REGULAR BAROCLINIC TRANSIENT WAVES IN A SIMPLIFIED GLOBAL CIRCULATIONMODEL OF THE MARTIAN ATMOSPHERE

Meteorological observations from the Viking landers showed that barocl inic waves an Mars are much more periodic than those seen on Earth. A global circulation model of the Martian atmosphere, with simplified pa rametrizations of heating and friction, has been used to examine baroc linic waves in the parameter range appropriate to the Martian atmosphe re. Two distinct flow regimes are found to be dependent on the magnitu de of thermal and frictional dissipation. The flow is composed of smal l-amplitude disturbances propagating around a zonally oriented midlati tude jet in an irregular fashion or a highly persistent large-amplitud e zonal wavenumber 3 wave. The large-amplitude waves are much like tho se seen in differentially heated rotating annuli. It is shown that the large amplitude waves are not the linearly most unstable modes of the system, rather nonlinear effects are dominant. Attractor reconstructi ons, using empirical orthogonal functions, show the regime transition as a bifurcation from a messy aperiodic attractor to, what is essentia lly, a limit cycle. Analysis of the potential vorticity budgets of the large-amplitude waves shows that a large free mode component is prese nt. A mechanism is suggested whereby the sources and sinks of potentia l vorticity balance departures from the free mode form, thus maintaini ng the large-amplitude waves.