Enhancing the resolution of sea ice in a global ocean GCM

Abstract

Open water in sea ice, such as leads and polynyas, has a considerable impact on the long-term global deep-ocean properties and circulation. Its representation in ocean general circulation models (GCMs) that are designed for studies of the long-term thermohaline circulation, however, bears large uncertainties. Here, an attempt has been made to reduce such uncertainties by enhancing the resolution of the sea-ice component, while keeping the ocean component at coarse resolution to preserve the necessary efficiency of the GCM. In this study, the higher-resolved sea-ice component has been restricted to the Southern Ocean. Compared to the original model, the new version yields more detailed structures, such as a more detailed representation of coastal polynyas, a realistically sharp ice edge, and an overall enhanced lead fraction. The latter gives rise to a somewhat enhanced rate of Antarctic Bottom Water formation through enhanced near-boundary convection, which is reflected in slightly cooler and fresher global deep-ocean properties and a reduced Antarctic Circumpolar Current as a result of reduced open-ocean convection. Sensitivity studies reveal that it is not the overall enhanced lead fraction but rather the coastal katabatic winds that lead to this behaviour of the higher-resolution model. Artifacts resulting from the coarse-grid coastline were minimized in a separate model version where fine surface grid cells of fast ice were introduced following the fine-grid land/ice-shelf—sea-ice/ocean boundary of satellite-derived microwave data. This study represents an intermediate step toward resolving the sea-ice component of a global coarse-resolution ocean GCM on a scale of about 30 km.