An Ekman layer is a viscous boundary layer in a rotating fluid. For constant kinematic viscosity and Coriolis parameter , its characteristic thickness is , and its velocity turns with distance from the boundary.
For , a stationary lower no-slip boundary condition below uniform exterior geostrophic flow produces . The upward shear component at the bottom is ; the bottom traction on the fluid is . This law cannot be assigned to an arbitrary prescribed surface wind stress without an additional boundary condition.
A surface Ekman layer is the stress-driven boundary correction below a rotating fluid’s upper boundary. For upward , , constant kinematic viscosity and imposed complex wind stress , its steady velocity is , . The underlying geostrophic flow is independent of unless an additional boundary velocity condition couples them.
A viscous no-slip boundary below a geostrophic current creates a bottom Ekman layer of thickness . Its depth-integrated ageostrophic transport has a component opposite the geostrophic current and a transverse component set by the sign of .
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The Ekman layer refers to a layer in the ocean or atmosphere that is influenced by the Earth's rotation and characterized by a specific current profile due to the balance of forces acting on the water or air. It is named after the Swedish oceanographer Vagn Walfrid Ekman, who studied the effects of the Coriolis force on wind-driven currents.