Abstract
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. Numerical 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 average and extreme wave heights in the Baltic Sea but almost no changes in the basinwide wave activity and wave periods. There has been a statistically significant decrease in the annual mean significant wave height by more than 10% between the islands of Öland and Gotland and in the southward 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 several 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 experienced 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.
Highlights
Studies into wind wave climate are gaining more importance in the context of global climate change
In this paper we focus on long-term changes and decadal variations in the numerically simulated wave climate in the Baltic Sea
The above discussion of the spatial patterns of changes in the overall wave activity suggests that these somewhat controversial results may be caused by an inappropriate choice of the relevant sites. For this reason we studied spatial variations in extreme wave heights based on the simulated values of the 99th percentile and the 95th percentile of significant wave height for each calendar year
Summary
Raamet: Long-term spatial variations in the Baltic Sea wave fields ignoring the ice conditions will lead to certain deviations of the resulting wave properties and statistics from the actual one. The reaction of wave fields to such changes is not obvious and it is not unexpected that the observed changes in the wave properties are quite different in different parts of the Baltic Sea. The annual mean wave heights have increased drastically at both the eastern and western coasts of the northern Baltic Proper (Broman et al, 2006; Soomere and Zaitseva, 2007; Soomere, 2008). The wave activity shows no considerable changes at the Lithuanian coasts (Kelpsaiteet al., 2008) but has decreased substantially at the south-eastern coast of the Gulf of Finland (Narva-Joesuu, Raamet et al, 2010) Another wave field characteristic of similar importance is the wave height in strong storms. Unlike Martucci et al (2010), we discuss both the magnitude and the extent of spatial variations in the linear trend in the annual mean and extreme wave heights together with their statistical significance
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