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Wijesekera, HW, Shroyer E, Tandon A, Ravichandran M, Sengupta D, Jinadasa SUP, Fernando HJS, Agrawal N, Arulananthan K, Bhat GS, Baumgartner M, Buckley J, Centurioni L, Conry P, Farrar TJ, Gordon AL, Hormann V, Jarosz E, Jensen TG, Johnston S, Lankhorst M, Lee CM, Leo LS, Lozovatsky I, Lucas AJ, MacKinnon J, Mahadevan A, Nash J, Omand MM, Pham H, Pinkel R, Rainville L, Ramachandran S, Rudnick DL, Sarkar S, Send U, Sharma R, Simmons H, Stafford KM, Laurent LS, Venayagamoorthy K, Venkatesan R, Teague WJ, Wang DW, Waterhouse AF, Weller R, Whalen CB.  2016.  ASIRI: An Ocean–Atmosphere Initiative for Bay of Bengal. Bulletin of the American Meteorological Society. 97:1859-1884.   10.1175/bams-d-14-00197.1   Abstract

Air–Sea Interactions in the Northern Indian Ocean (ASIRI) is an international research effort (2013–17) aimed at understanding and quantifying coupled atmosphere–ocean dynamics of the Bay of Bengal (BoB) with relevance to Indian Ocean monsoons. Working collaboratively, more than 20 research institutions are acquiring field observations coupled with operational and high-resolution models to address scientific issues that have stymied the monsoon predictability. ASIRI combines new and mature observational technologies to resolve submesoscale to regional-scale currents and hydrophysical fields. These data reveal BoB’s sharp frontal features, submesoscale variability, low-salinity lenses and filaments, and shallow mixed layers, with relatively weak turbulent mixing. Observed physical features include energetic high-frequency internal waves in the southern BoB, energetic mesoscale and submesoscale features including an intrathermocline eddy in the central BoB, and a high-resolution view of the exchange along the periphery of Sri Lanka, which includes the 100-km-wide East India Coastal Current (EICC) carrying low-salinity water out of the BoB and an adjacent, broad northward flow (∼300 km wide) that carries high-salinity water into BoB during the northeast monsoon. Atmospheric boundary layer (ABL) observations during the decaying phase of the Madden–Julian oscillation (MJO) permit the study of multiscale atmospheric processes associated with non-MJO phenomena and their impacts on the marine boundary layer. Underway analyses that integrate observations and numerical simulations shed light on how air–sea interactions control the ABL and upper-ocean processes.

Johnston, TMS, Chaudhuri D, Mathur M, Rudnick DL, Sengupta D, Simmons HL, Tandon A, Venkatesan R.  2016.  Decay mechanisms of near-inertial mixed layer oscillations in the Bay of Bengal. Oceanography. 29:180-191.   10.5670/oceanog.2016.50   AbstractWebsite

Winds generate inertial and near-inertial currents in the upper ocean. These currents dominate the kinetic energy and contain most of the vertical shear in horizontal currents. Subsequent shear instabilities lead to mixing. In the Bay of Bengal, the annual mean wind energy input and near-inertial mixed layer energy is almost as large as in the mid-latitude storm tracks. Also, mixing associated with these waves is known to affect mixed layer heat content, sea surface temperature, and, thus, precipitation in coupled global models. Therefore, the mechanisms leading to the decay of these currents in the mixed layer and below are of considerable importance. Two such decay mechanisms are examined here. One mechanism is the downward propagation of near-inertial internal waves, which is aided by the mesoscale circulation and is observed with a rapidly profiling float. In a few days (faster than at mid-latitudes), the near-inertial wave group propagated from the base of the mixed layer to 250 m depth in the stratified interior. Another decay mechanism is enhanced shear generation at the mixed layer base from periodic alignment of rotating, near-inertial current shear and winds, which is observed with a mooring and analyzed with a simple two-layer model.

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.

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.

