臺灣秋季降雨之低頻擾動變遷

Abstract

陳與汪(2000)的研究指出1930-1997年之臺灣秋季降雨有準20年振盪現象。本研究主要希望透過檢視臺灣全島秋季(9-11月)總降雨及兩種類型降雨(颱風降雨和非颱風降雨)在1945-2015年期間的變化，瞭解其相對應的低頻擾動特徵是否隨檢驗時間不同，而存在不同特性變化。主要研究議題分三個：(1)臺灣秋季降雨低頻擾動之時間特性，是否存在變化？ (2)臺灣秋季降雨低頻擾動是否存在區域性的差異？ (3)若臺灣秋季降雨低頻擾動存在變遷，其可能的原因為何？ 針對議題(1)，本研究發現臺灣秋季總降雨之低頻擾動的週期從前期(1945-1985年)的準20年振盪現象，轉換為後期(1990-2015年)的準10年振盪現象。此現象不但存在於「秋季總降雨量變化分析」，亦存在於「颱風降雨」及「非颱風降雨」分析。針對議題(2)，本研究透過分析10個中央氣象局局屬測站在1945-2015年期間的降雨資料，發現上述低頻擾動變遷現象為全島性現象，各地的振盪振幅雖有大小差異，但週期的變化卻是相近似的。針對議題(3)，本研究結果發現「臺灣秋季降雨之低頻擾動變遷現象」與「亞洲大陸高壓」和「太平洋副熱帶高壓」這兩個氣候因子的長期變化息息相關。其中，前期主要是受到亞洲大陸高壓和太平洋副熱帶高壓的影響，在兩個氣候因子的影響下，臺灣秋季降雨之低頻擾動為準20年振盪；而後期則是受到太平洋副熱帶高壓的影響，導致臺灣秋季降雨之低頻擾動轉變為準10年振盪。最後，針對臺灣附近的水氣通量變化進行分析，本研究發現「水氣通量的低頻擾動變遷現象」亦是影響「臺灣秋季降雨之低頻擾動變遷由準20年振盪變成準10年振盪」的主要原因之一。This study examines the characteristics of low-frequency variations of autumn (September, October and November) rainfall in Taiwan,during 1945-2015. Analyses focus on the contribution of rainfall from two different types: the typhoon-related-rainfall and the non-typhoon-related-rainfall. Research topics are divided into three: (1) Is there any change in the time characteristics of low-frequency variations of autumn rainfall in Taiwan? (2) Is there a regional difference in low-frequency variations of autumn rainfall in Taiwan? (3) If there is a change in low-frequency variations of autumn rainfall in Taiwan, what are the possible reasons? In response to the first issue, our results show that the autumn rainfall in Taiwan was dominated by a clear signal of quasi twenty-year oscillation before 1985. After 1990, the oscillation period for the low-frequency variations of autumn rainfall was changed to about 10 years. This phenomenon not only exists in the "total rainfall in autumn" but also in "the typhoon-related-rainfall" and "the non-typhoon-related-rainfall". In response to the second issue, by analyzing the rainfall data from 10 stations of the Central Weather Bureau during 1945-2015, we found that the above phenomenon of low-frequency variations is an island-wide phenomenon. Although the amplitudes of the oscillations vary from place to place, the oscillation period of autumn rainfall is similar for all examined stations in Taiwan. In response to the third issue, our examinations on the related mechanisms show that the changes of High over Asia Continent, Pacific Subtropical High, and moisture flux are three of important factors affecting the changes in the oscillation period of autumn rainfall (including typhoon-related and non-typhoon-related rainfall) in Taiwan.