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Mesoscale eddy dissipation by a “zoo” of submesoscale processes at a western boundary

Mesoscale eddy dissipation by a “zoo” of submesoscale processes at a western boundary
Mesoscale eddy dissipation by a “zoo” of submesoscale processes at a western boundary
Mesoscale eddies are ubiquitous dynamical features that tend to propagate westward and disappear along ocean western boundaries. Using a multiscale observational study, we assess the extent to which eddies dissipate via a direct cascade of energy at a western boundary. We analyze data from a ship‐based microstructure and velocity survey, and an 18‐month mooring deployment, to document the dissipation of energy in anticyclonic and cyclonic eddies impinging on the topographic slope east of the Bahamas, in the North Atlantic Ocean. These observations reveal high levels of turbulence where the steep and rough topographic slope modified the intensified northward flow associated with, in particular, anticyclonic eddies. Elevated dissipation was observed both near‐bottom and at mid depths (200–800 m). Near‐bottom turbulence occurred in the lee of a protruding escarpment, where elevated Froude numbers suggest hydraulic control. Energy was also radiated in the form of upward‐propagating internal waves. Elevated dissipation at mid depths occurred in regions of strong vertical shear, where the topographic slope modified the vertical structure of the northward eddy flow. Here, low Richardson numbers and a local change in the isopycnal gradient of potential vorticity (PV) suggest that the elevated dissipation was associated with horizontal shear instability. Elevated mid‐depth dissipation was also induced by topographic steering of the flow. This led to large anticyclonic vorticity and negative PV adjacent to the topographic slope, suggesting that centrifugal instability underpinned the local enhancement in dissipation. Our results provide a mechanistic benchmark for the realistic representation of eddy dissipation in ocean models.
direct energy cascade, eddy-topography interactions, energy, instability, mesoscale eddies, turbulence
2169-9275
Evans, Dafydd Gwyn
4f153214-2922-459c-9a34-661c851e2286
Frajka‐williams, Eleanor
2b2338d1-4a28-4785-9e13-c46c35a326b3
Naveira Garabato, Alberto C.
97c0e923-f076-4b38-b89b-938e11cea7a6
Polzin, Kurt L.
2424c950-d9b0-471c-95c0-b7b9d44132a1
Forryan, Alexander
4e753ae9-7f12-495f-933a-2c5a1f554a0e
Evans, Dafydd Gwyn
4f153214-2922-459c-9a34-661c851e2286
Frajka‐williams, Eleanor
2b2338d1-4a28-4785-9e13-c46c35a326b3
Naveira Garabato, Alberto C.
97c0e923-f076-4b38-b89b-938e11cea7a6
Polzin, Kurt L.
2424c950-d9b0-471c-95c0-b7b9d44132a1
Forryan, Alexander
4e753ae9-7f12-495f-933a-2c5a1f554a0e

Evans, Dafydd Gwyn, Frajka‐williams, Eleanor, Naveira Garabato, Alberto C., Polzin, Kurt L. and Forryan, Alexander (2020) Mesoscale eddy dissipation by a “zoo” of submesoscale processes at a western boundary. Journal of Geophysical Research: Oceans, 125 (11), [e2020JC016246]. (doi:10.1029/2020JC016246).

Record type: Article

Abstract

Mesoscale eddies are ubiquitous dynamical features that tend to propagate westward and disappear along ocean western boundaries. Using a multiscale observational study, we assess the extent to which eddies dissipate via a direct cascade of energy at a western boundary. We analyze data from a ship‐based microstructure and velocity survey, and an 18‐month mooring deployment, to document the dissipation of energy in anticyclonic and cyclonic eddies impinging on the topographic slope east of the Bahamas, in the North Atlantic Ocean. These observations reveal high levels of turbulence where the steep and rough topographic slope modified the intensified northward flow associated with, in particular, anticyclonic eddies. Elevated dissipation was observed both near‐bottom and at mid depths (200–800 m). Near‐bottom turbulence occurred in the lee of a protruding escarpment, where elevated Froude numbers suggest hydraulic control. Energy was also radiated in the form of upward‐propagating internal waves. Elevated dissipation at mid depths occurred in regions of strong vertical shear, where the topographic slope modified the vertical structure of the northward eddy flow. Here, low Richardson numbers and a local change in the isopycnal gradient of potential vorticity (PV) suggest that the elevated dissipation was associated with horizontal shear instability. Elevated mid‐depth dissipation was also induced by topographic steering of the flow. This led to large anticyclonic vorticity and negative PV adjacent to the topographic slope, suggesting that centrifugal instability underpinned the local enhancement in dissipation. Our results provide a mechanistic benchmark for the realistic representation of eddy dissipation in ocean models.

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2020JC016246 - Version of Record
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Accepted/In Press date: 2 November 2020
Published date: 21 November 2020
Keywords: direct energy cascade, eddy-topography interactions, energy, instability, mesoscale eddies, turbulence

Identifiers

Local EPrints ID: 445140
URI: http://eprints.soton.ac.uk/id/eprint/445140
ISSN: 2169-9275
PURE UUID: 431747cc-018b-40f6-9931-41e62006a104

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Date deposited: 23 Nov 2020 17:30
Last modified: 21 Nov 2021 21:08

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Contributors

Author: Dafydd Gwyn Evans
Author: Eleanor Frajka‐williams
Author: Kurt L. Polzin
Author: Alexander Forryan

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