Event-based wave statistics for the Baltic Sea

Abstract. Typical statistics, such as mean or percentiles, provide an excellent baseline for studying variations and changes in physical variables that have socioeconomic relevance. Nonetheless, they lack information on how often, and for how long, a certain wave height is exceeded, which might be needed for practical applications, such as planning marine operations. Using a 29-year wave hindcast, we determined the individual events where the significant wave height exceeded warning thresholds for Baltic Sea marine traffic (2.5, 4, and 7 m). During the summer months (JJA), the significant wave height exceeded 2.5 m less than twice a month. During the winter months (DJF), a significant wave height of 2.5 m was exceeded on average once a week in the larger Baltic Proper and Bothnian Sea sub-basins. Events with wave heights of over 7 m occurred roughly once every other year in the larger sub-basins. Our case study for fish-farm-related operations compared two sites that are located 10 and 30 km from the coast in the Bothnian Sea, where we determined wave events that could affect operations at the cage – meaning a significant wave height over 1 m. During the growth period of rainbow trout (May–October) there were roughly twice as many possibly disruptive events at the location further offshore than at the location closer to the coast. Even at the less-exposed location half of the wave events lasted more than 12 h, with a few events in September and October lasting around a week.