ASSOCIATIONS BETWEEN ANTARCTIC KATABATIC FLOW AND THE UPPER LEVEL WINTER VORTEX

We address the extent to which modifications to the surface wind field over Antarctica impact on the upper level winter vortex. This is approached by performing two sensitivity studies with a general circulation model. In the first experiment we have eliminated the katabatic outflow from the continent by removing the slope (i.e. we removed the topography of Antarctica). In the second experiment we weaken the surface flow by increasing the surface momentum drag coefficient over Antarctica. The results show a weakened upper level circulation in the no-topography experiment but a strengthened vortex in the increased drag experiment. The different response is related to the effect of the boundary forcings on the thermal structure of the atmosphere. In the no-topography simulation, significant warming occurs throughout the Antarctic troposphere and this is apparently associated with changes in cyclone behaviour. This appears to be the dominant effect on the upper level vortex. In the drag case the impact on temperature is much smaller but is still an important influence on the upper level circulation. We find that there is no simple, unambiguous relationship between the strength of the katabatic flow and the depth of the upper level vortex; it is clear that thermodynamic processes are important in determining the latter.