Asymmetry of AMOC Hysteresis in a State-of-the-Art Global Climate Model

Abstract: We study hysteresis properties of the Atlantic Meridional Overturning Circulation (AMOC) under a slowly-varying North Atlantic (20$^{\circ}$N -- 50$^{\circ}$N) freshwater flux forcing in state-of-the-art Global Climate Model (GCM), the Community Earth System Model. Results are presented of a full hysteresis simulation ($4,400$ model years) and show that there is a hysteresis width of about $0.4$ Sv. This demonstrates that an AMOC collapse and recovery do not only occur in conceptual and idealised climate models, but also in a state-of-the-art GCM. The AMOC recovery is about a factor six faster than the AMOC collapse and this asymmetry is due to the major effect of the North Atlantic sea-ice distribution on the AMOC recovery. The results have implications for projections of possible future AMOC behaviour and for explaining relatively rapid climate transitions in the geological past.