HIGH-FREQUENCY OROGRAPHICALLY FORCED VARIABILITY IN A SINGLE-LAYER MODEL OF THE MARTIAN ATMOSPHERE

A shallow water model with realistic topography and idealized zonal wi nd forcing is used to investigate orographically forced modes in the M artian atmosphere. Locally, the model produces barotropic modes with p eriods within the broad range of periods observed at the sites of Viki ng Lander I and II (VL1 and VL2) during the fall and spring seasons. I ts variability at those sites is dominated by an oscillation of 3 Mart ian solar days (sols) in the region of VL1 and by a 6-sol oscillation in that of VL2. These oscillations are forced by the zonal asymmetries of the Martian mountain field. Their robustness with respect to chang es of the fundamental model parameters is examined. Since the exhibite d periods occur for a barotropic forcing field that is highly idealize d, it is difficult to say whether they have much to do with the real M ars, but their resemblance to some of-the periodicities present in the observed Martian climatology deserves further investigation. The spat ial variability associated with the orographically forced oscillations is studied by means of extended empirical orthogonal function (EEOF) analysis. The 3-sol VL1 oscillation corresponds to a tropical, eastwar d traveling, zonal wavenumber one pattern. The 6-sol VL2 oscillation i s characterized by two midlatitude, eastward traveling, mixed zonal wa venumber one and two and zonal wavenumber three and four patterns, wit h respective periods near 6.1 and 5.5 sols. The corresponding phase sp eeds are in agreement with some of the conclusions drawn from the land er observations. A linear stability analysis of the zonally asymmetric climatology reveals that the two most unstable modes are associated w ith periods near 3 and 6 sols; with the corresponding eigenvectors sho wing patterns consistent with the results of the EEOF analyses.