositions (PD, PR, NP) efficiently enable the discrimination between surface manifestation of upper ocean dynamics and wind field variability. Applied to quad‐polarized SAR images, such decompositions further provide unique opportunities to more directly assess the cross‐polarized (CP for HV or VH) signal sensitivity to surface roughness changes. As demonstrated, such an analysis unambiguously demonstrates and quantitatively evaluates the relative impact of breakers on cross‐polarized signals under low to moderate wind conditions.
      PubDate: 2014-08-21T04:03:42.140894-05:
      DOI: 10.1002/2014JC010173
  • Winter sea ice melting in the Atlantic Water subduction area, Svalbard
    • Authors: V. Tverberg; O. A. N⊘st, C. Lydersen, K. M. Kovacs
      Pages: n/a - n/a
      Abstract: Herein we study a small area along the shelf west of Spitsbergen, near Prins Karls Forland, where warm, saline Atlantic Water of the West Spitsbergen Current currently first encounters sea ice. This sea ice is drifting in a coastal current that carries Arctic Water originating from the Barents Sea northward over the shelf. Our aim was to investigate whether melting of sea ice by Atlantic Water in this area might be a significant factor that could contribute to the formation of a cold halocline layer that isolates the sea ice from further melting from below. Observations of temperature and salinity profiles were collected during two winters, via CTD‐SRDL instruments deployed on harbor seals (Phoca vitulina), and fed into a heat and freshwater budget box model in order to quantify the importance of melting relative to other processes that could transform the shelf water mass during winter. Cross frontal exchange of Atlantic Water from the West Spitsbergen Current, driven by buoyancy forcing rather than Ekman upwelling, was determined to be the source of the heat that melted drift ice on the shelf. Some local sea ice formation did take place, but its importance in the total heat and freshwater budgets appeared to be minor. The data suggest that the production of a cold halocline layer was preceded by southerly winds and rapid drift ice melting.
      PubDate: 2014-08-21T03:54:07.624775-05:
      DOI: 10.1002/2014JC010013
  • Monitoring the occurrence of seasonal low‐oxygen events off the
           Changjiang Estuary through integration of remote sensing, buoy
           observations, and modeling
    • Authors: Jianyu Chen; Xiaobo Ni, Mingliang Liu, Jianfang Chen, Zhihua Mao, Haiyan Jin, Delu Pan
      Pages: n/a - n/a
      Abstract: Bottom water hypoxia occurs frequently during the summer off the Changjiang Estuary. To estimate its spatial extent and investigate how climatic variations and extreme events may affect its occurrence, we developed a regional statistical model that combines satellite and buoy observations via empirical and statistical relationships. The estimated results were validated using cruise data off the Changjiang Estuary and its adjacent areas. First, we quantified the linkage between the observed dissolved oxygen (DO) concentrations in the bottom water and the chlorophyll‐a (Chl‐a) concentrations at the surface. With the help of the model, the bottom layer DO concentrations in the region lacking in situ measurements were estimated using the observations near the buoys and the satellite‐derived Chl‐a concentrations. Comparisons between the modeled results and surveys conducted between 2006 and 2011 indicate that the error of the estimated extent of low‐oxygen bottom water was less than 26% and that the bias in the estimated minimum DO concentration was less than 0.5 mg L−1. Both buoy observations and modeled results indicate that the strength of water stratification and the amount of labile organic matter added to the bottom water are the two main factors that control the occurrence and the magnitude of seasonal low‐oxygen events off the Changjiang Estuary.
      PubDate: 2014-08-20T13:50:42.469134-05:
      DOI: 10.1002/2014JC010333
  • Hydrography and circulation in the Filchner Depression, Weddell Sea,
    • Authors: E. Darelius; K. Makinson, K. Daae, I. Fer, P. R. Holland, K. W. Nicholls
      Pages: n/a - n/a
      Abstract: Cold and dense Ice Shelf Water (ISW) emerging from the Filchner‐Ronne Ice Shelf cavity in the southwestern Weddell Sea flows northward through the Filchner Depression to eventually descend the continental slope and contribute to the formation of bottom water. New ship‐born observations of hydrography and currents from Filchner Depression in January 2013 suggest that the northward flow of ISW takes place in a mid‐depth jet along the eastern flank of the depression, thus questioning the traditional view with outflow along the western flank. This interpretation of the data is supported by results from a regional numerical model, which shows that ISW flowing northward along the eastern coast of Berkner Island turns eastward and crosses the depression to its eastern side upon reaching the Filchner ice front. The ice front represents a sudden change in the thickness of the water column and thus a potential vorticity barrier. Transport estimates of northward ISW flux based on observations ranges from 0.2‐1.0 Sv.
      PubDate: 2014-08-20T04:27:38.782472-05:
      DOI: 10.1002/2014JC010225
  • Insights on the OAFlux ocean surface vector wind analysis merged from
           scatterometers and passive microwave radiometers (1987 onward)
    • Authors: Lisan Yu; Xiangze Jin
      Pages: n/a - n/a
      Abstract: A high‐resolution global daily analysis of ocean surface vector winds (1987 onward) was developed by the Objectively Analyzed air‐sea Fluxes (OAFlux) project. This study addressed the issues related to the development of the time series through objective synthesis of 12 satellite sensors (two scatterometers and 10 passive microwave radiometers) using a least‐variance linear statistical estimation. The issues include the rationale that supports the multisensor synthesis, the methodology and strategy that were developed, the challenges that were encountered, and the comparison of the synthesized daily mean fields with reference to scatterometers and atmospheric reanalyses. The synthesis was established on the bases that the low and moderate winds (
      PubDate: 2014-08-19T15:36:01.703563-05:
      DOI: 10.1002/2013JC009648
  • On the nonequivalent barotropic structure of the Antarctic Circumpolar
           Current: An observational perspective
    • Authors: H. E. Phillips; N. L. Bindoff
      Pages: n/a - n/a
      Abstract: We examine the vertical structure of the horizontal flow and diagnose vertical velocities in the Antarctic Circumpolar Current (ACC) near the Kerguelen Plateau using EM‐APEX profiling floats. Eight floats measured horizontal velocity, temperature, and salinity profiles to 1600 dbar, with a vertical spacing of 3–5 dbar four times per day over a period of approximately 3 months. Horizontal velocity profiles show a complex vertical structure with strong rotation of the velocity vector through the water column. The distribution of rotation angles from 1247 profiles is approximately Gaussian and rotations of either sign are equally likely. Forty percent of profiles with speed greater than 5 cm s−1 have a depth‐integrated rotation of less than 15 degrees over 1300 dbar, while the other 60% demonstrate significantly stronger rotation. Consequently, most profiles do not conform to the equivalent barotropic model (deep flow parallel and proportional to the surface flow) used in simplified dynamical models and in Gravest Empirical Mode climatologies of the ACC. Nevertheless, since we find the mean rotation to be zero, an equivalent barotropic assumption is valid to first order. Vertical velocities inferred using conservation of mass and a gradient wind balance in natural coordinates have magnitudes on the order of 100 m/day. We find robust patterns of upwelling and downwelling phase‐locked to meanders in the flow, as found in earlier studies. With the advent of high‐resolution observations such as those presented here, and high‐resolution models, we can advance to a more complete understanding of the rich variability in ACC structure that is neglected in the equivalent barotropic model.
      PubDate: 2014-08-19T14:10:34.666649-05:
      DOI: 10.1002/2013JC009516
  • Variability of oceanic carbon cycle in the North Pacific from seasonal to
           decadal scales
    • Authors: Peng Xiu; Fei Chai
      Pages: n/a - n/a
      Abstract: Variability of upper‐ocean carbon cycle in the North Pacific during 1958–2010 period is investigated using a physical‐biogeochemical model. Comparisons with in situ data from five different oceanographic environments in the South China Sea, Monterey Bay, North Pacific gyre, northwestern Pacific, and Gulf of Alaska indicate that the model usually captures observed seasonal and interannual variability in both sea surface pCO2 and sea‐air CO2 flux. Seasonal variability of pCO2 and CO2 flux in the North Pacific follows the change in sea surface temperature (SST) closely with high and low values in summer and winter, respectively. Total CO2 modifies pCO2 seasonal pattern in an opposite manner with respect to SST, and surface wind speed modifies the magnitude of CO2 flux variations. On interannual and decadal time scales, sea surface pCO2 is primarily controlled by anthropogenic CO2, followed by modulations by the El Niño‐Southern Oscillation and the Pacific Decadal Oscillation (PDO), while sea‐air CO2 flux is significantly regulated by the PDO and the North Pacific Gyre Oscillation (NPGO). We show that anthropogenic CO2 tends to amplify the influence on CO2 flux from the PDO but to damp the influence from the NPGO.
      PubDate: 2014-08-19T14:07:26.324473-05:
      DOI: 10.1002/2013JC009505
  • Dense shelf water production in the Adélie Depression, East
           Antarctica, 2004–2012: Impact of the Mertz Glacier calving
    • Authors: Maité Lacarra; Marie‐Noëlle Houssais, Christophe Herbaut, Emmanuelle Sultan, Mickael Beauverger
      Pages: n/a - n/a
      Abstract: Summer repeated hydrographic surveys and 4 years of mooring observations are used to characterize for the first time the interannual variability of the bottom water in the Mertz Glacier Polynya (MGP) on the East Antarctic shelf (142°E–146°E). Until 2010, large interannual variability was observed in the summer bottom salinity with year‐to‐year changes reaching 0.12 in Commonwealth Bay, the region with the highest sea ice production. The summer variability was shown to be linked to the efficiency of the convection during the preceding winter. The recent freshening of the bottom waters subsequent to the Mertz Glacier calving was well beyond the range of the precalving interannual variability. Within 2 years after the event, the bottom water of the shelf became too light to possibly contribute to renewal of the Antarctic Bottom Water. Rough estimates of the freshwater budget of the Adélie Depression indicate that the freshening necessary to compensate for net sea ice production in the MGP did not change drastically after the Mertz calving. The year‐to‐year salinity changes appeared to respond to the MGP activity. Yet, prior to the calving, the convective system in the polynya was also partly controlled by the late winter bottom salinity through a mechanism leading to a sequence of alternatively fresher and more saline bottom waters over the period 2007–2010. Exceptional events like the Mertz calving seem to be able to switch over the system into a less stratified state where convection responds more directly to changes in the surface forcing.
      PubDate: 2014-08-19T13:14:24.497598-05:
      DOI: 10.1002/2013JC009124
  • Water mass analysis of the Coral Sea through an Optimum Multiparameter
    • Authors: Florent Gasparin; Christophe Maes, Joel Sudre, Veronique Garcon, Alexandre Ganachaud
      Pages: n/a - n/a
      Abstract: A water mass analysis of the Coral Sea thermocline waters provides a description of their distribution, pathways and mixture based on recent oceanographic cruises in this region of strong western boundary currents. The Optimum Multiparameter method is used to determine the relative contribution of core water masses based on their measured temperature, salinity and dissolved oxygen. The thermocline waters, carried by the broad South Equatorial Current (SEC), are essentially composed of four core water masses of different origins. Coming from the south, the South Pacic Tropical Water South (SPTWS, σ=25.3 kg.m−3) and the Western South Pa‐cic Central Water (WSPCW, σ=26.3 kg.m−3) enter the Coral Sea by the channel between the island of New Caledonia and the Vanuatu archipelago. Coming from the north, the South Pacic Tropical Water North (SPTWN, σ=24.5 kg.m−3) and the Pacic Equatorial Water (PEW, σ=26.3 kg.m−3) flow north of Vanuatu. The upper thermocline water that exits the Coral Sea equatorward, is mainly composed of SPTWN carried by the New Guinea Coastal Undercurrent. In contrast, upper thermocline waters exiting the Coral Sea poleward, in the East Australian Current, is dominated by SPTWS. The relative contributions are different in the lower thermocline where WSPCW dominates both western boundary currents. This refined description is consistent with the dynamics of the main currents, with a very strong depth dependence in the partitioning of incoming SEC waters.
      PubDate: 2014-08-19T03:41:16.580798-05:
      DOI: 10.1002/2014JC010246
  • Seasonal surface layer dynamics and sensitivity to runoff in a high Arctic
           fjord (Young Sound/Tyrolerfjord, 74°N)
    • Authors: Jørgen Bendtsen; John Mortensen, Søren Rysgaard
      Pages: n/a - n/a
      Abstract: Runoff from the Greenland Ice Sheet, local glaciers and snow melt along the northeastern Greenland coastline has a significant impact on coastal water masses flowing south towards Denmark Strait. Very few direct measurements of runoff currently exist in this large area and the water masses near the coast are also difficult to measure due to the presence of icebergs and sea ice. Measurements from the Zackenberg Research station, located in Young Sound/Tyrolerfjord in northeast Greenland (74° N), provide some of the few observations of hydrographic, hydrologic and atmospheric parameters from this remote area. Here we analyze measurements from the fjord and also measurements in the ambient water masses which are found in the outer fjord and between the fjord and the East Greenland Current and validate and apply a numerical model of the fjord. A model sensitivity study allows us to constrain runoff estimates for the area.We also show that a total runoff between 0.9 ‐ 1.4 km3 in 2006 is in accordance with observed surface salinities and calculated freshwater content in the fjord. This indicates that earlier reported runoff to the area is significantly underestimated and that melt from glaciers and the Greenland Ice Sheet in this region may be up to 50% larger than the current estimate. Model simulations indicate the presence of a cold low‐saline coastal water mass formed by runoff from fjords north of the Young Sound/Tyrolerfjord system. Simulations of passive and age tracers show that residence time of river water during the summer period is about one month in the inner part of the fjord.
      PubDate: 2014-08-19T03:37:54.431675-05:
      DOI: 10.1002/2014JC010077
  • Southern Ocean CO2 sink: The contribution of the sea ice
    • Authors: Bruno Delille; Martin Vancoppenolle, Nicolas‐Xavier Geilfus, Bronte Tilbrook, Delphine Lannuzel, Véronique Schoemann, Sylvie Becquevort, Gauthier Carnat, Daniel Delille, Christiane Lancelot, Lei Chou, Gerhard S. Dieckmann, Jean‐Louis Tison
      Pages: n/a - n/a
      Abstract: We report first direct measurements of the partial pressure of CO2 (pCO2) within Antarctic pack sea ice brines and related CO2 fluxes across the air‐ice interface. From late winter to summer, brines encased in the ice change from a CO2 large over‐saturation, relative to the atmosphere, to a marked under‐saturation while the underlying oceanic waters remains slightly oversaturated. The decrease from winter to summer of pCO2 in the brines is driven by dilution with melting ice, dissolution of carbonate crystals and net primary production. As the ice warms, its permeability increases, allowing CO2 transfer at the air‐sea ice interface. The sea ice changes from a transient source to a sink for atmospheric CO2. We upscale these observations to the whole Antarctic sea ice cover using the NEMO‐LIM3 large‐scale sea ice‐ocean, and provide first estimates of spring and summer CO2 uptake from the atmosphere by Antarctic sea ice. Over the spring‐summer period, the Antarctic sea ice cover is a net sink of atmospheric CO2 of 0.029 PgC, about 58% of the estimated annual uptake from the Southern Ocean. Sea ice then contributes significantly to the sink of CO2 of the Southern Ocean.
