Effects of surface freshwater flux induced by sea ice transport on the global thermohaline circulation

Abstract

Effects of surface freshwater flux induced by sea ice formation and melting on the thermohaline circulation are investigated by using a sea-ice-ocean coupled general circulation model forced by monthly climatology. Restoring to the observed sea surface salinity is not employed in order to evaluate precisely the sea ice effects. Because of the model's improper representation of interaction between sea ice and the Northern Hemisphere deep convection, the discussion is focused on the effect of sea ice in the Southern Hemisphere. In the control case, deep water formation around Antarctica occurs under compact sea ice cover, where positive annual mean sea ice production is an essential factor to induce deep convection. When sea ice motion is turned off, deep water formation is maintained primarily by thermal destabilization of water columns, as annual mean sea ice production is almost zero everywhere. Consequently, the deep ocean around Antarctica is warmer and less saline compared with that for the control case, and the Atlantic bottom circulation is weakened by 16%. Northward salt transport from the Southern Ocean in the bottom layer also decreases. The amount of the decrease is greater by 1 order of magnitude than that of the surface salt input associated with sea ice. In a case where the freshwater flux at the ice-ocean interface is turned off, results are similar to the case without sea ice transport, although surface heat and momentum fluxes significantly differ between them. This suggests that the influence of sea ice on freshwater flux is more important than on heat and momentum fluxes in affecting the global thermohaline circulation.