Extreme Mei-yu in 2020: Characteristics, causes, predictability and perspectives

The 2020 extreme rainfall was highly unusual with episodes of intensive rains and winds from the middle and lower reaches of the Yangtze River Valley to southern Japan. Given the severe implications and huge forecast spread among different models, the extreme Mei-yu has aroused widespread concern. This study is aimed at synthesizing the latest research on the characteristics and potential climate forcing factors of such extreme Meiyu and discusses the challenges and outlooks for prediction and numerical modeling. The distinct characteristics of the Yangtze River Valley summer rainfalls in 2020 included record-breaking accumulated precipitation, longest duration, earliest onset, and highest intensity. We summarize the majority of the studies investigating the diverse coupled ocean-atmospheric processes at different timescales. The research consensus is that the anomalous anticyclone spanning the western North Pacific and the mid-high latitude trough-ridge patterns are the two critical circulation features carrying tropical and mid-high latitude signals, jointly affecting the extreme Mei-yu. Potential mechanisms based on the two essential atmospheric circulations during the Mei-yu period are then highlighted. In addition, different climate model simulations are also introduced to reach an inter-model agreement despite certain model biases on the response of atmospheric circulations to these potential forcings among the state-of-the-art atmospheric and coupled general circulation models. This study provides a synthesis to promote the understanding, prediction, and disaster prevention of extreme Mei-yu.