      PubDate: 2014-08-19T03:26:32.244071-05:
      DOI: 10.1002/2014JC009941
  • Drivers of variability in Arctic sea‐ice drift speed
    • Authors: Einar Olason; Dirk Notz
      Pages: n/a - n/a
      Abstract: We explore the main drivers of seasonal and long‐term variations in basin‐scale Arctic sea‐ice drift speed. To do so, we examine the relationship between the observed time‐varying area‐mean ice drift‐speed in the central Arctic and observed thickness and concentration as well as surface wind‐stress. Drift speeds are calculated from the positions of drifting buoys, thickness is based on submarine observations, concentration on satellite observations and the wind stress comes from a global reanalysis. We find that seasonal changes in drift speed are correlated primarily with changes in concentration when concentration is low and with changes in thickness otherwise. The correlation between drift speed and concentration occurs because changing concentration changes how readily the ice responds to the synoptic‐scale forcing of the atmosphere. Drift speed is correlated with neither concentration nor thickness in April and May. We show this behaviour to be correlated with a decrease in the localisation of deformation. This indicates that the increase in drift speed is caused by newly formed fractures not refreezing, leading to an overall reduced ice‐cover strength without a detectable change in ice concentration. We show that a strong long‐term trend exists in months of relatively low ice concentration. Using our analysis of the seasonal cycle we show that the trend in concentration drives a significant portion of the drift‐speed trend, possibly reinforced by a trend in cyclone activity. Hence, the trend in drift‐speed in this period is primarily caused by increased synoptic‐scale movement of the ice pack.
      PubDate: 2014-08-19T03:17:12.3387-05:00
      DOI: 10.1002/2014JC009897
  • Simulating extreme total water levels using a time‐dependent,
           extreme value approach
    • Authors: Katherine A. Serafin; Peter Ruggiero
      Pages: n/a - n/a
      Abstract: Coastal flood hazard zones and the design of coastal defenses are often devised using the maximum recorded water level or a ‘design' event such as the 100‐year return‐level, usually projected from observed extremes. Despite technological advances driving more consistent instrumental records of waves and water levels, the observational record may be short, punctuated with intermittent gaps, and vary in quality. These issues in the record often preclude accurate and robust estimates of extreme return‐level events. Here we present a total water level full simulation model (TWL‐FSM) that simulates the various components of TWLs (waves, tides, and non‐tidal residuals) in a Monte Carlo sense, taking into account conditional dependencies that exist between the various components. Extreme events are modeled using non‐stationary extreme value distributions that include seasonality and climate variability. The resulting synthetic TWLs allow for empirical extraction of return level events and the ability to more robustly estimate and assess present‐day flood and erosion hazards. The approach is demonstrated along a northern Oregon, USA littoral cell but is applicable to beaches anywhere wave and water level records or hindcasts are available. Simulations result in extreme 100‐year TWL return‐levels as much as 1 m higher than those extrapolated from the “observational” record. At the Oregon site, this would result in 30% more coastal flooding than the “observational” 100‐year TWL return‐level projections. More robust estimates of extreme TWLs and tighter confidence bounds on return level events can aid coastal engineers, managers, and emergency planners in evaluating exposure to hazards.
      PubDate: 2014-08-19T03:10:22.928777-05:
      DOI: 10.1002/2014JC010093
  • Dispersion and nonlinear effects in the 2011 Tohoku‐Oki earthquake
    • Authors: Tatsuhiko Saito; Daisuke Inazu, Takayuki Miyoshi, Ryota Hino
      Pages: n/a - n/a
      Abstract: This study reveals the roles of the wave dispersion and nonlinear effects for the 2011 Tohoku‐Oki earthquake tsunami. We conducted tsunami simulations based on the nonlinear dispersive equations with a high‐resolution source model. The simulations successfully reproduced the waveforms recorded in the offshore, deep sea, and focal areas. The calculated inundation area coincided well with the actual inundation for the Sendai Plain, which was the widest inundation area during this event. By conducting sets of simulations with different tsunami equations, we obtained the followings insights into the wave dispersion, nonlinear effects, and energy dissipation for this event. Although the wave dispersion was neglected in most studies, the maximum amplitude was significantly overestimated in the deep sea if the dispersion was not included. The waveform observed at the station with the largest tsunami height (∼2 m) among the deep‐ocean stations also verified the necessity of the dispersion. It is well known that the nonlinear effects play an important role for the propagation of a tsunami into bays and harbors. Additionally, nonlinear effects need to be considered to accurately model later waves, even for offshore stations. In particular, including nonlinear terms rather than the inundation was more important when precisely modeling the waves reflected from the coast.
      PubDate: 2014-08-15T13:08:03.305314-05:
      DOI: 10.1002/2014JC009971
  • Quantifying uncertainty sources in the gridded data of sea surface CO2
           partial pressure
    • Authors: Guizhi Wang; Minhan Dai, Samuel S. P. Shen, Yan Bai, Yi Xu
      Pages: n/a - n/a
      Abstract: The bulk uncertainty in the gridded sea surface pCO2 data is crucial in assessing the reliability of the CO2 flux estimated from measurements of air‐sea pCO2 difference, because atmospheric pCO2 are relatively homogeneous and well defined. The bulk uncertainty results from three different sources: analytical error (Em), spatial variance ( σs2), and the bias from undersampling ( σu2). Common uncertainty quantification by standard deviation may mix up the different sources of uncertainty. We have established a simple procedure to determine these three sources of uncertainty using remote sensing‐derived and field‐measured pCO2 data. Em is constrained by the analytical method and data reduction procedures. σs2 is derived from the remotely sensed pCO2 field. σu2 is determined by spatial variance and the effective number of observations, considering, for the first time, the geometric bias introduced by pCO2 sampling. This approach is applied to 1° × 1° gridded pCO2 data collected from the East China Sea. We demonstrate that the spatial distribution of these biases is uneven and that none of them follow the same spatial trend as the standard deviation. σs2 contributes the most to the uncertainty in gridded pCO2 data over those grid boxes with good sampling coverage, while σu2 dominates the total uncertainty in the grid boxes with poor sampling coverage. Application of this procedure to other parts of the global ocean will help to better define the inherent spatial variability of the pCO2 field and thus better interpolate and/or extrapolate pCO2 data, and eventually better constrain air‐sea CO2 fluxes.
      PubDate: 2014-08-15T12:50:33.437005-05:
      DOI: 10.1002/2013JC009577
  • Low‐frequency ocean bottom pressure variations in the North Pacific
           in response to time‐variable surface winds
    • Authors: C. Petrick; H. Dobslaw, I. Bergmann‐Wolf, N. Schön, K. Matthes, M. Thomas
      Pages: n/a - n/a
      Abstract: One decade of time‐variable gravity field observations from the GRACE satellite mission reveals low‐frequency ocean bottom pressure (OBP) variability of up to 2.5 hPa centered at the northern flank of the subtropical gyre in the North Pacific. From a 145 year‐long simulation with a coupled chemistry climate model, OBP variability is found to be related to the prevailing atmospheric sea‐level pressure and surface wind conditions in the larger North Pacific area. The dominating atmospheric pressure patterns obtained from the climate model run allow in combination with ERA‐Interim sea‐level pressure and surface winds a reconstruction of the OBP variability in the North Pacific from atmospheric model data only, which correlates favorably (r=0.7) with GRACE ocean bottom pressure observations. The regression results indicate that GRACE‐based OBP observations are indeed sensitive to changes in the prevailing sea‐level pressure and thus surface wind conditions in the North Pacific, thereby opening opportunities to constrain atmospheric models from satellite gravity observations over the oceans.
      PubDate: 2014-08-15T12:45:45.854238-05:
      DOI: 10.1002/2013JC009635
  • Low calcium carbonate saturation state in an Arctic inland sea having
           large and varying fluvial inputs: The Hudson Bay system
    • Authors: Kumiko Azetsu‐Scott; Michel Starr, Zhi‐Ping Mei, Mats Granskog
      Pages: n/a - n/a
      Abstract: The Hudson Bay system (HBS) is a shallow inland sea in the Arctic, composed of Hudson Strait, Foxe Basin/Channel, James Bay and Hudson Bay. Dissolved inorganic carbon (DIC) and total alkalinity (TA) measurements were used to investigate the state of ocean acidification, specifically calcium carbonate saturation states (Ω) and pH. The freshwater sources were identified from the relationship between oxygen isotope composition (δ18O) and salinity to understand the role of freshwater in ocean acidification. The saturation state of seawater with respect to calcium carbonate (Ω) in surface water (2300 µmol/kg) were observed in the depths of central Hudson Bay with a pHtotal of 7.49 and Ωarg of 0.37. Over 67% and 22% of the bottom water of Hudson Bay was undersaturated with respect to aragonite and calcite respectively, despite Hudson Bay being very shallow (less than 250m deep). The aragonite saturation horizon in the central Hudson Bay was around 50m.
      PubDate: 2014-08-14T03:23:18.857362-05:
      DOI: 10.1002/2014JC009948
  • Scaling and stochastic cascade properties of NEMO oceanic simulations and
           their potential value for GCM evaluation and downscaling
    • Authors: Sébastien Verrier; Michel Crépon, Sylvie Thiria
      Pages: n/a - n/a
      Abstract: Spectral scaling properties have already been evidenced on oceanic numerical simulations and have been subject to several interpretations. They can be used to evaluate classical turbulence theories that predict scaling with specific exponents and to evaluate the quality of GCM outputs from a statistical and multiscale point of view. However, a more complete framework based on multifractal cascades is able to generalize the classical but restrictive second‐order spectral framework to other moment orders, providing an accurate description of probability distributions of the fields at multiple scales. The predictions of this formalism still needed systematic verification in oceanic GCM while they have been confirmed recently for their atmospheric counterparts by several papers. The present paper is devoted to a systematic analysis of several oceanic fields produced by the NEMO oceanic GCM. Attention is focused to regional, idealized configurations that permit to evaluate the NEMO engine core from a scaling point of view regardless of limitations involved by land masks. Based on classical multifractal analysis tools, multifractal properties were evidenced for several oceanic state variables (sea surface temperature and salinity, velocity components…). While first order structure functions estimated a different non‐conservativity parameter H in two scaling ranges, the multi‐order statistics of turbulent fluxes were scaling over almost the whole available scaling range. This multifractal scaling was then parameterized with the help of the Universal Multifractal framework, providing parameters that are coherent with existing empirical literature. Finally, we argue that the knowledge of these properties may be useful for oceanographers. The framework seems very well suited for the statistical evaluation of OGCM outputs. Moreover, it also provides practical solutions to simulate sub‐pixel variability stochastically for GCM downscaling purposes. As an independent perspective, the existence of multifractal properties in oceanic flows seems also interesting for investigating scale‐dependencies in remote‐sensing inversion algorithms.
      PubDate: 2014-08-14T02:57:58.50133-05:0
      DOI: 10.1002/2014JC009811
  • Spring carbonate chemistry dynamics of surface waters in the northern East
           China Sea: Water mixing, biological uptake of CO2, and chemical buffering
    • Authors: Wei‐dong Zhai; Jian‐fang Chen, Hai‐yan Jin, Hong‐liang Li, Jin‐wen Liu, Xian‐qiang He, Yan Bai
      Pages: n/a - n/a
      Abstract: We investigated sea surface total alkalinity (TAlk), dissolved inorganic carbon (DIC), dissolved oxygen (DO), and satellite‐derived chlorophyll‐a in the connection between the Yellow Sea and the East China Sea (ECS) during April to early May 2007. In spring, Changjiang dilution water (CDW), ECS offshore water, and together with Yellow Sea water (YSW) occupied the northern ECS. Using 16‐day composite satellite‐derived chlorophyll‐a images, several algal blooms were identified in the CDW and ECS offshore water. Correspondingly, biological DIC drawdown of 73 ± 20 μmol kg−1, oversaturated DO of 10–110 μmol‐O2 kg−1, and low fugacity of CO2 of 181–304 μatm were revealed in these two waters. YSW also showed CO2 uptake in spring, due to the very low temperature. However, its intrusion virtually counteracted CO2 uptake in the northern ECS. In the CDW and the ECS offshore water, Revelle factor was 9.3–11.7 and 8.9–10.6, respectively, while relatively high Revelle factor values of 11.4–13.0 were revealed in YSW. In the ECS offshore water, the observed relationship between DIC drawdown and oversaturated DO departed from the Redfield ratio, indicating an effect of chemical buffering capacity on the carbonate system during air–sea re‐equilibration. Given the fact that the chemical buffering capacity slows down the air–sea re‐equilibration of CO2, the early spring DIC drawdown may have durative effects on the sea surface carbonate system until early summer. Although our study is subject to limited temporal and spatial coverage of sampling, these insights are fundamental to understanding sea surface carbonate chemistry dynamics in this important ocean margin.
      PubDate: 2014-08-14T02:54:13.803042-05:
      DOI: 10.1002/2014JC009856
  • Temporal and spatial variability of tidal‐fluvial dynamics in the
           St. Lawrence fluvial estuary: An application of nonstationary tidal
           harmonic analysis
    • Authors: Pascal Matte; Yves Secretan, Jean Morin
      Pages: n/a - n/a
      Abstract: Predicting tides in upstream reaches of rivers is a challenge, because tides are highly nonlinear and nonstationary, and accurate short‐time predictions of river flow are hard to obtain. In the St. Lawrence fluvial estuary, tide forecasts are produced using a one‐dimensional model (ONE‐D), forced downstream with harmonic constituents and upstream with daily discharges using 30‐day flow forecasts from Lake Ontario and the Ottawa River. Although this operational forecast system serves its purpose of predicting water levels, information about nonstationary tidal‐fluvial processes that can be gained from it is limited, particularly the temporal changes in mean water level and tidal properties (i.e. constituent amplitudes and phases), which are function of river flow and ocean tidal range. In this paper, a harmonic model adapted to nonstationary tides, NS_TIDE, was applied to the St. Lawrence fluvial estuary, where the time‐varying external forcing is directly built into the tidal basis functions. Model coefficients from 13 analysis stations were spatially interpolated to allow tide predictions at arbitrary locations as well as to provide insights into the spatiotemporal evolution of tides. Model hindcasts showed substantial improvements compared to classical harmonic analyses at upstream stations. The model was further validated by comparison with ONE‐D predictions at a total of 32 stations. The slightly lower accuracy obtained with NS_TIDE is compensated by model simplicity, efficiency and capacity to represent stage and tidal variations in a very compact way, and thus represents a new means for understanding tidal rivers.
