Fourth Study Conference on BALTEX Scala Cinema Gudhjem

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Long-term Changes in Cyclone Trajectories in Northern Europe
Mait Sepp 1 , Piia Post 2 and Jaak Jaagus 1
1 Department of Geography, University of Tartu, Vanemuise 46, 51014 Tartu, Estonia, maitmss@ut.ee
2 Department of Environmental Physics, University of Tartu, Tähe 4, 51014 Tartu, Estonia
1. Introduction
The Global warming has been detected during the 20 th
century (Houghton et al., 2001). The warming has been
significant also in the Northern Europe, including Estonia.
Annual mean air temperature in Tartu (Estonia) has
increased by 0.7°C during the 20 th century. The most
remarkable warming in Estonia (by 1.7°C according to
linear trend) has taken place in the second half of the
century. Statistically significant trends in surface air
temperature have been observed in winter and especially in
spring. The highest increase in monthly mean temperature
has occurred in March when mean temperature has risen up
to 5°C during 1951-2000 (Jaagus, 2004).
The climatic conditions in Northern Europe are much
warmer then in Siberia or Canada at the same latitudes. The
main source of heat in Europe during the cold season is the
Atlantic Ocean. Heat of the Atlantic is transported to the
European inland by atmospheric circulation. Westerly
airflow transports warm and moist maritime air as far to the
east as to the Urals.
There are many ways to investigate the essence and
changes of atmospheric circulation. One of the most
popular approaches is to use circulation indices as the
North Atlantic Oscillation (NAO) indices (Hurrell 1995,
Jones et al. 1997) and the Arctic Oscillation (AO) index
(Thompson, Wallace, 1998). On the other hand a number of
subjective classifications of circulation types have been
worked out. The most prominent of them are the
Grosswetterlagen system (Gerstengarbe et al. 1993), and
the classifications elaborated by Dzerdzeevskij (1968),
Vangengeim and Girs (Girs, 1971) and Lamb (1972).
Numerous studies have demonstrated that there is a strong
correlation between variability of atmospheric circulation
and air temperature in Europe, especially in Northern
Europe (Hurrell 1995, Post et al., 2002; Sepp, Jaagus,
2002). Both circulation indices and types are
generalizations of circulation processes and do not allow to
make a detailed analyses. Circulation forms join together
genetically similar (cyclonic or anticyclonic) elementary
circulation processes. But the actual position of ridges and
lows may be different over a geographical location, which
affects air temperature differently, especially in spring and
autumn. For example, depending on the trajectory of
cyclones, warm Atlantic or cold Arctic air may be
transported into Estonia. Therefore, it is important to
investigate changes in cyclonic activity and in cyclone
trajectories.
The first attempt to investigate storm tracks over Europe
was made by van Bebber in 1882. He analyzed the
movement of depressions during 1876-80 and found five
major tracks (Barry, Perry, 1973). In the Soviet Union,
Krichak (1956) investigated trajectories of lows and highs
during the period 1930 - 1954 and distinguished ten main
tracks. Gulev et al. (2001) analyzed Northern Hemisphere
winter cyclone activity using NCEP/NCAR 6-hourly
reanalysis data for the period 1958 - 1999 using software
presented in Grigoriev et al. (2000). They concluded that
the total number of winter cyclones has decreased by 12
cyclones per decade on the Northern Hemisphere in years
1948-1999. Over the Atlantic Ocean, the number of
cyclones has decreased in mid latitudes but significantly
decreased in the region of the Icelandic minimum and in the
European Arctic till the early 1970s. A significant upward
trend (14 cyclones per decade) took place over Arctic from
the late 1970s to 1990s. In the 1980s and 1990s, intense
cyclones become more frequent while the number of weak
cyclones has decreased (Gulev et al., 2001).
Two hypotheses how the warming can be caused: 1) Due to
the increase in frequency of deep and long-living cyclones,
climate conditions in Northern Europe have become more
maritime because the increased number of cyclones have
moved inland of Northern Europe; 2) There have been
changes in frequency of cyclone trajectories. Probably,
cyclones in Northern Europe prefer more northern tracks
and less the southern ones. That causes a strong advection
of mild and moist air into Northern Europe.
Our task is to track the cyclones in the Atlantic-European
sector of the Northern Hemisphere and answer to these
abovementioned questions.
2. Data and preliminary results
At our disposal is the database of Northern Hemispheric
cyclones (Gulev, et al., 2001) and software presented in
Grigoriev et al. (2000). Changes in frequency and
trajectories of cyclones in the Atlantic-European sector
(30W-45E, 35N–75N) of the Northern Hemisphere for the
period 1948 - 2000 are analyzed.
The number of cyclones for the Atlantic-European sector is
calculated (Figure 1). During years 1948 – 2000 the total
number of cyclones has decreased over this area what
coincides also with the results of Gulev et al. (2001). The
breaking point in cyclonic activity at the beginning of
1970s can easily be followed.
Number of cyclones
625
575
525
475
1948
1958
1968
1978
1988
1998
Years
Figure 1. Time series of the total number of cyclones
(with linear trend line) in Atlantic-European sector (30W -
45E, 35N-75N) for period 1948-2000.
To concentrate more to the changes in cyclone activity over
Northern Europe, cyclones in circles with radii 500, 1000,
1500 and 2000 km are counted (Figure 2). The center of
those circles was chosen at 60 N, 22.5 E. All four lines look