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Recent multidecadal strengthening of the Walker circulation across the tropical Pacific


The Pacific Walker circulation is a large overturning cell that spans the tropical Pacific Ocean, characterized by rising motion (lower sea-level pressure) over Indonesia and sinking motion (higher sea level-pressure) over the eastern Pacific1,2. Fluctuations in the Walker circulation reflect changes in the location and strength of tropical heating, so related circulation anomalies have global impacts3,4. On interannual timescales, the El Niño/Southern Oscillation accounts for much of the variability in the Walker circulation, but there is considerable interest in longer-term trends and their drivers, including anthropogenic climate change5,6,7,8,9,10,11,12. Here, we examine sea-level pressure trends in ten different data sets drawn from reanalysis, reconstructions and in situ measurements for 1900–2011. We show that periods with fewer in situ measurements result in lower signal-to-noise ratios, making assessments of sea-level pressure trends largely unsuitable before about the 1950s. Multidecadal trends evaluated since 1950 reveal statistically significant, negative values over the Indonesian region, with weaker, positive trends over the eastern Pacific. The overall trend towards a stronger, La Niña-like Walker circulation is nearly concurrent with the observed increase in global average temperatures, thereby justifying closer scrutiny of how the Pacific climate system has changed in the historical record.

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Figure 1: SLP linear trends for 10-, 20-, 30-, 40-year moving windows from January 1900 to December 2011.
Figure 2: SLP linear trends over the eastern Pacific Ocean and over Indonesia for 10-, 20-, 30- and 40-year moving windows for January 1900–December 2011.
Figure 3: Residual SLP trends, with Niño-3.4 SST linearly removed, over the eastern Pacific Ocean and over Indonesia for 10-, 20-, 30- and 40-year moving windows for January 1900–December 2011.
Figure 4: Hovmoller diagrams of 30-year moving SLP linear trends, averaged between 10° S and 10° N, across the tropical Indo-Pacific Ocean.

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We thank Z-Z. Hu and W. Ebisuzaki for their comments and suggestions, and L. Zhang for directing us to some of the data. We also thank NOAA ESRL/PSD and the National Energy Research Scientific Computing Center for providing several data sets on their websites.

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All authors contributed to the ideas in this paper. M.L.L. carried out the analysis and wrote the paper. S.L. and B.L. offered their help with interpreting the analysis and also edited the paper.

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Correspondence to Michelle L. L’Heureux.

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L’Heureux, M., Lee, S. & Lyon, B. Recent multidecadal strengthening of the Walker circulation across the tropical Pacific. Nature Clim Change 3, 571–576 (2013).

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