Baroclinic Tripolar Geostrophic Vortices

In the Bay of Biscay, large anticyclonic vortices (called swoddies) have been observed to form surface-intensified tripoles. Here we idealize this process in a two-layer quasi-geostrophic model. First, we compute the growth rate of elliptic disturbances on circular baroclinic vortices. Such perturbed vortices, either with a continuous or a piecewise-constant potential vorticity profile, are then used as initial conditions for nonlinear simulations in a numerical quasi-geostrophic model at high Reynolds number. Finite-amplitude evolutions yield baroclinic dipoles in the case of strong instability, but also uniformly rotating states, such as surface-intensified or arch-shaped tripoles and orthogonal elliptical vortices, when the instability is moderate. These baroclinic tripoles are novel stationary solutions of the stratified geostrophic dynamics. Their long-term evolution is naturally stable, though an asymmetric breaking into a monopole and a dipole can be induced by increased dissipation or by beta-effect.