资源环境科技发展态势分析平台

Previous studies found that tropical cyclone (TC) formation is generally suppressed over the western North Pacific (WNP) following strong El Nino events. The 2015/2016 event is identified as one of the three major El Nino events since 1950. However, a climatological average of 26 named TCs occurred over the WNP in 2016. The plausible causes for this inconsistency are investigated in this study. By examining the historical records, we also found that 28 named TCs occurred over the WNP following 1991/1992 El Nino. For most strong El Nino cases, the suppressed TC formation in the ensuing early season (January-June) can persist to the peak season and lead to the negative TC frequency anomalies. However, TC formation turns to be active during August-October in 1992 and 2016, offsetting the suppressed TC formation in the early season and thus resulting in the climatological annual TC counts. It is found that anomalous sea surface temperature (SST) cooling over the north Indian Ocean and SST warming over the tropical North Pacific contribute to the enhanced TC formation in 1992 by stimulating an anomalous cyclonic circulation over the WNP, while the tri-polar SST pattern across the tropical Indo-western Pacific Ocean and the related convergence zone over 130 degrees-160 degrees E are responsible for the enhanced TC formation in 2016. The results indicate the crucial role of SST evolution over the north Indian Ocean and tropical Pacific in TC formation following strong El Nino events, which has important implication for the seasonal forecasting of TC activity over the WNP.