Deep-sea tide observations from the northeastern Pacific

Abstract

The instrument transducer is a multiturn Bourdon tube made of a ferro-nickel alloy in which thermoexpansion and thermoelastic coefficients are carefully balanced. The 0·3-cm water head resolution is achieved by means of a frictionless optical readout. A seven-day record of the tidal pressure fluctuations on the sea floor, 1150 km off northern Baja California, is analyzed (1) by means of a classic harmonic analysis based upon the main diurnal and semi-diurnal tidal frequencies and (2) by estimating the frequency dependent complex transfer functions between tidal fluctuations and gravitational forces as well as their uncertainty. Because of the extreme smoothness of the record, and in spite of its short duration, the harmonic method permits resolution of as many as four constituents in each species while the transfer functions estimates permit inference of the constituents amplitudes and phases with a much greater accuracy than is possible with coastal records of the same length. Offshore from California the semidiurnal tide is found to attenuate considerably faster than the diurnal tide. The observed amplitudes for both species are appreciably smaller than those predicted by Bogdanov's models suggesting the influence of the solid earth's tides. The absence of appreciable tidal energy at higher harmonic frequencies of the tides confirms the linearity of response of the ocean to gravitational excitation and the linearity of response of the instrument to pressure changes. The residual pressure fluctuation energy falls from 0·6 cm 2/cycles/hr at 2·0 cycles/hr into what appears to be the instrumental and reading of 0·5 × 10 −2 cm 2/cycles/hr at 10 cycles/hr.