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 also have a stronger AMOC. Intermodel variations in the overturning scale depth are also related to intermodel variations in North Atlantic surface buoyancy loss and stratification. We present a physically-motivated scaling relationship that links the scale-depth variations to buoyancy forcing and stratification in the North Atlantic, and thus connects North Atlantic surface processes to the interior ocean circulation. These results offer a framework for reducing mean-state AMOC biases in climate models.