Ekman spiral
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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).
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.
The diagram on the right shows the forces associated with the Ekman spiral. The force from above is in red (beginning with the wind blowing over the water surface), the Coriolis force (at right angles to the force from above) is in dark yellow, and the net resultant water movement is in pink, which then becomes the force from above for the layer below it, accounting for the gradual clockwise spiral motion as you move down.
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.
The null point is the point in which the water does not spin. Beyond the null point the water spins back upward in the direction opposite of the downward moving water.
[edit] References
- 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.
