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Amphidromic point
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An amphidromic point, also called a tidal node, is a geographical location where there is little or no difference in sea height between high tide and low tide; it has zero tidal amplitude for one harmonic constituent of the tide. The tidal range for that harmonic constituent increases with distance from this point, though not uniformly. As such, the concept of amphidromic points is crucial to understanding tidal behaviour. The term derives from the Greek words amphi ("around") and dromos ("running"), referring to the rotary tides which circulate around amphidromic points. It was first discovered by William Whewell, who extrapolated the cotidal lines from the coast of the North Sea and found that the lines must meet at some point.
Figure 1. The M2 tidal constituent, the amplitude indicated by color. The white lines are cotidal lines spaced at phase intervals of 30° (a bit over 1
Figure 1. The M2 tidal constituent, the amplitude indicated by color. The white lines are cotidal lines spaced at phase intervals of 30° (a bit over 1 hr). The amphidromic points are the dark blue areas where the lines come together.
Figure 2. Resonance between an incident and reflected wave and the resulting total wave. At certain points (nodes), the amplitude of the incident wave
Figure 2. Resonance between an incident and reflected wave and the resulting total wave. At certain points (nodes), the amplitude of the incident wave and the reflected wave cancel each other out. At other points (antinodes), the amplitude of the incident wave and the reflected wave amplify each other. The respective distance between the nodes and antinodes are shown in the bottom right of the Figure and expressed in terms of wavelength.
Figure 3. Amphidromic system of the M2 constituent in the North Sea. The light-blue lines are lines of equal tidal phase for the vertical tide (surfac
Figure 3. Amphidromic system of the M2 constituent in the North Sea. The light-blue lines are lines of equal tidal phase for the vertical tide (surface elevation) along such a line, and the amphidromic points are denoted by 1, 2 and 3.
Tide
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Tides are the rise and fall of sea levels caused by the combined effects of the gravitational forces exerted by the Moon and are also caused by the Earth and Moon orbiting one another.
In Maine (U.S.), low tide occurs roughly at moonrise and high tide with a high Moon, corresponding to the simple gravity model of two tidal bulges; at
In Maine (U.S.), low tide occurs roughly at moonrise and high tide with a high Moon, corresponding to the simple gravity model of two tidal bulges; at most places however, the Moon and tides have a phase shift.
Low tide at Bangchuidao scenic area, Dalian, Liaoning Province, China
Low tide at Bangchuidao scenic area, Dalian, Liaoning Province, China
Low tide at Ocean Beach in San Francisco, California, U.S.
Low tide at Ocean Beach in San Francisco, California, U.S.
Low tide at Bar Harbor, Maine, U.S. (2014)
Low tide at Bar Harbor, Maine, U.S. (2014)