Ekman Transport and the Ekman Spiral

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The mechanism by which wind blowing over the sea surface drives water not downwind but sideways. Each thin layer of water is dragged by the layer above it and deflected by the Earth's rotation, so velocity turns and weakens with depth in a spiral; integrated over the whole wind-driven layer the net transport is at right angles to the wind — to the right in the northern hemisphere, to the left in the southern. Fridtjof Nansen noticed during the Fram drift of 1893–1896 that Arctic sea ice moved at 20 to 40 degrees to the right of the wind and asked Vagn Walfrid Ekman to explain it mathematically; Ekman published the theory in 1905. Everything else about wind-driven circulation follows from this ninety-degree turn: where Ekman transports converge, water piles up and sinks; where they diverge, water is drawn up from below.