for Journals by Title or ISSN
for Articles by Keywords
help

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

Showing 1 - 17 of 17 Journals sorted alphabetically
Geochemistry, Geophysics, Geosystems     Full-text available via subscription   (Followers: 26, SJR: 2.439, h-index: 91)
Geophysical Research Letters     Full-text available via subscription   (Followers: 114, SJR: 3.323, h-index: 185)
Global Biogeochemical Cycles     Full-text available via subscription   (Followers: 15, SJR: 3.22, h-index: 136)
J. of Advances in Modeling Earth Systems     Open Access   (Followers: 4, SJR: 4.444, h-index: 18)
J. of Geophysical Research : Atmospheres     Partially Free   (Followers: 135)
J. of Geophysical Research : Biogeosciences     Full-text available via subscription   (Followers: 29)
J. of Geophysical Research : Earth Surface     Partially Free   (Followers: 51)
J. of Geophysical Research : Oceans     Partially Free   (Followers: 49)
J. of Geophysical Research : Planets     Full-text available via subscription   (Followers: 109)
J. of Geophysical Research : Solid Earth     Full-text available via subscription   (Followers: 44)
J. of Geophysical Research : Space Physics     Full-text available via subscription   (Followers: 124)
Paleoceanography     Full-text available via subscription   (Followers: 5, SJR: 3.067, h-index: 100)
Radio Science     Full-text available via subscription   (Followers: 38, SJR: 1.072, h-index: 59)
Reviews of Geophysics     Full-text available via subscription   (Followers: 34, SJR: 8.833, h-index: 107)
Space Weather     Full-text available via subscription   (Followers: 17, SJR: 1.341, h-index: 26)
Tectonics     Full-text available via subscription   (Followers: 14, SJR: 2.628, h-index: 96)
Water Resources Research     Full-text available via subscription   (Followers: 80, SJR: 2.661, h-index: 144)
Journal Cover Journal of Geophysical Research : Oceans
  [49 followers]  Follow
    
   Partially Free Journal Partially Free Journal
   ISSN (Online) 2169-9291
   Published by AGU Homepage  [17 journals]
  • Observation of oxygen ventilation into deep waters through targeted
           deployment of multiple Argo-O2 floats in the north-western Mediterranean
           Sea in 2013
    • Authors: L. Coppola; L. Prieur, I. Taupier-Letage, C. Estournel, P. Testor, D. Lefevre, S. Belamari, S. LeReste, V. Taillandier
      Abstract: During the winter 2013, an intense observation and monitoring was performed in the north-western Mediterranean Sea to study deep water formation process that drives thermohaline circulation and biogeochemical processes (HYMEX SOP2 and DEWEX projects). To observe intensively and continuously the impact of deep convection on oxygen (O2) ventilation, an observation strategy was based on the enhancement of the Argo-O2 floats to monitor the offshore dense water formation area (DWF) in the Gulf of Lion prior to and at the end of the convective period (December 2012 to April 2013). The intense O2 measurements performed through shipborne CTD casts and Argo-O2 floats deployment revealed an O2 inventory rapidly impacted by mixed layer (ML) deepening on the month scale. The open-sea convection in winter 2013 ventilated the deep waters from mid-February to the end of May 2013. The newly ventilated dense water volume, based on an Apparent Oxygen Utilization (AOU) threshold, was estimated to be about 1.5 1013 m3 during the DWF episode, increasing the deep O2 concentrations from 196 to 205 µmol kg−1 in the north-western basin.
      PubDate: 2017-07-18T11:05:39.275542-05:
      DOI: 10.1002/2016JC012594
       
  • Seasonal cycle of hydrography on the Eastern Shelf of the Filchner Trough,
           Weddell Sea, Antarctica
    • Authors: S. Ryan; T. Hattermann, E. Darelius, M. Schröder
      Abstract: New two-year long records from three moorings, located at 76°S along the eastern flank and shelf of the Filchner Trough, give insight in the seasonal cycle of hydrography to a region where Modified Warm Deep Water (MWDW) enters the southern Weddell Sea continental shelf, possibly reaching the Filchner Ronne Ice Shelf, the biggest ice shelf (by volume) in Antarctica. A persistent northward flow of Ice Shelf Water (ISW) is found along the slope of the trough at 600 m depth, while the data on the shelf indicate a seasonal cycle, characterized by four phases. A distinct warm inflow period (separated in two phases), with maximum temperatures of −1°C, appears to be related to the seasonal heaving of the Antarctic Slope Front thermocline along the continental shelf break further north and a seasonal extension of the ISW layer onto the Eastern Shelf. The density gradients between the ISW in the trough and the MWDW on the adjacent shelf drive the southward flow during these phases. A flow reversal is found in winter, ceasing the southward flow along the eastern flank of the trough. Weaker density gradients between the trough and the shelf during winter allow a westward flow, partly driven by a N-S density gradient, existing across the Eastern Shelf during this time. From spring through to summer the ISW layer in the trough extends onto the eastern shelf where it occupies the bottom layer at our moorings and it is associated with northward flow.
      PubDate: 2017-07-15T03:20:47.385037-05:
      DOI: 10.1002/2017JC012916
       
  • Understanding the spatial variation of sea level rise in the North Sea
           using satellite altimetry
    • Authors: Paul Sterlini; Dewi Le Bars, Hylke de Vries, Nina Ridder
      Abstract: This paper examines the spatial variation of sea surface height trends in the North Sea Basin as seen by satellite altimetry and assesses its underlying causes. Changes in the potential temperature and salinity of the North Sea are transposed into corresponding changes in sea surface height and regional anomalies of linear sea level trend calculated. The same is carried out for the meteorological processes which act on the sea surface. The steric and meteorological regional sea level rise anomalies are summed with those from contributions from land ice and compared against the values seen by satellite altimetry over the period 1993 - 2014. Results show that there is good agreement between the observations and the reconstruction. The local meteorological contribution appears to be most important in describing regional variation in linear sea level rise and is reinforced with a local halosteric contribution which shows a similar spatial pattern.
      PubDate: 2017-07-15T03:20:31.042185-05:
      DOI: 10.1002/2017JC012907
       
  • Modulation of frontogenetic plankton production along a meandering jet by
           zonal wind forcing: An application to the Alboran Sea
    • Authors: Temel Oguz; Baptiste Mourre, Joaquin Tintore
      Abstract: We present a coupled physical-biological modeling study to elucidate the changes in ageostrophic frontal dynamics and the frontogenetic plankton production characteristics of a meandering jet under the impacts of successive westerly/easterly wind events combined with seasonal variations in the upstream transport and buoyancy flux characteristics of the jet, using a case study for the Alboran Sea (Western Mediterranean). Their nonlinear coupling is shown to result in different forms of physical and biological characteristics of the background jet structure that follows a meandering path around two anticyclonic gyres in the western and eastern basins and a cyclonic eddy in between. The westerly, downfront wind events broaden the jet, and result in stronger cross-frontal density contrast and intensify ageostrophic cross-frontal secondary circulation. Thus, they improve the frontogenetic plankton production with respect to the no-wind case. They also support higher production along the northern coast in response to wind-induced coastal upwelling and spreading of resulting nutrient-rich, productive water by mesoscale stirring. These features weaken gradually as the jet transport reduces. In contrast, stronger and longer-lasting easterlies during the reduced jet transport phase weaken the currents and frontal density structure, change the circular Western Alboran Gyre to an elongated form, and shift the main axis of the jet towards the southern basin. Then, frontogenesis fails to contribute to phytoplankton production that becomes limited to the eddy pumping within cyclones. Apart from the frontogenetic production, eddy pumping, mesoscale stirring, and diapycnal mixing of nutrients support intermittent and localized phytoplankton patches over the basin.
      PubDate: 2017-07-13T03:59:40.489082-05:
      DOI: 10.1002/2017JC012866
       
  • Interannual variability of the evaporation duct over the South China Sea
           and its relations with regional evaporation
    • Authors: Kunde Yang; Qi Zhang, Yang Shi
      Abstract: The interannual variability of the evaporation duct over the South China Sea is investigated during the boreal winter and summer seasons using datasets from the National Centers for Environment Prediction Climate Forecast System Reanalysis. During winter, the distribution is characterized by a tripole-like structure, whereas in summer the pattern exhibits a west–east contrast. Special attention is paid to the links of the evaporation duct with the regional evaporation by analyzing the leading empirical orthogonal functions of both elements. Both the structural similarity in spatial patterns and the significant correlation in the principal components imply the inherent relevance between evaporation duct and evaporation. During both the winter and summer seasons, the empirical orthogonal function principal component 1 of the evaporation duct and evaporation are linked significantly with the regional atmospheric dynamics, the primary factor dominating the whole South China Sea. The partial correlation analysis reveals that, similar to the evaporation process, surface wind is a more important factor affecting evaporation duct than sea surface temperature. The results also indicate that the influence of surface wind and SST not only has an obvious seasonal dependence, but also displays a prominent south-north contrast.
      PubDate: 2017-07-13T03:59:08.182064-05:
      DOI: 10.1002/2017JC012683
       
  • Temporal evolution of near-surface chlorophyll over cyclonic eddy
           lifecycles in the Southeastern Pacific
    • Authors: Jie Huang; Fanghua Xu, Kuanbo Zhou, Peng Xiu, Yanluan Lin
      Abstract: Temporal evolution of near-surface chlorophyll (CHL) associated with mesoscale eddies over entire eddy lifespan is complicated. Based on satellite measurements and a reanalysis dataset, we identify and quantify major temporal and spatial CHL responses in cyclonic eddies in the southeastern Pacific, and explore the associated mechanisms. Only few temporal CHL variations can be directly linked to the four primary mechanisms: ‘eddy pumping', ‘eddy trapping', ‘eddy stirring' and ‘eddy-induced Ekman pumping'. About 80% of the temporal CHL variations are too complex to be explained by a single mechanism. Five characteristic CHL responses, including classic dipoles (CD), positive-dominant dipoles (PD), negative-dominant dipoles (ND), positive monopoles (PM), and negative monopoles (NM) are identified using the Self-Organizing Map (SOM). CD, a dominant response induced primarily by ‘eddy stirring', has a continued increasing of frequency of occurrence with time, although its contribution to the total CHL variability remains low. As the secondary prominent response, NM has two peaks of frequency of occurrence at eddy formation and maturation stages, mainly accounted by ‘eddy trapping' after eddy breakup and ‘eddy-induced Ekman pumping', respectively. The sum of frequency of occurrence of PD and PM are comparable to that of NM. The initial positive CHL at eddy formation stage is associated with ‘eddy trapping'. The significant positive CHL increase from the eddy intensification to early decay stage is mainly attributed to ‘eddy pumping'. Although the frequency of occurrence of ND is the smallest, its contribution to negative CHL anomalies is unnegligible.
      PubDate: 2017-07-13T03:58:27.244284-05:
      DOI: 10.1002/2017JC012915
       
  • Trends and interannual variability of mass and steric sea level in the
           Tropical Asian Seas
    • Authors: Marcel Kleinherenbrink; Riccardo Riva, Thomas Frederikse, Mark Merrifield, Yoshihide Wada
      Abstract: The mass and steric components of sea level changes have been separated in the Tropical Asian Seas (TAS) using a statistically optimal combination of Jason satellite altimetry, GRACE satellite gravimetry and ocean reanalyses. Using observational uncertainties, statistically optimally weighted time series for both components have been obtained in four regions within the TAS over the period January 2005 - December 2012.The mass and steric sea level variability is regressed with the first two principal components (PC1&2) of Pacific equatorial wind stress and the Dipole Mode Index (DMI). Sea level in the the South China Sea is not affected by any of the indices. Steric variability in the TAS is largest in the deep Banda and Celebes seas and is affected by both PCs and the DMI. Mass variability is largest on the continental shelves, which is primarily controlled by PC1. We argue that a water flux from the Western Tropical Pacific Ocean is the cause for mass variability in the TAS.The steric trends are about 2 mm yr−1 larger than the mass trends in the TAS. A signifcant part of the mass trend can be explained by the aforementioned indices and the nodal cycle. Trends obtained from fingerprints of mass redistribution are statistically equal to mass trends after subtracting the nodal cycle and the indices.Ultimately, the effect of omitting the TAS in global sea level budgets is estimated to be 0.3 mm yr−1.
      PubDate: 2017-07-13T03:57:34.308972-05:
      DOI: 10.1002/2017JC012792
       
  • Variability and trends in the Arctic Sea ice cover: Results from different
           techniques
    • Authors: Josefino C. Comiso; Walter N. Meier, Robert Gersten
      Abstract: Variability and trend studies of sea ice in the Arctic have been conducted using products derived from the same raw passive microwave data but by different groups using different algorithms. This study provides consistency assessment of four of the leading products namely: Goddard Bootstrap (SB2), Goddard NASA Team (NT1), EUMETSAT Ocean and Sea Ice Satellite Application Facility (OSI-SAF 1.2) and Hadley HadISST 2.2 data in evaluating variability and trends in the Arctic sea ice cover. All four provide generally similar ice patterns but significant disagreements in ice concentration distributions especially in the marginal ice zone and adjacent regions in winter and meltponded areas in summer. The discrepancies are primarily due to different ways the four techniques accounts for occurrences of new ice and meltponding. However, results show that the different products generally provide consistent and similar representation of the state of the Arctic sea ice cover. Hadley and NT1 data usually provide the highest and lowest monthly ice extents, respectively. The Hadley data also show the lowest trends in ice extent and ice area at -3.88%/decade and -4.37%/decade, respectively, compared to an average of -4.36%/decade and -4.57%/decade for all four. Trend maps also show similar spatial distribution for all four with the largest negative trends occurring at the Kara/Barents Sea and Beaufort Sea regions, where sea ice has been retreating the fastest. The good agreement of the trends especially with updated data provides strong confidence in the quantification of the rate of decline in the Arctic sea ice cover.
      PubDate: 2017-07-13T03:51:04.011607-05:
      DOI: 10.1002/2017JC012768
       
  • Using remote sensing to detect the polarized sunglint reflected from oil
           slicks beyond the critical angle
    • Authors: Yingcheng Lu; Yang Zhou, Yongxue Liu, Zhihua Mao, Weixian Qian, Mengqiu Wang, Minwei Zhang, Jiang Xu, Shaojie Sun, Peijun Du
      Abstract: The critical angle at which the brightness of oil slicks and oil-free seawater is reversed occurs under sunglint and is often shown as an area of uncertainty due to different roughness and surface Fresnel reflection parameters. Consequently, differentiating oil slicks from the seawater in these areas using optical sensors is a challenge. Polarized optical remote sensing techniques provide complementary information for intensity imagery with different physical properties and, thus, possess the ability to resolve this difficult problem. In the polarized reflectance model, the degree of linear polarization (DOLP) of sunglint depends on accurately knowing the Stokes parameter for the reflected light, and varies with the refractive index of the surface layer and the viewing angles. For the polarized detection of oil slicks, the highest sensitivity of the DOLP to the refractive index is located within the specular reflection direction where the sum of the solar and sensor zenith angles is 82.6°. The modeled results clearly indicate that the DOLP of oil slicks is weaker in comparison with oil-free seawater under sunglint. Using measurements from the space-borne Polarization and Anisotropy of Reflectances for Atmospheric Sciences coupled with Observations from a Lidar (PARASOL) over the Deepwater Horizon oil spill in the Gulf of Mexico, we illustrate that the PARASOL-derived DOLP difference between the oil spill and seawater is obvious and is in accordance with the modeled results. These preliminary results suggest that the potential of multi-angle measurement and feasibility of deriving refractive index of ocean surface from space-borne sensors need further researches.
      PubDate: 2017-07-13T03:50:35.378746-05:
      DOI: 10.1002/2017JC012793
       
  • Assessing variability in the size and strength of the North Atlantic
           subpolar gyre
    • Authors: Nicholas P. Foukal; M. Susan Lozier
      Abstract: Recent studies on the size and strength of the North Atlantic subpolar gyre (SPG) offer contrasting assessments of the gyre's temporal variability: studies that use empirical orthogonal function (EOF) analyses of satellite sea-surface height (SSH) report a rapid decline in SPG size and strength since 1992 (∼20% per decade), while concurrent in situ observations report either no trend or a slight decline. Here we investigate this discrepancy by analyzing the size and strength of the SPG with satellite SSH from 1993 to 2015 with two separate methods: indirectly via EOF analysis and more directly through measurements of the gyre center and boundary. We define the boundary of the gyre as the largest closed contour of SSH, the center as the minimum SSH, and the strength as the difference between the SSH at the boundary and the center. We identify a linear decline over the study period in the SPG strength (5.1% per decade), but find no statistically significant trend in the SPG area. The trend in the gyre strength is weaker than the EOF-based trend and is most likely below the level of detection of the in situ measurements. We conclude that the variability previously identified as a sharp decline in SPG circulation can be more appropriately attributed to basin-wide sea level rise during the satellite altimetry period. In addition, we find that the properties of the eastern SPG do not co-vary with the SPG size, suggesting that SPG dynamics do not control the strength of the inter-gyre throughput.
      PubDate: 2017-07-11T10:56:20.040974-05:
      DOI: 10.1002/2017JC012798
       
  • Behavior of flexural gravity waves on ice shelves: Application to the Ross
           Ice Shelf
    • Authors: O.V. Sergienko
      Abstract: Ocean waves continuously impact floating ice shelves and affect their stress regime. Low-frequency, long-period (75-400 s), ocean waves are able to reach ice-shelf cavities from distant sources and excite flexural gravity waves that represent coupled motion in the water of the cavity and the ice covering above. Analytic treatment of simplified geometric configuration and three-dimensional numerical simulations of these flexural gravity waves applied to the Ross Ice Shelf show that propagation and ice-shelf flexural stresses are strongly controlled by the geometry of the system, bathymetry of the ice-shelf cavity, and ice-shelf cavity thickness. The derived dispersion relationships, group and phase velocities of these waves can be used to infer poorly constrained characteristics of ice shelves from field observations. The results of numerical simulations show that the flexural gravity waves propagate as beams. The orientation of these beams is determined by the direction of the open ocean waves incident on the ice-shelf front. The higher frequency ocean waves cause larger flexural stresses, while lower frequency waves can propagate farther away from the ice-shelf front and cause flexural stresses in the vicinity of the grounding line.
      PubDate: 2017-07-10T10:46:42.040125-05:
      DOI: 10.1002/2017JC012947
       
  • Dense Shelf Water spreading from Antarctic coastal polynyas to the deep
           Southern Ocean: A regional circumpolar model study
    • Authors: Kazuya Kusahara; Guy D. Williams, Takeshi Tamura, Robert Massom, Hiroyasu Hasumi
      Abstract: The spreading of Dense Shelf Water (DSW) from Antarctic coastal margins to lower latitudes plays a vital role in the ocean thermohaline circulation and the global climate system. Through enhanced localized sea ice production in Antarctic coastal polynyas, cold and saline DSW is formed over the continental shelf regions as a precursor to Antarctic Bottom Water (AABW). However, the detailed fate of coastal DSW over the Southern Ocean is still unclear. Here, we conduct extensive passive tracer experiments using a circumpolar ocean-sea ice-ice shelf model to investigate pathways of the regional polynya-based DSW from the Antarctic margins to the deep Southern Ocean basins. In the numerical experiments, the Antarctic coastal margin is divided into nine regions, and a passive tracer is released from each region at the same rate as the local sea ice production. The modeled spatial distribution of the total concentration of the nine tracers is consistent with the observed AABW distribution and clearly demonstrates nine routes of the DSW over the Southern Ocean along its bottom topography. Furthermore, the model shows that while ∼50% of the total tracer is distributed northwards from the continental shelf to the deep ocean, ∼7% is transported polewards beneath ice shelf cavities. The comprehensive tracer experiments allow us to estimate the contribution of local DSW to the total concentration along each of the pathways.
      PubDate: 2017-07-08T03:16:11.594136-05:
      DOI: 10.1002/2017JC012911
       
  • Energetics of eddy-mean flow interactions in the Brazil Current between
           20°S and 36°S
    • Authors: F. C. Magalhães; J. L. L. Azevedo, L. R. Oliveira
      Abstract: The energetics of eddy-mean flow interactions in the Brazil Current (BC) between 20°S and 36°S are investigated in nineteen transects perpendicular to the 200 m isobath. Ten years (2000-2009) of output data from the Hybrid Coordinate Ocean Model (HYCOM) NCODA reanalysis, with a spatial resolution of 1/12.5° and five-day averages, are used. The mean kinetic energy (MKE) and eddy kinetic energy (EKE) fields presented the same subsurface spatial pattern but with reduced values. The EKE increases southward, with high values along the BC path and the offshore portion of the jet. The values of the barotropic conversion term (BTC) are highest in the surface layers and decreased with depth, whereas the values of the baroclinic conversion term (BCC) and the vertical eddy heat flux (VEHF) are highest in the subsurface. Despite the vertical thickening of the BC, the highest energy conversion rates are confined to the upper 700 m of the water column. The energetic analysis showed that the current features mixed instability processes. The vertical weighted mean of the BTC and BCC presented an oscillatory pattern related to the bathymetry. The eddy field accelerates the time-mean flow upstream and downstream of bathymetric features and drains energy from the time-mean flow over the features. The BC is baroclinically unstable south of 28°S, and the highest energy conversion rates occur in Cabo de São Tomé, Cabo Frio and the Cone do Rio Grande.
      PubDate: 2017-07-08T03:10:40.682192-05:
      DOI: 10.1002/2016JC012609
       
  • Large runup controls on a gently sloping dissipative beach
    • Authors: Gabriel García-Medina; H. Tuba Özkan-Haller, Rob A. Holman, Peter Ruggiero
      Abstract: Observations on a mildly sloping beach suggest that the largest runup events are related to bore-bore capture (BBC). A numerical model based on the Reynolds-averaged Navier-Stokes equation is implemented to evaluate the effects that BBC have on runup. From simulations with realistic sea states, BBC is found to be a necessary but not sufficient condition for large runup generation. The dominant dynamics leading to BBC are amplitude dispersion and interactions with infragravity waves in the outer and inner surf zone, respectively. When the effects of BBC are isolated it is found that the runup associated with the merging of two bores is at least 50% larger than that associated with the larger of the two waves in a monochromatic wave train. The phase difference of the incident waves with the infragravity wave can generate up to 30% variability of the runup maxima. The majority of the shoreward directed momentum flux, prior to runup initiation, is related to the interaction between the bores and the infragravity wave followed by that of the incident infragravity waves alone.
      PubDate: 2017-07-08T03:10:36.143424-05:
      DOI: 10.1002/2017JC012862
       
  • Impacts of preexisting ocean cyclonic circulation on sea surface
           Chlorophyll-a concentration off northeastern Taiwan following episodic
           typhoon passages
    • Authors: Fang-Hua Xu; Yao Yuan, Leo Oey, Yanluan Lin
      Abstract: Off northeastern Taiwan, enhancement of sea surface chlorophyll-a (Chl-a) concentration is frequently found after typhoon passages. From 1998 to 2013, forty-six typhoon events are analyzed to examine the variations in Chl-a concentration from satellite ocean color data. On average, Chl-a concentration increased by 38% after a typhoon passage. Noticeably, four remarkable Chl-a increases after typhoons coincide with pre-existing oceanic cyclones in the study area. The Chl-a increase is significantly anti-correlated (p
      PubDate: 2017-07-08T03:10:31.275942-05:
      DOI: 10.1002/2016JC012625
       
  • Relative Influences of the Metocean forcings on the drifting ice pack and
           estimation of internal ice stress gradients in the Labrador Sea
    • Authors: I.D. Turnbull; R.Z. Torbati, R.S. Taylor
      Abstract: Understanding the relative influences of the metocean forcings on the drift of sea ice floes is a crucial component to the overall characterization of an ice environment and to developing an understanding of the factors controlling the ice dynamics. In addition, estimating the magnitude of the internal stress gradients on drifting sea ice floes generated by surrounding ice cover is important for modeling operations, informing the design of offshore structures and vessels in ice environments, and for the proper calibration of Discrete Element Models (DEM) of fields of drifting ice floes. In the spring of 2015 and 2016, four sea ice floes offshore Makkovik, Labrador were tagged with satellite-linked ice tracking buoys along with one satellite-linked weather station on each floe to transmit wind speed and direction. Twenty satellite-linked Lagrangian surface ocean current tracking buoys were also deployed in the open water adjacent to the targeted ice floes. In this paper, the dynamics of the four ice floes are explored in terms of the relative proportions which were forced by the wind, current, sea surface topography, Coriolis, and internal stress gradients. The internal ice stress gradients are calculated as residuals between the observed accelerations of the floes as measured by the tracking buoys and the sums of the other metocean forcings. Results show that internal ice stress gradients accounted for up to 50% of the observed forcing on the floes, and may have reached up to around 0.19kPa.
      PubDate: 2017-07-08T03:10:24.773007-05:
      DOI: 10.1002/2017JC012805
       
  • A direct estimate of poleward volume, heat, and fresh water fluxes at
           59.5°N between Greenland and Scotland
    • Authors: T. Rossby; Gilles Reverdin, Leon Chafik, Henrik Søiland
      Abstract: The meridional overturning circulation (MOC) in the North Atlantic plays a major role in the transport of heat from low to high latitudes. In this study we combine recent measurements of currents from the surface to>700 m from a shipboard acoustic Doppler current profiler with Argo profiles (to 2000 m) to estimate poleward volume, heat and freshwater flux at 59.5°N between Greenland and Scotland. This is made possible thanks to the vessel Nuka Arctica that operates on a 3-week schedule between Greenland and Denmark. For the period late 2012 to early 2016 the de-seasoned mean meridional overturning circulation reaches a 18.4 ± 3.4 Sv maximum at the σθ = 27.55 kg m−3 isopycnal, which varies in depth from near the surface in the western Irminger Sea to 1000 m in Rockall Trough. The total heat and fresh water fluxes across 59.5°N = 399 ± 74 TW and -0.20 ± 0.04 Sv where the uncertainties are principally due to that of the MOC. Analysis of altimetric sea surface height variations along exactly the same route reveals a somewhat stronger geostrophic flow north during this period compared to the 23-year mean suggesting that for a long-term mean the above flux estimates should be reduced slightly to 17.4 Sv, 377 TW and -0.19 Sv, respectively, with the same estimate uncertainties. The ADCP program is ongoing.
      PubDate: 2017-07-05T12:50:33.877555-05:
      DOI: 10.1002/2017JC012835
       
  • Near-surface salinity and temperature structure observed with dual-sensor
           drifters in the subtropical South Pacific
    • Authors: Shenfu Dong; Denis Volkov, Gustavo Goni, Rick Lumpkin, Gregory R. Foltz
      Abstract: Three surface drifters equipped with temperature and salinity sensors at 0.2 m and 5 m depths were deployed in April/May 2015 in the subtropical South Pacific with the objective of measuring near-surface salinity differences seen by satellite and in situ sensors and examining the causes of these differences. Measurements from these drifters indicate that water at a depth of 0.2 m is about 0.013 psu fresher than at 5 m and about 0.024°C warmer. Events with large temperature and salinity differences between the two depths are caused by anomalies in surface freshwater and heat fluxes, modulated by wind. While surface freshening and cooling occurs during rainfall events, surface salinification is generally observed under weak wind conditions (≤4 m/s). Further examination of the drifter measurements demonstrates that (i) the amount of surface freshening and strength of the vertical salinity gradient heavily depend on wind speed during rain events, (ii) salinity differences between 0.2 m and 5 m are positively correlated with the corresponding temperature differences for cases with surface salinification, and (iii) temperature exhibits a diurnal cycle at both depths, whereas the diurnal cycle of salinity is observed only at 0.2 m when the wind speed is less than 6 m/s. The amplitudes of the diurnal cycles of temperature at both depths decrease with increasing wind speed. The mean diurnal cycle of surface salinity is dominated by events with winds less than 2 m/s.
      PubDate: 2017-07-05T12:50:25.848554-05:
      DOI: 10.1002/2017JC012894
       
