Variation Characteristics and Causes of the Flood and Drought in the Three Gorges Area in Summer

The Three Gorges area is located in the East Asian monsoon region. Under the influence of the monsoon activity, the precipitation of the Three Gorges area mainly concentrates on the period from June to August with relatively high variability, which is prone to the occurrence of the flood and drought disaster. The flood and drought of the Three Gorges area not only has a great impact on the lives of local residents and social activities, but also directly determines the operation and power generation efficiency of the Three Gorges Hydropower complex Project. As a result, the study of the variation characteristics and the formation of the flood and drought of the Three Gorges area is of great significance, providing a scientific decision base to prevent the disaster of flood and drought.
The temporal evolution of the drought and flood of the Three Gorges area in summer and its circulation patterns in the anomalous years are analyzed, obtaining a consistent result. The droughts occur frequently in the Three Gorges area in summer from 1951 to 1978. Following, there is a decadal abrupt change from the droughts to the floods in 1979. Then the floods are popular in the Three Gorges area from 1979 to 2000. Severe droughts take place frequently in the Three Gorges area after 2001. The circulation patterns are obviously different between in severe flood years and in severe drought years of the Three Gorges area. As for the severe flood years of the Three Gorges area, the South Asia High strengthens in 100 hPa, and there is a "－+－" wave train from the west to the east of the Eurasia high latitude area in 500 hPa height anomaly, which reflects that there are continuing block highs in the Okhotsk and Ural regions. Furthermore, the weaker convections over the Western Pacific Warm Pool make the West Pacific Subtropical High to shift southward. In addition, there is strengthening southwest water vapor transport from the Three Gorges area to the Indo China peninsula, which enhances the water vapor convergence in the Three Gorges area. These circulation patterns are advantageous to the genesis of the floods in the Three Gorges area.