Formation and stability of tripolar vortices in stratified geostrophic flows

The formation, stationarity and stability of tripolar vortices is investigated in a two-layer quasi-geostrophic model. On the f-plane, these tripoles form from the barotropic and baroclinic instabilities of circular isolated vortices. Various horizontal and vertical potential vorticity distributions, both piecewise-constant and continuous, are considered here for these circular vortices. First, their linear stability is computed for normal-mode perturbations. Then, their nonlinear evolution with an elliptical initial perturbation is studied by means of a pseudo-spectral and of a contour surgery code. Two types of baroclinic tripoles are found to result from these instabilities: surface-intensified structures for dominant barotropic instability, arch-shaped tripoles for more baroclinic vortices. The stationarity of these tripoles and their stability to initial disturbances or to the action of beta-effect is finally investigated.