Do Strong Winds Blow Waves Flat?

Seafarers have traditionally reported that under heavy seas, a rapid increase of wind force tends to reduce the height of the waves. This phenomenon is at variance with common scientifically-accepted ideas of the behavior of wind waves, since the wave height in a wind sea increases with wind speed, the rate of wave growth increases with wind stress and with sea-surface roughness, and wave breaking, which should increase during severe wind conditions, tends to increase the sea-surface roughness. However, it has been shown, for example by Banner and Phillips, that a vertically-sheared current reduces the maximum steepness a water wave can have before breaking, Since the shear in the surface current will respond more rapidly than the wave height to a sudden increase in wind speed, an increase in the intensity of wave breaking should then occur, leading to a temporary decrease in wave height until the sheared current distributes itself through the water column. The simple model described in this paper reproduces this effect, although the reduction in the wave height is predicted to be rather modest, of the order of 5 per cent. Although it is applied for deep water, the results are consistent with the laboratory observations of Douglass on the effect of on-shore and off-shore winds on nearshore wave breaking.