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Recent simulations performed with the Community Earth System Model (CESM)have suggested a crucial role of sea-ice processes in AMOC hysteresis behaviourunder varying surface freshwater forcing. Here, we further investigate thisissue using additional CESM simulations and a novel conceptual ocean-sea-icebox model. The CESM simulations show that the presence of sea ice gives rise tothe existence of statistical equilibrium states with a weak AMOC strength. Thisis confirmed in the conceptual model, which captures the same AMOC hysteresisbehaviour as in the CESM simulation and where steady states are computed versusforcing parameters. In the conceptual model, transition probabilities betweenthe different equilibrium states are determined using rare event techniques.The transition probabilities from a strong AMOC state to a weak AMOC stateincrease when considering sea-ice processes and indicate that sea ice promotesthese transitions. On the other hand, sea ice strongly reduces theprobabilities of the reverse transition from a weak AMOC state to a strong AMOCstate and this implies that sea ice also limits AMOC recovery. The results hereindicate that sea-ice processes play a dominant role in AMOC hysteresis widthand influence transition probabilities between the different equilibriumstates.

The Role of Sea-ice Processes on the Probability of AMOC Transitions