Satellite images have long revealed the surface expression of large amplitude internal waves that propagate along density interfaces beneath the sea surface1,2,3. Internal waves are typically the most energetic high-frequency events in the coastal ocean4,5,6, displacing water parcels by up to 100 m and generating strong currents and turbulence7 that mix nutrients into near-surface waters for biological utilization. While internal waves are known to be generated by tidal currents over ocean-bottom topography8,9,10,11,12,13, they have also been observed frequently in the absence of any apparent tide–topography interactions1,7,14. Here we present repeated measurements of velocity, density and acoustic backscatter across the Columbia River plume front. These show how internal waves can be generated from a river plume that flows as a gravity current into the coastal ocean. We find that the convergence of horizontal velocities at the plume front causes frontal growth and subsequent displacement downward of near-surface waters. Individual freely propagating waves are released from the river plume front when the front's propagation speed decreases below the wave speed in the water ahead of it. This mechanism generates internal waves of similar amplitude and steepness as internal waves from tide–topography interactions observed elsewhere11, and is therefore important to the understanding of coastal ocean mixing.
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We thank M. Neeley-Brown, R. Kreth and A. Perlin for their technical expertise. L. Kilcher, T. Kimura, R. Bjorkquist, A. Horner-Devine, T. Chisholm, and the captain and crew of the RV Pt. Sur made data collection possible. Satellite imagery was provided by P.T. Strub and P. Orton. Comments were provided by W.D. Smyth, G. Avicola and J. Klymak. This work was funded by the National Science Foundation and the Office of Naval Research.
Reprints and permissions information is available at npg.nature.com/reprintsandpermissions. The authors declare no competing financial interests.
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Nash, J., Moum, J. River plumes as a source of large-amplitude internal waves in the coastal ocean. Nature 437, 400–403 (2005). https://doi.org/10.1038/nature03936
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