Representation of Antarctic Coastal Polynyas in Ocean Climate Models: A Justification for Assimilation of Satellite-Derived Ice Concentration?

The representation of Antarctic coastal polynyas (AACPs) in global ice—ocean general circulation models (OGCMs) has a profound impact on long-term deep-ocean properties. Focus of this paper is on the model representation of the extent of coastal polynyas, in particular their local, regional and high- frequency behaviour. These are verified with the aid of daily ice concentration derived from satellite passive microwave data using the "NASA Team 2" (NT2) algorithm. Large systematic regional and temporal discrepancies arise, some of which are related to the type of convection parameterization used in the model. Arguing that the partially empirical model representation of ice concentration is generally the weakest part of a sea-ice component of an OGCM, the fresh-water flux forcing associated with melting and freezing in AACPs is being made more realistic by assimilating NT2 ice concentration where it comes to determining the open-water and ice-covered fraction in a model grid cell. For the case shown, the assimilated polynyas yield regionally substantially enhanced net freezing rates, slightly larger formation rates of Antarctic Bottom Water, and a more realistic strength of the Antarctic circumpolar current. The potential usefulness of such measure is discussed.