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  • 1
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    Diagnosing cross-shelf transport along an ocean front: an observational case study in the Gulf of Lion (2016)
    Wiley
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    Publication Date: 2016-08-30
    Description: Exchanges between coastal regions and the open ocean are often associated with intermittent and localized processes such as eddies, fronts and filaments. Since these features are difficult to observe, their impact has been predominantly investigated using numerical models and remote sensing. In this study, satellite sea surface temperature maps, Lagrangian surface drifter trajectories, and ship-based surveys of currents and hydrography from the Latex10 campaign are used to quantify cross-shelf exchanges associated with a temperature front in the western Gulf of Lion. Satellite imagery and thermosalinograph sections provide the characterization of the various water masses associated with the front. Lagrangian drifter trajectories are used to identify the main transport structures and to quantify the velocity components associated with near-inertial oscillations. These are removed from the instantaneous ADCP observations with which the cross-shelf exchanges are then computed. The results indicate an average outflow of 0.074 ±0.013 Sv and an inflow of 0.021 ±0.006 Sv. Integrated over the two-week lifetime of the front, such outflow induced a total export of ∼ 90 ± 14 km 3 of water, indicating that 3 to 4 of such events are sufficient to completely renew the surface waters of the Gulf of Lion. The total import was ∼ 25 ± 7 km 3 , suggesting larger inflows at depth or in the eastern part of the gulf to maintain its volume balance. These in-situ estimates represent a key term of comparison for the further development of numerical model- and satellite-based studies of cross-shelf exchanges associated with this type of processes. This article is protected by copyright. All rights reserved.
    Print ISSN: 0148-0227
    Topics: Geosciences , Physics
    Published by Wiley on behalf of American Geophysical Union (AGU).
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  • 2
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    Physical characteristics and dynamics of the coastal Latex09 Eddy derived from in situ data and numerical modeling (2013)
    Wiley
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    Publication Date: 2013-02-01
    Description: [1]  We investigate the dynamics of a coastal anticyclonic eddy in the western part of the Gulf of Lion (GoL) in the northwestern Mediterranean Sea during the Latex campaign in the summer 2009 (Latex09). The sampling strategy combines sea surface temperature satellite imagery, hull-mounted acoustic Doppler current profiler data, conductivity-temperature-depth casts, and drifter trajectories. Our measurements reveal an anticyclonic eddy (Latex09 eddy) with a diameter of ~23 km and maximum depth of 31 m, centered at 3°34′E, 42°33′N. We use a high resolution, three-dimensional, primitive equation numerical model to investigate its generation process and evolution. The model is able to reproduce the observed eddy, in particular its size and position. The model results suggest that the Latex09 eddy is induced by a large anticyclonic circulation in the northwestern part of the GoL, pushed and squeezed toward the coast by a meander of the Northern Current. This represents a new generation mechanism that has not been reported before. The post generation dynamics of the eddy is also captured by the model. The collision of the Latex09 eddy with Cape Creus results in a transient structure, which is depicted by the trajectories of two Lagrangian drifters during Latex09. The transient structure and its advection lead to a transfer of mass and vorticity from the GoL to the Catalan shelf, indicating the importance of mesoscale structures in modulating such exchanges in the region.
    Print ISSN: 0148-0227
    Topics: Geosciences , Physics
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  • 3
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    Vertical Motions and Their Effects on a Biogeochemical Tracer in a Cyclonic Structure Finely Observed in the Ligurian Sea (2019)
    Wiley
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    Publication Date: 2019
    Description: Abstract Vertical velocities can be estimated indirectly from in situ observations by theoretical frameworks like the ω‐equation. Direct measures of vertical exchanges are challenging due to their typically ephemeral spatiotemporal scales. In this study we address this problem with an adaptive sampling strategy coupling various biophysical instruments. We analyze the 3‐D organization of a cyclonic mesoscale structure finely sampled during the Observing Submesoscale Coupling At High Resolution cruise in the Ligurian Sea during fall 2015. The observations, acquired with a moving vessel profiler, highlight a subsurface low‐salinity layer (≃50 m), as well as rising isopycnals, generated by geostrophic cyclonic circulation, in the structure's center. Reconstructed 3‐D fields of density and horizontal velocities are used to estimate the vertical velocity field down to 250 m by applying the adiabatic QG ω‐equation, for the first time in this region. The vertical motions are characterized by multipolar patterns of downward and upward velocities on the edges of the structure and significantly smaller vertical velocities in its center. Both the 3‐D distribution of particles (size ≥100 μm), measured with a laser optical plankton counter, and the Synechococcus and Prochlorococcus abundances (cell per cubic meter) measured by flow cytometry are consistent with the 3‐D velocity field. In particular, a secondary vertical recirculation is identified that upwells particles (from 250 to 100 m) along isohalines to the structure's center. Besides demonstrating the effect of vertical patterns on biogeochemical distributions, this case study suggests to use particle matter as a tracer to assess physical dynamics.
