Future shoaling of the AMOC and its impact on oceanic heat transport to the subpolar North Atlantic

The Atlantic Meridional Overturning Circulation (AMOC) carries oceanic heat northward to the subpolar North Atlantic (SPNA), where the surface water cools and sinks to the deep ocean. According to a large-ensemble simulation under a medium-to-high emission scenario, the surface cooling and oceanic heat convergence in the SPNA may decrease to ~20% of their historical levels in the late 21st century. We show here that the projected weakening of the AMOC volume transport alone cannot explain such a large decrease in the heat convergence rate. Our analysis indicates that, due to the suppression of deep-water formation in the SPNA, the AMOC's lower limb becomes shallower, carrying relatively warm water southward away from the SPNA. This in turn accelerates the decrease in oceanic heat transport to the SPNA per unit AMOC weakening. These results are supported in other multi-ensemble models analyzed, despite large inter-model spreads.