Fourth Study Conference on BALTEX Scala Cinema Gudhjem

shortening of the winter seasons by 16 days was calculated.
All the long-term tendencies observed in the ice periods of
Estonian large lakes are in a good accordance with the trend
of increasing mean air temperature during winter and early
spring climatic seasons, and with the trend of decreasing
spatial mean snow cover duration (Jaagus et al 2003).
Our results indicate that the timing of ice break-up dates and
duration of ice cover were not associated with zonal
circulation indecies. The impact of the atmospheric
circulation on the ice cover duration acts indirectly through
air temperature and snow cover conditions in winter. Local
meteorlogical conditions such as type of snow cover and
melting-refreezing subperiods influenced the length of the
ice cover period more powerfully than the large-scale NAO.
In the same conditions, the air temperature and warm
rainfall significantly affect the timing of snow cover, but not
too much impact on the ice break-up in lake.
In general, the ice break-up dates of Lake Võrtsjärv
statistically cannot be related to large-scale atmospheric
circulation. Some statistically significant correlations
between zonal circulation indicies and ice cover parametres
detected only for ice thickness parametre (Fig. 2).
Ice thickness in the end of
January, cm
Ice thickness in the end of
February, cm
80
70
60
50
40
30
20
10
y = -7,81x + 39,49
R 2 = 0,16
0
-1,50 -0,50 0,50 1,50
Zonal circulation index in XI-I
80
70
60
50
40
30
20
10
0
y = -14,53x + 47,71
R 2 = 0,28
-1,50 -0,50 0,50 1,50
Zonal circulation index in XI-II
Figure 2. Correlation between zonal circulation indices and
ice-cover thickness.
These results can be explained with the influence of
meridional circulation index, which in most meterological
stations in Estonia statistically not correlated with the winter
temperatures (Tomingas, 2002). Also the correlation
between the snow cover data and the meridional index was
not statistically significant.
May be in some cases it is possible to obtain a better
understanding of the dependence of ice-freeze and ice cover
break-up dates and ice seasons by using daily circulation
indices. For example, an accumulated degree-day analysis
based on the daily temperatures gives a relatively good
correlation with the ice thickness (Fig. 3) and zonal monthly
circulation indices (Fig. 4). The formation and break-up of
the ice cover is assumed to occur when in every year the
accumulated degree-day reaches a critical value (Yoo and
D`Odorico, 2002).
- 119 -
Ice thickness, cm
100
80
60
40
y = -2E-05x2 - 0,067x + 12,24
R2 20
0
= 0,47
-1600 -1200 -800 -400 0
Sum of t0 from 01.11 to 31.03
4. Conclusion
Figure 3. The dependence of ice thickness from the air
temperature conditions.
Accumulated degree-day
from 01.11 to 31.03
0
-400
-800
-1200
-1600
-2000
y = 200x - 572
R 2 = 0,35
-2,00 -1,00 0,00 1,00 2,00 3,00
Zonal circulation index in XII-II
Figure 4. Correlation between zonal circulation index
and accumulated degree-day.
References
Blenckner, T., Chen, D. 2003. Comparison of the impact
of regional and North Atlantic atmospheric circulation
on an aquatic ecosystem. Climate Research 23: 131–
36.
Jaagus, J., Truu, J., Ahas, R., Aasa, A. 2003. Spatial and
temporal variability of climatic seasons on the East
European Plain in relation to large-scale atmospheric
circulation. Climate Research 23: 111-129.
Järvet, A. (1999) Influence of ice cover on the ecological
conditions of shallow Lake Võrtsjärv. Publicationes
Instituti Geographici Univesitatis Tartuensis. Vol. 84,
pp. 92-100.
Livingstone D. 1999. Ice break-up on southern Lake
Baikal and its relationship to local and regional air
temperatures in Sibiria and the North Atlantic
Oscillation. Limnol-Oceanography 44: 1486–1497.
Magnusson, J., J., Robertson, D.M, Benson, B.J., Wynne
R.H., Livingstone, D.M., Arai, T., Assel, R.A., Barry,
R.G., Card, V., Kuusisto, E., Granin, N.G., Prowse,
T.D., Stewart, K.M. and V.S. Vuglinski. (2000)
Historical trends in lake and River Ice cover in the
Northern hemisphere. Science, 289, 1743-1746.
Tomingas, O., Relationship between atmospheric
circulation indices and climate variability in Estonia,
Boreal Env. Res., 7, 463-469, 2002
Yoo, J.-C., D`Odorico, P. 2002. Trends and fluctuations in
the dates of ice break-up of lakes and rivers in
Northern Europe: the effect of the North Atlantic
Oscillation. Journal of Hydrology 268: issues 1–4,
100–112.