Johnston, TMS, Rudnick DL, Alford MH, Pickering A, Simmons HL.  2013.  Internal tidal energy fluxes in the South China Sea from density and velocity measurements by gliders. Journal of Geophysical Research-Oceans. 118:3939-3949.   10.1002/jgrc.20311   AbstractWebsite

Internal tidal energy fluxes were obtained from June 2011 to August 2011 using underwater gliders in the South China Sea. Spray gliders profiled every approximate to 2 h to 500 m, which is deep enough given the shallow thermocline to compute mode-1 fluxes from vertical mode fits to tidal displacements and currents. Westward, mode-1 diurnal and semidiurnal fluxes exceeded 40 and 30 kW m(-1). To our knowledge, these flux observations are the first from both velocity and density measurements by gliders. Fluxes compare well with a mooring near a generation site in southern Luzon Strait and a regional model. Furthermore, the zonal-depth structure of the internal tide is obtained by binning measurements, which cover four spring-neap cycles and over 100 km along 20 degrees 39N. Westward phase propagation is found for currents and displacements, while roughly constant phase is found along beams. Both these features of the phase suggest a narrow-banded internal tide. Semidiurnal energy density is largest along a raypath which coincides with generation sites on both the eastern and western ridges in Luzon Strait. Diurnal energy density is surface-intensified consistent with relatively shallower diurnal raypaths emanating from the eastern ridge.

Rudnick, DL, Johnston TMS, Sherman JT.  2013.  High-frequency internal waves near the {Luzon Strait} observed by underwater gliders. J. Geophys. Res. Oceans. 118   10.1002/jgrc.20083   Abstract
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.

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.

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.

Pallas-Sanz, E, Johnston TMS, Rudnick DL.  2010.  Frontal dynamics in a California Current System shallow front: 1. Frontal processes and tracer structure. Journal of Geophysical Research-Oceans. 115   10.1029/2009jc006032   AbstractWebsite

The three-dimensional dynamics in a shallow front are examined using density and current data from two surveys 100 km offshore of Monterey Bay, California. Survey 1 is forced by down-front winds, and both surveys have considerable cross-front density gradients and flow curvature. The maximum Rossby numbers on the dense side reached maxima of +0.60 in survey 1 and +0.45 in survey 2. Downwelling occurs in regions of confluence (frontogenesis) associated with potential vorticity (PV) change and thermal wind imbalance. Streamers of particulate matter and PV are advected southeastward by the frontal jet and downward. Nonlinear Ekman currents advect dense water over light water in the presence of down-front winds, which leads to upwelling along the front and downwelling on the light side of the front. At sites of active ageostrophic secondary circulation (ASC), induced by frontogenesis or Ekman effects, the observed cross-front ageostrophic velocity is consistent with the diagnosed vertical velocity. Furthermore, in survey 2, ageostrophic divergence may play an important role at the curved front, presumably counteracting quasi-geostrophic frontogenesis due to isopycnal confluence. Downward frictional vertical PV flux below the surface extracts PV from the pycnocline and reinforces the frontogenetic vertical PV flux. PV destruction at the surface is inferred from a low PV anomaly below the mixed layer in survey 2. Since the magnitude of the frontogenetic ASC is only twice the magnitude of Ekman suction, external forcing may have a considerable impact on the vertical heat and PV fluxes.

Pallas-Sanz, E, Johnston TMS, Rudnick DL.  2010.  Frontal dynamics in a California Current System shallow front: 2. Mesoscale vertical velocity. Journal of Geophysical Research-Oceans. 115   10.1029/2010jc006474   AbstractWebsite

