Analysis on the characteristics of Meiyu anomaly and prediction precursor signal in Anhui Province in 2020

Based on precipitation data at Anhui meteorological stations and NCEP reanalysis data, in this paper we analyze the characteristics and the causes of Meiyu climate anomaly in Anhui Province in 2020, and evaluate the skill of climate prediction and the effectiveness of prediction precursor signals. The main conclusions are as following. (1) In Anhui Province, Meiyu onset date was June 2 and outset date was August 1. Meiyu duration was 60 days. Meiyu precipitation was 1 057 mm in South along the Yangtze River and 810 mm between Yangtze River and Huaihe Rive, and the rainfall intensity in many places reached the historical extreme value in 2020. On the whole, the length of Meiyu period, the wide coverage, the large amount of accumulated rainfall and the intensity of Meiyu reached the top level since 1961. (2) The main reason for the abnormal Meiyu is that the blocking high is active near the Ural Mountain and the East Siberian-Okhotsk Sea from June to July. The EAP wave train on the 500 hPa geopotential height anomaly field along the East Asian coast is obvious. The summer monsoon is weak. The Western Pacific Subtropical High is abnormally strong and located too far to west and south, which is conducive to the convergence of cold and warm air in Anhui Province. The water vapor transport from the west side of the subtropical high to the province is extremely strong. The reason for the longer Meiyu period is that the ridge line of subtropical high is northward in June and southward in July, which leads to the early onset and late demise of Meiyu. (3) In the previous winter and spring, there was a weak El Niño event in the equatorial central Pacific Ocean. Although the indication of ist influence on Meiyu in Anhui Province is not clear, the response signal is that the Indian Ocean SST and subtropical high are abnormally warm and strong. According to historical statistics, if the equatorial Middle East Pacific Ocean is basically in El Niño state in previous winter, the probability of more intensive Meiyu in these years is very high. Therefore, the Indian Ocean SST and Western Pacific Subtropical High response factors can be used as the starting points to improve the prediction ability of Meiyu in weak El Niño years.