| Summary: | The hydrological regime is highly dependent on climatic conditions. In a boreal snow-driven region, such as Finland, changes in the winter and spring climatic seasonal conditions have a large effect on the hydrology. In this study, long-term changes and variability of the winter and spring season hydrological regime are investigated based on long-term ice phenology and river discharge time series in Finland. Ice cover and river discharge observations are among the oldest hydrological records in Finland and in some locations records cover 150–300 years. The analyses show statistically significant changes in both ice cover and discharge regime until the early 21st century. The ice freeze-up has become later, and the break-up is earlier, while the duration of the ice cover has shortened. In recent decades, changes have become even more apparent and the number of extremely early break-ups and late freeze-ups has increased. In ice phenology, new records have been set in the 21st century. Until 2002, there were both increasing and decreasing trends in the maximum lake ice thickness. Updated data until the winter 2017–2018 still show decreasing trends in the south, but no longer increasing trends in Central or Northern Finland. In the discharge regime, the seasonal winter and spring discharges have increased, and the spring peak has moved earlier up to 2004. The mean annual flows or annual high flows have not shown changes in general. Similar tendencies have even continued with updated data until the year 2017 as they did up to 2004. These observed seasonal winter and spring hydrological trends are in line with findings of annual and seasonal air temperature and precipitation changes and also with projected climate change scenarios until the late 21st century. In addition to long-term trend analysis, atmospheric circulation patterns and their relation to the seasonal winter and spring hydrological regimes were investigated. Both ice cover and river discharge are related to large scale atmospheric circulation patterns ...
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