From AMS Glossary
The mechanism by which the effects of boundary layer momentum fluxes are communicated directly to the neighboring (essentially inviscid) fluid.
The mechanism involves a forced secondary circulation referred to as Ekman pumping or Ekman suction, depending on its sign. The effect is important in both the atmosphere and the ocean.
Holton, J. R. 1992. An Introduction to Dynamic Meteorology. 3d edition, Academic Press, . 133–139.