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The Ekman spiral is a natural phenomenon where the apparent direction of a wind-driven ocean current veers to the right of the direction of the prevailing wind (or to the left in the Southern Hemisphere).

The phenomenon was first observed by famous Norwegian explorer and oceanographer Fridtjof Nansen who, during his famous ice-bound voyage on the Fram, noticed that the direction his ship (which was encased in the north polar ice cap) moved didn't correspond to the winds. He proposed that this aberration was due to the Coriolis effect.

The spiral was described in more detail by Swedish oceanographer V. Walfrid Ekman in 1905. At the surface, currents diverge anywhere from 30-45 degrees from the orientation of the surface winds. When constructing a depth profile, the amount of direction change increases, up to a maximum depth of about 100m. The velocity of the currents caused by the Ekman spiral, and thus the amount of mass transport (the volume of water moving) drops with increased depth. At the bottom of this Ekman layer, the current may flow upwind.

Ekman spirals help to explain some of the current patterns seen in the oceans today. The process has been used to explain patterns of upwelling, especially in equatorial regions. While the spiral has been reproduced under laboratory conditions, but only in the last 15 years has it been observed in the open ocean.

Univ. of Washington Dept of Atomospheric Sciences - http://www.atmos.washington.edu/gfd_exp/exp_e/exp/ek/1/res.htm
Dr. Anand Gnanadesikan - http://www.gfdl.gov/~a1g/Ekman_spiral.html
Don Garlick, Humboldt State University - http://www.humboldt.edu/~gdg1/Spiral.html

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