Interactions between thermohaline- and wind-driven circulations and their relevance to the dynamics of the Antarctic Circumpolar Current, in a coarse-resolution global ocean general circulation model

We present an analysis of the interaction between wind-driven and thermohaline-driven circulations in a coarse-resolution global ocean general circulation model of the Bryan-Cox code. A series of experiments is described in which the flow is driven by wind forcing only, thermohaline forcing only, or both. In a global ocean with topography, the circulation driven by wind alone is strongly influenced by contours of constant potential vorticity, which limit the barotropic transport to relatively small values. With the same geometry the flow driven by relaxing to observed surface temperature and salinity fields alone contains deep overturning circulation (producing North Atlantic Deep Water and Antarctic Bottom Water (AABW)) and a large barotropic Antarctic Circumpolar Current (ACC) generated by bottom form stress. The ACC is dependent on the overturning circulation, the deep density field and the bottom form stress and increases with A(TV), the vertical mixing coefficient. If wind is added to this flow, the additional circulation is also dependent on the baroclinic structure but decreases with increasing A(TV). This applies especially to the ACC, where wind-induced additions are much larger than when the:wind acts alone in a homogeneous ocean. An analysis of the dynamic balance of the ACC in the model shows that it is governed by lateral friction, bottom form stress, and wind. The mechanism for driving the ACC by deep convection and bottom topography is revealed in special experiments with simplified topography. In these runs, all topography is removed except for all or part of a submarine ridge across the Drake Passage; this topography alone causes an ACC with barotropic transport of 80 Sv, driven by a deep density difference and pressure gradient across the ridge. This ridge channels AABW formed in the Weddell Sea northward into the South Atlantic, and the ACC is driven by angular momentum conservation across these latitudes of the compensating southward flowing shallower flow (or, equivalently, by the Coriolis force acting on it) above the ridge. In most other parts of the model ocean, bottom form stress acts as a net drag on the zonal current.