Abstract
Abstract. The Agulhas leakage to the South Atlantic exhibits a strong anti-correlation with the mass flux of the Agulhas Current. When the Agulhas retroflection is in its normal position near Cape Agulhas, leakage is relatively high and the nearby South African coastal slant (angle of derivation from zonal) is very small and relatively invariant alongshore. During periods of strong incoming flux (low leakage), 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 and anti-correlation between Agulhas mass flux and leakage to the South Atlantic may be attributed to this kink. To do so, we develop a nonlinear analytical model for retroflection near a coastline that consists of two sections, a zonal western section and a strongly slanted eastern section. The principal difference between this and the model of a straight slanted coast (discussed in our earlier papers) is that, here, free purely westward propagation of eddies along the zonal coastline section is allowed. This introduces an interesting situation in which strong slant of the coast east of the kink prohibits the formation and shedding of rings, while the almost zonal coastal orientation west of the kink encourages shedding. Therefore, the kink "locks" the position of the retroflection, forcing it to occur just downstream of the kink. Rings are necessarily shed from the retroflection area in our kinked model, regardless of the degree of eastern coast slant. In contrast, a no-kink model with a coastline of intermediate slant indicates that shedding is almost completely arrested by that slant. We suggest that the observed difference in ring-shedding intensity during times of normal retroflection position and times when the retroflection is shifted eastward is due to the change in the retroflection location with respect to the kink. When the incoming flux detaches from the coast north of the kink, ring transport is small; when the flux detaches south of the kink, transport is large. Simple process-oriented numerical simulations are in fair agreement with our analytical results.
Highlights
The normal retroflection position of the Agulhas Current (AC) is to the southwest of Cape Agulhas (Lutjeharms and Van Ballegooyen, 1988a)
During strong incoming flux (SIF), retroflection occurs near a coastline that is strongly concave, whereas during times of normal position of retroflection (NPR), the nearby coastline is weakly concave
Using both analytical and numerical models, we showed that the South African coastline geometry exerts a fundamental control on the production of Agulhas eddies and, the leakage into the South Atlantic
Summary
The normal retroflection position of the Agulhas Current (AC) is to the southwest of Cape Agulhas (Lutjeharms and Van Ballegooyen, 1988a). During NPR, when the AC volume transport is low, the Agulhas retroflection protrudes westward and is located near a coastline of low slant and little curvature During these times, there is an increase of warm water influx (via rings) into the South Atlantic. (i.e., where the on-land angle between the two approximately straight coastlines is considerably smaller than 180◦) This is probably consistent with the observation that rings in the Agulhas region are typically shed about 5–6 times per year, but the period of their formation increases sometimes to almost half-a-year (e.g., Byrne et al, 1995; Gordon et al, 1987; Lutjeharms, 2006; Schouten et al, 2000; Van Aken et al, 2003). We can expect the time periods between consecutive La-Nina events to be comparable to the classic ENSO periods but with no regularity. (Note that in Biastoch et al, 2008, they are two years apart.) In addition, duration of La-Nina events in the Indian Ocean and the AC are much shorter than for the usual La-Nina conditions in the Pacific (see e.g., Van Sebille et al, 2009a, VSa, hereafter)
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