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Journal Cover Journal of Geophysical Research : Oceans
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   Published by American Geophysical Union (AGU) Homepage  [17 journals]
  • The characteristics and dynamics of wave‐driven flow across a
           platform coral reef in the Red Sea
    • Authors: S. J. Lentz; J. H. Churchill, K. A. Davis, J. T. Farrar, J. Pineda, V. Starczak
      Abstract: Current dynamics across a platform reef in the Red Sea near Jeddah, Saudi Arabia are examined using 18 months of current profile, pressure, surface wave, and wind observations. The platform reef is 700 m long, 200 m across with spatial and temporal variations in water depth over the reef ranging from 0.6 m to 1.6 m. Surface waves breaking at the seaward edge of the reef cause a 2 −10 cm setup of sea level that drives cross‐reef currents of 5 – 20 cm s−1. Bottom stress is a significant component of the wave setup balance in the surf zone. Over the reef flat, where waves are not breaking, the cross‐reef pressure gradient associated with wave setup is balanced by bottom stress. The quadratic drag coefficient for the depth‐average flow decreases with increasing water depth from Cda=0.17 in 0.4 m of water to Cda=0.03 in 1.2 m of water. The observed dependence of the drag coefficient on water depth is consistent with open channel flow theory and a hydrodynamic roughness of zo=0.06 m. A simple one‐dimensional model driven by incident surface waves and wind stress accurately reproduces the observed depth‐averaged cross‐reef currents and a portion of the weaker along‐reef currents over the focus reef and two other Red Sea platform reefs. The model indicates the cross‐reef current is wave‐forced and the along‐reef current is partially wind‐forced. This article is protected by copyright. All rights reserved.
      PubDate: 2016-01-29T08:33:09.29082-05:0
      DOI: 10.1002/2015JC011141
  • Wind and wave influences on sea ice floe size and leads in the Beaufort
           and Chukchi Seas during the summer‐fall transition 2014
    • Authors: Yu Wang; Benjamin Holt, W. Erick Rogers, Jim Thomson, Hayley H. Shen
      Abstract: Sea ice floe size distribution and lead properties in the Beaufort and Chukchi Seas are studied in the summer‐fall transition, 2014, to examine the impact on the sea ice cover from storms and surface waves. Floe size distributions are analyzed from MEDEA, Landsat8, and RADARSAT‐2 imagery, with a resolution span of 1 to 100 m. Landsat8 imagery is also used to identify the orientation and spacing of leads. The study period centers around three large wave events during August‐September 2014 identified by SWIFT buoys and WAVEWATCH III® model data. The range of floe sizes from different resolutions provides the overall distribution across a wide range of ice properties and estimated thickness. All cumulative floe size distribution curves show a gradual bending towards shallower slopes for smaller floe sizes. The overall slopes in the cumulative floe size distribution curves from Landsat8 images are lower than, while those from RADARSAT‐2 are similar to, previously reported results in the same region and seasonal period. The MEDEA floe size distributions appeared to be sensitive to the passage of storms. Lead orientations, regardless of length, correlate slightly better with the peak wave direction than with the mean wave direction. Their correlation with the geostrophic wind is stronger than with the surface wind. The spacing between shorter leads correlates well with the local incoming surface wavelengths, obtained from the model peak wave frequency. The information derived shows promise for a coordinated multi‐sensor study of storm effects in the Arctic marginal ice zone. This article is protected by copyright. All rights reserved.
      PubDate: 2016-01-28T18:18:15.51357-05:0
      DOI: 10.1002/2015JC011349
  • Sea level budget in the Bay of Bengal (2002–2014) from GRACE and
    • Authors: J. Kusche; B. Uebbing, R. Rietbroek, C. K. Shum, Z. H. Khan
      Abstract: Sea level rise is perceived as a major threat to the densely populated coast of the Bay of Bengal. Addressing future rise requires understanding the present‐day sea level budget. Using a novel method and data from the Gravity Recovery and Climate Experiment (GRACE) satellite, we partition altimetric sea level rise (6.1 mm/a over 2002–2014) into mass and steric components. We find that current mass trends in the Bay of Bengal are slightly above global mean, while steric trends appear much larger: 2.2–3.1 mm/a if we disregard a residual required to close the budget, and 4.3–4.6 mm/a if, as an upper bound, we attribute this residual entirely to steric expansion. Our method differs from published approaches in that it explains altimetry and GRACE data in a least squares inversion, while mass anomalies are parameterized through gravitationally self‐consistent fingerprints, and steric expansion through EOFs. We validate our estimates by comparing to Argo and modeling for the Indian Ocean, and by comparing total water storage change (TWSC) for the Ganges and Brahmaputra basins to the conventional GRACE approach. We find good agreement for TWSC, and reasonable agreement for steric heights, depending on the ocean region and Argo product. We ascribe differences to weaknesses of the Argo data, but we also find the inversion to be to some extent sensitive with respect to the EOFs. Finally, combining our estimates with CMIP5‐simulations, we estimate that Bay of Bengal absolute sea level may rise for additional 37 cm under the RCP4.5 scenario and 40 cm under RCP8.5 until 2050, with respect to 2005. This article is protected by copyright. All rights reserved.
      PubDate: 2016-01-27T10:29:01.183368-05:
      DOI: 10.1002/2015JC011471
  • Methods of calculation of the Atlantic meridional heat and volume
           transports from ocean models at 26.5°N
    • Authors: Vladimir N. Stepanov; Doroteaciro Iovino, Simona Masina, Andrea Storto, Andrea Cipollone
      Abstract: The Atlantic meridional overturning circulation (AMOC) and the associated meridional heat transport (MHT) at 26.5°N are investigated in two global ocean models at different resolutions and setup and compared with observational estimates from the Rapid Climate Change programme (RAPID). Three different methods of calculation are used to compute the modelled meridional transports: the first method (MOCmod) is based on simulated velocity fields, the second (MOC_endpoint) relies on the same assumptions as for the RAPID calculations, and the third (MOC_ff_baro) is also based on hydrostatic and geostrophic relationships, but relative to the model barotropic circulation. All methods correctly reproduce the time‐mean AMOC strength at 26.5°N, although some differences with observations are present at depth. Similar to other model results, despite the higher AMOC simulated by our eddy‐rich global model, the corresponding heat transport is significantly lower than the RAPID estimates. The above described differences between the deep structure of the modelled and observed AMOC impact the heat transport less than the discrepancies between the MOCmod and MOC_endpoint AMOC structure in the upper ∼500m layer. The comparison of the AMOC obtained by the three methods suggests that an inadequate representation of currents near the western boundary by geostrophy leads to underestimate the southward circulation in the upper‐mid ocean and largely impact the heat transport calculations. In our analysis, calculation based on RAPID assumptions (applied in MOC_endpoint) result in a higher mean heat transport (>25%) compared to MOCmod calculation. This article is protected by copyright. All rights reserved.
      PubDate: 2016-01-27T03:26:12.889439-05:
      DOI: 10.1002/2015JC011007
  • Issue Information
    • PubDate: 2016-01-27T03:13:39.023445-05:
      DOI: 10.1002/jgrc.20868
  • Global characteristics of coherent vortices from surface drifter
    • Authors: Rick Lumpkin
      Abstract: An algorithm is developed that can automatically identify loopers in Lagrangian trajectory data, i.e., looping trajectories that complete at least two orbits, in a significant update to Griffa et al. [2008; Geophys. Res. Lett., 35, L01608] . This algorithm is applied to the Global Drifter Program data set, and over 15,000 looping trajectory segments are identified worldwide. While 2/3 of these segments are 14—39 days long, some persist for hundreds of days; the longest looper in the record persisted for 287 days. The paths taken by the vortices at the center of these looper trajectory segments can be calculated from these data. The Lagrangian integral time scale can also be estimated for the looper segments, and is generally very close to the orbital period – a value several times larger than the integral time scales characterizing nonloopers. Fundamental time‐mean quantities such as total kinetic energy and velocity are shown to be significantly different between loopers and nonloopers. These results suggest that a careful approach to the data might require separately calculating means of the nonloopers and loopers, and only afterward combining the weighted results for an overall time‐mean picture. While many of the loopers with large radii orbit vortices identified in altimeter‐derived eddy census data, many with smaller radii do not match vortices resolved in altimetry. The data from this study are available at This article is protected by copyright. All rights reserved.
      PubDate: 2016-01-25T11:29:43.884048-05:
      DOI: 10.1002/2015JC011435
  • Climate controls multidecadal variability in U.S. extreme sea level
    • Authors: Thomas Wahl; Don P. Chambers
      Abstract: We investigate the links between multi‐decadal changes in extreme sea levels (expressed as 100‐year return water levels (RWLs)) along the United States coastline and large scale climate variability. We develop different sets of simple and multiple linear regression models using both traditional climate indices and tailored indices based on nearby atmospheric/oceanic variables (winds, pressure, sea surface temperature) as independent predictors. The models, after being tested for spatial and temporal stability, are capable of explaining large fractions of the observed variability, up to 96% at individual sites and more than 80% on average across the region. Using the model predictions as covariates in a quasi‐non‐stationary extreme value analysis also significantly reduces the range of change in the 100‐year RWLs over time, turning a non‐stationary process into a stationary one. This suggests that the models – when used with regional and global climate model output of the predictors – will also be capable of projecting future RWL changes. Such information is highly relevant for decision makers in the climate adaptation context in addition to projections of long‐term sea level rise. This article is protected by copyright. All rights reserved.
      PubDate: 2016-01-25T11:24:13.243099-05:
      DOI: 10.1002/2015JC011057
  • Dynamics of the Chesapeake Bay outflow plume: Realistic plume simulation
           and its seasonal and interannual variability
    • Authors: Long Jiang; Meng Xia
      Abstract: The three‐dimensional unstructured‐grid Finite Volume Coastal Ocean Model (FVCOM) was implemented for Chesapeake Bay and its adjacent coastal ocean to delineate the realistic Chesapeake Bay outflow plume (CBOP) as well as its seasonal and interannual variability. Applying the appropriate horizontal and vertical resolution, the model exhibited relatively high skill in matching the observational water level, temperature and salinity from 2003 to 2012. The simulated surface plume structure was verified by comparing output to the HF radar current measurements, earlier field observations and the MODIS and AVHRR satellite imagery. According to the orientation, shape and size of the CBOP from both model snapshots and satellite images, five types of real‐time plume behavior were detected, which implied strong regulation by wind and river discharge. In addition to the episodic plume modulation, horizontal and vertical structure of the CBOP exhibited variations on seasonal and interannual temporal scales. Seasonally, river discharge with a one‐month lag was primarily responsible for the surface plume area variation, while the plume thickness was mainly correlated to wind magnitude. On the interannual scale, river discharge was the predominant source of variability in both surface plume area and depth; however, the southerly winds also influenced the offshore plume depth. In addition, large‐scale climate variability, such as the North Atlantic Oscillation, could potentially affect the plume signature in the long term by altering wind and upwelling dynamics, underlining the need to understand the impacts of climate change on buoyant plumes, such as the CBOP. This article is protected by copyright. All rights reserved.
      PubDate: 2016-01-25T11:23:41.847275-05:
      DOI: 10.1002/2015JC011191
  • On the nature of the frontal zone of the Choctawhatchee Bay plume in the
           Gulf of Mexico
    • Abstract: River plumes often feature turbulent processes in the frontal zone and interfacial region at base of the plume, which ultimately impact spreading and mixing rates with the ambient coastal ocean. The degree to which these processes govern overall plume mixing is yet to be quantified with microstructure observations. A field campaign was conducted in a river plume in the northeast Gulf of Mexico in December 2013, in order to assess mixing processes that could potentially impact transport and dispersion of surface material near coastal regions. Current velocity, density and Turbulent Kinetic Energy Values, ε, were obtained using an Acoustic Doppler Current Profiler (ADCP), a Conductivity Temperature Depth (CTD) profiler, a Vertical Microstructure Profiler (VMP) and two Acoustic Doppler Velocimeters (ADVs). The frontal region contained ε values on the order of 10−5 m2 s−3, which were markedly larger than in the ambient water beneath (O 10−9 m2 s−3). An energetic wake of moderate ε values (O 10−6 m2 s−3) was observed trailing the frontal edge. The interfacial region of an interior section of the plume featured opposing horizontal velocities and a ε value on the order of 10−6 m2 s−3. A simplified mixing budget was used under significant assumptions to compare contributions from wind, tides, and frontal regions of the plume. The results from this order of magnitude analysis indicated that frontal processes (59%) in dominated overall mixing. This emphasizes the importance of adequate parameterization of river plume frontal processes in coastal predictive models. This article is protected by copyright. All rights reserved.
      PubDate: 2016-01-25T11:23:27.862946-05:
      DOI: 10.1002/2015JC010988
  • An evaluation of gas transfer velocity parameterizations during natural
           convection using DNS
    • Authors: S. T. Fredriksson; L. Arneborg, H. Nilsson, Q. Zhang, R. A. Handler
      Abstract: Direct numerical simulations (DNS) of free surface flows driven by natural convection are used to evaluate different methods of estimating air‐water gas exchange at no‐wind conditions. These methods estimate the transfer velocity as a function of either the horizontal flow divergence at the surface, the turbulent kinetic energy dissipation beneath the surface, the heat flux through the surface, or the wind speed above the surface. The gas transfer is modeled via a passive scalar. The Schmidt number dependence is studied for Schmidt numbers of 7, 150 and 600. The methods using divergence, dissipation and heat flux estimate the transfer velocity well for a range of varying surface heat flux values, and domain depths. The two evaluated empirical methods using wind (in the limit of no wind) give reasonable estimates of the transfer velocity, depending however on the surface heat flux and surfactant saturation. The transfer velocity is shown to be well represented by the expression, ks = A(Bν)1/4 Sc‐n, where A is a constant, B is the buoyancy flux, ν is the kinematic viscosity, Sc is the Schmidt number, and the exponent n depends on the water surface characteristics. The results suggest that A = 0.39 for n ≈ 1/2 and n ≈ 2/3 for slip and no‐slip boundary conditions at the surface, respectively. It is further shown that slip and no‐slip boundary conditions predict the heat transfer velocity corresponding to the limits of clean and highly surfactant contaminated surfaces, respectively. This article is protected by copyright. All rights reserved.
      PubDate: 2016-01-25T11:23:15.478417-05:
      DOI: 10.1002/2015JC011112
  • First dedicated hydrographic survey of the Comoros Basin
    • Authors: C. Collins; J.C. Hermes, C.J.C. Reason
      Abstract: The Comoros Basin, a region of eddies that propagate into the Mozambique Channel and subsequently the Agulhas Current, has not been well studied. This paper presents the findings from the first dedicated research cruise in the basin. ADCP data collected during the cruise show that the circulation in the Comoros Basin is dominated by the strong westward flowing North East Madagascar Current, which bifurcates twice: once upon encountering the Davie Ridge and again upon reaching the East African coast. One of the branches flows southward along the African coastline and appears to continue into the Mozambique Channel while the other branch turns northward to become the East African Coastal Current. The ADCP data also show evidence of a cyclonic eddy along the northwest coast of Madagascar. Water mass analysis indicates that all the major Indian Ocean water masses are present in the Comoros Basin. Surprisingly, North Atlantic Deep Water, a water mass previously assumed to only occur south of ∼16°S in the Mozambique Channel, was found to be present in the basin at depths below 2000m. Antarctic Intermediate Water is found to enter the basin via two routes; from the west within the North East Madagascar Current and from the south within the northward flowing Mozambique Undercurrent. This article is protected by copyright. All rights reserved.
      PubDate: 2016-01-25T10:20:24.027568-05:
      DOI: 10.1002/2015JC011418
  • Application of PMF receptor model merging with PAHs signatures for source
           apportionment of black carbon in the continental shelf surface sediments
           of the Bohai and Yellow Seas, China
    • Authors: Yin Fang; Yingjun Chen, Chongguo Tian, Tian Lin, Limin Hu, Jun Li, Gan Zhang
      Abstract: Black carbon (BC) and polycyclic aromatic hydrocarbons (PAHs) are byproducts generated from the incomplete combustion of organic materials, including fossil fuels and biomass. The similar production processes shared by BC and PAHs provide the possibility to infer the BC sources using the PAHs signatures. This study successfully utilized data sets of BC and PAHs analyzed from the continental shelf surface sediments of the Bohai and Yellow Seas to a standard receptor model of Positive Matrix Factorization (PMF) to apportion the sources of BC in the sediment matrix. Results showed that combustion of fossil fuels (i.e., coal and oil/petroleum) accounted for an average level of 83 ± 5% of the total BC preserved, which was significantly higher than that from the biomass burning (17 ± 5%). The spatial distributions of the fossil BC concentrations and percentages differed significantly from those of the biomass BC, implying their different geochemical behaviors in the continental shelf regimes and further emphasizing the importance to effectively differentiate between fossil BC and biomass BC. In addition to the relative proportions of the BC subtypes (char‐BC/soot‐BC), the regional specific hydrodynamic conditions, including the cold cyclonic eddy, re‐suspension and coastal current, also exerted a significant influence on these spatial variations. This article is protected by copyright. All rights reserved.
      PubDate: 2016-01-25T10:19:52.068455-05:
      DOI: 10.1002/2015JC011214
  • An extreme value model for maximum wave heights based on weather types
    • Abstract: Extreme wave heights are climate‐related events. Therefore, special attention should be given to the large‐scale weather patterns responsible for wave generation in order to properly understand wave climate variability. We propose a classification of weather patterns to statistically downscale daily significant wave height maxima to a local area of interest. The time‐dependent statistical model obtained here is based on the convolution of the stationary extreme value model associated to each weather type. The inter‐daily dependence is treated by a climate‐related extremal index. The modeĺs ability to reproduce different time scales (daily, seasonal and interanual) is presented by means of its application to three locations in the North Atlantic: Mayo (Ireland), La Palma island and Coruña (Spain). This article is protected by copyright. All rights reserved.
      PubDate: 2016-01-21T17:24:50.673488-05:
      DOI: 10.1002/2015JC010952
  • Tracking the attenuation and nonbreaking dissipation of swells using
    • Authors: Haoyu Jiang; Justin E. Stopa, He Wang, Romain Husson, Alexis Mouche, Bertrand Chapron, Ge Chen
      Abstract: A method for systematically tracking swells across oceanic basins is developed by taking advantage of high‐quality data from space‐borne altimeters and wave model output. The evolution of swells is observed over large distances based on 202 swell events with periods ranging from 12 to 18 s. An empirical attenuation rate of swell energy of about 4 × 10−7 m−1 is estimated using these observations, and the non‐breaking energy dissipation rates of swells far away from their generating areas are also estimated using a point source model. The resulting acceptance range of non‐breaking dissipation rates is ‐2.5 to 5.0 × 10−7 m−1, which corresponds to a dissipation e‐folding scales of at least 2000 km for steep swells, to almost infinite for small‐amplitude swells. These resulting rates are consistent with previous studies using in‐situ and synthetic aperture radar (SAR) observations. The frequency dispersion and angular spreading effects during swell propagation are discussed by comparing the results with other studies, demonstrating that they are the two dominant processes for swell height attenuation, especially in the near field. The resulting dissipation rates from these observations can be used as a reference for ocean engineering and wave modeling, and for related studies such as air‐sea and wind‐wave‐turbulence interactions. This article is protected by copyright. All rights reserved.
      PubDate: 2016-01-19T18:24:25.417395-05:
      DOI: 10.1002/2015JC011536
  • On the calculation of air‐sea fluxes of CO2 in the presence of
           temperature and salinity gradients
    • Abstract: The presence of vertical temperature and salinity gradients in the upper ocean and the occurrence of variations in temperature and salinity on time scales from hours to many years complicate the calculation of the flux of carbon dioxide (CO2) across the sea surface. Temperature and salinity affect the interfacial concentration of aqueous CO2 primarily through their effect on solubility with lesser effects related to saturated vapour pressure and the relationship between fugacity and partial pressure. The effects of temperature and salinity profiles in the water column and changes in the aqueous concentration act primarily through the partitioning of the carbonate system. Climatological calculations of flux require attention to variability in the upper ocean and to the limited validity of assuming “constant chemistry” in transforming measurements to climatological values. Contrary to some recent analysis, it is shown that the effect on CO2 fluxes of a cool skin on the sea surface is large and ubiquitous. An opposing effect on calculated fluxes is related to the occurrence of warm layers near the surface; this effect can be locally large but will usually coincide with periods of low exchange. A salty skin and salinity anomalies in the upper ocean also affect CO2 flux calculations, though these haline effects are generally weaker than the thermal effects. This article is protected by copyright. All rights reserved.
      PubDate: 2016-01-19T18:24:11.481402-05:
      DOI: 10.1002/2015JC011427
  • Dynamics of wind‐driven upwelling off the northeastern coast of
           Hainan Island
    • Authors: Peigen Lin; Peng Cheng, Jianping Gan, Jianyu Hu
      Abstract: Both observational and reanalysis sea surface temperature data reveal that upwelling occurs frequently off the northeastern coast of Hainan Island (downstream of the change in topography off Tongluo Cape), which cannot be attributed to the along‐shelf wind alone. To identify dynamics of the upwelling we conducts a numerical experiment using an idealized topography that is simplified from the actual topography off the eastern and northeastern coasts of Hainan Island. The result indicates that the upwelling downstream of the change in topograhy is associated with onshore cross‐isobath transport. Analysis of the vertically integrated momentum balance shows that the upwelling‐linked onshore transport is primarily intensified by the along‐isobath barotropic pressure gradient force (PGT), but is weakened by the along‐isobath baroclinic pressure gradient force (PGC). The along‐isobath PGT is linked to the advection of relative vorticity, the bottom stress curl and the gradient of momentum flux in vorticity equation. On the other hand, the PGC‐related process is diagnosed by potential vorticity (PV) balance. Similar to the negative PV term from wind stress, the negative PV terms of the joint effect of baroclinicity and relief and the baroclinic bottom pressure torque weaken the upwelling‐linked onshore transport downstream of the change in topography. The onshore transport is enhanced by the positive PV from bottom stress. In addition, the cross‐isobath forces play an important role in upwelling intensification in the shallow nearshore region. This article is protected by copyright. All rights reserved.
      PubDate: 2016-01-19T11:27:38.58027-05:0
      DOI: 10.1002/2015JC011000
  • Nearshore sandbar rotation at single‐barred embayed beaches
    • Authors: B. Blossier; K.R. Bryan, C.J. Daly, C. Winter
      Abstract: The location of a shore‐parallel nearshore sandbar derived from seven years of video imagery data at the single‐barred embayed Tairua Beach (NZ) is investigated to assess the contribution of barline rotation to the overall morphodynamics of sandbars in embayed environments and to characterise the process of rotation in relation to external conditions. Rotation induces cross‐shore barline variations at the embayment extremities on the order of magnitude of those induced by alongshore uniform cross‐shore migration of the bar. Two semi‐empirical models have been developed to relate the barline cross‐shore migration and rotation to external wave forcing conditions. The rotation model is directly derived from the cross‐shore migration model. Therefore its formulation advocates for a primary role of cross‐shore processes in the rotation of sandbars at embayed beaches. The orientation evolves towards an equilibrium angle directly related to the alongshore wave energy gradient due to two different mechanisms. Either the bar extremities migrate in opposite directions with no overall cross‐shore bar migration (pivotal rotation) or the rotation relates to an overall migration of the barline which is not uniform along the beach (migration‐driven rotation). Migration and rotation characteristic response times are similar, ranging from 10 to 30 days for mild and energetic wave conditions and above 200 days during very calm conditions or when the bar is located far offshore. This article is protected by copyright. All rights reserved.
