Dynamical drivers of the 2020 Mei Yu floods over China

Large parts of East and South Asia were affected by heavy precipitation and flooding during early summer 2020. This study provides both a statistical and dynamical characterisation of these events. By aggregating daily and monthly precipitation over river basins across Asia, it is shown that the Yangtze River Basin (YRB) is one of the areas that was particularly affected. June and July 2020 rainfalls were higher than in the previous 20 years, and the YRB experienced anomalously high rainfall across most of its sub-basins. An automated method detecting the daily position of the East Asian Summer Monsoon Front (EASMF) is applied to show that the anomalously high YRB precipitation was associated with an anomalously slow northward progression of the EASMF and prolonged Mei Yu conditions over the YRB lasting more than one month. Lagrangian trajectory analysis is employed to study the convergence of air masses in the EASMF during two 5-day heavy-precipitation episodes, 12-16 June and 4-8 July 2020. Despite heavy precipitation and the convergence of monsoonal and subtropical air masses seen in both episodes, clear differences are identified between these episodes in the location/strength of the Subtropical Westerly Jet and the location of the Western North Pacific Subtropical High. This study contextualises heavy precipitation in Asia in summer 2020 and showcases a number of analysis tools developed by the authors for the study of such events.