Why is the AMOC Monostable in Coupled General Circulation Models?

• Published in: Journal of Climate
• Main Authors: Liu, Wei, Liu, Zhengyu, Brady, Esther C.
• Other Authors: Liu, ZY (reprint author), 1225 W Dayton St, Madison, WI 53706 USA., Univ Wisconsin, Ctr Climat Res, Madison, WI USA., Univ Wisconsin, Dept Atmospher & Ocean Sci, Madison, WI USA., Peking Univ, Lab Climate Ocean & Atmosphere Studies, Beijing 100871, Peoples R China., Natl Ctr Atmospher Res, Climate & Global Dynam Div, Boulder, CO 80307 USA., 1225 W Dayton St, Madison, WI 53706 USA.
• Format: Journal/Newspaper
• Language: English
• Published: journal of climate 2014
• Subjects: Thermocline circulation; Ocean circulation; Meridional overturning circulation; Stability; Feedback; ATLANTIC THERMOHALINE CIRCULATION; MULTIPLE EQUILIBRIA REGIME; ABRUPT CLIMATE-CHANGE; MELTWATER PULSE 1A; FRESH-WATER; LAST DEGLACIATION; ATMOSPHERE MODELS; FLUX ADJUSTMENTS; OCEAN MODEL; North Atlantic; South Atlantic Ocean
• Online Access: https://hdl.handle.net/20.500.11897/389797; https://doi.org/10.1175/JCLI-D-13-00264.1

This paper is concerned with the question: why do coupled general circulation models (CGCM) seem to be biased toward a monostable Atlantic meridional overturning circulation (AMOC)? In particular, the authors investigate whether the monostable behavior of the CGCMs is caused by a bias of model surface climatology. First observational literature is reviewed, and it is suggested that the AMOC is likely to be bistable in the real world in the past and present. Then the stability of the AMOC in the NCAR Community Climate System Model, version 3 (CCSM3) is studied by comparing the present-day control simulation (without flux adjustment) with a sensitivity experiment with flux adjustment. It is found that the monostable AMOC in the control simulation is altered to a bistable AMOC in the flux-adjustment experiment because a reduction of the surface salinity biases in the tropical and northern North Atlantic leads to a reduction of the bias of freshwater transport in the 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 overstabilize the AMOC and therefore biases the AMOC from bistable toward monostable state. This conclusion is consistent with a further analysis of the stability indicator of two groups of IPCC Fourth Assessment Report (AR4) CGCMs: one without and the other with flux adjustment. Because the tropical bias is a common feature among all CGCMs without flux adjustment, the authors 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. Meteorology & Atmospheric Sciences SCI(E) EI 3 ARTICLE [e-mail] 6 2427-2443 27