What are internal lee waves?

Internal lee waves are generated by large-scale oceanic flows interacting with small-scale bottom topography (Figure 1). These lee waves have the potential to decelerate the time-mean flow of the ACC by applying a time-mean internal wave drag, which is a significant contributor to the momentum balance of the ocean circulation over the Southern Ocean (Naveira Garabato et al. 2013). Lee waves are also suggested to be a potential energy source for turbulent mixing in the Southern Ocean (e.g., Sheen et al. 2014). Lee-wave-driven mixing sustains water mass transformation in the deep Southern Ocean (Nikurashin and Ferrari 2013; De Lavergne et al. 2016).

Figure 1. Internal lee waves in the ocean (MacKinnon 2013). Internal lee waves are generated over topographic features (e.g., abyssal hills) that have horizontal scale of 600 m to 6 km in the presence of a strong bottom flow (~ 10 cm/s) in a stratified fluid.

What are the characteristics of lee waves?

The horizontal wavelength of lee waves is O(100 m - 10 km). The frequency of lee waves lies between local inertial (f) and buoyancy (N) frequency.

How much energy do lee waves have?

The total energy flux into internal lee waves is estimated to be 0.2 TW which is 20% of the global wind power input into the ocean. The geographical distribution of the energy flux is largest in the Southern Ocean which accounts for half of the total energy flux.

Lee waves have been found to extract 0.2 TW (Nikurashin and Ferrari 2011) to 0.49 TW (Scott et al. 2011) of energy from the total geostrophic flow globally.

Are lee waves important in the ocean?

The generation of internal gravity waves by the flow of a density stratified fluid over an obstacle is a problem of great geophysical interest, which has attracted considerable attention over the years (see e.g. Miles 1969; Zeytounian 1969a, b). Current interest in the problem derives in large measure from the fact that the momentum and energy transported by such waves may have a significant effect on large-scale geophysical flows (Lilly 1972).

The investigation of lee waves dates back to 1970s (e.g., Bell 1975a, b). Lee waves affect the large-scale circulation upon their generation and breaking. It has been under debate that whether we should consider lee waves in ocean general circulation models (OGCMs). Lee waves are potentially an important player in the ocean through their impacts on the large-scale flow. Lee waves carry momentum while radiating away from the generation sites, and deposit momentum back once they break; lee waves extract energy from the background flow and use part of them to drive irreversible mixing. The enhanced diapycnal mixing in the deep ocean changes properties of water masses.

As lee waves are too small to resolve in eddy-resolving OGCMs, their impacts on the large-scale circulation need to be parameterised. Although the momentum and energy in lee waves are not 'seen' by the models, through resolved flows, they are still able to regulate how the Southern Ocean responds to the changing westerly wind.

What are the differences between internal lee waves and internal tides? Internal tides are generated by the interaction of tidal flows (oscillating back-and-forth) and topography, while lee waves are generated by the quasi-steady flow.

References

1. Bell, T.H., 1975a. Topographically generated internal waves in the open ocean. Journal of Geophysical Research, 80(3), pp.320-327.

2. Bell, T.H., 1975b. Lee waves in stratified flows with simple harmonic time dependence. Journal of Fluid Mechanics, 67(4), pp.705-722.

3. De Lavergne, C., Madec, G., Le Sommer, J., Nurser, A.G. and Naveira Garabato, A.C., 2016. On the consumption of Antarctic Bottom Water in the abyssal ocean. Journal of Physical Oceanography, 46(2), pp.635-661.

4. MacKinnon, J., 2013. Oceanography: Mountain waves in the deep ocean. Nature, 501(7467), p.321.

5. Naveira Garabato, A.C., Nurser, A.G., Scott, R.B. and Goff, J.A., 2013. The impact of small-scale topography on the dynamical balance of the ocean. Journal of Physical Oceanography, 43(3), pp.647-668.

6. Nikurashin, M. and Ferrari, R., 2011. Global energy conversion rate from geostrophic flows into internal lee waves in the deep ocean. Geophysical Research Letters, 38(8).

7. Nikurashin, M. and Ferrari, R., 2013. Overturning circulation driven by breaking internal waves in the deep ocean. Geophysical Research Letters, 40(12), pp.3133-3137.

8. Scott, R.B., Goff, J.A., Naveira Garabato, A.C. and Nurser, A.J.G., 2011. Global rate and spectral characteristics of internal gravity wave generation by geostrophic flow over topography. Journal of Geophysical Research: Oceans, 116(C9).

9. Sheen, K.L., Garabato, A.N., Brearley, J.A., Meredith, M.P., Polzin, K.L., Smeed, D.A., Forryan, A., King, B.A., Sallée, J.B., Laurent, L.S. and Thurnherr, A.M., 2014. Eddy-induced variability in Southern Ocean abyssal mixing on climatic timescales. Nature Geoscience, 7(8), p.577.