After explaining the mechanism producing double diffusion, its representation analytically is developed and applied to linear stability analysis to determine conditions for double diffusion to occur. Laboratory observations of salt fingers are summarized, as well as the existence of thermohaline staircases produced by fingers in the thermocline. A similar development for the diffusive regime includes layering in Lake Kivu and in polar regions. Lastly, the role of double diffusion in thermohaline intrusions is explored.

The last chapter combines themes from earlier chapters to summarize what has been learned about mixing in the stratified ocean. The nature of finestructure is explored first, as it contains signatures of the processes producing the mixing, as well as modulating the development of turbulent patches. After exploring how patches are identified, their characteristics are discussed. Evidence for the other major mixing process, double diffusion, is presented, principally as signatures in horizontal tows. The chapter concludes with summaries of mixing in three important regions differing from the open ocean pycnocline: the Southern Ocean, the Arctic, and ocean ridges.