The effect of model freshwater flux biases on the multi-stable regime of the AMOC

Abstract

It is known that global climate models (GCMs) have substantial biases in the surface freshwater flux which forces the ocean component of these models. Using numerical bifurcation analyses on a global ocean model, we study here the effect of a specific freshwater flux bias on the multiple equilibrium regime of the Atlantic Meridional Overturning Circulation (AMOC). We find that a (positive) freshwater flux bias over the Indian Ocean shifts the multiple equilibrium regime to larger values of North Atlantic freshwater input but hardly affects the associated hysteresis width. The magnitude of this shift depends on the way the anomalous North Atlantic freshwater flux is compensated. We explain the changes in bifurcation diagrams using the freshwater balance over the Atlantic basin. The results suggest that state-of-the-art GCMs may have an AMOC multiple equilibrium regime, but that it is located in a parameter regime that is considered unrealistic and hence is not explored.