本研究第一部份主要針對秋季期間 (9月至11月)，通過西太平洋西北側海域（120°-130°E、20°-35°N）的颱風頻率年際與年代際變化進行診斷分析。結果顯示，秋季生成於西北太平洋地區的颱風，移動路徑於1998年發生轉變，通過20°N以北的頻率增加，整體颱風移動路徑有往北偏轉的趨勢。對應北太平洋環境場特徵則呈現負PDO相位，低層輻合中心位於西太平洋海洋大陸附近，誘發臺灣附近出現氣旋式環流距平，提高20°N以北的颱風生成機率，加上太平洋副高往西擴張延伸，受到高壓西南側東風及東南風導引，更容易讓颱風往西北方移動，提高颱風通過20°N以北、120°-130°E的機率。1998年之後，颱風活動特性也同時受ENSO影響。El Niño期間，颱風通過西北側海域機率較低，La Niña期間，若西太平洋輻合中心位於130°E以西，則通過上述區域的機率偏高，不過，若輻合中心位於130°E以東，則相對偏少。當颱風通過該區域的頻率較高(低)時，臺灣秋季的降雨量相對較多(少)，兩者之間有高度正相關。
第二部分主要針對臺灣降雨變化進行探討，透過氣象局及科技部TCCIP計畫所提供之降雨量資料，分析臺灣大雨 (日降雨量≧80 毫米) 和豪雨 (日降雨量≧200 毫米) 發生頻率變化，發現大 (豪) 雨事件主要出現在5至10月，其中又以7至10月發生頻率最高，日降雨量達80毫米 (含) 的大雨事件和200毫米 (含) 以上的豪雨事件發生頻率，分別占了年總發生頻率的61%和80%。夏秋季大雨和豪雨發生頻率變化，與該年受颱風影響頻率高低有關，海洋及大尺度環流條件則扮演關鍵角色。從年際變化來看，大雨和豪雨發生頻率偏多(少)年，太平洋西側副熱帶高壓脊線位置較為偏北(南)，颱風往臺灣移動機率較高(低)，全臺大雨發生頻率也相對增加(減少)。從長期變化來看，大雨和豪雨頻率增加，主要由颱風影響臺灣頻率高低所決定，非颱風系統的貢獻相對低。
The first part of this study examines interdecadal and interannual variabilities of tropical cyclone passage frequency during September-November over the northwestern North Pacific (NWNP) region (120°-130°E,20°-35°N). After 1998, the negative Pacific Decadal Oscillation (PDO) phase induces a low-level convergent center around the Maritime Continent that arouses a cyclonic circulation anomaly around Taiwan and raises TC formation in the region north of 20°N. Meanwhile, the westward extension of the western Pacific subtropical high, leading to increase TCs move northwestward and pass through the region between 120°-130°E along easterly and southeasterly flows at the southwestern boundary of the WPSH. On the interannual time scale, the frequency of TCs that pass through the NWNP region tends to decrease during El Niño events. During La Niña events, there is high (low) TC passage frequency around Taiwan when the low-level anomalous convergent center is located to the west of 130°E (east of 130°E). TC rainfall is then influenced in consequence.
The second part uses rainfall dataset developed from the Central Weather Bureau (CWB) and Taiwan Climate Change Projection and Information Platform Project (TCCIP) to examine the changes in heavy (≧80mm/day) and extremely heavy (≧200mm/day) rainfall frequencies in Taiwan. The major rainfall season in Taiwan generally occurs from May to October. However, about 61% of heavy and 80% of extremely heavy rainfall events occur between July and October. For the interannual variation, analytical results identified that typhoons are highly correlated with the western North Pacific subtropical high (WNPSH) ridge behavior. The northward movement of the WNPSH ridge causes typhoon activities to slip toward Taiwan. When the typhoon approaches Taiwan, the typhoon-associated heavy and extremely heavy rainfalls significantly affect the local area. In contrast, the southwestward extension of the WNPSH ridge decreases the impacts of typhoons on Taiwan. The increases in heavy and extremely heavy rainfall events are dominated by the increase in typhoon passage frequency on the long-term timescale.
