Amphidromic Points: tidal spiders in a real World
During the past week we have seen how tides are formed, and we have been discussing many details about them. We have seen how the gravity pull from the Moon and the Sun elongate the water, which bulges on one side, but we have described how an opposite bulge is also formed as Earth is also being pulled. Then we looked at how the different positions of the Sun and the Moon are responsible for spring and neap tides, and how the tilt of the Moon can make diurnal tides at places and semidiurnal tides at others.
However, up to this point, we have considered Earth as an idealised spheric planet completely covered by water, but that is not the case. In fact, the presence of continents, coastlines and the uneven shape of Earth determine the tides as we observe them in “real life”.
The complex system of tides in the North Sea. The spider-looking features are amphidromic systems or amphidromic points. They will be explained below. Source (http://isis.uwimona.edu.jm/uds/GEOHAZARDS_2001/coastal2001/NorthSeaamphidromKING.jpg)
To understand how that works, let’s start with a big closed basin surrounded by continents, picture the North Atlantic but completely surrounded by land. In this case, water cannot freely bulge and follow the Moon’s track. Actually, water will try to swell in a similar fashion, as it undergoes the same physics. So, as Europe rotates towards the Moon’s position –where gravity pull becomes larger- the water will start accumulating towards the European continent. Then, as the rest of the basin and the American continent follow towards the region of maximum pull, water will switch sides and accumulate towards America. Hence, water will propagate from one side to the other as a sloshing wave. In a similar way as filling a box with water and rocking it from side to side; water will generate high and low tides by the sides, but the centre will remain with the same water height. We call that a node, which are the regions that act as a swelling axis. In that case, a “big box” would allow for a big slosh, whereas a small one would have a smaller swell. That is why the Mediterranean sea has very small tides.
Sloshing wave in a cubic tank. This is how the water in a basin would bulge if Earth did not rotate. Source (http://science.kennesaw.edu/~jdirnber/oceanography/LecuturesOceanogr/LecTides/1024.jpg)
That is not everything! We still haven’t thrown in the effects of rotation; the Coriolis effect. As water is sloshing from side to side, it is flowing and therefore being deflected by Coriolis. So imagine a high tide by Europe that is propagating towards America, since we are in the North Atlantic (northern hemisphere), the deflection is towards the right (northwards). In the opposite case, when flowing from America to the European continent, the deflection towards the right will be southwards. As a consquence we end up with a circular sloshing situation, similar to a gyre but flowing counterclockwise (in the northern hemisphere). Now, instead of rocking our box with water, we are swinging a bowl with water in circles, making a wave that is propagating by the sides of the bowl countercloclwise. This time, our node is just a point in the centre of the basin where there are no tides; it is conserving the water height. The further away from that point, the higher amplitude tides will have. We call those points “Amphidromic Points”, and tides propagate in a circular way around them.
The effects of rotation (Coriolis) generates a standing rotating wave, like a sloshing wave travelling around the sides of a tank with a central point with no tide at all. Source (http://ocean.tamu.edu/wormuth/tides/standingwavemotion.gif)
That was in a perfect closed basin, but they are all somewhat connected, and they influence on each other. The irregular shapes of continents and coastlines make different sorts of amphidromic systems (like the ones pictured below). They all have similar characteristics, though. The node at which there is no tide is called Amphidromic Point. Then, the lines that spread away from that point as if spider legs are called cotidal lines, and they picture the time lag of the tide (where the position of the wave’s crest is found in time). Finally, the dashed lines that circle around the amphidromic points are corange lines, and they represent the amplitude of the tide (how high/low it will be). The further away from the amphidromic point, the higher the tide will be.
Examples of different basins and how tides adapt to their shape. They have common features (cotidal lines for the wave’s phase, corange lines for the magnitude of the tide and amphidromic points where there are no tides). Source (http://ffden-2.phys.uaf.edu/645fall2003_web.dir/Ellie_Boyce/cotidal%20corange%20fig.jpg)
Generally, amphidromic points would be located at the centre of perfect shaped basins, but as said before, the distribution of land is quite irregular. Thus, the effects of all of them together set a system of amphidromic points all over the planet. You can see in the picture below where those amphidromic points are found, and how tides rotate around them periodically (anticlockwise in the northern hemisphere, clockwise in the southern one). Also, the colour pictures the intensity of the tides; blue for very small ones, and dark red for the largest on Earth. Another amazing feature is that both Madagascar and New Zealand act as an amphidromic point themselves, and tides rotate around them every 12.5 hours. However, they allow for strong tides by their shores.
Global distribution of amphidromic points with their cotidal lines and relative coranges. Source (http://science.kennesaw.edu/~jdirnber/oceanography/LecuturesOceanogr/LecTides/1116.jpg)
Finally, this is a global picture, but then again we can find smaller scale features in certain straits and small basins (like the complex system of tides showed in the first picture). How can sea level rise affect those systems? As you can see, tides are very complex and hard to predict, although those spider-shaped systems, by which tides structure, are an amazing manifestation of planetary power.