  • Revisiting Ocean Color algorithms for chlorophyll a and particulate
           organic carbon in the Southern Ocean using biogeochemical floats
    • Authors: Nils Haëntjens; Emmanuel Boss, Lynne D. Talley
      Abstract: The Southern Ocean (SO) ecosystem plays a key role in the carbon cycle by sinking a major part (43%) of the ocean uptake of anthropogenic CO2, and being an important source of nutrients for primary producers. However, undersampling of SO biogeochemical properties limits our understanding of the mechanisms taking place in this remote area. The Southern Ocean Carbon and Climate Observions and Modeling project (SOCCOM) has been deploying a large number of autonomous biogeochemical floats to study the SO (as of December 2016, 74 floats out of 200 have been deployed). SOCCOM floats measurements can be used to extend remote sensing chlorophyll a (chla) and particulate organic carbon (POC) products under the clouds or during the polar night as well as adding the depth dimension to the satellite-based view of the SO.Chlorophyll a concentrations measured by fluorometers embedded on the floats and POC concentrations derived from backscattering coefficients were calibrated with samples collected during the floats' deployment cruise. Float chla and POC were compared with products derived from observations of MODIS and VIIRS sensors.We find the Ocean Color Index (OCI) global algorithm to agree well with the matchups (within 9%, on average, for the Visible Infrared Imaging Radioneter Suite (VIIRS) and 12%, on average, for the Moderate Resolution Imaging Spectroradiometer Aqua(MODIS)). SO specific algorithms estimating chla are offset by ∼45% south of the Sea Ice Extent Front (∼ 60°S). In addition, POC estimates based on floats agree well with NASA's POC algorithm.
      PubDate: 2017-07-05T12:45:20.454629-05:
      DOI: 10.1002/2017JC012844
       
  • How are warm and cool years in the California Current related to ENSO'
    • Authors: Paul C. Fiedler; Nathan J. Mantua
      Abstract: The tropical El Niño – Southern Oscillation (ENSO) is a dominant mode of interannual variability that impacts climate throughout the Pacific. The California Current System (CCS) in the northeast Pacific warms and cools from year to year, with or without a corresponding tropical El Niño or La Niña event. We update the record of warm and cool events in the CCS for 1950-2016 and use composite sea level pressure (SLP) and surface wind anomalies to explore the atmospheric forcing mechanisms associated with tropical and CCS warm and cold events. CCS warm events are associated with negative SLP anomalies in the NE Pacific – a strong and southeastward displacement of the wintertime Aleutian Low, a weak North Pacific High and a regional pattern of cyclonic wind anomalies that are poleward over the CCS. We use a first order auto-regressive model to show that regional North Pacific forcing is predominant in SST variations throughout most of the CCS, while remote tropical forcing is more important in the far southern portion of the CCS. In our analysis, cool events in the CCS tend to be more closely associated with tropical La Niña than are warm events in the CCS with tropical El Niño; the forcing of co-occurring cool events is analogous, but nearly opposite, to that of warm events.
      PubDate: 2017-07-05T08:27:01.895447-05:
      DOI: 10.1002/2017JC013094
       
  • Effects of a shallow flood shoal and friction on hydrodynamics of a
           multiple-inlet system
    • Authors: Mara M. Orescanin; Steve Elgar, Britt Raubenheimer, Levi Gorrell
      Abstract: Prior studies have shown that frictional changes owing to evolving geometry of an inlet in a multiple inlet-bay system can affect tidally driven circulation. Here, a step between a relatively deep inlet and a shallow bay also is shown to affect tidal sea-level fluctuations in a bay connected to multiple inlets. To examine the relative importance of friction and a step, a lumped element (parameter) model is used that includes tidal reflection from the step. The model is applied to the two-inlet system of Katama Inlet (which connects Katama Bay on Martha's Vineyard, MA to the Atlantic Ocean) and Edgartown Channel (which connects the bay to Vineyard Sound). Consistent with observations and previous numerical simulations, the lumped element model suggests that the presence of a shallow flood shoal limits the influence of an inlet. In addition, the model suggests an increasing importance of friction relative to the importance of the step as an inlet shallows, narrows, and lengthens, as observed at Katama Inlet from 2011 to 2014.
      PubDate: 2017-07-05T08:26:51.958833-05:
      DOI: 10.1002/2016JC012502
       
  • The evolution of scaling laws in the sea ice floe size distribution
    • Authors: Christopher Horvat; Eli Tziperman
      Abstract: The sub-gridscale floe size and thickness distribution (FSTD) is an emerging climate variable, playing a leading-order role in the coupling between sea ice, the ocean, and the atmosphere. The FSTD, however, is difficult to measure given the vast range of horizontal scales of individual floes, leading to the common use of power-law scaling to describe it. The evolution of a coupled mixed-layer-FSTD model of a typical marginal ice zone is explicitly simulated here, to develop a deeper understanding of how processes active at the floe scale may or may not lead to scaling laws in the floe size distribution. The time evolution of mean quantities obtained from the FSTD (sea ice concentration, mean thickness, volume) is complex even in simple scenarios, suggesting that these quantities, which affect climate feedbacks, should be carefully calculated in climate models. The emergence of FSTDs with multiple separate power-law regimes, as seen in observations, is found to be due to the combination of multiple scale-selective processes. Limitations in assuming a power-law FSTD are carefully analyzed, applying methods used in observations to FSTD model output. Two important sources of error are identified that may lead to model biases: one when observing an insufficiently small range of floe sizes, and one from the fact that floe-scale processes often do not produce power-law behavior. These two sources of error may easily lead to biases in mean quantities derived from the FSTD of greater than 100%, and therefore biases in modeled sea ice evolution.
      PubDate: 2017-07-05T08:26:41.477368-05:
      DOI: 10.1002/2016JC012573
       
  • Structure of the buoyant plume formed by Ob and Yenisei river discharge in
           the southern part of the Kara Sea during summer and autumn
    • Authors: A. A. Osadchiev; A. S. Izhitskiy, P. O. Zavialov, V. V. Kremenetskiy, A. A. Polukhin, V. V. Pelevin, Z. M. Toktamysova
      Abstract: This article focuses on the interaction between the Ob- and Yenisei-dominated parts of the large Ob-Yenisei buoyant plume formed in the southern part of the Kara Sea during ice-free periods. It was shown that certain wind forcing and river discharge conditions cause the formation of a specific structure of the Ob-Yenisei plume with significantly different properties of the Ob- and Yenisei-dominated water masses. Under these conditions, the Yenisei runoff generates a narrow coastal current propagating northward from the Yenisei Gulf along the Taymyr Peninsula, which is isolated by the Ob-dominated water mass from ambient sea water. As a result, the low-salinity Yenisei-dominated water mass occupies a relatively small area, while more saline Ob-dominated water mass spreads over a wide area between the Gulf of Ob and the Taymyr Peninsula. The formation of the “isolation” configuration of the Ob-Yenisei plume described above is presumed to be caused by the eastward Ekman transport and the resulting downwelling flow of the Ob-dominated waters under the low-saline and warm Yenisei-dominated waters along the Taymyr Peninsula. Based on satellite imagery, wind reanalysis, and river discharge data collected and derived for the period of 2005–2011, it was estimated that the “isolation” configuration is regularly formed during late summer and autumn when the Ob discharge to the Kara Sea exceeds the Yenisei discharge and the local atmospheric circulation is dominated by the northerly wind regime. Assessment of the frequency and duration of the occurrence of the “isolation” configuration showed their synoptic time scale and significant inter-annual variability.
      PubDate: 2017-07-05T08:26:38.885341-05:
      DOI: 10.1002/2016JC012603
       
  • Biogeochemical sensor performance in the SOCCOM profiling float array
    • Authors: Kenneth S. Johnson; Joshua N. Plant, Luke J. Coletti, Hans W. Jannasch, Carole M. Sakamoto, Stephen C. Riser, Dana D. Swift, Nancy L. Williams, Emmanuel Boss, Nils Haëntjens, Lynne D. Talley, Jorge L. Sarmiento
      Abstract: The Southern Ocean Carbon and Climate Observations and Modeling (SOCCOM) program has begun deploying a large array of biogeochemical sensors on profiling floats in the Southern Ocean. As of February 2016, 86 floats have been deployed. Here the focus is on 56 floats with quality controlled and adjusted data that have been in the water at least 6 months. The floats carry oxygen, nitrate, pH, chlorophyll fluorescence, and optical backscatter sensors. The raw data generated by these sensors can suffer from inaccurate initial calibrations and from sensor drift over time. Procedures to correct the data are defined. The initial accuracy of the adjusted concentrations is assessed by comparing the corrected data to laboratory measurements made on samples collected by a hydrographic cast with a rosette sampler at the float deployment station. The long-term accuracy of the corrected data is compared to the GLODAPv2 data set whenever a float made a profile within 20 km of a GLODAPv2 station. Based on these assessments, the fleet average oxygen data are accurate to 1±1%, nitrate to within 0.5±0.5 µmol kg−1, and pH to 0.005±0.01, where the error limit is 1 standard deviation of the fleet data. The bio-optical measurements of chlorophyll fluorescence and optical backscatter are used to estimate chlorophyll a and particulate organic carbon concentration. The particulate organic carbon concentrations inferred from optical backscatter appear accurate to with 35 mg C m−3 or 20%, whichever is larger. Factors affecting the accuracy of the estimated chlorophyll a concentrations are evaluated.
      PubDate: 2017-07-05T08:26:33.469098-05:
      DOI: 10.1002/2017JC012838
       
  • Equatorial Kelvin waves generated in the western tropical Pacific Ocean
           trigger mass and heat transport within the Middle America Trench off Costa
           Rica
    • Authors: Richard E. Thomson; Earl E. Davis
      Abstract: Sequences of correlated seafloor temperature, current velocity and acoustic backscatter events recorded at Ocean Drilling Program (ODP) sites at 4300 m depth in the Middle America Trench have been inferred to result from tidally-induced turbidity currents generated in the vicinity of the 3300-m deep sill at the southern end of the trench. New data from the borehole observatories extends the temperature records to 11 years (November 2002 to December 2013) and confirms the highly episodic nature of the events. We present satellite altimetry data and ocean circulation model results to show that event timing is correlated with intraseasonal Kelvin wave motions in the equatorial Pacific. The observed temperature events had a mean (±1standard deviation) occurrence interval of 61(±24) days, which spans the periods of the first two baroclinic modes. Lag times between peak bottom-water temperatures at the ODP sites and the passage of eastward propagating Kelvin wave crests at locations in the eastern equatorial Pacific are consistent with the time for mode-1 waves to propagate to the southern end of the trench at a mean phase speed of 2.0 m⋅s−1. Findings indicate that Kelvin wave currents augment tidal motions in the vicinity of the sill, triggering turbidity currents that travel northwestward along the trench axis at mean speeds of ∼0.1 m⋅s−1. We conclude that mode-1 (or, possibly, mixed mode-1 and mode-2) baroclinic Kelvin waves generated by large-scale atmospheric processes in the western tropical Pacific lead to heat and mass transport deep within Middle America Trench in the eastern tropical Pacific.
      PubDate: 2017-07-05T08:26:28.512886-05:
      DOI: 10.1002/2017JC012848
       
  • Testing a common ice-ocean parameterization with laboratory experiments
    • Authors: C. D. McConnochie; R. C. Kerr
      Abstract: Numerical models of ice-ocean interactions typically rely upon a parameterization for the transport of heat and salt to the ice face that has not been satisfactorily validated by observational or experimental data. We compare laboratory experiments of ice-saltwater interactions to a common numerical parameterization and find a significant disagreement in the dependence of the melt rate on the fluid velocity. We suggest a resolution to this disagreement based on a theoretical analysis of the boundary layer next to a vertical heated plate, which results in a threshold fluid velocity of approximately 4 cm/s at driving temperatures between 0.5 and 4°C, above which the form of the parameterization should be valid.
      PubDate: 2017-07-05T08:26:20.583958-05:
      DOI: 10.1002/2017JC012918
       
  • Air-sea interaction regimes in the sub-Antarctic Southern Ocean and
           Antarctic marginal ice zone revealed by icebreaker measurements
    • Authors: Lisan Yu; Xiangze Jin, Eric Schulz, Simon A. Josey
      Abstract: This study analyzed shipboard air-sea measurements acquired by the icebreaker Aurora Australis during its off-winter operation in December 2010 – May 2012. Mean conditions over 7 months (October-April) were compiled from a total of 22 ship tracks. The icebreaker traversed the water between Hobart, Tasmania and the Antarctic continent, providing valuable in situ insight into two dynamically important, yet poorly sampled, regimes: the sub-Antarctic Southern Ocean and the Antarctic marginal ice zone (MIZ) in the Indian Ocean sector. The transition from the open water to the ice-covered surface creates sharp changes in albedo, surface roughness, and air temperature, leading to consequential effects on air-sea variables and fluxes. Major effort was made to estimate the air-sea fluxes in the MIZ using the bulk flux algorithms that are tuned specifically for the sea-ice effects, while computing the fluxes over the sub-Antarctic section using the COARE3.0 algorithm. The study evidenced strong sea-ice modulations on winds, with the southerly airflow showing deceleration (convergence) in the MIZ and acceleration (divergence) when moving away from the MIZ. Marked seasonal variations in heat exchanges between the atmosphere and the ice margin were noted. The monotonic increase in turbulent latent and sensible heat fluxes after summer turned the MIZ quickly into a heat loss regime, while at the same time the sub-Antarctic surface water continued to receive heat from the atmosphere. The drastic increase in turbulent heat loss in the MIZ contrasted sharply to the non-significant and seasonally invariant turbulent heat loss over the sub-Antarctic open water.
      PubDate: 2017-07-05T08:26:18.2386-05:00
      DOI: 10.1002/2016JC012281
       
  • Three-dimensional transient rip currents: Bathymetric excitation of
           low-frequency intrinsic variability
    • Authors: Yusuke Uchiyama; James C. McWilliams, Cigdem Akan
      Abstract: The ROMS-WEC model [Uchiyama et al., 2010] based on an Eulerian wave-averaged vortex-force asymptotic theory of McWilliams et al. [2004] is applied to analyze 3D transient wave-driven rip currents and associated intrinsic very low-frequency (VLF) variability in the surf zone on a surveyed bathymetry under spatiotemporally uniform offshore incident waves. The 3D rip currents are substantially depth-dependent due to the vertical recirculation, composed of pairs of counter-rotating longitudinal overturning roll cells that promote surface convergence. The vortex force plays an important role in vorticity budget, preconditioning overall vorticity reduction. These rip currents are intrinsically unstable and contribute about 70% to kinetic energy (KE) as eddy kinetic energy (EKE), consistent with observations. The dominant fluctuation period fits the VLF band, at about 18 minutes. The current effect on waves (CEW) alters not only the mean rip structure, but also the associated turbulence as the modified cross-shore EKE profile with considerable accentuation in the inner surf zone. Increased alongshore bathymetric variability proportionally intensifies KE and intrinsic EKE, whereas it reduces the VLF period. With a guide of a pseudo 2D model, we reveal that vortex tilting effect due to the horizontal vorticity inherent in the 3D rip currents promotes collapse of the 3D eddies through an enhanced forward kinetic energy cascade, leading to short-lived, laterally-stretched 3D eddies resulting in elongated filaments that decay more quickly than coherent, long-lived, circular 2D eddies.
      PubDate: 2017-07-05T08:25:48.96565-05:0
      DOI: 10.1002/2017JC013005
       
  • On the imprint of surfactant-driven stabilization of laboratory breaking
           wave foam with comparison to oceanic whitecaps
    • Authors: A. H. Callaghan; G. B. Deane, M. D. Stokes
      Abstract: Surfactants are ubiquitous in the global oceans: they help form the materially-distinct sea surface microlayer (SML) across which global ocean-atmosphere exchanges take place, and they reside on the surfaces of bubbles and whitecap foam cells prolonging their lifetime thus altering ocean albedo. Despite their importance, the occurrence, spatial distribution, and composition of surfactants within the upper ocean and the SML remains under-characterized during conditions of vigorous wave breaking when in-situ sampling methods are difficult to implement. Additionally, no quantitative framework exists to evaluate the importance of surfactant activity on ocean whitecap foam coverage estimates. Here we use individual laboratory breaking waves generated in filtered seawater and seawater with added soluble surfactant to identify the imprint of surfactant activity in whitecap foam evolution. The data show a distinct surfactant imprint in the decay phase of foam evolution. The area-time-integral of foam evolution is used to develop a time-varying stabilization function, ϕ(t) and a stabilization factor, Θ, which can be used to identify and quantify the extent of this surfactant imprint for individual breaking waves. The approach is then applied to wind-driven oceanic whitecaps, and the laboratory and ocean Θ distributions overlap. It is proposed that whitecap foam evolution may be used to determine the occurrence and extent of oceanic surfactant activity to complement traditional in-situ techniques and extend measurement capabilities to more severe sea states occurring at wind speeds in excess of about 10 m/s. The analysis procedure also provides a framework to assess surfactant-driven variability within and between whitecap coverage datasets.
      PubDate: 2017-07-05T08:25:40.589311-05:
      DOI: 10.1002/2017JC012809
       
  • Three-dimensional pathways of water masses in the South China Sea: A
           modeling study
    • Authors: Zhiqiang Liu; Jianping Gan
      Abstract: We conduct a numerical Eulerian analysis on the Lagrangian characteristics of South China Sea (SCS) water masses. This is the first study to investigate the water pathway and residence time in response to time-dependent, three-dimensional basin circulation in the SCS. The circulation of the SCS is largely determined by westward intrusions of the northwestern Pacific waters in the upper layer (0-750 m) and deep layer (>1500 m), and by an eastward extrusion in the intermediate layer (750-1500 m) through Luzon Strait (LS). The waters in these three layers are interlinked to sustain the three-dimensional circulation in the SCS. The upper intrusive waters flow cyclonically and have a residence time of ∼3 years before they subduct into a deeper layer or exit the SCS through surrounding straits. The intrusive denser waters in the deep layer flow along the northern slope and reside for ∼25 years in the northern basin (north of 13°N), where a strong subduction occurs. A cyclonic circulation exists in the deep layer in the southern basin (south of 13°N), where a subduction of intermediate waters occurs over its eastern side and a deep upwelling exists over its western side. The water in the deep southern basin has a residence time of ∼40 years. The upwelled deep waters and the subducted waters from the upper layer form the intermediate water, where waters circulate anti-cyclonically towards LS and have a longest residence time (∼42 years). These findings are supported by the spatial variation in the observed potential density variance.
      PubDate: 2017-06-20T13:16:34.397722-05:
      DOI: 10.1002/2016JC012511
       
  • Black Sea thermohaline properties: Long-term trends and variations
    • Authors: S. Miladinova; A. Stips, E. Garcia-Gorriz, D. Macias Moy
      Abstract: The current knowledge about spatial and temporal dynamics of the Black Sea's thermohaline structure is incomplete because of missing data and sparse distribution of existing measurements in space and time. This study presents 56-year continuous simulations of the Black Sea's hydrodynamics using the 3D General Estuarine Transport Model (GETM), without incorporating any relaxation towards climatological or observational data fields. This property of the model allows us to estimate independent temporal trends, in addition to resolving the spatial structure. The simulations suggest that the intermediate layer temperature is characterized by a weak positive trend (warming), whereas the surface temperature does not show a clear linear trend. Different salinity trends have been established at the surface (negative), upper (weaker negative) and main halocline (positive). Three distinct dynamic periods are identified (1960-1970, 1970-1995, 1995-2015), which exhibit pronounced changes in the Black Sea's thermohaline properties and basin circulation. Strengthening of the main cyclonic circulation, accompanied by intensification of the mesoscale anticyclonic eddy formation is found. Both events strongly affect the sea surface salinity but contribute in opposing directions. Specifically, strong composite large-scale circulation leads to an increase in sea surface salinity, while enhanced formation of mesoscale anticyclones decreases it. Salinity evolution with time is thus the result of the competition of these two opposing yet interdependent processes.
      PubDate: 2017-06-20T13:14:55.23583-05:0
      DOI: 10.1002/2016JC012644
       
  • Using sea-ice deformation fields to constrain the mechanical strength
           parameters of geophysical sea ice
    • Authors: Amélie Bouchat; Bruno Tremblay
      Abstract: We investigate the ability of viscous-plastic (VP) sea-ice models with an elliptical yield curve and normal flow rule to reproduce the shear and divergence distributions derived from the RADARSAT Geophysical Processor System (RGPS). In particular, we reformulate the VP elliptical rheology to allow independent changes in the ice compressive, shear and isotropic tensile strength parameters (P*, S*, T* respectively) in order to study the sensitivity of the deformation distributions to changes in the ice mechanical strength parameters. Our 10-km VP simulation with standard ice mechanical strength parameters P*= 27.5 kNm−2, S* = 6.9 kNm−2, and T* = 0 kNm−2 (ellipse aspect ratio of e = 2) does not reproduce the large shear and divergence deformations observed in the RGPS deformation fields, and specifically lacks well-defined, active linear kinematic features (LKFs). Probability density functions (PDFs) for the shear and divergence of are nonetheless not Gaussian. Simulations with a reduced compressive or increased shear strength are in good agreement with RGPS-derived shear and divergence PDFs, with relatively more large deformations compared to small deformations. The isotropic tensile strength of sea ice on the other hand does not significantly affect the shear and divergence distributions. When considering additional metrics such as the ice drift error, mean ice thickness fields, and spatial scaling of the deformations, our results suggest that reducing the ice compressive strength is a better solution than increasing the shear strength when performing Arctic-wide simulations of the sea-ice cover with the VP elliptical rheology.
      PubDate: 2017-06-19T17:08:50.170636-05:
      DOI: 10.1002/2017JC013020
       
  • Modeling wave attenuation by salt marshes in Jamaica Bay, New York, using
           a new rapid wave model
    • Authors: Reza Marsooli; Philip M. Orton, George Mellor
      Abstract: Using a new rapid-computation wave model, improved and validated in the present study, we quantify the value of salt marshes in Jamaica Bay – a highly urbanized estuary located in New York City – as natural buffers against storm waves. We improve the MDO phase-averaged wave model by incorporating a vegetation-drag-induced energy dissipation term into its wave energy balance equation. We adopt an empirical formula from literature to determine the vegetation drag coefficient as a function of environmental conditions. Model evaluation using data from laboratory-scale experiments show that the improved MDO model accurately captures wave height attenuation due to submerged and emergent vegetation. We apply the validated model to Jamaica Bay to quantify the influence of coastal-scale salt marshes on storm waves. It is found that the impact of marsh islands is largest for storms with lower flood levels, due to wave breaking on the marsh island substrate. However, the role of the actual marsh plants, Spartina alterniflora, grows larger for storms with higher flood levels, when wave breaking does not occur and the vegetative drag becomes the main source of energy dissipation. For the latter case, seasonality of marsh height is important; at its maximum height in early fall, S. alterniflora causes twice the reduction as when it is at a shorter height in early summer. The model results also indicate that the vegetation drag coefficient varies one order of magnitude in the study area, suggest exercising extra caution in using a constant drag coefficient in coastal wetlands.
      PubDate: 2017-06-16T05:55:19.950197-05:
      DOI: 10.1002/2016JC012546
       
  • Tsunami and infragravity waves impacting Antarctic ice shelves
    • Authors: P.D. Bromirski; Z. Chen, R.A. Stephen, P. Gerstoft, D. Arcas, A. Diez, R.C. Aster, D.A. Wiens, A. Nyblade
      Abstract: The responses of the Ross Ice Shelf (RIS) to the September 16, 2015 8.3 (Mw) Chilean earthquake tsunami (> 75 s period) and to oceanic infragravity (IG) waves (50–300 s period) were recorded by a broadband seismic array deployed on the RIS from November 2014 to November 2016. Here we show that tsunami and IG-generated signals within the RIS propagate at gravity-wave speeds (∼70 m/s) as water-ice coupled flexural-gravity waves. IG band signals show measureable attenuation away from the shelf front. The response of the RIS to Chilean tsunami arrivals is compared with modeled tsunami forcing to assess ice shelf flexural-gravity wave excitation by very long period (VLP; > 300 s) gravity waves. Displacements across the RIS are affected by gravity-wave incident direction, bathymetry under and north of the shelf, and water layer and ice shelf thicknesses. Horizontal displacements are typically about 10 times larger than vertical displacements, producing dynamical extensional motions that may facilitate expansion of existing fractures. VLP excitation is continuously observed throughout the year, with horizontal displacements highest during the austral winter with amplitudes exceeding 20 cm. Because VLP flexural-gravity waves exhibit no discernable attenuation, this energy must propagate to the grounding zone. Both IG and VLP band flexural-gravity waves excite mechanical perturbations of the RIS that likely promote tabular iceberg calving, consequently affecting ice shelf evolution. Understanding these ocean-excited mechanical interactions is important to determine their effect on ice shelf stability to reduce uncertainty in the magnitude and rate of global sea level rise.
      PubDate: 2017-06-16T05:38:19.138053-05:
      DOI: 10.1002/2017JC012913
       
  • Physical response of a back-barrier estuary to a post-tropical cyclone
    • Authors: A. Beudin; N. K. Ganju, Z. Defne, A. L. Aretxabaleta
      Abstract: This paper presents a modeling investigation of the hydrodynamic and sediment transport response of Chincoteague Bay (VA/MD, USA) to Hurricane Sandy using the Coupled Ocean-Atmosphere-Wave-Sediment-Transport (COAWST) modeling system. Several simulation scenarios with different combinations of remote and local forces were conducted to identify the dominant physical processes. While 80% of the water level increase in the bay was due to coastal sea level at the peak of the storm, a rich spatial and temporal variability in water surface slope was induced by local winds and waves. Local wind increased vertical mixing, horizontal exchanges, and flushing through the inlets. Remote waves (swell) enhanced southward flow through wave setup gradients between the inlets, and increased locally generated wave heights. Locally generated waves had a negligible effect on water level but reduced the residual flow up to 70% due to enhanced apparent roughness and breaking-induced forces. Locally generated waves dominated bed shear stress and sediment resuspension in the bay. Sediment transport patterns mirrored the interior coastline shape and generated deposition on inundated areas. The bay served as a source of fine sediment to the inner shelf, and the ocean-facing barrier island accumulated sand from landward-directed overwash. Despite the intensity of the storm forcing, the bathymetric changes in the bay were on the order of centimeters. This work demonstrates the spectrum of responses to storm forcing, and highlights the importance of local and remote processes on back-barrier estuarine function.
      PubDate: 2017-06-16T05:38:02.780577-05:
      DOI: 10.1002/2016JC012344
       
