Seasonal simulation of the Southern Ocean coupled ice‐ocean system

Abstract

The Southern Ocean ice‐ocean system is investigated using a coupled model developed at Princeton University. The ocean model uses the primitive equations and solves the second‐moment closure for turbulent mixing; the snow‐ice model uses a three‐level thermodynamic scheme resembling Semtner's (1976) model. The focus here is to study the seasonal variability of the ice covered areas and water mass formation. The model simulates reasonably well the seasonal cycle of the ice cover in the Southern Ocean. The annual rapid advance of the ice extent is tied to the ice advection away from the coast toward the Antarctic Circumpolar Current (ACC), where it melts rapidly with the increase of radiative fluxes in the austral spring. The European Center for Medium‐Range Weather Forecasts climatology tends to produce too little ice in the Pacific sector, for which the primary cause is a weak meridional wind stress. The oceanic heat fluxes are up to 100 W m−2 in open water areas and along the coastal areas except in the Weddell and Ross Seas, which are occupied by a year‐round sea ice cover and have average heat flux of 20 W m−2 or less. However, there is a small region in the Ross Sea with high heat loss suggesting deep convection. The meridional and zonal heat fluxes are in reasonable agreement with observational estimates. The Atlantic meridional heat flux is zero at 34°S (northern boundary of the model), which is an inadequacy in the model. In the Indian Ocean sector the flux is southward, while in the Pacific sector it is weak and equatorward. From the water mass balance we can deduce that the Antarctic Bottom Water (AABW) (σt > 27.85) annual production is 12 Sv in the Atlantic sector and 5 Sv in the Ross Sea. The total production of 17 Sv is in excellent agreement with the observational estimates of 10–20 Sv. Intermediate water types are produced in the Indian Ocean sector, where some of the AABW is lost during that conversion. In the Atlantic sector, 6 Sv of the annually formed AABW is advected north across the ACC. In the Pacific sector the northward AABW flow is 7.5 Sv, which is in excess of the Ross Sea production, suggesting that a considerable portion of the AABW formed in the Weddell Sea is contributing to that flux.