Publications

Export 3 results:
Sort by: Author Title Type [ Year  (Desc)]
2015
Alford, MH, Peacock T, MacKinnon JA, Nash JD, Buijsman MC, Centuroni LR, Chao SY, Chang MH, Farmer DM, Fringer OB, Fu KH, Gallacher PC, Graber HC, Helfrich KR, Jachec SM, Jackson CR, Klymak JM, Ko DS, Jan S, Johnston TMS, Legg S, Lee IH, Lien RC, Mercier MJ, Moum JN, Musgrave R, Park JH, Pickering AI, Pinkel R, Rainville L, Ramp SR, Rudnick DL, Sarkar S, Scotti A, Simmons HL, St Laurent LC, Venayagamoorthy SK, Hwang Y, Wang J, Yang YJ, Paluszkiewicz T, Tang TY.  2015.  The formation and fate of internal waves in the South China Sea. Nature. 521:65-U381.   10.1038/nature14399   AbstractWebsite

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 photosynthesis(1), sediment and pollutant transport(2) and acoustic transmission(3); they also pose hazards for man-made structures in the ocean(4). Generated primarily by the wind and the tides, internal waves can travel thousands of kilometres from their sources before breaking(5), making it challenging to observe them and to include them in numerical climate models, which are sensitive to their effects(6,7). For over a decade, studies(8-11) 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.

2003
Johnston, TMS, Merrifield MA, Holloway PE.  2003.  Internal tide scattering at the Line Islands Ridge. Journal of Geophysical Research-Oceans. 108   10.1029/2003jc001844   AbstractWebsite

[ 1] Scattering of the M-2 mode one internal tide from the Line Islands Ridge is examined with a primitive equation numerical model. Model runs with baroclinic and barotropic forcing are performed to distinguish scattered from locally generated internal tides. TPXO. 5 tidal model sea surface elevations provide barotropic forcing, while for the run with baroclinic forcing a mode one M-2 energy flux of 1000 W m-(1) is used to represent energy fluxes emanating from the Hawaiian Ridge. Scattering redistributes more energy flux from mode one than is locally generated in mode one. For the higher modes, scattering and generation contribute equally in terms of the overall energy flux. Spatial and modal distributions of energy density and flux show internal tide scattering dominates at Hutchinson Seamount, while higher modes are generated locally at Sculpin Ridge. Hutchinson Seamount's slopes are steeper over a greater continuous area than Sculpin Ridge. Scattered energy is found downstream of the steepest topographies, similar to simulations with idealized Gaussian ridges. At the Line Islands Ridge, 37% of the incident mode one energy flux is lost because of scattering into modes 2 - 5 ( 19%), dissipation by the model's turbulence parameterization ( 15%), and nonlinear transfer to the M-4 internal tide ( 3%). Two TOPEX ground tracks pass through the model domain roughly normal to the ridge topography and confirm the general features of the modal and spatial distribution found in the model. In the topographically rough western Pacific, internal tide scattering may be a significant source of energy for mixing away from topography.

Johnston, TMS, Merrifield MA.  2003.  Internal tide scattering at seamounts, ridges, and islands. Journal of Geophysical Research-Oceans. 108   10.1029/2002jc001528   AbstractWebsite

[1] The scattering of mode-1 internal tides from idealized Gaussian topography in a nonrotating ocean with constant and realistic stratifications is examined with a primitive equation numerical model. Incident mode-1 energy fluxes of 20 and 2000 W m(-1) are used to examine the linear regime and a more realistic situation. Simulations using two-dimensional or infinite ridges compare well with ray tracing methods and illustrate how the size and shape of the topography influence wave scattering. The height affects energy transmission and reflection, while the slope and width determine the conversion of low-mode internal tides into beams or higher modes. Three-dimensional topographic scattering is considered for seamounts, finite-width ridges, and islands. Scattering from finite ridges focuses wave energy directly downstream, while scattering from seamounts produces azimuthal energy dispersion. Scattering to higher wave modes occurs in the lee of near-critical and supercritical seamounts and ridges. Nonlinear interactions transfer energy into the mode-1 M-4 internal tide. The Mellor-Yamada level-2.5 submodel parameterizes turbulent mixing. For the near-critical and supercritical ridges with realistic stratification, elevated mixing is found over the leading edge of the topography and along a tidal beam up to the first surface bounce. A transition from a beam structure near the topography to a low-mode structure farther away occurs due to an increased contribution from the mode-1 internal tide as it refracts around the topography and not due to turbulent dissipation. Internal tide scattering at topography leads to a loss of energy to mixing and to a redistribution of energy flux in space, frequency, and mode number.