Mesoscale atmospheric circulations over the southwestern Ross Sea sector, antarctica

Abstract

In this study the mesoscale atmospheric circulation over the southwestern Ross Sea sector during winter is examined. The hydrostatic meso-γ-scale atmospheric model MAR (Modèle Atmosphérique Régional) is used. Polar night is assumed, and an idealized large-scale situation is prescribed, with zero geostrophic forcing. The impact of a partial sea-ice cover on the atmospheric circulation is assessed by prescribing lead fractions in the range of the observed values (i.e., between 0% and 30%). Simulations show that the propagation of katabatic airstreams over Terra Nova Bay is facilitated by the presence of leads because the identity of cold, dense, katabatic air is better marked in warmer environmental maritime conditions. Boundary layer fronts and mesocyclonic activity are associated with the katabatic airstreams. They are enhanced 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 such a lead fraction is situated in the upper range of the observed values in the central Ross Sea during winter, these results suggest that winter Ross Sea mesocyclones could not always result from a pure mesoscale 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 agreement with the observations, which reveal a summer precipitation maximum at McMurdo Station on Ross Island. It is also found that the position of the simulated mesocyclone over Terra Nova Bay is not sensitive to the lead fraction. Furthermore, this meteorological situation favors the advection of relatively mild and moist maritime air over a long distance in the ice-sheet interior. This process, which is referred to as a moist-air intrusion, could affect the Antarctic ice-sheet mass balance. Corresponding author address: Dr. Hubert Gallée, Universite Catholique de Louvain, Institut d'Astronomie et de Géophysique G. Lemaître, Chemin du Cyclotron 2, B-1348 Louvain-la-Neuve, Belgium.