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2018
Carranza, MM, Gille ST, Franks PJS, Johnson KS, Pinkel R, Girton JB.  2018.  When mixed layers are not mixed. Storm-driven mixing and bio-optical vertical gradients in mixed layers of the Southern Ocean. Journal of Geophysical Research-Oceans. 123:7264-7289.   10.1029/2018jc014416   AbstractWebsite

Mixed layers are defined to have homogeneous density, temperature, and salinity. However, bio-optical profiles may not always be fully homogenized within the mixed layer. The relative timescales of mixing and biological processes determine whether bio-optical gradients can form within a uniform density mixed layer. Vertical profiles of bio-optical measurements from biogeochemical Argo floats and elephant seal tags in the Southern Ocean are used to assess biological structure in the upper ocean. Within the hydrographically defined mixed layer, the profiles show significant vertical variance in chlorophyll-a (Chl-a) fluorescence and particle optical backscatter. Biological structure is assessed by fitting Chl-a fluorescence and particle backscatter profiles to functional forms (i.e., Gaussian, sigmoid, exponential, and their combinations). In the Southern Ocean, which characteristically has deep mixed layers, only 40% of nighttime bio-optical profiles were characterized by a sigmoid, indicating a well-mixed surface layer. Of the remaining 60% that showed structure, approximate to 40% had a deep fluorescence maximum below 20-m depth that correlated with particle backscatter. Furthermore, a significant fraction of these deep fluorescence maxima were found within the mixed layer (20-80%, depending on mixed-layer depth definition and season). Results suggest that the timescale between mixing events that homogenize the surface layer is often longer than biological timescales of restratification. We hypothesize that periods of quiescence between synoptic storms, which we estimate to be approximate to 3-5days (depending on season), allow bio-optical gradients to develop within mixed layers that remain homogeneous in density. Storms influence high-latitude oceans by stirring the upper ocean nearly continuously. This wind mixing is usually expected to homogenize properties within the upper layer of the ocean, known as the mixed layer. New water column observations from floats and elephant seal tag confirm homogenization of hydrographic properties that determine density of seawater (e.g., temperature and salinity); however, biogeochemical properties are not necessarily homogenized. Most of the time optical measurements of biological properties within the mixed layer show vertical structure, which is indicative of phytoplankton biomass. These vertical inhomogenities are ubiquitous throughout the Southern Ocean and may occur in all seasons, often close to the base of the mixed layer. Within the mixed layer, observations suggest that biological processes create inhomogenities faster than mixing can homogenize. We hypothesize that 3- to 5-day periods of quiescence between storm events are long enough to allow bio-optical structure to develop without perturbing the mixed layers' uniform density. This may imply that phytoplankton in the Southern Ocean are better adapted to the harsh environmental conditions than commonly thought.

Waterhouse, AF, Kelly SM, Zhao Z, MacKinnon JA, Nash JD, Simmons H, Brahznikov D, Rainville L, Alford M, Pinkel R.  2018.  Observations of the Tasman Sea internal tide beam. Journal of Physical Oceanography. 48:1283-1297.   10.1175/jpo-d-17-0116.1   Abstract

AbstractLow-mode internal tides, a dominant part of the internal wave spectrum, carry energy over large distances, yet the ultimate fate of this energy is unknown. Internal tides in the Tasman Sea are generated at Macquarie Ridge, south of New Zealand, and propagate northwest as a focused beam before impinging on the Tasmanian continental slope. In situ observations from the Tasman Sea capture synoptic measurements of the incident semidiurnal mode-1 internal-tide, which has an observed wavelength of 183 km and surface displacement of approximately 1 cm. Plane-wave fits to in situ and altimetric estimates of surface displacement agree to within a measurement uncertainty of 0.3 cm, which is the same order of magnitude as the nonstationary (not phase locked) mode-1 tide observed over a 40-day mooring deployment. Stationary energy flux, estimated from a plane-wave fit to the in situ observations, is directed toward Tasmania with a magnitude of 3.4 ± 1.4 kW m−1, consistent with a satellite estimate of 3.9 ± 2.2 kW m−1. Approximately 90% of the time-mean energy flux is due to the stationary tide. However, nonstationary velocity and pressure, which are typically 1/4 the amplitude of the stationary components, sometimes lead to instantaneous energy fluxes that are double or half of the stationary energy flux, overwhelming any spring–neap variability. Despite strong winds and intermittent near-inertial currents, the parameterized turbulent-kinetic-energy dissipation rate is small (i.e., 10−10 W kg−1) below the near surface and observations of mode-1 internal tide energy-flux convergence are indistinguishable from zero (i.e., the confidence intervals include zero), indicating little decay of the mode-1 internal tide within the Tasman Sea.