      PubDate: 2014-08-14T02:52:13.611913-05:
      DOI: 10.1002/2014JC009791
  • Formation of fine sediment deposit from a flash flood river in the
           Mediterranean Sea
    • Authors: Manel Grifoll; Vicenç Gracia, Alfredo Aretxabaleta, Jorge Guillén, Manuel Espino, John C. Warner
      Pages: n/a - n/a
      Abstract: We identify the mechanisms controlling fine deposits on the inner‐shelf in front of the Besòs River, in the northwestern Mediterranean Sea. This river is characterized by a flash flood regime discharging large amounts of water (more than twenty times the mean water discharge) and sediment in very short periods lasting from hours to few days. Numerical model output was compared with bottom sediment observations and used to characterize the multiple spatial and temporal scales involved in offshore sediment deposit formation. A high‐resolution (50 m grid size) coupled hydrodynamic‐wave‐sediment transport model was applied to the initial stages of the sediment dispersal after a storm‐related flood event. After the flood, sediment accumulation was predominantly confined to an area near the coastline as a result of preferential deposition during the final stage of the storm. Subsequent reworking occurred due to wave‐induced bottom shear stress that resuspended fine materials, with seaward flow exporting them towards the mid‐shelf. Wave characteristics, sediment availability and shelf circulation determined the transport after the reworking, and the final sediment deposition location. One‐year simulations of the regional area revealed a prevalent southwestward average flow with increased intensity downstream. The circulation pattern was consistent with the observed fine‐deposit depocenter being shifted southwards from the river mouth. At the southern edge, bathymetry controlled the fine deposition by inducing near‐bottom flow convergence enhancing bottom shear stress. According to the short term and long term analyses a seasonal pattern in the fine deposit formation is expected.
      PubDate: 2014-08-14T02:40:25.707666-05:
      DOI: 10.1002/2014JC010187
  • Wind‐modulated buoyancy circulation over the Texas‐Louisiana
    • Authors: Zhaoru Zhang; Robert Hetland, Xiaoqian Zhang
      Pages: n/a - n/a
      Abstract: This numerical investigation examines buoyancy‐driven circulation on the Texas‐Louisiana shelf modulated by seasonal winds. In wintertime, with downcoast (in the direction of Kelvin wave propagation) wind forcing, the Mississippi‐Atchafalaya River plume exhibits a bottom‐advected pattern. The plume is fairly wide and the horizontal density gradients span almost across the entire shelf inshore of 50m. Within the plume, vertical shear of alongshore flow is in thermal wind balance with the cross‐shore density gradient, and the shear causes a slight reversal of alongshore flow near the bottom. An alongshore flow estimated by the thermal wind relation, along with an assumption of zero bottom velocity, generally well agrees with the actual flow near the surface in regions deeper than 20 m. In spring and summer, the thermal‐wind‐balance‐derived flow deviates from the actual alongshore flow, and an Ekman flow driven by strong onshore wind makes the major contribution to the deviation. Besides, the summertime upcoast wind component transforms the plume to a surface‐advected state, resulting in reduced cross‐shore density gradients and increasing the relative importance of wind‐driven, barotropic alongshore flow, which contributes to the remaining deviation.
      PubDate: 2014-08-14T02:38:49.412571-05:
      DOI: 10.1002/2013JC009763
  • Relative role of El Niño and IOD forcing on the southern tropical
           Indian Ocean Rossby waves
    • Authors: Soumi Chakravorty; C. Gnanaseelan, J. S. Chowdary, Jing‐Jia Luo
      Pages: n/a - n/a
      Abstract: The role of local air‐sea interactions over the tropical Indian Ocean (TIO) and remote forcing from the tropical Pacific Ocean in the formation and maintenance of southern TIO Rossby waves during El Niño and positive Indian Ocean Dipole (IOD) years is investigated. These Rossby waves are significantly intensified during the El Niño and IOD cooccurrence years, as compared to those during pure El Niño or IOD years. Coupled ocean‐atmosphere model sensitivity experiments reveal that air‐sea coupled processes in the TIO are responsible for the Rossby wave formation and its maintenance from boreal summer to fall, while remote forcing from the Pacific intensifies and maintains these waves up to the following spring. During the cooccurrence years, the Rossby waves are generated by both the persistent equatorial easterlies and off‐equatorial wind stress curl. During pure El Niño years, however, only off‐equatorial wind stress curl exists to drive weak Rossby wave. Asymmetric heating associated with IOD and the mean background easterly vertical wind shear (in the northern hemisphere) during summer and fall excite two symmetric anticyclones in both sides of the equator as atmospheric Rossby wave response, which are responsible for the anomalous equatorial surface easterlies. In contrast, symmetric heat sink over the Maritime Continent in winter associated with El Niño‐induced subsidence and mean easterly vertical shear (in southern hemisphere) are responsible for strong anticyclone in the southern TIO, which supports off‐equatorial wind stress curl.
      PubDate: 2014-08-12T15:46:30.725185-05:
      DOI: 10.1002/2013JC009713
  • Intraseasonal to interannual variability of the Atlantic meridional
           overturning circulation from eddy‐resolving simulations and
    • Authors: Xiaobiao Xu; Eric P. Chassignet, William E. Johns, William J. Schmitz, E. Joseph Metzger
      Pages: n/a - n/a
      Abstract: Results from two 1/12° eddy‐resolving simulations, together with data‐based transport estimates at 26.5°N and 41°N, are used to investigate the temporal variability of the Atlantic meridional overturning circulation (AMOC) during 2004–2012. There is a good agreement between the model and the observation for all components of the AMOC at 26.5°N, whereas the agreement at 41°N is primarily due to the Ekman transport. We found that (1) both observations and model results exhibit higher AMOC variability on seasonal and shorter time scales than on interannual and longer time scales; (2) on intraseasonal and interannual time scales, the AMOC variability is often coherent over a wide latitudinal range, but lacks an overall consistent coherent pattern over the entire North Atlantic; and (3) on seasonal time scales, the AMOC variability exhibits two distinct coherent regimes north and south of 20°N, due to different wind stress variability in the tropics and subtropics. The high AMOC variability south of 20°N in the tropical Atlantic comes primarily from the Ekman transport of the near‐surface water, and is modulated to some extent by the transport of the Antarctic Intermediate water below the thermocline. These results highlight the importance of the surface wind in driving the AMOC variability.
      PubDate: 2014-08-12T15:43:13.315502-05:
      DOI: 10.1002/2014JC009994
  • Effect of coastal‐trapped waves and wind on currents and transport
           in the Gulf of California
    • Authors: Manuel O. Gutiérrez; Manuel López, Julio Candela, Rubén Castro, Affonso Mascarenhas, Curtis A. Collins
      Pages: n/a - n/a
      Abstract: Subsurface pressure (SsP) observations from stations inside and outside of the Gulf of California (GC) are used to analyze the relationship between low‐frequency currents, temperature, and transport inside the GC and intraseasonal coastal‐trapped waves (CTWs), which propagate poleward along the coast toward the GC. Correlation functions and coherences of SsP stations were consistent with intraseasonal CTWs splitting in two at the mouth of the gulf: one part enters the gulf, propagates around the gulf, and eventually, toward the mouth, and another part that appears to “jump” the mouth of the gulf and travels poleward along the west coast of the peninsula. The correlation and coherence estimates of SsP at Manzanillo with currents showed that downwelling CTWs generated along‐gulf current anomalies toward the head of the gulf at the mainland shelf of the mouth, whereas at Ballenas Channel sill (San Lorenzo sill) these waves generated current anomalies toward the mouth near the surface (bottom). At the San Lorenzo (SL) sill, downwelling CTWs increased the near‐bottom (∼400 m) temperature and reduced the bottom transport of deep, fresher, and colder water that flows toward the head of the gulf. Cross‐Calibrated Multiplatform winds were used to investigate their relationship with currents. The first empirical orthogonal function of the along‐gulf wind stress showed that wind blowing toward the head of the gulf generated a reduction of bottom transport toward the head of the gulf through the SL sill, and intensified surface geostrophic current fluctuations toward the head of the gulf. There was also significant correlation between inflow bottom transport and outflow surface geostrophic velocities averaged across the gulf, consistent with the exchange pattern for the Northern Gulf.
      PubDate: 2014-08-12T15:34:21.144224-05:
      DOI: 10.1002/2013JC009538
  • Can we do better than the grid survey: Optimal synoptic surveys in
           presence of variable uncertainty and decorrelation scales
    • Authors: Sergey Frolov; Bartolame Garau, James Bellingham
      Pages: n/a - n/a
      Abstract: Regular grid (“lawnmower”) survey is a classical strategy for synoptic sampling of the ocean. Is it possible to achieve a more effective use of available resources if one takes into account a priori knowledge about variability in magnitudes of uncertainty and decorrelation scales? In this article, we develop and compare the performance of several path‐planning algorithms: optimized “lawnmower,” a graph‐search algorithm (A*), and a fully nonlinear genetic algorithm. We use the machinery of the best linear unbiased estimator (BLUE) to quantify the ability of a vehicle fleet to synoptically map distribution of phytoplankton off the central California coast. We used satellite and in situ data to specify covariance information required by the BLUE estimator. Computational experiments showed that two types of sampling strategies are possible: a suboptimal space‐filling design (produced by the “lawnmower” and the A* algorithms) and an optimal uncertainty‐aware design (produced by the genetic algorithm). Unlike the space‐filling designs that attempted to cover the entire survey area, the optimal design focused on revisiting areas of high uncertainty. Results of the multivehicle experiments showed that fleet performance predictors, such as cumulative speed or the weight of the fleet, predicted the performance of a homogeneous fleet well; however, these were poor predictors for comparing the performance of different platforms.
      PubDate: 2014-08-12T15:33:00.324955-05:
      DOI: 10.1002/2013JC009521
  • Impacts of nonbreaking wave‐stirring‐induced mixing on the
           upper ocean thermal structure and typhoon intensity in the South China Sea
    • Authors: Yineng Li; Shiqiu Peng, Jia Wang, Jing Yan
      Pages: n/a - n/a
      Abstract: To investigate the effect of nonbreaking wave‐induced mixing caused by surface wave stirring on the upper ocean thermal structure (UOTS) and the typhoon intensity, a simple nonbreaking wave‐stirring‐induced mixing parameterization (WMP) scheme is incorporated into a regional coupled atmosphere‐ocean model for the South China Sea (SCS), which couples the Princeton Ocean Model (POM) to the Weather Research and Forecasting (WRF) model using the OASIS3 coupler. The results of simulating two selected typhoon cases indicate that the nonbreaking wave‐stirring‐induced mixing has significant impacts on UOTS and the typhoon intensity, and the incorporation of the simple WMP scheme in the coupled model helps to improve the simulation of UOTS and thus the typhoon intensity. In the case that the typhoon intensity is underestimated by the atmosphere model alone, the improvement of initial UOTS by the ocean model with the WMP included can deepen the initial thermocline depth, reduce the effect of SST cooling, and prevent the typhoon intensity from undesired weakening. In the case that the typhoon intensity is overestimated (with strong winds), including the WMP in the ocean model significantly enhances the total vertical mixing rate in the upper ocean, which in turn enhances the SST cooling and thus reduces the typhoon intensity as desired. The results obtained in this study make a contribution to the ongoing efforts of improving the typhoon intensity forecast using a regional atmosphere‐ocean coupled model by worldwide researchers and forecasters, especially for the typhoons in the SCS regions.
      PubDate: 2014-08-12T15:13:54.607721-05:
      DOI: 10.1002/2014JC009956
  • Vertical diffusivity of the Western Arctic Ocean halocline
    • Authors: William J. Shaw; Timothy P. Stanton
      Pages: n/a - n/a
      Abstract: A nearly year‐long series of upper ocean temperature, conductivity, and temperature microstructure profiles were collected from an ice camp drifting in the Beaufort Gyre as part of the 1997–1998 Surface Heat Budget of the Arctic Experiment (SHEBA). Geographically, the record includes portions over the deep Canada Basin and the steep bathymetry of the Chukchi Borderlands region. Hydrographically, the record includes “cool,” Pacific‐origin haloclines, which contain a variety of subsurface temperature maxima, and cold haloclines typical of the Eurasian Basins. We present estimates of the vertical turbulent diffusivity derived from the dissipation rate of thermal variance and calculations of the associated vertical heat fluxes. We find that vertical diffusion proceeds at molecular rates in the deep basins and away from topographic features. While still relatively small, diffusivity is enhanced by 1 order of magnitude near and above the Chukchi Borderlands. The enhanced diffusivity is correlated to an increase in water column strain variance above the Borderlands, providing a linkage between bathymetry, internal wave activity and turbulence. The Chukchi Borderlands play a significant role in heat transport in the Western Arctic. They are a pathway for horizontal heat transport and a hot spot for vertical heat transport. Vertical fluxes make a substantial contribution to the energy balance of the sea ice cover in this region. Heat fluxes between the halocline and underlying Atlantic Water are shown to be small and lacking vertical connection near surface waters.
      PubDate: 2014-08-12T15:11:58.18475-05:0
      DOI: 10.1002/2013JC009598
  • Seasonal and interannual changes of the Kuroshio intrusion onto the East
           China Sea Shelf
    • Authors: Chau‐Ron Wu; Yi‐Chia Hsin, Tzu‐Ling Chiang, Yong‐Fu Lin, I‐Fong Tsui
      Pages: n/a - n/a
      Abstract: An advanced artificial neural network classification algorithm is applied to 20 years of multisatellite geostrophic velocity data to study the Kuroshio intrusion onto the southern shelf of the East China Sea. The results suggest that the on‐shelf intrusion may occur year round, but winter intrusion events are more frequent than summer events. Both stronger intrusion (which is closely correlated to surface heat flux gradient) and weaker intrusion (which is correlated to wind forcing) occur during wintertime, but the former dominates in late winter while the latter prevails in early winter. There is a significant year‐to‐year variation of the winter on‐shelf intrusion. Although on‐shelf intrusion is the major characteristic of the region during wintertime, seldom intrusion events have been identified in the winters of 1997–1998 and 2002–2003. Two conditions are responsible for the cause of the nonintrusion events. During the two nonintrusion winters, the upstream Kuroshio transport anomaly is insignificant (the Kuroshio is not weakened) and no significant winter cooling off northeast Taiwan. Thus, the Kuroshio tends to flow along the shelf break following the 200 m isobath and on‐shelf intrusion ceases. Those two nonintrusion events take place during the winters when both the Niño 3.4 index and the PDO index are large and positive.
