Zonally Symmetric Response to Atmospheric Tidal Forcing in the Dusty Martian Atmosphere

Abstract

We have computed a steady state, zonally symmetric response of an atmospheric circulation to the combined efffects of the very large zonal-mean diabatic heating and thermotidal forcing thought to exist in the dusty Martian atmosphere during one of its episodic global dust storms. The zonal-mean components of the tidal flux-convergences of momentum and heat are computed using an existing classical atmospheric tidal model constrained by the surface pressure observations at the two Viking Lander sites on Mars. The zonally symmetric response to the computed tidal flux-convergences and to the zonal-mean heating of the airborne dust is then computed by a nearly inviscid two dimensional (2-D) model previously used to study the Mars Hadley circulation. Tidally induced easterly forces at southern summer solstice can extend the core of the descending branch of the zonally symmetric circulation from 50° to 55°N, which is consistent with Viking Lander-2 surface pressure observations but not far enough poleward to produce an atmospheric warming like the one observed in 1977 by the Viking Orbiters. At low latitudes, the cross-equatorial (diabatic) circulation which would be driven by the zonal-mean diabatic beating alone is partially broken up, when viewed in the Eulerian framework, into several small circulation cells, stacked one above the other over the southern tropics. These cells are much less prominent in the residual-mean circulation, whose structure also differs from the diabatic circulation due in part to the ageostrophic tidal flux-convergence of meridional momentum, as well as to local thermotidal forcing and dissipation. For sufficiently large visible opacities (τ>2), the calculations suggest that the tidal flux-convergences will affect the zonal-mean advection of airborne dust into the Northern Hemisphere on Mars following the onset of a planetary-scale dust storm.