Fourth Study Conference on BALTEX Scala Cinema Gudhjem

obvious from temperature charts. However, differences
are evident for the studied rivers and the changes have
not been consistent for different time periods. For
example, a shift of ice break-up to an earlier time has not
been a typical feature for the entire period of
observations for the Daugava (Figure). The downward
trend was more expressed during the last 150 years,
especially for the last 30 years. No downward trend was
detected for the initial period, which includes Little Ice
Age. Lengths of periods are not equal and mild winters
can be followed by hard winters.
Ice break up data
140
120
100
80
60
40
20
0
y = 0,0239x + 92,796
y = -0,0895x + 115,04
1530
1579
1601
1662
1723
1743
1763
1783
1803
1823
1843
1863
1883
1903
1923
1943
1963
1983
2003
Figure. Time series of ice break-up dates in Daugava
River (dashed line shows trend from 1860-2003 and
continuous line from 1530-1859)
Similar trends of ice break-up were obtained using data
from the Lielupe, Salaca, Venta and Gauja rivers. The
applied Man-Kendall test verified that the number of
days during which a river is covered with ice has been
significantly decreasing. A downward trend was obtained
for all seven selected rivers, located in different parts of
Latvia. The trends were statistically significant (less
than –2) for the Salaca, Gauja and Bērze. Significant
trends for days with ice cover and NAO also were found
for the Daugava and Pededze rivers. The length of ice
cover on rivers was significantly negatively correlated
with NAO winter indexes (December- March) since the
middle of the 20 th century. However, there are some
periods when these correlations were insignificant (for
example, in the middle of 19 th century, and in the
transition period from 19 to 20 th century).
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4. Conclusions
The river discharge has undergone periodical fluctuations
and the obtained long-term trends for river discharges are
significant for winter season, especially during the last
30 years.
The study confirms increases of winter runoff in
proportion to the total runoff for the studied rivers.
A significant increasing temperature trend was observed
for the winter period (1.9 o C since 1795). A linear change
is pronounced, overlain with periodical oscillations.
Significant negative correlations and periodicity were
obtained between the sum of negative temperature and
NAO winter indexes. Moreover the negative correlations
between winter temperatures and NAO indexes have
become stronger during the last 100 yr. There is no
marked interconnection between temperatures in
summer, spring and NAO indexes, possible due to other
atmospheric circulation patterns.
The ice cover period in the selected rivers has been
decreasing. The reduction of the ice-covered period for
the last 30 years has been 2.8 up to 5.1 days every 10
years. The time of ice break-up depends not only on
meteorological conditions in a particular year and the
distance from Baltic Sea, but also on global climate
change. The trends are not consistent between periods
and changes of mild and hard winters are clearly seen.
The periodicity cannot be considered as a fixed cycle and
it more appears like a quasi-periodic process.
References
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