Droughtsfloods coexistence DFC during the normal summer monsoons in the mid and lower reaches of the Yangtze River

Previous studies suggest that the summer rainfall in the mid-lower reaches of the Yangtze River valley (MLYRV) has close relationship with the advance and retreat of the East Asian Summer Monsoon (Tao and Chen, 1987; Ding, 1992; Wang, 1994; Chang and Chen, 1995; Lau and Yang, 1997). It is also found out that the severe summer droughts or floods in the MLYRV are associated with the singularities of the large-scale atmospheric circulation (Wang and Xu, 1997; He et al., 2001; Nan and Li, 2003). The majority of the previous literatures on severe droughts and floods put their emphasis on the seasonal mean rainfall (Simmonds et al., 1999; Webster et al., 1998; Barlow et al., 2002; Matsumoto, 1997), while few consider the sub-seasonal variation of rainfalls that are also of great importance. For instance, if precipitation of some summer is predicted to be normal, it might be misunderstood that there would be neither droughts nor floods in the summer. Nevertheless, if both droughts and floods happen within the summer, the seasonal precipitation may also be normal. Such a misunderstanding is due to the ignorance of the sub-seasonal variation of precipitation. As a result, the decision maker and the public may not be warned and prepared for severe floods and droughts in advance, which would cause disastrous loss of lives and property.

The summer droughts-floods coexistence (DFC) in the MLYRV is just such an occasion as extremely large rainfalls varying on the sub-seasonal scale, yet with a normal seasonal mean. The DFC refers to the occurrence of both droughts and floods during the normal summer monsoons. In order to describe the DFC phenomenon quantitatively, a DFC index (DFCI) is defined as follows:

where SSTD and RSTD are, respectively, the normalized average summer no-

rain days and precipitation of the whole MLYRV. SSTD reflects the concentration intensity of the summer rainfall. The larger the SSTD is, the more concentrated the summer rainfall is, and vice versa. 10(05 -RstD) is the weight coefficient which magnifies the weight of the normal-precipitation summers and reduces that of the severe floods or droughts summers. It should be emphasized that those indices over 0.5 and under -0.5 standard deviations are defined as the high DFCI (or strong DFC) and low DFCI (or weak DFC) summers, respectively. The absolute magnitude of the summer precipitation anomalies within 0.5 standard deviations is regarded as normal, while the anomalies over 0.5 and under -0.5 standard deviations are defined as floods and droughts, respectively.

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