Cause analysis of Meiyu anomaly in Jianghuai region in 2020

Based on the observational data of national ordinary stations in Jiangsu and Anhui, the live sounding data, combined with the reanalysis data of ERA5, the abnormal characteristics of Meiyu in Jianghuai region in 2020 are analyzed and a study on the causes of the Meiyu anomaly is carried out. The results show:1) the Meiyu period in 2020 is longer than usual, the rainfall is significantly more, and the heavy rain process is frequent with the coverage wide. The Meiyu rain belt is basically centered on 30°-32°N, and experience 4 continuous north-south oscillations about biweekly cycle in the range of 28°N to 34°N. 2) The stable subtropical high at low latitudes, the stable double resistance situation at high latitudes in the early stage and the multi shallow trough fluctuations in the late stage are the key influencing systems for the super Meiyu. The stable maintenance of low-level jet is the key factor to cause the continuous heavy rainfall in Meiyu period, and the excess water vapor come from the South China Sea. 3) When the heavy rainfall located in the north of the Jianghuai region, the center of the cold vortex is generally located in the west of 120°E, and moves eastward or slightly northward; when the cold vortex is located in the east or south of 120°E, the precipitation is forced to the South. 4) The subtropical high experience 4 biweekly oscillations and 9 volatile jumps to the North during the Meiyu period, with 12 cold vortex activities. During the period of the northward rising of the subtropical high and the southward and eastward moving of the cold vortex, the anti-phase overlap and prolong, which provide a good system configuration for the formation of the super long Meiyu and the maintenance of the North-South oscillation of the rain belt in the Jianghuai region.