We use a reduced complexity climate model with a three-dimensional ocean component and realistic topography to investigate the effect of stratification-dependent mixing on the sensitivity of the North Atlantic subpolar gyre (SPG), and the Atlantic meridional overturning circulation (AMOC), to idealized CO2 increase and peaking scenarios. The vertical diffusivity of the ocean interior is parameterized as κ ∼ N −α, where N is the local buoyancy frequency. For all parameter values 0 ≤ α ≤ 3, we find the SPG, and subsequently the AMOC, to weaken in response to increasing CO2 concentrations. The weakening is significantly stronger for α ≥ αcr ≈ 1.5. Depending on the value of α, two separate model states develop. These states remain different after the CO2 concentration is stabilized, and in some cases even after the CO2 concentration has been decreased again to the pre-industrial level. This behaviour is explained by a positive feedback between stratification and mixing anomalies in the Nordic Seas, causing a persistent weakening of the SPG.