Coupled Effects of Climate Sensitivity and Aerosol Forcing on Future AMOC Stability: The IGSM–CESM Framework

Abstract: The future stability of the Atlantic Meridional Overturning Circulation (AMOC) remains highly uncertain, with its evolution influenced by the combined effects of greenhouse gas forcing, climate sensitivity, aerosol forcing, and internal variability. Here, we develop an emissions-driven IGSM–CESM framework using coordinated ensemble simulations that systematically span plausible climate sensitivities, sulfate aerosol forcing adjustments, and multiple initial conditions to quantify their combined influence on future AMOC changes. Initial-condition ensembles, generated by sampling different ocean states from a long control simulation, reveal distinct AMOC trajectories under identical external forcing. These divergent responses indicate that the pre-existing ocean state influences the timing and magnitude of future AMOC weakening and that internal variability contributes to uncertainty in projected circulation changes. Across climate sensitivity experiments, high-sensitivity simulations exhibit accelerated AMOC weakening compared with low-sensitivity simulations. These changes are accompanied by pronounced North Atlantic cooling by the end of the 21st century, consistent with reduced AMOC-driven northward ocean heat transport. By 2100, high-sensitivity simulations show an approximately 3 Sv greater reduction in AMOC strength at 26°N compared with low-sensitivity simulations, highlighting the strong influence of climate sensitivity on future AMOC stability. Sulfate aerosol forcing further modulates the magnitude and timing of these sensitivity-dependent AMOC responses. Our results demonstrate that future AMOC projections emerge from the coupled effects of greenhouse gas forcing, climate sensitivity, sulfate aerosol forcing, and internal ocean variability, including dependence on the initial ocean state, providing a robust framework for assessing future AMOC weakening and its potential climate impacts.