Climatic Factors Suppressing Tropical Cyclone Activity around Taiwan during October

During Octobers of 1970-2019, no tropical cyclones (TCs) affected Taiwan in 32 out of 50 years (64%). Suppressed TC activity in these years results from different modulating processes imposed by various climatic features. During Octobers of El Nino years, TC genesis in the western North Pacific (WNP) shifts eastward and decreases in the western WNP to the southeast of Taiwan. An anomalous anticyclone across the South China Sea (SCS) and Taiwan hinders TC movement toward Taiwan. In La Nina years, TC genesis increases in the region southeast of Taiwan. These TCs are guided by an anomalous cyclone centering in the SCS to have major TC tracks to the southwest of Taiwan toward the SCS. A year with a September-November value on the Oceanic Nino Index (ONI) of between 0 degrees-0.5 degrees C (-0.5 degrees-0 degrees C) is categorized as a positive (negative) Normal year. During the positive Normal years, an anomalous cyclone over the WNP enhances TC genesis in its southern section and guides these TCs northward along the regions east of Taiwan. An anomalous anticyclone across the SCS and Taiwan hinders TC movement toward Taiwan. During the negative Normal years, a westward elongation of warm sea surface temperature anomalies from the WNP into the eastern Indian Ocean forces an anomalous anticyclone to extend westward from the WNP toward the SCS. TC genesis to the south of this anomalous anticyclone decreases and is accompanied by reduced TC movement toward Taiwan.