MESOSCALE ATMOSPHERIC CIRCULATIONS OVER THE SOUTHWESTERN ROSS SEA SECTOR, ANTARCTICA

In this study the mesoscale atmospheric circulation over the southwest ern Ross Sea sector during winter is examined. The hydrostatic meso-ga mma-scale atmospheric model MAR (Modele Atmospherique Regional) is use d. Polar night is assumed, and an idealized large-scale situation is p rescribed, with zero geostrophic forcing. The impact of a partial sea- ice cover on the atmospheric circulation is assessed by prescribing le ad fractions in the range of the observed values (i.e., between 0% and 30%). Simulations show that the propagation of katabatic airstreams o ver Terra Nova Bay is facilitated by the presence of leads because the identity of cold, dense, katabatic air is better marked in warmer env ironmental maritime conditions. Boundary layer fronts and mesocyclone activity are associated with the katabatic airstreams. They are enhanc ed by the presence of leads. In particular, when the lead fraction is prescribed to be between 20% and 30%, the model simulates mesocyclone intensities comparable to those observed. Taking into account that suc h a lead fraction is situated in the upper range of the observed value s in the central Ross Sea during winter, these results suggest that wi nter Ross Sea mesocyclones could nor always result from a pure mesosca le forcing. In contrast to the summer situation, no snow precipitation occurs for the simulated winter case, probably because of the too-low absolute humidity content of the air. Such model behavior is in agree ment with the observations, which reveal a summer precipitation maximu m at McMurdo Station on Ross Island. It is also found that the positio n of the simulated mesocyclone over Terra Nova Bay is nor sensitive to the lead fraction. Furthermore, this meteorological situation favors the advection of relatively mild and moist maritime air over a long di stance in the ice-sheet interior. This process, which is referred to a s a moist-air intrusion, could affect the Antarctic ice-sheet mass bal ance.