Relationship between spring tropical cyclone frequency over the western North Pacific and El Niño‐Southern oscillation

Abstract

AbstractThe 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 to 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 vapour induced by the enhanced Walker Circulation, primarily favours TC development, while increased sea surface temperatures and positive low‐level relative vorticity anomalies appear to play a lesser role in impacting TC formation.