On the Vertical Structure of Wind-Driven Sea Currents

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 Ϫ1 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.

A Model for the Wind-Driven Current in the Wavy Oceanic Surface Layer: Apparent Friction Velocity Reduction and Roughness Length Enhancement

“…In this study a very simple model is developed, based on the partition of the shear stress in the surface layer between shear-related and wave-related parts, that reconciles all these results, explaining in particular the discrepancies between expected and observed values of u * and z 0 in the oceanic surface layer, purely due to the effect of non-breaking waves (unlike Kudryavtsev et al 2008). The model draws heavily on that developed by Teixeira (2012), which is inspired by Rapid Distortion Theory (RDT) calculations, and is much simpler than the one proposed by Kudryavtsev et al (2008), being essentially analytical, but produces more accurate results. It has the advantage of being formulated as a variant of Monin-Obuhov scaling, where instead of the Obukhov stability parameter, the key dimensionless parameters account for the effects of surface waves.…”

Effects of Bubbles and Sea Spray on Air–Sea Exchange in Hurricane Conditions

“…The effect of air-bubble buoyancy on turbulence is not expected to produce any significant contribution in the average current velocity profile in the wave-stirred layer (Kudryavtsev et al 2008). A moderate increase of shear due to the effect of air-bubble buoyancy is possible in the turbulence-diffusion layer located below the wave-stirred layer.…”

Dynamics of surface currents over Qingdao coastal waters in August 2008

“…According to Ye and Li [1991], the angle between wind and the surface currents should be about 30°, consistent with phases in Figure 6b. In addition, the energy transfer between the local wind and the surface currents, defined as the ratio of the surface sub‐tidal currents speed to wind speed, is about 1%–3%, and considerably agree with the continuity of the momentum flux through the air‐sea interface [ Kudryavtsev et al , 2008].…”