Abnormal warm sea-surface temperature in the Indian Ocean, active potential vorticity over the Tibetan Plateau, and severe flooding along the Yangtze River in summer 2020.

Excessive precipitation was observed throughout the Yangtze River Valley during the record-breaking Meiyu season in 2020. However, the mechanism of extreme precipitation over the upper reaches of the Yangtze River remains unclear. Our results show that the activities of high potential vorticity (PV) systems during their eastward propagation over the eastern Tibetan Plateau (TP) should be responsible for the above-normal rainfall not only over western China but also over eastern China. The activity of high-PV systems is characterized by a prominent diurnal cycle, and their formation is closely related to the thermal contrast between the near-surface and lower atmosphere. In the morning, surface sensible heating increases sharply after sunrise, leading to a decrease in diabatic heating with height. Accordingly, PV weakens in the lower atmosphere, and the formation of high-PV systems reaches a minimum. An increase in turbulence increases near-surface evaporation, which in turn reduces surface diabatic heating. At the same time, cloud formation increases diabatic heating at approximately 400 hPa. Consequently, the thermal contrast below 400 hPa leads to an increase in diabatic heating with height, favoring the generation of high-PV systems. Compared with the climatology, an excessive water vapor supply from the anomalous anticyclone over the northern Bay of Bengal, forced by the Indian Ocean warming in 2020, contributes to a stronger thermal contrast and enhanced activity of high-PV systems over the TP. The arrival timing of high-PV systems at the eastern flank of the TP plays an important role in the subsequent development of these systems. Early arrival in the afternoon or evening is generally accompanied by air convergence and sufficient water vapor supply downstream of the TP, which favors the systems moving off the TP and influencing precipitation downstream. [ABSTRACT FROM AUTHOR]