Asymmetric shifts in seasonal transition timing in a sub-Arctic river system under climate warming

Abstract. Climate warming is altering the timing of snow and ice processes across northern river systems, yet long-term shifts in their seasonal dynamics remain insufficiently resolved. Here, we analyze a 59-year daily record (1 October 1966–30 September 2025) from the River Oulankajoki in northeastern Finland to characterize proxy-based seasonal transitions in discharge, snow depth, and air temperature. Using a rule-based detection framework, we identify earlier shifts in springtime events across snow, discharge, and temperature timings (significant for discharge and temperature). In contrast, autumn/winter timings for those variables exhibit higher year-to-year variability and no consistent trends. The Cooling Dominant Period (CDP) has shortened. Over the record the thermal cooling transition shifted later than snow onset, and the thermal warming transition earlier than snow end. This indicates that thermal proxies are shifting faster than their surface-response counterparts at this site.