Coherent Interdecadal Variability of Tropical Cyclone Rainfall and Seasonal Rainfall in Taiwan during October

By separating total rainfall into tropical cyclone (TC) rainfall caused by TC passage and seasonal rainfall associated with moisture transport of prevailing seasonal flows, this study examines the interdecadal variability of these two rainfall components in Taiwan during fall. It is found that interdecadal variability of TC rainfall and seasonal rainfall tends to vary coherently in October. This coherent interdecadal rainfall mode features islandwide patterns in Taiwan and evident interdecadal oscillations. The associated large-scale regulating processes for the positive phase are characterized by warm sea surface temperature (SST) anomalies in the northern South China Sea and the western North Pacific, which in turn induce an anomalous anticyclonic circulation over the subtropical western Pacific to the east of Taiwan. On the western boundary of this anomalous anticyclone, anomalous southeasterly flows enhance mean moisture transport from the tropical western Pacific into Taiwan. Seasonal rainfall increases in Taiwan with stronger anomalies on the windward side over eastern Taiwan. Warm SST anomalies and accompanying large-scale convergence, ascendance, and positive relative vorticity anomalies provide favorable conditions for more TC formation in the Philippine Sea. More TCs tend to move along the western boundary of the anomalous anticyclone to cross the northern Philippines toward eastern Taiwan or toward the open oceans southwest of Taiwan. Taiwan is influenced by enhanced TC activity to have more TC rainfall with the largest anomalies being in the eastern parts.