MODULE 3b: TEXT ABOUT ANIMATION 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).