A node where the rise and fall of a given constituent cancels out, so the range is zero or nearly so. High water for that constituent sweeps around the point once per cycle, usually anticlockwise in the Northern Hemisphere, with cotidal lines radiating from it like spokes and range increasing outward. The North Sea, for example, holds three M2 amphidromic systems, which is why its coasts show such varied tides.
Station sits right by the amphidromic point, so M2 is tiny and the diurnal constituents run the show.
Close to an amphidromic point a short move brings a large change in tide timing, which makes tide zoning there notoriously difficult.
Also heard as
- amphidrome
- amphidromic system
- nodal point
Related terms
Cotidal line
A map line through every place where high water of a tide or constituent arrives at the same moment.
Corange line
A line joining places that share the same tidal range or the same constituent amplitude.
Kelvin wave
A long wave trapped against a coast by Earth's rotation, the form in which tides travel around ocean basins.
M2
The principal lunar semidiurnal constituent, period 12.42 hours, usually the largest part of the tide.