Ocean Science Long-term spatial variations in the Baltic Sea wave fields

Abstract. This study focuses on spatial patterns in linear trends of numerically reconstructed basic wave properties (average and extreme wave heights, wave periods) in the Baltic Sea under the assumption of no ice cover. Numeri-cal simulations of wave conditions for 1970–2007, using the WAM wave model and adjusted geostrophic winds, revealed extensive spatial variations in long-term changes in both av-erage and extreme wave heights in the Baltic Sea but almost no changes in the basinwide wave activity and wave peri-ods. There has been a statistically significant decrease in the annual mean significant wave height by more than 10 % be-tween the islands of ¨Oland and Gotland and in the south-ward sea area, and a substantial increase to the south-west of Bornholm, near the coast of Latvia, between the Åland Archipelago and the Swedish mainland, and between the Bothnian Sea and the Bothnian Bay. Variations in extreme wave heights (defined as the threshold for 1 % of the highest waves each year) show similar patterns of changes. In sev-eral areas the trends in average and extreme wave heights are different. Such a complicated pattern of changes indicates that (i) different regions of the Baltic Sea basin have expe-rienced widespread but essentially different changes in wind properties and (ii) many seemingly controversial trends and variations established in wave properties at different sites in the recent past may reflect the natural spatial variability in the Baltic Sea wave fields.