This was a curious question...
|The tide comes in near Gualala, California. P/C Dan.|
... which is my very favorite kind of question.
This Challenge really started from a chance observation ("the times of the tides are different here vs. there") and then a bit of a thought experiment ("what would it be like if the tides were the same everywhere").
When I thought it through, I realized that I'd never really considered how tides work at different places around the world. It was something to see that different places on the west coast of North America have different tides at different times. Of COURSE they must be different (I realized)--there's no way that high tide in Juneau, Alaska could happen at the same time as in Cabo San Lucas, Mexico (at the very bottom of the Baja peninsula).
And then, because this is the way thought experiments work (at least in my head), I wondered when high tide would happen if I looked westward. What time is the tide in Hawai'i or Japan? And of course, there's no way the tides on the east coast are the same as those on the west coast.
That's when I had my AHA moment--tides are complicated--and that's what led to this week's Challenge.
1. Is there some way to understand how the tides move around on the planet? If the time of high tide varies so much on the west coast of North America, there must be a similar variability in low tides at different locations. Is there some way to understand how tides vary over the course of a day? What's the best way to get an understanding of this global behavior?
I tell you what I did, and then review some of the responses in the comment thread.
My first thought was to get some kind of global visualization of the tides over time. That is, I was looking for some kind of animation to make the tidal changes over time AND space very apparent.
When I think of animation, I turn to YouTube. There, I did a search for:
[ global tides ]
and found this truly remarkable video from NASA:
Download the video from their site (Barotropic tides)
It's worth watching this video a couple of times to notice the patterns. If you focus in on the northern Pacific Ocean, you can see the tides rotating around the Pacific basin like clockwork. (White is high, black is low.) Notice how the white blob saunters up the west coast of North America (the high tide), followed by a black blob. If you look carefully, you can even see the large high tide followed one cycle later by a lower high tide.
The video is also worth listening to as they give a nice summary of what's going on here. This is the transcript (lightly edited) of the video:
Ocean tides are not simple. If our planet had no continents, tides would be hemispheric-sized bulges of water moving westward with the moon and sun.
This animation shows the tides as a complex system of rotating and trapped waves with a mixture of frequencies. In many oceans we see waves rotating clockwise or anticlockwise, with small amplitudes in the middle of the ocean and high amplitudes around the boundaries, especially along the coasts of northwest Europe and Britain.
Waves are trapped and rotating around New Zealand, causing a high tide on one side of the islands with a simultaneous low tide on the other side. The Topex/Poseidon and Jason satellite altimeter missions were designed to observe and record this complexity.
Altimeters on these missions acted as flying tide gauges. And after several years collecting data, researchers could analyze the signals at each ocean location to determine the tidal characteristics.
With that knowledge, plus near-perfect knowledge of the motion of the sun and moon, the tide can be predicted at any location and at any time in the future.
Note that the large short-scale waves on continental shelves and marginal seas, as well as polar regions outside the orbits of Topex and Jason, and are still inadequately known. Scientists expect that SWOT (Surface Water and Ocean Topography satellite) will provide wide-swath altimeter readings that will help improve tidal knowledge in these regions. (SWOT is due to launch in November, 2022)
As the narrator points out, take a close look at New Zealand--the tides really do race around the island, with high tides on one side and lows on the other. It's a truly remarkable difference over a relatively short distance.
Also take note of the crazy tides in northeastern bays of Canada (such as Hudson Bay) and off the east coast of Argentina near Tierra del Fuego. This side-by-side capture of two video frames (roughly 6 hours apart) shows the high/low tides, and you can clearly see the variation.
Bottom line: Tides are a world-wide phenomenon. You have to take a large-scale perspective to see what's going on... and why tides and time vary so much between San Diego and Juneau.
We had a bunch of comments on this topic, which happily surprised me. I had no idea so many people were interested in the tides. A few cherry-picked summary comments:
Arthur points out that "The specific features of a coastal location can affect how water moves, creating a lag that influences the times of the tide and other conditions. These variations give each coastal location a unique tidal pattern." He also points us to this remarkable time lapse of the extremely large tides in the Bay of Fundy (one of those NE Canadian bays). Worth a look:
Jon tells us that "On the west coast of North America the returning tide is forced by the Coriolis effect of Earth's rotation to be pushed to the right--landward-- as it races from the equatorial region at 700 mph to the north." (Details about the Coriolis effect, if you're a curious person. And no, the Coriolis effect doesn't make your bathroom drain turn one way north of the equator and the other way south of the equator.) Interesting thing to consider: Why doesn't the tidal motion go clockwise in the southern hemisphere? (Another Challenge?)
Remmij gave us the delightful discussion of oceanic whirlpools (e.g., the Old Sow, a whirlpool in the Bay of Fundy), and a pointer to how the big waves form off of Nazare, Portugal. (A video showing crazy surfers on those waves.)
Remmij also brought up the idea of creating mathematical models of tides with dynamic systems. A wonderful topic, but a bit beyond what we'll cover here. To see a model of the tides (and not the actual data), you might enjoy this animated tide model--compare it to the actual data visualization above.
|Dan standing on the rocks at high tide near Gualala, CA, pondering time and tides.|