      PubDate: 2016-01-18T10:31:47.259714-05:
      DOI: 10.1002/2015JC011031
  • Observations of the directional distribution of the wind energy input
           function over swell waves
    • Authors: Behnam Shabani; Alex V. Babanin, Tom E. Baldock
      Abstract: Field measurements of wind stress over shallow water swell travelling in different directions relative to the wind are presented. The directional distribution of the measured stresses is used to confirm the previously proposed but unverified directional distribution of the wind energy input function. The observed wind energy input function is found to follow a much narrower distribution (β ∝ cos3.6 θ) than the Plant [1982] cosine distribution. The observation of negative stress angles at large wind‐wave angles, however, indicates that the onset of negative wind shearing occurs at about θ ≈ 50°, and supports the use of the Snyder et al. [1981] directional distribution. Taking into account the reverse momentum transfer from swell to the wind, Snyder's proposed parameterisation is found to perform exceptionally well in explaining the observed narrow directional distribution of the wind energy input function, and predicting the wind drag coefficients. The empirical coefficient (ε) in Snyder's parameterisation is hypothesised to be a function of the wave shape parameter, with ε value increasing as the wave shape changes between sinusoidal, sawtooth, and sharp‐crested shoaling waves. This article is protected by copyright. All rights reserved.
      PubDate: 2016-01-14T12:32:26.900236-05:
      DOI: 10.1002/2015JC011225
  • The 2011 Tohoku tsunami south of Oahu: High‐frequency Doppler radio
           observations and model simulations of currents
    • Authors: L. R. Benjamin; P. Flament, K. F. Cheung, D. S. Luther
      Abstract: A 16‐MHz high‐frequency Doppler radio (HFDR) deployed on the south shore of Oahu (Hawaii) detected oscillatory radial currents following the arrival of the 2011 Tohoku tsunami. Observations of tsunami currents over a two‐dimensional area provided an opportunity to compare them with the spatial patterns of currents and the resonant modes predicted by a non‐hydrostatic model. Over the 50‐m deep Penguin Bank, extending west from Molokai, the observed currents are intensified in two areas: 43‐min period currents of 0.27 ms−1 lasting six hours are observed on the south part of the bank, while 27‐min period currents of 0.14 ms−1 lasting two hours are observed on the north. The spatial EOFs suggest that standing full‐waves and 3/2 waves formed over the bank. Modeled currents over Penguin Bank are similar to the observations but their north‐south asymmetry is less pronounced than observed. Near shore, observed alongshore currents showed long‐period oscillations of 43‐min that stretched along the entire coastline, while modeled currents show strong evidence for edge waves. EOF analysis of the near‐shore signal suggests that the HFDR and model reveal different processes. The discrepancy might be attributed to the fact that both the Penguin Bank and near‐shore observations are limited by HFDR sensitivity to azimuthal side lobe contamination and decreased angular resolution at high steering angles. This article is protected by copyright. All rights reserved.
      PubDate: 2016-01-12T10:41:42.045846-05:
      DOI: 10.1002/2015JC011207
  • Internal tides and tidal cycles of vertical mixing in western Long Island
    • Authors: Grant McCardell; James O'Donnell, Alejandro Souza, Matthew R. Palmer
      Abstract: In estuaries, tidal period variations in the rate of vertical mixing have been observed to result from various causes: in Liverpool Bay and the York River, they have been attributed to tidal straining of the along‐channel density gradient modulating stratification; in the Hudson River they arise from tidal modulation of the height of the tidal current bottom boundary layer (BBL). Along continental shelves, tidal period fluctuations in mixing have been observed to result from the dissipation of internal waves (IWs). Western Long Island Sound (WLIS) moored instrument records indicate that large near‐bottom increases in dissolved oxygen (DO) and heat and a decrease in salt occur during the middle of the flood tide: an analysis of water mass signatures indicates that the transport involved is vertical and not horizontal. Temperature data from a vertical thermistor array deployed in the WLIS for 16 days in August 2009 clearly shows a tidal cycle of IW activity creating a mean thermocline depression at mid‐flood of approximately 25% of the water depth with individual IW thermocline depressions of as much as 50% of the water depth. Contemporaneous ADCP measurements show increases in shear due to IWs during the flood. Near‐bottom internal wave activity is maximal at and after mid‐flood and is correlated with near‐bottom temperature and DO tendencies at both tidal and subtidal scales. We conclude that internal tides are an important vertical mixing mechanism in the WLIS through both increased shear from IWs and displacement of the pycnocline into the region of high shear in the BBL. This article is protected by copyright. All rights reserved.
      PubDate: 2016-01-12T10:35:11.74631-05:0
      DOI: 10.1002/2015JC010796
  • Dispersion of a tracer in the deep Gulf of Mexico
    • Authors: James R. Ledwell; Ruoying He, Zuo Xue, Steven F. DiMarco, Laura Spencer, Piers Chapman
      Abstract: A 25‐km streak of CF 3SF5 was released on an isopycnal surface approximately 1100 m deep, and 150 m above the bottom, along the continental slope of the northern Gulf of Mexico, to study stirring and mixing of a passive tracer. The location and depth of the release were near those of the deep hydrocarbon plume resulting from the 2010 Deepwater Horizon oil well rupture. The tracer was sampled between 5 and 12 days after release, and again 4 months and 12 months after release. The tracer moved along the slope at first but gradually moved into the interior of the Gulf. Diapycnal spreading of the patch during the first 4 months was much faster than it was between 4 and 12 months, indicating that mixing was greatly enhanced over the slope. The rate of lateral homogenization of the tracer was much greater than observed in similar experiments in the open ocean, again possibly enhanced near the slope. Maximum concentrations found in the surveys had fallen by factors of 104, 107, and 108, at 1 week, 4 months and 12 months, respectively, compared with those estimated for the initial tracer streak. A regional ocean model was used to simulate the tracer field and help interpret its dispersion and temporal evolution. Model‐data comparisons show the model simulation was able to replicate statistics of the observed tracer distribution that would be important in assessing the impact of oil releases in the mid‐depth Gulf. This article is protected by copyright. All rights reserved.
      PubDate: 2016-01-12T10:34:40.505501-05:
      DOI: 10.1002/2015JC011405
  • Probabilistic assessment of landslide tsunami hazard for the northern Gulf
           of Mexico
    • Abstract: The devastating consequences of recent tsunamis affecting Indonesia and Japan have prompted a scientific response to better assess unexpected tsunami hazards. Although much uncertainty exists regarding the recurrence of large‐scale tsunami events in the Gulf of Mexico (GoM), geological evidence indicates that a tsunami is possible and would most likely come from a submarine landslide triggered by an earthquake. This study customizes for the GoM a first‐order probabilistic landslide tsunami hazard assessment. Monte Carlo Simulation (MCS) is employed to determine landslide configurations based on distributions obtained from observational submarine mass failure (SMF) data. Our MCS approach incorporates a Cholesky decomposition method for correlated landslide size parameters to capture correlations seen in the data as well as uncertainty inherent in these events. Slope stability analyses are performed using landslide and sediment properties and regional seismic loading to determine landslide configurations which fail and produce a tsunami. The probability of each tsunamigenic failure is calculated based on the joint probability of slope failure and probability of the triggering earthquake. We are thus able to estimate sizes and return periods for probabilistic maximum credible landslide scenarios. We find that the Cholesky decomposition approach generates landslide parameter distributions that retain the trends seen in observational data, improving the statistical validity and relevancy of the MCS technique in the context of landslide tsunami hazard assessment. Estimated return periods suggest that probabilistic maximum credible SMF events in the north and northwest GoM have a recurrence of 5,000‐8,000 years, in agreement with age dates of observed deposits. This article is protected by copyright. All rights reserved.
      PubDate: 2016-01-12T10:33:52.241035-05:
      DOI: 10.1002/2015JC011261
  • The Antarctic Slope Current near 30°E
    • Authors: Jun Dong; Kevin Speer, Loic Jullion
      Abstract: The Antarctic Slope Current flows westward above the continental slope of Antarctica, entering the Weddell Sea near 30°E and supplying dense water to the deep overturning cell there, and contributing to global Antarctic Bottom Water formation. Observations from the 2008 I6S hydrographic section are used to investigate the strength of the Slope Current near 30°E. A prominent topographic feature, the Gunnerus Bank, diverts the Slope Current upstream of this longitude, and has a large effect on the current's structure, splitting it into a coastal and offshore component. The bank also enhances water mass mixing and lateral exchange across the slope. As part of the 2008 occupation, an additional line was made along the crest of the bank, forming a closed volume over the western side. By combining hydrographic and Lowered Acoustic Doppler Current Profiler measurements in this box using an inverse method, the Slope Current transport is estimated to be 9.6 ± 2.3Sv; the transport associated with the Antarctic Slope Front is 4.0 ± 0.3Sv, of which 1.8 ± 0.3Sv enters the Weddell Gyre as recently formed dense water. This article is protected by copyright. All rights reserved.
      PubDate: 2016-01-12T10:33:36.175666-05:
      DOI: 10.1002/2015JC011099
  • Characterizing the circulation off the Kenyan‐Tanzanian coast using
           an ocean model
    • Abstract: The Kenyan‐Tanzanian coastal region in the western Indian Ocean faces several environmental challenges including coral reef conservation, fisheries management, coastal erosion and near‐shore pollution. The region lacks hydrodynamic records and oceanographic studies at adequate spatial and temporal scales to provide information relevant to the local environmental issues. We have developed a 4 km horizontal resolution ocean circulation model of the region: The Kenyan‐Tanzanian Coastal Model (KTCM) that provides coastal circulation and hydrography with higher resolution than previous models and observational studies of this region. Comparisons to temperature profiles, satellite derived sea surface temperature and sea surface height anomaly fields, indicate that the model reproduces the main features of the regional circulation, while greatly increasing the details of the nearshore circulation. We describe the seasonal ocean circulation and hydrography of the Kenyan‐Tanzanian coastal region based on a climatology of 8 years (2000‐2007) of the KTCM simulations. The regional monsoon seasonality produces two distinct coastal circulation regimes: 1) During December to March there are relatively sluggish shelf flows; and 2) During April to November there are strong northward transports. Simulations from the model will be useful for examining dispersal of pollutants and spatial connectivity of coral reef species. This article is protected by copyright. All rights reserved.
      PubDate: 2016-01-12T10:33:32.515399-05:
      DOI: 10.1002/2015JC010860
  • Observed evidence of the anomalous South China Sea western boundary
           current during the summers of 2010 and 2011
    • Authors: Yeqiang Shu; Huijie Xue, Dongxiao Wang, Qiang Xie, Ju Chen, Jian Li, Rongyu Chen, Yunkai He, Daning Li
      Abstract: Seven years of directly measured current data from a mooring in the Xisha area of the South China Sea (SCS), together with shipboard ADCP and satellite data, have shown the western boundary current (WBC) anomaly and its vertical structure during the summers of 2010 and 2011. The observed WBC presented obvious year‐to‐year variability, especially in the summer. Overall, the summer mean velocity at the mooring site over 7‐year (2007 – 2013) was northeastward. The moored ADCP showed that the northeastward velocity was particularly strong in the summer of 2010, but the increase was confined in the upper 120 m. In contrast, the northeastward current disappeared throughout the observed depth range (from 50 to 450 m) in the summer of 2011. Even at the deepest observed position, the monthly velocity anomalies reached 14 cm s−1 westward and 12 cm s−1 southward in the zonal and meridional directions, respectively. Both the Vietnam offshore current (VOC) and double gyres in the western SCS disappeared and the southern anticyclonic gyre expanded to strengthened the northward WBC in the summer of 2010. However, in summer of 2011, the VOC intensified, and the northern cyclonic gyre enlarged with its northern edge reaching 18°N, slightly north of mooring site, which weakened the northeastward WBC. The observed SCS circulation anomalies during 2010 and 2011 were mainly induced by the basin‐scale wind field anomalies associated with the 2009/2010 El Niño and 2010/2011 La Niña. This article is protected by copyright. All rights reserved.
      PubDate: 2016-01-11T17:58:24.973793-05:
      DOI: 10.1002/2015JC011434
  • Seasonal variations of seawater pCO2 and sea‐air CO2 fluxes in a
           fringing coral reef, northern South China Sea
    • Authors: Hongqiang Yan; Kefu Yu, Qi Shi, Yehui Tan, Guohui Liu, Meixia Zhao, Shu Li, Tianran Chen, Yinghui Wang
      Abstract: Evidence based on four field surveys conducted between July 2009 and April 2011 indicates that both sea surface partial pressures of CO2 (pCO2) and sea–air CO2 fluxes at Luhuitou fringing reef in Sanya, Hainan Island, northern South China Sea (SCS) are subject to significant seasonal variations. The diurnal variation of seawater pCO2 ranges from 264 to 579 μatm in summer, which is much larger than that in autumn (152‐335 μatm), in winter (84‐260 μatm), and in spring (114‐228 μatm). The sea‐air CO2 flux in summer (∼9.6 mmol CO2 m−2 d−1) is also larger than that in other seasons (i.e.∼3 mmol CO2 m−2 d−1 in spring, ∼3.5 mmol CO2 m−2 d−1 in autumn, and ∼2.7 mmol CO2 m−2 d−1 in winter). The atmospheric pCO2 in this reef shows small diurnal and seasonal variations. The integration of the time‐series pCO2 data shows that the reef area is a weak source of atmospheric CO2 at ∼0.54 mol CO2 m−2 yr−1. Further analyses indicates that the seasonal variations of the surface seawater pCO2 in Luhuitou fringing reef are mainly affected by seasonally‐dependent biological metabolic processes (organic processe and inorganic process), and that the organic process play a more important role than the inorganic process. Seasonal sea surface temperature (SST) variations and hydrodynamic processes may also have some influence on seawater pCO2 variation. This article is protected by copyright. All rights reserved.
      PubDate: 2016-01-08T19:11:50.253786-05:
      DOI: 10.1002/2015JC011484
  • Diel vertical migration of zooplankton at the S1 biogeochemical mooring
           revealed from acoustic backscattering strength
    • Authors: Ryuichiro Inoue; Minoru Kitamura, Tetsuichi Fujiki
      Abstract: We examined the diel vertical migration of zooplankton by using the backscatter strength obtained from moored acoustic Doppler current profilers at mooring site S1 in the North Pacific subtropical gyre. There was seasonal variability in the vertical distribution and migration of the high‐backscatter layers in that they became deeper than the euphotic zone (
      PubDate: 2016-01-08T19:11:15.202168-05:
      DOI: 10.1002/2015JC011352
  • A wind‐driven, hybrid latent and sensible heat coastal polynya off
           Barrow, Alaska
    • Authors: Daisuke Hirano; Yasushi Fukamachi, Eiji Watanabe, Kay I. Ohshima, Katsushi Iwamoto, Andrew R. Mahoney, Hajo Eicken, Daisuke Simizu, Takeshi Tamura
      Abstract: The nature of the Barrow Coastal Polynya (BCP), which forms episodically off the Alaska coast in winter, is examined using mooring data, atmospheric re‐analysis data, and satellite‐derived sea‐ice concentration and production data. We focus on oceanographic conditions such as water mass distribution and ocean current structure beneath the BCP. Two moorings were deployed off Barrow, Alaska in the northeastern Chukchi Sea from August 2009 to July 2010. For sea‐ice season from December to May, a characteristic sequence of five events associated with the BCP has been identified; 1) dominant northeasterly wind parallel to the Barrow Canyon, with an offshore component off Barrow, 2) high sea‐ice production, 3) upwelling of warm and saline Atlantic Water beneath the BCP, 4) strong up‐canyon shear flow associated with displaced density surfaces due to the upwelling, 5) sudden suppression of ice growth. A baroclinic current structure, established after the upwelling, caused enhanced vertical shear and corresponding vertical mixing. The mixing event and open water formation occurred simultaneously, once sea‐ice production had stopped. Thus, mixing events accompanied by ocean heat flux from the upwelled warm water into the surface layer played an important role in formation/maintenance of the open water area (i.e., sensible heat polynya). The transition from a latent to a sensible heat polynya is well reproduced by a high‐resolution pan‐Arctic ice‐ocean model. We propose that the BCP, previously considered to be a latent heat polynya, is a wind‐driven hybrid latent and sensible heat polynya, with both features caused by the same northeasterly wind. This article is protected by copyright. All rights reserved.
      PubDate: 2016-01-08T04:39:52.639063-05:
      DOI: 10.1002/2015JC011318
  • A reconciliation of empirical and mechanistic models of the air‐sea
           gas transfer velocity
    • Abstract: Models of the air‐sea transfer velocity of gases may be either empirical or mechanistic. Extrapolations of empirical models to an unmeasured gas or to another water temperature can be erroneous if the basis of that extrapolation is flawed. This issue is readily demonstrated for the most well‐known empirical gas transfer velocity models where the influence of bubble ‐mediated transfer, which can vary between gases, is not explicitly accounted for. Mechanistic models are hindered by an incomplete knowledge of the mechanisms of air‐sea gas transfer. We describe a hybrid model that incorporates a simple mechanistic view – strictly enforcing a distinction between direct and bubble‐mediated transfer – but also uses parameterizations based on data from eddy flux measurements of dimethyl sulphide (DMS) to calibrate the model together with dual tracer results to evaluate the model. This model underpins simple algorithms that can be easily applied within schemes to calculate local, regional or global air‐sea fluxes of gases. This article is protected by copyright. All rights reserved.
      PubDate: 2016-01-05T09:47:41.761238-05:
      DOI: 10.1002/2015JC011096
  • Logarithmic velocity structure in the deep hypolimnetic waters of Lake
    • Authors: Cary Troy; David Cannon, Qian Liao, Harvey Bootsma
      Abstract: The characteristics of the bottom boundary layer are reported from a Lake Michigan field study carried out in deep hypolimnetic waters (55m depth) during the stratified period (June‐Sept., 2012). The sandy substrate at the measurement site was densely covered with invasive quagga mussels (mean size: 1.6 cm; mean density: 10,000 mussels m−2). The measurements reveal a sluggish, compact bottom boundary layer, with flow speeds at 1mab less than 5 cm s−1 for most of the period, and a dominance of subinertial energy. A downwelling event caused the largest currents observed during the deployment (10 cm s−1 at 1 mab) and a logarithmic layer thickness of 15 m. In spite of the weak flow, logarithmic profile fitting carried out on high‐resolution, near‐bed velocity profiles show consistent logarithmic structure (90% of profiles). Flow was dominated by subinertial energy but strong modified by near‐inertial waves. Fitted drag coefficients and roughness values are Cd1m = 0.004 and Z0 = 0.12cm, respectively. These values increase with decreasing flow speed, but approach canonical values for 1 mab flow speeds exceeding 4 cm s−1. The estimated vertical extent of the logarithmic region was compact, with a mean value of 1.2 m and temporal variation that is reasonably described by Ekman scaling, 0.07u*/f,and the estimated overall Ekman layer thickness was generally less than 10m. Near‐bed dissipation rates inferred from the law of the wall were 10−8−10−7 W kg−1 and turbulent diffusivities were 10−4‐10−3 m2s−1. This article is protected by copyright. All rights reserved.
      PubDate: 2015-12-30T20:28:44.122616-05:
      DOI: 10.1002/2014JC010506
  • Modulation of sea surface temperature warming in the Bay of Biscay by
           Loire and Gironde Rivers
    • Abstract: The influence of Loire and Gironde River discharges over the sea surface temperature (SST) in the eastern Bay of Biscay (0.6º‐36.6ºW, 44.2º‐47.8ºW) was analyzed by means of two complementary databases (MODIS and OISST1/4). The area influenced by river plume showed a different SST when compared with the adjacent oceanic area for the months when the plume attains its highest extension (December, January and February). Ocean was observed to warm at a rate of approximately 0.3 ºC dec−1 while temperature at the area influenced by the rivers cooled at a rate of ‐0.15 ºC dec−1 over the period 1982‐2014. The mere presence of a freshwater layer is able to modulate the warming observed at adjacent ocean locations since the coastal area is isolated from the rest of the Bay. This nearshore strip is the only part of the Bay where changes in SST depend on North Atlantic Oscillation (NAO) but not on North Atlantic SST represented by the Atlantic Multidecadal Oscillation (AMO). These different cooling‐warming trends are even more patent over the last years (2002‐2014) under atmospheric favourable conditions for plume enhancement. River runoff increased at a rate on the order of 120 m3s−1dec−1 over that period and southwesterly winds, which favor the confinement of the plume, showed a positive and significant trend both in duration and intensity. Thus, the coastal strip has been observed to cool at a rate of ‐0.5°C dec−1. This article is protected by copyright. All rights reserved.
      PubDate: 2015-12-30T20:28:30.355621-05:
      DOI: 10.1002/2015JC011157
  • Particle delivery to the benthos of coastal Lake Michigan
    • Authors: James T. Waples
      Abstract: A 2‐D non‐steady state model was applied to measured profiles of 234Th/238U and 90Y/90Sr disequilibria in a shallow (22 m) water column of coastal Lake Michigan. Downward fluxes of 234Th and 90Y were primarily driven by onshore horizontal advection. Concordance between 234Th and 90Y‐derived mass flux estimates from the water column could only be realistically achieved under a nuclide scavenging scenario dominated by direct sorption on bottom or near‐bottom sediment and vertical convection in the water column—not sinking particles. An estimated vertical 234Th/90Y flux ratio of ∼0.31 in the water column agreed with measured 234Th/90Y activity ratios on collected ejecta from bottom dwelling dreissenid mussels (0.26 ± 0.05) and not with water column particles (3.3 ± 1.3). A similar 238U/90Sr parent nuclide activity ratio of 0.30 ± 0.02 suggests that both 234Th and 90Y are scavenged in toto below the maximum sampling depth (17 m) and near the sediment/water interface. Determining the mechanism by which particles are transported to the bottom is important for understanding not only how benthos are supplied with water column material, but also how particle fluxes should be measured and calculated.
      PubDate: 2015-12-28T10:38:56.362369-05:
      DOI: 10.1002/2015JC011297
  • A 1.5‐D anisotropic sigma‐coordinate thermal stress model of
           landlocked sea ice in the Canadian Arctic Archipelago
    • Authors: Y. Hata; L. B. Tremblay
      Abstract: We present a 1.5‐D thermal stress model that takes into account the effect of land confinement, which causes anisotropy in thermal stresses. To this end, we fix the total strain in the direction perpendicular to the coastline to its value at landlocked ice onset. This prevents thermal expansion in the direction perpendicular to the coastline and therefore induces larger thermal stresses in this direction. The simulated stresses best match the observations, when a Young's Modulus of 0.5 GPa and a relaxation time constant of 8 days are used. This simulation gives root‐mean‐square errors of 13.0 and 13.1 kPa (∼15%) in the major and minor principal stresses, respectively. The simulated anisotropic component of thermal stress also generally agrees with observations. The optimal Young's Modulus is in the low range of reported values in the literature, and the optimal relaxation time constant (8 days) is larger than the largest relaxation time constant reported in the literature (5 days). A series of experiments are done to examine the model sensitivity to vertical resolution, snow cover, and the parameterizations of Young's Modulus and viscous creep. Results show that a minimum of one and three layers in the snow and ice, respectively, is required to simulate the thermal stresses within 15% error of the value assessed with the higher‐resolution control simulation. This highlights the importance of resolving the internal snow and ice vertical temperature profile in order to properly model the thermal stresses of sea ice.
