Baltic Sea climate: 200 yr of data on air temperature, sea level variation, ice cover, and atmospheric circulation

Abstract

Relevant observed time series for the Baltic Sea region from the last 2 centuries were used to investigate climate variations and trends. These time series were: Stockholm air temperature and magnitude of seasonal temperature cycle, Stockholm sea level data, Baltic Sea maximum ice cover, and circulation types based on regional air pressure data. The definition of climate was analysed by considering how each parameter varies with the time scale. We found that 90% of the variance was for time scales shorter than 15 yr, the period then used as the climate-averaging time for all studied parameters. The results indicate positive trends for air temperature, sea level, and fre- quencies of anti-cyclonic circulation and westerly wind types over the last 200 yr. Negative trends were found for the magnitude of seasonal temperature cycle, sea-ice cover, and frequency of south- westerly wind. The major climate changes of the late 19th century were probably associated with the end of the 'Little Ice Age' and characterized by an unusual high frequency of cyclonic circulation. In the 20th century, pronounced positive trends were observed in sea level variation and anti-cyclonic circulation. In the most recent studied climate period (1985-2000), air temperature and sea level climate anomalies were positive and lay outside the range of last 200 yr normal variations. The study indicates that increased frequencies of anti-cyclonic circulation and westerly winds have resulted in a slightly warmer climate with reduced seasonal amplitude and reduced ice cover. Thereby, we support the hypothesis that the long-term climate change in the Baltic Sea region is at least partly related to changes in the atmospheric circulation.