Publications

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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.

Johnston, TMS, Rudnick DL.  2015.  Trapped diurnal internal tides, propagating semidiurnal internal tides, and mixing estimates in the California Current System from sustained glider observations, 2006-2012. Deep-Sea Research Part Ii-Topical Studies in Oceanography. 112:61-78.   10.1016/j.dsr2.2014.03.009   AbstractWebsite

From 2006-2012, along 3 repeated cross-shore transects (California Cooperative Oceanic Fisheries Investigations lines 66.7, 80, and 90) in the California Current System, 33 609 shear and 39 737 strain profiles from 66 glider missions are used to estimate mixing via finescale parameterizations from a dataset containing over 52 000 profiles. Elevated diffusivity estimates and energetic diurnal (D-1) and semidiurnal (D-2) internal tides are found: (a) within 100 km of the coast on lines 66.7 and 80 and (b) over the Santa Rosa-Cortes Ridge (SRCR) in the Southern California Bight (SCB) on line 90. While finding elevated mixing near topography and associated with internal tides is not novel, the combination of resolution and extent in this ongoing data collection is unmatched in the coastal ocean to our knowledge. Both D-1 and D-2 internal tides are energy sources for mixing. At these latitudes, the D-1 internal tide is subinertial. On line 90, D-1 and D-2 tides are equally energetic over the SRCR, the main site of elevated mixing within the SCB. Numerous sources of internal tides at the rough topography in the SCB produce standing and/or partially-standing waves. On lines 66.7 and 80, the dominant energy source below about 100 m for mixing is the D-1 internal tide, which has an energy density of the D-2 internal tide. On line 80, estimated diffusivity, estimated dissipation, and D-1 energy density peak in summer. The D-1 energy density shows an increasing trend from 2006 to 2012. Its amplitude and phase are mostly consistent with topographically-trapped D-1 internal tides traveling with the topography on their right. The observed offshore decay of the diffusivity estimates is consistent with the exponential decay of a trapped wave with a mode-1 Rossby radius of 20-30 km. Despite the variable mesoscale, it is remarkable that coherent internal tidal phase is found. (C) 2014 Elsevier Ltd. All rights reserved.

2011
Johnston, TMS, Rudnick DL, Carter GS, Todd RE, Cole ST.  2011.  Internal tidal beams and mixing near Monterey Bay. Journal of Geophysical Research-Oceans. 116   10.1029/2010jc006592   AbstractWebsite

The spatial structure of velocity, density, and mixing in an internal tidal beam generated at a submarine ridge near Monterey Bay was observed using a combination of vessel-mounted acoustic Doppler current profilers, a towed conductivity-temperature-depth instrument (SeaSoar), and microconductivity sensors mounted on SeaSoar. Three <60 km meridional sections from the surface to 400-670 m in depth were occupied a total of 56 times during 16 days with the sampling pattern detuned from the M(2) tide. Averaging over all observations at a given latitude-depth bin produces a phase average of the M(2) internal tide. Observed velocity and displacement variances are scaled to estimate energy density. A beam in energy density originates from a submarine ridge and reflects with diminished amplitude at the surface. These results compare favorably with a numerical tidal model. The upward and downward beams show modestly elevated turbulence, which is patchy along the beam and has mean values about 50% larger than those outside of the beam. Peak values can be almost an order of magnitude larger in the beam. Dissipation increases with increasing shear and stratification similar to the MacKinnon-Gregg parameterization. Intermediate nepheloid layers were found in over half of the meridional sections. Their phasing and direction indicate that they originate at a secondary, weaker internal tidal generation site found in the model but not in the observations presumably due to mesoscale variability affecting stratification at the generation site and during wave propagation. The offshore movement of sediment is a result of westward mean current and internal wave-driven transport.

2010
Rainville, L, Johnston TMS, Carter GS, Merrifield MA, Pinkel R, Worcester PF, Dushaw BD.  2010.  Interference Pattern and Propagation of the M(2) Internal Tide South of the Hawaiian Ridge. Journal of Physical Oceanography. 40:311-325.   10.1175/2009jpo4256.1   AbstractWebsite

Most of the M(2) internal tide energy generated at the Hawaiian Ridge radiates away in modes 1 and 2, but direct observation of these propagating waves is complicated by the complexity of the bathymetry at the generation region and by the presence of interference patterns. Observations from satellite altimetry, a tomographic array, and the R/P FLIP taken during the Farfield Program of the Hawaiian Ocean Mixing Experiment (HOME) are found to be in good agreement with the output of a high-resolution primitive equation model, simulating the generation and propagation of internal tides. The model shows that different modes are generated with different amplitudes along complex topography. Multiple sources produce internal tides that sum constructively and destructively as they propagate. The major generation sites can be identified using a simplified 2D idealized knife-edge ridge model. Four line sources located on the Hawaiian Ridge reproduce the interference pattern of sea surface height and energy flux density fields from the numerical model for modes 1 and 2. Waves from multiple sources and their interference pattern have to be taken into account to correctly interpret in situ observations and satellite altimetry.

2003
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.