The dynamics of the Antarctic Circumpolar Current

The dynamics of the Antarctic Circumpolar Current (ACC) in a near-eddy-resolving model of the Southern Ocean (FRAM) are investigated. A streamwise coordinate system is used, rather than a more conventional approach of considering zonally averaged quantities. The motivation for this approach is the large deviation from a purely zonal flow made by the current. Comparisons are made with a zonal-mean analysis of the same model. It is found that the topographic form drag is the main sink of the momentum that is input by the wind. However, in contrast to a zonal-mean analysis other terms, namely, horizontal mixing, bottom friction, and advection of momentum, are no longer negligible. The total effect of transient eddies is to produce a drag on the mean how, again in contrast to the zonally averaged case. The vertical penetration of stress is considered. A generalized formula is derived for the interfacial form stress averaged along a convoluted path and that includes nonquasigeostrophic effects. The interfacial form stress is found to be related not only to the local wind stress but also to changes in stratification and the Coriolis parameter along the path of integration. The vertical gradient of the extra terms is found to be proportional to the quasi-meridional velocity averaged along an isopycnic surface. Using the model data, the nonquasigeostrophic effects are found to be important, particularly toward the northern flank of the ACC. Relating the vertical shear of the flow to the interfacial form stress, it is shown that the vertical structure of the how is set by a combination of the wind stress and the meridional overturning. There is, therefore, an intimate linking of the wind and thermohaline-driven circulations.