Agulhas Leakage changes in the Pliocene as a modulator of AMOC strength

Abstract:

The leakage of Agulhas Current water into the South Atlantic is now thought to be a major player in global climate change. Its volume is linked to the strength and position of southern westerlies. Past changes in the westerly winds over the southern ocean have been noted on glacial-interglacial timescales, in response to both Northern Hemispheric conditions and changes in Antarctic ice volume. The Pliocene to Pleistocene transition, associated cooling and Northern Hemisphere glaciation may have related to changes in Antarctica and the Southern Ocean, affecting both the position and strength of southern westerly winds. A northward shift in the westerlies, observed in past records of glaciation events, is thought to restrict the flow of warm, salty water from the Indian Ocean into the Atlantic, potentially impacting the Atlantic Meridional Overturning Circulation (AMOC) and North Atlantic SSTs. A weakening of the Agulhas Leakage therefore could transmit changes in the southern hemisphere to the northern hemisphere. Much of the Agulhas leakage is carried in small eddies rotating off the main flow south of Cape Horn. High ocean model resolution (< 1/2°) is therefore required to realistically simulate the leakage's response to the overlying wind field. Here we run a series of global high-resolution ocean model (1/6°) experiments using the MITgcm to test the effect of a shift in the southern hemisphere westerlies on the Agulhas Leakage, during a past climate (Pliocene) warmer than today. A prescribed perturbation of the winds near South Africa shows a significant increase in Agulhas eddies into the Atlantic. Following this, we have performed longer simulations (> 25 model years) on the simulated Pliocene Ocean reflecting past shifts in the wind field quantify changes in Agulhas Leakage transport and salinity anomalies into the South Atlantic. We then investigate whether there is any corresponding change in North Atlantic Deep Water formation and the overall response of the AMOC from the Agulhas Leakage changes.