Lemus‐Canovas et al. (2025) More intense heatwaves under drier conditions: a compound event analysis in the Adige River basin (Eastern Italian Alps)

Identification

• Journal: Hydrology and earth system sciences
• Year: 2025
• Date: 2025-11-28
• Authors: Marc Lemus‐Canovas, Alice Crespi, Elena Maines, Stefano Terzi, Massimiliano Pittore
• DOI: 10.5194/hess-29-6781-2025

Research Groups

Center for Climate Change and Transformation, Eurac Research, Bolzano-Bozen, Italy

Short Summary

This study analyzes a severe compound drought and heatwave (CDHW) event in the Adige River basin (Eastern Italian Alps) in May 2022, revealing that similar events are now significantly hotter (by 1–4 °C) and drier (with pronounced precipitation deficits) due to climate change, exacerbating water stress and shifting streamflow seasonality. It also highlights the inability of many regional climate models (EURO-CORDEX) to accurately reproduce these observed changes in both magnitude and sign.

Objective

• Identify the most severe compound drought and heatwave (CDHW) events in the Adige River catchment over recent decades using a composite indicator.
• Assess the relative intensity of the May 2022 hot and dry event and attribute observed changes in its meteorological drivers to regional warming trends and atmospheric circulation patterns.
• Evaluate the impacts of changes in snow dynamics associated with CDHW conditions on streamflow patterns.
• Evaluate the ability of EURO-CORDEX regional climate models to reproduce observed changes in drought and heat conditions (sign and magnitude) for future attribution analyses in mountainous regions.

Study Configuration

• Spatial Scale: Adige River basin (Eastern Italian Alps), Italy (basin area: 12,200 km²; elevation: 0 to 3905 m above sea level). Meteorological fields analyzed over the Eastern Alps (44–49° N, 7–15° E). Synoptic-scale atmospheric circulation analyzed over Western Europe (30–60° N, 10° W–30° E).
• Temporal Scale:
  • CDHW event ranking: 1950–2023.
  • Climate change attribution analysis (factual vs. counterfactual periods): 1992–2021 (moderate-high anthropogenic forcing) compared to 1951–1980 (low anthropogenic forcing).
  • Snow depth data: 1980–2018.
  • Streamflow data: 1951–2021 (Trento-Ponte San Lorenzo), 1951–2020 (HERA).
  • EURO-CORDEX simulations: Historical (1951–2005) merged with RCP8.5 (2006–2021).

Methodology and Data

• Models used:
  • Flow-analogue attribution approach based on geopotential height at 500 hPa (Z500).
  • EURO-CORDEX ensemble (25 members, Regional Climate Models driven by Global Climate Models) for historical and RCP8.5 scenarios.
• Data sources:
  • E-OBS v28.0 dataset: Daily maximum temperature (TX) and total precipitation (TP) at 0.1° spatial resolution (1950–2023).
  • ERA5-Land reanalysis: Daily TX and TP at 0.1° resolution.
  • ERA5 reanalysis: Daily geopotential height at 500 hPa (Z500) at 0.25° resolution.
  • Alpine Drought Observatory: Daily river discharge data from Trento-Ponte San Lorenzo gauging station (1951–2021).
  • HERA (High-resolution Pan-European Hydrological Reanalysis) dataset: Daily river discharge at 1.8 km resolution (1951–2020).
  • Historical observatories: Daily snow depth data from three stations in the upper Adige catchment (1980–2018).
  • ERA5-Land: Monthly snow cover for the Adige basin (1951–2020).
  • MODIS MOD10A1.061 product: Global snow cover phenology dataset (2000–2023).
  • NOAA/ERSSTv5 via KNMI's Climate Explorer: Monthly Arctic Oscillation (AO) and Atlantic Multidecadal Oscillation (AMO) indices.
  • Compound Drought and Heatwave (CDHW) indicator: Calculated using Standardised Precipitation Index at a 6-month scale (SPI-6) and daily maximum temperature (TX) above the 90th percentile.

Main Results

• The May 2022 CDHW event was the fifth most severe since 1950 and the most intense in the past 15 years, affecting over 80% of the Adige catchment.
• Flow-analogue attribution indicates that heatwaves comparable to the May 2022 event are now 1–4 °C hotter and occur under significantly drier conditions (precipitation deficits of 0.01–0.015 m) compared to 1951–1980.
• A strengthening of the large-scale atmospheric configuration (increased geopotential height at 500 hPa over central Europe) contributed to the heatwave's intensity.
• Earlier snowmelt, driven by rising temperatures, leads to increased streamflow in April but significant reductions in June (up to -50 to -60 m³/s per 30 years trend by mid-June), shifting the spring peak earlier and exacerbating summer low-flow conditions. Snow depth decreased by approximately 0.3–0.4 m per 30 years in mid-April.
• The timing of CDHW events is critical: events occurring in June now coincide with lower streamflow than in the past, intensifying hydrological impacts.
• Large-scale teleconnection patterns (AMO, AO) did not show statistically significant changes between the periods, suggesting regional and global warming trends are the primary drivers of observed changes.
• EURO-CORDEX regional climate models show significant limitations:
  • Only 56% of models reproduce the observed positive sign of temperature change in flow-conditioned reconstructions, and they strongly underestimate its magnitude (mostly < +0.5 °C compared to observed +1 to +4 °C).
  • Only 20% (flow-conditioned) and 16% (unconditioned) of models reproduce the observed drying trend for SPI-6.

Contributions

• First detailed compound event analysis (drought and heatwave) at a catchment scale in the Adige River basin, Eastern Italian Alps.
• Quantification of the exacerbation of heatwave intensity and drought conditions due to climate change using a flow-analogue attribution approach in a complex mountainous environment.
• Demonstration of the critical role of event timing and its interaction with snowmelt dynamics in shaping hydrological impacts (earlier snowmelt leading to reduced summer streamflow).
• Comprehensive evaluation of EURO-CORDEX regional climate models' ability to reproduce observed changes in CDHW conditions, revealing significant limitations in capturing both the sign and magnitude of changes, particularly for temperature and drought.
• Development and application of a composite indicator for ranking CDHW events based on intensity and spatial extent.

Funding

• European Space Agency (ESA) under the EO4MULTIHA project (contract number 4000141754/23/I-DT).
• Xunta de Galicia (Government of Galicia, Spain) for a postdoctoral contract (reference number: ED481B-2022-055).
• Department of Innovation, Research University and Museums of the Autonomous Province of Bozen/Bolzano for Open Access publication costs.

Citation

@article{LemusCanovas2025More,
  author = {Lemus‐Canovas, Marc and Crespi, Alice and Maines, Elena and Terzi, Stefano and Pittore, Massimiliano},
  title = {More intense heatwaves under drier conditions: a compound event analysis in the Adige River basin (Eastern Italian Alps)},
  journal = {Hydrology and earth system sciences},
  year = {2025},
  doi = {10.5194/hess-29-6781-2025},
  url = {https://doi.org/10.5194/hess-29-6781-2025}
}