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Isopycnal Momentum Budget of the Antarctic Circumpolar Current in the Fine Resolution Antarctic Model

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  • 1 Institute of Oceanographic Sciences, Deacon Laboratory, Wormley, Godalming, Surrey, United Kingdom
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Abstract

The momentum budget of the Antarctic Circumpolar Current (ACC) is analyzed using data from the Fine Resolution Antarctic Model (FRAM), using density as a vertical coordinate, since density is approximately conserved on streamlines. This steady budget is balanced. Volume, heat, and salt budgets are also computed within density layers, although these remain time dependent. The leading-order momentum balance is of approximate equality of the form drag on the top and bottom surfaces of each undulating density layer, but because density layers outcrop at the surface, this does not imply that the surface wind stress is simply transferred downward by form drag without change of amplitude. Restricting attention to the net form drag on a layer, this is found to be balanced by Coriolis force and, if the layer outcrops, the amount of wind stress put into the layer where it outcrops. Between 40% and 60% of the density layers at any latitude outcrop somewhere at the surface, so that wind stress can be moved directly into these layers, totally unlike the quasigeostrophic situation. Reynolds stress divergence and cross-isopycnal momentum transport are negligible. In layers dense enough to ground at the ocean floor, the form drag changes sign several times, following the sign changes in the northward volume flux through the Coriolis term. These north-south fluxes are produced by time-dependent filling or emptying of fluid layers south of the ACC. This shows that although FRAM and other marginally eddy resolving models reach apparent statistical steady dynamical states in about a decade, this is illusory: the long-time thermodynamic behavior affects the dynamics. It is shown that balances from a time-averaged dataset are not accurate guides to the time-averaged balances.

Abstract

The momentum budget of the Antarctic Circumpolar Current (ACC) is analyzed using data from the Fine Resolution Antarctic Model (FRAM), using density as a vertical coordinate, since density is approximately conserved on streamlines. This steady budget is balanced. Volume, heat, and salt budgets are also computed within density layers, although these remain time dependent. The leading-order momentum balance is of approximate equality of the form drag on the top and bottom surfaces of each undulating density layer, but because density layers outcrop at the surface, this does not imply that the surface wind stress is simply transferred downward by form drag without change of amplitude. Restricting attention to the net form drag on a layer, this is found to be balanced by Coriolis force and, if the layer outcrops, the amount of wind stress put into the layer where it outcrops. Between 40% and 60% of the density layers at any latitude outcrop somewhere at the surface, so that wind stress can be moved directly into these layers, totally unlike the quasigeostrophic situation. Reynolds stress divergence and cross-isopycnal momentum transport are negligible. In layers dense enough to ground at the ocean floor, the form drag changes sign several times, following the sign changes in the northward volume flux through the Coriolis term. These north-south fluxes are produced by time-dependent filling or emptying of fluid layers south of the ACC. This shows that although FRAM and other marginally eddy resolving models reach apparent statistical steady dynamical states in about a decade, this is illusory: the long-time thermodynamic behavior affects the dynamics. It is shown that balances from a time-averaged dataset are not accurate guides to the time-averaged balances.

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