The strongest high-temperature event since 1960 occurred in the middle and lower reaches of the Yangtze river in the summer of 2022, and its formation and maintenance are related to the anomalously strong Western Pacific Subtropical High (WPSH). Using station observational data, reanalysis data, and diagnostic analysis of different variables, the present study investigates the mechanism for the anomalously strong and stable maintenance of the WPSH during the high temperature period. Results are as follows: (1) During the high temperature period, stronger sinking movements are prevalent in the middle and lower reaches of the Yangtze river compared to that in typical high temperature years, and the perturbation vertical dynamic heating term is the main factor for maintaining high temperatures in this region. Anomalous downward shortwave radiation flux can lead to heat transfer from the ground to the lower atmosphere through upward sensible heat flux, promoting surface heating. (2) Unlike the situation in the typical high temperature years, the overlap of WPSH and South Asian High (SAH) in the summer of 2022 facilitated the maintenance of WPSH over the middle and lower reaches of the Yangtze river, providing a favorable circulation condition for the extreme high temperature event. The maintenance of WPSH intensity was related to local negative vorticity, i.e., the downward transport of negative vorticity disturbance and the negative anomaly of horizontal advection of relative vorticity. Especially, the vertical transport of negative vorticity from top to bottom played a crucial role on maintaining the WPSH over the middle and lower reaches of the Yangtze river. (3) Unlike that in typical high temperature years, the maintenance of the WPSH oceanic core was mainly related to anomalously low sea surface temperature near 180° around the equatorial central Pacific. The maintenance of the continental part of the WPSH was closely related to convection near the Philippine Islands, which was favorable for maintaining negative vorticity over the middle and lower reaches of the Yangtze river. The research results will deepen our understanding of the causes of WPSH variability and provide a theoretical basis for subsequent studies on the causes of persistent high temperature.

Based on the ERA5 reanalysis data, the typical flood years of 1999, 2016 and 2020 in the lower reaches of the Yangtze river are analyzed, and the quasi-biweekly oscillation component is extracted by the method of power spectrum analysis and Lanczos filtering. Common characteristics of different typical flood years are summarized, and their key influencing factors are respectively analyzed using in-phase composite study. The results are as follows: (1) 1999, 2016 and 2020 are the most typical flood years in the lower reaches of the Yangtze river, with a significant quasi-biweekly oscillation period of 10—20 d, especially from June to July. (2) In the middle and high latitudes, the low-frequency circulation is dispersed along the wave train disturbance, which is conducive to the maintenance of the large-scale background of "two ridges and one trough" and the enhancement of the disturbance in the lower reaches, causing precipitation anomalies by changing the low-frequency circulation and vertical movement over the lower reaches of the Yangtze river. The quasi-biweekly oscillation over the low-latitude region is another source of low-frequency in the lower reaches of the Yangtze river. The low-frequency circulation in 850 hPa is symmetrically distributed around 10°N, and the low-frequency divergent wind points from the South China Sea to the lower reaches of the Yangtze river. The low-frequency tropical convective activities can affect the precipitation anomaly in the lower reaches of the Yangtze river through vertical circulation. The interaction between circulations at the high and low latitudes, combined with the thermal forcing and the strong convergence of low-frequency water vapor movement, provides a favorable condition for anomalous precipitation in the lower reaches of the Yangtze river. (3) The distribution characteristics and key influencing factors of precipitation in the three typical flood years in the lower reaches of the Yangtze river are obviously different. In 1999, the low-frequency anticyclone circulation over the western North Pacific was located to the west of its normal position, the divergent wind pointed from the South China Sea to the lower reaches of the Yangtze river, and the low-frequency water vapor came from the Bay of Bengal and the South China Sea. In 2016, the coupling zone of the south-north low-frequency circulation system in the lower troposphere presented a northeast-southwest orientation. The low-latitude convective activities were vigorous, and the heating effect of the atmospheric heat source propagating northward was stronger. In 2020, the vertical circulation in the middle and high latitudes was more complete, and the southeasterly water vapor transport in the Northwest Pacific was the main source of water vapor. The results are of great significance to understand the anomalous variations of summer precipitation in the lower reaches of the Yangtze river.