Debunking myths about the Coriolis effect

Water does indeed drain clockwise in the Southern hemisphere. It also drains counterclockwise in the Southern hemisphere. In fact, it drains in whichever way it desires, regardless of geographical locale. I reluctantly came to accept this conclusion after spending a solid forty-five minutes last year draining and refilling my bathroom sink in Brisbane, Australia.

This myth, about the relationship between the Coriolis effect and the hemispheric drainage of toilet bowls, was debunked a long time ago. So here’s the conundrum: the Coriolis effect, with respect to the Earth, takes place on a rather massive scale. The spin of hurricanes, for examples, are largely influenced by the Earth’s spin. So how come the colossal inertial forces of our very planet seem to disappear at the smaller scale? And if not the drainage of water, then what does the Coriolis effect affect anyway?

The answers are all relative to the rotation. In fact, in a recent publication titled “Electomagnetically driven westward drift and inner-core superrotation in Earth’s core,” scientists have made conclusions about what direction Earth’s center spins. This may seem like an odd discovery to make so late until you take into effect the fact that different layers of the Earth spin in different directions. But more on that later.

First of all, what exactly is the Coriolis effect? The Coriolis effect results when a mass in a rotating system experiences a force that acts perpendicularly to the direction of motion.

In a common example, the Earth is the rotating system, and the atmosphere is the mass in question. You probably discussed the prevailing winds of the Earth back in some middle school science class (trade winds, westerlies, etc.). These winds are a result of the Coriolis effect and tend to dictate the movement and spin of major weather systems.

Hurricane systems from the lower Atlantic that drive upwards along the East Coast always turn counterclockwise. Cyclones in the Southern hemisphere rotate in the opposite direction. Of course, there are a variety of factors that affect the behavior of hurricane systems, but a combination of the major winds and the Coriolis effect are not small contributors.

Now that we’ve properly examined the Coriolis effect on the Earth’s atmosphere, let’s take a look at the Earth’s core. As was mentioned earlier, in recent news, scientists at the University of Leeds in the UK have come to a conclusion about the rotational behavior of both Earth’s inner and outer core. These findings were published in Proceedings of the National Academy of Sciences, not only because they solved a several-century old riddle, but because they have major implications for the future study of Earth’s core and flip-flopping of the geomagnetic field.

Scientists at the University of Leeds have been compiling data from seismometers measuring earthquakes for many years. It is from this data, that they discovered that the inner core, which is roughly the size of the moon and composed of solid iron, exhibits what researchers call, “superrotation,” in an eastward direction. The prefix “super” simply refers to the fact that it rotates at a speed greater than that of the Earth.

In contrast, the outer core, which is composed of molten iron, drifts in a vaguely westward direction. Scientists have finally linked the seemingly counterintuitive and conflicting spins of Earth’s layers through an examination of forces from the geomagnetic field. Does the Coriolis effect play a part as well?

In 1692, Edmund Halley, the prominent namesake of our favorite orbiting comet, pointed out a westward drift exhibited by the Earth’s geomagnetic field. A little over 300 years later, scientists have finally been able to link this behavior to the rotational dynamics of Earth’s core.

The geomagnetic field is created by the summation of induced magnetic fields. The movement of molten iron in Earth’s outer core creates electric currents, which, in turn, creates magnetic fields. The combined effects of temperature differentials, pressure gradients and — wait for it — the Coriolis effect all contribute to the movement and perpetuation of the geomagnetic field.

Even more interesting, changing geomagnetic field in turn induces electrical currents, which affect the fluid dynamics in the outer core. Researchers at the University of Leeds discovered that the geomagnetic field creates a force that pushes eastward on the inner core but also westward on the outer core. These opposite forces are a result of equal and opposite action. As you can see, the system is self-perpetuating.

So, if the Coriolis effect can play a part in the rotations of the Earth’s atmosphere and inner and outer cores, why doesn’t it affect water drainage?

The answer is actually rather simple. The Earth only rotates once in every 24 hour period. When put in this light, the rotation effects are actually very small and easily overcome by other forces that may be present. A tilted basin or a crooked faucet may be enough to create a tendency for water to spiral in a particular direction. As with all things, the effects of the Coriolis effect is simply relative.