Endeavors magazine: The flow below

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The flow below

by Margarite Nathe

Our planet depends on the waves we can’t see.

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Think about this next time you strap on your scuba gear: even if you dive deep enough to get away from the swells crashing on the surface, you won’t escape the waves completely. In fact, says marine scientist Alberto Scotti, you might get caught up in internal (or underwater) waves that sweep you back and forth, toward and then away from shore.

The ocean has two layers, Scotti says. The bottom layer is normally colder, heavier, and denser than the top is, and the two meet at what Scotti calls the interface. This means that there are actually two types of waves sloshing around out there—surface waves and internal waves.

And while the waves we see on the surface are wind-fed, internal waves get their energy from the tide pushing past obstacles such as undersea mountain ranges. Imagine river water rushing around rocks to create rapids, only underwater and on a grander scale.

Internal waves are a crucial link in the conveyor belt of global ocean circulation, Scotti says. So a change in the ocean’s internal waves could disturb all the earth’s climate. In short, they’re a pretty big deal.

Scotti develops computer models to figure out just how these underwater waves work. “We model how they’re generated and eventually ‘crash’ as the bottom shoals near the coast,” he says. The crashing creates powerful currents near the seafloor, and plays a big part in how the ocean mixes itself.

For example, the entire oceanic food chain is dependent on how different underwater currents bring nutrients up through the water. “In the ocean, everything valuable—meaning food, including nitrogen, phosphate, iron—has the despicable tendency to sink to the bottom,” Scotti says. “You need a physical mechanism, such as internal waves, to re-suspend it.” And since the ocean mixes itself more in some places than others, some parts of the ocean are jumping with life while others are basically underwater deserts.

“Location, location, location,” Scotti says.

A sweet ride

The word “plankton” comes from a Greek word that means wanderer, or drifter. It shouldn’t surprise us, then, if these microscopic critters actually use internal waves the way a landlubberly rambler might use a railroad car—they catch a ride.

While plankton can swim well vertically, they can only travel horizontally at the pace of a few inches per second, Scotti says. But he thinks that by surfing on internal waves, they could skim along at up to three or four miles an hour (which is a heck of a lot faster). And so far, Scotti says, all the evidence from his work with internal waves seems to point toward his surfer-plankton hypothesis.

And because plankton is a major food source for all the bigger beasts in the ocean, their movements are vital for every life form that has a stake in the ocean (including those of us who occasionally enjoy a nice seafood dinner).

Scotti’s computer models show, though, that as the oceans warm, internal waves may shrink and break farther off the coast, making it difficult for the plankton to travel and spread out. This means that fish and other organisms that live close to shore won’t be getting their own dinner, and may start to disappear.

Radar images and satellite photos taken from space reveal the surface expression of large-amplitude internal waves, which appear as relatively benign alternating bands of churning and smooth water running along the surface—what Scotti calls a slick.

But slicks are actually pretty strong. While working with his colleagues in the Massachusetts Bay, he says, their boat’s speed dropped by about one knot when they hit a slick—precisely the speed of the surface current generated by the internal waves. “It was like walking along an airport hallway,” he says, “and suddenly stepping on a moving walkway going in the opposite direction.”

Seafaring folk have known about large-amplitude internal waves for centuries, Scotti says. World War II submarines had a hard time getting into the Mediterranean Sea through the Strait of Gibraltar while they were submerged because the internal waves ran so strong against them; any chance of navigating the strait in that direction meant breaching, and risking exposure to enemies on the surface.

And even in Homer’s time, mariners knew that the Strait of Messina was treacherous; they told stories about the nightmarish monsters Scylla and Charybdis dwelling on either side. But now we know, Scotti says, that the danger to mariners in the Strait of Messina was most likely due to ruinously strong currents stirred up by the outsized internal waves.

So massive internal waves have been around forever, but only in the last thirty years have scientists begun to study them. It started when Al Osborn, then a scientist working for Exxon, was working on a drilling ship off the coast of the Andaman Sea, and happened to notice something fishy about the scenery: twice a day, his ship tended to drift some half a mile along a belt of churning water, all in the course of thirty minutes or so.

Energy flux

In 2002, Scotti and his colleagues weighed anchor for their third trip to the Massachusetts Bay to try to figure out exactly how much potential energy large internal waves take from the tides and carry with them—a measurement called energy flux. Knowing the energy flux would take us one step closer to understanding how the ocean—Earth’s final frontier—really works, Scotti says. Because the fact is there’s still a lot we don’t know, particularly about how and where ocean-mixing actually happens.

