A Diagnostic Indicator of the Stability of the Atlantic Meridional Overturning Circulation in CCSM3

A diagnostic indicator Delta M-ov is proposed in this paper to monitor the stability of the Atlantic meridional overturning circulation (AMOC). The Delta M-ov is a diagnostic for a basinwide salt-advection feedback and defined as the difference between the freshwater transport induced by the AMOC across the southern border of the Atlantic Ocean and the overturning liquid freshwater transport from the Arctic Ocean to the North Atlantic. As validated in the Community Climate System Model, version 3 (CCSM3), for an AMOC in the conveyor state, a positive Delta M-ov (freshwater convergence) in the Atlantic basin indicates a monostable AMOC and a negative Delta M-ov (freshwater divergence) indicates a bistable AMOC. Based on Delta M-ov, the authors investigate the AMOC stability in the Last Glacial Maximum (LGM) and analyze the modulation of the AMOC stability by an open/closed Bering Strait. Moreover, the authors estimate that the real AMOC is likely to be bistable in the present day, since some observations suggest a negative Delta M-ov (freshwater divergence) is currently in the Atlantic basin. However, this estimation is very sensitive to the choice of the observational data.