| 摘要: | 亞洲-太平洋夏季季風(APSM)環流主要結構中心為亞洲大陸副熱帶區的高層高壓系統和北太平洋上低層的副熱帶高壓系統。台灣處於兩高壓交界帶，氣候具有高度敏感性。考量到北太平洋高壓和亞洲高壓的季節性演變所導致的季風區域內環流場及雨量急劇的改變，本研究不採用夏季(JJA)平均場資料，而另外訂定三個季風轉換期: Phase 1 (五月15日-六月14日)-東亞季風、中南半島季風肇始；Phase 2 (六月15日-七月24日)-印度季風肇始、華中梅雨、日本Baiu季；Phase 3 (七月25日-八月31日)-亞洲季風衰弱，日本Baiu季的結束、颱風最盛期。Phase 1時北太平洋高壓座落於菲律賓東部海域(約15°N)；Phase 2時高壓脊線則是在台灣附近(約25°N左右)；最後，Phase 3時高壓西部北抬至日本南方。本研究使用中央氣象局10個氣象測站於1974-2010期間雨量代表台灣地區的降水情形，進行台灣乾濕年分類。台灣地區降水正距平年時，Phase 1期間所伴隨的氣候特徵為:北太平洋高壓增強向西南延伸，導致高壓北側的氣壓梯度和梅雨鋒面加強，而所引發的劇烈的對流加強了上層的亞洲高壓。然而增強的底層高壓抑制中南半島的底層南風，造成南北向的季風環流減弱；Phase 2多雨年時，北太平洋高壓減弱、東退，使得台灣脫離北太平洋高壓沉穩的下沉氣流控制，而且加強東亞-北太平洋西部(EA-WNP)的底層南風和垂直運動；Phase 3時北太平洋高壓增強、往西北延伸結束了日本的Baiu季。研究結果顯示台灣夏季降水主要受熱帶地區的北向氣流所影響，因而Phase 3時期位於中緯度帶的北太平洋高壓，對台灣降水影響不大。為了檢驗上層亞洲高壓和下層太平洋高壓強度、東西向移動，兩高壓脊線交會處定為敏感區域，計算200hPa和850hPa平均重力位高度值作相關性分析。Phase 1的敏感區域為[120-150°E; 10-30°N]; Phase 2為[120-150°E; 15-35°N]; Phase 3為[120-150°E; 25-40°N]。分析結果發現Phase 1和Phase 2時相關係數為正，表示亞洲高壓增強(減弱)並東伸(西退)時，北太平洋高壓增強(減弱)並西伸(東退)。Phase 3時微弱的相關性說明大尺度環流對台灣夏季降水無顯著的影響。雖然北太平洋高壓位置、亞洲高壓位置，以及上下對流層環流的特徵在各轉換期之間都不一樣，但是皆可看到台灣多雨時，南亞季風區對流減弱、低層的西風增強。另一方面，北太平洋高壓的變化加強EA-WNP地區底層的南風和垂直運動，因而構成活躍的夏季季風環流，導致包括台灣在內的EA-WNP低緯區的降水增加。The dominant structures of the Asian–Pacific summer monsoon (APSM) circulation are the upper-level high pressure system in the subtropical Asia and the lower-level high pressure system in the subtropical North Pacific (NP). Taiwan borders the two high pressure systems, and its climate is highly sensitive to the highs. In consideration of the impact of the seasonal evolution of the NP high and the Asian high on the rapid changes of the circulation and precipitation in the ASPM regions, the summer monsoon transitions is investigated in this study by classify the summer into three monsoon phases , instead of the JJA mean datasets : Phase 1 (May 15-June14)- the onset of the East Asia and Indochina monsoons; Phase 2 (June15-July24)- the onset of the Indian summer monsoon , the Meiyu season in central China and the Baiu season in Japan; Phase 3 (July25-August31)- the withdrawal of the Asian summer monsoon, the end of the Baiu season in Japan, and maximum tropical cyclone activity. In Phase 1, the western stretch of the NP high is located in the east of the Philippines Sea (~ 15°N). In Phase 2, the ridge of the high is near Taiwan (~25°N). In Phase 3, the western of the NP high shifts northward to southern Japan.In this study we used the (1974 to 2010) precipitation data collected at 10 CWB weather stations, and identified those years of anomalously high and low summer precipitation in Taiwan. It is found that in years when summer precipitation in Taiwan is anomalously high, the mean climate state are: during Phase 1 period, the strengthened and westward NP high intensifies gradient pressure and Meiyu frontal system to its north, which induce the deep convection and the enhanced upper-level Asia high. However, the intensified low-level high suppressed the surface southerly wind over Indochina weakens the meridional monsoon circulation; on Phase 2 when precipitation in Taiwan is anomalously high, the weakened and eastward retreat NP high causes that the climate in Taiwan is out of control form suppressed ascending motion of the NP high, and strengthened southerly wind and enhanced vertical motion in East Asia and the western NP (EA–WNP) region. On Phase 3, the NP high enhanced and extended northwestward ends the Baiu season. Our result demonstrates that the summer precipitation in Taiwan is mainly influenced by the northward flow from the tropic region. As a result, the impact of the NP high located in the mid-latitude zone is insignificant on the summer precipitation in Taiwan.To examine the strength and zonal shift of the upper-level Asia high and the low-level NP high, we do correlation analysis by computing the mean geopotential height at 850hPa and 200hPa over the overlapping of the two high ridges denoted as sensitivity regions. The sensitivity regions are [120-150°E; 10-30°N], [120-150°E; 15-35°N], and [120-150°E; 25-40°N], from Phase 1 to 3, respectively. It is found that the positive correlation coefficients on Phase 1 and Phase 2 represent the intensified (weakened) and extended eastward (retreat westward) Asia high and the intensified (weakened) and extended westward (retreat eastward) NP high. The lack of the correlation between the highs indicates that the large scale circulation does not have an apparent impact on the summer precipitation in Taiwan.It appears that the Asian monsoon circulation weakens in all three monsoon transition phases when precipitation in Taiwan is high, although the circulation characteristics of the three monsoon transitions are considerably different. The weakened convection in South Asia is associated with an increase of the surface westerly wind along the southern Asia. On the other hand, the low-level southerly wind and vertical motion in EA–WNP region enhances when precipitation in Taiwan is high, which implies an accelerated monsoon circulation surrounding South Asian and the EA-WNP region. |