      PubDate: 2014-08-12T15:10:00.406949-05:
      DOI: 10.1002/2013JC009748
  • Seasonal variability of the Red Sea, from satellite gravity, radar
           altimetry, and in situ observations
    • Authors: John Wahr; David A. Smeed, Eric Leuliette, Sean Swenson
      Pages: n/a - n/a
      Abstract: Seasonal variations of sea surface height (SSH) and mass within the Red Sea are caused mostly by exchange of heat with the atmosphere and by flow through the strait opening into the Gulf of Aden to the south. That flow involves a net mass transfer into the Red Sea during fall and out during spring, though in summer there is an influx of cool water at intermediate depths. Thus, summer water in the south is warmer near the surface due to higher air temperatures, but cooler at intermediate depths. Summer water in the north experiences warming by air‐sea exchange only. The temperature affects water density, which impacts SSH but has no effect on mass. We study this seasonal cycle by combining GRACE mass estimates, altimeter SSH measurements, and steric contributions derived from the World Ocean Atlas temperature climatology. Among our conclusions are: mass contributions are much larger than steric contributions; the mass is largest in winter, consistent with winds pushing water into the Red Sea in fall and out during spring; the steric signal is largest in summer, consistent with surface warming; and the cool, intermediate‐depth water flowing into the Red Sea in spring has little impact on the steric signal, because contributions from the lowered temperature are offset by effects of decreased salinity. The results suggest that the combined use of altimeter and GRACE measurements can provide a useful alternative to in situ data for monitoring the steric signal.
      PubDate: 2014-08-12T09:25:37.972322-05:
      DOI: 10.1002/2014JC010161
  • Pathways of basal meltwater from Antarctic ice shelves: A model study
    • Authors: Kazuya Kusahara; Hiroyasu Hasumi
      Pages: n/a - n/a
      Abstract: We investigate spreading pathways of basal meltwater released from all Antarctic ice shelves using a circumpolar coupled ice shelf‐sea ice‐ocean model that reproduces major features of the Southern Ocean circulation, including the Antarctic Circumpolar Current (ACC). Several independent virtual tracers are used to identify detailed pathways of basal meltwaters. The spreading pathways of the meltwater tracers depend on formation sites because the meltwaters are transported by local ambient ocean circulation. Meltwaters from ice shelves in the Weddell and Amundsen—Bellingshausen Seas in surface/subsurface layers are effectively advected to lower latitudes with the ACC. Although a large portion of the basal meltwaters is present in surface and subsurface layers, a part of the basal meltwaters penetrates into the bottom layer through active dense water formation along the Antarctic coastal margins. The signals at the seafloor extend along the topography, showing a horizontal distribution similar to the observed spreading of Antarctic Bottom Water. Meltwaters originating from ice shelves in the Weddell and Ross Seas and in the Indian sector significantly contribute to the bottom signals. A series of numerical experiments in which thermodynamic interaction between the ice shelf and ocean is neglected regionally demonstrates that the basal meltwater of each ice shelf impacts sea ice and/or ocean thermohaline circulation in the Southern Ocean.
      PubDate: 2014-08-12T02:39:34.773313-05:
      DOI: 10.1002/2014JC009915
  • Penetration depth of diapycnal mixing generated by wind stress and flow
           over topography in the northwestern Pacific
    • Authors: Ying Li; Yongsheng Xu
      Pages: n/a - n/a
      Abstract: The role of turbulent diapycnal mixing in the northwestern Pacific was estimated by employing a fine‐scale parameterization method based on 6,756 high‐resolution CTD profiles spanning a period of 8 years from the Japan Oceanography Data Center (JODC) and the Kuroshio Extension System Study (KESS). The rate of turbulent mixing in the upper ocean within 300‐1,800 m depth displayed a distinct seasonal cycle, bearing a statistically significant correlation to wind‐induced near‐inertial energy flux (hereafter denoted by WNEF). Enhanced turbulent mixing was also found near the rough seafloor relative to that over smooth topography. Enhanced dissipation at surface and bottom was found to be able to penetrate the ocean interior up to 1,800 m and 3,300 m, respectively, with penetration depths varying with the WNEF and topographic roughness. Our study here provides evidence for the important role of near‐inertial energy input from wind stress and the influence of bottom topography in maintaining mixing in the ocean interior.
      PubDate: 2014-08-12T02:31:00.597022-05:
      DOI: 10.1002/2013JC009681
  • Simulated tomographic reconstruction of ocean current profiles in a
           bottom‐limited sound channel
    • Authors: Naokazu Taniguchi; Chen‐Fen Huang
      Pages: n/a - n/a
      Abstract: Tomographic reconstruction of the vertical current profile in a bottom‐limited sound channel requires solving a difficult ray identification problem. An approach to deal with this problem is a ray group method in which received arrival pulses are divided into several ray groups according to the characteristics of the arrival patterns. The method is validated using numerically simulated reciprocal acoustic transmission in a synthetic ocean in the Luzon Strait, where the Kuroshio Current has speeds as high as 1.2 m/s, for both narrowband and broadband signals. Four ray groups are found for the synthetic data; these are chosen based on arrival time. The differential travel time is determined by pairing up the reciprocal arrival peaks and then averaging the differential travel times within the selected time windows. Compared with the narrowband case, the estimated broadband differential travel time is more consistent with that computed from the current magnitude in the synthetic ocean. The vertical current profile is reconstructed from the broadband differential travel times by a generalized Tikhonov regularization approach. The data weighting matrix includes observation error in picking and pairing travel times and model parameter error due to path length uncertainty. The time series of the reconstructed current agrees with the synthetic ocean current; the fractional residual variance is 0.013 for the surface layer and 0.01 for the entire water column. The ray group method mitigates the ray identification problem in the bottom‐limited environment and could offer valuable data regarding the range‐integrated current velocity.
      PubDate: 2014-08-11T09:40:52.278561-05:
      DOI: 10.1002/2014JC009885
  • Effects of super typhoons on cyclonic ocean eddies in the western North
           Pacific: A satellite data‐based evaluation between 2000 and 2008
    • Authors: Liang SUN; Ying‐Xin Li, Yuan‐Jian Yang, Qiaoyan Wu, Xue‐Tao Chen, Qiu‐Yang Li, Yu‐Bin Li, Xian Tao
      Pages: n/a - n/a
      Abstract: A composite time series of the merged satellite altimeters sea surface height anomaly (SSHA) data and satellite‐observed sea surface temperature (SST) data were used to identify eddies in the Western North Pacific Ocean (WNPO), where there were numbers of intense typhoons. This study systematically investigated 15 super typhoons during the period of 2000‐2008 in the WNPO to study their impacts on the pre‐typhoon ocean features, e.g., the cyclonic ocean eddy (COE) feature (closed contours of SSHA < ‐6 cm) and neutral condition (SSHA between ‐6 and 6 cm). Two new COEs are generated by two super typhoons, and 18 pre‐existing COEs are intensified by 13 super typhoons. 5 of the 13 super typhoons each influenced two pre‐exisiting COEs. Although the typhoon‐induced maximum cooling centers had a right bias along the tracks due to wind conditions, pre‐existing COEs also play a significant role in determining the strength and location of large SST cooling. Three possible factors (maximum wind speed, typhoon translation speed and the typhoon forcing time, Tf) are employed to explain the interactions. Above all, the changes of the COE geometric and physical parameters (e.g., effective radius, area, SST, SSHA, and eddy kinetic energy) were mostly related to the typhoon forcing time, Tf. This is because Tf is a parameter that is a combination of the typhoon’s translation speed, intensity and size. Although the typhoons may significantly impact COEs, such samples were not commonly observed. Thus, the impact of typhoon on the strength of COEs is generally inefficient.
      PubDate: 2014-08-11T06:58:40.337463-05:
      DOI: 10.1002/2013JC009575
  • The pattern and variability of winter Kuroshio intrusion northeast of
    • Authors: Xiaohui Liu; Changming Dong, Dake Chen, Jilan Su
      Pages: n/a - n/a
      Abstract: The variations of the Kuroshio path and velocity northeast of Taiwan are analyzed based on along‐track satellite altimeter data as well as high‐resolution model experiments. Observations reveal that in winter the Kuroshio intrusion into the East China Sea (ECS) at this location is manifested by a secondary maximum current core (SMCC) shoreward of the Kuroshio's main path. The SMCC varies significantly on interannual time scale, and its variability is strikingly out of phase with that of the Kuroshio entering the ECS, meaning that the stronger the Kuroshio, the weaker the SMCC, and vice versa. Model experiments corroborate the observational results and, more importantly, indicate that the Kuroshio intrusion here follows two primary routes, a large anticyclonic loop that separates from the Kuroshio at the northern end of Taiwan and moves forward to form the SMCC, and a straight northward path onto the shelf when the Kuroshio turns sharply eastward along the continental slope of the ECS. The intrusion is controlled by both local forcing and remote effect, with its pattern and variability depending mostly on the local heat flux and the inertia of the Kuroshio Current.
      PubDate: 2014-08-11T04:30:59.430707-05:
      DOI: 10.1002/2014JC009879
  • SAR observation and numerical modeling of tidal current wakes at the East
           China Sea offshore wind farm
    • Authors: XiaoMing Li; Lequan Chi, Xueen Chen, YongZheng Ren, Susanne Lehner
      Pages: n/a - n/a
      Abstract: A TerraSAR‐X (TS‐X) Synthetic Aperture Radar (SAR) image acquired at the East China Sea offshore wind farm presents distinct wakes at a kilometer scale on the lee of the wind turbines. The presumption was that these wakes were caused by wind movement around turbine blades. However, wind analysis using spaceborne radiometer data, numerical weather prediction, and in situ measurements suggest that the prevailing wind direction did not align with the wakes. By analyzing measurement at the tidal gauge station and modeling of the tidal current field, these trailing wakes are interpreted to have formed when a strong tidal current impinged on the cylindrical monopiles of the wind turbines. A numerical simulation was further conducted to reproduce the tidal current wake under such conditions. Comparison of the simulated surface velocity in the wake region with the TS‐X sea surface backscatter intensity shows a similar trend. Consequently, turbulence intensity (T.I.) of the tidal current wakes over multiple piles is studied using the TS‐X observation. It is found that the T.I. has a logarithmic relation with distance. Furthermore, another case study showing wakes due to wind movement around turbine blades is presented to discuss the differences in the tidal current wakes and wind turbine wakes. The conclusion is drawn that small‐scale wakes formed by interaction of the tidal current and the turbine piles could be also imaged by SAR when certain conditions are satisfied. The study is anticipated to draw more attentions to the impacts of offshore wind foundations on local hydrodynamic field.
      PubDate: 2014-08-11T04:11:30.957143-05:
      DOI: 10.1002/2014JC009822
  • Assessing the temporal variability in extreme storm‐tide time series
           for coastal flood risk assessment
    • Authors: N. Quinn; M. Lewis, M. P. Wadey, I. D. Haigh
      Pages: n/a - n/a
      Abstract: The probability of extreme storm‐tide events has been extensively studied; however, the variability within the duration of such events and implications to flood risk is less well understood. This research quantifies such variability during extreme storm‐tide events (the combined elevation of the tide, surge, and their interactions) at 44 national tide gauges around the UK. Extreme storm‐tide events were sampled from water level measurements taken every 15 min between 1993 and 2012. At each site, the variability in elevation at each time step, relative to a given event peak, was quantified. The magnitude of this time series variability was influenced both by gauge location (and hence the tidal and nontidal residual characteristics) and the time relative to high water. The potential influence of this variability on coastal inundation was assessed across all UK gauge sites, followed by a detailed case study of Portsmouth. A two‐dimensional hydrodynamic model of the Portsmouth region was used to demonstrate that given a current 1 in 200 year storm‐tide event, the predicted number of buildings inundated differed by more than 30% when contrasting simulations forced with the upper and lower bounds of the observed time series variability. The results indicate that variability in the time series of the storm‐tide event can have considerable influence upon overflow volumes, hence with implications for coastal flood risk assessments. Therefore, further evaluating and representing this uncertainty in future flood risk assessments is vital, while the envelopes of variability defined in this research provides a valuable tool for coastal flood modelers.
      PubDate: 2014-08-11T04:10:37.119922-05:
      DOI: 10.1002/2014JC010197
  • Interannual variability of the Kuroshio onshore intrusion along the East
           China Sea shelf break: Effect of the Kuroshio volume transport
    • Authors: Chuanyu Liu; Fan Wang, Xinping Chen, Jin‐Song von Storch
      Pages: n/a - n/a
      Abstract: The interannual variability of the Kuroshio onshore intrusion (KOI) across the East China Sea (ECS) shelf break and its response to the ECS Kuroshio volume transport (KVT) during 1993~2010 are studied based on a high resolution, real time and global ocean general circulation model. Since variability of KVT is mainly determined in the interior Pacific, it is rather a remote than a local factor for the local ECS KOI. On interannual time scales, KVT affects KOI not only in the net volume transport across the entire shelf break but also in the spatial pattern along the shelf break. When KVT increases, the intrusion decreases (increases) upstream (downstream) of the major intrusion region northeast of Taiwan, the retreating increases (decreases) upstream (downstream) of the main veering region southwest of Kyushu. These patterns are caused by cyclonic eddies induced by the seaward deflection of the Kuroshio axis from the shelf break, which ultimately results from the KVT increase. A diagnostic mechanism of KOI ~ M sin(θ) h–1 is proposed, where h is the bottom depth, θ is the angle between isobaths and the vertically averaged current, and M is the absolute volume transport at the shelf break. θ is large in the major intrusion/retreating regions and stable on interannual time scales while M changes with opposite signs between upstream and downstream of the major intrusion/retreating regions. The mechanism explains well the relation of local KVT and the spatial pattern of KOI along the shelf break, in both the mean state and the interannual variations.
