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

A1 Journal article (refereed)

Internal Authors/Editors

Publication Details

List of Authors: Norrgård Stefan, Helama, Samuli
Publication year: 2019
Journal: Holocene
Start page: 1
End page: 11


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.

Last updated on 2019-27-05 at 04:11