Near-Surface Turbulence in the Presence of Breaking Waves

Abstract

Observations with a three-axis pulse-to-pulse coherent acoustic Doppler profiler and acoustic resonators reveal the turbulence and bubble field beneath breaking waves in the open ocean at wind speeds up to 14 m s?1. About 55%?80% of velocity wavenumber spectra, calculated with Hilbert spectral analysis based on empirical mode decomposition, are consistent with an inertial subrange. Time series of turbulent kinetic energy dissipation at approximately 1 m beneath the free surface and 1-Hz sampling rate are obtained. High turbulence levels with dissipation rates more than four orders larger than the background dissipation are linked to wave breaking. Initial dissipation levels beneath breaking waves yield the Hinze scale of the maximum bubble size aH ? 2 ? 10?3 m. Turbulence induced by discrete breaking events was observed to decay as ε ? tn, where n = ?4.3 is close to the theoretical value for isotropic turbulence (?17/4). In the crest region above the mean waterline, dissipation increases as ε(z) ? z2.3. Depth-integrated dissipation in the crest region is more than 2 times the depth-integrated dissipation in the trough region. Adjusting the surface definition in common turbulence models to reflect the observed dissipation profile improves the agreement between modeled and observed dissipation. There is some evidence that turbulent dissipation increases above the background level prior to the air entrainment. The magnitude and occurrence of the prebreaking turbulence are consistent with wave?turbulence interaction in a rotational wave field.