High-Frequency Orographically Forced Variability in a Single-Layer Model of the Martian Atmosphere

Abstract

A shallow water model with realistic topography and idealized zonal wind forcing is used to investigate orographically forced modes in the Martian atmosphere. Locally, the model produces barotropic modes with periods within the broad range of periods observed at the sites of Viking Lander I and II (VILI and VL2) during the fall and spring seasons. Its variability at those sites is dominated by an oscillation of 3 Martian 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 changes of the fundamental model parameters is examined. Since the exhibited periods occur for a barotropic forcing field that is highly idealized, it is difficult to say whether they have much to do with the real Mars, but their resemblance to some of the periodicities present in the observed Martian climatology deserves further investigation. The spatial 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, eastward traveling, zonal wavenumber one pattern. The 6-sol VL2 oscillation is characterized by two midlatitude, eastward traveling, mixed zonal wavenumber one and two and zonal wavenumber three and four patterns, with respective periods near 6.1 and 5.5 sols. The corresponding phase speeds are in agreement with some of the conclusions drawn from the lander observations. A linear stability analysis of the zonally asymmetric climatology reveals that the two most unstable modes are associated with periods near 3 and 6 sols; with the corresponding eigen-vectors showing patterns consistent with the results of the EEOF analyses.