Generalized stability landscape of the Atlantic Meridional Overturning Circulation

The Atlantic meridional overturning circulation (AMOC) plays a crucial role in shaping climate conditions over the North Atlantic region and beyond, and its instability explains past abrupt climate transitions and millennial-scale glacial climate variability.
Here we use the fast Earth system model CLIMBER-X to explore the stability of the AMOC when faced with combined changes in atmospheric CO₂ concentrations and FWF in the North Atlantic under different configurations of the Northern Hemisphere (NH) ice sheets.
We find four different AMOC states associated with qualitatively different convection patterns. Apart from an Off AMOC state with no North Atlantic deep-water formation and a Modern-like AMOC with deep water forming in the Labrador and Nordic seas as observed at present, we find a Weak AMOC state with convection occurring south of 55°N and a Strong AMOC state characterized by deep-water formation extending into the Arctic. Generally, the equilibrium strength of the AMOC increases with increasing CO₂ and decreases with increasing FWF.
Our results show that Dansgaard-Oeschger (DO) oscillations originate from convective instability leading to transitions between the Weak and Modern AMOC states when the surface buoyancy flux integrated over the northern North Atlantic (>55°N) changes sign. Under typical mid-glacial conditions in terms of NH ice sheet size and atmospheric CO₂ the model produces pronounced internal millennial-scale climate variability that resembles observed DO oscillations.