Why Does the Deep Western Boundary Current “Leak” around Flemish Cap?

Type Article
Date 2020-07
Language English
Author(s) Solodoch Aviv1, McWilliams James C.1, Stewart Andrew L.1, Gula Jonathan2, Renault Lionel3
Affiliation(s) 1 : Dept. of Atmospheric and Oceanic Sciences, University of California, Los Angeles, California, USA
2 : Univ. Brest, CNRS, IRD, Ifremer, Laboratoire d’Océanographie Physique et Spatiale (LOPS), IUEM, Brest, France
3 : LEGOS, University of Toulouse, IRD, CNRS, CNES, UPS, Toulouse, France
Source Journal Of Physical Oceanography (0022-3670) (American Meteorological Society), 2020-07 , Vol. 50 , N. 7 , P. 1989-2016
DOI 10.1175/JPO-D-19-0247.1

The southward flowing deep limb of the Atlantic Meridional Overturning Circulation is comprised of both the Deep Western Boundary Current (DWBC) and interior pathways. The latter are fed by “leakiness” from the DWBC in the Newfoundland Basin. However, the cause of this leakiness has not yet been explored mechanistically. Here the statistics and dynamics of the DWBC leakiness in the Newfoundland Basin are explored using two float data sets and a high-resolution numerical model. The float leakiness around Flemish Cap is found to be concentrated in several areas (“hotspots”) that are collocated with bathymetric curvature and steepening. Numerical particle advection experiments reveal that the Lagrangian mean velocity is offshore at these hotspots, while Lagrangian variability is minimal locally. Furthermore, model Eulerian-mean streamlines separate from the DWBC to the interior at the leakiness hotspots. This suggests that the leakiness of Lagrangian particles is primarily accomplished by an Eulerian-mean flow across isobaths, though eddies serve to transfer around 50% of the Lagrangian particles to the leakiness hotspots via chaotic advection, and rectified eddy transport accounts for around 50% of the offshore flow along the Southern Face of Flemish Cap. Analysis of the model’s energy and potential vorticity budgets suggests that the flow is baroclinically unstable after separation, but that the resulting eddies induce modest modifications of the mean potential vorticity along streamlines. These results suggest that mean uncompensated leakiness occurs mostly through inertial separation, for which a scaling analysis is presented. Implications for leakiness of other major boundary current systems are discussed.

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