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 change5–12. 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 Niñalike 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. The Pacific Walker circulation is a key component of the global atmospheric circulation and its variability is associated with precipitation and temperature variations over disperse regions of the planet3,4,13. Its signature east–west sea-level pressure (SLP) differences across the tropical Pacific are linked with ascending motion and deep convection over the warm water surrounding Indonesia, contrasting with subsidence and net radiative cooling over the cooler water of the eastern Pacific Ocean1,2. Owing to the interrelationships between SLP, tropical rainfall14, winds15 and sea surface height16, studies assessing variability and trends in the Walker circulation extend beyond evaluations of SLP alone. Yet the surface expression of pressure is a fundamental aspect of the Walker circulation, with SLP records central to the studies by its namesake, Sir Gilbert Walker, when exploring the linkages between the South Asian monsoon and the tropical Pacific circulation back in the early twentieth century17. Most coupled model simulations reveal SLP trends indicative of a weakened Walker circulation over the twentieth century5,8,11. This weakening has been attributed to warming by anthropogenic greenhouse gases, which increases water vapour by roughly 7% ◦C−1, in agreement with the Clausius–Clapeyron relationship. These models also show precipitation increases at a slower rate of