      PubDate: 2014-08-08T11:59:35.237931-05:
      DOI: 10.1002/2013JC009653
  • Layered mixing on the New England Shelf in summer
    • Authors: Jianing Wang; Blair J. W. Greenan, Youyu Lu, Neil S. Oakey, William J. Shaw
      Pages: n/a - n/a
      Abstract: The layered structure of stratification and mixing on the New England Shelf (NES) in summer is examined by analyzing a comprehensive set of observations of hydrography, currents and turbulence. A clear distinction in mixing characteristics between the mid‐column water (consisting of sub‐surface stratification, mid‐depth weak stratification and lower‐layer stratification) and a well‐mixed bottom boundary layer (BBL) is revealed. The combination of subtidal Ekman onshore bottom transport and cross‐shore density gradient created a lower‐layer stratification that inhibited the upward extension of the BBL turbulence. The BBL mixing was related to strong shear generated by bottom stress, and the magnitude and periodic variation of BBL mixing was determined by both the tidal and subtidal flows. Mixing in the mid‐column water occurred under stably‐stratified conditions and showed correspondence with the occurrence of near‐inertial and semi‐diurnal internal waves. Positive correlations between buoyancy frequency squared (N2) and shear variance (S2), S2 and dissipation rate (ε), N2 and ε are established in the mid‐column, but not in the BBL. The mid‐column ε was reasonably described by a slightly modified MacKinnon‐Gregg (MG) model.
      PubDate: 2014-08-07T04:37:56.573516-05:
      DOI: 10.1002/2014JC009947
  • Periodicity and patterns of ocean wind and wave climate
    • Authors: Justin E. Stopa; Kwok Fai Cheung
      Pages: n/a - n/a
      Abstract: The Climate Forecast System Reanalysis (CFSR) provides a wealth of information spanning 1979 to 2009 for investigation of ocean wind and wave climate. Preprocessing of the data removes the dominant seasonal signals and creates time series of semi‐monthly averaged wind speed and significant wave height over a 0.5º global grid. We perform an empirical orthogonal function (EOF) analysis to extract the dominant space‐time patterns. The results for the three major ocean basins show strong zonal structures in the winds and saturation of the swells corroborating prior works with various datasets. We reexamine the CFSR data in the frequency domain to identify periodic signals associated with published climate indices. The Fourier transform generates spectra ranging from 1 month to 15 years period for an EOF analysis. The results demonstrate dominance of the Arctic Oscillation in the Atlantic basin with a broad range of intra‐annual signals off the European coasts. The Indian and Pacific Oceans are strongly influenced by inter‐annual cycles of the El Niño Southern Oscillation (ENSO) and the Antarctica Oscillation. The Indian Ocean also has strong intra‐annual components ranging from 50 to 80 day period. The ENSO proves to be a ubiquitous signal around the globe, and in particular, saturates the Pacific with strong influences in the Equatorial region and the Southern Hemisphere Westerlies. A commonality of all basins is that the magnitude and the spatial structure of the intra‐annual and inter‐annual signals are similar suggesting a wide range of periods in each of the climate cycles examined.
      PubDate: 2014-08-07T04:27:12.943945-05:
      DOI: 10.1002/2013JC009729
  • On the nature of the sea ice albedo feedback in simple models
    • Authors: W. Moon; J. S. Wettlaufer
      Pages: n/a - n/a
      Abstract: We examine the nature of the ice‐albedo feedback in a long standing approach used in the dynamic‐thermodynamic modeling of sea ice. The central issue examined is how the evolution of the ice area is treated when modeling a partial ice cover using a two‐category‐thickness scheme; thin sea ice and open water in one category and “thick” sea ice in the second. The problem with the scheme is that the area‐evolution is handled in a manner that violates the basic rules of calculus, which leads to a neglected area‐evolution term that is equivalent to neglecting a leading‐order latent heat flux. We demonstrate the consequences by constructing energy balance models with a fractional ice cover and studying them under the influence of increased radiative forcing. It is shown that the neglected flux is particularly important in a decaying ice cover approaching the transitions to seasonal or ice‐free conditions. Clearly, a mishandling of the evolution of the ice area has leading‐order effects on the ice‐albedo feedback. Accordingly, it may be of considerable importance to re‐examine the relevant climate model schemes and to begin the process of converting them to fully resolve the sea ice thickness distribution in a manner such as remapping, which does not in principle suffer from the pathology we describe.
      PubDate: 2014-08-05T02:56:16.22264-05:0
      DOI: 10.1002/2014JC009964
  • Observations of rain‐induced near‐surface salinity anomalies
    • Authors: William E. Asher; Andrew T. Jessup, Ruth Branch, Dan Clark
      Pages: n/a - n/a
      Abstract: Vertical salinity gradients in the top few meters of the ocean surface can exist due to the freshwater input from rain. If present, surface gradients complicate comparing salinity measured at depths of a few meters to salinities retrieved using L‐band microwave radiometers such as SMOS and Aquarius. Therefore, understanding the spatial scales and the frequency of occurrence of these vertical gradients and the conditions under which they form will be important in understanding sea surface salinity maps provided by microwave radiometers. Salinity gradients in the near‐surface ocean were measured using a towed profiler that profiled salinity in the top two meters of the ocean with a minimum measurement depth of 0.1 m. In addition, an underway salinity profiling system was installed on the R/V Thomas G. Thompson. This measured near‐surface salinity at depths of 1 m and 2 m. Both the towed profiler and the underway system found the occurrence of negative salinity anomalies (i.e., salinity decreasing towards the surface) was correlated with the presence of rain. The magnitude of the anomaly (i.e. the difference between salinity at 0.1 m and the salinity at 0.26 m) was proportional to the cube of the rain rate for rain rate, R, greater than 6 mm h−1. From this, for R > 15 to 22 mm h−1, depending on the areal extent of the salinity anomalies, rain can cause scene‐averaged salinity offsets that are as large as the accuracy goal for Aquarius of 0.1 o/oo.
      PubDate: 2014-08-05T02:54:01.952519-05:
      DOI: 10.1002/2014JC009954
  • Observed wintertime tidal and subtidal currents over the continental shelf
           in the northern South China Sea
    • Authors: Ruixiang Li; Changsheng Chen, Huayong Xia, Robert C. Beardsley, Maochong Shi, Zhigang Lai, Huichan Lin, Yanqing Feng, Changjian Liu, Qichun Xu, Yang Ding, Yu Zhang
      Pages: n/a - n/a
      Abstract: Synthesis analyses were performed to examine characteristics of tidal and subtidal currents at eight mooring sites deployed over the northern South China Sea (NSCS) continental shelf in the 2006‐2007 and 2009‐2010 winters. Rotary spectra and harmonic analysis results showed that observed tidal currents in the NSCS were dominated by baroclinic diurnal tides with phases varying both vertically and horizontally. This feature was supported by the CC‐FVCOM results, which demonstrated that the diurnal tidal flow over this shelf was characterized by baroclinic Kelvin waves with vertical phase differences varying in different flow zones. The northeasterly wind‐induced southwestward flow prevailed over the NSCS shelf during winter, with episodic appearances of meso‐scale eddies and a bottom‐intensified buoyancy‐driven slope water intrusion. The moored current records captured a warm‐core anticyclonic eddy, which originated from the southwestern coast of Taiwan and propagated southwestward along the slope consistent with a combination of β‐plane and topographic Rossby waves. The eddy was surface‐intensified with a swirl speed of >50 cm/s and a vertical scale of ˜400 m. In absence of eddies and onshore deep slope water intrusion, the observed southwestward flow was highly coherent with the northeasterly wind stress. Observations did not support the existence of the permanent wintertime South China Sea Warm Current (SCSWC). The definition of SCSWC, which was based mainly on thermal wind calculations with assumed level of no motion at the bottom, needs to be interpreted with caution since the observed circulation over the NSCS shelf in winter included both barotropic and baroclinic components.
      PubDate: 2014-08-04T02:27:53.870112-05:
      DOI: 10.1002/2014JC009931
  • Feedbacks between ice cover, ocean stratification, and heat content in
           Ryder Bay, western Antarctic Peninsula
    • Authors: Hugh J. Venables; Michael P. Meredith
      Pages: n/a - n/a
      Abstract: A multi‐year, all‐season time series of water column physical properties and sea ice conditions in Ryder Bay, at the western Antarctic Peninsula (WAP), is used to assess the effects on the ocean of varying ice cover. Reduced ice cover leads to increased mixing and heat loss in the winter. The reduction in stratification persists into the following summer, preconditioning the water column to a greater vertical extent of surface‐driven mixing. This leads to an increased amount of heat from insolation being mixed down, affecting approximately the top 100m. The increased heat uptake in summer exceeds the heat lost the preceding winter, giving the initially counter‐intuitive effect that enhanced winter cooling generates warmer temperatures in the following summer and autumn. This process is therefore a positive feedback on sea ice, as reduced sea ice leads to increased heat content in the ocean the following autumn. It also causes increased winter atmospheric temperatures due to the increased winter heat loss from the ocean. In the deeper part of the water column, heat and carbon stored in the Circumpolar Deep Water (CDW) layer are released by deep mixing events. At these depths, conditions are restored by advection and vertical mixing on multi‐year timescales. In recent years, stronger deep mixing events in winter have led to a persistent reduction in CDW temperatures at the study site. Ocean glider data demonstrate the representativeness of these results across the wider region of Marguerite Bay, within which Ryder Bay is situated.
      PubDate: 2014-08-04T02:22:08.109135-05:
      DOI: 10.1002/2013JC009669
  • Vertical gradient correction for the oceanographic Atlas of the East Asian
    • Authors: You‐Soon Chang; Hong‐Ryeol Shin
      Pages: n/a - n/a
      Abstract: Regional climatology around the East Asian Seas has been developed by an international collaboration between the National Oceanic Data Center and the Korea Oceanic Data Center. It provides reliable information on temperature and salinity climatological fields with high resolution (0.1 ° by 0.1 ° by 137 levels). However, there is a problem around near‐bottom areas where topographic change is steep and observations are not available near the bottom. This study resolves this problem using a vertical gradient correction method when the profile is statically unstable. The stability is determined based on the Brunt‐Väisälä frequency with individual temperature and salinity profiles. Topographic‐following mapping technique employing the potential vorticity constraint term is used to construct a vertical gradient database for the temperature and salinity at every grid point. The results show that the correction is effective for eliminating large erroneous vertical gradients around near‐bottom areas. In addition, we show the importance of the optimal length scale to construct a precise vertical gradient database in a particular area such as the northern shelf of Taiwan. We expect that our revised high resolution climatological mean fields will serve as important data for relevant studies around the East Asian Seas.
      PubDate: 2014-08-04T02:19:36.928533-05:
      DOI: 10.1002/2014JC009845
  • Interdecadal changes in snow depth on Arctic sea ice
    • Authors: Melinda A. Webster; Ignatius G. Rigor, Son V. Nghiem, Nathan T. Kurtz, Sinead L. Farrell, Donald K. Perovich, Matthew Sturm
      Pages: n/a - n/a
      Abstract: Snow plays a key role in the growth and decay of Arctic sea ice. In winter, it insulates sea ice from cold air temperatures, slowing sea ice growth. From spring into summer, the albedo of snow determines how much insolation is absorbed by the sea ice and underlying ocean, impacting ice melt processes. Knowledge of the contemporary snow depth distribution is essential for estimating sea ice thickness and volume, and for understanding and modeling sea ice thermodynamics in the changing Arctic. This study assesses spring snow depth distribution on Arctic sea ice using airborne radar observations from Operation IceBridge for 2009‐2013. Data were validated using coordinated in situ measurements taken in March 2012 during the BRomine, Ozone, and Mercury EXperiment (BROMEX) field campaign. We find a correlation of 0.59 and root‐mean‐square error of 5.8 cm between the airborne and in situ data. Using this relationship and IceBridge snow thickness products, we compared the recent results with data from the 1937, 1954‐1991 Soviet drifting ice stations. The comparison shows thinning of the snow pack, from 35.1 ± 9.4 cm to 22.2 ± 1.9 cm in the western Arctic, and from 32.8 ± 9.4 cm to 14.5 ± 1.9 cm in the Beaufort and Chukchi seas. These changes suggest a snow depth decline of 37 ± 29% in the western Arctic and 56 ± 33% in the Beaufort and Chukchi seas. Thinning is negatively correlated with the delayed onset of sea ice freeze‐up during autumn.
      PubDate: 2014-08-02T01:10:32.607412-05:
      DOI: 10.1002/2014JC009985
  • Validation of Aquarius sea surface salinity with in situ measurements from
           Argo floats and moored buoys
    • Authors: Wenqing Tang; Simon H. Yueh, Alexander G. Fore, Akiko Hayashi
      Pages: n/a - n/a
      Abstract: We validate sea surface salinity (SSS) retrieved from Aquarius instrument on SAC‐D satellite with in situ measurements by Argo floats and moored buoy arrays. We assess the error structure of three Aquarius SSS products: the standard product processed by Aquarius Data Processing System (ADPS) and two datasets produced at the Jet Propulsion Laboratory (JPL): the Combined Active‐Passive algorithm with and without rain correction, CAP and CAP_RC respectively. We examine the effect of various filters to prevent unreliable point retrievals from entering Level‐3 averaging, such as land or ice contamination, radio‐frequency‐interference (RFI), and cold water. Our analyses show that Aquarius SSS agrees well with Argo in a monthly average sense between 40°S and 40°N except in the Eastern Pacific Fresh Pool and Amazon River outflow. Buoy data within these regions show excellent agreement with Aquarius but have discrepancies with the Argo gridded products. Possible reasons include strong near surface stratification and sampling problems in Argo in regions with significant western boundary currents. We observe large root‐mean‐square (RMS) difference and systematic negative bias between ADPS and Argo in the tropical Indian Ocean and along the Southern Pacific Convergence Zone. Excluding these regions removes the suspicious seasonal peak in the monthly RMS difference between the Aquarius SSS products and Argo. Between 40°S and 40°N, the RMS difference for CAP is less than 0.22 PSU for all 28 months, CAP_RC has essentially met the monthly 0.2 PSU accuracy requirement, while that for ADPS fluctuates between 0.22 and 0.3 PSU.
      PubDate: 2014-08-02T00:49:13.424573-05:
      DOI: 10.1002/2014JC010101
  • Decadal trends in global pelagic ocean chlorophyll: A new assessment
           integrating multiple satellites, in situ data, and models
    • Authors: Watson W. Gregg; Cécile S. Rousseaux
      Pages: n/a - n/a
      Abstract: Quantifying change in ocean biology using satellites is a major scientific objective. We document trends globally for the period 1998‐2012 by integrating three diverse methodologies: ocean color data from multiple satellites, bias correction methods based on in situ data, and data assimilation to provide a consistent and complete global representation free of sampling biases. The results indicated no significant trend in global pelagic ocean chlorophyll over the 15 year data record. These results were consistent with previous findings that were based on the first 6 years and first 10 years of the SeaWiFS mission. However, all of the Northern Hemisphere basins (north of 10o latitude), as well as the Equatorial Indian basin, exhibited significant declines in chlorophyll. Trend maps showed the local trends and their change in percent per year. These trend maps were compared with several other previous efforts using only a single sensor (SeaWiFS) and more limited time series, showing remarkable consistency. These results suggested the present effort provides a path forward to quantifying global ocean trends using multiple satellite missions, which is essential if we are to understand the state, variability, and possible changes in the global oceans over longer time scales.