This is the second paper investigating the three-dimensional dynamics from two consecutive, quasi-synoptic surveys of a shallow front in the California Current System. The mesoscale vertical velocity (w) is obtained by solving a generalized omega equation using density and horizontal velocity observations. Highly nonlinear dynamics emerge from the ageostrophic forcing terms for w in an adiabatic generalized omega equation. The two main processes driving w are (1) wind-induced cross-frontal ageostrophic circulation (survey 1) and (2) ageostrophic kinematic deformation during frontogenesis (surveys 1 and 2). The horizontally averaged heat fluxes are positive in the whole water column with maxima at similar to 50 m, which warms (cools) the upper (lower) water column and upwells (downwells) light (dense) water. Wind-induced currents interact with the front, cooling the upper ocean and creating a divergent potential vorticity (PV) flux at the Ekman layer base which weakens the vertical heat and PV fluxes in survey 1. Vertical velocity extrema reach similar to 10 m d(-1) in both surveys. A diabatic omega equation is derived, which introduces an important new idea: the relation of the frictional w with the vertical diffusivity of the differential ageostrophic vorticity. This term is not found in the quasi-geostrophic omega equation. By including vertical mixing, |w| is enhanced by a factor of 2 in the upper similar to 100 m and reduced below. This effect is pronounced when the wind blows in the direction of the frontal jet, but it is sensitive to the vertical mixing parameterization.

Johnston, TMS, Rudnick DL.  2009.  Observations of the Transition Layer. Journal of Physical Oceanography. 39:780-797.   10.1175/2008jpo3824.1   AbstractWebsite

The transition layer is the poorly understood interface between the stratified, weakly turbulent interior and the strongly turbulent surface mixed layer. The transition layer displays elevated thermohaline variance compared to the interior and maxima in current shear, vertical stratification, and potential vorticity. A database of 91 916 km or 25 426 vertical profiles of temperature and salinity from SeaSoar, a towed vehicle, is used to define the transition layer thickness. Acoustic Doppler current measurements are also used, when available. Statistics of the transition layer thickness are compared for 232 straight SeaSoar sections, which range in length from 65 to 1129 km with typical horizontal resolution of; 4 km and vertical resolution of 8 m. Transition layer thicknesses are calculated in three groups from 1) vertical displacements of the mixed layer base and of interior isopycnals into the mixed layer; 2) the depths below the mixed layer depth of peaks in shear, stratification, and potential vorticity and their widths; and 3) the depths below or above the mixed layer depth of extrema in thermohaline variance, density ratio, and isopycnal slope. From each SeaSoar section, the authors compile either a single value or a median value for each of the above measures. Each definition yields a median transition layer thickness from 8 to 24 m below the mixed layer depth. The only exception is the median depth of the maximum isopycnal slope, which is 37 m above the mixed layer base, but its mode is 15-25 m above the mixed layer base. Although the depths of the stratification, shear, and potential vorticity peaks below the mixed layer are not correlated with the mixed layer depth, the widths of the shear and potential vorticity peaks are. Transition layer thicknesses from displacements and the full width at half maximum of the shear and potential vorticity peak give transition layer thicknesses from 0.11X to 0.22X the mean depth of the mixed layer. From individual profiles, the depth of the shear peak below the stratification peak has a median value of 6 m, which shows that momentum fluxes penetrate farther than buoyancy fluxes. A typical horizontal scale of 5-10 km for the transition layer comes from the product of the isopycnal slope and a transition layer thickness suggesting the importance of submesoscale processes in forming the transition layer. Two possible parameterizations for transition layer thickness are 1) a constant of 11-24 m below the mixed layer depth as found for the shear, stratification, potential vorticity, and thermohaline variance maxima and the density ratio extrema; and 2) a linear function of mixed layer depth as found for isopycnal displacements and the widths of the shear and potential vorticity peaks.

