ON THE FORCING OF SEASONAL-CHANGES IN SURFACE PRESSURE OVER ANTARCTICA

A 10-year record (1985-1994) of output statistics from the European Ce ntre for Medium-Range Weather Forecasts (ECMWF) model shows that profo und seasonal changes in surface pressure take place over the Antarctic continent. The most pronounced changes occur during the periods strad dling the brief Antarctic summer, fr-om September to December and agai n from January to April. Surface pressures atop the high Antarctic pla teau often display changes in excess of 20 hPa during these periods. T emperatures in the lower troposphere also exhibit marked changes durin g these transitional periods; surface temperature changes during these 3-month periods reach a maximum near 40 K over the high interior of A ntarctica. Hydrostatic considerations suggest that the thermal adjustm ent in the lowest levels of the atmosphere alters the vertical distrib ution of pressure with height and hence is consistent with the dramati c seasonal surface pressure changes over the elevated Antarctic ice sh eets. A strong interplay exists between the thermal forcing and the ka tabatic wind circulation over the continent. The large seasonal change s in solar insolation reaching the Antarctic ice surface modulate the intensity of the katabatic wind regime and thus the resulting mean mer idional circulation between the continent and the subpolar latitudes. It is proposed that the diabatic adjustment in the lower levels of the atmosphere over Antarctica disrupts the mean meridional circulation c reating a seasonal mass imbalance and hence surface pressure changes. It is through the meridional transports that the mass and wind fields reach a quasi-equilibrium adjusted state. The seasonal mass movement o ver Antarctica requires large-scale mass compensation over much of the southern hemisphere and shows that the diabatic influences at the Ant arctic surface have far-field impacts.