REACHING THE SURFACEInternal waves sometimes reach the surface and become visible, like these in the Red Sea. Internal waves can form from resonance created by tidal waves, researchers suggest in a new study.Zhang
Every day huge, underwater waves surge and ebb far outside
the rims of continents. Flowing much faster than the visible tides, these “internal
tides” usually don’t make it to the surface or close to the coast. But, because
they move around the sediment as it collects below, these waves play big roles
in shaping the edges of continental shelves.
Physicists may have now solved the riddle of the waves’
origin.
Around most coasts, relatively shallow waters cover the submerged
part of each continent. Farther offshore, sediment constantly falls from the
continental crust to the deeper oceanic floor. Internal tides come up at an
angle that’s influenced by the physics of deep ocean water. In turn, these internal
tides will shape the sediment at the edge of the crust into a continental slope
of the same angle.
Internal tides may be generated by the slower ordinary tides
as a resonance effect, similar to the way a repeated, gentle push can make a
child sitting on a swing go higher and higher, researchers suggest in the June
20 Physical Review Letters.
“To me, it’s fascinating because it provides an explanation
as to the origin of the internal waves,” comments Lincoln Pratson, a geologist
at DukeUniversity
in Durham, N.C.
Inside a one-meter–long pool, Hepeng Zhang and his
collaborators at the University of Texas at Austin
created a small-scale version of the ocean around a continent’s coastline. On
one side of the tank, a slanted surface represented the continental slope.
To simulate tides, the researchers set up their miniature
continental slope so they could move it in and out of the water. Although in a
real tide it’s the water that moves while the coast stays still, from the point
of view of the ocean’s surface, it’s as if the coast slides up and down.
The researchers also filled the tank with water that was
progressively saltier — and thus denser — the deeper it got, to simulate
differences in ocean water density, which changes as both salinity and
temperature change.
As they created their artificial tidal waves, Zhang and his
colleagues saw that a layer of water right above the continental slope started
flowing as much as 10 times faster than the simulated tides. The researchers
calculated that energy from the simulated tide could progressively accumulate
by resonance, generating an internal wave that would oscillate between water layers
of different densities.
Density differences allow waves to form, Zhang explains. For
example, surf waves arise at the interface between the air and the much denser
water. The same can happen at the interface between two liquids of different
densities, like water and oil.
In the case of the deep ocean, however, density varies
smoothly rather than abruptly, which allows waves to propagate at an angle
instead of just horizontally.
However, the team only saw the resonance, and thus the
internal waves, appear at a particular angle, an observation corroborated by
their theoretical calculations. This would explain why continental slopes don’t
get any steeper than about 3 degrees. The twice-daily shear of the internal
tides keeps removing any excess sediment.
Previous research into how internal waves affect continental
slopes assumed that the waves originated somewhere other than near the continental
slope. “In our picture, we don’t require this external source,” Zhang says.
David Cacchione, a retired USGS oceanographer in Camas, Wash., who helped
discover the internal tides’ effects on the continental slopes, says that a
more accurate experiment should also include the effects of the Earth’s
rotation.
Oceanic tides create underwater waves that shape
the sediment deposits off a continent’s coast. In this lab experiment, a
simulated tide (upper left) generates a faster-moving wave near the surface.
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