Long-term Changes of Waves at the German Baltic Sea Coast: Are There Trends from the Past?

For the development of future coastal and flood protection strategies and methods as well as the adaptation of today's existing protection structures, possible impacts of climate change on the long-term changes of the local wave conditions in the future and the past need to be analysed and assessed. In this study, two main research questions are raised: (i) Are there any long-term changes of the waves derived from numerical model data?, (ii) How well can the COSMO model reproduce the wave climate for the past? To answer those questions, two reanalysis wind data sets (COSMO-REA6 and coastDat2) for the past based on the COSMO model are used for the calculation of long-term time series of hourly wave parameters (SWAN) at the German Baltic Sea Coast. The long-term changes of the average wave conditions in the past are analysed and assessed for the coastDat2 data set for both average values and the frequency of occurrence of significant wave height and mean wave direction over time periods of 30 years. Moreover, the statistical significance of the changes of the frequency is assessed based on statistical hypothesis tests. In agreement with previous analysis of the future long-term changes of the wave climate, a large multi-(decadal), spatial and seasonal variability of the changes exist and the changes depend on the exposition of the coast towards the prevailing westerly winds. At westerly wind exposed locations, trends towards an increase of the 30 year annual averages of significant wave height up to 5 % were found, with possible larger changes of the 30 year seasonal averages, and trends towards more waves coming from westerly directions. The long-term changes of extreme wave conditions in the past are analysed based on extreme value statistics and over time periods of 30 years. Slightly increasing respectively decreasing trends between -5 % and +9 % (-0.16 m/+0.27 m) of significant wave heights with a return period of 200 years have been found at westerly respectively easterly wind exposed locations.