Tides are generated as a result of
gravitational attraction by the
Sun and
Moon. This gravitational attraction results in a
tidal force that acts on the
ocean.) in the ocean is much smaller (i.e.
D/
λ . The amplitude of the waves decreases further away from the coast and at certain points in the middle of the basin, the amplitude of the total wave becomes zero. Moreover, the phase of the tide seems to rotate around these points of zero amplitude. These points are called amphidromic points. The sense of rotation of the wave around the amphidromic point is in the direction of the Coriolis force; anticlockwise in the
northern hemisphere and clockwise in the
southern hemisphere.
Semi-enclosed basin In a semi-enclosed basin, such as the North Sea, Kelvin waves, though being the dominant tidal wave propagating in alongshore direction, are not able to propagate cross shore as they rely on the presence of lateral boundaries or the
equator. Tidal waves are not perfectly reflected, resulting in energy loss which causes a smaller reflected wave compared to the incoming wave. 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.
Amphidromic points and sea level rise 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. As the water depth increases, the wavelength of the tidal wave will increase. Consequently the position of the amphidromic points located at λ in semi-enclosed systems will move further away from the cross-shore coastal boundary. Furthermore, amphidromic points will move further away from each other as the interval of λ increases. 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. Moreover, the amount of sea-level rise differs per region. Some regions will be subject to a higher rate of sea-level rise than other regions and nearby amphidromic points will be more susceptible to change location. Lastly, sea-level rise results in less bottom friction and therefore less energy dissipation. This causes the amphidromic points to move further away from the coastal boundaries and more towards the centre its channel/basin. ==In the
M2 tidal constituent==