Swell hindcast statistics for the Baltic Sea

Abstract. Swell dominates the global sea state and therefore significantly contributes to processes at the air and seabed interfaces. Nonetheless, smaller enclosed seas are detached from the global swell climate. We present swell statistics for the Baltic Sea using 20 years of swell partitioned model data. The swell significant wave height was mostly under 2 m, and in the winter (DJF) the mean significant swell height was typically less than 0.4 m; higher swell was found at limited nearshore areas. Swell waves were typically short (under 5 s), with mean periods over 8 s being rare. In open-sea areas the average ratio of swell energy (to total energy) was below 0.4 – significantly less than in World Ocean. Certain coastal areas were swell dominated over half the times, mostly because of weak winds (U < 5 ms−1) rather than high swell heights. Swell dominated events with a swell height over 1 m typically lasted under 10 h. A cross-correlation analysis indicates that swell in the open sea is mostly generated from local wind-sea when wind decays (dominant time lag roughly 15 h). Near the coast, however, the results suggests that the swell is partially detached from the local wind-waves, although not necessarily from the weather system that generates them.