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The features of the large-scale circulation leading to catastrophic flood over the Huaihe river basin during the summer of 2003 and cause exploration

ZHANG Guo-cai, BI Bao-gui, BAO Yuan-yuan, LIAO Yao-ming (National Meteorological Centre, Beijing 100081, China)  
This paper analzyes the temporal and spatial distribution characteristics of the second greatest flood occurrence ever since 1954 in Huaihe river basin during the summer of 2003. Comparison is conducted with the previous data for the same period. The results show that 7 weather processes occurred with the total precipitation and water level and flux more than 1991 and less than 1954; that stable rain belt, intensive and explosive heavy rain are the main reason for the whole Huaihe river above the guaranteed water level. At the same time, by use of NCEP reanalysis data, large-scale atmospheric circulation and its formation reasons are discussed, which indicate that one of the important reasons for subtropical high noncontinually moving northward as a rule is that between the end of June and the beginning of July, the mid-high latitudes(60~180°E,30~50°S) of southern hemisphere experienced circulation change from low index(longitudinal pattern) to high index(latitudinal pattern) which lasted to the mid July with the cold masses and cross-equatorial flow at 110° E weaker than the normal. The other reasons are that between the third dekad of June and the second dekad of July, ITCZ was rather weak and further southward shifted than the normal, and shear between the west and the east wind is not obvious, and tropical cyclones formed at the western North Pacific Oceans and South China Sea; that high-level jet was southward shifted by 2 more degrees than the normal, whose coordination and coupling with low-level jet is the main reason for continuous torrential rain; that the secondary circulation caused by high and low jets played an important role in maintaining stable sub-tropical high and large-scale circulations in mid-high latitudes; and that the polar vortex in Asia is stronger than the normal.
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