Owen Cumming
Owen is a science communicator with a background in ecology and evolutionary biology. Owen enjoys surfing, hiking and convincing himself that his terrible woodworking has a “rustic” look. He firmly believes that quokkas’ smiles imply malicious intent.
Tides are the natural rise and fall of ocean levels that happen as a cycle every day, all around the world. Unlike waves at the beach, tides don’t depend on the weather: they operate like clockwork and can be predicted years into the future.
For many people, tides are a big deal. In coastal areas, they can control when and where people travel, what they do, even what food is available. Knowledge of tides has helped humans spread out to nearly every point on the globe. Being aware of the tides can be a matter of life and death, because tides can cause rip currents or flooding or expose ships to dangerous reefs!
If you happen to pay attention to the tides, you might have noticed that the tides in Perth are a bit odd.
THE EARTH AND ITS TIDES
Most places in Australia have semidiurnal tides, meaning there are two high tides and two low tides (almost) every day. This is due to the way the Moon and Sun’s gravities pull on the ocean.
But Perth has only one high tide and one low tide per day.
Strange, no?
What’s more, Perth’s tidal range (the height difference between high and low tide) is tiny compared with other parts of Australia.
Tides around Perth tend to only rise and fall by 1 metre or so, yet other areas regularly vary by 2–3 metres. In some places, the difference can be up to 11 metres!
So why are Perth’s tides so small and only half as frequent?
UNDERSTANDING PERTH’S TIDES
Well, to start with, while the Moon is responsible for moving the tides, it’s pulling water around a spinning planet covered in oddly shaped continents.
The rotation of the Earth, the placement of the continents and the shape of the ocean floor make the tides slosh back and forth like water in a bathtub or swirl around in enormous oceanic whirlpools.
All this sloshing and spinning creates spots where the tide barely moves the ocean levels at all. These spots are called amphidromic points, and there are only about a dozen in the whole world. One just happens to be off the coast of South West WA.
Essentially, the tides are dodging Perth.
That’s part of the reason why Perth has fewer tides, but there’s more.
The Earth spins on an angle of 23.5° compared to its orbit around the Sun, which allows the seasons to occur. But the Moon’s orbit around the Earth doesn’t match that tilt and is close to being in line with the Sun.
That means the tides don’t get pulled evenly around the equator. The Earth spins at an angle to how the tides move, so certain parts of the world can pass through one high tide bulge but not the other.
This phenomenon, called the diurnal inequality, along with South West WA’s amphidromic point, is the reason why there are only two tides a day in Perth rather than four!
TIDES ARE A COMPLICATED SCIENCE
Now there is more to it, but beyond this point, it starts to get really complicated – if it wasn’t already!
But next time you find yourself on the South West WA coast, just remember that Perth sits at the meeting point of global and astronomical phenomena! Our knowledge of these systems is, and always will be, a crucial part of surviving and thriving on our coasts.
Phew! That was a lot. I think I might go down to the beach and relax.
I wonder what the tides are doing?