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

Interannual variability of summer (June-August) tropical cyclones (TCs) in the northwestern North Pacific (NP) region (120 degrees-135 degrees E, 20 degrees-35 degrees N) and El Nino-Southern Oscillation (ENSO) exhibit four asymmetric ENSO-TC relationships. The major relationship types are El Nino-enhanced and La Nina-suppressed, while the minor relationship types are El Nino-suppressed and La Nina-enhanced. The major modulatory processes for the El Nino-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 Nino-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 Nina-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 Nina-enhanced type contains an anomalous anticyclone south of 20 degrees 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 Nino-enhanced and La Nina-enhanced types. ENSO is the major factor reducing TC frequency in the El Nino-suppressed and La Nina-suppressed types.