Waves travel astonishing distances

by Robert Krulwich on 19 Jul 2015 I'm standing on a beach and I see, a few hundred yards out, a mound of water heading right at me. It's not a wave, not yet, but a swollen patch of ocean, like the top of a moving beach ball, what sailors call a 'swell.' As it gets closer, its bottom hits the rising shore below, forcing the water up, then over, sending it tumbling onto the beach, a tongue of foam coming right up to my toes — and that's when I look down, as the wave melts into the sand and I say,

'Hi, I'm from New York. But what about you? Where are you from?'

Yes, I'm asking a wave to tell me where it was born. Can you do that? Crazily enough, you can. Waves do have birthplaces. Once upon a time, one of the world's greatest oceanographers asked this very question.

His name is Walter Munk, now in his 90s and a professor emeritus at the Scripps Institution of Oceanography in La Jolla, Calif. About 60 years ago, he was anchored off Guadalupe Island, on Mexico's west coast, watching swells come in, and, using an equation that he and others had devised to plot a wave's trajectory backward in time, he plotted the probable origins of those swells. But the answer he got was so startling, so over-the-top improbable, that he thought, 'No, there must be something wrong.'

His equations said that the swells hitting beaches in Mexico began some 9,000 miles away — somewhere in the southern reaches of the Indian Ocean, near Antarctica.

'Could it be?' he wrote in an autobiographical sketch. Could a storm halfway across the world produce a patch of moving water that traveled from near the South Pole, up past Australia, then past New Zealand, then across the vast expanse of the Pacific, arriving still intact — at a beach off Mexico?

He decided to find out for himself. That is why, in 1957, Walter Munk designed a global, real-life wave-watching experiment.

Professor Munk was not the first scientist to study swells. It was already well-established that weather moves water. When winds blow, energy from the sky gets transferred to the sea. On a quiet, sunny day, of course, the ocean is flat.

But, as I learned from Gavin Pretor-Pinney's The Wavewatcher's Companion, when breezes start to blow, 'tiny ripples dance across the surface, each no higher than a centimeter or so.' As the wind grows stronger, moving air pushes against these teeny mounds of water, making them taller, so the sea begins to rise, then fall. Energy is now passing from the sky into the water ...

As the wind stiffens, the peaks grow even taller, troughs even lower ...

The wilder the storm, the wilder the sea, with waves now crashing together, tumbling over each other, turning the sea a foamy white. These waves, says Gavin Pretor-Pinney, have 'badass written all over them.'

From forced To free

When the storm passes, you'd think the water would calm, settle and return to a quiet equilibrium, but the energy, oddly, doesn't dissipate. The storm has become a wave that now lives in a patch of sea, moving along with no need for a push from above. It is, says Pretor-Pinney, what scientists call a 'free wave,' no longer driven by wind (those are 'forced waves'). Now it is a moving bit of history, an old sea storm moving on, free to roam. It has become a 'swell.'

The astonishing thing is, you'd think it would bump into a million other waves that are coming at it from every direction; that it would pass through other storms, spreading, bumping, traveling, that all this travel would sap its momentum. But, as Walter Munk would discover, that's not what happens.

When two different swells approach each other, instead of, 'Uh oh, there's going to be a crash' ...

... 'they simply pass through each other, like friendly ghosts, before continuing on their way without having experienced any lasting interference,' writes Pretor-Pinney. 'The sea surface can look confused as the two swells cross, but they emerge on the other side, unaffected by the encounter.'

To be fair, swells will eventually lose a small bit of energy from white-capping (from air blowing against them), but can still travel largely intact across enormous distances — even distances that left Munk and his colleagues stupefied. But what they saw in 1957 is still good science today. It was also fun to do.

Walter took his wife and two daughters to Samoa, where they lived in a house built for them by a friendly island chief. Meanwhile, another member of the team went to Cape Palliser, in New Zealand, another to an uninhabited island in the South Pacific, another to Hawaii, another to a research ship up north. And his only grad student he sent (Walter says the guy 'volunteered') to a beach in Yakutat, Alaska.

There they wave-watched. Or, rather, swell-watched.

This wasn't an eyeball experiment. From a beach you can't see an old set of swells go by. They aren't that noticeable. Walter and his team had highly sensitive measuring devices that could spot swells that were very subtle, rising for a mile or two, then subsiding, with the peak being only a tenth of a millimeter high. Swells from a big storm travel in herds or groups. Long waves go faster than short waves. So when a group goes by, the fast ones come first, the shorter ones follow, getting shorter in a very characteristic way. That way you can say, 'That's our guy!' And when all six scientists reported in, Walter wrote, 'the results were spectacular.'

The swells they were tracking, when they reached Yakutat, Alaska, had indeed traveled halfway around the world. Working the data backward, Walter figured that the storm that had generated those swells had taken place two weeks earlier, in a remote patch of ocean near a bunch of snowy volcanic islands — Heard Island and the McDonald Islands, about 2,500 miles southwest of Perth, Australia.

It must have been a wild storm, with enormous waves like the ones you can see in Jan Porcellis' classic 1620 painting, Dutch Ships in a Gale ...

In a talk he gave at Scripps a couple of years ago, Walter told an audience that the southern Indian Ocean has a reputation for producing the highest waves in the world, with storms so violent that even two weeks later, when the imprint of that day had made its way across half the planet, and landed quietly on an Alaskan beach, it was still intact.

Had I been there to greet it on that day, asking my 'Hi, I'm from New York. What about you?' question, I can imagine the swell sighing, 'Ah, I was born far, far away ... '

'Tell me about it,' I hear myself saying.

And I see the wave looking at my 5-foot-11-ness, and my little body, and murmuring, 'Take it from me, you wouldn't want to have been there.'

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