Atmospheric response to spatial variations in concentration and size of polynyas in the Southern Ocean sea-ice zone

Although it is well known that sea-ice regions are important components of the Earth's climate system, the exchanges of energy between ocean, ice and atmosphere are not well understood. The majority of past observational and modelling studies of atmosphere-surface interactions over sea-ice regions w ere primarily concerned with airflow over a single, isolated area of open w ater. The more realistic situations of multiple polynyas within a sea-ice f ield and different areal concentrations of sea ice were studied here. Spati al structure of the atmospheric boundary layer in response to this surface was simulated using a high-resolution numerical model. A sea-ice concentrat ion of 80%, typical of the Southern Ocean sea-ice zone, was maintained with in a 100-km wide domain. The effects of three polynya characteristics were assessed: their horizontal extent; local concentration of sea ice (LCI); an d their arrangement with ice floes. Over polynyas of all sizes distinct plu mes of upward heat flux, their width and height closely linked to polynya w idth, resulted in mixed layers 600 to 1000 m deep over and downwind of the polynyas, their depth increasing with polynya width. Mean surface heat flux (MSHF) increased with size in polynyas less than 30 km wide. The air-to-ic e MSHF over the first 10 km of sea-ice downwind of each polynya and the dom ain-average surface heat flux increased linearly with polynya width. Turbul ent kinetic energy plumes occurred over all polynyas, their heights and wid ths increasing with polynya widths. Downward flux of high momentum air in t he plumes caused increased wind speeds over polynyas in the layer from abou t 300-1000 m above the surface, the depth varying directly with polynya wid th. MSHFs decreased as LCIs increased. The arrangement of polynyas had rela tively little effect on the overall depth of the modified layer but did inf luence the magnitude and spatial structure of vertical heat transfer. In th e two-polynya case the MSHF over the polynyas was larger when they were clo ser together. Although the MSHF over the sea ice between the polynyas decre ased in magnitude as their separation increased, the percentage of the poly nya-to-air heat recaptured by this ice floe increased fivefold.