Millennial-scale Agulhas Current variability and its implications for salt-leakage through the Indian-Atlantic Ocean Gateway

•Millennial-scale variability of the Agulhas Current.•Connection between Agulhas Current variability and Agulhas leakage.•Connection between Agulhas Current and bipolar seesaw.•Southern Hemisphere westerlies are one of the main controls on the Agulhas Current variability.

The inter-ocean exchange of warm and salt-enriched waters around South Africa (Agulhas leakage), may have played an important role in the mechanism of deglaciations. Paleoceanographic reconstructions from the Agulhas leakage corridor show that leakage maxima occurred during glacial terminations. Therefore enhanced leakage has been suggested as a forcing mechanism to shift the Atlantic Meridional Overturning Circulation into the interglacial mode of circulation. At present, studies have not considered that upstream changes in the properties of the Agulhas Current itself may, in part, explain the observed variability in the Agulhas leakage and play an important role in defining the upper ocean hydrography of the South Atlantic. Here, we present a multi-proxy record from a marine sediment core (CD154 17-17K) located in the main trajectory of the Agulhas Current that spans the past 100 kyr. The record shows considerable variability in reconstructed upper ocean temperatures and salinity. We also find that the relative abundance of tropical and sub-tropical planktic foraminifera, previously used as a proxy for Agulhas leakage fauna, shows considerable upstream variability, likely reflecting changes in the hydrography of the southwest Indian Ocean sub-gyre (SWIOSG) and upper ocean temperatures. Idealised numerical model simulations demonstrate that both a shifting and an intensification of the Southern Hemisphere westerlies modify the vigour of the SWIOSG. These changes also drive increased kinetic and eddy variability in the Agulhas Return Current that potentially enhances cross frontal mixing of southern sourced waters into the SWIOSG system. Our results suggest that variability in the upstream Agulhas Current hydrography is strongly linked to the dynamics of the Agulhas Return Current and strength of the SWIOSG and that downstream variability in the leakage area (Atlantic sector) at least partly reflects regional variations of the Agulhas Current as a whole.