Yoo, YD, Seong KA, Kim HS, Jeong HJ, Yoon EY, Park J, Kim JI, Shin W, Palenik B.  2018.  Feeding and grazing impact by the bloom-forming euglenophyte Eutreptiella eupharyngea on marine eubacteria and cyanobacteria. Harmful Algae. 73:98-109.   10.1016/j.hal.2018.02.003   AbstractWebsite

The phototrophic euglenophyte Eutreptiella eupharyngea often causes blooms in the coastal waters of many countries, but its mode of nutrition has not been assessed. This species has previously been considered as exclusively auxotrophic. To explore whether E. eupharyngea is a mixotrophic species, the protoplasm of E. eupharyngea cells were examined using light, epifluorescence, and transmission electron microscopy after eubacteria, the cyanobacterium Synechococcus sp., and diverse algal species were provided as potential prey. Furthermore, the ingestion rates of E. eupharyngea KR on eubacteria or Synechococcus sp. as a function of prey concentration were measured. In addition, grazing by natural populations of euglenophytes on natural populations of eubacteria in Masan Bay was investigated. This study is the first to report that E eupharyngea is a mixotrophic species. Among the potential prey organisms offered, E. eupharyngea fed only on eubacteria and Synechococcus sp., and the maximum ingestion rates of these two organisms measured in the laboratory were 5.7 and 0.7 cells predator(-1) h(-1), respectively. During the field experiments, the maximum ingestion rates and grazing impacts of euglenophytes, including E. eupharyngea, on natural populations of eubacteria were 11.8 cells predator(-1) h(-1) and 1.228 d(-1), respectively. Therefore, euglenophytes could potentially have a considerable grazing impact on marine bacterial populations. (C) 2018 Elsevier B.V. All rights reserved.

Zhao, ZX, Alford MH, Simmons HL, Brazhnikov D, Pinkel R.  2018.  Satellite investigation of the M-2 Internal Tide in the Tasman Sea. Journal of Physical Oceanography. 48:687-703.   10.1175/jpo-d-17-0047.1   AbstractWebsite

The M-2 internal tide in the Tasman Sea is investigated using sea surface height measurements made by multiple altimeter missions from 1992 to 2012. Internal tidal waves are extracted by two-dimensional plane wave fits in 180 km by 180 km windows. The results show that the Macquarie Ridge radiates three internal tidal beams into the Tasman Sea. The northern and southern beams propagate respectively into the East Australian Current and the Antarctic Circumpolar Current and become undetectable to satellite altimetry. The central beam propagates across the Tasman Sea, impinges on the Tasmanian continental slope, and partially reflects. The observed propagation speeds agree well with theoretical values determined from climatological ocean stratification. Both the northern and central beams refract about 158 toward the equator because of the beta effect. Following a concave submarine ridge in the source region, the central beam first converges around 45.5 degrees S, 155.5 degrees E and then diverges beyond the focal region. The satellite results reveal two reflected internal tidal beams off the Tasmanian slope, consistent with previous numerical simulations and glider measurements. The total energy flux from the Macquarie Ridge into the Tasman Sea is about 2.2 GW, of which about half is contributed by the central beam. The central beam loses little energy in its first 1000-km propagation, for which the likely reasons include flat bottom topography and weak mesoscale eddies.

