The Impact of Soil Moisture-Surface Temperature Coupling on the Compound Heat Wave Event in the Middle and Lower Reaches of the Yangtze River in August 2022

A compound heat wave in the middle and lower reaches of the Yangtze River in August 2022 had severe impacts on various aspects such as social-economics, human health, agricultural production and natural environment. Based on the daily maximum and minimum temperatures provided by the National Information Center from August 1979 to August 2022 and the soil moisture and other data from the European Centre for Medium-Range Weather Forecasts reanalysis datasets, this study defined an intensity index for the compound heat wave events and a corresponding soil moisture-surface temperature coupling index. Through segmented linear fitting and other methods, the research explored the characteristics of this compound heatwave and the impacts of land-atmosphere interactions. The results indicate that: (1) The intensity of the compound heat wave event in the middle and lower reaches of the Yangtze River in August 2022 is the strongest since 1979, accompanied by the most intense soil moisture-surface temperature coupling. The spatial distribution shows that the high heatwave index occurred in eastern Sichuan, western Chongqing, Hunan and other areas, where the soil moisture was severely dry, and evaporation was restricted, and the net radiation was mainly used for upward sensible heat flux at the surface. This led to a strong positive feedback process in soil moisture-surface temperature playing a crucial role in sustaining the heat wave. (2) The effect of soil moisture-surface air temperature coupling in the compound heatwave differs in physical forcing and processes between daytime and nighttime heat waves: during the day, the enhanced downward shortwave radiation at the surface forces strong soil moisture-surface temperature coupling, leading to daytime heat waves. At night, the heat accumulated in the soil during the day slows down the decrease in nighttime surface air temperature by reducing the divergence of atmospheric heat flux, resulting in nighttime heat wave. This process differs from previous studies on isolated nighttime heatwaves, which attribute them mainly to increased atmospheric column water vapor and enhanced atmospheric counter- radiation.