The influence of strongly varying topography on coastal‐trapped waves at 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 meter observations at the southern Great Barrier Reef (SGBR). The dominant 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. Previous observational and numerical studies indicate that the SGBR topography scatters mode 1 CTW energy into higher wave modes. In the present study, a spatially coherent CTW signal is found in a narrow frequency band near 0.1 cpd. Adjusted sea level and wind stress measurements from south of Fraser Island show that the similar alongshelf propagation 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 is derived to estimate the wave amplitude and phase in the alongshelf current records. The observed northward phase progression of the wave response is consistent with the scattering of mode 1 into mode 2 CTW energy by the SGBR topography. The strongest observed wave amplitudes are observed over the shelf slope at 300‐m depth below the East Australia Current, and near the shelf break at Fraser Island. The strong current response over the slope at the N‐line suggests a significant outgoing mode 2 wave.