Tropical Cyclone- and Monsoon-Induced Rainfall Variability in Taiwan

This study investigates the interannual variability of tropical cyclone (TC)- and monsoon-induced rainfall (P) in Taiwan during July September for the period 1950-2002. To examine the relative effects of TCs and monsoons, local rainfall in Taiwan is separated into two subcomponents: TC rainfall (P(TC)) and seasonal monsoon rainfall (P(SM)). The former is induced by TC passage across Taiwan, while the later is caused by large-scale monsoon circulation. Climatologically, P(TC) and P(SM) accounts for 47.5% and 52.5% of total rainfall in Taiwan, respectively, showing a comparable contribution. On an interannual time scale, P(TC) and P(SM) anomalies tend to vary inversely. Two dominant rainfall variability types are found in Taiwan: enhanced P(TC) but suppressed P(SM) (T+S-) and suppressed P(TC) but enhanced P(SM) (T-S+). The T+S- type features a low-level anomalous cyclone and enhanced upward motion southeast of Taiwan. This favorable environmental condition leads to more TC formation in the region. TCs are further steered by mean southeasterly flows toward Taiwan to increase P(TC) (T+). As Taiwan is located in the western boundary of the anomalous cyclone, anomalous northeasterly water vapor fluxes hinder moisture supplies from the South China Sea into Taiwan, resulting in decreased P(SM) (S-). The T-S+ type concurs with an anomalous cyclone over Taiwan. Its center enhances upward motion and moisture fluxes from the South China Sea into Taiwan, yielding increased P(SM) (S+). Meanwhile, weak relative vorticity anomalies occur to the southeast of Taiwan, suppressing TC formation in the region. Mean southerly steering flows tend to drive more TCs toward Japan and the North Pacific, resulting in decreased TC frequency and P(TC) in Taiwan (T-). The present approach provides a new perspective for studying and predicting interannual rainfall variability via the separation of rainfall into TC- and monsoon-induced rainfall subcomponents, rather than looking solely at total rainfall. The result shows that there are two ways to significantly increase total rainfall in Taiwan (T+S- and T-S+), but there is only one way to decrease it (T-S-). The composites of circulation anomalies based on two rainfall indexes have more significant and coherent dynamic patterns than those sorted based on the total rainfall index.