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°N–50°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 idealized 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 behavior and for explaining relatively rapid climate transitions in the geological past.