2002-08-25 04:59:04 -- http://www3.cosmiverse.com/newsimages/ocean_waves2.jpg

The sounds of the sea are one of the enduring great phenomena of nature. The crashing of the waves as they break at the shore or tumble down and self destruct on the way in are beloved by children and adults alike, and for many signal a return to memories of days in the sun and togetherness.

It has, until now, also remained largely a mystery as to how those sounds are produced. New research promises to shed valuable insight into just how the symphony of the waves is composed and executed.

Grant Deane and Dale Stokes of the Scripps Institution of Oceanography in La Jolla, California, looked at thousands of images of breaking waves both in a laboratory tank and off the coast of California. Counting bubble sizes by hand, the duo determined how many bubbles there are of different sizes.

In the August 22 issue of the journal Nature, the pair provide unprecedented insight into the characteristics and dynamics of bubbles inside breaking waves. The researchers used acoustical and optical observations, including data from a high-tech "BubbleCam," to develop a new depiction of bubble sizes and creation processes.

There is one law for bubble sizes up to about one millimeter across, and another for bubbles larger than this, the researchers found. Both distributions follow what are known as power laws -- larger bubbles are rarer than smaller ones. Power laws are common in nature, for example in the distribution of earthquake and landslide sizes.

But the rate at which big bubbles become rarer differs for the two size classes. There must therefore be two bubble-making processes, the researchers reasoned.

Big bubbles, they say, are produced when the wave tip-the jet-curls in front of the wave's face, creating the tube beloved of surfers. This cavity fragments into bubbles that range from about a millimeter to a centimeter in size.

Smaller bubbles are produced when the jet hits the wave face and creates a splash that forces air into the water. Smaller bubbles, say Deane and Stokes, produce a higher-pitched noise than larger ones.

Gas from the bubbles dissolves out and into the sea. Plant growth in the ocean is dependant on dissolved carbon dioxide, and therefore, the size of the bubble ultimately determines the ocean's ability to soak up this critical greenhouse gas from the atmosphere.

When bubbles burst at the surface, they throw water droplets into the air, which eventually find their way into clouds. Understanding breaking waves could therefore help to refine climate models.

All waves are not created equal. Waves come in two types: 'plunging' breakers turn over at their tip, whereas the crest of a 'spilling' wave runs down its face like an avalanche. The two types sound rather different, Steven Means and Richard Heitmeyer of the Naval Research Laboratory in Washington DC, have found, after monitoring the surf on the coast of North Carolina.

Plungers produce a crash with a sharp onset; whereas spillers generate a lower-pitched noise that begins more gradually and lasts longer, say the pair. In other words, you can tell by listening not just whether the surf is up, but also what it looks like-all without having to look at it at all.

Source: Scripps Institution of Oceanography; Naval Research Lab; Nature