    Print ISSN: 2169-9275
    Electronic ISSN: 2169-9291
    Topics: Geosciences , Physics
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  • 4
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    In situ estimates of submesoscale horizontal eddy diffusivity across an ocean front (2013)
    Wiley
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    Publication Date: 2013-12-11
    Description: In the last decade, the rapid advancements in computational power have favored the development of high-resolution numerical models capable of directly resolving small scale structures such as fronts and filaments. Such models have greatly improved our understanding of submesoscale dynamics. At the same time, the small dimensions and short duration of these structures still pose major challenges for small-scale dedicated field experiments. For this reason, submesoscale studies from in-situ observations are still relatively scarce and quantitative estimates of key physical parameters for high-resolution numerical models, such as horizontal eddy diffusivity, are still lacking. This study presents a novel approach for computing in-situ horizontal eddy diffusivity associated with frontal structures by combining cross-front widths derived from surface thermosalinograph sections with stirring rates estimated from Lagrangian drifter trajectories. The method is applied to the measurements collected across a frontal structure observed in the western part of the Gulf of Lion during the Latex10 campaign (LAgrangian Transport EXperiment, September 1-24, 2010). A total of 76 estimates of eddy diffusivity were obtained for strain rates of 0.70 and 1.21 day -1 and front widths (horizontal scales) ranging between 1 and 4 km. The estimates are log-normally distributed, with 70% of the values ranging between 0.4 and 5 m 2 s -1 . Further analysis based on high-resolution simulations and remote sensed observations, as well as dedicated field experiments will help to assess the robustness of some the assumptions at the base of the proposed approach, and to extend the results to different ocean regions.
    Print ISSN: 0148-0227
    Topics: Geosciences , Physics
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  • 5
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    Surface coastal circulation patterns by in-situ detection of Lagrangian coherent structures (2011)
    Wiley
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    Publication Date: 2011-09-07
    Description: Coastal transport and cross-shelf exchanges are important factors in controlling the dispersal of human and river discharged pollutants, as well as the advection of nutrients and larvae. Altimetry-based Lagrangian techniques provide accurate information on horizontal transport in the open ocean but are unreliable close to the coast. In order to circumvent this problem, during the Lagrangian Transport Experiment 2010 campaign (Latex10, 1–24 September 2010) transport structures in the western Gulf of Lion were investigated with an adaptive sampling strategy, combining satellite data, ship-based ADCP measurements, and iterative Lagrangian drifter releases. The sampling strategy was able to identify errors in the surface transport patterns derived from altimetry, and to track with in-situ observations attractive and repelling Lagrangian coherent structures for a period of 12 days. The structures maintained a corridor ∼10 km-wide, roughly parallel to the coast, along which waters from the continental shelf leave the gulf. This is confirmed by high-resolution SST imagery. The use of this sampling strategy to explore surface transport structures may provide important information for the environmental management of coastal regions, and may serve for validating future coastal altimetric products.
    Print ISSN: 0094-8276
    Electronic ISSN: 1944-8007
    Topics: Geosciences , Physics
    Published by Wiley on behalf of American Geophysical Union (AGU).
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  • 6
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    Numerical study of eddy generation in the western part of the Gulf of Lion (2011)
    Wiley
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    Publication Date: 2011-12-21
    Description: A realistic numerical model is used to investigate the generation process of anticyclonic eddies located in the western part of the Gulf of Lion. During 8 years of simulations from 2001 to 2008, 8 anticyclonic coastal eddies with a life duration longer than 15 days have been observed in the study area between July and early October. The formation process of eddies is linked to the wind stress curl in the area. Nonetheless a simpler criteria can be used examining the changes in the wind amplitude at a key coastal station. The influences of this factor and of the stratification conditions over the study area are investigated, alone or combined, regarding the eddy's formation process. Our results show that these eddies need two conditions to be generated: a persistent and strong northwest wind and a strong stratification. The Ekman transport associated to such a wind and the coastline shape characterized by the presence of capes can create a pressure gradient generating an anticyclonic circulation. At the same time, a strong stratification condition allows a better transfer of wind-induced potential energy to eddy kinetic energy. Persistent wind bursts are also required to sustain the eddy in size and intensity. The present work contributes to a better understanding of the hydrodynamics of the Gulf of Lion.
