Flow Across Large Amplitude Topography: Bottom PV injection and Watermass Transformation

Large amplitude, bottom ridge topography exerts a control on an inflow/outflow driven, uniformly rotating stratified flow with channel geometry resulting in an anticyclonic boundary current upstream from the crest, and a cyclonic boundary current downstream from the crest. In the absence of surface dynamics, the flow is bottom intensified, albeit with a depth influence that reaches the whole water column. In this idealized study we investigate the role of local bottom intensified dynamics that determine the pathway and modification of the flow using equilibrated non-linear simulations performed on a terrain-following primitive equation model. We find that bottom Ekman transport associated with the cyclonic boundary current (downstream from the ridge) promotes a bottom mixed layer front localized to the boundary current. The bottom mixed layer front maintained by the bottom Ekman transport continuously sheds mid-depth eddies downstream from the ridge, which then relax the mid-depth isopycnals advecting vanishingly low (Ertel) PV into the layer bounded by the tilting isopycnals within the mixed layer front. Associated with the process of destruction of PV localized to the cyclonic boundary current and the injection of PV within the bottom boundary layer at mid-depth downstream from the ridge by eddies, is the watermass transformation at mid-depth experienced by the large scale flow as it flows past large amplitude ridge topography.