  • Parameterization of mixing by secondary circulation in estuaries
    • Authors: N. B. Basdurak; K. Huguenard, A. Valle-Levinson, M. Li, Robert Chant
      Abstract: Eddy viscosity parameterizations that depend on a gradient Richardson number Ri have been most pertinent to the open ocean. Parameterizations applicable to stratified coastal regions typically require implementation of a numerical model. Two novel parameterizations of the vertical eddy viscosity, based on Ri, are proposed here for coastal waters. One turbulence closure considers temporal changes in stratification and bottom stress and is coined the ‘regular fit'. The alternative approach, named the ‘lateral fit', incorporates variability of lateral flows that are prevalent in estuaries. The two turbulence parameterization schemes are tested using data from a Self Contained Autonomous Microstructure Profiler (SCAMP) and an Acoustic Doppler Current Profiler (ADCP) collected in the James River Estuary. The ‘regular fit' compares favorably to SCAMP-derived vertical eddy viscosity values but only at relatively small values of gradient Ri . On the other hand, the ‘lateral fit' succeeds at describing the lateral variability of eddy viscosity over a wide range of Ri. The modifications proposed to Ri-dependent eddy viscosity parameterizations allow applicability to stratified coastal regions, particularly in wide estuaries, without requiring implementation of a numerical model.
      PubDate: 2017-06-16T05:37:59.426892-05:
      DOI: 10.1002/2016JC012328
       
  • Cost efficient environmental survey paths for detecting continuous tracer
           discharges
    • Authors: G. Alendal
      Abstract: Designing monitoring programs for detecting potential tracer discharges from unknown locations is challenging. The high variability of the environment may camouflage the anticipated anisotropic signal from a discharge, and there are a number of discharge scenarios. Monitoring operations may also be costly, constraining the number of measurements taken. By assuming that a discharge is active, and a prior belief on the most likely seep location, a method that uses Bayes' theorem combined with discharge footprint predictions is used to update the probability map. Measurement locations with highest reduction in the overall probability of a discharge to be active can be identified. The relative cost between reallocating and measurements can be taken into account. Three different strategies are suggested to enable cost efficient paths for autonomous vessels.
      PubDate: 2017-06-16T05:30:20.569643-05:
      DOI: 10.1002/2016JC012655
       
  • Development of upwelling on pathway and freshwater transport of Pearl
           River plume in northeastern South China Sea
    • Authors: Zhaoyun Chen; Yuwu Jiang, James T. Liu, Wenping Gong
      Abstract: In situ observations, satellite images, and numerical modeling results have shown that the Pearl River plume axis extends alongshore and passes through two separate upwelling regions—one off the Guangdong and Fujian coasts (the Yuedong upwelling) and the other in the Taiwan Bank during the initial and medium stages of the Yuedong upwelling, while it is directed offshore when the Yuedong upwelling is fully-developed. Model experiments are conducted to examine the effects of wind strength and baroclinicity on the upwelling and the corresponding pathway and freshwater transport of the Pearl River plume. The baroclinic effect is important to intensifying the horizontal velocity at the upwelling front and freshwater transport in the northeastern South China Sea. The freshwater transport flux is further decomposed into advection, vertical shear, and tidal pumping components, and advection is the dominant contributor. As the Yuedong upwelling develops, the zone with a relatively high pressure gradient moves offshore due to offshore Ekman transport and the shift in the upwelling front, which is responsible for the offshore transport of the river plume. When the river plume is transported to the outer-shelf, sometimes it can be further entrained into eddies, allowing its export to the open sea.
      PubDate: 2017-06-15T09:25:41.42345-05:0
      DOI: 10.1002/2016JC012411
       
  • Eastward-propagating decadal temperature variability in the South Atlantic
           and Indian Oceans
    • Authors: Yushi Morioka; Bunmei Taguchi, Swadhin K. Behera
      Abstract: The origin and structure of eastward-propagating decadal temperature variability in the South Atlantic and Indian Oceans are investigated by using long-term output of coupled general circulation model. Composite analysis during the warm Southwest Indian Ocean (SWIO) years shows that decadal temperature anomalies in the SWIO region originate in the South Atlantic and are associated with density anomalies. Since the density anomalies propagate at the speed of a few cm s−1, slower than the speed of the background eastward Antarctic Circumpolar Current, and exhibit a surface-intensified equivalent barotropic structure, the eastward propagation of the density anomalies may be attributed to quasi-stationary oceanic Rossby waves. The density anomalies are also accompanied with anomalous Ekman pumping, indicating an important contribution from the overlying atmospheric variability. The role of atmospheric variability is further examined by evaluating the mixed-layer heat balance. It is found that the warm temperature anomalies in the South Atlantic are due to anomalous entrainment and meridional advection. These results suggest that the atmospheric variability plays an important role through the ocean mixed-layer in generating the eastward propagation of decadal temperature variability from the South Atlantic besides the internal ocean variability as suggested in previous studies.
      PubDate: 2017-06-15T09:25:40.117275-05:
      DOI: 10.1002/2017JC012706
       
  • Quantifying phytoplankton productivity and photoinhibition in the Ross Sea
           Polynya with large eddy simulation of Langmuir circulation
    • Authors: Robyn L. Smyth; Cigdem Akan, Andrés Tejada-Martínez, Patrick J. Neale
      Abstract: Southern Ocean phytoplankton assemblages acclimated to low-light environments that result from deep mixing are often sensitive to ultraviolet and high photosynthetically available radiation. In such assemblages, exposures to inhibitory irradiance near the surface result in loss of photosynthetic capacity that is not rapidly recovered and can depress photosynthesis even after transport below depths penetrated by inhibitory irradiance. We used a coupled biophysical modeling approach to quantify the reduction in primary productivity due to photoinhibition based upon experiments and observations made during the spring bloom in Ross Sea Polynya (RSP). Large eddy simulation (LES) was used to generate depth trajectories representative of observed Langmuir circulation that were passed through an underwater light field to yield time series of spectral irradiance representative of what phytoplankton would have experienced in situ. These were used to drive an assemblage-specific photosynthesis-irradiance model with inhibition determined from a biological weighting function and repair rate estimated from shipboard experiments on the local assemblage. We estimate the daily depth-integrated productivity was 230 mmol C m−2. This estimate includes a 6-7% reduction in daily depth-integrated productivity over potential productivity (i.e., effects of photoinhibition excluded). When trajectory depths were fixed (no vertical transport), the reduction in productivity was nearly double. Relative to LES estimates, there was slightly less depth-integrated photoinhibition with random walk trajectories and nearly twice as much with circular rotations. This suggests it's important to account for turbulence when simulating the effects of vertical mixing on photoinhibition due to the kinetics of photodamage and repair.
      PubDate: 2017-06-12T11:10:32.272525-05:
      DOI: 10.1002/2017JC012747
       
  • A reflecting, steepening, and breaking internal tide in a submarine canyon
    • Authors: M. S. Alberty; S. Billheimer, M. M. Hamann, C. Y. Ou, V. Tamsitt, A. J. Lucas, M. H. Alford
      Abstract: Submarine canyons are common features of the coastal ocean. Although they are known to be hotspots of turbulence that enhance diapycnal transport in their stratified waters, the dynamics of canyon mixing processes are poorly understood. Most studies of internal wave dynamics within canyons have focused on a handful of canyons with along-axis slopes less steep than semidiurnal (D2) internal wave characteristics (subcritical). Here, we present the first tidally-resolving observations within a canyon with a steeply sloping axis (supercritical). A process study consisting of two 24-hour shipboard stations and a profiling mooring was conducted in the La Jolla Canyon off the coast of La Jolla, CA. Baroclinic energy flux is oriented up-canyon and decreases from 182 ± 18 W m−1 at the canyon mouth to 46 ± 5 W m−1 near the head. The ratio of horizontal kinetic energy to available potential energy and the observed group speed of each mode are lower than expected for freely propagating D2 internal waves at each station, indicating partial reflection. Harmonic analysis reveals that variance is dominated by the D2 tide. Moving up-canyon, the relative importance of D2 decreases and its higher harmonics are needed to account for a majority of the observed variance, indicating steepening. Steep internal tides cause large isopycnal displacements (∼50 m in 100m water depth) and high strain events. These events coincide with enhanced O(10−7 -10−5 m2 s−3) dissipation of turbulent kinetic energy at mid-depths.
      PubDate: 2017-06-09T11:10:20.824706-05:
      DOI: 10.1002/2016JC012583
       
  • The horizontal heat advection in the Middle Atlantic Bight and the
           cross-spectral interactions within the heat advection
    • Authors: Jin Sha; Xiao-Hai Yan, Xiaoming Li
      Abstract: The upper ocean horizontal heat advection over the middle and outer shelf of the Middle Atlantic Bight (MAB) is investigated using satellite and in-situ observations. In the upper mixed layer, the heat advection is mostly positive indicating that it decreases the shelf heat content by bringing cold water from upstream. The heat advection driven by the barotropic geostrophic current is one order larger relative to the density-driven geostrophic shear and the wind-driven current. The barotropic geostrophic advection components in the alongshore and offshore direction are of the same order. To investigate the temporal properties of the heat advection, the temperature and currents are decomposed into different time scales using Fast Multidimensional Ensemble Empirical Decomposition (FMEEMD). The cross-spectral interactions within the advection are quantitatively evaluated with major components identified. Due to the cross-spectral interaction, energy within the heat advection are found to be redistributed through different time scales, with at least 46.5% variation retained within the original band for the barotropic geostrophic advection. Our results help to better understand the temporal variability of the heat advection, provide a baseline of the non-linear energy transfer framework within different time scales in the heat advection, and imply the possibility of interplays between short and long term oceanic phenomena.
      PubDate: 2017-06-09T11:05:51.076334-05:
      DOI: 10.1002/2017JC013043
       
  • Episodic surface intrusions in the Yellow Sea during relaxation of
           northerly winds
    • Authors: Zifeng Hu; Dong-Ping Wang, Xianqiang He, Mingting Li, Jun Wei, Delu Pan, Yan Bai
      Abstract: The surface currents over the Yellow and East China Seas are mapped from the Geostationary Ocean Color Imager (GOCI). Based on a composite of six intrusion events in January–April, the strong northward surface current in the Yellow Sea is shown to be concentrated along the deep trough, accompanied by a broad northward surface current over the East China Sea. From the corresponding surface winds, the episodic northward surface flow bursts appear to be associated with abrupt changes from the strong northerly winds to weak southerly winds during cold front passages. A three-dimensional model driven with observed surface winds is used to simulate the observed shelf-wide response to northerly winds. There is an outstanding agreement between the simulated and observed surface currents. The surface intrusion in the Yellow Sea is shown to be driven primarily by a barotropic longitudinal surface slope, while the strong northward current in the East China Sea is associated with a coastal trapped wave. Moreover, the surface intrusion is associated with a large volume transport, suggesting that the transient intrusions could be important in the northward heat transport. The unprecedented capability of GOCI satellite in providing a regional circulation pattern, in conjunction with complementary model simulations, could contribute greatly to understanding of the dynamics of the Yellow and East China Seas.
      PubDate: 2017-06-09T11:05:47.895589-05:
      DOI: 10.1002/2017JC012830
       
  • Impact of data assimilation on Eulerian versus Lagrangian estimates of
           upper ocean transport
    • Authors: Ann Kristin Sperrevik; Johannes Röhrs, Kai Håkon Christensen
      Abstract: Using four-dimensional variational analysis, we produce an estimate of the state of, a coastal region in Northern Norway during the late winter and spring in 1984. We use satellite, sea surface temperature and in-situ observations from a series of intensive field campaigns, and obtain a more realistic distribution of water masses both in the horizontal and the, vertical than a pure downscaling approach can achieve. Although the distribution of Eulerian surface, current speeds are similar, we find that they are more variable and less dependent, on model bathymetry in our reanalysis compared to a hindcast produced using the same modeling system. Lagrangian drift currents on the other hand are significantly changed, with overall higher kinetic energy levels in the reanalysis than in the hindcast, particularly in the super-inertial, frequency band.
      PubDate: 2017-06-09T11:05:40.95441-05:0
      DOI: 10.1002/2016JC012640
       
  • Impact of Sea Spray on the Yellow and East China Seas Thermal Structure
           during the passage of Typhoon Rammasun (2002)
    • Authors: Lianxin Zhang; Xuefeng Zhang, P. C. Chu, Changlong Guan, Hongli Fu, Guofang Chao, Guijun Han, Wei Li
      Abstract: Strong winds lead to large amounts of sea spray in the lowest part of the atmospheric boundary layer. The spray droplets affect the air-sea heat fluxes due to their evaporation and the momentum due to the change of sea surface, and in turn change the upper ocean thermal structure. In this study, impact of sea spray on upper ocean temperatures in the Yellow and East China Seas (YES) during typhoon Rammasun's passage is investigated using the POMgcs ocean model with a sea spray parameterization scheme, in which the sea spray-induced heat fluxes are based on an improved Fairall's sea spray heat fluxes algorithm, and the sea spray-induced momentum fluxes are derived from an improved COARE version 2.6 bulk model.The distribution of the sea spray mediated turbulent fluxes was primarily located at Rammasun eye-wall region, in accord with the maximal wind speeds regions. When Rammasun enters the Yellow sea, the sea spray mediated latent (sensible) heat flux maximum is enhanced by 26% (13.5%) compared to that of the interfacial latent (sensible) heat flux. The maximum of the total air-sea momentum fluxes is enhanced by 43% compared to the counterpart of the interfacial momentum flux. Furthermore, the sea spray plays a key role in enhancing the intensity of the typhoon-induced “cold suction” and “heat pump” processes. When the effect of sea spray is considered, the maximum of the sea surface cooling in the right side of Rammasun's track is increased by 0.5°C, which is closer to the available satellite observations.
      PubDate: 2017-06-09T06:48:46.329082-05:
      DOI: 10.1002/2016JC012592
       
  • Tracking Labrador Sea Water property signals along the Deep Western
           Boundary Current
    • Authors: Isabela Astiz Le Bras; Igor Yashayaev, John Toole
      Abstract: Observations of the Deep Western Boundary Current (DWBC) at Line W on the western North Atlantic continental slope southeast of Cape Cod from 1995 to 2014 reveal water mass changes that are consistent with changes in source water properties upstream in the Labrador Sea. This is most evident in the cold, dense, and deep class of Labrador Sea Water (dLSW) that was created and progressively replenished and deepened by recurring winter convection during the severe winters of 1987-1994. The arrival of this record cold, fresh and low potential vorticity anomaly at Line W lags its formation in the Labrador Sea by 3-7 years. Complementary observations along the path of the DWBC provide further evidence that this anomaly is advected along the boundary and indicate that stirring between the boundary and the interior intensifies south of the Flemish Cap. Finally, the consistency of the data with realistic advective and mixing time scales is assessed using the Waugh and Hall [2005] model framework. The data are found to be best represented by a mean transit time of 5 years from the Labrador Sea to Line W, with a leading order role for both advection by the DWBC and mixing between the boundary flow and interior waters.
      PubDate: 2017-06-08T22:26:37.968258-05:
      DOI: 10.1002/2017JC012921
       
  • Seasonal variability and mechanisms regulating chlorophyll distribution in
           mesoscale eddies in the South China Sea
    • Authors: Mingxian Guo; Peng Xiu, Shiyu Li, Fei Chai, Huijie Xue, Kuanbo Zhou, Minhan Dai
      Abstract: This study shows that the response of satellite-observed chlorophyll (CHL) to eddy motion varies seasonally in the South China Sea (SCS). The spatial pattern of the CHL anomaly composite for eddies is a dipole in summer and a monopole in winter, indicating that sea surface CHL is largely regulated by the horizontal rotational velocity of the eddy in summer and by eddy pumping and trapping in winter. The dipole pattern for anticyclonic eddies was confirmed by in situ observations, which also show that the dipole pattern is mainly restricted to the mixed layer. The underlying mechanism was further investigated with a coupled physical–biogeochemical model. The key driver leading to the seasonal variation of the eddy effect is found to be the seasonal variation of the mixed layer depth. In summer when the mixed layer is shallow, the monopole nutrient change induced by eddy is restricted to the subsurface. The sea surface CHL distribution is thus mostly affected by eddy advection. In winter, the deepening of the mixed layer mixes the nutrients from subsurface where eddy significantly changes the nutrient levels, allowing the monopole pattern to be observed in the sea surface CHL anomaly.
      PubDate: 2017-06-07T10:35:42.173328-05:
      DOI: 10.1002/2016JC012670
       
  • Cascading off the West Greenland Shelf: A numerical perspective
    • Authors: Juliana M. Marson; Paul G. Myers, Xianmin Hu, Brian Petrie, Kumiko Azetsu-Scott, Craig M. Lee
      Abstract: Cascading of dense water from the shelf to deeper layers of the adjacent ocean basin has been observed in several locations around the world. The West Greenland Shelf (WGS), however, is a region where this process has never been documented. In this study, we use a numerical model with a 1/4° resolution to determine (i) if cascading could happen from the WGS; (ii) where and when it could take place; (iii) the forcings that induce or halt this process; and (iv) the path of the dense plume. Results show cascading happening off the WGS at Davis Strait. Dense waters form there due to brine rejection and slide down the slope during spring. Once the dense plume leaves the shelf, it gradually mixes with waters of similar density and moves northward into Baffin Bay. Our simulation showed events happening between 2003-2006 and during 2014; but no plume was observed in the simulation between 2007-2013. We suggest that the reason why cascading was halted in this period is related to: the increased freshwater transport from the Arctic Ocean through Fram Strait; the additional sea ice melting in the region; and the reduced presence of Irminger Water at Davis Strait during fall/early winter. Although observations at Davis Strait show that our simulation usually overestimates the seasonal range of temperature and salinity, they agree with the overall variability captured by the model. This suggests that cascades have the potential to develop on the WGS, albeit less dense than the ones estimated by the simulation.
      PubDate: 2017-06-07T10:35:35.08589-05:0
      DOI: 10.1002/2017JC012801
       
  • Distribution pattern and mass budget of sedimentary polycyclic aromatic
           hydrocarbons in shelf areas of the Eastern China Marginal Seas
    • Authors: C. L. Wang; X. Q. Zou, Y. F. Zhao, Y. L. Li, Q. C. Song, T. Wang, W. W. Yu
      Abstract: This study conducted the first extensive and comprehensive investigation of the regional-scale sedimentary polycyclic aromatic hydrocarbons (PAHs) concentration, flux, and budget in the continental shelves of the Eastern China Marginal Seas (ECMSs). Surface sediment samples from multiple sites were collected and assessed, and the latest data from current research was assessed. The spatial distribution pattern of PAHs in the ECMSs was significantly influenced by the regional hydrodynamics, sediment properties (grain-size, total organic carbon (TOC) content, and sedimentation rate), and anthropogenic impacts. Relatively higher PAHs concentrations occurred in areas with fine-grained sediment. Results of source apportionment found that the relative proportions of PAHs showed significant regional variation, mainly influenced by socio-economic differences between north and south China. The PAHs burial flux in the study area ranged from 11.2 to 1308 ng cm−2 y−1 with an average value of 101 ± 104 ng cm−2 y−1. The area-integrated sedimentary PAHs burial flux across the ECMSs was 494 t y−1. A mass budget calculation revealed that riverine input and atmospheric deposition were the most significant sources contributing, 28.4% and 71.6%, respectively. The study demonstrated that net PAHs transportation occurs between the Bohai Sea (BS) and Yellow Sea (YS), with a flux of approximately 10.2 t y−1. PAHs were also transported from YS to the East China Sea (ECS), due to water exchange between the YS and ECS. Additionally, substantial amounts of PAHs in the inner shelf of the ECS were transported out of the shelf area due to cross-shelf plume.
      PubDate: 2017-06-06T09:30:18.931251-05:
      DOI: 10.1002/2017JC012890
       
  • Nonlinear internal waves and plumes generated in response to sea loch
           outflow, AUV, and time-lapse photography observations
    • Authors: Matthew Toberman; Mark Inall, Tim Boyd, Estelle Dumount, Colin Griffiths
      Abstract: The tidally modulated outflow of brackish water from a sea loch forms a thin surface layer that propagates into the coastal ocean as a buoyant gravity current, transporting nutrients and sediments, as well as fresh water, heat and momentum. The fresh intrusion both propagates into and generates a strongly stratified environment which supports trains of nonlinear internal waves (NLIWs). NLIWs are shown to propagate ahead of this buoyancy input in response to propagation of the outflow water into the stratified environment generated by the previous release as well as in the opposing direction after the reflection from steep bathymetry. Oblique aerial photographs were taken and photogrammetric rectification led to the identification of the buoyant intrusion and the subsequent generation of NLIWs. An Autonomous Underwater Vehicle (AUV) was deployed on repeated reciprocal transects in order to make simultaneous CTD, ADCP and microstructure shear measurements of the evolution of these phenomena in conjunction with conventional mooring measurements. AUV-based temperature and salinity signals of NLIWs of depression were observed together with increased turbulent kinetic energy dissipation rates of over two orders of magnitude within and in the wake of the NLIWs. Repeated measurements allow a unique opportunity to investigate the horizontal structure of these phenomena. Simple metric scaling demonstrates that these these processes are likely to be feature of many fjordic systems located on the west coast of Scotland, but may also play a key role in the assimilation of the outflow from many tidally dominated fjordic systems throughout the world.
      PubDate: 2017-06-06T06:00:36.115435-05:
      DOI: 10.1002/2016JC012208
       
  • The Net energy budget at the ocean-atmosphere interface of the “Cold
           Tongue” region
    • Authors: Rachel T. Pinker; Abderrahim Bentamy, Banglin Zhang, Wen Chen, Yingtao Ma
      Abstract: Pacific “Cold Tongue” (PCT) Sea Surface Temperature (SST) experiences significant (>0.5° C) inter-annual variations forced by the El-Nino Southern Oscillations (ENSO) with global impacts on the Earth climate. In this study we estimate the PCT net heat budget known to be difficult to derive using numerical models. The main goal is to determine how accurately the net heat flux across the surface/atmosphere interface can currently be determined primarily, from satellite observations; these are first evaluated against the nearest available observations inside and outside the PCT of the Tropical Pacific Ocean,using buoy arrays such as the Tropical Atmosphere Ocean/Triangle Trans-Ocean Buoy Network (TAO/TRITON). It was found that the satellite based estimates of both turbulent and radiative fluxes are in better agreement with the observations than similar estimates from leading numerical models. The monthly mean satellite estimates of PCT SW during Jan/July 2009 were 273.07/170.14, for LW, latent heat and sensible heat they were 378.79/365.54, 95.52/130.31 9.89/20.67 respectively (all in W m−2). The estimated standard deviations for PCT SW were in the range of 7.2-7.8% of the mean and in the range of 2.0-2.5% for LW, at daily time scale. Satellite estimates of both PCT LHF and SHF exhibit much higher variability, characterized by standard deviations of 50% from the mean values.
      PubDate: 2017-06-05T11:01:21.375746-05:
      DOI: 10.1002/2016JC012581
       
  • Distribution of upper circumpolar Deep Water on the warming continental
           shelf of the West Antarctic Peninsula
    • Authors: N. Couto; D. G. Martinson, J. Kohut, O. Schofield
      Abstract: We use autonomous underwater vehicles to characterize the spatial distribution of Upper Circumpolar Deep Water (UCDW) on the continental shelf of the West Antarctic Peninsula (WAP) and present the first near-synoptic measurements of mesoscale features (eddies) containing UCDW on the WAP. Thirty-three subsurface eddies with widths on the order of 10 km were detected during four glider deployments. Each eddy contributed an average of 5.8 x 1016 J to the sub-pycnocline waters, where a cross-shelf heat flux of 1.37 x 1019 J yr−1 is required to balance the diffusive loss of heat to overlying winter water and to the near-coastal waters. Approximately two-thirds of the heat coming onto the shelf diffuses across the pycnocline and one-third diffuses to the coastal waters; long-term warming of the sub-pycnocline waters is a small residual of this balance. Sixty percent of the profiles that contained UCDW were part of a coherent eddy. Between 20 and 53% of the lateral onshore heat flux to the WAP can be attributed to eddies entering Marguerite Trough, a feature in the southern part of the shelf which is known to be an important conduit for UCDW. A northern trough is identified as additional important location for eddy intrusion.
      PubDate: 2017-06-05T10:56:28.756732-05:
      DOI: 10.1002/2017JC012840
       
  • Vertical energy flux at ENSO timescales in the subthermocline of the
           Southeastern Pacific
    • Authors: O. Vergara; B. Dewitte, M. Ramos, O. Pizarro
      Abstract: The question of how energy is redistributed in the ocean has renewed the interest for the processes leading to mid-latitude subthermocline variability at low frequency. Here we investigate a process that has been disregarded although potentially relevant for climatic studies dealing with the planetary energy budget. The focus is on the Southeastern Pacific where an efficient oceanic teleconnection takes place, linking the remote surface equatorial momentum forcing with the subthermocline through the vertical propagation of low-frequency long-wavelength extra-tropical Rossby waves (ETRW). A high-resolution model is used to document the vertical energy flux associated with ETRW at interannual to decadal timescales. The analysis of a long-term (1958-2008) simulation reveals that the vertical energy flux can be interpreted to a large extent as resulting from the coastally forced ETRW as far south as 35°S, so that heat content variability can be predicted along theoretical trajectories originating from the coast below the thermocline. It is shown that the vertical energy flux associated with the El Niño Southern Oscillation forms beams below the thermocline that account for a large fraction of the total vertical energy flux at interannual timescales. Extreme El Niño events are the dominant contributor to this flux, which is hardly impacted by mesoscale activity. The energy beams experience a dissipation processes in the ocean below 1000 m that is interpreted as resulting from vertical turbulent diffusion. Our results suggest that the ETRW at ENSO timescales are strongly dissipated at the surface but still can modulate the heat content in the deep ocean of the Southeastern Pacific.
      PubDate: 2017-06-02T07:15:35.836352-05:
      DOI: 10.1002/2016JC012614
       
  • Formation of sea ice bridges in narrow straits in response to wind and
           water stresses
    • Authors: Bhargav Rallabandi; Zhong Zheng, Michael Winton, Howard A. Stone
      Abstract: Ice bridges are rigid structures composed of sea ice that form seasonally in the many straits and channels of the Canadian Arctic Archipelago. Driven primarily by atmospheric stresses, these ice bridges are formed when sufficiently thick ice “jams” during the course of its flow between land masses, resulting in a region of stationary compacted ice that is separated from a region of flowing open water (a polynya) by a static arch. Using a continuum description of sea ice that is widely used in climate modeling, we present an asymptotic theory of the process of formation of such bridges in slender channels when the motion of the ice is driven by external wind and water stresses. We show that for an arbitrary channel shape, ice bridges can only form within a range of ice properties that is determined by the channel geometry and the external stress. We then compare the results of our theory with direct numerical simulations and observational evidence. Finally, we provide simple analytical expressions for the mean speed of the ice flow as a function of the channel shape, the properties of the ice, and the wind and water stresses along the channel.
      PubDate: 2017-06-02T07:15:30.129067-05:
      DOI: 10.1002/2017JC012822
       
  • The impacts of ocean bottom roughness and tidal flow amplitude on abyssal
           mixing
    • Authors: Toshiyuki Hibiya; Takashi Ijichi, Robin Robertson
      Abstract: Existing parameterizations of vertical mixing over a rough ocean bottom neglect the transformation of linear internal waves into quasi-steady lee waves, when tide-topography interactions strengthen. In the present study, we perform a series of eikonal calculations to investigate how upward propagating quasi-steady lee waves donate their energy to dissipation through nonlinear interactions with the background Garrett-Munk internal waves. Of special note with these upward propagating quasi-steady lee waves is that, for a fixed density stratification, the vertical group velocity becomes larger with the increase of the horizontal wavenumber of the bottom topography and also with the increase of the tidal flow amplitude, whereas the life time becomes smaller with the increase of the horizontal wavenumber of the bottom topography but becomes almost independent of the tidal flow amplitude. The resulting bottom-enhanced vertical mixing, therefore, extends further upward as the tidal flow amplitude increases, nearly independent of the bottom roughness. We also find a tradeoff between the fraction of energy dissipated at the ocean bottom and the vertical extent of the energy dissipation region above the ocean bottom.
      PubDate: 2017-06-02T07:05:54.53282-05:0
      DOI: 10.1002/2016JC012564
       
