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The Coriolis force, as it applies to water movement in the oceans, is a result of the fact that the vast majority of the ocean’s volume is only loosely coupled to the surface of the Earth. The “no slip” condition, discussed in the section on viscosity, applies only to the boundary formed by the ocean bottom. The remaining volume of the ocean moves with the Earth as it rotates, but there is slippage due to the near absence of frictional coupling.

Consider the fact that the Earth is a sphere rotating from west to east. Now imagine that the Earth is sliced in half at the equator and we are looking down at it. It would appear as a disk that rotates through 360° in a 24‐hour period. Now imagine taking another slice of the Earth about halfway between the equator and the North pole. This second slice will also appear as a disk but a smaller one than the disk described by the equator (Figure 1.7a). It will also rotate through 360° in a 24‐hour period but, because the circumference of this smaller disk is less than the equator’s disk, its velocity of rotation is also less. A city on the equator – Bogotá, Colombia, for example – is moving at a velocity of 1668 km h⁻¹ as the Earth rotates. Meanwhile Ottawa, Canada, at about 45 °N, is moving at 1180 km h⁻¹ (Figure 1.7b). The difference in relative velocities with latitude accounts for a difference of nearly 500 km traveled per hour and is at the heart of the Coriolis effect.

A classic example of the effect of the Coriolis force is to envision a missile or cannonball being fired northward from Bogotá at the equator. Let us stick with the missile. Because it was fired from the equator, the missile has an eastward velocity of 1668 km h⁻¹ when it leaves the ground, along with its northward velocity. As it speeds north the Earth is rotating beneath it, so the velocity at the Earth’s surface is declining with increasing distance from the equator. The result is that the missile, when viewed from the perspective of missile control at the equator, has veered to the right or clockwise (Figure 1.8). Imagine now a similar experiment with the missile being fired from latitude 45 °N toward the equator. The missile has an eastward velocity of 1180 km h⁻¹ when it leaves the launch pad and is heading south toward the equator, which is moving east at 1668 km h⁻¹. In this case, the Earth is literally moving east more quickly than the missile is moving as it heads south and, once again, the missile appears to veer to the right or clockwise.

This brings us to three general rules about Coriolis force. In the northern hemisphere, the Coriolis force deflects moving bodies, including fluids, in a clockwise direction: to the right. In the southern hemisphere, deflection is counterclockwise, to the left. Third, Coriolis force is nonexistent at the equator and strongest at the poles. Consider also that the influence of the Coriolis force will be very much greater on slow‐moving bodies such as parcels of water than on a quickly moving object such as our imaginary missile, which spends only minutes in the air.

The effect of the Coriolis force on ocean circulation is evident in Figure 1.5. In the North Atlantic gyre, the westerlies initially drive the water to the northeast, but the Coriolis force deflects the currents to the right, producing an easterly flowing current that is eventually deflected southward by the continental margins of Europe and Africa. The flow of the current is approximately 45° to the right of the wind direction. Its analogue in the South Atlantic, driven westward by the trade winds, is the North Equatorial Current. The circuit is completed by the powerful Gulf Stream on the western limb and the Canary Current on the eastern one.

Figure 1.7 Coriolis effect. Differences in velocity of Earth's surface as a function of latitude. (a) Equatorial view. (b) Polar view.