Boras et al. (2026) Changes in compound dry–hot extremes in Croatia: Spatio-temporal features and the dominant role of temperature

Identification

• Journal: Atmospheric Research
• Year: 2026
• Date: 2026-02-19
• Authors: Marijana Boras, Ivana Herceg Bulić, Zoran Pasarić
• DOI: 10.1016/j.atmosres.2026.108873

Research Groups

• Department of Geophysics, Faculty of Science, University of Zagreb, Croatia

Short Summary

This study investigates the spatio-temporal characteristics and intensity of compound dry and hot (DH) extreme events in Croatia during summer (June-August). It finds a significant increase in the frequency and intensity of these events across all regions of Croatia, primarily driven by rising temperatures, with distinct regional patterns in how temperature contributes to event intensity.

Objective

• To investigate the spatial and temporal characteristics of compound dry and hot (DH) extremes in Croatia, with particular emphasis on changes in the frequency and intensity of these events during summer (JJA).

Study Configuration

• Spatial Scale: Croatia, divided into three geographical groups: mountain (3 stations), inland (15 urban stations), and coastal (14 urban stations).
• Temporal Scale: 1963–2021, analyzed in two periods: P1 (1963–1992) and P2 (1993–2021), focusing on the summer season (June-August).

Methodology and Data

• Models used: Not applicable (observational study).
• Data sources: Daily maximum air temperature (Tx) and daily precipitation data from main meteorological stations maintained by the Croatian Hydrometeorological Service.
  • Compound DH event definition: Periods of consecutive dry days (precipitation < 1 mm) and the maximum Tx during those periods.
  • Extreme DH event definition: Simultaneous exceedance of the 90th percentile for dry spell duration (DSd) and maximum Tx (Max Tx) from the 1981–2010 reference period for the JJA season.
  • Cumulative Heat Intensity (CHI): Sum of positive daily maximum air temperature anomalies (ΔTi = Ti - Ti,p90, where Ti,p90 is the 90th percentile of Tx for the calendar day, calculated using a 15-day central sliding window during 1981–2010).
  • Statistical methods: Mann-Whitney U test (for significance of changes between periods), Sen's slope (for trends in temperature percentiles), Mann-Kendall (MK) test (for statistical significance of trends).

Main Results

• The occurrence of extreme DH events has increased at all stations in Croatia during the recent period (P2: 1993–2021) compared to the earlier period (P1: 1963–1992), with some stations showing a relative change of over 100%.
• The intensity of DH events, measured by Cumulative Heat Intensity (CHI), has significantly increased at almost all stations (e.g., Zagreb: ~3-fold increase, Split: ~6-fold increase).
• No significant change in the average duration of dry spells (DSd) was observed between the two periods at any station.
• Changes in CHI are primarily driven by increases in temperature-related parameters: the number of days exceeding the 90th percentile of daily maximum temperature (CHIndays) and the mean temperature anomaly during those days (CHImeanTa).
  • CHIndays increased significantly at all stations, more pronounced at coastal stations (e.g., Split: >5-fold increase).
  • CHImeanTa increased at all stations, with similar values across geographical groups in P2, though slightly higher at mountain and inland stations.
• For equally intense DH events (same CHI category):
  • Coastal stations are characterized by a greater number of days exceeding temperature thresholds (higher CHIndays) but with smaller average temperature anomalies (lower CHImeanTa).
  • Mountain and inland stations show the opposite pattern, with fewer exceedance days but larger average temperature anomalies.
• Analysis of summer maximum daily air temperature (Tx) distribution shows a continuous shift of the entire distribution towards higher values at all stations, with positive and statistically significant trends across all percentiles.
  • At most mountain and inland stations, higher percentiles (corresponding to extreme warm conditions) show greater trend values, indicating a more rapid increase in extreme temperatures.
  • At coastal stations, percentile trends are flatter, increasing up to approximately the 25th percentile and then stabilizing, suggesting a more uniform shift of the entire distribution in these ranges.

Contributions

• First comprehensive study of compound dry and hot extremes in Croatia, filling a significant research gap for the region.
• Identifies the dominant role of temperature increases as the primary driver of changes in DH extreme frequency and intensity, rather than changes in dry spell duration.
• Reveals distinct regional patterns in how temperature contributes to DH event intensity (number of hot days vs. magnitude of temperature anomaly) and in the shape of temperature percentile trend curves between inland/mountain and coastal areas of Croatia.
• Highlights the sensitivity of mountain regions to global warming and suggests potential moderating effects of local factors (e.g., sea breeze) in coastal areas on extreme temperatures.

Funding

• Croatian Science Foundation under the project HRZZ-IP-2022-10-4144 (CroClimExtremes)
• Croatian Science Foundation (HRZZ-DOK-2021-02-9704) for the work of M. Boras

Citation

@article{Boras2026Changes,
  author = {Boras, Marijana and Bulić, Ivana Herceg and Pasarić, Zoran},
  title = {Changes in compound dry–hot extremes in Croatia: Spatio-temporal features and the dominant role of temperature},
  journal = {Atmospheric Research},
  year = {2026},
  doi = {10.1016/j.atmosres.2026.108873},
  url = {https://doi.org/10.1016/j.atmosres.2026.108873}
}