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Fine, EC, MacKinnon JA, Alford MH, Mickett JB.  2018.  Microstructure observations of turbulent heat fluxes in a warm-core Canada Basin eddy. Journal of Physical Oceanography. 48:2397-2418.   10.1175/jpo-d-18-0028.1   AbstractWebsite

An intrahalocline eddy was observed on the Chukchi slope in September of 2015 using both towed CTD and microstructure temperature and shear sections. The core of the eddy was 6 degrees C, significantly warmer than the surrounding -1 degrees C water and far exceeding typical temperatures of warm-core Arctic eddies. Microstructure sections indicated that outside of the eddy the rate of dissipation of turbulent kinetic energy epsilon was quite low . Three different processes were associated with elevated epsilon. Double-diffusive steps were found at the eddy's top edge and were associated with an upward heat flux of 5 W m(-2). At the bottom edge of the eddy, shear-driven mixing played a modest role, generating a heat flux of approximately 0.5 W m(-2) downward. Along the sides of the eddy, density-compensated thermohaline intrusions transported heat laterally out of the eddy, with a horizontal heat flux of 2000 W m(-2). Integrating these fluxes over an idealized approximation of the eddy's shape, we estimate that the net heat transport due to thermohaline intrusions along the eddy flanks was 2 GW, while the double-diffusive flux above the eddy was 0.4 GW. Shear-driven mixing at the bottom of the eddy accounted for only 0.04 GW. If these processes continued indefinitely at the same rate, the estimated life-span would be 1-2 years. Such eddies may be an important mechanism for the transport of Pacific-origin heat, freshwater, and nutrients into the Canada Basin.

Alford, MH, Sloyan BM, Simmons HL.  2017.  Internal waves in the East Australian Current. Geophysical Research Letters. 44:12280-12288.   10.1002/2017gl075246   AbstractWebsite

Internal waves, which drive most ocean turbulence and add noise to lower-frequency records, interact with low-frequency current systems and topography in yet poorly known ways. Taking advantage of a heavily instrumented, 14 month mooring array, internal waves in the East Australian Current (EAC) are examined for the first time. Internal wave horizontal kinetic energy (HKE) is within a factor of 2 of the Garrett-Munk (1976) spectrum. Continuum internal waves, near-inertial waves, and internal tides together constitute a significant percentage of the total velocity variance. Mode-1 internal tide energy fluxes are southward and much smaller than energy times group velocity, consistent with reflection at the continental slope of incident waves generated from near New Caledonia and the Solomon Islands. Internal tide HKE is highly phase variable, consistent with refraction by the variable EAC. Mode-1 near-inertial wave energy fluxes are of comparable magnitude and are equatorward and episodic, consistent with generation by storms farther poleward. These processes are considered together in the complex environment of the EAC.

Alford, MH, McGinnis T, Howe BM.  2015.  An inductive charging and real-time communications system for profiling moorings. Journal of Atmospheric and Oceanic Technology. 32:2243-2252.   10.1175/jtech-d-15-0103.1   AbstractWebsite

This paper describes a system for providing power and communications to moored profiling vehicles. A McLane Moored Profiler (MP) was equipped with a rechargeable battery pack and an inductive charging system to allow it to move periodically to a charging dock at the top of a subsurface mooring. Power was provided from a large bank of alkaline batteries housed in two 0.94-m steel spheres. Data were transferred inductively from the profiler to a mooring controller, and from there back to shore via radio and Iridium satellite modems housed in a small surface communications float on an L tether. An acoustic modem provided backup communications to a nearby ship in the event of loss or damage to the surface float. The system was tested in a 180-m-deep fjord (Puget Sound, Washington) and at Station ALOHA (A Long-Term Oligotrophic Habitat Assessment), a 4748-m-deep open-ocean location north of Hawaii. Basic functionality of the system was demonstrated, with the profiler repeatedly recharging at about 225 W (with an overall efficiency of about 70%). Data were relayed back to shore via Iridium and to a nearby ship via the radio and acoustic modems. The system profiled flawlessly for the entire 6-week test in Puget Sound, but charging at the deep site stopped after only 9 days in the deep-ocean deployment owing to damage to the charging station, possibly by surface wave action.

