Processes and feedbacks for AMOC stability following freshwater-induced weakening

The stability of the Atlantic Meridional Overturning Circulation (AMOC) is a key uncertainty in climate projections. This study investigates the processes and feedbacks that determine whether the AMOC recovers or remains in a weak state following large freshwater perturbations, using coordinated hosing experiments from eight CMIP6-era climate models. The experiments apply a sustained surface freshwater input (hosing) over the North Atlantic and Arctic, weakening the AMOC. Results show that in half of the models, the AMOC does not recover within at least 100 years after hosingceases, indicating quasi-irreversible transitions to weak states. The key determinant of recovery is the state reached during hosing: models with the weakest AMOC and shallowest wintertime convection fail to recover. Analysis reveals that continued surface heat loss and dense water formation, particularly in the Greenland-Iceland-Norwegian (GIN) Seas, are essential for AMOC recovery. In models where these processes persist, the AMOC is maintained; where they cease, the AMOC remains weak. Feedbacks involving ocean heat and salt transport, surface fluxes, and sea ice extent are examined. In some models there is an increased advection of heat and salt into the GIN Seas, supporting deep convection and water mass transformation, and leading to a recovery of the AMOC. Other models show reduced advection of heat and salt, suppressing convection and preventing the AMOC recovery. These findings suggest that under strong freshwater forcing, some climate models can enter long-lasting weak AMOC states, with important implications for future climate risk.