|
 June 15, 2000 -- The Moon's gravity imparts
tremendous energy to the Earth, raising tides throughout the global
oceans. What happens to all this energy? This question has been pondered
by scientists for over 200 years, and has consequences ranging from the
history of the moon to the mixing of the oceans.
Richard Ray at
NASA's Goddard Space Flight Center, Greenbelt, MD., and Gary Egbert of the
College of Oceanic and Atmospheric Sciences, Oregon State University,
Corvallis, OR, studied six years of altimeter data from the TOPEX/Poseidon satellite to
address this question.
Above: The Moon's gravity tugs
at the Earth, causing ocean water to slosh back and forth in predictable
waves called tides. We can visibly observe some of that energy dissipate
at the beach, with waves rolling across coastal shallows and shoals. Most
of the energy dissipates due to friction between the water and the shallow
floor beneath it. This
Quicktime animation (6 Mb) showing tidal energy dissipation is
courtesy of the Scientific Visualization Studio at the Goddard Space
Flight Center. A 400 kb GIF
animation is also available.
According
to their report, appearing in the June 15 issue of Nature, about 1
trillion watts, or 25 to 30 percent of the total tidal energy dissipation,
occurs in the deep ocean. The remainder occurs in shallow seas, such as
the continental shelf off the southeast coast of South America.
"By
measuring sea level with the TOPEX/Poseidon satellite altimeter, our
knowledge of the tides in the global ocean has been remarkably improved,"
said Ray.
The accuracies are now so high that these data can be
used to map empirically the tidal energy dissipation. The deep-water tidal
dissipation occurs generally near rugged bottom topography (seamounts and
mid-ocean ridges).
"The observed pattern of deep-ocean dissipation
is consistent with topographic scattering of tidal energy into internal
motions within the water column, resulting in localized turbulence and
mixing," explained Egbert.
Below:
In order to search for missing tidal energy amid Earth's various
geophysical systems, researchers first had to map the ocean tides to a
precise degree. Using six years of data from TOPEX/Poseidon, they derived
a 16-day set of predictive data, showing a synthetic view of how the tides
move around the world's oceans. In this animation, which shows a snippet
of the full
16-day Quicktime animation (2 MB), blue signifies places where the
ocean level is lower than the average reference height, and red shows
areas where it's higher. [more
information]
 One important implication of this finding concerns
the possible energy sources needed to maintain the ocean's large-scale
"conveyor-belt" circulation and to mix upper ocean heat into the abyssal
depths. It is thought that 2 terawatts (2 trillion watts) are required for
this process. The winds supply about 1 terawatt, and there has been
speculation that the tides, by pumping energy into vertical water motions,
supply the remainder. However, all current general circulation models of
the oceans ignore the tides. "It is possible that properly accounting for
tidally induced ocean mixing may have important implications for long-term
climate modeling," Egbert said.
In the past, most geophysical
theories held that the only significant tidal energy sink was bottom
friction in shallow seas. Egbert and Ray find that this sink is indeed
dominant, but it is not the whole story. There had always been suggestive
evidence that tidal energy is also dissipated in the open ocean to create
internal waves, but published estimates of this effect varied widely and
had met with no general consensus before Topex/Poseidon.
TOPEX/Poseidon mission, a joint U.S.-French mission, is managed
by NASA’s Jet Propulsion Laboratory, Pasadena, CA, for NASA’s Office of
Earth Science, Washington, DC. The satellite was launched in August 1992,
and it continues to produce sea level measurements of the highest
quality. |
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