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La dynamique du courant profond de bord ouest dans l'Atlantique Nord et son influence sur la circulation méridienne moyenne

Abstract : The present study tackles the Deep Western Boundary Current (DWBC) dynamics in the North Atlantic basin as its impact on the AMOC. The DWBC advects dense water masses equatorward, produced in the subpolar gyre, and is one of the major component of the Atlantic Meridional Overturning Circulation (AMOC). This circulation contributes to the northward heat transport to high latitudes and allows to stabilise climate. When computing the AMOC in different ocean general circulation models (OGCM), results cover a wide range of intensity, spatial shape and temporal variability. Such response diversity is due to several factors. One of them is the remaining uncertainty on the link between dense water formation due to convection in the subpolar gyre, which contributes to connect the AMOC upper and lower branches, and the AMOC intensity at mid-latitudes. Those uncertainties are largely due to the knowledge gap of the deep circulation in North Atlantic because its direct observation is difficult and incorrectly reproduced in ocean models with a low spatial resolution. The methodology used rely on realistic numerical simulations based on the NEMO ocean general circulation model. Three configurations with an increasing spatial resolution have been developped using the grid refinement tool AGRIF : a global grid at 1/2◦ resolution (ORCA configuration), within which a first refined grid at 1/8◦ covering the whole North Atlantic (ERNA configuration) in which a second grid at 1/32◦ over the subpolar gyre (FER configuration). Both ERNA and FER are advanced and original by two aspects; they include a Sea-Ice model within embedded grids and FER reaches a high horizontal resolution over the subpolar gyre. We study the spatial horizontal resolution impact on the mean circulation in the North Atlantic with a focus on the AMOC contrasting simulations obtained with ORCA and ERNA solutions. Increasing the resolution improves the western boundary current dynamics at surface and depth. Indeed, the DWBC transport is intensified by 8Sv in the subpolar gyre partly due to a better representation of overflows coming from Nordic Seas through the Denmark Strait. Furthermore in ORCA the DWBC flows to the south along the Mid-Atlantic ridge ; in ERNA the flow along western continental shelf is dominant while a secondary circulation within the subpolar gyre arises being in better agreement with observations. The path followed by the DWBC along the continental shelf allows an interaction between surface and deep currents which seems to result both in an AMOC intensification and a maxima located close to 35 ̊N. This result tends to limit the influence of the convection, occuring at high latitudes, on the AMOC intensity at mid latitudes, often raised, and shed light on a modulation process of the AMOC intensity through the surface and deep currents interaction. We then addressed the thermohaline and the dynamical structure of the DWBC, asssocia- ted with the mesoscale representation, within the Labrador Sea using the FER configuration. With this configuration, which solved explicitly mesoscale eddies in the subpolar gyre, tempera- ture and salinity drift are clearly reduced compare to ERNA. Furthermore the vertical DWBC structure, especially its barotropisation from the eatstern to western side of the AR7W section within the Labrador Sea, is in very good agrement with observations. Using a simplified equation for relative vorticity, we try to identify the main processes handling the DWBC dynamics. The analysis reveals that the stretching associated with vertical velocities above topography and exchanges between isopycnal layers within boundary current dominate the vorticity balance. We also identify two areas within the DWBC where diapynal flux occur : along the Labrador Current on the western side of the Labrador Sea and seaward of Cape Desolation where eddy activity is marked. These results are close to two previous studies based on conceptual model and […]
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Submitted on : Tuesday, August 27, 2019 - 12:33:52 PM
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Claude Talandier. La dynamique du courant profond de bord ouest dans l'Atlantique Nord et son influence sur la circulation méridienne moyenne. Océanographie. Université de Bretagne occidentale - Brest, 2015. Français. ⟨NNT : 2015BRES0020⟩. ⟨tel-01252595v2⟩



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