東亞夏季季風肇始期間大尺度系統轉化特徵之診斷分析

The Evolution of Large-Scale Pattern during the Onset of East Asian Summer Monsoon

陳孟詩(Meng-Shin Chen) ； 王作臺(Jough-Tai Wang)

《大氣科學》 26卷2期 (1998/06) Pp. 109-131

摘要

台灣位於中、低緯度過渡之副熱帶地區，且地處歐亞大陸與太平洋交界地帶，在此最大陸地及最大海洋所產生的季風現象是非常特殊的，因此，台灣及其鄰近地區之天氣與氣候變化明顯的受到大尺度季風環流所影響。已有一些研究在探討東亞季風的問題，本研究則著重於分析東亞夏季季風肇始期間大尺度系統之演化特徵，經由各分量的空間結構與時間序列，探討彼此之間的關係及相對重要性，並了解各變數對季風肇始可能扮演的角色。經由時序分析，吾人認為，當季節由春季推進至夏季時，上對流層暖心自西太平洋暖水區向西北移至中南半島，造成經向溫度梯度逆轉，同時高層200 hPa南亞反氣旋向西北移動，其南方並出現東風。此外，東亞及印度越赤道流自赤道洋面上輸送暖溼水汽，導致熱帶大氣更加不穩定，但一切都必須等到西太平洋副熱帶高壓東退出南海，同時，因副高之東退使印度及東亞地區越赤道流得以進入南海，南海盛行西南氣流，深對流發展，造成東亞夏季季風肇始。換句話說，東亞夏季季風肇始的主要特徵之一為副高東退。此外，研究還發現每年春夏轉換期間，大氣環流會經歷二階段驟變，時間間隔約一個月，每次改變均伴隨著上對流層暖心大幅向西北移動。而由此分析四個個案年所得之結果顯示，若兩次變化分別發生於五月中旬及六月中旬，且西太平洋副熱帶高壓於第一次變化時即東退出南海，則東亞夏季季風肇始早於印度季風；若兩次變化分別發生於四月下旬及五月下旬，且副高於第一次變化時並未隨之東退出南海，則東亞夏季季風與印度季風幾乎同時肇始，此觀點亦值得進一步深入探討。

關鍵字

季風 ； 肇始 ； 轉化 ； 大尺度系統

並列摘要

The objective of this study is to understand the characteristics of monsoon onset, the evolution of large scale flow patterns during the onset of East Asian summer monsoon. The data used here are from ECMWF. Years chosen for the analyses are 1983, 1985, 1991 and 1992.From spring to summer, upper-level warm core moves northwestward from warm pool to Indochina Peninsula, thus the meridional temperature gradient reverses between land and sea. It was also found that the upper-level South Asian anticyclone moves northwestward with easterlies to the south, and westerly jet jumps northward consequently. The cross-equatorial flow also transfers moisture from equatorial ocean to the neighboring land surface. This process causes tropical atmosphere environment becomes more unstable. And it was concluded that the onset of East Asian summer monsoon will not occur until the Subtropical High withdrawed from the South China Sea and moved eastward.The large-scale circulation was also found to undergo two distinct stages of abrupt transitions annually. Those changes are associated with upper-level warm core moving northwestward suddenly. If those two changes happen on middle May and middle June, and Subtropical High withdraws at the first occurrence, then the onset of East Asian summer monsoon is earlier than those of Indian monsoon. However, if those two changes happen on late April and late May, and Subtropical High doesn't withdraw to the East at the first occurrence, then the onset of the East Asian and Indian monsoons occurs at the same time.

並列關鍵字

Monsoon ； Onset ； Evolution ； Large-scale pattern