The Atlantic Meridional Overturning Circulation (AMOC) is contributing considerably to the mild climate of north­ western Europe by transporting heat from the low latitudes northward. On long timescales, the AMOC is limited by the amount of potential energy that is put into the ocean by vertical mixing. By assuming a constant background rate of mixing, most state-­of-­the-­art climate models do not represent the effect of vertical mixing well, since theoretical arguments and mea­surements indicate that the vertical mixing strongly depends on local physical parameters ? above all, on stratification. We implemented stratification-­dependent mixing into a cou­pled climate model, and found the sensitivity of the AMOC to freshwater forcing to depend critically on the coupling be­ tween stratification and mixing. Weak coupling reproduces results from previous studies that assume constant vertical mixing. Stronger (and more realistic) coupling however al­lows for a positive feedback in the high northern latitudes stopping winter deep convection. Thus, the overturning north of the Greenland­Scotland Ridge is stopped, and the heat transport into that region is weakened.