For the first time, 100-foot tidal waves roaring at 70 miles-per-hour have been caught breaking and creating long trains of billowing turbulence more than 1,800 feet down along the ocean floor.
The discovery sheds light on the oceans' underappreciated internal waves and where they break in the deep sea — something that needs to be better understood to improve global models of the oceans and Earth's climate.
"In the deep sea, it has never been observed," said Hans Van Haren of the Royal Netherlands Institute for Sea Research. Van Haren and his colleague Louis Gostiaux have described their discovery in a paper in the journal Geophysical Research Letters.
The direct measurements of the 100-foot billows were made on the down-current slopes of a submerged seamount near the Canary Islands over which tidal currents rage every day. The researchers detected the billowing with a string of high-speed temperature probes that picked up the denser, colder water and the less dense, warmer water tripping over each other as the tides roared down the seamount slopes.
"It's a hefty current," observed Van Haren. "I wouldn't like to be down there: (The waves) are really vigorously breaking."
That kind of high energy is exactly what it takes to get waters of very different densities to blend, Van Haren explained. And that blending of cold and hot, or fresh and salty, is a vital element in explaining how the oceans push energy around the world.
"We saw the actual mechanism happening," said Van Haren. As the tidal current picked up speed down the seamount slope, it created unstable shear zones between the un-mixing waters.
In those shear zones, less dense, warm water was pushed under heavier colder water, somewhat like oil being forced under water. "It collapses the whole thing," said Van Haren.
And in the case of deep sea waters — unlike oil and water — the collapse means the waters mix in long trains of billowing turbulence.
What's more, despite this being the first time such mixing has been seen at great depth, there is nothing necessarily rare about it, he said.
"You only need a sloping bottom and a tidal current," Van Haren said. Even more amazing is that there are larger internal waves out there in the oceans breaking — some 100 meters (330 feet) high, he added.
That's exactly why more of them need to be measured: So they can be figured in as an important ocean parameter in the much larger scale models being developed by other researchers, said David Farmer oceanographer of the University of Rhode Island.