Control of the oscillations of the martian Northern Hemisphere polar vortex by the Hadley cell and topographic forcing

Abstract

The oscillations of the Martian Northern Hemisphere polar vortex are examined by using the Kida vortex model to interpret the dynamics of the vortex in reanalysis data. The vortex nutating around a fixed angle with little variability in its aspect ratio is shown to be controlled by the Hadley cell and wavenumber-2 planetary-scale waves that are characterised with a stationary component that is shown to be linked directly to Martian topography. The wave forcing and the Hadley cell structure are modelled as a strain and a rotational flow in Kida’s model. It is shown that the strain and rotational flow are strongly negatively correlated, acting against vortex elongation in Kida’s model. The result suggests that periods of high topographic forcing events are accompanied with a weakening in the Hadley cell winds and thus provides an explanation why vortex splits are rare on Mars.