Why the Spinning Earth Doesn't Impact Swells

Tony Butt

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Updated 48d ago

The Coriolis force is crucial when it comes to understanding the atmosphere and ocean, as I pointed out in a previous article (HERE) but there are also a few myths associated with it.

If you mention the word ‘Coriolis’ to anybody on the street, there is a good chance that they will tell you it is what makes the water go down the sink one way in the northern hemisphere and the other way in the southern hemisphere. Some people will claim to actually prove it. A few metres either side of the equator, typically in places like Kenya or Ecuador, you can find people who will show you how it works.

But of course, it’s a total myth. For a start, the Coriolis force is zero at the equator and very, very close to zero until you start getting hundreds of kilometres either side, so those demonstrations where people jump from one side of the line to the other are absurd. But even if you compared two sinks at high latitudes where the Coriolis force is significant, it still wouldn’t work, because the scale is all wrong. The Coriolis force depends on the Earth rotating underneath a moving object. In this case, the object (the water in the sink) moves such a short distance in such a short time that the rotation of the Earth underneath it isn’t nearly enough for any effect to be noticeable.

For the Coriolis force to have a significant effect the sink would have to be ridiculously large or the water flow would have to be ridiculously slow. More specifically, the diameter of the sink [in metres] divided by the velocity of the water [in m/s] would need to be about 10,000 before the Coriolis force started to have any effect. For example, with the water flowing at 1 m/s, the sink would have to be at least 10 km in diameter; or with a one-metre diameter sink, the water would have to flow slower than 0.1 millimetres per second.

Another thing you might be forgiven for thinking is that long-distance swells are affected by the Coriolis force – namely they swerve to the right in the northern hemisphere and to the left in the southern hemisphere. So, if you were tracking a swell thousands of kilometres from one side of the ocean to the other, you would need to take into account the Coriolis force. After all, the scale is big enough, right? Using the scaling argument above, the distance covered by ocean swells relative to their velocity is easily enough for the Coriolis force to be significant.

But no, swells are not affected by the Coriolis force. They don’t bend to the right in the northern hemisphere or to the left in the southern hemisphere. They travel in straight lines from the point of view of someone on the Earth’s surface.

The particles underneath a wave in deep water move in closed circles

The particles underneath a wave in deep water move in closed circles

Why? Because ocean swells do not carry any physical material from one place to another; they just carry energy. In the deep ocean, the particles beneath the waves just pass energy from one place to another, but the water itself doesn’t move any further than a few hundred metres. The particles under the waves move in closed circles similar in size to the wavelength of the waves, which is still too small for the Coriolis force to have much effect.

Ocean currents, on the other hand, are affected by the Coriolis force, because ocean currents are giant streams of water that creep around the Earth, transporting water from one part of the planet to the other.

Tides, too, are affected by the Coriolis force. The tide can be considered a wave, but one with a very long wavelength. Instead of a few hundred metres, the tide has a wavelength of thousands of kilometres. As a result, the movement of the particles beneath the surface is of such a large scale that it is easily deflected by the Coriolis force.

Cover shot by Helio Antonio.