Kalvāne et al. (2026) More frequent warm and dry spells along persistent cold and wet spells in the Baltics

Identification

• Journal: Theoretical and Applied Climatology
• Year: 2026
• Date: 2026-02-21
• Authors: Gunta Kalvāne, Andis Kalvāns, Agrita Briede
• DOI: 10.1007/s00704-026-06099-w

Research Groups

• Department of Geography, Faculty of Science and Technology, University of Latvia, Riga, Latvia
• Department of Geology, Faculty of Science and Technology, University of Latvia, Riga, Latvia

Short Summary

This study analyzed the temporal and spatial variability of warm, cold, wet, and dry spells in the Baltics (Estonia, Latvia, and Lithuania) from 1961 to 2020 using ERA5-Land reanalysis data. The findings reveal a significant increase in the frequency and duration of warm and dry spells, while cold and wet spells moderately decreased, indicating an amplification of extreme weather conditions in the region.

Objective

• To examine the temporal and spatial variability and changes in frequency and duration of warm, cold, wet, and dry spells across all seasons in Estonia, Latvia, and Lithuania from 1961 to 2020, comparing the 1961–1990 climatic reference period with the 1991–2020 recent climate normal period.

Study Configuration

• Spatial Scale: Estonia, Latvia, and Lithuania (Baltic region), covering grid cells with a resolution of 0.1 degrees latitude/longitude, approximately 11 km by 6 km.
• Temporal Scale: 1961–2020 (60 years) at a daily time step. Spells are defined as periods of at least two consecutive days.

Methodology and Data

• Models used:
  • FAO Penman–Monteith equation (for reference evapotranspiration, ET0)
  • Sen’s slope (for trend analysis)
  • Linear regression model (LM)
  • R packages: FAO56 v1.0, trend::sens.slope, stats::lm, tidyverse.
• Data sources:
  • ERA5-Land reanalysis data (Muñoz Sabater 2019, Muñoz-Sabater et al. 2021).
  • Meteorological parameters: mean air temperature (T), precipitation (P), reference evapotranspiration (ET0), and daily climatological water balance (CWB).
  • Spells were identified using the 10th and 90th percentiles of long-term daily distributions (31-day moving window). Dry days for precipitation were defined as less than 1 mm.

Main Results

• The mean annual temperature (T) in the Baltics increased from 5.85 °C (1961–1990) to 6.93 °C (1991–2020), at a rate of +0.038 °C per year (p < 0.01). The strongest warming occurred in winter, with an increase of +0.061 °C per year (p < 0.01).
• No statistically significant annual or seasonal trends were found for total precipitation (P) in the region.
• Reference evapotranspiration (ET0) showed a significant increasing trend (higher rates of evapotranspiration), with annual totals declining from -476 mm in the reference period to -527 mm in the recent normal, corresponding to a Sen’s slope of -0.004 mm per day per year (p < 0.01).
• The climatological water balance (CWB) decreased from 295 mm to 220 mm, primarily driven by ET0 changes, with a significant spring decline of -0.011 mm per day per year (p < 0.01), indicating increased drought risk.
• Warm spell frequency and duration doubled from 1991–2020 compared to 1961–1990 across all seasons. For example, the number of days with warm spells in winter increased at a rate of 0.22 days per year.
• Cold spell frequency and duration decreased across most seasons. The number of days with cold spells in winter decreased by 0.11 days per year.
• Precipitation (P) spells exhibited high variability. Winter wet spell frequency and duration slightly increased (p < 0.01), while spring dry spell days increased significantly (p < 0.05).
• ET0 and CWB dry spells strongly increased in spring and, to a lesser extent, in winter and summer. Summer ET0 dry spell days more than doubled from 3.6 to 8.2 days, and CWB dry spell days increased from 4.1 to 8.0 days.
• Regional variability was evident, with coastal zones (e.g., West Estonian archipelago) showing the most significant changes towards warmer and drier conditions. Southern-central areas also exhibited marked summer increases in dry spells.
• The study highlights an asymmetric increase in warm and dry spells relative to the decline in cold and wet events, indicating an amplification of extreme weather conditions.

Contributions

• Provides the first comprehensive examination of atypical weather conditions (spells) across all seasons in the Baltic region, comparing two distinct climate normal periods (1961–1990 and 1991–2020).
• Demonstrates an asymmetric amplification of extreme weather events, where the increase in warm and dry spells significantly outpaces the decrease in cold and wet spells.
• Emphasizes the critical role of increasing evapotranspiration in driving drying trends and decreasing climatological water balance, even in the absence of significant precipitation changes.
• Offers detailed spatial analysis, identifying coastal areas and specific sub-regions as hotspots for amplified warming and drying trends, which is crucial for targeted climate adaptation strategies.

Funding

• Ministry of Education and Science of the Republic of Latvia.
• Performance-based funding "Sustainable use of natural resources in the context of climate change" (ZD2010/AZ03) of the University of Latvia.

Citation

@article{Kalvāne2026More,
  author = {Kalvāne, Gunta and Kalvāns, Andis and Briede, Agrita},
  title = {More frequent warm and dry spells along persistent cold and wet spells in the Baltics},
  journal = {Theoretical and Applied Climatology},
  year = {2026},
  doi = {10.1007/s00704-026-06099-w},
  url = {https://doi.org/10.1007/s00704-026-06099-w}
}