MadSci Network: Earth Sciences

Re: Why do some places on Earth have 2 high tides per day?

Date: Wed Dec 6 11:08:41 2000
Posted By: Rob Campbell, Ph.D Candidate, Oceanography, University of British Columbia
Area of science: Earth Sciences
ID: 975253131.Es

Hi John:

Tides are a very complicated phenomenon! The tug of the moon's gravity is one of the strongest components to the tides, and in most places there are only two tides in a day. However, there are many forces acting on the oceans to produce tides. In addition to the pull of the moon (known to physical oceanographers as the M2 tidal component), the sun also has an affect on the tides (known as the S2 component). As well, the orbits of the moon around the earth, and the earth and moon around the sun are not perfectly circular, which causes the tides to vary over time. Furthermore, in shallow water the water tends to "slosh around" a bit- that affects the tides in varying ways, and can alter the resulting tidal patterns quite a lot (this would be the resonance you mentioned). Shallow water effects are generally responsible for odd tides (things like 4 tides in a day).

For more information, check out the following links:

  • Tides A good overview.
  • Ocean Tides More detail, this will give you an idea of some of the complexities of tidal modeling (don't let the math scare you!)
  • Glossary of tide terminology You'll find in here a list of all of the various tidal components beyond the M2 and S2 components that I mentioned- there are quite a lot: Q's, R's, T's, N's to name a few, and they all deal with various elliptical and shallow water effects. There's also good descriptions for other tidal terms.
  • Hope that helps!
    Rob Campbell, MAD Scientist

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