Influence of Intraseasonal–Interannual Oscillations on Tropical Cyclone Genesis in the Western North Pacific

Influences of intraseasonal–interannual oscillations on tropical cyclone (TC) genesis are evaluated by productivity of TC genesis (P TCG) from the developing (TCd) and nondeveloping (TCn) precursory tropical disturbances (PTDs). A PTD is identified by a cyclonic tropical disturbance with a strong-enough intensity, a large-enough maximum center, and a long-enough lifespan. The percentage value of PTDs evolving into TCd is defined as P TCG. The analysis is performed over the western North Pacific (WNP) basin during the 1990–2014 warm season (May–September). The climatological P TCG in the WNP basin is 0.35. Counted in a common period, mean numbers of PTDs in the favorable and unfavorable conditions of climate oscillations for TC genesis [such as equatorial Rossby waves (ERWs), the Madden–Julian oscillation (MJO), and El Niño–Southern Oscillation (ENSO)], all exhibit a stable value close to the climatological mean [∼31 (100 days)−1]. However, P TCG increases (decreases) during the phases of positive-vorticity (negative-vorticity) ERWs, the active (inactive)MJO, and El Niño (La Niña) years. P TCG varies from 0.17 in the most unfavorable environment (La Niña, inactive MJO, and negative-vorticity ERW) to 0.56 in the most favorable environment (El Niño, active MJO, and positive-vorticity ERW). ERWs are most effective in modulating TC genesis, especially in the negative-vorticity phases. Overall, increased P TCG is facilitated with strong and elongated 850-hPa relative vorticity overlapping a cyclonic shear line pattern, while decreased P TCG is related to weak relative vorticity. Relative vorticity acts as the most important factor to modulate P TCG, when compared with vertical wind shear and 700-hPa relative humidity.