Anomalous Features of Extreme Meiyu in 2020 over the Yangtze–Huai River Basin and Attribution to Large-Scale Circulations

Extremely anomalous features of Meiyu in 2020 over the Yangtze–Huai River basin (YHRB) and associated causes in perspective of the large-scale circulation are investigated in this study, based on the Meiyu operational monitoring information and daily data of precipitation, global atmospheric reanalysis, and sea surface temperature (SST). The main results are as follows. (1) The 2020 YHRB Meiyu exhibits extremely anomalous characteristics, which are the most prominent since the 1980s. The 2020 Meiyu season features the fourth earliest onset, the third latest retreat, the longest duration, the maximum Meiyu rainfall, the strongest mean rainfall intensity, and the maximum number of stations/days with rainstorm. (2) The extremely long duration of the 2020 Meiyu season lies in the farily early onset and late retreat of Meiyu in this particular year. The early onset of Meiyu is due to the earlier-than-normal first northward shift and migration of the key influential systems including the northwestern Pacific subtropical high (NWPSH) and the South Asian high (SAH) along with the East Asian summer monsoon, induced by weak cold air activities from late May to early mid-June. However, the extremely late retreat of Meiyu is because of later-than-normal second northward shift of the associated large-scale circulation systems accompanied with strong cold air activities, and extremely weak and southward located ITCZ over Northwest Pacific in July. (3) The extremely more than normal Meiyu rainfall is represented by its long duration and strong rainfall intensity. The latter is likely attributed to extreme anomalies of water vapor convergence and vertical ascending motion over the YHRB, resulting from the compound effects of the westward extended and enlarged NWPSH, the eastward extended and expanded SAH, and the strong water vapor transport associated with the low-level southerly wind. The extremely warm SST in the tropical Indian Ocean seems to be the key factor to induce the above-mentioned anomalous large-scale circulations. The results from this study serve to improve understanding of formation mechanisms of the extreme Meiyu in China and may help forecasters to extract useful large-scale circulation features from numerical model products to improve medium-extended-range operational forecasts.