The Motion in the Ocean

Walter Munk isn’t afraid to make waves. Thirty years ago, the oceanographer challenged his colleagues to solve a dark mystery: What drives ocean circulation? Today, he’s offering them a brilliant answer: The moon.

Despite their peacefulness on postcards, oceans are in constant motion. Winds, for instance, whip surface waters into major currents. What’s more, the North Atlantic Ocean is like a wet conveyor belt, with cold water constantly sinking in the polar regions and then traveling, deep in the ocean, back toward the tropics. Somewhere en route, scientists say, this colder, deep water must mix with warmer surface waters—otherwise, almost all the ocean would become cold and Earth’s climate would be strikingly different. What watery spoon stirs the deep sea—and how?

In a classic 1966 paper, Munk launched a continuing effort to quantify ocean mixing. At Scripps Institution of Oceanography in La Jolla, Calif., Munk’s first estimates of deep-sea mixing proved an order of magnitude too high, but his ideas influenced a generation of researchers who’ve gone searching for more precise descriptions of the power behind ocean circulation.

It hasn’t been easy. The deep sea makes an inconvenient workbench; it’s tricky to track water circling the globe so far down. Some researchers simply assume that ocean layers sort themselves out according to differences in their temperature and density. With few actual measurements at their fingertips, however, researchers have been forced to crunch equations and, basically, guess at what drives the deep.

That may now be starting to change. Armed with evolving models, finer satellite images, and a better understanding of the topsy-turvy seas, scientists are finding some surprises in the seascape.

Two years ago, Munk and oceanographer Carl Wunsch of the Massachusetts Institute of Technology argued that the moon’s orbital energy must cause far-reaching ocean tides that mix the open ocean’s warm and cold layers. Perhaps, the scientists suggested, this moony mix-up helps drive ocean circulation. But many researchers scoffed at the idea. “The moon’s relevance to ocean circulation was generally considered a lunatic suggestion,” jokes Munk.

Moon energy

Scientists agree that the orbiting moon dumps energy into the world’s oceans, causing the telltale ocean tides that make beachgoers inch backward as the day progresses. Most researchers, however, assume that these tides only stir shallow seas. For instance, tidal waves constantly crash against the continental shelf, losing energy and rolling back out to sea.

By contrast, the deep sea has always appeared calmer, with little tidal turbulence to mix the water. Still, Munk and Wunsch reasoned that up to a fourth of the ocean’s tidal energy might be swallowed by the deep sea, jumbling the ocean’s warm and cold layers. “We’ve been waiting for somebody to show us completely wrong,” Wunsch says.

Instead, a new study hints that they may be right. In the June 15 Nature, Gary D. Egbert of Oregon State University in Corvallis and Richard D. Ray at NASA’s Goddard Space Flight Center in Greenbelt, Md., report that about one terawatt (10¹² watts)—or 25 to 30 percent—of the total tidal energy deposited into the ocean is released into the deep sea.

To make that estimate, Egbert and Ray used sea-surface data from a U.S.-French satellite called TOPEX/Poseidon. The satellite measures the height of daily tides by bouncing radar beams off the ocean surface. With 7 years of TOPEX/Poseidon data in hand, Egbert says, the scientists could map tidal currents, looking to see where they scattered into underwater tides, presumably mixing deep water.

The maps suggest that open-ocean tides are spilling much energy—not all over but in hot spots, where tidal currents smash into undersea mountains and midocean ridges that jut from the seafloor. Normally, Egbert says, the stratified layers of ocean water—warm and light above, cold and heavy below—travel the oceans separately. When tides in the open ocean crash into an undersea mountain, however, some of the cold, deep water lurches into the upper ocean.

The chilled water then mixes with surrounding warm water as it sinks back down again. Repeated over and over in localized spots, Egbert says, this mixing might be enough to lighten and warm all that cold, deep water flowing along the Atlantic’s conveyor belt from the poles back toward the equator. “The spatial pattern is completely consistent with the idea that lots of energy is going into internal waves over rough topography in the ocean,” says Egbert. “These tides are losing the right amount of energy, in the right places, to be available for ocean mixing.”

Munk, for one, is convinced. “The new work strongly suggests two-thirds of tidal energy gets dissipated in the shallow seas but that about one-third is going into deep-ocean mixing,” he says.

Some of the mixing in the deep sea is controlled by wind, Munk adds—but a good half of it is driven by the moon’s motion. “I’d like to think we’ve won our battle to make the moon respectable,” he says. Wunsch agrees with his collaborator: “The tide-modeling work is quite supportive of what we suggest.”

But the implication for ocean circulation needs to be confirmed, cautions James C. McWilliams at the University of California, Los Angeles. Even though tidal dissipation in deep water is plausible, he says, it’s less clear that these rolling tides really drive circulation.

“Mixing induces circulation only when it occurs in the presence of strong current shears or density gradients,” McWilliams says. “If the mixing occurs in weakly stratified, slowly moving water—the usual situation in the abyss—then it may accomplish dissipation without inducing much circulation.”

Climate control

The deep sea isn’t just a workshop for oceanographers—it’s also one of Earth’s major tools for climate control. Along with the atmosphere, oceans even out the climate, shuttling heat between the equator and the poles. Without this circulation, temperatures would skyrocket in the tropics and plummet at higher latitudes like Europe. Some researchers worry that global warming will throw this circulation off, changing local climates—or even shut it down entirely, jerking the globe into the next ice age.

