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 (PTCG) 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 Nino-Southern Oscillation (ENSO)], all exhibit a stable value close to the climatological mean [similar to 31 (100 days)(-1)]. However, P-TCG increases (decreases) during the phases of positive-vorticity (negative-vorticity) ERWs, the active (inactive) MJO, and El Nino (La Nina) years. P-TCG varies from 0.17 in the most unfavorable environment (La Nina, inactive MJO, and negative-vorticity ERW) to 0.56 in the most favorable environment (El Nino, 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.