  • Semidiurnal internal tide incoherence in the equatorial Pacific
    • Authors: Maarten C. Buijsman; Brian K. Arbic, James G. Richman, Jay F. Shriver, Alan J. Wallcraft, Luis Zamudio
      Abstract: The jets in the equatorial Pacific Ocean of a realistically-forced global circulation model with a horizontal resolution of 1/12.5° cause a strong loss of phase coherence in semidiurnal internal tides that propagate equatorward from the French Polynesian Islands and Hawaii. This loss of coherence is quantified with a baroclinic energy analysis, in which the semidiurnal-band terms are separated into coherent, incoherent, and cross terms. For time scales longer than a year the coherent energy flux approaches zero values at the equator, while the total flux is ∼500 W/m. The time-variability of the incoherent energy flux is compared with the internal-tide travel-time variability, which is based on along-beam integrated phase speeds computed with the Taylor-Goldstein equation. The variability of monthly-mean Taylor-Goldstein phase speeds agrees well with the phase speed variability inferred from steric sea surface height phases extracted with a plane-wave fit technique. On monthly time scales, the loss of phase coherence in the equatorward beams from the French Polynesian Islands is attributed to the time variability in the vertically-sheared background flow associated with the jets and tropical instability waves. On an annual time scale, the effect of stratification variability is of equal or greater importance than the shear variability is to the loss of coherence. In the model simulations, low-frequency equatorial jets do not noticeably enhance the dissipation of the internal tide, but merely decohere and scatter it.
      PubDate: 2017-06-02T07:05:50.773271-05:
      DOI: 10.1002/2016JC012590
       
  • Observations of wave-induced pore pressure gradients and bed level
           response on a surf zone sandbar
    • Authors: Dylan Anderson; Dan Cox, Ryan Mieras, Jack A. Puleo, Tian-Jian Hsu
      Abstract: Horizontal and vertical pressure gradients may be important physical mechanisms contributing to onshore sediment transport beneath steep, near-breaking waves in the surf zone. A barred beach was constructed in a large-scale laboratory wave flume with a fixed profile containing a mobile sediment layer on the crest of the sandbar. Horizontal and vertical pore pressure gradients were obtained by finite-differences of measurements from an array of pressure transducers buried within the upper several centimeters of the bed. Co-located observations of erosion depth were made during asymmetric wave trials with wave heights between 0.10 m and 0.98 m, consistently resulting in onshore sheet flow sediment transport. The pore pressure gradient vector within the bed exhibited temporal rotations during each wave cycle, directed predominantly upwards under the trough and then rapidly rotating onshore and downwards as the wave front passed. The magnitude of the pore pressure gradient during each phase of rotation was correlated with local wave steepness and relative depth. Momentary bed failures as deep as 20 grain diameters were coincident with sharp increases in the onshore directed pore pressure gradients, but occurred at horizontal pressure gradients less than theoretical critical values for initiation of the motion for compact beds. An expression combining the effects of both horizontal and vertical pore pressure gradients with bed shear stress and soil stability is used to determine that failure of the bed is initiated at non-negligible values of both forces.
      PubDate: 2017-06-02T07:05:41.738935-05:
      DOI: 10.1002/2016JC012557
       
  • Impacts of El Niño events on the Peruvian upwelling system
           productivity
    • Authors: D. Espinoza-Morriberón; V. Echevin, F. Colas, J. Tam, J. Ledesma, L. Vásquez, M. Graco
      Abstract: Every 2 to 7 years, El Niño events trigger a strong decrease in phytoplankton productivity off Peru, which profoundly alters the environmental landscape and trophic chain of the marine ecosystem. Here we use a regional coupled physical-biogeochemical model to study the dynamical processes involved in the productivity changes during El Nino, with a focus on the strongest events of the 1958-2008 period. Model evaluation using satellite and in situ observations shows that the model reproduces the surface and subsurface interannual physical and biogeochemical variability. During El Niño, the thermocline and nutricline deepen significantly during the passage of coastal-trapped waves. While the upwelling-favorable wind increases, the coastal upwelling is compensated by a shoreward geostrophic near-surface current. The depth of upwelling source waters remains unchanged during El Niño but their nutrient content decreases dramatically, which, along with a mixed layer depth increase, impacts the phytoplankton growth. Offshore of the coastal zone, enhanced eddy-induced subduction during El Niño plays a potentially important role in nutrient loss.
      PubDate: 2017-06-02T07:05:34.262972-05:
      DOI: 10.1002/2016JC012439
       
  • Observation of tropical cyclone induced shallow water currents in Taiwan
           Strait
    • Authors: Junqiang Shen; Yun Qiu, Shanwu Zhang, Fangfang Kuang
      Abstract: The data from three stations equipped with Acoustic Doppler Current Profilers (ADCPs) deployed in the shallow water of the Taiwan Strait (TWS) were used to study the shallow coastal ocean response to five quasi-continuous tropical cyclone (TC) events in the late summer 2006. We revealed that, in the forced stage, when the large and strong TC (Bilis) transited, the geostrophic currents were formed which dominated the whole event, while the strong but relatively small one (Saomai) or the weak one (Bopha) primarily leaded to the generation of Ekman currents. In the relaxation stage, the barotropic subinertial waves and/or the baroclinic near-inertial oscillations (NIOs) were triggered. Typically, during the transit of the Saomai, subinertial waves were induced which demonstrated a period of 2.8-4.1 days and a mean alongshore phase velocity of 14.9±3.2 m/s in the form of free-barotropic continental shelf waves. However, the NIOs are only notable in the area in which the water column is stably stratified and also where the wind stress is dominated by the clockwise component and accompanied by high-frequency (near-inertial) variations. We also demonstrated that, due to the damping effects, the nonlinear wave-wave interaction (e.g., between NIO and semidiurnal tide in our case), together with the well-known bottom friction, led to the rapid decay of the observed TC-induced near-inertial currents, giving a typical e-folding time scale of 1-3 inertial periods. Moreover, such nonlinear wave-wave interaction was even found to play a major role during the spring tide in TWS.
      PubDate: 2017-06-02T07:05:31.080975-05:
      DOI: 10.1002/2017JC012737
       
  • Bed failure induced by internal solitary waves
    • Authors: Gustavo A. Rivera-Rosario; Peter J. Diamessis, James T. Jenkins
      Abstract: The pressure field inside a porous bed induced by the passage of an Internal Solitary Wave (ISW) of depression is examined using high-accuracy numerical simulations. Assuming a two-layer, continuously stratified water column, the velocity and density fields are obtained by solving the Dubreil-Jacotin-Long Equation. The total wave-induced pressure across the surface of the bed is computed by vertically integrating for the hydrostatic and nonhydrostatic contributions. The bed is assumed to be a continuum composed of either sand or silt, with a small amount of trapped gas. Results show variations in pore-water pressure penetrating deeper into more conductive materials and remaining for a prolonged period after the wave has passed. In order to quantify the potential for failure, the vertical pressure gradient is compared against the buoyant weight of the bed. The pressure gradient exceeds this weight for weakly conductive materials. Failure is further enhanced by a decrease in bed saturation, consistent with studies in surface-wave induced failure. In deeper water, the ISW-induced pressure is stronger, causing failure only for weakly conductive materials. The pressure associated with the free-surface displacement that accompanies ISWs is significant, when the depth of the water column is below 100m, but has little influence when it is above 100m, where the hydrostatic pressure due to the pycnocline displacement is much larger. Since the pore-pressure gradient reduces the specific weight of the bed, results show that particles are easier for the flow to suspend, suggesting that pressure contributes to the powerful resuspension events observed in the field.
      PubDate: 2017-05-30T11:20:43.766207-05:
      DOI: 10.1002/2017JC012935
       
  • Seaglider surveys at Ocean Station Papa: Diagnosis of upper-ocean heat and
           salt balances using least-squares with inequality constraints
    • Authors: Noel A. Pelland; Charles C. Eriksen, Meghan F. Cronin
      Abstract: Heat and salt balances in the upper 200 m are examined using data from Seaglider spatial surveys June 2008-January 2010 surrounding a NOAA surface mooring at Ocean Station Papa (OSP; 50°N, 145°W). A least-squares approach is applied to repeat Seaglider survey and moored measurements to solve for unknown or uncertain monthly three-dimensional circulation and vertical diffusivity. Within the surface boundary layer, the estimated heat and salt balances are dominated throughout the surveys by turbulent flux, vertical advection, and for heat, radiative absorption. When vertically-integrated balances are considered, an estimated upwelling of cool water balances the net surface input of heat, while the corresponding large import of salt across the halocline due to upwelling and diffusion is balanced by surface moisture input and horizontal import of fresh water. Measurement of horizontal gradients allows the estimation of unresolved vertical terms over more than one annual cycle; diffusivity in the upper-ocean transition layer decreases rapidly to the depth of the maximum near-surface stratification in all months, with weak seasonal modulation in the rate of decrease and profile amplitude. Vertical velocity is estimated to be on average upward but with important monthly variations. Results support and expand existing evidence concerning the importance of horizontal advection in the balances of heat and salt in the Gulf of Alaska, highlight time and depth variability in difficult-to-measure vertical transports in the upper ocean, and suggest avenues of further study in future observational work at OSP.
      PubDate: 2017-05-30T11:20:42.450795-05:
      DOI: 10.1002/2017JC012821
       
  • Internal wave generation by tidal flow over periodically and randomly
           distributed seamounts
    • Authors: Likun Zhang; Maarten C. Buijsman, Eva Comino, Harry L. Swinney
      Abstract: We examine numerically the conversion of barotropic tidal energy into internal waves by flow over an isolated seamount and over systems of periodically and randomly distributed 1100 m tall seamounts with Gaussian profiles. The simulations use the Massachusetts Institute of Technology general circulation model (MITgcm) to calculate for an infinitely deep ocean the dependence of the energy conversion on seamount slope, seamount separation, tidal direction, and the size and aspect ratio of the simulation domain. {For neighboring seamounts with a slope greater than the internal wave beam slope, wave interference reduces} the conversion relative to that calculated for an isolated seamount, and relative to that predicted by linear theory for a seamount of slope less than the beam slope. The conversion by an individual seamount in a system of random seamounts separated by an average distance of 18 km is found to be suppressed by 16% relative to the conversion by an isolated seamount. This study provides insight into tidal conversion by ocean seamounts modeled as Gaussian mountains with slopes both smaller and larger than the beam slope. We conclude that the total energy conversion by all seamounts (peak height ≥ 1000 m) and knolls (peak height 500-1000 m), taking into account interference affects, is of the order of 1% of the total barotropic to baroclinic energy conversion in the oceans, which is about twice as large as previous estimates.
      PubDate: 2017-05-30T11:20:41.054707-05:
      DOI: 10.1002/2017JC012884
       
  • Dynamics of simulated Atlantic upwelling annual cycle in CMIP5 models
    • Authors: Li-Chiao Wang; Fei-Fei Jin, Chau-Ron Wu
      Abstract: As one of the prominent climatic component of tropical climate system, the annual cycle of the equatorial Atlantic cold tongue region has been a subject of significant research. In this work, we examine the fidelity of the key part of the annual cycles, namely, the upwelling annual cycles, from the simulations of the Coupled Model Intercomparison Project Phase 5 (CMIP5). Utilizing the simple framework we have recently developed based on the Zebiak-Cane (ZC) ocean model, we first tested the validity of the simple framework of upwelling as a combination of the local wind-driven Ekman upwelling and nonlocal wind-driven wave upwelling. It is demonstrated that the theoretically reconstructed upwelling annual cycles to large extent are in good agreement with the simulations in these coupled climatic models. Comparing with observations, we show that the semi-annual component of the upwelling is systematically too strong. This significant bias is attributable to the dynamic amplification to the wind-driven wave upwelling originated from a less significant bias in the semi-annual harmonic component in the central equatorial wind field. The latter is dynamically amplified in the wind-driven wave upwelling owing to its relatively fast timescale. Thus the realistic simulations of equatorial annual cycles may sensitively depend on good simulations of the semi-annual harmonic components in equatorial wind fields.
      PubDate: 2017-05-30T11:20:34.258276-05:
      DOI: 10.1002/2017JC012781
       
  • The responses of cyclonic and anticyclonic eddies to typhoon forcing: The
           vertical temperature-salinity structure changes associated with the
           horizontal convergence/divergence
    • Authors: Shan-Shan Liu; Liang Sun, Qiaoyan Wu, Yuan-Jian Yang
      Abstract: The responses of the cyclonic eddies (CEs) and anti-cyclonic eddies (AEs) to typhoon forcing in the Western North Pacific Ocean (WNPO) are analyzed using Argo profiles. Both CEs and AEs have the primary cooling at the surface (0-10 m depth), and deep upwelling from the top of thermocline (200 m depth) down to deeper ocean shortly after typhoon forcing. Due to the deep upwelling, part of warm and fresh water at the top of AEs move out of the eddy, which leads to a colder and saltier subsurface in the AEs after the passage of the typhoon. In contrast, the inflow of warm and fresh water heats and freshens the subsurface in the CEs to compensate the cooling induced by the typhoon. This explains why the observed strong SST cooling were much less than modeled, since the AEs are more frequent than CEs in the WNPO. It indicates that there is divergence (convergence) of warm and fresh water in the surface of AEs (subsurface of CEs). The divergence-convergence effects of the AEs and CEs lead to the secondary cooling center locate at a shallow layer of 100 m in the AEs and a much deeper layer of 350 m in the CEs. This shallow divergence-convergence flow could lead a shallow overturning flow in upper oceans, which may potentially influences the large-scale ocean circulations and climates.
      PubDate: 2017-05-26T20:15:53.341101-05:
      DOI: 10.1002/2017JC012814
       
  • Eddy-driven nutrient transport and associated upper-ocean primary
           production along the Kuroshio
    • Authors: Yusuke Uchiyama; Yota Suzue, Hidekatsu Yamazaki
      Abstract: The Kuroshio is one of the most energetic western boundary currents accompanied by vigorous eddy activity both on meso- and submesoscale, which affects biogeochemical processes in the upper ocean. We examine the primary production around the Kuroshio off Japan using a climatological ocean modeling based on the Regional Oceanic Modeling System (ROMS) coupled with a nitrogen-based nutrient, phyto- and zooplankton, and detritus (NPZD) biogeochemical model in a submesoscale eddy-permitting configuration. The model indicates significant differences of the biogeochemical responses to eddy activities in the Kuroshio Region (KR) and Kuroshio Extension Region (KE). In the KR, persisting cyclonic eddies developed between the Kuroshio and coastline are responsible for upwelling-induced eutrophication. However, the eddy-induced vertical nutrient flux counteracts and promotes pronounced south- and downward diapycnal nutrient transport from the mixed layer down beneath the main body of the Kuroshio, which suppresses the near-surface productivity. In contrast, the KE has a 23.5% higher productivity than the KR, even at comparable eddy intensity. Upward nutrient transport prevails near the surface due to predominant cyclonic eddies, particularly to the north of the KE, where the downward transport barely occurs, except at depths deeper than 400 m and to a much smaller degree than in the KR. The eddy energy conversion analysis reveals that the combination of shear instability around the mainstream of the Kuroshio with prominent baroclinic instability near the Kuroshio front is essential for the generation of eddies in the KR, leading to the increase of the eddy-induced vertical nitrate transport around the Kuroshio.
      PubDate: 2017-05-26T20:00:37.872879-05:
      DOI: 10.1002/2017JC012847
       
  • Sedimentological regimes for turbidity currents: Depth-averaged theory
    • Authors: Thomas C. Halsey; Amit Kumar, Mauricio M. Perillo
      Abstract: Turbidity currents are one of the most significant means by which sediment is moved from the continents into the deep ocean; their properties are interesting both as elements of the global sediment cycle and due to their role in contributing to the formation of deep water oil and gas reservoirs. One of the simplest models of the dynamics of turbidity current flow was introduced three decades ago, and is based on depth-averaging of the fluid mechanical equations governing the turbulent gravity-driven flow of relatively dilute turbidity currents. We examine the sedimentological regimes of a simplified version of this model, focusing on the role of the Richardson number Ri (dimensionless inertia) and Rouse number Ro (dimensionless sedimentation velocity) in determining whether a current is net depositional or net erosional. We find that for large Rouse numbers, the currents are strongly net depositional due to the disappearance of local equilibria between erosion and deposition. At lower Rouse numbers, the Richardson number also plays a role in determining the degree of erosion vs. deposition. The currents become more erosive at lower values of the product Ro × Ri, due to the effect of clear water entrainment. At higher values of this product, the turbulence becomes insufficient to maintain the sediment in suspension, as first pointed out by Knapp and Bagnold. We speculate on the potential for two-layer solutions in this insufficiently turbulent regime, which would comprise substantial bedload flow with an overlying turbidity current.
      PubDate: 2017-05-26T20:00:31.811227-05:
      DOI: 10.1002/2016JC012635
       
  • Air-sea fluxes for Hurricane Patricia (2015): Comparison with supertyphoon
           Haiyan (2013) and under different ENSO conditions
    • Authors: Hsiao-Ching Huang; Julien Boucharel, I-I. Lin, Fei-Fei Jin, Chun-Chi Lien, Iam-Fei Pun
      Abstract: Hurricane Patricia formed on October 20th, 2015 in the Eastern Pacific and, in less than 3 days, rapidly intensified from a Tropical Storm to a record-breaking hurricane with maximum sustained winds measured around 185 knots. It is almost 15 knots higher than 2013's supertyphoon Haiyan (the previous strongest tropical cyclone (TC) ever observed). This research focuses on analyzing the air-sea enthalpy flux conditions that contributed to hurricane Patricia's rapid intensification, and comparing them to supertyphoon Haiyan's. Despite a stronger cooling effect, a higher enthalpy flux supply is found during Patricia, in particular due to warmer pre-TC sea surface temperature conditions. This resulted in larger temperature and humidity differences at the air-sea interface, contributing to larger air-sea enthalpy heat fluxes available for Patricia's growth (24% more than for Haiyan). In addition, air-sea fluxes simulations were performed for hurricane Patricia under different climate conditions to assess specifically the impact of local and large-scale conditions on storm intensification associated with six different phases and types of El Niño Southern Oscillation (ENSO) and long-term climatological summer condition. We found that the Eastern Pacific El Niño developing and decaying summers, and the Central Pacific El Niño developing summer are the three most favorable ENSO conditions for storm intensification. This still represents a 37% smaller flux supply than in October 2015, suggesting that Patricia extraordinary growth is not achievable under any of these typical ENSO conditions but rather the result of the exceptional environmental conditions associated with the build-up of the strongest El Niño ever recorded.
      PubDate: 2017-05-26T19:55:27.737665-05:
      DOI: 10.1002/2017JC012741
       
  • Dense Water Formation in the North-Western Mediterranean area during
           HyMeX-SOP2 in 1/36° ocean simulations: Ocean-atmosphere coupling impact
    • Authors: Cindy Lebeaupin Brossier; Fabien Léger, Hervé Giordani, Jonathan Beuvier, Marie-Noëlle Bouin, Véronique Ducrocq, Nadia Fourrié
      Abstract: The north-western Mediterranean Sea is a key location for the thermohaline circulation of the basin. The area is characterized by intense air-sea exchanges favoured by the succession of strong northerly and north-westerly wind situations (mistral and tramontane) in autumn and winter. Such meteorological conditions lead to significant evaporation and ocean heat loss that are well known as the main triggering factor for the Dense Water Formation (DWF) and winter deep convection episodes.During the HyMeX second field campaign (SOP2, 1 February to 15 March 2013), several platforms were deployed in the area in order to document the DWF and the ocean deep convection, as the air-sea interface conditions.This study investigates the role of the ocean-atmosphere coupling on DWF during winter 2012-2013. The coupled system, based on the NEMO-WMED36 ocean model (1/36° resolution) and the AROME-WMED atmospheric model (2.5 km-resolution), was run during two months covering the SOP2 and is compared to an ocean-only simulation forced by AROME-WMED real-time forecasts and to observations collected in the north-western Mediterranean area during the HyMeX SOP2.The comparison shows small differences in terms of net heat, water and momentum fluxes. On average, DWF is slightly sensitive to air-sea coupling. However fine-scale ocean processes, such as shelf DWF and export or eddies and fronts at the rim of the convective patch are significantly modified. The wind-current interactions constitute an efficient coupled process at fine scale, acting as a turbulence propagating vectors, producing large mixing and convection at the rim of the convective patch.
      PubDate: 2017-05-25T17:26:12.158651-05:
      DOI: 10.1002/2016JC012526
       
  • Real-time monitoring of nutrients in the Changjiang Estuary reveals
           short-term nutrient-algal bloom dynamics
    • Authors: Kui Wang; Jianfang Chen, Xiaobo Ni, Dingyong Zeng, Dewang Li, Haiyan Jin, Patricia M. Glibert, Wenxian Qiu, Daji Huang
      Abstract: The Changjiang Estuary is a large-river estuary ecosystem in the East China Sea, and its plume, the Changjiang Diluted Water (CDW), transports a large mass of nutrients (NO3-+NO2-, PO43-, SiO32-) to the shelf sea, leading to substantial eutrophication; the CDW also supports high primary production. However, relationships between nutrient delivery and phytoplankton responses have been difficult to establish, as many nutrient delivery events and algal blooms are episodic, and the CDW may expand or become detached with changing winds. To study the relationship between nutrient delivery events, algal blooms and estuarine metabolism dynamics, a buoy system was deployed in the CDW from 9 September to 10 October, 2013, with measurements of chlorophyll a and dissolved nutrients. Day-to-day nutrient increases co-varied with salinity decreases, regulated by both the spring-neap tidal cycle and wind events. Several specific nutrient injection periods were detected, each followed by nutrient draw-down and chlorophyll a accumulation (algal blooms). Each algal bloom had its own unique pattern of nutrient uptake based on change in nutrient ratios (ΔN:ΔP; ΔN:ΔSi) and appeared to be dominated by different algal groups. These events occurred under weak wind and stable hydrodynamic conditions. Ecosystem metabolism based on net community production (NCP) showed that the upper estuarine ecosystem was autotrophic when chlorophyll a accumulated, but heterotrophic when wind-induced mixing strengthened, and upwelling brought organic-rich water to the near surface. In spite of several short-lived algal blooms, the average NCPdaily was negative during the observation period, indicating a net source of CO2 to the atmosphere.
      PubDate: 2017-05-25T17:20:35.654708-05:
      DOI: 10.1002/2016JC012450
       
  • The effects of oceanic barrier layer on the upper ocean response to
           tropical cyclones
    • Authors: Youfang Yan; Li Li, Chunzai Wang
      Abstract: Previous studies of the barrier layer (BL) and tropical cyclone (TC) intensification show controversial results. Some studies suggest that the BL is favorable for TC intensification, whereas others demonstrate that the BL has little impact. The present paper reconciles these previous results by showing that the influence of the BL on TC intensification is complex dependent on the stages, TC intensity, TC forcing time and ocean stratification. The influence can be divided into three different stages. First, when a TC is weak or the TC-induced forcing can not break through the mixed layer (ML), the presence of the BL leads to a thin ML, results in a large ML cooling, and reduces the effective TC heat potential (ETCHP) by distributing the air-sea heat loss to the thin ML, thus suppresses TC growth. Second, when TC strengthens or wind forcing increases, the forcing becomes strong enough to break through the ML and penetrates into the BL. The entrainment of BL water, which is warmer than the cooled ML, compensates the surface heat loss and leads to an increase of the ML temperature, supporting TC growth. Third, when the TC-induced forcing is strong enough to break through the BL base and penetrates to the thermocline, the BL reduces the magnitudes of cooling temperature and of decreasing ETCHP compared to those without the BL, which is also favorable for TC intensification. Thus, whether the BL intensifies TCs depends the conditions associated with the upper layer ocean and TCs.
      PubDate: 2017-05-25T17:20:31.630247-05:
      DOI: 10.1002/2017JC012694
       
  • Phytoplankton bloom triggered by an anticyclonic eddy: The combined effect
           of eddy-Ekman pumping and winter mixing
    • Authors: Qingyou He; Haigang Zhan, Yiping Shuai, Shuqun Cai, Qian P. Li, Gaolong Huang, Junmin Li
      Abstract: Recent composite based researches reveal that anticyclonic eddies facilitate the growth of phytoplankton in the subtropical gyres. Two dynamical mechanisms, eddy-Ekman pumping and winter mixing, have been examined individually, but their relative and combined effects remain unclear. Using satellite observations and model simulations, this study investigated the process of a distinct phytoplankton bloom generated in an anticyclonic eddy in the nutrient-depleted southeastern Indian Ocean. The bloom propagated westward along with the eddy for more than 600 km from late April to August in 2010. The peak of surface chlorophyll concentration in the eddy is 2.2 times larger than the mean value of the ambient. The development of the bloom is dominated by the winter deepening of mixed layer, whose velocity in vertical nutrient flux is on average 3 times larger than that of eddy-Ekman pumping. The results of a 1-D physical-biogeochemical model demonstrate that the role of eddy-Ekman pumping is also indispensable, because it not only transports extra nutrients into the mixed layer, but also results in significant chlorophyll enrichment in subsurface water. The superposition of eddy-Ekman pumping on winter mixing triples the chlorophyll enrichment both at the surface and in the upper layer, and the entrainment of subsurface phytoplankton into the mixed layer contributes significantly to the surface bloom, especially in its initial stage. Both of the satellite observations and model simulation show that eddy-Ekman pumping can lead to an early occurrence of the bloom for more than two weeks.
      PubDate: 2017-05-25T17:20:28.999248-05:
      DOI: 10.1002/2017JC012763
       
  • OSSE quantitative assessment of rapid-response pre-storm ocean surveys to
           improve coupled tropical cyclone prediction
    • Authors: G. R. Halliwell; M. Mehari, L. K. Shay, V. H. Kourafalou, H. Kang, H-S Kim, J. Dong, R. Atlas
      Abstract: Ocean fields that initialize coupled TC prediction models must accurately represent the dynamics of mesoscale features and the associated distribution of upper-ocean temperature and salinity. They must also provide unbiased realizations of upper-ocean heat content and stratification. Ocean Observing System Simulation Experiments (OSSEs) are performed for three storms [Isaac (2012), Edouard (2014), Gonzalo (2014)] to assess the impact of rapid-response pre-storm ocean profile surveys on improving ocean model initialization. Two types of surveys are evaluated: airborne deployments of expendable profilers and deployments of in-situ thermistor chains along lines intersecting predicted storm paths. Assimilation of the existing ocean observing system substantially constrains mesoscale structure in dynamical fields, primarily because of the four available altimeters. However, these observations only modestly constrain mesoscale structure and bias in upper-ocean thermal fields. Adding rapid-response airborne surveys to these observing systems produces substantial additional correction in thermal fields, but minimal additional correction in dynamical fields. Without altimetry assimilation, rapid-response profiles produce large additional correction in both dynamical and thermal fields. Airborne CTDs sampling temperature and salinity over 1000 m versus XBTs sampling temperature over 400 m produce additional correction for dynamical fields, but not for upper-ocean thermal fields. Airborne surveys are generally more effective than thermistor chain deployments because they can sample a larger area at higher horizontal resolution, and because the latter only measures temperature over the upper ∼100m. Both airborne profile surveys and thermistor chain deployments effectively reduce upper-ocean thermal biases.
      PubDate: 2017-05-25T17:20:25.59152-05:0
      DOI: 10.1002/2017JC012760
       
