Topography and barotropic transport control by bottom friction

The problem of the stratified general circulation in the presence of topography is revisited. The novel effect examined here is that of localized, but large-scale, topographic anomalies on the wind-driven circulation, a problem whose relevance is found in the occurrence of many such features in the open ocean. Using the classical methods of homogenization theory, it is argued that the barotropic transport near topography can come under the direct control of bottom friction. This result differs substantively from either the well-known Sverdrup constraint (which applies to a flat-bottomed ocean, or to one with a resting deep layer) or its recent extensions that allow for planar bottom topographic profiles. Bottom friction emerges as a controlling parameter roughly in the event that the topography forms closed f/(H - h(b)) contours, where H - h(b) is the total fluid depth, although the theoretical minimum requirements are somewhat looser than this. Our analytical predictions are supported by numerical experimentation with a multi-layer quasi-geostrophic model, and we examine some mean flow observations from the North and South Atlantic in light of the theory. In particular; the theory can rationalize the 100 Sv transport observed recently around the Zapiola Drift.