Linear baroclinic instability in the Martian atmosphere: Primitive equation calculations

In this study, baroclinic-barotropic instability of the Martian atmosphere is studied for a zonal basic state based on Mariner 9 observations, using a spherical linear primitive equation model derived from a method of 3-D normal mode expansion. As a result of solving a matrix eigenvalue problem, a distinctly unstable mode at synoptic to planetary scales was found with a peak growth rate of 2.3 (1/sol) at zonal wavenumbers n = 5 to 6 with an eastward phase speed of 50 (°/sol). The unstable mode has a period of 2.4 sol for n = 3, which agrees well with Viking observations. The geopotential amplitude maximum is located at 50°N near the surface, and the phase tilts westward with height. The structure is similar to baroclinic instability of a Charney mode associated with the subtropical jet on Earth. It is found, however, that the mode on Mars, where the subtropical jet is absent, is not the ordinary Charney mode, but a different one that is referred to as a monopole Charney mode in previous study which is characterized by its overall northward eddy momentum flux.