Acoustic Backscatter Measurements from Breaking-wave Turbulence under High Winds

Breaking surface waves generate layers of bubble clouds as air parcels entrain into the upper-ocean by the action of turbulent motions. Acoustic-backscatter and current profiles along with temperature and salinity fields, collected on the shelf of the Gulf of Alaska near Kayak Island during late December 2012, were used to examine statistics of bubble penetration and turbulence. These observations were made when the ocean was experiencing winds and significant wave heights up to 22 m s^-1 and 9 m, respectively, and the water column was weakly stratified. Mean vertical profiles of acoustic backscatter show acoustic backscatter decayed exponentially from the wave surface and the bubble penetration depths were about 3 to 30 m. Spectral analyses of acoustic data show an increase in backscattering strength between 2cph and 30cph with a peak near 3.5cph. Velocity measurements show similar spectral peaks in vertical and horizontal motions. During high winds, the turbulent velocity field near the surface was approximately three dimensional although the velocity fluctuations in the along-wind direction were the strongest. Away from the surface, the along-wind velocity component decreased with depth, but the cross-wind velocity and vertical velocity components became stronger. Near-surface turbulent diffusivity estimated using bubble depth is parameterized as a function of the cube of the wind friction velocity with a proportionality coefficient that depends weakly on wave age.