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Long-term changes in frequencies of wind directions on the western coast
of Estonia
Jaak Jaagus
Institute of Geography, University of Tartu
Second International ASTRA Conference
Gdansk, 27 October 2006
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Objective
• Analyse long-term variability and trends in frequency of wind directions at Vilsandi meteorological station during different months and seasons
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Hypotheses
• Climate warming should be related to changes in frequencies of wind directions
• Increasing trend in NAO index in winter should increase a percentage of westerly and southwesterly winds
• Increase in frequency of wind directions bringing milder weather in winter (W, SW, NW), decrease in frequency of wind from the “cold” directions (N, NE, E, SE)
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Vilsandi 58°23’N, 21°49’E
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Data
• Ordinary observations 8 times per day• Period 1966-2005• Monthly, seasonal and annual frequencies of N,
NE, E, SE, S, SW, W and NW winds• Seasons: spring (MAM), summer (JJA), autumn
(SON), winter (DJF)• Weathercock (1966-1976 Oct)• Anemorhumbometer (1976 Nov – 2003 Aug)• Automatic weather station (since September 2003)
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Method
• Linear regression analysis
• Significance level P<0.05
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Annual windrose over Vilsandi
0
5
10
15
20N
NE
E
SE
S
SW
W
NW
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Seasonal windroses
0
5
10
15
20
25N
NE
E
SE
S
SW
W
NW
Kevad
Suvi
Sügis
Talv
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Changes in annual windrose
0
5
10
15
20
25
30
1966 1971 1976 1981 1986 1991 1996 2001
pe
r c
en
t
N
NE
E
SE
S
SW
W
NW
Linear (S)
Linear (N)
Linear (E)
Linear (W)
Linear (NE)
Linear (NW)
Linear (SE)
Linear (SW)
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Changes in annual windrose
0
5
10
15
20
25N
NE
E
SE
S
SW
W
NW
1966
2004
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Changes in spring windrose
0,00
5,00
10,00
15,00
20,00N
NE
E
SE
S
SW
W
NW
1966
2004
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Changes in summer windrose
0
5
10
15
20
25
30N
NE
E
SE
S
SW
W
NW
1966
2004
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Changes in autumn windrose
0
5
10
15
20N
NE
E
SE
S
SW
W
NW
1966
2004
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Changes in winter windrose
0
5
10
15
20
25N
NE
E
SE
S
SW
W
NW
1966
2004
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Percentage of SE and SW winds in winter
0
5
10
15
20
25
30
35
40
1967 1972 1977 1982 1987 1992 1997 2002
SE winds
SW winds
Linear (SW winds)
Linear (SE winds)
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Changes in frequency of monthly wind direction
• N winds: increase in March• NE winds: decrease in May and June• E winds: decrease in January• SE winds: decrease in January and February,
increase in August• SW winds: increase in January and February• W winds: increase in January and May• NW winds: increase in January, decrease in
September
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Conclusions
• During 1966-2005, a significant increase in annual percentage of SW winds have occurred
• Frequencies of NE, E and SE winds have slightly decreased
• Changes in windrose in spring, summer and autumn are mostly insignificant, however a shift to W in spring, to S in summer and to SE in autumn is evident
• Windroses in winter are changed dramatically, turning mostly from SE and E to SW and W