A third generation wave model has been run with constant wind speed over a numerical coastal current It is seen that the traditional kinematical results (with no sources and sinks of energy) of current refraction are overruled by the dynamical action of the wind, except for a transition zone at the shear zone of the current.


Wave theory has been a research topic for at least a century. Since the second World War, it has been a skill to forecast ocean waves, and since oil industry started offshore, it has also been useful to hindcast them for design purposes. Wave models have reached a relatively high level of confidence But even results are good overall, lt is important to work towards better theories to be able to also model the unpredicted happenings that occur on the ocean surface now and then. There are st111 situations where validation against measurements are showing bad results.

The kinematics of monochromatic waves is well known. Kinematics of waves describing a sea state by a sum of Fourier components are similar Geometrical optics approximation can for example be applied for varying currents and depths (Mel, 1989, Jonsson, 1990) The method of characteristics or ray tracing techniques have been used by many to calculate and visualize bottom and current refraction effects (see for example M Mathlesen, 1985, I.G. Jonsson, 1990) This method can demonstrate where there are possibilities of convergence or divergence of energy, but the method applies only for swell propagation since no wind action is taken account of

This article shows results from part of a Phd work dealing with the study of effects of currents In a third generation wave model, aiming at defining these effects both qualitatively and quantitatively, and especially studying the importance of the interplay between the dynamics and the kinematics Neither currents nor winds are affected In any way by the waves In this study. The model used is the model "WAVEWATCH", made and described by Tolman (1989) as Phd work. He demonstrated the impact of tidal currents on waves In the southern North Sea as recorded by instruments. Effects of the Gulf Stream on waves have also been modelled, demonstrating different impacts on swell and on high wind waves. In the storm situation over a Gulf stream ring, refraction causes considerable variations In significant wave height and short-crestedness, but is hardly affecting the mean wave direction. The effect is specially limited In the storm situation, whereas the impact of the gulf ring is more pronounced for swell.

The value of taking account of current effects In wave models has been put In doubt, especially given the reason that modelled current fields are not yet good enough to take account of. But while waiting for better current flelds and larger and faster computers, there is a good justification In studying further the interplay of waves, currents and winds.

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