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Amphidromic point

Posted to Maritime Musings (by on November 25, 2014

The point in an oceanic basin with zero tidal range

An amphidromic point or tidal node is a point of zero change of the amplitude or height harmonic constituent of the tide within an oceanic basin.  The tidal range (the difference between high tide and low tide) increases, but not uniformly, with distance from the amphidromic point.  Cotidal lines (lines connecting points experiencing high or low tide at the same time) can be plotted radiating from the amphidromic point.  Due to the Coriolis effect, the tide wave or kelvin wave  (as shown by the cotidal lines) bends to the right in the northern hemisphere, resulting in a counter-clockwise circulation.  Tide circulation is clockwise in oceanic basins in the southern hemisphere.  Because tide waves do not travel with constant speed due to varying depth and land mass obstructions, the cotidal lines are not evenly spaced or consistently shaped.  The tide wave progresses around the amphidromic point once each tidal period.  At each point in the oceanic basin, water levels vary sinusoidally due to a combination of factors.  Corange lines (lines connecting points experiencing the same tidal range) can be plotted forming irregular circles concentric around the amphidromic point for the basin.  Think of the oceanic basin as a very large bowl of water.  As the water sloshes around, high on one side and low on the opposite side and then reversed, there is a point near the center of this symmetrical bowl where the water level is relatively stable – that is the amphidromic point.  An oceanic basin, though, is not symmetrical.  Therefore, the amphidromic point is seldom at the center of the basin.  In addition, due to the numerous factors that influence tides (moon, sun, wind, atmospheric pressure, etc.), the amphidromic point moves somewhat as conditions change.  The oceanic basin under the influence of a single rotating tide wave is referred to as an amphidromic system.