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.

Changes in water vapor mass can obviously counteract seasonal changes of the interhemispheric oscillation. In the present paper, the outputs of CMIP6 models from January 2015 to December 2100 are used to analyze seasonal cycle characteristics of water vapor mass under four greenhouse gas emission scenarios and compare with the historical run from 1958 to 2015. It is found that the water vapor mass in both hemispheres show obvious seasonal cycles. In the Northern Hemisphere, water vapor mass is characterized by low value in winter and high value in summer, while the opposite is true in the Southern Hemisphere. Regardless of the Northern and Southern Hemispheres, the annual range of water vapor mass is the smallest under the SSP1-2.6 (Shared Socioeconomic Pathway) scenario, and large water vapor mass changes occur in winter and summer. With the increase of CO₂, the annual range of water vapor mass in the Northern Hemisphere under the SSP3-7.0 scenario is the largest, which increases by 26.49% compared with that of the historical run. The situation in the Southern Hemisphere is different to that in the Northern Hemisphere. With the increase of CO₂ after the SSP1-2.6 scenario, the annual range of water vapor mass in the Southern Hemisphere also increases, reaching the maximum under the SSP5-8.5 scenario. The annual range of water vapor mass IHO increases with the increase of CO₂ concentration, and reaches the maximum under the SSP5-8.5 scenario. However, the increase amplitude decreases. The change of CO₂ concentration has the most obvious influence on the abnormal change of water vapor mass near the Equator. Meanwhile, the closer to the Antarctic, the smaller the abnormal change of water vapor mass. However, the closer to the Arctic, the greater the abnormal change of water vapor mass in summer than in winter. In addition, the increase of CO₂ concentration will lead to gradual accumulation of water vapor mass in summer towards the mid-latitudes of the Northern Hemisphere. These conclusions are conducive to better understanding of the response of water vapor mass change to the increase in CO₂ concentration, and provide clues to future climate policy formulation on precipitation.