Some of the largest ocean waves in the world are nearly impossible to see. Unlike other large waves, such as tsunamis or rogue waves, these rollers, called internal waves, do not ride the ocean surface. Instead, they move underwater, undetectable without the use of satellite imagery or sophisticated monitoring equipment. Despite their hidden nature, internal waves are fundamental parts of ocean water dynamics, transferring heat to the ocean depths and bringing up cold water from below. And they can reach staggering heights—some as tall as skyscrapers.
Because these waves are involved in ocean mixing and thus the transfer of heat, understanding them is crucial to global climate modeling, says Tom Peacock, a researcher at the Massachusetts Institute of Technology. Most global models fail to fully take internal waves into account.* “If we want to have more and more accurate climate models, we have to be able to capture processes such as this,” Peacock says.
Peacock and his colleagues tried to do just that. Their study, published in November in Geophysical Research Letters, focused on internal waves generated in the Luzon Strait, which separates Taiwan and the Philippines. Internal waves in this region, thought to be some of the largest in the world, can reach around 500 meters high. “That’s the same height as the Freedom Tower that’s just been built in New York,” Peacock says.
Although scientists knew of this phenomenon in the South China Sea and beyond, they didn’t know exactly how internal waves formed. To find out, Peacock and a team of researchers from M.I.T. and Woods Hole Oceanographic Institution worked with France’s National Center for Scientific Research using a giant facility there called the Coriolis Platform. The rotating platform, about 15 meters in diameter, turns at variable speeds and can simulate Earth’s rotation. It also has walls, which means scientists can fill it with water and create accurate, large-scale simulations of various oceanographic scenarios.
Peacock and his team built a carbon-fiber resin scale model of the Luzon Strait, including the islands and surrounding ocean floor topography. Then they filled the platform with water of varying salinity to replicate the different densities found at the strait, with denser, saltier water below and lighter, less briny water above. Small particles were added to the solution and illuminated with lights from below in order to track how the liquid moved. Finally, they re-created tides using two large plungers to see how the internal waves themselves formed
The Luzon Strait’s underwater topography, with a distinct double ridge shape, turns out to be responsible for generating the underwater waves. As the tide rises and falls and water moves through the strait, colder, denser water is pushed up over the ridges into warmer, less dense layers above it. This action results in bumps of colder water trailed by warmer water that generate an internal wave. As these waves move toward land, they become steeper—much the same way waves at the beach become taller before they hit the shore—until they break on a continental shelf.
Matthieu Mercier, lead author of the paper and currently a postdoc at the Institute of Fluid Mechanics in Toulouse, explains that the crashing of internal waves against the continental shelf causes ocean waters to mix, pushing warmer waters down and pulling colder waters up. But he says there is also a biological component: “When you mix like that, you bring more nutrients” to organisms living in the area, he says, such as plankton and corals.
Another team of researchers from the same research program was also able to devise a mathematical model that describes the movement and formation of these waves.* Whereas the model is specific to the Luzon Strait, it can still help researchers understand how internal waves are generated in other places around the world. Eventually, this information will be incorporated into global climate models, making them more accurate. “It’s very clear, within the context of these [global climate] models, that internal waves play a role in driving ocean circulations,” Peacock says.*
*Editor's note (1/23/14): The three asterisked sentences were edited after posting: the first two were changed to correct errors in the original statements; the third to clarify the quote attribution.