365 Days of Climate Awareness 39: The Coriolis Effect

The rotation of the earth has major, fundamental effects on the motion of fluids on the planet. Fluid motion is what we would call "decoupled" from, or only weakly coupled, to the solid earth. Wind and water move move with greater freedom with respect to the earth's crust than terrestrial animals like us humans, who use the surface of earth as our basis for motion. Influences like convection and gravity drive the motion of wind and currents, and not interaction with the solid earth.

But the earth is (roughly) spherical, and rotating. Looking down from above the north pole, this rotation is counterclockwise, rotation we call "cyclonic". From above the south pole, the earth's rotation is clockwise. When something is moving independently of the solid planet, this rotation becomes important. The earth will spin underneath the moving object, giving the object an apparently curved trajectory. This effect of apparently bending ocean currents and winds is called the "Coriolis Effect" (sometimes incorrectly called a force), named for the French scientist Gaspard-Gustave de Coriolis.

                         

Look at the example in the first illustration, where a hypothetical missile is fired from the north pole toward the equator. The missile's target might have been zero longitude along the Equator, but in the intervening one hour, the planet has rotated 15 degrees to the east, giving the missile, when viewed from earth, an apparently westward trajectory. That trajectory is only apparent, in to the frame of reference of a person viewing the missile from the planet surface. The missile is in fact flying true south when viewed from space, but the planet beneath has spun beneath it.

This happens with wind and ocean currents as well. In the northern hemisphere, wind and water take an apparent rightward curve due to the planet's motion. In the southern hemisphere, wind and water take apparent leftward curves. The effect ranges from zero at the equator, where gravity pulls perpendicularly to the earth’s axis of rotation, to a maximum at the poles, where gravity pulls along the earth’s axis. The more poleward you travel from the equator, the stronger the Coriolis effect becomes.

These rotational effects happen only on a large, what we would call "geophysical", scale. Gimmicks like sinks or toilets which swirl one way or another depending on the hemisphere they're in, are hokum. There's a number in meteorology and oceanography called the "Rossby number", named for the meteorologist Karl Rossby, which accounts for latitude, speed and spatial scale of the fluid in motion, and predicts how important the Coriolis effect will be. For sinks and toilets, a baseball or a football, it's not. For ocean currents and hurricanes, it's hugely important.

Tomorrow: hurricanes.

Be well!