2017
Alford, MH, MacKinnon JA, Pinkel R, Klymak JM.  2017.  Space-time scales of shear in the North Pacific. Journal of Physical Oceanography. 47:2455-2478.   10.1175/jpo-d-17-0087.1   AbstractWebsite

The spatial, temporal, and directional characteristics of shear are examined in the upper 1400m of the North Pacific during late spring with an array of five profiling moorings deployed from 25 degrees to 37 degrees N (1330 km) and simultaneous shipboard transects past them. The array extended from a regime of moderate wind generation at the north to south of the critical latitude 28.8 degrees N, where parametric subharmonic instability (PSI) can transfer energy from semidiurnal tides to near-inertial motions. Analyses are done in an isopycnal-following frame to minimize contamination by Doppler shifting. Approximately 60% of RMS shear at vertical scales >20m (and 80% for vertical scales >80 m) is contained in near-inertial motions. An inertial back-rotation technique is used to index shipboard observations to a common time and to compute integral time scales of the shear layers. Persistence times are O(7) days at most moorings but O(25) days at the critical latitude. Simultaneous shipboard transects show that these shear layers can have lateral scales >= 100 km. Layers tend to slope downward toward the equator north of the critical latitude and are more flat to its south. Phase between shear and strain is used to infer lateral propagation direction. Upgoing waves are everywhere laterally isotropic. Downgoing waves propagate predominantly equatorward north and south of the critical latitude but are isotropic near it. Broadly, results are consistent with wind generation north of the critical latitude and PSI near it-and suggest a more persistent and laterally coherent near-inertial wave field than previously thought.

MacKinnon, JA, Alford MH, Ansong JK, Arbic BK, Barna A, Briegleb BP, Bryan FO, Buijsman MC, Chassignet EP, Danabasoglu G, Diggs S, Griffies SM, Hallberg RW, Jayne SR, Jochum M, Klymak JM, Kunze E, Large WG, Legg S, Mater B, Melet AV, Merchant LM, Musgrave R, Nash JD, Norton NJ, Pickering A, Pinkel R, Polzin K, Simmons HL, Laurent LSC, Sun OM, Trossman DS, Waterhouse AF, Whalen CB, Zhao Z.  2017.  Climate process team on internal-wave driven ocean mixing. Bulletin of the American Meteorological Society.   10.1175/bams-d-16-0030.1   Abstract

Recent advances in our understanding of internal-wave driven turbulent mixing in the ocean interior are summarized. New parameterizations for global climate ocean models, and their climate impacts, are introduced.Diapycnal mixing plays a primary role in the thermodynamic balance of the ocean and, consequently, in oceanic heat and carbon uptake and storage. Though observed mixing rates are on average consistent with values required by inverse models, recent attention has focused on the dramatic spatial variability, spanning several orders of magnitude, of mixing rates in both the upper and deep ocean. Away from ocean boundaries, the spatio-temporal patterns of mixing are largely driven by the geography of generation, propagation and dissipation of internal waves, which supply much of the power for turbulent mixing. Over the last five years and under the auspices of US CLIVAR, a NSF- and NOAA-supported Climate Process Team has been engaged in developing, implementing and testing dynamics-based parameterizations for internal-wave driven turbulent mixing in global ocean models. The work has primarily focused on turbulence 1) near sites of internal tide generation, 2) in the upper ocean related to wind-generated near inertial motions, 3) due to internal lee waves generated by low-frequency mesoscale flows over topography, and 4) at ocean margins. Here we review recent progress, describe the tools developed, and discuss future directions.

