BUOYANCY-DRIVEN CIRCULATION IN AN OCEAN-BASIN WITH ISOPYCNALS INTERSECTING THE SLOPING BOUNDARY

The dynamics that govern the spreading of a convectively formed water mass in an ocean with sloping boundaries are examined using an isopycn al model that permits the interface between the layers to intersect th e sloping boundaries. The simulations presented here use a two-layer c onfiguration to demonstrate some of the pronounced differences in a ba roclinically forced flow between the response in a basin with a flat b ottom and vertical walls and a more realistic basin bounded by a slopi ng bottom. Each layer has a directly forced signal that propagates awa y from the forcing along the potential vorticity (PV) contours of that layer. Paired, opposed boundary currents are generated by refracted t opographic Rossby waves, rather than Kelvin waves. It is impossible to decompose the flow into globally independent baroclinic and barotropi c modes; topography causes the barotropic (i.e., depth averaged) respo nse to buoyancy forcing to be just as strong as the baroclinic respons e. Because layer PV contours diverge, boundary currents are pulled apa rt at different depths even in weakly forced, essentially linear, case s. Such barotropic modes, often described as ''caused by the JEBAR eff ect,'' are actually dominated by strong free flow along PV contours. W ith both planetary vorticity gradients and topography, the two layers are linearly coupled. This coupling is evident in upper-layer circulat ions that follow upper-layer PV contours but originate in unforced reg ions of strong lower-layer flow. The interior ocean response is confin ed primarily to PV contours that are either directly forced or strongl y coupled at some point to directly forced PV contours of the other la yer. Even when the forcing is strong enough to generate a rich eddy fi eld in the upper layer, the topographic PV gradients in the lower laye r stabilize that layer and inhibit exchange of fluid across PV contour s. The dynamic processes explored in this study are pertinent to both nonlinear flows (strongly forced) and linear flows (weakly forced and forerunners of strongly forced). Both small (f plane) and large (full spherical variation of the Coriolis parameter) basins are included. Tr ansequatorial basins, in which the geostrophic contours are blocked, a re not described here.