Ocean Tides Lost and Found
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.
"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).
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 NASAs Jet Propulsion Laboratory, Pasadena, CA, for NASAs
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.
SCIENTISTS SOLVE MYSTERY OF THE DISAPPEARING OCEAN TIDES -- NASA/Goddard press release 00-71
More images and animations-- from the Scientific Visualization Studio at NASA/GSFC