FINESTRUCTURE, MICROSTRUCTURE, AND THIN LAYERS

W E ARE ALL FAMILIAR with the irregular profiles from modern, high resolution conductivity-temperature-depth profilers (commonly called CTDs) freely falling vertical profilers, and towed thermistor chains (Figs. 1 and 2). In fact sufficient resolution was available back in the 1930s with the advent of the Bathythermograph (BT) (Eckart, 1948) and even earlier through the use of the thermocouple (Schmidt, 1914; and Hacker, 1933). Figures 3 and 4 show thin layers of biological material. Fish and copepods which swim can easily form layers, but what about some of the particles which are very small, neutrally buoyant, and only swim slowly, if at all. Are their profiles related to the temperature, density, or their gradients? The easily measured profiles of temperature, salinity, density etc., carry a signature of the relevant physical processes. How much do they tell us about the formation of the biological and chemical layers? Microstructure refers to the signatures of oceanic turbulence at scales where moleulcar viscosity and diffusion are important. Quantitative measurements at these scales (millimeters to centimeters) provide estimates of the cross-isopycnal diffusion rates. Finestructure is the label for larger features where the stratification limits the motion to the horizontal plane. Signatures of this stirring motion have horizontal scales substantially greater than their vertical scales. Eckart (1948) created the paradigm of stirring and mixing, which shows the significance of the predominantly horizontal flow field, and the boundary conditions, in producing these irregular vertical distributions and layers. Thin layers are superficially like the physical finestructure features in thickness and extent. This similarity is a result of the