The influence of freshwater biases on AMOC stability and consequences for CMIP6 models

The Atlantic Meridional Overturning Circulation (AMOC) modulates global climate and has been identified as a potential tipping element that might collapse under future climate change. Such a collapse would have strong global consequences for the climate system, ecosystems and society. The IPCC AR6 report states that it is unlikely that the AMOC will collapse in the 21^st century which is mostly based on CMIP6 type Earth System Model results. However, these models have strong biases that can affect AMOC stability. If these models are biased towards a too stable AMOC, they might underestimate the probability of an AMOC collapse this century. To better understand the effects of freshwater biases on AMOC stability we perform experiments with the intermediate complexity Earth System Model CLIMBER-X. By introducing both positive and negative freshwater biases in the Atlantic and Indian Ocean we can gain a better understanding on how these biases affect AMOC stability. We find that introducing fresh biases in the Indian Ocean leads to an increase in stability, whereas fresh biases in the Atlantic Ocean lead to a decrease in stability. The combined effect of the biases in the Atlantic and Indian Ocean is near linear. We project the results of CLIMBER-X onto CMIP6 model biases such that we can assess whether CMIP6 models are likely simulating a too stable or too unstable AMOC.