Theory
Ekman theory explains the theoretical state of circulation if water currents were driven only by the transfer of momentum from the wind. In the physical world this is difficult to observe due to the presence of many other current driving forces (i.e. pressure and density gradients). Though the following theory technically applies to this ideal situation in which only wind drives circulation, Ekman motion describes the wind driven portion of surface circulation seen in the surface layer.
Surface currents flow at a 45 degree angle to the wind due to a balance between the Coriolis force and the drags generated by the wind and the water. If the ocean is divided vertically into thin layers, the magnitude of the velocity (the speed) decreases from a maximum at the surface until it dissipates. The direction also shifts slightly across each subsequent layer (right in the northern hemisphere and left in the southern hemisphere). This is called the Ekman spiral. The layer of water from the surface to the point of dissipation of this spiral is known as the Ekman layer. If all flow over the Ekman layer is integrated, the net transportation is at an angle of 90 degrees to the right (left) of the surface wind in the northern (southern) hemisphere.
Read more about this topic: Ekman Transport
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