Global Oceanic Overturning Circulation Forced by the Competition between Greenhouse Gases and Continental Ice Sheets during the Last Deglaciation

Abstract The deglacial change of Atlantic meridional overturning circulation (AMOC) since the Last Glacial Maximum (LGM; ~21 ka) has been studied extensively in both reconstructions and model simulations. While reconstructions suggest a shoaling of AMOC at the LGM, the strength of glacial AMOC relative to the modern day remains highly uncertain in both reconstructions and models. Using transient simulations of climate evolution forced by individual deglacial forcings since the LGM, this study shows that the uncertainties in glacial AMOC intensity can be caused by a competition between the elevated glacial Northern Hemisphere (NH) ice sheets and the reduced glacial greenhouse gases, in which the former tend to strengthen the AMOC while the latter play an opposite role. In spite of the dramatic difference of climate between the LGM and the present, the cancellation between the impacts of the two forcings leaves the strength of the glacial AMOC not too different from the modern day (0.5 Sv stronger in our study; 1 Sv ≡ 10 6 m 3 s −1 ). Furthermore, consistent with theoretical analysis, the response of the AMOC return flow to either forcing is predominantly compensated by an interbasin exchange between the Indo-Pacific (including the Indo-Pacific sector of Southern Ocean) and Atlantic via the Agulhas Current.