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Journal Cover Journal of Geophysical Research : Oceans
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  • A simulation of small to giant Antarctic iceberg evolution: Differential
           impact on climatology estimates
    • Authors: Thomas Rackow; Christine Wesche, Ralph Timmermann, Hartmut H. Hellmer, Stephan Juricke, Thomas Jung
      Abstract: We present a simulation of Antarctic iceberg drift and melting that includes small, medium-sized, and giant tabular icebergs with a realistic size distribution. For the first time, an iceberg model is initialized with a set of nearly 7000 observed iceberg positions and sizes around Antarctica. The study highlights the necessity to account for larger and giant icebergs in order to obtain accurate melt climatologies. We simulate drift and lateral melt using iceberg-draft averaged ocean currents, temperature, and salinity. A new basal melting scheme, originally applied in ice shelf melting studies, uses in situ temperature, salinity, and relative velocities at an iceberg's bottom. Climatology estimates of Antarctic iceberg melting based on simulations of small (≤ 2.2 km), 'small-to-medium'-sized (≤ 10 km), and small-to-giant icebergs (including icebergs > 10 km) exhibit differential characteristics: successive inclusion of larger icebergs leads to a reduced seasonality of the iceberg meltwater flux and a shift of the mass input to the area north of 58 ∘S, while less meltwater is released into the coastal areas. This suggests that estimates of meltwater input solely based on the simulation of small icebergs introduce a systematic meridional bias; they underestimate the northward mass transport and are, thus, closer to the rather crude treatment of iceberg melting as coastal runoff in models without an interactive iceberg model. Future ocean simulations will benefit from the improved meridional distribution of iceberg melt, especially in climate change scenarios where the impact of iceberg melt is likely to increase due to increased calving from the Antarctic ice sheet. This article is protected by copyright. All rights reserved.
      PubDate: 2017-03-23T18:50:40.453698-05:
      DOI: 10.1002/2016JC012513
  • Rip currents and alongshore flows in single channels dredged in the surf
    • Authors: Melissa Moulton; Steve Elgar, Britt Raubenheimer, John C. Warner, Nirnimesh Kumar
      Abstract: To investigate the dynamics of flows near non-uniform bathymetry, single channels (on average 30-m wide and 1.5-m deep) were dredged across the surf zone at five different times, and the subsequent evolution of currents and morphology was observed for a range of wave and tidal conditions. In addition, circulation was simulated with the numerical modeling system COAWST, initialized with the observed incident waves and channel bathymetry, and with an extended set of wave conditions and channel geometries. The simulated flows are consistent with alongshore flows and rip current circulation patterns observed in the surf zone. Near the offshore-directed flows that develop in the channel, the dominant terms in modeled momentum balances are wave breaking accelerations, pressure gradients, advection, and the vortex force. The balances vary spatially, and are sensitive to wave conditions and the channel geometry. The observed and modeled maximum offshore-directed flow speeds are correlated with a parameter based on the alongshore gradient in breaking-wave-driven-setup across the non-uniform bathymetry (a function of wave height and angle, water depths in the channel and on the sandbar, and a breaking threshold) and the breaking-wave-driven alongshore flow speed. The offshore-directed flow speed increases with dissipation on the bar and reaches a maximum (when the surf zone is saturated) set by the vertical scale of the bathymetric variability. This article is protected by copyright. All rights reserved.
      PubDate: 2017-03-23T18:39:12.110131-05:
      DOI: 10.1002/2016JC012222
  • The 2014 Lake Askja rockslide-induced tsunami: Optimization of numerical
           tsunami model using observed data
    • Authors: Sigríðhur Sif Gylfadóttir; Jihwan Kim, Jón Kristinn Helgason, Sveinn Brynjólfsson, Ármann Höskuldsson, Tómas Jóhannesson, Carl Bonnevie Harbitz, Finn Løvholt
      Abstract: A large rockslide was released from the inner Askja caldera into Lake Askja, Iceland on 21 July 2014. Upon entering the lake it caused a large tsunami that traveled about ∼3 km across the lake and inundated the shore with vertical run-up measuring up to 60–80 m. Following the event, comprehensive field data were collected, including GPS measurements of the inundation and multibeam echo soundings of the lake bathymetry. Using this exhaustive data set, numerical modeling of the tsunami has been conducted using both a nonlinear shallow water model and a Boussinesq-type model that includes frequency dispersion. To constrain unknown landslide parameters, a global optimization algorithm, Differential Evolution, was employed, resulting in a parameter set that minimized the deviation from measured inundation. The tsunami model of Lake Askja is the first example where we have been able to utilize field data to show that frequency dispersion is needed to explain the tsunami wave radiation pattern and that shallow water theory falls short. We were able to fit the trend in tsunami run-up observations around the entire lake using the Boussinesq model. In contrast, the shallow water model gave a different run-up pattern and produced pronounced offsets in certain areas. The well-documented Lake Askja tsunami thus provided a unique opportunity to explore and capture the essential physics of landslide tsunami generation and propagation through numerical modeling. Moreover, the study of the event is important because this dispersive nature is likely to occur for other subaerial impact tsunamis. This article is protected by copyright. All rights reserved.
      PubDate: 2017-03-23T18:39:08.377151-05:
      DOI: 10.1002/2016JC012496
  • Physical linkages between an offshore canyon and surf zone morphologic
    • Authors: Jeff E. Hansen; Britt Raubenheimer, Steve Elgar, Jeffrey H. List, Thomas C. Lippmann
      Abstract: The causes of surf zone morphologic changes observed along a sandy beach onshore of a submarine canyon were investigated using field observations and a numerical model (Delft3D/SWAN). Numerically simulated morphologic changes using four different sediment transport formulae reproduce the temporal and spatial patterns of net cross-shore integrated (between 0- and 6.5-m water depths) accretion and erosion observed in a ∼300-m alongshore region, a few hundred meters from the canyon head. The observations and simulations indicate that the accretion or erosion results from converging or diverging alongshore currents driven primarily by breaking waves and alongshore pressure gradients. The location of convergence or divergence depends on the direction of the offshore waves that refract over the canyon, suggesting that bathymetric features on the inner shelf can have first-order effects on short-term nearshore morphologic change. This article is protected by copyright. All rights reserved.
      PubDate: 2017-03-23T18:39:06.499398-05:
      DOI: 10.1002/2016JC012319
  • Bio-optical anomalies in the world's oceans: An investigation on the
           diffuse attenuation coefficients for downward irradiance derived from
           biogeochemical Argo float measurements
    • Authors: Emanuele Organelli; Hervé Claustre, Annick Bricaud, Marie Barbieux, Julia Uitz, Fabrizio D'Ortenzio, Giorgio Dall'Olmo
      Abstract: Identification of oceanic regions characterized by particular optical properties is extremely important for ocean color applications. The departure from globally established bio-optical models introduces uncertainties in the retrieval of biogeochemical quantities from satellite observations. Thanks to an array of 105 Biogeochemical Argo floats acquiring almost daily downward irradiance measurements at selected wavelengths in the UV and blue region of the spectrum, we re-examined the natural variability of the spectral diffuse attenuation coefficients, Kd(λ), among the world's oceans and compared them to previously established bio-optical models. The analysis of 2847 measurements of Kd(λ) at 380 and 490 nm, within the first optical depth, provided a classification of the examined regions into three groups. The first one included the Black Sea, a water body characterized by very high colored dissolved organic matter (CDOM) content. The second group was essentially composed by the subtropical gyres (Atlantic and Pacific Oceans), with optical properties consistent with previous models (i.e., no anomalies). High latitude (North Atlantic and Southern oceans) and temperate (Mediterranean Sea) seas formed the third group, in which optical properties departed from existing bio-optical models. Annual climatologies of the Kd(380)/Kd(490) ratio evidenced a persistent anomaly in the Mediterranean Sea, that we attributed to a higher-than-average CDOM contribution to total light absorption. In the North Atlantic subpolar gyre, anomalies were observed only in wintertime and were also attributed to high CDOM concentrations. In the Southern Ocean, the anomaly was likely related to high phytoplankton pigment packaging rather than to CDOM. This article is protected by copyright. All rights reserved.
      PubDate: 2017-03-23T18:38:58.314139-05:
      DOI: 10.1002/2016JC012629
  • The impact of storms and stratification on sediment transport in the Rhine
           region of freshwater influence
    • Authors: Raúl P. Flores; Sabine Rijnsburger, Alexander R. Horner-Devine, Alejandro J. Souza, Julie D. Pietrzak
      Abstract: We present measurements of along and across-shore sediment transport in a region of the Dutch coast 10 kilometers north of the Rhine River mouth. This section of the coast is characterized by strong vertical density stratification because it is within the mid-field region of the Rhine region of freshwater influence, where processes typical of the far-field, such as tidal straining, are modified by the passage of distinct freshwater lenses at the surface. The experiment captured two storms, and a wide range of wind, wave, tidal and stratification conditions. We focus primarily on the mechanisms leading to cross-shore sediment flux at a mooring location in 12m of water, which are responsible for the exchange of sediment between the near-shore and the inner shelf. Net transport during storms was directed offshore and influenced by cross-shelf winds, while net transport during spring tides was determined by the mean state of stratification. Tidal straining dominated during neap tides; however, cross-shore transport was negligible due to small sediment concentrations. The passage of freshwater lenses manifested as strong pulses of offshore transport primarily during spring tides. We observe that both barotropic and baroclinic processes are relevant for cross-shore transport at depth and, since transport rates due to these competing processes were similar, the net transport direction will be determined by the frequency and sequencing of these modes of transport. Based on our observations, we find that wind- and wave-driven transport during storms tends move fine sediment offshore, while calmer, more stratified conditions move it back onshore. This article is protected by copyright. All rights reserved.
      PubDate: 2017-03-23T18:38:34.093644-05:
      DOI: 10.1002/2016JC012362
  • A downslope propagating thermal front over the continental slope
    • Authors: Hans van Haren; Phil J. Hosegood
      Abstract: In the ocean, internal frontal bores above sloping topography have many appearances, depending on the local density stratification, and on the angle and source of generation of the carrier wave. However, their common characteristics are a backwards breaking wave, strong sediment resuspension and relatively cool (denser) water moving more or less upslope underneath warm (less dense) water. In this paper, we present a rare example of a downslope moving front of cold water moving over near-bottom warm water. Large backscatter is observed in the downslope moving front's trailing edge, rather than the leading edge as is common in upslope moving fronts. Time series observations have been made during a fortnight in summer, using a 101-m long array of high-resolution temperature sensors moored with an acoustic Doppler current profiler at 396 m depth in near-homogeneous waters, near a small canyon in the continental slope off the Malin shelf (West-Scotland, UK). Occurring between fronts that propagate upslope with tidal periodicity, the rare downslope propagating one resembles a gravity current and includes strong convective turbulence coming from the interior rather than the more usual frictionally-generated turbulence arising from interaction with the seabed. Its turbulence is 3-10 times larger than that of more common upslope propagating fronts. As the main turbulence is in the interior with a thin stratified layer close to the bottom, little sediment is resuspended by a downslope propagating front. The downslope propagating front is suggested to be generated by oblique propagation of internal (tidal) waves and flow over a nearby upstream promontory. This article is protected by copyright. All rights reserved.
      PubDate: 2017-03-23T03:55:58.485204-05:
      DOI: 10.1002/2017JC012797
  • Interannual surface salinity in Northwest Atlantic shelf
    • Authors: Semyon A. Grodsky; Nicolas Reul, Bertrand Chapron, James A. Carton, Frank O. Bryan
      Abstract: Sea surface salinity (SSS) from the Aquarius and SMOS satellite missions displays a steady increase of ∼1psu over the entire northwestern Atlantic shelf south of Nova Scotia during the 2011-2015. Put in the context of longer ocean profile data the results suggest that mixed layer salinity and temperature north of the Gulf Stream experience positively correlated shelf-wide interannual oscillations (1psu/2degC). Salty and warm events occur coincident with anomalous easterly-southeasterly winds and Ekman transport counteracting the mean southwestward shelf currents. They are coincident with the weakening of both branches of the Scotian Shelf Current (SSC), but only moderately correlate with shifts of the Gulf Stream North Wall. This suggests that salt advection by anomalous SSC acting on the mean salinity gradient is the primary driver regulating the transport of fresh/cold water from high latitudes. The advection mechanism imposes a connectedness of the larger-scale interannual variability in this region and its tie to atmospheric oscillations.In the second part, an analysis of 15-year long numerical simulations is presented which show 8 interannual salinity oscillations (positive and negative). Six of these are driven by the horizontal advection by slow varying currents (>2 months), while 2 events are driven by the horizontal eddy advection (
      PubDate: 2017-03-23T03:55:51.640341-05:
      DOI: 10.1002/2016JC012580
  • Deep sediment resuspension and thick nepheloid layer generation by
           open-ocean convection
    • Authors: X. Durrieu de Madron; S. Ramondenc, L. Berline, L. Houpert, A. Bosse, S. Martini, L. Guidi, P. Conan, C. Curtil, N. Delsaut, S. Kunesch, J. F. Ghiglione, P. Marsaleix, M. Pujo-Pay, T. Séverin, P. Testor, C. Tamburini,
      Abstract: The Gulf of Lions in the northwestern Mediterranean is one of the few sites around the world ocean exhibiting deep open-ocean convection. Based on 6 year long (2009–2015) time series from a mooring in the convection region, shipborne measurements from repeated cruises, from 2012 to 2015, and glider measurements, we report evidence of bottom thick nepheloid layer formation, which is coincident with deep sediment resuspension induced by bottom-reaching convection events. This bottom nepheloid layer, which presents a maximum thickness of more than 2000 m in the center of the convection region, probably results from the action of cyclonic eddies that are formed during the convection period and can persist within their core while they travel through the basin. The residence time of this bottom nepheloid layer appears to be less than a year. In situ measurements of suspended particle size further indicate that the bottom nepheloid layer is primarily composed of aggregates between 100 and 1000 µm in diameter, probably constituted of fine silts. Bottom-reaching open ocean convection, as well as deep dense shelf water cascading that occurred concurrently some years, lead to recurring deep sediments resuspension episodes. They are key mechanisms that control the concentration and characteristics of the suspended particulate matter in the basin, and in turn affect the bathypelagic biological activity.
      PubDate: 2017-03-22T03:35:45.413606-05:
      DOI: 10.1002/2016JC012062
  • New insights on tsunami genesis and energy source
    • Authors: Y. Tony Song; Ali Mohtat, Solomon C. Yim
      Abstract: Conventional tsunami theories suggest that earthquakes with significant vertical motions are more likely to generate tsunamis. In tsunami models, the vertical seafloor elevation is directly transferred to the sea-surface as the only initial condition. However, evidence from the 2011 Tohoku earthquake indicates otherwise; the vertical seafloor uplift was only 3∼5 meters, too small to account for the resultant tsunami. Surprisingly, the horizontal displacement was undeniably larger than anyone's expectation; about 60 meters at the frontal wedge of the fault plate, the largest slip ever recorded by in-situ instruments. The question is whether the horizontal motion of seafloor slopes had enhanced the tsunami to become as destructive as observed. In this study, we provide proof: (1) Combining various measurements from the 2011 Tohoku event, we show that the earthquake transferred a total energy of 3.1e+15 joule to the ocean, in which the potential energy (PE) due to the vertical seafloor elevation (including seafloor uplift/subsidence plus the contribution from the horizontal displacement) was less than a half, while the kinetic energy (KE) due to the horizontal displacement velocity of the continental slope contributed a majority portion; (2) Using two modern state-of-the-art wave flumes and a three-dimensional tsunami model, we have reproduced the source energy and tsunamis consistent with observations, including the 2004 Sumatra event. Based on the unified source energy formulation, we offer a competing theory to explain why some earthquakes generate destructive tsunamis, while others do not. This article is protected by copyright. All rights reserved.
      PubDate: 2017-03-20T11:20:40.567509-05:
      DOI: 10.1002/2016JC012556
  • Subregional characterization of mesoscale eddies across the
           Brazil-Malvinas Confluence
    • Authors: Evan Mason; Ananda Pascual, Peter Gaube, Simón Ruiz, Josep L. Pelegrí, Antoine Delepoulle
      Abstract: Horizontal and vertical motions associated with coherent mesoscale structures, including eddies and meanders, are responsible for significant global transports of many properties, including heat and mass. Mesoscale vertical fluxes also influence upper ocean biological productivity by mediating the supply of nutrients into the euphotic layer, with potential impacts on the global carbon cycle. The Brazil-Malvinas Confluence (BMC) is a western boundary current region in the South Atlantic with intense mesoscale activity. This region has an active role in the genesis and transformation of water masses and thus is a critical component of the Atlantic meridional overturning circulation. The collision between the Malvinas and Brazil Currents over the Patagonian shelf/slope creates an energetic front that translates offshore to form a vigorous eddy field. Recent improvements in gridded altimetric sea level anomaly fields allow us to track BMC mesoscale eddies with high spatial and temporal resolutions using an automated eddy tracker. We characterize the eddies across fourteen 5°×5° subregions. Eddy-centric composites of tracers and geostrophic currents diagnosed from a global reanalysis of surface and in situ data reveal substantial subregional heterogeneity. The in situ data are also used to compute the evolving quasi-geostrophic vertical velocity (QG-ω) associated with each instantaneous eddy instance. The QG- ω eddy composites have the expected dipole patterns of alternating upwelling/downwelling, however the magnitude and sign of azimuthally-averaged vertical velocity varies among subregions. Maximum eddy values are found near fronts and sharp topographic gradients. In comparison with regional eddy composites, subregional composites provide refined information about mesoscale eddy heterogeneity. This article is protected by copyright. All rights reserved.
      PubDate: 2017-03-20T11:20:39.23999-05:0
      DOI: 10.1002/2016JC012611
  • Generation of exchange flows in estuaries by tidal and gravitational eddy
           viscosity-fshear covariance (ESCO)
    • Authors: Yoeri M. Dijkstra; Henk M. Schuttelaars, Hans Burchard
      Abstract: We present a systematic analysis of generation mechanisms for exchange flows in partially stratified estuaries using water column (1DV) and width-averaged (2DV) numerical models. We focus on exchange flows generated by eddy viscosity - shear covariance (ESCO). We identify two distinctly different physical mechanisms. The first, tidal ESCO circulation, results from interactions between the barotropic tide and temporal variations of the eddy viscosity. While this flow is mostly generated by direct interactions between the tide and eddy viscosity variations at the main tidal frequency, a similarly important contribution can be attributed to indirect interactions. These are more complex interactions involving eddy viscosity variations at other frequencies than the main tidal frequency (e.g. M4). The second mechanism is called gravitational ESCO circulation. This results from an amplification of the gravitational circulation through indirect interactions between the gravitational circulation and temporal variations of the eddy viscosity at any time-scale. Tidal and gravitational ESCO circulation are generated by different mechanisms and have a different dependency on the phase and frequency of eddy viscosity variations and the density gradient.The relative contributions of gravitational circulation and tidal and gravitational ESCO circulation to the exchange flow are typically 1/3 each in tidally energetic well-mixed or partially stratified estuaries. The results are generalised using an idealised width-averaged model of the Scheldt River estuary. This model confirms the results of the water column model and additionally shows that temporal variations of turbulence not captured in the water column model have a significant effect on the exchange flow. This article is protected by copyright. All rights reserved.
      PubDate: 2017-03-20T11:15:37.45909-05:0
      DOI: 10.1002/2016JC012379
  • Large-scale laboratory study of breaking wave hydrodynamics over a fixed
    • Authors: Dominic A. van der A; Joep van der Zanden, Tom O'Donoghue, David Hurther, Iván Cáceres, Stuart J. McLelland, Jan S. Ribberink
      Abstract: A large-scale wave flume experiment has been carried out involving a T=4s regular wave with H=0.85 m wave height plunging over a fixed barred beach profile. Velocity profiles were measured at twelve locations along the breaker bar using LDA and ADV. A strong undertow is generated reaching magnitudes of 0.8 m/s on the shoreward side of the breaker bar. A circulation pattern occurs between the breaking area and the inner surf zone. Time-averaged turbulent kinetic energy (TKE) is largest in the breaking area on the shoreward side of the bar where the plunging jet penetrates the water column. At this location, and on the bar crest, TKE generated at the water surface in the breaking process reaches the bottom boundary layer. In the breaking area TKE does not reduce to zero within a wave cycle which leads to a high level of “residual” turbulence and therefore lower temporal variation in TKE compared to previous studies of breaking waves on plane beach slopes. It is argued that this residual turbulence results from the breaker bar-trough geometry, which enables larger length- and time-scales of breaking generated vortices and which enhances turbulence production within the water column compared to plane beaches. Transport of TKE is dominated by the undertow-related flux, whereas the wave-related and turbulent fluxes are approximately an order of magnitude smaller. Turbulence production and dissipation are largest in the breaker zone and of similar magnitude, but in the shoaling zone and inner surf zone production is negligible and dissipation dominates. This article is protected by copyright. All rights reserved.
      PubDate: 2017-03-20T11:15:36.069476-05:
      DOI: 10.1002/2016JC012072
  • Assessment of western Indian Ocean SST bias of CMIP5 models
    • Authors: Ibnu Fathrio; Satoshi Iizuka, Atsuyoshi Manda, Yasu-Masa Kodama, Sachinobu Ishida, Qoosaku Moteki, Hiroyuki Yamada, Yoshihiro Tachibana
      Abstract: The western Indian Ocean sea surface temperature (SST) is among the key factors that affect precipitation over India and East Africa. This study examined the western Indian Ocean SST biases among the Coupled Model Intercomparison Project phase 5 (CMIP5) models. It was found that the multi-model ensemble-mean SST biases over the western equatorial Indian Ocean are warmer than the observations during the summer monsoon season. However, about half the models show positive SST biases, whereas negative ones in the other half. The models with warmer SST biases exhibit a pattern similar to the Indian Ocean Dipole, with stronger equatorial easterly wind biases during fall and a deeper thermocline in the western equatorial Indian Ocean. In the models with cooler SST biases, negative SST biases are observed over the entire tropical Indian Ocean throughout the year and the wind biases over the equatorial Indian Ocean are southeasterly during summer and fall. Heat budget analysis revealed the importance of ocean currents in forming the early summer development of SST biases over the western equatorial Indian Ocean. The formation of SST biases is related to surface current biases induced by the weaker biases of southwesterly monsoon winds and SST biases over the southwestern equatorial Indian Ocean, which are advected by the East African Coastal Currents. On the other hand, almost of all the CMIP5 models show prominent cold SST biases over the northern Arabian Sea during the pre-monsoon season. The SST biases are induced by excess surface cooling during the winter monsoon season. This article is protected by copyright. All rights reserved.
      PubDate: 2017-03-20T11:15:29.324806-05:
      DOI: 10.1002/2016JC012443
  • Influence of the phytoplankton community structure on the spring and
           annual primary production in the North-Western Mediterranean Sea
    • Authors: Nicolas Mayot; Fabrizio D'Ortenzio, Julia Uitz, Bernard Gentili, Joséphine Ras, Vincenzo Vellucci, Melek Golbol, David Antoine, Hervé Claustre
      Abstract: Satellite ocean color observations revealed that unusually deep convection events in 2005, 2006, 2010 and 2013 led to an increased phytoplankton biomass during the spring bloom over a large area of the North-Western Mediterranean Sea (NWM). Here we investigate the effects of these events on the seasonal phytoplankton community structure, we quantify their influence on primary production, and we discuss the potential biogeochemical impact. For this purpose, we compiled in situ phytoplankton pigment data from five ship surveys performed in the NWM and from monthly cruises at a fixed station in the Ligurian Sea. We derived primary production rates from a light-photosynthesis model applied to these in situ data. Our results confirm that the maximum phytoplankton biomass during the spring bloom is larger in years associated with intense deep convection events (+ 51%). During these enhanced spring blooms, the contribution of diatoms to total phytoplankton biomass increased (+ 33%), as well as the primary production rate (+ 115%). The occurrence of a highly productive bloom is also related to an increase in the phytoplankton bloom area (+ 155%), and in the relative contribution of diatoms to primary production (+ 63%). Therefore, assuming that deep convection in the NWM could be significantly weakened by future climate changes, substantial decreases in the spring production of organic carbon and of its export to deep waters can be expected. This article is protected by copyright. All rights reserved.
      PubDate: 2017-03-17T19:15:43.10201-05:0
      DOI: 10.1002/2016JC012668
  • Remote sensing of the correlation between breakpoint oscillations and
           infragravity waves in the surf and swash zone
    • Authors: T. Moura; T. E. Baldock
      Abstract: A novel remote sensing methodology to determine the dominant infragravity mechanism in the inner surf and swash zone in the field is presented. Video observations of the breakpoint motion are correlated with the shoreline motion and inner surf zone water levels to determine the relationship between the time-varying breakpoint oscillations and the shoreline motion. The results of thirteen field data sets collected from three different beaches indicate that, inside the surf zone, the dominance of bound wave or breakpoint forcing is strongly dependent on the surf zone width and the type of short wave breaking. Infragravity generation by bound wave release was stronger for conditions with relatively narrow surf zones and plunging waves; breakpoint forcing was dominant for wider surf zones and spilling breaker conditions. This article is protected by copyright. All rights reserved.
      PubDate: 2017-03-17T11:50:35.809511-05:
      DOI: 10.1002/2016JC012233
  • A new look at ocean ventilation timescales and their uncertainties
    • Authors: Rana A. Fine; Synte Peacock, Mathew E. Maltrud, Frank O. Bryan
      Abstract: A suite of eddy-resolving ocean transient tracer model simulations are first compared to observations. Observational and model pCFC-11 ages agree quite well, with the eddy-resolving model adding detail. The CFC ages show that the thermocline is a barrier to interior ocean exchange with the atmosphere on timescales of 45 years, the measureable CFC transient, although there are exceptions. Next, model simulations are used to quantify effects on tracer ages of the spatial dependence of internal ocean tracer variability due to stirring from eddies and biases from non-stationarity of the atmospheric transient when there is mixing. These add to tracer age uncertainties and biases, which are large in frontal boundary regions, and small in subtropical gyre interiors. These uncertainties and biases are used to re-interpret observed temporal trends in tracer-derived ventilation timescales taken from observations more than a decade apart, and to assess whether interpretations of changes in tracer ages being due to changes in ocean ventilation hold water. For the southern hemisphere subtropical gyres, we infer that the rate of ocean ventilation 26-27.2 σθ increased between the mid-1990s and the decade of the 2000s. However, between the mid-1990s and the decade of the 2010s, there is no significant trend ‒ perhaps except for South Atlantic. Observed age/AOU/ventilation changes are linked to a combination of natural cycles and climate change, and there is regional variability. Thus, for the future it is not clear how strong or steady in space and time ocean ventilation changes will be. This article is protected by copyright. All rights reserved.
      PubDate: 2017-03-17T11:50:31.875048-05:
      DOI: 10.1002/2016JC012529
  • On nonhydrostatic coastal model simulations of shear instabilities in a
           stratified shear flow at high Reynolds number
    • Authors: Zheyu Zhou; Xiao Yu, Tian-Jian Hsu, Fengyan Shi, W. Rockwell Geyer, James T. Kirby
      Abstract: The non-hydrostatic surface and terrain-following coastal model NHWAVE is utilized to simulate a continually forced stratified shear flow in a straight channel, which is a generic problem to test the existing non-hydrostatic coastal models' capability in resolving shear instabilities in the field-scale. The resolved shear instabilities in the shear layer has a Reynolds number of about 1.4 × 106, which is comparable to field observed value. Using the standard Smagorinsky closure with a grid size close to the Ozmidov length scale, simulation results show that the resolved energy cascade exceeds one order of magnitude and the evolution and turbulent mixing characteristics are predicted well. Two different approaches are used to estimate the turbulent dissipation rate, namely using the resolved turbulent energy spectrum and the parameterized sub-grid turbulent dissipation rate, and the predicted results provide the upper and lower bounds that encompass the measured values. Model results show significantly higher turbulence in braids of shear instabilities, which is similar to field observations while both the subgrid turbulent dissipation rate and resolved vorticity field can be used as surrogates for measured high acoustic backscatter signals. Simulation results also reveal that the surface velocity divergence/convergence is an effective identifier for the front of the density current and the shear instabilities. To guide future numerical studies in more realistic domains, an evaluation on the effects of different grid resolutions and subgrid viscosity on the resolved flow field and subgrid dissipation rate are discussed. This article is protected by copyright. All rights reserved.
      PubDate: 2017-03-17T11:50:26.483557-05:
      DOI: 10.1002/2016JC012334
  • Surface wave breaking over sheared currents: Observations from the Mouth
           of the Columbia River
    • Authors: Seth Zippel; Jim Thomson
      Abstract: Measurements of waves and currents from freely drifting buoys are used to evaluate wave breaking parameterizations at the Mouth of the Columbia River, where breaking occurs in intermediate depths and in the presence of vertically sheared currents. Breaking waves are identified using images collected with cameras onboard the buoys, and the breaking activity is well-correlated with wave steepness. Vertical shear in the currents produces a frequency-dependent effective current that modifies the linear dispersion relation. Accounting for these sheared currents in the wavenumber spectrum is essential in calculating the correct wave steepness; without this, wave steepness can be over (under) estimated on opposing (following) currents by up to 20%. The observed bulk steepness values suggest a limiting value of 0.4. The observed fraction of breaking waves is in good agreement with several existing models, each based on wave steepness. Further, a semi-spectral model designed for all depth regimes also compares favorably with measured breaking fractions. In this model, the majority of wave breaking is predicted to occur in the higher frequency bands (i.e., short waves). There is a residual dependance on directional spreading, in which wave breaking decreases with increasing directional spread. This article is protected by copyright. All rights reserved.
      PubDate: 2017-03-17T11:45:25.17049-05:0
      DOI: 10.1002/2016JC012498
  • Parameterizing air-sea gas transfer velocity with dissipation
    • Authors: L. Esters; S. Landwehr, G. Sutherland, T.G. Bell, K. H. Christensen, E. S. Saltzman, S. D. Miller, B. Ward
      Abstract: The air-sea gas transfer velocity k is frequently estimated as an empirical function of wind speed. However, it is widely recognized that k depends on processes other than wind speed alone. The small-eddy model, which describes periodic events of small eddies disturbing the sea surface with water from below, suggests a direct relation between k and the dissipation rate of turbulent kinetic energy ε at the air-sea interface. This relation has been proven both in laboratories and in the field in various freshwater and coastal environments, but to date has not been verified in open ocean conditions. Here, concurrent North Atlantic field observations of ε and eddy covariance measurements of DMS and CO2 air-sea gas flux are presented. Using ε measurements, we compare the small-eddy model at various depths to previously published observations. Extrapolating the measured ε profiles to the thickness of the viscous sublayer allows us to formulate a function of k that depends solely on the water side friction velocity u*w, which can be inferred from direct eddy covariance measurements of the air side friction velocity u*a. These field observations are generally consistent with the theoretical small-eddy model. Utilizing a variable Schmidt number exponent in the model, rather than a constant value of 12 yields improved agreement between model and observations. This article is protected by copyright. All rights reserved.
      PubDate: 2017-03-17T11:40:41.294679-05:
      DOI: 10.1002/2016JC012088
  • Thin ice and storms: Sea ice deformation from buoy arrays deployed during
    • Authors: Polona Itkin; Gunnar Spreen, Bin Cheng, Martin Doble, Fanny Girard-Ardhuin, Jari Haapala, Nick Hughes, Lars Kaleschke, Marcel Nicolaus, Jeremy Wilkinson
      Abstract: Arctic sea ice has displayed significant thinning as well as an increase in drift speed in recent years. Taken together this suggests an associated rise in sea ice deformation rate. A winter and spring expedition to the sea ice covered region north of Svalbard – the Norwegian young sea ICE 2015 expedition (N-ICE2015) - gave an opportunity to deploy extensive buoy arrays and to monitor the deformation of the first- and second-year ice now common in the majority of the Arctic Basin. During the 5-month long expedition, the ice cover underwent several strong deformation events, including a powerful storm in early February that damaged the ice cover irreversibly. The values of total deformation measured during N-ICE2015 exceed previously measured values in the Arctic Basin at similar scales: At 100 km scale, N-ICE2015 values averaged above 0.1, day−1, compared to rates of 0.08 day −1 or less for previous buoy arrays. The exponent of the power law between the deformation length scale and total deformation developed over the season from 0.37 to 0.54 with an abrupt increase immediately after the early February storm, indicating a weakened ice cover with more free drift of the sea ice floes. Our results point to a general increase in deformation associated with the younger and thinner Arctic sea ice and to a potentially destructive role of winter storms. This article is protected by copyright. All rights reserved.
