Tidal-period internal waves in a tidal stream

Abstract

Some observations of thermocline depth in shallow water show a repeating pattern through the cycle of the semi-diurnal tide. About half the energy is apparently at the fundamental frequency, and about half at the second harmonic frequency with six-hour period. The pattern is due to the combined effects of tidal-period internal waves, and their advection in the tidal stream. In general these two phenomena will occur together, and when the ratio of maximum streaming velocity to internal wave velocity is large the apparent energy will concentrate in the higher harmonics of the tidal frequency. In the reported observations the ratio is 1·2 and the internal wave is deduced to be proceeding SW at about 0·35 metre/sec, towards the distant continental edge. It appears that other effects of the internal waves, such as fluctuations in acoustic transmission, should also show these higher frequency components.