西北太平洋热带云团发展与两类El Nio事件的联系

利用1982—2009年全球热带云团数据集、NCEP/NCAR再分析资料和英国Hadley中心海温资料,并引入热带云团生成率(Genesis Productivity,GP)来分析EP(Eastern Pacific)El Nio和CP(Central Pacific)El Nio事件与西北太平洋热带云团发展的相关性。研究表明,1)夏秋两季GP与Nio3指数在东南区(SE)均为显著正相关,在西南区(SW)仅秋季呈显著正相关;GP与EMI(El Nio Modoki Index)指数在夏季SE区域为显著正相关,在秋季南中国海(South China Sea,SCS)区呈负相关。2)在EP El Nio年,夏季SE区域的GP增大与低层涡度、高层散度以及低层相对湿度的相对增大一致。夏季SW区域与秋季SE区域的GP增大与有利的高低空配置相关,La Nia年则与之相反。3)在CP El Nio年,夏季SE区域的GP增大伴随着低层涡度和高层散度的增加,同时与充足的水汽及弱风切变相吻合;而秋季SCS区域的GP下降源于正涡度带、正散度带以及水汽带的东移。

Relationship between the development of tropical cloud clusters and two El Niño types over the Western North Pacific

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.