SENSITIVITY OF NUMERICAL PROGNOSES TO ANTARCTIC SEA-ICE DISTRIBUTION

To examine the effect of Antarctic sea ice concentration upon the atmosphere on synoptic timescales, a general circulation model was used hi conjunction with the Australian Bureau of Meteorology's Global Assimilation and Prediction system analyses and the Defense Meteorological Satellite Program's Special Sensor Microwave/Imager sea ice data to perform a suite of 5-day forecasts. As well as using a generic July sea ice concentration, further experiments were conducted with 0, 10, 25, 50, 80, and 100% sea ice concentrations. The 100% forecast was used as the control. Results show that the central pressure, structure and tracks of individual cyclones are sensitive to the ''switch on'' of different. sea ice conditions. Composites of all forecasts made with each concentration showed considerable and statistically significant anomalies in the surface temperatures and turbulent heat fluxes over the sea ice. The magnitudes of these changes varied monotonically with the area of open water. The largest changes were simulated closest to the coast for all concentrations, except for the generic July sea ice run, which displayed maxima over the outer pack. Significant westerly anomalies were also induced over the ice in all cases. Mean sea level pressure (MSLP) anomalies occurred in all forecasts. Again, the generic July sea ice run displayed a distribution of the MSLP anomaly different from all other runs, with maxima occurring in the central to outer pack. All other forecasts displayed maxima at the coast. These results suggest that sea ice concentration induces anomalies in the atmospheric parameters in timescales of less than 5 days. Further, use of a realistic distribution of sea-ice concentration produces results distinct from the constant concentration forecasts. Hence it is suggested that real-time Antarctic sea ice data may be of considerable benefit to numerical weather prediction and analysis.