Downward Wave Propagation on the Polar Vortex

This paper considers the propagation of waves on the edge of a stratospheric polar vortex, represented by a three-dimensional patch of uniform potential vorticity in a compressible quasigeostrophic system. Waves are initialized by perturbing the vortex from axisymmetry in the center of the vortex, and their subsequent upward and downward propagation is measured in terms of a nonlinear, pseudomomentumbased wave activity. Under conditions typical of the winter stratosphere, the dominant direction of wave propagation is downward, and wave activity accumulates in the lower vortex levels. The reason for the preferred downward propagation arises from a recent result of Scott and Dritschel, which showed that the three-dimensional Green’s function in the compressible system contains an anisotropy that causes a general differential rotation in a finite volume vortex. The sense of the differential rotation is to stabilize the upper vortex and destabilize the lower vortex. This mechanism is particularly interesting in view of recent interest in the downward influence of the stratosphere on the troposphere and also provides a possible conservative, balanced explanation of the formation of the robust dome plus annulus potential vorticity structure observed in the upper stratosphere.