Pickering, A, Alford M, Nash J, Rainville L, Buijsman M, Ko DS, Lim B.  2015.  Structure and variability of internal tides in Luzon Strait. Journal of Physical Oceanography. 45:1574-1594.   10.1175/jpo-d-14-0250.1   AbstractWebsite

The Luzon Strait is the generation region for strong internal tides that radiate westward into the South China Sea and eastward into the western Pacific. Intrusions of the Kuroshio and strong mesoscale variability in the Luzon Strait can influence their generation and propagation. Here, the authors use eight moorings and two numerical models to investigate these relationships by quantifying the coherence of the diurnal and semidiurnal internal tides in the Luzon Strait. This study finds that the level of coherence of internal tide generation, energy, and energy flux is quite variable, depending on the specific location within the Luzon Strait. Large-scale spatial patterns in internal tide pressure and velocity exist across the region, shaped by the bathymetry, mean flow, and stratification. Internal tide coherence is lower (<30%) near large gradients in this pattern (predominantly along the eastern ridge), which are shifted by the variable Kuroshio and mesoscale fields. At other locations within the Luzon Strait, the internal tide is largely coherent (>80%), and simple calculations suggest that remote sources of internal tides could account for these small decreases in coherence. To the west of the Luzon Strait (away from the primary generation regions), the model suggests that diurnal internal tide energy is more coherent than semidiurnal.

Hall, RA, Alford MH, Carter GS, Gregg MC, Lien RC, Wain DJ, Zhao ZX.  2014.  Transition from partly standing to progressive internal tides in Monterey Submarine Canyon. Deep-Sea Research Part Ii-Topical Studies in Oceanography. 104:164-173.   10.1016/j.dsr2.2013.05.039   AbstractWebsite

Monterey Submarine Canyon is a large, sinuous canyon off the coast of California, the upper reaches of which were the subject of an internal tide observational program using moored profilers and upward-looking moored ADCPs. The mooring observations measured a near-surface stratification change in the upper canyon, likely caused by a seasonal shift in the prevailing wind that favoured coastal upwelling. This change in near-surface stratification caused a transition in the behaviour of the internal tide in the upper canyon from a partly standing wave during pre-upwelling conditions to a progressive wave during upwelling conditions. Using a numerical model, we present evidence that either a partly standing or a progressive internal tide can be simulated in the canyon, simply by changing the initial stratification conditions in accordance with the observations. The mechanism driving the transition is a dependence of down-canyon (supercritical) internal tide reflection from the canyon floor and walls on the depth of maximum stratification. During pre-upwelling conditions, the main pycnocline extends down to 200 m (below the canyon rim) resulting in increased supercritical reflection of the up-canyon propagating internal tide back down the canyon. The large up-canyon and smaller down-canyon progressive waves are the two components of the partly standing wave. During upwelling conditions, the pycnocline shallows to the upper 50 m of the watercolumn (above the canyon rim) resulting in decreased supercritical reflection and allowing the up-canyon progressive wave to dominate. (C) 2013 Elsevier Ltd. All rights reserved.

Buijsman, MC, Klymak JM, Legg S, Alford MH, Farmer D, MacKinnon JA, Nash JD, Park JH, Pickering A, Simmons H.  2014.  Three-dimensional double-ridge internal tide resonance in Luzon Strait. Journal of Physical Oceanography. 44:850-869.   10.1175/jpo-d-13-024.1   AbstractWebsite

The three-dimensional (3D) double-ridge internal tide interference in the Luzon Strait in the South China Sea is examined by comparing 3D and two-dimensional (2D) realistic simulations. Both the 3D simulations and observations indicate the presence of 3D first-mode (semi)diurnal standing waves in the 3.6-km-deep trench in the strait. As in an earlier 2D study, barotropic-to-baroclinic energy conversion, flux divergence, and dissipation are greatly enhanced when semidiurnal tides dominate relative to periods dominated by diurnal tides. The resonance in the 3D simulation is several times stronger than in the 2D simulations for the central strait. Idealized experiments indicate that, in addition to ridge height, the resonance is only a function of separation distance and not of the along-ridge length; that is, the enhanced resonance in 3D is not caused by 3D standing waves or basin modes. Instead, the difference in resonance between the 2D and 3D simulations is attributed to the topographic blocking of the barotropic flow by the 3D ridges, affecting wave generation, and a more constructive phasing between the remotely generated internal waves, arriving under oblique angles, and the barotropic tide. Most of the resonance occurs for the first mode. The contribution of the higher modes is reduced because of 3D radiation, multiple generation sites, scattering, and a rapid decay in amplitude away from the ridge.

