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.

2011
Johnston, TMS, Rudnick DL, Pallas-Sanz E.  2011.  Elevated mixing at a front. Journal of Geophysical Research-Oceans. 116   10.1029/2011jc007192   AbstractWebsite

The mesoscale, submesoscale, and microscale structure of a front in the California Current was observed using a towed vehicle outfitted with microconductivity sensors. Thirteen >60 km cross-front sections from 0 to 350 m in depth were covered in 3.5 days. Objectively mapped data are fit via the Omega (omega) equation to obtain vertical velocity. A composite cross-front section shows elevated mixing on the dense side within 10-20 km of the front. Water downwells and gradients are elevated there: i.e., Rossby number (Ro), horizontal strain (alpha), spice gradients, and microscale thermal dissipation (chi). Thermal eddy diffusivity (K(T)) reaches 10 (3) m(2) s (1) and increases 3-10x from the anticyclonic to the cyclonic side with a depth mean of similar to 10 (4) m(2) s (1). The spatial structure of K(T), Ro, and alpha are similar on the dense side, suggesting an energy cascade from the mesoscale via the submesoscale to the microscale. However, it is unclear whether frontogenesis, internal wave blocking by elevated vorticity, or internal wave trapping by large a produces the elevated mixing. The mean turbulent heat flux opposes the mean restratifying, mesoscale heat flux of 10Wm(-2) and may allow the front to persist. Turbulent nitrate fluxes are 0.1-0.3 mmol m(-2) s(-1). Chlorophyll fluorescence and beam transmission reveal a <6 km wide, similar to 100 km long alongfront streamer which is a deep biomass maximum. Time scales for mixing and nutrient fluxes are 0.3-3 days, which are similar to phytoplankton growth rates and the time scale for frontal evolution.

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.