Internal Wave Dynamics in Oceanic Mixing

Internal waves arise where gravity acts on density stratification, transferring energy from large-scale tides and currents to small-scale turbulence. Their propagation along sloping topography and interaction with mesoscale features such as eddies instigate shear and convective instabilities that enhance diapycnal mixing, the process by which water masses and tracers cross density surfaces. This mixing underpins the global overturning circulation by returning deep waters to the surface, modulating heat, carbon and nutrient distributions, and influencing regional climates. Key generation mechanisms include barotropic tidal conversion over rough topography, lee-wave excitation by geostrophic flow, and wind-driven internal tides. Nonlinear processes—parametric subharmonic instability, elastic scattering and induced diffusion—mediate energy cascades from primary wave bands to turbulent scales. Observational advances using microstructure profilers, ship-borne acoustic Doppler current profilers and satellite altimetry have quantified mixing rates across mid-ocean ridges, continental slopes and frontal zones. In parallel, ocean and climate models now incorporate both explicit tidal forcing and parameterised mixing schemes calibrated against finescale shear data. An improved representation of internal wave dynamics is vital for accurate projection of ocean circulation, biogeochemical cycles and climate variability on seasonal to centennial timescales.

Research from Nature Portfolio

Recent studies have delineated the latitude-dependent generation of finescale turbulent shear in the upper Pacific, revealing distinct mixing peaks at equatorial, mid-tropical and subtropical latitudes driven respectively by vertically sheared currents, parametric subharmonic instability of diurnal tides and inertial chimney effects of eddies. This W-shaped distribution emphasises the need to incorporate geographically varying shear generation mechanisms into model parameterisations for improved climate simulations. In a complementary investigation, experiments with a global ocean-climate model have demonstrated that microscale mixing induced by deep Pacific tides reorganises Southern Ocean stratification, suppresses deep convection in the Ross Sea and alters sea-ice cover and westerly wind patterns. These findings underscore the influence of remote deep-ocean processes on Southern Ocean climate and the critical importance of representing tide-induced mixing in predictive models.

Research from all publishers

Observation-based estimates combined with numerical simulations have quantified diapycnal upwelling of North Atlantic Deep Water at rates of 0.5–8 Sv between 48° N and 32° S, demonstrating that internal wave breaking and near-boundary processes significantly alter tracer pathways within the Atlantic Meridional Overturning Circulation. Such mixing reshapes where heat and carbon return to the surface, with direct implications for global climate models. A detailed numerical evaluation of the Garrett–Munk internal wave spectrum has resolved long-standing questions about the mechanisms of energy transfer, attributing forward cascade primarily to local wave–wave interactions and elastic scattering, and confirming the magnitude of the downscale energy flux available for diapycnal mixing. In the Southern Ocean, satellite altimetry and Argo profiling have been used to composite anticyclonic and cyclonic eddies in a standing meander of the Antarctic Circumpolar Current, revealing that cyclonic eddies can elevate diapycnal diffusivity by up to two orders of magnitude to depths of 1200 dbar. These direct observations of eddy-induced mixing provide critical constraints for parameterisations in climate and circulation models.

Internal Wave Dynamics in Oceanic Mixing publication trend

The graph below shows the total number of articles in internal wave dynamics in oceanic mixing across all publications each year (not limited to Nature Index journals).

Technical terms

Internal wave: A gravity wave oscillating within a stratified fluid, propagating along density interfaces.

Diapycnal mixing: Mixing that crosses surfaces of constant density, facilitating water mass transformation.

Parametric subharmonic instability: A nonlinear mechanism by which a primary internal wave transfers energy to two waves of half frequency.

Garrett–Munk spectrum: A canonical model describing the statistical distribution of internal wave energy across frequencies and wavenumbers.

Diapycnal diffusivity: A coefficient quantifying the rate of turbulent mixing across density layers.

References

1. Significance of Diapycnal Mixing Within the Atlantic Meridional Overturning Circulation. AGU Advances (2023).
2. The tidal effects in the Finite-volumE Sea ice–Ocean Model (FESOM2.1): a comparison between parameterised tidal mixing and explicit tidal forcing. Geoscientific Model Development (2023).
3. Three-Dimensional Structure of Mesoscale Eddies and Their Impact on Diapycnal Mixing in a Standing Meander of the Antarctic Circumpolar Current. Remote Sensing (2024).
4. Energy cascade in the Garrett–Munk spectrum of internal gravity waves. Journal of Fluid Mechanics (2023).
5. Latitude-dependent finescale turbulent shear generations in the Pacific tropical-extratropical upper ocean. Nature Communications (2018).
6. Impact of deep ocean mixing on the climatic mean state in the Southern Ocean. Scientific Reports (2018).