      PubDate: 2015-12-28T10:38:38.866643-05:
      DOI: 10.1002/2015JC010820
  • Interannual variability of the Indonesian Throughflow transport: A revisit
           based on 30 year expendable bathythermograph data
    • Abstract: Based on 30 year repeated expendable bathythermograph (XBT) deployments between Fremantle, Western Australia, and the Sunda Strait, Indonesia, from 1984 to 2013, interannual variability of geostrophic transport of the Indonesian Throughflow (ITF) and its relationships with El Niño Southern Oscillation (ENSO) and the Indian Ocean Dipole (IOD) are investigated. The IOD induced coastal Kelvin waves propagate along the Sumatra‐Java coast of Indonesia, and ENSO induced coastal Kelvin waves propagate along the northwest coast of Australia, both influencing interannual variations of the ITF transport. The ITF geostrophic transport is stronger during La Niña phase and weaker during El Niño phase, with the Niño3.4 index leading the ITF variability by 7 months. The Indian Ocean wind variability associated with the IOD to a certain extent offset the Pacific ENSO influences on the ITF geostrophic transport during the developing and mature phases of El Niño and La Niña, due to the covarying IOD variability with ENSO. The ITF geostrophic transport experiences a strengthening trend of about 1 Sv every 10 years over the study period, which is mostly due to a response to the strengthening of the trade winds in the Pacific during the climate change hiatus period. Decadal variations of the temperature‐salinity relationships need to be considered when estimating the geostrophic transport of the ITF using XBT data.
      PubDate: 2015-12-28T10:37:57.880854-05:
      DOI: 10.1002/2015JC011351
  • Sea surface height and dynamic topography of the ice‐covered oceans
           from CryoSat‐2: 2011–2014
    • Authors: Ron Kwok; James Morison
      Abstract: We examine four years (2011‐2014) of sea surface heights (SSH) from CryoSat‐2 (CS‐2) over the ice‐covered Arctic and Southern Oceans. Results are from a procedure that identifies and determines the heights of sea surface returns. Along 25‐km segments of satellite ground tracks, variability in the retrieved SSHs is between ∼2‐3 cm (standard deviation) in the Arctic and slightly higher (∼3 cm) in the summer and the Southern Ocean. Average sea surface tilts (along these 25‐km segments) are 0.01±3.8 cm/10 km in the Arctic, and slightly lower (0.01±2.0 cm/10 km) in the Southern Ocean. Intra‐seasonal variability of CS‐2 dynamic ocean topography (DOT) in the ice‐covered Arctic is nearly twice as high as that of the Southern Ocean. In the Arctic, we find a correlation of 0.92 between three years of DOT and dynamic heights (DH) from hydrographic stations. Further, correlation of four years of area‐averaged CS‐2 DOT near the North Pole and time‐variable ocean‐bottom pressure from a pressure gauge and from GRACE, yields coefficients of 0.83 and 0.77, with corresponding differences of
      PubDate: 2015-12-28T04:52:32.774491-05:
      DOI: 10.1002/2015JC011357
  • Ecosystem model intercomparison of under‐ice and total primary
           production in the Arctic Ocean
    • Abstract: Previous observational studies have found increasing primary production (PP) in response to declining sea ice cover in the Arctic Ocean. In this study, under‐ice PP was assessed based on three coupled ice‐ocean‐ecosystem models participating in the Forum for Arctic Modeling and Observational Synthesis (FAMOS) project. All models showed good agreement with under‐ice measurements of surface chlorophyll‐a concentration and vertically integrated PP rates during the main under‐ice production period, from mid‐May to September. Further, modeled thirty‐year (1980‐2009) mean values and spatial patterns of sea ice concentration compared well with remote sensing data. Under‐ice PP was higher in the Arctic shelf seas than in the Arctic Basin, but ratios of under‐ice PP over total PP were spatially correlated with annual mean sea ice concentration, with higher ratios in higher ice concentration regions. Decreases in sea ice from 1980 to 2009 were correlated significantly with increases in total PP and decreases in the under‐ice PP/total PP ratio for most of the Arctic, but nonsignificantly related to under‐ice PP, especially in marginal ice zones. Total PP within the Arctic Circle increased at an annual rate of between 3.2 and 8.0 Tg C/year from 1980 to 2009. This increase in total PP was due mainly to a PP increase in open water, including increases in both open water area and PP rate per unit area, and therefore much stronger than the changes in under‐ice PP. All models suggested that, on a pan‐Arctic scale, the fraction of under‐ice PP declined with declining sea ice cover over the last three decades. This article is protected by copyright. All rights reserved.
      PubDate: 2015-12-28T03:25:55.124602-05:
      DOI: 10.1002/2015JC011183
  • Moored observation of abyssal flow and temperature near a hydrothermal
           vent on the Southwest Indian Ridge
    • Authors: Guanghong Liao; Beifeng Zhou, Chujin Liang, Huaiyang Zhou, Tao Ding, Wang Yuan, Changming Dong
      Abstract: Four moorings were deployed near “Dragon Flag”, an active hydrothermal vent in the valley of the Southwest Indian Ridge. The goal was to examine the variability of currents and temperature, which will guide the trajectory of spreading plumes. The mean current was cross‐isobath, and the circulation was characterized by a sub‐mesoscale circulation. Observed currents also showed fluctuations with periods of 1‐15 days. The inferred phase speed and wavelength for the wave with a period of 4.4 days are 10.4 km⋅d−1 and 45.8km, respectively, which are consistent with the topographic Rossby wave theory. The persistent warming tendency with corresponding variation of salinity based on background θ‐S properties may be caused by background circulation and divergence of the water column. The warming or cooling episodes were most likely as signatures of isopycnal surface depression or uplifting induced by the moving of mesoscale eddies. Well resolved rotary spectra exhibited important nonlinear interactions between inertial and semi‐diurnal tide in the velocity and temperature records. Amplification of near‐inertial currents in the near bottom are also exposed. These discoveries provided new evidence for the non‐linear interaction and trapped near‐inertial waves by the ridge, which occurred in the deep ocean of the Southern Hemisphere. Such nonlinear interaction may represent a significant energy loss pathway for the internal waves, and part of the decay of such motion would likely result in increased mixing to maintain the abyssal stratification. Enhanced near‐inertial motions can play a major role for the local advection of hydrothermal plumes. This article is protected by copyright. All rights reserved.
      PubDate: 2015-12-28T03:25:43.462678-05:
      DOI: 10.1002/2015JC011053
  • Effects of Submesoscale Turbulence on Ocean Tracers
    • Abstract: Ocean tracers such as carbon dioxide, nutrients, plankton, and oil advect, diffuse, and react primarily in the oceanic mixed layer where air‐sea gas exchange occurs and light is plentiful for photosynthesis. There can be substantial heterogeneity in the spatial distributions of these tracers due to turbulent stirring, particularly in the submesoscale range where partly geostrophic fronts and eddies and small‐scale three‐dimensional turbulence are simultaneously active. In this study, large eddy simulations spanning horizontal scales from 20km down to 5m are used to examine the effects of multiscale turbulent mixing on nonreactive passive ocean tracers from interior and sea‐surface sources. The simulations include the effects of both wave‐driven Langmuir turbulence and submesoscale eddies, and tracers with different initial and boundary conditions are examined in order to understand the respective impacts of small‐scale and submesoscale motions on tracer transport. Tracer properties are characterized using spatial fields and statistics, multiscale fluxes, and spectra, and the results detail how tracer mixing depends on air‐sea tracer flux rate, tracer release depth, and flow regime. Although vertical fluxes of buoyancy by submesoscale eddies compete with mixing by Langmuir turbulence, vertical fluxes of tracers are often dominated by Langmuir turbulence, particularly for tracers that are released near the mixed layer base or that dissolve rapidly through the surface, even in regions with pronounced submesoscale activity. Early in the evolution of some tracers, negative eddy diffusivities occur co‐located with regions of negative potential vorticity, suggesting that symmetric instabilities or other submesoscale phenomenon may act to oppose turbulent mixing. This article is protected by copyright. All rights reserved.
      PubDate: 2015-12-26T09:47:01.822188-05:
      DOI: 10.1002/2015JC011089
  • Greenland freshwater pathways in the sub‐Arctic Seas from model
           experiments with passive tracers
    • Abstract: Accelerating since the early 1990s, the Greenland Ice Sheet mass loss exerts a significant impact on thermohaline processes in the sub‐Arctic seas. Surplus freshwater discharge from Greenland since the 1990s, comparable in volume to the amount of freshwater present during the Great Salinity Anomaly events, could spread and accumulate in the sub‐Arctic seas, influencing convective processes there. However, hydrographic observations in the Labrador Sea and the Nordic Seas, where the Greenland freshening signal might be expected to propagate, do not show a persistent freshening in the upper ocean during last two decades. This raises the question of where the surplus Greenland freshwater has propagated. In order to investigate the fate, pathways, and propagation rate of Greenland meltwater in the sub‐Arctic seas, several numerical experiments using a passive tracer to track the spreading of Greenland freshwater have been conducted as a part of the Forum for Arctic Ocean Modeling and Observational Synthesis effort. The models show that Greenland freshwater propagates and accumulates in the sub‐Arctic seas, although the models disagree on the amount of tracer propagation into the convective regions. Results highlight the differences in simulated physical mechanisms at play in different models and underscore the continued importance of intercomparison studies. It is estimated that surplus Greenland freshwater flux should have caused a salinity decrease by 0.06—0.08 in the sub‐Arctic seas in contradiction with the recently observed salinification (by 0.15–0.2) in the region. It is surmised that the increasing salinity of Atlantic Water has obscured the freshening signal. This article is protected by copyright. All rights reserved.
      PubDate: 2015-12-26T09:44:38.680986-05:
      DOI: 10.1002/2015JC011290
  • Ice shelf basal melt rates around Antarctica from simulations and
    • Authors: M. P. Schodlok; D. Menemenlis, E. J. Rignot
      Abstract: We introduce an explicit representation of Antarctic ice shelf cavities in the Estimating the Circulation and Climate of the Ocean, Phase II (ECCO2) ocean retrospective analysis; and compare resulting basal melt rates and patterns to independent estimates from satellite observations. Two simulations are carried out: the first is based on the original ECCO2 vertical discretization; the second has higher vertical resolution particularly at the depth range of ice shelf cavities. The original ECCO2 vertical discretization produces higher than observed melt rates and leads to a misrepresentation of Southern Ocean water mass properties and transports. In general, thicker levels at the base of the ice shelves lead to increased melting because of their larger heat capacity. This strengthens horizontal gradients and circulation within and outside the cavities and, in turn, warm water transports from the shelf break to the ice shelves. The simulation with more vertical levels produces basal melt rates (1434 ± 54 Gt/a) and patterns that are in better agreement with observations. Thinner levels in the sub‐ice‐shelf cavities improve the representation of a fresh/cold layer at the ice shelf base and of warm/salty water near the bottom, leading to a sharper pycnocline and reduced vertical mixing underneath the ice shelf. Improved water column properties lead to more accurate melt rates and patterns, especially for melt/freeze patterns under large cold‐water ice shelves. At the 18‐km grid spacing of the ECCO2 model configuration, the smaller, warm‐water ice shelves cannot be properly represented, with higher than observed melt rates in both simulations. This article is protected by copyright. All rights reserved.
      PubDate: 2015-12-26T09:43:51.701132-05:
      DOI: 10.1002/2015JC011117
  • Dynamics of an idealized Beaufort Gyre: 1. the effect of a small beta and
           lack of western boundaries
    • Authors: Jiayan Yang; Andrey Proshutinsky, Xiaopei Lin
      Abstract: The Beaufort Gyre in the Arctic Ocean differs from a typical moderate‐latitude gyre in some major aspects of its dynamics. First, it is located in a basin without a western boundary, which is essential for closing mid‐latitude circulations. Second, the gradient in Coriolis parameter, β, is small and so the validity of the Sverdrup balance is uncertain. In this paper, we use an idealized two‐layer model to examine several processes that are related to these two issues. In a circular basin with closed geostrophic contours in interior, the variability of vorticity in the upper layer is dominated by eddies. But in the time‐mean circulation, the main dynamical balance in the basin's interior is between the curl of wind stress and the eddy vorticity fluxes. The torque of friction becomes important along the boundary where the rim current is strong. It is found that the smallness of β has only a relatively small impact in a circular basin without a meridional boundary. The gyre is considerably more sensitive to the existence of a meridional boundary. The time‐mean circulation weakens considerably when a peninsula is inserted between the model's center and the rim (one side of the peninsula is dynamically equivalent to a mid‐latitude western boundary). The gyre's sensitivity to β has also increased significantly when a meridional boundary is present. Subsurface ridges have similar effects on the gyre as a boundary, indicating that such topographic features may substitute, to some extents, the dynamical role of a western boundary. This article is protected by copyright. All rights reserved.
      PubDate: 2015-12-26T09:43:37.523027-05:
      DOI: 10.1002/2015JC011296
  • Simulating spatial and temporal varying CO2 signals from sources at the
           seafloor to help designing risk based monitoring programs
    • Abstract: Risk based monitoring requires quantification of the probability of the design to detect the potentially adverse events. A component in designing the monitoring program will be to predict the varying signal caused by an event, here detection of a gas seep through the seafloor from an unknown location. The Bergen Ocean Model (BOM) is used to simulate dispersion of CO2 leaking from different locations in the North Sea, focusing on temporal and spatial variability of the CO2 concentration. It is shown that the statistical footprint depends on seep location and that this will have to be accounted for in designing a network of sensors with highest probability of detecting a seep. As a consequence, heterogeneous probabilistic predictions of CO2 footprints should be available to sub‐sea geological CO2 storage projects in order to meet regulations. This article is protected by copyright. All rights reserved.
      PubDate: 2015-12-22T19:28:51.553211-05:
      DOI: 10.1002/2015JC011198
  • The separation of the East Australian Current: A Lagrangian approach to
           potential vorticity and upstream control
    • Authors: S.L. Ypma; E. van Sebille, A.E. Kiss, P. Spence
      Abstract: The East Australian Current (EAC) is the western boundary current flowing along the east coast of Australia separating from the coast at approximately 34°S. After the separation two main pathways can be distinguished, the eastward flowing Tasman Front and the extension of the EAC flowing southwards. The area south of the separation latitude is eddy‐rich and the separation latitude of the EAC is variable. Little is known of the properties of the water masses that separate at the bifurcation of the EAC. This paper presents new insights from the Lagrangian perspective, where the water masses that veer east and those that continue south are tracked in an eddy‐permitting numerical model. The transport along the two pathways is computed, and a 1:3 ratio between transport in the EAC extension and transport in the Tasman Front is found. The results show that the ‘fate' of the particles is to first order already determined by the particle distribution within the EAC current upstream of the separation latitude, where 85% of the particles following the EAC extension originate from below 460m and 90% of the particles following the Tasman Front originate from the top 460m depth at 28°S. The separation and pathways are controlled by the structure of the isopycnals in this region. Analysis of anomalies in potential vorticity show that in the region where the two water masses overlap, the fate of the water depends on the presence of anticyclonic eddies that push isopycnals down and therefore enable particles to travel further south. This article is protected by copyright. All rights reserved.
      PubDate: 2015-12-22T19:28:35.394743-05:
      DOI: 10.1002/2015JC011133
  • A Multi‐scale Climate Emulator for Long‐term Morphodynamics
    • Abstract: Interest in understanding long‐term coastal morphodynamics has recently increased as climate change impacts become perceptible and accelerated. Multi‐scale, behavior‐oriented and process‐based models, or hybrids of the two, are typically applied with deterministic approaches which require considerable computational effort. In order to reduce the computational cost of modeling large spatial and temporal scales, input reduction and morphological acceleration techniques have been developed. Here we introduce a general framework for reducing dimensionality of wave‐driver inputs to morphodynamic models. The proposed framework seeks to account for dependencies with global atmospheric circulation fields and deals simultaneously with seasonality, interannual variability, long‐term trends and autocorrelation of wave height, wave period and wave direction. The model is also able to reproduce future wave climate time series accounting for possible changes in the global climate system. An application of long‐term shoreline evolution is presented by comparing the performance of the real and the simulated wave climate using a one‐line model. This article is protected by copyright. All rights reserved.
      PubDate: 2015-12-22T19:28:23.008494-05:
      DOI: 10.1002/2015JC011107
  • Consequences of future increased arctic runoff on Arctic Ocean
           stratification, circulation, and sea ice cover
    • Authors: Aleksi Nummelin; Mehmet Ilicak, Camille Li, Lars H. Smedsrud
      Abstract: The Arctic Ocean has important freshwater sources including river runoff, low evaporation, and exchange with the Pacific Ocean. In the future, we expect even larger freshwater input as the global hydrological cycle accelerates, increasing high latitude precipitation and river runoff. Previous modelling studies show some robust responses to high latitude freshwater perturbations, including a strengthening of Arctic stratification and a weakening of the large‐scale ocean circulation; some idealized modelling studies also document a stronger cyclonic circulation within the Arctic Ocean itself. With the broad range of scales and processes involved, the overall effect of increasing runoff requires an understanding of both the local processes and the broader linkages between the Arctic and surrounding oceans. Here, we adopt a more comprehensive modelling approach by increasing river runoff to the Arctic Ocean in a coupled ice–ocean general circulation model, and show contrasting responses in the polar and subpolar regions. Within the Arctic, the stratification strengthens, the halocline and Atlantic Water layer warm, and the cyclonic circulation spins up, in agreement with previous work. In the subpolar North Atlantic, the model simulates a colder and fresher water column with weaker barotropic circulation. In contrast to the estuarine circulation theory, the volume exchange between the Arctic Ocean and the surrounding oceans does not increase with increasing runoff. While these results are robust in our model, we require experiments with other model systems and more complete observational syntheses to better constrain the sensitivity of the climate system to high latitude freshwater perturbations. This article is protected by copyright. All rights reserved.
      PubDate: 2015-12-18T17:59:39.349945-05:
      DOI: 10.1002/2015JC011156
  • Tracking the Hercules 265 marine gas well blowout in the Gulf of Mexico
    • Abstract: On July 23, 2013 a marine gas rig (Hercules 265) ignited in the northern Gulf of Mexico. The rig burned out of control for two days before being extinguished. We conducted a rapid‐response sampling campaign near Hercules 265 after the fire to ascertain if sediments and fishes were polluted above earlier baseline levels. A surface drifter study confirmed that surface ocean water flowed to the southeast of the Hercules site, while the atmospheric plume generated by the blowout was in eastward direction. Sediment cores were collected to the SE of the rig at a distance of ∼0.2 km, 8 km and 18 km using a multicorer, and demersal fishes were collected from ∼0.2 to 8 km SE of the rig using a longline (508 hooks). Recently deposited sediments document that only high molecular weight (HMW) polycyclic aromatic hydrocarbon (PAH) concentrations decreased with increasing distance from the rig suggesting higher pyrogenic inputs associated with the blowout. A similar trend was observed in the foraminifera Haynesina germanica, an indicator species of pollution. In red snapper bile, only HMW PAH metabolites increased in 2013 nearly double those from 2012. Both surface sediments and fish bile analyses suggest that, in the aftermath of the blowout, increased concentration of pyrogenically‐derived hydrocarbons were transported and deposited in the environment. This study further emphasizes the need for an ocean observing system and coordinated rapid‐response efforts from an array of scientific disciplines to effectively assess environmental impacts resulting from accidental releases of oil contaminants. This article is protected by copyright. All rights reserved.
      PubDate: 2015-12-18T17:59:26.083829-05:
      DOI: 10.1002/2015JC011037
  • Lateral variability of sediment transport in the Delaware Estuary
    • Authors: Jacqueline M. McSweeney; Robert J. Chant, Christopher K. Sommerfield
      Abstract: Lateral processes contribute significantly to circulation and material transport in estuaries. The mechanisms controlling transport may vary spatially such that shallow and deep regions of an estuary contribute differently to the total transport. An observational study was conducted to explore the importance of lateral variability in sediment transport mechanisms in the Delaware Estuary. Seven moorings were deployed across the channel in the region of the estuarine turbidity maximum (ETM) zone from April to August 2011. Timeseries of along‐channel sediment transport reveal a consistent pattern of sediment export across the entire estuary during periods of high river discharge, followed by a transition to import within the channel and export on the flanks during low river flow. There is a persistent divergence of across‐channel sediment fluxes on the Delaware side, where sediment from the flank is transported towards both the channel and wetland coast. Decomposition of the fluxes highlight that across‐channel sediment transport is driven by mean lateral circulation, whereas along‐channel transport is driven primarily by mean advection, with tidal pumping contributing to about 30% of total transport. The spatial and temporal variability of mean advection and tidal pumping were generally complementary, with both contributing to the observed sediment transport pathways. Tidal pumping, linked to tidal asymmetries in stratification and sediment resuspension, was shown to drive both ebb‐driven export and flood‐driven import depending on the tidal variability of stratification. The spatiotemporal patterns of sediment transport highlight the 3‐dimensional structure of the ETM and shed light on the variability of sediment transport mechanisms. This article is protected by copyright. All rights reserved.
      PubDate: 2015-12-18T17:59:13.913356-05:
      DOI: 10.1002/2015JC010974
  • Water renewals in the Saguenay Fjord
    • Abstract: Water renewals and renewal times of the Saguenay Fjord are investigated and classified according to their intrusion depth. Renewal dynamics are controlled by a shallow sill (∼ 20 m) at the fjord mouth, by large tides that are a distinguishing feature of the Saguenay Fjord and by large vertical mixing inside the inner basin (K∼ 10‐4, m2,s‐1). A mooring was deployed in the inner basin of the fjord to provide a clearer quantitative understanding of the complexity and seasonality of water renewals in this seasonally ice‐covered fjord. The mooring provided information on currents over nearly the entire water column, along with temperature‐salinity at a few discrete depths. Hydrographic temperature and salinity transects spanning multiple seasons and years as well as turbulence profiles were also collected. The observations show that the fjord dynamics are more complex than previously hypothesized, with large changes in renewal event depths leading to three different renewal regimes. Part of this renewal depth variability may be explained by the seasonality of the St. Lawrence estuarine circulation. Because of the large turbulence within the inner basin bottom layer, the density decreases over time such that new deep renewals can occur every year. The mechanisms behind the large vertical mixing can not yet be clearly identified but a statistically significant correlation (K ∝ N‐1.3) suggests that internal wave breaking may be a significant contributor to deep turbulence mixing in the inner basin. The renewal time of the inner basin waters is estimated to be between 1 and 6 months. This article is protected by copyright. All rights reserved.
      PubDate: 2015-12-18T17:59:01.218956-05:
      DOI: 10.1002/2015JC011085
  • Tidally discontinuous ocean forcing in bar‐built estuaries: The
           interaction of tides, infragravity motions, and frictional control
    • Authors: M.E. Williams; M.T. Stacey
      Abstract: Shallow, bar‐built estuaries on wave‐dominated coasts in Mediterranean climates experience an intermittent connection to the ocean. In the presence of low streamflow, their inlets may completely close as a result of nearshore sand transport, but even in the open condition these inlets remain constricted. Extensive field measurements in the highly salt‐stratified Pescadero estuary in Northern California show that the shallow mouth causes these estuaries to experience discontinuous tidal forcing. While the ocean and estuary are fully connected with near equal water levels, tidal velocities are slow but infragravity motions in the nearshore induce large velocity oscillations within the estuary. As the ocean tide falls, infragravity forcing is cut off because the estuarine mouth is perched above the low tide ocean water level, and ebbing velocities are set by bed friction. Observations reveal this oscillation between ocean‐forced and frictionally‐controlled conditions characterizes and sets estuarine hydrodynamics. Additional wave setup of the lagoon emphasizes the dependence of these estuaries on nearshore ocean conditions, but the diurnal or semidiurnal retreat of the ocean below the mouth cuts off this nearshore influence so it too is tidally varying. Here we present detailed observations and a framework for understanding hydrodynamics in small, shallow bar‐built estuaries. This article is protected by copyright. All rights reserved.
