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The Antarctic Circumpolar Current in equilibrium

  • Author(s): Gallego, B
  • Cessi, Paola
  • McWilliams, J C
  • et al.

A simple channel-flow model is used to examine the equilibrium upper-ocean dynamics and thermodynamics of the Antarctic Circumpolar Current (ACC). The model consists of two zonally averaged, variable-temperature layers-a surface boundary layer and a thermocline layer-separated by a turbulent interface. Weak air-sea heat flux, determined by relaxation to a prescribed atmospheric temperature, determines the leading-order temperature structure in the oceanic surface layer. The equilibrium thermal structure in the interior is mostly determined by a dominant balance between the meridional transport due to the wind-driven Eulerian mean circulation and the heat flux due to the baroclinic eddies. The resulting latitudinal temperature gradient depends on both the wind and the atmospheric temperature forcing and sustains the geostrophic zonal flow. Consideration of the next-order balance for the oceanic surface temperature results in an air-sea heat flux proportional to the magnitude of the residual flow. The residual meridional circulation ( Eulerian mean plus eddy-induced) is necessary to balance small diabatic sources and sinks of heat. Therefore, it depends on the processes of vertical diffusion, boundary layer entrainment/detrainment, and, on the polar flank, convection. In the absence of substantial lateral diffusion, the leading-order balance of weak residual circulation implies a very weak meridional heat transport across the ACC and a correspondingly weak differential heat exchange to the atmosphere. This limitation can be eased if the lateral diffusive flux of temperature in the surface layer becomes as large as the adiabatic eddy transport.

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