Observed Multidecadal Variability of Low-Level Convergence over the Western North Pacific in the Tropical Cyclone Season

Low-level convergence is one of important features for the genesis of tropical cyclone (TC) over the Western North Pacific (WNP) in the tropical cyclone season. By using the reanalysis data, the analysis results show that the intensity of intraseasonal low-level convergence in WNP has a timing variation, fluctuating between July and August with a frequency of around 20 years; this fluctuation then further influences the variation of TC genesis. Further analysis shows this multidecadal variation of low-level convergence over WNP is associated with the feature of eastward propagation of tropical intraseasonal oscillation. This mutidecadal variation is not related to noted atmospheric and oceanic variations. A preliminary investigation reveals that the sea surface temperature variation over WNP has similar multidecadal variation in the transition season. The possible mechanism might result from the oceanic natural variations.