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

Abstract

The polar regions play an important role in the climate system but energy exchanges between the atmosphere and surface in polar sea ice regions are not 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 (widths of 10, 20, 30, 40, and 50 km) surrounded by sea ice in the Southern Ocean, rather than the colder Arctic environment which is the focus of most past work. Previous work has concentrated on surface turbulent fluxes. Here we present data on momentum, heat, and moisture fluxes up to 800 m. Shear and 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 convergence and ascent. Surface drag on the atmosphere over the water increased due to the accelerated wind and increased drag coefficient. The surface heat flux increased for distances up to 20 km over open water owing to the increase in the heat transfer coefficient and wind speed. Beyond the point where the wind speed reached a maximum, the decrease in surface-air temperature difference, which decreased with distance from the upwind ice edge, became 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. Copyright 1999 by the American Geophysical Union.