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 the intraseasonal oscillation (ISO) of ITCZ convection during May to October in the period 1979–2008. The phases of the ITCZ ISO were determined based on 30–80-day filtered OLR anomalies averaged over the region (5°–20°N, 120°–150°E). The number of TCs during the active phases was nearly 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 relative humidity anomalies, and larger (smaller) vertical gradient anomalies of relative vorticity associated with enhanced (weakened) ITCZ convection anomalies. During the active phases, TCs tended to form in the center of the ITCZ region.