      PubDate: 2015-12-18T17:53:50.658572-05:
      DOI: 10.1002/2015JC011166
  • Impact of storm‐induced cooling of sea surface temperature on large
           turbulent eddies and vertical turbulent transport in the atmospheric
           boundary layer of Hurricane Isaac
    • Authors: Ping Zhu; Yuting Wang, Shuyi S. Chen, Milan Curcic, Cen Gao
      Abstract: Roll vortices in the atmospheric boundary layer (ABL) are important to oil operation and oil spill transport. This study investigates the impact of storm‐induced sea surface temperature (SST) cooling on the roll vortices generated by the convective and dynamic instability in the ABL of Hurricane Isaac (2005) and the roll induced transport using hindcasting large eddy simulations (LESs) configured from the multiply nested Weather Research & Forecasting model. Two experiments are performed: one forced by the Unified Wave INterface ‐ Coupled Model and the other with the SST replaced by the NCEP FNL analysis that does not include the storm‐induced SST cooling. The simulations show that the roll vortices are the prevalent eddy circulations in the ABL of Isaac. The storm‐induced SST cooling causes the ABL stability falls in a range that satisfies the empirical criterion of roll generation by dynamic instability, whereas the ABL stability without considering the storm‐induced SST cooling meets the criterion of roll generation by convective instability. The ABL roll is skewed and the increase of convective instability enhances the skewness. Large convective instability leads to large vertical transport of heat and moisture; whereas the dominant dynamic instability results in large turbulent kinetic energy but relatively weak heat and moisture transport. This study suggests that failure to consider roll vortices or incorrect initiation of dynamic and convective instability of rolls in simulations may substantially affect the transport of momentum, energy, and pollutants in the ABL and the dispersion/advection of oil spill fume at the ocean surface. This article is protected by copyright. All rights reserved.
      PubDate: 2015-12-18T17:53:38.182977-05:
      DOI: 10.1002/2015JC011320
  • Surface gravity wave transformation across a platform coral reef in the
           Red Sea
    • Authors: S. J. Lentz; J. H. Churchill, K. A. Davis, J. T. Farrar
      Abstract: The transformation of surface gravity waves across a platform reef in the Red Sea is examined using eighteen months of observations and a wave transformation model developed for beaches. The platform reef is 200 m across, 700 m long and the water depth varies from 0.3 to 1.2 m. Assuming changes in wave energy flux are due to wave breaking and bottom drag dissipation, the wave transformation model with optimal parameters characterizing the wave breaking (γμ=0.25) and bottom drag (hydrodynamic roughness zo=0.08 m) accounts for 75% ‐ 90% of the observed wave‐height variance at four sites. The observations and model indicate that wave breaking dominates the dissipation in a 20 – 30 m wide surf zone while bottom drag dominates the dissipation over the rest of the reef. Friction factors (drag coefficients) estimated from the observed wave energy balance range from fw=0.5 to fw=5 and increase as wave‐orbital displacements decrease. The observed dependence on wave‐orbital displacement is roughly consistent with extrapolation of an empirical relationship based on numerous laboratory studies of oscillatory flow. As a consequence of the dependence on wave‐orbital displacement, wave friction factors vary temporally due to changes in water depth and incident wave heights, and spatially across the reef as the waves decay. This article is protected by copyright. All rights reserved.
      PubDate: 2015-12-18T04:10:35.394079-05:
      DOI: 10.1002/2015JC011142
  • Contrasting spatial patterns in the diurnal and semidiurnal temperature
           variability in the Santa Barbara Channel, California
    • Abstract: The Santa Barbara Channel, California, experiences large temperature fluctuations during summer that have been associated with the input of nutrients to the euphotic zone. We studied the temperature fluctuations in the diurnal and semidiurnal bands, which account for as much as 65% of the total variance. We analyzed data from 25 moorings along the mainland and the Northern Channel Islands deployed at depths 8‐18 m during 1999‐2012. In the diurnal band the temperature fluctuations vary almost simultaneously within two distinct regions, with a lag of 5 hours between the regions: the mainland east of Pt. Conception and the west part of the Channel exposed to the large‐scale winds. The two regions of in‐phase temperature variability are in agreement with a previously published division of zones according to the wind characteristics. The portion of the diurnal temperature variance that is wind‐driven does not propagate along the coastline, but rather is directly forced by the wind. The semidiurnal temperature oscillations are more substantial in the Northern Channel Islands. These findings are consistent with a numerical study that predicted that the steep slopes of the Santa Cruz Basin, located south of the Channel Islands, are a source of semidiurnal internal tides. We conclude that the contrast between the spatial patterns of the diurnal and semidiurnal temperature oscillations on scales of 10s of km reflects the spatial distribution of the main forcing in each band, namely the diurnal wind and the locally generated semidiurnal internal tide. This article is protected by copyright. All rights reserved.
      PubDate: 2015-12-18T04:09:35.299604-05:
      DOI: 10.1002/2015JC011239
  • The formation of a subsurface anticyclonic eddy in the Peru‐Chile
           Undercurrent and its impact on the near‐coastal salinity, oxygen and
           nutrient distributions
    • Authors: Soeren Thomsen; Torsten Kanzow, Gerd Krahmann, Richard J. Greatbatch, Marcus Dengler, Gaute Lavik
      Abstract: The formation of a subsurface anticyclonic eddy in the Peru‐Chile Undercurrent (PCUC) in January and February 2013 is investigated using a multi‐platform four‐dimensional observational approach. Research vessel, multiple glider and mooring‐based measurements were conducted in the Peruvian upwelling regime near 12°30'S. The dataset consists of > 10000 glider profiles and repeated vessel‐based hydrography and velocity transects. It allows a detailed description of the eddy formation and its impact on the near‐coastal salinity, oxygen and nutrient distributions. In early January, a strong PCUC with maximum poleward velocities of ∼ 0.25 m/s at 100 to 200 m depth was observed. Starting on January 20 a subsurface anticyclonic eddy developed in the PCUC downstream of a topographic bend, suggesting flow separation as the eddy formation mechanism. The eddy core waters exhibited oxygen concentrations 
      PubDate: 2015-12-18T03:22:44.814648-05:
      DOI: 10.1002/2015JC010878
  • Wave transformation and shoreline water level on Funafuti Atoll, Tuvalu
    • Abstract: The influence of sea swell (SS) waves, infragravity (IG) waves, and wave setup on maximum runup (Rmax) is investigated across different tidal stages on Fatato Island, Funafuti Atoll, Tuvalu. Field results illustrate that SS waves are tidally modulated at the shoreline, with comparatively greater wave attenuation and setup occurring at low tide versus high tide. A shoreward increase in IG wave height is observed across the 100 m wide reef flat at all tidal elevations, with no tidal modulation of IG wave height at the reef flat or island shoreline. A 1D shock‐capturing Green‐Naghdi solver is used to replicate the field deployment and analyse Rmax. Model outputs for SS wave height, IG wave height and setup at the shoreline match field results with model skill > 0.96. Model outputs for Rmax are used to identify the temporal window when geomorphic activity can occur on the beach face. During periods of moderate swell energy, waves can impact the beach face at spring low tide, due to a combination of wave setup and strong IG wave activity. Under mean wave conditions, the combined influence of setup, IG waves and SS waves results in interaction with island sediment at mid‐tide. At high tide, SS and IG waves directly impact the beach face. Overall, wave activity is present on the beach face for 71% of the study period, a significantly longer duration than is calculated using mean water level and topographic data. This article is protected by copyright. All rights reserved.
      PubDate: 2015-12-18T03:19:16.934773-05:
      DOI: 10.1002/2015JC011246
  • Beach steepness effects on nonlinear infragravity‐wave interactions:
           A numerical study
    • Authors: A.T.M. de Bakker; M.F.S. Tissier, B.G. Ruessink
      Abstract: The numerical model SWASH is used to investigate nonlinear energy transfers between waves for a diverse set of beach profiles and wave conditions, with a specific focus on infragravity waves. We use bispectral analysis to study the nonlinear triad interactions, and estimate energy transfers to determine energy flows within the spectra. The energy transfers are divided into four types of triad interactions, with triads including either one, two or three infragravity‐frequency components, and triad interactions solely between sea‐swell wave frequencies. The SWASH model is validated with a high‐resolution laboratory data set on a gently‐sloping beach, which shows that SWASH is capable of modeling the detailed nonlinear interactions. From the simulations, we observe that especially the beach slope affects nonlinear infragravity‐wave interactions. On a low‐sloping beach, infragravity‐wave energy dominates the water motion close to shore. Here, infragravity‐infragravity interactions dominate and generate higher harmonics that lead to the steepening of the infragravity wave and eventually breaking, causing large infragravity energy dissipation. On the contrary, on a steep‐sloping beach, sea‐swell wave energy dominates the water motion everywhere. Here, infragravity frequencies interact with the spectral peak and spread energy to a wide range of higher frequencies, with relatively less infragravity energy dissipation. Although both beach types have different nonlinear interaction patterns during infragravity‐wave dissipation, the amount of infragravity‐wave reflection can be estimated by a single parameter, the normalized bed slope. This article is protected by copyright. All rights reserved.
      PubDate: 2015-12-18T03:19:00.947082-05:
      DOI: 10.1002/2015JC011268
  • Spectral albedo and transmittance of thin young Arctic sea ice
    • Abstract: Spectral albedo and transmittance in the range 400nm to 900nm were measured on three separate dates on less than 15cm thick new Arctic sea ice growing on Kongsfjorden, Svalbard, at 78.9°N 11.9°E. Inherent optical properties, including absorption coefficients of particulate and dissolved material, were obtained from ice samples and fed into a radiative transfer model, which was used to analyze spectral albedo and transmittance, and to study the influence of clouds and snow on these. Integrated albedo and transmittance for photosynthetically active radiation (400nm‐700nm) were in the range 0.17‐0.21 and 0.77‐0.86, respectively. The average albedo and transmittance of the total solar radiation energy were 0.16 and 0.51, respectively. Values inferred from the model indicate that the ice contained possibly up to 40% brine and only 0.6% bubbles. Angular redistribution of solar radiation by clouds and snow was found to influence both the wavelength integrated value and the spectral shape of albedo and transmittance. In particular, local peaks and depressions in the spectral albedo and spectral transmittance were found for wavelengths within atmospheric absorption bands. Simulated and measured transmittance spectra were within 5% for most of the wavelength range, but deviated up to 25% in the vicinity of 800nm, indicating the need for more optical laboratory measurements of pure ice, or improved modeling of brine optical properties in this near‐infrared wavelength region. This article is protected by copyright. All rights reserved.
      PubDate: 2015-12-18T03:18:48.546509-05:
      DOI: 10.1002/2015JC011254
  • Modeled sensitivity of the northwestern Pacific upper‐ocean response
           to tropical cyclones in a fully coupled climate model with varying ocean
           grid resolution
    • Authors: Hui Li; Ryan L. Sriver, Marlos Goes
      Abstract: Tropical cyclones (TCs) actively contribute to Earth's climate, but TC‐climate interactions are largely unexplored in fully‐coupled models. Here we analyze the upper‐ocean response to TCs using a high resolution Earth system model, in which a 0.5° atmosphere is coupled to an ocean with two different horizontal resolutions: 1° and 0.1°. Both versions of the model produce realistic TC climatologies for the northwestern Pacific region, as well as the transient surface ocean response. We examined the potential sensitivity of the coupled modeled responses to ocean grid resolution by analyzing TC‐induced sea surface cooling, latent heat exchange and basin‐scale ocean heat convergence. We find that sea surface cooling and basin‐scale aggregated ocean heat convergence are relatively insensitive to the horizontal ocean grid resolutions considered here, but we find key differences in the post‐storm restratification processes related to mesoscale ocean eddies. We estimate the annual basin‐scale TC‐induced latent heat fluxes are 1.70 ± 0.16 • 1021 J and 1.43 ± 0.16 • 1021 J for the high‐resolution and low‐resolution model configurations, respectively, which account for roughly 45% of the total TC‐induced ocean heat loss from the upper ocean. Results suggest that coupled modeling approaches capable of capturing ocean‐atmosphere feedbacks are important for developing a complete understanding of the relationship between TCs and climate. This article is protected by copyright. All rights reserved.
      PubDate: 2015-12-18T03:18:37.900042-05:
      DOI: 10.1002/2015JC011226
  • Optimum interpolation analysis of Aquarius sea surface salinity
    • Authors: Oleg Melnichenko; Peter Hacker, Nikolai Maximenko, Gary Lagerloef, James Potemra
      Abstract: A new high‐resolution sea surface salinity (SSS) analysis has been produced using Aquarius satellite observations from September 2011 to June 2015. The motivation for the new product is twofold: to produce level‐4 SSS analysis that is consistent with existing in‐situ observations such as from Argo profile data, and to reduce the large‐scale satellite biases that have existed in all versions of the standard level‐3 Aquarius products. The new product is a weekly SSS analysis on a nearly‐global 0.5‐degree grid. The analysis method is optimum interpolation (OI) that takes into account analyzed errors of the observations, specific to the Aquarius instrument. The method also includes a large‐scale correction for satellite biases, filtering of along‐track SSS data prior to OI, and the use of realistic correlation scales of SSS anomalies. All these features of the analysis are shown to result in more accurate SSS maps. In particular, the method reduces the effects of relative biases between the Aquarius beams and eliminates most of the large‐scale, space‐ and time‐varying satellite biases relative to in‐situ data, including spurious annual signals. Statistical comparison between the weekly OI SSS maps and concurrent buoy data demonstrates that the global root‐mean‐square error of the analysis is smaller than 0.2 pss for nearly all weeks over the ∼4‐year period of comparison. The utility of the OI SSS analysis is also exemplified by the derived patterns of regional SSS variability. This article is protected by copyright. All rights reserved.
      PubDate: 2015-12-18T03:18:24.039802-05:
      DOI: 10.1002/2015JC011343
  • An examination of double‐diffusive processes in a mesoscale eddy in
           the Arctic Ocean
    • Abstract: Temperature and salinity measurements of an Atlantic Water mesoscale eddy in the Arctic Ocean's Canada Basin are analyzed to understand the effects of velocity shear on a range of double‐diffusive processes. Double‐diffusive structures in and around the eddy are examined through the transition from low shear (outside the eddy and within its solid body core) to high geostrophic shear zones at the eddy flanks. The geostrophic Richardson number takes large values where a double‐diffusive staircase is observed and lowest values at the eddy flanks where geostrophic velocity is largest and a well‐formed staircase is not present. A Thorpe scale analysis is used to estimate turbulent diffusivities in the flank regions. Double‐diffusive and turbulent heat, salt and buoyancy fluxes from the eddy are computed, and used to infer that the eddy decays on timescales of around 4 to 9 years. Fluxes highlight that Atlantic Water heat within the eddy can be fluxed downwards into deeper water layers by means of both double‐diffusive and turbulent mixing. Estimated lateral variations in vertical fluxes across the eddy allow for speculation that double diffusion speeds up the eddy decay, having important implications for the transfer of Atlantic Water heat in the Arctic Ocean. This article is protected by copyright. All rights reserved.
      PubDate: 2015-12-15T10:17:02.708126-05:
      DOI: 10.1002/2015JC011105
  • Interannual variability in the South‐East Atlantic Ocean, focusing
           on the Benguela Upwelling System: Remote versus local forcing
    • Abstract: We investigate the respective roles of equatorial remote (Equatorial Kelvin Waves) and local atmospheric (wind, heat fluxes) forcing on coastal variability in the South‐East Atlantic Ocean extending up to the Benguela Upwelling System (BUS) over the 2000‐2008 period. We carried out a set of 6 numerical experiments based on a regional ocean model, that differ only by the prescribed forcing (climatological or total) at surface and lateral boundaries. Results show that at sub‐seasonal timescales (
      PubDate: 2015-12-15T10:16:52.652935-05:
      DOI: 10.1002/2015JC011168
  • Evidence of enhanced double‐diffusive convection below the main
           stream of the Kuroshio Extension
    • Authors: Takeyoshi Nagai; Ryuichiro Inoue, Amit Tandon, Hidekatsu Yamazaki
      Abstract: In this study, a Navis‐MicroRider microstructure float and an EM‐APEX float were deployed along the Kuroshio Extension Front. The observations deeper than 150 m reveal widespread interleaving thermohaline structures for at least 900 km along the front, presumably generated through mesoscale stirring and near‐inertial oscillations. In these interleaving structures, microscale thermal dissipation rates χ are very high (>10−7 K2s−1), while turbulent kinetic energy dissipation rates ϵ are relatively low (10−10‐10−9 Wkg−1), with effective thermal diffusivity Kθ of (10−3 m2s−1) consistent with the previous parameterizations for double‐diffusion, and, Kθ is two orders of magnitude larger than the turbulent eddy diffusivity for density Kρ. The average observed dissipation ratio Γ in salt finger and diffusive convection favorable conditions are 1.2 and 4.0, respectively, and are larger than that for turbulence. Our results suggest that mesoscale subduction/obduction and near‐inertial motions could catalyze double‐diffusive favorable conditions, and thereby enhancing the diapycnal tracer fluxes below the Kuroshio Extension Front. This article is protected by copyright. All rights reserved.
      PubDate: 2015-12-15T10:13:17.639794-05:
      DOI: 10.1002/2015JC011288
  • Causes for Intraseasonal Sea Surface Salinity Variability in the Western
           Tropical Pacific Ocean and Its Seasonality
    • Authors: Yuanlong Li; Weiqing Han
      Abstract: Pronounced intraseasonal variability (ISV; 20‐90 day) of sea surface salinity (SSS) with a standard deviation of 0.12‐0.20 psu is detected in the western tropical Pacific Ocean (PO) from measurements of Aquarius/SAC‐D satellite. These variations are not spatially uniform but show distinct regional features. The Hybrid Coordinate Ocean Model (HYCOM) well simulated the observed SSS variations, and a suite of parallel experiments were performed to understand the underlying physical processes. Surface forcing by atmospheric intraseasonal oscillations which are dominated by the Madden‐Julian oscillation (MJO) is largely responsible for producing the SSS ISV, while ocean internal variability plays a secondary role. Impact of atmospheric forcing is primarily through precipitation and wind stress‐driven oceanic processes. Their relative importance shows spatial variations. They have approximately equal importance in the western equatorial PO west of 155°E and the southwestern tropical PO. Wind stress effect dominates SSS ISV in the equatorial PO east of 155°E, while precipitation effect is larger in the northwestern tropical PO. In comparison, the effect of evaporation induced by wind speed change is smaller. The SSS ISV also shows evident seasonality in some areas, particularly in the far western equatorial basin and southwestern tropical PO. During boreal summer (winter), SSS ISV is enhanced (weakened) in the northwestern PO and weakened (enhanced) in the southwestern PO. Comparing with the strength of atmospheric forcing, seasonal variation of the ocean state, especially the mixed layer depth, is generally more important in causing such seasonality. This article is protected by copyright. All rights reserved.
      PubDate: 2015-12-14T18:16:33.865446-05:
      DOI: 10.1002/2015JC011413
  • Subduction of Pacific Antarctic Intermediate Water in an
           Eddy‐resolving Model
    • Authors: Yuri Hiraike; Yukio Tanaka, Hiroyasu Hasumi
      Abstract: The subduction process of Pacific Antarctic Intermediate Water (PAAIW) in the Pacific is investigated using output from an eddy‐resolving ocean model. Focus is on contribution of eddies to the subduction process. To separate the subduction rate into contributions by eddies and mean flows, the temporal residual mean (TRM) velocity is used. In the mean subduction rate, lateral induction caused by the strong eastward flow of the Antarctic Circumpolar Current (ACC) is dominant. The largest rate is located in the Drake Passage. The estimated eddy‐induced subduction rate is comparable with the mean subduction rate, and it tends to cancel the vertical mean component in many regions. In the west of the Drake Passage, however, the eddy‐induced subduction is larger than the vertical mean component, and this eddy‐induced subduction was not detected in previous studies using the thickness diffusion parameterization and an eddy‐permitting model. Results of idealized sensitivity studies to model resolution suggest that the subduction rate would be larger using a model with higher vertical resolution. Therefore, the vertical resolution should be paid more attention in model studies investigating eddy‐induced subduction, and not just the horizontal resolution. This article is protected by copyright. All rights reserved.
      PubDate: 2015-12-14T18:15:04.526036-05:
      DOI: 10.1002/2015JC010802
  • Spatial analysis of trends in primary production and relationship with
           large‐scale climate variability in the Ross Sea, Antarctica
    • Authors: Casey M. S. Schine; Gert van Dijken, Kevin R. Arrigo
      Abstract: Recent studies have documented an increase in sea ice extent and the duration of the ice season in the Ross Sea, Antarctica. We conducted a satellite‐based study to quantify changes in net primary production (NPP) and chlorophyll a (Chl a) in response to the observed changes in ice dynamics in the Ross Sea south of 60°S. Our study covers a 16‐year time period (1997‐2013) and incorporates both the shelf and off‐shelf regions of the Ross Sea. We observed significant secular changes in NPP from 1997 to 2013 in the off‐shelf region, with NPP increasing on the eastern side and decreasing on the western side of our study area. The changes we observed in NPP are consistent with the changes we observed in sea surface temperature (SST) and open water days (OWDs), decreasing (increasing) on the western (eastern) side of our study area. Finally, we examined the influence of the Southern Annular Mode (SAM) and the El Niño Southern Oscillation (ENSO), on SST, OWDs, Chl a, and NPP in the Ross Sea and observed a significant relationship between the state of the SAM and ENSO and SST, OWDs, and NPP across the study region. The response of OWDs, SST, and NPP to atmospheric forcing by SAM and ENSO was opposite for the shelf and off‐shelf regions, such that during a positive phase of SAM or negative phase of ENSO (La Niña), SST, OWDs, and NPP increased on the shelf and decreased in the off‐shelf region. This article is protected by copyright. All rights reserved.
      PubDate: 2015-12-14T18:15:01.246801-05:
      DOI: 10.1002/2015JC011014
  • The Future of the Subsurface Chlorophyll‐a Maximum in the Canada
           Basin ‐ A Model Intercomparison
    • Authors: N. S. Steiner; T. Sou, C. Deal, J. M. Jackson, M. Jin, E. Popova, W. Williams, A. Yool
      Abstract: Six Earth system models and three ocean‐ice‐ecosystem models are analyzed to evaluate magnitude and depth of the subsurface Chl‐a maximum (SCM) in the Canada Basin and ratio of surface to subsurface Chl‐a in a future climate scenario. Differences in simulated Chl‐a are caused by large intermodel differences in available nitrate in the Arctic Ocean and to some extent by ecosystem complexity. Most models reproduce the observed SCM and nitracline deepening and indicate a continued deepening in the future until the models reach a new state with seasonal ice free waters. Models not representing a SCM show either too much nitrate and hence no surface limitation or too little nitrate with limited surface growth only. The models suggest that suppression of the nitracline and deepening of the SCM are caused by enhanced stratification, likely driven by enhanced Ekman convergence and freshwater contributions with primarily large scale atmospheric driving mechanisms. The simulated ratio of near surface Chl‐a to depth integrated Chl‐a is slightly decreasing in most areas of the Arctic ocean due to enhanced contributions of subsurface Chl‐a. Exceptions are some shelf areas and regions where the continued ice thinning leaves winter ice too thin to provide a barrier to momentum fluxes, allowing winter mixing to break up the strong stratification. Results confirm that algorithms determining vertically integrated Chl‐a from surface Chl‐a need to be tuned to Arctic conditions, but likely require little or no adjustments in the future. This article is protected by copyright. All rights reserved.
