Amphidromic point

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

[
₂ tidal constituent the amplitude indicated by color. The white lines are cotidal differing by 1 hr. [ Picture credit: R. Ray, [http://svs.gsfc.nasa.gov/stories/topex/tides.html 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] ] Amphidromic points occur because of the coriolis effect and interference within oceanic basins 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.

Amphidromic points occur at Tahiti, off the coast of Newfoundland, near the South Shetlands, midway between Rio de Janeiro and Angola, at three points in the North Sea and in general at several more points in the Pacific Ocean, Atlantic Ocean and Indian Ocean. The island of Madagascar is an amphidromic point, as is New Zealand.

"M"₂ 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, and the low tide lags or leads by 1 hr 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