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摘要
Western North Pacific (WNP) summer monsoon and tropical cyclone (TC) activity are supposed to be declined during a strong El Nino decaying summer. The 2015-2016 event, which had a Nino 3.4 sea surface temperature (SSTA) similar to the 1997-1998 event, was classified as a strong El Nino. 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 Nino 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 Nino 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 Nino decaying spring to summer, which may have an effect on offsetting the El Nino-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.
Plain Language Summary Western North Pacific (WNP) summer monsoon and tropical cyclone (TC) activity are supposed to be declined during a strong El Nino decaying summer. The 2015-2016 event, which had a Nino 3.4 sea surface temperature (SST) anomaly similar to the 1997-1998 event, was classified as a strong El Nino. However, WNP summer monsoon and TC activity were normal and even stronger than the climatological mean during the decaying summer. This study addressed why the 2015-2016 El Nino event exerted distinct effects on the WNP's climate compared with the 1997-1998 event. We demonstrated the distinct impacts in the two strong El Ninos are primarily attributed to the oceanic forcing of the subtropical warm SST in the eastern North Pacific.
- 出版日期2018-5