      PubDate: 2015-12-14T10:25:07.930096-05:
      DOI: 10.1002/2015JC011232
  • Derivation of the specific optical properties of suspended mineral
           particles and their contribution to the attenuation of solar irradiance in
           offshore waters by ocean color remote sensing
    • Authors: Catherine Mitchell; Alex Cunningham, David McKee
      Abstract: Two independently‐derived algorithms which had previously been validated against in situ data were applied to eight years of MODIS observations of the Irish Sea to obtain (i) concentrations of lithogenic mineral particles (MSSl) in surface waters and (ii) the specific backscattering and absorption coefficients for these particles in the 488 nm waveband (the values obtained were a*MSSl488 =0.031 m2 g−1 and bb*MSSl488 = 0.010 m2 g−1). This information was used to calculate the mean attenuation coefficient for downward irradiance (488) in the surface mixed layer, and the fraction of this coefficient that was attributable to suspended mineral particles. Mineral particles at relatively low concentrations (
      PubDate: 2015-12-14T10:23:47.21653-05:0
      DOI: 10.1002/2015JC011056
  • Physical processes contributing to an ice free Beaufort Sea during
           September 2012
    • Authors: D.G. Babb; R.J. Galley, D.G. Barber, S. Rysgaard
      Abstract: During the record September 2012 sea ice minimum the Beaufort Sea became ice free for the first time during the observational record. Increased dynamic activity during late winter enabled increased open water and seasonal ice coverage that contributed to negative sea ice anomalies and positive solar absorption anomalies which drove rapid bottom melt and sea ice loss. As had happened in the Beaufort Sea during previous years of exceptionally low September sea ice extent, anomalous solar absorption developed during May, increased during June, peaked during July and persisted into October. However in situ observations from a single floe reveal less than 78% of the energy required for bottom melt during 2012 was available from solar absorption. We show that the 2012 sea ice minimum in the Beaufort was the result of anomalously large solar absorption that was compounded by an arctic cyclone and other sources of heat such as solar transmission, oceanic upwelling and riverine inputs, but was ultimately made possible through years of preconditioning towards a younger, thinner ice pack. Significant negative trends in sea ice concentration between 1979 and 2012 from June to October, coupled with a tendency towards earlier sea ice reductions have fostered a significant trend of +12.9 MJ m−2 year−1 in cumulative solar absorption, sufficient to melt an additional 4.3 cm m−2 year−1. Overall through preconditioning towards a younger, thinner ice pack the Beaufort Sea has become increasingly susceptible to increased sea ice loss that may render it ice free more frequently in coming years. This article is protected by copyright. All rights reserved.
      PubDate: 2015-12-14T03:32:18.27323-05:0
      DOI: 10.1002/2015JC010756
  • On the majestic seasonal upwelling system of the Arafura Sea
    • Abstract: A three‐dimensional hydrodynamic model is applied to explore the development of phytoplankton blooms in the Arafura Sea during the southeast monsoon (June‐November). Before this season, blooms are restricted to near‐shore waters covering a total area of ∼25,000 km2. Satellite data indicate that the bloom area grows substantially to >90,000 km2 during the southeast monsoon covering most of the northwestern Arafura Sea. Findings confirm that the southeast monsoon creates undercurrents via the classical lee effect driving nutrient‐rich Banda Sea slope water into this region. This nutrient‐rich slope water is driven over vast distances (∼300 km) into the northwestern Arafura Sea where it upwells and/or is entrained into the surface mixed layer. The associated overturning circulation is slow but continuous and it takes 1‐2 months before nutrient‐rich water appears in surface waters of the region. The predicted pathways of nutrient‐rich inflows across the shelf break both north and south of the Aru Islands agree with observational evidence. This article is protected by copyright. All rights reserved.
      PubDate: 2015-12-14T03:32:08.407958-05:
      DOI: 10.1002/2015JC011197
  • The impact of wave‐induced Coriolis‐Stokes forcing on
           satellite‐derived Ocean Surface Currents
    • Authors: Zhenli Hui; Yongsheng Xu
      Abstract: Ocean surface currents estimated from the satellite data consist of two terms: Ekman currents from the wind stress and geostrophic currents from the sea surface height (SSH). But the classical Ekman model doesn't consider the wave effects. By taking the wave‐induced Coriolis‐Stokes forcing into account, the impact of waves (primarily the Stokes drift) on ocean surface currents is investigated and the wave‐modified currents are formed. The products are validated by comparing with OSCAR currents and Lagrangian drifter velocity. The result shows that our products with the Stokes drift are better adapted to the in situ Lagrangian drifter currents. Especially in the Southern Ocean region (40° S‐65° S), 90% (91%) of the zonal (meridional) currents have been improved compared with currents that don't include Stokes drift. The correlation (RMSE) in the Southern Ocean has also increased (decreased) from 0.78 (13) to 0.81 (10.99) for the zonal component and 0.76 (10.87) to 0.79 (10.09) for the meridional component. This finding provides the evidence that waves indeed play an important role in the ocean circulation, and need to be represented in numerical simulations of the global ocean circulation. This article is protected by copyright. All rights reserved.
      PubDate: 2015-12-14T03:31:00.642137-05:
      DOI: 10.1002/2015JC011082
  • High‐resolution Sr/Ca ratios in a Porites lutea coral from
           Lakshadweep Archipelago, southeast Arabian Sea: An example from a region
           experiencing steady rise in the reef temperature
    • Abstract: Here we present the first record of Sr/Ca variability in a massive Porites lutea coral from the Lakshadweep Archipelago, Arabian Sea. The annual mean sea surface temperature (SST) in this region and the surrounding areas has increased steadily in the recent past. During some major El Niño events, SSTs are even higher, imposing additional thermal‐stress on corals, episodically leading to coral bleaching. We infer from the coral‐Sr/Ca record (1981 to 2008) that during some of these events high and persistent SSTs lead to a dampening of the temperature signal in coral‐Sr/Ca, impairing the coral's ability to record full scale warming. Thus, coral‐Sr/Ca may provide a history of past El Niño Southern‐Oscillation (ENSO) induced thermal‐stress episodes, which are a recurrent feature also seen in cross‐spectral analysis between coral‐Sr/Ca and the Nino3.4 index. Despite the impact of episodical thermal‐stress during major El Niño events, our coral proxy faithfully records the seasonal monsoon‐induced summer cooling on the order of ∼2.3 ˚C. Calibration of coral‐Sr/Ca with instrumental grid‐SST data shows significant correlation to regional SST and monsoon variability. Hence, massive Porites corals of this region are highly valuable archives for reconstructing long‐term changes in SST, strongly influenced by monsoon variability on seasonal scales. More importantly, our data show that this site with increasing SST is an ideal location for testing the future effects of the projected anthropogenic SST increase on coral reefs that are already under thermal‐stress worldwide. This article is protected by copyright. All rights reserved.
      PubDate: 2015-12-13T03:41:55.238705-05:
      DOI: 10.1002/2015JC010821
  • Enhanced warming of the northwest Atlantic Ocean under climate change
    • Authors: Vincent S. Saba; Stephen M. Griffies, Whit G. Anderson, Michael Winton, Michael A. Alexander, Thomas L. Delworth, Jonathan A. Hare, Matthew J. Harrison, Anthony Rosati, Gabriel A. Vecchi, Rong Zhang
      Abstract: The Intergovernmental Panel on Climate Change (IPCC) fifth assessment of projected global and regional ocean temperature change is based on global climate models that have coarse (∼100‐km) ocean and atmosphere resolutions. In the Northwest Atlantic, the ensemble of global climate models has a warm bias in sea surface temperature due to a misrepresentation of the Gulf Stream position; thus, existing climate change projections are based on unrealistic regional ocean circulation. Here we compare simulations and an atmospheric CO2 doubling response from four global climate models of varying ocean and atmosphere resolution. We find that the highest resolution climate model (∼10‐km ocean, ∼50‐km atmosphere) resolves Northwest Atlantic circulation and water mass distribution most accurately. The CO2 doubling response from this model shows that upper‐ocean (0‐300 m) temperature in the Northwest Atlantic Shelf warms at a rate nearly twice as fast as the coarser models and nearly three times faster than the global average. This enhanced warming is accompanied by an increase in salinity due to a change in water mass distribution that is related to a retreat of the Labrador Current and a northerly shift of the Gulf Stream. Both observations and the climate model demonstrate a robust relationship between a weakening Atlantic Meridional Overturning Circulation (AMOC) and an increase in the proportion of Warm‐Temperate Slope Water entering the Northwest Atlantic Shelf. Therefore, prior climate change projections for the Northwest Atlantic may be far too conservative. These results point to the need to improve simulations of basin and regional‐scale ocean circulation. This article is protected by copyright. All rights reserved.
      PubDate: 2015-12-13T03:41:24.23359-05:0
      DOI: 10.1002/2015JC011346
  • Observations and model simulations of wave‐current interaction on
           the inner shelf
    • Authors: Julia Hopkins; Steve Elgar, Britt Raubenheimer
      Abstract: Wave directions and mean currents observed for two 1‐month‐long periods in 7‐ and 2‐m water depths along 11 km of the southern shoreline of Martha's Vineyard, MA have strong tidal modulations. Wave directions are modulated by as much as 70° over a tidal cycle. The magnitude of the tidal modulations to the wave field decreases alongshore to the west, consistent with the observed decrease in tidal currents from 2.1 to 0.2 m/s along the shoreline. A numerical model (SWAN and Deflt3D‐FLOW) simulating waves and currents reproduces the observations accurately. Model simulations with and without wave‐current interaction and tidal depth changes demonstrate that the observed tidal modulations of the wave field primarily are caused by wave‐current interaction and not by tidal changes to water depths over the nearby complex shoals. This article is protected by copyright. All rights reserved.
      PubDate: 2015-12-13T03:38:38.901515-05:
      DOI: 10.1002/2015JC010788
  • Rain effects on the hurricane observations over the ocean by C‐band
           Synthetic Aperture Radar
    • Authors: Guosheng Zhang; Xiaofeng Li, William Perrie, Biao Zhang, Lei Wang
      Abstract: A composite radar scattering model composed of the atmosphere radiative transfer model and the ocean surface Bragg wave theory is developed to analyze the impact of hurricane rain on the normalized radar‐backscatter cross‐section (NRCS) measured in the VV‐ and cross‐polarized C‐band Synthetic Aperture Radar (SAR) channels. The model results are validated against SAR and SFMR measured wind speeds and rain rates for two hurricane cases. The contribution of rain to the NRCS is backscatter from two parts: the atmosphere column and the ocean surface. In the atmosphere, microwave attenuation and the rain‐induced volume backscattering are simulated by the model. We find that the impact of raindrops in the atmosphere is almost negligible for the VV polarization, but important for the cross‐polarization. On the ocean surface, comparisons between our model and other existing models without rain lead to the conclusion that the VV polarization NRCS can be simulated reasonably well without considering the non‐Bragg scattering mechanisms. Similar to the wave breaking mechanism, the microwave diffraction on the craters, crowns and stalks, produced by rain drops, are also negligible for VV polarization. However, the non‐Bragg scattering is important for the cross‐polarized NRCS simulations. Finally, we performed simulations to understand the VV‐polarized NRCS behavior under different wind speeds at various rain rates. This article is protected by copyright. All rights reserved.
      PubDate: 2015-12-13T03:38:33.804233-05:
      DOI: 10.1002/2015JC011044
  • Wave dynamics of a Pacific atoll with high frictional effects
    • Authors: Justin S. Rogers; Stephen G. Monismith, David A. Koweek, Robert B. Dunbar
      Abstract: We report field measurements of waves and currents made from Sept‐2011 to Jul‐2014 on Palmyra Atoll in the central Pacific that were used in conjunction with the SWAN wave model to characterize the wave dynamics operant on the atoll. Our results indicate that wave energy is primarily from the north during the northern hemisphere winter and from the south in the northern hemisphere summer. Refraction of waves along the reef terraces due to variations in bathymetry leads to focusing of waves in specific locations. Bottom friction, modeled with a modified bottom roughness formulation, is the significant source of wave energy dissipation on the atoll, a result that is consistent with available observations of wave damping on Palmyra. Indeed modeled wave dissipation rates from bottom friction are on average larger than dissipation rates due to breaking and are an order of magnitude larger than what has been observed on other, less geometrically complex reefs, a result which should be corroborated with future in situ measurements. The SWAN wave model with a modified bottom friction formulation better predicts bulk wave energy properties than the existing formulation at our measurement stations. The near bed squared velocity, a proxy for bottom stress, shows strong spatial variability across the atoll and exerts control over geomorphic structure and benthic community composition. This article is protected by copyright. All rights reserved.
      PubDate: 2015-12-13T03:38:31.562779-05:
      DOI: 10.1002/2015JC011170
  • Do deep‐ocean kinetic energy spectra represent deterministic or
           stochastic signals?
    • Authors: Hans van Haren
      Abstract: In analogy with previous analyses of shallow‐water tide‐gauge records, in which tides and their higher harmonics are modified by sea level changes induced by atmospheric disturbances, it is shown that deep‐sea currents can be interpreted as motions at predominantly inertial‐tidal harmonic frequencies modified by slowly varying background conditions. In this interpretation, their kinetic energy spectra may not be smoothed into a quasi‐stochastic continuum for (random‐)statistic confidence. Instead, they are considered as quasi‐deterministic line‐spectra. Thus, the climatology of the internal wave field and its slowly varying background can be inferred from line spectra filling the cusps around nonlinear tidal‐inertial harmonics, as suggested previously. This article is protected by copyright. All rights reserved.
      PubDate: 2015-12-13T03:38:24.387307-05:
      DOI: 10.1002/2015JC011204
  • Mobile mud dynamics in the East China Sea elucidated using 210Pb,137Cs,
           7Be, and 234Th as tracers
    • Authors: Jinlong Wang; Jinzhou Du, Mark Baskaran, Jing Zhang
      Abstract: “Mobile mud” (MM) which has fine grain‐size distribution (> 90% clay + silt, and  2.5 in south inshore indicates that the sediment focusing resulted in the increased mass flux. The residence time of MM is estimated as 3‐6 years both by mass balance of MM and 210Pbex in MM . This article is protected by copyright. All rights reserved.
      PubDate: 2015-12-13T03:38:15.909654-05:
      DOI: 10.1002/2015JC011300
  • Forcing and variability of nonstationary rip currents
    • Abstract: Surface wave transformation and the resulting nearshore circulation along a section of coast with strong alongshore bathymetric gradients outside the surf zone are modeled for a consecutive four‐week time period. The modeled hydrodynamics are compared to in situ measurements of waves and currents collected during the Nearshore Canyon Experiment and indicate that for the entire range of observed conditions, the model performance is similar to other studies along this stretch of coast. Strong alongshore wave height gradients generate rip currents that are observed by remote sensing data and predicted qualitatively well by the numerical model. Previous studies at this site have used idealized scenarios to link the rip current locations to undulations in the offshore bathymetry but do not explain the dichotomy between permanent offshore bathymetric features and intermittent rip current development. Model results from the month‐long simulation are used to track the formation and location of rip currents using hourly statistics, and results show that the direction of the incoming wave energy strongly controls whether rip currents form. In particular, most of the offshore wave spectra were bi‐modal and we find that the ratio of energy contained in each mode dictates rip current development, and the alongshore rip current position is controlled by the incident wave period. Additionally, model simulations performed with and without updating the nearshore morphology yield no significant change in the accuracy of the predicted surf zone hydrodyanmics indicating that the large scale offshore features (e.g. submarine canyon) predominately control the nearshore wave‐circulation system. This article is protected by copyright. All rights reserved.
      PubDate: 2015-12-13T03:37:53.416066-05:
      DOI: 10.1002/2015JC010990
  • On the specification of background errors for wave data assimilation
    • Abstract: In this paper a new methodology is proposed for the computation of Background Errors in wave data assimilation systems. Background errors define the spatial influence of an observation in the model domain. Since at present the directional wave spectrum is the fundamental variable of both state‐of‐the‐art numerical models and most modern instrumentation, this is at the core of the proposed methodology. The advantage of the spectral approach is that the wave spectrum contains detailed information of the different wave systems and physical processes at work (e.g., wind sea or swells). These systems have different origins and may be driven by different mechanisms, having therefore different spatial structures, length scales and sensitivity to local wind conditions. The presented method enables making consistent and specific corrections to each component of the spectrum, in time and space. The innovations presented here require an integral look at the data assimilation algorithm for which a suitable scheme is also proposed. Examples of computed background errors are presented for shelf and oceanic basins showing the spatial structures of the different wave systems active in these areas. This article is protected by copyright. All rights reserved.
      PubDate: 2015-12-13T03:36:01.143373-05:
      DOI: 10.1002/2015JC011309
  • The inflow of Atlantic water at the Fram Strait and its interannual
    • Authors: Takao Kawasaki; Hiroyasu Hasumi
      Abstract: The heat influx of the Atlantic water and its interannual variability through the Fram Strait toward the Arctic Ocean are examined by using a realistically configured ice‐ocean general circulation model. The modeled routes of the Atlantic water and high eddy activity around the Fram Strait are consistent with many observations. Two‐thirds of the heat transported by the Atlantic water passing through the Fram Strait (78°N) is lost by the westward transport and the sea surface cooling, and the other one‐third is injected to the Arctic Ocean. The contribution of oceanic eddy to the westward heat transport is 5% of that of mean current. The variability of sea level pressure anomaly centered at the Nordic Seas explains the interannual variability of the heat passing through the Fram Strait, transported westward, and cooled at the sea surface in the north of the Fram Strait. The interannual variabilities of these heat fluxes have significant correlations with the NAO. The interannual variability of heat transported by the Atlantic water and entering the Arctic Ocean is caused by the variability of the Siberian high. This article is protected by copyright. All rights reserved.
      PubDate: 2015-12-13T03:35:52.900935-05:
      DOI: 10.1002/2015JC011375
  • Parameterization of intrawave ripple‐averaged sediment pickup above
           steep ripples
    • Abstract: Near‐bed sediment pickup is critical for predictions of intrawave suspension and in turn net sediment transport in coastal models. In the present study, numerical results from a two‐dimensional Reynolds‐averaged Navier‐Stokes model are used to assess the functional relationship of intrawave ripple‐averaged sediment pickup above steep ripples. The numerical model provides intrawave time histories of ripple‐averaged near‐bed velocities and turbulence, which are qualitatively interrogated to determine pickup functional relationships. Several specific sediment pickup formulations are implemented within the numerical model: expressions relating pickup to near‐bed velocity or near‐bed turbulent kinetic energy via the bed shear stress; and expressions relating pickup to near‐bed shear production of turbulent kinetic energy. These are then tested via model‐data comparisons of near‐bed suspended sediment concentration. The results show that the traditional functions relating sediment pickup to near‐bed velocity cannot lead to reasonable intrawave suspension predictions above vortex ripples under a ripple‐averaged framework. Instead, relating sediment pickup to near‐bed turbulence quantities, such as turbulent kinetic energy or shear production of turbulent kinetic energy, significantly improves the numerical predictions for these conditions. This article is protected by copyright. All rights reserved.
      PubDate: 2015-12-13T03:35:33.919062-05:
      DOI: 10.1002/2015JC011185
  • Antarctic icebergs distributions 1992‐2014
    • Abstract: Basal melting of floating ice shelves and iceberg calving constitute, the two almost equal paths of freshwater flux between the Antarctic, ice cap and the Southern Ocean. The largest icebergs (>100km2), transport most of the ice volume but their basal melting is small, compared to their breaking into smaller icebergs that constitute thus, the major vector of freshwater. The archives of nine altimeters have, been processed to create a small icebergs (
      PubDate: 2015-12-13T03:35:32.235578-05:
      DOI: 10.1002/2015JC011178
  • Ocean processes underlying surface clustering
    • Authors: Gregg A. Jacobs; Helga S. Huntley, A. D. Kirwan, Bruce L. Lipphardt, Timothy Campbell, Travis Smith, Kacey Edwards, Brent Bartels
      Abstract: Ageostrophic ocean processes such as frontogenesis, submesoscale mixed‐layer instabilities, shelf break fronts and topographic interactions on the continental shelf produce surface‐divergent flows that affect buoyant material over time. This study examines the ocean processes leading to clustering, i.e. the increase of material density over time, on the ocean surface. The time series of divergence along a material trajectory, the Lagrangian divergence (LD), is the flow property driving clustering. To understand the impacts of various ocean processes on LD, numerical ocean model simulations at different resolutions are analyzed. Although the relevant processes differ, patterns in clustering evolution from the deep ocean and the continental shelf bear similarities. Smaller‐scale ocean features are associated with stronger surface divergence, and the surface material clustering is initially dominated by these features. Over time, the effect of these small‐scale features becomes bounded, as material traverses small‐scale regions of both positive and negative divergence. Lower frequency flow phenomena, however, continue the clustering. As a result, clustering evolves from initial small‐scale to larger‐scale patterns. This article is protected by copyright. All rights reserved.
      PubDate: 2015-12-13T03:35:29.25891-05:0
      DOI: 10.1002/2015JC011140
  • Initial tsunami source estimation by inversion with an intelligent
           selection of model parameters and time delays
    • Authors: Iyan E. Mulia; Toshiyuki Asano
      Abstract: We propose a method for accurately estimating the initial tsunami source. Our technique is independent of the earthquake parameters, because we only use recorded tsunami waveforms and an auxiliary basis function, instead of a fault model. We first use the measured waveforms to roughly identify the source area using backwards‐propagated travel times, and then infer the initial sea surface deformation through inversion analysis. A computational intelligence approach based on a genetic algorithm combined with a pattern search was used to select appropriate least squares model parameters and time delays. The proposed method significantly reduced the number of parameters and suppressed the negative effect of regularization schemes that decreased the plausibility of the model. Furthermore, the stochastic approach for deriving the time delays is a more flexible strategy for simulating actual phenomena that occur in nature. The selected parameters and time delays increased the accuracy, and the model's ability to reveal the underlying physics associated with the tsunami generating processes. In this paper, we applied the method to the 2011 Tohoku‐Oki tsunami event and examined its effectiveness by comparing the results to those using the conventional method. This article is protected by copyright. All rights reserved.
      PubDate: 2015-12-13T03:34:18.061911-05:
      DOI: 10.1002/2015JC010877
  • The response of the central Arctic Ocean stratification to freshwater
    • Authors: P. Pemberton; J. Nilsson
      Abstract: Using a state‐of‐the‐art coupled ice‐ocean circulation model, we perform a number of sensitivity experiments to examine how the central Arctic Ocean stratification responds to changes in river runoff and precipitation. The simulations yield marked changes in the cold halocline and the Arctic Atlantic layer. Increased precipitation yields a warming of the Atlantic layer, which primarily is an advective signal, propagated through the St. Anna Trough, reflecting air–sea heat flux changes over the Barents Sea. As the freshwater supply is increased, the anti‐cyclonic Beaufort Gyre is weakened and a greater proportion of the Arctic Ocean freshwater is exported via the Fram Strait, with nearly compensating export decreases through the Canadian Arctic Archipelago. The corresponding reorganization of the freshwater pool appears to be controlled by advective processes, rather than by the local changes in the surface freshwater flux. A simple conceptual model of the Arctic Ocean, based on a geostrophically‐controlled discharge of the low salinity water, is introduced and compared with the simulations. Key predictions of the conceptual model are that the halocline depth should decrease with increasing freshwater input and that the Arctic Ocean freshwater storage should increase proportionally to the square root of the freshwater input; which are in broad qualitative agreement with the sensitivity experiments. However, the model‐simulated rate of increase of the freshwater storage is weaker, indicating that effects related to wind forcing and rerouting of the freshwater‐transport pathways play an important role for the dynamics of the Arctic Ocean freshwater storage. This article is protected by copyright. All rights reserved.