Finding clues to deep-sea circulation, says MIT atmospheric scientist Kerry Emanuel, will help scientists understand and model climate change far better.

Emanuel may have an important clue of his own: tropical cyclones. These storms—which can become hurricanes in the Atlantic Ocean and typhoons in the Pacific—violently mix the ocean when they strike, thrashing the water column down to about 150 meters. When the storms pass, they leave behind a sprawling stretch of surface water that’s been mixed with cold, deep water from below. This cold wake typically extends a few hundred kilometers across. When Hurricane Hilda raged through the Gulf of Mexico in 1964, for instance, surface waters over which the storm had passed dropped by as much as 6ºC.

Alone, a single tropical cyclone would do just temporary damage to the sea’s swirl. But in an upcoming study, Emanuel argues that the 80 or more storms that hit worldwide each year begin to add up. “The issue is, what happens to all these cold wakes?” he says.

Emanuel notes that the surface waters that are left cold in each wake gradually warm in the sun, so the ocean ends up with more heat than it had before the storm. Transported by the oceans’ natural conveyor belt, this newly warmed water flows north or south, out of the tropics and toward the poles, where it will grow cold, sink, and start the journey back.

To estimate the importance of tropical cyclones to ocean circulation, Emanuel turned to data gathered during and after 67 such storms in 1996. A few of the storms were well documented, including measures of the cold wake that followed. Using those cyclones, Emanuel crafted a coupled atmosphere-ocean model to estimate the net ocean heating generated by all 67 storms. He concludes that the storms may have induced up to 1.4 petawatts (10¹⁵ watts)—close to half the amount of heat energy transported on the ocean conveyor belt toward polar regions in any given year. “It’s quite conceivable that tropical cyclones account for a lot of the deep overturning of the ocean,” Emanuel says. “I expect this will be quite controversial, since it’s so far from what everyone else is thinking.”

Tropical cyclones

There’s one big problem with the cyclone-circulation idea, says J. Robert Toggweiler at the National Oceanic and Atmospheric Administration’s Geophysical Fluid Dynamics Laboratory in Princeton, N.J. Tropical cyclones shake up the upper ocean, he notes, while it’s the deeper water—below 1,000 m—that’s central to ocean circulation.

While Emanuel acknowledges this limit, he adds that during field work on hurricanes in the Gulf of Mexico, research teams have documented waves as far as 530 m below the sea surface. The scientists collect data from current-measuring sensors attached at various depths to steel cables running down into the ocean from buoys. A compilation of measurements at several ocean depths suggests that tropical cyclones set off waves reaching deep into the sea, Emanuel says. “If it could be demonstrated that this is going on,” Toggweiler responds, “then, he’s got something.”

Still, Toggweiler questions whether the ocean really needs mixing to make the deep sea circulate. According to his own ocean models, he says, cold water could sink because of its own weight in the North Atlantic and settle comfortably in the deep sea. Then, it would drift into the Drake Passage, a 1,000-km-wide waterway connecting the Atlantic and Pacific Oceans between South America and the Antarctic Peninsula. There, the cold water could be drawn by strong winds to the sea surface, where it would warm in the sun before floating on to high latitudes in the North Atlantic, only to grow cold and repeat the cycle.

“The chief advantage of this idea is its simplicity,” says Toggweiler. “It can explain all the main features of [deep-sea] circulation in a way that is independent of interior mixing.” Other scientists agree that ocean mixing remains a poorly understood piece of the circulation puzzle. “The hot spot hypothesis was born of desperation, since scientists haven’t found any kind of uniform mixing to explain deep-sea circulation,” remarks oceanographer Robert Pinkel of Scripps.

To see if the hot spot idea holds up, Pinkel and his colleagues at Scripps and four other institutions have designed HOME—the Hawaiian Oceanic Mixing Experiment. Beginning this year, the scientists will study the ocean tide as it propagates across the Hawaiian Ridge, a 2,000-km volcanic mountain chain that stretches from the Big Island of Hawaii to Midway Island. They’ll try to map the internal waves that emerge and measure the mixing that happens when these waves break.

Pinkel already knows that the ridge can spur serious tidal waves. Internal tides 100 m in amplitude have been reported just a few kilometers off Hawaii’s Waikiki Beach. “Now, we’re trying to determine if these waves break locally, mixing the ocean, or whether the energy just propagates away into the deep sea,” he says. “We also need to find the spots where the surface tide is losing energy and identify the key aspects of the topography that make it happen.”

Once researchers understand the physics of ocean mixing at the Hawaiian Ridge, he adds, they’ll have some idea of what’s happening near other undersea mountains worldwide.

No matter how deep-sea circulation plays out, Wunsch says, studies like these are giving scientists a whole new take on the seascape. “People once thought of the ocean as unchanging and steady,” he says. “We now know the ocean changes, violently, from day to day. We know that tides and winds may change what’s happening near the seafloor, that maybe hurricanes are important, that maybe there are hot spots of mixing. Ocean circulation is a lot more complex—and more interesting—than we ever thought.”

Kathryn Brown is a freelance science writer based in Alexandria, Va.