Fingerprints of AMOC Decline Are Sensitive to External and Mechanistic Forcing

The Atlantic meridional overturning circulation (AMOC) plays a crucial role in past, present, and future climate, and there is substantial interest in using sea surface temperature (SST) as a fingerprint of past AMOC strength. Using a hierarchy of climate model ensembles, we find that the decline in AMOC, and its SST fingerprint within the North Atlantic warming hole region, are sensitive to external forcing level and wind driven ocean forcing. Once external forcing reaches a level at which sea ice melt increases the Labrador Sea vertical salinity gradient, localized cooling and resulting expansion of the sea ice edge decrease vertical mechanical stirring. Under greenhouse gas only forcing, this mechanism plays a large role and under SSP3.70 forcing, it plays a relatively minor role due to larger buoyancy forcing. This implies that an AMOC fingerprint developed from one simulation or external forcing level cannot be applied to other scenarios.