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

We study hysteresis properties of the Atlantic Meridional Overturning Circulation (AMOC) under a slowly-varying North Atlantic (20 degrees N-50 degrees 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. The Atlantic Meridional Overturning Circulation (AMOC) is considered to be a tipping element in the climate system. We here simulate AMOC tipping events by slowly varying the North Atlantic freshwater forcing in a state-of-the-art global climate model. The AMOC collapses when this forcing is sufficiently large and, when reversing this forcing, the recovery occurs at much smaller values of the forcing than the collapse, giving rise to hysteresis behavior. The AMOC changes are much faster during its recovery than during its collapse and this asymmetry is caused by the effect of the North Atlantic sea-ice distribution on the AMOC recovery. The results demonstrate that AMOC tipping does not only occur in simplified climate models, but in a large hierarchy of climate models. A wide Atlantic Meridional Overturning Circulation (AMOC) hysteresis is found in a state-of-the-art global climate modelThe AMOC recovery is about six times faster than the AMOC collapseThe North Atlantic sea-ice cover plays a dominant role in the hysteresis asymmetry