Retroflection from a double slanted coastline - a model for the Agulhas leakage variability.

The Agulhas leakage to the South Atlantic (SA) exhibits strong anti-correlations with the mass flux of the Agulhas Current. This is accompanied by the migration of the Agulhas retroflection whose normal position (NPR) is near Cape Agulhas, where the slant of the South African coast is very small. During periods of strong incoming flux (SIF), the retroflection shifts upstream to Port Elizabeth or East London, where the coastline shape has a "kink", i.e., the slant changes abruptly from small on the west side, to large (about 55°) on the east side. Here, we show that the variability of rings shedding maybe attributed to this kink. To do so, we develop a nonlinear analytical model for retroflection near a coastline that consists of two sections, one strongly slanted (corresponding to the east side) and the other zonal (corresponding to the west side). The principal difference between this and the model of a single straight slanted coast discussed in our earlier papers is that a free purely westward propagation of eddies along the zonal coastline section is allowed in the kinked case. This introduces the interesting situation where the strong slant of the coast east of the kink prohibits the formation and shedding of rings whereas the coast west of the kink encourages such shedding. Therefore, the kink model "locks" the position of the retroflection forcing it to occur just downstream of the kink. That is, rings are necessarily shed from the retroflection area in our kinked model, regardless of the eastern coast slant. By contrast, the application of "no-kink" model for an "averaged" slant (at the same point as the kink) leads to the conclusion that shedding is almost completely arrested by the slant. We suggest that the difference between the intensities of rings shedding during NPR and SIF is due to the shift in the zero curl line in respect to the kink. When the zero curl intersects the coast north of the kink the transport is small but it is large when the zero curl is situated south of the kink. Simple process-oriented numerical simulations are in fair agreement with our results. [ABSTRACT FROM AUTHOR]