Fourth Study Conference on BALTEX Scala Cinema Gudhjem

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Trends in Wind Speed over the Gulf of Finland 1961-2000
Sirje Keevallik 1 and Tarmo Soomere 2
1 Estonian Maritime Academy, Luise 1/3, 10142 Tallinn, Estonia, sirje.keevallik@emara.ee
2 Marine Systems Institute at Tallinn Technical University, Akadeemia tee 21, 12618 Tallinn, Estonia
1. Introduction
During the recent years, much attention has been paid to
the analysis of wind properties. This attention is mostly
connected with the outlooks of the usage of wind energy.
On the other hand, wind is one of the most important
indicators of the climate change. Thus, trends in the
wind speed and/or direction may give particularly
valuable information on the processes going on in the
atmosphere.
Orviku et al. (2003) have shown that storm frequency in
Estonia has increased, especially during the 1980s and
1990s. They relate this finding to intensified westerlies
due to warming during the cold period. On the other
hand, investigations of the circulation patterns over
Estonia show that most serious changes in the air flow in
the free atmosphere during the second half of the 20 th
century have taken place in late winter and spring.
Jaagus (2003) has shown that Westerly circulation
increased in February and Northerly circulation
decreased in March. Keevallik (2003) has shown that
wind speed on the 850 and 500 hPa isobaric levels
increased and the wind vectors turned from WNW or
NW to SW or WSW.
The purpose of the present paper is to find out if these
changes in the large scale circulation influence also the
wind speed at the surface.
2. Data
The analysis is mostly based on data from routine
meteorological measurements at eight weather stations.
Utö, Hanko, and Kotka measurement sites are located at
the northern coast of the Gulf of Finland and Pakri and
Kunda at the southern coast. Among the sites, Utö is
located virtually in the Baltic Proper. Together with
Vilsandi it serves as a reference for wind properties in
the Baltic Proper. Kalbådagrund is situated in the centre
part of the Gulf of Finland.
The wind speed has been filed eight times per day at all
sites as an average over a 2-10 minutes at every three
hours with the resolution of 1 m/s.
An important change in the measurement routine at
Estonian stations was an extension of the measurement
interval from 2 minutes to 10 minutes. Namely, at the
end of the 1970s the traditional weather vanes were
replaced by anemorhumbometers. The older version of
the routine caused a certain overestimation of the
maximum wind speed in the older part of the time series,
the relatively large dispersion of wind speed in general
and a certain roughness of the distribution of wind
speed. However, no systematic overestimation of the
wind speed occurred and time series of e.g. monthly
mean wind parameters can be assumed as homogeneous.
By these reasons, we do not distinguish the older part of
the time series.
The total length of the analysed times series is 40 years
at Utö, Hanko, Inkoo, Kotka (1961-2000) and 35 years
at Pakri and Kunda (1966-2000) that generally is
considered long enough to make climatological
conclusions. The length of the recordings is 30 years
(1969-1999) at Vilsandi and 20 years (1981-2000) at
Kalbådagrund.
3. Monthly trends of the mean wind speed
Table 1 shows the calculated trend slopes at all eight
stations. We must confess that mostly the trends are not
statistically significant. However, they give an intriguing
view on the possible changes in the wind speed.
The first thing to be noticed is that wind speed seems to
have opposite trends on the northern and southern coasts
of the Gulf of Finland. It increases at the Finnish coast
and decreases at the Estonian coast. Of course, this
discrepancy can reflect certain systematic error due to
the changes in the measurement method at the Estonian
meteorological stations. But the Kalbådagrund data also
show negative trends. At least, the wind speed decrease
from August to January at this site does not coincide
with the wind speed tendencies at the northern coast.
One may also suppose that there must be a systematic
error in Pakri data that show the largest decrease during
35 years round the year. We have scrutinised the
situation at this station but we did not detect any evident
source that could produce erroneous trends. Therefore,
this question remains open.
Another important conclusion from Table 1 is that the
positive trend in the wind speed at the Finnish coast is
stronger in the east (Kotka) than in the west (Utö).
The most interesting analysis can be carried out on the
basis of Kalbådagrund data. It seems that during the cold
half-year (more precisely from August to January) the
wind speed decreased and from February to July
increased. To check this, an additional analysis was
carried out.
Niros et al. (2002) have shown that average wind speed
at Kalbådagrund is at minimum in summer and increases
rapidly from August to October. We combined the data
of October, November and December when the winds
are the strongest. The time series of three-month
averages shows a trend slope of –0.06 that is statistically
significant at least at the 0.1 level.