Increased heating of the land surface as hot-dry events persist

Abstract. Compound hot-dry events have devastating effects on ecosystems as well as societies. Combinations of more incoming shortwave radiation (SW_down) and drying soil moisture lead to the build-up of high temperatures during dry periods. In this process, evaporation (ET) plays an important role in coupling temperature and soil moisture, and thus can lead to feedback loops and more drying. While both atmospheric contributors (SW_down) and the land surface (ET) are known to influence temperature during dry periods, it remains unclear how their relevance for high temperatures varies throughout a dry event, i. e. from the build-up of heat to its persistence during ongoing dryness. Furthermore, the contributions of ET and SW_down to heat onset and persistence during dryness are likely to differ across space and over the last decades. In this study, we investigate SW_down and ET changes as two contributing factors to heat accumulation throughout dry events and across recent decades using reanalysis data. We determine periods of soil dryness accompanied by high temperatures using weekly timescale data. Within the detected hot-dry weeks, we distinguish between heat onset and heat persistence by evaluating the continuity of high temperatures. By mapping changes in ET and SW_down during heat onset and heat persistence, we find that radiation increases contribute to the onset of heat globally but are less dominant for heat persistence. Evaporative cooling mitigates radiation-driven temperature increases during the onset of heat in humid regions. By contrast, this effect vanishes during persistent high temperatures. While the general occurrence of hot conditions during dry events increased from 14 % to 28 % from the 1980s to the 2010s, the evolution of ET and SW_down throughout hot-dry events shows no clear trend over the last few decades. Our study emphasizes that contributors to heat development and/or persistence vary during the lifetime of a dry event which should be considered when analyzing compound extremes.