Stratification-dependent mixing may increase sensitivity of a wind-driven Atlantic overturning to surface freshwater flux

Abstract

Stratification-dependent mixing is employed in a coupled climate model of intermediate complexity with a 3-dimensional ocean component. Oceanic vertical diffusivity is calculated as κ ∼ N-α, where N is the local buoyancy frequency. The sensitivity of the Atlantic meridional overturning circulation (AMOC) to freshwater forcing is tested for exponents 0 ≤ α ≤ 2 by first slowly increasing, then decreasing the freshwater forcing over the North Atlantic, keeping the model close to equilibrium. The surface fresh anomaly imposed between 20°N and 50°N in the Atlantic reaches the deep ocean by vertical diffusion, and by AMOC advection via the northern convection sites. The fresh anomaly leads to enhanced stratification and thereby reduces vertical mixing stronger for higher values of α. Consequently, the freshwater anomaly reaches the northern deep water formation regions less diluted, and reduces the AMOC more strongly compared to lower values of α. Our findings indicate that modeled changes in the AMOC depend critically on the details of the mixing parameterization employed in the model. Copyright 2009 by the American Geophysical Union.