Competing Roles of Heat and Freshwater Flux in Forcing Thermohaline Oscillations

We describe the physical mechanisms causing century-scale Southern Ocean thermohaline oscilla-tions in the Hamburg Large Scale Geostrophic model, a primitive equation oceanic general circulation model. The oscillations have been shown to occur on a 320-year time scale when random fluctuations are added to the fresh water flux field which forces the model; we extend this result to show that they occur in a variety of situations, including ones without added noise. The oscillations involve move-ment between two model states: one characterized by strong convection and an active thermohaline circulation, and the other with a halocline around Antarctica capping off the water column, preventing convection. The physical mechanism which forces the model from the quiescent state to an actively convecting one is subsurface (300 m) heating around Antarctica, which destabilizes the water column; the ultimate source of this heat is advected North Atlantic Deep Water. This leads to a teleconnection between forcing conditions in the North Atlantic and the thermohaline structure of the Southern Ocean. The mechanism which shuts off convection is surface freshening, primarily by precipitation, in the re-gion poleward of the Antarctic Circumpolar Current. The oscillations are analyzed in terms of a simple “flip-flop ” model which indicates that non-linearities in sea water’s 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 way in which different surface forcings cause the model to oscillate. 1.