MARS SURFACE PRESSURE TIDES AND THEIR BEHAVIOR DURING GLOBAL DUST STORMS

Simulations of Mars' atmospheric tides with the NASA-Ames Mars general circulation model are presented and analyzed. In an annual simulation , diurnal and semidiurnal tides in the model's surface pressure field are shown to compare well with tides observed by the Viking landers th roughout the Mars year. Focused simulations of global dust storms duri ng the northern winter season show a semidiurnal tide that responds ve ry strongly to the increased dust load during the storms, again matchi ng well with observations. Analysis of the structure and behavior of t his tide during global storms verifies that it is dominated by the gra vest symmetric Hough mode (H-2(2)),as has been noted previously. Howev er, it is shown here that additional symmetric and asymmetric modes pl ay an important role in accounting for the structure of the tide and i ts variation during the dust storms. The phase advance observed betwee n the Viking lander sites (earlier semidiurnal phase at the higher-lat itude site) is explained by the presence of these higher-order modes. Finally, decomposition of the semidiurnal tide during a global dust st orm into its component Hough modes shows that the amplitude of the H-2 (2) mode mirrors changes in the observed overhead visible dust opacity , while variations in higher-order modes are indicative of the zonal ( and, possibly, longitudinal) distribution of airborne dust.