The Mindanao and Halmahera Eddies—Twin Eddies Induced by Nonlinearities

Abstract

It is shown analytically that a nonlinear collision of northward- and southward-flowing western boundary currents (WBC) on a β 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.