Why is the AMOC Monostable in Coupled General Circulation Models?

Observations suggest that the AMOC is likely to be bistable in the real world, whereas the AMOC is biased toward monostable in coupled general circulation models (CGCMs). The reason for such inconsistency lies in the bias of model surface climatology. Based on a refined stability indicator, we compare the AMOC stability in two simulations: the NCAR CCSM3 present-day control run and a sensitivity experiment with flux adjustment. The monostable AMOC in the control simulation alters to a bistable AMOC in the flux-adjustment experiment owing to a reduction of the surface salinity biases in the tropical and northern North Atlantic. In particular, the tropical bias associated with the double ITCZ reduces salinity in the upper South Atlantic Ocean and, in turn, the AMOC freshwater export, which tends to over-stabilize the AMOC and therefore biases the AMOC from bistable toward monostable state. Given the tropical bias as a common issue in CGCMs without flux adjustment, we propose that the surface climate bias, notably the tropical bias in the Atlantic, may contribute significantly to the monostability of AMOC behavior in current CGCMs.