Jun 14, 2000

Ocean Tides Lost and Found

see caption, click for animation
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

is also available.

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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]

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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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More images and animations-- from the Scientific Visualization Studio at NASA/GSFC