Impact of diurnal temperature and relative humidity hysteresis on atmospheric dryness in changing climates

Vapor pressure deficit (VPD), a key indicator of atmospheric dryness, is strongly influenced by diurnal cycles of temperature (<i>T</i>) and relative humidity (RH). While these cycles are typically inversely locked in-phase, recent research has identified diurnal hysteresis, characterized by a time lag between <i>T</i> and RH; yet, its impact on VPD under changing climates remains poorly understood. In this study, we examine how diurnal <i>T</i>/RH hysteresis modulates VPD across different climates using observational data alongside high-resolution reanalysis and simulations. Here, we find that regions exhibiting strong diurnal <i>T</i>/RH hysteresis, especially in some waterside and montane regions, experience earlier daily VPD peaks. We also demonstrate that global warming weakens diurnal <i>T</i>/RH hysteresis, leading to amplified VPD increasing trends and greater ecosystem stress. These results highlight the need for improved representation of diurnal <i>T</i>/RH interactions in climate models to better predict atmospheric dryness and its impacts on land-atmosphere feedbacks, ecosystems, and regional water cycles.