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5.1 Tides and Tidal Forcing     continued...

While the rise and fall of sea level due to tides is the most apparent aspect of tides, it is the tidal currents that are the direct manifestation of tides, and the sea level rise or fall is due to resulting convergences and divergences. The sun and the moon are the only important celestial bodies in producing terrestrial tides. While the moon is much smaller than the sun, it is nevertheless more important for tidal processes, because of its proximity to the Earth. It is the small imbalance between the centrifugal force and gravitational attraction of the moon on the water column that give rise to horizontal tractive forces, causing water motions that tend to cause two bulges in the sea surface, one immediately underneath and the other on the other side of the globe. These bulges tend to rotate around the globe along with the moon resulting in semi-diurnal tides with a period of half a lunar day (12.4 hours), even though the Earth's rotation is diurnal (period of 24 hours). To model tidal motions then, it is necessary to prescribe the astronomical forcing. The tidal potential term in the equations of motion can be expressed as a Fourier series, with each term representing a tidal constituent. While there are tens of such constituents in a general expansion, usually in the deep ocean, only 11 so-called primary components are important. Of these, four are semi-diurnal, M2, N2, S2 and K2; four are diurnal, K1, O1, P1 and Q1; and the rest, Mf, Mm and Ss are long-term.