Characteristics of the Recent 40-year Flood/Drought over the Middle Reaches of the Yangtze

Using the Z-index method and the 50-station monthly mean rainfall data over the middle reaches of the Yangtze from Jan 1960 to Dec 2001, the monthly flood/drought grades of each station and the whole region are calculated. Through analyzing the monthly regional flood/drought grades combined with the monthly standard deviation of precipitation, results show that both flood and drought events occur frequently over the middle reaches of the Yangtze during the recent 40 years with average about 1.3 drought months and 1.6 flood months per year, but the frequency and location of flood and drought are quite different from season to season. They mainly happen in midsummer and autumn with a large scale and the Dongting Lake is always among those flood or drought area likely owing to its special landform.Using the EOF (empirical orthogonal function) method, the monthly flood/drought grades of 50 stations from Jan 1960 to Dec 2001 are studied. Based on the first four S-EOFs, the floods/droughts over the middle reaches of the Yangtze are mainly divided into the following four patterns: large scale flood in the whole region, flood in the northwest while drought in the southeast, flood in the southwest while drought in the northeast and drought in the center but flood all around, among which the first pattern is the dominating one whose deviation is much greater than those of the other three. That is to say flood over the middle reaches of the Yangtze is the major one in climate events.The MSSA (multi-channel singular spectrum analysis) method is used to analyze the first 10 S-PCs (spatialprincipal components) generated by the above EOF. It shows that the inter-decadal and inter-annual variations of flood/drought over the middle reaches of the Yangtze are very clear. There is a sharp turning point in the early of 1980s, there are more droughts before and more floods after that period. So, since 1980s, floods occur frequently and become one of the biggest weather disasters over the middle reaches of the Yangtze. At the same time, the flood and drought in the middle reaches of the Yangtze have periodic characteristics of quasi-eightyear, quasi-five-year, quasi-biennial and quasi-quadrennial oscillations. Focusing on the details of the quasi-biennial oscillation of the flood/drought, it is found that it is different from other researches. The flood/drought's quasi-biennial oscillation in the target region is quite regular though its magnitude decreases gradually before the 1980s, but it is not clear after the 1980s.