      PubDate: 2017-03-17T11:40:37.589288-05:
      DOI: 10.1002/2016JC012403
  • The variations of sediment transport patterns in the outer Changjiang
           Estuary and Hangzhou Bay over the last 30 years
    • Authors: Dongfeng Xie; Cunhong Pan, Xiuguang Wu, Shu Gao, Zhengbing Wang
      Abstract: The research objective is to investigate the variations of sediment transport in outer Changjiang Estuary and adjacent Hangzhou Bay, induced by the decline of Changjiang River sediment discharge and massive land reclamation in the last three decades. A synchronous hydrographic survey was conducted along two transects (at the bay-mouth and outer Changjiang Estuary, respectively) during the spring-neap tides of January and July 2014. The results show that the suspended sediment grain size, current velocity, suspended sediment concentration (SSC), and the water and sediment fluxes varied with the tidal cycles. Quantitative correlations with the tidal range were found for SSC and fluxes. These data have been compared with those at the same hydrographic stations in the summer and winter of the early 1980s. Along the outer Changjiang Estuary transect, the SSCs and sediment fluxes decreased in the winter, but no apparent changes occurred in the summer. The SSCs in the northern Hangzhou Bay decreased in both summer and winter, while the southern bay mouth has evolved from a low SSC region to a high SSC region. The findings clarify that the SSC and sediment flux changes in this area have only an indirect connection to the dramatic riverine sediment decline, because the sediment resuspension by the strong tidal currents provided a major source. At the present stage, the impact of the riverine sediment decline is insignificant for the SSC variation off the Changjiang River mouth. Finally, a sediment flux model is proposed to explain and predict the morphological evolution trends. This article is protected by copyright. All rights reserved.
      PubDate: 2017-03-17T11:40:33.407591-05:
      DOI: 10.1002/2016JC012264
  • Space and time scales of sea surface salinity and freshwater forcing
           variability in the global ocean (60°S–60°N)
    • Authors: Frederick M. Bingham; Tong Lee
      Abstract: Using Aquarius Version-4.0 data, we have investigated the time and space scales of sea surface salinity (SSS) over the global ocean between 60°S and 60°N. Decorrelation time scales of SSS were found to be divided among less than 80 days (covering 1/2 of ocean area), 80-100 days (1/3) and greater than 100 days (remainder). Once the seasonal cycle is removed, shorter time scales (less than 80 days) dominate. Spatial scales are largest in the tropics along the intertropical convergence zones of all oceans and the South Pacific convergence zone in the South Pacific. Time scales were also calculated for time-integrated (cumulative) surface freshwater forcing (CFWF) using precipitation from Tropical Rainfall Measurement Mission and evaporation from OAFlux data. These showed little spatial pattern, but a dominance of the seasonal and longer time scales over the globe. The lack of correspondence between dominant temporal and spatial scales of SSS and CFWF highlights the importance of ocean processes in regulating SSS variability. This article is protected by copyright. All rights reserved.
      PubDate: 2017-03-17T11:40:25.877062-05:
      DOI: 10.1002/2016JC012216
  • Observations of waves and currents during barrier island inundation
    • Authors: A. Engelstad; B.G. Ruessink, D. Wesselman, P. Hoekstra, A. Oost, M. van der Vegt
      Abstract: Overwash and inundation on barrier islands can transport sediment onshore, leading to vertical accretion. These processes could ensure barrier island growth in times of sea-level rise, but wave and current fields during overwash and inundation are not well understood. Field data of water levels, waves, and currents were collected on a barrier island in the Netherlands to investigate the hydrodynamics during island inundation. Observations show that even in shallow water depths (< 0.5 m) wave energy was not completely dissipated as waves propagated from the North Sea onshore. Additionally, locally generated wind waves entered the field area from the Wadden Sea and propagated offshore. Infragravity waves were an important part of the wave field, particularly onshore of the beach crest. They were observed to be onshore progressive and displayed a bore-like shape when water depths were shallow. Wave breaking was the dominant dissipation mechanism for high-frequency waves as well as for infragravity waves, which is in agreement with prior research on infragravity wave energy dissipation on mild sloping (closed-boundary) beaches. A large-scale offshore directed water level gradient between the Wadden Sea and the North Sea side, caused by elevated water levels in the Wadden Sea during the storms, frequently drove an offshore flow if it was large enough to exceed the cross-shore gradient due to wave set-up. In addition, elevated water levels in the Wadden Sea decreased current velocities due to a decrease in water level gradients. This study highlights the influence of back-barrier processes on the hydrodynamics during inundation. This article is protected by copyright. All rights reserved.
      PubDate: 2017-03-17T11:40:24.032003-05:
      DOI: 10.1002/2016JC012545
  • Effects of sea-ice and biogeochemical processes and storms on under ice
           water fCO2 during the winter-spring transition in the high Arctic Ocean:
           Implications for sea-air CO2 fluxes
    • Authors: Agneta Fransson; Melissa Chierici, Ingunn Skjelvan, Are Olsen, Philipp Assmy, Algot K. Peterson, Gunnar Spreen, Brian Ward
      Abstract: We performed measurements of carbon dioxide fugacity (fCO2) in the surface water under Arctic sea ice from January to June 2015 during the Norwegian young sea ICE (N-ICE2015) expedition. Over this period, the ship drifted with four different ice floes and covered the deep Nansen Basin, the slopes north of Svalbard and the Yermak Plateau. This unique winter-to-spring dataset includes the first winter-time under-ice water fCO2 observations in this region. The observed under-ice fCO2 ranged between 315 µatm in winter and 153 µatm in spring, hence was undersaturated relative to the atmospheric fCO2. Although the sea ice partly prevented direct CO2 exchange between ocean and atmosphere, frequently occurring leads and breakup of the ice sheet promoted sea-air CO2 fluxes. The CO2 sink varied between 0.3 and 86 mmol C m−2 d−1, depending strongly on the open-water fractions (OW) and storm events. The maximum sea-air CO2 fluxes occurred during storm events in February and June. In winter, the main drivers of the change in under-ice water fCO2 were dissolution of CaCO3 (ikaite) and vertical mixing. In June, in addition to these processes, primary production and sea-air CO2 fluxes were important. The cumulative loss due to CaCO3 dissolution of 0.7 mol C m−2 in the upper 10 m played a major role in sustaining the undersaturation of fCO2 during the entire study. The relative effects of the total fCO2 change due to CaCO3 dissolution was 38%, primary production 26%, vertical mixing 16%, sea-air CO2 fluxes 16%, and temperature and salinity insignificant. This article is protected by copyright. All rights reserved.
      PubDate: 2017-03-16T11:06:18.658027-05:
      DOI: 10.1002/2016JC012478
  • Buoyancy-driven coastal current blocks ventilation of an anoxic fjord on
           the Pacific coast of Canada
    • Authors: Richard E. Thomson; David J. Spear, Maxim V. Krassovski, Roy A.S. Hourston, Tamás A. Juhász, Steven F. Mihály
      Abstract: Shallow sills restrict the ventilation of deep coastal fjords. Dense oceanic water seaward of the sill and lower density water within the receiving basin are generally required for oxygenated water to cross the sill and descend deep into the fjord. Here, we use concurrent 10-year time series from current meters in the fjord and on the continental shelf to examine ventilation of the 120-m deep, anoxic inner basin of Effingham Inlet on the west coast of Vancouver Island. Whereas density currents traverse the 40 m-deep sill and flow into the inner basin at mid-depth at quasi-fortnightly tidal intervals, only five current intrusions descended to the bottom of the basin over the decade-long measurement period. The deep intrusions had a mean (±SD) return interval of 1.75 (±1.33) years and induced bottom-water changes that persisted for 1-2 months. We show that, in addition to conditions within the inlet, deep ventilation of the inner basin is dependent on a coordinated sequence of external processes: (1) The onset of upwelling winds along the outer coast; (2) reversal of the buoyancy-driven coastal current that normally flows poleward over the inner shelf off Vancouver Island; and (3) reversal of the estuarine circulation in the channel connecting the inlet to the ocean. As the observed ventilation intervals are short compared to the decadal intervals derived from the spacing of “homogenites” (sedimentation sequences disrupted by intruding bottom currents), the use of homogenites as proxies for past upwelling conditions in the northeast Pacific may need to be re-examined. This article is protected by copyright. All rights reserved.
      PubDate: 2017-03-16T11:01:07.645158-05:
      DOI: 10.1002/2016JC012512
  • Seasonal cycle of cross-equatorial flow in the central Indian Ocean
    • Authors: Yi Wang; Michael J. McPhaden
      Abstract: This study investigates the seasonal cycle of meridional currents in the upper layers of central equatorial Indian Ocean using acoustic Doppler current profiler (ADCP) and other data over the period 2004-2013. The ADCP data set collected along 80.5°E is the most comprehensive collection of direct velocity measurements in the central Indian Ocean to date, providing new insights into the meridional circulation in this region. We find that mean volume transport is southwards across the equator in the central Indian Ocean in approximate Sverdrup balance with the wind stress curl. In addition, mean westerly wind stress near the equator drives convergent Ekman flow in the surface layer and subsurface divergent geostrophic flow in the thermocline at 50-150 m depths. In response to a mean northward component of the surface wind stress, the maximum surface layer convergence is shifted off the equator to 0.75°N. Evidence is also presented for the existence of a shallow equatorial roll consisting of a northward wind-driven surface drift overlaying the southward-directed subsurface Sverdrup transport. Seasonal variations are characterized by cross equatorial transports flowing from the summer to the winter hemisphere in quasi-steady Sverdrup balance with the wind stress curl. In addition, semi-annually varying westerly monsoon transition winds lead to semi-annual enhancements of surface layer Ekman convergence and geostrophic divergence in the thermocline. These results quantify expectations from ocean circulation theories for equatorial Indian Ocean meridional circulation patterns with a high degree of confidence given the length of the data records. This article is protected by copyright. All rights reserved.
      PubDate: 2017-03-16T11:00:47.586481-05:
      DOI: 10.1002/2016JC012537
  • Submesoscale processes accelerate seasonal restratification in the
           Subantarctic Ocean
    • Authors: M. du Plessis; S. Swart, I. J. Ansorge, A. Mahadevan
      Abstract: Traditionally, the mechanism driving the seasonal restratification of the Southern Ocean mixed layer (ML) is thought to be the onset of springtime warming. Recent developments in numerical modelling and North Atlantic observations have shown that submesoscale ML eddies (MLE) can drive a restratifying flux to shoal the deep winter ML prior to solar heating at high latitudes. The impact of submesoscale processes on the intra-seasonal variability of the Subantarctic ML is still relatively unknown. We compare five months of glider data in the Subantarctic Zone to simulations of a 1-D mixing model to show that the magnitude of restratification of the ML cannot be explained by heat, freshwater and momentum fluxes alone. During early spring, we estimate that periodic increases in the vertical buoyancy flux by MLEs caused small increases in stratification, despite predominantly down-front winds that promote the destruction of stratification. The timing of seasonal restratification was consistent between 1-D model estimates and the observations. However, during up-front winds, the strength of springtime stratification increased over two-fold compared to the 1-D model, with a rapid shoaling of the MLD from >200 m to
      PubDate: 2017-03-14T03:35:39.709338-05:
      DOI: 10.1002/2016JC012494
  • A joint active/passive physical model of sea surface microwave signatures
    • Authors: William J. Plant; Vladimir Irisov
      Abstract: Active and passive microwave signatures of the ocean can only depend on the ocean wave spectrum if bound and breaking waves are neglected. However, history has not been kind to attempts to explain both radiometer brightness temperatures (Tb) and normalized radar cross sections (σo) of the sea using the same ocean wave spectrum without bound and breaking waves. In this paper, we show that if bound and breaking waves are included in physical models of radiometer and scatterometer microwave signatures of the ocean, a single wave spectrum can explain both Tb and σo to reasonable accuracy. Bound waves are the roughness produced by gently breaking, or crumpling, waves that travel near the speed of the parent wave. Bound wave modeling is based on earlier work by Plant [1997] but using additional information about the slope probability distributions of the bound waves' parents. Breaking wave and foam modeling both build on the function Λ(cb) introduced by Phillips [1985], which describes the average length of breaking wave fronts on the ocean per unit area as a function of breaker velocity, cb. We model Λ(cb) as documented in our previous work [Irisov and Plant, 2016] where we show that the wave spectrum completely determines Λ(cb). We thus show here that the result of including bound and breaking waves in radiometer and scatterometer models of oceanic signatures is a much closer fit to data over a wide range of microwave frequencies and incidence angles using a single wave spectrum. This article is protected by copyright. All rights reserved.
      PubDate: 2017-03-14T03:35:34.766044-05:
      DOI: 10.1002/2017JC012749
  • Sea-level rise impacts on the temporal and spatial variability of extreme
           water levels: A case study for St. Peter-Ording, Germany
    • Authors: S. Santamaria-Aguilar; A. Arns, A. T. Vafeidis
      Abstract: Both the temporal and spatial variability of storm surge water level (WL) curves are usually not taken into account in flood risk assessments as observational data are often scarce. In addition, sea-level rise (SLR) can further affect the variability of WLs. We analyze the temporal and spatial variability of the WL curve of 75 historical storm surge events that have been numerically simulated for St. Peter-Ording at the German North Sea coast, considering the effects induced by three SLR scenarios (RCP 4.5, RCP 8.5 and a RCP 8.5 high end scenario). We assess potential impacts of these scenarios on two parameters related to flooding: overflow volumes and fullness. Our results indicate that due to both the temporal and spatial variability of those events the resulting overflow volume can be two or even three times greater. We observe a steepening of the WL curve with an increase of the tidal range under the three SLR scenarios, although SLR induced effects are relatively higher for the RCP 4.5. The steepening of the WL curve with SLR produces a reduction of the fullness, but the changes in overflow volumes also depend on the magnitude of the storm surge event. This article is protected by copyright. All rights reserved.
      PubDate: 2017-03-11T04:25:27.709871-05:
      DOI: 10.1002/2016JC012579
  • Validation of sea ice models using an uncertainty-based distance metric
           for multiple model variables
    • Authors: Jorge R. Urrego-Blanco; Elizabeth C. Hunke, Nathan M. Urban, Nicole Jeffery, Adrian K. Turner, James R. Langenbrunner, Jane M. Booker
      Abstract: We implement a variance-based distance metric (Dn) to objectively assess skill of sea ice models when multiple output variables or uncertainties in both model predictions and observations need to be considered. The metric compares observations and model data pairs on common spatial and temporal grids improving upon highly aggregated metrics (e.g. total sea ice extent or volume) by capturing the spatial character of model skill. The Dn metric is a gamma-distributed statistic that is more general than the χ2 statistic commonly used to assess model fit, which requires the assumption that the model is unbiased and can only incorporate observational error in the analysis. The Dn statistic does not assume that the model is unbiased, and allows the incorporation of multiple observational data sets for the same variable and simultaneously for different variables, along with different types of variances that can characterize uncertainties in both observations and the model. This approach represents a step to establish a systematic framework for probabilistic validation of sea ice models. The methodology is also useful for model tuning by using the Dn metric as a cost function and incorporating model parametric uncertainty as part of a scheme to optimize model functionality. We apply this approach to evaluate different configurations of the standalone Los Alamos sea ice model (CICE) encompassing the parametric uncertainty in the model, and to find new sets of model configurations that produce better agreement than previous configurations between model and observational estimates of sea ice concentration and thickness. This article is protected by copyright. All rights reserved.
      PubDate: 2017-03-11T03:50:32.426884-05:
      DOI: 10.1002/2016JC012602
  • Downstream evolution of the Kuroshio's time-varying transport and velocity
    • Authors: M. Andres; V. Mensah, S. Jan, M.-H. Chang, Y.-J. Yang, C.M. Lee, B. Ma, T.B. Sanford
      Abstract: Observations from two companion field programs—Origins of the Kuroshio and Mindanao Currents (OKMC) and Observations of Kuroshio Transport Variability (OKTV)—are used here to examine the Kuroshio's temporal and spatial evolution. Kuroshio strength and velocity structure were measured between June 2012 and November 2014 with pressure-sensor equipped inverted echo sounders (PIESs) and upward-looking acoustic Doppler current profilers (ADCPs) deployed across the current northeast of Luzon, Philippines and east of Taiwan with an eight-month overlap in the two arrays' deployment periods. The time-mean net (i.e., integrated from the surface to the bottom) absolute transport increases downstream from 7.3 Sv (±4.4 Sv standard error) northeast of Luzon to 13.7 Sv (±3.6 Sv) east of Taiwan. The observed downstream increase is consistent with the return flow predicted by the simple Sverdrup relation and the mean wind stress curl field over the North Pacific (despite the complicated bathymetry and gaps along the North Pacific western boundary). Northeast of Luzon, the Kuroshio–bounded by the 0 m s−1 isotach–is shallower than 750 dbar, while east of Taiwan areas of positive flow reach to the seafloor (3000 m). Both arrays indicate a deep counterflow beneath the poleward-flowing Kuroshio (-10.3 ±2.3 Sv by Luzon and -12.5 ±1.2 Sv east of Taiwan). Time-varying transports and velocities indicate the strong influence at both sections of westward propagating eddies from the ocean interior. Topography associated with the ridges east of Taiwan also influences the mean and time-varying velocity structure there. This article is protected by copyright. All rights reserved.
      PubDate: 2017-03-11T03:45:30.45504-05:0
      DOI: 10.1002/2016JC012519
  • Composition and variability of the Denmark Strait overflow water in a
           high-resolution numerical model hindcast simulation
    • Authors: Erik Behrens; Kjetil Våge, Benjamin Harden, Arne Biastoch, Claus W. Böning
      Abstract: The upstream sources and pathways of the Denmark Strait Overflow Water and their variability have been investigated using a high-resolution model hindcast. This global simulation covers the period from 1948 to 2009 and uses a fine model mesh (1/20°) to resolve mesoscale features and the complex current structure north of Iceland explicitly. The three sources of the Denmark Strait Overflow, the shelfbreak East Greenland Current (EGC), the separated EGC, and the North Icelandic Jet, have been analyzed using Eulerian and Lagrangian diagnostics. The shelfbreak EGC contributes the largest fraction in terms of volume and freshwater transport to the Denmark Strait Overflow and is the main driver of the overflow variability. The North Icelandic Jet contributes the densest water to the Denmark Strait Overflow and shows only small temporal transport variations. During summer the net volume and freshwater transports to the south are reduced. On interannual timescales these transports are highly correlated with the large-scale wind stress curl around Iceland and, to some extent, influenced by the North Atlantic Oscillation, with enhanced southward transports during positive phases. The Lagrangian trajectories support the existence of a hypothesized overturning loop along the shelfbreak north of Iceland, where water carried by the North Icelandic Irminger Current is transformed and feeds the North Icelandic Jet. Monitoring these two currents and the region north of the Iceland shelfbreak could provide the potential to track long-term changes in the Denmark Strait Overflow and thus also the AMOC. This article is protected by copyright. All rights reserved.
      PubDate: 2017-03-11T03:40:36.636995-05:
      DOI: 10.1002/2016JC012158
  • Response of the Kuroshio Extension path state to near-term global warming
           in CMIP5 experiments with MIROC4h
    • Authors: Rui Li; Zhao Jing, Zhaohui Chen, Lixin Wu
      Abstract: In this study, responses of the Kuroshio Extension (KE) path state to near-term (2006 − 2035) global warming are investigated using a Kuroshio-resolving atmosphere-ocean coupled model. Under the representative concentration pathway 4.5 (RCP4.5) forcing, the KE system is intensified and its path state tends to move northward and becomes more stable. It is suggested that the local anticyclonic wind stress anomalies in the KE region favor the spin-up of the southern recirculation gyre, and the remote effect induced by the anticyclonic wind stress anomalies over the central and eastern mid-latitude North Pacific also contributes to the stabilization of the KE system substantially. The dominant role of wind stress forcing on KE variability under near-term global warming is further confirmed by adopting a linear 1.5-layer reduced-gravity model forced by wind stress curl field from the present climate model. It is also found that the main contributing longitudinal band for KE index (KEI) moves westward in response to the warmed climate. This results from the northwestward expansion of the large-scale sea level pressure (SLP) field. This article is protected by copyright. All rights reserved.
      PubDate: 2017-03-11T03:40:34.857054-05:
      DOI: 10.1002/2016JC012468
  • Detailed spatiotemporal impacts of El Niño on phytoplankton biomass
           in the South China Sea
    • Authors: Eko Siswanto; Haijun Ye, Dai Yamazaki, DanLing Tang
      Abstract: The lagging and leading correlations among satellite observations, reanalyzed biogeophysical data, and the Nino3.4 El Niño index were investigated to reveal the impacts of El Niño on the phytoplankton biomass (chlorophyll-a [Chl-a]) in the South China Sea (SCS), in an attempt to identify the probable responsible factors in greater spatiotemporal detail. A basin-scale high Chl-a concentration during the developing phase of El Niño changed to basin-scale low Chl-a during the weakening phase. Cyclonic wind circulation in the northern basin, increased wind speed in the southern basin, and strengthened upwelling off the Vietnamese coast likely caused a basin-scale nutrient increase during the developing phase of an El Niño event; the opposite conditions led to low nutrient levels during the weakening phase. Decreases in Chl-a east of the Vietnamese coast and northwest of Borneo Island were due to decreases in nutrients supplied by rivers. These spatiotemporal changes are considered biogeophysical responses to a variety of types of El Niño. Regardless of the El Niño type, reanalyzing biogeophysical datasets during central Pacific warming separately from those during eastern Pacific warming is recommended for a more robust understanding of the detailed spatiotemporal impacts of different El Niño types on the biogeophysical environment of the SCS. This article is protected by copyright. All rights reserved.
      PubDate: 2017-03-11T03:40:32.557452-05:
      DOI: 10.1002/2016JC012276
  • Importance of mesoscale eddies and mean circulation in ventilation of the
           Southern Ocean
    • Authors: Igor Kamenkovich; Zulema Garraffo, Romain Pennel, Rana A. Fine
      Abstract: This study examines the relative importance of the mean advection and mesoscale currents in the property exchange between the Southern Ocean mixed layer and downstream in the upper 2000 meters; this exchange is referred to as ventilation. A new, highly efficient offline tracer model employed here uses pre-calculated velocities to advect dynamically passive tracers. Two idealized tracers are considered: the Boundary Impulse Response (BIR) tracer, which helps to determine the ventilation pathways and time scales, and the Transient Surface Tracer (TST), which is relevant to transient atmospheric tracers. The importance of eddies is isolated by contrasting the control simulation with a simulation without mesoscale currents. The analysis reveals complex three-dimensional ventilation pathways, controlled by the interplay between the mean advection and eddy-induced spreading. The mean currents carry the tracers eastward within ACC and contribute to the formation of the Antarctic Intermediate Water (AAIW) in the South Pacific and South Atlantic. The main effect of eddies is to disperse tracers away from the mean pathways, and this dispersion acts to retain the BIR tracer in the Atlantic and Indian sectors and reduce the upstream influence of these regions on the South Pacific. In addition, the eddy-induced along-isopycnal spreading within ACC increases the ventilated depth and the inventory of TST. The results can be used to interpret distribution of tracers in the ocean in numerical simulations and observations. This article is protected by copyright. All rights reserved.
      PubDate: 2017-03-11T03:40:26.199629-05:
      DOI: 10.1002/2016JC012292
  • Striations and preferred eddy tracks triggered by topographic steering of
           the background flow in the eastern South Pacific
    • Authors: Ali Belmadani; Emilio Concha, David Donoso, Alexis Chaigneau, François Colas, Nikolai Maximenko, Emanuele Di Lorenzo
      Abstract: In recent years, persistent quasi-zonal jets or striations have been ubiquitously detected in the world ocean using satellite and in situ data as well as numerical models. This study aims at determining the role of mesoscale eddies in the generation and persistence of striations off Chile in the eastern South Pacific. A 50-year climatological integration of an eddy-resolving numerical ocean model is used to assess the long-term persistence of striations. Automated eddy tracking algorithms are applied to the model outputs and altimetry data. Results reveal that striations coincide with both polarized eddy tracks and the offshore formation of new eddies in the subtropical front and coastal transition zone, without any significant decay over time that discards random eddies as a primary driver of the striations. Localized patches of vortex stretching and relative vorticity advection, alternating meridionally near the eastern edge of the subtropical front, are associated with topographic steering of the background flow in the presence of steep topography, and with baroclinically and barotropically unstable meridional flow. These sinks and sources of vorticity are suggested to generate the banded structure further west, consistently with a β-plume mechanism. On the other hand, zonal/meridional eddy advection of relative vorticity and the associated Reynolds stress covariance are consistent with eddy deformation over rough topography and participate to sustain the striations in the far field. Shear instability of mean striations is proposed to feedback onto the eddy field, acting to maintain the subtropical front eddy streets and thus the striations. This article is protected by copyright. All rights reserved.
      PubDate: 2017-03-11T03:35:31.994929-05:
      DOI: 10.1002/2016JC012348
  • Using a 1-D model to reproduce the diurnal variability of SST
    • Authors: Ioanna Karagali; Jacob L. Høyer, Craig Donlon
      Abstract: A wide range of applications, from air-sea interaction studies to fisheries and biological modeling, need accurate, high resolution SST which requires that the diurnal signal is known; for many applications diurnal estimates are necessary and should be included in blended SST products. A widely preferred approach to bridge the gap between in situ and remotely sensed measurements and obtain diurnal warming estimates at large spatial scales, is modeling of the upper ocean temperature. This study uses the 1 dimensional General Ocean Turbulence Model (GOTM) to resolve diurnal signals identified from satellite SSTs and in situ measurements. Focus is given on testing and validation of different parameterizations of the basic physical processes known to influence the generation of a warm surface layer. GOTM is tested and validated using in situ measurements obtained at three locations, two in the Atlantic Ocean and one in the Baltic Sea, where different oceanographic and atmospheric conditions occur, in order to obtain an insight into its general performance. It is found that the model, with a 9 band solar absorption model rather than the standard 2 band scheme, performs well when using 3-hourly NWP forcing fields and is able to resolve daily SST variability seen both from satellite and in situ measurements. As such, and due to its low computational cost, it is proposed as a candidate model for diurnal variability estimates. This article is protected by copyright. All rights reserved.
      PubDate: 2017-03-11T03:35:27.31997-05:0
      DOI: 10.1002/2016JC012542
  • Bay of Bengal Salinity Stratification and Indian Summer Monsoon
           Intraseasonal Oscillation: 2. Impact on SST and convection
    • Authors: Yuanlong Li; Weiqing Han, Wanqiu Wang, M. Ravichandran, Tong Lee, Toshiaki Shinoda
      Abstract: The Indian summer monsoon intraseasonal oscillations (MISOs) induce pronounced intraseasonal sea surface temperature (SST) variability in the Bay of Bengal (BoB), which has important feedbacks to atmospheric convection. An ocean general circulation model (OGCM) is employed to investigate the upper-ocean processes affecting intraseasonal SST variability and its feedback to the MISO convection. In the BoB the MISO induces intraseasonal SST variability predominantly through surface heat flux forcing with comparable contributions from shortwave radiation and turbulent heat flux, and to a much smaller extent through wind-driven ocean mixed layer entrainment. The ocean salinity stratification, represented by mixed layer depth (MLD) and barrier layer thickness (BLT), has a strong control on SST but weak impact on convection of the MISO. The MLD is critical for the amplitude of SST response to various forcing processes, while the BLT mainly affects entrainment by determining the temperature difference between the mixed layer and the water below. From May to mid-June, the shallow MLD and thin barrier layer greatly enhance intraseasonal SST anomalies, which can amplify convection fluctuations of the MISO through air-sea interaction and leads to intense but short-duration post-convection break spells. When either the MLD or the BLT is large, intraseasonal SSTs tend to be weak. Further investigation reveals that freshwater flux of the monsoon gives rise to the shallow MLD and thick barrier layer, and its overall effect on intraseasonal SSTs is a 20% enhancement. These results provide implications for improving the simulation and forecast of the MISO in climate models. This article is protected by copyright. All rights reserved.
      PubDate: 2017-03-11T03:30:26.898227-05:
      DOI: 10.1002/2017JC012692
  • Behavior of a wave-driven buoyant surface jet on a coral reef
    • Authors: Liv M. M. Herdman; James L. Hench, Oliver Fringer, Stephen G. Monismith
      Abstract: A wave-driven surface buoyant jet exiting a coral reef was studied in order to quantify the amount of water re-entrained over the reef crest. Both moored observations and Lagrangian drifters were used to study the fate of the buoyant jet. To investigate in detail the effects of buoyancy and along-shore flow variations, we developed an idealized numerical model of the system. Consistent with previous work, the ratio of along-shore velocity to jet-velocity and the jet internal Froude number were found to be important determinants of the fate of the jet. In the absence of buoyancy, the entrainment of fluid at the reef crest, creates a significant amount of retention, keeping 60% of water in the reef system. However, when the jet is lighter than the ambient ocean-water, the net effect of buoyancy is to enhance the separation of the jet from shore, leading to a greater export of reef water. Matching observations, our modeling predicts that buoyancy limits retention to 30% of the jet flow for conditions existing on the Moorea reef. Overall, the combination of observations and modeling we present here shows that reef-ocean temperature gradients can play an important role in reef-ocean exchanges. This article is protected by copyright. All rights reserved.
      PubDate: 2017-03-07T19:40:28.076378-05:
      DOI: 10.1002/2016JC011729
  • Large-scale anomalies in sea-surface temperature and air-sea fluxes during
           wind relaxation events off the United States West Coast in summer
    • Authors: Kayla R. Flynn; Melanie R. Fewings, Christopher Gotschalk, Kelly Lombardo
      Abstract: In summertime along the United States West Coast, the winds exhibit a 3-stage cycle spanning ∼12 days. The prevailing upwelling-favorable winds weaken (relax) or reverse off the Pacific Northwest, then reintensify, then weaken off central California. We study the sea-surface temperature (SST) response to these “northern” and “southern” wind relaxations. (1) Satellite data indicate northern wind relaxations result in SST anomalies O(+1°C) extending ∼2000 km offshore. Surface heat flux reanalyses indicate the warm anomaly is mainly from decreased latent cooling. (2) After the winds reintensify, SST becomes anomalously cold along central and southern California. (3) During the southern wind relaxations, the cold SST anomaly persists but the SST warms with time. This warming is not driven by surface heat flux. The latent cooling is reduced, yet unlike during the northern relaxation, this change is canceled by a decrease in solar radiation due to increased cloudiness. In the region south of Point Conception, reduced southward advection of cold water and increased northward advection of warm water by the coastal countercurrent could explain the warming. Reduced Ekman pumping likely contributes to the warming trend during the southern relaxations, and reduced wind-driven entrainment at the base of the mixed layer likely contributes to the warming during both relaxations. Whether the net surface heat flux is the main driver of SST anomalies during wind relaxation depends on the regional response of clouds. Southern wind relaxations follow episodes of enhanced surface cooling, which may contribute to greater cloudiness during southern than northern wind relaxations. This article is protected by copyright. All rights reserved.