Johnston, TMS, Cheriton OM, Pennington JT, Chavez FP.  2009.  Thin phytoplankton layer formation at eddies, filaments, and fronts in a coastal upwelling zone. Deep-Sea Research Part Ii-Topical Studies in Oceanography. 56:246-259.   10.1016/j.dsr2.2008.08.006   AbstractWebsite

On two cruises in August and September 2003 (hereafter cruises 2 and 3) during wind relaxations and transitions to upwelling conditions, thin layers of phytoplankton were observed in or a few meters below the stratified transition layer at the mixed layer base and in regions of sheared flow on the flanks of eddies, filaments, and fronts near Monterey Bay, California. On an earlier cruise in August (cruise 1), no thin layers were found after a prolonged wind relaxation. Chlorophyll concentrations and shear were both an order of magnitude less than on cruises 2 and 3. Our vertical profiles were made using a fluorometer mounted on a conductivity-temperature-depth package, which was lowered from the ship as slowly as 0.25 m s(-1) every 10 km on five similar to 80-km cross-shore transects. Remotely sensed sea-surface temperature, chlorophyll, and currents are required to understand the temporal and spatial evolution of the circulation and to interpret the quasi-synoptic in situ data. Decorrelation scales are similar to 20km from lagged temperature and salinity covariances. Objectively mapped sections of the in situ data indicate the waters containing thin layers were recently upwelled at either the Point Sur or Point Ano Nuevo upwelling centers. Spatially limited distributions of phytoplankton at the coastal upwelling centers (similar to 40 km alongshore, 20 km cross-shore, and 30 m thick) were transformed into thin layers by current shear and isolated from wind-driven vertical mixing by the stratification maximum of the transition layer. Vertically sheared horizontal currents on the flanks of the eddies, filaments, and fronts horizontally stretched and vertically thinned phytoplankton distributions. These thin, elongated structures were then observed as thin layers of phytoplankton in vertical fluorescence profiles at four stations on cruise 2 and eight stations on cruise 3. Light winds during relaxations did not mix away these thin layers. On cruise 2, thin layers were found at eddies at the inshore and offshore ends of a 100 km-long filament, while broader subsurface chlorophyll maxima were found along the filament. This result suggests that higher-resolution sampling along and across a filament may find thin layers forming and dissipating along its length. On cruise 3, thin layers were found at three adjacent stations across an upwelling front and may have extended continuously for > 20 km, but neither species composition nor bio-optical data are available to confirm this conjecture. The thin layers were 1-5 m thick in the vertical at full width half maximum and had peak fluorescence values from 7-30 mg m(-3). (Bottle chlorophyll samples showed fluorometer chlorophyll readings may have been 1.3-1.5 x too large, but the scatter in this relation was considerable especially at the larger fluorescence values detected in thin layers.) While sheared currents thinned an initially thick subsurface chlorophyll maximum into thin layers, the peak values in these thin layers exceeded concentrations in the upwelled source waters and were unexplained by our data. (C) 2008 Elsevier Ltd. All rights reserved.

MacKinnon, JA, Johnston TMS, Pinkel R.  2008.  Strong transport and mixing of deep water through the Southwest Indian Ridge. Nature Geoscience. 1:755-758.   10.1038/ngeo340   AbstractWebsite

The Indian Ocean harbours an important but poorly understood part of the global meridional ocean overturning circulation, which transports heat to high latitudes(1). Understanding heat exchange in the Indian Ocean requires knowledge of the magnitudes and locations of both meridional deep-water transport and mixing, but in particular the latter is poorly constrained at present(2,3). Here we present detailed measurements of transport and mixing in the Atlantis II fracture zone in the Southwest Indian Ridge, one of the main conduits for equatorward-flowing deep water(4,5). We observe a northward jet of deep and bottom water extending 1,000 m vertically with a transport rate of 3 x 10(6) m(3) s(-1). Turbulent diffusivity within the jet was up to two orders of magnitude above typical deep ocean levels in our measurements. Our results quantify the flow through this narrow fracture zone to 20 to 30% of the total meridional overturning circulation in the Indian Ocean, and provide an example of elevated turbulence in a deep sheared flow that is not hydraulically controlled, in contrast to many other fracture zones(6-9).

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.

