Review of southern hemisphere mid-latitude water mass formation processes in ocean circulation models

During the last decade, the global thermohaline circulation attracted much scientific attention in the context of climate change research. This overturning of the global ocean is understood to be driven by deep ocean convection in the Labrador and Greenland Seas. Much of the debate relies on the accuracy of ocean circulation models used in prognostic climate change studies. For example, it has been predicted that deep ocean convection may cease under present-day greenhouse gas forcing. In such a situation, the thermohaline circulation is likely to collapse with severe disturbances to present climatic conditions.

Climatic changes may already be underway and first indicative oceanic observations have been made in intermediate water depths of the Southern Ocean. Subantarctic Mode and Antarctic Intermediate Water ventilate these depths, which are identified through their temperature and salinity characteristics. The particular signature of a water mass is a result of heat and freshwater fluxes in water mass formation regions; hence, any changes in atmospheric conditions are preserved in the temperature and salinity signal of mode and intermediate water.

Antarctic Intermediate Water and its likely precursor Subantarctic Mode Water are formed in the mid-latitudes of the Southern Ocean. The large-scale distribution and temporal variability of their water mass characteristics is well established from observations. Yet, no clear picture of the underlying water mass formation processes and their formation sites has emerged from those observations. The traditional hypothesis was proposed in the early 1940s and argued for a circumpolar formation of intermediate water by subduction along the Subantarctic Front. This idea was challenged in the early 1980s proposing instead localized formation by mid-latitude convection in the southeast Pacific Ocean.

In this paper, several investigations are being reviewed to elucidate the formation process of Subantarctic Mode Water and Antarctic Intermediate Water as simulated by state-of-the-art ocean general circulation models. The ocean modeling studies may lead to further details about the nature of mid-latitude water mass formation processes in the Southern Hemisphere.