Resonant Third-Degree Diurnal Tides in the Seas off Western Europe

Abstract

Third-degree diurnal tides are estimated from long time series of sea level measurements at three North Atlantic tide gauges. Although their amplitudes are only a few millimeters or less, their admittances are far larger than those of second-degree diurnal tides, just as Cartwright discovered for the M1 constituent. The tides are evidently resonantly enhanced owing to high spatial correlation between the third-degree spherical harmonic of the tidal potential and a near-diurnal oceanic normal mode that is most pronounced in the North Atlantic. By estimating the ocean tidal response across the diurnal band (five tidal constituents plus nodal modulations), the period and Q of this mode and one nearby mode are estimated.