05/06/2015
CORVALLIS, Ore. – Once a day, a wave as tall as the Empire State
Building and as much as a hundred miles wide forms in the waters between
Taiwan and the Philippines and rolls across the South China Sea – but
on the surface, it is hardly noticed.
These daily monstrosities are called “internal waves” because they are
beneath the ocean surface and though scientists have known about them
for years, they weren’t really sure how significant they were because
they had never been fully tracked from cradle to grave.
But a new study, published this week in Nature Research Letter,
documents what happens to internal waves at the end of their journey and
outlines their critical role in global climate. The international
research project was funded by the Office of Naval Research and the
Taiwan National Science Council.
“Ultimately, they are what mixes heat throughout the ocean,” said Jonathan Nash,
an Oregon State University oceanographer and co-author on the study.
“Without them, the ocean would be a much different place. It would be
significantly more stratified – the surface waters would be much warmer
and the deep abyss colder.
“It’s like stirring cream into your coffee,” he added. “Internal waves are the ocean’s spoon.”
Internal waves help move a tremendous amount of energy from Luzon Strait
across the South China Sea, but until this project, scientists didn’t
know what became of that energy. As it turns out, it’s a rather
complicated picture. A large fraction of energy dissipates when the wave
gets steep and breaks on the deep slopes off China and Vietnam, much
like breakers on the beach.
But part of the energy remains, with waves reflecting from the coast and rebounding back into the ocean in different directions.
The internal waves are caused by strong tides flowing over the topography, said Nash, who is in OSU’sCollege of Earth, Ocean, and Atmospheric Sciences.
The waves originating in Luzon Strait are the largest in the world,
based on the region’s tidal flow and topography. A key factor is the
depth at which the warm- and cold-water layers of the ocean meet – at
about 1,000 meters.
The waves can get as high as 500 meters tall and 100-200 kilometers wide before steepening.
“You can actually see them from satellite images,” Nash said. “They will
form little waves at the ocean surface, and you see the surface
convergences piling up flotsam and jetsam as the internal wave sucks the
water down. They move about 2-3 meters a second.”
The waves also have important global implications. In climate models,
predictions of the sea level 50 years from now vary by more than a foot
depending on whether the effects of these waves are included.
“These are not small effects,” Nash said.
This new study, which was part of a huge international collaboration
involving OSU researchers Nash and James Moum – as well as 40 others
from around the world – is the first to document the complete life cycle
of these huge undersea waves.
