Structure and Generation of Turbulence at Interfaces Strained by Internal Solitary Waves Propagating Shoreward over the Continental Shelf

Detailed observations of the structure within internal solitary waves propagating shoreward over Oregon’s continental shelf reveal the evolving nature of interfaces as they become unstable and break, creating turbulent flow. A persistent feature is high acoustic backscatter beginning in the vicinity of the wave trough and continuing through its trailing edge and wake. This is demonstrated to be due to enhanced density microstructure. Increased small-scale strain ahead of the wave trough compresses select density interfaces, thereby locally increasing stratification. This is followed by a sequence of overturning, high density microstructure and turbulence at the interface, which is coincident with the high acoustic backscatter. Richardson number estimated from observations is larger than 1/4, indicating that the interface is stable. However, density profiles reveal these pre-turbulent interfaces to be O(10 cm) thick, much thinner than can be resolved with shipboard velocity measurements. By assuming that streamlines parallel isopycnals ahead of the wave trough, we infer a velocity profile in which the shear is sufficiently high to create explosively-growing, small wavelength shear instabilities. We argue that this is the generation mechanism for the observed turbulence and hence the persistent structure of high acoustic backscatter in these internal solitary waves.