Fig.1a. Schematic of the present-day Agulhas system (after Lutjeharms, 1996). The meandering Agulhas current retroflects into the Agulhas Return Current, and a relatively small part of the AC (10 Sv out of 80 Sv) invades the South Atlantic in the form of anticyclonic eddies.

Fig.1b. Southeast Atlantic SST (at the Cape Basin Region core which was situated at 35° S and 18° E, slightly northwest of Astrid) for the past 360 Kyr as determined from paleoceanographic proxies (adapted from Peeters et al. 2004). Note the dramatic increase in temperature in the end of each glaciations period (vertical shaded blue). These increases were attributed by Peeters et al. (2004) to an increase in ring production which induces a transfer of warm and salty Indian Ocean water to the colder South Atlantic. Other proxies unrelated to temperature show the same tendencies.

Fig. 2. A diagram of the Agulhas Current and its retroflection. We hypothesize that, due to a northward shift in the position of the vanishing wind stress curl, the retroflection during gla ciation periods occurred in a l ower latitude where the orientation of the coast is close to meridional (situation I). During de-glaciation, the retroflection occurs farther to the south, where the coastline inclination is almost zero (situation II). We hypothesize that almost no ring detachment occurs in situation I but plenty of rings detach in situation II.

Fig. 3. Schematic diagram of the Agulhas retroflection integration region (ABCDD1D3D2A1A), showing the upstream an d downstream jets and the gradually evolving ring.

Fig. 4. The preliminary analytical solution for the nonlinear Agulhas ring production problem. Q and q are the volume fluxes of the upstream and retroflecting currents so that Q-q is the rings’ volume flux. The coastline angle relative to the east-west direction is g and the upstream vorticity is a f. Note that, for large values of g, the production of rings is relatively small. It increases dramatically as the angle is reduced. For convenience, we define a critical angle as the one corresponding to the maximum gradient in (Q-q)/Q. It is around 30°.

Fig. 5. The preliminary Agulhas numerical simulations with a reduced-gravity model. The t wo panels on th e left show the depth contours (for day 80 and 190) in the case of steep coastline angle ( = 60° ) whereas the two panels on the right show the contours for the sam e times but in the case of small coastline angle ( = 15° ). The upstream relative vorticity is approximately 0.3 f. Note that, no rings are shed in the steep slant case (left) but rings are regularly shed in the small slant case (right). This is consistent with the analytics (Fig. 4) which predicts an almost zero ring production in the steep case but about 25% production in the small slant case. Thus, in the small slant case (right), 25% of the Agulhas current transport (70 Sv), or about 17 Sv, penetrate into the South Atlantic in the form of rings.