For a full and detailed answer go to
http://www.taiwan.com.au/Envtra/Geography/Geo/repo
rt01.html
The gist of what they say, as far as I can work out: being in the path of warm ocean currents, with steep mountain ranges, and in the sub-tropics, Taiwan gets both oceanic and subtropical monsoons. It's greatly affected by the seasonal exchange of air masses between the continent and the ocean, and talking of the seasons, it gets both summer and winter monsoons. Between them, they last up to 11 months of the year. But I'm no expert - go to that site and read the whole thing!