Observations of Large-Amplitude Mode-2 Nonlinear Internal Waves on the Australian North West Shelf

Large-amplitude mode-2 nonlinear internal waves were observed in 250-m-deep water on the Australian North West shelf. Wave amplitudes were derived from temperature measurements using three through-the-water-column moorings spaced 600 m apart in a triangular configuration. The moorings were deployed for 2 months during the transition period between the tropical monsoon and the dry season. The site had a 25–30-m-amplitude mode-1 internal tide that essentially followed the spring–neap tidal cycle. Regular mode-2 nonlinear wave trains with amplitudes exceeding 25 m, with the largest event exceeding 50 m, were also observed at the site. Overturning was observed during several mode-2 events, and the relatively high wave Froude number and steepness (0.15) suggested kinematic (convective) instability was likely to be the driving mechanism. The presence of the mode-2 waves was not correlated with the tidal forcing but rather occurred when the nonlinear steepening length scale was smaller than the distance from the generation region to the observation site. This steepening length scale is inversely proportional to the nonlinear parameter in the Korteweg–de Vries equation, and it varied by at least one order of magnitude under the evolving background thermal stratification over the observation period. Despite the complexity of the internal waves in the region, the nonlinear steepening length was shown to be a reliable indicator for the formation of large-amplitude mode-2 waves and the rarer occurrence of mode-1 large-amplitude waves. A local mode-2 generation mechanism caused by a beam interacting with a pycnocline is demonstrated using a fully nonlinear numerical solution.