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Climate Dynamics

, Volume 42, Issue 11–12, pp 3323–3336 | Cite as

Response of the Atlantic meridional overturning circulation to a reversal of greenhouse gas increases

  • L. C. JacksonEmail author
  • N. Schaller
  • R. S. Smith
  • M. D. Palmer
  • M. Vellinga
Article

Abstract

The reversibility of the Atlantic meridional overturning circulation (AMOC) is investigated in multi-model experiments using global climate models (GCMs) where CO2 concentrations are increased by 1 or 2 % per annum to 2× or 4× preindustrial conditions. After a period of stabilisation the CO2 is decreased back to preindustrial conditions. In most experiments when the CO2 decreases, the AMOC recovers before becoming anomalously strong. This "overshoot" is up to an extra 18.2Sv or 104 % of its preindustrial strength, and the period with an anomalously strong AMOC can last for several hundred years. The magnitude of this overshoot is shown to be related to the build up of salinity in the subtropical Atlantic during the previous period of high CO2 levels. The magnitude of this build up is partly related to anthropogenic changes in the hydrological cycle. The mechanisms linking the subtropical salinity increase to the subsequent overshoot are analysed, supporting the relationship found. This understanding is used to explain differences seen in some models and scenarios. In one experiment there is no overshoot because there is little salinity build up, partly as a result of model differences in the hydrological cycle response to increased CO2 levels and partly because of a less aggressive scenario. Another experiment has a delayed overshoot, possibly as a result of a very weak AMOC in that GCM when CO2 is high. This study identifies aspects of overshoot behaviour that are robust across a multi-model and multi-scenario ensemble, and those that differ between experiments. These results could inform an assessment of the real-world AMOC response to decreasing CO2.

Keywords

Climate AMOC GCM 

Notes

Acknowledgments

This work was supported by the Joint DECC/Defra Met Office Hadley Centre Climate Programme (GA01101). The FAMOUS experiments were integrated on HECToR, the UK National Supercomputing resource. Advice on the CCSM3.5 and CESM experiments from J. Sedláček is gratefully acknowledged. We acknowledge the World Climate Research Programme’s Working Group on Coupled Modelling, which is responsible for CMIP, and we thank the climate modeling groups (listed in Table 2 of this paper) for producing and making available their model output. For CMIP the U.S. Department of Energy’s Program for Climate Model Diagnosis and Intercomparison provides coordinating support and led development of software infrastructure in partnership with the Global Organization for Earth System Science Portals. We would also like to thank J. Kettleborough, I. Edmond and J. Gregory for developing tools and code for the downloading, archiving and analysis of CMIP5 data at the Met Office. Finally we wish to thank two anonymous reviewers for their comments which helped to improve this manuscript.

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Copyright information

© Crown Copyright 2013

Authors and Affiliations

  • L. C. Jackson
    • 1
    Email author
  • N. Schaller
    • 2
  • R. S. Smith
    • 3
  • M. D. Palmer
    • 1
  • M. Vellinga
    • 1
  1. 1.Met Office Hadley CentreExeterUK
  2. 2.Institute for Atmospheric and Climate Science, ETH ZurichZurichSwitzerland
  3. 3.NCAS-Climate, Department of Meteorology, University of ReadingReadingUK

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