Historical trends in spring ice breakup for the Aura River in Southwest Finland, AD 1749–2018

Ice breakup (IBU) observations have been recorded for Aura River in Turku, southwest Finland, since the mid-18th century. The latest long-term investigation covered the period 1749–1906, but the series contains several biases. In this article, we present a fully revised and extended IBU series (1749–2018) of Aura River for climate research purposes. Instead of typical univariate IBU data (a single time series with one data point per year), this series includes three different types of IBU data. These are the initial observations of the ice starting to break up (IBU1), the river recorded as free from ice within city limits (IBU2) and, finally, the arrival of the ice from behind the Halinen dam, the so-called Halinen ice (IBU3). The IBU1 series shows negative, statistically significant long-term trends since 1749. While the other two phenological events also exhibit negative long-term trends, indicative of earlier and warmer springs towards the present day, their trends are not significant over multi-centurial scales. The IBU2 series is the only series covering the entire period, and it shows a significant negative trend towards earlier breakups (16 days) between 1919 and 2018. Consequently, breakups in March, which were extremely rare before the 1900s, have become common, while breakups in May (previously once per decade) have not occurred since 1881. These results are indicative of sub-seasonal trends in the breakups. Our results also confirm defects in the previous time series, which shows a bias of up to 16 days.