WESTERN BOUNDARY CURRENTS IN THE MARTIAN ATMOSPHERE - NUMERICAL SIMULATIONS AND OBSERVATIONAL EVIDENCE

Western boundary currents (hereafter WBCs) are an intensification of meridional fluid flow along the eastward facing flank of a boundary. They occur in the Earth's oceans, the best known instance being the Gulf Stream, and in the terrestrial atmosphere, an example of which is the East African Jet. We have investigated WBCs in numerical simulations of the Martian atmosphere, where they occur in the presence of large longitudinal topographical gradients, combined with the beta effect. We suggest that WBCs have already been simulated by other Martian atmospheric models, although not identified as such. The intensity of these currents is dependent on basic model parameters, notably surface drag. We show that for physically reasonable values of drag, frictional forces dominate the WBCs' behavior. The requirement of zero net cross-equatorial time-mean mass flow produces a ''return flow'' in simulations with relatively low surface drag, which acts in opposition to the Tharsis WBC. We also show that the intense flow associated with WBCs advects potential vorticity across the equator, although the presence of strong radiative damping inhibits low-level inertial instabilities that might result from this. Slope winds are found to have a profound effect on WBC structure, especially where they have a component parallel to the jet. In these cases, slope winds can cancel out or reinforce WBCs, depending on their direction. Enhanced low-level winds associated with WBCs may be a factor that influences the locations of dust storm generation. The high vertical shear associated with WBCs may also play a role in the maintenance and subsequent decay of these dust storms. Some observational evidence may exist for WBCs in the form of the alignment of bright depositional dust streaks, wave clouds, and in wind measurements taken during the parachute descent phase of Viking Lander 1 through the planetary boundary layer.