Abstract
Internal gravity waves, the subsurface analogue of the familiar surface gravity waves that break on beaches, are ubiquitous in the ocean. Because of their strong vertical and horizontal currents, and the turbulent mixing caused by their breaking, they affect a panoply of ocean processes, such as the supply of nutrients for photosynthesis1, sediment and pollutant transport2 and acoustic transmission3; they also pose hazards for man-made structures in the ocean4. Generated primarily by the wind and the tides, internal waves can travel thousands of kilometres from their sources before breaking5, making it challenging to observe them and to include them in numerical climate models, which are sensitive to their effects6,7. For over a decade, studies8,9,10,11 have targeted the South China Sea, where the oceans’ most powerful known internal waves are generated in the Luzon Strait and steepen dramatically as they propagate west. Confusion has persisted regarding their mechanism of generation, variability and energy budget, however, owing to the lack of in situ data from the Luzon Strait, where extreme flow conditions make measurements difficult. Here we use new observations and numerical models to (1) show that the waves begin as sinusoidal disturbances rather than arising from sharp hydraulic phenomena, (2) reveal the existence of >200-metre-high breaking internal waves in the region of generation that give rise to turbulence levels >10,000 times that in the open ocean, (3) determine that the Kuroshio western boundary current noticeably refracts the internal wave field emanating from the Luzon Strait, and (4) demonstrate a factor-of-two agreement between modelled and observed energy fluxes, which allows us to produce an observationally supported energy budget of the region. Together, these findings give a cradle-to-grave picture of internal waves on a basin scale, which will support further improvements of their representation in numerical climate predictions.
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Acknowledgements
This article is dedicated to the memory of author T.-Y. Tang. Our work was supported by the US Office of Naval Research and the Taiwan National Science Council. We are indebted to the captains and crew of all of the research vessels that supported this work, as well as to the technical staff of the seagoing institutions. Without the skill and hard work of all of these people, these observations would not have been possible.
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All authors contributed to the paper in multiple ways. Primary writing: M.H.A., T. Peacock, J.A.M. & J.D.N. Synthesis and overall coordination: T. Paluszkiewicz & T.-Y.T. Energy flux calculations: M.H.A. & A.I.P. Energy budget calculation: M.H.A., M.C.B., M.-H.C., R.-C.L., J.M.K. & L.C.S.L. Near-field moorings and calculations: M.H.A., A.I.P., L.R., J.D.N., J.N.M. & M.-H.C. Far-field moorings and calculations: L.R.C., M.-H.C., R.-C.L., S.R.R., Y.J.Y. & T.-Y.T. Near-field CTD measurements (Fig. 2d): R.P. & R.M. Near-field lowered acoustic Doppler current profiler measurements: M.H.A., J.D.N., J.A.M., L.R., H.L.S., A.I.P & R.M. Pressure inverted echo sounder measurements: D.M.F., J.-H.P., Y.J.Y. & M.H.A. Microstructure measurements: L.C.S.L., K.-H.F., H.L.S. & Y.-H.W. Remote sensing: C.R.J. & H.C.G. Theory: K.R.H. & D.M.F. Glider measurements: T.M.S.J. & D.L.R. Regional contextualization and logistical support: S.-Y.C., I-H.L., S.R.R., J.W., Y.J.Y. & T.-Y.T. Far-field modelling: S.J. & H.L.S. Two-dimensional modelling: J.M.K., S.S., S.M.J., A.S., R.M. & K.V. Near-field modelling: M.C.B., O.B.F., S.L. & S.M.J. Kuroshio modelling: P.C.G., S.J. & D.S.K. Laboratory measurements: T. Peacock & M.J.M.
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Extended Data Figure 1 Comparison of observed and model energy flux.
Left, Scatter plot of flux magnitude F from observations (x axis) and far-field (UAF; University of Alaska Fairbanks) model (y axis). Error bars are ±20% for observed values and ±10% for model values (see Methods). Right, As for the left panel, but for direction θ; error bars are ±30°. See source data and ref. 15 for station locations. Black, semi-diurnal; red, diurnal.
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Alford, M., Peacock, T., MacKinnon, J. et al. The formation and fate of internal waves in the South China Sea. Nature 521, 65–69 (2015). https://doi.org/10.1038/nature14399
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DOI: https://doi.org/10.1038/nature14399
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