      PubDate: 2017-03-07T19:35:42.23695-05:0
      DOI: 10.1002/2016JC012613
  • Flow paths and variability of the North Atlantic Current: A comparison of
           observations and a high-resolution model
    • Authors: Tilia Breckenfelder; Monika Rhein, Achim Roessler, Claus W. Böning, Arne Biastoch, Erik Behrens, Christian Mertens
      Abstract: The North Atlantic Current (NAC) is subject to variability on multiannual to decadal time scales, influencing the transport of volume, heat and freshwater from the subtropical to the eastern subpolar North Atlantic (NA). Current observational time series are either too short or too episodic to study the processes involved. Here we compare the observed continuous NAC transport time series at the western flank of the Mid-Atlantic Ridge (MAR) and repeat hydrographic measurements at the OVIDE line in the eastern Atlantic with the NAC transport and circulation in the high-resolution (1/20°) ocean model configuration VIKING20 (1960-2008). The modeled baroclinic NAC transport relative to 3400 m (24.5 ± 7.1 Sv) at the MAR is only slightly lower than the observed baroclinic mean of 27.4 ± 4.7 Sv from 1993 until 2008, and extends further north by about 0.5°. In the eastern Atlantic, the western NAC (WNAC) carries the bulk of the transport in the model, while transport estimates based on hydrographic measurements from five repeated sections point to a preference for the eastern NAC (ENAC). The model is able to simulate the main features of the subpolar NA, providing confidence to use the model output to analyze the influence of the North Atlantic Oscillation (NAO). Model based velocity composites reveal an enhanced NAC transport across the MAR of up to 6.7 Sv during positive NAO phases. Most of that signal (5.4 Sv) is added to the ENAC transport, while the transport of the WNAC was independent of the NAO. This article is protected by copyright. All rights reserved.
      PubDate: 2017-03-07T19:35:36.794082-05:
      DOI: 10.1002/2016JC012444
  • Requirement of minimal signal-to-noise ratios of ocean color sensors and
           uncertainties of ocean color products
    • Authors: Lin Qi; Zhongping Lee, Chuanmin Hu, Menghua Wang
      Abstract: Using simulations, error propagation theory, and measurements from the Moderate Resolution Imaging Spectroradiometer (MODIS), we determined the minimal signal-to-noise ratio (SNR) required for ocean color measurements and product uncertainties at different spatial and temporal scales. First, based on typical top-of-atmosphere (TOA) radiance over the ocean, we evaluate the uncertainties in satellite-derived Rrs in the visible wavelengths (ΔRrs(vis)) due to sensor noise in both the near-infrared (NIR) and the visible bands. While the former induces noise in Rrs(vis) through atmospheric correction, the latter has a direct impact on Rrs(vis). Such estimated uncertainties are compared with inherent ΔRrs(vis) uncertainties from in situ measurements and from the operational atmosphere correction algorithm. The comparison leads to a conclusion that once SNR(NIR) is above 600:1, an SNR(vis) better than 400:1 will not make a significant reduction in product uncertainties at pixel level under typical conditions for a solar zenith angle of 45°. Then, such uncertainties are found to decrease significantly in data products of oceanic waters when the 1-km pixels from individual images are binned to lower spatial resolution (e.g., 4-km) or temporal resolution (e.g., monthly). Although these findings do not suggest that passive ocean color sensors should have SNR(vis) around 400:1, they do support the argument for more trade space in higher spatial and/or spectral resolutions once this minimal 400:1 SNR(vis) requirement is met. This article is protected by copyright. All rights reserved.
      PubDate: 2017-03-07T19:35:31.496328-05:
      DOI: 10.1002/2016JC012558
  • The role of the Indian Ocean sector for prediction of the coupled
           Indo-Pacific system: Impact of atmospheric coupling
    • Authors: E.C. Hackert; A.J. Busalacchi, J. Carton, R. Murtugudde, P. Arkin, M. N. Evans
      Abstract: Indian Ocean (IO) dynamics impact ENSO predictability by influencing wind and precipitation anomalies in the Pacific. To test if the upstream influence of the IO improves ENSO validation statistics, a combination of forced ocean, atmosphere, and coupled models are utilized. In one experiment, the full tropical Indo-Pacific region atmosphere is forced by observed interannual SST anomalies. In the other, the IO is forced by climatological SST. Differences between these two forced atmospheric model experiments spotlight a much richer wind response pattern in the Pacific than previous studies that used idealized forcing and simple linear atmospheric models. Weak westerlies are found near the equator similar to earlier literature. However, at initialization strong easterlies between 30°S to 10°S and 0°N to 25°N and equatorial convergence of the meridional winds across the entire Pacific are unique findings from this paper. The large-scale equatorial divergence west of the dateline and northeasterly-to-northwesterly cross-equatorial flow converging on the equator east of the dateline in the Pacific are generated from interannual IO SST coupling. In addition, off-equatorial downwelling curl impacts large-scale oceanic waves (i.e. Rossby waves reflect as western boundary Kelvin waves). After 3 months, these downwelling equatorial Kelvin waves propagate across the Pacific and strengthen the NINO3 SST. Eventually Bjerknes feedbacks take hold in the eastern Pacific which allows this warm anomaly to grow. Coupled forecasts for NINO3 SST anomalies for 1993-2014 demonstrate that including interannual IO forcing significantly improves predictions for 3-9 month lead times. This article is protected by copyright. All rights reserved.
      PubDate: 2017-03-07T19:35:27.474965-05:
      DOI: 10.1002/2016JC012632
  • Observed basin-wide propagation of Mediterranean water in the Black Sea
    • Authors: Anastasia Falina; Artem Sarafanov, Emin Özsoy, Ufuk Utku Turunҫoğlu
      Abstract: Mediterranean water entering the Black Sea through the Bosphorus Strait forms mid-depth intrusions that contribute to the salt, heat, and volume balances of the sea, ventilate its water column at intermediate depths and restrain the upward flux of hydrogen sulfide from deeper layers. Despite the importance for the Black Sea environment, the circulation of Mediterranean-origin water in the basin is fundamentally underexplored. Here we use hydrographic data collected from ships and Argo profiling floats to identify pathways of the Mediterranean intrusions in the general circulation system of the sea. While earlier the intrusions were observed primarily near the Bosphorus Strait, we present an evidence for their intermittent extensive propagation throughout the basin. We find that the main conduit for the intrusions is the southern limb of the Rim Current that carries the intruded water from the Bosphorus Strait eastwards. A part of this eastward flow recirculates cyclonically into the interior of the sea, where traces of the intrusions gradually disappear because of mixing. We put forward the hypothesis that the formation of the most prominent intrusions is associated with strong cyclonic storms over the Bosphorus Strait, which lead to abnormally large influx of Mediterranean water. This article is protected by copyright. All rights reserved.
      PubDate: 2017-03-07T19:30:30.516987-05:
      DOI: 10.1002/2017JC012729
  • On the effect of topography and wind on warm water inflow: An idealized
           study of the southern Weddell Sea continental shelf system
    • Authors: K. Daae; T. Hattermann, E. Darelius, I. Fer
      Abstract: An idealized eddy-resolving numerical model, with topographic features common to the southern Weddell Sea, is constructed to study mechanisms through which warm deep water enters a wide continental shelf with a trough. The open ocean, represented by a 1700 m deep channel, is connected to a 400 m deep shelf with a continental slope. The shelf is narrow (50 km) in the east, but widens to 300 km at the center of the model domain. Over the narrow shelf, the slope front is balanced by wind-driven Ekman downwelling and counteracting eddy overturning, favoring on-shelf transport of warm water in summer scenarios when fresher surface water is present. Over the wide shelf, the Ekman downwelling ceases, and the mesoscale eddies relax the front. Inflow of warm water is sensitive to along-shelf salinity gradients, and is most efficient when denser water over the wide shelf favors up-slope eddy transport along isopycnals of the V-shaped slope front. Inflow along the eastern side of the trough cannot penetrate the sill region due to potential vorticity constraints, while along the western trough flank, eddy-induced inflow crosses the sill and reaches the ice front. The warm inflow into the trough is sensitive to the density of the outflowing dense shelf water. For weaker winds, absence of the dense water outflow leads to a reversal of the trough circulation and a strong inflow of warm water, while for stronger winds, baroclinic effects become less important and the inflow is similar to experiments including dense water outflow. This article is protected by copyright. All rights reserved.
      PubDate: 2017-03-02T10:55:50.572698-05:
      DOI: 10.1002/2016JC012541
  • Nitrous oxide production in surface waters of the midlatitude North
           Atlantic Ocean
    • Authors: Qixing Ji; Bess B. Ward
      Abstract: The ocean is a major source of atmospheric nitrous oxide (N2O), an important greenhouse gas and ozone depleting agent. The oceanic flux of N2O varies regionally, and in the mid-latitude North Atlantic, the production of N2O is poorly constrained. Incubation experiments with 15N-ammonium and 15N-nitrite revealed active N2O production from ammonium oxidation and nitrite reduction in the surface ocean, suggesting the mid-latitude North Atlantic could be a net source for N2O, with a flux density of 0.06 µmol-N2O m−2 d−1 in the top 120 meters. The peak of N2O production was detected in the upper 100 m, shallower than the depth at which highest rates of ammonium oxidation to nitrite occurred. Oxygen was not depleted in the water column, but its concentration minimum corresponded to highest N2O oversaturation and low in situ N2O production. The apparent N2O yield, i.e. the molar ratio of N2O-N production over nitrite production, was 1.7% at the peak N2O production depths in the surface layer, and decreased to less than 0.1% at peak ammonium oxidation depths. The majority of N2O production was apparently through “hybrid formation”, in which ammonium and nitrite each contribute one nitrogen atom to N2O formation, a process that is proposed to be mediated by ammonia oxidizing archaea. This article is protected by copyright. All rights reserved.
      PubDate: 2017-03-02T10:45:40.535632-05:
      DOI: 10.1002/2016JC012467
  • Frequency content of sea surface height variability from internal gravity
           waves to mesoscale eddies
    • Authors: Anna C. Savage; Brian K. Arbic, James G. Richman, Jay F. Shriver, Matthew H. Alford, Maarten C. Buijsman, J. Thomas Farrar, Hari Sharma, Gunnar Voet, Alan J. Wallcraft, Luis Zamudio
      Abstract: High horizontal-resolution (1/12.5° and 1/25°) 41-layer global simulations of the HYbrid Coordinate Ocean Model (HYCOM), forced by both atmospheric fields and the astronomical tidal potential, are used to construct global maps of sea surface height (SSH) variability. The HYCOM output is separated into steric and non-steric, and into subtidal, diurnal, semidiurnal, and supertidal frequency bands. The model SSH output is compared to two datasets that offer some geographical coverage and that also cover a wide range of frequencies–a set of 351 tide gauges that measure full SSH, and a set of 14 in-situ vertical profilers from which steric SSH can be calculated. Three of the global maps are of interest in planning for the upcoming Surface Water and Ocean Topography (SWOT) two-dimensional swath altimeter mission: (1) maps of the total and (2) non-stationary internal tidal signal (the latter calculated after removing the stationary internal tidal signal via harmonic analysis), with an average variance of 1.05 cm2 and 0.43 cm2 respectively for the semidiurnal band, and (3) a map of the steric supertidal contributions, which are dominated by the internal gravity wave continuum, with an average variance of 0.15 cm2. Stationary internal tides (which are predictable), non-stationary internal tides (which will be harder to predict), and non-tidal internal gravity waves (which will be very difficult to predict), may all be important sources of high-frequency “noise” that could mask lower-frequency phenomena in SSH measurements made by the SWOT mission. This article is protected by copyright. All rights reserved.
      PubDate: 2017-03-02T10:45:38.191442-05:
      DOI: 10.1002/2016JC012331
  • Factors responsible for the limited inland extent of sand deposits on
           Leyte Island during 2013 Typhoon Haiyan
    • Authors: M. Watanabe; J. D. Bricker, K. Goto, F. Imamura
      Abstract: Previous geological studies suggest that the maximum inland extent of storm-induced sand deposits is shorter, but their thickness is larger, than those of tsunami-induced sand deposits. However, factors that determine the maximum extent and thickness of storm deposits are still uncertain. We conducted numerical simulations of storm surge, waves, and sediment transport during Typhoon Haiyan in order to understand the distribution and sedimentary processes responsible for storm deposits. Numerical results showed that wave-induced currents slightly offshore were strong, but attenuated rapidly in the inland direction after wave breaking. Therefore, sediments were not transported far inland by waves and storm surge. Consequently, the maximum inland extent of storm deposits was remarkably shorter than the inland extent of inundation. We also revealed that vegetation (roughness coefficient) and typhoon intensity greatly affect the calculation of maximum extent and thickness distribution of storm deposits. As the duration of wave impact on a coast is relatively long during a storm (hours, compared to minutes for a tsunami), sediments are repeatedly supplied by multiple waves. Therefore, storm deposits tend to be thicker than tsunami deposits, and multiple layers can form in the internal sedimentary structure of the deposits. We infer that limitation of the sand deposit to within only a short distance inland from the shoreline and multiple layers found in a deposit can be used as appropriate identification proxies for storm deposits. This article is protected by copyright. All rights reserved.
      PubDate: 2017-03-02T10:45:36.209764-05:
      DOI: 10.1002/2016JC012023
  • Temperature flux carried by individual eddies across 47°N in the
           Atlantic Ocean
    • Authors: Vasco Müller; Dagmar Kieke, Paul G. Myers, Clark Pennelly, Christian Mertens
      Abstract: Surface geostrophic velocity fields derived from satellite-altimetry between January 1993 and April 2014 are used to detect and investigate eddies in the North Atlantic between 40°-55°N and 60°-10°W. Focus is on a zonal section along 47°N, roughly at the boundary between the subpolar and the subtropical gyres. Sea surface temperature data are used to quantify the temperature anomalies associated with eddies and the respective surface temperature fluxes related to these eddies. Identified eddy pathways across 47°N are related to the mean background velocity from full-depth ship observations carried out on 11 cruises between 2003 and 2014. The analysis is repeated in two model simulations with 1/4° and 1/12° horizontal resolution, respectively for the period 2002-2013. The analysis reveals almost 37000 altimeter-derived eddies with a lifetime longer than one week in the area. The highest number of eddies is found along the pathway of the North Atlantic Current, roughly following the 4000 m isobath, and on the Grand Banks of Newfoundland. Time series of temperature fluxes by eddies crossing 47°N reveal that single isolated eddies with large SST signatures contribute ∼25% to the surface temperature flux. Relating the observed eddies to the observed top-to-bottom velocity distribution at 47°N points to the existence of eddy pathways across 47°. The highest temperature fluxes are linked to the fastest and most pronounced current branches in the western Newfoundland Basin. While there are fewer eddies in both model simulations, the key findings are consistent between the observations and the two model simulations. This article is protected by copyright. All rights reserved.
      PubDate: 2017-03-02T10:45:33.557385-05:
      DOI: 10.1002/2016JC012175
  • A modeling study of the impact of major storms on seabed shear stress and
           sediment transport on the Grand Banks of Newfoundland
    • Authors: Michael Z. Li; Yongsheng Wu, Guoqi Han, Robert H. Prescott, Charles C. L. Tang
      Abstract: Waves, current and sediment transport processes in major storms on the Grand Banks of Newfoundland were simulated using integrated wave, three-dimensional tide and circulation, and combined-flow sediment transport models. While the tidal and non-tidal currents are generally low and cause little sediment transport, storm-induced waves and currents enhance bed shear velocity by more than 5 times and cause significant sediment transport over the entire Grand Banks. The impact of storms on shear stress and transport strongly depends on water depths and the greatest impact occurs over the bathymetric highs on southeastern Grand Bank where the maximum shear velocity reaches 15 cm s−1 and the maximum transport rates are >5 kg m−1 s−1. The direction of sediment transport rotates clockwise progressively through nearly 360° during the passage of a storm. Although peak transport typically occurs on central and southeastern Grand Bank with a southeastward direction, the magnitude, direction and timing of peak transport show strong spatial and temporal variability. The variability of the peak transport largely depends on the timing and relative intensity of the waves and the total bottom currents which in turn depends on the addition of the storm-induced and tidal currents. The calculation of the maximum transport potential suggests that sediments as coarse as small pebbles are mobile in water depths 
      PubDate: 2017-03-02T10:45:30.440671-05:
      DOI: 10.1002/2016JC012215
  • On the role of sea-state in bubble-mediated air-sea gas flux during a
           winter storm
    • Authors: Jun-Hong Liang; Steven R. Emerson, Eric A. D'Asaro, Craig L. McNeil, Ramsey R. Harcourt, Peter P. Sullivan, Bo Yang, Meghan F. Cronin
      Abstract: Oceanic bubbles play an important role in the air-sea exchange of weakly soluble gases at moderate to high wind speeds. A Lagrangian bubble model embedded in a large eddy simulation model is developed to study bubbles and their influence on dissolved gases in the upper ocean. The transient evolution of mixed-layer dissolved oxygen and nitrogen gases at Ocean Station Papa (50ºN, 145ºW) during a winter storm are reproduced with the model. Among different physical processes, gas bubbles are the most important in elevating dissolved gas concentrations during the storm, while atmospheric pressure governs the variability of gas saturation anomaly (the relative departure of dissolved gas concentration from the saturation concentration). For the same wind speed, bubble-mediated gas fluxes are larger during rising wind with smaller wave age than during falling wind with larger wave age. Wave conditions are the primary cause for the bubble gas flux difference: When wind strengthens, waves are less-developed with respect to wind, resulting in more frequent large breaking waves. Bubble generation in large breaking waves is favorable for a large bubble-mediated gas flux. The wave age dependence is not included in any existing bubble-mediated gas flux parameterizations. This article is protected by copyright. All rights reserved.
      PubDate: 2017-03-02T10:45:25.669572-05:
      DOI: 10.1002/2016JC012408
  • Meteorological influence on summertime baroclinic exchange in the straits
           of Mackinac
    • Authors: Eric J. Anderson; David J. Schwab
      Abstract: Straits flows can impose a complex hydrodynamic environment with high seasonal variability and significant impacts to nearby water bodies. In the Straits of Mackinac, exchange flow between Lake Michigan and Lake Huron influences water quality and ecological processes, as well as the transport of any contaminants released in or near the Straits. Although previous work has shown that a Helmholtz mode is responsible for the barotropic flow oscillations in the Straits, baroclinic effects impose opposite surface and subsurface flows during the summer months. In this study, we use observations of currents and water temperatures from instruments deployed in the Straits to validate a hydrodynamic model of the combined Lake Michigan-Huron system and then use the model results to investigate the baroclinic flow and determine the forcing mechanisms that drive exchange flow in the Straits of Mackinac. Analysis shows that although the Helmholtz mode drives a 3-day oscillation throughout the year, thermal stratification in the summer establishes a bi-directional flow that is governed by a shift from regional- to local-scale meteorological conditions. These results detail the seasonal variability in the Straits, including the barotropic and baroclinic contributions to exchange flow and the influence of local atmospheric forcing on transport through the Straits of Mackinac. This article is protected by copyright. All rights reserved.
      PubDate: 2017-02-28T03:41:02.762388-05:
      DOI: 10.1002/2016JC012255
  • Concentrations and cycling of DMS, DMSP, and DMSO in coastal and offshore
           waters of the Subarctic Pacific during summer, 2010–2011
    • Authors: Elizabeth Asher; John W. Dacey, Debby Ianson, Angelica Peña, Philippe D. Tortell
      Abstract: Concentrations of dimethylsulfide (DMS), measured in the Subarctic Pacific during summer 2010 and 2011, ranged from ∼ 1 – 40 nM, while dissolved DMSO concentrations (range 13 - 23 nM) exceeded those of dissolved DMSP (range 1.3 – 8.8 nM). Particulate DMSP dominated the reduced sulfur pool, reaching maximum concentrations of 100 nM. Coastal and off shore waters exhibited similar overall DMS concentration ranges, but sea-air DMS fluxes were lower in the oceanic waters due to lower wind speeds. Surface DMS concentrations showed statistically significant correlations with various hydrographic variables including the upwelling intensity (r2 = 0.52, p 
      PubDate: 2017-02-28T03:35:34.563188-05:
      DOI: 10.1002/2016JC012465
  • Comparison of full and empirical Bayes approaches for inferring sea-level
           changes from tide-gauge data
    • Authors: Christopher G. Piecuch; Peter Huybers, Martin P. Tingley
      Abstract: Tide-gauge data are one of the longest instrumental records of the ocean, but these data can be noisy, gappy, and biased. Previous studies have used empirical Bayes methods to infer the sea-level field from tide-gauge records, but have not accounted for uncertainty in the estimation of model parameters. Here we compare to a fully Bayesian method that accounts for uncertainty in model parameters, and demonstrate that empirical Bayes methods underestimate the uncertainty in sea level inferred from tide-gauge records. We use a synthetic tide-gauge dataset to assess the skill of the empirical and full Bayes methods. The empirical-Bayes credible intervals on the sea-level field are narrower and less reliable than the full-Bayes credible intervals: the empirical-Bayes 95% credible intervals are 42.8% narrower on average than are the full-Bayes 95% credible intervals; full-Bayes 95% credible intervals capture 95.6% of the true field values, while the empirical-Bayes 95% credible intervals capture only 77.1% of the true values, showing that parameter uncertainty has an important influence on the uncertainty of the inferred sea-level field. Most influential are uncertainties in model parameters for data biases (i.e., tide-gauge datums); letting data-bias parameters vary along with the sea-level process, but holding all other parameters fixed, the 95% credible intervals capture 92.8% of the true synthetic-field values. Results indicate that full Bayes methods are preferable for reconstructing sea-level estimates in cases where complete and accurate estimates of uncertainty are warranted. This article is protected by copyright. All rights reserved.
      PubDate: 2017-02-28T03:35:26.325932-05:
      DOI: 10.1002/2016JC012506
  • Hydrographic and fish larvae distribution during the “Godzilla El Niño
           2015-2016” in the northern end of the shallow oxygen minimum zone of the
           eastern tropical Pacific Ocean
    • Authors: L. Sánchez-Velasco; E. Beier, V.M. Godínez, E.D. Barton, E. Santamaría-del-Angel, S.P.A. Jiménez-Rosemberg, S.G. Marinone
      Abstract: Based on hydrographic data and vertical distributions of tropical species of fish larvae (Diogenichthys laternatus, Vinciguerria lucetia, Bregmaceros bathymaster and Auxis spp), effects of “Godzilla El Niño 2015-2016” in the shallow oxygen minimum zone off Mexico were analyzed. Zooplankton samples were collected during four cruises, before (February 2010, April 2012) and during (June 2015, March 2016) the warm event. Temporal series of sea surface temperature revealed that June 2015 was the warmest June of the last years. Conservative Temperature was > 2°C higher than normal in the surface mixed layer, and the suboxic layer (4.4 µmol/kg) reached as shallow as 100 m depth. Unexpected results were that larval abundances were relatively high during the warm event, unlike zooplankton volumes, which declined. Before the warm event, V. lucetia and Auxis spp were more abundant in the surface mixed layer, while B. bathymaster and D. laternatus dominated in the thermocline and shallow hypoxic layer (44 µmol/kg). However, during the event in June 2015, all species were most abundant in the surface mixed layer, which implied that the species adapted to hypoxia had inverted their normal pattern of distribution, possibly as consequence of the rise of the suboxic layer; but further observations are required to confirm this generality. Results showed no dramatic change in the total larval abundance during the warm event. Nevertheless, a differential response in their vertical distribution was evident in association with changes in the depth of the shallow hypoxic and suboxic layers. This might indicate adaptability of tropical species to prolonged periods of warming in the oceans. This article is protected by copyright. All rights reserved.
      PubDate: 2017-02-28T03:34:14.762393-05:
      DOI: 10.1002/2016JC012622
  • Climate change impacts on southern Ross Sea phytoplankton composition,
           productivity, and export
    • Authors: Daniel E. Kaufman; Marjorie A. M. Friedrichs, Walker O. Smith, Eileen E. Hofmann, Michael S. Dinniman, John C. P. Hemmings
      Abstract: The Ross Sea, a highly productive region of the Southern Ocean, is expected to experience warming during the next century along with reduced summer sea ice concentrations and shallower mixed layers. This study investigates how these climatic changes may alter phytoplankton assemblage composition, primary productivity and export. Glider measurements are used to force a one-dimensional biogeochemical model, which includes diatoms and both solitary and colonial forms of Phaeocystis antarctica. Model performance is evaluated with glider observations, and experiments are conducted using projections of physical drivers for mid- and late-21st century. These scenarios reveal a 5% increase in primary productivity by mid-century and 14% by late-century and proportional increase in carbon export, which remains approximately 18% of primary production. In addition, scenario results indicate diatom biomass increases while P. antarctica biomass decreases in the first half of the 21st century. In the second half of the century, diatom biomass remains relatively constant and P. antarctica biomass increases. Additional scenarios examining the independent contributions of expected future changes (temperature, mixed layer depth, irradiance, and surface iron inputs from melting ice) demonstrate that earlier availability of low light due to reduction of sea ice early in the growing season is the primary driver of productivity increases over the next century; shallower mixed layer depths additionally contribute to changes of assemblage composition and export. This study further demonstrates how glider data can be effectively used to facilitate model development and simulation, and inform interpretation of biogeochemical observations in the context of climate change. This article is protected by copyright. All rights reserved.
      PubDate: 2017-02-27T18:35:39.128723-05:
      DOI: 10.1002/2016JC012514
  • A tale of two eddies: The biophysical characteristics of two contrasting
           cyclonic eddies in the East Australian Current System
    • Authors: M. Roughan; S. R. Keating, A. Schaeffer, P. Cetina Heredia, C. Rocha, D. Griffin, R. Robertson, I.M. Suthers
      Abstract: Mesoscale cyclonic eddies are known to be highly productive. Less well-known are the dynamics and productivity of smaller cyclonic eddies, known as frontal eddies, that form on the landward side of western boundary currents. In this study we investigate the physical and biogeochemical properties of two contrasting cyclonic eddies in the East Australian Current (EAC). The first (“Murphy”), a mesoscale cyclonic eddy that formed at ∼28°S with a diameter of ∼160 km and high surface chlorophyll-a concentrations, which lived ∼47 days. The second (“Freddy”), a smaller frontal eddy (∼35 km diameter) that formed from a shelf water billow ∼7 days prior to sampling at ∼31.5°S and was advected off the shelf along the EAC front (from ∼200m to 4000m of water). Both eddies were at least 1000m deep with a similar steric height anomaly. We introduce and employ ‘the method of closest approach' using shipboard ADCP velocities to estimate the eddy centers, which reveals significant tilting through the water column. We estimate rotation rates of 4-10 days and 1-9 days and Rossby numbers 0.25-0.1 and 0.6-0.1, from the surface to 600m for Murphy and Freddy respectively. High-resolution altimetry measurements from the SARAL/AltiKA satellite provide estimates of the ageostrophic component of rotation. Our results show that the frontal eddy is significantly more ageostrophic, energetic and productive than the mesoscale cyclone, despite its small size and short life (∼4 weeks). We suggest that frontal eddies have potential to contribute significantly to the net productivity of the Tasman Sea region. This article is protected by copyright. All rights reserved.
      PubDate: 2017-02-23T07:25:37.509634-05:
      DOI: 10.1002/2016JC012241
  • Tidal currents and Kuroshio transport variations in the Tokara Strait
           estimated from ferryboat ADCP data
    • Authors: Xiao-Hua Zhu; Hirohiko Nakamura, Menghong Dong, Ayako Nishina, Toru Yamashiro
      Abstract: From 2003 through 2011, current surveys, using an acoustic Doppler current profiler (ADCP) mounted on the Ferry Naminoue, were conducted across the Tokara Strait (TkS). 1,234 of the resulting velocity sections were used to estimate major tidal current constituents in the TkS. The semi-diurnal M2 tidal current (maximum amplitude 27 cm s−1) was dominant among all the tidal constituents, and the diurnal K1 tidal current (maximum amplitude 21 cm s−1) was the largest among all the diurnal tidal constituents. Over the section, the ratios, relative to M2, of averaged amplitudes of M2, S2, N2, K2, K1, O1, P1, and Q1 tidal currents were 1.00:0.44:0.21:0.12:0.56:0.33:0.14:0.10. Tidal currents estimated from the ship-mounted ADCP data were in good agreement with those from the mooring ADCP data. Their root-mean-square difference for the M2 tidal current amplitude was 2.0 cm s−1. After removing the tidal currents, the annual-mean of the net volume transport (NVT) through the TkS ± its standard derivation was 23.03 ± 3.31 Sv (Sv = 106 m3 s−1). The maximum (minimum) monthly-mean NVT occurred in July (November) with 24.60 (21.47) Sv. NVT values from the ship-mounted ADCP were in good agreement with previous geostrophic volume transports calculated from conductivity temperature depth data, but the former showed much finer temporal structure than those from the geostrophic calculation. This article is protected by copyright. All rights reserved.
      PubDate: 2017-02-23T07:20:57.820966-05:
      DOI: 10.1002/2016JC012329
  • Low-mode internal tides and balanced dynamics disentanglement in
           altimetric observations: Synergy with surface density observations
    • Authors: Aurélien L. Ponte; Patrice Klein, Michael Dunphy, Sylvie Le Gentil
      Abstract: The performance of a tentative method that disentangles the contributions of a low-mode internal tide on sea level from that of the balanced mesoscale eddies is examined using an idealized high resolution numerical simulation. This disentanglement is essential for proper estimation from sea level of the ocean circulation related to balanced motions. The method relies on an independent observation of the sea surface water density whose variations are 1/dominated by the balanced dynamics and 2/correlate with variations of potential vorticity at depth for the chosen regime of surface-intensified turbulence. The surface density therefore leads via potential vorticity inversion to an estimate of the balanced contribution to sea level fluctuations. The difference between instantaneous sea level (presumably observed with altimetry) and the balanced estimate compares moderately well with the contribution from the low mode tide. Application to realistic configurations remains to be tested. These results aim at motivating further developments of reconstruction methods of the ocean dynamics based on potential vorticity dynamics arguments. In that context, they are particularly relevant for the upcoming wide-swath high resolution altimetric missions (SWOT). This article is protected by copyright. All rights reserved.
      PubDate: 2017-02-23T07:20:48.748741-05:
      DOI: 10.1002/2016JC012214
  • Nutrient and phytoplankton dynamics on the inner shelf of the eastern
           Bering Sea
    • Authors: Calvin W. Mordy; Allan Devol, Lisa B. Eisner, Nancy Kachel, Carol Ladd, Michael W. Lomas, Peter Proctor, Raymond N. Sambrotto, David H. Shull, Phyllis J. Stabeno, Eric Wisegarver
      Abstract: The nitrogen cycle on the inner shelf of the southeastern Bering Sea is complicated due to limited nutrient replenishment across this broad shelf, and substantial nitrogen loss through sedimentary processes. While diffusion at the inner front may periodically support new production, the shelf is generally hypothesized to be a regenerative system. This study uses a combination of hydrographic surveys, and measurements of nitrogen assimilation and benthic fluxes to examine nitrogen cycling on the inner shelf, and connectivity between the middle and inner shelves of the southern and central Bering Sea. Results establish the inner shelf as primarily a regenerative system even in spring, although new production can occur at the inner front. Results also identify key processes that influence nutrient supply to the inner shelf, and reveal coupling between the middle shelf nutrient pool and production on the inner shelf. This article is protected by copyright. All rights reserved.
      PubDate: 2017-02-23T07:20:42.77849-05:0
      DOI: 10.1002/2016JC012071
  • Impact of mesoscale eddies on Kuroshio intrusion variability northeast of
    • Authors: Yuqi Yin; Xiaopei Lin, Ruoying He, Yijun Hou
      Abstract: Abundant mesoscale eddies propagate to the east of Taiwan and affect the Kuroshio Current and its onshore intrusion northeast of Taiwan. But the dynamical process of how eddy activities modulate the Kuroshio intrusion is still unclear. In this study, in situ ocean velocity observations, drifter trajectories, tide gauge measurements, satellite sea surface height data, and assimilative Hybrid Coordinate Ocean Model (HYCOM) reanalysis outputs were used to analyze a cyclonic eddy-induced large Kuroshio onshore intrusion during late autumn 2008. Along with nine anticyclonic eddy events and seven cyclonic eddy events extracted from long-term HYCOM reanalysis outputs, the variability of the eddy-induced intrusion were quantified. The impacts of the cyclonic eddy and the anticyclonic eddy were opposite. Under the impact of cyclonic (anticyclonic) eddy, the total onshore intrusion between 122°E and 124°E across the 200 m isobaths was decreased (increased) by 15.7% (7.3%), while the onshore intrusion west of 122.6°E and the northeastward current on the outer shelf was increased (decreased) by 30.6% (4.7%) and 31% (10.1%), respectively. The arrival of cyclonic (anticyclonic) eddy brought positive (negative) potential vorticity (PV) flux and modulated the local ocean vertical stratification, which weakened (enhanced) the cross-slope PV gradient and the shelf slope constraint on the upper layer current, therefore, favored (inhibited) the Kuroshio onshore intrusion. This article is protected by copyright. All rights reserved.
