Tides on the Australian north-west shelf

Abstract

Harmonic analysis of the records from 14 current meters and three offshore tide gauges from the Australian north-west shelf and continental slope regions and a number of coastal stations shows that tidal motion is dominated by the principal lunar (M₂) and principal solar (S₂) constituents. The tidal velocities are essentially barotropic with flow predominantly in the cross-shelf direction at the shelf break changing to predominantly along-shelf near the coast. The elevations of the M₂ and S₂ constituents are amplified across the shelf off Dampier by 1 .3 times and by approximately five times along the length of the shelf, a distance of around 1000 km. Several features of the semidiurnal shelf tide are discussed with the aid of a simple analytical model. The cross-shelf bathymetry is shown to produce cross-shelf amplification of the semidiurnal tide in good agreement with the observations off Dampier. An along-shelf pressure gradient results from the change in elevation of the tide along the coast and this pressure gradient is shown to be the main driving mechanism of the along-shelf component of the M₂ and S₂ velocities. The gradient also influences, but to a lesser extent, the cross-shelf component of the velocity and the cross-shelf amplification of the semidiurnal tidal elevation. Results of a world ocean tidal model (Schwiderski 1979) are presented and indicate the existence of a gradient in the shelf-edge M₂ tide along the shelf and this is shown to have an important effect on the shelf tide.