Fourth Study Conference on BALTEX Scala Cinema Gudhjem

- 118 -
Changes in Lake Võrtsjärv Ice Regime During the Second Half of the 20 th
Century Characterized by Monthly Zonal Circulation Index
Arvo Järvet
Department of Geography, University of Tartu, Vanemuise 46, 51014 Tartu, Estonia, ajarvet@ut.ee
1. Introduction
Ice cover is a complex climatic phenomena that is an even
more important factor characterizing ecological conditions
of lakes in wintertime. Winter is the period with the highest
variation and change in the Northern Hemisphere and has
therefor a potentially high impact on the lake ecosystem.
Shift towards earlier spring events in lake ice cover is
observed globally in the Northern hemisphere lakes
(Magnusson et al. 2000). On the global scale, the lake ice
parameters such as thickness, freeze-up and break-up dates,
ice cover duration are good indicators of regional climate
change in high-latitude regions. Earlier findings suggested a
substantial impact of the NAO on winter ice cover
(Livingstone 1999). Significant temporal variability of
climate seasons in winter period in Estonia was detected. In
particular, trends and fluctuations are analysed in detail,
searching for possible connections with the North Atlantic
Oscillation (NAO).
The use of ice regime of lake as climatic indicators are more
useful than those for rivers because a) human activity on
lakes is comparatively neglible, and b) the hydrodynamic
conditions inluence much more ice formation in rivers as in
lakes. The study of dates and duration of ice cover in lakes
provides an interesting assessment of the impact of
interannual ice climate variability on the regime of seasonal
temperature. In particular, the interannual variability of latewinter
and early-spring temperature in Europe shows
association with the phases of the NAO. The positive phases
of NAO are associated with warmer and rainy late-winters,
and early-spring in Estonia. The circulation indices for
Estonia calculated by O. Tomingas (2003) were used as a
measure for regional-scale circulation. The zonal circulation
index are calculated using daily 5x5 degree gridded sealevel
pressure data. It is difference between standardized
average pressure anomalies at three stations south (52.5°N)
and north (62.5°N) of Estonia. Positive values of the zonal
index represent higher than normal westerly circulation, and
negative values indicate lower than normal westerlies or
even easterly airflow.
In this study we compared the impact of local climate
variability by regional circulation indices to examine
atmospheric circulation influence on a Lake Võrtsjärv ice
regime. Lake Võrtsjärv is a naturally eutrophic lake in
southern Estonia at 34 m above sea level with a surface area
of 270 km 2 , a maximum depth of 6 m, a mean depth of 2.8
m and a turnover time of 1 year. The lake is always icecovered
in winter and the ice break-up, registered since
1924, occurs in general between mid of March and the mid
of April. Shallowness of the lake causes the winter ice cover
great importance for its ecosystem. The ice thickness affects
the active water volume and the ice period duration affects
the annual cycle of primary production, especially in
shallow lakes (Järvet 1999).
2. Data
The observation data from the Estonian Meteorological and
Hydrological Institute (EMHI) of ice regime in the 78-year
period of 1923–2000 were used. The observation
programme included information about the date of the first
freezing, the formation and the end of the permanent ice
cover, the final disappearance of the ice and the thickness
of the ice cover. Ice thickness is measured 100 m from
coast in stations after every fifth day. The collected data
materials were published in hydrological yearbooks and
preserved to the present day in the archive of EMHI. The
circulation indices related to ice cover characteristics
analyzed. Regression analysis was employed to establish
relationships between any variables of interest.
3. Results and discussion
The ice thickness typically increases until late February –
early March, and maximum mean thickness is 53±13 cm
in an average, ranging 50-60 cm in normal winters and
over 80 cm even in the second half of March in a harsh
winter (Fig. 1). The thickest ice cover in Lake Võrtsjärv –
98 cm – was measured in 1942 (Table 1).
Table 1. Statistics of temporal variability for start date of
ice periods Lake Võrtsjärv by linear trend line in 1924–
2000.
Statistics Beginning
of ice
phenomena
Beginning
of ice
cover
End of End of ice
ice cover phenomena
Mean 14. nov. 28. nov. 07. apr. 20. apr.
St. Dev 13,9 14,7 14,0 11,8
Earliest 15. oct. 03. nov. 14.feb 21.march
Year 1976 1940 1989 1990
Latest 24. dec. 03. jan. 03. may 12. may
Year 1929 1933 1955 1956
Slope -0,158 -0,109 -0,058 0,063
in 1924 20. nov. 02. dec 09. apr. 17. apr.
in 2000 08. nov. 24. nov. 05. apr. 21. apr.
Change -12 -8 -4 5
p-value 0,029 0,159 0,442 0,323
Ice thickness, cm
100
90
80
70
60
50
40
30
20
10
0
1925 1935 1945 1955 1965 1975 1985 1995
Figure 1. Long-period change of annual maximum ice
thickness in 1925–2000.
Results of linear regression analysis of long-term ice
observation series indicate the presence of some long-term
changes in the beginning dates of ice periods. The most
important of them is the shortening of ice cover period and
the shifting earlier of the ice break-up date in spring. The