Numerical study of the air sea interactions in the Antarctic coastal zone and their implications on deep sea formation in the case of katabatic wind

Antarctica plays an important and particular role in the climate system. lts location (South Pole) is responsible of the formation of an ice sheet which covers entirely the continent. Because of the intense infrared radiative heat losses, cold dense air forms near the surface and flows down to the coast where its daily average speed reaches 200 km/hour at some places. Katabatic flow can thwart the synoptic weather situation and can maintain coastal water areas free of ice. In these ice free zones, strong heat exchanges occur between the cold atmosphere and the relatively warm ocean, with enhanced sea ice formation and salt rejection in the water column. This mechanism plays an important role in the deep sea water formaton and, because the ocean is an important reservoir of heat and CO2, it can affect the climate over long time scales. The purpose of the present work is the study of the ocean -atmosphere interactions in the Antarctic coastal zone under polar night conditions which are particularly favorable to katabatic winds development. The spatial evolution of katabatic winds along idealized and realistic slopes representative of Antarctic terrain is investigated using an atmospheric two-dimensional primitive equation model written in terrain following δ- coordinate and with high resolution. The surface heat balance of snow (which is assumed to recover sea ice and continental ice) is computed using a simple soil model. Three first simulations are done with the ocean being recovered by sea ice.The slope is either constant, either realistic (Adélie Land topography is then chosen). Much of the flow evolution is completed by 15 h integration period. Then a downslope momentum forces analysis of the katabatic flow is made, emphasizing on physical mechanisms in the coastal zone. The importance of the reversal of the pressure gradient force in the coastal zone, causing the sudden decay of katabatic winds and the onset of a "sea ice breeze" flow, is discussed. It is shown that the cause of this phenomenon is the pilling up of cold katabatic air in the coastal region. Second the consequences of the absence of sea ice cover on the whole ocean or in a small coastal polynia, are examined. The terrain profile is that of Adélie Land. It is been that a small water area free of ice does not influence the atmospheric circulation signficantly. On the contrary, the heating of the air by an underlaying completely ice free ocean is such that the " sea ice breeze ftow" in the coastal zone is weakened. Finally a polynia model is forced by the output of the atmospheric model in the case of an ocean free of ice. The computed salt fluxes in the polynia are sufficient so that deep sea water can form if the atmospheric circulation is maintained during 2 month