On the Semidiurnal Internal Tide at a Shelf-Break Region on the Australian North West Shelf

Abstract

The properties of the semidiurnal internal tide, in the region of the shelf-break, at a location on the Australian North West Shelf are discussed. Information is derived from an analysis of thermistor chain and current meter data, collected over six months at the shelf-break and slope locations. The work is an extension of an earlier study. The internal tide is described in terms of modes, finding that the firm mode dominates, propagating onshore at an angle of ?30° from normal to the bathymetry, with a rapid decay in amplitude of nearly five times from the shelf-slope to the shelf-break, a distance of 22.5 km or approximately one wavelength. The loss of energy flux from this decay gives rise to vertical mixing with a vertical eddy viscosity of 1.4 ? 10?4 m2 s?1. The amplitude at the M2 tidal frequency dominates over the S2 amplitude giving an S2/M2 amplitude ratio significantly smaller than for the barotropic tidal motion. The internal tide appears to have a three-dimensional structure at the shelf-slope location with the ratio of alongshore over onshore wavenumbers ?0.8. There are large variations in amplitude of the motion with time, showing a gradual buildup in amplitude over summer, reaching a maximum at the M2 frequency of 25 m (an average over a 14½-day segment) in 123 m depth of water. The analysis results support the suggestion by Holloway that this measurement region is not the generation site for the internal tides. A region with steep bathymetric features to the northwest is suggested as a likely site.