An amphidromic point is a point within a tidal system where the tidal range is almost zero. The tidal range (the height difference between high tide and low tide) is zero at the amphidromic point and increases with distance from this point.[1] [2][3] These points are called nodes.[4]
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.[6][7] At the amphidromic point, there is no vertical movement from tidal action.[8] There can be tidal currents as the water levels on either side of the amphidromic point are not the same.
In most locations M2 is the largest (semidiurnal) tidal constituent, with an amplitude of roughly 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.
Based on the accompanying figure, the set of clockwise amphidromic points includes:
Anti-clockwise amphidromic points include:
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.