      PubDate: 2014-07-31T04:03:08.962624-05:
      DOI: 10.1002/2014JC010158
  • Is weaker Arctic sea ice changing the Atlantic water circulation?
    • Authors: Polona Itkin; Michael Karcher, Rüdiger Gerdes
      Pages: n/a - n/a
      Abstract: With a numerical model we test the sensitivity of the Arctic Ocean circulation at mid‐depth (212‐1200 m) to the change in the sea ice rheology parameter P* that controls the sea ice compressive strength. We show that the reduction of the sea ice strength via P* within commonly used envelope reduces the sea ice extent and consequently enhances the ocean surface heat loss in the marginal ice zone. This leads to cooling of the Atlantic water inflow into the Arctic Ocean. As a result Eurasian Basin and Amerasian Basin temperatures are in average cooled by 0.1 °C and 0.05 °C, respectively. An increased sea ice drift speed in the central Arctic leads to an enhanced circulation of the anticyclonic Beaufort Gyre of the Amerasian Basin, which in turn weakens the cyclonic Atlantic water circulation below and enhances the recirculation of the Atlantic water in the Eurasian Basin. Consequently the balance of the volume fluxes through the Arctic gateways changes. Fram Strait net outflow increases by 0.46 Sv, Barents Sea Opening net inflow increases by 0.19 Sv and Davis Strait net outflow decreases by 0.28 Sv. This can spread the effects of the sea ice strength change beyond the limits of the Arctic Ocean and into the deep water convection zones in the North Atlantic. These substantial effects should be considered also in the model optimization efforts where P* is commonly used as one of the tuning parameters to achieve better sea ice simulations, whereas the effects on the ocean circulation are rarely taken into account.
      PubDate: 2014-07-25T23:32:34.515752-05:
      DOI: 10.1002/2013JC009633
  • Role of the western tropical Pacific in the North Pacific regime shift in
           the winter of 1998/1999
    • Authors: Hyun‐Su Jo; Sang‐Wook Yeh, Ben P. Kirtman
      Pages: n/a - n/a
      Abstract: In this study we examine the role of the western tropical Pacific in the North Pacific regime shift during the winter (December‐January‐February) of 1998/99. The North Pacific regime shift in the winter of 1998/99 is characterized by a dipole‐like structure along 40°N where significant warming is prominent in the western and central North Pacific. This shift might have been associated with an abrupt warming in the western tropical Pacific after 1998/99, in the North Equatorial Current (NEC) bifurcation region (8°N‐18°N, 125°E‐160°E) in particular. The NEC bifurcation region experiences a regime shift from cooling to warming during the 1998/99. Consequently, the Kuroshio current, which originates in the NEC bifurcation region, could transport the anomalous warm water into the North Pacific. In addition, a sudden shift in the NEC bifurcation latitude to the south also contributes to the 1998/99 regime shift in the North Pacific. We suggest a possible mechanism that could have caused the regime shift in the NEC bifurcation region during the 1998/99. An abrupt deepening of the thermocline depth in the NEC bifurcation region from 1997/98 to 1998/99, which is associated with the biggest El Niño in 1997/98 and a subsequent La Niña event in 1998/99, plays a role in initiating the shift in the NEC bifurcation region. Subsequently, the frequent occurrence of the La Niña events after 1998/99 plays a role in maintaining the regime shift toward warming by sustaining a deepening of the thermocline depth.
      PubDate: 2014-07-25T23:18:28.213633-05:
      DOI: 10.1002/2013JC009527
  • Coherent sea level variability on the North Atlantic western boundary
    • Authors: P. R. Thompson; G. T. Mitchum
      Pages: n/a - n/a
      Abstract: Interannual to decadal sea level variability on the North Atlantic western boundary is surprisingly coherent over substantial distances stretching from the Caribbean to Nova Scotia. The physical mechanisms responsible for this basin‐scale, low‐frequency coherence are explored in a diagnosis of simulated ocean fields from GECCO, which reproduces the observations to good approximation. Coastal sea level variability on the western boundary is known to be influenced by meridional divergence in the boundary current resulting in a geostrophic tilting of the sea surface. This mechanism is found to be of leading order along some stretches of the boundary, but it does not account for the coherence spanning the western North Atlantic. Instead, the coherence along the entire boundary is accounted for by vertical divergence resulting in the uniform rise and fall of the sea surface west of the 295°E meridian. The vertical divergence is found to be due to net vertically integrated zonal transport across this meridian resulting from meridional variation in the Sverdrup transport over the basin interior.
      PubDate: 2014-07-25T05:04:30.969259-05:
      DOI: 10.1002/2014JC009999
  • Ocean response to volcanic eruptions in Coupled Model Intercomparison
           Project 5 (CMIP5) simulations
    • Authors: Yanni Ding; James A. Carton, Gennady A. Chepurin, Georgiy Stenchikov, Alan Robock, Lori T. Sentman, John P. Krasting
      Pages: n/a - n/a
      Abstract: We examine the oceanic impact of large tropical volcanic eruptions as they appear in ensembles of historical simulations from eight Coupled Model Intercomparison Project Phase 5 models. These models show a response that includes lowering of global average sea surface temperature by 0.1‐0.3 K, comparable to the observations. They show enhancement of Arctic ice cover in the years following major volcanic eruptions, with long‐lived temperature anomalies extending to the mid‐depth and deep ocean on decadal to centennial timescales. Regional ocean responses vary, although there is some consistent hemispheric asymmetry associated with the hemisphere in which the eruption occurs. Temperature decreases and salinity increases contribute to an increase in the density of surface water and an enhancement in the overturning circulation of the North Atlantic Ocean following these eruptions. The strength of this overturning increase varies considerably from model to model and is correlated with the background variability of overturning in each model. Any cause/effect relation between eruptions and the phase of El Niño is weak.
      PubDate: 2014-07-24T07:30:27.173756-05:
      DOI: 10.1002/2013JC009780
  • Investigation of the causes of historical changes in the subsurface
           salinity minimum of the South Atlantic
    • Authors: Marlos Goes; Ilana Wainer, Natalia Signorelli
      Pages: n/a - n/a
      Abstract: In this study we investigate the sub‐surface salinity changes on decadal timescales across the Subtropical South Atlantic Ocean using two ocean reanalysis products, the latest version of the Simple Ocean Data Assimilation and the Estimating the Circulation and Climate of the Ocean, Phase II, as well as with additional climate model experiments. Results show that there is a recent significant salinity increase at the core of the salinity minimum at intermediate levels. The main underlying mechanism for this sub‐surface salinity increase is the lateral advective (gyre) changes due to the Southern Annular mode variability, which conditions an increased contribution from the Indian Ocean high salinity waters into the Atlantic. The global warming signal has a secondary but complementary contribution. Latitudinal differences at intermediate depth in response to large‐scale forcing are in part caused by local variation of westward propagation features, and by compensating contributions of salinity and temperature to density changes.
      PubDate: 2014-07-24T04:37:41.348152-05:
      DOI: 10.1002/2014JC009812
  • A physically based parameterization of gravity drainage for sea‐ice
    • Authors: David W. ReesJones; M. Grae Worster
      Pages: n/a - n/a
      Abstract: We incorporate a physically‐derived parameterization of gravity drainage, in terms of a convective upwelling velocity, into a one‐dimensional, thermodynamic sea‐ice model of the kind currently used in coupled climate models. Our parameterization uses a local Rayleigh number to represent the important feedback between ice salinity, porosity, permeability and desalination rate. It allows us to determine salt fluxes from sea ice and the corresponding evolution of the bulk salinity of the ice, in contrast to older, established models that prescribe the ice salinity. This improves the predictive power of climate models in terms of buoyancy fluxes to the polar oceans, and also the thermal properties of sea ice, which depend on its salinity. We analyze the behaviour of existing fixed‐salinity models, elucidate the physics affecting ice growth and compare against our dynamic‐salinity model, both in terms of laboratory experiments and also deep‐ocean calculations. These comparisons explain why the direct effect of ice salinity on growth is relatively small (though not always negligible, and sometimes different from previous studies), and also highlight substantial differences in the qualitative pattern and quantitative magnitude of salt fluxes into the polar oceans. Our study is particularly relevant to growing first‐year ice, when gravity drainage is the dominant mechanism by which ice desalinates. We expect that our dynamic model, which respects the underlying physics of brine drainage, should be more robust to changes in polar climate and more responsive to rapid changes in oceanic and atmospheric forcing.
      PubDate: 2014-07-24T03:52:57.946899-05:
      DOI: 10.1002/2013JC009296
  • Impact of climate change on the Northwestern Mediterranean Sea pelagic
           planktonic ecosystem and associated carbon cycle
    • Authors: Marine Herrmann; Claude Estournel, Fanny Adloff, Frédéric Diaz
      Pages: n/a - n/a
      Abstract: The Northwestern Mediterranean Sea (NWMS) is biologically one of the most productive Mediterranean regions. NWMS pelagic planktonic ecosystem is strongly influenced by hydrodynamics, in particular by deep convection, that could significantly weaken under the influence of climate change. Here we investigate the response of this ecosystem and associated carbon cycle to the long‐term evolution of oceanic and atmospheric circulations. For that we developed a tridimensional coupled physical‐biogeochemical model and performed two groups of annual simulations under the climate conditions of respectively the XXth and the end of XXIst century. Our results suggest that the evolution of oceanic and atmospheric circulations does not modify the NWMS pelagic planktonic ecosystem and associated carbon cycle at a first order. However differences mainly induced by the deep convection weakening and the surface warming are obtained at a second order. The spring bloom occurs one month earlier. Resulting from the decrease in nutrients availability, the bottom up control of phytoplankton development and bacteria growth by the nitrogen and phosphorus availability strengthens and the microbial loop intensifies as the small‐sized plankton biomass increases. Carbon net fixation and deep export do not change significantly. The choice of the biogeochemical initial and boundary conditions does not change the representation of the ecosystem seasonal cycle, but the associated uncertainty range can be one order of magnitude larger than the predicted interannual and long term variabilities. The uncertainty range of long‐term trends associated to the physical forcing (hydrological, atmospheric, hydrodynamical and socio‐economic) is much smaller (
      PubDate: 2014-07-21T22:56:50.233667-05:
      DOI: 10.1002/2014JC010016
  • Multiscale dynamical analysis of a high resolution numerical model
           simulation of the Solomon Sea circulation
    • Authors: Bughsin' Djath; Jacques Verron, Angelique Melet, Lionel Gourdeau, Bernard Barnier, Jean‐Marc Molines
      Pages: n/a - n/a
      Abstract: A high 1/36° resolution numerical model is used to study the ocean circulation in the Solomon Sea. An evaluation of the model with (the few) available observation shows that the 1/36° resolution model realistically simulates the Solomon Sea circulations. The model notably reproduces the high levels of mesoscale eddy activity observed in the Solomon Sea. With regard to previous simulations at 1/12° resolution, the average eddy kinetic energy levels are increased by up to ~30‐40% in the present 1/36° simulation, and the enhancement extends at depth. At the surface, the eddy kinetic energy level is maximum in March‐April‐May and is minimum in December‐January‐February. The high subsurface variability is related to the variability of the western boundary current (New Guinea Coastal Undercurrent). Moreover, the emergence of submesoscales is clearly apparent in the present simulations. A spectral analysis is conducted in order to evidence and characterize the modeled submesoscale dynamics and to provide a spectral view of scales interactions. The corresponding spectral slopes show a strong consistency with the Surface Quasigeostrophic turbulence theory.
      PubDate: 2014-07-17T04:09:28.480867-05:
      DOI: 10.1002/2013JC009695
  • Impact of an extreme flood event on optical and biogeochemical properties
           in a sub‐tropical coastal peri‐urban embayment (Eastern
    • Authors: Kadija Oubelkheir; Phillip W. Ford, Lesley A. Clementson, Nagur Cherukuru, Gary Fry, Andrew D.L. Steven
      Pages: n/a - n/a
      Abstract: Major floods impacted the city of Brisbane, eastern Australia, in January 2011, delivering large amounts of dissolved and particulate materials and nutrients into the adjacent coastal embayment, Moreton Bay. The resulting spatially‐resolved changes in biogeochemical and optical properties in Moreton Bay were examined 1, 2, 6, 19 and 49 weeks after the main freshwater discharge. One week post‐flood, total suspended matter (TSM) and chlorophyll a (TChla) concentrations varied over one order of magnitude throughout Moreton Bay, the particle scattering coefficient at 555 nm varied by a factor of 20, and the total absorption coefficient and coloured dissolved organic matter (CDOM) absorption coefficient at 440 nm varied by a factor of 5. The largest changes in biogeochemical and optical properties observed during our study were from one to two weeks after the floods: near the Brisbane River mouth, TSM decreased by a factor of 3, CDOM by a factor of 2, while TChla increased by a factor of 3. Within a year, optical and biogeochemical properties recovered to levels similar to non‐flood conditions. The strong changes in the characteristics of the particulate and dissolved material following the flood event and subsequent biological and photochemical processes led to a large spatial and temporal variability in the relative contribution of different constituents to the total absorption coefficient at 440 nm, the particle single scattering albedo, and the specific inherent optical properties. This work has significant implications for the accuracy of standard ocean colour remote sensing algorithms in coastal waters during flood events.
      PubDate: 2014-07-14T19:23:20.886476-05:
      DOI: 10.1002/2014JC010205
  • Modeling the winter‐spring transition of first‐year ice in the
           western Weddell Sea
    • Authors: N. Jeffery; E. C. Hunke
      Pages: n/a - n/a
      Abstract: A new halodynamic scheme is coupled with the Los Alamos sea ice model to simulate western Weddell Sea ice during the winter‐spring transition. One‐dimensional temperature and salinity profiles are consistent with the warming and melt stages exhibited in first‐year ice cores from the 2004 Ice Station POLarstern (ISPOL) expedition. Results are highly sensitive to snowfall. Simulations which use reanalysis precipitation data do not retain a snow cover beyond mid‐December, and the warming transition occurs too rapidly. Model performance is greatly improved by prescribing a snowfall rate based on reported snow thicknesses. During ice growth prior to ISPOL, simulations indicate a period of thick snow and upper ice salinity enrichment. Gravity drainage model parameters impact the simulation immediately, while effects from the flushing parameter (snow porosity at the ice top) appear as the freeboard becomes negative. Simulations using a snow porosity of 0.3, consistent with that of wet snow, agree with salinity observations. The model does not include lateral sources of sea‐water flooding, but vertical transport processes account for the high upper‐ice salinities observed in ice cores at the start of the expedition. As the ice warms, a fresh upperice layer forms, and the high salinity layer migrates downwards. This pattern is consistent with the early spring development stages of high‐porosity layers observed in Antarctic sea ice that are associated with rich biological production. Future extensions of the model may be valuable in Antarctic icebiogeochemical applications.
