Distinct Effects of the Two Strong El Niño Events in 2015–2016 and 1997–1998 on the Western North Pacific Monsoon and Tropical Cyclone Activity: Role of Subtropical Eastern North Pacific Warm SSTA
Distinct Effects of the Two Strong El Niño Events in 2015–2016 and 1997–1998 on the Western North Pacific Monsoon and Tropical Cyclone Activity: Role of Subtropical Eastern North Pacific Warm SSTA
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DOI：
10.1002/2018jc013798
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发表时间：
2018-05
影响因子：
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通讯作者：
Yi-Kai Wu;C. Hong;Cheng-Ta Chen
中科院分区：
文献类型：
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作者：
Yi-Kai Wu;C. Hong;Cheng-Ta Chen
西北太平洋(WNP)夏季风和热带气旋(TC)活动应该在一个强烈的厄尔尼诺现象衰败的夏季期间减弱。2015-2016年的厄尔尼诺事件的海面温度(SSTA)为3.4，与1997-1998年的厄尔尼诺事件相似，被归类为强厄尔尼诺现象。然而，在衰败的夏季，WNP夏季风和TC活动正常，甚至强于气候平均水平。这项研究阐述了为什么2015-2016年的厄尔尼诺事件与1997-1998年的厄尔尼诺事件对西北地区的气候产生了不同的影响。这两次事件在海洋条件上的主要不同之处在于，2015-2016年在北太平洋东部观测到了西南-东北倾斜的副热带暖海温，并伴随着副热带北太平洋明显的西风异常。对海温异常与低空风场协方差的奇异值分解(SVD)分析表明，北太平洋东部副热带暖海温异常存在类似于太平洋经向模(PMM)的第二模。相反，前导型(即厄尔尼诺海温异常)的贡献不大。观测分析表明，与PMM相关的海温异常与厄尔尼诺衰变过程中西北部大尺度低层气旋性环流异常显著相关，这可能起到抵消厄尔尼诺引起的西北部反气旋的作用，从而使西北部夏季风和TC活动恢复正常。与气旋性环流异常相关的PMM-SST由于季节内振荡的活跃期而进一步增强。数值试验进一步支持了与PMM相关的海温对夏季风和TC活动的可能影响。
Western North Pacific (WNP) summer monsoon and tropical cyclone (TC) activity are supposed to be declined during a strong El Niño decaying summer. The 2015–2016 event, which had a Niño 3.4 sea surface temperature (SSTA) similar to the 1997–1998 event, was classified as a strong El Niño. However, WNP summer monsoon and TC activity were normal or even stronger than the climatological mean during the decaying summer. This study addressed why the 2015–2016 El Niño event exerted distinct effects on the WNP's climate compared with the 1997–1998 event. The major difference in oceanic conditions between the two events is that a southwest‐northeast‐tilted subtropical warm SSTA in the eastern North Pacific associated with a pronounced westerly anomaly in the subtropical North Pacific was observed in 2015–2016. Singular value decomposition (SVD) analysis of the covariance of the SSTA and low‐level wind indicated that second mode, resembling the Pacific meridional mode (PMM), accounted for the subtropical warm SSTA in the eastern North Pacific. Conversely, the contribution of leading mode (i.e., El Niño SSTA) was insignificant. Observational analysis indicated that the PMM‐associated SSTA is significantly correlated with a large‐scale low‐level cyclonic circulation anomaly in the WNP during the El Niño decaying spring to summer, which may have an effect on offsetting the El Niño‐induced anticyclone in the WNP and therefore returns WNP summer monsoon and TC activity to normal. The PMM‐SST correlated with cyclonic circulation anomaly was further enhanced by an active phase of intraseasonal oscillation. The possible effect of PMM‐associated SST on the summer monsoon and TC activity was further supported by numerical experiments.