The sound of waves lapping the shore tends to have a calming effect. But the source of the surf's noise is by no means tranquil. As a wave breaks, dense plumes of tiny bubbles form within the surface water. It is the size distribution of these air bubbles, researchers report today in the journal Nature, that determines what a wave sounds like. Because the bubbles' proportions also affect climate-influencing processes such as gas exchange between the ocean and the air, the findings could help scientists produce better climate models.
The fate of bubbles within waves tens to hundreds of seconds after their formation is quite well understood. But Grant D. Deane and M. Dale Stokes of the Scripps Institution of Oceanography in California wanted to know what was happening in the immediate aftermath of wave breaking. Using a specially designed BubbleCam to document the laboratory tank waves and their open-ocean counterparts as they broke (see image), the scientists counted the bubbles and categorized them according to size. They determined that the distribution of small bubbles (less than one millimeter across) differed from that of large ones (greater than one millimeter across). Because the two classes do not behave the same way, the scientists posit that they are created by separate processes. When a wave curls over itself (which creates the tube favored by surfers) big bubbles form. Smaller bubbles, in contrast, arise from the splash that occurs when the wave's tip collides with the wave face. "On the surface, breaking waves seem to be very complicated," Deane says. "But underneath there is a very appealing and simple process driving this."