THE MARTIAN ZONAL-MEAN CIRCULATION - ANGULAR-MOMENTUM AND POTENTIAL VORTICITY STRUCTURE IN GCM SIMULATIONS

Analysis of simulations performed with the NASA/Ames Mars GCM shows th at under dusty conditions the Northern Hemisphere winter solstice circ ulation becomes characterized by a zonally averaged state in which the potential vorticity at upper levels is very small outside of high lat itudes. The available observational data-in particular the 15-mu m obs ervations obtained by the Viking IRTM during the 1977 winter solstice global dust storm-provide evidence for changes in the Martian circulat ion that are basically like those found in the GCM. In the Mars GCM si mulations for dusty solstice conditions, an extremely intense and appr oximately angular-momentum-conserving Hadley circulation is responsibl e for creating the low potential vorticity configuration. This can be contrasted with the Venus-Titan numerical simulations discussed by All ison et al. in which quasi-barotropic eddies appear to be largely resp onsible for the existence of low potential vorticity in lower and midl atitudes. At a near-equinox season the simulated Mars circulation is g reatly weakened in comparison to that for solstice conditions, angular momentum is not approximately conserved by the mean meridional circul ation, and potential vorticity increases relatively smoothly away from the equator.