Atlantic meridional overturning circulation weakening in CMIP6: Labrador Sea surface fluxes and salt transport feedback

Abstract

Future Atlantic Meridional Overturning Circulation (AMOC) weakening remains a robust but poorly understood projection, with substantial uncertainty in the underlying regional mechanisms and inter-model spread. This study analyzes 30 Coupled Model Intercomparison Project Phase 6 (CMIP6) models to identify regional surface-forcing sensitivities associated with the inter-model spread of AMOC weakening. Multi-model analysis shows that inter-model differences in AMOC weakening are tightly linked to net surface heat flux (NSHF), SST, and salinity anomalies in the Labrador Sea, reflecting coupled ocean–atmosphere adjustment in the subpolar North Atlantic. While observations suggest the Labrador Sea contributes modestly to the mean overturning, these results indicate it is a region of disproportionate sensitivity in CMIP6 models. To assess this sensitivity, we conduct idealized coupled experiments using regional atmospheric nudging. Heat flux forcing over the Labrador Sea increases downward NSHF by ~60 W m −2 and weakens the AMOC by 7–9 Sv, accompanied by sustained subpolar freshening through reduced northward salt transport. When comparable forcing is applied to the Irminger–Greenland Seas, the simulated AMOC response is smaller despite similar heat uptake. These results indicate that salt transport feedback varies with the location of surface forcing and that the pronounced Labrador Sea sensitivity in CMIP6 models can contribute to the inter-model spread of future AMOC projections.