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.