Controls on the Strength and Structure of the Atlantic Meridional Overturning Circulation in Climate Models

State-of-the-art climate models simulate a large spread in the mean-state Atlantic meridional overturning circulation (AMOC), with strengths varying between 12 and 25 Sv. Here, we introduce a framework for understanding this spread by assessing the balance between the thermal-wind expression and surface water mass transformation in the North Atlantic. The intermodel spread in the mean-state AMOC strength is shown to be related to the overturning scale depth: climate models with a larger scale depth tend to have a stronger AMOC. We present a physically motivated scaling relationship that links intermodel variations in the scale depth to surface buoyancy fluxes and stratification in the North Atlantic, and thus connects North Atlantic surface processes to the interior overturning circulation. Climate models with a larger scale depth tend to have stronger surface buoyancy loss and weaker stratification in the North Atlantic. These results offer a framework for reducing mean-state AMOC biases in climate models. The Atlantic meridional overturning circulation-a branch of ocean currents confined to the Atlantic basin-strongly influences regional climate by redistributing heat, freshwater and carbon throughout the ocean. Understanding the processes that control the strength of this circulation feature, particularly in state-of-the-art climate models, remains an active area of research. In this study, we introduce a conceptual framework to understand the processes that contribute to a large spread in the strength of the Atlantic meridional overturning circulation across climate models. We find climate models that exhibit a stronger circulation also have a deeper circulation. We introduce another expression to show that climate models with a deeper circulation also have stronger surface buoyancy loss and weaker stratification in the North Atlantic, which allows for more formation of dense waters that supply the southward flowing component of the Atlantic meridional overturning circulation. These results provide a framework for reducing biases in simulating the present-day Atlantic meridional overturning circulation in climate models. The thermal-wind expression captures the intermodel spread in mean-state AMOC strength across climate models Intermodel variations in the AMOC strength are related to intermodel variations in the overturning scale depth Models with a stronger AMOC have a larger scale depth, and stronger surface buoyancy loss and weaker stratification in the North Atlantic