Three-dimensional measurements of air entrainment and enhanced bubble transport during wave breaking

We experimentally investigate the depth distributions and dynamics of air bubbles entrained by breaking waves in a wind-wave channel. The forcing frequency of the wavemaker and the speed of the wind above the water surface are varied independently in order to achieve a range of breaking wave conditions. Bubbles are imaged with both a high-resolution camera, which provides information on their depths, and a two-camera stereo tracking system, which provides three-dimensional bubble trajectories and velocities.

 

Below the wave troughs, we find that the bubble concentration decays exponentially with depth. Further, patches of entrained sub-surface bubbles are identified for each breaking wave, and bubble transport velocities are computed from the three-dimensional bubble data for each entrainment event. Aggregating our results, we find that the typical stream-wise transport of bubbles is faster than both the Stokes drift and modified Stokes drift for buoyant particles associated with non-breaking waves of the same slope, which is an effect not accounted for in current models of bubble transport. This enhancement in transport is attributed to the action of wave breaking and is relevant for the transport of bubbles, oil droplets, and microplastics at the ocean surface.