Baroclinic toroidal quasi-geostrophic vortices

We investigate the stability and nonlinear evolution of two tori of opposite-signed uniform potential vorticity, located one above the other, in a three-dimensional, continuously stratified, quasi-geostrophic flow. We focus on the formation of hetons as a result of the destabilization of the tori of potential vorticity. Hetons are pairs of vortices of opposite signs lying at different depths capable of transporting heat, momentum, and mass over large distances. Particular attention is paid to the condition under which the hetons move away from their region of formation. We show that their formation and evolution depend on the aspect ratio of the tori, as well as the vertical gap separating them. The aspect ratio of a torus is the ratio of its major (center line) radius to its minor (cross-sectional) radius. Pairs of thin opposite-signed potential vorticity tori self-organize into a large number of hetons. On the other hand, increasing the vertical gap between the tori decreases the coupling between the opposite-signed vortices forming the hetons. This results in a more convoluted dynamics where the vortices remain near the center of the domain.