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 reproduces well the climatology at the sites of Viking Lander I and II (VLl and VL2)
as inferred from the Viking Lander fall and spring observations. Its variability at those
sites is dominated by a 3-sol (Martian solar day) oscillation in the region of VLl and by
a 6-sol oscillation in that of VL2. These oscillations are forced by the zonal asymmetries
of the Martian mountain field. It is suggested that they contribute to the observed
variability by reinforcing the baroclinic oscillations with nearby periods identified in
observational studies.
The spatial variability associated with the orographically forced oscillations is
studied by means of extended empirical orthogonal function 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 arc in agreement with the
conclusions drawn from the VL2 observations
