The Martian Zonal-Mean Circulation: Angular Momentum and Potential Vorticity Structure in GCM Simulations

Abstract

Analysis of simulations performed with the NASA/Ames Mars GCM shows that under dusty conditions the Northern Hemisphere winter solstice circulation becomes characterized by a zonally averaged state in which the potential vorticity at upper levels is very small outside of high latitudes. The available observational data-in particular the 15-µm observations obtained by the Viking IRTM during the 1977 winter solstice global dust storm-provide evidence for changes in the Martian circulation that are basically like those found in the GCM. In the Mars GCM simulations for dusty solstice conditions, an extremely intense and approximately angular-momentum-conserving Hadley circulation is responsible for creating the low potential vorticity configuration. This can be contrasted with the Venus-Titan numerical simulations discussed by Allison et al. in which quasi-barotropic eddies appear to be largely responsible for the existence of low potential vorticity in lower and midlatitudes. At a near-equinox season the simulated Mars circulation is greatly weakened in comparison to that for solstice conditions, angular momentum is not approximately conserved by the mean meridional circulation, and potential vorticity increases relatively smoothly away from the equator.