Wind-Driven Ocean Currents

Wind-driven ocean currents behave in ways that, at first appearance do not make sense.

In thinking about the wind blowing over the ocean's surface, one easily recognizes that the moving air exerts a force on the top of the water in the direction the wind blows. Seems obvious that the water should move that direction, too, doesn't it? In fact, the water at the surface will move at an angle to the wind direction.

The reason for the deviation is that the daily rotation of the earth about its axis exerts an additional force on the moving water. That force is at right angles to the direction of the water's motion and is directly proportional to the speed of the water. The combination of this so-called Coriolis force and the force of the wind results in the water surface moving at a 45 degree angle to the wind direction--to the right in the Northern Hemisphere and to the left in the Southern Hemisphere.

To make matters more complicated, the moving surface water tries to force the water below to move also. Going down in depth in the Northern Hemisphere, the water is driven more and more to the right, until at a depth of about 60 meters, the water actually is driven opposite the direction of the wind. Averaged over the top 60 meters, the wind-driven current in the Northern Hemisphere is 90 degrees to the right of the wind direction.

Thus, a west wind blowing against a north-south segment of the Alaskan coast will drive a current along the coast to the south. A wind blowing parallel to the coast or the edge of the arctic ice pack in the Bering Sea will drive water either toward or away from the coastline or the ice pack. If the water is driven away, the result is upwelling to replace the water forced away. Food production rates are highest where upwelling occurs; these are the ocean's richest places.