The tidal signal in inverted echo-sounder records

Abstract

Four IES records of several months duration from the western equatorial Atlantic are analysed with principal interest in their tidal content. Spectral noise level in the tidal bands is some two orders of magnitude higher than in comparable sea-level records, but the main constituents of both diurnal and semi-diurnal tides stand out with usable coherence with the tidal potential. A mid-ocean record, FLAVIA, gives amplitudes and phases that correspond closely with the surface tide, but three other records in a region of disturbed bathymetry near the continental shelf give amplitudes and phases which differ from the expected surface effect, indicating relatively strong coherent internal tides in the region as well as an evident incoherent tidal signal. Two of the latter records also show second-harmonic distortion, which is characteristic of internal tides, in the present case corresponding to a steeppened forward face of the internal wave. Theory, following the analysis of Long (1972 Tellus, 24, 88–89), suggests that this form of wave steepening is due to the steady shear in the surface layer. The physical theory of acoustic time-delay in vertical transmission through long internal waves with and without surface elevation is analysed quantitatively. In the region studied a pure internal tide of amplitude 10 m at 200-m depth would produce the observed changes in tidal signal. Wave amplitudes resulting from tidal flow over a 2.4-km high ridge, computed from the linear theory of Zeilon (1912 Kungliga Svenska Vetenskapsakademiens Handligar, 47, 1–45), are only about 1.2 m maximum, but the theory does suggest a likely mechanism for producing coherent internal tidal motion, possibly by invoking the shelf edge.