An Echo-Sounding Record and a Sound Transmission Record Showing Internal Waves

摘要

An echo-sounding record taken from R.V. CLIONE in shallow coastal water showed a strong return from a 'cloud' in the lower half of the water column, the cloud apparently consisting of zooplankton and larval fish. Its upper surface varied sinusoidally in depth with amplitude about 10 m and period about 7 min. Two hours earlier the level of 2 kHz sound, received on CLIONE from a bottom-laid source 2 km distant, varied through several dB with period about 5 1/2 min. The regularity of the records, together with other evidence, suggests internal waves controlled by the varying temperature structure and current flow through the tidal cycle. The apparent wavelength reduces with time from about 100 m to about 40 m, and perhaps less. The measured shear flow in the tidal stream is of the right order to affect the stability of the water column, but very extensive measurements would be needed for a detailed check with theory. (Author)