Impact of Antecedent Soil Moisture Anomalies over the Indo-China Peninsula on the Super Meiyu Event in 2020

In the summer of 2020, a super Meiyu event occurred in the Yangtze River basin (YRB), causing enormous economic losses and human casualties. Recent studies have investigated the possible causes of this super Meiyu event from the perspective of anomalous atmospheric circulation activities and sea surface temperature (SST) anomalies; however, the influence of land surface processes has not garnered considerable attention. This study investigates the possible contributions of land surface processes to this extreme event based on observational analysis and numerical simulations, and shows that antecedent soil moisture (SM) anomalies over the Indo-China Peninsula (ICP) may have had a vital influence on the super Meiyu in 2020. Negative SM anomalies in May over the ICP increased the surface temperature and sensible heat flux. The “memory” of soil allowed the anomalies to persist into the Meiyu period. The heating of the lower atmosphere by the surface strengthened the western Pacific subtropical high, which caused an anomalous anticyclone from the ICP to Northwest Pacific and thus enhanced the southwesterly winds and vertical motion over the YRB. Consequently, the water vapor flux and convergence were strengthened. Sensitivity experiments based on the Weather Research and Forecasting (WRF) model further confirmed the results of observational analysis and indicated that the warm air heated by the ICP surface significantly warmed the lower troposphere from the ICP to Northwest Pacific under the influence of the background wind, thus increasing the geopotential height and inducing an anticyclone. The results of the sensitivity experiments showed that the SM anomalies in May over the ICP increased the precipitation by 10.6% from June to July over the YRB. These findings can improve our understanding of the mechanism of the super Meiyu event in 2020 and facilitate the prediction of extreme Meiyu events.