Modulation of Abnormal Soil Moisture in the Qinghai-Xizang Plateau in Spring and Early Summer on the High Temperature in the Yangtze River Basin in the Summer of 2022

In the summer of 2022, the Yangtze River basin experienced a rare extreme high-temperature weather event since 1961, with the obvious characteristics of a combination of high temperature and drought disasters. Specifically, it was characterized by the widest coverage area, longest duration, and highest comprehensive intensity of the high temperature. This study found a strong lag correlation between the soil moisture changes in the northwest of the Qinghai-Xizang Plateau in May and June 2022 and the frequency of high-temperature heat waves in the Yangtze River basin during summer. The positive feedback mechanism of the abnormally moist soil in the northwest of the plateau in June led to the formation of dipole modes of western cyclones and eastern anticyclones over the plateau, which in turn caused Kelvin waves to propagate eastward over South Asia and the plateau, resulting in anomalous anticyclones in the Bay of Bengal and suppressing the transport of southwest monsoon wind and air to the Yangtze River basin. In addition, a meridional wave train was associated with the anomalous anticyclone in the Bay of Bengal, which formed an anticyclone anomaly in the Yangtze River basin and supported the westward extension and maintenance of the subtropical high pressure in the northwest Pacific. It also showed a lagged negative correlation between soil moisture in the northwest Qinghai-Xizang Plateau in June and the western ridge of the subtropical high pressure over the northwest Pacific during summer. The abnormal anticyclone in the Yangtze River basin increased shortwave radiation, evaporation, soil drying, and land–atmosphere coupling during the rainy season, thereby strengthening surface temperatures through land–atmosphere feedback. Moreover, the adiabatic heating of descending motion under the control of the subtropical high pressure jointly promoted high-temperature heat waves during both the rainy and drought seasons.