On the representation of sea ice in global ocean general circulation models

The performance of the sea ice component of two ocean general circulation models (OGCMs) is investigated under quasi-identical forcing and boundary conditions and compared with the performance of a state-of-the-art stand-alone sea ice model. The latter reproduced realistic sea ice characteristics under the same external conditions. All three sea ice models employ a viscous-plastic constitutive law to describe the variation in internal ice stress in the momentum balance. The individual thermodynamic formulations were unified to provide consistent reference versions for this investigation. The sea ice models are compared under various conditions to detect first-order discrepancies. Finally, the treatment of the sea ice component in global OGCMs is discussed in a more general context, illustrating the effect of some simplifications commonly used in OGCMs. We focus on the Southern Ocean, where sea ice plays a critical role in bottom water formation. Our studies show that sea ice in present-day global OGCMs can be formulated with the same quality as stand-alone sea ice models designed for specific regional studies, without the sacrifice of notable extra computation time. A standardization of both the dynamic and the thermodynamic part of an OGCM sea ice component turns out to be necessary to prevent unrealistic upper boundary conditions in forced OGCM simulations and distorted sensitivities in coupled simulations.