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

20 N 15 10 5 0 NE W NW E SW SE S

W

Seasonal windroses

NW 25 N 20 15 10 5 0 NE E Kevad Suvi Sügis Talv SW SE S

Changes in annual windrose

30 25 20 15 10 5 N NE E SE S SW W NW Linear (S) Linear (N) Linear (E) Linear (W) Linear (NE) Linear (NW) Linear (SE) Linear (SW) 0 1966 1971 1976 1981 1986 1991 1996 2001

Changes in annual windrose W NW 25 N 20 15 10 5 0 SW S NE SE E 1966 2004

Changes in spring windrose

20,00 N 15,00 10,00 5,00 0,00 NE W NW E 1966 2004 SW SE S

Changes in summer windrose

30 25 20 N 15 10 5 0 NE W NW E 1966 2004 SW SE S

Changes in autumn windrose

20 N 15 10 5 0 NE W NW E 1966 2004 SW SE S

Changes in winter windrose

25 N 20 15 10 5 0 NE W NW E 1966 2004 SW SE S

Percentage of SE and SW winds in winter 40 35 30 25 20 15 10 5 0 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