      PubDate: 2017-02-23T07:20:32.284365-05:
      DOI: 10.1002/2016JC012263
  • Sedimentation from buoyant muddy plumes in the presence of interface
           mixing: An experimental study
    • Authors: Mohamad Rouhnia; Kyle Strom
      Abstract: A series of flume experiments were conducted to study the sediment removal rates, characterized by an effective settling velocity, from buoyant muddy plumes overriding clear saltwater at steady state conditions under different Richardson numbers and initial suspended sediment concentrations. The experiments were all carried out in the subcritical regime, leading to cusps style instabilities at the interface of the two fluids. Flocculation in the experiments was not suppressed, yet flocs remained small while in the plume layer. Data from the experiments allowed for calculation of the individual floc settling velocity, Ws,f, and the overall effective settling velocity, Ws,eff, i.e., that velocity needed to predict the downward flux with CWs,eff. Results showed that Ws,eff was greater than the floc settling velocity in all runs. Ws,eff was found to increase with plume velocity and interface mixing but not with plume concentration. The difference between the effective and floc settling velocities, was therefore attributed to turbulent diffusion brought on by mixing at the interface and not to convective sedimentation or leaking. The turbulence enhanced settling velocity, Ws,t, was found to be a strong function of the Richardson number, and conceptual and empirical equations are presented to predict Ws,t as a function of the plume surface velocity, Ups, and Richardson number, Ri; this analysis showed that Ws,t ∝UpsRi−2. In the runs with low Richardson number, Ws,t was approximately equal to the floc settling velocity, leading to an overall doubling of the effective settling velocity relative to that of the individual particles. This article is protected by copyright. All rights reserved.
      PubDate: 2017-02-23T07:20:24.901036-05:
      DOI: 10.1002/2016JC012053
  • Boundary layers at a dynamic interface: Air-sea exchange of heat and mass
    • Authors: Andrew J. Szeri
      Abstract: Exchange of mass or heat across a turbulent liquid-gas interface is a problem of critical interest, especially in air-sea transfer of natural and anthropogenic gases involved in the study of climate. The goal in this research area is to determine the gas flux from air to sea or vice versa. For sparingly soluble non-reactive gases, this is controlled by liquid phase turbulent velocity fluctuations that act on the thin species concentration boundary layer on the liquid side of the interface. If the fluctuations in surface-normal velocity wʹ and gas concentration cʹ are known, then it is possible to determine the turbulent contribution to the gas flux. However, there is no suitable fundamental direct approach in the general case where neither wʹ nor cʹ can be easily measured. A new approach is presented to deduce key aspects about the near-surface turbulent motions from measurements that can be taken by an infrared (IR) camera. An equation is derived with inputs being the surface temperature and heat flux, and a solution method developed for the surface–normal strain experienced over time by boundary layers at the interface. Because the thermal and concentration boundary layers experience the same near-surface fluid motions, the solution for the surface-normal strain determines the gas flux or gas transfer velocity. Examples illustrate the approach in the cases of complete surface renewal, partial surface renewal, and insolation. The prospects for use of the approach in flows characterized by sheared interfaces or rapid boundary layer straining are explored. This article is protected by copyright. All rights reserved.
      PubDate: 2017-02-22T03:40:43.452951-05:
      DOI: 10.1002/2016JC012312
  • Influence of snow depth and surface flooding on light transmission through
           Antarctic pack ice
    • Authors: Stefanie Arndt; Klaus M. Meiners, Robert Ricker, Thomas Krumpen, Christian Katlein, Marcel Nicolaus
      Abstract: Snow on sea ice alters the properties of the underlying ice cover as well as associated physical and biological processes at the interfaces between atmosphere, sea ice and ocean. The Antarctic snow cover persists during most of the year and contributes significantly to the sea-ice mass due to the widespread surface flooding and related snow-ice formation. Snow also enhances the sea-ice surface reflectivity of incoming shortwave radiation and determines therefore the amount of light being reflected, absorbed, and transmitted to the upper ocean.Here, we present results of a case study of spectral solar radiation measurements under Antarctic pack ice with an instrumented Remotely Operated Vehicle in the Weddell Sea in 2013. In order to identify the key variables controlling the spatial distribution of the under-ice light regime, we exploit under-ice optical measurements in combination with simultaneous characterization of surface properties, such as sea-ice thickness and snow depth. Our results reveal that the distribution of flooded and non-flooded sea-ice areas dominates the spatial scales of under-ice light variability for areas smaller than 100m-by-100m. However, the heterogeneous and highly metamorphous snow on Antarctic pack ice obscures a direct correlation between the under-ice light field and snow depth. Compared to the Arctic, light levels under Antarctic pack ice are extremely low during spring (< 0.1%). This is mostly a result of the distinctly different dominant sea ice and snow properties with seasonal snow cover (including strong surface melt and summer melt ponds) in the Arctic and a year-round snow cover and widespread surface flooding in the Southern Ocean. This article is protected by copyright. All rights reserved.
      PubDate: 2017-02-21T03:30:31.989957-05:
      DOI: 10.1002/2016JC012325
  • Impact of the Ice Strength Formulation on the Performance of a Sea Ice
           Thickness Distribution Model in the Arctic
    • Authors: Mischa Ungermann; L. Bruno Tremblay, Torge Martin, Martin Losch
      Abstract: The impact of a subgrid-scale ice thickness distribution (ITD) and two standard ice strength formulations on simulated Arctic sea ice climate is investigated. To this end different model configurations with and without an ITD were tuned by minimizing the weighted mean error between the simulated and observed sea ice concentration, thickness and drift speed with an semi-automatic parameter optimization routine. The standard ITD and ice strength parameterization lead to larger errors when compared to the simple single-category model with an ice strength parameterization based on the mean ice thickness. Interestingly, the simpler ice strength formulation, which depends linearly on the mean ice thickness, also reduces the model-observation error when using an ITD. For the ice strength parameterization that makes use of the ITD, the effective ice strength depends strongly on the number of thickness categories, so that introducing more categories can lead to overall thicker ice that is more easily deformed. This article is protected by copyright. All rights reserved.
      PubDate: 2017-02-20T07:15:41.250358-05:
      DOI: 10.1002/2016JC012128
  • Long term impacts of ocean wave-dependent roughness on global climate
    • Authors: Tomoya Shimura; Nobuhito Mori, Tetsuya Takemi, Ryo Mizuta
      Abstract: Ocean surface waves can play an active role in climate systems, but they are often ignored in Global Climate Models (GCMs). Wave-dependent surface roughness was implemented within the Atmospheric GCM (MRI-AGCM) using the spectral wave model WAVEWATCH III. Two types of wave-dependent roughness, due to wave steepness and to wave age, were considered. Climate simulations with wave-dependent roughness were compared to simulations with just wind speed-dependent roughness. In climate simulation with wave steepness-dependent roughness, the spatial distribution of roughness is correlated to that of swell dominance. In simulation with wave age-dependent roughness, the spatial distribution of roughness is correlated to that of wind direction stationarity. Both simulations show reduced roughness in the tropics, which leads to an enhancement of surface wind speeds by up to 15%; these enhanced wind speeds are closer to observations compared with the baseline simulation with just wind speed-dependent roughness. We find that the reduced roughness and the enhanced wind speeds in the tropics lead to significant changes in atmospheric circulation, as in Hadley circulation and precipitation. The characteristic responses of the Hadley circulation and precipitation to changing sea surface roughness are presented. This article is protected by copyright. All rights reserved.
      PubDate: 2017-02-20T07:15:40.051059-05:
      DOI: 10.1002/2016JC012621
  • Observations and a linear model of water level in an interconnected
           inlet-bay system
    • Authors: Alfredo L. Aretxabaleta; Neil K. Ganju, Bradford Butman, Richard P. Signell
      Abstract: A system of barrier islands and back-barrier bays occurs along southern Long Island, New York and in many coastal areas worldwide. Characterizing the bay physical response to water level fluctuations is needed to understand flooding during extreme events and evaluate their relation to geomorphological changes. Offshore sea level is one of the main drivers of water level fluctuations in semi-enclosed back-barrier bays. We analyzed observed water levels (Oct. 2007-Nov. 2015) and developed analytical models to better understand bay water level along southern Long Island. An increase (∼0.02 m change in 0.17 m amplitude) in the dominant M2 tidal amplitude (containing the largest fraction of the variability) was observed in Great South Bay during mid-2014. The observed changes in both tidal amplitude and bay water level transfer from offshore were related to the dredging of nearby inlets and possibly the changing size of a breach across Fire Island caused by Hurricane Sandy (after Dec. 2012). The bay response was independent of the magnitude of the fluctuations (e.g., storms) at a specific frequency. An analytical model that incorporates bay and inlet dimensions reproduced the observed transfer function in Great South Bay and surrounding areas. The model predicts the transfer function in Moriches and Shinnecock bays where long-term observations were not available. The model is a simplified tool to investigate changes in bay water level and enables the evaluation of future conditions and alternative geomorphological settings. This article is protected by copyright. All rights reserved.
      PubDate: 2017-02-15T10:45:39.49788-05:0
      DOI: 10.1002/2016JC012318
  • Wind modulation of upwelling at the shelf-break front off Patagonia:
           Observational evidence
    • Authors: M. M. Carranza; S. T. Gille, A. R. Piola, M. Charo, S. I. Romero
      Abstract: The South-Atlantic Patagonian shelf is the largest chlorophyll-a (Chl-a) hot spot in Southern Ocean color images. While a persistent 1500-km long band of high Chl-a along the shelf-break front (SBF) is indicative of upwelling, the mechanisms that drive it are not entirely known. Along-front wind oscillations can enhance upwelling and provide a nutrient pumping mechanism at shelf break fronts of western boundary currents. Here, we assess wind-induced upwelling at the SBF off Patagonia from daily satellite Chl-a and winds, historical hydrographic observations, cross-shelf Chl-a fluorescence transects from two cruises, and in situ winds and water column structure from a mooring site.Satellite Chl-a composites segregated by along-front wind direction indicate that surface Chl-a is enhanced at the SBF with southerly winds and suppressed with northerly winds. Northerly winds also result in enhanced Chl-a further offshore (∼25-50 km). Synoptic transects as well as mean hydrographic sections segregated by along-front winds show isopycnals tilted upward for southerly winds. Spring observations from the mooring also suggest that southerly winds destratify the water column and northerly winds restratify, in agreement with Ekman transport interacting with the front. Moreover, changes in water column temperature lag along-front wind forcing by 2-4 days. Our results suggest that oscillations in along-front winds, on timescales typical of atmospheric storms (2-10 days), can significantly modulate the upwelling and Chl-a concentrations at the SBF off Patagonia, revealing the importance of wind-induced upwelling for shelf-slope exchange at shelf-break fronts of western boundary currents. This article is protected by copyright. All rights reserved.
      PubDate: 2017-02-15T10:40:42.038874-05:
      DOI: 10.1002/2016JC012059
  • The Atlantic Water boundary current north of Svalbard in late-summer
    • Authors: M. Dolores Pérez-Hernández; Robert S. Pickart, Vladimir Pavlov, Kjetil Våge, Randi Ingvaldsen, Arild Sundfjord, Angelika H. H. Renner, Daniel J. Torres, Svetlana Y. Erofeeva
      Abstract: Data from a shipboard hydrographic/velocity survey carried out in September 2013 of the region north of Svalbard in the Nansen Basin are analyzed to characterize the Atlantic Water (AW) boundary current as it flows eastward along the continental slope. Eight meridional transects across the current, spanning an alongstream distance of 180 km, allow for a detailed description of the current and the regional water masses. During the survey the winds were light and there was no pack-ice. The mean section reveals that the boundary current was O(40 km) wide, surface-intensified, with a maximum velocity of 20 cm/s. Its mean transport during the survey was 3.11 ± 0.33 Sv, of which 2.31 ± 0.29 Sv was AW. This suggests that the two branches of AW entering the Arctic Ocean via Fram Strait – the Yermak Plateau branch and the Svalbard branch – have largely combined into a single current by 30oE. At this location the boundary current meanders with a systematic change in its kinematic structure during offshore excursions. A potential vorticity analysis indicates that the flow is baroclinically unstable, consistent with previous observations of AW anti-cyclones offshore of the current as well as the presence of a near-field cyclone in this data set. Our survey indicates that only a small portion of the boundary current is diverted into the Kvitøya Trough (0.17 ± 0.08 Sv), and that the AW temperature/salinity signal is quickly eroded within the trough. This article is protected by copyright. All rights reserved.
      PubDate: 2017-02-15T10:40:32.0751-05:00
      DOI: 10.1002/2016JC012486
  • Evolution of the East Greenland Current from Fram Strait to Denmark
           Strait: Synoptic measurements from summer 2012
    • Authors: L. Håvik; R. S. Pickart, K. Våge, D. Torres, A. M. Thurnherr, A. Beszczynska-Möller, W. Walczowski, W.-J. von Appen
      Abstract: We present measurements from two shipboard surveys conducted in summer 2012 that sampled the rim current system around the Nordic Seas from Fram Strait to Denmark Strait. The data reveal that, along a portion of the western boundary of the Nordic Seas, the East Greenland Current (EGC) has three distinct components. In addition to the well-known shelfbreak branch, there is an inshore branch on the continental shelf as well as a separate branch offshore of the shelfbreak. The inner branch contributes significantly to the overall freshwater transport of the rim current system, and the outer branch transports a substantial amount of Atlantic-origin Water equatorward. Supplementing our measurements with historical hydrographic data, we argue that the offshore branch is a direct recirculation of the western branch of the West Spitsbergen Current in Fram Strait. The total transport of the shelfbreak EGC (the only branch sampled consistently in all of the sections) decreased towards Denmark Strait. The estimated average transport of dense overflow water (σθ > 27.8 kg/m3 and Θ > 0°C) in the shelfbreak EGC was 2.8 ± 0.7 Sv, consistent with previous moored measurements. For the three sections that crossed the entire EGC system the freshwater flux, relative to a salinity of 34.8, ranged from 127 ± 13∼mSv to 81 ± 8 mSv. The hydrographic data reveal that, between Fram Strait and Denmark Strait, the core of the Atlantic-origin Water in the shelfbreak EGC cools and freshens but changes very little in density. This article is protected by copyright. All rights reserved.
      PubDate: 2017-02-15T10:40:26.986928-05:
      DOI: 10.1002/2016JC012228
  • Salt wedge dynamics lead to enhanced sediment trapping within
           side-embayments in high-energy estuaries
    • Authors: Brian Yellen; Jonathan D. Woodruff, David K. Ralston, D. G. MacDonald, D. S. Jones
      Abstract: Off-river coves and embayments provide accommodation space for sediment accumulation, particularly for sandy estuaries where high energy in the main channel prevents significant long-term storage of fine-grained material. Seasonal sediment inputs to Hamburg Cove in the Connecticut River Estuary (USA) were monitored to understand the timing and mechanisms for sediment storage there. Unlike in freshwater tidal coves, sediment was primarily trapped here during periods of low discharge, when the salinity intrusion extended upriver to the cove entrance. During periods of low discharge and high sediment accumulation, deposited sediment displayed geochemical signatures consistent with a marine source. Numerical simulations reveal low discharge conditions provide several important characteristics that maximize sediment trapping. First, these conditions allow the estuarine turbidity maximum (ETM) to be located in the vicinity of the cove entrance, which increases sediment concentrations during flood tide. Second, the saltier water in the main channel can enter the cove as a density current, enhancing near-bed velocities and resuspending sediment, providing an efficient delivery mechanism. Finally, higher salinity water accumulates in the deep basin of the cove, creating a stratified region that becomes decoupled from ebb currents, promoting retention of sediment in the cove. This process of estuarine-enhanced sediment accumulation in off-river coves will likely extend upriver during future sea-level rise. This article is protected by copyright. All rights reserved.
      PubDate: 2017-02-15T10:35:31.920647-05:
      DOI: 10.1002/2016JC012595
  • Subtidal circulation in a deep-silled fjord: Douglas Channel, British
    • Authors: Di Wan; Charles G. Hannah, Michael G.G. Foreman, Stan Dosso
      Abstract: Douglas Channel, a deep fjord on the west coast of British Columbia, Canada, is the main waterway in the fjord system that connects the town of Kitimat to Queen Charlotte Sound and Hecate Strait. A 200 m depth sill divides Douglas Channel into an outer and an inner basin. This study examines the low-frequency (from seasonal to meteorological bands) circulation in Douglas Channel from data collected at three moorings deployed during 2013 – 2015. The deep flows are dominated by a yearly renewal that takes place from May/June to early September. A dense bottom layer with a thickness of 100 m that cascades through the system at the speed of 0.1 – 0.2 m s−1, which is consistent with gravity currents. Estuarine flow dominates the circulation above the sill-depth, and the observed landward net volume flux suggests that it is necessary to include the entire complex channel network to fully understand the estuarine circulation in the system. The influence of the wind forcing on the sub-tidal circulation is not only at the surface, but also at mid-depth. The along-channel wind dominates the surface current velocity fluctuations and the sealevel response to the wind produces a velocity signal at 100 – 120 m in the counter-wind direction. Overall, the circulation in the seasonal and the meteorological bands is a mix of estuarine flow, direct wind driven flow, and the barotropic and baroclinic responses to changes to the surface pressure gradient caused by the wind stress. This article is protected by copyright. All rights reserved.
      PubDate: 2017-02-15T10:35:29.040964-05:
      DOI: 10.1002/2016JC012022
  • Slope-induced tidal straining: Analysis of rotational effects
    • Authors: Kirstin Schulz; Takahiro Endoh, Lars Umlauf
      Abstract: Tidal straining is known to be an important factor for the generation of residual currents and transports of suspended matter in the coastal ocean. Recent modeling studies and field experiments have revealed a new type of “slope-induced” tidal straining, in which the horizontal density gradient required for this process is induced by the presence of a slope rather than by river runoff (as in classical tidal straining). Slope-induced tidal straining is investigated here with the help of an idealized numerical model, and results are compared to a recent data set from the East China Sea providing first direct observational evidence. The focus of this study is on the effect of rotation that was ignored in previous investigations. The model is shown to reproduce the key features of the observations, in particular the strain-induced generation of unstable stratification in the bottom boundary layer during periods of upslope flow. Rotation effects are found to significantly reduce the upslope tidal pumping of suspended material but also give rise to a newly identified pumping mechanism that results in a vigorous transport of suspended material along the slope. It is shown that slope-induced tidal straining is likely to be relevant for a wide range of oceanic slopes exposed to tidal motions. This article is protected by copyright. All rights reserved.
      PubDate: 2017-02-15T10:30:25.207758-05:
      DOI: 10.1002/2016JC012448
  • Comparison of the ocean surface vector winds from atmospheric reanalysis
           and scatterometer-based wind products over the Nordic Seas and the
           northern North Atlantic and their application for ocean modeling
    • Authors: Dmitry S. Dukhovskoy; Mark A. Bourassa, Guðrún Nína Petersen, John Steffen
      Abstract: Ocean surface vector wind fields from reanalysis data sets and scatterometer-derived gridded products are analyzed over the Nordic Seas and the northern North Atlantic for the time period from 2000 to 2009. The data sets include the National Center for Environmental Prediction Reanalysis 2 (NCEPR2), Climate Forecast System Reanalysis (CFSR), Arctic System Reanalysis (ASR), Cross-Calibrated Multi-Platform (CCMP) wind product version 1.1 and recently released version 2.0, and QuikSCAT. The goal of the study is to assess discrepancies across the wind vector fields in the data sets and demonstrate possible implications of these differences for ocean modeling. Large-scale and mesoscale characteristics of winds are compared at interannual, seasonal, and synoptic timescales. A cyclone tracking methodology is developed and applied to the wind fields to compare cyclone characteristics in the data sets. Additionally, the winds are evaluated against observations collected from meteorological buoys deployed in the Iceland and Irminger Seas. The agreement among the wind fields is better for longer time and larger spatial scales. The discrepancies are clearly apparent for synoptic timescales and mesoscales. CCMP, ASR, and CFSR show the closest overall agreement with each other. Substantial biases are found in the NCEPR2 winds. Numerical sensitivity experiments are conducted with a coupled ice-ocean model forced by different wind fields. The experiments demonstrate differences in the net surface heat fluxes during storms. In the experiment forced by NCEPR2 winds, there are discrepancies in the large-scale wind-driven ocean dynamics compared to the other experiments. This article is protected by copyright. All rights reserved.
      PubDate: 2017-02-15T03:20:36.788653-05:
      DOI: 10.1002/2016JC012453
  • Semidiurnal internal tide energy fluxes and their variability in a global
           ocean model and moored observations
    • Authors: Joseph K. Ansong; Brian K. Arbic, Matthew H. Alford, Maarten C. Buijsman, Jay F. Shriver, Zhongxiang Zhao, James G. Richman, Harper L. Simmons, Patrick G. Timko, Alan J. Wallcraft, Luis Zamudio
      Abstract: We examine the temporal means and variability of the semidiurnal internal tide energy fluxes in 1/25° global simulations of the HYbrid Coordinate Ocean Model (HYCOM) and in a global archive of 79 historical moorings. Low-frequency flows, a major cause of internal tide variability, have comparable kinetic energies at the mooring sites in model and observations. The computed root-mean-square (RMS) variability of the energy flux is large in both model and observations and correlates positively with the time-averaged flux magnitude. Outside of strong generation regions, the normalized RMS variability (the RMS variability divided by the mean) is nearly independent of the flux magnitudes in the model, and of order 23% or more in both the model and observations. The spatially averaged flux magnitudes in observations and the simulation agree to within a factor of about 1.4 and 2.4 for vertical modes-1 and -2 respectively. The difference in energy flux computed from the full-depth model output versus model output subsampled at mooring instrument depths is small. The global historical archive is supplemented with six high-vertical resolution moorings from the Internal Waves Across the Pacific (IWAP) experiment. The model fluxes agree more closely with the high-resolution IWAP fluxes than with the historical mooring fluxes. The high variability in internal tide energy fluxes implies that internal tide fluxes computed from short observational records should be regarded as realizations of a highly variable field, not as “means” that are indicative of conditions at the measurement sites over all time. This article is protected by copyright. All rights reserved.
      PubDate: 2017-02-14T03:35:39.009494-05:
      DOI: 10.1002/2016JC012184
  • Observations of flooding and snow-ice formation in a thinner Arctic sea
           ice regime during the N-ICE2015 campaign: Influence of basal ice melt and
    • Authors: Christine Provost; Nathalie Sennéchael, Jonas Miguet, Polona Itkin, Anja Rösel, Zoé Koenig, Nicolas Villacieros-Robineau, Mats A. Granskog
      Abstract: Seven ice mass balance instruments deployed near 83°N on different first-year and second-year ice floes, representing variable snow and ice conditions, documented the evolution of snow and ice conditions in the Arctic Ocean north of Svalbard in Jan-Mar 2015. Frequent profiles of temperature and thermal diffusivity proxy were recorded to distinguish changes in snow depth and ice thickness with 2 cm vertical resolution. Four instruments documented flooding and snow-ice formation. Flooding was clearly detectable in the simultaneous changes in thermal diffusivity proxy, increased temperature and heat propagation through the underlying ice. Slush then progressively transformed into snow-ice. Flooding resulted from two different processes; i) after storm-induced break-up of snow-loaded floes and ii) after loss of buoyancy due to basal ice melt. In the case of break-up, when the ice was cold and not permeable, rapid flooding, probably due to lateral intrusion of seawater, led to slush and snow-ice layers at the ocean freezing temperature (-1.88°C). After the storm the instruments documented basal sea-ice melt over warm Atlantic waters and ocean-to-ice heat flux peaked at up to 400 Wm−2. The warm ice was then permeable and flooding was more gradual probably involving vertical intrusion of brines and led to colder slush and snow-ice (-3°C). The N-ICE2015 campaign provided the first documentation of significant flooding and snow-ice formation in the Arctic ice pack as the slush partially refroze. Snow-ice formation may become a more-frequently observed process in a thinner-ice Arctic. This article is protected by copyright. All rights reserved.
      PubDate: 2017-02-14T03:35:35.750566-05:
      DOI: 10.1002/2016JC012011
  • Variability and change of sea level and its components in the Indo-Pacific
           region during the altimetry era
    • Authors: Quran Wu; Xuebin Zhang, John A. Church, Jianyu Hu
      Abstract: Previous studies have shown that regional sea level exhibits interannual and decadal variations associated with the modes of climate variability. A better understanding of those low-frequency sea level variations benefits the detection and attribution of climate change signals. Nonetheless, the contributions of thermosteric, halosteric, and mass sea level components to sea level variability and trend patterns remain unclear. By focusing on signals associated with dominant climate modes in the Indo-Pacific region, we estimate the interannual and decadal fingerprints and trend of each sea level component utilizing a multivariate linear regression of two adjoint-based ocean reanalyses. Sea level interannual, decadal, and trend patterns primarily come from thermosteric sea level (TSSL). Halosteric sea level (HSSL) is of regional importance in the Pacific Ocean on decadal time scale, and dominates sea level trends in the northeast subtropical Pacific. The compensation between TSSL and HSSL is identified in their decadal variability and trends. The interannual and decadal variability of temperature generally peak at subsurface around 100 m, but that of salinity tend to be surface intensified. Decadal temperature and salinity signals extend deeper into the ocean in some regions than their interannual equivalents. Mass sea level (MassSL) is critical for the interannual and decadal variability of sea level over shelf seas. Inconsistencies exist in MassSL trend patterns among various estimates. This study highlights regions where multiple processes work together to control sea level variability and change. Further work is required to better understand the interaction of different processes in those regions. This article is protected by copyright. All rights reserved.
      PubDate: 2017-02-14T03:30:34.857658-05:
      DOI: 10.1002/2016JC012345
  • Estimating surface water mixing ratios using salinity and potential
           alkalinity in the Kuroshio-Oyashio mixed water regions
    • Authors: Shigeho Kakehi; Shin-ichi Ito, Taku Wagawa
      Abstract: We used salinity and potential alkalinity data from hydrographic observations to investigate surface mixing ratios in the Kuroshio-Oyashio mixed water region in the western North Pacific. In addition to mixing between the Kuroshio Extension (KE) and Oyashio, we assessed freshwater input/removal. A mixing scenario with three end members was assumed in the surface layer ≤ 100 m. The results indicate that water masses near the sea surface in the Kuroshio-Oyashio mixed water region were mainly the result of mixing between the KE and Oyashio. The freshwater contribution was approximately 2.2% at depth 10 m. The volume of freshwater estimated from this percentage was consistent with surface water budgets estimated from reanalysis precipitation and evaporation data. The estimated mixing ratio of the KE (rk) along the quasi-stationary jet in the western North Pacific, which splits from the KE and flows northeastward toward the subarctic region, decreased downstream from 95% to 27% in only 42 days, suggesting that water properties were changed rapidly by mixing. Correlation between rk around the quasi-stationary jet and nutrients was significantly negative in the layer where photosynthesis was negligible, indicating that the mixing between the KE and Oyashio is an important determinant of the horizontal distribution of nutrients in this area. This article is protected by copyright. All rights reserved.
      PubDate: 2017-02-14T03:30:26.392904-05:
      DOI: 10.1002/2016JC012268
  • The study of droplet-laden turbulent airflow over waved water surface by
           direct numerical simulation
    • Authors: O.A. Druzhinin; Yu.I. Troitskaya, S.S. Zilitinkevich
      Abstract: The objective of the present paper is to elucidate possible effects of sea spray on the momentum transfer in marine boundary layer under strong wind-forcing conditions by performing direct numerical simulation (DNS) of turbulent, droplet-laden air-flow over a waved water surface. Three-dimensional, turbulent Couette air-flow is considered in DNS as a model of a constant-flux layer in the atmospheric surface layer. Two-dimensional stationary waves at the water surface are prescribed and assumed to be unaffected by the air-flow and/or droplets. Droplets are considered as non-deformable spheres and tracked in a Lagrangian framework, and their impact on the carrier flow is modeled with the use of a point-force approximation. The results show that drops dynamics and their impact on the carrier air-flow is controlled by the drops velocity at injection, the ratio of drops gravitational settling velocity versus the product of air friction velocity and Karman constant (Vg/κu∗), and the wave slope, ka. Drops injected into the flow with the surrounding air-flow velocity reduce the turbulent air-stress and increase mean air velocity as compared to the droplet-free case. On the other hand, the opposite effect is observed for drops injected with velocity equal to the water surface velocity, which increase the turbulent air-stress and reduce the mean wind velocity. This modification of the air-flow by drops is most pronounced for the ratio Vg/κu∗≈1, increases with drops mass loading and is reduced for steeper waves and smaller settling velocity. This article is protected by copyright. All rights reserved.
      PubDate: 2017-02-11T03:16:27.038481-05:
      DOI: 10.1002/2016JC012134
  • Ocean circulation and sea-ice thinning induced by melting ice shelves in
           the Amundsen Sea
    • Authors: Nicolas C. Jourdain; Pierre Mathiot, Nacho Merino, Gaël Durand, Julien Le Sommer, Paul Spence, Pierre Dutrieux, Gurvan Madec
      Abstract: A 1/12° ocean model configuration of the Amundsen Sea sector is developed to better understand the circulation induced by ice shelf melt and the impacts on the surrounding ocean and sea ice. Eighteen sensitivity experiments to drag and heat exchange coefficients at the ice shelf/ocean interface are performed. The total melt rate simulated in each cavity is function of the thermal Stanton number, and for a given thermal Stanton number, melt is slightly higher for lower values of the drag coefficient. Sub ice shelf melt induces a thermohaline circulation that pumps warm circumpolar deep water into the cavity. The related volume flux into a cavity is 100 to 500 times stronger than the melt volume flux itself. Ice-shelf melt also induces a coastal barotropic current that contributes 45±12% of the total simulated coastal transport. Due to the presence of warm circumpolar deep waters, the melt-induced inflow typically brings 4 to 20 times more heat into the cavities than the latent heat required for melt. For currently observed melt rates, approximately 6% to 31% of the heat that enters a cavity with melting potential is actually used to melt ice shelves. For increasing sub ice shelf melt rates, the transport in the cavity becomes stronger, and more heat is pumped from the deep layers to the upper part of the cavity then advected towards the ocean surface in front of the ice shelf. Therefore, more ice shelf melt induces less sea ice volume near the ice sheet margins. This article is protected by copyright. All rights reserved.
      PubDate: 2017-02-11T03:16:23.324722-05:
      DOI: 10.1002/2016JC012509
  • On the response of the Lorenz energy cycle for the Southern Ocean to
           intensified westerlies
    • Authors: Yang Wu; Zhaomin Wang, Chengyan Liu
      Abstract: The potential impact of intensified westerlies on the Lorenz Energy Cycle for the Southern Ocean is examined by employing a global eddy-permitting ocean-sea ice model. Two idealized sensitivity experiments are designed for this purpose: one is driven by 1992 forcing with weaker westerlies and the other driven by 1998 forcing with stronger westerlies. The intensified westerlies lead to the most significant increase of about 30% in the eddy kinetic energy (EKE) reservoir, followed by the mean kinetic energy (MKE) reservoir increase (17.9%), eddy available potential energy (EAPE) reservoir increase (8.6%), and mean available potential energy (MAPE) reservoir increase (6.5%). In contrast, the increases in the generations of kinetic energy and available potential energy are quite similar, ranging from 21% for EAPE generation to 26% for MKE generation. There are considerablly increased energy transfers from MKE to MAPE (about 75%) and from MAPE to EAPE (about 78%), reflecting greatly enhanced baroclinic instability pathway.The conversion rates are strongly influenced by large topography; in particular, a relatively large energy conversion from EKE to MKE exists in the regions associated with large topography, in contrast to the energy flow from MKE to EKE over the broad Southern Ocean. Under stronger wind forcing, all energy conversions are enhanced, and the increases in the conversion rates from EAPE to EKE and from EKE to MKE are more prominent than the increases from MKE to MAPE and from MAPE to EAPE near large topography. This article is protected by copyright. All rights reserved.
