Extreme precipitation trends in Bulgaria in conditions of a warming climate

When the skies over Bulgaria open, they increasingly open with force. A new study published in Theoretical and Applied Climatology by Peter Nojarov and colleagues at the Climate, Atmosphere and Water Research Institute of the Bulgarian Academy of Sciences has revealed that extreme precipitation events—defined as daily precipitation totals equal to or greater than 20 mm—show predominantly positive trends across Bulgaria during the period 1979–2025, according to both ground-based instrumental observations and the ERA5-Land reanalysis dataset. The research, which covers the second half of the 20th century and the beginning of the 21st century, provides one of the most comprehensive analyses of extreme precipitation dynamics in southeastern Europe to date.

The study employed two complementary data sources: ground-based measurements from weather stations across Bulgaria and the ERA5-Land reanalysis product, which provides gridded hourly data from 1950 to the present. The researchers found that the two datasets show good agreement in terms of the frequency of extreme precipitation events, but not in terms of their average values. Notably, the ERA5-Land reanalysis systematically underestimates heavy precipitation events, a finding consistent with previous evaluations of ERA5 precipitation over Spain and other regions.

To detect trends, the team applied a suite of statistical methods including Sen’s slope estimator, the Mann-Kendall test, and correlation analysis. Sen’s slope estimator provides a robust nonparametric measure of trend magnitude, while the Mann-Kendall test assesses the statistical significance of monotonic trends in time series data. These methods are particularly well-suited to precipitation data, which often violate the distributional assumptions of parametric tests.

The results show that the frequency of extreme precipitation events—the number of cases per year—increases in almost all studied stations and grid cells during the period 1979–2025, according to both instrumental observations and ERA5-Land reanalysis. However, when the analysis is extended to the longer period 1950–2025 using only the reanalysis data, no trend is detected in frequency. This discrepancy suggests that the intensification of extreme precipitation is a relatively recent phenomenon, likely linked to the accelerated warming of recent decades.

The average values of daily extreme precipitation also show predominantly positive trends for the period 1979–2025 according to both datasets, and weak positive or no trends for the longer 1950–2025 period. In some stations and grid cells, these trends are statistically significant. The most pronounced increases in extreme precipitation are observed in southern and, especially, southeastern Bulgaria, a region that appears to be experiencing a disproportionate intensification of heavy rainfall.

One of the most striking findings of the study is an increase in the 21st century in the duration of extreme precipitation periods that extend beyond a single day. Multi-day extreme precipitation episodes are particularly dangerous, as they can saturate soils and overwhelm drainage infrastructure, leading to severe flooding. This finding aligns with broader research across the Balkans, where studies in Serbia, Montenegro, Bosnia and Herzegovina, Romania, and Greece have documented similar trends toward more intense and persistent heavy rainfall under a warming climate.

The physical mechanisms underlying these trends are consistent with the expectations of climate science. As the atmosphere warms, the Clausius-Clapeyron relation dictates that its water-holding capacity increases by approximately 7% per degree Celsius of warming. This means that when precipitation systems develop, they have more moisture available to convert into rainfall, increasing the likelihood of extreme daily totals. Changes in atmospheric circulation patterns over the Mediterranean and the Balkans, which Nojarov has documented in previous work, may also contribute to the observed intensification.

The spatial heterogeneity of the trends is noteworthy. While positive trends dominate across most of the country, the concentration of the most significant increases in southern and southeastern Bulgaria suggests that regional factors—including proximity to the Aegean Sea, orographic effects, and the influence of Mediterranean cyclone tracks—play a crucial role in modulating how global warming translates into local extreme precipitation. This finding has important implications for water management, flood preparedness, and infrastructure planning in the region.

The methodological rigor of the study deserves emphasis. By comparing ground-based observations with reanalysis data over the same period, the researchers were able to validate the reliability of their trend detections and identify the limitations of the reanalysis product. The finding that ERA5-Land underestimates heavy precipitation events is particularly valuable for the research community, as it cautions against relying solely on reanalysis data for studies of precipitation extremes—a caution echoed by other evaluations of ERA5 and ERA5-Land precipitation in Europe.

As Bulgaria and its neighbors continue to experience the effects of a warming climate, the findings of this study provide crucial evidence for understanding how extreme precipitation is evolving in southeastern Europe. The predominance of positive trends in both the frequency and intensity of extreme precipitation events during recent decades, combined with the emergence of longer-duration extreme precipitation periods in the 21st century, suggests that the region faces increasing risks of flooding and water-related hazards. Continued monitoring, improved high-resolution modeling, and adaptive water management strategies will be essential for building resilience in the face of these changing precipitation patterns.

Subject of Research: Extreme precipitation trends in Bulgaria in conditions of a warming climate

Article Title: Extreme precipitation trends in Bulgaria in conditions of a warming climate

Article References: Nojarov, P., Svetozarevic, J., Parvanova, D., & Belev, G. (2026). Extreme precipitation trends in Bulgaria in conditions of a warming climate. Theoretical and Applied Climatology, 157(11), Article 727. https://doi.org/10.1007/s00704-026-06655-4

Image Credits: AI Generated

DOI: 10.1007/s00704-026-06655-4

Keywords: extreme precipitation, Bulgaria, climate change, ERA5-Land, Mann-Kendall test, Sen's slope estimator, Theoretical and Applied Climatology, Bulgarian Academy of Sciences, southeastern Europe, hydrology, Extreme, precipitation