Characteristics of land-atmosphere coupling in compound dry-hot events in Northwestern China

Northwest China comprises Xinjiang, Qinghai, Gansu, Ningxia, and Shaanxi provinces. Compound dry-hot events (CDHEs) pose substantially higher risks to agricultural, ecological, and socio-economic systems than individual extreme events. However, their daily-scale characteristics and associated land-atmosphere coupling mechanisms remain inadequately characterized, particularly in arid/semi-arid regions. In this study, we developed a novel compound dry-hot magnitude index (CDHMI) by integrating daily standardized precipitation evapotranspiration index (SPEI) and heatwave intensity data. Then, we quantified the frequency, duration, and magnitude of CDHEs across Northwest China during 1962–2020. Furthermore, we conducted an in-depth investigation into the characteristics of land-atmosphere coupling processes during CDHEs. The frequency, duration, and magnitude of CDHEs in Northwest China exhibited increasing trends from 1962 to 2020, particularly pronounced in the eastern parts of the study area. Deficits in soil moisture and evapotranspiration promote the conversion of a greater proportion of net surface radiation into sensible heat, thereby elevating near-surface air temperatures and exacerbating soil desiccation. These land-atmosphere interactions can amplify the likelihood of CDHEs, as exemplified by the extreme CDHE in 1997. Land surface meteorological and energy variables shifted toward more severe conditions during 1991–2020 relative to 1962–1990, indicating that enhanced land-atmosphere coupling can contribute to more CDHEs. In the eastern parts of the study area, daily mean surface meteorological and energy variables under CDHEs exhibited more severe patterns during the warm recent (1991–2020) relative to the past period (1962–1990). Furthermore, progressive intensification of land surface meteorological and energy variables was observed under drought-only, heatwave-only, CDHE, and extreme CDHE conditions, respectively. These findings can provide critical insights into the characteristics and impacts of CDHEs in arid and semi-arid regions, underscoring the necessity for further research on land-atmosphere coupling and its role in exacerbating compound events. • Compound dry-hot events (CDHEs) exhibited upward trends, especially in the eastern regions. • Intensified land-atmosphere coupling can amplify the severity of CDHEs. • Surface variables under CDHEs worsen more severely than individual extremes.