|MadSci Network: Earth Sciences|
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).
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Hope that helps!
Rob Campbell, MAD Scientist
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