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Character of ice regime in Latvian rivers
49
NATURE RESEARCH
Agrita Briede, Māris Klaviņš, Valērijs Rodinovs, and Lita Lizuma
Abstract
Different aspects of the ice regime of rivers in Latvia were studied. For all rivers in Latvia
the ice cover periods have been shorter during the last decades. However, the long records of
collected ice data for the Daugava River (started from 1529) indicate that oscillation around
mean values is normal. Climate during the cold season, and thus also the ice regime, is very
much influenced by oscillation patterns of the North Atlantic.
Key words: ice regime of rivers, long-term changes, NAO indexes
Introduction
There is a need to apply a multitude of different factors to the evaluation of climate
change and the patterns of processes in the environment. Records of ice freeze-up and break-up
times in rivers allow for assessment of long-term and seasonal variability of the climate,
especially in regard to climate change [Livingstone 1997; Magnuson et al. 2000). There are
three major reasons why ice regime studies are important: (a) since calendar dates of freezing
and thawing of lakes and rivers have been recorded in many rivers (also in Latvia) well before
scientific observations in Latvia began, the data covers a longer period than that of other
hydrological factors; (b) the ice regime influence the hydrological regime during the period of
maximal discharge of accumulated atmospheric precipitation; (c) ice conditions are sensitive
and reliable indicators of climate.
Winter ice regimes have been observed to be related to the North Atlantic Oscillation
(NAO) pattern [Hurell 1995; Osborn et al. 1999] of large-scale anomalies in the North Atlantic
atmospheric circulation. Moreover, the Southern Oscillation has been proven to also be able to
influence the ice regime of lakes and rivers in the Northern Hemisphere [Robertson et al.2000].
The so-called positive phases of NAO (associated with strong westerly winds and increased
flow of warm and moist air to Western Europe) cause warmer and later winters, and early
springs [Hurrell 1995; Paeth et al. 1999; Chen and Hellström 1999]. The air temperature regime
and the occurrence of warm rainfall, influenced by airflows from the North Atlantic (indicated
by NAO), significantly affect the ice regime [Loewe and Koslowski 1998], and hence also the
river discharge pattern [Hurrell and van Loon 1997]. Furthermore, global warming processes
may constitute another major factor influencing the ice regime [Ruosteenoja 1986; Singh et al.
2000]. Break-up dates for the last two centuries on rivers in the Northern Hemisphere provide
consistent evidence of later freezing and earlier break-up [Magnuson et al. 2000]. Since easily
identifiable parameters describing ice break-up have been recorded for a long period of time
[Beltaos 1997; Benson et al. 2000; Likens 2000; Arai 2000] several studies have analyzed ice
regime trends for inland waters. These studies have clearly shown long-term climate changes, as
well as the dependence of natural processes and the ice regimes of surface waters in Northern
Europe on the NAO [Yoo and D’Odorico 2002]. The ice regime of the Baltic Sea has been
previously analyzed using a historical time series of ice break-up at the port of Riga [Jevrejeva
2001] in order to reconstruct climatic history [Tarand and Nordli 2001].
It is the aim of this study to analyse the different aspects of the ice regime in Latvia, in
order to put the recent shortening of the ice cover into a long-term perspective.
Sources and methods
The first time series of ice break-up data for the River Daugava was published by P.
Stakle (1931), which also has been used for this study. Data on temperature records for the
period 1795 to 2002 was obtained from the Meteorological Station at the University of Latvia in
Riga. Before analyses the standard homogeneity test was applied on the data set [Lizuma and
Briede 2003].