      PubDate: 2014-07-11T03:08:03.088371-05:
      DOI: 10.1002/2013JC009634
  • An in situ‐satellite blended analysis of global sea surface salinity
    • Authors: P. Xie; T. Boyer, E. Bayler, Y. Xue, D. Byrne, J. Reagan, R. Locarnini, F. Sun, R. Joyce, A. Kumar
      Pages: n/a - n/a
      Abstract: Ablended monthly sea‐surface salinity (SSS) analysis, called the NOAA “Blended Analysis of Surface Salinity” (BASS), is constructed for the four year period from 2010 to 2013. Three data sets are employed as inputs to the blended analysis: in situ SSS measurements aggregated and quality controlled by NOAA/NODC, and passive microwave (PMW) retrievals from both the National Aeronautics and Space Administration's (NASA) Aquarius/SAC‐D and the European Space Agency's (ESA) Soil Moisture – Ocean Salinity (SMOS) satellites. The blended analysis comprises two steps. First, the biases in the satellite retrievals are removed through probability distribution function (PDF) matching against temporally‐spatially co‐located in situ measurements. The blended analysis is then achieved through optimal interpolation (OI), where the analysis for the previous time step is used as the first guess while the in situ measurements and bias‐corrected satellite retrievals are employed as the observations to update the first guess. Cross‐validations illustrate improved quality of the blended analysis, with reduction in bias and random errors over most of the global oceans as compared to the individual inputs. Large uncertainty, however, remains in high‐latitude oceans and coastal regions where the in situ networks are sparse and current‐generation satellite retrievals have limitations. Our blended SSS analysis shows good agreements with the NODC in situ ‐ based analysis over most of the tropical and sub‐tropical oceans, but large differences are observed for high‐latitude oceans and along coasts. In the tropical oceans, the BASS is shown to have coherent variability with precipitation and evaporation associated with the evolution of the El Niño – Southern Oscillation (ENSO).
      PubDate: 2014-07-11T00:14:08.446024-05:
      DOI: 10.1002/2014JC010046
  • Effects of mangroves and tidal flats on suspended‐sediment dynamics:
           Observational and numerical study of Darwin Harbour, Australia
    • Authors: Li Li; Xiao Hua Wang, Fernando Andutta, David Williams
      Pages: n/a - n/a
      Abstract: The suspended‐sediment dynamics in Darwin Harbour, Australia were investigated using field measurements and numerical modeling. The model suspended‐sediment concentration (SSC) agreed well with observation; the root‐mean‐square error was less than 0.02kgm‐3 and the anomaly‐correlation‐coefficient greater than 0.6. Model results indicate that the tide is the dominant forcing for suspended‐sediment transport: total sediment transport was seaward in the channel and landward at the East and Middle Arm entrances, dominated by the Eulerian residual current. Further numerical experiments indicate that mangroves and tidal flats play key roles in redistributing suspended sediment and affecting total sediment transport by modulating the tides and the tidal asymmetry. In Darwin Harbour, if these areas were reclaimed, there would be a significant transport of sediment into the inner harbor. However, the water in East Arm would be less turbid, with about 70% lower bottom SSC during spring tides. The landward sediment flux at its entrance would decrease by 99%, because of reduced currents in the Arm due to a weakened tidal choking effect. Tidal pumping would then dominate sediment transport in the channel and at the entrances of East and Middle Arms. Dredging for the East Arm Wharf affected the SSC upstream in East Arm. According to the model, material from dredging disposed of at a location outside the harbor will be transported back into the outer harbor, generating higher SSC values there. Although this study is site‐specific, the findings may be applicable to suspended‐sediment dynamics in other harbors and estuaries with extensive tidal flats and mangroves.
      PubDate: 2014-06-26T14:18:20.614901-05:
      DOI: 10.1002/2014JC009987
  • SMOS salinity in the subtropical north Atlantic salinity maximum: 1.
           Comparison with Aquarius and in situ salinity
    • Authors: O. Hernandez; J. Boutin, N. Kolodziejczyk, G. Reverdin, N. Martin, F. Gaillard, N. Reul, J.L. Vergely
      Pages: n/a - n/a
      Abstract: Sea surface salinity (SSS) measured from space by the Soil Moisture and Ocean Salinity (SMOS) mission is validated in the subtropical North Atlantic Ocean. 39 transects of ships of opportunity equipped with thermosalinographs (TSG) crossed that region from 2010 to 2012, providing a large database of ground truth SSS. SMOS SSS is also compared to Aquarius SSS. Large seasonal biases remain in SMOS and Aquarius SSS. In order to look at the capability of satellite SSS to monitor spatial variability, especially at scales less than 300 km (not monitored with the Argo network), we first apply a monthly bias correction derived from satellite SSS and In Situ Analysis System (ISAS) SSS differences averaged over the studied region. Ship SSS averaged over 25 km is compared with satellite and ISAS SSS. Similar statistics are obtained for SMOS, Aquarius and ISAS products (root mean square error of about 0.15 and global correlation coefficient r of about 0.92). However, in the above statistics, SSS varies due to both large scale and mesoscale (here, for scales around 100 km) variability. In order to focus on mesoscale variability, we consider SSS anomalies with respect to a monthly climatology. SMOS SSS and Aquarius SSS anomalies are more significantly correlated (r > 0.5) to TSG SSS anomaly than ISAS. We show the effective gain of resolution and coverage provided by the satellite products over the interpolated in situ data. We also show the advantage of SMOS (r=0.57) over Aquarius (r=0.52) to reproduce SSS mesoscale features.
      PubDate: 2014-06-25T21:41:39.780213-05:
      DOI: 10.1002/2013JC009610
  • Data assimilative modeling investigation of Gulf Stream Warm Core Ring
           interaction with continental shelf and slope circulation
    • Authors: Ke Chen; Ruoying He, Brian S. Powell, Glen G. Gawarkiewicz, Andrew M. Moore, Hernan G. Arango
      Pages: n/a - n/a
      Abstract: A data assimilative ocean circulation model is used to hindcast the interaction between a large Gulf Stream Warm Core Ring (WCR) with the Mid‐Atlantic Bight (MAB) shelf and slope circulation. Using the recently developed Incremental Strong constraint 4D Variational (I4D‐Var) data assimilation algorithm, the model assimilates mapped satellite sea surface height (SSH), sea surface temperature (SST), in situ temperature and salinity profiles measured by expendable bathythermograph, Argo floats, shipboard CTD casts, and glider transects. Model validations against independent hydrographic data show 60% and 57% error reductions in temperature and salinity, respectively. The WCR significantly changed MAB continental slope and shelf circulation. The mean cross‐shelf transport induced by the WCR is estimated to be 0.28 Sv offshore, balancing the mean along‐shelf transport by the shelfbreak jet. Large heat/salt fluxes with peak values of ‐8900 Watt m‐2/‐4×10‐4 Kg m‐2 s‐1 are found when the WCR was impinging upon the shelf break. Vorticity analysis reveals the nonlinear advection term, as well as the residual of joint effect of baroclinicity and bottom relief (JEBAR) and advection of potential vorticity (APV) play important roles in controlling the variability of the eddy vorticity.
      PubDate: 2014-06-14T03:39:41.892753-05:
      DOI: 10.1002/2014JC009898
  • Similarity scaling of turbulence in a temperate lake during fall cooling
    • Authors: Edmund W. Tedford; Sally MacIntyre, Scott D. Miller, Matthew J. Czikowsky
      Pages: 4689 - 4713
      Abstract: Turbulence, quantified as the rate of dissipation of turbulent kinetic energy (ε), was measured with 1400 temperature‐gradient microstructure profiles obtained concurrently with time series measurements of temperature and current profiles, meteorology, and lake‐atmosphere fluxes using eddy covariance in a 4 km2 temperate lake during fall cooling. Winds varied from near calm to 5 m s−1 but reached 10 m s−1 during three storm events. Near‐surface values of ε were typically on the order of 10−8 to 10−7 m2 s−3 and reached 10−5 m2 s−3 during windy periods. Above a depth equal to LMO , the Monin‐Obukhov length scale, turbulence was dominated by wind shear and dissipation followed neutral law of the wall scaling augmented by buoyancy flux during cooling. During cooling, εz = 0.56 u*w3/kz + 0.77 JB0 and during heating εz = 0.6 u*w3/kz, where u*w is the water friction velocity computed from wind shear stress, k is von Karman's constant, z is depth, and JB0 is surface buoyancy flux. Below a depth equal to LMO during cooling, dissipation was uniform with depth and controlled by buoyancy flux. Departures from similarity scaling enabled identification of additional processes that moderate near‐surface turbulence including mixed layer deepening at the onset of cooling, high‐frequency internal waves when the diurnal thermocline was adjacent to the air‐water interface, and horizontal advection caused by differential cooling. The similarity scaling enables prediction of near‐surface ε as required for estimating the gas transfer coefficient using the surface renewal model and for understanding controls on scalar transport.
      PubDate: 2014-08-01T15:04:43.234817-05:
      DOI: 10.1002/2014JC010135
  • Sea level extremes in the Caribbean Sea
    • Authors: R. Ricardo Torres; Michael N. Tsimplis
      Pages: 4714 - 4731
      Abstract: Sea level extremes in the Caribbean Sea are analyzed on the basis of hourly records from 13 tide gauges. The largest sea level extreme observed is 83 cm at Port Spain. The largest nontidal residual in the records is 76 cm, forced by a category 5 hurricane. Storm surges in the Caribbean are primarily caused by tropical storms and stationary cold fronts intruding the basin. However, the seasonal signal and mesoscale eddies also contribute to the creation of extremes. The five stations that have more than 20 years of data show significant trends in the extremes suggesting that flooding events are expected to become more frequent in the future. The observed trends in extremes are caused by mean sea level rise. There is no evidence of secular changes in the storm activity. Sea level return periods have also been estimated. In the south Colombian Basin, where large hurricane‐induced surges are rare, stable estimates can be obtained with 30 years of data or more. For the north of the basin, where large hurricane‐induced surges are more frequent, at least 40 years of data are required. This suggests that the present data set is not sufficiently long for robust estimates of return periods. ENSO variability correlates with the nontidal extremes, indicating a reduction of the storm activity during positive ENSO events. The period with the highest extremes is around October, when the various sea level contributors' maxima coincide.
      PubDate: 2014-08-04T13:44:42.511988-05:
      DOI: 10.1002/2014JC009929
  • Estimating satellite salinity errors for assimilation of Aquarius and SMOS
           data into climate models
    • Authors: Nadya T. Vinogradova; Rui M. Ponte, Ichiro Fukumori, Ou Wang
      Pages: 4732 - 4744
      Abstract: Constraining dynamical systems with new information from ocean measurements, including observations of sea surface salinity (SSS) from Aquarius and SMOS, requires careful consideration of data errors that are used to determine the importance of constraints in the optimization. Here such errors are derived by comparing satellite SSS observations from Aquarius and SMOS with ocean model output and in situ data. The associated data error variance maps have a complex spatial pattern, ranging from less than 0.05 in the open ocean to 1–2 (units of salinity variance) along the coasts and high latitude regions. Comparing the data‐model misfits to the data errors indicates that the Aquarius and SMOS constraints could potentially affect estimated SSS values in several ocean regions, including most tropical latitudes. In reference to the Aquarius error budget, derived errors are less than the total allocation errors for the Aquarius mission accuracy requirements in low and midlatitudes, but exceed allocation errors in high latitudes.
      PubDate: 2014-08-04T13:45:55.425233-05:
      DOI: 10.1002/2014JC009906
  • Mesoscale activity in the Comoros Basin from satellite altimetry and a
           high‐resolution ocean circulation model
    • Authors: C. Collins; J. C. Hermes, C. J. C. Reason
      Pages: 4745 - 4760
      Abstract: Recently it has been shown that anticyclonic eddies are generated in the Comoros Basin contesting the long‐held notion of a single large anticyclonic cell, the Comoros Gyre. Limited knowledge exists about the mesoscale activity within the basin, a potential key source of variability for the Mozambique Channel and subsequently the Agulhas Current. In this paper an automated eddy tracking scheme, applied to satellite altimetry data and a high‐resolution model simulation, is used to determine the characteristics of the anticyclonic eddies generated in the Comoros Basin. The generation and characteristics of cyclonic eddies are also investigated. The eddy tracking scheme revealed that anticyclonic eddies are primarily generated west of the tip of Madagascar due to barotropic instabilities whereas cyclonic eddies are mainly generated along the northwest coast of Madagascar as a result of baroclinic instabilities. Anticyclonic eddies, with a mean lifespan of about 3 months, reside in the basin for half their lifespan before propagating into the Mozambique Channel. On the other hand, the majority of cyclonic eddies, with a similar mean lifespan, dissipate within the basin. Initially, the anticyclones, with translation speeds of 6–8 km d−1 and mean radii of 80–100 km, follow the trajectory of the North East Madagascar Current and turn south upon reaching the African coast. The cyclonic eddies tend to be smaller (∼60 km) and have slower translation speeds (2.5–3.5 km d−1) than their anticyclonic counterparts.
      PubDate: 2014-08-05T15:15:28.01769-05:0
      DOI: 10.1002/2014JC010008
  • Southern Ocean velocity and geostrophic transport fields estimated by
           combining Jason altimetry and Argo data
    • Authors: Michael Kosempa; Don P. Chambers
      Pages: 4761 - 4776
      Abstract: Zonal geostrophic velocity fields above 1975 dbar have been estimated for the Southern Ocean from 2004 into 2011 based on sea surface topography observed by Jason altimetry and temperature/salinity measured by Argo autonomous floats. The velocity at 1000 dbar estimated with the method has been compared to Argo drift trajectory at the same pressure level available from the Asia Pacific Data Research Center (APDRC). The inferred velocities agree with those from the Argo drift within the estimated sampling error of the latter, but have fewer gaps in space and time. The velocity has also been integrated from depth to surface to determine the mean and time‐variable zonal geostrophic transport in the Southern Ocean between 29.5°S and 58.5°S, primarily in the South Atlantic and South Indian Ocean basins, due to limitations in coverage of Argo. Analysis shows errors can be reduced by >70% by averaging gridded results over wide areas. Zonal transport averaged over the entire Indian Ocean basin shows a significant correlation with the Antarctic Oscillation (AAO) at low frequencies: transport is higher than normal during a positive phase of the AAO, and lower during the negative phase.
