Publications

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2015
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.

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.