ON THE REPRESENTATION OF SEA-ICE IN GLOBAL OCEAN GENERAL-CIRCULATION MODELS

The performance of the sea ice component of two ocean general circulat ion models (OGCMs) is investigated under quasi-identical forcing and b oundary conditions and compared with the performance of a state-of-the -art stand-alone sea ice model. The latter reproduced realistic sea ic e characteristics under the same external conditions. All three sea ic e models employ a viscous-plastic constitutive law to describe the var iation in internal ice stress in the momentum balance. The individual thermodynamic formulations were unified to provide consistent referenc e versions for this investigation. The sea ice models are compared und er various conditions to detect first-order discrepancies. Finally, th e 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 tha t sea ice in present-day global OGCMs can be formulated with the same quality as stand-alone sea ice models designed for specific regional s tudies, without the sacrifice of notable extra computation time. A sta ndardization of both the dynamic and the thermodynamic part of an OGCM sea ice component turns out to be necessary to prevent unrealistic up per boundary conditions in forced OGCM simulations and distorted sensi tivities in coupled simulations.