Yavaşlı (2025) Thermal persistence index (TPI): a novel measure of prolonged heat stress in the Mediterranean, 1950–2024

Identification

• Journal: Theoretical and Applied Climatology
• Year: 2025
• Date: 2025-12-20
• Authors: Doğukan Doğu Yavaşlı
• DOI: 10.1007/s00704-025-05972-4

Research Groups

Department of Geography, Kırşehir Ahi Evran University, Kırşehir, Turkey.

Short Summary

This study introduces the Thermal Persistence Index (TPI) to quantify intra-daily heat stress persistence in the Mediterranean from 1950 to 2024, revealing widespread, statistically significant increases in prolonged heat stress, particularly since the 1990s.

Objective

• To introduce the Thermal Persistence Index (TPI) as a novel hourly metric for quantifying human-relevant heat stress persistence.
• To analyze long-term trends, regime shifts, and spatial hotspots of TPI-derived metrics (seasonal mean, seasonal maximum, and frequency of days with TPI ≥ 6 hours) across the Mediterranean basin from 1950 to 2024.

Study Configuration

• Spatial Scale: Mediterranean basin, approximated by a rectangular window spanning 30–45° N and − 10–40° E, at approximately 9-kilometer resolution.
• Temporal Scale: May-September for the period 1950–2024.

Methodology and Data

• Models used: Universal Thermal Climate Index (UTCI) computed using the operational UTCI polynomial (based on the Fiala multi-node thermoregulation model), Theil-Sen median slope, Mann-Kendall test, Pettitt change-point test, local Getis-Ord Gi* statistic, and Benjamini-Hochberg false discovery rate (FDR) procedure.
• Data sources: ERA5-Land reanalysis dataset at hourly, approximately 9-kilometer resolution, providing 2-meter air temperature, 2-meter dewpoint temperature, 10-meter winds, downward shortwave and longwave radiation at the surface, and surface pressure.

Main Results

• Widespread, statistically significant increases were observed in all three TPI metrics (seasonal mean, seasonal maximum, and frequency of days with TPI ≥ 6 hours) across the Mediterranean basin from 1950 to 2024.
• Consensus hotspots for intensifying heat stress persistence, where all three TPI metrics show significant positive trends, are concentrated in eastern Spain, northern Italy, the Aegean, and the Levant.
• The 1990s, particularly 1997, emerged as the dominant period for widespread and abrupt regime shifts in thermal persistence, indicating a basin-wide transition to a new thermal regime.
• The seasonal mean TPI (tpi_mean) increased by a median of approximately 0.10 hours per day per decade across the basin, with some areas experiencing increases up to 0.30 hours per day per decade.
• The seasonal maximum TPI (tpi_max) increased by a median of approximately 0.3 hours per day per decade, with the strongest signals exceeding 0.5 hours per day per decade in eastern Spain, central and southern Italy, and along the southern coast of Turkey.
• The frequency of days with TPI ≥ 6 hours (tpi_ge6days) showed strong increases, with a median of approximately 1.5 days per decade, and hotspots exceeding 2.5 days per decade in the Po Valley, mainland Greece, and the Aegean and western Turkish coasts.

Contributions

• Introduction of the Thermal Persistence Index (TPI), a novel metric that explicitly quantifies the maximum number of consecutive hours within a day during which the Universal Thermal Climate Index (UTCI) exceeds +32 °C, addressing a gap in existing daily-scale heat stress indices.
• First systematic application of an hourly persistence index at the Mediterranean basin scale, providing a comprehensive assessment of long-term trends, regime shifts, and spatial hotspots of prolonged intra-daily heat stress.
• Identification of the 1990s (especially 1997) as a critical decade for a basin-wide regime shift towards increased thermal persistence, offering insights into the timing of accelerated climate change impacts in the region.
• Provision of a concise framework for assessing climate-driven changes in heat stress duration, supporting targeted adaptation strategies in public health, labor, and agriculture.

Funding

Not applicable. The authors received no specific funding for this study.

Citation

@article{Yavaşlı2025Thermal,
  author = {Yavaşlı, Doğukan Doğu},
  title = {Thermal persistence index (TPI): a novel measure of prolonged heat stress in the Mediterranean, 1950–2024},
  journal = {Theoretical and Applied Climatology},
  year = {2025},
  doi = {10.1007/s00704-025-05972-4},
  url = {https://doi.org/10.1007/s00704-025-05972-4}
}