广西后汛期不同等级降水变化及突变机理探讨

STUDY ON THE VARIATION OF DIFFERENT GRADES OF PRECIPITATION INTENSITY IN THE ANNUALLY LATER FLOOD SEASON OF GUANGXI AND ITS ABRUPT CHANGE MECHANISM

利用1961—2010年广西后汛期(7—9月)逐日降水资料进行降水分级统计,采用线性倾向估计、经验正交函数(EOF)等方法,对各等级降水日数及降水量进行了趋势变化和型态分布研究。结果表明,桂东小雨日数的比例高于桂西,中雨和大雨日数的比例则低于桂西,而暴雨以上日数的比例则是沿海高于内地。从各级降水对总降水的贡献来看,只占降水日数15%的大雨等级以上降水量的贡献接近60%。近50年来小雨至大雨等级降水呈现下降趋势,而暴雨以上等级降水则呈上升趋势,但这种特征具有明显的区域差异。EOF分析表明后汛期降水型态分布基本可以归结为大雨以上等级降水的分型。在此基础上,对广西后汛期各级降水在1990年代初出现由少到多的突变进行机理分析,研究表明,南半球澳大利亚高压和马斯克林高压的增强及西太平洋副高的合理配置是这次突变的重要原因,突变后来自印度洋及南海的水汽增强,西太平洋副高明显加强且位置偏南、偏西,大量的水汽在广西区域辐合上升,造成了降水的增加。通过对澳大利亚高压指数与前期海温的相关分析得知,印度洋暖位相由南向北传播及赤道太平洋与副热带南太平洋海温的反相变化将会导致后期澳大利亚高压强度的变化,这对降水预测有一定的参考意义。

The patterns and linear trends of rain days and rainfall in different grades in the annually later flood season （July to September） of Guangxi from 1961 to 2010 are examined by using the daily precipitation data. The results indicated that the percentage of light rain days （moderate and heavy rain days） in the eastern part is higher （lower） than in the western part of Guangxi, and the proportion of torrential rain days in coastal areas is higher than in the mainland. Based on the contribution of rainfall of different grades to the total rainfall, it is known that the rainfall of heavy rain, which appears in 15% of the rain days, contributes to nearly 60% of the total rainfall. Light rain and moderate rain show downward trends in the past 50 years while the grades above torrential rain take a significant upward trend with significant regional differences. EOF analysis shows that the pattern of total rainfall in the annually later flood season can be basically attributed to the heavy rainfall. On this basis, we focus on the abrupt change of rainfall in the early 1990s and compare differences in the atmospheric circulation before and after this shift. It was revealed that the enhancement of the Australian high and the Mascarene high, together with the right allocation of the western Pacific subtropical high, is the main reason for this abrupt change of rainfall. After the shift, moisture from the Indian Ocean and the South China Sea is enhanced and the Western Pacific subtropical high is significantly strengthened with its location shifted south-westward. As a result, vertical flows over the Guangxi area are enhanced, causing large amount of rain. Analysis of the correlation between the Australian high index with preceding sea surface temperatures （SSTs） revealed that the south-to-north spread of the warm phase of the Indian Ocean and out-of-phase variation of SSTs between the equatorial Pacific and the subtropical South Pacific wi11lead to the strengthening of the Australian high, which is of significance for the prediction of rainfall.