Spatio‐Temporal Variability and Trends in Frost‐Free Season Parameters Across Finland: Associations With Large‐Scale Atmospheric Circulation Patterns

ABSTRACT Climate warming substantially influences surface air temperature (SAT)‐related seasonal characteristics across high‐latitude regions, including the timing and duration of frost‐free conditions. However, comprehensive assessments of long‐term variability and trends in frost‐free season (FFS) parameters across Finland and their relationships with large‐scale atmospheric circulation patterns (ACPs) remain relatively limited. This study investigated the historical variability and trends in FFS start (FFSS), end (FFSE) and length (FFSL) across Finland during 1961–2025 using gridded (10 × 10 km 2 ) daily minimum SAT ( T min ) datasets. Relationships between FFS parameters and seven major ACPs were also evaluated using Spearman's rank correlation analysis. On the national scale, FFSL exhibited a statistically significant ( p < 0.05) increasing trend of 4.2 days decade −1 during 1961–2025, because of earlier FFSS (−1.8 days decade −1 ) and later FFSE (2.5 days decade −1 ). Considerable spatial variability was also observed across Finland, with substantially longer FFS conditions occurring in southern and southwestern regions and shorter FFS dominating northern Lapland. The strongest positive FFSL trends were mainly detected in southern and central Finland, primarily driven by negative and positive trends in FFSS and FFSE, respectively. The East Atlantic/West Russia (EA/WR) and East Atlantic (EA) patterns showed the strongest associations with FFS parameters across most parts of Finland in the original annual series, although the national‐scale relationships generally weakened after linear detrending. Significant ACP–FFS correlations were generally strongest in southern and central Finland, whereas northern regions exhibited weaker and more spatially heterogeneous relationships. Overall, the findings indicate substantial long‐term changes in FFS parameters across Finland associated with large‐scale atmospheric circulation variability and ongoing climatic warming, with potential implications for agriculture, ecosystems, hydrology and other climate‐sensitive environmental systems in the boreal environment of northern Europe.