位在中南半島的越南之天氣，除在季節轉換期間受到熱帶擾動影響外，主要仍受制於東亞冬季及夏季季風系統的影響。大部分嚴重的水災事件，經常在秋天的雨季裡發生在越南中部。2007年因連續豪雨造成此區域重大水患災害，引起眾多的關注。本研究據此著重於氣候思維，從事探討越南中部2007年10-11月豪雨的可能發生原因，特別是年際變化的聖嬰卅南方振盪影響。 運用經驗正交函數分析越南全國163個雨量站資料，結果發現越南降水具有兩特殊分佈特性：秋季（10-11月）集中在越南中部，夏季（5-10月）則分別於北部及南部呈現多雨情形。其中秋季雨量分佈佔有48.31%的總變異數。另，越南中部秋季降水與聖嬰卅南方振盪顯示具有相當好的關係。通常於反聖嬰年時，此區域會發生較多的豪雨，而在聖嬰年時則呈現偏乾少雨現象。此外，在反聖嬰年時，越南中部外海常有一氣旋式環流盤據在西北太平洋海面溫度相對較高的海域上。同時，低層大氣伴隨水氣傳輸的輻合，將溫暖海域上的水氣帶來，成為引發越南中部秋季（10-11月）豪雨的重要因素。反之，於聖嬰年時，上述環境場反過來，以致此區域呈偏乾少雨的情形。 The weather of Vietnam, a subtropical country located along the east coast of the Indochina Peninsula on the north-western Pacific Ocean, is greatly affected by both East Asian winter and summer monsoon systems beside the tropical activities occurred during the seasonal transition period. However, the most severe flooding events frequently appear in the central Vietnam (120N-190N) during fall rainy season. The 2007 disastrous damages in this region caused by sequential heavy rainfall had been attracted much attention. Thus, the major purpose of this study is to investigate the possible causes of heavy rainfall events in central Vietnam during Oct–Nov 2007 from climate perspective, especially the interannual variation effect of ENSO. By using EOF analysis of 163 daily rainfall station data in Vietnam, two significant rainfall regimes were identified: the fall rainfall regime (Oct - Nov) in central Vietnam and summer rainfall regime (May - Oct) in the northern and southern regions of this country. The fall rainfall regime, the most important component in 2007, explained 48.31% of the total variation. It is interesting that a close relationship between the Oct-Nov precipitation in central Vietnam and ENSO was distinctly exhibited. In general, heavy rainfall would occur in this region during cold phase (La Nina), while the appearance of dry conditions is corresponded to warm phase (El Nino). Over the positive (negative) SST anomaly at northwestern Pacific Ocean, an anomalous cyclonic circulation adjoined to the coast of central Vietnam together with a positive convergent anomaly and strong water vapor transport from the warm sea surface during La Nina events played an important role for inducing heavy rainfall in central Vietnam during Oct-Nov, while the opposite situation in El Nino years were responsible for the dry conditions in central Vietnam.
