Variability of the ice-ocean system in the Pacific sector of the Southern Ocean: Numerical model studies

Abstract

In the framework of BRIOS (Bremerhaven Ice Ocean Simulations) a coupled ice-ocean model of the Southern Ocean was adapted to investigate the variability of the ice-ocean system in the Pacific sector of the Southern Ocean with a particular focus on the factors affecting dense water formation on the Ross Sea continental shelf. The sea ice regimes in the western and eastern Ross Sea were shown to be decoupled from each other with sea ice characteristics in the western Ross Sea determined predominantly by the local atmospheric conditions and those in the eastern Ross Sea by ice import from the Amundsen Sea. The resulting strong gradient in sea ice formation provides the thermohaline driving force for the shelf circulation. The cooling, salt input and subsequent deep convection during the seasonal cycle modifies the waters of the shelf inflow so that their density is sufficient participate in Antarctic Bottom Water formation.Model results show that the region around Ross Island and McMurdo Sound has a key role in controlling the exchange between the ice shelf cavity and the open ocean in the Ross Sea. Drainage of High Salinity Shelf Water through McMurdo Sound into the cavity in winter prevents brine accumulation and thus lowers High Salinity Shelf Water salinities to the range observed. This also affects the balance of Ice Shelf Water and High Salinity Shelf Water, the two parent water masses for the formation of Antarctic Bottom Water in the Ross Sea. The interannual variability of dense water characteristics is, however, predominantly controlled by variations in the shelf inflow through a sub-surface salinity and a deep temperature signal whose origin can be traced into the Amundsen and Bellingshausen Seas. The temperature anomalies are induced in the western Bellingshausen Sea where the meridional transport of Circumpolar Deep Water causes temperature anomalies at the continental shelf break.