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A Geostrophic Vortex over a Slope*
ABSTRACT
Nonlinear, quasigeostrophic, f-plane vortices in two layers over a topographic slope are considered. Scaling arguments suggest two parameters that dictate the effective strength of the slope: the first indicates the likelihood of dispersion at depth, and the second relates to baroclinic stability. If the deep flow is only weakly dispersive (weak slopes), an initially barotropic vortex can translate barotropically across the isobaths, provided the vortex scale exceeds the deformation scale. Over stronger slopes, the vortex separates into topographic waves and a stationary, surface-trapped vortex. An initially surface-trapped vortex larger than deformation scale becomes unstable over a weak slope, as it does over a flat bottom. However, a strong slope can stabilize the vortex to small perturbations, despite the large vortex scale. The effective slope parameters depend not only on topographic grade, but on vortex strength and size, and on the ambient stratification. Parameters obtained with representative oceanic values suggest that topographically induced vertical decoupling may be quite common.
1. Introduction
It is well known that there are eddies in the ocean, and that these features are often quite large, energetic, and possess significant vertical structure. They are important in terms of local current variability, and perhaps to the lateral transport of momentum and tracers. Gulf Stream rings and Agulhas eddies are two notable examples: both are of the order of 100 km in scale, are baroclinic, and possibly transport heat in the North and South Atlantic Oceans. Eddies such as these often interact with topography, either the continental shelf, ridges and canyons or even isolated seamounts, which in turn may lead to drastic structural alterations, and perhaps even to destruction. As such, a complete understanding of how eddies redistribute properties requires a knowledge of how topography affects their dynamics. The present work seeks a broad understanding of the effect of topography on such vortices.
Observations of eddy-topography interactions are numerous. An eddy may be substantially weakened while passing over steep topography, as seen by Cheney and Richardson (1976) for the case of a cold core ring south of the Gulf Stream, and Vidal et al. (1994) with a Loop Current eddy in the Gulf of Mexico. Vortices may possibly break up after hitting steep relief; Richardson...