本文根據臺灣 6 個有百年以上日雨量記錄的測站資料，設計了一個可以代表臺灣有異常偏長乾期現象的極端乾期指數（DSidx），藉此衡量臺灣極端乾期事件是否發生與嚴重程度。極端乾期事件的判斷是以連續不降雨日數達到超越機率小於 1％的水準為依據。分析結果顯示，若以 11 年為一個窗區，近百年臺灣極端乾期事件發生頻率有明顯 20～30 年尺度的年代際變化，並有季節差異。整體來說，1960 年之後極端乾期發生頻率變多，1920～1940 年代期間極端乾期發生頻率較低。1960～1980 年代異常乾期主要發生在夏、秋二季，梅雨季的異常偏乾事件在 1970～1980 年較其他年代稍多。1990 年代之後，11月至隔年 4 月冬半年的臺灣極端乾期發生頻率則有明顯增多趨勢，近 20 年極端乾期事件發生在乾季的頻率明顯高於雨季。分析極端乾期事件發生的大尺度環流條件，發現梅雨季和夏季太平洋副熱帶高壓強度和高壓脊位置是影響極端乾期的重要因素，冬季極端乾期發生頻率則受到東亞冬季風強弱以及印度洋和太平洋上熱帶對流系統活躍程度的影響。

An index DSidx is proposed to represent the overall dry extremes in Taiwan based on the 110 years of daily rainfall data from 1901-2010. The extreme dry event is identified as an event with less than 1% of the exceeding probability based on the climatology of longest dry spells in a 90-day window on daily basis at a station. DSidx is the total count of the stations that show extremely prolonged dry spells. The value of DSidx is from 0 to 6 because the total number of stations is 6. The daily DSidx are summed up using the 11-year window to illustrate variations on the multi-decadal time scale. The result suggests clear decadal-scale variations of the dry spell extremity in Taiwan. The overall dry spell extremity has increased after 1960. The extremity is more sever during the summer months before 1990, while it is more sever during the winter months after 1990. The change of the dry extremes enhances the general concern about dry season becomes drier and wet season becomes wetter, although there is no correlation between the variations of DSidx and rainfall amount. The summer dry spell extremes can be caused by the stronger and more westward extension of the western Pacific subtropical high pressure system. The winter dry spell extremes can be caused by weak East Asian winter monsoon.