The anomalous "Dragon_boat rainfall" variability in South China linked to atmospheric circulation and SST anomalies

The dragon boat race period around the Dragon Boat Festival every year is the period with the highest and most concentrated precipitation during the pre-flood season in South China, and heavy rainfall often leads to serious flood disasters. Analyzing climatic causes of abnormal intensity of the Dragon_boat rainfall in South China in the past 62 years is imperative for disaster prevention and mitigation. Using daily precipitation data collected at 192 national meteorological observation stations in South China from 1961 to 2022, a yearly index is constructed to characterize the intensity of the Dragon_boat rainfall in South China. Linear trend analysis, wavelet analysis, and other methods are used to analyze its changing characteristics. Using NCEP/NCAR reanalysis data and SST data, correlation and composite analysis methods are used to analyze the relationship between the intensity anomaly of the Dragon_boat rainfall in South China over the past 62 years and atmospheric circulation anomalies during the same period and preceding SST anomalies. The results show that the annual intensity index of Dragon_boat rainfall in South China has obvious interannual and interdecadal changes, while an obvious linear upward trend can be found in the past 62 years. The annual intensity index of Dragon_boat rainfall has shifted from weak before 1993 to strong afterwards. There was mainly an interannual variation cycle of 4—5 a before 1993, and there is also an interdecadal cycle of about 16 a in addition to the interannual cycles of about 2 and 6 a after 1993. Related analysis shows that during the Dragon_boat rainfall period (21 May—20 June), geopotential height in the mid-latitude region of the North Pacific decreases in the upper troposphere, the southern section of the East Asian trough in the middle troposphere is stronger, and geopotential height in northern part of South China decreases, and the southern branch trough is stronger. The presence of abnormal anticyclones to the east of the Philippines in the lower troposphere, the strengthening of the block high in the surface over the Okhotsk sea, and the weakening of the high pressure in the Northeast Pacific all are conducive to abnormally strong Dragon_boat rainfall in South China. In different interdecadal backgrounds before and after 1993, there are significant differences in the atmospheric circulation between abnormally strong and abnormally weak years during the Dragon_boat rainfall period in South China. After 1993, upper-level divergence over South China becomes stronger, and water vapor transport from the Western Pacific, the South China Sea and the bay of Bengal to southern China becomes more prominent. Meanwhile, cold air from the north to the south is stronger than before, leading to abnormally strong water vapor convergence and upward movement in southern China and large Dragon_boat rainfall intensity after 1993. Related analysis shows that the tropical Atlantic SST remains low in summer, autumn and winter of the previous year, and the maritime continent SST is also low from summer to autumn of the previous year. SST is high in the Northeast Pacific but low near the tropical North Pacific Date Line in the spring of the current year, which is conducive to strong intensity of the Dragon_boat rainfall in South China. The relationship between the strong and weak years of the Dragon_boat rainfall in South China and SST in the equatorial central and eastern Pacific is asymmetric. The strong years are mainly years of El Niño persistence or attenuation before 1993, while they were mainly years of La Niña persistence or attenuation after 1993.