  Recent simulations performed with the Community Earth System Model (CESM)
have suggested a crucial role of sea-ice processes in AMOC hysteresis behaviour
under varying surface freshwater forcing. Here, we further investigate this
issue using additional CESM simulations and a novel conceptual ocean-sea-ice
box model. The CESM simulations show that the presence of sea ice gives rise to
the existence of statistical equilibrium states with a weak AMOC strength. This
is confirmed in the conceptual model, which captures the same AMOC hysteresis
behaviour as in the CESM simulation and where steady states are computed versus
forcing parameters. In the conceptual model, transition probabilities between
the different equilibrium states are determined using rare event techniques.
The transition probabilities from a strong AMOC state to a weak AMOC state
increase when considering sea-ice processes and indicate that sea ice promotes
these transitions. On the other hand, sea ice strongly reduces the
probabilities of the reverse transition from a weak AMOC state to a strong AMOC
state and this implies that sea ice also limits AMOC recovery. The results here
indicate that sea-ice processes play a dominant role in AMOC hysteresis width
and influence transition probabilities between the different equilibrium
states.
