Complexity of the tides: Actual tide patterns

c. Actual tidal patterns

Now consider the effect of the continents. The tidal bulge is actually not free to sweep around the globe, but instead encounters land masses. What happens when the tide encounters land masses? Two things happen:

(a) The water gets shallower as the tide approaches the land, slowing down the wave. But since the edge of the land is irregular, the slowing of the wave is irregular, too.

(b) The wave reflects from the edge of the land, and interacts with the incoming wave, creating very complex patterns.

Very rarely do we see an "ideal" pattern, such as the top picture on the right. Instead, the usual effect is that the tides sweep around the ocean basins like the hand of a clock.

This is something like the way water sloshes around in a tub.

Here is a diagram showing the tidal patterns for most of the world. Here's how it was created: For any ocean basin, take all the points where it's high tide at 12:00 on a given day and connect them with a line. Then draw a different line connecting all the points where it is high tide at 1:00; then another line connecting all the points where it is high tide at 2:00; and so on. (These lines are called isotidal lines)

You can clearly see how the tides sweep around the ocean basins like the hand of a clock.
A very clear pattern can be seen, for example, in the Pacific Ocean off the west coast of the United States. Look carefully, and you will see that the high tide sweeps up the west coast of the U.S. from south to north. At 1:00 it is down in Mexico, by 8:00 it has reached the west coast of Canada.
Another neat thing you will notice is that in every ocean basin there is a point where all the isotidal lines meet. At this point, there are no tides at all! This is a fairly astonishing result.
Can you find the places in the middle of the Atlantic and Pacific oceans where there are no tides?

Certain bays and other bodies of water have extraordinary tide ranges from low to high. The classic example is the Bay of Fundy in Canada, where the tide range is 12-15 m (40-50 ft), creating effects such as this:

This occurs when the bay is just the right size and shape to amplify the natural tide period and amplitudes. This is something like pushing a person on a swing: if you give a push at just the right time, you amplify the swing. Read more HERE.

Interesting effects happen at such places: sometimes you get a tidal bore, a little wall of water coming up a river or bay; or in places you can hear a roar caused by the tidal current.

Car on pier at low tide

SCSP

Water on Earth The Science of Tides

2. The complexity of tide patterns

c. Actual tidal patterns