台灣五—七月豪（大）雨之中長期變化與季內振蕩

The Medium-long Range Variability of Heavy Rainfall in Taiwan and Intraseasonal Oscillation during May to July

鄒治華(Chih-Hua Tsou) ； 徐邦琪(Pang-Chi Hsu) ； 柯文雄(Wen-Shung Kan)

《大氣科學》 30卷1期 (2002/03) Pp. 1-20

摘要

台灣西部地區豪（大）雨的發生頻率最大值出現在六月，次高峰出現在七月或八月，顯示台灣西部地區豪（大）雨深受其背景環流東亞夏季季風之影響。本研究的目的即在探討東亞夏季季風的低頻變化一季內振盪與台灣西部地區豪（大）雨之關係。
本研究採用CDC所分析之1979～1998年外逸長波輻射(OLR)，及NCEP重新分析之1979～1998年200mb、850mb水平風場資料，並以在頻率及時間域中皆有良好區域性的小波轉換(Wavelet)，進行資料的濾波。比較10～60天對流振盪與總變異的分布可知，10～60天OLR波動變異量的高值區分布與總變異量一致，即亞洲季風區變異量最高，東太平洋次之。亞洲季風區之季內振盪又以30～60天波動的貢獻較大，其中孟加拉灣為全球30～60天變異量之冠，而南海地區為東亞地區季內對流振盪變異量最大的地區。南海地區其季內對流變化主要以30～60天振盪的貢獻為主。
南海地區五-七月季內振盪的年際變化與台灣西部地區五-七月豪（大）雨發生次數的年際變化成正相關。分析顯示當南海地區30～60天對流振盪變異量為正距平時，台灣地區的豪（大）雨次數較多，反之南海地區30～60天振盪變異量為負距平時，台灣五、六、七月豪（大）雨次數偏少，且此期間豪（大）雨次數多發生於南海地區30～60天OLR波動之第一波低值區由負轉正後。而相對的10～20天的季內對流振盪與台灣地區豪（大）雨兩者之間並無顯著的相關性。
由延遲相關分析與合成分析發現，30～60天OLR波動到達南海前，最大上升區在印度洋至孟加拉灣地區，南海為下沈區。30～60天波動到達南海後，短暫滯留強度增強，對流強度達到最大時，東亞地區南北方向波列結構明顯，波動轉向北及東北移至華南／台灣及日本東南方。此時台灣附近30～60天渦度由負轉正，風場由東風轉為西南風，伴隨30～60天波動之上升運動、西南氣流及氣旋式渦度，有利於台灣、華南地區的對流成長，進而可能形成豪（大）雨。台灣豪（大）雨之高峰期、與季內對流振盪由南海向北及東北移至台灣、華南的發生時間一致。監測南海地區30～60天振盪的結構與移動，將有助於掌握台灣地區豪（大）雨中期變化。

關鍵字

豪（大）雨 ； 季內振蕩 ； 夏季季風

並列摘要

The maximum and secondary maximum occurrence of heavy rainfall over western Taiwan appears in June and in July or August, respectively. Apparently, the heavy rainfall in Taiwan is strongly influenced by its background flow EASM (East Asian Summer Monsoon). The purpose of this study is to diagnostic the low frequency-Intraseasonal Oscillation of EASM and its relationship with Taiwan heavy rainfall.
The OLR(Outgoing Longwave Radiation)analyzed by the Climate Diagnostic Center from 1979 to1998 and circulation fields of NECP reanalyzed data were adopted in this study. These original data were decomposed into intraseasonal time scale by wavelet transform that provides a local value of amplitude and phase. The result show that ISO (Intraseasonal Oscillation) is closely related to the onset and variability of EASM. The maximum climatological variance of ISO in East Asia occurs in South China Sea. Its climatological variance of ISO is dominated by 30-60 day oscillation.
The interannual variability of ISO in South China Sea is positively correlated with the interannual variability of heavy rainfall in Taiwan. When ISO in South China Sea has positive anomalies, the occurrence of heavy rainfall in Taiwan is above normal. The interannual variability of ISO in South China Sea is also dominated by 30-60 day oscillation. The distribution of the interannual variability of 30-60 day oscillation with heavy rainfall in Taiwan is very similar to that of 10-60 day oscillation, while the heavy rainfall in Taiwan is less correlated with 10-20 day oscillation.
As the 30-60 day OLR propagated from South China Sea to Taiwan and Southern China, the 30-60day vorticity in nearby area of Taiwan changed from negative to positive. The 30-60 day wind fields changed from easterly to westerly. The upward motion, cyclonic vorticity, and low level moisture associated with the 30-60 day convection is favorable to the growth of deep convection in Taiwan and Southern China. Subsequently, this favorable condition may lead to the heavy precipitation in Taiwan and Southern China. Monitoring the structure and propagation of 30-60 day convective oscillation in South China Sea, may be helpful to the medium range forecast of heavy rainfall in Taiwan.

並列關鍵字

Heavy rainfall ； Intraseasonal oscillation ； Summer Monsoon