Mixing by Oceanic Lee Waves

Abstract

Oceanic lee waves are generated in the deep stratified ocean by the flow of ocean currents over sea floor topography, and when they break, they can lead to mixing in the stably stratified ocean interior. While the theory of linear lee waves is well established, the nonlinear mechanisms leading to mixing are still under investigation. Tidally driven lee waves have long been observed in the ocean, along with associated mixing, but observations of lee waves forced by geostrophic eddies are relatively sparse and largely indirect. Parameterizations of the mixing due to ocean lee waves are now being developed and implemented in ocean climate models. This review summarizes current theory and observations of lee wave generation and mixing driven by lee wave breaking, distinguishing between steady and tidally oscillating forcing. The existing parameterizations of lee wave–driven mixing informed by theory and observations are outlined, and the impacts of the parameterized lee wave–driven mixing on simulations of large-scale ocean circulation are summarized. Expected final online publication date for the Annual Review of Fluid Mechanics, Volume 53 is January 7, 2021. Please see http://www.annualreviews.org/page/journal/pubdates for revised estimates.

Shear-induced breaking of internal gravity waves

“…A key open question remains of how much mixing can be attributed to each of these sources, such as tidal beams (Dauxois et al. 2018), lee waves (Legg 2021) and near-inertial waves (Alford et al. 2016).…”

“…In this idealised study, we do not specify the source of the internal gravity wave, but simply choose appropriate parameters to remain consistent with the observations. We acknowledge that for many oceanographic applications, it is useful to quantify mixing associated with specific generation mechanisms, such as oceanic lee waves (Legg 2021).…”

Surface reflection of bottom generated oceanic lee waves

“…Observations of lee waves are sparse due to their unpredictable generation by the time varying eddy field, difficulty in taking measurements at the bottom of the ocean, and their steady nature (Legg 2021). However, the available observational evidence indicates that linear predictions of energy flux exceed the levels of dissipation in the bottom by up to an order of magnitude (Brearley et al.…”

Exploiting self-organized criticality in strongly stratified turbulence

“…Indeed, the fluid dynamical core of these computational models generally comprises the so-called hydrostatic primitive equations (Miller 2007; Klein 2010). Nevertheless, small-scale non-hydrostatic stratified mixing events – often although not exclusively associated with the breakdown of internal waves (MacKinnon et al 2017; Legg 2021) – ultimately exert strong feedbacks on the resolved large-scale flow, particularly to close the global buoyancy budget (Ferrari & Wunsch 2004; Gregg et al 2018); therefore, suitable parametrizations must be developed and implemented. Generally, these parametrizations are physically motivated but ultimately ad hoc and thus raise the attendant issues over robustness, accuracy and reliability.…”