Abstract:Based on NCEP/NCAR reanalysis data and conventional meteorological and hydrological observation data, the meteorological and hydrological characteristics as well as circulation anomalies and possible causes of the autumn floods in Hanjiang River in 2021 are analyzed. The results show that the precipi tation in Hanjiang River in 2021 was more in the west and less in the east. The accumulated rainfall in the upper reaches of Hanjiang River was 400-985 mm, and the accumulated areal rainfall ranked the first since 1961. During the autumn flood season, the Danjiangkou Reservoir experienced the flood peak processes of more than 10000 m3·s-1 for 7 times. The accumulated alarm time of the major hydrological control stations in the middle and lower reaches of the Hanjiang River got to 151-580 h, with the maximum amplitude beyond warning line being 0.29-1.56 m, showing the characteristics of the longer alarm time, the higher amplitude over warnig line in the lower reaches. The comparison with the hydrological conditions in 2011 and 2017 shows that in 2021 the flood peak in the upper reaches of Hanjiang River came the earliest, the number of flood peaks was the largest, the duration of high water level in the upper reaches was the longest, the accumulated alarm time in the middle and lower reaches of Hanjiang River was also the longest, and the abnormal degree of water regime was significantly much stronger. In terms of atmospheric circulation, the low trough of Lake Balkhash and Lake Baikal region, the high ridge from the east of mainland China to the Northwest Pacific, the low trough in the areas to the east of Japan Island, the subtropical high, the South Asian high and the East Asia subtropical westerly jet were all abnormally stronger than those in the same period in history. The formations of abnormal low level flow field and water vapor convergence center, combined with the divergence and suction of high level flow field, were the large scale circulation background favorable for the continuous and enhancement of precipitation. When the inflow of Danjiangkou Reservoir was close to 2000 m3·s-1, the 5 d accumulated areal rainfall exceeding 60 mm in the upper reaches of Hanjiang River was closely related to the appearance of flood peaks above 10000 m3·s-1. After the upstream inflow increased, the narrowing river coarse of the middle and lower reaches of the Hanjiang River was the main reason for the over alarm line floods in this area where the precipitation was not so heavy.