The correlation between summertime Niño3.4 index and western North Pacific (WNP) summer monsoon index has strikingly enhanced since the early 1990s, with nonsignificant correlation before the early 1990s but significant correlation afterward. This observed interdecadal change around the 1990s may be associated with more frequent occurrences of central Pacific (CP) El Niño and the interdecadal changes in ENSO-associated SST anomalies. During the post-1990s period (the pre-1990s period), highly noticeable tropical Atlantic (Indian) Ocean SST anomalies tend to co-occur with the summertime Niño3.4 SST anomalies. The concurrent tropical Atlantic (Indian) Ocean SST anomalies could constructively reinforce (destructively mitigate) the WNP monsoon circulation anomalies induced by the summertime Niño3.4 SST, thus boosting (muting) the correlation between summertime Niño3.4 SST and WNP monsoon. In addition, the faster decaying pace of preceding-winter El Niño after the 1990s, which may have been mainly induced by the influences from the spring tropical North Atlantic SST anomalies, could also have contributed to the enhanced correlation between the summertime Niño3.4 index and WNP monsoon. These results suggest that the enhanced influences from the tropical Atlantic SST may have triggered the intensified correlation between summertime ENSO and WNP monsoon since the early 1990s. 摘要 西北太平洋夏季风指数与同时期夏季的Niño3.4指数的相关系数在90年代初期以后出现显著的 增强。两者相关系数的这种年代际变化可能跟中太平洋型ENSO的更频繁发生以及ENSO相关联 的其他热带洋盆的海表面温度异常有关。在90年代之后(之前)，显著的热带大西洋(印度 洋)海表面温度异常倾向伴随夏季Niño3.4海表面温度异常同时出现，并且能够增强(减弱) Niño3.4海表面温度异常所引起的西北太平洋夏季风异常，从而能够增强(减弱)Niño3.4海表 面温度与西北太平洋夏季风指数之间的相关系数。另一方面，由春季北大西洋海表面温度异常 所引起的前一年冬季El Niño更快的衰减速率，也能够促进夏季Niño3.4指数与西北太平洋夏季 风之间相关系数在90年代之后的增加。这些结果表明来自大西洋海温影响的增强可能导致了夏 季Niño3.4指数与西北太平洋夏季风指数之间的相关系数在90年代之后的显著增加。 ARTICLE HISTORY

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Tropical cloud clusters (TCCs) are embryos of tropical cyclones (TCs) and may have the potential to develop into TCs. The genesis productivity (GP) of TCCs is used to quantify the proportion of TCCs that can evolve into TCs. Recent studies have revealed a decrease in GP of western North Pacific (WNP) TCCs during the extended boreal summer (July–October) since 1998. Here, we show that the changing tendencies in GP of WNP TCCs have obvious seasonality. Although most months could see recent decreases in GP of WNP TCCs, with October experiencing the strongest decreasing trend, May is the only month with a significant recent increasing trend. The opposite changing tendencies in May and October could be attributed to different changes in low-level atmospheric circulation anomalies triggered by different sea surface temperature (SST) configurations across the tropical oceans. In May, stronger SST warming in the tropical western Pacific could prompt increased anomalous westerlies associated with anomalous cyclonic circulation, accompanied by the weakening of the WNP subtropical high and the strengthening of the WNP monsoon. Such changes in background atmospheric circulations could favor the enhancement of atmospheric eddy kinetic energy and barotropic energy conversions, resulting in a recent intensified GP of WNP TCCs in May. In October, stronger SST warming in the tropical Atlantic and Indian Oceans contributed to anomalous easterlies over the tropical WNP associated with anomalous anticyclonic circulation, giving rise to the suppressed atmospheric eddy kinetic energy and recent weakened GP of WNP TCCs. These results highlight the seasonality in recent changing tendencies in the GP of WNP TCCs and associated large-scale atmospheric-oceanic conditions.

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The intensity of interannual variability (IIV) of the monsoon and monsoon-ENSO biennial relationship (MEBR) were examined and compared for both the Indian summer monsoon (ISM) and western North Pacific summer monsoon (WNPSM) during 1958-2018. Covariability of the IIV and MEBR were identified for the two monsoons. When the MEBR was strong (weak), the IIV of the monsoon was observed to be large (small). This rule applied to both the ISM and WNPSM. Out-ofphase relationships were found between the ISM and the WNPSM. When the IIV and MEBR of the ISM were strong (weak), those of the WNPSM tended to be weak (strong). During the period with a stronger (weaker) ENSO-Atlantic coupling after (before) the mid-1980s, the IIV and MEBR of the WNPSM (ISM) were observed to be stronger. The increasing influences from the tropical Atlantic sea surface temperature (SST) may trigger the observed seesaw pattern of the ISM and WNPSM in terms of the IIV and MEBR multidecadal variability. The results imply that tropical Atlantic SST may need to be given more attention and consideration when predicting future monsoon variability of the ISM and WNPSM.