FORCING OF THE AUSTRAL AUTUMN SURFACE PRESSURE CHANGE OVER THE ANTARCTIC CONTINENT

Pronounced seasonal variations in the surface pressure field are prese nt over the Antarctic continent. Surface pressures over the ice sheet decrease during the austral autumn period January-April and increase d uring the austral springtime months September-December. The largest ch anges are found over the highest portions of the Antarctic ice sheets where seasonal surface pressure changes of up to 20 hPa are common. Th e outstanding feature of these surface pressure changes is that typica lly the isallobaric contours closely follow the Antarctic orography du ring both transition periods, suggesting a strong seasonal diabatic ad justment within the lower troposphere. During austral autumn, the pron ounced cooling of the lower atmosphere adjacent to the ice sheets lead s to an enhancement of the Antarctic katabatic wind regime and hence t he lower branch of the mean meridional circulation over the high south ern latitudes. The mass transport provided by these drainage flows is proposed as the mechanism behind the autumn pressure falls. Numerical simulations of the evolution of the Antarctic katabatic wind regime in dicate that the radiative cooling of the sloping ice fields and attend ant mass transport result in a modification of the temperature and pre ssure fields in the lower troposphere similar to what is seen during t he early austral autumn period.