On reading about tide formation and the role of moon's gravitational force, I came across the term amphidromic point. Could someone please elaborate this concept?

  • $\begingroup$ Have your read the wikipedia article? If so, what exactly did you not understand? $\endgroup$ – Erik Jun 25 '19 at 8:54
  • $\begingroup$ The article mentions,"Amphidromic points occur because the Coriolis effect and interference within oceanic basins, seas and bays creates a wave pattern " I am not getting an idea on what leads to this interference and how exactly we get a point where tidal amplitude is zero? $\endgroup$ – William Gilbert Jun 25 '19 at 10:53
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    $\begingroup$ The existence of basins/continents itself is the interference. Tidal waves can't move freely but are blocked, the water "sloshes" around in any given ocean/sea. If the body of water is big enough, the Coriolis effect changes the linear sloshing (due to rotation of the Earth and the movement of the Moon) to a circular movement, which rotates around the amphidromic point. One side of the body of water has a high tide, the other low tide - and right in the middle there is no significant change in the water depth. $\endgroup$ – Erik Jun 25 '19 at 11:20

This will be a bare-bones answer, because I don't have time to do more. If somebody wants to do a more complete answer, with a figure or two, feel free; otherwise I'll try to come back and expand this in a day or two

Good question! If one is familiar with the simple explanations of tides (bulges of water following the moon around the earth), this doesn't make sense. However, that way of thinking only gives us equilibrium tides, which are what we would have if Earth was an ocean planet. In reality, we have continents, and they get in the way.

Most tidal movement actually consists of water "sloshing" around ocean basins - e.g. across the Atlantic between North America and Europe. But because the planet is spinning, Coriolis force applies and tends to impart rotation to straight-line movements. This results in the tide tending to form two-dimensional waves that go around ocean basins in a roughly circular pattern. You can get a similar wave going in a hot-tub, with a bit of effort! At the outside of the basin, where the wave meets the land, the amplitude tends to be the greatest - but in the middle, it's close to zero.

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  • $\begingroup$ Thanks for the update, Simon. Could you please explain with an example of the formation of an amphidromic point and how the direction of circular motion becomes clockwise or anticlockwise. It's mentioned in reference materials that the circular motion is anticlockwise in Northern hemisphere. But, Coriolis force deflects right in Northern hemisphere. If so, a wave travelling from west to east of ocean in N.hemisphere should get deflected towards the south and the circular motion around the amphidromic point should be clockwise, right? $\endgroup$ – William Gilbert Jun 26 '19 at 5:40
  • $\begingroup$ @WilliamGilbert Does earthscience.stackexchange.com/q/370/39 help re direction of circular motion in cyclones, etc? The way it's explained there doesn't quite apply for tidal systems. Re your question: That depends where it's deflected south. If it's deflected soon enough, on the western side of the ocean, it'll be a counterclockwise motion. But I'm not sure that this is a meaningful way of talking about this, since these systems are long-term periodic ones and don't "form" or disperse much. Maybe others can explain better... $\endgroup$ – Semidiurnal Simon Jun 26 '19 at 20:24
  • $\begingroup$ @John no it isn't! :-) $\endgroup$ – Semidiurnal Simon Jun 26 '19 at 20:24

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