An event-based approach to understanding decadal fluctuations in the Atlantic meridional overturning circulation

Abstract

Many previous studies have shown that unforced climate model simulations exhibit decadal-scale fluctuations in the Atlantic meridional overturning circulation (AMOC), and that this variability can have impacts on surface climate fields. However, the robustness of these surface fingerprints across different models is less clear. Furthermore, with the potential for coupled feedbacks that may amplify or damp the response, it is not known whether the associated climate signals are linearly related to the strength of the AMOC changes, or if the fluctuation events exhibit nonlinear behaviour with respect to their strength or polarity. To explore these questions, we introduce an objective and flexible method for identifying the largest natural AMOC fluctuation events in multicentennial/multimillennial simulations of a variety of coupled climate models. The characteristics of the events are explored, including their magnitude, meridional coherence and spatial structure, as well as links with ocean heat transport and the horizontal circulation. The surface fingerprints in ocean temperature and salinity are examined, and compared with the results of linear regression analysis. It is found that the regressions generally provide a good indication of the surface changes associated with the largest AMOC events. However, there are some exceptions, including a nonlinear change in the atmospheric pressure signal, particularly at high latitudes, in HadCM3. Some asymmetries are also found between the changes associated with positive and negative AMOC events in the same model. Composite analysis suggests that there are signals that are robust across the largest AMOC events in each model, which provides reassurance that the surface changes associated with one particular event will be similar to those expected from regression analysis. However, large differences are found between the AMOC fingerprints in different models, which may hinder the prediction and attribution of such events in reality.

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Acknowledgments

This work was funded by the UK Natural Environment Research Council (NERC) RAPID-WATCH RAPIT project. We are grateful to Laura Jackson for many helpful discussions and to Jon Robson and David Ferreira for comments on the manuscript. We thank MPI, IPSL, GFDL and the Met Office for making their model data available. We are grateful to Robin Smith, Daniela Matei, Juliette Mignot and Bert Wouters for the simulations of FAMOUS, ECHAM5/MPI-OM, IPSL CM4, and EC-EARTH respectively. We acknowledge the Program for Climate Model Diagnosis and Intercomparison (PCMDI) and the WCRP’s Working Group on Coupled Modelling (WGCM) for their roles in making available the WCRP CMIP3 multi-model dataset. Support of this dataset is provided by the Office of Science, U.S. Department of Energy. We thank two anonymous reviewers for comments and suggestions that improved the manuscript.

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Correspondence to Lesley Allison.

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Allison, L., Hawkins, E. & Woollings, T. An event-based approach to understanding decadal fluctuations in the Atlantic meridional overturning circulation. Clim Dyn 44, 163–190 (2015). https://doi.org/10.1007/s00382-014-2271-9

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Keywords

  • Atlantic meridional overturning circulation
  • Thermohaline circulation
  • Decadal variability
  • Events
  • Nonlinear response