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

TL;DR

This study uses bifurcation analysis on a global ocean model to examine how biases in freshwater flux influence the multiple equilibrium states of the Atlantic Meridional Overturning Circulation, revealing shifts caused by biases.

Contribution

It demonstrates how specific freshwater flux biases in climate models affect the stability regimes of the AMOC, providing insights into model biases and their impacts.

Findings

Freshwater flux bias shifts the AMOC multiple equilibrium regime.

The hysteresis width remains largely unaffected by the bias.

Bias effects depend on how North Atlantic freshwater flux is compensated.

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.