      PubDate: 2015-12-13T03:33:09.046518-05:
      DOI: 10.1002/2015JC011003
  • Marine seismic observation of internal solitary wave packets in the
           northeast South China Sea
    • Authors: Qunshu Tang; Richard Hobbs, Dongxiao Wang, Longtao Sun, Chan Zheng, Jiabiao Li, Chongzhi Dong
      Abstract: Recently the novel seismic oceanography method has been reported to be an effective way to study the energetic internal solitary waves (ISWs) in the northern South China Sea. An optimized seismic‐oceanographic cruise was carried out to observe such near surface ISWs on Dongsha Plateau in July 2014. Several soliton trains rather than single solitons were captured using the seismic technique. After seismic data processing, one prototypical rank‐ordered ISW packet on northeast side of Dongsha Island was clearly identified for further analysis. This included waveforms, propagation velocities, and vertical velocities for individual solitons. In this study, an improved scheme was applied to derive the transient phase velocities from the seismic data which is verified from independent satellite and hydrographic data. Analytical predictions from Korteweg–de Vries equation fit better than the extended Korteweg–de Vries equation ignoring background currents. Our results show that the seismic method can be successfully used to image targets in shallow water below 40 m, and that seismic oceanography is a promising technique for studying near surface phenomena with high spatial resolution. This article is protected by copyright. All rights reserved.
      PubDate: 2015-12-09T06:40:52.218503-05:
      DOI: 10.1002/2015JC011362
  • On the vertical phytoplankton response to an ice free Arctic Ocean
    • Authors: J. Lawrence; E. Popova, A. Yool, M. Srokosz
      Abstract: Rapidly retreating sea ice is expected to influence future phytoplankton production in the Arctic Ocean by perturbing nutrient and light fields, but poor understanding of present phytoplankton distributions and governing mechanisms make projected changes highly uncertain. Here we use a simulation that reproduces observed seasonal phytoplankton chlorophyll distributions and annual nitrate to hypothesize that surface nitrate limitation in the Arctic Ocean deepens vertical production distributions where light‐dependent growth rates are lower. We extend this to interpret depth‐integrated production changes projected by the simulation for an ice free Arctic Ocean. Future spatial changes correspond to patterns of reduced surface nitrate and increased light. Surface nitrate inventory reductions in the Beaufort Gyre and Atlantic inflow waters drive co‐located production distributions deeper to where light is lower, offsetting increases in light over the water column due to reduced ice cover and thickness. Modest production increases arise, 10% in a seasonally ice free Arctic Ocean and increasing to 30% by the end of the century, occurring at depth. This article is protected by copyright. All rights reserved.
      PubDate: 2015-12-09T06:40:06.033221-05:
      DOI: 10.1002/2015JC011180
  • Observed ocean thermal response to Hurricanes Gustav and Ike
    • Authors: Patrick C. Meyers; Lynn K. Shay, Jodi K. Brewster, Benjamin Jaimes
      Abstract: The 2008 Atlantic hurricane season featured two hurricanes, Gustav and Ike, crossing the Gulf of Mexico (GOM) within a two week period. Over 400 airborne expendable bathythermographs (AXBTs) were deployed in a GOM field campaign before, during, and after the passage of Gustav and Ike to measure the evolving upper ocean thermal structure. AXBT and drifter deployments specifically targeted the Loop Current (LC) complex, which was undergoing an eddy‐shedding event during the field campaign. Hurricane Gustav forced a 50 m deepening of the ocean mixed layer (OML), dramatically altering the pre‐storm ocean conditions for Hurricane Ike. Wind‐forced entrainment of colder thermocline water into the OML caused sea surface temperatures to cool by over 5°C in GOM common water, but only 1 to 2°C in the LC complex. Ekman pumping and a near‐inertial wake were identified by fluctuations in the 20°C isotherm field observed by AXBTs and drifters following Hurricane Ike. Satellite estimates of the 20° and 26°C isotherm depths and ocean heat content were derived using a two‐layer model driven by sea surface height anomalies. Generally, the satellite estimates correctly characterized pre‐storm conditions, but the two‐layer model inherently could not resolve wind‐forced mixing of the OML. This study highlights the importance of a coordinated satellite and in‐situ measurement strategy to accurately characterize the ocean state before, during, and after hurricane passage, particularly in the case of two consecutive storms traveling through the same domain. This article is protected by copyright. All rights reserved.
      PubDate: 2015-12-09T02:19:00.616588-05:
      DOI: 10.1002/2015JC010912
  • Estimation of critical shear stress for erosion in the Changjiang Estuary:
           A synergy research of observation, GOCI sensing, and modeling
    • Authors: Jianzhong Ge; Fang Shen, Wenyun Guo, Changsheng Chen, Pingxing Ding
      Abstract: Simulating the sediment transport in a high‐turbidity region with spatially varying bed properties is challenging. A comprehensive strategy that integrates multiple methods is applied here to retrieve the critical shear stress for erosion, which plays a major role in suspended sediment dynamics in the Changjiang Estuary (CE). Time‐series of sea surface suspended sediment concentration (SSC) were retrieved from the Geostationary Ocean Color Imager (GOCI) satellite data at hourly intervals (for 8 h each day) and combined with hydrodynamic modeling of high‐resolution CE Finite‐Volume Community Ocean Model (CE‐FVCOM) to estimate the near‐bed critical shear stress in the clay‐dominated bed region (plasticity index > 7%). An experimental algorithm to determine the in‐situ critical shear stress via the plasticity index method was also used to verify the GOCI‐derived critical shear stress. Implemented with this new critical shear stress, the sediment transport model significantly improved the simulation of the distribution and spatial variability of the SSC during the spring and neap tidal cycles in the CE. The results suggest that a significant lateral water exchange between channels and shoals occurred during the maximum ebb tide, which led to a broader high‐SSC area in the CE throughout the water column. This article is protected by copyright. All rights reserved.
      PubDate: 2015-12-09T02:15:35.499145-05:
      DOI: 10.1002/2015JC010992
  • Refinement of the critical angle calculation for the contrast reversal of
           oil slicks under sunglint
    • Authors: Yingcheng Lu; Shaojie Sun, Minwei Zhang, Brock Murch, Chuanmin Hu
      Abstract: It has long been observed that oil slicks under sunglint can reverse their optical contrast against nearby oil‐free seawater. Such a phenomenon has been described through both empirical statistical analysis of the sunglint strength and modeled theoretically using a critical angle concept. The critical angle, in this model, is the angle at which the image pixels show no or negligible contrast between oiled and non‐oiled seawater. Pixels away from this critical angle show either positive or negative contrast from the oil‐free pixels. Although this concept has been fully demonstrated in the published literature, its calculation needs to be further refined to take into account: 1) the different refractive indices of oil slicks (from natural seeps) and seawater; and, 2) atmospheric effects in the sensor‐measured radiance. Using measurements from the Moderate Resolution Imaging Spectroradiometer (MODIS) over oil films in the Gulf of Mexico, we show improvement in the modeled and MODIS‐derived reflectance over oil slicks originated from natural seeps after incorporating these two factors in the model. Specifically, agreement between modeled and measured sunglint reflectance is found for both negative‐ and positive‐contrasting oil slicks. These results indicate that surface roughness and reflectance from oil films can be estimated given any solar/viewing geometry and surface wind. Further, this model might be used to correct the sunglint effect on thick oil under similar illumination conditions. Once proven possible, it may allow existing laboratory‐based models, which estimate oil thickness after such corrections, to be applied to remote sensing imagery. This article is protected by copyright. All rights reserved.
      PubDate: 2015-12-08T18:11:20.221284-05:
      DOI: 10.1002/2015JC011001
  • Meteotsunami occurrences and causes in Lake Michigan
    • Authors: Adam J. Bechle; David A.R. Kristovich, Chin H. Wu
      Abstract: The occurrence of meteotsunamis in Lake Michigan is quantified at 10 locations from up to 20 years of historical water level records. Meteotsunami height data are fit with Pareto Type 1 and Generalized Pareto Distributions to estimate exceedance probabilities. The annual meteotsunami return level exceeds 0.25 m at all but two stations, with the largest annual return level of 0.62 m at Calumet Harbor. Analysis of radar imagery indicates that Lake Michigan meteotsunamis are associated primarily with convective storm structures, with a considerable contribution from frontal storms as well. Meteotsunami association with convective storm structures is more prevalent in southern Lake Michigan while frontal storm structures have a greater association with meteotsunamis in northern Lake Michigan. Water depths in southern Lake Michigan are conducive to Proudman resonance with convective storms while the northern Lake Michigan is too deep to meet Proudman resonance criteria, suggesting Greenspan edge wave resonance as the likely generation mechanism. Interestingly, meteotsunami events occur primarily in the late spring and early summer, approximately one month before the peak convective storm season but after the peak cyclone season. Overall, this statistical analysis provides valuable insight into the spatial and temporal trends in meteotsunami occurrence in Lake Michigan needed to estimate the risk posed by these dangerous coastal hazards. This article is protected by copyright. All rights reserved.
      PubDate: 2015-12-08T10:48:42.268996-05:
      DOI: 10.1002/2015JC011317
  • Uniform asymptotic approximations for transient waves due to an initial
    • Abstract: In this work, we first present a semi‐analytical method for the evolution of linear fully‐dispersive transient waves generated by an initial surface displacement and propagating over a constant depth. The procedure starts from Fourier and Hankel transforms and involves a combination of the method of stationary phase, the method of uniform asymptotic approximations and various Airy integral formulations. Secondly, we develop efficient convolution techniques expressed as single and double summations over the source area. These formulations are flexible, extremely fast and highly accurate even for the dispersive tail of the transient waves. To verify the accuracy of the embedded dispersion properties, we consider test cases with sharp‐edged disturbances in 1D and 2D. Furthermore, we consider the case of a relatively blunt Gaussian disturbance in 2D. In all cases the agreement between the convolution results and simulations with a high‐order Boussinesq model is outstanding. Finally, we make an attempt to extend the convolution methods to geophysical tsunami problems taking into account e.g. uneven bottom effects. Unfortunately, refraction/diffraction effects cannot easily be incorporated, so instead we focus on the incorporation of linear shoaling and its effect on travel time and temporal evolution of the surface elevation. The procedure is tested on data from the 2011 Japan tsunami. Convolution results are likewise compared to model simulations based on the nonlinear shallow water equations and both are compared with field observations from 10 deep water DART buoys. The near‐field results are generally satisfactory, while the far‐field results leave much to be desired. This article is protected by copyright. All rights reserved.
      PubDate: 2015-12-08T10:47:46.000762-05:
      DOI: 10.1002/2015JC011155
  • A new technique for the retrieval of near‐surface vertical current
           shear from marine X‐band radar images
    • Authors: B. Lund; H. C. Graber, H. Tamura, C. O. Collins, S. M. Varlamov
      Abstract: A new method to determine near‐surface vertical current shear from non‐coherent marine X‐band radar (MR) data is introduced. A three‐dimensional fast Fourier transform is employed to obtain the wavenumber‐frequency spectrum of a MR image sequence. Near‐surface currents are estimated from the Doppler‐shifted surface gravity wave signal within the spectrum. They represent a weighted mean of the upper ocean flow. The longer the ocean waves on which the current estimates are based, the greater their effective depth. The novelty lies in the wavenumber‐dependent retrieval method, yielding ∼100 independent current estimates at effective depths from ∼2–8∼m per ∼12∼min measurement period. First MR near‐surface vertical current shear measurements are presented using data collected from R/V Roger Revelle during the 2010 Impact of Typhoons on the Ocean in the Pacific experiment in the Philippine Sea. Shipboard acoustic Doppler current profiler (ADCP) and anemometer measurements as well as WAVEWATCH∼III (WW3) model results are used to demonstrate that results are in accord with physical expectations. The wind‐ and wave‐driven Ekman flow is obtained by subtracting ADCP‐based background currents from the radar measurements. At ∼2∼m it is on average ∼1.6% of the wind speed and ∼38.9° to the right of the wind. With increasing effective depth, the speed factor decreases and the deflection angle increases. Based on WW3 results, the MR‐sensed Stokes drift speed is ∼50% of the Ekman flow at ∼2∼m and ∼25% at ∼8∼m. These findings are consistent with previous observations and Ekman theory. This article is protected by copyright. All rights reserved.
      PubDate: 2015-12-07T11:19:12.462058-05:
      DOI: 10.1002/2015JC010961
  • Interannual variability of South Equatorial Current bifurcation and
           western boundary currents along the Madagascar coast
    • Authors: Y. Yamagami; T. Tozuka
      Abstract: The South Equatorial Current (SEC) in the southern Indian Ocean bifurcates at the east coast of Madagascar into the Northeast and Southeast Madagascar Currents (NEMC and SEMC, respectively). In observational and reanalysis data, interannual variations of the NEMC and SEMC transports are strongly correlated with those of the SEC transport, rather than those of the SEC bifurcation latitude (SBL). Their dynamical mechanisms are then examined based on the time‐dependent Island Rule for the first time. It is shown that interannual anomalies of the SBL as well as the NEMC and SEMC transports are predominantly a response to the anomalous inflow from the ocean interior that is determined by the meridional interior transport. This, in turn, is a result of westward propagating Rossby waves induced by wind stress curl anomalies mainly in 60°E‐90°E. The above mechanism is contrasted with that of the seasonal variation, where the local transport driven by wind stress around the island plays a role. Furthermore, the interannual variations of the SBL and the NEMC and SEMC transports are significantly correlated with the Niño 3.4 index with 5‐15 months lag. It is suggested that diabatic heating anomalies associated with the El Niño/Southern Oscillation (ENSO) along with a local process in the southeastern Indian Ocean may generate wind stress curl anomalies over the southern Indian Ocean. This article is protected by copyright. All rights reserved.
      PubDate: 2015-12-07T11:16:41.547042-05:
      DOI: 10.1002/2015JC011069
  • Natural and unnatural oil slicks in the Gulf of Mexico
    • Abstract: When wind speeds are 2 – 10 m s−1, reflective contrasts in the ocean surface make oil slicks visible to synthetic aperture radar (SAR) under all sky conditions. Neural network analysis of satellite SAR images quantified the magnitude and distribution of surface oil in the Gulf of Mexico from persistent, natural seeps and from the Deepwater Horizon (DWH) discharge. This analysis identified 914 natural oil seep zones across the entire Gulf of Mexico in pre‐2010 data. Their ∼0.1 µm slicks covered an aggregated average of 775 km2. Assuming an average volume of 77.5 m3 over an 8 – 24 h lifespan per oil slick, the floating oil indicates a surface flux of 2.5 – 9.4 × 104 m3 y−1. Oil from natural slicks was regionally concentrated: 68%, 25%, 7%, and
      PubDate: 2015-12-01T06:25:41.614325-05:
      DOI: 10.1002/2015JC011062
  • Temporal and vertical distributions of anthropogenic 236U in the Japan Sea
           using a coral core and seawater samples
    • Authors: Aya Sakaguchi; Tomoya Nomura, Peter Steier, Robin Gloser, Keiichi Sasaki, Tsuyoshi Watanabe, Tomoeki Nakakuki, Yoshio Takahashi, Hiroya Yamano
      Abstract: The input history of 236U to the surface water of the Japan Sea was reconstructed through measurement of the 236U/238U atom ratio in annual bands of a coral skeleton which was collected at Iki Island in the entrance to the Japan Sea. The 236U/238U atom ratios and concentrations of U isotopes were successfully measured for the period 1935‐2010 using AMS and ICP‐MS. The 236U/238U atom ratios revealed three prominent peaks: 4.51 × 10−9 in 1955, 6.15 × 10−9 in 1959 and 4.14 × 10−9 in 1963; thereafter the isotope ratios gradually decreased over the next several decades, attaining a value of ca.1.3 × 10−9 for the present day. A simplified depth profile model for 236U in the Japan Sea, using the reconstructed 236U value for the surface water together with observed depth profiles for 236U in the water column, yielded diffusion coefficients of 3.4‐5.6 cm2/s for 6 sampling points. The constants obtained for the northern stations were relatively large and fitting uncertainty was also large. Results for the distribution of 236U in the water columns have been influenced not only by diffusion but also by subduction of the surface water in the Japan Sea. This article is protected by copyright. All rights reserved.
      PubDate: 2015-11-30T04:08:42.477248-05:
      DOI: 10.1002/2015JC011109
  • Temporal variation and stoichiometric ratios of organic matter
           remineralization in bottom waters of the northern Gulf of Mexico during
           late spring and summer
    • Abstract: An improved extended optimum multi‐parameter (eOMP) analysis was applied to hydrographic (temperature and salinity), and water chemistry data, including dissolved oxygen (O2), nutrients (nitrate plus nitrite, phosphate, and silicate), dissolved inorganic carbon (DIC), and total alkalinity (TAlk) data collected during late spring and summer from 2006 to 2012 in bottom waters off the Louisiana coast, to explore the dynamics and stoichiometry of DIC production during the development and maintenance of summer hypoxia. Our analysis demonstrated that DIC in bottom water was relatively low from April to June, but increased significantly in July, peaked in August, and dropped slightly in September. Furthermore, DIC production resulted from both aerobic organic carbon (OC) respiration and denitrification, as well as substantial loss due to vertical mixing with surface water. The average summer gross OC respiration rate was estimated to be 0.19 g C m−2 d−1, with the highest values occurring in late summer when hypoxic conditions dominated. We also found that Corg/N/P/‐O2 remineralization ratios for aerobic respiration were generally consistent with the classic Redfield ratio (106/16/1/138) except individual C/N and C/P ratios were slightly lower, indicating that marine OC was the major source of the DIC production in the bottom water. This study quantified the role of temporal bottom‐water microbial respiration to seasonal DIC dynamics and provided a means for studying the stoichiometry of biogeochemical processes in coastal waters. This article is protected by copyright. All rights reserved.
      PubDate: 2015-11-25T17:44:47.081679-05:
      DOI: 10.1002/2015JC011453
  • Short‐term sea ice forecasting: An assessment of ice concentration
           and ice drift forecasts using the U.S. Navy's Arctic Cap Nowcast/Forecast
    • Authors: David A. Hebert; Richard A. Allard, E. Joseph Metzger, Pamela G. Posey, Ruth H. Preller, Alan J. Wallcraft, Michael W. Phelps, Ole Martin Smedstad
      Abstract: In this study the forecast skill of the U.S. Navy operational Arctic sea ice forecast system, the Arctic Cap Nowcast/Forecast System (ACNFS), is presented for the period Feb 2014 – June 2015. ACNFS is designed to provide short term, 1‐7 day forecasts of Arctic sea ice and ocean conditions. Many quantities are forecast by ACNFS; the most commonly used include ice concentration, ice thickness, ice velocity, sea surface temperature, sea surface salinity, and sea surface velocities. Ice concentration forecast skill is compared to a persistent ice state and historical sea ice climatology. Skill scores are focused on areas where ice concentration changes by ±5% or more, and are therefore limited to primarily the marginal ice zone. We demonstrate that ACNFS forecasts are skillful compared to assuming a persistent ice state, especially beyond 24 hours. ACNFS is also shown to be particularly skillful compared to a climatologic state for forecasts up to 102 hours. Modeled ice drift velocity is compared to observed buoy data from the International Arctic Buoy Programme. A seasonal bias is shown where ACNFS is slower than IABP velocity in the summer months and faster in the winter months. In February 2015 ACNFS began to assimilate a blended ice concentration derived from Advanced Microwave Scanning Radiometer 2 (AMSR2) and the Interactive Multisensor Snow and Ice Mapping System (IMS). Preliminary results show that assimilating AMSR2 blended with IMS improves the short‐term forecast skill and ice edge location compared to the independently derived National Ice Center Ice Edge product. This article is protected by copyright. All rights reserved.
      PubDate: 2015-11-23T18:26:40.532518-05:
      DOI: 10.1002/2015JC011283
  • Seasonal variability of nutrient concentrations in the Mediterranean Sea:
           Contribution of Bio‐Argo floats
    • Abstract: In 2013, as part of the French NAOS (Novel Argo Oceanic observing System) program, five profiling floats equipped with nitrate sensors (SUNA‐V2) together with CTD and bio‐optical sensors were deployed in the Mediterranean Sea. At present day, more than 500 profiles of physical and biological parameters were acquired, and significantly increased the number of available nitrate data in the Mediterranean Sea. Results obtained from floats confirm the general view of the basin, and the well‐known west‐to‐east gradient of oligotrophy. At seasonal scale, the north western Mediterranean displays a clear temperate pattern sustained by both deep winter mixed layer and shallow nitracline. The other sampled areas follow a subtropical regime (nitracline depth and mixed layer depth are generally decoupled). Float data also permit to highlight the major contribution of high frequency processes in controlling the nitrate supply during winter in the north western Mediterranean Sea, and in altering the nitrate stock in subsurface in the eastern basin. This article is protected by copyright. All rights reserved.
      PubDate: 2015-11-19T10:39:14.016906-05:
      DOI: 10.1002/2015JC011103
  • Influence of varying upper ocean stratification on coastal
           near‐inertial currents
    • Authors: Sung Yong Kim; Alexander L. Kurapov, P. Michael Kosro
      Abstract: The influence of varying horizontal and vertical stratification in the upper layer [O (10) m] associated with riverine waters and seasonal atmospheric fluxes on coastal near‐inertial currents is investigated with remotely sensed and in‐situ observations of surface and subsurface currents, and realistic numerical model outputs off the coast of Oregon. Based on numerical simulations with and without the Columbia River (CR) during summer, the directly wind‐forced near‐inertial surface currents are enhanced by 30% to 60% when the near‐surface layer has a stratified condition due to riverine water inputs from the CR. Comparing model results without the CR for summer and winter conditions indicates that the directly wind‐forced near‐inertial surface current response to a unit wind forcing during summer are 20% to 70% stronger than those during winter depending on the cross‐shore location, which is in contrast to the seasonal patterns of both mixed layer depth and amplitudes of near‐inertial currents. The model simulations are used to examine aspects of coastal inhibition of near‐inertial currents, manifested in their spatial coherence in the cross‐shore direction, where the phase propagates upward over the continental shelf, bounces at the coast, and continues increasing upward offshore (toward surface) and then downward offshore at the surface, with magnitudes and length scales in the near‐surface layer increasing offshore. This pattern exhibits a particularly well‐organized structure during winter. Similarly, the ray paths of clockwise near‐inertial internal waves are consistent with the phase propagation of coherence, showing the influence of upper layer stratification and coastal inhibition. This article is protected by copyright. All rights reserved.
