Introduction

Woods' method of studying the breaking of internal waves using divers to photograph dye movements described in Section 3.1 was constrained by weather, limited to the working depth of divers, and confined to scales of order 20 m because of the rapid attenuation of underwater visibility. It was soon recognized that a more thorough, quantitative investigation was required to observe and record the small-scale temperature and velocity structure of the water column, whilst insulating instruments from extraneous vibrations such as those induced by waves or ships' engines.

The first documented measurements of turbulence within the thermocline were published at almost the same time as Woods' observations. They were made by Grant et al. (1968) off the west coast of Vancouver Island using hot film anemometers mounted on a submarine. Turbulence was found to be continuous in the mixed layer at 15 m depth, with measured mean values of the turbulence dissipation rate, ε, of 2.5 × 10g−6 W kg−1 estimated by fitting the observed one-dimensional spectra to the theoretical spectrum (1.14). The rate of loss of temperature variance, χT, was 5.6 × 10−7 K2 s−1. In and below the seasonal thermocline, turbulence was patchy with mean values, ε = 1.5 × 10−8 W kg−1 and χT = 7.2 × 10−8 K2 s−1, at 90 m depth.

Except in boundary layers, such ‘patchiness’ has proved to be a usual characteristic of oceanic turbulence.