Robust Strengthening and Westward Shift of the Tropical Pacific Walker Circulation during 1979-2012: A Comparison of 7 Sets of Reanalysis Data and 26 CMIP5 Models

In this study, the zonal mass streamfunction Psi, which depicts intuitively the tropical Pacific Walker circulation (PWC) structure characterized by an enclosed and anticlockwise rotation cell in the zonal-vertical section over the equatorial Pacific, was used to study the changes of PWC spatial structure during 1979-2012. To examine the robustness of changes in PWC characteristics, the linear trends of PWC were evaluated and compared among the current seven sets of reanalysis data, along with a comparison to the trends of surface climate variables. The spatial pattern of Psi trend exhibited a strengthening and westward-shifting trend of PWC in all reanalysis datasets, with the significantly positive Psi dominating the western Pacific and negative Psi controlling the eastern Pacific. This kind of change is physically in agreement with the changes of the sea level pressure (SLP), surface winds, and precipitation derived from both the reanalyses and independent observations. Quantitative analyses of the changes in the PWC intensity and western edge, defined based on the zonal mass streamfunction, also revealed a robust strengthening and westward-shifting trend among all reanalysis datasets, with a trend of 15.08% decade(-1) and 3.70 degrees longitude decade(-1) in the ensemble mean of seven sets of reanalysis data, with the strongest (weakest) intensification of 17.53% decade(-1) (7.96% decade(-1)) in the Twentieth Century Reanalysis (NCEP-2) and largest (smallest) westward shift of -4.68 degrees longitude decade(-1) (-2.55 degrees longitude decade(-1)) in JRA-55 (JRA-25). In response to the recent observed La Nina-like anomalous SST forcing, the ensemble simulations from phase 5 of the Coupled Model Intercomparison Project (CMIP5), with 26 models in the ensemble, reasonably reproduced the observed strengthening and westward-shifting trend of PWC, implying the dominant forcing of the La Nina-like SST anomalies to the recent PWC change.