Alford, MH, Klymak JM, Carter GS.  2014.  Breaking internal lee waves at Kaena Ridge, Hawaii. Geophysical Research Letters. 41:906-912.   10.1002/2013GL059070   AbstractWebsite

Shallow water oscillatory flows and deep ocean steady flows have both been observed to give rise to breaking internal lee waves downstream of steep seafloor obstacles. A recent theory also predicts the existence of high-mode oscillatory internal lee waves in deep water, but they have not previously been directly observed. Here we present repeated spatial transects of velocity, isopycnal displacement, and dissipation rate measured with towed instruments on the south flank of a supercritical ridge in Hawaii known as Kaena Ridge and compare them with predictions from a 3-D numerical model with realistic tidal forcing, bathymetry, and stratification. The measured and modeled flow and turbulence agree well in their spatial structure, time dependence, and magnitude, confirming the existence and predicted nature of high-mode internal lee waves. Turbulence estimated from Thorpe scales increases 2 orders of magnitude following downslope tidal flow, when the internal lee wave begins to propagate upslope and breaks.

Alford, MH, MacCready P.  2014.  Flow and mixing in Juan de Fuca Canyon, Washington. Geophysical Research Letters. 41:1608-1615.   10.1002/2013GL058967   AbstractWebsite

We report breaking internal lee waves, strong mixing, and hydraulic control associated with wind-driven up-canyon flow in Juan de Fuca Canyon, Washington. Unlike the flow above the canyon rim, which shows a tidal modulation typical on continental shelves, the flow within the canyon is persistently up-canyon during our observations, with isopycnals tilted consistent with a geostrophic cross-canyon momentum balance. As the flow encounters a sill near the canyon entrance at the shelf break, it accelerates significantly and undergoes elevated mixing on the upstream and downstream sides of the sill. On the downstream side, a strong lee wave response is seen, with displacements of O(100 m) and overturns tens of meters high. The resulting diffusivity is O(10−2 m2 s−1), sufficient to substantially modify coastal water masses as they transit the canyon and enter the Salish Sea estuarine system.

Alford, MH, Gregg MC, Zervakis V, Kontoyiannis H.  2012.  Internal wave measurements on the Cycladic Plateau of the Aegean Sea. Journal of Geophysical Research: Oceans. 117:C01015.   10.1029/2011JC007488   AbstractWebsite

The internal wave climate in the southern Aegean Sea is examined with an array of two bottom-mounted acoustic Doppler current profilers and three profiling moorings deployed on the northern continental slope of the Cretan Sea for 3 months. Frequency spectra indicate an extremely weak internal wave continuum, about 4–10 times weaker than the Garrett-Munk and Levine reference levels. Spectra are instead dominated by semidiurnal internal tides and near-inertial waves, which are examined in detail by bandpass filtering. In the semidiurnal band, a barotropic tidal flow of ≈2 cm s−1 is observed, with a pronounced spring/neap modulation in phase with the lunar fortnightly cycle. One to two days following several of these spring tide periods, a distinct internal tide featuring 10–20 m vertical displacements and 15–20 cm s−1baroclinic velocities is detectable propagating upward and to the southeast. Time-mean energy increases a factor of 2–5 within about 100 m from the bottom, implying generation and/or scattering from the bottom, whose slope is nearly critical to semidiurnal internal waves over much of the array. Several strong, downward propagating near-inertial events are also seen, each of which occurs following a period of work done by the wind on the mixed layer as estimated from a nearby surface mooring. The high-frequency internal wave continuum is more temporally constant but increases substantially toward the end of the deployment. Significant but unexplained differences in kinetic energy occur between successive spring tide periods in the case of the internal tides and between successive wind events in the case of the near-inertial signals. Substantial variability is observed in the low-frequency flows, which likely contributes to the time variability of the internal wave signals.