2016
Klymak, JM, Simmons HL, Braznikov D, Kelly S, MacKinnon JA, Alford MH, Pinkel R, Nash JD.  2016.  Reflection of linear internal tides from realistic topography: The Tasman continental slope. Journal of Physical Oceanography. 46:3321-3337.   10.1175/jpo-d-16-0061.1   AbstractWebsite

The reflection of a low-mode internal tide on the Tasman continental slope is investigated using simulations of realistic and simplified topographies. The slope is supercritical to the internal tide, which should predict a large fraction of the energy reflected. However, the response to the slope is complicated by a number of factors: the incoming beam is confined laterally, it impacts the slope at an angle, there is a roughly cylindrical rise directly offshore of the slope, and a leaky slope-mode wave is excited. These effects are isolated in simulations that simplify the topography. To separate the incident from the reflected signal, a response without the reflector is subtracted from the total response to arrive at a reflected signal. The real slope reflects approximately 65% of themode-1 internal tide asmode 1, less than two-dimensional linear calculations predict, because of the three-dimensional concavity of the topography. It is also less than recent glider estimates, likely as a result of along-slope inhomogeneity. The inhomogeneity of the response comes from the Tasman Rise that diffracts the incoming tidal beam into two beams: one focused along beam and one diffracted to the north. Along-slope inhomogeneity is enhanced by a partially trapped, superinertial slope wave that propagates along the continental slope, locally removing energy from the deep-water internal tide and reradiating it into the deep water farther north. This wave is present even in a simplified, straight slope topography; its character can be predicted from linear resonance theory, and it represents up to 30% of the local energy budget.

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.

Lucas, AJ, Nash JD, Pinkel R, MacKinnon JA, Tandon A, Mahadevan A, Omand MM, Freilich M, Sengupta D, Ravichandran M, Le Boyer A.  2016.  Adrift upon a salinity-stratified sea: A view of upper-ocean processes in the Bay of Bengal during the southwest monsoon. Oceanography. 29:134-145.   10.5670/oceanog.2016.46   AbstractWebsite

The structure and variability of upper-ocean properties in the Bay of Bengal (BoB) modulate air-sea interactions, which profoundly influence the pattern and intensity of monsoonal precipitation across the Indian subcontinent. In turn, the bay receives a massive amount of freshwater through river input at its boundaries and from heavy local rainfall, leading to a salinity-stratified surface ocean and shallow mixed layers. Small-scale oceanographic processes that drive variability in near-surface BoB waters complicate the tight coupling between ocean and atmosphere implicit in this seasonal feedback. Unraveling these ocean dynamics and their impact on air-sea interactions is critical to improving the forecasting of intraseasonal variability in the southwest monsoon. To that end, we deployed a wave-powered, rapidly profiling system capable of measuring the structure and variability of the upper 100 m of the BoB. The evolution of upper-ocean structure along the trajectory of the instrument's roughly two-week drift, along with direct estimates of vertical fluxes of salt and heat, permit assessment of the contributions of various phenomena to temporal and spatial variability in the surface mixed layer depth. Further, these observations suggest that the particular "barrier-layer" stratification found in the BoB may decrease the influence of the wind on mixing processes in the interior, thus isolating the upper ocean from the interior below, and tightening its coupling to the atmosphere above.

MacKinnon, JA, Nash JD, Alford MH, Lucas AJ, Mickett JB, Shroyer EL, Waterhouse AF, Tandon A, Sengupta D, Mahadevan A, Ravichandran M, Pinkel R, Rudnick DL, Whalen CB, Alberty MS, Lekha JS, Fine EC, Chaudhuri D, Wagner GL.  2016.  A tale of two spicy seas. Oceanography. 29:50-61.   10.5670/oceanog.2016.38   AbstractWebsite

Upper-ocean turbulent heat fluxes in the Bay of Bengal and the Arctic Ocean drive regional monsoons and sea ice melt, respectively, important issues of societal interest. In both cases, accurate prediction of these heat transports depends on proper representation of the small-scale structure of vertical stratification, which in turn is created by a host of complex submesoscale processes. Though half a world apart and having dramatically different temperatures, there are surprising similarities between the two: both have (1) very fresh surface layers that are largely decoupled from the ocean below by a sharp halocline barrier, (2) evidence of interleaving lateral and vertical gradients that set upper-ocean stratification, and (3) vertical turbulent heat fluxes within the upper ocean that respond sensitively to these structures. However, there are clear differences in each ocean's horizontal scales of variability, suggesting that despite similar background states, the sharpening and evolution of mesoscale gradients at convergence zones plays out quite differently. Here, we conduct a qualitative and statistical comparison of these two seas, with the goal of bringing to light fundamental underlying dynamics that will hopefully improve the accuracy of forecast models in both parts of the world.

