I recently moved to Prince Edward Island (PEI), and have noticed what I find to be a strange (and annoying) phenomenon with the tides. In Charlottetown, on the coast facing the mainland, the tide times are nearly exactly opposite to the tides in, say, North Rustico, directly on the other side of the island. In Charlottetown tomorrow, high tides are at around 0400 and 1530, and low tides are at 0930 and 2200. In North Rustico, high tides are at 1030 and 2350, and low tides are at 0420 and 1700. These locations are only approximately 25 km away from one another as the crow flies. As someone from the interior, I had always been taught that tides moved around the world more or less uniformly, with some minor variations for geography and ocean currents. Is that a major simplification of what happens? Is this sort of variation over a very short distance normal? Why would they be that different? Maybe something to do with the way the tides flow around the island and through the Northumberland strait?
It appears that parts of PEI have diurnal tides, or only on high/low tide per day. So there are odd things going on around PEI and New Brunswick, and that probably adds to the irregular nature of the local times.
http://www.zollitschcanoeadventures.com/articles/diurnaltides.html
Likely this. The usual model we are taught about tides simplifies things significantly. Clearly water is not rushing about the planet fast enough to keep pace with the rotation of the Earth and maintain the bulge. Water has mass and viscosity, and the sea is, relative to the size of the Earth almost infinitesimally shallow. (I remember the point being made as - if the Earth was an orange, you would not notice it was wet.) So tides flow, but the water never reaches the point where the bulge points at (or away from) the moon. There is a lag. And there is water in partly enclosed regions, or water where the flow is restricted in some form. A large body of water connected to open ocean through a restriction will be emptying and filling with a significant phase lag behind the ocean’s rise and fall, and may develop local oscillations that result in very odd tidal movements. Where I live we get dodge tides: occasionally an entire tidal cycle just doesn’t happen and the water stays at the same level. We live on a relatively shallow gulf connected to the Southern Ocean by a narrow straight. The geometry of the system results in a very large local oscillator coupled to the ocean and its periodic tides.
Another place for insane tides is the English Channel. One of my favourite machines ever is Lord Kelvin’s tidal prediction machine and its partner analysis machine. There are mechanical analog computers that computed then used the Fourier coefficients of measured tides to then predict tides into the future. With enough measurements and enough coefficients calculated it would auiomatically take into account sun, moon, and local features such as islands and bathymetry and create tables good for years to come.
@Francis_Vaughan is on the money. The oceans can be thought of as a complicated damped non-linear system, being driven at a couple of primary frequencies (twice per lunar day & twice per solar day). This system resonates at the driving frequencies but also at some of their harmonics, sum and difference frequencies, etc. Each oscillation at a particular frequency can be looked at separately, and these are called the “constituent tides”.
Here’s a map of the largest component of the tides: the so-called M2 constituent, which oscillates twice per lunar day. You can see that it’s not nearly as straightforward as “all points at the same latitude are at the same point in the tide cycle.” In particular, note that New Zealand & Madagascar are in the same situation as PEI: when it’s high tide on one side of the island(s), it’s low tide on the other. I believe this is the case for the British Isles as well, though it’s hard to tell from this animation alone.
Ah, here’s a map for the British Isles. The solid lines tell you where a given point will be in the tide cycle at a given time; so a point at “2h” will be two hours “ahead” of a point marked “0h”. The tides do “rotate” around the isles to a certain extent, but the presence of the Irish Sea makes things even more delightfully complicated.
One more and I’ll stop: there are maps of the phases and amplitudes of the tidal constituents around PEI available on pages 16, 18, and 19 of this PDF. This PDF from 1958 was a study of how tides would be affected if a completely solid causeway (!) was built across the Northumberland Strait, but it includes assessments of how the tides around PEI behave naturally as well.
Ireland has a similar situation to PEI. The tidal wave coming in from the Atlantic diverges at the south-western tip of the country (near the Skellig islands) to flow north along the west coast and east along the south coast. The two streams flowing clockwise around the island meet at the north-east coast (near Saint John’s point) about 7 hours later, by which time it is low tide at the Skelligs.
As pointed out above, the tidal wave doesn’t have a wavelength of 12,500 miles or half the earth’s circumference (which would give rise to absurd speeds); it’s more like 300 miles, comparable with the size of Ireland.
Thanks all! Yes, it makes sense that the long skinny island and the strait would mess with the water flow, I was just surprised there was such a difference over such a short distance. I suppose that the islands shape just makes it very noticeable, since you’re never more than 20 or 30 minutes from the other side of the island.