On the Vertical Structure of Wind-Driven Sea Currents

Abstract

Abstract The vertical structure of wind-driven sea surface currents and the role of wind-wave breaking in its formation are investigated by means of both field experiments and modeling. Analysis of drifter measurements of surface currents in the uppermost 5-m layer at wind speeds from 3 to 15 m s−1 is the experimental starting point of this study. The velocity gradients beneath the surface are found to be 2 to 5 times weaker than in the “wall” boundary layer. Surface wind drift (identified via drift of an artificial slick) with respect to 0.5-m depths is about 0.7%, which is even less than the velocity defect over the molecular sublayer in the wall boundary layer at a smooth surface. To interpret the data, a semiempirical model describing the effect of wave breaking on wind-driven surface currents and subsurface turbulence is proposed. The model elaborates on the idea of direct injection of momentum and energy from wave breaking (including microscale breaking) into the water body. Momentum and energy transported by breaking waves into the water significantly enhance the turbulent mixing and considerably decrease velocity shears as compared to the wall boundary layer. No “artificial” surface roughness scale is needed in the model. From the experimental fact of the existence of cool temperature skin at the sea surface, it is deduced that there is a molecular sublayer at the water side of the sea surface with a thickness that depends on turbulence intensity just beneath the surface. The model predictions are consistent with the reported and other available experimental data.