The mixed-layer depth is the depth of the oceanic mixed layer, generally ranging from 40-150 meters below the ocean surface.
The oceanic mixed layer is a nearly isothermal surface layer of around 40 to 150 m depth caused by wind stirring and convection. It has been qualitatively defined as as the
"envelope" of maximum depths reached by the mixing layer on time scales of a day or more, i.e. the zone that has been mixed in the recent past. It generally corresponds to the zone above the top of the seasonal pycnocline. The depth of the mixed layer is related to phytoplankton primary productivity due to its relationship with the availability of nutrients in well-mixed and stratified water columns.
Oceanic primary productivity
Air-sea gas exchange
Other sources for data