Studying the Thermohaline Circulation with a Coupled Higher-Resolution Sea-Ice - Coarse-Resolution Ocean GCM

High-latitude Southern Ocean surface conditions have a profound impact on long-term deep-ocean properties and circulation. Due to the long adjustment time of the deep ocean, such impact can only be investigated with coarse-resolution (ice -) ocean general circulation models (GCMs). Critical processes such as lead formation and plume convection are thus not resolved by the governing physics of the model. The parameterization of such subgrid-scale features is associated with major uncertainties. To reduce the uncertainties associated with the model representation of leads, an attempt has been made to enhance the resolution of only the sea-ice component of a global ice - ocean GCM, thereby preserving the required efficiency of the GCM.

Preliminary experiments reveal various finer-scale structures in the Southern Ocean sea-ice pack, such as locally reduced ice concentration lee-ward of coastal extensions, regionally enhanced lead fraction, and, in association with this, enhanced net freezing rates, all compared to the reference simulation where the resolution of the sea-ice and the ocean component is identical. As a result, the formation rate of Antarctic Bottom Water, together with its intrusion into the world's ocean is slightly enhanced in the higher sea-ice resolution case. Sensitivity studies show that interpolation of the coarse-grid wind field to the finer sea-ice grid makes a crucial difference. This is due to the nonlinear ice dynamics being a function of the local wind stress gradients. An atmospheric boundary-layer feedback is proposed that modifies the momentum flux over the finer ice - ocean texture as a function of the finer scale leads, with implications for atmosphere - ice - ocean coupling on different spatial and temporal scales.