This study discusses major impacts of the Madden-Julian oscillation (MJO) on the winter (NovemberApril) rainfall in Taiwan. The results show that Taiwan has more rainfall in MJO phases 3 and 4 (MJO convectively active phase in the Indian Ocean and the western part of the Maritime Continent), and less rainfall in phases 7 and 8 (the western Pacific warm pool area). Mechanisms associated with the MJO are suggested as follows. 1) The tropics to midlatitude wave train: when the MJO moves to the middle Indian Ocean, a Matsuno-Gill-type pattern is induced. The feature of this tropical atmospheric response to the MJO diabatic heating is a pair of upper-level anomalous anticyclones symmetric about the equator to the west of the heating. The northern anomalous anticyclone over the Arabian Sea and northern India induces a northeastward-propagating wave train to the midlatitudes. The wave pattern consists of a cyclonic anomaly centered at East Asia that enhances the winter rainfall in Taiwan. 2) Increase of moisture supply from the South China Sea: when the MJO convection approaches Sumatra and Java of the Maritime Continent, the eastward penetration of equatorial convection enhances a low-level southerly flow that transports the moisture northward to Taiwan and southern China. As a consequence, with the increase of moisture supply from the south, more winter monsoon rainfall is observed in Taiwan.

This study discusses the boreal summer intraseasonal oscillation (BSISO) impact on the western North Pacific (WNP) typhoons and the summer rainfall in Taiwan. The real time BSISO1 and BISISO2 indices are created using the first two and the third and fourth principal components of the multivariate empirical orthogonal function analysis, based on outgoing long-wave radiation and zonal wind at 850 hPa from Lee et al. (2013). The results show that heavy rainfall in Taiwan and the associated WNP typhoon frequency patterns are closely related to the 10 -30 days BSISO2 phases during the typhoon season (July -October). Taiwan has larger rainfall during BSISO2 phases 3, 4, and 5 when the major BSISO2 convection moves northwestward from the Philippine Sea to the Taiwan area. During phases 3 and 4 the anomalous low-level cyclonic flow and the increased typhoon frequency directly result in larger rainfall in Taiwan. Phase 5 exhibits enhanced low-level southwesterly flow which transports the moisture to Taiwan responsible for more summer rainfall on the island.