30-60天季內振盪現象對臺灣夏季降雨特色的影響

Abstract

本研究主要是探討2007~2015年6月至8月期間30~60天季內振盪現象對臺灣夏季降雨特色的影響。研究過程中採用澳洲氣象局網站所提供之逐日即時多變數季內振盪指數 (Real-time Multivariate Madden-Julian Oscillation index，簡稱MJO指數)，針對強MJO事件(MJO指數振幅>0.9)下的臺灣夏季降雨特色變化進行討論。研究結果顯示，當MJO對流中心抵達中印度洋(相位2)時，臺灣降雨偏少；而當MJO對流中心逐漸離開海洋大陸進入西太平洋(相位6)時，臺灣降雨則偏多。進一步針對「臺灣降雨型態在MJO相位2和相位6時期中之改變」這個議題進行分析，則發現四種常出現的降雨型態(含午後熱對流降雨、鋒面降雨、颱風降雨、及其它因西南氣流增強帶來的降雨)中，僅午後熱對流及颱風降雨對總降雨天數的貢獻百分比，在相位2和相位6時期出現明顯差異。而透過環流場的分析本研究發現，因受到「西太平洋上空副熱帶高壓，在相位2時期比相位6時期更為西伸增強」以及「臺灣島上海陸風環流結構，在相位2時較相位6時期顯著」這兩因素影響，造成「臺灣午後熱對流降雨天數佔總降雨天數的比例」在MJO相位2時期高於相位6時期。反觀相位6時期，在「夏季季風槽增強」加上「南來水氣增加」的配合之下，提供了颱風生成及發展的良好環境，使得「臺灣颱風降雨天數佔總降雨天數的比例」及「臺灣颱風降雨的日平均降雨量」，在MJO相位6時期較相位2時期大。這些研究成果，對我們在瞭解強MJO事件下30~60天季內振盪造成臺灣夏季降雨型態的可能改變，至為重要。

This study investigates the influence of the 30~60 day intra-seasonal oscillation on the characteristics of summer (June to August) rainfall in Taiwan during 2007~2015. Analyses focus on the changes of rainfall types in Taiwan for the stronger Madden-Julian oscillation (MJO) events (defined as MJO index is greater than 0.9). Results show that Taiwan has less precipitation when MJO is in phase 2 (i.e. the active convections located over the middle tropical Indian ocean), and has more precipitation when MJO is in phase 6 (i.e. the active convections located over the tropical Pacific Ocean). Diagnoses show that affected by the intensification of western Pacific subtropical high and the intensification of island-scale diurnal circulation, the percentage of diurnal convection events to the total rainfall events is larger in Taiwan when MJO is in phase 2 than in phase 6. In contrast, due to the intensification of monsoon trough and the intensification of southerly moisture supply over the East Asian-western North Pacific region, both (1) the percentage of tropical convection events to the total rainfall events and (2) the intensify of typhoon type of rainfall are larger in Taiwan when MJO is in phase 6 than in phase 2. These findings can help us understand how the 30~60 day intra-seasonal oscillation identified by the MJO index impacts on the characteristics of summer rainfall in Taiwan.