Spatiotemporal Patterns of Compound Heatwave–Drought Severity Across Europe

Compound dry and hot events (CDHEs), which arise from the co‑occurrence of heatwaves and droughts, represent one of the most critical and rapidly intensifying climate‑related hazards worldwide, particularly in climate change hotspots like the Mediterranean Europe. The consequences of these CDHEs often exceed those associated with isolated occurrences.

Despite growing recognition of their importance, CDHEs remain challenging to characterize due to their multivariate structure, requiring methodological approaches that differ from those typically employed in univariate analyses. As a result, advancing the study of CDHEs is essential, especially given expectations of their increasing frequency and severity under continued warming.

In this study, we employ a compound severity index based on the product of marginal probabilities of individually standardized hot and dry indicators, providing a meaningful measure of compound hot–dry severity across Europe. These indicators rely on well‑established metrics for defining heatwaves and drought conditions, including commonly used heatwave indices and the SPI, both derived from ERA5 data. The severity index is used to evaluate the spatial and temporal patterns of CDHEs for the period 1979–2025, with particular emphasis on distinct severity classes and the percentage of area affected by events.

The results show distinct spatial and temporal variations in CDHE severity and in the extent of the areas impacted. This perspective on joint magnitude and spatial extent allows for a consistent comparison of events, helping to identify those that were both exceptionally strong and unusually widespread across the domain, uncovering information that would be missed by analyses limited to event frequency.

Overall, this investigation advances the emerging field of compound‑event research by providing a detailed climatological assessment of heatwaves, droughts, and their co‑occurrence in a region already experiencing substantial climate pressures. The proposed framework offers a robust way to improve the representation of multivariate hazard characteristics and is expected to offer useful insights for climate‑impact assessment and risk management under continued warming. It further provides a solid starting point for expanding the analysis to include additional variables and processes linked to compound events, supporting more comprehensive evaluations of climate‑related risks.

                             

Acknowledgements: This work is supported by FCT, I.P./MCTES through national funds (PIDDAC): LA/P/0068/2020 - https://doi.org/10.54499/LA/P/0068/2020, UID/50019/2025,  https://doi.org /10.54499/UID/PRR/50019/2025 ,UID/PRR2/50019/2025, and DHEFEUS (https://doi.org/10.54499/2022.09185.PTDC).