Standing on the footbridge above the wave tank in Trondheim, Osborne is safer than any ship captain. Better yet, he can order up his own seas. “With computers around, everybody says we just gather the data and run numerical models,” he says. “But first you have to physically understand the phenomena. Something’s going on, and we don’t know what it is.”
A rogue sea is an unstable sea. Along the coast of South Africa, for instance, the powerful Agulhas Current runs headlong into wind-driven waves from Antarctica, creating highly energetic conditions that often give rise to rogues. The North Sea is even more violent. “You get these abrupt storms,” Osborne says. “I mean they are monsters. A wall of wind—wham!—smashes into the ocean and brings these waves up very quickly. You get nonlinear effects like crazy and rogues just jumping up out of nowhere.” Waves in shallow seas tend to “feel” the bottom and draw stability from it. But in areas with offshore drilling rigs, the North Sea is roughly 400 feet deep.
The secret to making rogue waves is to work fast, Osborne says. “You’ll see. I’ll start up a normal sine wave with a little bitty baby modulation. But the wave is unstable. The modulation will grow and—boom!—the rogue wave will rise right up. I could start with modest waves and by the end of the tank have them three to five times bigger. And then they go away again. The incredible thing is that even with the instability, the dynamics are exactly solvable.”Osborne and his crew begin by feeding their wave parameters into the tank’s computers. When they’re done, a steel paddle the size of a barn door slowly begins to move, sending waves down the length of the tank. At his point, Osborne says, the wave pattern is like a radio signal. “If you turn the dial, you get some frequency. That’s your sine wave, your carrier wave. The sounds that come out—the voices—are modulations of the wave’s amplitude. You’re piggybacking information on top of a wave
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If water waves were as stable as electromagnetic waves, Osborne could modulate them the same way. He could encode a message into the waves as he sent them out, and someone at the other end of the tank, equipped with some kind of water-wave receiver/transformer could decode the modulations to read the message. But deepwater waves are not stable. They grow exponentially, and as they do, their modulations grow distorted and harder to decipher. If the signals sent on radio waves did the same thing, he says, the sound would suddenly grow three to five times as loud—that’s the nonlinearities kicking in—return to the original volume, and then suddenly spike again.
“These large spikes are the rogues jumping out of a deepwater wave field,” Osborne says. “Water, the stuff we drink, is nonlinear! So in the end the exotic case is the more natural. Isn’t that pretty?”
As he talks, the water undulates down the tank in a lustrous black ribbon, then washes up on the concrete beach at the end of the pool. Osborne clambers down from the bridge and continues his commentary, almost coaching the water now. “The first wave starts to live its own life. Then it eats from the other waves.” A wave lifts. “There. That’s got to be the leading-edge effect.” Then, two-thirds of the way down the tank, a wave rises higher than the ones before or the ones behind. It has a steep face and a narrow crest. “There!”
As he speaks, the wave jumps the pool wall.
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