Preprints
https://doi.org/10.5194/tc-2021-326
https://doi.org/10.5194/tc-2021-326

  08 Nov 2021

08 Nov 2021

Review status: this preprint is currently under review for the journal TC.

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

Stefan Norrgård1 and Samuli Helama2 Stefan Norrgård and Samuli Helama
  • 1Department of History, Åbo Akademi University, Turku, FI-20500, Finland
  • 2Natural Resources Institute Finland, Rovaniemi, FI-96200, Finland

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 21st 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 21st century occurred only five days earlier than the earliest breakup date in the 18th 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.

Stefan Norrgård and Samuli Helama

Status: open (until 03 Jan 2022)

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Stefan Norrgård and Samuli Helama

Stefan Norrgård and Samuli Helama

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Short summary
We examine changes in the dates of ice breakups in three Finnish rivers since the 1600s. Our analyses show that ice breakups nowadays occur earlier in spring than in previous centuries. The changes are pronounced in the two southern rivers, both of which experienced years without a complete ice cover in the 21st century. These events occurred during exceptionally warm winters and show that climate change affects river-ice in southern Finland differently than in the north.