Chinn, BS, Girton JB, Alford MH.  2012.  Observations of internal waves and parametric subharmonic instability in the Philippines archipelago. Journal of Geophysical Research: Oceans. 117:C05019.   10.1029/2011JC007392   AbstractWebsite

Internal waves contain a significant fraction of the kinetic energy in the ocean and are important intermediaries between the forcing (by wind and tide) and interior diapycnal mixing. We report here on measurements from Mindoro Strait in the Philippines (connecting the South China Sea to the Sulu Sea) of an internal wave field with a number of surprising properties that point to previously-unrecognized processes at work in the region. Continuum spectral levels are very close to typical “background” values found in the open ocean, but internal tide energy in both the diurnal and semidiurnal frequency bands is significantly elevated—and higher at the northern mooring (MP1) than the southern (MP2). Two particularly energetic depth ranges stand out at MP1: an upper layer centered near 300 m, and one at the bottom of the water column, near 1800 m. The upper layer contains both internal tides and a near-inertial band with upward and downward propagating waves and an apparent spring-neap cycle. The combination is suggestive of Parametric Subharmonic Instability as the forcing for the near-inertial band—a conclusion supported by bicoherence estimates. Mixing, estimated from density overturns, is weak over much of the water column but enhanced by about an order of magnitude in the deep layer and closely tied to the internal tide—both diurnal and semidiurnal. Near-inertial currents in this deep layer are dominantly rectilinear and not well-correlated with the mixing. Bulk mixing rates at the two sites are less than required to produce property changes seen in hydrography, suggesting additional enhancement elsewhere in the archipelago.

Alford, MH.  2008.  Observations of parametric subharmonic instability of the diurnal internal tide in the South China Sea. Geophysical Research Letters. 35:L15602.   10.1029/2008GL034720   AbstractWebsite

Shipboard observations are presented that suggest the occurrence of parametric subharmonic instability (PSI) of diurnal K1 and O1 internal tides at “critical” latitudes of 14.52°N and 13.44°N, respectively. In a transect spanning 12.5–18°N, depth-mean shear squared shows sharp peaks at 14.52°N (elevated relative to that at 15°N by a factor of ten) and at 13.44°N (by a factor of 7). Wind speed measured from the ship and Quikscat scatterometer during and before the transect was <10 m s−1 at these latitudes. Eight-hour time series (about 1/6 of an inertial period) of shear and isopycnal depth at 14.52°N are sufficient to associate the elevated shear with alternating, clockwise-rotating layers analogous to those observed at the M2 critical latitude of 28.8°N.

Martini, KI, Alford MH, Nash JD, Kunze E, Merrifield MA.  2007.  Diagnosing a partly standing internal wave in Mamala Bay, Oahu. Geophysical Research Letters. 34:L17604.   10.1029/2007GL029749   AbstractWebsite

An internal partly standing wave in Mamala Bay, Hawaii is studied using new observations and the Princeton Ocean Model (POM). Previous work suggested a convergence in the bay of east- and westbound waves emanating from Kaena Ridge and Makapuu Point, respectively. New energy flux measurements with shipboard ADCP/CTD confirm that Makapuu Point is the eastern source. After validating the POM results against observations, the model output is modally decomposed and compared with the expected theoretical patterns of kinetic and available potential energy, energy flux, and group velocity for a partly standing wave. Agreement is seen for the first baroclinic mode, which also contains most of the energy. The results confirm previous suggestions of standing wave dynamics in Mamala Bay.

Nash, JD, Alford MH, Kunze E, Martini K, Kelly S.  2007.  Hotspots of deep ocean mixing on the Oregon continental slope. Geophysical Research Letters. 34:L01605.   10.1029/2006GL028170   AbstractWebsite

Two deep ocean hotspots of turbulent mixing were found over the Oregon continental slope. Thorpe-scale analyses indicate time-averaged turbulent energy dissipation rates of ε > 10−7 W/kg and eddy diffusivities of Kρ ∼ 10−2 m2/s at both hotspots. However, the structure of turbulence and its generation mechanism at each site appear to be different. At the 2200-m isobath, sustained >100-m high turbulent overturns occur in stratified fluid several hundred meters above the bottom. Turbulence shows a clear 12.4-h periodicity proposed to be driven by flow over a nearby 100-m tall ridge. At the 1300-m isobath, tidally-modulated turbulence of similar intensity is confined within a stratified bottom boundary layer. Along-slope topographic roughness at scales not resolved in global bathymetric data sets appears to be responsible for the bulk of the turbulence observed. Such topography is common to most continental slopes, providing a mechanism for turbulence generation in regions where barotropic tidal currents are nominally along-isobath.

