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 nor mal mode expansion. As a result of solving a matrix eigenvalue problem, a d istinctly unstable mode at synoptic to planetary scales was found with a pe ak growth rate of 2.3 (1/sol) at zonal wavenumbers n = 5 to 6 with an eastw ard phase speed of 50 (degrees/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 degrees N near the surface, and the pha se tilts westward with height. The structure is similar to baroclinic insta bility of a Charney mode associated with the subtropical jet on Earth. It i s found, however, that the mode on Mars, where the subtropical jet is absen t, is not the ordinary Charney mode, but a different one that is referred t o as a monopole Charney mode in previous study which is characterized by it s overall northward eddy momentum flux.