Brix, H, Hench JL, Johnson HL, Johnston TMS, Polton JA, Roughan M, Testor P.  2003.  An international perspective on graduate education in physical oceanography. Oceanography. 16:128-133.   10.5670/oceanog.2003.43   Abstract

During the inaugural Physical Oceanography Dissertation Symposium in June 2002 we found that the graduate school experience varied markedly amongst the 20 international participants. The diversity of backgrounds led to lively discussion about the differences between physical oceanography programs. Here we review the length, content, and quality of education for graduate programs in Australia, France, Germany, the UK, and the U.S. We also comment on the financial, social, and scientific status of graduate students in these countries. Graduate programs in physical oceanography face the challenge of introducing students to the wide range of tools and techniques which define the field, ranging from observational work and remote sensing, through dynamical theory and laboratory experiments, to numerical modeling. While individual character largely determines the success of the Ph.D. experience, a graduate education in physical oceanography should include the following factors to best serve students in their future career: solid mentorship, regular department level progress checks, course work, summer schools, field work, practice in communication skills, scientific and social integration, international exchange, and stable and sufficient funding. We propose a model four year physical oceanography graduate degree structure, distilled from the best aspects of international physical oceanography programs.

Merrifield, MA, Holloway PE, Johnston TMS.  2001.  The generation of internal tides at the Hawaiian Ridge. Geophysical Research Letters. 28:559-562.   10.1029/2000gl011749   AbstractWebsite

Previous analyses of altimeter and acoustic data indicate that the Hawaiian Ridge is a significant generation site for semidiurnal (M-2 period) internal waves owing to tidal flow over topography. Such waves likely play an important role connecting the dissipation of tidal energy to ocean mixing. Here a primitive equation numerical model with realistic stratification, bathymetry, and tidal forcing is used to simulate the generation of these waves in a manner consistent with altimeter sea surface observations. The highest amplitude waves originate at three generation sites located at both the island and seamount portions of the chain. These sites have in common both enhanced flows across ridge-shaped topography and maxima in model calculated body force which couples barotropic and baroclinic motions. The model vertical structure suggests the presence of multiple dynamical modes within 100 km of the ridge and not just the lowest modes that have been detected at the sea surface.

Johnston, TMS, Merrifield MA.  2000.  Interannual geostrophic current anomalies in the near-equatorial western Pacific. Journal of Physical Oceanography. 30:3-14.   10.1175/1520-0485(2000)030<0003:igcait>;2   AbstractWebsite

A network of island ride gauges is used to estimate interannual geostrophic current anomalies (GCAs) in the western Pacific from 1975 to 1997. The focus of this study is the zonal component of the current averaged between 160 degrees E and 180 degrees and 2 degrees to 7 degrees north and south of the equator in the mean flow regions associated with the North Equatorial Countercurrent (NECC) and the South Equatorial Current (SEC), respectively. The tide gauge GCA estimates agree closely with similarly derived currents from TOPEX/Poseidon sea level anomalies. The GCAs in the western Pacific relate to a basin-scale adjustment associated with the El Nino-Southern Oscillation, characterized here using empirical orthogonal functions of tide gauge and supporting sea surface temperature and heat storage data, The dominant EOF mode describes the mature phase of ENSO events and correlates (0.8) with the GCA south of the equator. The second mode describes transitions to and from ENSO events and correlates (0.9) with the GCA north of the equator. The typical scenario then is for the NECC to intensify about 6 months prior to the peak of an El Nino, to remain near mean conditions during the peak stage of El Nino, and to later weaken about 6 months following the peak. In contrast, the SEC generally weakens throughout an El Nino displaying eastward anomalies. This equatorial asymmetry in the GCAs is consistent with a similar asymmetry in the wind field over the western Pacific. The phase differences between the NECC and SEC are less apparent during La Nina events. The GCA results provide further evidence that transitional phases of ENSO are more active north than south of the equator in the warm pool region.

