The Stability of the AMOC During Heinrich Events Is Not Dependent on the AMOC Strength in an Intermediate Complexity Earth System Model Ensemble

Abstract

Previous studies that used Earth system models of intermediate complexity showed that stronger background winds drove a more vigorous and stable Atlantic Meridional Overturning Circulation (AMOC), while those with weaker winds had a more sluggish and unstable AMOC. In other studies, ensembles under vertical mixing uncertainty showed the opposite effect, where the simulations with a stronger AMOC were more unstable. To tackle this conundrum, we produce a model ensemble featuring uncertainties related to wind forcing and vertical mixing to understand the role of feedbacks on the AMOC stability. We show that the stability of the AMOC is not influenced by vertical mixing and the AMOC strength, and rather, it is determined by the strength of the Northern Hemisphere winds. Paleoproxies indicate an AMOC shutdown during the last Heinrich Stadial. Our comparisons to sea surface temperature proxies show a better fit with the simulations under a stable AMOC, which corresponds to a forced off-state. The sign of the AMOC-driven freshwater transport in the South Atlantic, which is regarded as an index for its stability, is shown not to be an absolute measure, although its evolution agrees with the salt advection feedback.