Stratification-dependent mixing may increase sensitivity of Atlantic overturning to global warming

Full text: We use the earth system model of intermediate complexity CLIMBER-3 to investigate the effect of stratification-dependent mixing on the stability of the Atlantic Meridional Overturning Circulation (AMOC) under an idealized CO2 increase scenario. The vertical diffusivity κ of the ocean is parameterized as κ ∼ N-α, where N is the local buoyancy frequency. For all parameter values 0 ≤ α ≤ 3, we find the AMOC to decrease in response to increased CO2 concentrations. The sensitivity of the AMOC is significantly stronger for α ≥ αcr ∼ 1.5, also after stabilization of the CO2 concentration. This threshold behavior 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. Our findings indicate that climate models using time-invariant vertical mixing may underestimate the sensitivity of the AMOC to global warming. (author)