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A Simple Model for Sheddies: Ocean Eddies Formed from Shed Vorticity

O. R. Southwick Department of Mathematics, University College London, London, United Kingdom

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E. R. Johnson Department of Mathematics, University College London, London, United Kingdom

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N. R. McDonald Department of Mathematics, University College London, London, United Kingdom

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Print Publication:
01 Oct 2016
DOI:
https://doi.org/10.1175/JPO-D-15-0251.1
Page(s):
2961–2979
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Abstract

Recent studies show that vertical eddy diffusivity is sufficient on its own to introduce intense horizontal shear layers at sloping ocean margins (Molemaker et al.; Gula et al.; Dewar et al.). These layers influence mesoscale energy and potential vorticity budgets but cannot be fully represented in models without sloping boundaries, no-slip boundary conditions, and sufficiently high resolution. This paper investigates the detachment of these shear layers and their subsequent rolling up into concentrated eddies. These shed eddies, or “sheddies,” may have significant oceanographic impacts. Their growth is considered using a simple point vortex model that adapts the Brown–Michael model of vortex shedding to quasigeostrophic flow and allows detailed consideration of the vorticity fluxes. The model shows good qualitative agreement with observations and experimental and numerical results. It is applied to a number of examples of well-known cases of sheddy formation, including the Agulhas cyclones, California Undercurrent, and Canary Eddy Corridor, and also is used to investigate the effects of shed vorticity in the growth of the Cook Strait eddy and the interaction of the North Brazil Current rings with the islands of the Lesser Antilles.

Denotes Open Access content.

This article is licensed under a Creative Commons Attribution 4.0 license.

Corresponding author address: Oliver Southwick, Department of Mathematics, University College London, Gower St., London, WC1E 6BT, England. E-mail: oliver.southwick.11@ucl.ac.uk

Abstract

Recent studies show that vertical eddy diffusivity is sufficient on its own to introduce intense horizontal shear layers at sloping ocean margins (Molemaker et al.; Gula et al.; Dewar et al.). These layers influence mesoscale energy and potential vorticity budgets but cannot be fully represented in models without sloping boundaries, no-slip boundary conditions, and sufficiently high resolution. This paper investigates the detachment of these shear layers and their subsequent rolling up into concentrated eddies. These shed eddies, or “sheddies,” may have significant oceanographic impacts. Their growth is considered using a simple point vortex model that adapts the Brown–Michael model of vortex shedding to quasigeostrophic flow and allows detailed consideration of the vorticity fluxes. The model shows good qualitative agreement with observations and experimental and numerical results. It is applied to a number of examples of well-known cases of sheddy formation, including the Agulhas cyclones, California Undercurrent, and Canary Eddy Corridor, and also is used to investigate the effects of shed vorticity in the growth of the Cook Strait eddy and the interaction of the North Brazil Current rings with the islands of the Lesser Antilles.

Denotes Open Access content.

This article is licensed under a Creative Commons Attribution 4.0 license.

Corresponding author address: Oliver Southwick, Department of Mathematics, University College London, Gower St., London, WC1E 6BT, England. E-mail: oliver.southwick.11@ucl.ac.uk
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