Voila! :: Internal Tidal Wave

Internal tides are generated as the surface tides move stratified water up and down sloping topography, which produces a wave in the ocean interior. So internal tides are internal waves at a tidal frequency. The other major source of internal waves is the wind which produces internal waves near the inertial frequency. When a small water parcel is displaced from its equilibrium position, it will return either downwards due to gravity or upwards due to buoyancy. The water parcel will overshoot its original equilibrium position and this disturbance will set off an internal gravity wave. Munk (1981) notes, "Gravity waves in the ocean's interior are as common as waves at the sea surface-perhaps even more so, for no one has ever reported an interior calm." [1]

The largest internal tides are generated at steep, midocean topography such as the Hawaiian Ridge, Tahiti, the Macquarie Ridge, and submarine ridges in the Luzon Strait [2]. Continental slopes such as the Australian North West Shelf also generate large internal tides [3]. These internal tide may propagate onshore and dissipate much like surface waves. Or internal tides may propagate away from the topography into the open ocean. For tall, steep, midocean topography, such as the Hawaiian Ridge, it is estimated that about 85% of the energy in the internal tide propagates away into the deep ocean with about 15% of its energy being lost within about 50 km of the generation site. The lost energy contributes to turbulence and mixing near the generation sites[4, 5].It is not clear where the energy that leaves the generation site is dissipated, but there are 3 possible processes: 1) the internal tides scatter and/or break at distant midocean topography, 2) interactions with other internal waves remove energy from the internal tide, or 3) the internal tides shoal and break on continental shelves.

Internal tides generated by tidal semidiurnal currents impinging on steep submarine ridges in island passages, or near the shelf edge, can enhance turbulent dissipation and internal mixing near the generation site. The development of Kelvin-Helmholtz instability during the breaking of the internal tide can explain the formation of high diffusivity patches that generate a vertical flux of nitrate (NO₃⁻) into the photic zone and can sustain new production locally [7, 8]. Another mechanism for higher nitrate flux at spring tides results from pulses of strong turbulent dissipation associated with high frequency internal soliton packets [9]. Some internal soliton packets are the result of the nonlinear evolution of the internal tide.

Fig. Internal tidal wave detected within Labani Channel, Makassar Strait during Widya Nusantara Expedition, LIPI, 2013 [10].

References:

1. Munk, W. (1981). B. A. Warren and C. Wunsch, ed. "Internal Waves and Small-Scale Processes". Evolution of Physical Oceanography (MIT Press): 264–291.
2. Simmons, H. L., R. W. Hallberg, and B. K. Arbic (2004). "Internal wave generation in a global baroclinic tide model". Deep-Sea Res. II 51 (25–26): 3043–3068.
3. Holloway, P. E. (2001). "A regional model of the semidiurnal tide on the Australian North West Shelf". J. Geophys. Res. 106 (C9): 19,625–19,638.
4. Carter, G. S., M. A. Merrifield, J. M. Becker, K. Katsumata, M. C. Gregg, D. S. Luther, M. D. Levine, T. J. Boyd, and Y. L. Firing (2008). "Energetics of M2 Barotropic-to-Baroclinic Tidal Conversion at the Hawaiian Islands". J. Phys. Oceanogr. 38 (10): 2205–2223.
5. Klymak, J. M., J. N. Moum, J. D. Nash, E. Kunze, J. B. Girton, G. S. Carter, C. M. Lee, T. B. Sanford, and M. C. Gregg (2006). "An Estimate of Tidal Energy Lost to Turbulence at the Hawaiian Ridge". J. Phys. Oceanogr. 36 (6): 1148–1164.
6. Briscoe, M. (1975). "Introduction to a collection of papers on oceanographic internal waves". J. Geophys. Res. 80 (3): 289–290.
7. Alfonso-Sosa, E. (2002). Variabilidad temporal de la producción primaria fitoplanctonica en la estación CaTS (Caribbean Time-Series Station): Con énfasis en el impacto de la marea interna semidiurna sobre la producción. Ph. D. Dissertation. Department of Marine Sciences, University of Puerto Rico, Mayagüez, Puerto Rico. UMI publication AAT 3042382. p. 407. Retrieved 2014-08-25.
8. Alfonso-Sosa, E., J. M. Lopez, J. E. Capella, A. Dieppa and J. Morell (2002). "Internal Tide-induced Variations in Primary Productivity and Optical Properties in the Mona Passage, Puerto Rico". Retrieved 2015-01-01.
9. Sharples, J., J. F. Tweddle, J. A. M. Green, M. R. Palmer, Y. Kim, A. E. Hickman,P. M. Holligan, C. M. Moore, T. P. Rippeth, J. H. Simpson and V. Krivtsov (2007). "Spring–neap modulation of internal tide mixing and vertical nitrate fluxes at a shelf edge in summer". Limnol. Oceanogr. 52 (5): 1735–1747. doi:10.4319/lo.2007.52.5.1735. Retrieved 2014-08-25.
10. Purwandana, A. Turbulent mixing in Labani Channel, Makassar Strait. Oseanologi dan Limnologi di Indonesia (OLDI), 2014 Vol. 40 (2):155-169.