      PubDate: 2017-02-11T03:16:02.623644-05:
      DOI: 10.1002/2016JC012539
  • Estimating the distribution of colored dissolved organic matter during the
           Southern Ocean gas exchange experiment using four-dimensional variational
           data assimilation
    • Authors: C. E. Del Castillo; S. Dwivedi, T. W. N. Haine, D. T. Ho
      Abstract: We diagnosed the effect of various physical processes on the distribution of mixed-layer colored dissolved organic matter (CDOM) and a sulfur hexafluoride (SF6) tracer during the Southern Ocean Gas Exchange Experiment (SO GasEx). The biochemical upper ocean state estimate uses in situ and satellite biochemical and physical data in the study region, including CDOM (absorption coefficient and spectral slope), SF6, hydrography, and sea level anomaly. Modules for photo-bleaching of CDOM and surface transport of SF6 were coupled with an ocean circulation model for this purpose. The observed spatial and temporal variations in CDOM were captured by the state estimate without including any new biological source term for CDOM, assuming it to be negligible over the 26 days of the state estimate. Thermocline entrainment and photobleaching acted to diminish the mixed layer CDOM with time scales of 18 and 16 days, respectively. Lateral advection of CDOM played a dominant role and increased the mixed layer CDOM with a time scale of 12 days, whereas lateral diffusion of CDOM was negligible. A Lagrangian view on the CDOM variability was demonstrated by using the SF6 as a weighting function to integrate the CDOM fields. This and similar data assimilation methods can be used to provide reasonable estimates of optical properties, and other physical parameters over the short term duration of a research cruise, and help in the tracking of tracer releases in large scale oceanographic experiments, and in oceanographic process studies. This article is protected by copyright. All rights reserved.
      PubDate: 2017-02-11T03:15:57.046331-05:
      DOI: 10.1002/2016JC012406
  • Distribution of water masses and meltwater on the continental shelf near
           the Totten and Moscow University ice shelves
    • Authors: Alessandro Silvano; Stephen R. Rintoul, Beatriz Peña-Molino, Guy D. Williams
      Abstract: Warm waters flood the continental shelf of the Amundsen and Bellingshausen seas in West Antarctica, driving rapid basal melt of ice shelves. In contrast, waters on the continental shelf in East Antarctica are cooler and ice shelves experience relatively low rates of basal melt. An exception is provided by the Totten and Moscow University ice shelves on the Sabrina Coast, where satellite-derived basal melt rates are comparable to West Antarctica. Recent oceanographic observations have revealed that relatively warm (∼ -0.4°C) modified Circumpolar Deep Water (mCDW) enters the cavity beneath the Totten Ice Shelf through a 1100 m deep trough, delivering sufficient heat to drive rapid basal melt. Here we use observations from a recent summer survey to show that mCDW is widespread on the continental shelf of the Sabrina Coast, forming a warm (up to 0.3°C) and saline (34.5-34.6) bottom layer overlaid by cold (∼ freezing point) and fresh (salinity ∼ 34.3) Winter Water. Dense Shelf Water is not observed. A 1000 deep m trough allows water at -1.3°C to reach the Moscow University ice-shelf cavity to drive basal melt. Freshening by addition of glacial meltwater is widespread on the southern shelf at depths above 300-400 m, with maximum meltwater concentrations up to 4-5 ml l−1 observed in outflows from the ice-shelf cavities. Our observations indicate that the ocean properties on the Sabrina Coast more resemble those found on the continental shelf of the Amundsen and Bellingshausen seas than those typical of East Antarctica. This article is protected by copyright. All rights reserved.
      PubDate: 2017-02-11T03:15:51.487122-05:
      DOI: 10.1002/2016JC012115
  • Seasonal and interannual variability of the eastern tropical Pacific fresh
    • Authors: S. Guimbard; N. Reul, B. Chapron, M. Umbert, C. Maes
      Abstract: The Eastern Pacific Fresh Pool (EPFP) is a large region of low Sea Surface Salinity (SSS) defined by values lower than 34 practical salinity scale within [5°S-30°N, 75°W-180°W]. The fresh pool dynamically responds to strong regional and seasonally varying ocean-atmosphere-land interactions (including monsoon rain, trade and gap winds and strong currents). Using more than five years of Soil Moisture and Ocean Salinity (SMOS) satellite sea surface salinity (SSS) and complementary satellite wind, rain, currents, and sea surface temperature data together with a historical ensemble of in situ products, the present study explores the seasonal and interannual dynamics of the fresh pool over the period 2004-2015. An important interannual variability of the maximal surface extension of the EPFP over the past decade is revealed with two extreme events (2012, 2015) occurring during the SMOS satellite period. These extremes are found to be related to the El Niño-Southern Oscillation (ENSO) phases and associated anomalies of precipitation, surface currents and trade wind in the central Pacific. In 2012 (La Niña), stronger trade winds coupled with a deficit of precipitation induced a minimum extension of the pool during the rainy season. Whereas, during the strong El Niño 2014-2015, the EPFP extension reached an unprecedented maximum value. A modification of the atmospheric freshwater fluxes and ocean surface currents during winter 2014 are found to have favored the onset of this abnormal fresh event. This article is protected by copyright. All rights reserved.
      PubDate: 2017-02-11T03:10:34.407101-05:
      DOI: 10.1002/2016JC012130
  • One-dimensional evolution of the upper water column in the Atlantic sector
           of the Arctic Ocean in winter
    • Authors: Ilker Fer; Algot K. Peterson, Achim Randelhoff, Amelie Meyer
      Abstract: A one-dimensional model is employed to reproduce the observed time evolution of hydrographic properties in the upper water column during winter, between 26 January and 11 March 2015, in a region north of Svalbard in the Nansen Basin of the Arctic Ocean. From an observed initial state, vertical diffusion equations for temperature and salinity give the hydrographic conditions at a later stage. Observations of microstructure are used to synthesize profiles of vertical diffusivity, K, representative of varying wind forcing conditions. The ice-ocean heat and salt fluxes at the ice-ocean interface are implemented as external source terms, estimated from the salt and enthalpy budgets, using friction velocity from the Rossby similarity drag relation, and the ice core temperature profiles. We are able to reproduce the temporal evolution of hydrography satisfactorily for two pairs of measured profiles, suggesting that the vertical processes dominated the observed changes. Sensitivity tests reveal a significant dependence on K. Variation in other variables, such as the temperature gradient of the sea ice, the fraction of heat going to ice melt, and the turbulent exchange coefficient for heat are relatively less important. The increase in salinity as a result of freezing and brine release is approximately 10%, significantly less than that due to entrainment (90%) from beneath the mixed layer. Entrainment was elevated during episodic storm events, leading to melting. The results highlight the contribution of storms to mixing in the upper Arctic Ocean and its impact on ice melt and mixed-layer salt and nutrient budgets. This article is protected by copyright. All rights reserved.
      PubDate: 2017-02-11T03:10:33.062768-05:
      DOI: 10.1002/2016JC012431
  • Understanding feedbacks between ocean acidification and coral reef
    • Authors: Yuichiro Takeshita
      Abstract: Biogeochemical feedbacks from benthic metabolism have been hypothesized as a potential mechanism to buffer some effects of ocean acidification on coral reefs. The article in JGR-Oceans by DeCarlo et al. demonstrates the importance of benthic community health on this feedback from Dongsha Atoll in the South China Sea. This article is protected by copyright. All rights reserved.
      PubDate: 2017-02-11T03:10:25.005992-05:
      DOI: 10.1002/2017JC012740
  • Physical forcing of late summer chlorophyll-a blooms in the oligotrophic
           eastern North Pacific
    • Authors: Takahiro Toyoda; Suguru Okamoto
      Abstract: We investigated physical forcing of late summer chlorophyll-a (chl-a) blooms in the oligotrophic eastern North Pacific Ocean by using ocean reanalysis and satellite data. Relatively large chl-a blooms as defined in this study occurred in August–October following sea surface temperature (SST) anomaly (SSTA) decreases, mixed layer deepening, and temperature and salinity increases at the bottom of the mixed layer. These physical conditions were apparently induced by the entrainment of subsurface water resulting from the destabilization of the surface layer caused by anomalous northward Ekman transport of subtropical waters of higher salinity. Salinity-normalized total alkalinity data provide supporting evidence for nutrient supply by the entrainment process. We next investigated the impact of including information about the entrainment on bloom identification. The results of analyses using reanalysis data and of those using only satellite data showed large SSTA decreases when the northward Ekman salinity transports were large, implying that the entrainment of subsurface water is well represented in both types of data. After surface-destabilizing conditions were established, relatively high surface chl-a concentrations were observed. The use of SST information can further improve the detection of high chl-a concentrations. Although the detection of high chl-a concentrations would be enhanced by finer data resolution and the inclusion of biogeochemical parameters in the ocean reanalysis, our results obtained by using existing reanalysis data as well as recent satellite data are valuable for better understanding and prediction of lower trophic ecosystem variability. This article is protected by copyright. All rights reserved.
      PubDate: 2017-02-09T10:56:13.757425-05:
      DOI: 10.1002/2016JC012423
  • From interannual to decadal—17 years of boundary current transports at
           the exit of the Labrador Sea
    • Authors: R. Zantopp; J. Fischer, M. Visbeck, J. Karstensen
      Abstract: Over the past 17 years, the western boundary current system of the Labrador Sea has been closely observed by maintaining the 53°N observatory (moorings and shipboard station data) measuring the top-to-bottom flow field offshore from the Labrador shelf break. Volume transports for the North Atlantic Deep Water (NADW) components were calculated using different methods, including gap filling procedures for deployment periods with suboptimal instrument coverage. On average the Deep Western Boundary Current (DWBC) carries 30.2 ± 6.6 Sv of NADW southward, which are almost equally partitioned between Labrador Sea Water (LSW, 14.9 Sv ± 3.9) and Lower North Atlantic Deep Water (LNADW, 15.3 Sv ± 3.8). The transport variability ranges from days to decades, with the most prominent multi-year fluctuations at interannual to near decadal time scales (± 5 Sv) in the LNADW overflow water mass. These long-term fluctuations appear to be in phase with the NAO-modulated wind fluctuations. The boundary current system off Labrador occurs as a conglomerate of nearly independent components, namely the shallow Labrador Current, the weakly sheared LSW range, and the deep baroclinic, bottom-intensified current core of the LNADW, all of which are part of the cyclonic Labrador Sea circulation. This structure is relatively stable over time, and the 120 km wide boundary current is constrained seaward by a weak counterflow which reduces the Deep Water export by 10 to 15%. This article is protected by copyright. All rights reserved.
      PubDate: 2017-02-09T10:51:26.67299-05:0
      DOI: 10.1002/2016JC012271
  • Eastward salinity anomaly propagation in the intermediate layer of the
           North Pacific
    • Authors: Shinya Kouketsu; Satoshi Osafune, Yuichiro Kumamoto, Hiroshi Uchida
      Abstract: An objective mapping with the data of profiling float array, maintained under the Argo project, revealed eastward propagation of long-term (>5 years) salinity anomalies in the subsurface and deep neutral density (γ) layers of 27.0–27.6 γ along the subarctic front in the North Pacific after 2000. Such propagation was previously inferred from water property variations along a few observation lines and from numerical simulations, mainly for shallow layers. In the western North Pacific, the signs of the anomalies were the same on and below the 27.0γ, whereas in the eastern North Pacific the sign on 27.0γ was opposite to those on 27.4γ. This difference was attributed mainly to slower advection in the deeper layers. These changes were larger than the standard errors inferred from the objective mapping at least. Furthermore, the variation revealed by the float array was similar to decadal changes observed along repeat ship-based observation lines, and they were also associated with changes in apparent oxygen utilization especially along 165°E. The small salinity changes in the deeper layers inferred from the float array were also detected as decadal differences in highly accurate trans-basin observations. Furthermore, because the extension of small changes into the subtropical gyre was also captured by the float and ship-based observations, the influence of the decadal changes on the isopycnal surfaces off the coast of Japan could appear relatively quickly, even in deeper layers (27.0–27.4 γ) in the North Pacific. This article is protected by copyright. All rights reserved.
      PubDate: 2017-02-09T10:51:20.162827-05:
      DOI: 10.1002/2016JC012118
  • A semianalytical MERIS green-red band algorithm for identifying
           phytoplankton bloom types in the East China Sea
    • Authors: Bangyi Tao; Zhihua Mao, Hui Lei, Delu Pan, Yan Bai, Qiankun Zhu, Zhenglong Zhang
      Abstract: A new bio-optical algorithm based on the green and red bands of the Medium Resolution Imaging Spectrometer (MERIS) is developed to differentiate the harmful algal blooms of Prorocentrum donghaiense Lu (P. donghaiense) from diatom blooms in the East China Sea (ECS). Specifically, a novel green-red index (GRI), actually an indicator for a(510) of bloom waters, is retrieved from a semi-analytical bio-optical model based on the green and red bands of phytoplankton-absorption and backscattering spectra. In addition, a MERIS-based diatom index (DIMERIS) is derived by adjusting a Moderate Resolution Imaging Spectroradiometer (MODIS) diatom index algorithm to the MERIS bands. Finally, bloom types are effectively differentiated in the feature spaces of the green-red index and DIMERIS. Compared with three previous MERIS-based quasi-analytical algorithm (QAA) algorithms and three existing classification methods, the proposed GRI and classification method have the best discrimination performance when using the MERIS data. Further validations of the algorithm by using several MERIS image series and near-concurrent in-situ observations indicate that our algorithm yields the best classification accuracy and thus can be used to reliably detect and classify P. donghaiense and diatom blooms in the ECS. This is the first time that the MERIS data have been used to identify bloom types in the ECS. Our algorithm can also be used for the successor of the MERIS, the Ocean and Land Color Instrument, which will aid the long-term observation of species succession in the ECS. This article is protected by copyright. All rights reserved.
      PubDate: 2017-02-09T10:51:05.256344-05:
      DOI: 10.1002/2016JC012368
  • The importance of lateral variability on exchange across the inner shelf
           south of Martha's Vineyard, MA
    • Authors: A. R. Kirincich; S. J. Lentz
      Abstract: This study examines the spatial variability of transport within the inner shelf south of Martha's Vineyard Massachusetts, its time and space dependence, and its importance to the total volume exchanged between the nearshore and the coastal ocean. The exchange of water across the inner shelf is often considered to be driven primarily by wind forcing, yet the effects of small-scale O(1-10 km) variability on the total exchange have not been well quantified. Using a combination of high-resolution HF radar-based surface currents and a dense array of moorings to document the lateral variability of across-shelf exchange, the cumulative wind-driven across-shelf transport over the summer stratified period was less than the volume of the inner-shelf onshore of the 25-m isobath. Along-shelf variations in the wind-driven exchange were as large as the spatial mean of the wind-driven exchange. A spatially varying time-averaged circulation caused by tidal rectification resulted in across-shelf exchange larger in magnitude than, and independent of, the integrated wind-forced exchange. Coherent submesoscale eddies also occurred frequently within the domain due to flow-topography effects onshore and horizontal density gradients offshore, generally with lifespans shorter than 10 hours, diameters smaller than 6 km, and vertical depths shallower than 10 meters. The across-shelf volume transport due to eddies, estimated by seeding particles within the surface current fields, was more than half the wind-driven depth-dependent exchange. Thus, accounting for the potential coherent along-shelf variability present over the inner-shelf can significantly increase estimates of the across-shelf transfer of water masses and particles. This article is protected by copyright. All rights reserved.
      PubDate: 2017-02-09T10:45:32.041551-05:
      DOI: 10.1002/2016JC012491
  • Zooplankton community response to the winter 2013 deep convection process
           in the NW Mediterranean Sea
    • Authors: Katty Donoso; François Carlotti, Marc Pagano, Brian P. V. Hunt, Rubén Escribano, Léo Berline
      Abstract: The Gulf of Lion is an important area of deep convection, where intense winter vertical mixing brings nutrients up from deeper layers, promoting the largest bloom in the Mediterranean at the end of winter/early spring. In DEWEX program conducted cruises in February and April 2013 to investigate the ecosystem level impacts of deep water convection. Zooplankton data were collected through net sampling and imaging with an Underwater Vision Profiler. In winter, low zooplankton abundance and biomass were observed in the Deep Convection Zone (DCZ) and higher values on its periphery. In spring, this pattern reversed with high biomass in the DCZ and lower values on the periphery. On average for the whole area, the potential grazing impact was estimated to increase by one order of magnitude from winter to spring. In April, all areas except the DCZ incurred top-down control by zooplankton on the phytoplankton stock. In the DCZ, the chlorophyll-a values remained high despite the high zooplankton biomass and carbon demand, indicating a sustained bottom-up control. The zooplankton community composition was comparable for both periods, typified by high copepod dominance, but with some differences between the DCZ and peripheral regions. In spring the DCZ was characterized by a strong increase in herbivorous species such as Centropages typicus and Calanus helgolandicus, and an increase in the number of large zooplankton individuals. Our study indicates that the DCZ is likely an area of both enhanced energy transfer to higher trophic levels and organic matter export in the North Western Mediterranean Sea. This article is protected by copyright. All rights reserved.
      PubDate: 2017-02-08T11:01:05.074732-05:
      DOI: 10.1002/2016JC012176
  • Semiannual variability of the California undercurrent along the Southern
           California current system: A tropical generated phenomenon
    • Authors: Felipe Gómez-Valdivia; Alejandro Parés-Sierra, Ana Laura Flores-Morales
      Abstract: We used a high-resolution numerical model implementation to analyze the California Undercurrent (CU) dynamics along the Southern California Current System. In agreement with reported observations, the modeled CU was stronger during June-July and December-January, when it flowed continuously along Baja California and Southern California reaching long-term averages up to 6 cms−1. Previous research has associated the biannual CU intensification to the local dynamics off Southern California. Our results evidenced, however, that the passage of remote Semiannual Coastally Trapped Waves (SCTW) primarily explained the semiannual CU variability. The CU was stronger 2-3 months after the passage of the upwelling SCTW phase, when the offshore propagation of Rossby waves, brought about by the SCTW transit, induced an energetic cross-shore pressure gradient that strengthened the subsurface poleward circulation along the continental slope. The SCTW were independent of the local wind; they corresponded to the northward extension of semiannual equatorial Kelvin waves that have been observed along the northeastern tropical Pacific. This article is protected by copyright. All rights reserved.
      PubDate: 2017-02-07T11:35:46.6912-05:00
      DOI: 10.1002/2016JC012350
  • Variability of upper ocean characteristics and tropical cyclones in the
           South West Indian Ocean
    • Authors: D. Mawren; C. J. C Reason
      Abstract: Track and intensity are key aspects of tropical cyclone behaviour. Intensity may be impacted by the upper-ocean heat content relevant for TC intensification (known as Tdy) and barrier layer thickness (BLT). Here, the variability of Tdy and BLT in the South West Indian Ocean and their relationships with tropical cyclones are investigated. It is shown that rapid cyclone intensification is influenced by large Tdy values, thick barrier layers and the presence of anticyclonic eddies. For TC generation in the South West Indian Ocean, the parameter Tdy was found to be important.Large BLT values overlay with large Tdy values during summer. Both fields are modulated by the westward propagation of Rossby waves, which are often associated with ENSO. For example, the 1997-1998 El Niño shows a strong signal in Tdy, SST and BLT over the South West Indian Ocean. After this event, an increasing trend in Tdy occurred over most of the basin which may be associated with changes in atmospheric circulation. Increasing SST, Power Dissipation Index and frequency of Category-5 tropical cyclones also occurred from 1980 to 2010.To further examine the links between tropical cyclones, Tdy and BLT, the ocean response to Category 5 Tropical Cyclone Bansi that developed near Madagascar during January 2015 was analysed. Its unusual track was found to be linked with the strengthening of the monsoonal north westerlies while its rapid intensification from Category-2 to Category-4 was linked to a high- Tdy region, associated with a warm core eddy and large BLT. This article is protected by copyright. All rights reserved.
      PubDate: 2017-02-07T11:35:41.486144-05:
      DOI: 10.1002/2016JC012028
  • Sensitivity of typhoon modeling to surface waves and rainfall
    • Authors: Biao Zhao; Fangli Qiao, Luigi Cavaleri, Guansuo Wang, Luciana Bertotti, Li Liu
      Abstract: Improving intensity simulation and forecast of tropical cyclones has always been a challenge, although in recent years the track forecasts have been remarkably improved. In this study, we explore the sensitivity of typhoon simulation to three physical processes using a fully coupled atmosphere-ocean-wave model. Two storms, a strong and a weak one, have been chosen. The effects of wave breaking induced sea spray, ocean vertical mixing associated with non-breaking surface waves, and sea surface cooling due to intense rainfall are assessed by means of a set of numerical experiments. The results show and confirm that sea spray leads to an increase of typhoon intensity by enhancing the air-sea heat flux, while non-breaking wave-induced vertical mixing and rainfall lead to a decrease. Each process can be relevant, depending on wind and wave conditions. These can vary dramatically when typhoons interact with not sufficiently well-defined coastal areas, typically an archipelago. Compared with the control runs, when all the three physical processes are considered, the (absolute) difference between the modeled sea level pressure and best track data is reduced from 26.05 to 0.70 hPa for typhoon Haiyan, and from -9.42 to -8.67 hPa for typhoon Jebi. We have found a steady overestimate of the dimensions of the typhoons. We have verified an extreme sensitivity to the initial conditions, especially when small differences in the typhoon track may imply different relevance of the physical processes, like the ones we have considered, governing the evolution of the storm. This article is protected by copyright. All rights reserved.
      PubDate: 2017-02-07T11:35:34.512451-05:
      DOI: 10.1002/2016JC012262
  • A scale-aware subgrid model for quasigeostrophic turbulence
    • Authors: Scott D. Bachman; Baylor Fox-Kemper, Brodie Pearson
      Abstract: This paper introduces two methods for dynamically prescribing eddy-induced diffusivity, advection, and viscosity appropriate for primitive equation models with resolutions permitting the forward potential enstrophy cascade of quasigeostrophic dynamics, such as operational ocean models and high-resolution climate models with O(25)km horizontal resolution and finer. Where quasigeostrophic dynamics fail (e.g., the equator, boundary layers, deep convection), the method reverts to scalings based on a matched two-dimensional enstrophy cascade. A principle advantage is that these subgrid models are scale-aware, meaning that the model is suitable over a range of grid resolutions: from mesoscale grids that just permit baroclinic instabilities to grids below the submesoscale where ageostrophic effects dominate. Two approaches are presented here using Large Eddy Simulation (LES) techniques adapted for three-dimensional rotating, stratified turbulence. The simpler approach has one non-dimensional parameter, Λ, which has an optimal value near 1. The second approach dynamically optimizes Λ during simulation using a test filter. The new methods are tested in an idealized scenario by varying the grid resolution, and their use improves the spectra of potential enstrophy and energy in comparison to extant schemes. The new methods keep the gridscale Reynolds and Péclet numbers near one throughout the domain, which confers robust numerical stability and minimal spurious diapycnal mixing. Although there are no explicit parameters in the dynamic approach, there is strong sensitivity to the choice of test filter. Designing test filters for heterogeneous ocean turbulence adds cost and uncertainty, and we find the dynamic method does not noticeably improve over setting Λ = 1. This article is protected by copyright. All rights reserved.
      PubDate: 2017-02-07T03:20:43.590989-05:
      DOI: 10.1002/2016JC012265
  • Current-induced dissipation in spectral wave models
    • Authors: H. Rapizo; A. V. Babanin, D. Provis, W. E. Rogers
      Abstract: Despite many recent developments of the parameterization for wave dissipation in spectral models, it is evident that when waves propagate onto strong adverse currents the rate of energy dissipation is not properly estimated. The issue of current-induced dissipation is studied through a comprehensive data set in the tidal inlet of Port Phillip Heads, Australia. The wave parameters analyzed are significantly modulated by the tidal currents. Wave height in conditions of opposing currents (ebb tide) can reach twice the offshore value, whereas during co-flowing currents (flood) it can be reduced to half. The wind-wave model SWAN is able to reproduce the tide-induced modulation of waves and the results show that the variation of currents is the dominant factor in modifying the wave field. In stationary simulations the model provides an accurate representation of wave height for slack and flood tides. During ebb tides, wave energy is highly overestimated over the opposing current-jet. None of the four parameterizations for wave dissipation tested performs satisfactorily. A modification to enhance dissipation as a function of the local currents is proposed. It consists of the addition of a factor that represents current-induced wave steepening and it is scaled by the ratio of spectral energy to the threshold breaking level. The new term asymptotes to the original form as the current in the wave direction tends to zero. The proposed modification considerably improves wave height and mean period in conditions of adverse currents, whereas the good model performance in co-flowing currents is unaltered. This article is protected by copyright. All rights reserved.
      PubDate: 2017-02-07T03:15:33.779653-05:
      DOI: 10.1002/2016JC012367
  • Generation of internal solitary waves over a large sill: From Knight Inlet
           to Luzon Strait
    • Authors: Zhiwu Chen; Yuhua Nie, Jieshuo Xie, Jiexin Xu, Yinghui He, Shuqun Cai
      Abstract: A fully nonlinear, nonhydrostatic numerical model is utilized to investigate the generation of Internal Solitary Waves (ISWs) upstream of the Knight Inlet sill. While an upstream hydraulic jump initiates the ISW generation and both hydraulic jump and upstream influence contribute to the generation, it is found that upstream influence is dominant and the hydraulic jump is not necessary for the ultimate generation of ISWs. Decreasing the tidal forcing or upstream sill width may render the flow subcritical (i.e., the hydraulic jump disappears) and ISWs can be generated by nonlinear steepening of long wave disturbances induced by upstream influence. Increasing the tidal forcing or upstream sill width may generate a hydraulic jump blocking strong upstream propagating disturbances. The jump subsequently becomes a turbulent bore and later disperses into a train of ISWs as the tide relaxes. Further increase in the tidal forcing may sweep the turbulent bore downstream and a train of ISWs is emitted upstream towards the end of waning tide. By reducing the stratification strength by one order of magnitude, the near-sill flow is in the transcritical regime and ISWs are resonantly generated over the lee side slope. Connections to the internal tide release mechanism at Luzon Strait and to the unsteady lee wave model are also discussed. The present work provides some more insights into the ISW generation process at Knight Inlet and the connection between the generation mechanism at Knight Inlet and that at Luzon Strait is identified. This article is protected by copyright. All rights reserved.
      PubDate: 2017-02-07T03:15:31.652976-05:
      DOI: 10.1002/2016JC012206
  • Sea-ice thickness from field measurements in the northwestern Barents Sea
    • Authors: Jennifer King; Gunnar Spreen, Sebastian Gerland, Christian Haas, Stefan Hendricks, Lars Kaleschke, Caixin Wang
      Abstract: The Barents Sea is one of the fastest changing regions of the Arctic, and has experienced the strongest decline in winter-time sea-ice area in the Arctic, at −23±4% decade–1. Sea-ice thickness in the Barents Sea is not well studied. We present two previously unpublished helicopter-borne electromagnetic (HEM) ice thickness measurements from the north-western Barents Sea acquired in March 2003 and 2014. The HEM data are compared to ice thickness calculated from ice draft measured by ULS deployed between 1994-96. These data show that ice thickness varies greatly from year to year; influenced by the thermodynamic and dynamic processes that govern local formation vs long-range advection. In a year with a large inflow of sea-ice from the Arctic Basin the Barents Sea ice cover is dominated by thick multiyear ice; as was the case in 2003 and 1995. In a year with an ice cover that was mainly grown in-situ the ice will be thin and mechanically unstable; as was the case in 2014. The HEM data allow us to explore the spatial and temporal variability in ice thickness. In 2003 the dominant ice class was more than 2 years old; and modal sea-ice thickness varied regionally from 0.6 - 1.4 m, with the thinner ice being either first-year ice, or multiyear ice which had come into contact with warm Atlantic water. In 2014 the ice cover was predominantly locally-grown ice less than 1 month old (regional modes of 0.5 - 0.8 m). These two situations represent two extremes of a range of possible ice thickness distributions that can present very different conditions for shipping traffic; or have a different impact on heat transport from ocean to atmosphere. This article is protected by copyright. All rights reserved.
      PubDate: 2017-02-03T20:07:20.515122-05:
      DOI: 10.1002/2016JC012199
  • Variability of the Pacific North equatorial current from 1993 to 2012
           based on a 1/8˚ Pacific model simulation
    • Authors: Zhichun Zhang; Huijie Xue, Fei Chai, Yi Chao
      Abstract: Based on a multi-decadal Pacific basin model simulation, inter-annual variations of the North Equatorial Current (NEC) are investigated. The model reproduces well the characteristics of the NEC and its inter-annual variability. The NEC transport is calculated as an integral of the westward velocity from 6º N to 21º N and from the surface to the 1200 m depth. The magnitude and standard deviation of the NEC transport increase from 46.5 and 3.9 Sv at 175ºE to 66.7 and 6.5 Sv at 130º E, respectively, and both peak around 132º E prior to entering the bifurcation region. The NEC transport tends to be higher during positive Oceanic Niño Index (ONI) years but lower during negative ONI years with the maximum difference of more than 20 Sv. The inter-annual variability of the NEC transport is closely related to changes of the sea surface height in the tropical Pacific Ocean, and the increase of the NEC is mostly balanced by the increase in the North Equatorial Counter Current (NECC) on the tropical gyre side. The present study further suggests a long-term decline of the NEC transport from 1993 to 2012, which is consistent with the patterns in the trend of wind stress curl.Transport anomalies reconstructed from the normal modes of zonal velocity suggest that the 1st baroclinic mode captures about 95% of the variance in the NEC transport, while the 2nd mode adds only additional 3-4%. A 1.5-layer reduced gravity model further reveals that the 1st (2nd) baroclinic mode is driven primarily by the wind (thermal) forcing. The wind forcing plays a predominant role in determining the inter-annual variability in the NEC transport while the effect of the thermal forcing is rather limited. This article is protected by copyright. All rights reserved.
      PubDate: 2017-02-03T20:00:43.746693-05:
      DOI: 10.1002/2016JC012143
  • The coastal streamflow flux in the regional Arctic system model
    • Authors: Joseph Hamman; Bart Nijssen, Andrew Roberts, Anthony Craig, Wieslaw Maslowski, Robert Osinski
      Abstract: The coastal streamflow flux from the Arctic drainage basin is an important driver of dynamics in the coupled ice-ocean system. Comprising more than one-third of the total freshwater flux into the Arctic Ocean, streamflow is a key component of the regional and global freshwater cycle. To better represent the coupling of the streamflow flux to the ocean, we have developed and applied the RVIC streamflow routing model within the Regional Arctic System Model (RASM). The RASM is a high-resolution regional Earth System Model whose domain includes all of the Arctic drainage basin. In this paper, we introduce the RVIC streamflow routing model, detailing its application within RASM and its advancements in terms of representing high-resolution streamflow processes. We evaluate model simulated streamflow relative to in-situ observations and demonstrate a method for improving model performance using a simple optimization procedure. We also present a new, spatially and temporally consistent, high-resolution dataset of coastal freshwater fluxes for the Arctic drainage basin and surrounding areas that is based on a fully-coupled RASM simulation and intended for use in Arctic Ocean modeling applications. This dataset is evaluated relative to other coastal streamflow datasets commonly used by the ocean modeling community. We demonstrate that the RASM-simulated streamflow flux better represents the annual cycle than existing datasets, especially in ungauged areas. Finally, we assess the impact that streamflow has on the coupled ice-ocean system, finding that the presence of streamflow leads to reduced sea surface salinity, increased sea surface temperatures, and decreased sea ice thickness. This article is protected by copyright. All rights reserved.
