Shear-induced breaking of internal gravity waves

Howland, Christopher J.; Taylor, John R.; Caulfield, C. P.

doi:10.1017/jfm.2021.506

Cited by 16 publications

(16 citation statements)

References 53 publications

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“…We anticipate that this reasoning will apply to more general overturning structures in the ocean. Indeed, Howland et al (2021) find a similar competition between shear and convective instabilities in the interaction between a large-scale breaking gravity wave and a background sinusoidal shear flow.…”

Section: Discussionmentioning

confidence: 71%

“…Indeed, Howland et al. (2021) find a similar competition between shear and convective instabilities in the interaction between a large-scale breaking gravity wave and a background sinusoidal shear flow.…”

Section: Discussionmentioning

confidence: 91%

“…The transition to and subsequent evolution of fully developed turbulence in a simple two-layer KHI set-up with constant background vertical shear is 'fuelled' by the shear, in the sense that it provides the dominant source of energy for inherently three-dimensional turbulent fluctuations to grow (Caulfield & Peltier 2000). In oceanographic flows, regions of intensified local shear arise due to large-scale forcing by internal gravity waves (Woods 1968) and their interaction with existing background shear (Howland, Taylor & Caulfield 2021). However, it is reasonable to expect that the background shear induced by a gravity wave might not be constant, and it is therefore natural to ask whether KHI turbulence that mixes vigorously can develop in the absence of the shear that acted to form the initial instability.…”

Section: Introductionmentioning

confidence: 99%

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Stratified turbulent mixing in oscillating shear flows

Lewin

Caulfield

2022

J. Fluid Mech.

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Motivated by the variation of local shear produced by internal waves in the ocean, we use direct numerical simulations to investigate the effect of a time-dependent shear forcing on the evolution and mixing of turbulence produced by Kelvin–Helmholtz instability (KHI) at high Reynolds number. The forcing is implemented using a tilting coordinate system which causes the background shear to accelerate and decelerate periodically. We demonstrate that, with suitable timing between development of instability and the shear oscillation cycle, turbulence produced by KHI with a decelerating shear mixes in a distinctly different way from the flow with constant background shear, specifically with the energy for turbulent motions extracted from alternative sources. As a result, the total amount of mixing as measured by the change in background potential energy can in fact be significantly larger for flows in which the shear is decelerated, despite the fact that the total kinetic energy in the flow is significantly smaller. The mixing has characteristics more in common with convectively driven rather than shear-driven flows, supporting the argument for an underlying change in the mechanisms triggering the turbulence.

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Section: Discussionmentioning

confidence: 71%

Section: Discussionmentioning

confidence: 91%

Section: Introductionmentioning

confidence: 99%

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Stratified turbulent mixing in oscillating shear flows

Lewin

Caulfield

2022

J. Fluid Mech.

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“…Field studies have shown that there is considerable spatial variation in the mixing, typically being weaker in the ocean interior and more intense near the ocean floor and boundaries (Polzin et al 1997;Waterhouse et al 2014). The process of mixing involves a combination of internal wave breaking and shear-driven mixing (Winters et al 1995;Howland, Taylor & Caulfield 2021;Mashayek, Caulfield & Alford 2021), as well as localised boundary mixing (Thorpe 2007;Echeverri et al 2009;Winters 2015;Jagannathan, Winters & Armi 2017;Polzin & McDougall 2022) which then interacts with the remainder of the fluid. From a theoretical perspective, turbulent mixing in a stratified fluid involves a number of complex processes, and depends in detail on the balance between the dissipation of kinetic energy, the generation of potential energy, the ambient stratification and both the mean velocity and the shear (Osborn 1980).…”

Section: Introductionmentioning

confidence: 99%

“…The process of mixing involves a combination of internal wave breaking and shear-driven mixing (Winters et al. 1995; Caulfield 2021; Howland, Taylor & Caulfield 2021; Mashayek, Caulfield & Alford 2021), as well as localised boundary mixing (Thorpe 2007; Echeverri et al. 2009; Winters 2015; Jagannathan, Winters & Armi 2017; Polzin & McDougall 2022) which then interacts with the remainder of the fluid.…”

Section: Introductionmentioning

confidence: 99%

Boundary mixing. Part 1. Transport of buoyancy and passive tracer

Li,

Woods

2023

J. Fluid Mech.

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Diapycnal mixing plays a key role in the thermohaline circulation of the deep ocean. Field observations have suggested that this mixing is intensified over the rough topography along the boundaries of the ocean. In this study, we experimentally explore the transport of salinity and tracer in a horizontally stirred, stratified fluid with a steady vertical buoyancy flux. The mechanical mixing occurs either uniformly across the tank or near a sidewall. To compare uniform and boundary mixing, first we explore the steady state dynamics and find that, in both cases, the surfaces of constant density are horizontal. With uniform mixing, vertical transport of buoyancy occurs uniformly throughout the tank while, with boundary mixing, transport is confined to the turbulent region and the interior space remains quiescent and does not mix or have any net movement. We then explore the transient evolution of the stratification when the source of buoyancy is removed from the base of the system. In the boundary mixing case, the resulting divergence of the turbulent diffusive flux in the boundary region leads to a reduction in the buoyancy of the fluid in the boundary region, and a net upflow develops in the boundary region. In turn, this drives a downwelling in the interior. Vertical gradients in the rate of downwelling lead to stretching of isopycnals and, together, these processes enable the interior stratification to evolve. The experiments highlight that, with boundary mixing, the main transport of buoyancy occurs near the boundaries even though the interior is stratified; independent measurements of tracer mixing in the interior show that this fluid may be quiescent and stratified even though there is a large flux being transported in the boundary region. This has important implications for the interpretation of mixing data in the ocean. In a companion paper, (Li and Woods Part 2), we explore the interaction of a net upwelling with such boundary mixing.

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