Ekman Layer
The Ekman layer is a kind of viscous boundary layer in a rotating system, and refers to the area to which force applied to a horizontal boundary is transmitted. The thickness of this boundary layer is given by a depth to which the force is transmitted during one revolution of the earth, so as the viscosity coefficient grows it becomes thicker, and as the effect of rotation grows it becomes thinner.
In the Ekman layer caused by winds blowing across the sea's surface, if the viscosity coefficient is not dependent on depth, the current on the sea's surface will point 45 degrees to the right of the wind's direction (in the Northern hemisphere), and as you go deeper, the current speed decreases gradually while turning in a clockwise direction. If you link the ends of these current vectors, you get a spiral, so it is called Ekman spiral.
The net current direction, obtained by vertically integrating the current direction of an Ekman spiral is called Ekman transport, and in the Northern hemisphere, this is perpendicularly right of the wind direction.
If you think of this relationship in terms of the balance of forces acting on the Ekman layer as a whole, Coriolis' force acting on the net direction of the current is canceling out the pulling force of the wind on the surface of the water. In other words, Coriolis' force and the pulling power of the wind on the surface are acting in opposite directions and are equal.
In the actual sea, it is necessary to think in terms of the turbulent diffusion rather than the molecular viscosity. This variable relies on the strength of turbulence at a given place, and is not necessarily a constant, however the qualitative shape of the Ekman layer does not change. The depth of the Ekman layer in the sea varies from place to place, but is normally to the order of about 100m2, 7).
