The effect of freshwater biases on AMOC stability across the model complexity spectrum

A collapse of the Atlantic Meridional Overturning Circulation (AMOC) would have strong consequences for the global climate system. Assessing whether the AMOC will collapse in the future is difficult since current Earth System Models (ESMs) have biases. An earlier study using an intermediate complexity Earth system model (EMIC) showed the potential effect of freshwater biases on AMOC stability. However, the used model has a limited ocean model with respect to the used resolution and processes represented compared to ESMs. Here, we supplement the EMIC simulations with simulations of an ocean-only model using the same resolution as is typically used in ESMs. This allows us to study the effect of ocean resolution on the physical mechanism controlling the effect of freshwater biases on AMOC stability. We find that both the intermediate complexity and the ocean-only model behave qualitatively similar. In both models freshwater biases influence AMOC stability where negative (positive) biases in the Indian Ocean tend to stabilize (destabilize) the AMOC, whereas the opposite applies to biases in the Atlantic Ocean. Based on the freshwater biases present in most ESMs, our results suggest that most ESMs have a too stable AMOC and might therefore underestimate the probability of an AMOC collapse under future emission scenarios.