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Robust decrease in El Niño/Southern Oscillation amplitude under long-term warming

Abstract

El Niño/Southern Oscillation (ENSO) is the primary mode of interannual climate variability, and understanding its response to climate change is critical, but research remains divided on the direction and magnitude of that response. Some twenty-first-century simulations suggest that increased CO2 strengthens ENSO, but studies suggest that on palaeoclimate timescales higher temperatures are associated with a reduced ENSO amplitude and a weaker Pacific zonal temperature gradient, sometimes termed a ‘permanent El Niño’. Internal variability complicates this debate by masking the response of ENSO to forcing in centennial-length projections. Here we exploit millennial-length climate model simulations to disentangle forced changes to ENSO under transient and equilibrated conditions. On transient timescales, models show a wide spread in ENSO responses but, on millennial timescales, nearly all of them show decreased ENSO amplitude and a weakened Pacific zonal temperature gradient. Our results reconcile differences among twenty-first-century simulations and suggest that CO2 forcing dampens ENSO over the long term.

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Fig. 1: Methods used to analyse ENSO in LongRunMIP simulations.
Fig. 2: Percentage changes in ENSO amplitude after a sustained increase in CO2.
Fig. 3: Evolution of ENSO amplitude under increased CO2 for three representative LongRunMIP models.
Fig. 4: Relationship between zonal temperature gradient change and ENSO amplitude change.

Data availability

All processed data required to reproduce the results of this study are available at https://github.com/ccallahan45/Callahan-et-al_NCC_2021/, archived on Zenodo at https://doi.org/10.5281/zenodo.4718010 (ref. 74). Raw LongRunMIP data are not provided due to large file sizes, but these data are publicly available at https://data.iac.ethz.ch/longrunmip/, with further information available at http://www.longrunmip.org.

Code availability

Analysis code required to reproduce the results of this study is available at https://github.com/ccallahan45/Callahan-et-al_NCC_2021/, archived on Zenodo at https://doi.org/10.5281/zenodo.4718010 (ref. 74).

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Acknowledgements

We thank M. Jansen, N. Maher, J. Franke and K. Schwarzwald for helpful discussions and insights. This research was performed as part of the Center for Robust Decision-making on Climate and Energy Policy at the University of Chicago, funded by NSF through the Decision Making Under Uncertainty programme (grant no. SES-1463644 to E.J.M.). Computing resources were provided by the University of Chicago Research Computing Center and Dartmouth College Research Computing. M.R. is funded by the Alexander von Humboldt foundation. This project received funding from the European Research Council under the European Union’s Horizon 2020 research and innovation programme (grant agreement no. 786427, project Couplet) to J.B.-J. Finally, this work would not have been possible without the efforts of the contributors to the LongRunMIP project.

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C.W.C., C.C. and E.J.M. conceptualized the study. C.W.C. and C.C. performed the analyses. J.B.-J., M.R., S.Y. and E.J.M. provided data, analysis tools and feedback on results. C.W.C., C.C. and E.J.M. wrote the manuscript, with all authors providing feedback.

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Correspondence to Chen Chen or Elisabeth J. Moyer.

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Callahan, C.W., Chen, C., Rugenstein, M. et al. Robust decrease in El Niño/Southern Oscillation amplitude under long-term warming. Nat. Clim. Chang. 11, 752–757 (2021). https://doi.org/10.1038/s41558-021-01099-2

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