• Main
• Construction

Ekman Layer Historical Observations

First Observations:

The first observations of what became known as the Ekman spiral were made by many people but not understood until a Norwegian explorer and Oceanographer, Fridtjof Nansen, observed the surface currents of the ocean were not the same just below the surface as the wind, but angled slightly. Icebergs in the norther hemisphere were observed to travel at an angle 20 to 40 degrees to the right of the prevailing winds. He took his findings to his student, Vagn Walfrid Ekman, who determined that the combination of viscous drag at a fluid boundary and the Coriolis effect were the cause. Ekman's solution was presented in 1902 as his doctoral thesis.

More Observations:

It was only until recently that sensitive enough instrumentation was developed to measure the Ekman transport in real geophysical systems such as the ocean surface. Additionally, visualization of the Ekman layer at the surface of the ocean is difficult because of the Ekman scale height being on the order of only tens of meters and the wind-driven surface waves interfering with measurements. However, there are many visualizations in experiment that demonstrate. The most famous of these is the convergence of tea leaves (or other precipitant) in the bottom of stirred tea.

Stirring the tea sets up a bulk rotation of tea fluid. Since the Ekman layer is taller than the fluid depth, the Ekman number is large and the transport force is strong. This creates a converging horizontal flow at the bottom boundary (of the teacup) and a diverging flow at the top fluid boundary. The incompressibility of water demands that there be vertical flows to keep continuity of fluid, thus there is a strong upwelling in the middle of the teacup. Since the tea leaves at the bottom of the teacup are heavy, they collect at the middle at the bottom of the teacup. Many think of this as a paradox, initially thinking that the centrifugal effects should serve to push the tea leaves radially outward. The key here is that Coriolis effects dominate over centrifugal effects.

Additionally, the Ekman layer and associated Ekman pumping is thought to be the driver of coastal upwelling and downwelling. Wind following the coastline forces the fluid in the Ekman layer as discussed in the derivation. In the northern hemisphere, if the wind flow has the coast on its left an upwelling occurs; a downwelling occurs when the wind flow has the coast on its right. The opposite is true in the southern hemisphere, e.g. the western coast of Peru. Here, the wind blows north, with the coast on its right, inducing an upwelling in the Ekman layer. Water flows upward along the coast to replenish that taken by the Ekman flow bringing with it fish, creating one of the most active fishing locations in the world.

Upwelling and downwelling in the ocean is also exited by the equatorial trade winds. In this geometry, the winds are blowing either east or west which induces a transport of warmer water toward the pole or a transport of cool water away from the pole, respectively. This can affect and contribute to the existence of ocean gyres, as observed by Harald Sverdrup.

Many geophysical exhibitions of Ekman pumping are difficult to see directly in real systems due to the presence of many other phenomena associated with sheared flows. If the flow is sufficient enough, as is often present in real-world flows and experiment, turbulence can make observing the Ekman spiral difficult and even serve to disrupt the Ekman layer itself.

• Apparatus
• Experiments