Magnitude, Scale, and Dynamics of the 2020 Mei-yu Rains and Floods over China

Abstract

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 characterization of rains and floods affecting the Yangtze River Basin (YRB). By aggregating daily and monthly precipitation over river basins across Asia, it is shown that the YRB is one of the areas that was particularly affected. June and July 2020 rainfall was higher than in the previous 20 years, and the YRB experienced anomalously high rainfall across most of its sub-basins. YRB discharge also attained levels not seen since 1998/1999. 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 a halted northward progression of the EASMF and prolonged mei-yu conditions over the YRB lasting more than one month. Two 5-day heavy-precipitation episodes (12–16 June and 4–8 July 2020) are selected from this period for dynamical characterization, including Lagrangian trajectory analysis. Particular attention is devoted to the dynamics of the airstreams converging at the EASMF. Both episodes display heavy precipitation and convergence of monsoonal and subtropical air masses. However, clear differences are identified in the upper-level flow pattern, substantially affecting the balance of airmass advection towards the EASMF. This study contextualizes heavy precipitation in Asia in summer 2020 and showcases several analysis tools developed by the authors for the study of such events.

Present-day and future projection of East Asian summer monsoon in Coupled Model Intercomparison Project 6 simulations

“…Rainband expansion is associated with increased moisture and the northward advance of the Pacific anticyclone. Subsequently, a significant northward movement is observed around mid-June, where the rainband abruptly progressed northward to northern China and the Korean Peninsula [ 1 , 52 , 53 ]. The EASM begins to retreat after mid-August as the rainband moves southward with a relatively weaker precipitation.…”

Intensification of mesoscale convective systems in the East Asian rainband over the past two decades

“…For example, in 2020, large regions of East Asia experienced an exceptionally wet rainy season (Figure S1b in Supporting Information S1) and intense MCS precipitation (Figure 1b), which led to severe impacts and consequential economic losses (Clark et al, 2021). In the Yangtze River Valley, 2020 was the wettest summer since 1998 (Ding et al, 2021;Volonté et al, 2021), but no significant El Niño event was observed, which is widely considered as a dominant factor at the inter-annual timescale (Liu et al, 2020;Volonté et al, 2021). Hence, it is important to see whether the MCSs over East Asia have changed in the past and whether the anomaly in 2020 may reflect part of the long-term change.…”

Trends in Warm Season Mesoscale Convective Systems Over Asia in 2001–2020

“…Remarkably, the linear trends for both MαCSs and MβCSs in these three regions are all statistically significant at the 99% confidence level. The year 2020 features the highest MCS frequency in EA and NEA, likely because of the abnormally abundant moisture transported to the Yangtze river basin (T. Chen et al., 2020; Clark et al., 2021; Ding et al., 2021; Volonté et al., 2021; L. Zhang et al., 2021) and Northeast China (Y. Liu et al., 2021) in the summer that year.…”