An idealised model of Martian polar vortex variability

The time averaged winter polar vortex on Mars has been observed to have an annular structure, with a potential vorticity (PV) local minimum at the pole and a surrounding region of higher PV. This structure is known to be barotropically unstable; latent heat released by condensation of atmospheric CO₂ is thought to be the major forcing mechanism responsible for maintaining it. Whilst the time-averaged polar vortex is seen to take a smooth annular structure, reanalysis data suggest the instantaneous polar vortex is spatially patchy with localised regions of higher and lower PV rotating around the pole. Polar vortices are typically seen to have strong mixing barriers on their equatorward edges, where large PV gradients are present, however it is not known whether this differs for a patchy polar vortex such as on Mars. Given the close correlation between PV gradients and atmospheric horizontal mixing properties, it is thought that this patchiness may have significant effects on the transport of dust and trace gases within Mars’ polar regions.

Here we present results from a novel modelling approach aiming to represent a potential driver of polar vortex patchiness and its impacts on atmospheric mixing. The shallow water equations are solved on a sphere, with additional terms representing a zonally symmetric radiative forcing, and spatially variable CO₂ condensation. A new finite element model, Gusto, is used; this has the potential for future work to utilise adaptive resolution meshes. The effect of the spatially variable latent heating representation is analysed, in the context of the resulting PV patchiness, using metrics such as the eddy enstrophy. A passive tracer is included in the model, allowing a visualisation of horizontal transport across the polar vortex. Differences in mixing properties arising from differing extents of patchiness in the vortex may help explain temporal variations in dust deposits across polar regions, which are visible in the polar layered deposits and may help increase knowledge of Mars’ paleoclimate.