Transport and mixing across Mars's annular polar vortices

Steep potential vorticity (PV) gradients at the edge of Earth's stratospheric polar vortices cause a partial barrier to the mixing of polar and midlatitude air. This barrier is, for instance, instrumental in the formation of the Antarctic ozone hole. It has been recently shown that Mars's polar vortices, unlike Earth's, consist of a persistent local minimum in PV at the winter pole, with a region of high PV encircling it. This annular structure is barotropically unstable, but thought to be maintained by fast radiative time scales and latent heating caused by the condensation of CO₂, the major atmospheric constituent.

Here we present an analysis of transport and mixing properties of Mars's annular polar vortices using atmospheric reanalysis data, a comprehensive general circulation model (GCM), and idealized numerical integrations of the rotating shallow water equations. A "mean age" tracer within the GCM reveals that the formation of the polar PV minimum around the winter solstice is accompanied by a major decrease in the age of polar air, indicative of increased mixing. Horizontal mixing is further diagnosed using contour advection calculations, which reveal two regions of high equatorward filamentation, at the inner and outer edges of the high-PV ring, with a local mixing minimum between. The major transport properties identified in the reanalysis and GCM are reproduced in the shallow water system. We use this system to study the dependence of transport properties on the PV structure and relaxation timescales, and so to understand its seasonally-dependent behavior. Finally, we discuss implications for the transport of dust to polar regions, including connections to the structure of the polar layered deposits.