Visually observed wave climate in the Gulf of Riga

The basic properties of wave fields in the Gulf of Riga, the third largest semi-enclosed sub-basin of the Baltic Sea, are evaluated using recently digitized records of coastal visual wave observations from the island of Ruhnu (57°47' N, 23°15'32” E) in the central part of the gulf and from a site near the south-eastern tip of the Sõrve Peninsula site (57°54'04“ N, 22°03'28” E) for 1954-2011. The basic features of the wave climate in the Gulf of Riga are similar to those in other parts of the Baltic Sea. The long-term average wave heights are relatively low, about 0.5 m at Ruhnu and 0.35 m at Sõrve. Waves are shorter than in several other parts of the Baltic Sea: the typical periods are 3-4 s at Ruhnu and below 3 s at Sõrve. The annual course of wave heights at Ruhnu is more pronounced than in the open parts of the Baltic Sea but this course at Sõrve is relatively modest. The data sets reveal no distinct trend in the annual mean wave heights. While the Ruhnu data set exhibits extensive interannual and decadal variations in the wave height, the observations from Sõrve (that is much more sheltered against waves exited by predominant winds) show no substantial variations. The correlation between annual mean wave heights at these two sites and with data from the open Baltic Sea coast is modest. The results reflect substantial anisotropy of the wave climate of the Gulf of Riga that has relatively large wave intensity in its central and eastern regions.