Impact of Atmospheric Quasi-periodic Oscillations on Autumn Precipitation in West China During Flood and Drought Years Over the Last 40 Years

WEN Yu; LI Liping

doi:10.16032/j.issn.1004-4965.2024.074

Abstract：This study examines the influence of atmospheric quasi-periodic oscillations on autumn precipitation in West China during flood and drought years over the past 40 years.Using the CN05.1 gridded daily precipitation data and NCEP/NCAR daily reanalysis data from 1980 to 2021,we compare and analyze the characteristics of low-frequency precipitation in typical flood and drought years,as well as its relationship with low-frequency atmospheric circulation at different latitudes.Moreover,we provide early forecast signals based on our findings.The results indicate the following:(1)Autumn precipitation in West China during flood and drought years exhibits significant low-frequency oscillation periods of 10-20 days,20-30 days,and 30-60 days,with the 10-20 day period being the most prominent.(2)Convective development occurs in West China during both flood and drought years.However,in flood years,the subtropical westerly jet over the region is stronger,resulting in more active convection,stronger vertical upward motion,and vertical meridional circulation.(3)Regardless of flood or drought years,the low-frequency circulation pattern over Eurasia consists of two ridges and two troughs.The key low-frequency circulation systems that impact autumn rainfall in West China include the low-frequency Lake Balkhash-Baikal ridge(referred to as"Ba-Bei Lake"),the East Asian trough,the Bay of Bengal anticyclone,and the western Pacific subtropical high(South China and coastal anticyclone).However,these low-frequency circulation systems are stronger and more westward(southward)in flood years compared to drought years,resulting in a convergence of more abundant dry-cold and warm-humid airflows in West China.(4)The low-frequency water vapor circulation systems that influence precipitation during flood and drought years mainly include the mid-high latitude"Ba-Bei Lake"low-frequency anticyclone circulation,the low-frequency cyclone circulation near North China-Japan Islands,the low-frequency anticyclone circulation in the Bay of Bengal,and the low-frequency cyclone circulation in the South China Sea-Western Pacific.In flood years,the dry-cold(wet-cold)airflow from the north(northeast)of the mid-high latitudes is more northward and eastward,originating from Novaya Zemlya(Sea of Okhotsk and Sea of Japan).In drought years,these airflows originate from Central Siberia(the Northwest Pacific Ocean to the east of the Japanese Islands).Furthermore,in flood years,there are more warm and humid water vapor flows from the Bay of Bengal and the tropical North Pacific Central and South China Sea,while in drought years,warm and humid water vapor flows mainly come from the South China Sea,with a smaller proportion from the Bay of Bengal and the Western Pacific.(5)Regardless of flood or drought years,early monitoring of low-frequency forecast signals related to the"Ba-Bei Lake"ridge,the East Asian trough,and the West Pacific subtropical high is crucial.In flood years,these signals appear on the-10th day and primarily propagate along the northwest-southeast path.In drought years,the signals appear on the 8th day,with slightly more complex propagation paths for each signal.