Multiscale Variability of Autumn Precipitation in Eastern Taiwan Modulated by ENSO, ISO, and TC Activity

This study investigates the multiscale variability of rainfall over eastern Taiwan during October–November, focusing on the companion effect of tropical cyclone (TC) activity in the South China Sea (SCS) and northeasterly monsoon flow. The interannual variation of autumn rainfall is significantly influenced by the ENSO Phase. During La Niña years, the moisture transport from the SCS-Philippine Sea to eastern Taiwan is enhanced by the anomalous southeasterly winds owing to the cyclonic flow over the SCS. The response of autumn rainfall to ENSO is contributed by intraseasonal variability and the associated TC activity in SCS. During La Niña years, the Madden–Julian Oscillation (MJO) convective areas during phases 4–7 shift into the SCS-Philippine Sea, and the quasi bi-weekly oscillation (QBWO) convective activity is enhanced around the north of Luzon Island. We categorize TCs moving westward into or forming within the SCS into the groups causing significant rainfall in eastern Taiwan or not (the rainfall and non-rainfall groups). The rainfall group predominantly occurs during La Niña years in MJO phases 5. Both groups have similar average TC intensities, but the rainfall group’s path and the associated cyclonic circulation are placed more northward. Both groups of TCs coincide with QBWO’s cyclonic circulation, but the cyclonic circulation associated with the rainfall group stretched from the SCS to the Ryukyu Islands, favoring the moisture transport from the Philippine Sea to eastern Taiwan. We concluded that, excluding direct TC influences, the most favorable conditions for heavy rainfall in eastern Taiwan in Autumn are La Niña years during MJO phases 4–5, when the coinciding of TCs with appropriately structured QBWOs passing through the Bashi Channel or the Northern Philippines into the SCS. A regression model is developed based on the diagnostics in this study using vertically integrated moisture transport and divergence from 1000–700 hPa, which provide the basis of the storyline approach to estimate autumn rainfall over eastern Taiwan from the future projection of global climate models.