On early warning of full versus partial Atlantic overturning circulation collapse

Climate models indicate a significant slowdown of the Atlantic Meridional Overturning Circulation (AMOC) in the future, and some suggest it may collapse irreversibly to a substantially weakened state. The global warming threshold where this may happen is highly uncertain. An alternative to model-based threshold estimation is early warning signals (EWS) in AMOC fingerprints, which can predict destabilization of a steady state (a saddle-node bifurcation) from generic changes in statistical properties. But an AMOC collapse may be a sequence of partial weakenings with shutdown of deep water formation in distinct regions. A conceptual model featuring sequential tipping points in two such regions is presented. Since the system only follows the expected saddle-node normal form when very close to each tipping point, a variety of trends in EWS are seen for different observables. This makes it hard to determine what type of collapse (partial or full) will follow, and when it will happen.