Ekman Notes

Ekman Transport
The great Norwegian polar explorer, Fridtjof Nansen, during his 1893-96 Fram
Expedition toward the North Pole, noticed that the icebound Fram drifted at a
consistently different angle from that of what the wind was blowing. The
thinking at the time was that the drift should be exactly in the downwind
direction. Nansen thought this deviation was due to the earth’s rotation, or the
Coriolis Effect upon the ocean water. Furthermore, he theorized that the direction
and magnitude of this deviation would change with depth in the water as friction
of successive water layers caused differences in shear forces. But Nansen needed
mathematical assistance in the formulation of his theory. He found this help from
the Swedish scientist W.V. Ekman of Stockholm in late 1901. Ekman
mathematically substantiated the physics that predicted “the effect of wind on the
surface of the sea produced currents which ‘formed something like a spiral
staircase… down towards the depths’.” (pg, 389, 1997, Roland Huntford, Nansen,
Duckworth, London, 610 pp.) Our animation shows this Ekman Spiral. At the
surface the water currents are 45 degrees to the right (in the Northern
Hemisphere) of the wind direction. Strangely, the much-reduced current at the
bottom of the Ekman spiral moves in the direction opposite to that of the surface
flow. When the vector deviations are averaged over the water depth through
which they act (about 50 m), the net shift is 90 degrees to the right of the direction
of the wind. This cumulative flow is called Ekman transport and is an important
rule of thumb used by all oceanographers.
Ekman’s explanation of the motion of near-surface waters due to the winds was a
seminal formulation. It appeared at the time when oceanography was turning
from an empirical to a mathematical science. It is now seen to be a principle of
the behavior of all fluids and gases and is one of the foundations of modern
meteorology and oceanography (ibidem).