Observations of wave-breaking turbulence and whitecap coverage, and the relation to wind stress and wave slope

Measurements of turbulent dissipation rate and whitecap coverage from two research cruises to Station P in the North Pacific Ocean are compared with observations of wind stress and wave frequency spectra. Both turbulent dissipation and whitecap coverage have frequently been parameterized in terms of the cubed wind friction velocity, u_*³. Such scalings fit the data at moderate wind speeds, but are less successful for wind speeds above 15 m/s, where both whitecap coverage and turbulent dissipation are less than predicted. Overall, dissipation is better described by the product of u_*² and an effective phase speed of order 1 m/s. The increase in the mean square slope of the equilibrium waves (mss) also slows above 15 m/s. Since a large portion of the whitecaps are produced by the waves of the equilibrium range, the similarity between their steepness with whitecap coverage and turbulent dissipation is consistent. The relationship between mss and whitecap coverage is further strengthened by normalizing mss by wave directional spread and equilibrium frequency bandwidth. Although both whitecap coverage and turbulent dissipation are related to the total rate of wave energy dissipation, a direct comparison of the two measurements shows significant scatter and an apparent bias with wave age. Finally, a phase-resolved view of the dissipation profiles shows the episodic nature of the breaking dissipation and the vertical advection of strong turbulence by the wave orbital motion. These are important for interpretation of the average dissipation and the vertical scaling of turbulent diffusion, respectively.