Musgrave, RC, Pinkel R, MacKinnon JA, Mazloff MR, Young WR.  2016.  Stratified tidal flow over a tall ridge above and below the turning latitude. Journal of Fluid Mechanics. 793:933-957.: Cambridge University Press   10.1017/jfm.2016.150   Abstract

The interaction of the barotropic tide with a tall, two-dimensional ridge is examined analytically and numerically at latitudes where the tide is subinertial, and contrasted to when the tide is superinertial. When the tide is subinertial, the energy density associated with the response grows with latitude as both the oscillatory along-ridge flow and near-ridge isopycnal displacement become large. Where $f\neq 0$ , nonlinear processes lead to the formation of along-ridge jets, which become faster at high latitudes. Dissipation and mixing is larger, and peaks later in the tidal cycle when the tide is subinertial compared with when the tide is superinertial. Mixing occurs mainly on the flanks of the topography in both cases, though a superinertial tide may additionally generate mixing above topography arising from convective breaking of radiating waves.

Musgrave, RC, MacKinnon JA, Pinkel R, Waterhouse AF, Nash J.  2016.  Tidally driven processes leading to near-field turbulence in a channel at the crest of the Mendocino Escarpment*. Journal of Physical Oceanography. 46:1137-1155.   10.1175/jpo-d-15-0021.1   AbstractWebsite

In situ observations of tidally driven turbulence were obtained in a small channel that transects the crest of the Mendocino Ridge, a site of mixed (diurnal and semidiurnal) tides. Diurnal tides are subinertial at this latitude, and once per day a trapped tide leads to large flows through the channel giving rise to tidal excursion lengths comparable to the width of the ridge crest. During these times, energetic turbulence is observed in the channel, with overturns spanning almost half of the full water depth. A high-resolution, nonhydrostatic, 2.5-dimensional simulation is used to interpret the observations in terms of the advection of a breaking tidal lee wave that extends from the ridge crest to the surface and the subsequent development of a hydraulic jump on the flanks of the ridge. Modeled dissipation rates show that turbulence is strongest on the flanks of the ridge and that local dissipation accounts for 28% of the energy converted from the barotropic tide into baroclinic motion.

2015
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.

Klymak, JM, Crawford W, Alford MH, MacKinnon JA, Pinkel R.  2015.  Along-isopycnal variability of spice in the North Pacific. Journal of Geophysical Research-Oceans. 120:2287-2307.   10.1002/2013jc009421   AbstractWebsite

Two hydrographic surveys in the Gulf of Alaska and the North Pacific subtropical gyre are presented. Both surveys are roughly perpendicular to lateral temperature gradients, and were collected in the summer when there was a shallow mixed layer and a seasonal thermocline. Isopycnal displacements and horizontal velocities are dominated by internal waves. Spice anomalies along isopycnals are examined to diagnose lateral stirring mechanisms. The spectra of spice anomaly gradients along near-surface isopycnals roughly follow power laws of similar to k(X)(0.6+/-0.2) (variance spectra power laws similar to k(X)(1.4+/-0.2)), and in most cases, the spectra become redder at depth. The near-surface spectra are possibly consistent with the predictions of quasi-geostrophic turbulence theory (when surface buoyancy effects are accounted for), but the spectra at depth are inconsistent with any quasi-geostrophic theory. Probability distributions of spice gradients exhibit a large peak at low gradients and long tails for large gradients, symptomatic of fronts. Vertical coherence of the spice signal falls off with a decorrelation depth scale that has a maximum of about 80 m at 100 km wavelengths and depends on horizontal wavelength with a power law of approximately k(x)(-1/2). Lateral decorrelation length scales are 20-40 km, close to the baroclinic Rossby radius. Lateral stirring occurs over large scales, with average lateral displacements of about 200 km in the upper 75 m, decreasing to 100 km at greater depths. The depth variation of the statistics indicates that time history of tracer stirring on each isopycnal is important, or that there are unconsidered depth-dependent stirring mechanisms.

