Competing Roles of Heat and Freshwater Flux in Forcing Thermohaline Oscillations

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  • 1 Climate Research Division, Scripps Institution of Oceanography, La Jolla, California
  • | 2 Max-Planck-Institut fuer Meteorologie, Hamburg, Germany
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Abstract

The physical mechanisms causing century-scale Southern Ocean thermohaline oscillations in a primitive equation oceanic general circulation model are described. The oscillations have been shown to occur on a 320-year timescale when random fluctuations am added to the freshwater flux field that forces the model; this result is extended to show that they occur in a variety of situations, including ones without added noise. The oscillations involve movement between two model states: one characterized by strong convection and an active thermohaline circulation. and the other with a holocline around Antarctica capping off the water column, thus preventing convection. The physical mechanism that 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 that shuts off convection is surface freshening, primarily by precipitation, in the region poleward of the Antarctic Circumpolar Current. The oscillations are analyzed in terms of a simple “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 way in which different surface forcings cause the model to oscillate.

Abstract

The physical mechanisms causing century-scale Southern Ocean thermohaline oscillations in a primitive equation oceanic general circulation model are described. The oscillations have been shown to occur on a 320-year timescale when random fluctuations am added to the freshwater flux field that forces the model; this result is extended to show that they occur in a variety of situations, including ones without added noise. The oscillations involve movement between two model states: one characterized by strong convection and an active thermohaline circulation. and the other with a holocline around Antarctica capping off the water column, thus preventing convection. The physical mechanism that 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 that shuts off convection is surface freshening, primarily by precipitation, in the region poleward of the Antarctic Circumpolar Current. The oscillations are analyzed in terms of a simple “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 way in which different surface forcings cause the model to oscillate.

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