Earth-orbiting satellites routinely measure tides over the deep-ocean. The U.S.-French TOPEX/POSEIDON satellite is equipped with a radar-altimeter that bounces microwave signals off the sea surface and precisely measures sea level. With such data, it is possible to determine what happens to tide waves and their energy as they travel across deep ocean basins. As shallow-water waves, tides lose energy through frictional drag with the ocean floor, especially in the ocean's shallow seas and along continental margins. Satellite measurements show that about three quarters of the global tidal energy dissipates in shallow seas bordering northern Europe, in the Yellow Sea off Asia, in the shallow seas around Australia, near Argentina, and in Canada's Hudson Bay. Open-ocean tides are important in mixing deep-ocean water. Ocean scientists long assumed that wind was the principal mixing agent of the open ocean, but satellite altimeter data now show that tidal mixing in the deep ocean is about as important as the wind. Perhaps as much as half of the tidal energy in the ocean is dissipated in mixing processes when tidal currents in the deep ocean flow over seamounts, ridges, and other rugged features on the ocean floor or weave through passages between islands. Tidal currents flowing over topographic irregularities on the ocean floor generate internal waves that propagate away from their source. These internal waves arise from the fact that water density increases gradually with increasing depth. As tidal currents encounter a seamount or submarine ridge, relatively dense water is forced upward into slightly less dense water. Then to the lee of the obstacle gravity pulls the denser water downward. However, the descending water gains momentum and over shoots its equilibrium level and descends into denser water. The water then ascends thereby forming an oscillating wave that propagates horizontally. Because these waves are generated by tides, they occur at tidal frequencies and are called internal tides. Internal tide waves can travel thousands of kilometers beyond the obstruction that formed them and can have very large wave heights. They also break, like surf on a beach but under water, locally mixing waters above and below the internal wave. Internal tides are important in mixing cold bottom waters with warmer surface waters as part of the global oceanic conveyer belt circulation.
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Recently ocean scientists gathered evidence that internal tides influence the gradient of the continental slope. The inclination of the continental slope varies from very gentle (as small as one degree) to precipitous (up to 25 degrees where submarine canyons cut into the slope). About 80% of the continental slope is inclined at less than 8 degrees and the average inclination is about 4 degrees. According to geological studies, however, the sediments supplied to the continental slope (mostly by rivers) would support a stable average slope of perhaps 15 degrees or greater. Data acquired from model studies, dives in piloted submersible vessels, and moored instruments show that internal tides produce strong currents that prevent accumulation of sediment that would make the continental slope steeper. In fact, the internal waves ascending the continental slope apparently behave very much like ordinary sea waves entering the shoaling waters of a coastal area (with changes in amplitude, wavelength, and water velocity). Whereas the influence of internal tides is widespread along the continental slope, turbidity currents and tectonic forces can be important locally and regionally in shaping the slope. A vast amount of energy is involved in ocean tides and waves so it is not surprising that considerable interest has focused on developing technologies to tap this energy to generate electricity. Although the potential is enormous, very little of this energy resource has been developed to date. |
