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Reconciling opposing Walker circulation trends in observations and model projections

Abstract

A strengthening of the Pacific Walker circulation (PWC) over recent decades triggered an intense debate on the validity of model-projected weakening of the PWC in response to anthropogenic warming. However, limitations of in situ observations and reanalysis datasets have hindered an unambiguous attribution of PWC changes to either natural or anthropogenic causes. Here, by conducting a comprehensive analysis based on multiple independent observational records, including satellite observations along with a large ensemble of model simulations, we objectively determine the relative contributions of internal variability and anthropogenic warming to the emergence of long-term PWC trends. Our analysis shows that the satellite-observed changes differ considerably from the model ensemble-mean changes, but they also indicate substantially weaker strengthening than implied by the reanalyses. Furthermore, some ensemble members are found to reproduce the observed changes in the tropical Pacific. These findings clearly reveal a dominant role of internal variability on the recent strengthening of the PWC.

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Fig. 1: Model-simulated forced response of PWC to anthropogenic warming.
Fig. 2: Comparison of model-simulated PWC changes with reanalyses and in situ observations.
Fig. 3: Model-simulated versus satellite-observed PWC changes.
Fig. 4: Model-simulated versus observed PWC intensity changes over the period 1979–2014.
Fig. 5: Impacts of internal variability on the PWC changes over the period 1979–2014.

Data availability

The CESM Large Ensemble Project simulation output can be obtained from http://www.cesm.ucar.edu/projects/community-projects/LENS/data-sets.html. The CMIP5 model output analysed in this study is available from the Earth System Grid Federation server (https://esgf-node.llnl.gov/search/cmip5/). HadSLP2 data can be downloaded from the Met Office Hadley Centre (http://www.metoffice.gov.uk/hadobs/hadslp2/). WASWind data are available from http://www.dpac.dpri.kyoto-u.ac.jp/tokinaga/waswind.html. A bias-corrected version of EECRA data can be obtained from ref. 38. HIRS channel 12 brightness temperature data can be downloaded from a NOAA website (https://www.ncei.noaa.gov/data/hirs-brightness-temperature/access/). ERA-Interim and ERA-20C reanalysis data are accessible via the ECMWF data server (http://apps.ecmwf.int/datasets/). ECMWF ORA-S4 data can be obtained from the University of Hamburg Integrated Climate Data Center (http://icdc.cen.uni-hamburg.de/). MERRA-2 reanalysis data are available from the NASA Goddard Earth Sciences Data and Information Services Center (https://gmao.gsfc.nasa.gov/reanalysis/MERRA-2/data_access/). JRA-55 reanalysis data are accessible via the JMA Data Dissemination System (http://jra.kishou.go.jp/JRA-55/index_en.html#download). NOAA Interpolated OLR data, NOAA 20th Century Reanalysis V2c data, ERSST version 5 SST data and GPCP precipitation data are available from the NOAA/OAR/ESRL PSD website (https://www.esrl.noaa.gov/psd/data/gridded/data.interp_OLR.html, http://www.esrl.noaa.gov/psd/data/gridded/data.20thC_ReanV2c.html, https://www.esrl.noaa.gov/psd/data/gridded/data.noaa.ersst.v5.html and https://www.esrl.noaa.gov/psd/data/gridded/data.gpcp.html).

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Acknowledgements

The authors thank the National Center for Atmospheric Research for providing the CESM Large Ensemble Project output. We acknowledge the World Climate Research Programme’s Working Group on Coupled Modeling, which is responsible for the CMIP, and we thank the climate modelling groups for producing and making available their model output. For CMIP, the US Department of Energy’s Program for Climate Model Diagnosis and Intercomparison provides coordinating support and led the development of software infrastructure in partnership with the Global Organization for Earth System Science Portals. E.-S.C., A.T. and K.-J.H. were supported by Institute for Basic Science under grant IBS-R028-D1. B.J.S. was supported by grants from the NOAA Climate Program Office.

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E.-S.C., A.T. and B.J.S. designed the study, performed the analysis and wrote the manuscript. K.-J.H., L.S. and V.O.J. contributed to the writing of the manuscript. All authors contributed to interpreting results and improvements of the manuscript.

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Correspondence to Eui-Seok Chung.

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Journal peer review information Nature Climate Change thanks Remy Roca, Tobias Bayr and other anonymous reviewer(s) for their contribution to the peer review of this work.

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Chung, ES., Timmermann, A., Soden, B.J. et al. Reconciling opposing Walker circulation trends in observations and model projections. Nat. Clim. Chang. 9, 405–412 (2019). https://doi.org/10.1038/s41558-019-0446-4

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