Changes in the Timing of the Thermal Spring Season Across Finland and Its Turning Point Over the Past Six Decades.

Thermal season variations contribute to shaping natural hydrological processes in Nordic regions. Although changes in seasonal hydro-climatological factors due to global warming at both global and regional scales have been widely studied, there remains a limited understanding of the timing characteristics of these seasonal shifts. Given the critical role of the annual temperature transition from the cold to the warm phase in controlling hydrological events in Nordic regions, this research focuses on the temporal variation of the thermal spring season across Finland. We investigate how the timing of the thermal spring has changed over the past six decades across Finland and how the changes vary spatially. We also identify temporal turning points in these transitions.

This research utilizes high-resolution (1x1 km) daily mean temperature data over Finland, spanning past six decades, publicly provided by the Finnish Meteorological Institute. Several indices were calculated based on a fixed thermal threshold to track both spatial and temporal variations in the thermal spring season, and to identify possible trends and correlations using various statistical methods. Temporal changes in the indices were analyzed using Mann-Kendall test, while the Theil-Sen estimator was applied to determine the slope of the observed trends. To mitigate the influences of potential autocorrelation in the dataset, the Trend-Free Pre-Whitening (TFPW) method was employed. For spatial analysis, Empirical Orthogonal Function (EOF) decomposition was used to identify dominant spatial pattens. To separate significant physical signals from noise in the estimated spatial patterns, the North significance test was used. Furthermore, the Pettitt test was applied to assess the turning points in spatial behavior of timing indices. By analyzing an extensive dataset covering Finland, coupled with a long data period, this research provides valuable insights into temporal shifts in the thermal spring season and their potential connections to other hydro-climatological factors.