The structure of the thermocline

Abstract

A temperature-gradient meter and dye tracers have been used to extend Tully and G iovando's (1963) description of summer stratification to the finest scale. The principal new feature is a series of 3- 4 m thick layers of weak shear and temperature gradient separated by thin sheets of strong shear and temperature gradient and containing a laminar flow zone at the centre. A not able feature of the sheets are long coherent trains of steep internal gravity waves with wavelengths lying within a narrow band about \0 m : the strong shear at the crests of these waves has been observed to give Kelvin-Helmhotz instability. A physical model is proposed, in which the near surface sheets are formed by the classic process of transient formation, but deeper sheets are generated by an internal mechanism associated with shear instability. Weak turbulence in the layers maintains the sheets, and internal waves control their thickness.