Available online: October 12, 2020 , DOI:
The Meiyu in 2020 is a typical super Meiyu, with the characteristics of the earlier onset, later end, longer rainy season, wider rainfall range, abundant accumulated rainfall, and more heavy storm processes than normal. Through the analysis of simultaneous atmospheric circulation systems, it is found that the average positions of the key East Asian monsoon circulation systems are relatively stable, with significant quasi biweekly oscillation (QBWO) in the Meiyu period of 2020. The onset and end of Meiyu, the northward shift and stagnation of the rainfall belt, and the occurrence and persistence of heavy rainstorm processes have a good relationship with the QBWO. During the Meiyu period, the western Pacific subtropical high experienced six periodic oscillations of northward uplift and southward retreat. Meanwhile, the upper- and low- level monsoon circulation systems strengthened for five times corresponding with the QBWO in the Meiyu season, especially the strengthening of the southwesterly low-level jet, the repeated establishment of the large value centers of southerly with relatively consistent latitude, which made the water vapor transport from the tropics strengthened again and again, and the water vapor convergence and upward movement developed repeatedly, and led to the long-term persistence of Meiyu in the YHRV, with frequent occurrences of the rainstorm. On the other hand, two blocking highs are maintaining in the middle and high latitudes, which is characterized by the spatial pattern of "two ridges and one trough" over Eurasia, and the low trough along the East Asian coast is also active. The cold air falls continuously southward through the northwestern and/or northeastern paths, frequently merging with the repeatedly strengthened warm moisture from the low latitude over the YHRV, which is another important factor causing the super Meiyu in 2020.
Moreover, we compared the temporal and spatial distribution of Meiyu, and the corresponding flood disaster in 2020 with other super Meiyu years since 1951. It is shown that the super Meiyu in 2020 is weaker than that of 1954, but stronger than that of 1998 and 1991. Benefits from the improvement of both the accuracy of climate prediction and the ability of disaster prevention and mitigation, the flood and disaster losses caused by this super Meiyu in the YHRV are significantly decreased than before.