Alford, MH, Pinkel R.  2000.  Observations of overturning in the thermocline: The context of ocean mixing. Journal of Physical Oceanography. 30:805-832.   10.1175/1520-0485(2000)030<0805:oooitt>;2   AbstractWebsite

The time sequence of events that lead to internal wave breaking and ocean turbulence is investigated. Data are obtained from depths 100-400 m with a repeat profiling CTD and a coded-pulse Doppler sonar. The instruments were deployed from R/P FLIP during February-March 1995 while stationed 30 km west of Point Arguello, California, as an aspect of the Marine Boundary Layer Experiment. Although the water depth at the site is 1500 m, both rms shear and diapycnal diffusivity, as inferred from the average rate and size of overturning events, increase with depth below 250 m. A deep source of wave energy is implied. Depth-time series of 6.4-m shear S, 2-m strain (gamma = (N) over bar(2)/N-2, where N is the buoyancy frequency), 6.4-m gradient Richardson number Ri - N-2/S-2, and 2-m "effective strain rate" partial derivative (w) over cap/partial derivative z = 1 partial derivative gamma/gamma partial derivative t (the depth derivative of CTD-inferred vertical velocity (w) over cap) are obtained at 4 minute intervals over a 9-day, 100-400 m domain. The occurrence of overturns, static instabilities of vertical scale greater than or equal to 2 m in the observed density profiles, is monitored. Data are examined in both Eulerian and isopycnal-following (semi-Lagrangian) reference frames. Over two thousand overturns are detected and mapped relative to the background shear, strain, Ri, and strain rate fields. Correspondence between low values of 6.4-m Ri and overturns is indeed significant. However, 2-m strain and effective strain rate appear to be better indices of overturning: gamma greater than or equal to 2 in 60% of all overturning events, and \partial derivative (w) over cap/partial derivative z\ is greater than average in 80% of overturns. Depth-time maps of effective strain rate show wavelike features on vertical scales 5-20 m, which extend coherently across isopycnal surfaces. Time series of depth-averaged partial derivative (w) over cap/partial derivative z variance (expressed as a dissipation rate), and depth-averaged dissipation rate epsilon(T), estimated from observed overturning scales are highly correlated. Time-mean <(epsilon(T))over bar> = 8.6 x 10(-9) W kg(-1), implying an eddy diffusivity <(K-rho)over bar> = 0.89 x 10(-4) m(2) s(-1). Mean dissipation rate and diffusivity computed from 10-m shear display comparable magnitude (<(epsilon(IW))over bar> = 6.8 x 10(-9) W kg(-1), <(K-rho)over bar> = 0.70 x 10(-4) m(2) s(-1)) and similar dependence upon mean stratification and shear to the overturn-inferred quantities. It is suggested that the overturns seen at this site result from breaking of these high partial derivative (w) over cap/partial derivative z waves.

Alford, M, Pinkel R.  2000.  Patterns of turbulent and double-diffusive phenomena: Observations from a rapid-profiling microconductivity probe. Journal of Physical Oceanography. 30:833-854.   10.1175/1520-0485(2000)030<0833:potadd>;2   AbstractWebsite

Throughout much of the ocean interior, the diapycnal buoyancy Bur is maintained by both mechanical and double-diffusive processes. Assessing the relative roles of each is a challenge, particularly in complex coastal environments. During February-March 1995, a repeat-profiling CTD system, equipped with a dual-needle microconductivity probe, was deployed off the central California coast (35 degrees N, 121 degrees W) from the research platform FLIP. The probe's vertical resolution (8 cm) appears sufficient to resolve the low wavenumbers of the turbulent inertial subrange. This paper presents depth-time maps, spanning 12 days and 100-400 m, of temperature dissipation rate <(chi)over cap>, and Cox number (C) over cap: High <(chi)over cap> and (C) over cap values tend to occur in layers, on a variety of spatial scales. Simultaneously, finescale (6.4-m) Richardson number, effective strain rate, and Turner angle are measured. The occurrence of intense microstructure fluctuations is correlated with all three quantities, affirming that both mechanical turbulence and double diffusion are active at the site. Depth-averaged dissipation rate epsilon(mu) is inferred from the <(chi)over cap> records under the assumption that a Batchelor spectrum for scalars obtains and that the buoyancy flux J(b) and dissipation epsilon are related through a constant mixing efficiency Gamma, J(b) = Gamma epsilon. Time series of epsilon(mu) are highly correlated with dissipation rate computed from Thorpe scales (epsilon(r)). estimated from large (2 m and greater) density overturns (except during periods when large portions of the water column are double-diffusively unstable: epsilon(mu) >> epsilon(T) in these regions, suggesting enhanced fluxes due to double diffusion.