Stratification-dependent mixing is employed in a coupled climate model of intermediate complexity with a 3-dimensional ocean component. The vertical diusivity in the ocean is calculated as k ~ N^(a), where N is the local buoyancy frequency. The sensitivity of the Atlantic meridional overturning circulation (AMOC) to freshwater forcing is tested by first slowly increasing, then decreasing the freshwater forcing over the North Atlantic. The fresh anomaly imposed at the surface reaches the deep ocean by vertical diffusion, and by AMOC advection via the northern convection sites. Higher values of a lead to enhanced stratification and thereby reduced vertical mixing in the subtropical forcing region. Consequently, the freshwater anomaly reaches the northern deep water formation regions less diluted, and reduces the AMOC more strongly compared to lower values of a.