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Slowdown of the Walker circulation driven by tropical Indo-Pacific warming

Abstract

Global mean sea surface temperature (SST) has risen steadily over the past century1,2, but the overall pattern contains extensive and often uncertain spatial variations, with potentially important effects on regional precipitation3,4. Observations suggest a slowdown of the zonal atmospheric overturning circulation above the tropical Pacific Ocean (the Walker circulation) over the twentieth century1,5. Although this change has been attributed to a muted hydrological cycle forced by global warming5,6, the effect of SST warming patterns has not been explored and quantified1,7,8. Here we perform experiments using an atmospheric model, and find that SST warming patterns are the main cause of the weakened Walker circulation over the past six decades (1950–2009). The SST trend reconstructed from bucket-sampled SST and night-time marine surface air temperature features a reduced zonal gradient in the tropical Indo-Pacific Ocean, a change consistent with subsurface temperature observations8. Model experiments with this trend pattern robustly simulate the observed changes, including the Walker circulation slowdown and the eastward shift of atmospheric convection from the Indonesian maritime continent to the central tropical Pacific. Our results cannot establish whether the observed changes are due to natural variability or anthropogenic global warming, but they do show that the observed slowdown in the Walker circulation is presumably driven by oceanic rather than atmospheric processes.

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Figure 1: Observed and simulated patterns of SLP and SST changes for 1950–2009.
Figure 2: Observed and simulated changes in cloudiness, surface wind and precipitation for 1950–2009.
Figure 3: Scatter diagram of ΔSST and ΔSLP changes in AGCM and CMIP3 model simulations and reanalysis data sets.
Figure 4: Uncertainty in HadSST3 trends.

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Acknowledgements

We thank A. Timmermann for discussions, and X. T. Zheng and J. Ma for data processing. The work was supported by JAMSTEC, the National Basic Research Program of China (2012CB955600), NASA, NSF and NOAA. NCAR is sponsored by NSF.

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Authors

Contributions

H.T., Y.K. and Y.M.O. designed and performed model experiments. S.-P.X. and C.D. supervised the work. H.T. analysed observations and model simulations. H.T., S.-P.X., C.D. and Y.M.O. wrote the manuscript, with feedback from all authors.

Corresponding authors

Correspondence to Hiroki Tokinaga or Shang-Ping Xie.

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The authors declare no competing financial interests.

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Tokinaga, H., Xie, SP., Deser, C. et al. Slowdown of the Walker circulation driven by tropical Indo-Pacific warming. Nature 491, 439–443 (2012). https://doi.org/10.1038/nature11576

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