Temperature Microstructure in a Fresh Water Thermocline

Abstract

RECENT measurements of microstructure in the ocean1–4 have led to considerable speculation about physical processes that lead to a steplike distribution of temperature, salinity and velocity. Briefly, one school of thought explains the microstructure in terms of interleaving of layers originating at different locations1,2. The other school suggests that the transport processes responsible for the vertical diffusion of heat, salt and momentum may inevitably result in the development of layers; the most striking theory, the so-called salt finger mechanism developed by Turner, Stommel and Stern5–7, depends on the difference between the molecular diffusivities for salt and heat. This mechanism works only in a statically stable ocean where the temperature gradient is stable, but the salinity gradient is unstable, or in the reverse situation. Tait and Howe3 have recently published profiles that strongly favour the salt-fingering process for layers under the Mediterranean outflow, but regular steps in temperature and salt of this kind are rare: the great majority of ocean microstructure is much more variable. Dr R. W. Stewart (private communication) has suggested that this microstructure is caused by the difference between the (turbulent) transport coefficients for buoyancy and momentum. No adequate theory for this buoyancy–momentum process has yet been developed, but the low Reynolds number turbulent spots discovered by Woods8,9 may be important. Temperature microstructure in a freshwater lake at least demonstrates that salinity gradients are not an essential factor in the generation of ocean microstructure. The occurrence of such microstructure in freshwater lakes seems likely from transparency observations made by Whitney11.