Sensitivity of Southern Ocean sea-ice simulations to different atmospheric forcing algorithms

Abstract

Sea ice is sensitively dependent on the fluxes of energy, mass and momentum between the ocean and the atmosphere, making it worth investigating the modification of these fluxes by the respective boundary layers. Complementary to earlier investigations with a coupled sea-ice—oceanic mixed-layer model for the Southern Ocean, the atmospheric forcing in the present investigation is changed from monthly, observational data to daily, essentially modelled values computed by an operational numerical weather-prediction model. Applying these computations directly as atmospheric surface forcing to the sea-ice—oceanic mixed-layer model yields (in first order) encouraging results, indicating the general reliability of these data. As a supplement to the oceanic mixed-layer model, the fluxes derived from the atmospheric forcing are modified in a first step to include the stability dependency of the atmospheric surface-layer. Compared to the application of usual adjustment practices, this leads to improved results, especially with respect to the ice velocities in divergent ice fields. In the next step, the atmospheric forcing level is raised to the geostrophic level thus incorporating the entire atmospheric boundary layer. While the forcing fields become less dependent on the prescribed boundary conditions of the weather-prediction model, the simulations appear to be reasonable only when the near-surface wind forcing is applied, the overall roughness length is increased and the large-scale stability is reduced. This leads to important implications for coupled atmosphere—sea-ice—ocean models.