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

Objective

• Analyse long-term variability and trends in frequency of wind directions at Vilsandi meteorological station during different months and seasons

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)

Vilsandi 58°23’N, 21°49’E

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)

Method

• Linear regression analysis

• Significance level P<0.05

Annual windrose over Vilsandi

0

5

10

15

20N

NE

E

SE

S

SW

W

NW

Seasonal windroses

0

5

10

15

20

25N

NE

E

SE

S

SW

W

NW

Kevad

Suvi

Sügis

Talv

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)

Changes in annual windrose

0

5

10

15

20

25N

NE

E

SE

S

SW

W

NW

1966

2004

Changes in spring windrose

0,00

5,00

10,00

15,00

20,00N

NE

E

SE

S

SW

W

NW

1966

2004

Changes in summer windrose

0

5

10

15

20

25

30N

NE

E

SE

S

SW

W

NW

1966

2004

Changes in autumn windrose

0

5

10

15

20N

NE

E

SE

S

SW

W

NW

1966

2004

Changes in winter windrose

0

5

10

15

20

25N

NE

E

SE

S

SW

W

NW

1966

2004

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)

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

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