      PubDate: 2017-02-03T20:00:38.778681-05:
      DOI: 10.1002/2016JC012323
  • Signature of mesoscale eddies in satellite sea surface salinity data
    • Authors: Oleg Melnichenko; Angel Amores, Nikolai Maximenko, Peter Hacker, James Potemra
      Abstract: A persistent signature of coherent mesoscale eddies in sea surface salinity (SSS) is revealed by analyzing the relationship between satellite SSS and sea surface height (SSH) variability in an eddy-following reference frame. Our analysis focuses on mid-ocean eddies in two representative regions, the southern Indian Ocean and the North Atlantic subtropical gyre. The resulting composite averages reveal a clear signature of mesoscale eddies in satellite SSS with typical SSS anomalies of 0.03-0.05 psu. The spatial structure of eddy-induced SSS perturbations can be characterized as a superposition of a dipole structure, arising from horizontal advection of the background SSS gradient by eddy velocity field, and a monopole structure related to the eddy core. The observed relationships between SSS and SSH anomalies are used to provide a regional assessment of the role of mesoscale eddies in the ocean freshwater transport in the North Atlantic subtropical gyre. This article is protected by copyright. All rights reserved.
      PubDate: 2017-02-03T20:00:35.711208-05:
      DOI: 10.1002/2016JC012420
  • Impact of wave breaking on upper ocean turbulence
    • Authors: Yongqing Cai; Yuanqiao Wen, Lichuan Wu, Chunhui Zhou, Fan Zhang
      Abstract: Previous studies have demonstrated that surface wave breaking can impact upper-ocean turbulence through wave-breaking-induced turbulence kinetic energy (TKE) flux and momentum flux. Wave-breaking-induced momentum flux decays approximately exponentially with depth, and the decay exponent depends on both the wind speed and wave age. With increasing wave age, the decay speed of wave-breaking-induced momentum flux first decreases, reaching a minimum around a wave age of 16, and then increases. In this study, a wave-breaking-induced momentum flux parameterization was proposed based on wave age and wind-speed dependence. The new proposed parameterization was introduced into a one-dimensional (1-D) ocean model along with a wave-age-dependent wave-breaking-induced TKE flux parameterization. The simulation results showed that the wave-breaking impact on the ocean mainly affected the upper-ocean layer. Adding the wave-age impact to the wave-breaking-induced TKE flux and momentum flux improved the 1-D model performance concerning the sea temperature. Moreover, the wave-breaking-induced momentum flux had a larger impact on the simulation results than the wave-breaking-induced TKE flux. This article is protected by copyright. All rights reserved.
      PubDate: 2017-02-03T20:00:31.973187-05:
      DOI: 10.1002/2016JC012654
  • Vertical structure anomalies of oceanic eddies in the Kuroshio Extension
    • Authors: Wenjin Sun; Changming Dong, Ruyun Wang, Yu Liu, Kai Yu
      Abstract: Using collocated altimetry sea surface height anomalies (SSHA) and Argo profiles within detected eddies, we investigated structures of temperature, salinity, potential density, geostrophic current, mixed layer depth (MLD), potential vorticity (PV), and buoyancy frequency (N) in the Kuroshio Extension (KE) region under the influences of oceanic eddies. We identified 54,302 oceanic eddies (snapshots) in the KE region during the period of 1999-2013. The composite analysis showed that changes in physical parameters modulated by the climatological composite eddies (hereinafter referred as composite eddies) were mainly confined in the upper 800 m. At the eddy core, the maximum cooling in the composite cyclonic eddy (CE) reaches 2.00°C at ∼360 m, with maximum salinity change of −0.13 psu at ∼260 m and maximum potential density change of +0.27 kg/m3 at ∼310 m. In contrast, the maximum warming in the composite anticyclonic eddy (AE) reaches 1.78°C at ∼410 m of the eddy core, with maximum salinity change of 0.12 psu at ∼260 m and maximum potential density change of −0.22 kg/m3 at ∼410 m. There were obvious anti-clockwise and clockwise geostrophic current anomalies associated with the composite CE and AE, respectively. The seasonal mean eddy-modulated MLD anomaly had significant seasonal variations. Besides, they could modulate opposite PV changes, the magnitude of which varied with depth. This article is protected by copyright. All rights reserved.
      PubDate: 2017-01-31T18:35:44.516564-05:
      DOI: 10.1002/2016JC012226
  • Investigation of turbulent momentum flux in the typhoon boundary layer
    • Authors: Ziqiang Duan; Xiaohong Yao, Yongping Li
      Abstract: Turbulent momentum flux in the typhoon boundary layer was measured at a turbulent flux tower in Southeast China using an eddy covariance technique during the passage of typhoons Fanapi and Megi in 2010. The anemometers were mounted on the cantilever bracket at the height of 26.6 m, 42.4 m, 60.4 m and 82.9 m. The momentum flux in the typhoon boundary layer decreased with increasing height during the two typhoon passages. The decrease was nearly linear during the passage of the Typhoon Fanapi. However, in the boundary layer of the Typhoon Megi, the momentum flux at 26.6 m height was much greater than that at 42.4 m height. The cospectra of wind components showed that the enhanced flux at the bottom of the boundary layer was caused by small scale turbulence with frequency in the range of 0.1–1 Hz and can be explained by the turbulent energy accumulation of small scale turbulence. This article is protected by copyright. All rights reserved.
      PubDate: 2017-01-31T18:35:40.826977-05:
      DOI: 10.1002/2016JC012169
  • Long-term monitoring of ocean deep convection using multisensors altimetry
           and ocean color satellite data
    • Authors: Marine Herrmann; Pierre-Amael Auger, Caroline Ulses, Claude Estournel
      Abstract: Deep convection occurs in oceanic regions submitted to strong atmospheric buoyancy losses and results in the formation of deep water masses (DWF) of the ocean circulation. It shows a strong interannual variability, and could drastically weaken under the influence of climate change. In this study, a method is proposed to monitor quantitatively deep convection using multi-sensors altimetry and ocean color satellite data. It is applied and evaluated for the well observed Northwestern Mediterranean Sea (NWMS) case study. For that, a coupled hydrodynamical-biogeochemical numerical simulation is used to examine the signature of DWF on sea level anomaly (SLA) and surface chlorophyll concentration. Statistically significant correlations between DWF annual indicators and the areas of low surface chlorophyll concentration and low SLA in winter are obtained, and linear relationships between those indicators and areas are established. These relationships are applied to areas of low SLA and low chlorophyll concentration computed respectively from a 27-year altimetry dataset and a 19-year ocean color dataset. The first long time series (covering the last 2 decades) of DWF indicators obtained for the NWMS from satellite observations are produced. Model biases and smoothing effect induced by the low resolution of gridded altimetry data are partly taken into account by using corrective methods. Comparison with winter atmospheric heat flux and previous modeled and observed estimates of DWF indicators suggests that those DWF indicators time series capture realistically DWF interannual variability in the NWMS. The advantages as well as the weaknesses and uncertainties of the method are finally discussed. This article is protected by copyright. All rights reserved.
      PubDate: 2017-01-31T06:41:31.033425-05:
      DOI: 10.1002/2016JC011833
  • Turbulent heat and momentum fluxes in the upper ocean under Arctic Sea Ice
    • Authors: Algot K. Peterson; Ilker Fer, Miles G. McPhee, Achim Randelhoff
      Abstract: We report observations of heat and momentum fluxes measured in the ice-ocean boundary layer from four drift stations between January and June 2015, covering from the typical Arctic basin conditions in the Nansen Basin to energetic spots of interaction with the warm Atlantic Water branches near the Yermak Plateau and over the North Spitsbergen slope. A wide range of oceanic turbulent heat flux values are observed, reflecting the variations in space and time over the five month duration of the experiment. Oceanic heat flux is weakly positive in winter over the Nansen Basin during quiescent conditions, increasing by an order of magnitude during storm events. An event of local upwelling and mixing in the winter-time Nansen basin highlights the importance of individual events. Spring-time drift is confined to the Yermak Plateau and its slopes, where vertical mixing is enhanced. Wind events cause an approximate doubling of oceanic heat fluxes compared to calm periods. In June, melting conditions near the ice edge lead to heat fluxes of O(100 W m−2). The combination of wind forcing with shallow Atlantic Water layer and proximity to open waters leads to maximum heat fluxes reaching 367 W m−2, concurrent with rapid melting. Observed ocean-to-ice heat fluxes agree well with those estimated from a bulk parameterization except when accumulated freshwater from sea ice melt in spring probably causes the bulk formula to overestimate the oceanic heat flux. This article is protected by copyright. All rights reserved.
      PubDate: 2017-01-31T06:41:29.530446-05:
      DOI: 10.1002/2016JC012283
  • Sun glitter imagery of surface waves: 2. Waves Transformation on Ocean
    • Authors: Vladimir Kudryavtsev; Maria Yurovskaya, Bertrand Chapron, Fabrice Collard, Craig Donlon
      Abstract: Under favourable imaging conditions, the Sentinel-2 Multi-Spectral Instrument (MSI) can provide spectacular and novel quantitative ocean surface wave directional measurements in satellite Sun Glitter Imagery (SSGI). Owing to a relatively large-swath with high spatial resolution (10 m) ocean surface roughness mapping capabilities, changes in ocean wave energy and propagation direction can be precisely quantified at very high resolution, across spatial distances of 10 km and more. This provides unique opportunities to study ocean wave refraction induced by spatial varying surface currents. As expected and demonstrated over the Grand Agulhas current area, the mesoscale variability of near-surface currents, documented and reconstructed from satellite altimetry, can significantly deflect in-coming south-western swell systems. Based on ray-tracing calculations, and unambiguously revealed from the analysis of Sentinel-2 MSI SSGI measurements, the variability of the near-surface current explains significant wave-current refraction, leading to wave-trapping phenomenon and strong local enhancement of the total wave energy. In addition to its importance for wave modelling and hazard prediction, these results open new possibilities to combine different satellite measurements and greatly improve the determination of the upper ocean mesoscale vorticity motions. This article is protected by copyright. All rights reserved.
      PubDate: 2017-01-31T06:41:26.763297-05:
      DOI: 10.1002/2016JC012426
  • Sun glitter imagery of ocean surface waves: 1. Directional spectrum
           retrieval and validation
    • Authors: Vladimir Kudryavtsev; Maria Yurovskaya, Bertrand Chapron, Fabrice Collard, Craig Donlon
      Abstract: A practical method is suggested to quantitatively retrieve directional spectra of ocean surface waves from high-resolution satellite sun glitter imagery (SSGI). The method builds on direct determination of the imaging transfer function from the large-scale smoothed shape of sun glitter. Observed brightness modulations are then converted into sea surface elevations to perform directional spectral analysis. The method is applied to the Copernicus Sentinel-2 Multi-Spectral Instrument (MSI) measurements. Owing to the specific instrumental configuration of MSI (which has a primary mission dedicated to mapping land surfaces), a physical angular difference between channel detectors on the instrument focal plane array can be used to efficiently determine the surface brightness gradients in two directions, i.e. in sensor zenith and azimuthal directions. In addition, the detector configuration of MSI means that a small temporal lag between channel acquisitions exists. This feature can be exploited to detect surface waves and infer their space-time characteristics using cross-channel correlation. We demonstrate how this can be used to remove directional ambiguity in 2D detected wave spectra, and to obtain information describing local dispersion of surface waves. Directional spectra derived from Sentinel-2 MSI SSGI are compared with in situ buoy measurements. We report an encouraging agreement between SSGI-derived wave spectra and in situ measurements. This article is protected by copyright. All rights reserved.
      PubDate: 2017-01-31T06:41:15.983637-05:
      DOI: 10.1002/2016JC012425
  • Three-dimensional simulation of high-frequency nonlinear internal wave
           dynamics in Cayuga Lake
    • Authors: Abbas Dorostkar; Leon Boegman, Andrew Pollard
      Abstract: Three-dimensional (3D) hydrostatic and nonhydrostatic versions of the MITgcm were applied to simulate the dynamics of the internal wave field (basin-scale seiches, nonlinear surges and high-frequency nonlinear internal waves, NLIWs) in Cayuga Lake, NY. The simulations were performed using up to 226 million computational cells with several horizontal grid resolutions, varying from 450 × 450 m to 22 × 22 m. Vertical grid spacing was not varied and ranged from 0.5 m to 2.95 m. The 22 × 22 m nonhydrostatic grid reproduced qualitatively the formation, propagation and shoaling of observed NLIWs using >10 grid points along the wavelength and a grid lepticity λ of O(1). This ensured, respectively, that the waves were not aliased and physical dispersion predominated over numerical dispersion. Using a sensitivity analysis, we generalize that correctly simulating NLIWs in real domains, using second-order discretization, requires grid resolutions that are an order of magnitude smaller than the wavelength and amplitude with λ ∼ 2; consistent with published work on idealized domains. Local gyre-like circulation was simulated, near mid-basin headlands, and transverse shoaling of NLIW packets on lateral boundaries was associated with topographic reflection and refraction, in agreement with published field observations from estuaries, which show NLIW propagation in long narrow quasi-2D systems (e.g., Finger Lakes, lochs, fjords, estuaries and straits) is fundamentally 3D. These results, therefore, help fill the gap in understanding and correctly modelling the multiscale 3D dynamics of NLIWs in complex natural systems. This article is protected by copyright. All rights reserved.
      PubDate: 2017-01-21T04:30:33.627065-05:
      DOI: 10.1002/2016JC011862
  • Impact of recently upwelled water on productivity investigated using in
           situ and incubation-based methods in Monterey Bay
    • Authors: Cara C. Manning; Rachel H.R. Stanley, David P. Nicholson, Jason M. Smith, J. Timothy Pennington, Melanie R. Fewings, Michael E. Squibb, Francisco P. Chavez
      Abstract: Photosynthetic conversion of CO2 to organic carbon and the transport of this carbon from the surface to the deep ocean is an important regulator of atmospheric CO2. To understand the controls on carbon fluxes in a productive region impacted by upwelling, we measured biological productivity via multiple methods during a cruise in Monterey Bay, California. We quantified net community production and gross primary production from measurements of O2/Ar and O2 triple isotopes (17Δ), respectively. We simultaneously conducted incubations measuring the uptake of 14C, 15NO3- and 15NH4+, and nitrification, and deployed sediment traps. At the start of the cruise (Phase 1) the carbon cycle was at steady state and the estimated net community production was 35(10) and 35(8) mmol C m−2 d−1 from O2/Ar and 15N incubations respectively, a remarkably good agreement. During Phase 1, net primary production was 96(27) mmol C m−2 d−1 from C uptake, and gross primary production was 209(17) mmol C m−2 d−1 from 17Δ. Later in the cruise (Phase 2), recently upwelled water with higher nutrient concentrations entered the study area, causing 14C and 15NO3- uptake to increase substantially. Continuous O2/Ar measurements revealed submesoscale variability in water mass structure and likely productivity in Phase 2 that was not evident from the incubations. These data demonstrate that O2/Ar and 15N incubation-based NCP estimates can give equivalent results in an N-limited, coastal system, when the non-steady state O2 fluxes are negligible or can be quantified. This article is protected by copyright. All rights reserved.
      PubDate: 2017-01-21T04:25:35.999603-05:
      DOI: 10.1002/2016JC012306
  • Percolation blockage: A process that enables melt pond formation on first
           year Arctic Sea ice
    • Authors: Chris Polashenski; Kenneth M. Golden, Donald K. Perovich, Eric Skyllingstad, Alexandra Arnsten, Carolyn Stwertka, Nicholas Wright
      Abstract: Melt pond formation atop Arctic sea ice is a primary control of shortwave energy balance in the Arctic Ocean. During late spring and summer, the ponds determine sea ice albedo and how much solar radiation is transmitted into the upper ocean through the sea ice. The initial formation of ponds requires that melt water be retained above sea level on the ice surface. Both theory and observations, however, show that first year sea ice is so highly porous prior to the formation of melt ponds that multi-day retention of water above hydraulic equilibrium should not be possible. Here we present results of percolation experiments that identify and directly demonstrate a mechanism allowing melt pond formation. The infiltration of fresh water into the pore structure of sea ice is responsible for blocking percolation pathways with ice, sealing the ice against water percolation, and allowing water to pool above sea level. We demonstrate that this mechanism is dependent on fresh water availability, known to be predominantly from snowmelt, and ice temperature at melt onset. We argue that the blockage process has the potential to exert significant control over inter-annual variability in ice albedo. Finally, we suggest that incorporating the mechanism into models would enhance their physical realism. Full treatment would be complex. We provide a simple temperature threshold-based scheme that maybe used to incorporate percolation blockage behavior into existing model frameworks. This article is protected by copyright. All rights reserved.
      PubDate: 2017-01-16T06:25:48.394936-05:
      DOI: 10.1002/2016JC011994
  • Observed cold filaments associated with mesoscale eddies in the South
           China Sea
    • Authors: Jiaxun Li; Guihua Wang, Xiaoming Zhai
      Abstract: Unusual cold filaments are uncovered during the spring intermonsoon season in the South China Sea (SCS) using a suite of satellite observations. They have a width of about 100 km on average and extend several hundreds of kilometers offshore on the sea surface, providing significant cross-shelf transport of heat and nutrients. The eastward current associated with mesoscale eddies in spring in the western SCS is found to play an important role in the filament formation by advecting coastal cold waters far offshore. The meridional location of the cold filament displays considerable interannual variability ranging between 9oN and 18oN, which can be attributed to the interannual south-north shift of the eastward current associated with eddies. It is also found that in the spring, cold filaments have profound effects on the chlorophyll a concentration in the upper ocean and the overlying atmosphere. These findings provide new insights into the role of eddies in cross-shelf exchange and mesoscale air-sea interaction in the marginal seas.
      PubDate: 2017-01-10T17:30:24.776939-05:
      DOI: 10.1002/2016JC012353
  • Community production modulates coral reef pH and the sensitivity of
           ecosystem calcification to ocean acidification
    • Authors: Thomas M. DeCarlo; Anne L. Cohen, George T.F. Wong, Fuh-Kwo Shiah, Steven J. Lentz, Kristen A. Davis, Kathryn E.F. Shamberger, Pat Lohmann
      Abstract: Coral reefs are built of calcium carbonate (CaCO3) produced biogenically by a diversity of calcifying plants, animals and microbes. As the ocean warms and acidifies, there is mounting concern that declining calcification rates could shift coral reef CaCO3 budgets from net accretion to net dissolution. We quantified net ecosystem calcification (NEC) and production (NEP) on Dongsha Atoll, northern South China Sea, over a two-week period that included a transient bleaching event. Peak daytime pH on the wide, shallow reef flat during the non-bleaching period was ∼8.5, significantly elevated above that of the surrounding open ocean (∼8.0-8.1) as a consequence of daytime NEP (up to 112 mmol C m−2 hr−1). Diurnal-averaged NEC was 390 ± 90 mmol CaCO3 m−2 day−1, higher than any other coral reef studied to date despite comparable calcifier cover (25%) and relatively high fleshy algal cover (19%). Coral bleaching linked to elevated temperatures significantly reduced daytime NEP by 29 mmol C m−2 hr−1. pH on the reef flat declined by 0.2 units, causing a 40% reduction in NEC in the absence of pH changes in the surrounding open ocean. Our findings highlight the interactive relationship between carbonate chemistry of coral reef ecosystems and ecosystem production and calcification rates, which are in turn impacted by ocean warming. As open-ocean waters bathing coral reefs warm and acidify over the 21st century, the health and composition of reef benthic communities will play a major role in determining on-reef conditions that will in turn dictate the ecosystem response to climate change.
      PubDate: 2017-01-10T17:25:31.868009-05:
      DOI: 10.1002/2016JC012326
  • Three-compartment structure of subsurface-intensified mesoscale eddies in
           the ocean
    • Authors: Zhengguang Zhang; Yu Zhang, Wei Wang
      Abstract: Mesoscale eddies are energetically dominant and pervasive over most of the world's oceans. Among them, many are subsurface intensified with strongest signals in the ocean interior such as mode water eddies, which trap water masses with distinctive properties and carry them over long distances. With both Argo profiling floats and atmospheric reanalysis data we showed that the structure of these eddies obeys a universal rule. Hence their three-dimensional hydrographic fields can be readily reconstructed from very limited information. More interestingly, the volume of water trapped and moved by a mode water eddy is much greater than previously thought; it has a three-compartment structure in the vertical with the mode water being sandwiched between two layers of notably different properties and accounting for only a portion of the total trapped volume. This article is protected by copyright. All rights reserved.
      PubDate: 2017-01-05T04:05:22.89151-05:0
      DOI: 10.1002/2016JC012376
  • Issue Information
    • Pages: 1 - 3
      PubDate: 2017-02-23T06:31:20.223721-05:
      DOI: 10.1002/jgrc.21925
  • The fate of terrigenous dissolved organic carbon on the Eurasian shelves
           and export to the North Atlantic
    • Authors: K. Kaiser; R. Benner, R. M. W. Amon
      Pages: 4 - 22
      Abstract: Dissolved lignin phenols, chromophoric dissolved organic matter (CDOM) absorption, and fluorescence were analyzed along cross-slope mooring locations in the Barents, Laptev, and East Siberian Seas to gain a better understanding of terrigenous dissolved organic carbon (tDOC) dynamics in Arctic shelf seas and the Arctic Ocean. A gradient of river water and tDOC was observed along the continental shelf eastward into the East Siberian Sea. Correlations of carbon-normalized yields of lignin-derived phenols supplied by Siberian rivers with river water fractions and known water residence times yielded in situ decay constants of 0.18–0.58 yr−1. Calculations showed ∼50% of annual tDOC discharged by Siberian rivers was mineralized in estuaries and on Eurasian shelves per year indicating extensive removal of tDOC. Bioassay experiments and in situ decay constants indicated a reactivity continuum for tDOC. CDOM parameters and acid/aldehyde ratios of vanillyl (V) and syringyl (S) lignin phenols showed biomineralization was the dominant mechanism for the removal of tDOC. Characteristic ratios of p-hydroxy (P), S, and V phenols (P/V, S/V) also identified shelf regions in the Kara Sea and regions along the Western Laptev Sea shelf where formation of Low Salinity Halocline Waters (LSHW) and Lower Halocline Water (LHW) occurred. The efficient removal of tDOC demonstrates the importance of Eurasian margins as sinks of tDOC derived from the large Siberian Rivers and confirms tDOC mineralization has a major impact on nutrients budgets, air-sea CO2 exchange, and acidification in the Siberian Shelf Seas.
      PubDate: 2017-01-05T05:45:26.77094-05:0
      DOI: 10.1002/2016JC012380
  • Coherent mesoscale eddies in the North Atlantic subtropical gyre: 3-D
           structure and transport with application to the salinity maximum
    • Authors: Angel Amores; Oleg Melnichenko, Nikolai Maximenko
      Pages: 23 - 41
      Abstract: The mean vertical structure and transport properties of mesoscale eddies are investigated in the North Atlantic subtropical gyre by combining historical records of Argo temperature/salinity profiles and satellite sea level anomaly data in the framework of the eddy tracking technique. The study area is characterized by a low eddy kinetic energy and sea surface salinity maximum. Although eddies have a relatively weak signal at surface (amplitudes around 3–7 cm), the eddy composites reveal a clear deep signal that penetrates down to at least 1200 m depth. The analysis also reveals that the vertical structure of the eddy composites is strongly affected by the background stratification. The horizontal patterns of temperature/salinity anomalies can be reconstructed by a linear combination of a monopole, related to the elevation/depression of the isopycnals in the eddy core, and a dipole, associated with the horizontal advection of the background gradient by the eddy rotation. A common feature of all the eddy composites reconstructed is the phase coherence between the eddy temperature/salinity and velocity anomalies in the upper ∼300 m layer, resulting in the transient eddy transports of heat and salt. As an application, a box model of the near-surface layer is used to estimate the role of mesoscale eddies in maintaining a quasi-steady state distribution of salinity in the North Atlantic subtropical salinity maximum. The results show that mesoscale eddies are able to provide between 4 and 21% of the salt flux out of the area required to compensate for the local excess of evaporation over precipitation.
      PubDate: 2017-01-05T05:40:31.35061-05:0
      DOI: 10.1002/2016JC012256
  • The use of a wave boundary layer model in SWAN
    • Authors: Jianting Du; Rodolfo. Bolaños, Xiaoli Guo Larsén
      Pages: 42 - 62
      Abstract: A wave boundary layer model (WBLM) is implemented in the third-generation ocean wave model SWAN to improve the wind-input source function under idealized, fetch-limited condition. Accordingly, the white capping dissipation parameters are recalibrated to fit the new wind-input source function to parametric growth curves. The performance of the new pair of wind-input and dissipation source functions is validated by numerical simulations of fetch-limited evolution of wind-driven waves. As a result, fetch-limited growth curves of significant wave height and peak frequency show close agreement with benchmark studies at all wind speeds (5–60 m s−1) and fetches (1–3000 km). The WBLM wind-input source function explicitly calculates the drag coefficient based on the momentum and kinetic energy conservation. The modeled drag coefficient using WBLM wind-input source function is in rather good agreement with field measurements. Thus, the new pair of wind-input and dissipation source functions not only improve the wave simulation but also have the potential of improving air-sea coupling systems by providing reliable momentum flux estimation at the air-sea interface.
      PubDate: 2017-01-05T05:45:40.650223-05:
      DOI: 10.1002/2016JC012104
  • Tidal flow separation at protruding beach nourishments
    • Authors: Max Radermacher; Matthieu A. de Schipper, Cilia Swinkels, Jamie H. MacMahan, Ad J.H.M. Reniers
      Pages: 63 - 79
      Abstract: In recent years, the application of large-scale beach nourishments has been discussed, with the Sand Motor in the Netherlands as the first real-world example. Such protruding beach nourishments have an impact on tidal currents, potentially leading to tidal flow separation and the generation of tidal eddies of length scales larger than the nourishment itself. The present study examines the characteristics of the tidal flow field around protruding beach nourishments under varying nourishment geometry and tidal conditions, based on extensive field observations and numerical flow simulations. Observations of the flow field around the Sand Motor, obtained with a ship-mounted current profiler and a set of fixed current profilers, show that a tidal eddy develops along the northern edge of the mega-nourishment every flood period. The eddy is generated around peak tidal flow and gradually gains size and strength, growing much larger than the cross-shore dimension of the coastline perturbation. Based on a 3 week measurement period, it is shown that the intensity of the eddy modulates with the spring-neap tidal cycle. Depth-averaged tidal currents around coastline perturbations are simulated and compared to the field observations. The occurrence and behavior of tidal eddies is derived for a large set of simulations with varying nourishment size and shape. Results show that several different types of behavior exist, characterized by different combinations of the nourishment aspect ratio, the size of the nourishment relative to the tidal excursion length, and the influence of bed friction.
      PubDate: 2017-01-09T00:35:34.118542-05:
      DOI: 10.1002/2016JC011942
  • Direct estimates of friction factors for a mobile rippled bed
    • Authors: S. Rodríguez-Abudo; D. L. Foster
      Pages: 80 - 92
      Abstract: New friction factor estimates are computed from the total momentum transfer applied to a rippled sediment bed. The total time-dependent momentum flux is achieved by implementing the double-averaged horizontal momentum equation on the nearbed flow field collected with PIV. Time-independent friction factors are obtained by regressing the total momentum flux to the common quadratic stress law given by 12ρu∞ u∞ . The resulting friction factors compare favorably with available analysis techniques including energy dissipation, vertical turbulence intensity, and maximum shear stress, but can be 2-6 times smaller than estimates determined with the model by Madsen (1994) and the formula of Swart (1974) using the ripple roughness.
      PubDate: 2017-01-10T10:20:31.090911-05:
      DOI: 10.1002/2016JC012055
  • Liquid freshwater transport estimates from the East Greenland Current
           based on continuous measurements north of Denmark Strait
    • Authors: L. de Steur; R. S. Pickart, A. Macrander, K. Våge, B. Harden, S. Jónsson, S. Østerhus, H. Valdimarsson
      Pages: 93 - 109
      Abstract: Liquid freshwater transports of the shelfbreak East Greenland Current (EGC) and the separated EGC are determined from mooring records from the Kögur section north of Denmark Strait between August 2011 and July 2012. The 11 month mean freshwater transport (FWT), relative to a salinity of 34.8, was 65 ± 11 mSv to the south. Approximately 70% of this was associated with the shelfbreak EGC and the remaining 30% with the separated EGC. Very large southward FWT ranging from 160 mSv to 120 mSv was observed from September to mid-October 2011 and was foremost due to anomalously low upper-layer salinities. The FWT may, however, be underestimated by approximately 5 mSv due to sampling biases in the upper ocean. The FWT on the Greenland shelf was estimated using additional inshore moorings deployed from 2012 to 2014. While the annual mean ranged from nearly zero during the first year to 18 mSv to the south during the second year, synoptically the FWT on the shelf can be significant. Furthermore, an anomalous event in autumn 2011 caused the shelfbreak EGC to reverse, leading to a large reduction in FWT. This reversed circulation was due to the passage of a large, 100 km wide anticyclone originating upstream from the shelfbreak. The late summer FWT of −131 mSv is 150% larger than earlier estimates based on sections in the late-1990s and early-2000s. This increase is likely the result of enhanced freshwater flux from the Arctic Ocean to the Nordic Seas during the early 2010s.
      PubDate: 2017-01-10T10:20:46.153891-05:
      DOI: 10.1002/2016JC012106
  • Internal wave generation from tidal flow exiting a constricted opening
    • Authors: Caixia Wang; Richard Pawlowicz
      Pages: 110 - 125
      Abstract: The southern Strait of Georgia, British Columbia, often contains packets of large, near-surface internal waves. Wave crests at the leading edge of the packet, spaced a few hundred meters apart, can have a longitudinal extent of more than 10 km. It has long been assumed that these waves are generated by tidal flow through narrow passages and channels at the Strait's southern boundaries, but no actual link has ever been made between these waves and a specific passage or generation mechanism. Here we identify the location and extent of a number of these large packets at specific times using mosaics of photogrammetrically rectified oblique air photos. Wave speeds are determined by analyzing a time sequence of images, with water column measurements used to subtract effects of tidal advection. The location and extent of these internal waves are then compared with the predicted location and extent of hypothetical waves generated in different passages, at different stages of the tide, which are then propagated through a predicted time-varying barotropic flow field. It is found that the observed waves are most likely generated near or after the time of the peak flood tide, or peak inflow into the Strait. They are therefore inconsistent with generation mechanisms involving the release and upstream propagation of waves by the relaxation of an ebb tide. Instead they are probably associated with the nonlinear adjustment of conditions at the edge of an inflowing injection of relatively weakly stratified water.
      PubDate: 2017-01-10T10:20:39.527447-05:
      DOI: 10.1002/2016JC011765
  • Effects of an Arctic under-ice bloom on solar radiant heating of the water
    • Authors: Torbjørn Taskjelle; Mats A. Granskog, Alexey K. Pavlov, Stephen R. Hudson, Børge Hamre
      Pages: 126 - 138
      Abstract: The deposition of solar energy in the upper Arctic Ocean depends, among other things, on the composition of the water column. During the N-ICE2015 expedition, a drift in the Arctic pack ice north of Svalbard, an under-ice phytoplankton bloom was encountered in May 2015. This bloom led to significant changes in the inherent optical properties (IOPs) of the upper ocean. Mean values of total water absorption in the upper 20 m of the water column were up to 4 times higher during the bloom than prior to it. The total water attenuation coefficient increased by a factor of up to around 7. Radiative transfer modeling, with measured IOPs as input, has been performed with a coupled atmosphere-ice-ocean model. Simulations are used to investigate the change in depth-dependent solar heating of the ocean after the onset of the bloom, for wavelengths in the region 350–700 nm. Effects of clouds, sea ice cover, solar zenith angle, as well as the average cosine for scattering of the ocean inclusions are evaluated. An increase in energy absorption in the upper 10 m of about 36% is found under 25 cm ice with 2 cm snow, for bloom conditions relative to prebloom conditions, which may have implications for ice melt and growth in spring. Thicker clouds and lower sun reduce the irradiance available, but lead to an increase in relative absorption.