  • Circulation, hydrography, and transport over the summit of Axial Seamount,
           a deep volcano in the Northeast Pacific
    • Authors: G. Xu; J. W. Lavelle
      Abstract: A numerical model of ocean flow, hydrography, and transport is used to extrapolate observations of currents and hydrography and infer patterns of material flux in the deep ocean around Axial Seamount, a destination node of NSF's Ocean Observatories Initiative's Cabled Array. Using an inverse method, the model is made to approximate measured deep ocean flow around this site during a 35-day time period in the year 2002. The model is then used to extract month-long mean patterns and examine smaller-scale spatial and temporal variability around Axial. Like prior observations, model month-long mean currents flow anti-cyclonically around the seamount's summit in toroidal form with maximum speeds at 1500m depth of 10-11 cm/s. As a time mean, the temperature (salinity) anomaly distribution takes the form of a cold (briny) dome above the summit. Passive tracer material continually released at the location of the ASHES vent field exits the caldera primarily through its southern open end before filling the caldera. Once outside the caldera, the tracer circles the summit in clockwise fashion, fractionally re-entering the caldera over lower walls at its north end, while gradually bleeding southwestward during the modeled time period into the ambient ocean. A second tracer release experiment using a source of only two-day duration inside and near the CASM vent field at the northern end of the caldera suggests a residence time of the fluid at that locale of 8-9 days.
      PubDate: 2017-05-24T07:25:28.151435-05:
      DOI: 10.1002/2016JC012464
       
  • Photophysiological and light absorption properties of phytoplankton
           communities in the river-dominated margin of the northern Gulf of Mexico
    • Authors: Sumit Chakraborty; Steven E. Lohrenz, Kjell Gundersen
      Abstract: Spatial and temporal variability in photophysiological properties of phytoplankton were examined in relationship to phytoplankton community composition in the river-dominated continental margin of the northern Gulf of Mexico (NGOM). Observations made during five research cruises in the NGOM included phytoplankton photosynthetic and optical properties and associated environmental conditions and phytoplankton community structure. Distinct patterns of spatial and temporal variability in photophysiological parameters were found for waters dominated by different phytoplankton groups. Photophysiological properties for locations associated with dominance by a particular group of phytoplankton showed evidence of photoacclimation as reflected by differences in light absorption and pigment characteristics in relationship to different light environments. The maximum rate of photosynthesis normalized to chlorophyll (PmaxB) was significantly higher for communities dominated (> 60% biomass) by cyanobacteria+prochlorophyte (cyano+prochl). The initial slope of the photosynthesis-irradiance (P-E) curve normalized to chlorophyll (αB) was not clearly related to phytoplankton community structure and no significant differences were found in PmaxB and αB between different geographic regions. In contrast, maximum quantum yield of carbon fixation in photosynthesis (Φcmax) differed significantly between regions and was higher for diatom-dominated communities. Multiple linear regression models, specific for the different phytoplankton communities, using a combination of environmental and bio-optical proxies as predictor variables showed considerable promise for estimation of the photophysiological parameters on a regional scale. Such an approach may be utilized to develop size-class or phytoplankton group-specific primary productivity models for the NGOM.
      PubDate: 2017-05-24T07:20:38.868577-05:
      DOI: 10.1002/2016JC012092
       
  • Warm layer and cool skin corrections for bulk water temperature
           measurements for air-sea interaction Studies
    • Authors: Denny P. Alappattu; Qing Wang, Ryan Yamaguchi, Richard J. Lind, Mike Reynolds, Adam J. Christman
      Abstract: The sea surface temperature (SST) relevant to air-sea interaction studies is the temperature immediately adjacent to the air, referred to as skin SST. Generally, SST measurements from ships and buoys are taken at depths varies from several centimeters to five meters below the surface. These measurements, known as bulk SST, can differ from skin SST up to O(1°C).Shipboard bulk and skin SST measurements were made during the Coupled Air-Sea Processes and Electromagnetic ducting Research east coast field campaign (CASPER-East). An Infrared SST Autonomous Radiometer (ISAR) recorded skin SST, while R/V Sharp's Surface Mapping System (SMS) provided bulk SST from one-meter water depth. Since the ISAR is sensitive to sea spray and rain, missing skin SST data occurred in these conditions. However, SMS measurement is less affected by adverse weather and provided continuous bulk SST measurements. It is desirable to correct the bulk SST to obtain a good representation of the skin SST, which is the objective of this research.Bulk-skin SST difference has been examined with respect to meteorological factors associated with cool skin and diurnal warm layers. Strong influences of wind speed, diurnal effects and net longwave radiation flux on temperature difference are noticed. A three-step scheme is established to correct for wind effect, diurnal variability and then for dependency on net longwave radiation flux. Scheme is tested and compared to existing correction schemes. This method is able to effectively compensate for multiple factors acting to modify bulk SST measurements over the range of conditions experienced during CASPER-East.
      PubDate: 2017-05-24T07:20:29.369419-05:
      DOI: 10.1002/2017JC012688
       
  • Seasonal and interannual variability in along-slope oceanic properties off
           the US West Coast: Inferences from a high-resolution regional model
    • Authors: A. B. Kurapov; N. A. Pelland, D. L. Rudnick
      Abstract: A 6-year, 2009-2014 simulation using a 2-km horizontal resolution ocean circulation model of the Northeast Pacific coast is analyzed with focus on seasonal and interannual variability in along-slope subsurface oceanic properties. Specifically, the fields are sampled on the isopycnal surface σ = 26.5 kg m−3 that is found between depths of 150 and 300 m below the ocean surface over the continental slope. The fields analyzed include the depth z26.5, temperature T26.5, along-slope currents ν26.5 and the average potential vorticity PV between σ = 26.5 and 26.25 kg m−3. Each field is averaged in the cross-shore direction over the continental slope and presented as a function of the alongshore coordinate and time. The seasonal cycle in z26.5 shows a coherent upwelling-downwelling pattern from Mexico to Canada propagating to the north with a speed of 0.5 m s−1. The anomalously deep (−20 m) z26.5 displacement in spring-summer 2014 is forced by the southern boundary condition at 24°N as a manifestation of an emerging strong El Niño. The seasonal cycle in T26.5 is most pronounced between 36-53°N indicating that subarctic waters are replaced by warmer Californian waters in summer with the speed close 0.15 m s−1, which is consistent with earlier estimates of the undercurrent speed and also present ν26.5 analyses. The seasonal patterns and anomalies in z26.5 and T26.5 find confirmation in available long-term glider and ship-borne observations. The PV seasonality over the slope is qualitatively different to the south and north of the southern edge of Heceta Bank (43.9°N).
      PubDate: 2017-05-24T07:20:24.900531-05:
      DOI: 10.1002/2017JC012721
       
  • Standing infragravity waves over an alongshore irregular rocky bathymetry
    • Authors: G. Winter; R.J. Lowe, G. Symonds, J.E. Hansen, A.R. van Dongeren
      Abstract: Rocky reef coastlines typically feature highly variable and often abrupt cross- and alongshore changes in bathymetry. The effects of this irregular rocky bathymetry on the dynamics of infragravity waves are largely unknown. Most models of infragravity wave dynamics have been developed and validated on smooth alongshore-uniform bathymetries, which may break down over these highly variable bathymetries. A two-week field experiment was conducted on a rocky reef-fringed beach to investigate how the variable bathymetry affects the spatial and temporal variability of infragravity waves. The height of short (sea-swell) waves decreased over the shallow reef due to breaking, whereas the height of infragravity waves increased towards the shoreline. Both during a storm event (Hm0 = 2.3 m) and under moderate wave conditions (Hm0 = 1.0-1.8 m), the infragravity waves formed a persistent cross-shore standing wave pattern along the entire shoreline, despite the irregular bathymetry. In addition, the alongshore components of infragravity waves refracted by the presence of the nearshore reef, were observed to propagate in opposite directions up and down the coast resulting in a local alongshore standing wave pattern. Thus, the presence of highly variable nearshore bathymetry, which commonly occurs along rocky reef coastlines, may produce both cross- and alongshore standing wave patterns.
      PubDate: 2017-05-23T05:15:25.144911-05:
      DOI: 10.1002/2016JC012242
       
  • On the dynamics of flow past a cylinder: Implications for CTD package
           motions and measurements
    • Authors: D. R. Munday; M. P. Meredith
      Abstract: During the collection of ship-based observations, heaving of the vessel may lead to variation in the descent rate of a CTD package. This can result in the package being pulled upwards through previously sampled water, leading to difficult to quantify errors due to the complex wake. To reduce this problem to one of manageable stature, we use the simple paradigm of two dimensional flow past a cylinder. By using a tracer with a gradient along the flow, we quantify the effect of the cylinder on its distribution and the impact of post processing. At high Reynolds numbers, over 200, uniform translation leads to a small error in the tracer value. This error is likely negligible at the much higher Reynolds number of the ocean. When the flow is oscillated longitudinally, there are two main sources of error; attached vortices may propagate around the cylinder and/or shed vortices may translate into the path of the cylinder. Post processing by removing records from previous pressure levels removes much of the first error, due to it occurring as the package ascends. However, the second source of error is more difficult to remove, due to it occurring when the package is once again descending. In general, results indicate that long period oscillations are preferable. Whilst the magnitude of the errors are comparable to those from short period oscillations, they are spread farther apart in time and space and the overall effect is to localise the errors in small regions of the final depth profile.
      PubDate: 2017-05-22T05:40:47.920308-05:
      DOI: 10.1002/2017JC012708
       
  • On the dynamics of the Mouth of Columbia River: Results from a
           three-dimensional fully coupled wave-current interaction model
    • Authors: Çiğdem Akan; Saeed Moghimi, H. Tuba Özkan-Haller, John Osborne, Alexander Kurapov
      Abstract: Numerical simulations were performed using a 3D ocean circulation model (ROMS) two-way coupled to a phase-averaged wave propagation model (SWAN), in order to expand our understanding of the dynamics of wave-current interactions at the Mouth of the Columbia River (MCR). First, model results are compared with water elevations, currents, temperature, salinity and wave measurements obtained by the U.S. Army Corp of Engineers during the Mega-Transect Experiment in 2005. We then discuss the effects of the currents on the waves and vice versa. Results show that wave heights are intensified notably at the entrance of the mouth in the presence of the tidal currents, especially in ebb flows. We also find non-local modifications to the wave field because of wave focusing processes that re-direct wave energy towards the inlet mouth from adjacent areas, resulting in the presence of a tidal signatures in areas where local currents are weak. The model also suggests significant wave amplification at the edge of the expanding plume in the later stages of ebb, some tens of kilometers offshore of the inlet mouth, with potential implications for navigation safety. The effect of waves on the location of the plume is also analyzed, and results suggest that the plume is shifted in the down-wave direction when wave effects are considered, and that this shift is more pronounced for larger waves, and consistent with the presence of alongshore advection terms in the salt advection equation, which are related to the Stokes velocities associated with waves.
      PubDate: 2017-05-22T05:40:34.398229-05:
      DOI: 10.1002/2016JC012307
       
  • Land adjacency effects on MODIS Aqua top-of-atmosphere radiance in the
           shortwave infrared: Statistical assessment and correction
    • Authors: Lian Feng; Chuanmin Hu
      Abstract: Satellite measurements of coastal or inland waters near land/water interfaces suffer from land adjacency effects (LAEs), particularly in the short wave infrared (SWIR) wavelengths. Here, a statistical method was developed to quantify the LAEs as the ratio of top-of-atmosphere (TOA) total radiance (Lt, W m−2 µm−1 sr−1) between near-shore pixels and LAE-free offshore pixels (>12 pixels away from land). The calculations were conducted using MODIS Aqua images between 2003 and 2012 over the Madagascar Island, with results showing the dependency of LAEs on different environmental and observational factors. The LAEs decrease dramatically with increasing distance from shoreline, and increase with decreasing aerosol optical thickness at 869 nm (τ869). The nearby land surface albedo also plays a role in modulating the LAEs, but the impact is only prominent under low-aerosol conditions. Based on these observations, a look-up-table (LUT) to formulate a correction scheme was established. Tests of the correction scheme using satellite observations over the Hawaii Islands and using in situ measurements in the Chesapeake Bay show significant improvements in Lt (LAEs much closer to 1 than uncorrected data) and retrieved surface chlorophyll-a concentration (Chl-a, mg m−3), respectively. Furthermore, the number of Chl-a retrievals within the range of 0 – 64 mg m−3 also increases by >60%. While the ultimate solution of correcting the LAEs for coastal/inland water applications still requires further work, these preliminary results suggest that the method proposed here deserves further tests in other estuaries and lakes.
      PubDate: 2017-05-22T05:40:32.150019-05:
      DOI: 10.1002/2017JC012874
       
  • Eddy-induced sea surface temperature gradients in Eastern Boundary Current
           Systems
    • Authors: Yeping Yuan; Renato M. Castelao
      Abstract: Many processes are known to influence the distribution of sea surface temperature (SST) fronts in Eastern Boundary Current Systems (EBCS), including wind forcing, mesoscale activity, flow instabilities and flow-topography interactions. Here, we used satellite observations to quantify the relative importance of one of these processes, mesoscale eddy activity, on the distribution of SST gradients in EBCS. Eddies are characterized by large SST and SST gradient anomalies in all EBCS. At the end of the upwelling season, eddy-induced SST gradients are dominant components of the full SST gradient offshore of 300 km from the coast, especially in regions of high eddy activity. Comparisons between eddy-induced and full SST gradients in the California Current System indicate that the offshore migration of mesoscale eddies plays a significant role on the seasonal widening of the region of high frontal activity. SST gradients associated with anticyclonic eddies are largest within 400 km from shore, while the signature of cyclones can extend for up to 700 km from the coast.
      PubDate: 2017-05-22T05:35:32.822335-05:
      DOI: 10.1002/2017JC012735
       
  • Rapid shelf-wide cooling response of a stratified coastal ocean to
           hurricanes
    • Authors: Greg Seroka; Travis Miles, Yi Xu, Josh Kohut, Oscar Schofield, Scott Glenn
      Abstract: Large uncertainty in the predicted intensity of tropical cyclones (TCs) persists compared to the steadily improving skill in the predicted TC tracks. This intensity uncertainty has its most significant implications in the coastal zone, where TC impacts to populated shorelines are greatest. Recent studies have demonstrated that rapid ahead-of-eye-center cooling of a stratified coastal ocean can have a significant impact on hurricane intensity forecasts. Using observation-validated, high-resolution ocean modeling, the stratified coastal ocean cooling processes observed in two U.S. Mid-Atlantic hurricanes were investigated: Hurricane Irene (2011)—with an inshore Mid-Atlantic Bight (MAB) track during the late summer stratified coastal ocean season—and Tropical Storm Barry (2007)—with an offshore track during early summer. For both storms, the critical ahead-of-eye-center depth-averaged force balance across the entire MAB shelf included an onshore wind stress balanced by an offshore pressure gradient. This resulted in onshore surface currents opposing offshore bottom currents that enhanced surface to bottom current shear and turbulent mixing across the thermocline, resulting in the rapid cooling of the surface layer ahead-of-eye-center. Because the same baroclinic and mixing processes occurred for two storms on opposite ends of the track and seasonal stratification envelope, the response appears robust. It will be critical to forecast these processes and their implications for a wide range of future storms using realistic 3D coupled atmosphere-ocean models to lower the uncertainty in predictions of TC intensities and impacts and enable coastal populations to better respond to increasing rapid intensification threats in an era of rising sea levels.
      PubDate: 2017-05-20T11:20:26.414635-05:
      DOI: 10.1002/2017JC012756
       
  • Large-scale experimental observations of sheet flow on a sandbar under
           skewed-asymmetric waves
    • Authors: Ryan S. Mieras; Jack A. Puleo, Dylan Anderson, Daniel T. Cox, Tian-Jian Hsu
      Abstract: A novel large wave flume experiment was conducted on a fixed, barred beach with a sediment pit on the sandbar, allowing for the isolation of small-scale bed response to large-scale forcing. Concurrent measurements of instantaneous sheet layer sediment concentration profiles and near-bed velocity profiles were obtained on a sandbar for the first time. Two sediment distributions were used with median grain diameters, d50, of 0.17 mm and 0.27 mm. Sheet flow occurred primarily under wave crests, where sheet thickness increased with increasing wave height. A proportionality constant, Λ, was used to relate maximum Shields parameter to maximum sheet thickness (normalized by d50), with bed shear stress computed using the quadratic drag law. An enhanced sheet layer thickness was apparent for the smaller sediment experiments (Λ = 18.7), when directly compared to closed-conduit oscillatory flow tunnel data (Λ = 10.6). However, Λ varied significantly (5 
      PubDate: 2017-05-20T11:15:25.389222-05:
      DOI: 10.1002/2016JC012438
       
  • Modification of sea surface temperature by chlorophyll concentration in
           the Atlantic upwelling systems
    • Authors: O. Hernandez; J. Jouanno, V. Echevin, O. Aumont
      Abstract: The influence of the chlorophyll on the upper Tropical Atlantic ocean is investigated with long term (1979-2012) regional oceanic simulations with 1/4° horizontal resolution based on the NEMO3.6 model. The model solar radiation penetration scheme depends on the chlorophyll concentration. Simulations with time and spatially varying concentrations obtained from satellite ocean color observations are compared with a simulation forced with constant chlorophyll concentration of 0.05 mg m−3, representative of chlorophyll depleted waters. Results indicate that regions of the Tropical Atlantic with chlorophyll concentrations larger than in the reference simulation (i.e. [chl] > 0.05 mg m−3) get warmer at the surface, with the exception of the main upwelling regions where high chlorophyll concentrations are associated with a significant cooling of the sea surface (∼1°C in the Benguela upwelling). The analysis of the model heat balance shows that the biological differential heating causes negative temperature anomalies in subsurface source waters prior to their upwelling at the coast. The shallow mixed-layer in the eastern equatorial and tropical Atlantic favors the persistence of these subsurface anomalies and may explain why the Benguela is particularly sensitive to the biological differential heating. In spite of the presence of high chlorophyll concentrations in the upwelling regions, both the larger amount of shortwave radiation captured in the surface layers and the modifications of the horizontal and vertical advection at the coast are found to play a secondary role in the SST change in the upwelling region.
      PubDate: 2017-05-17T03:26:22.749651-05:
      DOI: 10.1002/2016JC012330
       
  • Role for Atlantic inflows and sea ice loss on shifting phytoplankton
           blooms in the Barents Sea
    • Authors: L. Oziel; G. Neukermans, M. Ardyna, C. Lancelot, J-L. Tison, P. Wassmann, J. Sirven, D. Ruiz-Pino, J. C. Gascard
      Abstract: Phytoplankton blooms in the Barents Sea are highly sensitive to seasonal and interannual changes in sea ice extent, water mass distribution, and oceanic fronts. With the ongoing increase of Atlantic Water inflows, we expect an impact on these blooms. Here, we use a state-of-the-art collection of in situ hydro-geochemical data for the period 1998-2014, which includes ocean color satellite-derived proxies for the biomass of calcifying and non-calcifying phytoplankton. Over the last 17 years, sea ice extent anomalies were evidenced having direct consequences for the spatial extent of spring blooms in the Barents Sea. In years of minimal sea ice extent, two spatially distinct blooms were clearly observed: one along the ice edge and another in ice-free water. These blooms are thought to be triggered by different stratification mechanisms: heating of the surface layers in ice-free waters and melting of the sea ice along the ice edge. In years of maximal sea ice extent, no such spatial delimitation was observed. The spring bloom generally ended in June when nutrients in the surface layer were depleted. This was followed by a stratified and oligotrophic summer period. A coccolithophore bloom generally developed in August, but was confined only to Atlantic Waters. In these same waters, a late summer bloom of non-calcifying algae was observed in September, triggered by enhanced mixing, which replenishes surface waters with nutrients. Altogether, the 17-year time-series revealed a northward and eastward shift of the spring and summer phytoplankton blooms.
      PubDate: 2017-05-16T05:21:01.331179-05:
      DOI: 10.1002/2016JC012582
       
  • Morphological characterization of coral reefs by combining LiDAR and MBES
           data: A case study from Yuanzhi Island, South China Sea
    • Authors: Kai Zhang; Fanlin Yang, Hande Zhang, Dianpeng Su, QianQian Li
      Abstract: The correlation between seafloor morphological features and biological complexity has been identified in numerous recent studies. This research focused on the potential for accurate characterization of coral reefs based on high-resolution bathymetry from multiple sources. A standard deviation (STD) based method for quantitatively characterizing terrain complexity was developed that includes robust estimation to correct for irregular bathymetry and a calibration for the depth-dependent variablity of measurement noise. Airborne LiDAR and shipborne sonar bathymetry measurements from Yuanzhi Island, South China Sea, were merged to generate seamless high-resolution coverage of coral bathymetry from the shoreline to deep water. The new algorithm was applied to the Yuanzhi Island surveys to generate maps of quantitive terrain complexity, which were then compared to in situ video observations of coral abundance. The terrain complexity parameter is significantly correlated with seafloor coral abundance, demonstrating the potential for accurately and efficiently mapping coral abundance through seafloor surveys, including combinations of surveys using different sensors.
      PubDate: 2017-05-16T05:20:53.81525-05:0
      DOI: 10.1002/2016JC012507
       
  • The sea-level budget along the Northwest Atlantic coast: GIA, mass changes
           and large-scale ocean dynamics
    • Authors: Thomas Frederikse; Karen Simon, Caroline A. Katsman, Riccardo Riva
      Abstract: Sea-level rise and decadal variability along the northwestern coast of the North Atlantic Ocean are studied in a self-consistent framework that takes into account the effects of solid-earth deformation and geoid changes due to large-scale mass redistribution processes. Observations of sea and land level changes from tide gauges and GPS are compared to the cumulative effect of GIA, present-day mass redistribution, and ocean dynamics over a 50-year period (1965-2014). GIA explains the majority of the observed sea-level and land motion trends, as well as almost all inter-station variability. Present-day mass redistribution resulting from ice melt and land hydrology causes both land uplift and sea-level rise in the region.We find a strong correlation between decadal steric variability in the Subpolar Gyre and coastal sea level, which is likely caused by variability in the Labrador Sea that is propagated southward. The steric signal explains the majority of the observed decadal sea-level variability and shows an upward trend and a significant acceleration, which are also found along the coast.The sum of all contributors explains the observed trends in both sea level rise and vertical land motion in the region, as well as the decadal variability. The sum of contributors also explains the observed acceleration within confidence intervals. The sea-level acceleration coincides with an accelerating density decrease at high latitudes.
      PubDate: 2017-05-16T05:18:04.544712-05:
      DOI: 10.1002/2017JC012699
       
  • Modeling modified Circumpolar Deep Water intrusions onto the Prydz Bay
           continental shelf, East Antarctica
    • Authors: Chengyan Liu; Zhaomin Wang, Chen Cheng, Ruibin Xia, Bingrui Li, Zelin Xie
      Abstract: An eddy-resolving coupled regional ocean-sea ice-ice shelf model is employed to locate the hot spots where modified Circumpolar Deep Water (mCDW) intrudes onto the continental shelf within Prydz Bay, and locate the paths through which mCDW is transported to the Amery Ice Shelf (AIS) calving front. Evaluation of the model output is with satellite, hydrographic and borehole data. Two critical windows responsible for mCDW intrusions are identified. The first is the eastern branch of the cyclonic Prydz Bay gyre (PBG) that carries mCDW to the ice front line, accounting for an annual mean heat transport of ∼8.7 ×1011 J s−1. The second is located to the east of the Four Ladies Bank (FLB) where mCDW is channeled through submarine troughs, accounting for an annual mean heat transport of ∼16.2 ×1011 J s−1. The eddy-induced heat transport accounts for ∼23% in the path of the PBG and ∼52% in the path of the eastern coastal current, with respect to their total onshore heat transport. The seasonal pulsing of mCDW intrusions is greatly dependent on the seasonal cycle of the Antarctic Slope Current (ASC) that peaks with a maximum of ∼29.3 Sv at 75°E in June. In austral winter, mCDW is allowed to access the eastern flank of the AIS calving front with potential consequences for the basal mass balance of the AIS. The dynamic effects of small-scale troughs on the longshore ASC play an important role in the onshore mCDW transport.
      PubDate: 2017-05-15T20:45:38.28924-05:0
      DOI: 10.1002/2016JC012336
       
  • Hydraulics and mixing in a laterally divergent channel of a highly
           stratified estuary
    • Authors: W. Rockwell Geyer; D.K. Ralston, R.C. Holleman
      Abstract: Estuarine mixing is often intensified in regions where topographic forcing leads to hydraulic transitions. Observations in the salt-wedge estuary of the Connecticut River indicate that intense mixing occurs during the ebb tide in regions of supercritical flow that is accelerated by lateral expansion of the channel. The zones of mixing are readily identifiable based on echo-sounding images of large-amplitude shear instabilities. The gradient Richardson number (Ri) averaged across the mixing layer decreases to a value very close to 0.25 during most of the active mixing phase. The along-estuary variation in internal Froude number and interface elevation are roughly consistent with a steady, inviscid, two-layer hydraulic representation, and the fit is improved when a parameterization for interfacial stress is included. The analysis indicates that the mixing results from lateral straining of the shear layer, and that the rapid development of instabilities maintains the overall flow near the mixing threshold value of Ri=0.25, even with continuous, active mixing. The entrainment coefficient can be estimated from salt conservation within the interfacial layer, based on the finding that the mixing maintains Ri=0.25. This approach leads to a scaling estimate for the interfacial mixing coefficient based on the lateral spreading rate and the aspect ratio of the flow, yielding estimates of turbulent dissipation within the pycnocline that are consistent with estimates based on turbulence-resolving measurements.
      PubDate: 2017-05-15T20:41:01.837936-05:
      DOI: 10.1002/2016JC012455
       
  • The formation processes of phytoplankton growth and decline in mesoscale
           eddies in the western North Pacific Ocean
    • Authors: Yu-Lin Chang; Yasumasa Miyazawa, Lie-Yauw Oey, Tsubasa Kodaira, Shihming Huang
      Abstract: In this study, we investigate the processes of phytoplankton growth and decline in mesoscale eddies in the western North Pacific Ocean based on the in-situ chlorophyll data obtained from 52 cruises conducted by the Japan Meteorological Agency together with idealized numerical simulations. Both the observation and model results suggest that chlorophyll/phytoplankton concentrations are higher in cold than in warm eddies in near-surface water (z > –70 m). In the idealized simulation, the isopycnal movements associated with upwelling/downwelling transport biomass to different vertical depths during eddy formation (stage A). Biomass in cold eddies is transported toward shallower waters while those in warm eddies move toward deeper waters. In the period after the eddy has formed (stage B), sunlight and initially upwelled nutrients together promote the growth of phytoplankton in cold eddies. Phytoplankton in warm eddies decays due to insufficient sunlight in deeper waters. In stage B, upwelling and downwelling coexist in both warm and cold eddies, contributing nearly equally to vertical displacement. The upwelling/downwelling-induced nitrate flux accounts for a small percentage (∼3%) of the total nitrate flux in stage B. The vertical velocity caused by propagating eddies, therefore, is not the primary factor causing differences in phytoplankton concentrations between stage-B warm and cold eddies.
      PubDate: 2017-05-15T20:40:38.214171-05:
      DOI: 10.1002/2017JC012722
       
