Surface Constraints On The Strength And Structure Of The Atlantic Meridional Overturning Circulation In Coupled Climate Models

The Atlantic Meridional Overturning Circulation (AMOC), a branch of the ocean's global overturning circulation (GOC), plays an important role in regulating Earth's climate by transporting heat northward and ventilating the upper 2000 meters of the ocean. State-of-the-art general circulation models (GCMs), however, exhibit large mean-state biases in the strength and structure of the AMOC. For instance, in pre-industrial control simulations from CMIP6, the AMOC strength varies between 10 and 30 Sv and the AMOC depth varies between 1500 and 3000 meters. The exact reason for the intermodel spread in mean-state AMOC biases remains unclear. Here, we introduce a framework for understanding differences in the strength and structure of the AMOC in GCMs by assessing the surface buoyancy fluxes and the associated meridional buoyancy transport. We find that the strength of the AMOC in each GCM is strongly related to the magnitude of the surface buoyancy fluxes: stronger buoyancy gain in the low-latitudes is related to stronger buoyancy loss in the high-latitude Atlantic and a stronger AMOC. Additionally, we find that the low-latitude Atlantic and Indo-Pacific basins account for approximately 80% of the intermodel variations in the surface buoyancy gain, and that the heat and freshwater components contribute equally. Our results highlight the unique role that low-latitude surface processes play in setting the strength and structure of the AMOC, and that low-latitude surface heat and freshwater fluxes processes may provide a so-called emergent constraint on AMOC changes under greenhouse-gas forcing.