On the Formation of Antarctic Intermediate and Bottom Water in Ocean General Circulation Models

Abstract

A series of coarse-resolution models were integrated with a view to determining the most appropriate representation of the largest-scale water masses formed in the Southern Ocean. In particular, it was hoped that the models could realistically simulate Antarctic Bottom and Intermediate Water. The ocean model employed has a global domain with a realistic approximation of the continental outlines and bottom bathymetry. The subgrid-scale variation of bottom bathymetry is removed by spatial averaging over each grid box. The annual mean forcing at the sea surface is derived from climatological fields of temperature, salinity, and wind stress. It is found that the salinity of shelf water in the Weddell and Ross seas is critical if the model is to appropriately simulate the world's intermediate and bottom water masses. If the surface layer is too fresh in the Weddell and Ross seas, any bottom water formed adjacent to Antarctica is significantly less dense than in the real ocean. Furthermore, surface water at about 60°S (normally the region of intermediate water formation) strongly contributes to the model ocean's bottom water. This leaves the simulated bottom water too fresh and warm. On the other hand, with sufficiently salty bottom-water formed in the extreme Southern Ocean, a low-salinity tongue of intermediate water develops at 60°S. It is suggested that the sea-ice component of climate models is critical if the simulation is to capture the high-salinity shelf water and bottom-water formation adjacent to Antarctica and, in turn, allow for a realistic tongue of low-salinity Antarctic Intermediate Water (AAIW). The bathymetry of the Drake Passage is shown to determine the shape and strength of an intense meridional overturning cell in the Southern Ocean. By properly representing the northward extent of the Drake Passage, the formation and equatorward spreading of AAIW is simulated realistically. The scheme of AAIW formation obtained is quite different from the classical notion of circumpolar subduction of surface water at the polar front.