Evaluation of the mechanisms acting on the Atlantic Meridional Overturning Circulation in CESM2 for the 1pctCO2 experiment

Abstract The Atlantic Meridional Overturning Circulation (AMOC) is a crucial component of the Earth’s climate system due to its fundamental role in the uptake and distribution of important tracers such as heat, carbon, and oxygen. Evaluating changes in AMOC in a warming climate is paramount to understanding the potential influence on global climate. In this study, we utilized outputs produced by the Community Earth System Model for Phase 6 of the Coupled Model Intercomparison Project, in addition to in situ data, to investigate changes in AMOC and the associated physical processes. Two experiments were carried out: a control experiment and an experiment with an annual increase of 1% CO2. Our analysis revealed a significant decrease in AMOC, with the upper branch weakening more than 60% and shallowing more than 600 m, compared to in situ data, leading to a reduction in heat transport to high latitudes in the North Atlantic. This decline was associated with changes in mixed layer depth and buoyancy in high latitudes of the North Atlantic, resulting in the shutdown of deep convection and potentially affecting the formation of North Atlantic Deep Water and Antarctic Bottom Water. Furthermore, the increase in CO2 levels altered the pattern of several parameters and the way they determine the flow of freshwater to the North Atlantic Ocean. Our findings suggest that continued warming may further weaken AMOC, with significant implications for global climate.