The Net Advective Effect of a Vertically Sheared Current on a Coherent Vortex

Abstract

The nonlinear interaction of a localized vortex with a vertically sheared mean flow has been studied using numerical and asymptotic methods in a multilayer quasigeostrophic model on the beta plane. Numerical solutions indicate that baroclinic large-scale flows have a weak influence on the translation of coherent vortices, even when the advective effect of the mean flow is important. This is opposed to what is observed in general for the dynamics of linear Rossby waves, which are sensitive to the presence of a baroclinic current. Thus, the nonlinear nature of vortices has to be taken into account to explain this reduction of the net advective effect of a vertically sheared current on a coherent vortex. The asymptotic method of Sutyrin and Morel is generalized to describe analytically the development of the beta gyres and corresponding vortex motion in the presence of a vertically sheared current. The initial vortex structure is prescribed as a piecewise constant potential vorticity anomaly in one or two layers with no motion in the lower layer. Three major effects are shown to contribute to the vortex translation: advection by the mean current, beta-gyre development due to planetary and mean flow potential vorticity gradients, and deformation of the vortex core. The analytical model predicts that the initial uniform gradient associated with the background current is strongly distorted and eventually ?homogenized,? leading to the cancellation of the current net advective effect. In other words, the part of beta gyres associated with the mean-flow potential vorticity gradient compensates most of the advection by the baroclinic part of the current. Thus, the vortex is advected mainly by the planetary beta gyres (and the barotropic part of the flow if any). The influence of the vortex size and strength on this compensation mechanism is evaluated.