No longer on schedule, the pattern Is breaking apart: The Loss of Seasonal Synchrony in a Sub-Arctic River System Under Warming Climate

Abstract. Climate warming is altering the timing and synchrony of snow and ice processes across northern river systems, yet long-term shifts in their seasonal dynamics remain insufficiently resolved. Here, we analyze a 57-year daily record (1966–2023) from the River Oulankajoki in northeastern Finland to characterize freeze-up, break-up, snow accumulation and melt, and key atmospheric temperature transition points. Using a process-based detection tool, we identify significant advances in spring-related events, including snow melt, ice break-up, and the seasonal shift from cold to warm temperatures. In contrast, autumn transitions such as freeze-up and snow onset exhibit higher year-to-year variability and no consistent trends. The durations of cold season, ice cover, and snow melt periods have shortened, while warm and open-water seasons have lengthened. Moreover, the temporal gap between atmospheric warming and surface responses has increased in spring but contracted in autumn. These findings suggest not only a shift in seasonal timing but also a growing desynchronization between atmospheric conditions and cryo-hydrological processes, with implications for Arctic river ecology, ice forecasting, and flood risk under continued climate change.