Roles of Soil Moisture–Air Temperature Coupling in Three Types of Heatwaves over the Greater Bay Area of China

China’s Greater Bay Area (GBA) is one of the fastest urbanizing regions in the world, featured by its complex land surface and unique geography. In this study, heatwaves (HWs) in the GBA during the summers (June, July, and August) of 1961–2020 are analyzed by using observational and reanalysis datasets. The results indicate that 70% of daytime HWs occur in the northern forested areas of the GBA, 65% of nighttime HWs are observed in the cropland and forest areas around the GBA, and 75% of compound HWs occur in the urban and southern coastal areas of the GBA. Daytime HWs are featured by lower near-surface specific humidity and drier soil moisture, while nighttime HWs are often accompanied by relatively wetter conditions. For compound HWs, they are jointly affected by the conditions of the above two types. During daytime and compound HWs, soil moisture dries and recovers quickly, exacerbating the high temperatures of HWs through strong soil moisture–air temperature coupling that far exceeds the climatology. Nighttime HWs lack this coupling and are primarily driven by atmospheric factors, with high temperatures maintained mainly by increased water vapor.