Internal Waves in the Clyde Sea

Temperature measurements were made with 15 s and 1 min sampling intervals in the thermocline of the Clyde Sea to assess the variability associated with internal waves on time scales from min to hours. The data were collected during shallow water acoustics experiments which took place near Troon during the summer of 1997. The data reveal clear evidence of internal wave activity which was also apparent in a sequence of rapid CTD yo-yo profiles. Intermittent high frequency internal wave events displayed a period of about 10 min, which is comparable to the local buoyancy period. Vertical amplitudes of 3 m were observed, corresponding to sound speed fluctuations of about ±2 m s−1. Vertical displacement spectra calculated from the temperature data suggest that the observed energy levels did not differ significantly from the generic Garrett and Munk deep water internal wave spectrum. A stochastic internal wave model, based on the Garrett and Munk spectrum, was used to calculate dynamically consistent internal wave fields from which estimates of acoustic variability were derived. Three numerical schemes were used to simulate the vertical advection of the density field: an Eulerian method, a Lagrangian method, and a time-varying mode method. The Eulerian method, based on linear dynamics, performed as well as the more sophisticated techniques and good agreement was obtained between the observed and simulated variability of the sound channel. The observations suggest that the internal wave activity, and hence the acoustic variability, was phase locked to the surface tide.