Disconnection of the late Pliocene Agulhas Leakage from Atlantic Meridional Overturning Circulation

Abstract

The strength of the Atlantic Meridional Overturning Circulation (AMOC), in terms of streamfunction, is considered to depend partly on salt supply from the Indian Ocean through Agulhas Leakage (AL). In turn, AL is regulated by the position of the Southern Ocean subtropical front, which creates the pathway of the leaking around the African continent. Here, to evaluate the links between the subtropical front, AL and AMOC, we present latitudinal shifts of the subtropical front, and thereby potential AL changes, using dinoflagellate cysts and biomarkers in sediment cores from the Agulhas Plateau covering the late Pliocene (3,600–2,580 thousand years ago (ka)). We subsequently compare our frontal migration and AL reconstruction with ocean surface and bottom records along the AMOC pathway, and climate model simulations, to evaluate the influence of the AL on the AMOC. We find that the northward migration of the subtropical front gradually reduced the volume and salt supply of AL from 3,600 ka to 3,300 ka, whereas the AMOC intensified. We conclude that the causal link between the AL and AMOC was disconnected during the late Pliocene, challenging the presumed role of the AL in modulating the AMOC. Northward movement of the Southern Ocean subtropical front diminished salt transport by Agulhas Leakage in the late Pliocene, although this had no apparent influence on the strength of the Atlantic Meridional Overturning Circulation, according to proxy records from marine sediment cores.