Shortening of the thermal winter in Spain (1953-2022): changes in onset, end, and duration

Global climate change is increasingly driving the alteration of seasonal cycles, leading to significant shifts in the timing and duration of thermal periods across the planet. This study investigates the long-term trends of the thermal winter in Spain, including the Balearic Islands and the autonomous cities of Ceuta and Melilla, over the period 1953-2022. The analysis was based on daily maximum and minimum temperature data from a dense network of meteorological stations. Thermal winter was defined using an observatory-specific threshold corresponding to the 25th percentile of the 1961-1990 climatological annual cycle. Trends in thermal winter onset, end, and total duration were evaluated using Sen’s slope and the modified Mann–Kendall test. The results reveal a generalized and significant contraction of the thermal winter season across the study area. Regionally, thermal winter onset has experienced a mean delay of approximately 11.0 days (+1.6 days/decade), while the end of thermal winter has advanced by 14.9 days (-2.15 days/decade). Consequently, the total duration of thermal winter has shortened by an average of 25-26 days throughout the 70-year period, with a significant decadal trend of -3.69 days. Spatially, these trends are most pronounced and statistically robust in the Mediterranean facade and the northeastern regions. Notably, the most recent decade (2013-2022) recorded the shortest thermal winters in the entire series, with a mean duration of 50.2 days compared to 87.8 days in the 1963-1972 period. In conclusion, the thermal winter in Spain is undergoing a drastic reduction, driven primarily by a robust advance in the transition to spring conditions. These findings provide clear evidence of the accelerating impact of climate change on seasonal dynamics in the Mediterranean region.