Formation and maintenance of a polynya in the Weddell Sea

A dynamic-thermodynamic sea ice-mixed layer model for the Weddell Sea is co mplemented by a simple, diagnostic model to account for local sea ice-atmos phere interaction. To consider the atmospheric influence on the oceanic mix ed layer. the pycnocline upwelling velocity is calculated using the theory of Ekman pumping. In several experiments, formation and conservation of a p olynya in the Weddell Sea are investigated. Intrusion of heat into the lowe r atmosphere above the polynya area is assumed to cause a thermal perturbat ion and a cyclonic thermal wind field. Superposed with daily ECMWF surface winds, this modified atmospheric forcing field intensifies oceanic upwellin g and induces divergent ice drift. Simulation results indicate that in case of a weak atmospheric cross-polynya flow the formation of a thermal wind f ield can significantly extend the lifetime of a large polynya. The repeated occurrence of the Weddell polynya in the years 1974-76 thus appears to be an effect of feedback mechanisms between sea ice, atmosphere, and oceanic m ixed layer.