Polychroni et al. (2025) Compound extremes of air temperature and precipitation over the Mediterranean region. Understanding the influence of atmospheric circulation

Identification

• Journal: Atmospheric Research
• Year: 2025
• Date: 2025-10-10
• Authors: Iliana Polychroni, P. T. Nastos, Maria Hatzaki, John Kapsomenakis
• DOI: 10.1016/j.atmosres.2025.108529

Research Groups

• Laboratory of Climatology and Atmospheric Environment, Department of Geology and Geoenvironment, National and Kapodistrian University of Athens, Athens, Greece
• Research Centre for Atmospheric Physics and Climatology, Academy of Athens, Athens, Greece

Short Summary

This study quantifies the spatiotemporal variability and trends of four compound extreme climate indices (Cold/Dry, Cold/Wet, Warm/Dry, Warm/Wet) across the Mediterranean region from 1950 to 2018, revealing a significant shift towards warmer and drier conditions modulated by the North Atlantic Oscillation (NAO) and North Sea–Caspian Pattern (NCP).

Objective

• To quantify and evaluate the seasonal and annual means and trends of four compound extreme climate indices (Cold/Dry, Cold/Wet, Warm/Dry, Warm/Wet) over the Mediterranean region during the period 1950–2018, including analysis over consecutive 30-year periods.
• To assess the influence of large-scale atmospheric circulation patterns, specifically the North Atlantic Oscillation (NAO) and the North Sea–Caspian Pattern (NCP), on these compound extreme indices.

Study Configuration

• Spatial Scale: Wider Mediterranean region (30° N–50° N latitude, 10° W–37° E longitude) at a spatial resolution of 0.1° × 0.1°.
• Temporal Scale: Daily data from January 1, 1950, to December 31, 2018, with analysis of annual and seasonal means and trends, and consecutive 30-year moving windows. A reference period of 1981–2005 was used for calculating percentile thresholds.

Methodology and Data

• Models used: Not applicable (observational study). Data processing and spatio-temporal analysis were performed using the R-project, including the RNetCDF library. Pearson correlation analysis was used to assess relationships with teleconnection patterns. The North Sea-Caspian Pattern Index (NCPI) was calculated using equations from Kutiel and Benaroch (2002).
• Data sources: High-resolution gridded daily mean temperature and precipitation datasets from the E-OBS dataset (v.19e) provided by the European Climate Assessment & Dataset (ECA&D). The North Atlantic Oscillation Index (NAOI) was obtained from the National Weather Service Climate Prediction Center.

Main Results

• The Mediterranean region experienced significantly more dry days (Cold/Dry and Warm/Dry) than wet days (Cold/Wet and Warm/Wet) between 1950 and 2018.
• Cold compound indices (Cold/Dry and Cold/Wet) showed statistically significant negative trends across the Mediterranean, while warm compound indices (Warm/Dry and Warm/Wet) exhibited statistically significant positive trends, indicating a shift towards warmer conditions.
• The Cold/Dry index showed annual negative trends of -0.8 to -3.2 days per year in the central and western Mediterranean. The Warm/Dry index displayed widespread annual positive trends exceeding 1.6 days per year, reaching up to 2.8 days per year, signifying an increase in warm and dry conditions.
• The most intense decreases in Cold/Dry days and increases in Warm/Dry days occurred after the 1970s, suggesting a dominant role of anthropogenic climate forcing.
• The North Atlantic Oscillation (NAO) significantly influenced compound extremes, with positive NAO phases in winter associated with increased Warm/Dry days and decreased Cold/Dry and Cold/Wet days, particularly in the northern Mediterranean.
• The North Sea–Caspian Pattern (NCP) also modulated compound extremes, showing a statistically significant anticorrelation with Warm/Dry days in the southeastern Mediterranean during winter and summer, and a negative correlation with Warm/Wet days across most of the study area in winter.

Contributions

• Provides a comprehensive spatiotemporal assessment of compound temperature and precipitation extremes across the entire Mediterranean region, moving beyond previous studies focused on smaller areas.
• Utilizes a high-resolution (0.1° × 0.1°) observational dataset (E-OBS), enabling detailed analysis of sub-regional contrasts in extreme event evolution.
• Incorporates a moving 30-year window analysis to capture subtle temporal variations and long-term shifts in compound extreme frequencies over nearly seven decades.
• Integrates the analysis of large-scale atmospheric circulation patterns (NAO and NCP) to provide a physically consistent characterization of the drivers of compound extremes in the region.
• Bridges the gap between local-scale assessments and global perspectives, offering a more nuanced understanding of climate change impacts in a highly vulnerable region.

Funding

The authors acknowledge the E-OBS dataset from the EU-FP6 Project UERRA and the Copernicus Climate Change Service, and the data providers in the ECA&D Project. No specific funding projects for this research were explicitly listed.

Citation

@article{Polychroni2025Compound,
  author = {Polychroni, Iliana and Nastos, P. T. and Hatzaki, Maria and Kapsomenakis, John},
  title = {Compound extremes of air temperature and precipitation over the Mediterranean region. Understanding the influence of atmospheric circulation},
  journal = {Atmospheric Research},
  year = {2025},
  doi = {10.1016/j.atmosres.2025.108529},
  url = {https://doi.org/10.1016/j.atmosres.2025.108529}
}