Competing Roles of Heat and Freshwater Flux in Forcing Thermohaline Oscillations

Abstract

The physical mechanisms causing century-scale Southern Ocean thermohalineoscillations in a primitive equation oceanic general circulationmodel are described. The oscillations have been shown to occur ona 320-year timescale when random fluctuations are added to the freshwaterflux field that forces the model; this result is extended to showthat they occur in a variety of situations, including ones withoutadded noise, The oscillations involve movement between two modelstates: one characterized by strong convection and an active thermohalinecirculation, and the other with a halocline around Antarctica cappingoff the water column, thus preventing convection. The physical mechanismthat forces the model from the quiescent state to an actively convectingone is subsurface (300 m) heating around Antarctica, which destabilizesthe water column; the ultimate source of this heat is advected NorthAtlantic Deep Water. This leads to a teleconnection between forcingconditions in the North Atlantic and the thermohaline structure ofthe Southern Ocean. The mechanism that shuts off convection is surfacefreshening, primarily by precipitation, in the region poleward ofthe Antarctic Circumpolar Current. The oscillations are analyzedin terms of a simple ''flip-flop'' model, which indicates that nonlinearitiesin the seawater equation of state are necessary for the oscillationsto occur. The spatial pattern of convection around Antarctica affectsthe time evolution of the Southern Ocean's thermohaline overturningand the way in which different surface forcings cause the model tooscillate.