The AMOC Stability Paradigm: Review and Synthesis
The Atlantic Meridional Overturning Circulation (AMOC) redistributes significant amounts of heat, and is an important component of the climate system. Paleoclimatological studies suggest that collapses of the AMOC may have contributed to rapid climate fluctuations during the Pleistocene. Indeed, theoretical and modeling studies indeed suggest that the AMOC could have two modes of operation, one with a strong overturning circulation, and one with a reversed or collapsed circulation cell. Models still disagree on whether the AMOC in the current climate has multiple stable equilibria, and if so, how close we are to the stability thresholds. Here we review the literature and present a synthesis on the so-called AMOC stability paradigm.
This is the most comprehensive review of the AMOC stability paradigm to date. Our main conclusions are:
• It cannot be ruled out that the current or future AMOC is in a regime of multiple equilibria;
• Determining the proximity of the AMOC to stability thresholds is a main research challenge;
• Further research is needed to identify observable metrics that distinguish between an AMOC slowdown and a collapse.
There is still a lot of uncertainty about whether the AMOC has multiple equilibria, and if so, where the stability thresholds are. This uncertainty results from persistent model biases, imperfect parameterizations, and unresolved physics; as well as the inherent non-linearity of the system. The mere possibility that the AMOC could be subject to stability thresholds should be a strong reason for concern in an era where human activity is forcing significant changes to the Earth system.