Interannual variability of summer tropical cyclone activity in the northwestern North Pacific modulated by El Niño‐Southern oscillation and intraseasonal oscillation

Abstract Interannual variability of summer (June–August) tropical cyclones (TCs) in the northwestern North Pacific (NP) region (120°–135°E, 20°–35°N) and El Niño‐Southern Oscillation (ENSO) exhibit four asymmetric ENSO‐TC relationships. The major relationship types are El Niño‐enhanced and La Niña‐suppressed, while the minor relationship types are El Niño‐suppressed and La Niña‐enhanced. The major modulatory processes for the El Niño‐enhanced type feature an elongated anomalous cyclone extending from the tropical western NP (WNP) northwestward toward the northwestern NP. This feature intensifies TC formation in the tropical WNP and guides them northward/northwestward increasing TC frequency in the northwestern NP. The El Niño‐suppressed type has its anomalous cyclone displacing eastward in the tropical WNP with an accompanying anomalous anticyclone across the northwestern NP. TC formation reduces in the tropical WNP and TC movement is blocked toward the northwestern NP, resulting in decreased TC frequency. The La Niña‐suppressed type features a dominant elongated anomalous anticyclone in the WNP to suppress TC formation and movement toward the northwestern NP, yielding reduced TC frequency. The La Niña‐enhanced type contains an anomalous anticyclone south of 20°N in the tropical WNP in company with an anomalous cyclone over the northwestern NP. Under this anomalous cyclone, enhanced TC formation and movement in the northwestern NP result in increased TC frequency. For all relationship types, 30–60‐day intraseasonal oscillation (ISO) anomalies feature a dominant anomalous cyclone over the WNP providing favourable conditions that guide TC movement toward its central region over the northwestern NP. Both ENSO and ISO make positive contributions that increase TC frequency in the northwestern NP for the El Niño‐enhanced and La Niña‐enhanced types. ENSO is the major factor reducing TC frequency in the El Niño‐suppressed and La Niña‐suppressed types.