Stability of the AMOC during the Last Glacial Maximum under realistic boundary conditions

Abstract:

The response of the Atlantic Meridional Overturning Circulation (AMOC) to freshwater perturbations critically depends on its mean-state. Large swaths of icebergs melting in the North Atlantic during the last deglaciation constituted such perturbations, and thus can provide important constraints on the stability of the AMOC. Yet, the mean AMOC state during the Last Glacial Maximum (LGM), preceding the rapid disintegration of the ice-sheets during the deglaciation, as well as its response to these perturbations remain debated. Here we investigate the evolution of the AMOC responding to freshwater perturbations under realistic LGM boundary conditions in the Bern3D intermediate complexity model. Particularly, we consider the effect of an open versus a closed Bering Strait. The substantially more vigorous and deeper AMOC under these glacial boundary conditions, consistent with previous model simulations, reacts more strongly to North Atlantic freshwater forcings than under pre-industrial conditions. This increased sensitivity, also reflected in a stronger AMOC hysteresis, is mostly related to the closed Bering Strait that cuts off the freshwater escape route through the Arctic into the Pacific, thus facilitating faster accumulation of freshwater in the North Atlantic halting deep water formation. Proxy reconstructions of the LGM AMOC are largely in conflict with the particularly strong and deep circulation states coherently simulated with ocean circulation models for the LGM, instead indicating prevalence of a weaker and possibly shallower AMOC than today. Simulations with reduced deep water formation, as a consequence of potentially increased continental runoff from ice-sheet melt, more closely resemble the overturning circulation inferred from proxies. Yet, no AMOC states are found here that either comprise an extreme shoaling or vigorous and concurrent shallow overturning as previously proposed.