Introducing katabatic winds in global ERA40 fields to simulate their impacts on the Southern Ocean and sea-ice

摘要

A medium resolution (10-20 km around Antarctica) global ocean/sea-ice model is used to evaluate the impact of katabatic winds on sea-ice and hydrography. A correction is developed to compensate for the drastic underestimation of these katabatic winds in the ERA40 reanalysis. This correction derives from a comparison over 1980-1989 between wind stress in ERA40 and those downscaled from ERA40 by the MAR regional atmospheric model. The representation in MAR of the continental orography surrounding the ocean, like the Transantarctic Mountains, and a specific parameterisation of roughness length in the planetary boundary layer yield a major improvement in the representation of katabatic winds along the coast of Antarctica. Wind stress directions at the first ocean point are remarkably similar in ERA40 and MAR, but MAR wind stress amplitudes are much greater. From this comparison, a scale factor constant in time (i.e. no seasonal variation) but spatially varying (decreasing off-shore over a distance of about 150 km) is created for the meridional and zonal wind stress components and adapted to the wind vector. The correction thus consists of a local amplification of the amplitude of the 6-hourly ERA40 wind vector components at ocean points near the coast. The impact of katabatic wind correction is investigated in 40-year long twin simulations of a global ocean/sea-ice model. The wind stress over polynyas is increased by a factor of 2, and amplitudes of sensible and latent air-sea heat exchanges are increased by 28% and 18%, respectively. Sea-ice thickness and ice-fraction near the coast of Antarctica show a marked decrease. The amplified katabatic winds also increase the extent of coastal polynyas by 24% (i.e. the total polynya area is augmented by 60,000 km(3) around Antarctica), and the winter sea-ice production in polynyas is greater by 42%. Outside polynyas, the impact is a reduction of sea-ice production in the Southern Ocean sea-ice pack. Impacts on the ocean circulation are also marked. Katabatic wind amplification strengthens the local overturning in coastal polynyas with a more intense transformation of Antarctic Surface Waters into colder and denser shelf waters (in total over all polynyas around Antarctica, the overturning reaches 4.7 Sv in annual mean, an increase of 1.8 Sv, and peaks to 6 Sv in winter). The modification of shelf water properties and of the zonal surface winds yields an increase of the amplitude of the seasonal cycle of the Antarctic Coastal Current.