Characteristics and causes analysis of abnormal Meiyu rainfall in the middle and lower reaches of Yangtze River Valley in 2020

Based on rainfall data during Meiyu season in the middle and lower reaches of the Yangtze River, NCEP/NCAR reanalysis dataset and NOAA sea surface temperature dataset from 1961 to 2020, the circulation anomaly characteristics and precursor influence signals of the abnormally excessive rainfall during the Meiyu season in the middle and lower reaches of the Yangtze River in 2020 are analyzed. Results show that in 2020 the Meiyu season in the middle and lower reaches of the Yangtze River started earlier and ended later, with the rainy season lasted for 52 days. The rainfall during the rainy season is 753.9 mm, which is 1.5 times more than normal, and the largest in history since 1961. From June to July, the Eurasian mid-high latitudes showed a "two troughs and two ridges" pattern. Cold air activities in mid-latitudes between 30-50°N are frequent, the East Asian summer monsoon is abnormally weak, and the West Pacific subtropical high is abnormally strong and westward, which leads the water vapor from the South China Sea and the diverted water vapor from the Northwest Pacific to transport along the periphery of the subtropical high to the Yangtze River basin, resulting in abnormally high precipitation in the middle and lower reaches of the Yangtze River. The warmer SST in key areas of the Indian Ocean and the central Pacific El Niño in the previous winter play an important role in the strong and western Pacific subtropical high and the abnormally weak East Asian summer monsoon. They are the important influencing factors for the abnormally high rainfall during the Meiyu season in the middle and lower reaches of the Yangtze River.