      PubDate: 2017-01-11T23:45:39.585641-05:
      DOI: 10.1002/2016JC012187
  • Submarine groundwater discharge and associated nutrient fluxes into the
           Southern Yellow Sea: A case study for semi-enclosed and oligotrophic
           seas-implication for green tide bloom
    • Authors: Jian'an Liu; Ni Su, Xilong Wang, Jinzhou Du
      Pages: 139 - 152
      Abstract: The biogenic elements concentrations in a coastal bay/estuary are strongly influenced not only by riverine input but also by submarine groundwater discharge (SGD) which has been identified as a typical process of land/ocean interactions in coastal zones. To assess the role of SGD in nutrient fluxes in the Southern Yellow Sea (SYS), 228Ra activities were measured in seawater collected in May 2014. Analyzing the sources and sinks of 228Ra, the flux of excess 228Ra through SGD was estimated to be (2.2 ± 1.0) ×1015 dpm yr−1. Based on the 228Ra mass balance model, we estimated the average SGD flux to be approximately (1.3 ± 0.87) ×1012 m3 yr−1 over the entire SYS, which is at least 3.3 times the estimated annual delivery from the Changjiang River into the SYS (∼1.3 × 1011 m3 yr−1). The SGD-derived biogenic elements loads (dissolved inorganic nitrogen (DIN), phosphorus (DIP) and silicon (DSi)) were estimated as (487 ± 384) × 109 mol yr−1, (2.8 ± 2.2) × 109 mol yr−1, and (313 ± 259) × 109 mol yr−1, respectively, which are approximately 18 times, 7 times and 13 times the riverine input from both mainland China and Korea. The accumulation nutrient fluxes derived by SGD may play one of the most important roles in the green tide bloom that originated from the Subei Shoal zone in the SYS. Additionally, DIN and DIP via SGD can provide the necessary amounts of nutrient for recovering nutrient concentrations to normal levels after the green tide bloom is terminated.
      PubDate: 2017-01-11T01:25:42.780532-05:
      DOI: 10.1002/2016JC012282
  • Vertical structure of pore pressure under surface gravity waves on a
           steep, megatidal, mixed sand-gravel-cobble beach
    • Authors: Tristan B. Guest; Alex E. Hay
      Pages: 153 - 170
      Abstract: The vertical structure of surface gravity wave-induced pore pressure is investigated within the intertidal zone of a natural, steeply sloping, megatidal, mixed sand-gravel-cobble beach. Results from a coherent vertical array of buried pore pressure sensors are presented in terms of signal phase lag and attenuation as functions of oscillatory forcing frequency and burial depth. Comparison of the observations with the predictions of a theoretical poro-elastic bed response model indicates that the large observed phase lags and attenuation are attributable to interstitial trapped air. In addition to the dependence on entrapped air volume, the pore pressure phase and attenuation are shown to be sensitive to the hydraulic conductivity of the sediment, to the changing mean water depth during the tidal cycle, and to the redistribution/rearrangement of beach face material by energetic wave action during storm events. The latter result indicates that the effects on pore pressure of sediment column disturbance during instrument burial can persist for days to weeks, depending upon wave forcing conditions. Taken together, these results raise serious questions as to the practicality of using pore pressure measurements to estimate the kinematic properties of surface gravity waves on steep, mixed sand-gravel beaches.
      PubDate: 2017-01-11T23:45:48.135985-05:
      DOI: 10.1002/2016JC012257
  • Nitrous oxide during the onset of the Atlantic cold tongue
    • Authors: D. L. Arévalo-Martínez; A. Kock, T. Steinhoff, P. Brandt, M. Dengler, T. Fischer, A. Körtzinger, H. W. Bange
      Pages: 171 - 184
      Abstract: The tropical Atlantic exerts a major influence in climate variability through strong air-sea interactions. Within this region, the eastern side of the equatorial band is characterized by strong seasonality, whereby the most prominent feature is the annual development of the Atlantic cold tongue (ACT). This band of low sea surface temperatures (∼22–23°C) is typically associated with upwelling-driven enhancement of surface nutrient concentrations and primary production. Based on a detailed investigation of the distribution and sea-to-air fluxes of N2O in the eastern equatorial Atlantic (EEA), we show that the onset and seasonal development of the ACT can be clearly observed in surface N2O concentrations, which increase progressively as the cooling in the equatorial region proceeds during spring-summer. We observed a strong influence of the surface currents of the EEA on the N2O distribution, which allowed identifying “high” and “low” concentration regimes that were, in turn, spatially delimited by the extent of the warm eastward-flowing North Equatorial Countercurrent and the cold westward-flowing South Equatorial Current. Estimated sea-to-air fluxes of N2O from the ACT (mean 5.18 ± 2.59 μmol m−2 d−1) suggest that in May–July 2011 this cold-water band doubled the N2O efflux to the atmosphere with respect to the adjacent regions, highlighting its relevance for marine tropical emissions of N2O.
      PubDate: 2017-01-13T02:55:54.19799-05:0
      DOI: 10.1002/2016JC012238
  • On the observed synoptic signal in the Mississippi-Alabama slope flow
    • Authors: E. V. Maksimova
      Pages: 185 - 192
      Abstract: This communication discusses a strong near-barotropic current signal and its dynamics observed on the Mississippi-Alabama upper slope in the northeastern Gulf of Mexico. When the variability related to mesoscale eddies is not present or removed, the subinertial current is found to be controlled by the synoptic-scale wind fluctuations, qualitatively in agreement with coastally trapped wave theory. Specifically, the along-isobath synoptic velocity component on the Mississippi-Alabama slope is correlated with the wind stress component in the direction along the Florida peninsula. Moreover, the observed along-isobath flows on the Mississippi-Alabama slope and on the west Florida shelf are highly coherent. These relations are pronounced in wintertime but less obvious during summertime, less energetic, wind forcing.
      PubDate: 2017-01-13T02:55:37.041657-05:
      DOI: 10.1002/2016JC012320
  • Large-eddy simulation of wave-breaking induced turbulent coherent
           structures and suspended sediment transport on a barred beach
    • Authors: Zheyu Zhou; Tian-Jian Hsu, Daniel Cox, Xiaofeng Liu
      Pages: 207 - 235
      Abstract: To understand the interaction between wave-breaking induced turbulent coherent structures and suspended sediment transport, we report a Large-Eddy Simulation (LES) study of wave-breaking processes over a near-prototype scale barred beach. The numerical model is implemented using the open-source CFD toolbox, OpenFOAM®, in which the incompressible three-dimensional filtered Navier-Stokes equations for the water and air phases are solved with a finite volume scheme. A volume of fluid (VOF) method is used to capture the evolution of the water-air interface. The numerical model is validated with measured free surface elevation, turbulence-averaged flow velocity, turbulent intensity, and for the first time, the intermittency of breaking wave turbulence. Simulation results confirm that as the obliquely descending eddies (ODEs) approach the bottom, significant bottom shear stress is generated. Remarkably, the collapse of ODEs onto the bed can also cause drastic spatial and temporal changes of dynamic pressure on the bottom. By allowing sediment to be suspended from the bar crest, intermittently high sediment suspension events and their correlation with high turbulence and/or high bottom shear stress events are investigated. The simulated intermittency of sediment suspension is similar to previous field and large wave flume observations. Coherent suspension events account for only 10% of the record but account for about 50% of the sediment load. Model results suggest that about 60%∼70% of coherent bottom stress events are associated with surface-generated turbulence. Nearly all the coherent sand suspension events are associated with coherent turbulence events due to wave-breaking turbulence approaching the bed.
      PubDate: 2017-01-17T01:21:56.554498-05:
      DOI: 10.1002/2016JC011884
  • Bottom-slope-induced net sheet-flow sediment transport rate under
           sinusoidal oscillatory flows
    • Authors: Jing Yuan; Zhiwei Li, O. S. Madsen
      Pages: 236 - 263
      Abstract: It is generally believed that the slope of beaches can lead to a net downslope (usually offshore) sediment transport rate under shoaling waves, but very few high-quality measurements have been reported for a quantitative understanding of this phenomenon. In this study, full-scale (1:1) experiments of bottom-slope-induced net sheet-flow sediment transport rate under sinusoidal oscillatory flows are conducted using a tilting oscillatory water tunnel. The tests cover a variety of flow-sediment conditions on bottom slopes up to 2.6°. A laser-based bottom profiler system is developed for measuring net transport rate based on the principle of mass conservation. Experimental results suggest that for a given flow-sediment condition the net transport rate is in the downslope direction and increases linearly with bottom slope. A conceptual model is presented based on the idea that gravity helps bottom shear stress drive bedload transport and consequently enhances (reduces) bedload transport and suspension when the flow is in the downslope (up-slope) direction. The model predicts both the measured net sediment transport rates and the experimental linear relationship between net transport rates and bottom slope with an accuracy generally better than a factor of 2. Some measured net transport rates in this study are comparable to those due to flow skewness obtained in similar sheet-flow studies, despite that our maximum slope could be milder than the actual bottom slope in surf zones, where sheet-flow conditions usually occur. This shows that the slope effect may be as important as wave nonlinearity in producing net cross-shore sheet-flow sediment transport.
      PubDate: 2017-01-17T01:55:43.338507-05:
      DOI: 10.1002/2016JC011996
  • A comparison of the climates of the Medieval Climate Anomaly, Little Ice
           Age, and Current Warm Period reconstructed using coral records from the
           northern South China Sea
    • Authors: Wenfeng Deng; Xi Liu, Xuefei Chen, Gangjian Wei, Ti Zeng, Luhua Xie, Jian-xin Zhao
      Pages: 264 - 275
      Abstract: For the global oceans, the characteristics of high-resolution climate changes during the last millennium remain uncertain because of the limited availability of proxy data. This study reconstructs climate conditions using annually resolved coral records from the South China Sea (SCS) to provide new insights into climate change over the last millennium. The results indicate that the climate of the Medieval Climate Anomaly (MCA, AD 900–1300) was similar to that of the Current Warm Period (CWP, AD 1850-present), which contradicts previous studies. The similar warmth levels for the MCA and CWP have also been recorded in the Makassar Strait of Indonesia, which suggests that the MCA was not warmer than the CWP in the western Pacific and that this may not have been a globally uniform change. Hydrological conditions were drier/saltier during the MCA and similar to those of the CWP. The drier/saltier MCA and CWP in the western Pacific may be associated with the reduced precipitation caused by variations in the Pacific Walker Circulation. As for the Little Ice Age (LIA, AD 1550–1850), the results from this study, together with previous data from the Makassar Strait, indicate a cold and wet period compared with the CWP and the MCA in the western Pacific. The cold LIA period agrees with the timing of the Maunder sunspot minimum and is therefore associated with low solar activity. The fresher/wetter LIA in the western Pacific may have been caused by the synchronized retreat of both the East Asian Summer Monsoon and the Australian Monsoon.
      PubDate: 2017-01-17T01:40:24.584364-05:
      DOI: 10.1002/2016JC012458
  • Satellite observation of particulate organic carbon dynamics on the
           Louisiana continental shelf
    • Authors: Chengfeng Le; John C. Lehrter, Chuanmin Hu, Hugh MacIntyre, Marcus W. Beck
      Abstract: Particulate organic carbon (POC) plays an important role in coastal carbon cycling and the formation of hypoxia. Yet, coastal POC dynamics are often poorly understood due to a lack of long term POC observations and the complexity of coastal hydrodynamic and biogeochemical processes that influence POC sources and sinks. Using field observations and satellite ocean color products, we developed a new multiple regression algorithm to estimate POC on the Louisiana Continental Shelf (LCS) from satellite observations. The algorithm had reliable performance with mean relative error (MRE) of ∼40% and root mean square error (RMSE) of ∼50% for MODIS and SeaWiFS images for POC ranging between ∼80 and ∼1200 mg m−3, and showed similar performance for a large estuary (Mobile Bay). Substantial spatio-temporal variability in the satellite-derived POC was observed on the LCS, with high POC found on the inner shelf (< 10 m depth) and lower POC on the middle (10-50 m depth) and outer shelf (50-200 m depth), and with high POC found in winter (January to March) and lower POC in summer to fall (August to October). Correlation analysis between long-term POC time series and several potential influencing factors indicated that river discharge played a dominant role in POC dynamics on the LCS, while wind and surface currents also affected POC spatial patterns on short time scales. This study adds another example where satellite data with carefully developed algorithms can greatly increase the spatial and temporal observations of important biogeochemical variables on continental shelf and estuaries. This article is protected by copyright. All rights reserved.
      PubDate: 2016-12-29T18:35:44.221314-05:
      DOI: 10.1002/2016JC012275
  • Importance of the Equatorial Undercurrent on the Sea Surface Salinity in
           the Eastern Equatorial Atlantic in boreal spring
    • Authors: C. Y. Da-Allada; J. Jouanno, F. Gaillard, N. Kolodziejczyk, C. Maes, N. Reul, B. Bourlès
      Abstract: The physical processes implied in the sea surface salinity (SSS) increase in the equatorial Atlantic Cold Tongue (ACT) region during boreal spring and the lag observed between boreal spring SSS maximum and sea surface temperature (SST) summer minimum are examined using mixed-layer salinity budgets computed from observations and model during the period 2010-2012. The boreal spring SSS maximum is mainly explained by an upward flux of high salinity originating from the core of the Equatorial Undercurrent (EUC) through vertical mixing and advection. The vertical mixing contribution to the mixed-layer salt budget peaks in April-May. It is controlled primarily by i) an increased zonal shear between the surface South Equatorial Current and the subsurface EUC and ii) the presence of a strong salinity stratification at the mixed-layer base from December to May. This haline stratification that is due to both high precipitations below the Inter Tropical Convergence Zone and zonal advection of low-salinity water from the Gulf of Guinea, explains largely the seasonal cycle of the vertical advection contribution to the mixed-layer salt budget. In the ACT region, the SST reaches its maximum in March/April and minimum in July/August. This SST minimum appears one month after the maximum of SSS. The 1-month lag observed between the maximum of SSS in June and the minimum of SST in July is explained by the shallowing of the EUC salinity core in June, then the weakening/erosion of the EUC in June-July which dramatically reduces the lateral subsurface supply of high saline waters. This article is protected by copyright. All rights reserved.
      PubDate: 2016-12-29T18:25:30.928408-05:
      DOI: 10.1002/2016JC012342
  • Multiscale Climate Emulator of Multimodal Wave Spectra: MUSCLE-spectra
    • Authors: Ana Rueda; Christie A. Hegermiller, Jose A.A. Antolinez, Paula Camus, Sean Vitousek, Peter Ruggiero, Patrick L. Barnard, Li H. Erikson, Antonio Tomás, Fernando J. Mendez
      Abstract: Characterization of multimodal directional wave spectra is important for many offshore and coastal applications, such as marine forecasting, coastal hazard assessment, and design of offshore wave energy farms and coastal structures. However, the multivariate and multiscale nature of wave climate variability makes this complex problem tractable using computationally-expensive numerical models. So far, the skill of statistical-downscaling model-based parametric (unimodal) wave conditions is limited in large ocean basins such as the Pacific. The recent availability of long-term directional spectral data from buoys and wave hindcast models allows for development of stochastic models that include multimodal sea-state parameters. This work introduces a statistical-downscaling framework based on weather types to predict multimodal wave spectra (e.g., significant wave height, mean wave period, and mean wave direction from different storm systems, including sea and swells) from large-scale atmospheric pressure fields. For each weather type, variables of interest are modeled using the categorical distribution for the sea-state type, the Generalized Extreme Value (GEV) distribution for wave height and wave period, a multivariate Gaussian copula for the interdependence between variables, and a Markov chain model for the chronology of daily weather types. We apply the model to the Southern California coast, where local seas and swells from both the Northern and Southern Hemispheres contribute to the multimodal wave spectrum. This work allows attribution of particular extreme multimodal wave events to specific atmospheric conditions, expanding knowledge of time-dependent, climate-driven offshore and coastal sea-state conditions that have a significant influence on local nearshore processes, coastal morphology, and flood hazards. This article is protected by copyright. All rights reserved.
      PubDate: 2016-12-29T18:20:35.14266-05:0
      DOI: 10.1002/2016JC011957
  • Mapping the Non-Stationary Internal Tide with Satellite Altimetry
    • Authors: Edward D. Zaron
      Abstract: Temporal variability of the internal tide has been inferred from the 23-year-long combined records of the TOPEX/Poseidon, Jason-1, and Jason-2 satellite altimeters by combining harmonic analysis with an analysis of along-track wavenumber spectra of sea-surface height (SSH). Conventional harmonic analysis is first applied to estimate and remove the stationary components of the tide at each point along the reference ground tracks. The wavenumber spectrum of the residual SSH is then computed, and the variance in a neighborhood around the wavenumber of the mode-1 baroclinic M2 tide is interpreted as the sum of noise, broadband non-tidal processes, and the non-stationary tide. At many sites a bump in the spectrum associated with the internal tide is noted, and an empirical model for the noise and non-tidal processes is used to estimate the non-stationary semidiurnal tidal variance. The results indicate a spatially inhomogeneous pattern of tidal variability. Non-stationary tides are larger than stationary tides throughout much of the Equatorial Pacific and Indian Oceans. This article is protected by copyright. All rights reserved.
      PubDate: 2016-12-29T18:20:26.400285-05:
      DOI: 10.1002/2016JC012487
  • Mixing efficiency in the thermocline of lakes observed from eddy
           correlation flux measurements
    • Authors: Julika Weck; Andreas Lorke
      Abstract: Vertical mixing in the thermocline of lakes is poorly understood and most of the current knowledge is based on ex situ methods like laboratory measurements and simulations. Here, we used the eddy correlation technique (EC) to directly measure oxygen and buoyancy fluxes in the thermocline of two lakes (Lake Scharmützelsee and Lake Arendsee in 2012 and 2013, respectively). Additionally, sets of temperature microstructure profiles (SCAMP) were measured during the EC deployments. We used these data to quantify the mixing efficiency as well as the turbulent diffusivity. The derived turbulent diffusivities from EC for the Prandtl number of DO were one order of magnitude higher than predicted by commonly applied parameterization, while the diffusivities for the Prandtl number of heat confirmed the parameterization. The results from EC and SCAMP showed strong differences which we attribute to the fact that SCAMP measurements reflect snapshots of the instantaneous turbulence field while EC provides a temporal average of the prevailing turbulence. Finally, we discuss problems of the EC and the inertial dissipation method in a strongly stratified environment and propose how they could be improved to resolve the full temporal variability of mixing in thermoclines. This article is protected by copyright. All rights reserved.
      PubDate: 2016-12-29T05:41:31.942552-05:
      DOI: 10.1002/2016JC012188
  • Tsunamis generated by long and thin granular landslides in a large flume
    • Authors: Garrett S. Miller; W. Andy Take, Ryan P. Mulligan, Scott McDougall
      Abstract: In this experimental study granular material is released down slope to investigate landslide-generated waves. Starting with a known volume and initial position of the landslide source, detailed data are obtained on the velocity and thickness of the granular flow, the shape and location of the submarine landslide deposit, the amplitude and shape of the near-field wave, the far-field wave evolution, and the wave runup elevation on a smooth impermeable slope. The experiments are performed on a 6.7 m long 30° slope on which gravity accelerates the landslides into a 2.1 m wide and 33.0 m long wave flume that terminates with a 27° runup ramp. For a fixed landslide volume of 0.34 m3, tests are conducted in a range of still water depths from 0.05-0.50 m. Observations from high-speed cameras and measurements from wave probes indicate that the granular landslide moves as a long and thin train of material, and that only a portion of the landslide (termed the ‘effective mass') is engaged in activating the leading wave. The wave behaviour is highly dependent on the water depth relative to the size of the landslide. In deeper water the near-field wave behaves as a stable solitary-like wave, while in shallower water the wave behaves as a breaking dissipative bore. Overall, the physical model observations are in good agreement with the results of existing empirical equations when the effective mass is used to predict the maximum near-field wave amplitude, the far-field amplitude and the runup of tsunamis generated by granular landslides. This article is protected by copyright. All rights reserved.
      PubDate: 2016-12-29T05:41:00.929071-05:
      DOI: 10.1002/2016JC012177
  • The influence of ENSO on an oceanic eddy pair in the South China Sea
    • Authors: Xiaoqing Chu; Changming Dong, Yiquan Qi
      Abstract: An eddy pair off the Vietnam coast is one of the most important features of the summertime South China Sea circulation. Its variability is of interest due to its profound impact on regional climate, ecosystems, biological processes, and fisheries. This study examines the influence of the El Niño–Southern Oscillation (ENSO), a basin-scale climatic mode, on the interannual variability of this regional eddy pair using satellite observational data and historical hydrographic measurements. Over the last three decades, the eddy pair strengthened in 1994 and 2002, and weakened in 2006, 2007, and 2008. It was absent in 1988, 1995, 1998, and 2010, coinciding with strong El Nino-to-La Nina transitions. Composite analyses showed that the strong transition events of ENSO led to radical changes in the summer monsoon, through the forcing of a unique sea surface temperature anomaly structure over the tropical Indo-Pacific basin. With weaker zonal wind, a more northward wind direction, and the disappearance of a pair of positive and negative wind stress curls, the eastward current jet turns northward along the Vietnam coast and the eddy pair disappears. This article is protected by copyright. All rights reserved.
      PubDate: 2016-12-29T05:40:26.947209-05:
      DOI: 10.1002/2016JC012642
  • A new real-time tsunami detection algorithm
    • Authors: Francesco Chierici; Davide Embriaco, Luca Pignagnoli
      Abstract: Real-time tsunami detection algorithms play a key role in any Tsunami Early Warning System. We have developed a new algorithm for tsunami detection based on the real-time tide removal and real-time band-pass filtering of sea-bed pressure recordings. The algorithm greatly increases the tsunami detection probability, shortens the detection delay and enhances detection reliability with respect to the most widely used tsunami detection algorithm, while containing the computational cost. The algorithm is designed to be used also in autonomous early warning systems with a set of input parameters and procedures which can be reconfigured in real time. We have also developed a methodology based on Monte Carlo simulations to test the tsunami detection algorithms. The algorithm performance is estimated by defining and evaluating statistical parameters, namely the detection probability, the detection delay, which are functions of the tsunami amplitude and wavelength, and the occurring rate of false alarms. Pressure data sets acquired by Bottom Pressure Recorders in different locations and environmental conditions have been used in order to consider real working scenarios in the test. We also present an application of the algorithm to the tsunami event which occurred at Haida Gwaii on October 28th, 2012 using data recorded by the Bullseye underwater node of Ocean Networks Canada. The algorithm successfully ran for test purpose in year-long missions onboard abyssal observatories, deployed in the Gulf of Cadiz and in the Western Ionian Sea. This article is protected by copyright. All rights reserved.
      PubDate: 2016-12-27T05:25:26.25192-05:0
      DOI: 10.1002/2016JC012170
  • Significant wave heights from Sentinel-1 SAR: Validation and applications
    • Authors: J. E. Stopa; A. Mouche
      Abstract: Two empirical algorithms are developed for wave mode images measured from the synthetic aperture radar aboard Sentinel-1 A. The first method, called CWAVE_S1A, is an extension of previous efforts developed for ERS2 and the second method, called Fnn, uses the azimuth cutoff amongst other parameters to estimate significant wave heights and average wave periods without using a modulation transfer function. Neural networks are trained using co-located data generated from WAVEWATCH III and independently verified with data from altimeters and in-situ buoys. We use neural networks to relate the nonlinear relationships between the input SAR image parameters and output geophysical wave parameters. CWAVE_S1A performs well and has reduced precision compared to Fnn with Hs root mean square errors within 0.5 and 0.6 m respectively. The developed neural networks extend the SAR's ability to retrieve useful wave information under a large range of environmental conditions including extra-tropical and tropical cyclones. This article is protected by copyright. All rights reserved.
      PubDate: 2016-12-26T19:50:44.472802-05:
      DOI: 10.1002/2016JC012364
  • Methane and nitrous oxide distributions across the North American Arctic
           Ocean during summer, 2015
    • Authors: Lindsay Fenwick; David Capelle, Ellen Damm, Sarah Zimmermann, Bill Williams, Svein Vagle, Philippe D. Tortell
      Abstract: We collected Arctic Ocean water column samples for methane (CH4) and nitrous oxide (N2O) analysis on three separate cruises in the summer and fall of 2015, covering a ∼10,000 km transect from the Bering Sea to Baffin Bay. This provided a three-dimensional view of CH4 and N2O distributions across contrasting hydrographic environments, from the oligotrophic waters of the deep Canada Basin and Baffin Bay, to the productive shelves of the Bering and Chukchi Seas. Percent saturation relative to atmospheric equilibrium ranged from 30-800% for CH4 and 75-145% for N2O, with the highest concentrations of both gases occurring in the northern Chukchi Sea. Nitrogen cycling in the shelf sediments of the Bering and Chukchi Seas likely constituted the major source of N2O to the water column, and the resulting high N2O concentrations were transported across the Arctic Ocean in eastward-flowing water masses. Methane concentrations were more spatially heterogeneous, reflecting a variety of localized inputs, including likely sources from sedimentary methanogenesis and sea ice processes. Unlike N2O, CH4 was rapidly consumed through microbial oxidation in the water column, as shown by the 13C enrichment of CH4 with decreasing concentrations. For both CH4 and N2O, sea-air fluxes were close to neutral, indicating that our sampling region was neither a major source nor sink of these gases. Our results provide insight into the factors controlling the distribution of CH4 and N2O in the North American Arctic Ocean, and an important baseline data set against which future changes can be assessed. This article is protected by copyright. All rights reserved.
      PubDate: 2016-12-26T19:50:36.424758-05:
      DOI: 10.1002/2016JC012493
  • Modeling the intense 2012-2013 dense water formation event in the
           northwestern Mediterranean Sea: Evaluation with an ensemble simulation
    • Authors: Robin Waldman; Samuel Somot, Marine Herrmann, Anthony Bosse, Guy Caniaux, Claude Estournel, Loic Houpert, Louis Prieur, Florence Sevault, Pierre Testor
      Abstract: The northwestern Mediterranean Sea is a well-observed ocean deep convection site. Winter 2012-2013 was an intense and intensely documented dense water formation (DWF) event. We evaluate this DWF event in an ensemble configuration of the regional ocean model NEMOMED12. We then assess for the first time the impact of ocean intrinsic variability on DWF with a novel perturbed initial state ensemble method. Finally, we identify the main physical mechanisms driving water mass transformations.NEMOMED12 reproduces accurately the deep convection chronology between late January and March, its location off the Gulf of Lions although with a southward shift and its magnitude. It fails to reproduce the Western Mediterranean Deep Waters salinification and warming, consistently with too strong a surface heat loss.The Ocean Intrinsic Variability modulates half of the DWF area, especially in the open-sea where the bathymetry slope is low. It modulates marginally (3-5\%) the integrated DWF rate, but its increase with time suggests its impact could be larger at interannual timescales. We conclude that ensemble frameworks are necessary to evaluate accurately numerical simulations of DWF.Each phase of DWF has distinct diapycnal and thermohaline regimes: during preconditioning, the Mediterranean thermohaline circulation is driven by exchanges with the Algerian basin. During the intense mixing phase, surface heat fluxes trigger deep convection and internal mixing largely determines the resulting deep water properties. During restratification, lateral exchanges and internal mixing are enhanced. Finally, isopycnal mixing was shown to play a large role in water mass transformations during the preconditioning and restratification phases. This article is protected by copyright. All rights reserved.
      PubDate: 2016-12-26T19:50:27.646395-05:
      DOI: 10.1002/2016JC012437
  • Bioavailable dissolved organic matter and biological hot spots during
           austral winter in Antarctic waters
    • Authors: Yuan Shen; Ronald Benner, Alison E. Murray, Carla Gimpel, B. Greg Mitchell, Elliot L. Weiss, Christian Reiss
      Abstract: Primary production and heterotrophic bacterial activity in the Antarctic Ocean are generally low during the austral winter. Organic carbon is considered to be a major factor limiting bacterial metabolism, but few studies have investigated the bioavailability of organic matter during winter. Herein, the chemical composition and bioavailability of dissolved organic matter (DOM) were investigated in surface (5-100 m) and mesopelagic (200-750 m) waters off the northwestern Antarctic Peninsula during August 2012. Concentrations of dissolved organic carbon (DOC) were low (42±4 µmol L−1) and showed no apparent spatial patterns. By contrast, the composition of DOM exhibited significant spatial trends that reflected the varying ecosystem productivity and water masses. Surface distributions of chlorophyll-a and particulate organic carbon depicted a southward decline in primary productivity from open waters (60.0˚S-61.5˚S) to ice-covered regions (61.5˚S-62.5˚S). This trend was evident from concentrations and DOC-normalized yields of dissolved amino acids in the surface waters, indicating decreasing DOM bioavailability with increasing latitude. A different pattern of DOM bioavailability was observed in the mesopelagic water masses, where amino acids indicated highly altered DOM in the Circumpolar Deep Water and bioavailable DOM in the Transitional Weddell Water. Depth distributions of amino acid yields and compositions revealed hot spots of elevated bioavailable DOM at ∼75 m relative to surrounding waters at most ice-free stations. Relatively low mole percentages of bacterially-derived d-amino acids in hot spots were consistent with an algal source of bioavailable DOM. Overall, these results reveal spatial heterogeneity of bioavailable substrates in Antarctic waters during winter. This article is protected by copyright. All rights reserved.
      PubDate: 2016-12-26T19:45:50.326231-05:
      DOI: 10.1002/2016JC012301
  • Surface flux and ocean heat transport convergence contributions to
           seasonal and interannual variations of ocean heat content
    • Authors: C. D. Roberts; M. D. Palmer, R. P. Allan, D.G.D. Desbruyeres, P. Hyder, C. Liu, D. Smith
      Abstract: We present an observation-based heat budget analysis for seasonal and interannual variations of ocean heat content (H) in the mixed layer (Hmld) and full depth ocean (Htot). Surface heat flux and ocean heat content estimates are combined using a novel Kalman smoother-based method. Regional contributions from ocean heat transport convergences are inferred as a residual and the dominant drivers of Hmld and Htot are quantified for seasonal and interannual time scales. We find that non-Ekman ocean heat transport processes dominate Hmld variations in the equatorial oceans and regions of strong ocean currents and substantial eddy activity. In these locations, surface temperature anomalies generated by ocean dynamics result in turbulent flux anomalies that drive the overlying atmosphere. In addition, we find large regions of the Atlantic and Pacific oceans where heat transports combine with local air-sea fluxes to generate mixed layer temperature anomalies. In all locations except regions of deep convection and water mass transformation, interannual variations in Htot are dominated by the internal rearrangement of heat by ocean dynamics rather than the loss or addition of heat at the surface. Our analysis suggests that, even in extra-tropical latitudes, initialization of ocean dynamical processes could be an important source of skill for interannual predictability of Hmld and Htot. Furthermore, we expect variations in Htot (and thus thermosteric sea level) to be more predictable than near surface temperature anomalies due to the increased importance of ocean heat transport processes for full-depth heat budgets. This article is protected by copyright. All rights reserved.
      PubDate: 2016-12-26T19:45:38.7027-05:00
      DOI: 10.1002/2016JC012278
  • Observed mesoscale eddy signatures in Southern Ocean surface mixed-layer
    • Authors: U. Hausmann; Dennis J. McGillicuddy, John Marshall
      Abstract: Combining satellite altimetry with Argo profile data a systematic observational estimate of mesoscale eddy signatures in surface mixed-layer depth (MLD) is provided across the Southern Ocean (SO). Eddy composite MLD anomalies are shallow in cyclones, deep in anticyclones, and increase in magnitude with eddy amplitude. Their magnitudes show a pronounced seasonal modulation roughly following the depth of the climatological mixed layer. Weak eddies of the relatively quiescent SO subtropics feature peak late-winter perturbations of ±10 m. Much larger MLD perturbations occur over the vigorous eddies originating along the Antarctic Circumpolar Current (ACC) and SO western boundary current systems, with late-winter peaks of −30 m and +60 m in the average over cyclonic and anticyclonic eddy cores (a difference of ≈ 100 m). The asymmetry between modest shallow cyclonic and pronounced deep anticyclonic anomalies is systematic and not accompanied by corresponding asymmetries in eddy amplitude. Nonetheless the net deepening of the climatological SO mixed layer by this asymmetry in eddy MLD perturbations is estimated to be small (few meters). Eddies are shown to enhance SO MLD variability with peaks in late winter and eddy-intense regions. Anomalously deep late-winter mixed layers occur disproportionately within the cores of anticyclonic eddies, suggesting the mesoscale heightens the frequency of deep winter surface-mixing events along the eddy-intense regions of the SO. The eddy modulation in MLD reported here provides a pathway via which the oceanic mesoscale can impact air-sea fluxes of heat and carbon, the ventilation of water masses, and biological productivity across the SO. This article is protected by copyright. All rights reserved.
