We use the Earth System Model of Intermediate Complexity CLIMBER­3alpha to investigate the effect of stratification-­dependent mixing on the stability of the Atlantic Meridional Overturning Cir­culation under an idealized CO2 increase scenario. The vertical dif­fusivity of the ocean is parameterized as k ~ N^(-a), where N is the local buoyancy frequency. For all parameter values of 0 < a < 3, we find the AMOC to decrease in response to increasing CO2 concentrations. However, the decrease is significantly stronger for a > a_cr = 1.5. Depending on the value of a, two separate model states develop that remain different even after the CO2 concentrattions are stabilizied. This is explained by a halt of dense water formation in the subpolar gyre, which is caused by a positive feedback between stratification and mixing anomalies.