This special issue of Oceanography presents a survey of recent work on internal waves in the ocean. The undersea analogue to the surface waves we see breaking on beaches, internal waves play an important role in transferring heat, energy, and momentum in the ocean. When they break, the turbulence they produce is a vital aspect of the ocean’s meridional overturning circulation. Numerical circulation models must parameterize internal waves and their breaking because computers will likely never be powerful enough to simultaneously resolve climate and internal wave scales. The demonstrated sensitivity of these models to the magnitude and distribution of internal wave-driven mixing is the primary motivation for the study of oceanic internal waves. Because internal waves can travel far from their source regions to where they break, progress requires understanding not only their generation but also their propagation through the eddying ocean and the processes that eventually lead to their breaking. Additionally, in certain regions such as near coasts and near strong generation regions, internal waves can develop into sharp fronts wherein the thermocline dramatically shoals hundreds of meters in only a few minutes. These “nonlinear” internal waves can have horizontal currents of several knots (1 knot is roughly 2 meters per second), and are strong enough to significantly affect surface navigation of vessels. Vertical current anomalies often reach one knot as well, posing issues for subsurface navigation and engineering structures associated with offshore energy development. Finally, the upwelling and turbulent mixing supported by internal waves can be vital for transporting nutrient-rich fluid into coastal ecosystems such as coral reefs. Below, we provide a very brief introduction to some of the central concepts discussed in the 14 articles that make up the special issue section, and then put each of these articles in context.

St. Laurent, L., M.H. Alford, and T. Paluszkiewicz. 2012. An introduction to the special issue on internal waves. Oceanography 25(2):15–19, https://doi.org/10.5670/oceanog.2012.37.

Alford, M.H. 2001. Internal swell generation: The spatial distribution of energy flux from the wind to mixed-layer near-inertial motions. Journal of Physical Oceanography 31:2,359–2,368, https://doi.org/10.1175/1520-0485(2001)031<2359:ISGTSD>2.0.CO;2.
1. Alford, M.H. 2003a. Improved global maps and 54-year history of wind-work on ocean inertial motions. Geophysical Research Letters 30, 1424, https://doi.org/10.1029/2002GL016614.
2. Alford, M.H. 2003b. Redistribution of energy available for ocean mixing by long-range propagation of internal waves. Nature 423:159–162, https://doi.org/10.1038/nature01628.
3. Apel, J.R. 2003. A new analytical model for internal solitons in the ocean. Journal of Physical Oceanography 33:2,247–2,269, https://doi.org/10.1175/1520-0485(2003)033<2247:ANAMFI>2.0.CO;2.
4. Arbic, B., S. Garner, R. Hallberg, and H. Simmons. 2004. The accuracy of surface elevations in forward near-global barotropic and baroclinic tidal models. Deep-Sea Research Part II 51:3,069–3,101, https://doi.org/10.1016/j.dsr2.2004.09.014.
5. Cushman-Roisin, B., and J.-M. Beckers. 2011. Introduction to Geophysical Fluid Dynamics: Physical and Numerical Aspects, 2nd ed. Academic Press, 875 pp.
6. Egbert, G., and S. Erofeeva. 2002. Efficient inverse modeling of barotropic ocean tides. Journal of Atmospheric and Oceanic Technology 19:183–204, https://doi.org/10.1175/1520-0426(2002)019<0183:EIMOBO>2.0.CO;2.
7. Egbert, G., and R. Ray. 2000. Significant dissipation of tidal energy in the deeßp ocean inferred from satellite altimeter data. Nature 405:775–778, https://doi.org/10.1038/35015531.
8. Garrett, C., and W.H. Munk. 1972. Space-time scales of internal waves. Geophysical Fluid Dynamics 2:225–264, https://doi.org/10.1080/03091927208236082.
9. Gill, A.E. 1982. Atmosphere-Ocean Dynamics. Academic Press, 662 pp.
10. Gregg, M.C. 1987. Diapycnal mixing in the thermocline: A review. Journal of Geophysical Research 92:5,249–5,286, https://doi.org/10.1029/JC092iC05p05249.
11. Jayne, S.R., and L.C. St. Laurent. 2001. Parameterizing tidal dissipation over rough topography. Geophysical Research Letters 28:811–814, https://doi.org/10.1029/2000GL012044.
12. Jayne, S.R., L.C. St. Laurent, and S.T. Gille. 2004. Connections between ocean bottom topography and Earth’s climate. Oceanography 17(1):65–74, https://doi.org/10.5670/oceanog.2004.68.
13. McComas, C.H., and F.P. Bretherton. 1977. Resonant interaction of oceanic internal waves. Journal of Geophysical Research 82:1,397–1,412, https://doi.org/10.1029/JC082i009p01397.
14. Müller, P., G. Holloway, F. Henyey, and N. Pomphrey. 1986. Nonlinear interactions among internal gravity waves. Reviews of Geophysics 24:493–536, https://doi.org/10.1029/RG024i003p00493.
15. Munk, W. 1981. Internal waves and small-scale processes. Pp. 264–291 in Evolution of Physical Oceanography. B.A. Warren and C. Wunsch, eds, The MIT Press.
16. Pedlosky, J. 1987. Geophysical Fluid Dynamics. Springer-Verlag, Berlin, 710 pp.
17. Plueddemann, A.J., and J.T. Farrar. 2006. Observations and models of the energy flux from the wind to mixed-layer inertial currents. Deep-Sea Research Part II 53:5–30, https://doi.org/10.1016/j.dsr2.2005.10.017.
18. Simmons, H., M.-H. Chang, Y.-T. Chang, S.-Y. Chao, O. Fringer, C.R. Jackson, and D.S. Ko. 2011. Modeling and prediction of internal waves in the South China Sea. Oceanography 24(4):88–99, https://doi.org/10.5670/oceanog.2011.97.
19. St. Laurent, L., H. Simmons, T.Y. Tang, and Y.H. Wang. 2011. Turbulent properties of internal waves in the South China Sea. Oceanography 24(4):78–87, https://doi.org/10.5670/oceanog.2011.96.
20. Sutherland, B. 2010. Internal Gravity Waves. Cambridge University Press, 394 pp.
21. Thorpe, S.A. 2007. An Introduction to Ocean Turbulence. Cambridge University Press, 240 pp.

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