Tidal resonance
Encyclopedia
In oceanography
, a tidal resonance
occurs when the tide
excites one of the resonant modes of the ocean
.
The effect is most striking when a continental shelf
is about a quarter wavelength wide. Then an incident tidal wave can be reinforced by reflections between the coast and the shelf edge, the result producing a much higher tidal range
at the coast.
Famous examples of this effect are found in the Bay of Fundy
, where the world's highest tides are reportedly found, and in the Bristol Channel
. Large tides due to resonances are also found on the Patagonian Shelf
and on the N.W. Australian continental shelf.
The speed of long waves
in the ocean is given, to a good approximation, by
, where g is the acceleration of gravity and h is the depth of the ocean
.
For a typical continental shelf with a depth of 100 m, the speed is approximately 30 m/s. So if the tidal period is 12 hours, a quarter wavelength shelf will have a width of about 300 km.
With a narrower shelf, there is still a resonance but it has less effect at tidal frequencies. However the effect is still enough to partly explain why tides along a coast lying behind a continental shelf are often higher than at offshore islands in the deep ocean. The strong tidal currents associated with resonances also mean that the resonant regions are the areas where most tidal energy is dissipated.
In the deep ocean, where the depth is typically 4000 m, the speed of long waves increases to approximately 200 m/s. The difference in speed, when compared to the shelf, is responsible for the reflections at the continental shelf edge. Away from resonance this can stop tidal energy moving onto the shelf. However near a resonant frequency the phase relationships between the wave on the shelf and in the deep ocean can have the effect of drawing energy onto the shelf.
The increased speed of long waves in the deep ocean means that the tidal wavelength there is of order 10,000 km. As the ocean basins have a similar size, they also have the potential of being resonant
.
In practice deep ocean resonances are difficult to observe, probably because the deep ocean loses tidal energy too rapidly to the resonant shelves.
resulting from tides, called tidal locking
, which causes a moon
's rotational period to coincide with the period of its revolution around the planet that it orbits, so that one side of the moon always faces the planet.
Oceanography
Oceanography , also called oceanology or marine science, is the branch of Earth science that studies the ocean...
, a tidal resonance
Resonance
In physics, resonance is the tendency of a system to oscillate at a greater amplitude at some frequencies than at others. These are known as the system's resonant frequencies...
occurs when the tide
Tide
Tides are the rise and fall of sea levels caused by the combined effects of the gravitational forces exerted by the moon and the sun and the rotation of the Earth....
excites one of the resonant modes of the ocean
.
The effect is most striking when a continental shelf
Continental shelf
The continental shelf is the extended perimeter of each continent and associated coastal plain. Much of the shelf was exposed during glacial periods, but is now submerged under relatively shallow seas and gulfs, and was similarly submerged during other interglacial periods. The continental margin,...
is about a quarter wavelength wide. Then an incident tidal wave can be reinforced by reflections between the coast and the shelf edge, the result producing a much higher tidal range
Tidal range
The tidal range is the vertical difference between the high tide and the succeeding low tide. Tides are the rise and fall of sea levels caused by the combined effects of the gravitational forces exerted by the Moon and the Sun and the rotation of the Earth...
at the coast.
Famous examples of this effect are found in the Bay of Fundy
Bay of Fundy
The Bay of Fundy is a bay on the Atlantic coast of North America, on the northeast end of the Gulf of Maine between the Canadian provinces of New Brunswick and Nova Scotia, with a small portion touching the U.S. state of Maine...
, where the world's highest tides are reportedly found, and in the Bristol Channel
Bristol Channel
The Bristol Channel is a major inlet in the island of Great Britain, separating South Wales from Devon and Somerset in South West England. It extends from the lower estuary of the River Severn to the North Atlantic Ocean...
. Large tides due to resonances are also found on the Patagonian Shelf
and on the N.W. Australian continental shelf.
The speed of long waves
WAVES
The WAVES were a World War II-era division of the U.S. Navy that consisted entirely of women. The name of this group is an acronym for "Women Accepted for Volunteer Emergency Service" ; the word "emergency" implied that the acceptance of women was due to the unusual circumstances of the war and...
in the ocean is given, to a good approximation, by
, where g is the acceleration of gravity and h is the depth of the ocean.
For a typical continental shelf with a depth of 100 m, the speed is approximately 30 m/s. So if the tidal period is 12 hours, a quarter wavelength shelf will have a width of about 300 km.
With a narrower shelf, there is still a resonance but it has less effect at tidal frequencies. However the effect is still enough to partly explain why tides along a coast lying behind a continental shelf are often higher than at offshore islands in the deep ocean. The strong tidal currents associated with resonances also mean that the resonant regions are the areas where most tidal energy is dissipated.
In the deep ocean, where the depth is typically 4000 m, the speed of long waves increases to approximately 200 m/s. The difference in speed, when compared to the shelf, is responsible for the reflections at the continental shelf edge. Away from resonance this can stop tidal energy moving onto the shelf. However near a resonant frequency the phase relationships between the wave on the shelf and in the deep ocean can have the effect of drawing energy onto the shelf.
The increased speed of long waves in the deep ocean means that the tidal wavelength there is of order 10,000 km. As the ocean basins have a similar size, they also have the potential of being resonant
.
In practice deep ocean resonances are difficult to observe, probably because the deep ocean loses tidal energy too rapidly to the resonant shelves.
Tidal locking
The above concept of tidal resonance differs from another sort of resonanceResonance
In physics, resonance is the tendency of a system to oscillate at a greater amplitude at some frequencies than at others. These are known as the system's resonant frequencies...
resulting from tides, called tidal locking
Tidal locking
Tidal locking occurs when the gravitational gradient makes one side of an astronomical body always face another; for example, the same side of the Earth's Moon always faces the Earth. A tidally locked body takes just as long to rotate around its own axis as it does to revolve around its partner...
, which causes a moon
Moon
The Moon is Earth's only known natural satellite,There are a number of near-Earth asteroids including 3753 Cruithne that are co-orbital with Earth: their orbits bring them close to Earth for periods of time but then alter in the long term . These are quasi-satellites and not true moons. For more...
's rotational period to coincide with the period of its revolution around the planet that it orbits, so that one side of the moon always faces the planet.
See also
- SeicheSeicheA seiche is a standing wave in an enclosed or partially enclosed body of water. Seiches and seiche-related phenomena have been observed on lakes, reservoirs, swimming pools, bays, harbors and seas...
- Severn BarrageSevern BarrageThe Severn Barrage refers to a range of ideas for building a barrage from the English coast to the Welsh coast over the Severn tidal estuary. Ideas for damming or barraging the Severn estuary have existed since the 19th century. The building of such a barrage would be a huge engineering feat,...
(proposed for the Bristol Channel). - Standing waveStanding waveIn physics, a standing wave – also known as a stationary wave – is a wave that remains in a constant position.This phenomenon can occur because the medium is moving in the opposite direction to the wave, or it can arise in a stationary medium as a result of interference between two waves traveling...
- Cavity resonator