      PubDate: 2016-12-26T19:45:31.602686-05:
      DOI: 10.1002/2016JC012225
  • Decadal changes in salinity in the oceanic subtropical gyres
    • Authors: Bryce A. Melzer; Bulusu Subrahmanyam
      Abstract: We analyzed spatial and temporal salinity trends in five subtropical gyre regions over the past six decades using Simple Ocean Data Assimilation (SODA) reanalysis with a focus on the subsurface salinity of the upper 1000 m of the ocean. Our results indicate an overall salinity increase within the mixed layer, and a salinity decrease at depths greater than 200m in the global subtropical gyres over 61 years, of which each individual gyre was analyzed in further detail. We determine that freshwater fluxes at the air-sea interface are the primary drivers of the sea surface salinity (SSS) signature over these open ocean regions by quantifying the advective contribution within the surface layer. This was demonstrated through a mixed layer salinity budget in each subtropical gyre based on the vertically integrated advection and entrainment of salt. Our analysis of decadal variability of fluxes into and out of the gyres reveals little change in the strength of the mean currents through this region despite an increase in the annual export of salt in all subtropical gyres, with the meridional component dominating the zonal. This study reveals that the salt content of E-P maximum waters advected into the subtropical gyres is increasing over time. A combination of increasing direct evaporation over the regions with increasing remote evaporation over nearby E-P maxima is believed to be the main driver in increasing salinity of the subtropical oceans, suggesting an intensification of the global water cycle over decadal timescales. This article is protected by copyright. All rights reserved.
      PubDate: 2016-12-26T19:45:26.679908-05:
      DOI: 10.1002/2016JC012243
  • Water Mass Modification and Mixing Rates in a 1/12° Simulation of the
           Canadian Arctic Archipelago
    • Authors: Kenneth G. Hughes; Jody M. Klymak, Xianmin Hu, Paul G. Myers
      Abstract: Strong spatial differences in diapycnal mixing across the Canadian Arctic Archipelago are diagnosed in a 1/12° basin-scale model. Changes in mass flux between water flowing into or out of several regions are analyzed using a volume-integrated advection–diffusion equation, and focus is given to denser water, the direct advective flux of which is mediated by sills. The unknown in the mass budget, mixing strength, is a quantity seldom explored in other studies of the Archipelago, which typically focus on fluxes. Regionally averaged diapycnal diffusivities and buoyancy fluxes are up to an order of magnitude larger in the eastern half of the Archipelago relative to those in the west. Much of the elevated mixing is concentrated near sills in Queens Channel and Barrow Strait, with stronger mixing particularly evident in the net shifts of the densest water to lower densities as it traverses these constrictions. Associated with these shifts are areally averaged buoyancy fluxes up to 10−8 m2 s−3 through the 1027 kg m−3 isopycnal surface, which lies at approximately 100m depth. This value is similar in strength to the destabilizing buoyancy flux at the ocean surface during winter. Effective diffusivities estimated from the buoyancy fluxes can exceed 10−4 m2 s−1, but are often closer to 10−5 m2 s−1 across the Archipelago. Tidal forcing, known to modulate mixing in the Archipelago, is not included in the model. Nevertheless, mixing metrics derived from our simulation are of the same order of magnitude as the few comparable observations. This article is protected by copyright. All rights reserved.
      PubDate: 2016-12-23T06:56:23.849476-05:
      DOI: 10.1002/2016JC012200
  • Nitrogen fixation in the eastern Atlantic reaches similar levels in the
           southern and northern hemisphere
    • Authors: Debany Fonseca-Batista; Frank Dehairs, Virginie Riou, François Fripiat, Marc Elskens, Florian Deman, Natacha Brion, Fabien Quéroué, Maya Bode, Holger Auel
      Abstract: Euphotic layer dinitrogen (N2) fixation and primary production (PP) were measured in the eastern Atlantic Ocean (38°N–21°S) using 15N2 and 13C bicarbonate tracer incubations. This region is influenced by Saharan dust deposition and waters with low nitrogen to phosphorus (N/P) ratios originating from the Subantarctic and the Benguela upwelling system. Depth-integrated rates of N2 fixation in the north (0–38°N) ranged from 59 to 370 µmol N m−2 d−1, with the maximal value at 19°N under the influence of the northwest African upwelling. Diazotrophic activity in the south (0–21°S), though slightly lower, was surprisingly close to observations in the north, with values ranging from 47 to 119 µmol N m−2 d−1. Our North Atlantic N2 fixation rates correlate well with dust deposition, while those in the South Atlantic correlate strongly with excess phosphate relative to nitrate. There the necessary iron is assumed to be supplied from the Benguela upwelling system. When converting N2 fixation to carbon uptake using a Redfield ratio (6.6), we find that N2 fixation may support up to 9% of PP in the subtropical North Atlantic (20–38°N), 5% in the tropical North Atlantic (0–20°N) and 1% of PP in the South Atlantic (0–21°S). Combining our data with published datasets, we estimate an annual N input of 27.6 ± 10 Tg N yr−1 over the open Atlantic Ocean, 11% of which enters the region between 20° to 50°N, 71% between 20°N and 10°S and 18% between 10 and 45°S. This article is protected by copyright. All rights reserved.
      PubDate: 2016-12-22T04:50:34.570514-05:
      DOI: 10.1002/2016JC012335
  • Turbulent and numerical mixing in a salt wedge estuary: Dependence on grid
           resolution, bottom roughness, and turbulence closure
    • Authors: David K. Ralston; Geoffrey W. Cowles, W. Rockwell Geyer, Rusty C. Holleman
      Abstract: The Connecticut River is a tidal salt wedge estuary, where advection of sharp salinity gradients through channel constrictions and over steeply sloping bathymetry leads to spatially heterogeneous stratification and mixing. A 3-d unstructured-grid finite-volume hydrodynamic model (FVCOM) was evaluated against shipboard and moored observations, and mixing by both the turbulent closure and numerical diffusion were calculated. Excessive numerical mixing in regions with strong velocities, sharp salinity gradients, and steep bathymetry reduced model skill for salinity. Model calibration was improved by optimizing both the bottom roughness (z0), based on comparison with the barotropic tidal propagation, and the mixing threshold in the turbulence closure (steady-state Richardson number, Rist), based on comparison with salinity. Whereas a large body of evidence supports a value of Rist ∼ 0.25, model skill for salinity improved with Rist ∼ 0.1. With Rist = 0.25, numerical mixing contributed about 1/2 the total mixing, while with Rist = 0.10 it accounted for ∼2/3, but salinity structure was more accurately reproduced. The combined contributions of numerical and turbulent mixing were quantitatively consistent with high-resolution measurements of turbulent mixing. A coarser grid had increased numerical mixing, requiring further reductions in turbulent mixing and greater bed friction to optimize skill. The optimal Rist for the fine grid case was closer to 0.25 than for the coarse grid, suggesting that additional grid refinement might correspond with Rist approaching the theoretical limit. Numerical mixing is rarely assessed in realistic models, but comparisons with high-resolution observations in this study suggest it is an important factor. This article is protected by copyright. All rights reserved.
      PubDate: 2016-12-22T04:46:07.28881-05:0
      DOI: 10.1002/2016JC011738
  • Extension of the prognostic model of sea surface temperature to
           rain-induced cool and fresh lenses
    • Authors: H. Bellenger; K. Drushka, W. Asher, G. Reverdin, M. Katsumata, M. Watanabe
      Abstract: The Zeng and Beljaars (2005) sea surface temperature prognostic scheme, developed to represent diurnal warming, is extended to represent rain-induced freshening and cooling. Effects of rain on salinity and temperature in the molecular skin layer (first few hundred micrometers) and the near-surface turbulent layer (first few meters) are separately parameterized by taking into account rain-induced fluxes of sensible heat and freshwater, surface stress, and mixing induced by droplets penetrating the water surface. Numerical results from this scheme are compared to observational data from two field studies of near-surface ocean stratifications caused by rain, to surface drifter observations and to previous computations with an idealized ocean mixed layer model, demonstrating that the scheme produces temperature variations consistent with in situ observations and model results. It reproduces the dependency of salinity on wind and rainfall rate and the lifetime of fresh lenses. In addition, the scheme reproduces the observed lag between temperature and salinity minimum at low wind speed and is sensitive to the peak rain rate for a given amount of rain. Finally, a first assessment of the impact of these fresh lenses on ocean surface variability is given for the near-equatorial western Pacific. In particular, the variability due to the mean rain-induced cooling is comparable to the variability due to the diurnal warming so that they both impact large-scale horizontal surface temperature gradients. The present parameterization can be used in a variety of models to study the impact of rain-induced fresh and cool lenses at different spatial and temporal scales. This article is protected by copyright. All rights reserved.
      PubDate: 2016-12-22T04:46:01.203328-05:
      DOI: 10.1002/2016JC012429
  • Hydrodynamic influences on acoustical and optical backscatter in a
           fringing reef environment
    • Authors: G. Pawlak; M. A. Moline, E. J. Terrill, P. L. Colin
      Abstract: Observations of hydrodynamics along with optical and acoustical water characteristics in a tropical fringing reef environment reveal a distinct signature associated with flow characteristics and tidal conditions. Flow conditions are dominated by tidal forcing with an offshore component from the reef flat during ebb. Measurements span variable wave conditions enabling identification of wave effects on optical and acoustical water properties.High frequency acoustic backscatter (6 MHz) is strongly correlated with tidal forcing increasing with offshore directed flow and modulated by wave height, indicating dominant hydrodynamic influence. Backscatter at 300 kHz and 1200 kHz is predominantly diurnal suggesting a biological component. Optical backscatter is closely correlated with high frequency acoustic backscatter across the range of study conditions. Acoustic backscatter frequency dependence is used along with changes in optical properties to interpret particle size variations. Changes across wave heights suggest shifts in particle size distributions with increases in relative concentrations of smaller particles for larger wave conditions. Establishing a connection between the physical processes of a fringing tropical reef and the resulting acoustical and optical signals allows for interpretation and forecasting of the remote sensing response of these phenomena over larger scales. This article is protected by copyright. All rights reserved.
      PubDate: 2016-12-22T04:45:56.961567-05:
      DOI: 10.1002/2016JC012497
  • Sea surface pCO2 and O2 dynamics in the partially ice-covered Arctic Ocean
    • Authors: Fakhrul Islam; Michael D. DeGrandpre, Cory M. Beatty, Mary-Louise Timmermans, Richard A. Krishfield, John M. Toole, Samuel R. Laney
      Abstract: Understanding the physical and biogeochemical processes that control CO2 and dissolved oxygen (DO) dynamics in the Arctic Ocean (AO) is crucial for predicting future air-sea CO2 fluxes and ocean acidification. Past studies have primarily been conducted on the AO continental shelves during low-ice periods and we lack information on gas dynamics in the deep AO basins where ice typically inhibits contact with the atmosphere. To study these gas dynamics, in situ time-series data have been collected in the Canada Basin during late summer to autumn of 2012. Partial pressure of CO2 (pCO2), DO concentration, temperature, salinity, and chlorophyll-a fluorescence (Chl-a) were measured in the upper ocean in a range of sea ice states by two drifting instrument systems. Although the two systems were on average only 222 km apart, they experienced considerably different ice cover and external forcings during the 40-50 d periods when data were collected. The pCO2 levels at both locations were well below atmospheric saturation whereas DO was almost always slightly supersaturated. Modeling results suggest that air-sea gas exchange, net community production (NCP) and horizontal gradients were the main sources of pCO2 and DO variability in the sparsely ice-covered AO. In areas more densely covered by sea ice, horizontal gradients were the dominant source of variability, with no significant NCP in the surface mixed layer. If the AO reaches equilibrium with atmospheric CO2 as ice cover continues to decrease, aragonite saturation will drop from a present mean of 1.00 ± 0.02 to 0.86 ± 0.01. This article is protected by copyright. All rights reserved.
      PubDate: 2016-12-22T04:45:47.136148-05:
      DOI: 10.1002/2016JC012162
  • Monitoring remote ocean waves using P-wave microseisms
    • Authors: Jennifer Neale; Nicholas Harmon, Meric Srokosz
      Abstract: Oceanic microseisms are generated by the interaction of opposing ocean waves and subsequent coupling with the seabed, so microseisms should contain information on the ocean conditions that generated them. This leads to the possibility of using seismic records as a proxy for the ocean gravity wavefield. Here we investigate the P-wave component of microseisms, which has previously been linked to areas of high wave interaction intensity in mid-ocean regions. We compare modeled P-wave microseismic sources with those observed at an array in California, and also investigate the relationship between observed sources and significant wave height. We found that the time-varying location of microseism sources in the North Pacific, mapped from beamforming and backprojection of seismic data, was accurate to ≤10° in 90% of cases. The modeled sources were found to dominate at ∼0.2 Hz which was also reflected in the seismic observations. An empirical relationship between observed beampower and modeled source power allowed sources during an independent data period to be estimated with a correlation coefficient of 0.63. Likewise, significant wave height was also estimated with a correlation coefficient of 0.63. Our findings suggest that with improvements in resolution and amplitude retrieval from beamforming, correlations up to 0.78 should be possible between observed P-wave microseisms and significant wave height in remote ocean regions. This article is protected by copyright. All rights reserved.
      PubDate: 2016-12-22T04:45:34.545346-05:
      DOI: 10.1002/2016JC012183
  • Interannual variability of tropical Pacific Sea level from 1993 to 2014
    • Authors: Xiaoting Zhu; Richard J. Greatbatch, Martin Claus
      Abstract: A multi-mode, linear reduced-gravity model, driven by ERA-Interim monthly mean wind stress anomalies, is used to investigate interannual variability in tropical Pacific sea level as seen in satellite altimeter data. The model output is fitted to the altimeter data along the equator, in order to derive the vertical profile for the model forcing, showing that a signature from modes higher than mode six cannot be extracted from the altimeter data. It is shown that the model has considerable skill at capturing interannual sea level variability both on and off the equator. The correlation between modelled and satellite-derived sea level data exceeds 0.8 over a wide range of longitudes along the equator and readily captures the observed ENSO events. Overall, the combination of the first, second, third and fifth modes can provide a robust estimate of the interannual sea level variability, the second mode being dominant. A remarkable feature of both the model and the altimeter data is the presence of a pivot point in the western Pacific on the equator. We show that the westward displacement of the pivot point from the centre of the basin is strongly influenced by the fact that most of the wind stress variance is found in the western part of the basin. We also show that the Sverdrup transport is not fundamental to the dynamics of the recharge/discharge mechanism in our model, although the spatial structure of the wind forcing does play a role in setting the amplitude of the “warm water volume”. This article is protected by copyright. All rights reserved.
      PubDate: 2016-12-22T04:45:27.105334-05:
      DOI: 10.1002/2016JC012347
  • Seasonal controls of aragonite saturation states in the Gulf of Maine
    • Authors: Zhaohui Aleck Wang; Gareth L. Lawson, Cynthia H. Pilskaln, Amy E. Maas
      Abstract: The Gulf of Maine (GoME) is a shelf region especially vulnerable to ocean acidification (OA) due to natural conditions of low pH and aragonite saturation states (Ω-Ar). This study is the first to assess the major oceanic processes controlling seasonal variability of the carbonate system and its linkages with pteropod abundance in Wilkinson Basin in the GoME. Two years of seasonal sampling cruises suggest that water-column carbonate chemistry in the region undergoes a seasonal cycle, wherein the annual cycle of stratification-overturn, primary production, respiration-remineralization and mixing all play important roles, at distinct spatiotemporal scales. Surface production was tightly coupled with remineralization in the benthic nepheloid layer during high production seasons, which results in occasional aragonite undersaturation. From spring to summer, carbonate chemistry in the surface across Wilkinson Basin reflects a transition from a production-respiration balanced system to a net autotropic system. Mean water-column Ω-Ar and abundance of large thecosomatous pteropods show some correlation, although patchiness and discrete cohort reproductive success likely also influence their abundance. Overall, photosynthesis-respiration is the primary driving force controlling Ω-Ar variability during the spring-to-summer transition as well as over the seasonal cycle. However, calcium carbonate (CaCO3) dissolution appears to occur near bottom in fall and winter when bottom water Ω-Ar is generally low but slightly above 1. This is accompanied by a decrease in pteropod abundance that is consistent with previous CaCO3 flux trap measurements. The region might experience persistent subsurface aragonite undersaturation in 30-40 years under continued ocean acidification. This article is protected by copyright. All rights reserved.
      PubDate: 2016-12-20T18:15:29.583734-05:
      DOI: 10.1002/2016JC012373
  • Using Landsat 8 data to estimate suspended particulate matter in the
           Yellow River estuary
    • Authors: Zhongfeng Qiu; Cong Xiao, William Perrie, Deyong Sun, Shengqiang Wang, Hui Shen, Dezhou Yang, Yijun He
      Abstract: The distribution of suspended particulate matter (SPM) and its variations in estuary regions are key to promoting carbon, oxygen and nutrient cycling in coastal regions and nearby seas. This study presents SPM estimations for the Yellow River estuary from Landsat 8 Operational Land Imager (L8/OLI) data from 2013 to 2016. L8/OLI-measured remote sensing reflectance (Rrs) was cross-validated with Moderate Resolution Imaging Spectroradiometer (MODIS) measurements, and SPM concentrations calculated from the tuned retrieval model, were validated with in situ observations. The validation shows that L8/OLI can provide reasonably Rrs, which can be used to quantify SPM distributions and variations in the Yellow River estuary. Three-year averaged SPM maps show that highly turbid waters are mostly found in an ovate area surrounding the mouth of the Yellow River. The corresponding area proportion is less than 30%, with SPM concentrations greater than 100 g m−3. High variations of SPM distributions are consistent with high SPM concentrations, and vice versa. Significant difference is observed between dry and wet seasons. Higher SPM in the dry season are observed both in range and intensity compared to those of the wet season. Furthermore, multiyear averaged SPM distributions with high concentrations are mainly attributable to currents. Significant seasonal variations are mainly controlled by sediment re-suspension processes driven by wind-wave forces. Due to human interventions, seasonal variability in river runoff and sediment discharge from the Yellow River has decreased in recent years. Accordingly, seasonal variability in SPM distributions in the Yellow River estuary due to sediment discharge has decreased. This article is protected by copyright. All rights reserved.
      PubDate: 2016-12-20T18:15:27.342577-05:
      DOI: 10.1002/2016JC012412
  • Satellite assessment of particulate matter and phytoplankton variations in
           the Santa Barbara Channel and its surrounding waters: Role of surface
    • Authors: Fernanda Henderikx Freitas; David A. Siegel, Stéphane Maritorena, Erik Fields
      Abstract: Satellite observations of chlorophyll in coastal waters are often described in terms of changes in productivity in response to regional upwelling processes while optical backscattering coefficients are more often linked to episodic inputs of suspended sediments from storm runoff. Here we show that the surface gravity wave resuspension of sediments has a larger role in controlling backscatter than previously considered. Almost 18 years of SeaWiFS, MODIS, MERIS and VIIRS satellite imagery of the Santa Barbara Channel, California and its surrounding waters spectrally-merged with the Garver-Siegel-Maritorena bio-optical model were used to assess the controls on suspended particle distributions. Analysis revealed that chlorophyll blooms in the warmer portions of the domain occur in phase with SST minima, usually in early spring, while blooms in the cooler regions lag SST minima and occur simultaneously to the strongest equatorward winds every year, often in the summer. Tight coupling between the optical variables was seen in offshore areas, as expected for productive waters. However, values of backscatter near the coast were primarily modulated by surface waves. This relationship holds throughout all seasons and is stronger within the 100-m isobath, but often extends tens of kilometers offshore. This forcing of particle resuspension by surface waves is likely a feature ubiquitous in all coastal oceans characterized by fine sediments. The implication of surface wave processes determining suspended particle loads far beyond the surf zone has large consequences for the interpretation of satellite ocean color signals in coastal waters and potentially redefines the extent of the littoral zone. This article is protected by copyright. All rights reserved.
      PubDate: 2016-12-20T18:15:24.861734-05:
      DOI: 10.1002/2016JC012152
  • On the hydrography of Denmark Strait
    • Authors: Dana Mastropole; Robert S. Pickart, Héðinn Valdimarsson, Kjetil Våge, Kerstin Jochumsen, James Girton
      Abstract: Using 111 shipboard hydrographic sections across Denmark Strait occupied between 1990-2012, we characterize the mean conditions at the sill, quantify the water mass constituents, and describe the dominant features of the Denmark Strait Overflow Water (DSOW). The mean vertical sections of temperature, salinity, and density reveal the presence of circulation components found upstream of the sill, in particular the shelfbreak East Greenland Current (EGC) and the separated EGC. These correspond to hydrographic fronts consistent with surface-intensified southward flow. Deeper in the water column the isopycnals slope oppositely, indicative of bottom-intensified flow of DSOW. An end-member analysis indicates that the deepest part of Denmark Strait is dominated by Arctic-Origin Water with only small amounts of Atlantic-Origin Water. On the western side of the strait the overflow water is a mixture of both constituents, with a contribution from Polar Surface Water. Weakly stratified “boluses” of dense water are present in 41% of the occupations, revealing that this is a common configuration of DSOW. The bolus water is primarily Arctic-Origin Water and constitutes the densest portion of the overflow. The boluses have become warmer and saltier over the 22-year record, which can be explained by changes in end member properties and their relative contributions to bolus composition. This article is protected by copyright. All rights reserved.
      PubDate: 2016-12-20T13:20:44.897778-05:
      DOI: 10.1002/2016JC012007
  • A periodic freshwater patch detachment process from the block Island sound
           estuarine plume
    • Authors: Qianqian Liu; Lewis M. Rothstein, Yiyong Luo
      Abstract: Previous observations suggest periodic freshwater patches separating from the Block Island Sound (BIS) estuarine plume. In this study, the dynamics of the separation process is investigated through a series of numerical experiments using the Regional Ocean Modeling System (ROMS). In addition, we explore the seasonal variability of the freshwater patches and their response to river discharge and ambient current. The model results indicate that episodic freshwater patches are triggered by small changes in tidal currents over the spring-neap tidal cycle. The spring-neap variation in tidal currents causes significant, monthly fluctuations in turbulent mixing and vertical stratification in BIS, modulating the freshwater discharge thereby generating episodic freshwater patches that move both downstream along the southern shore of Long Island and toward Rhode Island Sound (RIS). The realistically configured model shows that the freshwater patches experience strong seasonal variability. They are largest in spring when the river discharge peaks, and smallest in summer due to the weak river discharge and a robust upstream ambient current from RIS. According to the analysis of the freshwater transport out of BIS, we conclude that such detachment occurs at tidal mixing fronts. This article is protected by copyright. All rights reserved.
      PubDate: 2016-12-20T13:20:37.919238-05:
      DOI: 10.1002/2015JC011546
  • Deep temperature variability in Drake passage
    • Authors: Yvonne L. Firing; Elaine L. McDonagh, Brian A. King, Damien G. Desbruyères
      Abstract: Observations made on 21 occupations between 1993 and 2016 of GO-SHIP line SR1b in eastern Drake Passage show an average temperature of 0.53°C deeper than 2000 dbar, with no significant trend, but substantial year-to-year variability (standard deviation 0.08°C). Using a neutral density framework to decompose the temperature variability into isopycnal displacement (heave) and isopycnal property change components shows that approximately 95% of the year-to-year variance in deep temperature is due to heave. Changes on isopycnals make a small contribution to year-to-year variability but contribute a significant trend of -1.4±0.6 m°C per year, largest for density (γn) > 28.1, south of the Polar Front (PF). The heave component is depth-coherent and results from either vertical or horizontal motions of neutral density surfaces, which trend upward and northward around the PF, downward for the densest levels in the southern section, and downward and southward in the Subantarctic Front and Southern Antarctic Circumpolar Current Front (SACCF). A proxy for the locations of the Antarctic Circumpolar Current (ACC) fronts is constructed from the repeat hydrographic data and has a strong relationship with deep ocean heat content, explaining 76% of deep temperature variance. The same frontal position proxy based on satellite altimeter-derived surface velocities explains 73% of deep temperature variance. The position of the PF plays the strongest role in this relationship between ACC fronts and deep temperature variability in Drake Passage, although much of the temperature variability in the southern half of the section can be explained by the position of the SACCF. This article is protected by copyright. All rights reserved.
      PubDate: 2016-12-20T13:20:32.809602-05:
      DOI: 10.1002/2016JC012452
  • Combined observations of Arctic sea ice with near-coincident colocated X,
           C, and L-band SAR satellite remote sensing and helicopter-borne
    • Authors: A.M. Johansson; J.A. King, A.P. Doulgeris, S. Gerland, S. Singha, G. Spreen, T. Busche
      Abstract: In this study we compare co-located near-coincident X-, C- and L-band fully polarimetry SAR satellite images with helicopter-borne ice thickness measurements acquired during the Norwegian Young sea ICE 2015 (N-ICE2015) expedition in the region of the Arctic Ocean north of Svalbard in April 2015. The air-borne surveys provide near-coincident snow plus ice thickness, surface roughness data and photographs. This unique dataset allows us to investigate how the different frequencies can complement one another for sea ice studies; but also to raise awareness of limitations. X- and L-band satellite scenes were shown to be a useful complement to the standard SAR frequency for sea ice monitoring (C-band) for lead ice and young ice identification. This may be in part be due to the frequency but also the high spatial resolution of these sensors. Importantly this is true even when the scenes are outside their nominal performance range. We found a relatively low correlation between snow plus ice thickness and surface roughness. Therefore, ice thickness cannot directly be observed by SAR which has important implications for operational ice charting based on automatic segmentation. This article is protected by copyright. All rights reserved.
      PubDate: 2016-12-13T12:45:02.140353-05:
      DOI: 10.1002/2016JC012273
  • Tropical Cyclone asymmetry - development and evaluation of a new
           parametric model
    • Authors: M. Olfateh; David P. Callaghan, Peter Nielsen, Tom E. Baldock
      Abstract: A new parametric model is developed to describe the asymmetry commonly observed in Tropical Cyclones or Hurricanes. Observations from 21 Hurricanes from the Gulf of Mexico basin and TC Roger in the Coral Sea are analysed to determine the azimuthal and radial asymmetry typical in these mesoscale systems. On the basis of the observations a new asymmetric directional wind model is proposed which adjusts the widely used Holland (1980) axisymmetric wind model to account for the action of blocking high pressure systems, boundary layer friction and forward speed. The model is tested against the observations and demonstrated to capture the physical features of asymmetric cyclones and provides a better fit to observed winds than the Holland model. Optimum values and distributions of the model parameters are derived for use in statistical modelling. Finally, the model is used to investigation of the asymmetric character of TC systems, including the azimuth of the maximum wind speed, the degree of asymmetry and the re-lationship between asymmetry and forward speed. This article is protected by copyright. All rights reserved.
      PubDate: 2016-11-28T14:36:02.040704-05:
      DOI: 10.1002/2016JC012237
  • The ocean mixed-layer under Southern Ocean sea-ice: Seasonal cycle and
    • Authors: Violaine Pellichero; Jean-Baptiste Sallée, Sunke Schmidtko, Fabien Roquet, Jean-Benoît Charrassin
      Abstract: The oceanic mixed-layer is the gateway for the exchanges between the atmosphere and the ocean; in this layer all hydrographic ocean properties are set for months to millennia. A vast area of the Southern Ocean is seasonally capped by sea-ice, which alters the characteristics of the ocean mixed-layer. The interaction between the ocean mixed-layer and sea-ice plays a key role for water-mass transformation, the carbon cycle, sea-ice dynamics, and ultimately for the climate as a whole. However, the structure and characteristics of the under-ice mixed-layer are poorly understood due to the sparseness of in-situ observations and measurements. In this study, we combine distinct sources of observations to overcome this lack in our understanding of the Polar Regions. Working with Elephant Seal-derived observations, ship-based and Argo float observations, we describe the seasonal cycle of the ocean mixed-layer characteristics and stability of the ocean mixed-layer over the Southern Ocean and specifically under sea-ice. Mixed-layer heat and freshwater budgets are used to investigate the main forcing mechanisms of the mixed-layer seasonal cycle. The seasonal variability of sea surface salinity and temperature are primarily driven by surface processes, dominated by sea-ice freshwater flux for the salt budget, and by air-sea flux for the heat budget. Ekman advection, vertical diffusivity and vertical entrainment play only secondary roles.Our results suggest that changes in regional sea-ice distribution and annual duration, as currently observed, widely affect the buoyancy budget of the underlying mixed-layer, and impact large-scale water-mass formation and transformation with far reaching consequences for ocean ventilation. This article is protected by copyright. All rights reserved.
      PubDate: 2016-11-28T14:35:55.289042-05:
      DOI: 10.1002/2016JC011970
  • Role of wind in erosion–accretion cycles on an estuarine mudflat
    • Authors: B.W. Shi; S.L. Yang, Y.P. Wang, G.C. Li, M.L. Li, P. Li, C. Li
      Abstract: Wind is an important regulator of coastal erosion and accretion processes that have significant ecological and engineering implications. Nevertheless, previous studies have mainly focused on storm−generated changes in the bed level. This paper aims to improve the understanding of wind−induced erosion–accretion cycles on intertidal flats under normal (non−stormy) weather conditions using data that relates to the wave climate, near−bed 3D flow velocity, suspended sediment concentration, and bed−level changes on a mudflat at the Yangtze Delta front. The following parameters were calculated at 10−minute intervals over 10 days: the wind wave orbital velocity (Ûδ), bed shear stress from combined current–wave action, erosion flux, deposition flux, and predicted bed−level change. The time series of measured and predicted bed−level changes both show tidal cycles and a 10−day cycle. We attribute the tidal cycles of bed−level changes to tidal dynamics, but we attribute the 10−day cycle of bed−level changes to the interaction between wind speed/direction and neap−spring cyclicity. We conclude that winds can significantly affect bed−level changes in mudflats even during non−stormy weather and under macro−mesotidal conditions and that the bed−level changes can be predicted well using current–wave–sediment combined models. This article is protected by copyright. All rights reserved.
      PubDate: 2016-11-28T14:35:48.454254-05:
      DOI: 10.1002/2016JC011902
  • Storm surge along the Pacific Coast of North America
    • Authors: Peter D. Bromirski; Reinhard E. Flick, Arthur J. Miller
      Abstract: Storm surge is an important factor that contributes to coastal flooding and erosion. Storm surge magnitude along eastern North Pacific coasts results primarily from low sea level pressure (SLP). Thus coastal regions where high surge occurs identify the dominant locations where intense storms make landfall, controlled by storm track across the North Pacific. Here storm surge variability along the Pacific coast of North America is characterized by positive non-tide residuals at a network of tide gauge stations from southern California to Alaska. The magnitudes of mean and extreme storm surge generally increase from south to north, with typically high amplitude surge north of Cape Mendocino and lower surge to the south. Correlation of mode 1 non-tide principal component (PC1) during winter months (Dec.-Feb.) with anomalous SLP over the Northeast Pacific indicates that the dominant storm landfall region is along the Cascadia/British Columbia coast. Although empirical orthogonal function spatial patterns show substantial interannual variability, similar correlation patterns of non-tide PC1 over the 1948-1975 and 1983-2014 epochs with anomalous SLP suggest that, when considering decadal-scale time periods, storm surge and associated tracks have generally not changed appreciably since 1948. Non-tide PC1 is well-correlated with PC1 of both anomalous SLP and modeled wave height near the tide gauge stations, reflecting the inter-relationship between storms, surge, and waves. Weaker surge south of Cape Mendocino during the 2015-16 Niño compared with 1982-83 may result from changes in Hadley circulation. Importantly from a coastal impacts perspective, extreme storm surge events are often accompanied by high waves. This article is protected by copyright. All rights reserved.
      PubDate: 2016-11-11T17:03:03.014741-05:
      DOI: 10.1002/2016JC012178
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