Vortex–Vortex Interactions in the Winter Stratosphere

Abstract

This paper examines the interaction of oppositely signed vortices in the compressible (non-Boussinesq) quasigeostrophic system, with a view to understanding vortex interactions in the polar winter stratosphere. A series of simplifying approximations leads to a two-vortex system whose dynamical properties are determined principally by two parameters: the ratio of the circulation of the vortices and the vertical separation of their centroids. For each point in this two-dimensional parameter space a family of equilibrium solutions exists, further parameterized by the horizontal separation of the vortex centroids, which are stable for horizontal separations greater than a critical value. The stable equilibria are characterized by vortex deformations that generally involve stronger deformations of the larger and/or lower of the two vortices. For smaller horizontal separations, the equilibria are unstable and a strongly nonlinear, time-dependent interaction takes place, typically involving the shedding of material from the larger vortex while the smaller vortex remains coherent. Qualitatively, the interactions resemble previous observations of certain stratospheric sudden warmings that involved the interaction of a growing anticyclonic circulation with the cyclonic polar vortex.