中国南方地区冬季风降水异常的分析

AN ANALYSIS ON ANOMALOUS PRECIPITATION IN SOUTHERN CHINA DURING WINTER MONSOONS

1997/1998和1998/1999年冬季是中国南方典型的多雨年和少雨年,它们分别发生在El Ni^no年和La Nina年。为了了解这两个冬季降水异常的原因,通过对比分析方法对这两个冬季的大气环流和水汽输送的差异进行了研究。结果表明:多雨年与ENSO事件的暖期相联系,西风带槽脊偏东偏弱,东亚冬季风减弱,副热带高压增强,对流层低层距平流场上呈现两个反气旋和一个气旋性环流,中心位于长江流域的气旋性环流的垂直结构和形成机理与菲律宾海反气旋不同。少雨年赤道海温的距平分布及高低层环流系统都与多雨年几乎相反。研究还揭示,冬季中国南方地区的水汽主要来自南支西风带低槽前部的西南气流和南海—中南半岛上空的转向气流,水汽输送通道随高度有明显的变化。1997/1998年冬季加强的南支西风气流和菲律宾海异常反气旋有利于水汽向中国大陆输送;1998/1999年南支西风气流弱,中国东南沿海低层为冷性高压控制,两支水汽输送带都大大减弱。这种水汽输送的明显年际变化是造成这2年冬季南方降水明显差异的一个关键因子。

The winters of 1997/1998 and 1998/1999, corresponding to E1 Nifio and La Nina episodes, respectively, were two typical rain-abundant and -scarce seasons for the southern part of China. In order to understand the anomalous precipitation in the two winters, the comparative analysis technique has been employed to investigate the differences of general circulation and moisture transfer between the two seasons. The results show that the abundant rainfall in the winter of 1997/1998 was associated with the warm episode of ENSO event, weak and eastward shifted westerly trough/ridge, weakened East Asian winter monsoon, strengthened subtropical high, and the presence of two anti-cyclonic circulations over Hokkaido, and the Philippine Sea, respectively, and one cyclonic circulation over the Changjiang River Basin in the anomalous wind fields in the lower troposphere. In the rain-scarce winter, however, the patterns of equatorial sea surface temperature anomalies and the circulation systems both in upper and lower levels were almost opposite to those in the rain-abundant winter. It is also revealed that the water vapor over southern China in the winters came mainly from the southwesterly flow ahead of south branch westerly troughs, and the turning flow over the South China Sea-Indo-China peninsula area; and the channels of moisture transfer varied significantly with height. The intensified south branch westerly flow and the anti-cyclonic circulation anomaly over the Philippine Sea in the winter of 1997/1998 were favorable for moisture transfer towards the continental China, however the two moisture transfer streams were dramatically weakened in the winter of 1998/1999 due to the weak westerly flow and the dominance of a cold high system in the lower level over the southeast coast of China. Such a significant inter-annual change of moisture transfer is a key factor resulting in the obvious difference in precipitation between the two winters.