Local Dynamics of Baroclinic Waves in the Martian Atmosphere

The paper investigates the processes that drive the spatiotemporal evolution of baroclinic transient waves in the Martian atmosphere by a simulation experiment with the Geophysical Fluid Dynamics Laboratory (GFDL) Mars General Circulation Model (GCM). The main diagnostic tool of the study is the (local) eddy kinetic energy equation. Results are shown for an early period of the cold season, in which a deep baroclinic wave, extending from the surface (about 800~Pa on average) to 10~Pa, of zonal wavenumber two circles around the planet at an eastward phase speed of about 70$ degree/ Sol (Martian day) in the latitude belt 55 degree N--85 degree N. The regular structure of the wave gives the impression that the classical models of baroclinic instability, which describe the underlying process by a temporally unstable global wave (e.g., Eady-model and Charney model), may have a direct relevance for the description of the Martian baroclinic waves. The results of the diagnostic calculations show, however, that while the Martian waves remain zonally global features at all times (their amplitude is larger than zero everywhere along a latitude circle), there are large spatiotemporal changes in their amplitude. The most intense episodes of baroclinic energy conversion, which typically take place in the two great plain regions (Acidalia Planitia and Utopia Planitia), are strongly localized in both space and time. In addition, similar to the situation for terrestrial baroclinic waves, ageostrophic geopotential fluxes play an important role in the local dynamics of the waves. These properties of the Martian baroclinic waves suggest that their seeming similarity to the waves predicted by the classical models is deceptive, as some of the inherent limitations of those idealized models are not less important for Mars than Earth.