  • Storm-induced changes in pCO2 at the sea surface over the northern South
           China Sea during Typhoon Wutip
    • Authors: Haijun Ye; Jinyu Sheng, Danling Tang, Eko Siswanto, Muhsan Ali Kalhoro, Yi Sui
      Abstract: In-situ oceanographic measurements were made before and after the passage of Typhoon Wutip in September 2013 over the northern South China Sea. The surface geostrophic circulation over this region inferred from satellite altimetry data features a large-size anti-cyclonic eddy, a small-size cyclonic eddy, and smaller-size eddies during this period. Significant typhoon-induced changes occurred in the partial pressure of CO2 at the sea surface (pCO2sea) during Wutip. Before the passage of Wutip, pCO2sea was about 392.92±1.83, 390.31±0.50, and 393.04±4.31 μatm over the cyclonic eddy water, the anti-cyclonic eddy water, and areas outside two eddies, respectively. The entire study region showed a carbon source (1.31±0.46 mmol CO2 m−2 d−1) before Wutip. In the cyclonic eddy water after Wutip, high sea surface salinity (SSS), low sea surface temperature (SST), and high pCO2sea (413.05±7.56 μatm) made this area to be a carbon source (3.30±0.75 mmol CO2 m−2 d−1). In the anti-cyclonic eddy water after Wutip, both the SSS and SST were lower, pCO2sea was also lower (383.03±3.72 μatm), and this area became a carbon sink (-0.11±0.55 mmol CO2 m−2 d−1), in comparison with the pre-typhoon conditions. The typhoon-induced air-sea CO2 flux reached about 0.03 mmol CO2 m−2 d−1. Noticeable spatial variations in pCO2sea were affected mainly by the typhoon-induced mixing/upwelling and vertical stratifications. This study suggests that the local air-sea CO2 flux in the study region was affected significantly by oceanographic conditions during the typhoon.
      PubDate: 2017-05-12T10:40:50.280544-05:
      DOI: 10.1002/2016JC012643
       
  • Small-scale sea ice deformation during N-ICE2015: From compact pack ice to
           marginal ice zone
    • Authors: Annu Oikkonen; Jari Haapala, Mikko Lensu, Juha Karvonen, Polona Itkin
      Abstract: We studied small scale (50 m to 5 km) sea ice deformation from ship radar images recorded during the N-ICE2015 campaign. The campaign consisted of 4 consecutive drifting ice stations (Floes 1 to 4) north of Svalbard, with a total duration of nearly 5 months. Deformation was calculated using 5 different time intervals from 10 min to 24 h, and the deformation rate was found to depend strongly on the time scale. Floes 1 to 3 had a mean deformation rate within the range of 0.06 to 0.07 h– 1 with the interval of 10 min, and 0.03 to 0.04 h– 1 with the interval of 1 h. Floe 4 represented marginal ice zone (MIZ) with very high deformation rate, 0.14/0.08 h– 1 with the interval of 10 min/1 h. Deep in the ice pack, high deformation rates occurred only with high wind and drift speed, while in MIZ they were found also during calm conditions. The deformation rates were found to follow power law scaling with respect to length and time scale even on this small scales and in small domain (15 km × 15 km). The length scale dependence of deformation rate depends on the time scale: the power law scaling exponent β of the whole study period decreases from 0.82 to 0.52 with the time interval increasing from 10 min to 24 h. Ship radar images reveal the importance of the deformation history of the ice pack, since the deformation events were initialized along the lines of previous damages.
      PubDate: 2017-05-09T18:45:26.413209-05:
      DOI: 10.1002/2016JC012387
       
  • Short-term variability of aragonite saturation state in the central
           Mid-Atlantic Bight
    • Authors: Yuan-Yuan Xu; Wei-Jun Cai, Yonghui Gao, Rik Wanninkhof, Joseph Salisbury, Baoshan Chen, Janet J. Reimer, Stephen Gonski, Najid Hussain
      Abstract: The uptake of anthropogenic carbon dioxide (CO2) from the atmosphere has resulted in a decrease in seawater aragonite saturation state (Ωarag), which affects the health of carbonate-bearing organisms and the marine ecosystem. A substantial short-term variability of surface water Ωarag, with an increase of up to 0.32, was observed in the central Mid-Atlantic Bight off the Delaware and the Chesapeake Bays over a short period of 10 days in summer 2015. High-frequency underway measurements for temperature, salinity, percentage saturation of dissolved oxygen, oxygen to argon ratio, pH, fCO2, and measurements based on discrete samples for pH, dissolved inorganic carbon and total alkalinity are used to investigate how physical and biogeochemical processes contribute to the changes of Ωarag. Quantitative analyses show that physical advection and mixing processes are the dominant forces for higher Ωarag in slope waters while biological carbon removal and CO2 degassing contribute to increased Ωarag in shelf waters.
      PubDate: 2017-05-09T18:35:36.704397-05:
      DOI: 10.1002/2017JC012901
       
  • Evaluation of anthropogenic influences on the Luhuitou fringing reef via
           spatial and temporal analyses (from Isotopic Values)
    • Authors: D. Cao; W. Cao, K. Yu, G. Wu, J. Yang, X. Su, F. Wang
      Abstract: Coral reefs have suffered remarkable declines worldwide. Nutrient over-enrichment is considered to be one of the primary local causes. The Luhuitou fringing reef in southern China is a well-known tourist destination that is subject to enormous coastal renovation. The mean δ13C, δ15N value and carbon over nitrogen ratio (C/N) of particulate organic matter were - 21.56 ± 1.94 ‰, 7.04 ± 3.81 ‰ and 5.81 ± 1.86, respectively, suggesting mixed sources of carbon and nitrogen. The IsoError calculations suggested that marine phytoplankton and marine benthic algae dominated the majority of carbon sources, while anthropogenic and terrestrial organic nitrogen dominated the nitrogen sources. A tendency towards greater terrestrial detritus and anthropogenic-derived discharges was found during dry seasons, and greater marine-derived organic matter during wet seasons. These results demonstrated the existence of anthropogenic influences and high dissolved inorganic nitrogen concentrations and C/N ratios. Anthropogenic nutrient discharge moderated nitrogen limitation, whereas phosphorus became more important to the reef ecosystem. Despite the marine carbon sources dominated, freshwater and terrestrial derived organic carbon sources were also very important. Meanwhile, anthropogenic and terrestrial organic nitrogen sources were dominant. Therefore, pollution from more extensive region and anthropogenic activities from riverine sewage discharges adjacent to reefs should be focused to effectively reduce human-derived nutrients on reefs.
      PubDate: 2017-05-09T18:35:29.699504-05:
      DOI: 10.1002/2017JC012871
       
  • Bay of Bengal salinity stratification and Indian summer monsoon
           intraseasonal oscillation: 1. Intraseasonal variability and causes
    • Authors: Yuanlong Li; Weiqing Han, M. Ravichandran, Wanqiu Wang, Toshiaki Shinoda, Tong Lee
      Abstract: The huge freshwater flux of the Indian summer monsoon (ISM; May-October) gives rise to strong salinity stratification in the Bay of Bengal (BoB), causing a shallow mixed layer and a thick barrier layer, which potentially affects intraseasonal oscillations of the monsoon (MISOs). In this study intraseasonal variability of the mixed layer depth (MLD) and barrier layer thickness (BLT) is investigated using in-situ observations from Argo floats and moored buoys and an ocean general circulation model (OGCM). The average MLD in the BoB is typically 20-30 m during the ISM, while the BLT increases from ∼10 m in May-June to 20-40 m in September-October. MISOs induce in-phase variations in MLD and isothermal layer depth (ILD), both of which are deepened by 8-15 m during MISO active phase, while the change of BLT is small and within the error range of Argo data sampling. In the northern (southern) bay, BLT increases by ∼5 m (2 m) during MISOs owing to a larger deepening of ILD than MLD. OGCM experiments are performed to understand the underlying mechanism. In the BoB intraseasonal variations of MLD, ILD and BLT arise largely from ocean internal instability, whereas those induced by MISOs are weaker. The in-phase variations of MLD and ILD during MISOs are induced by different processes. The MLD deepening is primarily caused by wind stress forcing, while the ILD deepening is driven by surface heat fluxes via surface cooling. The limited variability of BLT is due to the offsetting of different forcing processes.
      PubDate: 2017-05-04T17:49:41.530302-05:
      DOI: 10.1002/2017JC012691
       
  • Tropical cyclone sensitivity to ocean coupling in the ECMWF coupled model
    • Authors: Kristian S. Mogensen; Linus Magnusson, Jean-Raymond Bidlot
      Abstract: We present an investigation of the performance of the ECMWF coupled atmosphere-waves-ocean model for different ocean and atmosphere resolutions on a series of tropical cyclones in the Western Pacific with the aim to better understand the coupled feedback mechanisms in these exterme conditions.For some of the test cases, we only find little impact of coupling the atmosphere to the ocean, while in others, we observe a very large impact. To further understand these differences, we have selected two tropical cyclones (TCs) as case studies: TC Haiyan (with small impact of coupling) and TC Neoguri (with large impact of coupling). The comparison between these two cases suggests that the upper ocean stratification is the key in determining the strength of the coupled feedback. A strong coupled feedback is found whenever the ocean heat content of the upper layer is low while a very weak coupled feedback is found whenever the ocean has a thick warm mixed layer.The oceanographic response to tropical cyclones for the two storms has been compared to sea surface temperature and derived surface currents from drifting buoys and to subsurface observations from Argo and ship launched XBT's. These comparisons show that we are able to realistically reproduce the atmospheric and oceanographic interaction during tropical cyclone conditions which gives us confidence that the coupled modelling system is physically sound.
      PubDate: 2017-05-04T17:49:09.323699-05:
      DOI: 10.1002/2017JC012753
       
  • A submesoscale coherent vortex in the Ligurian Sea: From dynamical
           barriers to biological implications
    • Authors: Anthony Bosse; Pierre Testor, Nicolas Mayot, Louis Prieur, Fabrizio D'Ortenzio, Laurent Mortier, Hervé Le Goff, Claire Gourcuff, Laurent Coppola, Héloïse Lavigne, Patrick Raimbault
      Abstract: In June 2013, a glider equipped with oxygen and fluorescence sensors has been used to extensively sample an anticyclonic Submesoscale Coherent Vortex (SCV) in the Ligurian Sea (NW Mediterranean Sea). Those measurements are complemented by full-depth CTD casts (T, S, Oxygen) and water samples documenting nutrients and phytoplankton pigments within the SCV and outside. The SCV has a very homogeneous core of oxygenated waters between 300 m and 1200 m formed 4.5 months earlier during the winter deep convection event. It has a strong dynamical signature with peak velocities at 700 m depth of 13.9 cm s-1 in cyclogeostrophic balance. The eddy has a small radius of 6.2 km corresponding to high Rossby number of −0.45. The vorticity at the eddy center reaches −0.8f. Cross-stream isopycnic diffusion of tracers between the eddy core and the surroundings is found to be very limited due to dynamical barriers set by the SCV associated with a diffusivity coefficient of about 0.2 m2 s-1. The deep core is nutrients-depleted with concentrations of nitrate, phosphate and silicate 13–18% lower than the rich surrounding waters. However, the nutriclines are shifted of about 20–50 m toward the surface thus increasing the nutrients availability for phytoplankton. Chlorophyll-a concentrations at the deep chlorophyll maximum are subsequently about twice bigger as compared to outside. Pigments further reveal the predominance of nanophytoplankton inside the eddy and an enhancement of the primary productivity. This study demonstrate the important impact of post-convective SCVs on nutrients distribution and phytoplankton community, as well as on the subsequent primary production and carbon sequestration.
      PubDate: 2017-05-04T17:47:09.545253-05:
      DOI: 10.1002/2016JC012634
       
  • Impact of typhoons on the Changjiang plume extension in the Yellow and
           East China Seas
    • Authors: Joon Ho Lee; Il-Ju Moon, Jae-Hong Moon, Sung-Hun Kim, Yeong Yun Jeong, Jun-Ho Koo
      Abstract: It is well known that river discharges, winds, ocean currents, and tides are major dynamical factors that determine the distribution and extension of the Changjiang plume (CP) in the Yellow and East China Seas (YECS). Using observations and numerical experiments, this study demonstrates that, in addition to these factors, typhoons in the YECS also play a crucial role in the extension of the CP during the summer season. The hydrographic data observed at the Ieodo Ocean Research Station (IORS) and by a research vessel during the period of Typhoons Ewiniar (0603) and Dianmu (1004) showed that the typhoon-induced strong vertical mixing modified spatial distribution of the CP significantly, resulting in the delay of the CP's extension by as much as up to 20 days. A series of numerical experiments for Typhoon Dianmu also showed that the typhoon plays a blocking role for the extension of CP for up to 17 days through the vertical mixing process and the change of background winds. In particular, it is found that the delay due to Dianmu in 2010 contributed to the avoidance of potential mass mortality of marine life by preventing the low-salinity water from spreading to the aquaculture regions near Jeju Island.
      PubDate: 2017-05-04T17:45:48.608816-05:
      DOI: 10.1002/2017JC012754
       
  • Importance of Ekman transport and gyre circulation change on seasonal
           variation of surface dissolved iron in the western subarctic North Pacific
           
    • Authors: Takuya Nakanowatari; Tomohiro Nakamura, Keisuke Uchimoto, Jun Nishioka, Humio Mitsudera, Masaaki Wakatsuchi
      Abstract: Iron (Fe) is an essential nutrient for marine phytoplankton and it constitutes an important element in the marine carbon cycle in the ocean. This study examined the mechanisms controlling seasonal variation of dissolved Fe (dFe) in the western subarctic North Pacific (WSNP), using an ocean general circulation model coupled with a simple biogeochemical model incorporating a dFe cycle fed by two major sources (atmospheric dust and continental shelf sediment). The model reproduced the seasonal cycle of observed concentrations of dFe and macro nutrients at the surface in the Oyashio region with maxima in winter (February–March) and minima in summer (July–September), although the simulated seasonal amplitudes are a half of the observed values. Analysis of the mixed-layer dFe budget indicated that both local vertical entrainment and lateral advection are primary contributors to the wintertime increase in dFe concentration. In early winter, strengthened northwesterly winds excite southward Ekman transport and Ekman upwelling over the western subarctic gyre, transporting dFe-rich water southward. In mid- to late winter, the southward western boundary current of the subarctic gyre and the outflow from the Sea of Okhotsk also bring dFe-rich water to the Oyashio region. The contribution of atmospheric dust to the dFe budget is several times smaller than these ocean transport processes in winter. These results suggest that the westerly wind-induced Ekman transport and gyre circulation systematically influence the seasonal cycle of WSNP surface dFe concentration.
      PubDate: 2017-05-03T05:50:50.719547-05:
      DOI: 10.1002/2016JC012354
       
  • Altered inherent optical properties and estimates of the underwater light
           field during an Arctic under ice bloom of Phaeocystis pouchetii
    • Authors: Alexey K. Pavlov; Torbjørn Taskjelle, Hanna M. Kauko, Børge Hamre, Stephen R. Hudson, Philipp Assmy, Pedro Duarte, Mar Fernández-Méndez, C.J. Mundy, Mats A. Granskog
      Abstract: In spring 2015, we observed an extensive phytoplankton bloom of Phaeocystis pouchetii, with chlorophyll a concentrations up to 7.5 mg m−3, under compact snow-covered Arctic sea ice at 80-81˚N during the Norwegian young sea ICE (N-ICE2015) expedition. We investigated the influence of the under-ice bloom on inherent optical properties (IOPs) of the upper ocean. Absorption and scattering in the upper 20 m of the water column at visible wavebands increased threefold and tenfold, respectively, relative to pre-bloom conditions. The scattering-to-absorption ratio during the Phaeocystis under-ice bloom was higher than in previous Arctic studies investigating diatom blooms. During the bloom, absorption by colored dissolved organic matter (at 375 nm), seemingly of autochthonous origin, doubled. Total absorption by particles (at 440 nm), dominated by phytoplankton (> 90%), increased tenfold. Measured absorption and scattering in the water were used as inputs for a 1D coupled atmosphere-ice-ocean radiative transfer model (AccuRT) to investigate effects of altered IOPs on the under-ice light field. Multiple scattering between sea ice and phytoplankton in the ocean led to an increase in scalar irradiance in the photosynthetically active radiation range (Eo(PAR)) at the ice-ocean interface by 6–7% compared to pre-bloom situation. This increase could have a positive feedback on ice-algal and under-ice phytoplankton productivity. The ratio between Eo(PAR) and downwelling planar irradiance (Ed(PAR)) below sea ice reached 1.85. Therefore, the use of Ed(PAR) might significantly underestimate the amount of PAR available for photosynthesis underneath sea ice. Our findings could help to improve light parameterizations in primary production models.
      PubDate: 2017-04-27T18:30:54.434852-05:
      DOI: 10.1002/2016JC012471
       
  • Amundsen and Bellingshausen Seas simulation with optimized ocean, sea ice,
           and thermodynamic ice shelf model parameters
    • Authors: Y. Nakayama; D. Menemenlis, M. Schodlok, E. Rignot
      Abstract: Recent studies suggest that the thickness of Winter Water (WW), that is, water with potential temperature below ∼-1°C located below Antarctic Surface Water and above Circumpolar Deep Water (CDW) is critical in determining the ice shelf melt rate, especially for the Pine Island Glacier (PIG). Existing model studies, however, misrepresent WW thickness and properties in the Amundsen Sea (AS). Here, we adjust a small number of model parameters in a regional Amundsen and Bellingshausen Seas configuration of the Massachusetts Institute of Technology general circulation model in order to reproduce properties and thickness of WW and CDW close to observations, with significant improvement for WW compared to previous studies. The cost, which is defined as weighted model-data difference squared, is reduced by 23%. Although a previous modeling study points out that the local surface heat loss upstream from Pine Island Polynya could be the reason for the observed 2012 PIG melt decline and WW thickening, they did not show WW freshening, which was observed at the same time. Model sensitivity experiments for surface heat loss, PIG melt rate, and precipitation fail to replicate WW freshening concurrent with PIG melt decline, implying that these processes can not fully explain the observed PIG melt decrease.
      PubDate: 2017-04-25T10:50:46.06782-05:0
      DOI: 10.1002/2016JC012538
       
  • On the spatial coherence of the Atlantic Water inflow across the Nordic
           Seas
    • Authors: Christophe Herbaut; Marie-Noëlle Houssais, Sally Close, Anne-Cécile Blaizot
      Abstract: The co-variability of the Atlantic Water (AW) branches in the Nordic Seas is investigated over the period 1979-2012 using an eddy permitting model. A noticeable circulation change is found in the mid-1990s. Prior to the mid-1990s, the leading mode of variability defines a large scale pattern, with concomitant variations in the Atlantic Water (AW) inflow in the Faroe-Shetland Channel (FSC), the Norwegian Atlantic Slope Current (NwASC), the AW inflow to the Barents Sea and the West Spitsbergen Current (WSC). After the mid-1990s, the co-variability between the NwASC and the AW inflow in both the FSC and the WSC is lost. Consequently, the northern Barents Sea circulation anomaly pattern, which is present throughout the full period, becomes the leading mode of circulation in the northern Nordic Seas after the mid-1990s. The circulation change of the mid-1990s appears to be linked to a weakening of the southwesterly wind anomalies in Norwegian Sea, as the northern center of action of the first mode of sea level pressure (NAO) weakens. Passive tracer experiments suggest that this circulation change may be accompanied by increased heat transfer from the AW current to the interior Nordic Seas. This in turn may have limited the influence of the recently observed AW warming in the Iceland-Scotland Passage on the NwASC downstream.
      PubDate: 2017-04-25T10:45:41.849468-05:
      DOI: 10.1002/2016JC012566
       
  • Isopycnal eddy mixing across the Kuroshio extension: Stable versus
           unstable states in an eddying model
    • Authors: Ru Chen; Sarah T. Gille, Julie L. McClean
      Abstract: The Kuroshio Extension (KE) jet transitions between stable and unstable states on interannual time scales. Cross-jet eddy mixing in the two states is contrasted in the KE region (28° – 40°N, 125 – 165°E), using a global eddying 0.1° configuration of the Parallel Ocean Program with online numerical particles. The four-year period chosen (June 1994 to May 1998) covers a full cycle of the stable state, unstable state and the transition period. Large values of cross-jet eddy diffusivities within the KE jet are concentrated in the upper 1000m. In the upper ocean, elevated cross-jet mixing within the KE jet is mainly concentrated in the downstream part of the KE jet, where the jet is weak but eddy activity is strong. The simulated time-mean KE jet is more intense and extends further east in the stable state than in the unstable state. Consequently, strong cross-jet mixing within the KE jet is located west of 150oE during June 1996-May 1997 (a typical unstable state), but east of 150oE during June 1995-May 1996 (a typical stable state). However, average mixing within the KE jet is indistinguishable in the typical stable and unstable states. In the deep ocean, mixing is strongly influenced by topography, and thus their horizontal structures have less inter-annual variability than in the upper ocean. One caveat is that results here cover one representative cycle of the two states. To obtain the climate mean mixing structures for the stable or unstable state, one would need numerical output covering a period much longer than four years.
      PubDate: 2017-04-24T10:21:20.133761-05:
      DOI: 10.1002/2016JC012164
       
  • Biases of five latent heat flux products and their impacts on mixed-layer
           temperature estimates in the South China Sea
    • Authors: Xin Wang; Rong-Wang Zhang, Jian Huang, Li-Li Zeng, Fei Huang
      Abstract: Five latent heat flux (LHF) products are evaluated based on in situ observations in the South China Sea (SCS), including the ECWMF ERA-Interim (ERA-I), the NCEP2, the Objectively Analyzed air-sea Fluxes (OAFlux), the Japanese 55-year Reanalysis (JRA55), and the TropFlux datasets. The results show that there are good correlations between the LHF products and observations, ranging from 0.68 to 0.74. However, mean biases of -8 to 40 W m−2 exist in the LHF products with respect to the observations. For root-mean-square errors, the OAFlux dataset is the closest to the observations, followed by ERA-I and TropFlux, while the NCEP2 dataset shows significant overestimation. It is found that the biases in the near-surface specific humidity are most correlated with the biases in the LHF products, followed by the biases in the near-surface wind speed, air temperature and sea surface temperature. The biases in the LHF products have a prominent seasonal variation that is 25 W m−2 higher in boreal winter than in summer. Using the thermal equation, it is shown that the tendency errors of the mixed-layer temperature estimated by the biases in the LHF products vary from -2.0 to 3.5°C/month in the SCS. When all of the products are averaged, the errors are reduced to a range of -0.7 to 1.5°C/month. It is noteworthy that the errors in summer are more obvious than those in winter, since a thinner mixed layer in the summer can amplify the effect of even a small bias in the LHF.
      PubDate: 2017-04-24T10:21:13.962667-05:
      DOI: 10.1002/2016JC012332
       
  • Coastal-trapped behavior of the diurnal internal tide at O ‘ahu,
           Hawai‘i
    • Authors: Katharine A. Smith; Mark A. Merrifield, Glenn S. Carter
      Abstract: The influence of rotation on the structure and propagation of internal tides around O‘ahu, Hawai‘i is investigated using in situ observations and a tidally forced, primitive equation model with realistic bathymetry and stratification. Particular attention is given to the diurnal internal tide, which largely has been de-emphasized in previous studies of the region because of the dominance of the semidiurnal internal tide but has been determined by recent studies to be a significant contributor to baroclinic variability. Though both diurnal and semidiurnal internal tides are generated primarily over Ka‘ena Ridge to the northwest of the island, the diurnal internal tide propagates clockwise around the island as an imperfectly trapped wave, while the semidiurnal internal tide propagates away from the ridge, unaffected by rotation. The diurnal and semidiurnal internal tides fall into the superinertial frequency range; however, the diurnal frequency apparently is sufficiently close to inertial (∼ 1.4f) for rotation to affect internal tide propagation. The in situ observations support the model finding that diurnal trapping provides the primary source of baroclinic variability along the eastern coast of the island, a stretch of coastline otherwise sheltered from the internal tide energy generated over the Hawaiian Ridge. The findings in Hawai‘i suggest that coastal trapping of superinertial internal tides may be a significant source of variability and mixing in other nearshore systems around the world.
      PubDate: 2017-04-24T10:21:12.698717-05:
      DOI: 10.1002/2016JC012436
       
  • Stable isotopes in surface waters of the Atlantic Ocean: Indicators of
           ocean-atmosphere water fluxes and oceanic mixing processes
    • Authors: M. Benetti; G. Reverdin, G. Aloisi, Sveinbjörnsdóttir Árny Erla
      Abstract: The surface ocean hydrological cycle is explored based on ∼300 new δ18O and δD measurements from surface waters of the Atlantic Ocean and the Mediterranean Sea over the period 2010-2016. Our approach combines these surface observations with salinity (S) and stable isotope measurements of atmospheric water vapor. The distinct regional S-δ distributions are used to identify different surface water masses and their horizontal advection. Moreover, based on assumptions on the δ-S characteristics of seawater sources and the isotope composition of the evaporative (δe) and meteoric water (δMW) fluxes, the δ-S distribution is used to indicate the relative importance of evaporation (E) and meteoric water inputs (MW). Here, δe is estimated from the Craig and Gordon's equation using 120 days of measurements of the ambient air above the Atlantic Ocean collected during three cruises. To provide quantitative estimates of the E:MW ratio, we use the box model from Craig and Gordon (1965). This identifies the subtropical gyre as a region where E:MW ∼2 and the tropical ocean as a region were MW:E ∼2. Finally, we show that the δ18O-δD distribution is better represented by a linear fit than the δ-S relationship, even in basins governed by different hydrological processes. We interpret the δ18O-δD distribution considering the kinetic fractionation processes associated with evaporation. In the tropical region where MW exceeds E, the δ18O-δD distribution identifies the MW inputs from their kinetic signature, whereas in regions where E exceeds MW, the δ18O-δD distribution traces the humidity at the sea surface.
      PubDate: 2017-04-24T10:20:32.60819-05:0
      DOI: 10.1002/2017JC012712
       
  • Short-crested waves in the surf zone
    • Authors: Zhangping Wei; Robert A. Dalrymple, Munan Xu, Roland Garnier, Morteza Derakhti
      Abstract: This study investigates short-crested waves in the surf zone by using the mesh-free Smoothed Particle Hydrodynamics model, GPUSPH. The short-crested waves are created by generating intersecting wave trains in a numerical wave basin with a beach. We first validate the numerical model for short-crested waves by comparison with large-scale laboratory measurements. Then short-crested wave breaking over a planar beach is studied comprehensively. We observe rip currents as discussed in Dalrymple [1975] and undertow created by synchronous intersecting waves. The wave breaking of the short-crested wave field created by the nonlinear superposition of intersecting waves and wave-current interaction result in the formation of isolated breakers at the ends of breaking wave crests. Wave amplitude diffraction at these isolated breakers gives rise to an increase in the alongshore wave number in the inner surf zone. Moreover 3D vortices and multiple circulation cells with a rotation frequency much lower than the incident wave frequency are observed across the outer surf zone to the beach. Finally, we investigate vertical vorticity generation under short-crested wave breaking and find that breaking of short-crested waves generates vorticity as pointed out by Peregrine [1998]. Vorticity generation is not only observed under short-crested waves with a limited number of wave components, but also under a directional wave spectra.
      PubDate: 2017-04-13T15:10:54.943308-05:
      DOI: 10.1002/2016JC012485
       
