用湿位涡定义的南海西南季风指数及其与我国区域降水的关系研究

A Study on the South China Sea Monsoon Index and the Relationship Between the Index and Regional Rainfalls of China

使用美国NCEP/NCAR 195 8 1997年逐日资料 ,对南海季风爆发前后的气象要素场作了分析 .结果表明 :南海季风爆发时 ,南海南、北部要素场变化有差异 ,北部西南季风爆发特征更为显著 .针对南海西南季风爆发特征 ,提出了用湿位势涡度定义季风指数 ,它能很好地反映夏季风爆发的特征 .其中 ,用湿位势涡度定义的季风指数对长江中下游及华北华南地区的旱涝具有一定的预示性 .相关分析表明 :前一年冬季的季风指数和秋季的季风指数分别与华北次年夏季降水、华南次年春季降水有显著的相关 。

The Asian summer monsoon consists of two systems, the South Asian monsoon and the East Asian monsoon. The South China Sea (SCS) monsoon is one of the important subsystems of the latter. Tao and Chen suggest that the Asian summer monsoon usually starts first in the northern SCS in the middle May while the Indian summer monsoon starts in the middle June, one month later than that of the SCS summer monsoon. In this paper we will give an index of the SCS Summer monsoon and discuss the onset time of the SCS summer monsoon. Based on NCEP/NCAR reanalysis data(1957-1997),the features of the meteorological element field are studied. The results show that the variations of the meteorological element field at the onset of the SCS monsoon are different between the northern SCS and the southern SCS. The features of the onset of the northern SCS monsoon are more remarkable. According to these features , several new SCS monsoon indexes are developed by using vorticity and potential vorticity which reveal the changes of the summer monsoon very well, whereas the indx of potential vorticity indicates that the earliest onset date is the 5 th pentad of April, the latest onset date is the 2 nd pentad of June and the mean onset date is the 4 th pentad of May. The onset date of the SCS summer monsoon determined by using potential vorticity is basically consistent with that by using TBB,OLR etc. The indx of potential vorticity relats closely to OLR with the correlation coefficient of 0.47 and to vorticity. Then the indx can reveal the variations of the stream field and the humidity field very well. The monsoon index of potential vorticity better indicates the drought/flood in the middle-low regions of the Yangtze River, south China and north China. The major findings of the correlation analysis are as follows: the previous winter and autumn monsoon index bears significant corelationship with the next summer rainfall of the North China and the next spring rainfall of south China, whereas the summer monsoon index of the year reveals significant positive corelationship with the autumn rainfall in the middle-low regions of the Yangtze River.