THE DROUGHT/FLOOD IN THE LOW REACHES OF YANG-TZE RIVER AND THE INTERANNUAL VARIATIONS OF THE 30-60 DAY OSCILLATION OF EURASIAN CIR-CULATION DURING SUMMER HALF YEAR

The relat ionships between the interannual variations on the intensity of LFO(30-60 days) of Eurasian circulation during summer half years and the rainfall in summer and the number of heavy rains in the low reaches of Yangtze River have been investigated by using wavelet analysis and rotated principal component analysis based on the data of the pentad mean 500 hPa height field over the Eastern Hemisphere for the 25 years period 1971-1995. The results indicated that there exists a dominant negative correlation between the intensity of LFO(30-60 d) for Eurasian pattern (EU) from June to October and the number of heavy rains in the low reacher of Yangtze River. Meantime, the contrast function analysis (CFA) is proposed with identifying nonlinear relationships between the two variables. In addition, it is found that the number of heavy rains is enhanced and the floods are caused easily when the intensity of the Western Pacific pattern (WP) enhances during the period of May-August by means of the CFA, but the effect is less significant when WP pattern reduces. The rainfall in summer is reduced signifiantly, only when the intensity of the Southen Asia pattern (SA) is enhanced but the number of heavy rains increased, only when the lintensity of SA is reduced during the period of March-May. Hence, the linear or nonlinear correlation may exist simultaneously between the drought/flood and low frequency flow patterns.