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

The Atlantic Meridional Overturning Circulation (AMOC) is a key component of the Earth’s climate system, and has been suggested 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. Here, we look for CSD in historical 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 SST-based AMOC index (ASSTI) based on averaging SSTs in the subpolar gyre region. No model shows CSD in the ASSTI, which is in marked disagreement with observations. This lack of CSD is reflected in the AMOC streamfunctions in most models, although individual ensemble members in some models do show signs of CSD even under a conservative significance calculation. We thus conclude that: 1) The historical AMOC in CMIP6 models is not losing stability, 2) studies of AMOC stability should consider an ensemble of realisations, 3) there is no non-AMOC-related physical process in CMIP6 models which could cause signs of CSD in North-Atlantic SSTs in the 1850–2014 period, and thus the CSD in the observed ASSTI is likely a sign of a change in the AMOC. This final result suggests that observed changes in the ASSTI could indicate a loss of stability in the real-world AMOC.