W. Kendall Melville
Scripps Inst. of Oceanogr., Univ. of California, San Diego, La Jolla, CA 92093-0213
Ocean surface processes affecting high-frequency acoustics are reviewed. The generation, propagation, and attenuation of high-frequency sound at and near the ocean surface is of interest for many current problems in acoustics and its applications in oceanography. While models of surface scattering by short surface gravity waves and even shorter gravity-capillary waves can be formulated, the scatterers are not homogeneously distributed on the surface. Nonlinear interactions with longer waves and swell, and direct forcing by the wind lead to modulations of the scatterers that are poorly understood. Nonlinear effects may not be weak, with the surface slope becoming infinite in breaking waves. Breaking waves also generate sound as the entrained air breaks up into smaller bubbles that radiate as they relax towards their equilibrium shape. The bubble plumes and clouds scatter and attenuate sound but field measurements of the acoustic properties of the bubble layer near the surface vary significantly. Bubble clouds may align with coherent Langmuir circulations. The surface layers may also be regions of strong temperature gradients (with concomitant sound-speed gradients), which are broken up by thermal convection and breaking waves. Natural and man-made surface slicks can suppress the waves and thereby modify both active and passive acoustics. These and other upper ocean processes will be reviewed.