100 years of atmospheric and marine observations at the Finnish Utö Island in the Baltic Sea

Abstract. The Utö Atmospheric and Marine Research Station introduced in this paper is located on Utö Island (59∘46.84′ N, 21∘22.13′ E) at the outer edge of the Archipelago Sea, by the Baltic Sea towards the Baltic Proper. Meteorological observations at the island started in 1881 and vertical profiling of seawater temperature and salinity in 1900. Since 1980, the number of observations at Utö has rapidly increased, with a large number of new meteorological, air quality, aerosol, optical and greenhouse gas parameters, and recently, a variety of marine observations. In this study, we analyze long-term changes of atmospheric temperature, cloudiness, sea salinity, temperature and ice cover. Our main dataset consists of 248 367 atmospheric temperature observations, 1632 quality-assured vertical seawater temperature and salinity profiles and 8565 ice maps, partly digitized for this project. We also use North Atlantic Oscillation (NAO), major Baltic inflow (MBI) and Baltic Sea river runoff data from the literature as reference variables to our data. Our analysis is based on a statistical method utilizing a dynamic linear model. The results show an increase in the atmospheric temperature at Utö, but the increase is significantly smaller than on land areas and has taken place only since the early 1980s, with a rate of 0.4 ∘C decade−1 during the last 35 years. We also see an increase in seawater temperatures, especially on the surface, with an increase of 0.3 ∘C decade−1 for the last 100 years. In deeper water layers, the increase is smaller and influenced by vertical mixing, which is modulated by inflow of saline water from the North Sea and freshwater inflow from rivers and by wind-driven processes influenced by the local bathymetry. The date when air temperature in the spring exceeds +5 ∘C became 5 days earlier from the period 1951–1980 to the period 1981–2010 and the date when sea surface water temperature exceeds +4 ∘C changed to 9 days earlier. Sea ice cover duration at Utö shows a decrease of approximately 50 % during the last 35 years. Based on the combined results, it is possible that the climate at Utö has changed into a new phase, in which the sea ice no longer reduces the local temperature increase caused by the global warming.