Particle movement in wind waves in deep water is on orbital paths in a vertical plane. One way of replacing the simple assumption of constant A v by a more realistic description is therefore to assume an exponential decrease of A v with depth and make the decrease dependent on wind speed. Implementation of this idea is not trivial, and we shall not pursue the details further. We only note that the effect of an exponential decrease of A v is to concentrate most of the mixing in the upper wave zone, thereby reducing the Ekman layer depth. Waves are not always the most important turbulence- generating mechanism.
Current shear tends to produce eddies.
Acting against the formation of overturning eddies is the stratification, since it is more difficult to move water up or down in the water column against a strong density gradient. It is possible to quantify the tendency for the formation of turbulence by comparing a measure for the stratification with a measure of the vertical current shear. The Richardson number Ri is a non- dimensional number which achieves this. The larger Ri, the larger the relative role of stratification and the less likely the presence of active turbulence. Inversely, the smaller Ri, the larger the relative role of current shear and the more likely the presence of turbulence.