Study of Mechanisms for the Decadal Variation of the Relationship between the Frenquency of Northwest Pacific Intense TC and ENSO

In this study, we mainly analyze the reason for interdecadal variation of the correlation between the annual frequency of the western North Pacific intense TC (TC4, 5) and Nino3.4 index. Many studies suggested the tropical cyclone (TC) activities in western North Pacific (WNP) are related to ENSO. TC formation is enhanced remarkably in the southeast quadrant in El nino developing year and enhanced in the northwest in La Nina developing year. On the other hand, we found that during 1949-1968 (first stage) and 1989-2008 (third stage), the TC frequency and the ENSO are closely related, whereas during 1969-1988 (second stage), these factors are unrelated. Moreover, in the second stage, the intenseTCs frequency is lower, with an average of 4.85 per year. In the first and third stages, the intense TCs is higher, with an average of 7.60 and 7.75 per year, respectively. The seasonal change of the strong TC frequency in the August-October during the second stage is evidently lower than that of the first and third stages, which exhibits a double-peak profile on July and October during 1969-1988. Using the monthly mean reanalysis data from the National Centers for Environmental Prediction (NCEP)/National Center for Atmospheric Research (NCAR) during 1949-2008 as well as the sea surface temperature (SST) data from Hadley Centre Sea Ice and SST data set's (HadISST), the meteorological element fields are analyzed for August-October of the three stages. The result suggests that the lower intense TCs frequency in the second stage compared with the first stage is mainly caused by the higher temperature of the Indian Ocean during August-October of the second stage, which causes the anomalous anticyclone in the southeastern part of the northwestern Pacific Ocean. The higher pressure at the sea surface, lower water vapor in the middle layer, lower temperature in the upper layer, stronger vertical wind shear, and upper level convergence/lower level divergence make the TC difficult to generate in the southeastern part of the northwestern Pacific Ocean during the second stage. Compared with the first stage, the maximum possible intensity (MPI) and genesis potential index (GPI) indexes of the second stage in the southeastern part of the northwestern Pacific Ocean were negative anomalies; therefore, the intense TC frequency is lower in the second stage. Comparing the second and third stages, the equatorial mid-Pacific Ocean SST is noticeably warmer during August-October during the third stage and causes the “Matsuno-Gill pattern” response of the atmosphere. The Cyclonic Rossby wave is excited northwest of the heat source (i.e., northwestern Pacific Ocean). This large-scale northwest-southeast cyclonic circulation facilitates the formation and strengthening of the TC in the northwestern Pacific Ocean, which is the reason for the higher intense TCs frequency during the third stage and the close correlation with the ENSO.