Role of thermocline properties on mode-2 nonlinear internal wave formation on a continental shelf

Matt Rayson1, Yankun Gong2, Nicole L Jones1 and Gregory N Ivey3, (1)University of Western Australia, Oceans Graduate School and Oceans Institute, Crawley, WA, Australia, (2)University of Western Australia, Crawley, WA, Australia, (3)University Western Australia, Oceans Graduate School and Oceans Institute, Crawley, Western Australia, Australia
Moored observations at many sites along the outer region of the Australian North West Shelf (water depths 200 – 500 m) have revealed the presence of large-amplitude mode-2 nonlinear internal waves (NLIW). The formation of these waves is explained using weakly nonlinear internal wave evolution theory. We show that variations in the density stratification properties, namely the thermocline thickness, help predict the occurrence of mode-2 NLIW in the region. The thermocline thickness varies between 50 and 150 m on the shelf at both seasonal time scales, due to Equatorial wind stress changes, and weekly time scales, due to passing tropical squalls and cyclones. We observed density overturns within the core of several mode-2 NLIW with a high-resolution (dz = 10 m, dt = 0.5 s) thermistor chain suggesting that they drive enhanced vertical mixing within the thermocline away from the bottom boundary. One particularly large overturning mode-2 NLIW occurred after a tropical cyclone-induced submesoscale eddy which strained the thermocline structure causing it to thin and deepen. Observational evidence of events like these suggest that mixing on the shelf is modulated by mode-2 NLIW formation, which is driven by thermocline variations.