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Article
Article Type:
Research Article

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

Shuangmei Ma State Key Laboratory of Numerical Modeling for Atmospheric Sciences and Geophysical Fluid Dynamics, Institute of Atmospheric Physics, Chinese Academy of Sciences, and University of Chinese Academy of Sciences, Beijing, China

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Tianjun Zhou State Key Laboratory of Numerical Modeling for Atmospheric Sciences and Geophysical Fluid Dynamics, Institute of Atmospheric Physics, Chinese Academy of Sciences, and Joint Center for Global Change Studies, Beijing, China

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Print Publication:
01 May 2016
DOI:
https://doi.org/10.1175/JCLI-D-15-0398.1
Page(s):
3097–3118
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Abstract

In this study, the zonal mass streamfunction Ψ, which depicts intuitively the tropical Pacific Walker circulation (PWC) structure characterized by an enclosed and clockwise 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 Ψ trend exhibited a strengthening and westward-shifting trend of PWC in all reanalysis datasets, with the significantly positive Ψ dominating the western Pacific and negative Ψ 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° 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° longitude decade−1 (−2.55° longitude decade−1) in JRA-55 (JRA-25). In response to the recent observed La Niña–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 Niña–like SST anomalies to the recent PWC change.

Publisher’s Note: This article was revised on 8 June 2016 to replace the word “anticlockwise” in the abstract and in section 3a, with “clockwise”, which is what should have appeared when originally published.

Corresponding author address: Dr. Tianjun Zhou, LASG, Institute of Atmospheric Physics, Chinese Academy of Sciences, HuaYanLi No. 40, QiJiaHuozi, Chaoyang District, Beijing 100029, China. E-mail: zhoutj@lasg.iap.ac.cn

Abstract

In this study, the zonal mass streamfunction Ψ, which depicts intuitively the tropical Pacific Walker circulation (PWC) structure characterized by an enclosed and clockwise 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 Ψ trend exhibited a strengthening and westward-shifting trend of PWC in all reanalysis datasets, with the significantly positive Ψ dominating the western Pacific and negative Ψ 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° 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° longitude decade−1 (−2.55° longitude decade−1) in JRA-55 (JRA-25). In response to the recent observed La Niña–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 Niña–like SST anomalies to the recent PWC change.

Publisher’s Note: This article was revised on 8 June 2016 to replace the word “anticlockwise” in the abstract and in section 3a, with “clockwise”, which is what should have appeared when originally published.

Corresponding author address: Dr. Tianjun Zhou, LASG, Institute of Atmospheric Physics, Chinese Academy of Sciences, HuaYanLi No. 40, QiJiaHuozi, Chaoyang District, Beijing 100029, China. E-mail: zhoutj@lasg.iap.ac.cn
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  • Zhou, T., R. Yu, H. Li, and B. Wang, 2008: Ocean forcing to changes in global monsoon precipitation over the recent half-century. J. Climate, 21, 3833–3852, doi:10.1175/2008JCLI2067.1.

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