Numerical modeling of atmospheric response to polynyas in the Southern Ocean sea ice zone

The polar regions play an important role in the climate system but energy e xchanges between the atmosphere and surface in polar sea ice regions are no t well understood, particularly in the observation-sparse Southern Ocean. A high resolution mesoscale numerical model of the planetary boundary layer was used to simulate mean winter atmospheric conditions over open water (wi dths of 10, 20, 30, 40, and 50 km) surrounded by sea ice in the Southern Oc ean, rather than the colder Arctic environment which is the focus of most p ast work. Previous work has concentrated on surface turbulent fluxes. Here we present data on momentum, heat, and moisture fluxes up to 800 m. Shear a nd increase in buoyancy above the polynya generated a turbulent plume that caused downward mixing of high momentum air. The consequent acceleration of the surface wind and horizontal divergence contributed to descent, despite thermal buoyancy, while downwind of the polynya, there was a region of con vergence and ascent. Surface drag on the atmosphere over the water increase d due to the accelerated wind and increased drag coefficient. The surface h eat flux increased for distances up to 20 km over open water owing to the i ncrease in the heat transfer coefficient and wind speed. Beyond the point w here the wind speed reached a maximum, the, decrease in surface-air tempera ture difference, which decreased with distance from the upwind ice edge, be came the dominant influence on heat flux. Transfer of heat from air to the ice downwind of a polynya increased with polynya width despite an increase in thermal stability of the lower atmosphere.