Publications

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2015
Johnston, TMS, Rudnick DL, Kelly SM.  2015.  Standing internal tides in the Tasman Sea observed by gliders. Journal of Physical Oceanography. 45:2715-2737.   10.1175/jpo-d-15-0038.1   AbstractWebsite

Low-mode internal tides are generated at tall submarine ridges, propagate across the open ocean with little attenuation, and reach distant continental slopes. A semidiurnal internal tide beam, identified in previous altimetric observations and modeling, emanates from the Macquarie Ridge, crosses the Tasman Sea, and impinges on the Tasmanian slope. Spatial surveys covering within 150 km of the slope by two autonomous underwater gliders with maximum profile depths of 500 and 1000 m show the steepest slope near 43 degrees S reflects almost all of the incident energy flux to form a standing wave. Starting from the slope and moving offshore by one wavelength (similar to 150 km), potential energy density displays an antinode-node-antinode-node structure, while kinetic energy density shows the opposite. Mission-mean mode-1 incident and reflected flux magnitudes are distinguished by treating each glider's survey as an internal wave antenna for measuring amplitude, wavelength, and direction. Incident fluxes are 1.4 and 2.3 kW m(-1) from the two missions, while reflected fluxes are 1.2 and 1.8 kW m(-1). From one glider surveying the region of highest energy at the steepest slope, the reflectivity estimates are 0.8 and 1, if one considers the kinetic and potential energy densities separately. These results are in agreement with mode-1 reflectivity of 0.7-1 from a model in one horizontal dimension with realistic topography and stratification. The direction of the incident internal tides is consistent with altimetry and modeling, while the reflected tide is consistent with specular reflection from a straight coastline.

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.

Zhao, ZX, Alford MH, Girton J, Johnston TMS, Carter G.  2011.  Internal tides around the Hawaiian Ridge estimated from multisatellite altimetry. Journal of Geophysical Research-Oceans. 116   10.1029/2011jc007045   AbstractWebsite

Satellite altimetric sea surface height anomaly (SSHA) data from Geosat Follow-on (GFO) and European Remote Sensing (ERS), as well as TOPEX/Poseidon (T/P), are merged to estimate M(2) internal tides around the Hawaiian Ridge, with higher spatial resolution than possible with single-satellite altimetry. The new estimates are compared with numerical model runs. Along-track analyses show that M(2) internal tides can be resolved from both 8 years of GFO and 15.5 years of ERS SSHA data. Comparisons at crossover points reveal that the M(2) estimates from T/P, GFO, and ERS agree well. Multisatellite altimetry improves spatial resolution due to its denser ground tracks. Thus M(2) internal tides can be plane wave fitted in 120 km x 120 km regions, compared to previous single-satellite estimates in 4 degrees lon x 3 degrees lat or 250 km x 250 km regions. In such small fitting regions the weaker and smaller-scale mode 2 M(2) internal tides can also be estimated. The higher spatial resolution leads to a clearer view of the M(2) internal tide field around the Hawaiian Ridge. Discrete generation sites and internal tidal beams are clearly distinguishable, and consistent with the numerical model runs. More importantly, multisatellite altimetry produces larger M(2) internal tidal energy fluxes, which agree better with model results, than previous single-satellite estimates. This study confirms that previous altimetric underestimates are partly due to the more widely spaced ground tracks and consequently larger fitting region. Multisatellite altimetry largely overcomes this limitation.

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, 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.