Internal gravity waves, the subsurface analogue of the familiar surface gravity waves that break on beaches, are ubiquitous in the ocean. Because of their strong Internal gravity waves are propagating disturbances of the ocean's density stratification. Their physics resembles that of surface gravity waves but with buoyancy rather than gravity providing their restoring force -making them much larger (10's to 100's of meters instead of 1 to 10 meters) and slower (hours instead of seconds). Generated primarily by tidal flow past seafloor topography and winds blowing on the sea surface, and typically having multi-kilometer-scale horizontal wavelengths, their estimated 1 TW of deep-sea dissipation is understood to play a crucial role in the ocean's global redistribution of heat and momentum 12 . A major challenge is to improve understanding of internal wave generation, propagation, steepening and dissipation, so that the role of internal waves can be more accurately incorporated in climate models.The internal waves that originate from the Luzon Strait on the eastern margin of the South China Sea (SCS) are the largest documented in the global oceans ( Figure 1).As the waves propagate west from the Luzon Strait they steepen dramatically ( Figure 1a), producing distinctive solitary wave fronts evident in sun glint and synthetic aperture radar (SAR) images from satellites ( Figure 1b). When they shoal onto the continental slope to the west, the downward displacement of the ocean's layers associated with these solitary waves can exceed 250 m in 5 minutes 8 . On such a scale, these waves pose hazards for underwater navigation and offshore drilling 4 , and supply nutrients from the deep ocean that nourish coral reefs 1 and pilot whale populations that forage in their wakes 13 .Over the past decade a number of field studies have been conducted in the region; this work has been comprehensively reviewed 10,11 . All of these studies, however, focused on the propagation of the internal waves across the SCS and their interactions with the continental shelf of China. Until the present study there had been no substantial in situ data gathered at the generation site of the Luzon Strait, in large part because of the extremely challenging operating conditions. A consequence has been persistent 5 confusion regarding the nature of the generation mechanism 11 ; an underlying cause being the sensitivity of the models employed to the system parameters, such as the chosen transect for a two-dimensional model, the linear internal wave speed or the assumed location of the waves' origin within the Luzon Strait. Furthermore, the lack of in situ data from the Luzon Strait has meant an inability to test numerical predictions of energy budgets 9 and no knowledge of the impact of the Kuroshio on the emergence of internal solitary waves 11 .The goal of IWISE is to obtain the first comprehensive in situ data set from the Luzon Strait, which in combination with high-resolution three-dimensional numerical modeling supports a cradle-to-grave picture ...

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. The Richardson number estimated from observations is larger than 1/4, indicating that the interface is stable. However, density profiles reveal these preturbulent 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, a velocity profile is inferred in which the shear is sufficiently high to create explosively growing, small wavelength shear instabilities. It is argued that this is the generation mechanism for the observed turbulence and hence the persistent structure of high acoustic backscatter in these internal solitary waves.

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The gravitational exchange of two fluids with different densities between reservoirs connected by a channel of constant depth and slowly varying breadth is analysed as a problem of internal hydraulics. It is shown that maximal two-way exchange with a net barotropic flow requires the presence of two controls, one at the narrrowest section and a second or ‘virtual’ control lying to one side of the narrowest section. The two controls are connected by a subcritical region, but are separated from subcritical conditions in the reservoirs by supercritical flow and stationary internal bores. Solutions are found for maximal exchange without a net barotropic component, in which case the problem is similar to that first examined by Stommel & Farmer (1953). The Stommel & Farmer analysis is shown to be a rather special limiting example of submaximal exchange, not generally applicable to natural flows. The addition of a net barotropic flow yields a range of different flow types, including maximal exchange, one-layer baroclinic flow, one-layer barotropic flow, submaximal flow governed by a reservoir condition and two-layer unidirectional flow. The maximal-exchange solution is integrated for periodic barotropic flow.