COMPETING ROLES OF HEAT AND FRESH-WATER FLUX IN FORCING THERMOHALINE OSCILLATIONS

The physical mechanisms causing century-scale Southern Ocean thermohal ine oscillations in a primitive equation oceanic general circulation m odel are described. The oscillations have been shown to occur on a 320 -year timescale when random fluctuations are added to the freshwater f lux field that forces the model; this result is extended to show that they occur in a variety of situations, including ones without added no ise, The oscillations involve movement between two model states: one c haracterized by strong convection and an active thermohaline circulati on, and the other with a halocline around Antarctica capping off the w ater column, thus preventing convection. The physical mechanism that f orces the model from the quiescent state to an actively convecting one is subsurface (300 m) heating around Antarctica, which destabilizes t he water column; the ultimate source of this heat is advected North At lantic Deep Water. This leads to a teleconnection between forcing cond itions in the North Atlantic and the thermohaline structure of the Sou thern Ocean. The mechanism that shuts off convection is surface freshe ning, primarily by precipitation, in the region poleward of the Antarc tic Circumpolar Current. The oscillations are analyzed in terms of a s imple ''flip-flop'' model, which indicates that nonlinearities in the seawater equation of state are necessary for the oscillations to occur . The spatial pattern of convection around Antarctica affects the time evolution of the Southern Ocean's thermohaline overturning and the wa y in which different surface forcings cause the model to oscillate.