Analysis of precipitation characteristics and causes of autumn flood season in the Hanjiang River Basin in 2021 and 2023 in ENSO events

Serious autumn floods occurred in the autumn flood season(September-October) in the Hanjiang River Basin in 2021 and 2023 under different ENSO backgrounds,this paper compared and analyzed the characteristics and impacts of precipitation during the autumn flood season in 2021 and 2023,as well as the similarities and differences between atmospheric circulation and external SST forcing in these two years by using correlation analysis and regression analysis,based on the NCEP reanalysiss data,and the NOAA sea temperature data.The results show that:The autumn flood season precipitation in 2021 and 2023 in the Hanjiang River basin ranked third and fifth in intensity in the same period in history,respectively.The atmospheric circulations in 2021 and 2023 were similar,and the northeastern hemisphere circulation field presented a "low-high" distribution from west to east.The anomaly center is located from Lake Balkhash to the west of Lake Baikal and from the Bohai Sea to the northwest Pacific Ocean.The west Pacific subtropical high is stronger,and the westward extension is obvious.The sea temperature varied obviously in the equatorial eastern Pacific Ocean and in the tropical North Atlantic Ocean(TNA).In 2021,the developed La Niña SST distribution was reflected,with the warmer TNA led to the emergence of the autumn flood rainy circulation.In 2023,TNA reached the highest temperature in history,and the synergistic effect of the two had stronger influence than that of El Niño.Therefore,the Bohai Sea to the Japanese archipelago in China was controlled by an obvious anticyclone,and the Balkhash to Baikal Lake presented a low trough,showing an obvious autumn flood precipitation circulation configuration pattern.This research findings can provide references for the prediction of precipitation of autumn flood season in Hanjiang River Basin under the different backgrounds of external SST forcing.