Changes in surface wind directions in Estonia during 1966–2008 and their relationships with large-scale atmospheric circulation.docx

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1、J Jaagus and A. Kull: Changes in wind directions in EstoniaChanges in surface wind directions in Estonia during 1966-2008 andtheir relationships with large-scale atmospheric circulationJaak Jaagus and Ain KullInstitute of Ecology and Earth Sciences University of Tartu Vanemuise 4 51014 Tartu Estonia

2、; jaak jaagus uteeReceived 7 February 2011, accepted 9 August 2011A bstract Changes in the percentages of eight main surface wind directions at 14 meteorological stations in Estonia NortheastEurope, were studied during 1966-200& Long-term changes in wind directions are rekted to variations in the la

3、rge-scaleat mospheric circulation but partly also to changes in the surroundings of the stations and in wind obstacles Significant alterationsin wind directions were determined, and found to be the strongest in the winter season The percentages of W and S W winds haveclecir positive trend while SE,

4、E and NE winds are characterized by negative tendencies in winter In conclusion, wind directionshave probably been shifted frcm east to west Differences in trends between the stations are explained by changes in windobstacles around the stations The trends in rind roses in Estonia were caused by the

5、 intensification of the westerly circulationover the AtlanticEopcan sector dining the wi ntcr seasonKey words: wind directions climate change atmospheric circulation, windrose Estonia235INTRODUCTIONNear-surfcice wind is an importnt climate ptrameter;which has a strong influence on human activities W

6、indspeed and direction significantly affect transport -aviation, marine navigation, road tiansport, etc Wind isan important natural factor also in other kinds of humanactidt as well as in everyday life of humansW i nd spee and especially wind direction, determinethe general character of weather cond

7、itions In thetemperate latitudes N wind is usually related to colderand drier weather; and S wind to warmer weather Winddirections have different effects on winter weatherconditions in Europe. Warmer air is ming from thewestern directions from the ocean, and lder air iscoming from the eastern direct

8、ions Long-term changesin wind regi me might have a substantial effect on cli maticconditions as a wholeEstonia is located in the transitional zone between amaritime climate in the west and a continental climatein the east Substantial alternations of air masses ofdifferent origin have been observer!

9、which cause differentweather conditions in different years Atmosphericcirculation is the most i mportant cli matic factor inEstonia during the cold season Qaagus 2006). GenerallyW, S W and NW winds bring warmer aifim the NorthAtlantic; while E NE and SE winds bring colder airfrom the East European P

10、lain. During the other seasonsthe thermal influence of the wind directions is differentW hile studying climate change, main attention hasbeen paid to changes in the large-scale atmosphericcirculation They are induced by changes in the sea-level pressure (SLP) patte The most pronouncedSLP change in t

11、he Northern lie mi sphere has beenobserved in the Arctic; where mean SLP has decreasedsignificantly during the second half of the 20th century(Serreze et al. 2000). t the same time an increase inmean SLP was observed over the subtropical NorthAtlantic and southern Europe As a consequence, the airpre

12、ssure gradient has increased in mid-latitudes It hasinduced an intensification of westerlies ie the air owfrom the North Atlantic to Europe has become strongerIt is well reflected by trends in winter North AtlanticOscillation (NA 0) and Arctic Oscillation (A 0) indices(Hurrcll 1995; Huell & van Loon

13、 1997; Jones et al1997; Slonosky et aL 2001; Thompson et aL 2000;Feldstein 2002).The N0 index is closely rrelated with the otherregional circulation indices in the Baltic Sea region likeFennoscandian and Central European zonal indices(Heino et al 2008). The winter warming in the BailieSea region has

14、 been caused by the changes in large-scale atmospheric circulation (Lehmann et aL 2011).The NAO index has started to fall during the lastdecade and reached its highest, negative value in thewinter of 2009/2010, which has caused winter coolingin Europe Considering the variations in the N 0 indexand t

15、he oceanic cirulation variables prevailing of theNAO negative phases temperature decrease and colderwinters are predicted for the next few decades (Keenlysideet al. 2008; Semenov et al. 2010).Large-scale atmospheric circulation generally formsthe character of local wind directions Unfortunately,few studies have been centred on changes in thedirections of near-surface winds. Mostly much widerregions are studied, with a focus on changes in cycloneactivrit storm tracks and storminess (lexanderssonet aL 1998, 2000; Bijl et aL 1999; 0rvik et al 2003;Barring & von Storch 2004; M atulla et