Abstract

The number of tropical cyclones (TCs) over the western North Pacific (WNP) during spring (March-May) has a significant inverse correlation with concurrent El Niño-Southern Oscillation (ENSO) conditions during the period from 1979-2018. This relationship is different from the previously-documented weak relationship between TC frequency and ENSO during the climatologically most active portion of the TC season. In general, TCs seldom occur in El Niño years during March-May, whereas they frequently form over the western part of the WNP, particularly to the southeast of the Philippines, in La Niña years. This difference can be largely explained by ENSO-driven differences in the genesis potential index as derived from environmental variables. In La Niña years, the abnormally moist mid-troposphere, which relates to the strengthened vertical transport of water vapor induced by the enhanced Walker Circulation, primarily favors TC development, while increased sea surface temperatures and positive low-level relative vorticity anomalies appear to play a lesser role in impacting TC formation.