CORVALLIS, Ore. - The moon's gravity imparts tremendous energy to the Earth, raising tides throughout the global oceans. What happens to all this energy is a question that has been pondered by scientists for more than 200 years, and has consequences ranging from the history of the moon to the mixing of the oceans.
Richard Ray of NASA's Goddard Space Flight Center in Maryland and Gary Egbert of the College of Oceanic and Atmospheric Sciences at Oregon State University studied six years of altimeter data from the TOPEX/POSEIDON satellite to address this question.
Their findings, reported in the June 15 issue of Nature, show that about 1 "terawatt," or 25 to 30 percent of the total tidal energy dissipation, occurs in the deep ocean. The remainder occurs in shallow seas, such as on the Patagonian Shelf. A terawatt is enough energy to light 10 billion 100-watt light bulbs.
"By measuring sea level with the TOPEX/POSEIDON satellite altimeter, our knowledge of the tides in the global ocean has been remarkably improved," said Richard Ray, a geophysicist at Goddard. The accuracy is now so high that this data can be used to map empirically the tidal energy dissipation. The deep-water tidal dissipation occurs generally near rugged bottom topography such as 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", said Egbert, an associate professor at OSU.
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 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 the sink is 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.
The TOPEX/POSEIDON mission, a joint U.S.-French initiative, is managed by the Jet Propulsion Laboratory 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. For supporting images: ftp://geodesy.gsfc.nasa.gov/dist/ray/Lynn/.