MIT researchers deployed intricate contraptions, including cables that run to the sea floor and an autonomous submarine, to measure internal weaves around the South China Sea. The researchers followed and measured these waves from their origin, until they dissipated, and in doing so have recorded the “largest waves documented in the global oceans.”

The water in the oceans and seas is laid down in layers, each of different density and other varying physical quantities like temperature, salinity and so forth. These interfaces extend horizontally and oscillations of the water across these layers are called internal waves, which in many respects aren’t that different from the surface waves that hit beaches and surfers use to ride them. Unlike surface waves, though, these are very large and with long periods.

In the South China Sea where the researchers concentrated their efforts, internal waves are due to tidal currents raking back and forth across sharp features of bathymetry. Ridges in the Luzan Strait are particularly effective at creating these sort of large amplitude waves. These waves can travel thousands of kilometres from their sources before breaking, making it very difficult to observe and measure.

To measure them, Thomas Peacock along with colleagues at MIT deployed cables that stretched more than 3000m until the rock bottom sea floor. These were attached to large  buoys and along the cables various sensors were fixed at certain depths to measure physical properties. An autonomous underwater vehicles were also deployed to gather more data in the surrounding waters.

“Internal waves are the lumbering giants of the ocean,” Peacock says. “They move fairly slowly but they are very large in amplitude and carry a lot of energy.”

According to the paper published in Nature, the researchers found: 

• the waves begin as sinusoidal disturbances rather than arising from sharp hydraulic phenomena
• there are >200-metre-high breaking internal waves in the region of generation that give rise to turbulence levels >10,000 times that in the open ocean. The biggest one was larger than 500m.
• the Kuroshio western boundary current noticeably refracts the internal wave field emanating from the Luzon Strait
• there’s a factor-of-two agreement between modelled and observed energy fluxes, which allows us to produce an observationally supported energy budget of the region.

“Together, these findings give a cradle-to-grave picture of internal waves on a basin scale, which will support further improvements of their representation in numerical climate predictions,” the researchers write.

Understanding how inner waves form and crash is important for a lot of reasons. For one, nowadays the oceans are littered with hundreds of thousands of underwater cables and understanding how these are affected by the ocean’s currents and waves is paramount for engineers seeking to design better solutions. Secondly, these are important mechanisms that need to be accounted for in global climate models; these blend the various ocean layers and mix matter.

Internal waves are also involved in making the moon move away from Earth. The moon recedes at a rate of around 1.78cm per year. That means it is now around 1.75m more distant than it was on the first moon landing. Maybe that’s why we’re not landing spacecraft that often, not to mention humans, on the moon anymore (/joke).

 “To cut a long story short, it’s not unreasonable to say internal waves play a role in the moon moving away or receding from the Earth,” he says. “They are big enough that they affect large-scale celestial motions.”

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