Most surfers have experienced the inside of a wave--along with either the ecstasy of riding through the tube, or the agony of being spun headlong under the brine. But neither these wave motion experts nor their more classically trained counterparts know exactly what happens as a wave turns white and breaks. Now, physicists have described for the first time how bubbles form and fragment during this crucial moment. Although the finding won't keep surfers on their boards, it is an essential step in understanding how the ocean absorbs greenhouse gases.
To study the guts of a wave, two land-lubbing physicists--Grant Deane and Dale Stokes of the Scripps Institution of Oceanography in La Jolla, California--videotaped breakers in a laboratory flume and in the open ocean. Their findings, published in the 22 August issue of Nature, offer a narrative of a crashing wave. As the crest of a wave curls to meet the wave face, it captures an air pocket, creating a tube of large bubbles. When the watery peak plunges into the wave, it creates a frothy jet that spins around the air-filled cavity then collapses into a cloud of smaller bubbles.
Large and small bubbles follow distinct life cycles, Deane and Stokes found. Bubbles with a radius bigger than 1 millimeter are quickly broken up by turbulence, while smaller bubbles remain stable due to surface tension. As it turns out, bubble size is critical to improving models that describe how the atmosphere and the ocean exchange gases; large bubbles generally determine carbon dioxide absorption and small ones release aerosols that lead to cloud formation. So knowing how bubbles are born and grow could help scientists determine a wealth of climate information simply by measuring wind speed, Deane says.
In addition to charting the life history of wave bubbles, Deane and Stokes also calculated the probability of finding certain sizes of bubbles--a poorly understood feature of waves. "This paper is certainly a significant contribution," says marine scientist Edward Monahan of the University of Connecticut at Avery Point in Groton.