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Internal waves impinging on sloping topography can generate mixing through the formation of near-bottom bores and overturns in what has been called the “internal swash” zone. Here, we investigate the mixing generated during these breaking events and the subsequent ventilation of the bottom boundary layer across a realistic nondimensional parameter space for the ocean using three-dimensional large-eddy simulations. Waves overturn and break at two points during a wave period: when the downslope velocity is strongest and during the rapid onset of a dense, upslope bore. From the first overturning bore to the expulsion of fluid into the interior, there is a strong dependence on the effective wave height, a length scale defined by the ratio of wave velocity over the background buoyancy frequency, an upper bound on the vertical parcel displacement an internal wave can cause. While a similar energetically motivated vertical length scale is often seen in the context of lee-wave generation over topography, the results discussed here suggest this readily measurable parameter can be used to estimate the size of near-boundary overturns, the strength of the ensuing turbulent mixing, and the vertical scale of the along-isopycnal intrusions of fluid ejected from the boundary layer. Examining a volume budget of the near-boundary region highlights spatial and temporal variability that must be considered when determining the water mass transformation during this process.
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Direct measurements reveal instabilities and turbulence within large amplitude internal solitary waves beneath the ocean
Abstract Internal solitary waves are ubiquitous in coastal regions and marginal seas of the world’s oceans. As the waves shoal shoreward, they lose the energy obtained from ocean tides through globally significant turbulent mixing and dissipation and consequently pump nutrient-rich water to nourish coastal ecosystem. Here we present fine-scale, direct measurements of shoaling internal solitary waves in the South China Sea, which allow for an examination of the physical processes triggering the intensive turbulent mixing in their interior. These are convective breaking in the wave core and the collapse of Kelvin–Helmholtz billows in the wave rear and lower periphery of the core, often occurring simultaneously. The former takes place when the particle velocity exceeds the wave’s propagating velocity. The latter is caused by the instability induced by the strong velocity shear overcoming the stratification. The instabilities generate turbulence levels four orders of magnitude larger than that in the open ocean.
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- Award ID(s):
- 1753317
- PAR ID:
- 10213723
- Date Published:
- Journal Name:
- Communications Earth & Environment
- Volume:
- 2
- Issue:
- 1
- ISSN:
- 2662-4435
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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- Free Publicly Accessible Full Text
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Accepted Manuscript1.0
- Journal Article:
- https://doi.org/10.1038/s43247-020-00083-6
