
%0 Journal Article
%A Amin, M 
%T Changing mean sea level and tidal constants on the west coast of Australia
%J Marine and Freshwater Research
%D 1993/12/01
%V 44
%N 6
%P 911-925
%K 
%R https://doi.org/10.1071/MF9930911
%U https://www.publish.csiro.au/paper/MF9930911 
%X Long time-series of sea-level observations spanning 21 years (1966-86) from four ports  on the west coast of Australia are analysed in yearly sets to study the secular trends and  periodic changes in the tidal harmonic constituents. Mean sea level and five harmonic  consistuents-Sa from low frequency band, O1 and K1 from the diurnal band, M2 and S2  from the semi-diurnal band-are investigated for secular trend and other variations that  are not accounted for by conventional methods. Secular trends at Darwin, Wyndham  and Geraldton are such that the amplitudes of M2 and S2 tides are decreasing and their  phases are retarding. The changes observed at Fremantle are similar in magnitude but  opposite in sign. In the diurnal band, variations in the phases of 01 and K1 are similar to  those of the M2 and S2 tides. The observed trends of the diurnal and semidiurnal tides of  Wyndham and Fremantle are well above standard error. Secular changes in the annual tide  are not significant. In addition to the secular trends, these constituents are also modulated  by terms which are not present in the tide-generating potential and they cannot be resolved  from observations of less than 18.61 years. For example, in the spectrum of the equilibrium  tide, there is no nodal term associated with the S2 constituent because it is of the solar origin,  whereas in the observed tide two nodal terms appear as side bands. At some ports the amplitudes of the new nodal terms are large enough to modulate the principal tide by 2%.  The mean sea level has been rising at the average rate of 1.73 mm year-1 over this period.  It is modulated by a tidal signal of 18.61 years' cycle and non-tidal signals which are spatially  consistent but aperiodic. A strong correlation between the residual component of the annual  mean sea level and Southern Oscillation Index suggests that non-tidal long-period perturbations  of mean sea level (MSL) are mainly due to El Nino-Southern Oscillation effects. 
