THE INFLUENCE OF STRONGLY VARYING TOPOGRAPHY ON COASTAL-TRAPPED WAVESAT THE SOUTHERN GREAT-BARRIER-REEF

Coastal-trapped waves (CTWs) that propagate into a region of strongly varying topography are investigated using 6 months of moored current m eter observations at the southern Great Barrier Reef (SGBR). The domin ant topographic variations in this region include a large sand island that extends from the coast to the shelf break, and an abrupt doubling in shelf width over an alongshelf distance of approximately 100 km. P revious observational and numerical studies indicate that the SGBR top ography scatters mode 1 CTW energy into higher wave modes. In the pres ent study, a spatially coherent CTW signal is found in a narrow freque ncy band near 0.1 cpd. Adjusted sea level and wind stress measurements from south of Fraser Island show that the similar alongshelf propagat ion speeds of coastal winds at 10-day timescales and mode 1 CTWs lead to the narrow-banded CTW signal. A proxy for the incident CTW signal i s derived to estimate the wave amplitude and phase in the alongshelf c urrent records. The observed northward phase progression of the wave r esponse is consistent with the scattering of mode 1 into mode 2 CTW en ergy by the SGBR topography. The strongest observed wave amplitudes ar e observed over the shelf slope at 300-m depth below the East Australi a Current, and near the shelf break at Fraser Island. The strong curre nt response over the slope at the N-line suggests a significant outgoi ng mode 2 wave.