Droughts and floods in China In Reaches of Yangtze and Huaihe Rivers
Most of the flood/drought events in the eastern part of China are due to the anomalous Meiyu precipitation in June and July in reaches of Yangtze and Huaihe rivers. Meiyu is known as a phenomena designated by a rain belt that elongates in zonal and moves in a quasi-stationary way in meridional. This rain belt is always related to some fronts with sometime the strong convections around them. Meiyu can be influenced by ENSO. In different stages of El Niño events, rainfall in the eastern part of China shows its different statistics (Ye, 1996; Jin et al, 1999; Wang, 1992). Generally, the flood/drought events occur usually in the developing or decaying period of the El Niño/La Niña events. In the developing stage of the El Niño events a rain belt stretches steadily in reaches of the Yangtze and Huaihe rivers, and in regions of South Korea and Japan, causing more rainfall in these regions. Meanwhile, less than normal rainfall is received in the northern part of China and in areas south of Yangtze River. In the decaying stage of the El Niño events, much less rainfall, even the drought events are observed in reaches of Yangtze and Huaihe rivers and in regions of South Korea. In areas south of Yangtze River, especially in regions of Dongting and Panyang lakes, flood events are observed. However, during the La Niña events, the scenarios of the rainfall anomalies as mentioned above seem to be oppositely signed in those regions.
It seems that there are some lagging influences of ENSO on the summer rainfall in China. If the El Niño events develop in fall and winter seasons, the precipitation in the following summer tends to be more in reaches of Yangtze and Huaihe rivers. If the El Niño events develop in spring and summer seasons, the rainfall in the following summer tends to be less there (Li, et al, 2000). The ENSO occurred in 1997/1998 could be an example case. The 1997/1998 El Niño was the strongest ENSO event in the 20th century. It had strong impacts on the summer climate of China. Ob servational studies (Tao et al, 1998; Huang, et al, 1998; Yan, 1998 ) show that the occurrences of the extraordinary flood events in Yangtze River Valley in 1998 and the drought event in the northern part of China in 1997 are closely related to the 1997/1998 El Niño event.
These lagging influences could be related to the ENSO effects on Indian Ocean variabilities. Some studies (Wu, 1996; Sun, et al, 2003) suggested that the Meiyu rainfall should be more (less) when the anomalously higher (lower) temperatures were observed in both the central and eastern equatorial Pacific and the tropical Indian Ocean, along with the anomalously lower (higher) temperature in the western Pacific.
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