Inter-decadal change of the inter-annual relationship between the frequency of intense tropical cyclone over the western North Pacific and ENSO

The inter-decadal change of the inter-annual relationship between the frequency of intense tropical cyclone (TC) over the western North Pacific (WNP) and the El Nino-Southern Oscillation (ENSO) is studied based on the TC best track and global reanalysis data. The results show that during 1983-2014 (the recent period), the frequency of intense TC and the November-January (NDJ) Nino3.4 are closely correlated, whereas during 1950-1971 (the prior period), their correlation is not statistically significant. The westward shift of positive sea-surface temperature anomaly SSTA over the tropical Pacific in the recent El Nino developing years is a key factor that enhanced the relationship between the Nino3.4 index and annual frequency of intense TC. The enhancement of positive (negative) MPI, positive (negative) relative humidity and negative (positive) vertical wind shear anomalies in the southeastern quadrant of the tropical WNP in the El Nino (La Nina) developing years during the recent period are favourable (unfavourable) to TC genesis in the southeastern quadrant of the tropical WNP and enhanced the inter-annual relationship between the frequency of intense TC and ENSO in the recent period. This is because the TCs forming in the southeastern quadrant are more likely to move over the warm tropical ocean and intensify into intense TCs. In the recent period, more intense TCs formed in the southeastern quadrant in the El Nino developing years, moreover, the difference of the genesis location and lifespan of intense TCs between the El Nino developing years and La Nina developing years has been significantly enlarged in the recent period, compared to that in the prior period.