      PubDate: 2015-11-18T14:07:51.235153-05:
      DOI: 10.1002/2015JC011153
  • Arctic pathways of Pacific Water: Arctic Ocean model intercomparison
    • Abstract: Pacific Water (PW) enters the Arctic Ocean through Bering Strait and brings heat, fresh water and nutrients from the northern Bering Sea. The circulation of PW in the central Arctic Ocean is only partially understood due to the lack of observations. In this paper pathways of PW are investigated using simulations with six state‐of‐the art regional and global Ocean General Circulation Models (OGCMs). In the simulations PW is tracked by a passive tracer, released in Bering Strait. Simulated PW water spreads from the Bering Strait region in three major branches. One of them starts in the Barrow Canyon, bringing PW along continental slope of Alaska into the Canadian Straits and then into Baffin Bay. The other initiates in the vicinity of the Herald Canyon and transports PW along the continental slope of the East‐Siberian Sea into the transpolar drift, and then through Fram Strait and the Greenland Sea. The third branch begins near the Herald Shoal and the central Chukchi shelf and brings PW waters into the Beaufort Gyre. Models suggest that the spread of PW through the Arctic Ocean depends on the atmospheric circulation. In the models the wind, acting via Ekman pumping, drives the seasonal and interannual variability of PW in the Canadian Basin of the Arctic Ocean. The wind effects the simulated PW pathways by changing vertical shear of the relative vorticity of the ocean flow in the Canada Basin. This article is protected by copyright. All rights reserved.
      PubDate: 2015-10-16T03:30:57.993879-05:
      DOI: 10.1002/2015JC011299
  • Loitering of the retreating sea ice edge in the Arctic Seas
    • Authors: Michael Steele; Wendy Ermold
      Pages: 7699 - 7721
      Abstract: Each year, the arctic sea ice edge retreats from its winter maximum extent through the Seasonal Ice Zone (SIZ) to its summer minimum extent. On some days, this retreat happens at a rapid pace, while on other days, parts of the pan‐arctic ice edge hardly move for periods of days up to 1.5 weeks. We term this stationary behavior “ice edge loitering,” and identify areas that are more prone to loitering than others. Generally, about 20–25% of the SIZ area experiences loitering, most often only one time at any one location during the retreat season, but sometimes two or more times. The main mechanism controlling loitering is an interaction between surface winds and warm sea surface temperatures in areas from which the ice has already retreated. When retreat happens early enough to allow atmospheric warming of this open water, winds that force ice floes into this water cause melting. Thus, while individual ice floes are moving, the ice edge as a whole appears to loiter. The time scale of loitering is then naturally tied to the synoptic time scale of wind forcing. Perhaps surprisingly, the area of loitering in the arctic seas has not changed over the past 25 years, even as the SIZ area has grown. This is because rapid ice retreat happens most commonly late in the summer, when atmospheric warming of open water is weak. We speculate that loitering may have profound effects on both physical and biological conditions at the ice edge during the retreat season.
      PubDate: 2015-12-03T07:06:12.333284-05:
      DOI: 10.1002/2015JC011182
  • Near‐inertial ocean response to tropical cyclone forcing on the
           Australian North‐West Shelf
    • Authors: M. D. Rayson; G. N. Ivey, N. L. Jones, R. J. Lowe, G. W. Wake, J. D. McConochie
      Pages: 7722 - 7751
      Abstract: The Regional Ocean Modeling System (ROMS) was applied to the Australian North‐West Shelf (NWS) to hindcast the ocean response to four intense historical tropical cyclones (TCs). While the four cyclones had very different trajectories across the NWS, all passed within 150 km of a long‐term vertical mooring located on the continental shelf in 125 m depth. The observed ocean response at this relatively shallow, Southern Hemisphere shelf site was characterized by the development of a peak in the counter‐clockwise (CCW) near‐inertial kinetic energy, mixed layer deepening, and subsequent restratification. Strong near‐inertial isotherm oscillations were also observed following two of the cyclones. ROMS reproduced these features and also showed that the peak in the near‐inertial CCW kinetic energy was observed on the left side of each cyclone trajectory. The time rate of change of near‐inertial kinetic energy depended strongly on the storm Rossby number, i.e., defined based on the storm speed, the storm length scale, and the Coriolis frequency. The shallow water depth on the NWS resulted in first, a more rapid decay of near‐inertial oscillations than in the deep ocean, and second a generation efficiency (the ratio of near‐inertial power to the rate of wind work) of up to 10%, smaller than found for cyclones propagating across deeper water. The total energy put into near‐inertial motions is nevertheless large compared to the background tidal energy. The rapid decay of near‐inertial motions emphasizes the importance of frictional effects in characterizing the response to cyclone forcing in shallow seas.
      PubDate: 2015-12-07T09:15:49.251993-05:
      DOI: 10.1002/2015JC010868
  • Influence of post‐Tehuano oceanographic processes in the dynamics of
           the CO2 system in the Gulf of Tehuantepec, Mexico
    • Pages: 7752 - 7770
      Abstract: This investigation reports, for the first time, results of CO2 system variables in the Gulf of Tehuantepec, located in the Mexican tropical Pacific. We quantified the post‐Tehuano concentration of dissolved inorganic carbon (DIC) and pH (April 2013). These values were used to calculate pCO2, aragonite saturation (ΩAr), and air‐sea CO2 fluxes (FCO2). The intense vertical stratification was found to contribute to the biogeochemical processes in surface waters (
      PubDate: 2015-12-09T04:45:17.549042-05:
      DOI: 10.1002/2015JC011249
  • Future change in ocean productivity: Is the Arctic the new Atlantic?
    • Authors: A. Yool; E. E. Popova, A. C. Coward
      Pages: 7771 - 7790
      Abstract: One of the most characteristic features in ocean productivity is the North Atlantic spring bloom. Responding to seasonal increases in irradiance and stratification, surface phytopopulations rise significantly, a pattern that visibly tracks poleward into summer. While blooms also occur in the Arctic Ocean, they are constrained by the sea‐ice and strong vertical stratification that characterize this region. However, Arctic sea‐ice is currently declining, and forecasts suggest this may lead to completely ice‐free summers by the mid‐21st century. Such change may open the Arctic up to Atlantic‐style spring blooms, and do so at the same time as Atlantic productivity is threatened by climate change‐driven ocean stratification. Here we use low and high‐resolution instances of a coupled ocean‐biogeochemistry model, NEMO‐MEDUSA, to investigate productivity. Drivers of present‐day patterns are identified, and changes in these across a climate change scenario (IPCC RCP 8.5) are analyzed. We find a globally significant decline in North Atlantic productivity (> −20%) by 2100, and a correspondingly significant rise in the Arctic (> +50%). However, rather than the future Arctic coming to resemble the current Atlantic, both regions are instead transitioning to a common, low nutrient regime. The North Pacific provides a counterexample where nutrients remain high and productivity increases with elevated temperature. These responses to climate change in the Atlantic and Arctic are common between model resolutions, suggesting an independence from resolution for key impacts. However, some responses, such as those in the North Pacific, differ between the simulations, suggesting the reverse and supporting the drive to more fine‐scale resolutions.
      PubDate: 2015-12-12T05:50:57.354911-05:
      DOI: 10.1002/2015JC011167
  • Seasonal and interannual variability of fast ice extent in the
           southeastern Laptev Sea between 1999 and 2013
    • Authors: V. Selyuzhenok; T. Krumpen, A. Mahoney, M. Janout, R. Gerdes
      Pages: 7791 - 7806
      Abstract: Along with changes in sea ice extent, thickness, and drift speed, Arctic sea ice regime is characterized by a decrease of fast ice season and reduction of fast ice extent. The most extensive fast ice cover in the Arctic develops in the southeastern Laptev Sea. Using weekly operational sea ice charts produced by Arctic and Antarctic Research Institute (AARI, Russia) from 1999 to 2013, we identified five main key events that characterize the annual evolution of fast ice in the southeastern Laptev Sea. Linking the occurrence of the key events with the atmospheric forcing, bathymetry, freezeup, and melt onset, we examined the processes driving annual fast ice cycle. The analysis revealed that fast ice in the region is sensitive to thermodynamic processes throughout a season, while the wind has a strong influence only on the first stages of fast ice development. The maximal fast ice extent is closely linked to the bathymetry and local topography and is primarily defined by the location of shoals, where fast ice is likely grounded. The annual fast ice cycle shows significant changes over the period of investigation, with tendencies toward later fast ice formation and earlier breakup. These tendencies result in an overall decrease of the fast ice season by 2.8 d/yr, which is significantly higher than previously reported trends.
      PubDate: 2015-12-12T05:48:29.255622-05:
      DOI: 10.1002/2015JC011135
  • A comparative assessment of coastal mean dynamic topography in Norway by
           geodetic and ocean approaches
    • Authors: Vegard Ophaug; Kristian Breili, Christian Gerlach
      Pages: 7807 - 7826
      Abstract: The ocean's mean dynamic topography (MDT) is the surface representation of ocean circulation. It may be determined by the ocean approach, using numerical ocean circulation models, or by the geodetic approach, where MDT is the height of the mean sea surface (MSS), or mean sea level (MSL), above the geoid. Using new geoid models, geodetic MDT profiles based on tide gauges, dedicated coastal altimetry products, and conventional altimetry are compared with six ocean MDT estimates independent of geodetic data. Emphasis is put on the determination of high‐resolution geoid models, combining ESA's fifth release (R5) of GOCE satellite‐only global gravity models (GGMs) with a regional geoid model for Norway by a filtering technique. Differences between MDT profiles along the Norwegian coast together with Taylor diagrams confirm that geodetic and ocean MDTs agree on the ∼3–7 cm level at the tide gauges, and on the ∼5–11 cm level at the altimetry sites. Some geodetic MDTs correlate more with the best‐performing ocean MDT than do other ocean MDTs, suggesting a convergence of the methods. While the GOCE R5 geoids are shown to be more accurate over land, they do not necessarily show the best agreement over the ocean. Pointwise monomission altimetry products give results comparable with the multimission DTU13MSS grid on the ∼5 cm level. However, dedicated coastal altimetry products generally do not offer an improvement over conventional altimetry along the Norwegian coast.
      PubDate: 2015-12-12T05:48:08.964359-05:
      DOI: 10.1002/2015JC011145
  • Accuracy of short‐term sea ice drift forecasts using a coupled
           ice‐ocean model
    • Authors: Axel J. Schweiger; Jinlun Zhang
      Pages: 7827 - 7841
      Abstract: Arctic sea ice drift forecasts of 6 h–9 days for the summer of 2014 are generated using the Marginal Ice Zone Modeling and Assimilation System (MIZMAS); the model is driven by 6 h atmospheric forecasts from the Climate Forecast System (CFSv2). Forecast ice drift speed is compared to drifting buoys and other observational platforms. Forecast positions are compared with actual positions 24 h–8 days since forecast. Forecast results are further compared to those from the forecasts generated using an ice velocity climatology driven by multiyear integrations of the same model. The results are presented in the context of scheduling the acquisition of high‐resolution images that need to follow buoys or scientific research platforms. RMS errors for ice speed are on the order of 5 km/d for 24–48 h since forecast using the sea ice model compared with 9 km/d using climatology. Predicted buoy position RMS errors are 6.3 km for 24 h and 14 km for 72 h since forecast. Model biases in ice speed and direction can be reduced by adjusting the air drag coefficient and water turning angle, but the adjustments do not affect verification statistics. This suggests that improved atmospheric forecast forcing may further reduce the forecast errors. The model remains skillful for 8 days. Using the forecast model increases the probability of tracking a target drifting in sea ice with a 10 km × 10 km image from 60 to 95% for a 24 h forecast and from 27 to 73% for a 48 h forecast.
      PubDate: 2015-12-12T05:47:43.963692-05:
      DOI: 10.1002/2015JC011273
  • CO2 sink/source characteristics in the tropical Indonesian seas
    • Authors: A. R. Kartadikaria; A. Watanabe, K. Nadaoka, N. S. Adi, H. B. Prayitno, S. Soemorumekso, M. Muchtar, I. Triyulianti, A. Setiawan, S. Suratno, E. N. Khasanah
      Pages: 7842 - 7856
      Abstract: Two distinct CO2 sink/source characteristics are found in the tropical Indonesian seas from the compilation of observed data for the period 1984–2013. The western region persistently emits CO2 to the atmosphere, whereas the eastern region is dynamic and acts either as a small source or sink of CO2 to the atmosphere, depending on sites. The segregation is proximal to the Makassar Strait, which is located over the continental shelf and is one of the main routes of the Indonesian Throughflow (ITF). Lower salinity and higher silicate were found in the western region, suggesting a terrestrial influence in this area. Temperature has a limited influence in controlling different CO2 sink/source characteristics in the west and east. However, an SST change of −2.0°C during La Niña events effectively reduces the pCO2 difference between the atmosphere and surface seawater by 50% compared to normal year conditions. During La Niña events, higher wind speeds double the CO2 flux from the ocean to the atmosphere compared to that of a normal year. In the continental shelf area where the CO2 sink area was found, data of over 29 years show that the seawater pCO2 increased by 0.6–3.8 μatm yr−1. Overall, the seawater pCO2 of the Indonesian seas is supersaturated relative to the atmosphere by 15.9 ± 8.6 μatm and thus acts as a source of CO2 to the atmosphere.
      PubDate: 2015-12-12T05:47:19.6424-05:00
      DOI: 10.1002/2015JC010925
  • DopSCAT: A mission concept for simultaneous measurements of marine winds
           and surface currents
    • Pages: 7857 - 7879
      Abstract: A radar scatterometer operates by transmitting a pulse of microwave energy toward the ocean's surface and measuring the normalized (per‐unit‐surface) radar backscatter coefficient (σ°). The primary application of scatterometry is the measurement of near‐surface ocean winds. By combining σ° measurements from different azimuth angles, the 10 m vector wind can be determined through a Geophysical Model Function (GMF), which relates wind and backscatter. This paper proposes a mission concept for the measurement of both oceanic winds and surface currents, which makes full use of earlier C‐band radar remote sensing experience. For the determination of ocean currents, in particular, the novel idea of using two chirps of opposite slope is introduced. The fundamental processing steps required to retrieve surface currents are given together with their associated accuracies. A detailed description of the mission proposal and comparisons between real and retrieved surface currents are presented. The proposed ocean Doppler scatterometer can be used to generate global surface ocean current maps with accuracies better than 0.2 m/s at a spatial resolution better than 25 km (i.e., 12.5 km spatial sampling) on a daily basis. These maps will allow gaining some insights on the upper ocean mesoscale dynamics. The work lies at a frontier, given that the present inability to measure ocean currents from space in a consistent and synoptic manner represents one of the greatest weaknesses in ocean remote sensing.
      PubDate: 2015-12-12T05:54:59.432335-05:
      DOI: 10.1002/2015JC011011
  • Observational validation of the diffusive convection flux laws in the
           Amundsen Basin, Arctic Ocean
    • Authors: John D. Guthrie; Ilker Fer, James Morison
      Pages: 7880 - 7896
      Abstract: The low levels of mechanically driven mixing in many regions of the Arctic Ocean make double diffusive convection virtually the only mechanism for moving heat up from the core of Atlantic Water towards the surface. In an attempt to quantify double diffusive heat fluxes in the Arctic Ocean, a temperature microstructure experiment was performed as a part of the North Pole Environmental Observatory (NPEO) 2013 field season from the drifting ice station Barneo located in the Amundsen Basin near the Lomonosov Ridge (89.5°N, 75°W). A diffusive convective thermohaline staircase was present between 150 and 250 m in nearly all of the profiles. Typical vertical heat fluxes across the high‐gradient interfaces were consistently small, O(10−1) W m−2. Our experiment was designed to resolve the staircase and differed from earlier Arctic studies that utilized inadequate instrumentation or sampling. Our measured fluxes from temperature microstructure agree well with the laboratory derived flux laws compared to previous studies, which could find agreement only to within a factor of two to four. Correlations between measured and parameterized heat fluxes are slightly higher when using the more recent Flanagan et al. [2013] laboratory derivation than the more commonly used derivation presented in Kelley [1990]. Nusselt versus Rayleigh number scaling reveals the convective exponent, η, to be closer to 0.29 as predicted by recent numerical simulations of single‐component convection rather than the canonical 1/3 assumed for double diffusion. However, the exponent appears to be sensitive to how convective layer height is defined.
      PubDate: 2015-12-12T05:54:35.120702-05:
      DOI: 10.1002/2015JC010884
  • On the flow of Atlantic water and temperature anomalies in the Nordic Seas
           toward the Arctic Ocean
    • Pages: 7897 - 7918
      Abstract: The climatic conditions over the Arctic Ocean are strongly influenced by the inflow of warm Atlantic water conveyed by the Norwegian Atlantic Slope Current (NwASC). Based on sea surface height (SSH) data from altimetry, we develop a simple dynamical measure of the NwASC transport to diagnose its spatio‐temporal variability. This supports a dynamical division of the NwASC into two flow regimes; the Svinøy Branch (SvB) in the southern Norwegian Sea, and the Fram Strait Branch (FSB) west of Spitsbergen. The SvB transport is well correlated with the SSH and atmospheric variability within the Nordic Seas, factors that also affect the inflow to the Barents Sea. In contrast, the FSB is influenced by regional atmospheric conditions around Svalbard and northern Barents Sea. Using a composite analysis, we further relate anomalous strong SvB flow events to temperature fluctuations along the core of Atlantic water. A warm composite anomaly is found to propagate northward, with a tendency to amplify enroute, after these events. A roughly 12 months delayed temperature signal is identified in the FSB. However, also in the Lofoten Basin interior a delayed temperature signal is found, which appears to originate from the NwASC. This study suggests that hydrographic anomalies both upstream from the North Atlantic, and locally generated in the Norwegian Sea, are important for the oceanic heat and salt transport that eventually enters into the Arctic.
      PubDate: 2015-12-13T14:35:13.677613-05:
      DOI: 10.1002/2015JC011012
  • Topographic form stress in the Southern Ocean State Estimate
    • Authors: Jessica Masich; Teresa K. Chereskin, Matthew R. Mazloff
      Pages: 7919 - 7933
      Abstract: We diagnose the Southern Ocean momentum balance in a 6 year, eddy‐permitting state estimate of the Southern Ocean. We find that 95% of the zonal momentum input via wind stress at the surface is balanced by topographic form stress across ocean ridges, while the remaining 5% is balanced via bottom friction and momentum flux divergences at the northern and southern boundaries of the analysis domain. While the time‐mean zonal wind stress field exhibits a relatively uniform spatial distribution, time‐mean topographic form stress concentrates at shallow ridges and across the continents that lie within the Antarctic Circumpolar Current (ACC) latitudes; nearly 40% of topographic form stress occurs across South America, while the remaining 60% occurs across the major submerged ridges that underlie the ACC. Topographic form stress can be divided into shallow and deep regimes: the shallow regime contributes most of the westward form stress that serves as a momentum sink for the ACC system, while the deep regime consists of strong eastward and westward form stresses that largely cancel in the zonal integral. The time‐varying form stress signal, integrated longitudinally and over the ACC latitudes, tracks closely with the wind stress signal integrated over the same domain; at zero lag, 88% of the variance in the 6 year form stress time series can be explained by the wind stress signal, suggesting that changes in the integrated wind stress signal are communicated via rapid barotropic response down to the level of bottom topography.
      PubDate: 2015-12-17T04:18:46.93439-05:0
      DOI: 10.1002/2015JC011143
  • Modeled ocean circulation in Nares Strait and its dependence on
           landfast‐ice cover
    • Pages: 7934 - 7959
      Abstract: Two simplified ocean simulations are used to study circulation and transport within Nares Strait. The simulations are similar, except that one included a coupled sea ice model that effectively established a landfast ice cover throughout the simulation year. Comparison between the ocean‐only and ocean‐ice simulations reveals a systematic change in the current structure, reminiscent of the seasonal shift under mobile and landfast ice previously observed in Nares Strait. A surface‐intensified jet, which carries low‐salinity water along the strait's centerline, develops within the ocean‐only simulation. The current structure under landfast ice is characterized by a subsurface jet located along the western side with low‐salinity surface water distributed along the eastern side of the strait. Intermediate salinity water is offset to the west in the ice‐ocean simulation relative to the ocean‐only simulation, while high‐salinity water (>34.8) is constrained to recirculations that are located north and south of a sill in Kane Basin. The simulations, combined with an idealized, semianalytical model, suggest that the structural shift is caused by the surface Ekman layer beneath the landfast ice and the associated eastward advection of near‐surface low‐salinity water and westward movement of the jet. Temporal variability in the ocean‐ice simulation is dominated by the remote response to the time‐dependent northern boundary conditions. In contrast, the ocean‐only simulation favors an instability and additionally responds to local surface wind forcing, which enhances the variability within the strait above that imposed at the boundaries.
      PubDate: 2015-12-17T09:43:08.480227-05:
      DOI: 10.1002/2015JC011091
  • Can we map the interannual variability of the whole upper Southern Ocean
           with the current database of hydrographic observations?
    • Pages: 7960 - 7978
      Abstract: With the advent of Argo floats, it now seems feasible to study the interannual variations of upper ocean hydrographic properties of the historically undersampled Southern Ocean. To do so, scattered hydrographic profiles often first need to be mapped. To investigate biases and errors associated both with the limited space‐time distribution of the profiles and with the mapping methods, we colocate the mixed‐layer depth (MLD) output from a state‐of‐the‐art 1/12° DRAKKAR simulation onto the latitude, longitude, and date of actual in situ profiles from 2005 to 2014. We compare the results obtained after remapping using a nearest neighbor (NN) interpolation and an objective analysis (OA) with different spatiotemporal grid resolutions and decorrelation scales. NN is improved with a coarser resolution. OA performs best with low decorrelation scales, avoiding too strong a smoothing, but returns values over larger areas with large decorrelation scales and low temporal resolution, as more points are available. For all resolutions OA represents better the annual extreme values than NN. Both methods underestimate the seasonal cycle in MLD. MLD biases are lower than 10 m on average but can exceed 250 m locally in winter. We argue that current Argo data should not be mapped to infer decadal trends in MLD, as all methods are unable to reproduce existing trends without creating unrealistic extra ones. We also show that regions of the subtropical Atlantic, Indian, and Pacific Oceans, and the whole ice‐covered Southern Ocean, still cannot be mapped even by the best method because of the lack of observational data.
      PubDate: 2015-12-17T09:42:20.516844-05:
      DOI: 10.1002/2015JC011115
  • A new application of conditional nonlinear optimal perturbation approach
           to boundary condition uncertainty
    • Authors: Qiang Wang; Mu Mu
      Pages: 7979 - 7996
      Abstract: The conditional nonlinear optimal perturbation (CNOP) approach was mainly used to investigate the effects of the uncertainties in initial condition and model parameters on model results. This study presents a new application of the CNOP approach to the investigation of the effects of boundary condition uncertainty. Specifically, we first give the general formulation of the CNOP approach for uncertainties of initial and boundary conditions and model parameters. The method is then applied to analyze the effects of nutrient perturbations at the bottom boundary of the water column on the modeled deep chlorophyll maximum (DCM) using an ocean ecosystem model. The results show that nutrient perturbations at the bottom boundary have significant impacts on the modeled DCM. Interestingly, the approach also reveals that nonlinear processes play important roles in the evolution of phytoplankton perturbations caused by nutrient perturbations at the bottom boundary. This implies that the CNOP approach is useful for investigating the effects of boundary condition uncertainty.
