Amphidromic point

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An amphidromic point is a point within a tidal system where the tidal range is almost zero.

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).^[1] The amphidromic points are the dark blue areas where the lines come together.

Amphidromic points occur because of the Coriolis effect and interference within oceanic basins, seas and bays creating a wave pattern — called an amphidromic system — which rotates around the amphidromic point. At the amphidromic point, there is almost no vertical movement. There can be tidal currents as the water levels on either side of the amphidromic point are not the same.

Based on the accompanying figure, the set of clockwise amphidromic points includes:

• north of the Seychelles
• near Enderby Land
• off Perth
• east of New Guinea
• south of Easter Island
• west of the Galapagos Islands
• north of Queen Maud Land

Anti-clockwise amphidromic points include:

• near Sri Lanka
• north of New Guinea
• at Tahiti
• between Mexico and Hawaii
• near the Leeward Islands
• east of Newfoundland
• midway between Rio de Janeiro and Angola
• east of Iceland

The islands of Madagascar and New Zealand are amphidromic points in the sense that the tide goes around them (counterclockwise in both cases) in about 12 and a half hours, but the amplitude of the tides on their coasts is in some places large.

M₂ is the largest (semidiurnal) tidal constituent. The amplitude is half of the full tidal range. Cotidal points means they reach high tide at the same time and low tide at the same time. In the accompanying figure, the low tide lags or leads by 1 hr 2 min from its neighboring lines. Where the lines meet are amphidromes and the tide rotates around them; for example: along the Chilean coast, and from southern Mexico to Peru the tide propagates southward, while from Baja California to Alaska the tide propagates northward.

References and notes

1. ^ Picture credit: R. Ray, TOPEX/Poseidon: Revealing Hidden Tidal Energy GSFC,NASA. Redistribute with credit to R. Ray, as well as NASA-GSFC, NASA-JPL, Scientific Visualization Studio, and Television Production NASA-TV/GSFC