Observations of internal tide propagation on the Australian North West Shelf

The propagation of internal tides (12-h period internal waves) is observed over a continental shelf break region on the Australian North West Shelf. The observed phase speeds of the waves, calculated from thermistor chain and current meter data, are shown to agree closely with simple linear theory for phase velocity of freely propagating waves. Even over the relatively steep bathymetry of a continental shelf break region (ratio of bathymetric slope to slope of internal wave characteristics reaching 0.27), a simple modal model for phase speed is found to be accurate, providing that slowly varying depth is taken into account. The direction of wave propagation is determined from the orientation of baroclinic current ellipses derived from current meter data; a two-dimensional, horizontal field of phase information; and visual observations of surface slicks. Observations also demonstrate that substantial temporal variations in phase are a result of variations in the thermal stratification of the water column. Both the direction of internal tide propagation and the phase values, when averaged over some months of data, are consistent from year to year. Orientation of the wave fronts is relatively constant over an area of approximately 60 x 60 km super(2), centered around the shelf break, and they are aligned approximately 10 degree off parallel to the local bathymetric contours.