Impacts of Land–Atmosphere Coupling on Summer Extreme Hot-Humid Compound Events over Southern Eurasia under Different Sea Surface Temperature Backgrounds

Land–atmosphere coupling and sea surface temperature (SST) anomalies both have essential impacts on weather and climate extremes. Based on the ERA5 reanalysis dataset and the CESM1.2.2 model, this study investigates the influence of land–atmosphere coupling on summer extreme hot-humid events (EHHE) over southern Eurasia under different SST backgrounds. The results suggest that coupling causes near-surface air temperature increases that exceed 0.5°C. From 1961 to 2020, the frequency of EHHE has continuously increased, and is closely related to soil moisture anomalies in the northern Indian Peninsula (IDP) and the middle and lower reaches of the Yangtze River (YRB). Numerical simulations further demonstrate that land–atmosphere coupling raises the risk of EHHE by 25.4%. In a typical El Niño SST background state, intensified land–atmosphere coupling tends to produce notable increases in the frequency of EHHE. The dominant processes that land–atmosphere coupling affects the EHHE variations are evidently different between these two regions. Land surface thermal anomalies predominate in the IDP, while moisture conditions are more critical in the YRB. When warm SST anomalies exist, dry soil anomalies in the IDP are prominent, and evaporation is constrained, increasing sensible heat flux. Positive geopotential height anomalies are significant, combined with adiabatic warming induced by descending motion and a noticeable warm center in the near-surface atmosphere. The southward shift of the westerly jet enhances divergence over YRB. The anticyclonic circulation anomalies over the western Pacific are conducive to guiding moisture transport to the YRB, providing a favorable circulation background for the development of summer EHHE.