Another example for a shallow surface Ekman layer as a result of a shallow thermocline is indicated in Figure 3.3 . The fact that current speed and direction do not change between 25 m and 35 m from the bottom in 74 m water depth suggests an Ekman layer of less than 39 m extent, significantly less than what is normally observed. The shallowness of the thermocline is the result of coastal upwelling, which lifts the thermocline towards the surface, reducing the Ekman layer thickness. The principle that the magnitude and variation with depth of the Austausch coefficient depends on the character of the turbulence applies at the ocean floor as well. In contrast to the free surface, the sea floor imposes a rigid boundary to all movement.
Larger eddies can occur further away from the sea floor.
If this is expressed in terms of turbulent viscosity it is found that A v is zero at the bottom and grows linearly with distance, until it reaches the value typical for turbulence in the ocean interior. This results in a velocity profile where the current increases logarithmically in strength with distance from the sea floor and does not change direction. In the atmosphere, this logarithmic boundary layer determines the wind profile in the first tens of metres from the ground upward. In the ocean it rarely exceeds a few metres in thickness. It occurs as a modification of the Ekman layer over the first few metres and is of no consequence to the direction and magnitude of the total Ekman layer transport discussed below.