Joint modulations of Taiwan rainfall by tropical cyclone, northeast monsoon, and intraseasonal oscillation in October

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 degrees - 122 degrees 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 degrees N) and weak for those with a southern track (south of 19 degrees 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.