The Effect of Alternative Wind Forcing from Operational Analyses on the Simulations of Antarctic Coastal Sea Ice

An accurate representation of Antarctic coastal sea ice in Earth System Models is important as it affects the rate of Antarctic Bottom Water formation. Here we investigate the effect of using relatively high-resolution (~20 km) near-surface wind fields of two different operational analyses (from the European Centre for Medium-Range Weather Forecasts (ECMWF), and the Antarctic Mesoscale Prediction System (AMPS)) to force a sea-ice - ocean model. We focus on the wind impact on coastal polynya formation in winter. Even though the horizontal resolution of both analyses is about the same, striking differences emerge in simulated coastal sea ice and thus the representation of coastal polynyas and net freezing rates. In AMPS, offshore winds are up to 10 m s^-1 stronger than those of ECMWF, and mostly overestimated compared to weather station data. This leads to major differences in simulated coastal ice drift, concentration, and thickness, as well as discrepancies in the annual net freezing rate of up to 5 m yr^-1. To identify the reasons for the discrepancies in the coastal wind fields of the analyses we scrutinized their orography, boundary-layer stratification, sea-ice boundary conditions, large-scale atmospheric circulations, data assimilation and initialization schemes, and their parameterization of subgrid-scale orographic effects. The final factor explained most of the discrepancies along steep coastlines, whereas the differences in boundary-layer stratification are mainly a consequence of the differences in surface winds. The data assimilation and initialization procedure biased the AMPS wind fields toward those of the NCEP analyses, which probably overestimate offshore winds as a result of coarse-resolution orography. The sea-ice boundary conditions and the large-scale circulations turned out to have a minimal effect.