Bubbling with ideas on sound of surf

By SIMON COLLINS

A childhood fascination for the sea has led former Auckland scientist Grant Deane to new discoveries about how bursting bubbles cause the "roar" of crashing waves.

As a teenager, Dr Deane used to drive down to Baylys Beach on summer evenings from the dairy farm where he grew up at Tangowahine, east of Dargaville.

He would sit on the rocks, listen to the waves and wonder.



Now, a quarter-century later, he is a researcher at San Diego's famous Scripps Institution of Oceanography. He and a colleague, Dr Dale Stokes, have come up with what they call "the big bang theory of bubbles" to explain the number and size of bubbles in breaking waves.

Their research, published in the British journal Nature, is significant because these bubbles also have a central role in the world's carbon dioxide cycle.

They contain carbon dioxide emitted by the activities of people and animals, which is absorbed into the ocean, where it helps to feed plankton. Fish eat the plankton and start the cycle all over again.

The next step is to use the information to calculate how much carbon dioxide the sea absorbs, and this research will be done off Cape Cod in November.

Apparently, most of the sound that we call the sea pounding on the beach is caused by millions of air bubbles being created and bursting as incoming waves form, and then crush, tubes of air.

Dr Deane and his team have now spent years diving into the surf, watching from underneath as the waves break, taking photos and then painstakingly counting the bubbles and noting their sizes, using computerised equipment they invented.



They have done the same research where the sea is churned up by storms on the ocean, using an extraordinary ship called Flip which literally flips 90 degrees. The bulk of the ship fills with water to become a deep, vertical underwater tank stabilising the vessel, while the bow - the only part left above water - becomes a five-storey tower.

At sea and on the shore, waves create millions of bubbles.

"Each bubble is highly deformed. It expands and contracts very rapidly and radiates sound as it does so," Dr Deane said.

"Imagine a spherical bell - it would emit a tone. The bubbles do that.

"Each bubble has a characteristic sound and the ensemble of all of them - millions of bubbles being created and destroyed in a second or so - the cacophony of that is the roar of the breaking wave."







Dr Deane, 40, went to Dargaville High School, Auckland Grammar and Auckland University. He did his doctorate at Oxford, and has been at Scripps since 1990.

He and his ex-wife share home-schooling of their daughter Darcy, 7.

"She's learning about bubbles too."

He is still a New Zealand citizen and returns at least once a year to visit his parents, David and Fay Deane, on the Kaipara Harbour. He also does joint research with his former lecturer at Auckland University, Professor Chris Tindle.

But Scripps, with 200 scientists and 2000 support staff, offers opportunities which rule out returning permanently any time soon. And San Diego has its own attractions.

"The climate is fantastic," he said.

"As I drive into work I go over this hill. There is the ocean laid out before me and my office is right there [by the sea]."

Scripps Institution of Oceanography