Understanding simulated AMOC changes in single-forcing historical experiments

A potential forced Atlantic Meridional Overturning Circulation (AMOC) slowdown is widely debated, and its historical drivers and future evolution remain uncertain. Using a multi-model ensemble from the Large Ensemble Single Forcing Model Intercomparison Project, we assess model responses to greenhouse gases (GHGs) and anthropogenic aerosols (AAs) forcing, focusing on the AMOC and related processes in the high-latitude North Atlantic (NA). We find robust AMOC weakening under increasing GHGs and strengthening under increasing AAs, accompanied by largely antisymmetric spatial responses between GHGs-only and AAs-only simulations. Our results suggest that radiative forcing modifies Arctic sea-ice mass, leading to consistent changes in upper-ocean salinity over key deep-water formation regions. In turn, upper-ocean salinity and temperature modulate changes in upper-ocean density and stratification that contribute to the simulated AMOC trends. We discuss key mechanisms involved, including feedback mechanisms and the impacts of AMOC variations on the Subpolar NA. This study highlights the model responses to single external forcings on NA ocean processes, offering insights into model limitations in the representation of coupled processes that are essential for reliable climate projections.