The Record-breaking Meiyu in 2020 and Associated Atmospheric Circulation and Tropical SST Anomalies

The record-breaking Meiyu in the Yangtze-Huaihe River valley (YHRV) in 2020 is characterized by early onset, delayed retreat, long duration, wide meridional rainbelt, abundant precipitation, and frequent heavy rainstorm processes. It is noted that the East Asian monsoon circulation system presents a significant quasi-biweekly oscillation (QBWO) during the Meiyu season of 2020. That is associated with the onset and retreat of Meiyu, the northward shift and stagnation of the rainbelt, and the occurrence and persistence of heavy rainstorm processes. Correspondingly during the Meiyu season, the monsoon circulation subsystems, including the western Pacific subtropical high (WPSH), the East Asian westerly jet in the upper level, and the southwesterly jet in the lower level, experience periodic oscillations linked with the QBWO. Especially, the repeated establishment of the large southerly center with relatively stable latitude leads to moisture convergence and ascending developed repeatedly, and the long-term duration of the Meiyu rainfall in YHRV, with frequent occurrences of the rainstorm processes. Moreover, two blocking highs present in the mid-high latitudes over Eurasia, and the low trough along the East Asian coast is also active, which is conducive to the cold air activities moving southward through the northwestern and/or northeastern paths, and frequently merging with the warm and wet moisture from the low latitudes and converging over YHRV. The persistent warming in the tropical Indian Ocean is an important external contributor to an EAP/PJ-like teleconnection pattern over East Asia and the intensified and southward displaced WPSH, which is favorable to excessive rainfall over YHRV.