Climatologically, tropical cyclone (TC) genesis in the South China Sea (SCS; 108°–120°E, 8°–22°N) and westernmost North Pacific (WmNP; 122°–140°E, 8°–22°N) exhibit different seasonal variability. Maximum TC genesis occurs in July over the WmNP, but in September over the SCS. Based upon genesis potential index (GPI) analysis for the period 1979–2018, the dominant cause of increased TC genesis in the SCS from July to September is a reduction of vertical wind shear of total winds (VWS), while decreased 600‐hPa relative humidity (RH600) acts to suppress TC genesis. Reduced VWS in the SCS is associated with weakening of southwesterly flows and cross‐equatorial southerly flows at 850 hPa and the Tibetan high at 200 hPa. In the WmNP, decreased TC genesis from July to September results from a minor increase in its northern section (north of 15°N) and a major decrease in its southern section (south of 15°N). Decreased TC genesis in the southern section is primarily affected by reduced RH600 and secondly by enhanced VWS. Reduced RH600 is associated with a decrease in total humidity and an increase in 600‐hPa temperature from July to September. Enhanced VWS is mainly caused by intensified entrances of 850‐hPa westerly/southwesterly flows from the SCS and cross‐equatorial flows from tropical oceans in company with weakening of the Pacific subtropical high. The effects of 850‐hPa absolute vorticity and potential intensity associated with the possible maximum wind speed on seasonal variability of TC genesis are insignificant and minor.
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.
Taiwan experiences a distinct seasonal transition in fall from the summer to winter monsoon. In October, the northeasterly monsoon impinges on Taiwan's Central Mountain Range producing heavy rainfall in eastern Taiwan. In addition to monsoonal influences, tropical cyclones (TCs) move along the Western Pacific warm pool to the south and southwest of Taiwan over the 115 o -122 o E region.These are found to influence rainfall over eastern Taiwan by enhancing the meridional pressure gradient with the resultant northeasterly flows moving toward Taiwan. During TC warning periods, this meridional pressure gradient on maximum rainfall day over northeastern Taiwan (Ilan) is strong for TCs with a northern track (north of 19 o N) and weak for those with a southern track (south of 19 o N). For TCs with a northern track, intensified northeasterly flows merge with cyclonic flows in the northern sector of a TC-related low centered in the South China Sea (SCS). Flow confluences and moisture flux convergence occur over Taiwan causing strong rainfall over eastern Taiwan, but immediately to the west of Taiwan in relation to weak rainfall over eastern Taiwan. For TCs with a southern track, moisture flux convergence shifts southward over the SCS, leading to very weak rainfall over Taiwan. Rainfall variability in different interaction types is mainly determined by the 3-10-day transient mode. The major effect of 30-60-day intraseasonal oscillation (ISO) mode is to provide favorable conditions due to large-scale cyclonic anomalies across the SCS and tropical western North Pacific. These conditions steer westward TC movement over oceans to the south and southwest of Taiwan.
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