High-Resolution Sea Ice in Long-Term Global Ocean GCM Integrations

The resolution of the sea-ice component of a coarse-resolution global ocean general circulation model (GCM) has been enhanced to about 22 km in the Southern Ocean. The ocean GCM is designed for long-term integrations suitable for investigations of the deep-ocean equilibrium response to changes in southern hemisphere high-latitude processes. The space and time scales of the high-resolution sea-ice component are commensurate with those of the resolution of satellite passive-microwave sea-ice data. This provides the opportunity for a rigorous evaluation of simulated sea-ice characteristics. It is found that the satellite-derived continuous high ice concentration of the interior winter ice pack can only be captured when vertical oceanic mixing is modified in a way that less local, intermittent convection occurs. Furthermore, the width and the variability of the coastal polynyas around the Antarctic continent and its ice shelves is best captured when some form of ice-shelf melting is accounted for. The width of the winter-time ice edge is reasonably reproduced, while its fine-scale variability remains underestimated, closely following the coarse-grid pattern of the ocean model due to its high dependence on the ocean temperature. Additional variability besides daily winds, e.g. in an idealized form of tidal currents, improves the temporal and spatial ice-edge variability, while leads in the interior ice pack become more abundant, closer resembling the fine-scale satellite-derived texture. The coast- or ice shelf line is described on the fine grid based on satellite passive-microwave data. This method requires parts of a coarse coastal ocean grid cell to be covered by an inert layer of "fast ice" or "ice shelf". Reasonable long-term global deep-ocean properties can only be achieved when these areas are not inert, i.e. are exposed to heat flux and ice growth, or when the vertical mixing parameterization allows for excessive open-ocean convection. The model area exposed to cold high-latitude atmospheric conditions being most decisive for a realistic representation of the global thermohaline circulation suggests that high-latitude coastlines are definitely in need of being represented at high resolution, including ice sheets and their effects on the heat and freshwater flux for the ocean.