Tricentennial trends in spring ice breakups in three rivers in northern Europe

Abstract. In Finland, ice breakup observations have been recorded for centuries for Aura River (1749–2020), Torne River (1693–2020) and Kokemäki River (1793–2020). The Kokemäki River is a newly revised, extended, and updated ice breakup series from Pori. The Spearman analysis shows that the correlation between Aura and Kokemäki rivers is strong, while the correlation between the two southern rivers (Aura and Kokemäki) and Torne River is weaker. The difference is attributed to the longitudinal distance between the rivers. Temperature correlations are strong for all three rivers and the long-term trends towards earlier breakups are statistically significant. Aura and Kokemäki rivers show considerable changes. Aura and Kokemäki river have had two respectively three years without a complete ice cover in the 21^st century. These are the first non-freeze events in over 270 years of recorded observations. In Torne River, however, the earliest recorded breakup date has changed only marginally the last 100 years. Moreover, the earliest recorded breakup date in the 21^st century occurred only five days earlier than the earliest breakup date in the 18^th century. Kokemäki River did not escape the hydroelectric power plant boom in the mid-1900s, and this has speeded up the breakup process. A qualitative analysis shows that exceptionally late ice breakups occurred in all three rivers in 1807, 1810 and 1867. There are noticeable clusters of late events in the early 1800s in all three series, while an exceptionally early breakup event occurred in Aura and Kokemäki rivers in 1822.