Cooperative Institute for Research in Environmental Sciences

Forcing of the wintertime antarctic boundary layer winds from the NCEP-NCAR global reanalysis

Antarctica is noted for strong and persistent winds in the lower atmosphere. The wind directions are controlled by the underlying ice terrain and are deflected in general 20 degrees -50 degrees to the left of the fall line. The Antarctic surface wind regime is thought to be the result of the dual influences of diabatic cooling of the terrain, responsible for the infamous katabatic winds, and the synoptic pressure gradient force in the free atmosphere. The relative importance of pressure gradients associated with katabatic and synoptic processes in forcing the wintertime Antarctic boundary layer winds is evaluated using output from the National Centers for Environmental Prediction-National Center for Atmospheric Research (NCEP-NCAR) global reanalysis program for June, July, and August of 1997. Both katabatic and synoptic forces are found to be significant in shaping the near-surface winter winds over the Antarctic ice slopes. Analyses show that the synoptic force is influenced by the underlying ice terrain such that the net force over Antarctica is directed primarily downslope. This result reflects the adjustment of the large-scale ambient pressure gradient to the continental orography. The synoptic force over Antarctica thus differs significantly in both magnitude and direction from that found over the oceanic regions to the north. The adjustment of the synoptic force over the ice sheets enable even the nonwinter Antarctic winds to attain a high directional constancy and resemble a gravity-driven flow. This process also suggests that direction alone is insufficient in classifying Antarctic flows as katabatic.