Occurrence of droughts and floods during the normal summer monsoons in the mid- and lower reaches of the Yangtze River

[1] The daily precipitation data at 720 stations over China for the 1957-2000 period during summer (May-August) are used to investigate the droughts-floods coexistence (DFC) phenomenon during the normal summer monsoons. A droughts-floods coexistence index on seasonal timescale over the mid- and lower reaches of the Yangtze River (MLYRV) is defined to quantify this phenomenon and the associated ocean-atmospheric features in the strong DFC years are examined statistically. Results demonstrate that the occurrence of the strong summer DFC in the MLYRV is of an increasing trend for the period of 1957-2000. The strong summer DFC in the MLYRV is often accompanied by the anomalously subseasonal oscillation of the western Pacific subtropical high, the low-level westerly winds anomalies over the equatorial oceanic areas from the Indian Ocean to the western Pacific and the northward cross-equatorial winds anomalies near Sumatra and Somalia during summer, the strong Southern Hemisphere annual mode during the preceding November through January, high sea surface temperature in the oceanic areas from the Arabian Sea to the South China Sea, and El Nino or the developing phase of El Nino in the 6 preceding months. All these offer some predictive signals for the summer DFC in the MLYRV.