This is the first in a series of papers that will discuss Mars atmospheric dynamics as simulated by the NASA Ames General Circulation Model (GCM). This paper describes the GCM's zonal-mean circulation and how it responds to seasonal variations and dust loading. The results are compared to Mariner 9 and Viking observations, and the processes responsible for maintaining the simulated circulation are discussed. At the solstices the zonal-mean circulation consists of a single cross-equatorial Hadley circulation between 30¿S and 30¿N. For relatively modest dust loadings (&tgr;=0.3), the associated peak mass flux is 100¿108 kg s-1 at northern winter solstice and 55¿108 kg s-1 at southern winter solstice. At both seasons, westerlies dominate the winter hemisphere, and easterlies dominate the summer hemisphere. Maximum zonal winds occur near the model top (~47 km) and are about the same at both seasons: 120 m s-1 in the winter hemisphere and 60 m s-1 in the summer hemisphere. Mean surface westerlies of 10--20 m s-1 are predicted at the middle and high latitudes of the winter hemisphere, as well as in the summer hemisphere near the rising branch of the Hadley circulation. The latter has the structure of a ''jet'' and is particularly strong (>20 m s-1) at northern winter solstice.

With increasing amounts of dust (up to &tgr;=5), the zonal mean circulation at northern winter solstice intensifies and gives no indication of a negative feedback. Dust can easily double the mass flux of the Hadley circulation. In the solstice simulations, the mean meridional circulation is the main dynamical contributor to the heat and momentum balance; the eddies play a relatively minor role. There is no evidence in these simulations for a polar warming. At the equinoxes the zonal mean circulation is more Earth-like and consists of two roughly symmetric Hadley cells with westerly winds in the mid-latitudes of each hemisphere and easterlies in the tropics. The simulated zonal winds are about half as strong as they are at solstice. However, the strength of the mean meridional circulation is much less than at solstice and averages between 5 and 10¿108 kg s-1. At these seasons, the eddies and mean circulation make comparable, but opposing, contributions to the heat and momentum balances. ¿ American Geophysical Union 1993