Vortex-vortex interactions in the winter stratosphere

Traditionally, stratospheric sudden warmings, rapid reductions in the circulation of the polar vortex, have been studied within the framework of wave-mean flow interaction, whereby planetary waves are generated in the troposphere and propagate vertically upward before breaking in the upper stratosphere. Here we investigate an alternative paradigm of stratospheric sudden warmings as vortex--vortex interactions, involving the nonlinear interaction of a strong cyclonic polar vortex with a weaker (but still significant) anticyclone, the latter associated with e.g. the Aleutian high or the systematic poleward transport of low-latitude air by irreversible processes.

We proceed by considering vortex--vortex interactions in the simplest possible relevant dynamical model: the 3D, nonlinear, compressible quasi-geostrophic equations on a polar f-plane, with oppositely signed spheroidal vortex patches representing the polar vortex and Aleutian high. The volume, vertical aspect ratio, circulation, and vertical offset of the vortices are all varied as external control parameters. Using an adiabatic technique we first obtain a family of stable equilibria of the two-vortex system, in which the vortices are flattened in the direction of alignment and tilt away from each other with increasing height, the larger (or lower for offset vortices) vortex always exhibiting the strongest deformation and often becoming concave towards the smaller (or higher) vortex. This behaviour is understood by consideration of the Green's function for the system.

As the horizontal separation of the vortices decreases the equilibria loose stability and we track the unsteady, nonlinear evolution using high resolution contour dynamics. We find that deformations of the polar vortex are larger and more destructive than those of the smaller anticyclone, which tends to remain coherent and approximately ellipsoidal. For certain parameter values, the shedding of material from the polar vortex results in the rapid reduction of the circulation that qualitatively resembles the stratospheric sudden warming. For a given circulation ratio of the two vortices the magnitude of the interaction, in terms of polar vortex deceleration, is peaked sharply with vertical offset, maximizing when the centroid of the anticyclone is slightly below that of the polar vortex. We find that even a relatively weak anticyclone (circulation one-fifth that of the polar vortex) at the appropriate vertical offset can induce significant polar vortex deceleration not dissimilar to that observed during minor stratospheric warmings.