ASSESING HISTORICAL (1926-1996) NEAR-SURFACE WIND SPEED VARIABILITY ACROSS SWEDEN

Abstract: Homogenized near-surface wind speed (NSWS) series of 7 stations from rescued observational journals have been analyzed for the 70-years time-period 1926-1996 to investigate past changes and variations across Sweden. During the whole period, there has been a statistically significant (at p < 0.05) decreasing trend of -0.11 m s-1 decades-1. In particular, there has been a steep slowdown in wind (-0.27 m s-1 decades-1) for 1945-1960, followed by a long period of stabilization until 1990. Complementary data from 1956-2013 and 1997-2019 also reveal a stilling period between 1990-2005, followed by a reversal which is in line with previous studies from Scandinavia. Summer and fall are the season which show the largest decreasing trends (- 0.11 and -0.12 m s-1 decades-1). Winter shows a greater interannual variation, and reports the same, smaller decreasing trend as spring (-0.7 m s-1 decades-1). In addition, wind speed series have been correlated with 4 different large-scale circulation, namely North Atlantic Oscillation (NAO), Artic Oscillation (AO), Scandinavian Pattern (SCA) and East Atlantic Pattern (EA). Correlation with circulation patterns has wide seasonal differences. Correlation with NAO generally shows a positive correlation, which is highest during winter (0.5); AO shows the largest positive correlation in spring and winter (0.45 and 0.5 respectively); and EA has the highest positive correlation with wind speed during spring (0.45). SCA is the only teleconnection system which displays statistically significant (p < 0.05) negative correlation, largest in summer (-0.27). A possible cause for the seasonal variation in wind speed trends is the tendency to a positive phase in NAO, AO and EA during winter and spring, which increases the westerly winds strength; during summer, when large-scale circulation has less tendency for positive phase (i.e., less pressure difference), smaller more-regional and local patterns may affect, for example, the uneven warming in the high latitudes, causing less pressure gradient and leading to decreased wind speeds.