长江流域中下游梅雨时期500毫巴环流形势的分析

本文对1954—1962年5—7月500毫巴流型作了分析,确定了长江中下游各年的梅雨期,划分了梅雨期500毫巴环流型。分析指出,西太平洋副热带高压和东亚上空西风气流的变化,是决定入梅和出梅的重要因素,其中东经110°到125°之间的副热带高压脊线的变化更重要。当这个脊线从低纬度向北移动,越过北纬20°时,梅雨开始,再次北移越过北纬25°时,梅雨结束。500毫巴西风气流的变化是另一个指标。90°E上南支强西风消失,东风突然向北推进,是季节转换的标志,这以后,当115°—125°E上南支强西风北撤到北纬30°以北时,梅雨开始,再次北撤越过北纬35°时,梅雨就结束。

AN ANALYSIS ON THE 500 MB CIRCULATION DURING MAI-YÜ PERIOD IN THE LOWER AND MIDDLE YANTZE

In this paper we have analysed the 500 mb flow patterns from May to July, 1954-1962, to decide the Mai-yü period of the lower and middle Yantze.It is found that the variations of the subtropical high of Western Pacific and the westerlies over East Asia are the essential elements to determine the beginning and the end of Mai-yü. Especially, the variation of the ridge line of the subtropical high in the region 110°-125°E is more important in determining the Mai-yu period. If the ridge line moves across 20°N from lower latitude, Mai-yü will begin; if it moves further north- ward and crosses 25°N, Mai-yü will terminate. The variation of westerlies on 500 mb is another index. The disappearance of the subtropical jet and the sudden northward push of the easterly along 90°E indicate exchange of seasons. After this variation Maiyü will begin when the subtropical jet in the band of longitudes 115°-125°E retreats northward to the north of 30°N and terminates when the jet acrosses 35°N.