The weakening AMOC under extreme climate change

Abstract

Changes in the Atlantic Meridional Overturning Circulation (AMOC) in the quadrupled CO₂ experiments conducted under the sixth Coupled Model Intercomparison Project (CMIP6) are examined. Increased CO₂ triggers extensive Arctic warming, causing widespread melting of sea ice. The resulting freshwater spreads southward, frst from the Labrador Sea and then the Nordic Seas, and proceeds along the eastern coast of North America. The freshwater enters the subpolar gyre north of the separated Gulf Stream, the North Atlantic Current. This decreases the density gradient across the current and the current weakens in response, reducing the infow to the deepwater production regions. The AMOC cell weakens in tandem, frst near the North Atlantic Current and then spreading to higher and lower latitudes. This contrasts with the common perception that freshwater caps the convection regions, stifing deepwater production; rather, it is the infow to the subpolar gyre that is suppressed. Changes in surface temperature have a much weaker effect, and there are no consistent changes in local or remote wind forcing among the models. Thus an increase in freshwater discharge, primarily from the Labrador Sea, is the precursor to AMOC weakening in these simulations.