Tides

The contribution of the continents and various ocean depths will have an great affect on the tidal wave. Because the continents separate the world into three north-south basins, it is impossible for tides to move west-east across Earth as forced waves.

Tidal wave

Only around Antarctica can tides move all the way around Earth. This restriction imposed by the continents adds to the complexity of open-ocean tides. In addition, because the waters in an open-ocean tide move over substantial distances for long times, the tide is modified by the Coriolis effect. To see how open-ocean tides behave, we can have an look at the tide in the Atlantic Ocean. The northern basin in the Atlantic Ocean, is open to its south but nearly closed off at its northern end. As the tide moves northward into the North Atlantic basin, the water is deflected to the right because of the Coriolis effect, causing a tilted water surface, as shown in the figures below.

Atlantic ocean

As the tidal wave continues to rotate in the basin, the surface tilts in the opposite sense. The tilted surface rotates around the basin, once during each tidal wave period. Near the center of such a system is a point where the water level does not change, the amphidromic point. The tidal wave in the North Atlantic travel around an amphidromic point in a form which approximates to a Kelvin wave. The wave moves from Portugal along the edge of the north west European continental shelf towards Iceland and from there west and south past Greenland to Newfoundland (see figure).

There is considerable leakage of tidal wave energy to the surrounding continental shelves and to the Arctic Ocean, so the wave that is reflected in a southerly direction is much weaker than the wave travelling northwards along the European coast.

Every basin has a natural period for standing waves determined by its dimensions. If that natural period is near 12 hours, the standing-wave component of the tide is well developed. Because of their natural periods, ocean basins respond more readily to certain constituents of the tide-generating forces than others. The Gulf of Mexico, for example, has natural period of about 24 hours. Therefore, it responds more to daily tidal constituents than to semidaily constituents, and as a result, much of the Gulf has a daily tide. The Atlantic Ocean responds readily to the semidaily constituent of the tide-generating forces and has semidaily tide. The Pacific and Indian oceans respond to both daily and semidaily forces and these basins have mixed tides.

The tide in a bay, harbour, or sea is greatly influenced by the magnitude of the ocean tide at its mouth. It is also influenced by the natural period of the basin. A tidal wave advancing through a bay or harbour is reflected by the coast of the basin. Where the natural period of the basin is near the tidal period, it is possible to set up a large standing wave, giving rise to large tidal ranges.

Bay of Fundy

One example is the Bay of Fundy on the Canadian Atlantic coast, the natural period is apparently about 12 hours. Spring tides in the inner part of the bay have ranges of 16 meters, which is largest in the world.

Another factor influencing the tide in a bay, harbour, or sea is the cross section of the opening through which the tidal wave must pass. For example the small tidal range (less than 0.6 meter) in the Mediterranean Sea results from the small opening through the narrow strait of Gibraltar from the Atlantic Ocean

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