|The simple model
The easiest way to understand tides is shown in this drawing. The earth turns around the sun and is kept in orbit by the gravitational pull between them. Likewise the moon is kept in orbit around the earth by the gravitational pull between these two. Each causes a bulge of water on the nearest side and an equal bulge on the other side. The tide is thus composed of two bulges of water (four, in fact), travelling around the world as the world spins round. When moon and sun are aligned, their respective tide bulges add together to a spring tide every two weeks. When sun and moon are at right angles (the smaller drawing), it causes the bulge of the sun to add to the low tide, resulting in an overall higher low tide but lower high tide. This is called the neap tide, every two weeks in between spring tides.
Seen from the north pole, the earth rotates in a counter-clockwise direction: a point on the equator moves east-ward; the sun rises in the east. The tidal bulge thus moves westward. In our solar system, all heavenly bodies rotate in approximately the same plane and in the same direction. The rotation of earth and moon is in the same direction, earth doing its spin in exactly 24 hours and the moon in about 28 days. This difference delays the tide each day by 1/28th day or about 51 minutes. The solar tide is 24 hours.
This simple model has been used for centuries to calculate tide levels all over the world but it has a number of insurmountable problems.
|Tide tables are constructed
by analysing a long tidal record for a particular locality and finding
the contributions to the tidal curve made by its various components. It
is in fact a method of breaking the tidal wave form into its components
or causes. The main component is the moon, then the sun, then the elliptical
motion of the moon around the earth, and others, as shown in the table
From the relative sizes of each component, it can be seen
that, should all components work together, the tide could almost double
its size during special spring tides.
|The buckling crust
In the early seventies, as computers were able to model wave behaviour, they were able to show that tide waves would run in ways to prevent loss of energy. Instead of running east to west, tide waves run around in circles (clockwise and CCW on both hemispheres) around islands, and certain points in the sea, called nodes. The map shown here shows the tides circling around their nodes in 30º increments (12 parts to 25/2 hours) in the directions of the solid arrows. Each solid line connects tide levels in the ocean that are at the same phase. The dotted lines show tide amplitude (half tide height) in cm. This early model has been shown to agree with reality and, for its time, has been a remarkable achievement of computational mathematics. From this map one can see that some places in the world (the nodes) have no tides, others two (12 lines) and a few places one (24 lines). (Picture from Van Dorn, 1974)
This new model does away with the objections mentioned for the simple model:
As has been observed with other stellar objects rotating in close proximity, their round shapes become distorted by gravity. Likewise the earth is pulled into a slightly oval form, independently by both the moon and the sun. As the earth rotates, these bulges (body tides) travel round the planet in the times calculated above. So the simple two-bulge model is true for the earth's crust. Being 2.8 times denser than water, and much deeper than the oceans, a body tide can easily run as a gravity wave at the calculated speeds without losing much energy. The amplitude of the earth's body tide is typically 0.1-0.4m (0.2-0.8m wave height), which is quite negligible compared to the earth's diameter of 12,600,000m. The Earth's mantle has enough flexibility to allow such a wave to pass effortlessly. [Note that this body tide is also estimated at 0.1m amplitude or 0.2m high to low, as measured by satellites]
Since the TOPEX/Poseidon satellite has been measuring surface height data, the oscillating surface of the oceans
due to moon tides could be measured and mapped. Note that the nodes correlate with areas of no tide change,
except where these rotate around islands such as New Zealand and Madagascar. Highest tidal ranges are found
where continental coasts distort the tide wave. http://svs.gsfc.nasa.gov/stories/topex/images/TidalPatterns_hires.tif
Once the actual oscillation of the sea could be measured, one was able
to compare it with calculations from computer models to see if any differences
occurred. The above picture shows where such tide anomalies occur: around
islands where the tide wave distorts most (NZ, Madagascar), and around
deep sea ridges and chains of seamounts (Hawaii, Kermadecs). It is now
thought that these anomalies give rise to deep eddies that transport nutrients
from the deep to the surface, thereby giving rise to unanticipated marine
productivity. Note also how some anomalies correspond with already notable
fishing grounds. Note also that these computer-generated results must be
|Tides dissipate 3.75 ± 0.08 TW (TeraWatt= 1E12 Watt= 1 million million Watt) of power (Kantha,1998), of which 3.5 TW are dissipated in the ocean, and much smaller amounts in the atmosphere and solid earth. The dissipation increases the length of day by about 2.07 milliseconds per century, it causes the semimajor axis of moon’s orbit to increase by 3.86 cm/yr, and it mixes water masses in the ocean.
|Tides around New Zealand
New Zealand, as has been seen above, forms a node around which the tide runs twice daily. Recent measurements have shown that some of the nodes of the component waves, are located east of New Zealand
|Tides and the environment
Would the world be different had there been no tides, no moon? Fortunately, we can find an answer to this question by simply visiting the Mediterranean Sea. This sea is so enclosed and relatively small, that tides don't exist (less than 20cm). Life (for people) is quite pleasant there and the underwater environment counts a high number of species, but the fishery does not sustain large volumes.