Abstract
Currents in the Baltic Sea are generally weak, but during strong winds they can grow high enough to affect the surface wave propagation and evolution. To evaluate the significance of wave-current interactions in the Baltic Sea, we conducted a study using the wave model WAM, comparing a run without surface currents to one with current forcing from a NEMO hydrodynamical model simulation. The overall changes to the wave field caused by currents were quite small. Changes of over 10 cm in significant wave height (SWH) or 1 s in the peak period (Tp) occurred only in some areas and typically less than 3% of the time. Current refraction changed the SWH annual mean by up to 2 cm, but changes up to 60 cm were seen in the maximum values. Tp had occasionally large changes due to shifts in the peak energy in two-peaked swell and wind-sea spectra. Including currents typically led to a stronger changes in swell energy compared to the changes in wind sea energy. A comparison with a wave buoy in the Gulf of Finland showed that this change in the swell energy improved the accuracy of the simulation in this narrow gulf. Current-induced refraction was most prominent near the coastal areas, where current speed occasionally exceeded 0.3 m/s. In general, SWH decreased in the coastal areas with strong currents and slightly increased in adjacent open sea areas. The current effects were most frequent in the Gulf of Finland, the Western Gotland Basin and the Åland Sea.
Highlights
Surface currents affect surface waves through many different mechanisms that induce changes in the wave frequency, wave height, and the propagation direction of the waves
The most notable differences can be seen at the Gulf of Finland (GoF) wave buoy, where the bias of the peak period changes from −0.505 s to −0.624 s
We performed two different wave model runs to see how much, where and how often sea surface currents affected the wave fields in the Baltic Sea. This was done by comparing two WAM model runs, one with and one without hourly forcing from sea surface currents for the period of 7 June 2016–1 July 2018
Summary
Surface currents affect surface waves through many different mechanisms that induce changes in the wave frequency, wave height, and the propagation direction of the waves. Varying currents cause refractionrelated changes in the propagation direction of the waves. In areas with strong currents, such as western boundary currents, the wave-current interaction has been quite evident. These areas have been a focus of many modelling and measurements studies Sugimori 1973, McKee 1977, Hayes 1980, Holthuijsen and Tolman 1991, Wang et al 1994). These strong shear currents are able to refract waves, inducing significant changes to the wave direction and the wave length and energy. According to them, including surface currents in a wave model changed the monthly mean SWH up to 0.2 m and T p up to 0.5 s (relative changes varied up to 20%)
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