Fourth Study Conference on BALTEX Scala Cinema Gudhjem

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Storminess on the Western Coast of Estonia in Relation to Large-scale
Atmospheric Circulation
Jaak Jaagus 1 , Piia Post 2 and Oliver Tomingas 1
1 Department of Geography, University of Tartu, Vanemuise 46, 51014 Tartu, Estonia, jjaagus@ut.ee
2 Department of Environmental Physics, University of Tartu, Tähe 4, 51014 Tartu, Estonia
1. Introduction
Storms are important atmospheric phenomena that influence
significantly human activity in the Baltic Sea area. They
affect sea transport as well as coastal management. Coastal
erosion caused by severe storms leads to destruction of
harbors and sandy beaches.
Storms in extra-tropical belt are densely related to cyclonic
activity. There is no general conclusion concerning
relationship between global warming and changes in storm
activity (WASA Group 1998; Bijl et al. 1999; Houghton et
al. 2001; Gulev et al. 2001). Nevertheless, some evidence
demonstrating increasing winter storminess in Northern
Europe since the 1960s is obtained (Alexandersson et al.
1998; Paciorek et al. 2002; Pryor and Barthelmie, 2003).
Studies on the western coast of Estonia have detected a
remarkable intensification of coastal processes caused by
climate change (Orviku et al. 2003). The number of storm
days has increased significantly in all three coastal stations
during the second half of the 20 th century.
The most severe events of coastal erosion occur in
combination of a number of circumstances: high wind speed,
high sea level, and lack of ice cover. Climate warming in
winter in the Baltic Sea region is inevitably related to higher
influence of warm air from the North Atlantic, lower
pressure, higher cyclonic activity, less snow cover and sea
ice. It leads to higher storminess and coastal damages.
The objective of this study is to analyze relationships
between storms observed at Vilsandi station and large-scale
atmospheric circulation. On the base of long-term
circulation time series, this knowledge allows reconstructing
storms and coastal erosion events back into the 19 th century.
On the other hand, it allows estimating possible future
changes in storminess induced by changes in circulation.
2. Data
Storm data are obtained from the catalogue of storms in
Vilsandi, described in Orviku et al. (2003). A storm is
defined when mean wind speed of 15 m/s or above was
measured during at least one observation time a day. The
catalogue of storms contains maximum mean wind speed,
wind directions and duration of storm during 1948-2003.
Vilsandi station (58°23´N, 21°49´E) is the westernmost
inhabited place in Estonia located on a small island near the
western coast of the Saaremaa Island with highest mean
wind speed and maximum frequency of storm days in
Estonia. The observation site is partly shaded by forest from
the eastern side and by a lighthouse from the western side.
Wind rose of storms in Vilsandi (Figure 1) demonstrates two
main directions of storm winds: southwest and northwest, in
line with results of the previous studies (Soomere, 2001;
Soomere, Keevallik, 2001).
Large-scale atmospheric circulation over the Baltic Sea and
Estonia is described in this study by circulation indices
elaborated by Tomingas (2002) and by circulation patterns
according to Post et al. (2002).
The circulation indices are calculated using daily 5 x 5
degree gridded sea-level pressure data for years 1881-1997.
Zonal circulation index is a difference between standardized
pressure anomalies at three grid cells south (52.5°N) and
north (62.5°N) of Estonia (Tomingas, 2002). Positive
values of the zonal index represent higher than normal
westerly circulation, and negative values indicate lower
than normal westerlies or even easterly airflow.
Meridional circulation index is calculated as a difference
between standardized average pressure anomalies at three
grid cells east (35°E) and west (15°E) of Estonia. Indeed,
positive values of the meridional index correspond to
southerly circulation and negative values to northerlies. In
addition, circulation indices for intermediate directions are
used. SW-NE and SE-NW circulation indices express an
intensity of airflow from southwest and southeast (positive
values), and from northeast and northwest (negative
values), correspondingly (Tomingas, 2002).
Circulation weather types for Estonia were elaborated
using the methodology worked out by Jenkinson and
Collison (1977). It was designed as an automatic version
of Lamb classification initially used for description of
atmospheric circulation in the British Isles region. The
daily circulation pattern is described using the location of
the centers of high and low pressure that determine the
direction of geostrophic airflow. Daily gridded sea level
pressure in surroundings of Estonia (with the center at
60°N, 22.5°E) is used as initial data (Post et al., 2002).
Using the geostrophic resultant flow F and vorticity Z, 27
weather types are distinguished: cyclonic (C), anticyclonic
(A), types according to the direction of airflow (N, NO, O,
SO, S, SW, W, NW), and hybrid types according to the
direction of atmospheric rotation and the direction of the
airflow, i.e. types of cyclonic (CN, CNO, CO, CSO, CS,
CSW, CW, CNW) and anticyclonic airflow (AN, ANO,
AO, ASO, AS, ASW, AW, ANW). An unclassified type is
also used (Post et al., 2002).
WNW
W
WSW
NW
SW
NNW
SSW
N
S
NNE
SSE
Figure 1. Wind rose of storms in Vilsandi.
NE
SE
ENE
E
ESE