How the Ocean Surface Moves: Ekman Transport
In 1893, Norwegian explorer Fridtjof Nansen froze his ship into Arctic sea ice. He was trying to reach the North Pole by drifting in sea ice as it bobbed along in the surface ocean currents. Nansen and his ship drifted for close to three years in the ice yet they never got to the North Pole. Out in the ice for so long, Nansen had a lot of time to make observations of the environment. He noticed that the ice was not moving in the same direction as the wind. It was moving consistently to the right of the wind direction – a concept that, several years later, would become known as Ekman transport.
As wind blows across the ocean, it moves ocean water because of friction at the ocean surface. If Earth did not rotate, the water would move in the same direction as the wind. However, the Earth does rotate, which complicates things a bit. Earth’s rotation causes surface water to move to the right of the wind direction in the Northern Hemisphere and to the left of the wind direction in the Southern Hemisphere because of the Coriolis effect.
The speed and direction of the moving water changes with depth. For example, ocean water at the surface move to the right of the wind in the Northern Hemisphere, then the water under the surface water turns even more to the right. The water below turns even more. The result is a spiral of moving water called an Ekman spiral. The wind doesn’t have much impact on ocean water below about 100 to 150 m (330 to 500 ft).
The average direction of all this turning water turns out to be about a right angle from the wind direction. This average is Ekman transport.
The Ekman spiral and Ekman transport are named for Swedish scientist V. Walfrid Ekman who first described the spiral mathematically in 1905. To help him make a model of these movements, Ekman used the observations that Fridtjof Nansen made while frozen into the Arctic sea ice.