  • Lagrangian and Eulerian characterization of two counterrotating
           submesoscale eddies in a western boundary current
    • Authors: Alessandra Mantovanelli; Shane Keating, Lucy R. Wyatt, Moninya Roughan, Amandine Schaeffer
      Abstract: In recent decades, high-spatial resolution ocean radar and satellite imagery measurements have revealed a complex tangle of submesoscale filaments and eddies, in the surface velocity, temperature and chlorophyll-a fields. We use a suite of high resolution data to characterize two counter-rotating, short-lived eddies formed at the front between the warm East Australian Current (EAC) and temperate coastal waters (30°S, Eastern Australia). In this region, submesoscale filaments and short-lived eddies are dynamically generated and decay at time scales of hours to days. Dominant cyclonic filaments of O(1) Rossby number formed along frontal jets and eddy boundaries, generating localized ageostrophic circulations at the submesoscale. Measurements of over-ocean wind direction and surface currents from high-frequency radars reveal the influence of the short-term, small-scale wind forcing on the surface circulation, enhancement of the horizontal shear, frontal jet destabilization and the generation and decay of the cyclonic eddy. By contrast, the anticyclonic eddy formation was most likely associated with EAC mesoscale instability and anticyclonic vorticity. Lagrangian tracks show that surface particles can be temporarily trapped in the eddies and frontal convergent zones, limiting their transport. Mixing between EAC-derived and coastal waters was increased along the frontal regions, and particles starting at the divergent regions around the eddies experienced significant dispersion at submesoscales. The cyclonic cold-core eddy entrained high chlorophyll-a shelf waters on its convergent side, suggesting spiral eddy cyclogenesis.
      PubDate: 2017-04-13T15:05:29.858331-05:
      DOI: 10.1002/2016JC011968
       
  • Equatorial Stokes drift and Rossby rip currents
    • Authors: Jan Erik H. Weber
      Abstract: The Stokes drift in long baroclinic equatorial Rossby waves is investigated theoretically by using eigenfunction expansions in the vertical. These waves are non-dispersive and propagate westward along the equator. Particular attention is paid to the first baroclinic, first meridional Rossby wave mode which has been observed in the equatorial Pacific. It is demonstrated that the Stokes drift depends very much on the depth-variation of the Brunt-Väisälä frequency. Even more importantly, it is found that, for arbitrary stable stratification, the total zonal Stokes volume transport induced by the Rossby wave mode (1,1) is identically zero. The eastward drift due reflected wave energy in the form of internal equatorial Kelvin waves is also addressed. Due to the very long period of the incident Rossby wave mode (1,1), the reflected equatorial Kelvin wave must at least be a 2. mode component in the vertical. The corresponding Stokes drift only induces a minor change near the surface of the total westward drift velocity at the equator. The implication for the existence of compensating Rossby rip currents along the equator is discussed.
      PubDate: 2017-03-31T13:51:24.609701-05:
      DOI: 10.1002/2016JC012653
       
  • Modeling postconvective submesoscale coherent vortices in the northwestern
           Mediterranean Sea
    • Authors: P. Damien; A. Bosse, P. Testor, P. Marsaleix, C. Estournel
      Abstract: For the first time, the formation of Submesoscale Coherent Vortices (SCVs) during intermediate and deep convection events is documented in a realistic high resolution (1 km) numerical simulation of the oceanic circulation in the northwestern Mediterranean Sea. Winter intermediate and deep convection leads to the formation of anticyclonic and cyclonic eddies with lifetimes exceeding one year. By focusing on three typical eddies, the main characteristics of such vortices are discussed. The anticyclonic eddies are typical of SCVs observed in deep convection areas so far. They are characterized by a small radius (∼ 6.5 km) and orbital peak velocities of about 7 cm/s located at great depth (∼ 1500 m) or intermediate depth (∼ 500 m). The cyclonic vortices show very similar characteristics, such as a high Rossby number (∼ 0.4), but with surface-intensified structures. The long lifetimes of both anticyclonic and cyclonic eddies reflect very slow diffusive processes between their core and their surroundings and a strong resistance to external perturbations. These long-lived eddies are found to participate in the spreading of a significant portion (from 15 to 35%) of the convected waters in the Gulf of Lions and contribute to the ventilation of the deep basin.
      PubDate: 2017-03-31T13:51:16.311955-05:
      DOI: 10.1002/2016JC012114
       
  • Wind-driven ocean dynamics impact on the contrasting sea-ice trends around
           West Antarctica
    • Authors: Sang-Ki Lee; Denis L. Volkov, Hosmay Lopez, Woo Geun Cheon, Arnold L. Gordon, Yanyun Liu, Rik Wanninkhof
      Abstract: Since late 1978, Antarctic sea-ice extent in the East Pacific has retreated persistently over the Amundsen and Bellingshausen Seas in warm seasons, but expanded over the Ross and Amundsen Seas in cold seasons, while almost opposite seasonal trends have occurred in the Atlantic over the Weddell Sea. By using a surface-forced ocean and sea-ice coupled model, we show that regional wind-driven ocean dynamics played a key role in driving these trends. In the East Pacific, the strengthening Southern Hemisphere (SH) westerlies in the region enhanced the Ekman upwelling of warm upper Circumpolar Deep Water and increased the northward Ekman transport of cold Antarctic surface water. The associated surface ocean warming south of 68°S and the cooling north of 68°S directly contributed to the retreat of sea ice in warm seasons and the expansion in cold seasons, respectively. In the Atlantic, the poleward shifting SH westerlies in the region strengthened the northern branch of the Weddell Gyre, which in turn increased the meridional thermal gradient across it as constrained by the thermal wind balance. Ocean heat budget analysis further suggests that the strengthened northern branch of the Weddell Gyre acted as a barrier against the poleward ocean heat transport, and thus produced anomalous heat divergence within the Weddell Gyre and anomalous heat convergence north of the gyre. The associated cooling within the Weddell Gyre and the warming north of the gyre contributed to the expansion of sea ice in warm seasons and the retreat in cold seasons, respectively.
      PubDate: 2017-03-31T13:51:09.142908-05:
      DOI: 10.1002/2016JC012416
       
  • Physical and biogeochemical controls of the phytoplankton blooms in
           North-Western Mediterranean Sea: A multiplatform approach over a complete
           annual cycle (2012–2013 DEWEX experiment)
    • Authors: Nicolas Mayot; Fabrizio D'Ortenzio, Vincent Taillandier, Louis Prieur, Orens Pasqueron de Fommervault, Hervé Claustre, Anthony Bosse, Pierre Testor, Pascal Conan
      Abstract: The North Western Mediterranean Sea exhibits recurrent and significant autumnal and spring phytoplankton blooms. The existence of these two blooms coincide with typical temperate dynamics. To determine the potential control of physical and biogeochemical factors on these phytoplankton blooms, data from a multiplatform approach (combining ships, Argo and BGC-Argo floats, and bio-optical gliders) were analyzed in association with satellite observations in 2012-2013. The satellite framework allowed a simultaneous analysis over the whole annual cycle of in situ observations of mixed layer depth, photosynthetical available radiation, particle backscattering, nutrients (nitrate and silicate) and chlorophyll-a concentrations. During the year 2012-2013, satellite ocean color observations, confirmed by in situ data, have revealed the existence of two areas (or bioregions) with comparable autumnal blooms but contrasting spring blooms. In both bioregions, the ratio of the euphotic zone (defined as the isolume 0.415 mol photons m−2 d−1, Z0.415) and the MLD identified the initiation of the autumnal bloom, as well as the maximal annual increase in [Chl-a] in spring. In fact, the autumnal phytoplankton bloom might be initiated by mixing of the summer shallowing deep chlorophyll maximum, while the spring restratification (when Z0.415/MLD ratio became > 1) might induce surface phytoplankton production that largely overcomes the losses. Finally, winter deep convection events that took place in one of the bioregions induced higher net accumulation rate of phytoplankton in spring associated with a diatom-dominated phytoplankton community principally. We suggest that very deep winter MLD lead to an increase in surface silicates availability, which favored the development of diatoms.
      PubDate: 2017-03-31T13:50:32.70281-05:0
      DOI: 10.1002/2016JC012052
       
  • Observation-based parameterization of air-sea fluxes in terms of wind
           speed and atmospheric stability under low-to-moderate wind conditions
    • Authors: Zhongshui Zou; Dongliang Zhao, Bin Liu, Jun A Zhang, Jian Huang
      Abstract: This study explores the behavior of the exchange coefficients for wind stress (CD), sensible heat flux (CH), and water vapor flux (CE) as functions of surface wind speed (U10) and atmospheric stability using direct turbulent flux measurements obtained from a platform equipped with fast-response turbulence sensors in a low-to-moderate wind region. Turbulent fluxes are calculated using the eddy correlation method with extensive observations. The total numbers of quality-controlled 30-min flux runs are 12,240, 5,813, and 5,637 for estimation of CD, CH, and CE, respectively. When adjusted to neutral stability using the Monin-Obukhov similarity theory (MOST), we found that CDN, CHN, and CEN decrease with neutral-adjusted wind speed when wind speed is less than 5 m/s. CDN is constant over the range 5 m/s 
      PubDate: 2017-03-31T13:50:27.588045-05:
      DOI: 10.1002/2016JC012399
       
  • Turbulent entrainment in a strongly stratified barrier layer
    • Authors: H. T. Pham; S. Sarkar
      Abstract: Large-eddy simulation (LES) is used to investigate how turbulence in the wind-driven ocean mixed layer erodes the stratification of barrier layers. The model consists of a stratified Ekman layer that is driven by a surface wind. Simulations at a wide range of N0/f are performed to quantify the effect of turbulence and stratification on the entrainment rate. Here, N0 is the buoyancy frequency in the barrier layer and f is the Coriolis parameter. The evolution of the mixed layer follows two stages: a rapid initial deepening and a late-time growth at a considerably slower rate. During the first stage, the mixed layer thickens to the depth that is proportional u*/fN0 to where u* is the frictional velocity. During the second stage, the turbulence in the mixed layer continues to entrain further into the barrier layer, and the turbulent length scale is shown to scale with u*/N0, independent of f. The late-time entrainment rate E follows the law of E= 0.035Ri*−1/2 where Ri* is the Richardson number. The exponent is identical and the coefficient of 0.035 is much smaller relative to the corresponding power law in the non-rotating boundary layer. Simulations using the KPP model (version applicable to this simple case without additional effects of Langmuir turbulence or surface buoyancy flux) also yield the entrainment scaling of E∝Ri*−1/2; however, the proportionality coefficient varies with the stratification. The structure of the Ekman current is examined to illustrate the strong effect of stratification in the limit of large N0/f. This article is protected by copyright. All rights reserved.
      PubDate: 2017-03-29T12:19:35.71541-05:0
      DOI: 10.1002/2016JC012357
       
  • 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
       
  • 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
       
  • 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
       
  • 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
       
  • 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
       
  • 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
       
  • 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
       
  • 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
       
  • Subtidal circulation in a deep-silled fjord: Douglas Channel, British
           Columbia
    • 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
       
  • 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
           storms
    • 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
       
  • Winter to summer oceanographic observations in the Arctic Ocean north of
           Svalbard
    • Authors: Amelie Meyer; Arild Sundfjord, Ilker Fer, Christine Provost, Nicolas Villacieros Robineau, Zoe Koenig, Ingrid H. Onarheim, Lars H. Smedsrud, Pedro Duarte, Paul A. Dodd, Robert M. Graham, Sunke Schmidtko, Hanna M. Kauko
      Abstract: Oceanographic observations from the Eurasian Basin north of Svalbard collected between January and June 2015 from the N-ICE2015 drifting expedition are presented. The unique winter observations are a key contribution to existing climatologies of the Arctic Ocean, and show a ∼100m deep winter mixed layer likely due to high sea ice growth rates in local leads. Current observations for the upper ∼200m show mostly a barotropic flow, enhanced over the shallow Yermak Plateau. The two branches of inflowing Atlantic Water are partly captured, confirming that the outer Yermak Branch follows the perimeter of the plateau, and the inner Svalbard Branch the coast. Atlantic Water observed to be warmer and shallower than in the climatology, is found directly below the mixed layer down to 800m depth, and is warmest along the slope, while properties inside the basin are quite homogeneous. From late May onwards, the drift was continually close to the ice edge and a thinner surface mixed layer and shallower Atlantic Water coincided with significant sea ice melt being observed. This article is protected by copyright. All rights reserved.
      PubDate: 2017-01-31T06:41:22.912925-05:
      DOI: 10.1002/2016JC012391
       
  • Air-Sea heat flux climatologies in the Mediterranean Sea: Surface energy
           balance and its consistency with ocean heat storage
    • Authors: Xiangzhou Song; Lisan Yu
      Abstract: This study provides an analysis of the Mediterranean Sea surface energy budget using nine surface heat flux climatologies. The ensemble mean estimation shows that the net downward shortwave radiation (192±19 Wm−2) is balanced by latent heat flux (-98±10 Wm−2), followed by net longwave radiation (-78±13 Wm−2) and sensible heat flux (-13±4 Wm−2). The resulting net heat budget (Qnet) is 2±12 Wm−2 into the ocean, which appears to be warm biased. The annual-mean Qnet should be -5.6±1.6 Wm−2 when estimated from the observed net transport through the Strait of Gibraltar. To diagnose the uncertainty in nine Qnet climatologies, we constructed Qnet from the heat budget equation by using historic hydrological observations to determine the heat content changes and advective heat flux. We also used the Qnet from a data-assimilated global ocean state estimation as an additional reference. By comparing with the two reference Qnet estimates, we found that seven products (NCEP 1, NCEP 2, CFSR, ERA-Interim, MERRA, NOCSv2.0 and OAFlux+ISCCP) overestimate Qnet, with magnitude ranging from 6 to 27 Wm−2, while two products underestimate Qnet by -6 Wm−2 (JRA55) and -14 Wm−2 (CORE.2). Together with the previous warm pool work of Song and Yu, [2013], we show that CFSR, MERRA, NOCSv2.0, and OAFlux+ISCCP are warm biased not only in the western Pacific warm pool but also in the Mediterranean Sea, whilst CORE.2 is cold biased in both regions. The NCEP 1, 2 and ERA-Interim are cold biased over the warm pool but warm biased in the Mediterranean Sea. This article is protected by copyright. All rights reserved.
      PubDate: 2017-01-21T04:25:27.820129-05:
      DOI: 10.1002/2016JC012254
       
  • Issue Information
    • Pages: 3501 - 3504
      PubDate: 2017-06-27T22:36:34.426009-05:
      DOI: 10.1002/jgrc.21928
       
  • Targeted ocean sampling guidance for tropical cyclones
    • Authors: Sue Chen; James A. Cummings, Jerome M. Schmidt, Elizabeth R. Sanabia, Steven R. Jayne
      Pages: 3505 - 3518
      Abstract: A 3-D variational ocean data assimilation adjoint approach is used to examine the impact of ocean observations on coupled tropical cyclone (TC) model forecast error for three recent hurricanes: Isaac (2012), Hilda (2015), and Matthew (2016). In addition, this methodology is applied to develop an innovative ocean observation targeting tool validated using TC model simulations that assimilate ocean temperature observed by Airborne eXpendable Bathy Thermographs and Air-Launched Autonomous Micro-Observer floats. Comparison between the simulated targeted and real observation data assimilation impacts reveals a positive maximum mean linear correlation of 0.53 at 400–500 m, which implies some skill in the targeting application. Targeted ocean observation regions from these three hurricanes, however, show that the largest positive impacts in reducing the TC model forecast errors are sensitive to the initial prestorm ocean conditions such as the location and magnitude of preexisting ocean eddies, storm-induced ocean cold wake, and model track errors.
      PubDate: 2017-05-13T00:31:20.171443-05:
      DOI: 10.1002/2017JC012727
       
  • Downstream evolution of the Kuroshio's time-varying transport and velocity
           structure
    • Authors: M. Andres; V. Mensah, S. Jan, M.-H. Chang, Y.-J. Yang, C. M. Lee, B. Ma, T. B. Sanford
      Pages: 3519 - 3542
      Abstract: Observations from two companion field programs—Origins of the Kuroshio and Mindanao Current (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 8 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.
      PubDate: 2017-05-02T10:26:32.459625-05:
      DOI: 10.1002/2016JC012519
       
  • 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
      Pages: 3543 - 3564
      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 (i.e., anomaly) 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 reexamined 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 a very high content of colored dissolved organic matter (CDOM). 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.
      PubDate: 2017-05-02T10:27:00.580254-05:
      DOI: 10.1002/2016JC012629
       
  • Transport and thermohaline variability in Barrow Canyon on the
           Northeastern Chukchi Sea Shelf
    • Authors: Thomas J. Weingartner; Rachel A. Potter, Chase A. Stoudt, Elizabeth L. Dobbins, Hank Statscewich, Peter R. Winsor, Todd D. Mudge, Keath Borg
      Pages: 3565 - 3585
      Abstract: We used a 5 year time series of transport, temperature, and salinity from moorings at the head of Barrow Canyon to describe seasonal variations and construct a 37 year transport hindcast. The latter was developed from summer/winter regressions of transport against Bering-Chukchi winds. Seasonally, the regressions differ due to baroclinicity, stratification, spatial, and seasonal variations in winds and/or the surface drag coefficients. The climatological annual cycle consists of summer downcanyon (positive and toward the Arctic Ocean) transport of ∼0.45 Sv of warm, freshwaters; fall (October–December) upcanyon transport of ∼−0.1 Sv of cooler, saltier waters; and negligible net winter (January–April) mass transport when shelf waters are saline and near-freezing. Fall upcanyon transports may modulate shelf freezeup, and negligible winter transports could influence winter water properties. Transport variability is largest in fall and winter. Daily transport probability density functions are negatively skewed in all seasons and seasonal variations in kurtosis are a function of transport event durations. The latter may have consequences for shelf-basin exchanges. The climatology implies that the Chukchi shelf circulation reorganizes annually: in summer ∼40% of the summer Bering Strait inflow leaves the shelf via Barrow Canyon, but from fall through winter all of it exits via the western Chukchi or Central Channel. We estimate a mean transport of ∼0.2 Sv; ∼50% less than estimates at the mouth of the canyon. Transport discrepancies may be due to inflows from the Beaufort shelf and the Chukchi shelfbreak, with the latter entering the western side of the canyon.
      PubDate: 2017-05-02T10:26:45.269567-05:
      DOI: 10.1002/2016JC012636
       
  • A statistical study on the subthermocline submesoscale eddies in the
           northwestern Pacific Ocean based on Argo data
    • Authors: Cheng Li; Zhiwei Zhang, Wei Zhao, Jiwei Tian
      Pages: 3586 - 3598
      Abstract: In this study, statistical characteristics and spatiotemporal variability of the cold-core subthermocline eddies (STEs) discovered by Zhang et al. (2015) in the northwestern Pacific are investigated with 14 years of Argo temperature/salinity (T/S) data between 2002 and 2015. By applying an objective identification procedure to Argo T/S profiles, a total of 337 lens-like STEs that trap cold and fresh homogeneous water are obtained. The STEs' cores are primarily located between 26.5 and 26.9σ0 (300–800 m) and their shapes share a uniform Gaussian structure in vertical. Vertical and horizontal scale of the STEs is generally between 100 and 200 m and 5 and 15 km, respectively, suggesting that they are in the category of submesoscale processes. According to their different densities and seasonalities, the STEs are classified into upper and lower types with density interface of 26.7σ0. The number of upper-type STEs is much larger in spring and summer than in autumn and winter while that of lower-type ones show relatively uniform seasonal distributions. Spatially, the STEs are primarily distributed within the region between the Kuroshio Extension (KE) and subarctic front (SAF) in meridional and 140°E–155°E in zonal direction. Based on STEs' spatial distributions and vertical locations, we propose to name these STEs as KE intermediate-layer eddies (Kiddies). Water property analysis suggests that the Kiddies most likely originated from the SAF region. For the upper-type Kiddies, they are probably generated through subduction associated with mixed-layer frontogenesis, which at least contribute to 0.25 Sv annual subduction rate for the layers 26.2–26.7σ0.
      PubDate: 2017-05-02T10:27:15.664862-05:
      DOI: 10.1002/2016JC012561
       
  • An idealized study of near equatorial river plumes
    • Authors: Elbio D. Palma; Ricardo P. Matano
      Pages: 3599 - 3620
      Abstract: The dynamics of near equatorial river plumes (NERPs) are investigated using a highly idealized model. The spreading of a NERP from an eastern boundary is characterized by a continuous shedding of westward propagating eddies. This process transfers the bulk of the freshwater discharge to the deep ocean, thus distinguishing NERPs from their midlatitude counterparts. In the long-term limit, a NERP can be rationalized as a β-plume emanating from a coastal source. The evolution of NERPs in an unstratified basin is quite different from that in a stratified one. The spin-up in an unstratified basin is characterized by the formation of an anticyclonic bulge, which spreads westward thus creating a density stratification that favors the subsequent development of smaller and faster moving secondary eddies. The collision of the secondary eddies with the leading bulge arrests the effects of mixing thus allowing the further spreading of the buoyancy anomaly. In a stratified basin, the generation of anticyclonic eddies is accompanied by a concurrent generation of cyclones, which pump saltier waters to the surface hence leading to smaller sea surface salinity (SSS) anomalies. NERPs are sensitive to variations of the freshwater flux (Qfw) and the geomorphological setting. Larger Qfw generates bigger eddies, which spread at a rate proportional to the square root of the normalized flux. Wide shelves allow the interaction of the eddies with the bottom, thus promoting a cyclonic shift of the axis of the eddy train. The inclination of the coast affects the dynamical balance controlling the near-field behavior of NERPs.
      PubDate: 2017-05-02T10:26:39.180575-05:
      DOI: 10.1002/2016JC012554
       
  • Impacts of oil spills on altimeter waveforms and radar backscatter cross
           section
    • Authors: Yongcun Cheng; Jean Tournadre, Xiaofeng Li, Qing Xu, Bertrand Chapron
      Pages: 3621 - 3637
      Abstract: Ocean surface films can damp short capillary-gravity waves, reduce the surface mean square slope, and induce “sigma0 blooms” in satellite altimeter data. No study has ascertained the effect of such film on altimeter measurements due to lack of film data. The availability of Environmental Response Management Application (ERMA) oil cover, daily oil spill extent, and thickness data acquired during the Deepwater Horizon (DWH) oil spill accident provides a unique opportunity to evaluate the impact of surface film on altimeter data. In this study, the Jason-1/2 passes nearest to the DWH platform are analyzed to understand the waveform distortion caused by the spill as well as the variation of σ0 as a function of oil thickness, wind speed, and radar band. Jason-1/2 Ku-band σ0 increased by 10 dB at low wind speed (
      PubDate: 2017-05-02T10:26:54.213051-05:
      DOI: 10.1002/2016JC012568
       
  • Interannual surface salinity on Northwest Atlantic shelf
    • Authors: Semyon A. Grodsky; Nicolas Reul, Bertrand Chapron, James A. Carton, Frank O. Bryan
      Pages: 3638 - 3659
      Abstract: Sea surface salinity (SSS) from the Aquarius and SMOS satellite missions displays a steady increase of ∼1 psu over the entire northwestern Atlantic shelf south of Nova Scotia during 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 (1 psu/2°C). Salty and warm events occur coincident with anomalous easterly-southeasterly winds and Ekman transport counteracting the mean southwestward shelf currents. They are coincident with 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 eight interannual salinity oscillations (positive and negative). Six of these are driven by horizontal advection by slow varying currents (>2 months), while two events are driven by horizontal eddy advection (
      PubDate: 2017-05-04T06:17:02.368456-05:
      DOI: 10.1002/2016JC012580
       
  • The wave-driven current in coastal canopies
    • Authors: Maryam Abdolahpour; Magnus Hambleton, Marco Ghisalberti
      Pages: 3660 - 3674
      Abstract: Wave-driven flows over canopies of aquatic vegetation (such as seagrass) are characterized by the generation of a strong, shoreward mean current near the top of the canopy. This shoreward drift, which is observed to be up to 75% of the RMS above-canopy orbital velocity, can have a significant impact on residence times within coastal canopies. There have been limited observations of this current and an accurate formulation of its magnitude is still lacking. Accordingly, this study aims to develop a practical relationship to describe the strength of this current as a function of both wave and canopy characteristics. A simple model for the Lagrangian drift velocity indicates that the magnitude of the wave-driven current increases with the above-canopy oscillatory velocity, the vertical orbital excursion at the top of the canopy, and the canopy density. An extensive laboratory study, using both rigid and (dynamically scaled) flexible model vegetation, was carried out to evaluate the proposed model. Experimental results reveal a strong agreement between predicted and measured current velocities over a wide and realistic range of canopy and wave conditions. The validity of this model is also confirmed through available field measurements. Characterization of this wave-induced mean current will allow an enhanced capacity for predicting residence time, and thus key ecological processes, in coastal canopies.
      PubDate: 2017-05-04T06:00:56.853779-05:
      DOI: 10.1002/2016JC012446
       
  • The role of ocean-atmosphere interaction in Typhoon Sinlaku (2008) using a
           regional coupled data assimilation system
    • Authors: Akiyoshi Wada; Masaru Kunii
      Pages: 3675 - 3695
      Abstract: For improving analyses of tropical cyclone (TC) and sea surface temperature (SST) and thereby TC simulations, a regional mesoscale strongly coupled atmosphere-ocean data assimilation system was developed with the local ensemble transform Kalman filter (LETKF) implemented with the Japan Meteorological Agency's nonhydrostatic model (NHM) coupled with a multilayer ocean model and the third-generation ocean wave model. The NHM-LETKF coupled data assimilation system was applied to Typhoon Sinlaku (2008) along with the original NHM-LETKF system to investigate the sensitivity of Sinlaku to SST assimilation with the Level 2 Pre-processed (L2P) standard product of satellite SST. SST calculated in the coupled-assimilation experiment with the coupled data assimilation system and the satellite SST (CPL) showed a better correlation with Optimally Interpolated SST than SST used in the control experiment with the original NHM-LETKF (CNTL) and SST calculated in the succession experiment with the coupled system without satellite SST (SUCC). The time series in the CPL experiment well captured the variation in the SST observed at the Kuroshio Extension Observation buoy site. In addition, TC-induced sea surface cooling was analyzed more realistically in the CPL experiment than that in the CNTL and SUCC experiments. However, the central pressure analyzed in each three experiments was overestimated compared with the Regional Specialized Meteorological Center Tokyo best-track central pressure, mainly due to the coarse horizontal resolution of 15 km. The 96 h TC simulations indicated that the CPL experiment provided more favorable initial and boundary conditions than the CNTL experiment to simulate TC tracks more accurately.
      PubDate: 2017-05-04T06:00:45.004936-05:
      DOI: 10.1002/2017JC012750
       
