It is shown analytically that a nonlinear collision of northward-and southward-flowing western boundary currents (WBC) on a ␤ plane produces both an anticyclonic and a cyclonic eddy. (On an f plane no eddies are established; similarly, no eddies are established in the linear limit.) The length scales of both the anticyclonic and cyclonic eddies are larger than most eddies in the ocean. Furthermore, the anticyclone scale is larger than the cyclone length scale because of the higher upstream momentum flux. A reduced-gravity numerical model is used to validate these analytical results. The balance of forces and the eddy size estimates (derived from the numerical simulations) agree with the analytical results. Based on the above collision problem, it is argued that the Halmahera and Mindanao eddies are required to balance the nonlinear momentum fluxes of their colliding parent currents, the southward-flowing Mindanao Current (MC) and the northward-flowing South Equatorial Current (SEC). Assuming that the interior is in Sverdrup balance, it is further argued that neither of the eddies would have been present had the Indonesian Throughflow not been active.

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The dynamics of current retroflection and rings shedding are not yet fully understood. In this paper, the authors develop an analytical model of the Agulhas Current retroflection dynamics using three simple laws: conservation of volume, momentum balance, and Bernoulli's principle. This study shows that, for a retroflecting current with a small Rossby number, this theoretical model is in good agreement with numerical simulations of a reducedgravity isopycnal model. Otherwise, the retroflection position becomes unstable and quickly propagates upstream, leaving a chain of eddies in its path. On the basis of these findings, the authors hypothesize that the westward protrusion of the Agulhas retroflection and the local ''zonalization'' of the Agulhas Current after it passes the Agulhas Bank are stable only for small Rossby numbers. Otherwise, the retroflection shifts toward the eastern slope of the Agulhas Bank, where its position stabilizes due to the slanted configuration of the slope. This study shows that this scenario is in good agreement with several high-resolution numerical models.