More intense heatwaves under drier conditions: a compound event analysis in the Adige River basin (Eastern Italian Alps)

Abstract. The Adige River basin has been affected several times in recent years by concurrent very hot and dry conditions. In summers 2015, 2017, and more recently in 2021–2022, severe hydrological droughts compounded and cascaded with wildfire and heatwave events. The chained effect of snow deficit in winter, higher-than-normal temperatures in early spring and heatwaves during summer caused multiple drought impacts. Despite the severe consequences, the role of observed climate change in exacerbating the intensity of the drivers leading to specific hot and dry events and their potential impacts in this region remains poorly understood. A ranking of compound drought and heatwave events (CDHW) occurring in the Adige River basin between 1950 and 2023 was built using E-OBS precipitation and temperature observations. The ranking was based on a composite index considering both event intensity and the spatial extent of the affected area, derived from the Standardised Precipitation Index at a 6 month scale (SPI-6) and a heatwave definition based on daily maximum temperature (TX). The major 2022 CDHW event, which caused severe environmental and societal impacts in the Adige River catchment, stood out. Occurring in late spring (10–28 May 2022), it ranked fifth out of 119 events detected since 1950 and was the most intense CDHW event in the past 15 years. As one of the most recent and severe CDHW events, the 2022 event was selected for an in-depth characterisation and a climate change attribution analysis of both its meteorological drivers and hydrological impacts. The changing characteristics of CDHW events similar to that of May 2022 were investigated through a flow-analogue attribution approach based on the reconstruction of its atmospheric conditions using ERA5 geopotential height at 500 hPa. By comparing May 2022 CDHW flow analogues from 1951–1980 (low anthropogenic forcing) and 1992–2021 (moderate-high anthropogenic forcing), we found that heatwaves comparable to the one in May 2022 are now significantly hotter – by 1–4 °C – than historical analogues and occur in a much drier context, characterised by pronounced precipitation deficits. These conditions, along with earlier snowmelt and persistent precipitation deficits, might have exacerbated river flow reductions and water stress in recent years. Also, shifts in the timing of a CDHW event were found to significantly influence the severity of its potential consequences. However, extracting a reliable signal of future changes in the characteristics of CDHW events from climate projections remains challenging. Based on flow-conditioned analogues of the May 2022 event from 25 EURO-CORDEX simulations, more than half of the models failed to reproduce the observed sign of change in temperature and drought conditions. Unconditioned reconstructions showed closer agreement with observations, particularly for temperature patterns, but critical aspects such as the magnitude of the changes remained underestimated.