Ocean Tides Lost and Found
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.
"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]
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.
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