Long-term trends in near-surface flow over the Baltic

We report an analysis of trends in 850 hPa wind speed, as manifest in the NCEP-NCAR reanalysis fields, over the Baltic region during the latter half of the 20th century. The results indicate that annual mean wind speeds over the Baltic significantly increased over the period 1953-99 with the majority of the increase being associated with increases in the upper quartile of the wind speed distribution. Accordingly, much of the change is focused on the winter season. The trends in annual and seasonal mean wind speeds are greatest in relative and absolute sense in the southwest of the Baltic basin, where they are in excess of 0.25 m s(-1) per decade for the annual mean. The extremes of the wind-speed distribution also increased by up to 5 m s(-1) over the study period for the wind speed with a 50 year return period, again with the largest magnitude changes in the southwestern Baltic. These changes in wind speed are strongly linked to changes in the synoptic-scale circulation. The majority of the increase in wintertime wind speeds is attributable to an increase in westerly anticyclonic, westerly cyclonic and northwesterly cyclonic circulation types as manifest in the Grosswetterlagen catalogue, which are in turn related to the recent prevalence of the positive phase of the North Atlantic oscillation. Copyright (C) 2003 Royal Meteorological Society.