Decadal shift of the interannual relationship between western North Pacific tropical cyclone genesis frequency and South China Sea monsoon trough around 1980s

Abstract

AbstractThe present study investigates the decadal shift of the interannual relationship between the South China Sea (SCS) monsoon trough (MT) and the genesis frequency of tropical cyclone (TC) over the western North Pacific during June–October 1949–2015. Significant change was found around the 1980s. The weak TC (categories 1–3) number has a decreasing relationship with the SCS MT. Their correlation coefficients are statistically remarkable during 1949–1978 but reduce to around 0.1. In contrast, the connection between strong TC (categories 4–5) number and SCS MT is strengthened after 1980s with correlation reaching up to 0.7. Such a shift can be explained by the changes in large‐scale environmental factors associated with MT movement. After 1980s, when the SCS MT extends southeastward, there exits enhanced cyclonic relative vorticity in lower level, increased moisture in middle level, reduced vertical wind shear and intensified divergence in upper level. These conditions are favourable for weak TC intensifying and developing into strong TC. In addition, more strong TC occurred in the southwest quadrant of WNP after 1980s; thus, the SCS MT would affect the TC for a longer time. Therefore, the genesis frequency of weak TCs has diminished, while more strong TCs are formed along with the SCS MT extending southeastward, leading to their strengthened interannual relationship.