The Stability of Mars' Annular Polar Vortex

In common with Earth and several other planetary bodies, Mars' atmosphere exhibits regions of high potential vorticity (PV) near the winter pole, known as polar vortices. On Earth, PV generally increases monotonically from the equator to pole, however, on Mars there is a local minimum at the pole, with an annulus of high PV encircling it. Recently produced reanalyses of the Martian atmospheric circulation have confirmed that this annular vortex is a persistent feature, forming in autumn and lasting until spring. This finding is surprising since an isolated band of PV is barotropically unstable, a result going back to Rayleigh. Here we investigate the stability of an annular vortex using numerical integrations of the rotating shallow water equations. We show that the mode of instability and its growth rate strongly depend upon the latitude and width of the annulus. By introducing thermal relaxation with a time scale similar to that of the instability we are able to simulate a persistent annular vortex with similar characteristics as that observed in the Martian atmosphere. This time scale, typically 1-2 sols, is similar to thermal relaxation timescales which have been estimated for the Martian atmosphere. We also demonstrate that the persistence of an annular vortex is robust to topographic forcing, as long as it is below a certain amplitude. We hence propose that the persistence of this barotropically unstable annular vortex is permitted due to the combination of short radiative relaxation time scales and relatively weak topographic forcing in Mars' polar atmosphere. Implications of this annular vortex structure for the transport of trace species, dust, and aerosols into polar regions are also discussed.