Ekman's paper1).

Nansen came to the conclusion that the tendency to drift to the right was the result of the earth's rotation. Ekman was the first to prove this mathematically. According to his theory1), if a diffusion coefficient is not dependent on depth, then surface sea water will flow 45 degrees to the right of the wind's direction (in the Northern hemisphere), and at a greater depth, although the speed of the current will decrease, the direction of the current will deviate even further from the wind's direction. Eventually, at a certain depth, the sea water will even flow upwind. The velocity changes with depth in a spiral pattern, so this velocity profile is called an Ekman Spiral.