Abstract:In order to improve the accuracy of tropical cyclogenesis forecasting,it is vitally important to study how Tropical Cloud Clusters(TCCs) develop into Tropical Cyclones(TCs),and the related climatic characteristics.TC genesis is poorly understood,since most previous studies have focused on the time after genesis.In the present study,global tropical cloud cluster data,NCEP/NCAR reanalysis data,Hadley Centre sea surface temperature data,and Genesis Productivity(GP) data (representing the ratio of TCCs that develop into TCs) were used to analyze the different impacts of two El Niño types on the evolution of TCCs in the Western North Pacific (WNP) from 1982 to 2009.The results indicate that GP is significantly and positively correlated with the Niño3 index in the southeastern sub-region of the WNP during summer.During fall,GP is significantly and positively correlated with the Niño3 index in the southwestern and southeastern sub-regions.The El Niño Modoki Index(EMI) has a markedly positive(negative) correlation with GP in the southeastern(South China Sea) sub-region during summer(fall).All these results are consistent with the large-scale environmental flow that affects TCC activity.Eastern Pacific(EP) El Niño events are associated with a significant increase in 850 hPa relative vorticity,200 hPa divergence and 700 hPa relative humidity in the southeastern sub-region during summer.The increase of GP in the southwestern sub-region during fall occurs because the relative vorticity and divergence are favorable for TCCs in EP El Niño years,whereas the opposite situation occurs during La Niña years.Central Pacific(CP) El Niño events enhance(suppress) GP in the southeastern sub-region(South China Sea) during summer(fall) because relative vorticity,divergence,relative humidity and vertical zonal wind shear are favorable(unfavorable) for the development of TCCs.In summary,the large-scale environmental flow anomalies,caused by El Niño events,create distinct effects on TCC activity in the WNP.Positive feedback is produced by CP El Niño events and EP El Niño events in the southeastern sub-region,but the effects of the two El Niño types are completely different in the southwestern sub-region and South China Sea.After comparing the different impacts of the two El Niño types on TCC activity,it was found that the changes in large-scale environmental factors produce different feedback effects on the development of TCCs in the WNP.Specifically,it is in the eastern part of the WNP that EP El Niño events affect TCC activity,but for CP El Niño events the impacts on the evolution of TCCs spread to the South China Sea.And it is in the South China Sea that the influences of the two El Niño types on TCCs are opposite.