A New Regime of the Agulhas Current Retroflection: Turbulent Choking of Indian-Atlantic leakage

An analysis of the Indian Ocean circulation and the Agulhas Current retroflection is carried out using a primitive equation model with simplified coastline and flat bottom. Four configurations with 0.25 degrees and 0.1 degrees horizontal resolution and in barotropic and baroclinic cases are considered. The wind stress is taken as control parameter to increase the inertia of the currents. The volume transport of the Indonesian Throughflow, Mozambique Channel, and Agulhas Current are found to increase linearly with the wind stress strength, and three nonlinear retroflection regimes are found. A viscous and an inertial regime had already been documented, but a new turbulent regime appears at large wind stress amplitude. In this turbulent regime, the volume of Agulhas leakage reaches a plateau because of strong mesoscale variability and, in contrast to the other regimes, does not depend on the wind stress magnitude. The physical mechanism causing the plateau is shown to be associated with the cross-jet exchange of Indian Ocean water and water from the Antarctic Circumpolar Current. In the turbulent regime, the permeability of the Agulhas Return Current to material transport increases and the Indian Ocean water available for the Agulhas leakage decreases.