Observed and modelled changes in the drivers that trigger compound drought and heatwave events in the Adige River basin (Eastern Italian Alps) with a focus on May 2022.

The Adige River basin has been affected several times in recent years by the concurrence of 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.

For these reasons, we initially developed a procedure to identify and rank past compound drought and heatwave events (CDHW) which occurred in the basin using precipitation and temperature observations from EOBS covering 1950 to 2023. CDHWs were identified combining two criteria: while heatwaves were defined as periods of at least three consecutive days with daily maximum temperatures (TX) exceeding the 90th percentile of the calendar day. A CDHW event was identified when both conditions occurred simultaneously over at least 60% of the catchment area. Detected CDHWs are then sorted by severity, as combination of their intensity and spatial extent. This ranking allowed us to characterise and determine the temporal extent of the major CDHW event that occurred in the late spring of 2022 (10–28 May). This event was selected for subsequent analyses since it is one of the most recent episodes when compound hot and dry conditions in spring and summer caused severe water use restrictions in the basin area affecting key sectors such as water supply, agriculture, and hydropower, among others. It was identified as the most intense CDHW events of the last 15 years and ranked sixth out of 119 events recorded since 1950.

To better understand the drivers of the May 2022 event, we reconstructed its atmospheric conditions using a flow-analogue attribution approach based on ERA5 Z500. We compared the characteristics of the event in two different periods: 1951–1980 (low anthropogenic forcing) and 1991–2020 (moderate-high anthropogenic forcing). Our analysis shows that heatwaves like 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 have also exacerbated river flow reductions and water stress in the recent period compared to the past.

We also evaluated the ability of 25 EURO-CORDEX climate model simulations to reproduce the observed changes in TX and SPI-6 through flow-conditioned reconstructions based on the May 2022 event. The assessment reveals that EURO-CORDEX models mostly fail to capture the observed signal and magnitude of changes in flow-analogue attribution experiments. Specifically, when identifying flow-conditioned analogues, nearly half of the models fail to reproduce the observed temperature increases, either in terms of sign or magnitude. Similarly, for drought conditions, models fail to reproduce both the direction and magnitude of observed changes.

 

This research was supported by the European Space Agency (ESA) under the EO4MULTIHA project (2023–2025), contract number 4000141754/23/I-DT.