Modulation of Western North Pacific Tropical Cyclone Genesis by Intraseasonal Oscillation of the ITCZ: A Statistical Analysis

The present study investigates modulation of western North Pacific (WNP)tropical cyclone (TC) genesis in relation to different phases of theintraseasonal oscillation (ISO) of ITCZ convection during May to October inthe period 1979--2008. The phases of the ITCZ ISO were determined based on30--80-day filtered OLR anomalies averaged over the region (5^o--20^oN,120^o--150^oE). The number of TCs during the active phases wasnearly three times more than during the inactive phases. The active(inactive) phases of ISO were characterized by low-level cyclonic(anticyclonic) circulation anomalies, higher (lower) midlevel relativehumidity anomalies, and larger (smaller) vertical gradient anomalies ofrelative vorticity associated with enhanced (weakened) ITCZ convectionanomalies. During the active phases, TCs tended to form in the center of theITCZ region.Barotropic conversion from the low-level mean flow is suggested to be themajor energy source for TC formation. The energy conversion mainly dependedon the zonal and meridional gradients of the zonal flow during the activephases. However, barotropic conversion weakened greatly during the inactivephases. The relationship between the meridional gradient of absolutevorticity and low-level zonal flow indicates that the sign of the absolutevorticity gradient tends to be reversed during the two phases, whereas thesame sign between zonal flow and the absolute vorticity gradient is moreeasily satisfied in the active phases. Thus, the barotropic instability oflow-level zonal flow might be an important mechanism for TC formation overthe WNP during the active phases of ISO.