    Print ISSN: 0148-0227
    Topics: Geosciences , Physics
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  • 7
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    Impacts of mesoscale activity on the water masses and circulation in the Coral Sea (2016)
    Wiley
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    Publication Date: 2016-09-16
    Description: The climatological vision of the circulation within the Coral Sea is today well established with the westward circulation of two main jets, the North Caledonian Jet (NCJ) and the North Vanuatu Jet (NVJ) as a consequence of the separation of the South Equatorial Current (SEC) on the islands of New Caledonia, Vanuatu and Fiji. Each jet has its own dynamic and transports different water masses across the Coral Sea. The influence of mesoscale activity on mean flow and on water mass exchanges is not yet fully explored in this region of intense activity. Our study relies on the analysis of in situ , satellite and numerical data. Indeed we first use in situ data from the Bifurcation cruise and from an Argo float, jointly with satellite-derived velocities, to study the eddy influence on the Coral Sea dynamics. We identify an anticyclonic eddy as participating in the transport of NVJ-like water masses into the theoretical pathway of NCJ waters. This transfer from the NVJ to the NCJ is confirmed over the long-term by a Lagrangian analysis. In particular, this numerical analysis shows that anticyclonic eddies can contribute up to 70% to 90% of the overall eddy transfer between those seemingly independent jets. Finally, transports calculated using S-ADCP measurements (0-500 m) show an eddy-induced sensitivity that can reach up to 15 Sv, i.e, the order of the transport of the jets. This article is protected by copyright. All rights reserved.
    Print ISSN: 0148-0227
    Topics: Geosciences , Physics
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  • 8
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    Modeling the Wake of the Marquesas Archipelago (2018)
    Wiley
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    Publication Date: 2018-01-20
    Description: In this study, a high-resolution (∼2.5 km) numerical model was set up to investigate the fine-scale activity within the region of the Marquesas archipelago. This has never been performed before. The robustness of the model results is assessed by comparison with remote sensing and in situ observations. Our results highlight regions of warm waters leeward of the different islands with high eddy kinetic energy ( EKE ) on their sides. The analysis of energy conversion terms reveals contributions to EKE variability by wind, baroclinic and barotropic instabilities. The use of a geometry-based eddy detection algorithm reveals the generation of cyclonic and anticyclonic eddies in the wake of the largest islands, with both an inshore and offshore effect. Maximum eddy activity occurs in austral winter following the seasonality of both wind stress and EKE intensity. Most eddies have a radius between 20 and 30 km and are generally cyclonic rather than anticyclonic. Significant vertical velocities are observed in the proximity of the islands, associated with topographically-induced flow separation. Eddy trapping inshore waters are advected offshore in the wake of the islands. The overall influence of these fine-scale dynamics could explain the strong biological enhancement of the archipelago.
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    Topics: Geosciences , Physics
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  • 9
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    Role of Iron in the Marquesas Island Mass Effect (2019)
    Wiley
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    Publication Date: 2019
    Description: Abstract A remarkable chlorophyll‐a concentration (Chl, a proxy of phytoplankton biomass) plume can be noticed on remotely sensed ocean color observations at the boundary separating the equatorial mesotrophic from the subtropical oligotrophic waters in the central South Pacific Ocean. This prominent biological feature is known as the island mass effect of the Marquesas archipelago. Waters surrounding these islands present high macronutrient concentrations but an iron depletion. In this study, the origin of Chl enhancement is investigated using a modeling approach. Four simulations based on identical physical and biogeochemical forcings but with different iron sources are conducted and analyzed. Only simulations considering an iron input from the island sediments present similar patterns (despite being too weak) of vertical and horizontal Chl distributions as compared to biogeochemical‐Argo profiling float and satellite observations. In addition, simulations with no other iron input than the boundary forcings reveal the relative importance of remote processes in modulating the seasonal pattern of Chl around the archipelago through horizontal advection of nutrient‐rich waters from the equator toward the archipelago and vertical mixing uplifting deep nutrient‐rich waters toward the upper lit layer.
    Print ISSN: 2169-9275
    Electronic ISSN: 2169-9291
    Topics: Geosciences , Physics
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