Toward more frequent winter rain-on-snow events in the Pyrenees

Rain-on-snow (ROS) events represent a compound hydroclimatic hazard whose frequency and characteristics are being modified by climate change. This study presents a spatio-temporal analysis of winter ROS events in the Pyrenees over the period 1960–2024. Results reveal a statistically significant increase in ROS frequency (approximately 1 day/decade; p ≤ 0.05) across all analyzed elevation bands (1800, 2100, 2400, and 2700 m). Winter snow depth exhibits a statistically significant and widespread decline (approximately 0.5 to 10 cm/decade; p ≤ 0.05). Nevertheless, snowpack persistence remains sufficient to sustain ROS events at all elevations. Decreasing ROS rainfall and winter precipitation suggest that the observed increase in ROS frequency is primarily driven by rising temperatures during ROS days and an increasing fraction of precipitation falling as rain, rather than by enhanced rainfall amounts. Atmospheric circulation analysis shows that extreme ROS days (above the 95th percentile) are dominated by a limited number of circulation weather types (CWTs). In particular, CWTs 6, 10, and 11 account for up to ~70% of extreme ROS events in recent periods. These circulation patterns are characterized by strong southwesterly flow, elevated mid-tropospheric temperatures, and intensified moisture transport, leading to spatially extensive rainfall. Overall, the results indicate that the Pyrenees are currently in a transitional climatic regime in which warming enhances winter ROS frequency despite an overall decline in snowpack. These findings improve understanding of ROS processes in mid-latitude mountain regions and have important implications for hazard management.