Fix the mistake: Motion of the Ocean and Atmosphere

The Coriolis Force and Geostrophic Wind

Because Earth rotates, an apparent force acts when the atmosphere or seawater moves. This Coriolis force forms the foundation of global atmospheric circulation.

Basic Knowledge

The Coriolis force (deflection force) is an apparent force acting on objects moving on the rotating Earth. In the Northern Hemisphere, objects are deflected to the right of their direction of motion; in the Southern Hemisphere, they are deflected to the left. The magnitude of the Coriolis force is proportional to latitude and speed: it is 0 at the equator and greatest at the poles.
The pressure-gradient force acts from areas of high pressure toward areas of low pressure. When the pressure-gradient force and Coriolis force balance in the upper atmosphere of the Northern Hemisphere, wind blows parallel to the isobars. This is called geostrophic wind. Around a low-pressure center in the Northern Hemisphere, geostrophic wind blows counterclockwise (cyclonic circulation).
Near the surface, friction also acts, so wind crosses the isobars and blows toward the low-pressure side (gradient wind and surface wind). Around a Northern Hemisphere low-pressure system, air converges counterclockwise, producing upward air currents at the center.

Further Study (Background and Significance)

The mathematical expression for the Coriolis force is F = 2mΩv sinφ (Ω: angular velocity of rotation, v: object speed, φ: latitude). At latitude 45°, the Coriolis acceleration is on the order of approximately 10⁻⁴ rad/s × v, a magnitude that cannot be ignored for large-scale atmospheric flows.
The geostrophic-wind approximation is well established in the free atmosphere above 5 km. In the Northern Hemisphere, it is useful to remember that wind moves with high pressure (high pressure aloft) on its right, similar to Buys Ballot's law.
At low latitudes, the Coriolis force is weak, so the geostrophic-wind approximation breaks down. Disturbances in the Intertropical Convergence Zone (tropical cyclones that can develop into typhoons) occur at locations more than 5° from the equator. Cyclonic vortices do not form at the equator, where the Coriolis force is zero.

Points of Caution (Often Confused)

① The Coriolis force is not real (it is an apparent force), but it is always perceived by observers on Earth. ② Do not reverse Northern Hemisphere low pressure = counterclockwise (clockwise in the Southern Hemisphere). ③ Geostrophic wind concerns the upper atmosphere; friction cannot be ignored at the surface. ④ The Coriolis force is proportional to speed, so faster winds experience greater deflection.

Practice

1. At what latitude is the Coriolis force greatest?
2. Which two forces balance to produce geostrophic wind?
3. In which direction does wind blow around a Northern Hemisphere low-pressure system?