No critical slowing down in the Atlantic Overturning Circulation in historical CMIP6 simulations

Abstract. The Atlantic Meridional Overturning Circulation (AMOC) is a key component of the Earth’s climate system, and is theorized to have multiple stable states. Critical slowing down (CSD) can detect stability changes in Earth system components, and has been found in sea-surface temperature (SST) based fingerprints of the AMOC. We look for CSD in simulations from 27 models from the sixth Climate Model Intercomparison Project (CMIP6). We calculate three different CSD indicators for the AMOC streamfunction strengths at 26.5° N and 35° N, as well as for a previously suggested AMOC fingerprint based on averaging SSTs in the subpolar gyre region. We find mixed results. Most models do not have a statistically significant sign of CSD, and no model has a conclusive sign of CSD in all ensemble members. However, some models exhibit a number of significant increases in the CSD indicators of the streamfunction strength that is highly unlikely to occur by chance (p<0.05). In addition, the number of significant increases in the AMOC SST fingerprint are as would be expected by random chance. Since we do not know whether or not the AMOC in these models is approaching a critical transition, we cannot deny or confirm the validity of CSD for detecting an upcoming AMOC collapse. However, we can confirm that the AMOC SST fingerprint is not prone to false positives.