The Atlantic Coast by Harry Thurston

Author:Harry Thurston
Language: eng
Format: epub
Tags: ebook, book
Publisher: Douglas & McIntyre
Published: 2011-08-06T16:00:00+00:00

Maine’s many islands– some 4,600 of them– boost productivity in the gulf by creating local upwelling of nutrients from the sea bottom.

As coast dwellers know, tides vary in height during different times of the month and year, and this variation also relates largely to the Earth-moon celestial relationship, with the added influence of the Sun. At 40 percent that of the moon, the sun’s effect on tides is relatively large, given that it is so much farther away. Every two weeks, the sun, moon, and Earth are aligned, maximizing their combined effect. The result is the highest tides of the month—the spring tides. (The term does not apply to the seasons but derives from the Anglo-Saxon word “springen”—to leap up.) Conversely, at the first and last quarter of the moon, the solar and lunar influences are at right angles to each other, diminishing their combined effect and producing the smallest tides of the month, or neap tides. The extremes of tides, both high and low, vary over the course of the year, again as a result of the relative positions of, and distances between, sun, moon, and Earth.

These astronomical factors influence the range of tides the world over. It follows, then, that the conditions that produce the extreme tides in the Bay of Fundy have a local origin. The laws of physics tell us that every basin has a characteristic period of oscillation, which means that once set in motion, waters within it will slosh back and forth with a regular rhythm. Where an open bay communicates with the sea, however, the influence of the lunar-dominated tides must also be taken into account. If the basin and ocean tides are rocking back and forth in harmony with one another, they are said to be in resonance. This is what happens in the Bay of Fundy and largely accounts for the great tides experienced there.

When applied to water in motion, resonance is more commonly referred to as the “bathtub effect.” A home experiment can easily demonstrate what happens on a grand scale in the Bay of Fundy. Draw a shallow bath and give a push to the water at one end of the tub. A wave will travel to the other end and be reflected back. Just before it reaches your hand, give another push and you will observe that the water begins to slosh back and forth quite dramatically. In the bay, the moon acts as the hand in the bathtub analogy; it has been shown, however, that it is not the bay alone but the Bay of Fundy and the Gulf of Maine acting as a single oceanographic system that produces this effect. The period of time it takes for the tidal wave, or bulge, to travel from the mouth of the bay to the head is only nine hours—not long enough to cause resonance. When the bay and the gulf are considered together, the resonant period is 13.3 hours, close enough to be in resonance with the 12.42-hour, moon-forced tidal system.

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