Abstract:

Based on the daily precipitation data from 75 rainfall gauging stations covering 1960-2015 in the middle and lower reaches of the Yangtze River Basin, we analyzed the temporal and spatial distribution characteristics of dry-wet abrupt alternation (DWAA) events during the summer (from May to August) and its relationship with ENSO by defining the daily scale dry-wet abrupt alternation index (DWAAI) based on the modification of original index. The results showed that: (1) Modified DWAAI, which was defined by taking into account the differences of dry-wet degree between the earlier and later periods as well as how slowly or quickly the process changes from dry to wet in the abrupt alteration period, could be used to identify DWAA events accurately and effectively. (2) On the whole, areas where DWAA events occurred had expanded gradually since 1960. Meanwhile, the frequencies and intensities of such events had gradually increased over time. DWAA events mainly occurred in May-June, and the Hanjiang River watershed sub-basin, the middle reaches of the Yangtze River, the northern Dongting Lake watershed sub-basin and the northwestern Poyang Lake watershed sub-basin were high-incidence areas of such events. (3) There were some relationships between DWAA events and phenomena of continuously low SST in Nino3.4 region before such events occurred. Specifically, La Ni?a early-warning reacted to the occurrence of DWAA events. About 41.04% of such events occurred during decline stages of La Ni?a or within the first 8 months after La Ni?a ended. In terms of intensity, there were significant negative correlations between DWAAI at all the stations and SST anomalies in Nino 3.4 region within 6 months before DWAA events occurred, especially in the Poyang Lake watershed sub-basin and the middle reaches of the Yangtze River. The conclusions indicated that these methods and results were meaningful for the fighting against drought and flood in the Yangtze River Basin.

Key words: dry-wet abrupt alternation, the middle and lower reaches of the Yangtze River Basin, spatial-temporal evolution characteristics, La Ni?a