"Stability of the Atlantic meridional overturning circulation: A model intercomparison," by Andrew Weaver et al., GRL 39 (2012); doi:10.1029/2012GL053763

Geophysical Research Letters, 39 (2012) L20709; doi:10.1029/2012GL053763

Stability of the Atlantic meridional overturning circulation: A model intercomparison

Key Points

• All climate models project very similar behavior during the 21st century
• No model exhibits an abrupt change of the MOC
• More than 1/2 of the models are in the bistable regime ==> not overly stable

Andrew J. Weaver (School of Earth and Ocean Sciences, University of Victoria, Victoria, BC, Canada), Jan Sedláček (Institute for Atmospheric and Climate Science, ETH, Zurich, Switzerland), Michael Eby (School of Earth and Ocean Sciences, University of Victoria, Victoria, BC, Canada), Kaitlin Alexander (School of Earth and Ocean Sciences, University of Victoria, Victoria, BC, Canada), Elisabeth Crespin (Georges Lemaître Centre for Earth and Climate Research, Earth and Life Institute, Université Catholique de Louvain, Louvain-La-Neuve, Belgium), Thierry Fichefet (Georges Lemaître Centre for Earth and Climate Research, Earth and Life Institute, Université Catholique de Louvain, Louvain-La-Neuve, Belgium), Gwenaëlle Philippon-Berthier (Georges Lemaître Centre for Earth and Climate Research, Earth and Life Institute, Université Catholique de Louvain, Louvain-La-Neuve, Belgium), Fortunat Joos (Climate and Environmental Physics, Physics Institute, University of Bern, Bern, Switzerland, Oeschger Centre for Climate Change Research, University of Bern, Bern, Switzerland), Michio Kawamiya (Research Institute for Global Change, JAMSTEC, Yokohama, Japan), Katsumi Matsumoto (Department of Earth Sciences, University of Minnesota, Twin Cities, Minneapolis, MN, USA), Marco Steinacher (Climate and Environmental Physics, Physics Institute, and the Oeschger Centre for Climate Change Research, University of Bern, Bern, Switzerland), Kaoru Tachiiri (Research Institute for Global Change, JAMSTEC, Yokohama, Japan), Kathy Tokos (Department of Earth Sciences, University of Minnesota, Twin Cities, Minneapolis, MN, USA), Masakazu Yoshimori (Atmosphere and Ocean Research Institute, University of Tokyo, Tokyo, Japan), Kirsten Zickfeld (Department of Geography, Simon Fraser University, Vancouver, BC, Canada)

Abstract 

The evolution of the Atlantic Meridional Overturning Circulation (MOC) in 30 models of varying complexity is examined under four distinct Representative Concentration Pathways. The models include 25 Atmosphere-Ocean General Circulation Models (AOGCMs) or Earth System Models (ESMs) that submitted simulations in support of the 5th phase of the Coupled Model Intercomparison Project (CMIP5) and 5 Earth System Models of Intermediate Complexity (EMICs). While none of the models incorporated the additional effects of ice sheet melting, they all projected very similar behaviour during the 21st century. Over this period the strength of MOC reduced by a best estimate of 22% (18%–25%; 5%–95% confidence limits) for RCP2.6, 26% (23%–30%) for RCP4.5, 29% (23%–35%) for RCP6.0 and 40% (36%–44%) for RCP8.5. Two of the models eventually realized a slow shutdown of the MOC under RCP8.5, although no model exhibited an abrupt change of the MOC. Through analysis of the freshwater flux across 30°–32°S into the Atlantic, it was found that 40% of the CMIP5 models were in a bistable regime of the MOC for the duration of their RCP integrations. The results support previous assessments that it is very unlikely that the MOC will undergo an abrupt change to an off state as a consequence of global warming.

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Received 4 September 2012; accepted 24 September 2012; published 24 October 2012.

Weaver, A. J., et al. (2012), Stability of the Atlantic meridional overturning circulation: A model intercomparison, Geophys. Res. Lett., 39, L20709, doi:10.1029/2012GL053763.

http://www.agu.org/pubs/crossref/2012/2012GL053763.shtml