Vertical Alignment of Geostrophic Vortices

In a two-layer quasi-geostrophic numerical model, we study the vertical alignment of geostrophic vortices; we compare its efficiency for RVI conditions (a disk of constant relative vorticity in each layer initially) to that for PVI conditions (a disk of constant potential vorticity in each layer; Polvani, 1991). The disks of radius R are initially separated by a distance d, and R d is the internal radius of deformation. At small R d R and large d/R, a multipolar potential vorticity distribution is associated with these RVI conditions. This distribution is responsible for a much weaker efficiency of the alignment process in the RVI case than in the PVI case: indeed, the formation of horizontal or of vertical dipoles prevents alignment. Even more interesting is the case where alignment does occur: under PVI conditions, the end state is a simple vertical column of potential vorticity, while for RVI vortices it is a steadily rotating baroclinic tripole, a novel stationary feature of stratified geostrophic dynamics.