On early-warning of full versus partial Atlantic overturning circulation collapse

Climate models indicate a possible collapse of the Atlantic Meridional Overturning Circulation (AMOC) even for moderate climate change scenarios. There is considerable uncertainty in its likelihood for a given scenario and the critical global warming threshold. An alternative are early-warning signals (EWS) in AMOC fingerprints, which leverage generic statistical properties before destabilization of a steady state (a saddle-node bifurcation). But an AMOC collapse may be a sequence of partial collapses with shutdown of deep water formation in distinct regions. A conceptual model is presented featuring a sequential shutdown in two deep water formation regions. Multiple tipping points are present that do not follow the saddle-node normal form. As a result, the choice of the observable used to monitor EWS dramatically influences the prediction of the collapse via EWS. Various trends in EWS for different observables make it hard to determine what type of collapse (partial or full) will follow.