  • How well does wind speed predict air-sea gas transfer in the sea ice
           zone' A synthesis of radon deficit profiles in the upper water column
           of the Arctic Ocean
    • Authors: B. Loose; R. P. Kelly, A. Bigdeli, W. Williams, R. Krishfield, M. Rutgers van der Loeff, S. B. Moran
      Pages: 3696 - 3714
      Abstract: We present 34 profiles of radon-deficit from the ice-ocean boundary layer of the Beaufort Sea. Including these 34, there are presently 58 published radon-deficit estimates of air-sea gas transfer velocity (k) in the Arctic Ocean; 52 of these estimates were derived from water covered by 10% sea ice or more. The average value of k collected since 2011 is 4.0 ± 1.2 m d−1. This exceeds the quadratic wind speed prediction of weighted kws = 2.85 m d−1 with mean-weighted wind speed of 6.4 m s−1. We show how ice cover changes the mixed-layer radon budget, and yields an “effective gas transfer velocity.” We use these 58 estimates to statistically evaluate the suitability of a wind speed parameterization for k, when the ocean surface is ice covered. Whereas the six profiles taken from the open ocean indicate a statistically good fit to wind speed parameterizations, the same parameterizations could not reproduce k from the sea ice zone. We conclude that techniques for estimating k in the open ocean cannot be similarly applied to determine k in the presence of sea ice. The magnitude of k through gaps in the ice may reach high values as ice cover increases, possibly as a result of focused turbulence dissipation at openings in the free surface. These 58 profiles are presently the most complete set of estimates of k across seasons and variable ice cover; as dissolved tracer budgets they reflect air-sea gas exchange with no impact from air-ice gas exchange.
      PubDate: 2017-05-05T00:30:33.732599-05:
      DOI: 10.1002/2016JC012460
       
  • Erosion of a California Undercurrent eddy by bottom topography
    • Authors: Hector S. Torres; Jose Gomez-Valdes
      Pages: 3715 - 3735
      Abstract: Subsurface eddies are ubiquitous features in eastern boundary current systems. These phenomena tend to modulate the across-shore distribution of heat and biogeochemical tracers. A California Undercurrent eddy was detected by shipboard observations in October 2009 off the northern Baja California continental slope. The spatiotemporal variation in the California Undercurrent eddy is investigated by using a mesoscale-resolving hindcast ocean simulation. A poleward coastal current that is driven by an upwelling-wind relaxation event and the coastline geometry instigated the separation of the California Undercurrent from the slope, forming a meander-like structure, which evolved as a mesoscale eddy-like structure. The latter structure evolved as a subsurface eddy with a warm anomaly core, a distinctive feature of eddies that form from the California Undercurrent. During the initial stage, the subsurface eddy presented a cone-shape form, with the maximum amplitude of the relative vorticity in the upper section. The inviscid effect of the irregular bottom topography altered both the initial direction of propagation and the initial eddy shape: the propagation direction of the eddy changed from north-south to southwest, and the shape changed from a cone shape to a subsurface lens shape. The strong eddy-topography interactions triggered a sign change in the heat flux direction from the environment toward the eddy and vice versa through the horizontally divergent component of the velocity field, which accelerated the eddy's decay. This study shows the relevance of the synoptic wind stress events and the irregularity of the bottom topography on the mesoscale eddy activity in the southern portion of the California Current.
      PubDate: 2017-05-05T04:17:01.383169-05:
      DOI: 10.1002/2016JC011870
       
  • Seagrass blade motion under waves and its impact on wave decay
    • Authors: M. Luhar; E. Infantes, H. Nepf
      Pages: 3736 - 3752
      Abstract: The hydrodynamic drag generated by seagrass meadows can dissipate wave-energy, causing wave decay. It is well known that this drag depends on the relative motion between the water and the seagrass blades, yet the impact of blade motion on drag and wave-energy dissipation remains to be fully characterized. In this experimental study, we examined the impact of blade motion on wave decay by concurrently recording blade posture during a wave cycle and measuring wave decay over a model seagrass meadow. We also identified a scaling law that predicts wave decay over the model meadow for a range of seagrass blade density, wave period, wave height, and water depth scaled from typical field conditions. Blade flexibility led to significantly lower drag and wave decay relative to theoretical predictions for rigid, upright blades. To quantify the impact of blade motion on wave decay, we employed an effective blade length, le, defined as the rigid blade length that leads to equivalent wave-energy dissipation. We estimated le directly from images of blade motion. Consistent with previous studies, these estimates showed that the effective blade length depends on the dimensionless Cauchy number, which describes the relative magnitude of the wave hydrodynamic drag and the restoring force due to blade rigidity. As the hydrodynamic forcing increases, the blades exhibit greater motion. Greater blade motion leads to smaller relative velocities, reducing drag, and wave-energy dissipation (i.e., smaller le).
      PubDate: 2017-05-05T00:30:37.939621-05:
      DOI: 10.1002/2017JC012731
       
  • Spatial variability of upper ocean POC export in the Bay of Bengal and the
           Indian Ocean determined using particle-reactive 234Th
    • Authors: S. Subha Anand; R. Rengarajan, V. V. S. S. Sarma, A. K. Sudheer, R. Bhushan, S. K. Singh
      Pages: 3753 - 3770
      Abstract: The northern Indian Ocean is globally significant for its seasonally reversing winds, upwelled nutrients, high biological production, and expanding oxygen minimum zones. The region acts as sink and source for atmospheric CO2. However, the efficiency of the biological carbon pump to sequester atmospheric CO2 and export particulate organic carbon from the surface is not well known. To quantify the upper ocean carbon export flux and to estimate the efficiency of biological carbon pump in the Bay of Bengal and the Indian Ocean, seawater profiles of total 234Th were measured from surface to 300 m depth at 13 stations from 19.9°N to 25.3°S in a transect along 87°E, during spring intermonsoon period (March–April 2014). Results showed enhanced in situ primary production in the equatorial Indian Ocean and the central Bay of Bengal and varied from 13.2 to 173.8 mmol C m−2 d−1. POC export flux in this region varied from 0 to 7.7 mmol C m−2 d−1. Though high carbon export flux was found in the equatorial region, remineralization of organic carbon in the surface and subsurface waters considerably reduced organic carbon export in the Bay of Bengal. Annually recurring anticyclonic eddies enhanced organic carbon utilization and heterotrophy. Oxygen minimum zone developed due to stratification and poor ventilation was intensified by subsurface remineralization. 234Th-based carbon export fluxes were not comparable with empirical statistical model estimates based on primary production and temperature. Region-specific refinement of model parameters is required to accurately predict POC export fluxes.
      PubDate: 2017-05-05T00:25:39.721505-05:
      DOI: 10.1002/2016JC012639
       
  • A new look at ocean ventilation time scales and their uncertainties
    • Authors: Rana A. Fine; Synte Peacock, Mathew E. Maltrud, Frank O. Bryan
      Pages: 3771 - 3798
      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 time scales 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 nonstationarity 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 reinterpret observed temporal trends in tracer-derived ventilation time scales 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.
      PubDate: 2017-05-08T00:39:58.435136-05:
      DOI: 10.1002/2016JC012529
       
  • Rip currents and alongshore flows in single channels dredged in the surf
           zone
    • Authors: Melissa Moulton; Steve Elgar, Britt Raubenheimer, John C. Warner, Nirnimesh Kumar
      Pages: 3799 - 3816
      Abstract: To investigate the dynamics of flows near nonuniform 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 nonuniform 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.
      PubDate: 2017-05-08T00:43:11.734614-05:
      DOI: 10.1002/2016JC012222
       
  • Seasonal cycle of cross-equatorial flow in the central Indian Ocean
    • Authors: Yi Wang; Michael J. McPhaden
      Pages: 3817 - 3827
      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 southward 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 between 0.5° and 1°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, semiannually varying westerly monsoon transition winds lead to semiannual 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.
      PubDate: 2017-05-09T01:16:01.202563-05:
      DOI: 10.1002/2016JC012537
       
  • Fjord light regime: Bio-optical variability, absorption budget, and
           hyperspectral light availability in Sognefjord and Trondheimsfjord, Norway
           
    • Authors: V. J. Mascarenhas; D. Voß, J. Wollschlaeger, O. Zielinski
      Pages: 3828 - 3847
      Abstract: Optically active constituents (OACs) in addition to water molecules attenuate light via processes of absorption and scattering and thereby determine underwater light availability. An analysis of their optical properties helps in determining the contribution of each of these to light attenuation. With an aim to study the bio-optical variability, absorption budget and 1% spectral light availability, hydrographical (temperature and salinity), and hyperspectral optical (downwelling irradiance and upwelling radiance) profiles were measured along fjord transects in Sognefjord and Trondheimsfjord, Norway. Optical water quality observations were also performed using Secchi disc and Forel-Ule scale. In concurrence, water samples were collected and analyzed via visible spectrophotometry, fluorometry, and gravimetry to quantify and derive inherent optical properties of the water constituents. An absorption model (R2 = 0.91, n = 36, p 
      PubDate: 2017-05-09T01:15:45.071822-05:
      DOI: 10.1002/2016JC012610
       
  • Modeling temperature inversion in southeastern Yellow Sea during winter
           2016
    • Authors: Ig-Chan Pang; Jae-Hong Moon, Joon-Ho Lee, Ji-Seok Hong, Sung-Jun Pang
      Pages: 3848 - 3860
      Abstract: A significant temperature inversion with temperature differences larger than 3°C was observed in the southeastern Yellow Sea (YS) during February 2016. By analyzing in situ hydrographic profiles and results from a regional ocean model for the YS, this study examines the spatiotemporal evolution of the temperature inversion and its connection with wind-induced currents in winter. Observations reveal that in winter, when the northwesterly wind prevails over the YS, the temperature inversion occurs largely at the frontal zone southwest of Korea where warm/saline water of a Kuroshio origin meets cold/fresh coastal water. Our model successfully captures the temperature inversion observed in the winter of 2016 and suggests a close relation between northwesterly wind bursts and the occurrence of the large inversion. In this respect, the strong northwesterly wind drove cold coastal water southward in the upper layer via Ekman transport, which pushed the water mass southward and increased the sea level slope in the frontal zone in southeastern YS. The intensified sea level slope propagated northward away from the frontal zone as a shelf wave, causing a northward upwind flow response along the YS trough in the lower layer, thereby resulting in the large temperature inversion. Diagnostic analysis of the momentum balance shows that the westward pressure gradient, which developed with shelf wave propagation along the YS trough, was balanced with the Coriolis force in accordance with the northward upwind current in and around the inversion area.
      PubDate: 2017-05-09T01:10:34.78412-05:0
      DOI: 10.1002/2017JC012718
       
  • Boussinesq modeling of wave-induced hydrodynamics in coastal wetlands
    • Authors: Agnimitro Chakrabarti; Steven R. Brandt, Qin Chen, Fengyan Shi
      Pages: 3861 - 3883
      Abstract: In this paper, an improved formulation of the vegetation drag force, applicable for the fully nonlinear Boussinesq equations and based on the use of the depth-varying, higher-order expansion of the horizontal velocity, in the quadratic vegetation drag law has been presented. The model uses the same numerical schemes as FUNWAVE TVD but is based on the CACTUS framework. The model is validated for wave height and setup, against laboratory experiments with and without vegetation cover. The wave attenuation results using the improved formulation were compared with those using the first-order reference velocity as well as with analytical solutions using linear wave theory. The analytical solution using the depth-varying velocity, predicted by the linear wave theory, was shown to match the model results with the fully expanded velocity approach very well for all wave cases, except under near-emergent and emergent conditions (when the ratio of stem height to water depth is greater than 0.75) and when the Ursell (Ur) number is less than 5. Simulations during peak storm waves, during Hurricane Isaac, showed that vegetation is very effective in reducing setup on platforms and in reducing the wave energy within the first few hundred meters.
      PubDate: 2017-05-12T23:57:24.21689-05:0
      DOI: 10.1002/2016JC012093
       
  • Flow variability within the Alaska Coastal Current in winter
    • Authors: Ewa Jarosz; David Wang, Hemantha Wijesekera, W. Scott Pegau, James N. Moum
      Pages: 3884 - 3906
      Abstract: Coastal circulation off Kayak Island in the northern Gulf of Alaska was explored in wintertime (October 2012 to March 2013) by deploying nine moorings within the Alaska Coastal Current (ACC). Hydrographic, bottom-pressure, and velocity observations depicted well the winter variability of the ACC. Atmospheric observations showed a net loss of heat, 30 W m−2 or more, from the ocean to the atmosphere and indicated that storms with downwelling-favorable winds over 10 m s−1 frequently passed over the area. Due to vigorous mixing during storms, the waters were well-mixed or weakly stratified whereas bottom-pressure anomalies were mainly related to surface-elevation fluctuations and indicated that there was also a cross-shelf surface-elevation gradient. Current observations showed along-shelf nearly barotropic subtidal flow of 40 cm s−1 or more throughout the water column. They also indicated that along-shelf flow was primarily driven by the cross-shelf pressure gradient resulting from the cross-shelf surface-elevation gradient and not by wind stress. Analyses suggested that flow dynamics within the ACC in winter were well-described by vertically averaged momentum equations and showed a dominance of the cross-shelf pressure gradient that was mainly balanced by the Coriolis term. Observations also showed that when winds relaxed, cold low-salinity waters moved offshore and stratification was reestablished. Consequently, near-shore waters were less dense, i.e., cooler and fresher than offshore waters resulting in the cross-shelf density gradient that may have contributed to the along-shelf flow by generating near-surface currents of ∼20 cm s−1.
      PubDate: 2017-05-13T00:37:49.842834-05:
      DOI: 10.1002/2016JC012102
       
  • Strongly nonlinear internal solitons: Models and applications
    • Authors: L. A. Ostrovsky; V. G. Irisov
      Pages: 3907 - 3916
      Abstract: A strongly nonlinear evolution equation based on an approximate Hamiltonian is suggested and applied to the description of nonstationary processes of formation and evolution of strong internal solitons and their groups (solibores). Comparison with experimental data shows that such a simplified approach can be effectively applied to the description and prediction of strong internal waves’ evolution in the upper ocean.
      PubDate: 2017-05-13T01:13:12.865755-05:
      DOI: 10.1002/2017JC012762
       
  • Impact of sea level rise on tidal range in Chesapeake and Delaware Bays
    • Authors: Serena Blyth Lee; Ming Li, Fan Zhang
      Pages: 3917 - 3938
      Abstract: Coastal inundation is affected not only by rising mean sea level but also by changing tides. A numerical model is developed to investigate how sea level rise and coastline changes may impact tides in two coastal-plain estuaries, Chesapeake Bay and Delaware Bay. Despite their different tidal characteristics, the two estuaries display similar responses to the sea level rise and shoreline management scenarios. When hypothetic sea walls are erected at the present coastline to prevent low-lying land from flooding, tidal range increases, with greater amplification in the upper part of the two estuaries. When low-lying land is allowed to become permanently inundated by higher sea level, however, tidal range in both estuaries decreases. Analyses of the tidal energy budget show that the increased dissipation over the shallow water and newly inundated areas compensates for the reduced dissipation in deep water, leading to smaller tidal range. The changes in the tidal range are not proportional to the changes in the mean sea level, indicating a nonlinear tidal response to sea level rise. The ratio of tidal range change to sea level rise varies between −0.05 and 0.1 in Chesapeake Bay and between −0.2 and 0.25 in Delaware Bay. The model results suggest a potential adaptation strategy that uses inundation over low-lying areas to reduce tidal range at up-estuary locations.
      PubDate: 2017-05-13T01:13:20.415135-05:
      DOI: 10.1002/2016JC012597
       
  • Nonbreaking wave-induced mixing in upper ocean during tropical cyclones
           using coupled hurricane-ocean-wave modeling
    • Authors: S. Aijaz; M. Ghantous, A. V. Babanin, I. Ginis, B. Thomas, G. Wake
      Pages: 3939 - 3963
      Abstract: The effects of turbulence generated by nonbreaking waves have been investigated by testing and evaluating a new nonbreaking wave parameterization in a coupled hurricane-ocean-wave model. The MPI version of the Princeton Ocean Model (POM) with hurricane forcing is coupled with the WAVEWATCH-III (WW3) surface wave model. Hurricane Ivan is chosen as the test case due to its extreme intensity and availability of field data during its passage. The model results are validated against field observations of wave heights and sea surface temperatures (SSTs) from the National Data Buoy Centre (NDBC) during Hurricane Ivan and against limited in situ current and bottom temperature data. A series of numerical experiments is set up to examine the influence of the nonbreaking wave parameterization on the mixing of upper ocean. The SST response from the modeling experiments indicates that the nonbreaking wave-induced mixing leads to significant cooling of the SST and deepening of the mixed layer. It was found that the nondimensional constant b1 in the nonbreaking wave parameterization has different impacts on the weak and the strong sides of the storm track. A constant value of b1 leads to improved predictions on the strong side of the storm while a steepness-dependent b1 provides a better agreement with in situ observations on the weak side. A separate simulation of the intense tropical cyclone Olwyn in north-west Australia revealed the same trend for b1 on the strong side of the tropical cyclone.
      PubDate: 2017-05-15T02:41:33.742769-05:
      DOI: 10.1002/2016JC012219
       
  • Characterizing frontal eddies along the East Australian Current from HF
           radar observations
    • Authors: Amandine Schaeffer; A. Gramoulle, M. Roughan, A. Mantovanelli
      Pages: 3964 - 3980
      Abstract: The East Australian Current (EAC) dominates the ocean circulation along south-eastern Australia, however, little is known about the submesoscale frontal instabilities associated with this western boundary current. One year of surface current measurements from HF radars, in conjunction with mooring and satellite observations, highlight the occurrence and propagation of meanders and frontal eddies along the inshore edge of the EAC. Eddies were systematically identified using the geometry of the high spatial resolution (∼1.5 km) surface currents, and tracked every hour. Cyclonic eddies were observed irregularly, on average every 7 days, with inshore radius ∼10 km. Among various forms of structures, frontal eddies associated with EAC meanders were characterized by poleward advection speeds of ∼0.3–0.4 m/s, migrating as far as 500 km south, based on satellite imagery. Flow field kinematics show that cyclonic eddies have high Rossby numbers (0.6–1.9) and enhance particle dispersion. Patches of intensified surface divergence at the leading edge of the structures are expected to generate vertical uplift. This is confirmed by subsurface measurements showing temperature uplift of up to 55 m over 24 h and rough estimates of vertical velocities of 10s of meters per day. While frontal eddies propagate through the radar domain independently of local wind stress, upfront wind can influence their stalling and growth, and can also generate large cold core eddies through intense shear. Such coherent structures are a major mechanism for the transport and entrainment of nutrient rich coastal or deep waters, influencing physical and biological dynamics, and connectivity over large distances.
      PubDate: 2017-05-15T02:35:40.739407-05:
      DOI: 10.1002/2016JC012171
       
  • Energy-optimal path planning in the coastal ocean
    • Authors: Deepak N. Subramani; Patrick J. Haley, Pierre F. J. Lermusiaux
      Pages: 3981 - 4003
      Abstract: We integrate data-driven ocean modeling with the stochastic Dynamically Orthogonal (DO) level-set optimization methodology to compute and study energy-optimal paths, speeds, and headings for ocean vehicles in the Middle-Atlantic Bight (MAB) region. We hindcast the energy-optimal paths from among exact time-optimal paths for the period 28 August 2006 to 9 September 2006. To do so, we first obtain a data-assimilative multiscale reanalysis, combining ocean observations with implicit two-way nested multiresolution primitive-equation simulations of the tidal-to-mesoscale dynamics in the region. Second, we solve the reduced-order stochastic DO level-set partial differential equations (PDEs) to compute the joint probability of minimum arrival time, vehicle-speed time series, and total energy utilized. Third, for each arrival time, we select the vehicle-speed time series that minimize the total energy utilization from the marginal probability of vehicle-speed and total energy. The corresponding energy-optimal path and headings are obtained through the exact particle-backtracking equation. Theoretically, the present methodology is PDE-based and provides fundamental energy-optimal predictions without heuristics. Computationally, it is 3–4 orders of magnitude faster than direct Monte Carlo methods. For the missions considered, we analyze the effects of the regional tidal currents, strong wind events, coastal jets, shelfbreak front, and other local circulations on the energy-optimal paths. Results showcase the opportunities for vehicles that intelligently utilize the ocean environment to minimize energy usage, rigorously integrating ocean forecasting with optimal control of autonomous vehicles.
      PubDate: 2017-05-15T00:11:23.194897-05:
      DOI: 10.1002/2016JC012231
       
  • Warm water pathways toward Nioghalvfjerdsfjorden Glacier, Northeast
           Greenland
    • Authors: Janin Schaffer; Wilken-Jon von Appen, Paul A. Dodd, Coen Hofstede, Christoph Mayer, Laura de Steur, Torsten Kanzow
      Pages: 4004 - 4020
      Abstract: Nioghalvfjerdsfjorden Glacier (79NG) is the largest of three marine-terminating outlet glaciers draining the Northeast Greenland Ice Stream. To understand how Atlantic waters supply waters in the cavity beneath the floating 79NG, we analyze historic and recent bathymetric, hydrographic, and velocity observations obtained on the Northeast Greenland continental shelf. The bathymetry is characterized by a trough system, consisting of the Westwind Trough and the Norske Trough in the northern and southern part of the continental shelf, respectively. Atlantic waters recirculating in Fram Strait cross the shelf break and enter the trough system at its southeastern inlet toward the inner shelf. Warm Atlantic Intermediate Water (AIW) present below 200 m in the Norske Trough shows large contributions of the recirculating Atlantic water. We found that the bathymetry is sufficiently deep to provide a direct subsurface pathway for warm AIW between the shelf break and the 79NG cavity via the Norske Trough. Likewise, based on the hydrographic data, we show that the Norske Trough supplies AIW warmer than 1°C to the 79NG, which is not present in the Westwind Trough. Our moored and lowered velocity measurements indicate that a boundary current carries warm AIW along the northeastern slope of Norske Trough toward the 79NG. We suggest that anomalies in Atlantic water temperatures in Fram Strait could reach 79NG within less than 1.5 years, thereby modifying the glacier's basal melt rates.
      PubDate: 2017-05-15T02:30:41.635083-05:
      DOI: 10.1002/2016JC012462
       
  • Spatial patterns of mixing in the Solomon Sea
    • Authors: M. S. Alberty; J. Sprintall, J. MacKinnon, A. Ganachaud, S. Cravatte, G. Eldin, C. Germineaud, A. Melet
      Pages: 4021 - 4039
      Abstract: The Solomon Sea is a marginal sea in the southwest Pacific that connects subtropical and equatorial circulation, constricting transport of South Pacific Subtropical Mode Water and Antarctic Intermediate Water through its deep, narrow channels. Marginal sea topography inhibits internal waves from propagating out and into the open ocean, making these regions hot spots for energy dissipation and mixing. Data from two hydrographic cruises and from Argo profiles are employed to indirectly infer mixing from observations for the first time in the Solomon Sea. Thorpe and finescale methods indirectly estimate the rate of dissipation of kinetic energy (ϵ) and indicate that it is maximum in the surface and thermocline layers and decreases by 2–3 orders of magnitude by 2000 m depth. Estimates of diapycnal diffusivity from the observations and a simple diffusive model agree in magnitude but have different depth structures, likely reflecting the combined influence of both diapycnal mixing and isopycnal stirring. Spatial variability of ϵ is large, spanning at least 2 orders of magnitude within isopycnal layers. Seasonal variability of ϵ reflects regional monsoonal changes in large-scale oceanic and atmospheric conditions with ϵ increased in July and decreased in March. Finally, tide power input and topographic roughness are well correlated with mean spatial patterns of mixing within intermediate and deep isopycnals but are not clearly correlated with thermocline mixing patterns.
      PubDate: 2017-05-15T02:30:38.386844-05:
      DOI: 10.1002/2016JC012666
       
  • Tide-induced residual circulation in a bay with laterally asymmetric depth
    • Authors: Bong-Gwan Kim; Yang-Ki Cho
      Pages: 4040 - 4050
      Abstract: The tide-induced residual current is a crucial determinant of the distribution of sediment in a tidally dominated bay. There have been only a few studies of residual currents in bays with laterally asymmetric depth, although the residual circulation in bays with symmetric lateral depth variation has been studied extensively. Gomso Bay is a typical bay with laterally asymmetric depth. The bay is characterized by a deep channel to the north and a wide shoal to the south. This study reports the tide-induced residual current in Gomso Bay and explains its dynamics using numerical and analytical models. The results from the models are consistent with observations. Residual flows show a clockwise gyre inside the bay and counterclockwise circulation near the mouth. Three physical causes induce residual circulation: (1) bottom friction coupled with a tidal height, (2) advection causing inward residual transport, and (3) a pressure gradient due to a surface slope induced by subtidal flow. The maximum inward residual current is shifted toward the wider shoal. This is caused by across-channel advection due to the laterally asymmetric depth.
      PubDate: 2017-05-15T02:35:28.708274-05:
      DOI: 10.1002/2016JC012473
       
  • Intraseasonal-to-semiannual variability of sea-surface height in the
           astern, equatorial Indian Ocean and southern Bay of Bengal
    • Authors: Xuhua Cheng; Julian P. McCreary, Bo Qiu, Yiquan Qi, Yan Du
      Pages: 4051 - 4067
      Abstract: Intraseasonal-to-semiannual variability of sea-surface height (SSH) in the eastern, equatorial Indian Ocean (EEIO) and southern Bay of Bengal (BoB) is investigated using altimetric data, and solutions to 1½ layer (first baroclinic mode) and linear, continuously stratified (LCS; multibaroclinic-mode) models. The amplitude and dominant periods of SSH variability differ regionally. Large-amplitude variability is found along the west coast of Sumatra, in a zonal band across the BoB centered along 5°N, east of Sri Lanka, and in the northwestern BoB, respectively. Along the Sumatran west coast, SSH variability peaks at 30–60, 90, and 180 days. Along 5°N and east of Sri Lanka, the 30–60 day variability is dominant. Sensitivity experiments using a nonlinear version of the 1½ layer model forced by realistic winds reproduce the observed patterns of intraseasonal variability in the southern BoB. At 30–60 days, the solutions show that eddies (nonlinear Rossby waves) propagating from the east, rather than local wind forcing, account for most of the variance east of Sri Lanka; furthermore, they demonstrate that the variance is significantly enhanced by the nonlinear transfer of 90–120 day energy into the intraseasonal band of 30–60 days. The LCS solutions show that the first two baroclinic modes explain most of the SSH variance at 90–180 days. The second baroclinic mode dominates the SSH variance at 180 days, a consequence of basin resonance and strong wind forcing.
      PubDate: 2017-05-15T00:10:51.902978-05:
      DOI: 10.1002/2016JC012662
       
  • 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
      Pages: 4088 - 4109
      Abstract: A wave-driven surface-buoyant jet exiting a coral reef was studied in order to quantify the amount of water reentrained 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 alongshore flow variations, we developed an idealized numerical model of the system. Consistent with previous work, the ratio of alongshore 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.
      PubDate: 2017-05-16T01:25:43.156636-05:
      DOI: 10.1002/2016JC011729
       
 
 
JournalTOCs
School of Mathematical and Computer Sciences
Heriot-Watt University
Edinburgh, EH14 4AS, UK
Email: journaltocs@hw.ac.uk
Tel: +00 44 (0)131 4513762
Fax: +00 44 (0)131 4513327
 
Home (Search)
Subjects A-Z
Publishers A-Z
Customise
APIs
Your IP address: 23.20.193.33
 
About JournalTOCs
API
Help
News (blog, publications)
JournalTOCs on Twitter   JournalTOCs on Facebook

JournalTOCs © 2009-2016