Breaking waves and breaking bubbles
When a wave crashes on the shore, there's a lot more going on than meets the eye and now US oceanographers have made an important discovery about how bubbles form, then break, beneath the wave.
Dr Grant Deane and colleague Dale Stokes, from the Scripps Institution of Oceanography in California, have developed a model explaining the number and size of bubbles created in waves.
The size of bubbles has a major influence on the way gases move between the atmosphere and the ocean thereby affecting greenhouse gas transfer, underwater noises, and weather patterns.
"We believe we've discovered something fundamental about the physical processes occurring in waves," said Dr Deane, a New Zealander by birth.
The results appeared in a recent issue of Nature.
When waves break, they make numerous small bubbles that trap air from the atmosphere into the ocean. When those bubbles break, they produce aerosols that affect cloud and cyclone formation.
And bubble size is an important factor in whether or not the bubble breaks.
Deane and Stokes developed a unique instrument, the 'BubbleCam', to track the bubble size spectrum. They observed breaking waves in a laboratory tank full of sea water, then counted bubbles of different sizes by hand.
They found that two distinct processes are responsible for creating different-sized bubbles one process for bubbles smaller than about one millimetre, and another for bubbles larger than this.
Smaller bubbles are created by the splash of the wave's tip hitting its face, while larger bubbles are formed when the wave curls over onto itself and the tube of air collapses and fragments.
"What you end up with is a core of the bubbles created by the collapsed tube, ringed by the bubbles created by the overturning jet," said Dr Deane.
"And all this is occurring under the leading edge of the wave as it rolls up onto the beach. In fact surfers, when they go underneath the waves, would have seen the dense plumes of air that get formed once the wave has broken and passed by."
"They look a bit like thunderclouds when you look up, because they scatter the light out, and they are quite dark and ominous."
According to Dr Deane, this is the first study to come up with an explanation of the number and sizes of bubbles created in a wave.
"Larger bubbles are fragmented by turbulence in the water column as the wave breaks. Smaller bubbles are stabilised by surface tension and don't fragment, and the length scale at which this trade-off occurs is called the Hinze scale, and it's about a millimetre," he explained.
If their model proves to be accurate, it could be one more tool in the quest to predict global warming, as it could help better understand movement between greenhouse gasses and the ocean. The ocean is a major 'sink' for carbon dioxide.
Dr Deane said that bubbles larger than half a millimetre are know to be important in dissolving carbon dioxide, while smaller bubbles deal with nitrogen and oxygen.
The sound made by a breaking wave also depends on the size of the bubbles formed. Every bubble 'rings' like a little bell, and each has its own signature tone.
Millions of little bells together make the thunderous sound of the crash of the wave.