A Case Study of Antarctic Katabatic Wind Interaction with Large-Scale Forcing*

Surface pressure decreases of up to 20 hPa occurred over much of the Antarctic continent during a 4-day midwinter period of 1988. The widespread change in the pressure field accompanied intense cyclonic activity to the north of the ice sheet. The equatorward mass transport across the Antarctic coastline resulted in a redistribution of atmospheric pressure that extended to the subtropics of the Southern Hemisphere. Most of the mass flux from Antarctica was the result of low-level processes and appears tied to the katabatic wind circulation. The observed surface pressure decrease over the continent reflects a perturbation of the mean meridional circulation between Antarctica and the subpolar latitudes by synoptic-scale processes. Zonally averaged circulations over Antarctica were examined using output from the European Centre for Medium-Range Weather Forecasts model. Results suggest that only a poorly defined return branch of the meridional circulation exists in the middle and upper troposphere. This southward-directed flow does not compensate for the northward mass transport provided by the katabatic wind outflow in the lower atmosphere. Isallobaric contours over the Antarctic ice sheet roughly match the area of the large-scale drainage catchment associated with katabatic wind transport through the Ross Sea sector. An intense extratropical cyclone was present in the circumpolar oceanic belt to the north of the continent. The horizontal pressure gradient associated with the cyclone prompted enhanced drainage off the high interior plateau. The resulting katabatic flow issued from the continent through a narrow corridor across the Ross Ice Shelf and out over the Southern Ocean.