Temporal variations of tides on the west coast of Great Britain

Abstract

Summary. Temporal variations of tides are investigated by analysing tidal observations from five ports on the west coast of Great Britain. Variations considered here are those occurring in the semi-diurnal species daily, in the tidal constituents resolved from analyses of 29 day sets over a period of two years, 1980–81, and in the tidal constituents resolved from 1 yr sets over a period of 19 yr. The time variations of tidal constituents are mainly due to inseparable terms grouping with the principal term due to the short length of data and the insufficiency of the procedures to account for them. Surges and surge-tide interaction are affecting the resolution of tides. Time variations of constituents of the diurnal species induced by non-tidal forces are spatially consistent. The use of equilibrium relationships to account for modulations of principal constituents by waves separable in an 18.61 yr nodal cycle improves the analysis. However, the modulation is smaller in the real tide than in the equilibrium tide. When the application of this principle is extended to a wider frequency band to cover constituents which cannot be resolved from one month of observations, it may produce the opposite effect. These problems start compounding with decreasing length of data, i.e. increasing band width of resolution. Functional approximations of interaction coefficients (bi-admittance) of non-linear tides, similar to the response (admittance) of linear tides are computed for semi-diurnal, fourth-diurnal and sixth-diurnal bands. These functional estimates may be used to interpolate unseparable constituents to improve the quality of analyses. The results are consistent with the fact that the response of the sea does not change in a narrow frequency band covering a nodal cycle of the Moon. The change in the equilibrium relationship between the principal term and its nodal term can be interpreted in terms of the damping effect of bottom friction.