Interannual Weakening of the Tropical Pacific Walker Circulation Due to Strong Tropical Volcanism
In order to examine the response of the tropical Pacific Walker circulation (PWC) to strong tropical volcanic eruptions (SVEs), we analyzed a three-member long-term simulation performed with HadCM3, and carried out four additional CAM4 experiments. We found that the PWC shows a significant interannual weakening after SVEs. The cooling effect from SVEs is able to cool the entire tropics. However, cooling over the Maritime Continent is stronger than that over the central-eastern tropical Pacific. Thus, non-uniform zonal temperature anomalies can be seen following SVEs. As a result, the sea level pressure gradient between the tropical Pacific and the Maritime Continent is reduced, which weakens trade winds over the tropical Pacific. Therefore, the PWC is weakened during this period. At the same time, due to the cooling subtropical and midlatitude Pacific, the Intertropical Convergence Zone (ITCZ) and South Pacific convergence zone (SPCZ) are weakened and shift to the equator. These changes also contribute to the weakened PWC. Meanwhile, through the positive Bjerknes feedback, weakened trade winds cause El Niño-like SST anomalies over the tropical Pacific, which in turn further influence the PWC. Therefore, the PWC significantly weakens after SVEs. The CAM4 experiments further confirm the influences from surface cooling over the Maritime Continent and subtropical/midlatitude Pacific on the PWC. Moreover, they indicate that the stronger cooling over the Maritime Continent plays a dominant role in weakening the PWC after SVEs. In the observations, a weakened PWC and a related El Niño-like SST pattern can be found following SVEs.
Key wordsPacific Walker circulation strong tropical volcanic eruptions cooling effect trade winds
为了研究太平洋沃克环流(PWC)对热带强火山喷发(SVEs)的响应, 本文分析了耦合模式HadCM3模拟的3组火山强迫数值试验结果. 结果表明, SVEs对PWC的年际变化具有重要的调制作用. 在SVEs之后的一年里, 火山气溶胶制冷效应造成整个热带地区低层显著降温, 其中海洋性大陆地区及周缘海域降温比热带中东太平洋更强, 进而导致热带太平洋东西向海平面气压梯度减小, 信风减弱. 同时, 副热带和中纬度太平洋海表温度变冷导致热带辐合带和南太平洋辐合带减弱并且向赤道位移. 在上述两者的共同作用下, PWC在火山喷发之后显著减弱. 此外, 通过皮叶克尼斯反馈作用, 信风的减弱导致类厄尔尼诺型海温异常, 这反过来进一步减弱了信风及PWC. 我们进一步开展的4组CAM4敏感性试验证明由火山强迫导致的海洋性大陆及周缘海域表层强降温对PWC的年际减弱起着主导性作用. 在观测资料中, 我们也发现了SVEs之后类似的PWC减弱现象及过程.
关键词沃克环流 热带强火山喷发 降温效应 信风
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We thank the two anonymous reviewers and editor for their valuable comments and suggestions, which helped improve the quality of this paper significantly. This research was supported by the National Key R&D Program of China (Grant No. 2016YFA0600701), the National Natural Science Foundation of China (Grant Nos. 41661144005, 41575086 and 41320104007), and the CAS–PKU Joint Research Program. The authors are grateful to Dr. Schurer A. P. for providing the coupled model output.
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