東亞夏季季風年際變化：江淮及台灣地區觀點之探討

A Study on the East Asian Summer Monsoon Interannual Variability: Viewpoints from Different Region

潘琦(Chi Pan) ； 王作臺(Jough-Tai Wang)

《大氣科學》 24卷1-2期 (1996/06) Pp. 89-122

摘要

東亞夏季季風的年際變化，會影響地區性雨量的分佈，進而對民生經濟產生巨大之影響，探討季風之年際變化，是一個非常重要的問題。本研究使用1983、1985、1991及1992四個個案年3～7月資料，探討東亞季風在江淮地區（6～7月）及台灣地區（5～6月）的特性，並進一步了解這兩個地區年際變化時，各因素之分佈特徵，及其受高層熱力作用之影響。研究結果顯示，對於江淮和台灣地區梅雨期雨量相位之關係，當五、六月東亞地區500 hPa西太平洋副熱帶高壓與該月份之氣候平均比較，一直持續地偏強或偏弱時，兩地之雨量乃呈現反相位關係，吾人認為此為正常狀態；而由於副高的異常發展，五、月出現不同的正或負偏差，即先增強然後減弱，並不是隨著季節而持續增強，以及ENSO或其他因素的影響，則出現正相位之關係。由氣象場時序分析結果，高對流層陸續於85°E以東及85°E以西出現經向溫度梯度逆轉（5°N～25°N）之現象，使得高層西風噴流、南亞高壓、及低層西南氣流分別在85°E東、西側隨之發生二階段地急遽變化－北跳／西移、組織發展。以200 hPa南亞高壓來判斷，則當江淮地區為乾旱時，南亞高壓於第一階段暖化時才穩定出現，且很快就西移。另外，對噴流北跳而言，於第二階段暖化時，高層西風噴流於乾年時仍繼續北跳，江淮地區洪澇時卻未出現北跳情形，此乃乾溼年最大差異處。此外，暖化時間間隔的長短，可能導致環流明顯地年際變化特徵，造成江淮及台灣地區不同地季風／梅雨特性。

關鍵字

夏季季風 ； 年際變化

並列摘要

Interannual variability of the East Asian summer monsoon affects the distribution of regional rainfall, thus it is a very important scientific problem. The ECMWF data from March to July for 1983, 1985, 1991 and 1992 are used to investigate the characteristics of the East Asian summer monsoon in the Yangtze and Huaihe Rivers (June-July) and in Taiwan area (May-June). The spatial distribution of several important components related to the interannual variability of the East Asian monsoon are examined especially the upper-level thermal effect. It is found that the phase of Mei-Yu rainfall between the Yangtze-Huaihe River valley and Taiwan is out of phase, when 500 hPa western Pacific subtropical high prevails continuously stronger or weaker. However, when the influence of irregular development of the western pacific subtropical high that possess differently positive or negative anomalies in May and June, then those two areas rainfall have in phase correlation. According to the time series's analysis, the reversal of the meridional temperature gradient (5°N-25°N) occurs first over the longitudes east of 85°E and then over the longitudes west of 85°E at the upper troposphere. The heating structure is responsible for the dramatic change of two stages of the upper-level westerly jet, the South Asian anticyclone and the low-level southwesterly. When the Yangtze-Hualihe River vally is dry, the South Asian anticyclone doesn't establish until the warming process of the first stage occurs and move westward rapidly. In the dry year, the upper-level westerly jet jumps continuously northward even in the second stage of heating. But it is not so in the wet year. This is the distinct difference between the dry and wet years. Besides, the length of warming period is also potentially responsible for the interannual variability of rainfall between the Yangtze-Huaihe River valley and Taiwan.

並列關鍵字

East Asian summer monsoon ； interannual variability