Forcing of the Austral Autumn Surface Pressure Change over the Antarctic Continent*

Abstract

Pronounced seasonal variations in the surface pressure field are present over the Antarctic continent. Surface pressures over the ice sheet decrease during the austral autumn period January–April and increase during the austral springtime months September–December. The largest changes are found over the highest portions of the Antarctic ice sheets where seasonal surface pressure changes of up to 20 hPa are common. The outstanding feature of these surface pressure changes is that typically the isallobaric contours closely follow the Antarctic orography during both transition periods, suggesting a strong seasonal diabatic adjustment within the lower troposphere. During austral autumn, the pronounced cooling of the lower atmosphere adjacent to the ice sheets leads to an enhancement of the Antarctic katabatic wind regime and hence the lower branch of the mean meridional circulation over the high southern 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 indicate that the radiative cooling of the sloping ice fields and attendant mass transport result in a modification of the temperature and pressure fields in the lower troposphere similar to what is seen during the early austral autumn period.