The Coriolis Effect: How Earth's Rotation Shapes Winds, Oceans, and Storms

The Coriolis force, or Coriolis effect, named for the French scientist Gaspard-Gustave de Coriolis (1792-1843), is a curious consequence of momentum’s conservation in the special case of a rotating system. Imagine a cannon at the equator. If we aim at a distant point due north from our position and fire, the cannonball will, bizarrely, appear to curve rightward away from our target, despite the absence of any apparent force acting on it. This apparent bending of a longitudinal trajectory is called the Coriolis effect.

This results from Earth’s rotation; the Earth is a spinning sphere, much wider at its center than at its poles, and points at the equator are moving at a strictly greater velocity than points northward or southward. Put another way, any point at Earth’s equator must cover a longer distance in a day than do points closer to the poles. Due to Earth’s eastward rotation, we, and our cannon at the equator, are moving eastward at tremendous velocity, and the cannonball maintains this eastward velocity when we fire it, hence why it seems to slide to our right. Similarly, if we were at the North Pole and fired our cannon straight southward, our target at the equator would have sped away to the east (our left) when the projectile reached it, and we would miss. Essentially, an object moving northward from the equator will end up over ground moving more slowly than its starting position and retain its eastward momentum, causing the perceived (not actual) curving of its trajectory above Earth’s surface.

Several important consequences of the Coriolis effect influence Earth’s climate. First, as winds blow away from the high-pressure areas over Earth’s poles, the Coriolis effect bends their straight longitudinal trajectory into various curvatures, such that Earth’s high-altitude prevailing winds follow curving paths. This in turn creates phenomena like trade winds and Hadley circulation cells, among others. The circulation of the world’s oceans depends heavily on prevailing wind currents, and as such, the curving of wind by the Coriolis effect bends ocean currents as well. This results in an ocean gyre, a system of major ocean currents in a sort of rotating vortex. The Indian Ocean features a gyre, and the Atlantic and Pacific each have two; one in the Northern and one in the Southern Hemisphere. The Northern Pacific gyre is partially responsible for the creation of a vast collection of plastic and chemical sludge referred to as the Great Pacific Garbage Patch.

Finally, the Coriolis effect is responsible for the rotating storm systems we call tropical cyclones or hurricanes. At the heart of a major storm system is an area of low pressure, into which air tends to flow. Any given parcel of air will simply flow straight from high to low pressure; however, the Coriolis effect deflects this straight movement, such that our air parcel will miss the low-pressure area (the so-called “eye of the storm”). When many such air currents are distorted by the Coriolis effect, the result is a sort of vortex of storms rotating around a common center (clockwise in the Southern Hemisphere and counterclockwise in the Northern Hemisphere). Peter Whitesell and Rainer F. Buschmann

FURTHER READING:Butterworth-Heinemann and the Open University. 2001. Ocean Circulation. Oxford: Oxford University Press.

Henderson-Sellers, A., H. Zhang, G. Berz, K. Emanuel, W Gray, C. Landsea, G. Holland, J. Lighthill, S.-L. Shieh, P Webster, and K. McGuffie. 1998. “Tropical Cyclones and Global Climate Change: A Post-IPCC Assessment.” Bulletin of the American Meteorological Society 79 (1): 19-38.

Holton, J. R. 2004. An Introduction to Dynamic Meteorology. London: Elsevier University Press.