Amphidromic point

[3] The term derives from the Greek words amphi ("around") and dromos ("running"), referring to the rotary tides which circulate around amphidromic points.

The waves reflect due to changes in water depth (for example when entering shelf seas) and at coastal boundaries.

[8] This side-way component of the flow due to the Coriolis force causes a build-up of water that results in a pressure gradient.

In an idealized situation, amphidromic points can be found at the position of these nodes of the total tidal wave.

Locations with more shallow water depth have their amphidromic points closer to each other as the distance of the interval (1⁄2λ) of the nodes decreases.

Secondly, energy losses due to friction in shallow seas and coastal boundaries result in additional adjustments of the tidal pattern.

[8] Consequently, on the northern hemisphere, the amphidromic point will be displaced from the centre line of the channel towards the left of the direction of the incident wave.

Furthermore, a study has shown than there is a pattern of amphidrome movement related to spring-neap cycles in the Irish Sea.

[15] The maximum displacement of the amphidrome from the centre coincides with spring tides, whereas the minimum occurs at neaps.

As a result, the reflection coefficient α is smaller and the displacement of the amphidromic point from the centre is larger.

Similar amphidromic movement is expected in other seas where energy dissipation due to friction is high.

[15][16][17] In this case, the amplitude and the phase of the tidal wave will still rotate around an inland point, which is called a virtual or degenerate amphidrome.

The position of amphidromic points and their movement predominantly depends on the wavelength of the tidal wave and friction.

As a result of enhanced greenhouse gas emissions, the oceans in the world are becoming subject to sea-level rise.

Consequently the position of the amphidromic points located at 1⁄4λ in semi-enclosed systems will move further away from the cross-shore coastal boundary.

This effect will be more pronounced in shallow seas and coastal regions, as the relative water depth increase due to sea-level rise will be larger, when compared to the open ocean.

Figure 1. The M 2 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.
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 M 2 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.