Walker Circulation Slowdown and Indo-Pacific Climate Over the Past Six Decades

1. Walker Circulation Slowdown and Indo-Pacific Climate Over the Past Six Decades H. Tokinaga, S.-P. Xie, A. Timmermann, T. Ogata (IPRC), S. McGregor (UNSW), H. Kubota (JAMSTEC), and Y. M. Okumura (NCAR) (J. Climate, 2012, & highlighted in Nature Geoscience) Observed changes in a) surface wind (m/s), b) SLP (hPa), c) cloudiness, and d) surface air temperature (C) from 1950 to 2009. A synthesis of physically coherent in situ observations of surface temperature, clouds, sea level pressure (SLP), surface wind, and subsurface ocean temperature shows changes over the last six decades in the tropical Indo-Pacific climate that are consistent with ocean model simulations. The new bias-corrected WAS Wind dataset shows westerly trends over the western tropical Pacific and easterly trends over the tropical Indian Ocean, indicating a Walker circulation slowdown (Fig. a), consistent with observed increasing SLP over the Maritime Continent and decreasing SLP over the central equatorial Pacific (Fig. b). Suppressed moisture convergence over the Maritime Continent, largely due to surface wind changes, contributes to observed decreases in marine cloudiness and land precipitation there (Fig. c). Although major sea surface temperature reconstructions vary because of observational biases, the zonal surface air temperature gradient (Fig. d) shows a flattening as does the observed zonal subsurface temperature gradient in both observations and model simulations. These many independent observations provide robust evidence that ocean–atmosphere coupling has led to a weaker Walker circulation over the past six decades.