Oceanographers rely on a well-known mathematical formula to measure energy flux in small-amplitude waves, Scotti says; you just need to know the velocity, depth, and pressure variables of the conditions that make up the wave’s immediate environment. This way you can measure how much work an internal wave is doing to push against the weight of the water on top of it.

But there’s one important thing this formula doesn’t tell us: how much potential energy an internal wave is carrying on its own. This leaves a pretty big gap in the information marine scientists use to make their computer models, Scotti says.

So while they were sailing around the Massachusetts Bay, Scotti and his colleagues dodged a few Nor’easters, did a little whale-watching, and finally figured out a new formula to calculate what they call the available potential energy (APE) in internal waves.

They used a gadget called an acoustic Doppler current profiler—a two-and-a-half-foot long cylinder that, from the ocean floor, draws on acoustic signals to take measurements of the currents above it. From the data, Scotti’s team estimated the direction and speed of the internal waves. But that wasn’t enough. One of the essential variables in their calculations—background potential energy (BPE)—was still missing.

BPE, Scotti says, is like the minimum balance in your bank account; it’s the amount of potential energy that’s always there in the ocean, but can’t be used up. And scientists have always assumed that to calculate it, they needed simultaneous measurements of several different variables across an entire section of ocean; this is impossible, Scotti says, so historically, efforts to determine APE have ended here.

But using what measurements they did have, Scotti and his team came up with a range of estimates for the variables involved in BPE, and then calculated what the energy flux—or the amount of energy expended in a given unit of time—would be for each variable. This gave them a range of possible fluxes, Scotti says. And using the range taken from field measurements at isolated moorings in the Massachusetts Bay, they showed that you actually can estimate APE using a series of other estimates.

Scotti’s new formula won’t benefit only marine scientists. The beauty of fluid dynamics, he says, is that new formulas and models can solve problems in entirely different fields. For example: mathematically speaking, the way waves act near solid boundaries is pretty similar to what happens in our large blood vessels, he says. That is, rather than a continuous flow, blood pumps in waves, and knowing how this works can help doctors better treat their patients. And engineers can apply some of the same ideas to, say, the unsteady pulse of a turbine or helicopter blades to ultimately improve how the machinery functions.

Movin’ on up

Thanks to a hefty grant from the National Science Foundation, Scotti will soon have all the benefits of studying from a boat in the Massachusetts Bay—just a few floors below his own office.

Right now, he and his colleagues are dreaming about the basement-level lab that should be complete before the year’s out, and the water-filled channel that will span the entire length of Chapman Hall—about a hundred and thirty feet.

Massachusetts Bay is one of the best places to study internal waves because of its relatively stable conditions, Scotti says, but there are still plenty of conditions and variables that they can’t control. “But in the lab, we can control them,” he says.

Researchers in the marine sciences department will share the lab with those in the applied mathematics department, and they’re all keen to get their feet wet. “This way, we can do lab, field, and computer models,” Scotti says. “And we need all three of them to connect the dots.”

They’re out there

In their new lab, Scotti and his colleagues will study not only the waves under the ocean’s surface, but also the ninety-foot ocean monsters known as rogue waves.

Lots of people think of rogue waves as big fish stories—never as big as eyewitnesses claim, if they ever existed at all. But the thing is that there are hardly ever any witnesses who live to tell the story. Rogue waves can take down a ship built to sustain forty-five-foot waves in a matter of minutes, Scotti says; that’s not even enough time to send out a distress signal, in many cases.

So how do we know they’re really out there?

Satellite images show us storms out in the open ocean; they also show us all the resulting rogue waves tossing and turning over the seas. And remnants from shattered ships tell us more of the story. For example, the Queen Elizabeth II, the flagship of the British Cunard Line, was walloped in 1995 by a ninety-five-foot rogue wave that had no trouble smashing the windows of her bridge and carrying away her forward whistle mast.

“Of course, you can make a ship that can sustain a ninety-foot wave,” Scotti says, “but it’ll cost a lot more.” And shipping companies don’t want their insurance rates to skyrocket even higher than they already are.

Because all you have to do is take a close look at insurance claim records for shipping companies to know that “ships go down at a frightening rate,” Scotti says. “The public’s not much concerned, though, mostly because we don’t go to sea much anymore. If airplanes were to come down at the same rate that ships go down, people would be up in arms.

“But I don’t want to discourage you from going on your next cruise,” he says.

Alberto Scotti is an associate professor in the Department of Marine Sciences. The Office of Naval Research and the National Science Foundation funded his work in the Massachusetts Bay.

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