A simplified (quasi-geostrophic, beta-plane) dynamical model of the zonal-mean flow is used to investigate the possible effects of breaking internal gravity waves on the circulation in the middle atmospheric region (~10--100 km altitude) of Mars. The gravity wave parameterization employed is basically the saturation parameterization of Lindzen (1981) which has been utilized in several terrestrial modeling studies. Numerical simulations are performed for a range of plausible values of the gravity wave parameters and for different representations of the mean flow thermal forcing, with a focus on the effects of topographically forced stationary gravity waves on the winter hemisphere circulation. The results of the model simulations show that breaking gravity waves (of intermediate horizontal scales, ~100--1000 km wavelengths) could act to ''close off'' the winter westerly circumpolar jet in the ~40--80 km altitude region and induce strong high-latitude warming at these levels. The latter could explain recent thermal measurements indicating that Martian winter polar latitudes may be very warm, far above radiative equilibrium, in this middle atmospheric region <Deming et al., 1986>.

For the most plausible range of parameter values breaking gravity waves do not induce much warming below ~40 km, consistent with available data showing the winter high latitudes are normally very cold at lower levels. Several ''dust storm'' cases indicate that breaking gravity waves possibly could act to induce a very large polar warming at relatively low altitudes, such as was observed during the 1977 winter solstice dust storm. However, quite large gravity wave amplitudes and/or relatively long horizontal wavelengths are required for such strong low-level effects; thermal forcing associated with dust heating might be a source for such anomalous gravity during a global dust storm. Summer simulations indicate that breaking gravity waves might alter the mean flow at somewhat higher levels than in the winter hemisphere, as a consequence of the presence of westerlies in the lower atmosphere. This could explain the presence of a warm summer pole in the recent 50--80 km thermal measurements of Deming et al. Several aspects of the results relating to the middle atmospheric circulation and the polar warming phenomenon are discussed. The possible importance of breaking gravity waves for constituent transport in the middle atmosphere of Mars is also briefly considered. ¿ American Geophysical Union 1990