S1: A small but perplexing component of the tide

Abstract

S1, of exact diurnal periodicity, is detectable in most water level records. Although small in magnitude, it shows itself to be much larger than expected, considering its almost negligible contribution to the field of tidal forces. When numerous years of continuous observations are available, S1 emerges as a sharply defined component, as steady and as stable as its more powerful companions in the diurnal band. It is therefore an authentic tidal component, unrelated to extraneous effects like the sea breeze. Its unexpected and persistent presence in almost all records probably is due to a diurnal inequality in the tidal forces actually exerted on the Earth, caused by a slight rotational asymmetry in the shape of the geoid and being felt during the course of each rotation. © 1990 Deutsches Hydrographisches Institut.