Accelerating onset of heatwaves after the Meiyu termination in the middle-lower Yangtze River basin

This study employs station observation and Meiyu monitoring data in the middle-lower Yangtze River (MLYR) basin to investigate a distinctive heatwave phenomenon, characterized by an accelerated onset following the Meiyu termination in 2001–2022. In the basin, the proportion of rapid heatwaves (transition duration less than two days) is 30 % in the period 1981–2000, while it increased markedly to 77 % in the period 2001–2022. Furthermore, the accelerating onset of heatwaves in the basin is not limited to the surface. The results of the statistical analysis demonstrate the significant influence of circulations in the middle-low troposphere. In comparison to the circulations observed during the period between 1981 and 2000, the geopotential height (GPH) over the basin displays a considerable increase between 300 and 700 hPa following the termination of Meiyu in the later period. Additionally, there is a notable acceleration in the warming of the lower troposphere. The abrupt evolution of the circulation patterns resulted in the formation of a rapid heat-dome effect over the entire basin. This effect persists even when the warming trend of air temperatures and the enhancing trend of GPHs are removed. The heat-dome effect is closely associated with the rapid shift of the western Pacific subtropical high (WPSH), particularly its western boundary. The rapid westward extension of the WPSH after the Meiyu event and its subsequent control of the MYLR basin form an “Ω”-shape circulation structure, which block the cold air invasion from high latitudes and enhance the geopotential heights by downward shortwave radiation, to induce the heat dome over the basin. This, in turn, will accelerate the onset and persistence of heatwaves.