Johnston, TMS, Chow KH, Dunsiger S, Duty T, Kiefl RF, Koster E, Macfarlane WA, Morris GD, Sonier J, Williams DLI.  1997.  The giant muon Knight shift in antimony: Evidence for a Kondo impurity. Hyperfine Interactions. 106:71-76.   10.1023/a:1012665118803   AbstractWebsite

A detailed study has been undertaken of the muon Knight shift in high purity antimony single crystals. No periodic variations with magnetic field (de Haas-van Alphen oscillations) are observed. The temperature dependence below 175 K is close to that expected for a Kondo-like impurity with an anisotropic muon-electron hyperfine interaction. At higher temperatures the paramagnetic state becomes unstable and a transition occurs to a second state. The longitudinal relaxation rate rises from an apparently non-zero value at T = 0 to a maximum at 50 K, followed by a slow decline. This leads to a Korringa product which is strongly temperature dependent.

Macfarlane, WA, Kiefl RF, Chow KH, Dunsiger S, Duty TL, Johnston TMS, Schneider JW, Sonier J, Brard L, Strongin RM, Fischer JE, Smith AB.  1994.  Hebel-Slichter peak and superconducting energy gap in Rb3C60 observed by muon spin relaxation. Hyperfine Interactions. 86:467-472.   10.1007/bf02068935   AbstractWebsite

Muon spin relaxation has been observed in both the normal and superconducting states of Rb3C60 (T(c) = 29.3K). The field dependence of the T1 spin relaxation rate is due to muonium undergoing spin-exchange scattering with conduction electrons, making this the first observation of muonium in a metal. The temperature dependence of T1(-1) shows a Hebel-Slichter coherence peak just below T(c) which is not seen in C-13 spin relaxation. The peak can be fit assuming spin relaxation due to interaction with the quasiparticle excitations of a BCS superconductor provided the density of states is broadened relative to that of BCS. Such fits yield a value for the zero temperature energy gap, DELTA0/k(B), of 53(4)K, consistent with weak-coupling BCS.

Storchak, V, Brewer JH, Hardy WN, Johnston S, Kreitzman SR, Morris GD.  1994.  Inhomogeneous muonium diffusion in solid nitrogen. Hyperfine Interactions. 85:103-108.   10.1007/bf02069409   AbstractWebsite

The spin dynamics of the muonium (Mu) atom diffusing quantum mechanically in solid nitrogen (s-N-14(2)) has been studied using the technique of Mu spin relaxation. A strong relationship between longitudinal (T1(-1)) and transverse (T2(-1)) relaxation rates (familiar in NMR) has been experimentally demonstrated for the first time for muonium relaxation. At low temperatures the results are inconsistent with diffusion models using a single correlation time tau(c); this is taken as evidence for the intrinsic inhomogeneity of the problem. The temperature dependence of the average Mu hop rate tau(c)-1 gives clear evidence that Mu quantum diffusion in s-N2 is governed by the two-phonon interaction.

Chow, KH, Kiefl RF, Schneider JW, Estle TL, Hitti B, Lichti RL, Schwab C, Kreitzman SR, Duvarney RC, Senba M, Sonier J, Johnston TMS, Macfarlane WA.  1994.  Muonium dynamics in doped Si above room temperature studied by longitudinal field — μSR. Hyperfine Interactions. 86:693-698.   10.1007/bf02068965   AbstractWebsite

We report longitudinal field muSR 1/T1 measurements in Si from room temperature to 850 K. The data in pure Si and Si:B (p-type) can be explained in a two-state model where muonium cycles between its positive and neutral charge states. Within this model, the average muon-electron hyperfine parameter in the neutral state is consistent with muonium at the tetrahedral interstitial site, indicating that at the highest temperatures measured, neutral muonium spends a significant amount of time away from the bond centered site, the calculated potential minimum. Although this is also true for Si:P (n-type) at high temperatures, the data in the region between 300-450 K indicates that at least one other state is involved in the dynamics.