Winter polar warmings and the meridional transport on Mars simulated with a general circulation model

Winter polar warmings in the middle atmosphere of Mars occur due to the adiabatic heating associated with the downward branch of the cross-equatorial meridional circulation. Thus, they are the manifestation of the global meridional transport rather than of local radiative effects. We report on a series of numerical experiments with a recently developed general circulation model of the martian atmosphere to examine the relative roles of the mechanical and thermal forcing in the meridional transport. The experiments were focused on answering the question of whether the martian circulation is consistent with the thermally driven nearly inviscid Hadley cell, as was pointed out by some previous studies, or it is forced mainly by zonally asymmetric eddies. It is demonstrated that, under realistic conditions in the middle atmosphere, the meridional transport is maintained primarily by dissipating large-scale planetary waves and solar tides. This mechanism is similar to the “extratropical pump” in the middle atmosphere on Earth. Only in the run with artificially weak zonal disturbances, was the circulation reminiscent of thermally induced Hadley cells. In the experiment with an imposed dust storm, the modified atmospheric refraction changes the vertical propagation of the eddies. As the result, the Eliassen-Palm fluxes convergence increases in high winter latitudes of the middle atmosphere, the meridional transport gets stronger, and the polar temperature rises. Additional numerical experiments demonstrated that insufficient model resolution, increased numerical dissipation, and, especially, neglect of non-LTE effects for the 15 μm CO₂ band could weaken the meridional transport and the magnitude of polar warmings in GCMs.