Seasonal variations in low level flow in the NASA-Ames Mars GCM

The Martian Hadley circulation consists of two cells of different sizes at most times of the year, with one cell dominating near the solstices. The lower branch of this flow is strongly topographically controlled, especially east of the Tharsis plateau and Syrtis Major. Simulations using idealized topography show that longitudinal asymmetries in heating also play a role in forming the jets. Nearer the equinoxes, the total cross-equatorial flow intensity is reduced and the effect of the seasonal condensation flow is felt more strongly. Because of effects such as topography, zero cross-equatorial flow is not actually achieved at the equinoxes, but is slightly offset in time. Even when the mean flow is zero, the two jets are still present but flow in opposite directions: the Tharsis jet flows south and the Syrtis jet north. The latter is a result of the time-averaged effect of slope wind circulations. The two jets are still relatively intense at this time. For example, with an optical depth τ of 0.3, and at a height of 250 m, v̄ at L = 270° is about 9ms southward, while at 45°W v is 26 m s. At L = 350°, v̄ is 0.5 m s, while at 45°W it is still above 8 m s. The effect of atmospheric dust loading is also examined.