Ekman spirals and Ekman pumping

Friction dampens the movement of air and water, and this creates interesting and important water movement systems, such as the Ekman spirals, discovered in 1905.

1905 was the eventful year of the Russian revolution and the dissolution of the Norway-Sweden Union, and was also the year when Albert Einstein published his revolutionary theories. Old ideas within meteorology and oceanography were also questioned.
The same year the Swedish oceanographer Vagn Walfrid Ekman introduced a theory that contradicted both accepted opinions and common sense, as is often the case for real science.

The Norwegian Nansen’s ideas

It started, not as a Swedish, but as a Norwegian idea. In 1893 the Norwegian scientist and explorer Fritjof Nansen (1861-1930) headed through the Arctic Ocean on board the purpose-built ship “Fram”.
Nansen was a respected scientist and good observer. In the Arctic Ocean he noted that the icebergs were not driven in the same direction as the wind but at an angle of 20-40 degrees to the right. Nansen assumed that this had something to do with the affect of the earth’s rotation, which is to the right in the northern hemisphere.

Nansen continued to thing about this phenomenon. When the wind, due to friction against the water surface, drives the water in the top layer a little to the right, then this water layer will in turn, through internal friction, drive the layer under it a little more to the right, and so it continues through underlying layers.
The action of the wind is therefore spread down through a water mass with increased deviation so that it is even possible that water at a certain depth moves in the opposite direction to the wind.

Ekman explains the phenomenon

Nansen got a complete explanation after he had presented his observation for his compatriot Vilhelm Bjerknes (1862- 1951), professor of physics at Stockholm University. He passed on the problem to one of his young students, Vagn Walfrid Ekman (1874-1954).
Ekman used equations to prove Nansen’s hypothesis which suggestively described how the rotation of the earth causes the direction of water movement to change with increasing depth, in a converging spiral which later became known as the Ekman spiral.
Today both Swedish and international scientists talk about the “Ekman layer” which is where this effect is noticeable. However at the time, Ekman’s work was not appreciated. His results were controversial and created as much debate within the fields of oceanography and meteorology as Albert Einstein’s articles on relativity the same year.
Ekman’s conclusions (which are really Nansen’s) disagreed with the dominating opinions that the currents in the sea were not affected by the earth’s rotation, and that they had the same direction from the surface down to the bottom.

Ekman’s legacy

The extent of the Ekman layer above the Earth’s surface is decided not only by the turbulence caused by the underlying ruggedness but also by the temperature distribution in the layer, as well as, surprisingly, the affect of the Earth’s rotation.

The ability of the Coriolis effect to limit air currents is also shown. In the sea it limits the vertical effect of the wind, and in the atmosphere it limits the vertical exchange of air.

Ekman’s name is associated with more phenomena and mechanisms than another Swedish oceanography Rossby, who has also given his name to many scientific concepts. The article Circulation in the tropics shows how the Earth’s rotation causes the Trade winds to force cold water to upwell to the surface along the equator in the eastern Pacific.

The scientific term for this is “Ekman pumping” or “Ekman transport”, despite the fact that it is actually due to the Coriolis effect. There is also an “Ekman number” and anyone unsure of the turning properties of the Coriolis effect can refer to “Ekman veering”.