Interactive momentum flux forcing over sea ice in a global ocean GCM
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abstract
The sensitivity of Southern Ocean sea ice to the strength of the atmospheric momentum forcing is investigated in the framework of a global ocean general circulation model. In contrast to the usual approach of having the momentum flux just depend on the wind speed and a constant drag coefficient, the newly introduced momentum flux driving sea ice considers the local stratification and roughness over ice in one case, and the flux-aggregated stratification and roughness using the blending-height concept in the other case. While both cases thus allow for an interactive feedback, only the latter case accounts for the subgrid-scale heterogeneity of the sea-ice pack. In particular, the sea-ice feedback is in the former case only provided by the simulated ice thickness, affecting the surface temperature and local stratification, while in the latter case it is also determined by the ice concentration. Both parameterizations yield predominantly statically stable, but dynamically unstable conditions at any instant over the wintertime sea-ice pack. In the winter mean, statically and dynamically unstable conditions prevail over coastal polynyas, and lead to a positive feedback with increased momentum flux. The larger momentum flux enhances the along and offshore ice drift, leading to corresponding changes in the winter-mean ice-thickness distribution, a reduction in coastal ice concentration, and an increase of heat loss due to sensible heat flux. In the case where surface heterogeneity is accounted for, the impact of the lower coastal ice concentration leads to a larger momentum flux than in the homogeneous case. The long-term deep-ocean properties are only affected when in the heterogeneous case the form drag is raised by increasing the ice freeboard and decreasing the maximum ice concentration. Only the combination of both yields a significant increase of Antarctic Bottom Water formation, as reflected by a long-term cooling and freshening of the global deep-ocean properties. Copyright 2008 by the American Geophysical Union.