      PubDate: 2015-12-17T04:10:13.306576-05:
      DOI: 10.1002/2015JC011095
  • Internal lee wave closures: Parameter sensitivity and comparison to
    • Pages: 7997 - 8019
      Abstract: This paper examines two internal lee wave closures that have been used together with ocean models to predict the time‐averaged global energy conversion rate into lee waves and dissipation rate associated with lee waves and topographic blocking: the Garner (2005) scheme and the Bell (1975) theory. The closure predictions in two Southern Ocean regions where geostrophic flows dominate over tides are examined and compared to microstructure profiler observations of the turbulent kinetic energy dissipation rate, where the latter are assumed to reflect the dissipation associated with topographic blocking and generated lee wave energy. It is shown that when applied to these Southern Ocean regions, the two closures differ most in their treatment of topographic blocking. For several reasons, pointwise validation of the closures is not possible using existing observations, but horizontally averaged comparisons between closure predictions and observations are made. When anisotropy of the underlying topography is accounted for, the two horizontally averaged closure predictions near the seafloor are approximately equal. The dissipation associated with topographic blocking is predicted by the Garner (2005) scheme to account for the majority of the depth‐integrated dissipation over the bottom 1000 m of the water column, where the horizontally averaged predictions lie well within the spatial variability of the horizontally averaged observations. Simplifications made by the Garner (2005) scheme that are inappropriate for the oceanic context, together with imperfect observational information, can partially account for the prediction‐observation disagreement, particularly in the upper water column.
      PubDate: 2015-12-17T04:08:40.275412-05:
      DOI: 10.1002/2015JC010892
  • Reconstruction of a meteotsunami in Lake Erie on 27 May 2012: Roles of
           atmospheric conditions on hydrodynamic response in enclosed basins
    • Authors: Eric J. Anderson; Adam J. Bechle, Chin H. Wu, David J. Schwab, Greg E. Mann, Kirk A. Lombardy
      Pages: 8020 - 8038
      Abstract: On 27 May 2012, atmospheric conditions gave rise to two convective systems that generated a series of waves in the meteotsunami band on Lake Erie. The resulting waves swept three swimmers a 0.5 mi offshore, inundated a marina, and may have led to a capsized boat along the southern shoreline. Analysis of radial velocities from a nearby radar tower in combination with coastal meteorological observation indicates that the convective systems produced a series of outflow bands that were the likely atmospheric cause of the meteotsunami. In order to explain the processes that led to meteotsunami generation, we model the hydrodynamic response to three meteorological forcing scenarios: (i) the reconstructed atmospheric disturbance from radar analysis, (ii) simulated conditions from a high‐resolution weather model, and (iii) interpolated meteorological conditions from the NOAA Great Lakes Coastal Forecasting System. The results reveal that the convective systems generated a series of waves incident to the southern shore of the lake that reflected toward the northern shoreline and reflected again to the southern shore, resulting in spatial wave focusing and edge wave formation that combined to impact recreational users near Cleveland, OH. This study illustrates the effects of meteotsunami development in an enclosed basin, including wave reflection, focusing, and edge wave formation as well as temporal lags between the causative atmospheric conditions and arrival of dangerous wave conditions. As a result, the ability to detect these extreme storms and predict the hydrodynamic response is crucial to reducing risk and building resilient coastal communities.
      PubDate: 2015-12-18T07:47:08.804987-05:
      DOI: 10.1002/2015JC010883
  • Assessing changes in extreme sea levels along the coast of China
    • Authors: Jianlong Feng; Hans von Storch, Wensheng Jiang, Ralf Weisse
      Pages: 8039 - 8051
      Abstract: Hourly tide‐gauge data along the coast of China are used to evaluate changes in extreme water levels in the past several decades. Mean sea level, astronomical tide, nontidal component and the tide‐surge interaction was analyzed separately to assess their roles in the changes of extreme sea levels. Mean sea level at five tide gauges, Kanmen, Keelung, Zhapo, Xiamen and Quarrybay, show significant increasing trends during the past decades (1954–2013) with a rate of about 1.4–3.5 mm/yr. At Keelung, Kaohsiung and Quarrybay the mean high waters increased during 1954–2013 with a rate from 0.6 to 1.8 mm/yr, while the annual mean tidal range rose at the same time by 0.9 to 3.8 mm/yr. In terms of storm surge intensities, there is interannual variability and decadal variability but five tide gauges show significant decreasing trends, and three gauges, at Keelung, Xiamen and Quarrybay, exhibited significant increases of extreme sea levels with trends of 1.5–6.0 mm/yr during 1954–2013. Significant tide‐surge interactions were found at all 12 tide gauges, but no obvious change was found during the past few decades. The changes in extreme sea levels in this area are strongly related to the changes of mean sea levels (MSL). At gauges, where the tide‐surge interaction is large, the astronomic tides are also an important factor for the extreme sea levels, whereas tide gauges with little tide‐surge interaction, the changes of wind driven storm surge component adds to the change of the extreme sea levels.
      PubDate: 2015-12-18T02:21:47.420257-05:
      DOI: 10.1002/2015JC011336
  • An ocean‐biology‐induced negative feedback on ENSO as derived
           from a hybrid coupled model of the tropical Pacific
    • Pages: 8052 - 8076
      Abstract: Biological conditions in the tropical Pacific Ocean (e.g., phytoplankton biomass) are strongly regulated by physical changes that are associated with the El Niño‐Southern Oscillation (ENSO). The existence and variation of phytoplankton biomass act to modulate the vertical penetration of the incoming sunlight into the upper ocean, which causes an ocean‐biology‐induced heating (OBH) effect on the climate system. Previously, the penetration depth of solar radiation in the upper ocean (Hp) has been defined to describe the related bioclimate connections. An empirical model for interannual Hp variability that is parameterized in terms of its relationship with the sea surface temperature (SST) in the tropical Pacific was derived from remotely sensed ocean color data and is incorporated into a hybrid coupled model (HCM) to represent the OBH effects. In this paper, several HCM experiments are performed to demonstrate the biofeedback onto the ENSO, including a climatological Hp run (in which Hp is prescribed as only seasonally varying), interannual Hp runs (with different intensities of the interannually varying OBH effects), and a run in which the sign of the OBH effect is reversed. Significant modulating impacts on the interannual variability are found in the HCM and are characterized by a negative feedback between the ocean biology and the climate system in the tropical Pacific; stronger the OBH feedback, weaker the interannual variability. The processes that are involved in the feedback are analyzed. The SST is modulated indirectly by dynamic ocean processes that are induced by OBH. The significance and implication of the OBH effects are discussed in terms of their roles in ENSO variability and the model biases in the tropical Pacific.
      PubDate: 2015-12-18T02:23:14.256444-05:
      DOI: 10.1002/2015JC011305
  • Regional dependence in the timing of onset of rapid decline in Arctic sea
           ice concentration
    • Pages: 8077 - 8098
      Abstract: Arctic sea ice concentration from satellite passive microwave measurements is analyzed to assess the form and timing of the onset of decline of recent ice loss, and the regional dependence of the response. The timing of the onset is estimated using an objective method, and suggests differences of up to 20 years between the various subregions. A clear distinction can be drawn between the recent onset times of the Atlantic sector (beginning in 2003) and the much earlier onset times associated with the Pacific sector, where the earliest transition to rapid loss is found in 1992. Rates of decline prior to and following the transition points are calculated, and suggest that the postonset rate of loss is greatest in the Barents Sea, and weakest in the Pacific sector. Covariability between the seasons is noted in the SIC response, both at interannual and longer time scales. For two case regions, potential mechanisms for the onset time transitions are briefly analyzed. In the Barents Sea, the onset time coincides with a redistribution of the pathways of ice circulation in the region, while along the Alaskan coast, the propagation of the regional signal can be traced in the age of the sea ice. The results presented here indicate a series of spatially self‐consistent regional responses, and may be useful in understanding the primary drivers of recent sea ice loss.
      PubDate: 2015-12-18T02:21:32.058136-05:
      DOI: 10.1002/2015JC011187
  • Do East Australian Current anticyclonic eddies leave the Tasman Sea?
    • Authors: Gabriela S. Pilo; Peter R. Oke, Tatiana Rykova, Richard Coleman, Ken Ridgway
      Pages: 8099 - 8114
      Abstract: Using satellite altimetry and high‐resolution model output we analyze the pathway of large, long‐lived anticyclonic eddies that originate near the East Australian Current (EAC) separation point. We show that 25–30% of these eddies propagate southward, around Tasmania, leave the Tasman Sea, and decay in the Great Australian Bight. This pathway has not been previously documented owing to poor satellite sampling off eastern Tasmania. As eddies propagate southward, they often “stall” for several months at near‐constant latitude. Along the pathway eddies become increasingly barotropic. Eddy intensity is primarily influenced by merging with other eddies and a gradual decay otherwise. Surface temperature anomaly associated with anticyclonic eddies changes as they propagate, while surface salinity anomaly tends to remain relatively unchanged as they propagate.
      PubDate: 2015-12-18T02:22:44.15225-05:0
      DOI: 10.1002/2015JC011026
  • Long‐term variations in global sea level extremes
    • Pages: 8115 - 8134
      Abstract: Decadal to multidecadal variations in sea level extremes unrelated to mean sea level changes have been investigated using long tide gauge records distributed worldwide. A state space approach has been applied that provides robust solutions and uncertainties of the time evolving characteristics of extremes, allowing for data gaps and uneven sampling, both common features of historical sea level time series. Two different models have been formulated for the intensity and for the occurrence of extreme sea level events and have been applied independently to each tide gauge record. Our results reveal two key findings: first, the intensity and the frequency of occurrence of extreme sea levels unrelated to mean sea level vary coherently on decadal scales in most of the sites examined (63 out of 77) and, second, extreme sea level changes are regionally consistent, thus pointing toward a common large‐scale forcing. This variability of extremes associated with climate drivers should be considered in the framework of climate change studies.
      PubDate: 2015-12-18T07:46:11.061278-05:
      DOI: 10.1002/2015JC011173
  • Time variability in the annual cycle of sea ice thickness in the
           Transpolar Drift
    • Pages: 8135 - 8150
      Abstract: The annual cycle of modal and mean sea ice thickness was derived from upward looking sonar ice thickness observations (1990–2011) in Fram Strait. The average annual peak‐to‐trough amplitude of the mode of 0.54 m is superimposed on interannual variability with peak‐to‐trough amplitudes of 0.73 m on time scales of 6–8 years, which again is superimposed on a long‐term trend of −0.55 m/decade over the observation period. The long‐term trend is stronger for April than for August, the average months of maximum and minimum modal thickness. As a result, the annual peak‐to‐trough modal thickness amplitude was reduced by 30% between the 1990s and the 2000s. The average annual peak‐to‐trough amplitude of the mean ice thickness of 1.20 m is also superimposed on interannual variability, with as much as 0.97 m thickness change over only 3 years. These two modes of variability are superimposed on a long‐term trend of −0.35 m/decade through the entire data set. In contrast to the modal thickness, the long‐term trend is weaker for the average month of maximum mean thickness (June), than for the average month of minimum (September). Therefore, the annual peak‐to‐trough amplitude of the mean ice thickness increased by 14% between the 1990s and the 2000s.
      PubDate: 2015-12-18T02:21:08.844129-05:
      DOI: 10.1002/2015JC011102
  • Remote alongshore winds drive variability of the California Undercurrent
           off the British Columbia‐Washington coast
    • Authors: Richard E. Thomson; Maxim V. Krassovski
      Pages: 8151 - 8176
      Abstract: The California Undercurrent transports warm, salty, nutrient‐rich, oxygen‐depleted water along the continental slope from the equatorial Pacific to the Aleutian Islands. We use multiyear acoustic Doppler current profiler records collected simultaneously at two mooring sites off Vancouver Island to detail the regional structure of the undercurrent and to show that much of its variability is attributable to the passage of remotely forced, coastal‐trapped waves. We also document two subsurface currents missed by earlier current measurements. The undercurrent becomes evident in spring, intensifies through summer and fall, and merges with the wind‐driven poleward surface flow in winter. During intensification at the southern mooring site (A1), the undercurrent shoals from 250 ± 50 m in early summer to 150 ± 50 m depth in late fall. At the northern site (BP2), 225 km to the northwest of A1, the current is weaker and maintains a year‐round depth of 150 ± 50 m. Temporal variability in the undercurrent velocity attains highest coherence with winds along the southern Oregon‐northern California coast, with peak coherence occurring for “synoptic” (10–40 day period) alongshore winds off Cape Blanco in southern Oregon. The undercurrent lag of 3 ± 2 days relative to the Cape Blanco winds at synoptic periods is consistent with low mode, poleward propagating, coastally trapped waves. For periods >40 days, the wind‐current coherence remains high for winds off the Oregon‐California coast but lags are often negative, indicating possible forcing by alongshore baroclinic pressure gradients. At interannual time scales, the undercurrent variations have links to climate‐scale processes in the equatorial Pacific.
      PubDate: 2015-12-18T09:08:46.456663-05:
      DOI: 10.1002/2015JC011306
  • The Gas Transfer through Polar Sea ice experiment: Insights into the rates
           and pathways that determine geochemical fluxes
    • Authors: A. Lovely; B. Loose, P. Schlosser, W. McGillis, C. Zappa, D. Perovich, S. Brown, T. Morell, D. Hsueh, R. Friedrich
      Pages: 8177 - 8194
      Abstract: Sea ice is a defining feature of the polar marine environment. It is a critical domain for marine biota and it regulates ocean‐atmosphere exchange, including the exchange of greenhouse gases such as CO2 and CH4. In this study, we determined the rates and pathways that govern gas transport through a mixed sea ice cover. N2O, SF6, 3He, 4He, and Ne were used as gas tracers of the exchange processes that take place at the ice‐water and air‐water interfaces in a laboratory sea ice experiment. Observation of the changes in gas concentrations during freezing revealed that He is indeed more soluble in ice than in water; Ne is less soluble in ice, and the larger gases (N2O and SF6) are mostly excluded during the freezing process. Model estimates of gas diffusion through ice were calibrated using measurements of bulk gas content in ice cores, yielding gas transfer velocity through ice (kice) of ∼5 × 10−4 m d−1. In comparison, the effective air‐sea gas transfer velocities (keff) ranged up to 0.33 m d−1 providing further evidence that very little mixed‐layer ventilation takes place via gas diffusion through columnar sea ice. However, this ventilation is distinct from air‐ice gas fluxes driven by sea ice biogeochemistry. The magnitude of keff showed a clear increasing trend with wind speed and current velocity beneath the ice, as well as the combination of the two. This result indicates that gas transfer cannot be uniquely predicted by wind speed alone in the presence of sea ice.
      PubDate: 2015-12-19T10:22:25.079894-05:
      DOI: 10.1002/2014JC010607
  • Vertical scales and dynamics of eddies in the Arctic Ocean's Canada Basin
    • Pages: 8195 - 8209
      Abstract: A decade of moored measurements from the Arctic Ocean's northwestern Beaufort Gyre (collected as a component of the Beaufort Gyre Exploration Project) are analyzed to examine the range of mesoscale eddies over the water column and the dynamical processes that set eddy vertical scales. A total of 58 eddies were identified in the moored record, all anticyclones with azimuthal velocities ranging from 10 to 43 cm/s. These are divided into three classes based on core depths. Shallow eddies (core depths around 120 m) are shown to be vertically confined by the strong stratification of the halocline; typical thicknesses are around 100 m. Deep eddies (core depths around 1200 m) are much taller (thicknesses around 1300 m) owing to the weaker stratification at depth, consistent with a previous study. Eddies centered around mid‐depths all have two cores (vertically aligned and separated in depth) characterized by velocity maxima and anomalous temperature and salinity properties. One core is located at the base of the halocline (around 200 m depth) and the other at the depth of the Atlantic Water layer (around 400 m depth). These double‐core eddies have vertical scales between those of the shallow and deep eddies. The strongly decreasing stratification in their depth range motivates a derivation for the quasi‐geostrophic adjustment of a nonuniformly stratified water column to a potential vorticity anomaly. The result aids in interpreting the dynamics and origins of the double‐core eddies, providing insight into transport across a major water mass front separating Canadian and Eurasian Water.
      PubDate: 2015-12-19T02:21:21.582435-05:
      DOI: 10.1002/2015JC011251
  • Upper‐ocean mixing due to surface gravity waves
    • Pages: 8210 - 8228
      Abstract: Surface gravity waves play an important role in the lower layer of the atmosphere and the upper layer of the ocean. Surface waves effect upper‐ocean mixing mainly through four processes: wave breaking, Stokes drift interaction with the Coriolis force, Langmuir circulation, and stirring by nonbreaking waves. We introduce the impact of these four processes into a 1‐D k−ϵ ocean turbulence model. The parameterizations used are based mainly on existing investigations. Comparison of simulation results and measurements demonstrates that considering all the effects of waves, rather than just one effect, significantly improves model performance. The nonbreaking‐wave‐induced mixing and Langmuir turbulence are the most important terms when considering the impact of waves on upper‐ocean mixing. Under high‐wave conditions, the turbulent mixing induced by nonbreaking waves can be of the same order of magnitude as the viscosity induced by other terms at the surface. Nonbreaking waves contribute very little to shear production and their impact is negligible in the models. Sensitivity experiments demonstrate that the vertical profile of the Stokes drift calculated from the 2‐D wave spectrum improves model performance significantly compared with other methods of introducing wave effects.
      PubDate: 2015-12-25T01:27:04.953807-05:
      DOI: 10.1002/2015JC011329
  • PDO and ENSO modulations intensified decadal sea level variability in the
           tropical Pacific
    • Pages: 8229 - 8237
      Abstract: According to long‐term sea level reconstruction and steric sea level data, regional sea levels in the tropical Pacific have oscillated between east and west on a decadal time scale over the past 60 years, but the oscillation has been intensified significantly in the last three decades. Using conditional composite analysis, we show that the recent intensification in sea level variability is caused by modulation between the Pacific Decadal Oscillation (PDO) and El Niño‐Southern Oscillation (ENSO), i.e., an El Niño in a positive PDO or a La Niña in a negative PDO phase. Our analysis of meteorological fields indicates that atmospheric circulation associated with the changes in ENSO‐PDO phase relationship plays a positive role in enhancing the decadal sea level oscillation. The intensified sea level oscillation, when superimposed on the global trend of sea level rise, will have profound implications for coastal communities, therefore, the combined effect of PDO and ENSO should be taken into account in the decadal sea level prediction in the tropical Pacific.
      PubDate: 2015-12-25T01:26:33.955729-05:
      DOI: 10.1002/2015JC011139
  • Shelf/fjord exchange driven by coastal‐trapped waves in the Arctic
    • Authors: Mark E. Inall; Frank Nilsen, Finlo R. Cottier, Ragnhild Daae
      Pages: 8283 - 8303
      Abstract: In this article, we show that the class of low frequency (subinertial) waves known as coastal‐trapped waves (CTWs) are a significant agent of water volume exchange in a west Svalbard fjord, and by extension more widely along the west Svalbard and east Greenland margins where similar conditions prevail. We show that CTWs generated by weather systems passing across the sloping topography of the shelf break propagate into the fjord, steered by the topography of an across‐shelf trough. The CTWs have characteristic periods of ∼2 days, set by the passage time of weather systems. Phase speeds and wavelengths vary seasonally by a factor of two, according to stratification: winter (summer) values are Cp = 0.25 ms−1 (0.5 ms−1) and λ = 40 km (84 km). CTW‐induced flow velocities in excess of 0.2 ms−1 at 100 m water depth are recorded. Observationally scaled CTW model results allow their explicit role in volume exchange to be quantified. Of the estimated exchange terms, estuarine exchange is weakest ( Qest=0.62×103 m3s−1), followed by barotropic tidal pumping ( Qbt=2.5×103 m3s−1), with intermediary exchange dominating ( Qi=2.4×104 m3s−1). Oscillatory flows display greatest activity in the 1–5 day period band, and CTW activity is identified as the likely source of variability in the 40–60 h period band. Within that band, intermediary exchange driven by CTWs is estimated as QiCTW_ave=0.82×104 m3s−1; an exchange rate exceeding both barotropic and estuarine exchange estimates.
      PubDate: 2015-12-28T10:37:17.229221-05:
      DOI: 10.1002/2015JC011277
  • Observations of whitecap coverage and the relation to wind stress, wave
           slope, and turbulent dissipation
    • Authors: Michael Schwendeman; Jim Thomson
      Pages: 8346 - 8363
      Abstract: Shipboard measurements of whitecap coverage are presented from two cruises in the North Pacific, and compared with in situ measurements of wind speed and friction velocity, average wave steepness, and near‐surface turbulent dissipation. A threshold power law fit is proposed for all variables, which incorporates the flexibility of a power law with the threshold behavior commonly seen in whitecapping. The fit of whitecap coverage to wind speed, U10, closely matches similar relations from three recent studies, particularly in the range of 6–14 m/s. At higher wind speeds, the whitecap coverage data level off relative to the fits, and an analysis of the residuals shows some evidence of reduced whitecapping in rapidly developing waves. Wave slope variables are examined for potential improvement over wind speed parameterizations. Of these variables, the mean square slope of the equilibrium range waves has the best statistics, which are further improved after normalizing by the directional spread and frequency bandwidth. Finally, the whitecap coverage is compared to measurements of turbulent dissipation. Though still statistically significant, the correlation is worse than the wind or wave relations, and residuals show a strong negative trend with wave age. This may be due to an increased influence of microbreaking in older wind seas.
      PubDate: 2015-12-28T10:36:34.396718-05:
      DOI: 10.1002/2015JC011196
  • Tilt‐induced biases in sediment trap functioning
    • Authors: Stephen M. Chiswell; Scott D. Nodder
      Pages: 8381 - 8391
      Abstract: We present data from two sediment traps moored 5 km apart ∼150 km off the east coast of New Zealand. Both traps were at 1500 m in 3100 m of water. One trap was attached to a 3038 m tall mooring and exhibited tilts measured by a tilt sensor ranging from 5° to 20°, the other trap was attached to a 1659 m tall mooring and exhibited tilts ranging from 2° to 4°. The computed relative trapping efficiency depends on which flux constituent (mass, particulate organic carbon, biogenic silica, etc.) is used to compute it. We suggest that this physically unrealistic result arises because of relatively high noise in the measurements due to patchiness, sampling and/or measurement errors. On average, the trap exhibiting most tilt collected 30–50% less material than the trap exhibiting least tilt. For the total mass flux, efficiencies appear to be lower at higher trap tilt. However, with only 5 months of data, we do not have enough samples to accurately model the relationship between trap tilt and efficiency, except to show that with even moderate tilt, the sediment trap trapping efficiency can be reduced by a factor of nearly two.
      PubDate: 2015-12-28T10:36:11.599142-05:
      DOI: 10.1002/2015JC011350
  • Coexistence of nitrous oxide undersaturation and oversaturation in the
           surface and subsurface of the western Arctic Ocean
    • Authors: Jiexia Zhang; Liyang Zhan, Liqi Chen, Yuhong Li, Jianfang Chen
      Pages: 8392 - 8401
      Abstract: The nitrous oxide (N2O) distributions in a shelf‐slope‐basin section from the Chukchi Sea shelf to the Chukchi Abyssal Plain (CAP) in the western Arctic Ocean were observed for the first time during the fourth Chinese National Arctic Research Expedition (4th CHINARE) in late August 2010. The N2O concentrations were 11.4–16.9 nmol L−1, corresponding to saturation values of 92–118% in the surface water. In the surface, biological factor was negligible to N2O production, and physical process dominated the surface N2O distribution. The N2O undersaturation may be the result of dilution by sea ice meltwater. The oversaturation may be caused by the diffusing of N2O‐rich shelf bottom water. Below the surface, N2O concentrations ranged from 11.4 to 21.4 nmol L−1. On the Chukchi Sea shelf, N2O concentrations increased from the surface to the near‐bottom water, indicating sediment N2O produced by nitrification or denitrification emission may be a significant source. The subsurface maxima in the upper halocline layer (UHL) may receive partial contribution from nitrification production but was primarily caused by the spreading of shelf water.
      PubDate: 2015-12-28T10:35:51.579777-05:
      DOI: 10.1002/2015JC011245
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