      PubDate: 2014-08-05T15:33:09.273936-05:
      DOI: 10.1002/2014JC009853
  • A SEEK filter assimilation of sea surface salinity from Aquarius in an
           OGCM: Implication for surface dynamics and thermohaline structure
    • Authors: Abhisek Chakraborty; Rashmi Sharma, Raj Kumar, Sujit Basu
      Pages: 4777 - 4796
      Abstract: Singular Evolutive Extended Kalman (SEEK) filter has been used to assimilate Aquarius‐derived sea surface salinity (SSS) in a near‐global ocean general circulation model (OGCM). Advanced Very High Resolution Radiometer (AVHRR)‐derived sea surface temperature (SST) has also been assimilated in conjunction. The primary aim of the study is to investigate the improvement in simulation of global ocean surface currents as a result of this assimilation. The route of empirical orthogonal function (EOF) analysis has been taken for an efficient assessment of this impact separately in the space and time domains and satellite‐derived surface current has been used as a benchmark. As expected, the assimilation has been found to impart significant positive impact in both the domains. Also, joint assimilation of SSS and SST has been found to be better than standalone SSS assimilation. These results have been further corroborated by a comparison with buoy‐derived surface currents. Further emphasis has been laid on the simulation of Wyrtki and monsoon jets in the equatorial Indian Ocean, because of their importance in the climate of this region and again it has been found that assimilation guides the simulation toward realism in both the cases. Finally, impact on the SSS and SST fronts and their zonal displacements in the western Pacific has been investigated and here again the assimilation has led to an improvement in simulation of these features.
      PubDate: 2014-08-05T15:13:32.268866-05:
      DOI: 10.1002/2014JC009984
  • Dense intermediate water outflow from the Cretan Sea: A salinity driven,
           recurrent phenomenon, connected to thermohaline circulation changes
    • Authors: Dimitris Velaoras; George Krokos, Kostas Nittis, Alexander Theocharis
      Pages: 4797 - 4820
      Abstract: Data collected from different platforms in the Cretan Sea during the 2000s decade present evidence of gradually increasing salinity in the intermediate and deep intermediate layers after the middle of the decade. The observed gradual salt transport toward the deeper layers indicates contributions of dense water masses formed in various Aegean Sea subbasins. The accumulation of these saline and dense water masses in the Cretan Sea finally led to outflow from both Cretan Straits, with density greater than typical Levantine/Cretan Intermediate water but not dense enough to penetrate into the deep layers of the Eastern Mediterranean. We name this outflowing water mass as dense Cretan Intermediate Water (dCIW). A retrospective analysis of in situ data and literature references during the last four decades shows that similar events have occurred in the past in two occasions: (a) in the 1970s and (b) during the Eastern Mediterranean Transient (EMT) onset (1987–1991). We argue that these salinity‐driven Aegean outflows are mostly attributed to recurrent changes of the Eastern Mediterranean upper thermohaline circulation that create favorable dense water formation conditions in the Aegean Sea through salinity preconditioning. We identify these phenomena as “EMT‐like” events and argue that in these cases internal thermohaline mechanisms dominate over atmospheric forcing in dense water production. However, intense atmospheric forcing over an already salinity preconditioned basin is indispensable for creating massive deep water outflow from the Cretan Sea, such as the EMT event.
      PubDate: 2014-08-05T15:27:23.946021-05:
      DOI: 10.1002/2014JC009937
  • Uncertainty of Aquarius sea surface salinity retrieved under rainy
           conditions and its implication on the water cycle study
    • Authors: Wenqing Tang; Simon H. Yueh, Alexander G. Fore, Akiko Hayashi, Tong Lee, Gary Lagerloef
      Pages: 4821 - 4839
      Abstract: The uncertainty of Aquarius sea surface salinity (SSS) retrieved under rain is assessed. Rain not only has instantaneous impact on SSS but also interferes with the microwave remote sensing signals, making the task to retrieve SSS under rainy conditions difficult. A rain correction model is developed based on analysis of the L‐band radiometer/scatterometer residual signals after accounting for roughness due to wind and flat surface emissivity. The combined active passive algorithm is used to retrieve SSS in parallel with (CAP_RC) or without rain correction (CAP). The CAP bias against individual ARGO floats increases with rain rate with slope of −0.14 PSU per mm h−1, which reduced to near zero in CAP_RC. On the global monthly basis, CAP_RC is about 0.03 PSU higher than CAP. RMSD against ARGO is slightly smaller for CAP_RC than CAP. Regional biases are examined in areas with frequent rain events. As expected, results show that ΔSSS (CAP_RC‐CAP) is highly correlated with the seasonal precipitation pattern, reaching about 0.2–0.3 PSU under heavy rain. However, ΔSSS shows no correlation with the difference pattern between ARGO and CAP or CAP_RC. This, along with regional analyses, suggests that the difference between ARGO and Aquarius' SSS is likely caused by the different spatial and temporal sampling, in addition to near surface stratification depicted by radiometer and ARGO at different depths. The effect of ΔSSS on water cycle in terms of mixed‐layer salt storage tendency is about 10% in areas where evaporation‐minus‐precipitation is the dominant process driving the variability of near surface salinity.
      PubDate: 2014-08-07T15:40:57.931154-05:
      DOI: 10.1002/2014JC009834
  • Global observations of quasi‐zonal bands in microwave sea surface
    • Authors: C. E. Buckingham; P. C. Cornillon, F. Schloesser, K. M. Obenour
      Pages: 4840 - 4866
      Abstract: Global observations of quasi‐zonal jet‐like structures have recently been reported in estimates of upper ocean circulation. To date, these observations have come primarily from float‐derived and altimeter‐derived estimates of zonal velocity. Here, we explore the existence of similar structures in the ocean using satellite‐derived estimates of sea surface temperature (SST) from the Advanced Microwave Scanning Radiometer for the Earth Observing System (AMSR‐E). Applying an ocean front detection algorithm globally to microwave measurements of SST, we find that repeated ocean fronts occur along quasi‐zonal bands in a multiyear (2002–2011) average of detections. Such a pattern is also observed in SST gradient magnitude. Composite analyses of SST, sea surface height (SSH), and upper ocean temperatures from Argo profiling floats suggest repeated fronts in the subtropics occur as a result of neighboring anticyclonic and cyclonic eddies. Horizontal advection in the presence of a background temperature gradient likely plays a role as evidenced by the tilt of temperature anomalies with depth. High gradient events found within the bands are observed to propagate westward with speed comparable to mesoscale eddies and we estimate these events explain 20% of the observed variance in SST gradient magnitude (2002–2011). In a final analysis, we regress the decay of the bands with averaging period and observe mild‐to‐strong persistence throughout much of the World Ocean. These findings support the view that propagating eddies help give rise to the bands. Whether or not eddies follow preferred paths remain unanswered.
      PubDate: 2014-08-08T14:05:21.923121-05:
      DOI: 10.1002/2014JC010088
  • Mesoscale eddy effects on the subduction of North Pacific mode waters
    • Authors: Lixiao Xu; Shang‐Ping Xie, Julie L. McClean, Qinyu Liu, Hideharu Sasaki
      Pages: 4867 - 4886
      Abstract: Mesoscale eddy effects on the subduction of North Pacific mode waters are investigated by comparing observations and ocean general circulation models where eddies are either parameterized or resolved. The eddy‐resolving models produce results closer to observations than the noneddy‐resolving model. There are large discrepancies in subduction patterns between eddy‐resolving and noneddy‐resolving models. In the noneddy‐resolving model, subduction on a given isopycnal is limited to the cross point between the mixed layer depth (MLD) front and the outcrop line whereas in eddy‐resolving models and observations, subduction takes place in a broader, zonally elongated band within the deep mixed layer region. Mesoscale eddies significantly enhance the total subduction rate, helping create remarkable peaks in the volume histogram that correspond to North Pacific subtropical mode water (STMW) and central mode water (CMW). Eddy‐enhanced subduction preferentially occurs south of the winter mean outcrop. With an anticyclonic eddy to the west and a cyclonic eddy to the east, the outcrop line meanders south, and the thermocline/MLD shoals eastward. As eddies propagate westward, the MLD shoals, shielding the water of low potential vorticity from the atmosphere. The southward eddy flow then carries the subducted water mass into the thermocline. The eddy subduction processes revealed here have important implications for designing field observations and improving models.
      PubDate: 2014-08-08T10:28:21.411177-05:
      DOI: 10.1002/2014JC009861
  • An intercomparison of Arctic ice drift products to deduce uncertainty
    • Authors: Hiroshi Sumata; Thomas Lavergne, Fanny Girard‐Ardhuin, Noriaki Kimura, Mark A. Tschudi, Frank Kauker, Michael Karcher, Rüdiger Gerdes
      Pages: 4887 - 4921
      Abstract: An intercomparison of four low‐resolution remotely sensed ice‐drift products in the Arctic Ocean is presented. The purpose of the study is to examine the uncertainty in space and time of these different drift products. The comparison is based on monthly mean ice drifts from October 2002 to December 2006. The ice drifts were also compared with available buoy data. The result shows that the differences of the drift vectors are not spatially uniform, but are covariant with ice concentration and thickness. In high (low) ice‐concentration areas, the differences are small (large), and in thick (thin) ice‐thickness areas, the differences are small (large). A comparison with the drift deduced from buoys reveals that the error of the drift speed depends on the magnitude of the drift speed: larger drift speeds have larger errors. Based on the intercomparison of the products and comparison with buoy data, uncertainties of the monthly mean drift are estimated. The estimated uncertainty maps reasonably reflect the difference between the products in relation to ice concentration and the bias from the buoy drift in relation to drift speed. Examinations of distinctive features of Arctic sea ice motion demonstrate that the transpolar drift speed differs among the products by 13% (0.32 cm s−1) on average, and ice drift curl in the Amerasian Basin differs by up to 24% (3.3 × 104 m2 s−1). These uncertainties should be taken into account if these products are used, particularly for model validation and data assimilation within the Arctic.
      PubDate: 2014-08-08T14:05:53.760363-05:
      DOI: 10.1002/2013JC009724
  • Observed bottom boundary layer transport and uplift on the continental
           shelf adjacent to a western boundary current
    • Authors: A. Schaeffer; M. Roughan, J. E. Wood
      Pages: 4922 - 4939
      Abstract: Western boundary currents strongly influence the dynamics on the adjacent continental shelf and in particular the cross‐shelf transport and uplift through the bottom boundary layer. Four years of moored in situ observations on the narrow southeastern Australian shelf (in water depths of between 65 and 140 m) were used to investigate bottom cross‐shelf transport, both upstream (30°S) and downstream (34°S) of the separation zone of the East Australian Current (EAC). Bottom transport was estimated and assessed against Ekman theory, showing consistent results for a number of different formulations of the boundary layer thickness. Net bottom cross‐shelf transport was onshore at all locations. Ekman theory indicates that up to 64% of the transport variability is driven by the along‐shelf bottom stress. Onshore transport in the bottom boundary layer was more intense and frequent upstream than downstream, occurring 64% of the time at 30°S. Wind‐driven surface Ekman transport estimates did not balance the bottom cross‐shelf flow. At both locations, strong variability was found in bottom water transport at periods of approximately 90–100 days. This corresponds with periodicity in EAC fluctuations and eddy shedding as evidenced from altimeter observations, highlighting the EAC as a driver of variability in the continental shelf waters. Ocean glider and HF radar observations were used to identify the bio‐physical response to an EAC encroachment event, resulting in a strong onshore bottom flow, the uplift of cold slope water, and elevated coastal chlorophyll concentrations.
      PubDate: 2014-08-11T04:07:29.19617-05:0
      DOI: 10.1002/2013JC009735
  • Active‐passive synergy for interpreting ocean L‐band
           emissivity: Results from the CAROLS airborne campaigns
    • Authors: A. C. H. Martin; J. Boutin, D. Hauser, E. P. Dinnat
      Pages: 4940 - 4957
      Abstract: The impact of the ocean surface roughness on the ocean L‐band emissivity is investigated using simultaneous airborne measurements from an L‐band radiometer (CAROLS) and from a C‐band scatterometer (STORM) acquired in the Gulf of Biscay (off‐the French Atlantic coasts) in November 2010. Two synergetic approaches are used to investigate the impact of surface roughness on the L‐band brightness temperature (Tb). First, wind derived from the scatterometer measurements is used to analyze the roughness contribution to Tb as a function of wind and compare it with the one simulated by SMOS and Aquarius roughness models. Then residuals from this mean relationship are analyzed in terms of mean square slope derived from the STORM instrument. We show improvement of new radiometric roughness models derived from SMOS and Aquarius satellite measurements in comparison with prelaunch models. Influence of wind azimuth on Tb could not be evidenced from our data set. However, we point out the importance of taking into account large roughness scales (>20 cm) in addition to small roughness scale (5 cm) rapidly affected by wind to interpret radiometric measurements far from nadir. This was made possible thanks to simultaneous estimates of large and small roughness scales using STORM at small (7–16°) and large (30°) incidence angles.
      PubDate: 2014-08-11T04:06:08.279911-05:
      DOI: 10.1002/2014JC009890
  • Near‐surface temperature gradient in a coastal upwelling regime
    • Authors: H. Maske; J. Ochoa, C. O. Almeda‐Jauregui, M. C. Ruiz‐de la Torre, R. Cruz‐López, J. R. Villegas‐Mendoza
      Pages: 4972 - 4982
      Abstract: In oceanography, a near homogeneous mixed layer extending from the surface to a seasonal thermocline is a common conceptual basis in physics, chemistry, and biology. In a coastal upwelling region 3 km off the coast in the Mexican Pacific, we measured vertical density gradients with a free‐rising CTD and temperature gradients with thermographs at 1, 3, and 5 m depths logging every 5 min during more than a year. No significant salinity gradient was observed down to 10 m depth, and the CTD temperature and density gradients showed no pronounced discontinuity that would suggest a near‐surface mixed layer. Thermographs generally logged decreasing temperature with depth with gradients higher than 0.2 K m−1 more than half of the time in the summer between 1 and 3 m, 3 and 5 m and in the winter between 1 and 3 m. Some negative temperature gradients were present and gradients were generally highly variable in time with high peaks lasting fractions of hours to hours. These temporal changes were too rapid to be explained by local heating or cooling. The pattern of positive and negative peaks might be explained by vertical stacks of water layers of different temperatures and different horizontal drift vectors. The observed near‐surface gradient has implications for turbulent wind energy transfer, vertical exchange of dissolved and particulate water constituents, the interpretation of remotely sensed SST, and horizontal wind‐induced transport.
      PubDate: 2014-08-11T04:08:38.524388-05:
      DOI: 10.1002/2014JC010074
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