The Variability of Drag Cofficients over the Polar Oceans Caused by Sea Ice Morphology and Atmospheric Stability

The interaction between atmosphere, sea ice and ocean is goverened by the near-surface fluxes of energy and momentum which depend on the atmospheric transfer coefficients. Over the polar oceans the latter are strongly influenced by sea ice morphology parameters like sea ice concentration, length of floes and melt ponds, ice freeboard and by the atmospheric stability. We apply a parameterization of drag coefficients distinguishing skin drag and form drag by floe and melt pond edges to satellite observed sea ice distributions and discuss first the distribution of neutral drag coefficients over the Antarctic and Arctic Ocean. Due to this parameterization the effective form drag coefficients in a model's grid cell depend on the stratification over sea ice and over open water with stability functions weighted with the respective ice and water fraction. Considered stability functions are those used in typical climate and weather prediction models (like ECHAM, ECMWF model) written as a function of the bulk Richardson number (Rib) following the Louis approach. We include here also a new Louis type formulation reproducing stability functions that have been derived on the basis of data from the Surface Heat Budget of the Arctic Ocean (SHEBA) campaign over the Beaufort Sea. It is shown that form drag reacts very sensitive on the used stability function. Differences between results for various stability functions are especially large for strongly stable conditions. Then the effect of stability on the drag coefficients and form drag alone is larger than the geographic variability. It is also shown that during summer with only weak stratification, the stability impact should not be neglected as well.