AMOC Evolution in the Last Deglaciation: Forcing Mechanism, Thermohaline Instability and Implications

The forcing mechanism and instability of the Atlantic Meridional Overtuning Circulation (AMOC) over the last 21,000 years is studied using transient simulations under realistic forcings in the NCAR-CCSM3. First, in addition to the strong millennial AMOC variability forced by melting water fluxes, the background AMOC is determined by two opposing effects: the intensification by the rising atmospheric CO2 and the reduction by the retreating ice sheet, both through the sea ice feedbacks in the North Atlantic. As a result, the AMOC strength does not change significantly after the deglaciation. Second, the model AMOC exhibits a monostable behavior. This monostable AMOC, which has been observed in almost all state-of-art coupled general climate models (CGCMs), is likely to be caused by a systematic model bias that is associated with the tropical bias, the resulted freshwater flux and AMOC freshwater export. This AMOC over-stabilization bias needs to be improved in these CGCMs to allow for a credible projection of AMOC evolution in the future.