(German)  Ekman spiral effect. This picture is incorrect. Surface current should be at 45° to right (left) of wind direction in the northern (southern) hemisphere. Coriolis force should be at 90° to right (left) of the current in the northern (southern) hemisphere at each depth. Decay of current speed with depth should be exponential.

The Ekman spiral refers to a structure of currents or winds near a horizontal boundary in which the flow direction rotates as one moves away from the boundary. It derives its name from the Swedish oceanographer Vagn Ekman. The deflection of surface currents was first noticed by the Norwegian oceanographer Fridtjof Nansen during the Fram expedition (1893-1896). Download high resolution version (494x825, 19 KB) Wikipedia does not have an article with this exact name. ... Download high resolution version (494x825, 19 KB) Wikipedia does not have an article with this exact name. ... Thermohaline circulation Oceanography (from Ocean + Greek γράφειν = write), also called oceanology or marine science is the study of the Earths oceans and seas. ... Vagn Walfrid Ekman (May 3, 1874 - March 9, 1954) was a Swedish oceanographer. ... Fridtjof Nansen Fridtjof Nansen (born October 10, 1861 in Store Frøen, near Kristiania, now Oslo - died May 13, 1930 in Lysaker, outside Oslo) was a Norwegian explorer, scientist and diplomat. ...

The effect is a consequence of the Coriolis effect which causes objects to move to the right of applied forces in the northern hemisphere and to the left in the Southern Hemisphere. Thus when the wind blows on the ocean surface in the northern hemisphere, the surface current moves to the right of the wind. As the water at the surface pushes on water below it, this water moves yet further to the right of the wind, and so on. As the current deflects, the velocity becomes progressively weaker. The depth to which the Ekman spiral penetrates is determined by how far turbulent mixing can penetrate over the course of a pendulum day. This low pressure system over Iceland spins counter-clockwise due to the Coriolis effect. ...

The picture on the right shows the force balances associated with the Ekman spiral. The velocities are in red, the frictional stresses in pink, and the Coriolis force (at right angles to the velocity) in dark yellow.

The classic Ekman spiral has been observed under sea ice, but is not found in most open-ocean conditions. This is due both to the fact that the turbulent mixing in the surface layer of the ocean has a strong diurnal cycle and to the fact that surface waves can destabilize the Ekman spiral. Ekman spirals are, however, found in the atmosphere. Surface winds in the Northern Hemisphere tend to blow to the left of winds aloft.

References

Wikimedia Commons has media related to:

Ekman spiral

• AMS Glossary, mathematical description
• A. Gnanadesikan and R.A. Weller, 1995 · "Structure and instability of the Ekman spiral in the presence of surface gravity waves" · Journal of Physical Oceanography  25(12), pp.3148-3171.

• J.F. Price, R.A. Weller and R. Pinkel, 1986 · "Diurnal cycling: Observations and models of the upper ocean response to diurnal heating, cooling and wind mixing" · Journal of Geophysical Research  91, pp.8411-8427.

• J.G. Richman, R. deSzoeke and R.E. Davis, 1987 · "Measurements of near-surface shear in the ocean" · Journal of Geophysical Research  92, pp.2851-2858.