Non-linear interactions between tides and planetary waves resulting in periodic tidal variability

Data obtained from a meteor radar at Sheffield, U.K. (53°27'N, 1°35'W) between 30 June 1989 and 6 October 1994 have been used to study the variability of tidal amplitudes and the possibility that this is caused by non-linear interactions with planetary waves. Instances of regular variations in tidal amplitude with periods corresponding to those of simultaneously-observed planetary waves are investigated. The observed tidal variability is analysed in the light of a current theory of non-linear interaction between tides and planetary waves, which predicts the presence of secondary waves having frequencies that are the sum and difference of the primary waves involved in the interaction. Three case studies are presented in which periodogram analysis reveals the presence of these secondary waves and a bispectral analysis of the same data is used to confirm these findings. The suitability and limitations of this latter technique as a detector of quadratic phase coupling in data sets of the type used in this study are discussed.