2014
Pinkel, R.  2014.  Vortical and internal wave shear and strain. Journal of Physical Oceanography. 44:2070-2092.   10.1175/jpo-d-13-090.1   AbstractWebsite

Depth-time records of isopycnal vertical strain have been collected from intensive CTD profiling programs on the research platform (R/P) Floating Instrument Platform (FLIP). The associated vertical wavenumber frequency spectrum of strain, when viewed in an isopycnal-following frame, displays a clear spectral gap at low vertical wavenumber, separating the quasigeostrophic (vortical) strain field and the superinertial internal wave continuum. This gap enables both model and linear-filter-based methods for separating the submesoscale and internal wave strain fields. These fields are examined independently in six field programs spanning the period 1983-2002. Vortical and internal wave strain variances are often comparable in the upper thermocline, of order 0.2. However, vortical strain tends to decrease with increasing depth (decreasing buoyancy frequency N-2 = -g/rho(d rho/dz) as similar to(N-2)(1/2), while internal wave strain variance increases as similar to(N-2)(-1/2), exceeding vortical variance by a factor of 5-10 at depths below 500 m. In contrast to strain, the low-frequency spectral gap in the shear spectrum is largely obscured by Doppler-smeared near-inertial motions. The vertical wavenumber spectrum of anticyclonic shear exceeds the cyclonic shear and strain spectra at all scales greater than 10 m. The frequency spectrum of anticyclonic shear exceeds that of both cyclonic shear and strain to frequencies of 0.5 cph, emphasizing the importance of lateral Doppler shifting of near-inertial shear. The limited Doppler shifting of the vortical strain field implies surprisingly small submesoscale aspect ratios: k(H)/k(z) similar to 0.001, Burger numbers Br = k(H)N/k(z)f similar to 0.1. Submesoscale potential vorticity is dominated by vertical straining rather than the vertical component of relative vorticity. The inferred rms fluctuation of fluid vorticity is far less for the vortical field than for the internal wavefield.

Waterhouse, AF, MacKinnon JA, Nash JD, Alford MH, Kunze E, Simmons HL, Polzin KL, St Laurent LC, Sun OM, Pinkel R, Talley LD, Whalen CB, Huussen TN, Carter GS, Fer I, Waterman S, Garabato ACN, Sanford TB, Lee CM.  2014.  Global patterns of diapycnal mixing from measurements of the turbulent dissipation rate. Journal of Physical Oceanography. 44:1854-1872.   10.1175/jpo-d-13-0104.1   AbstractWebsite

The authors present inferences of diapycnal diffusivity from a compilation of over 5200 microstructure profiles. As microstructure observations are sparse, these are supplemented with indirect measurements of mixing obtained from(i) Thorpe-scale overturns from moored profilers, a finescale parameterization applied to (ii) shipboard observations of upper-ocean shear, (iii) strain as measured by profiling floats, and (iv) shear and strain from full-depth lowered acoustic Doppler current profilers (LADCP) and CTD profiles. Vertical profiles of the turbulent dissipation rate are bottom enhanced over rough topography and abrupt, isolated ridges. The geography of depth-integrated dissipation rate shows spatial variability related to internal wave generation, suggesting one direct energy pathway to turbulence. The global-averaged diapycnal diffusivity below 1000-m depth is O(10(-4))m(2) s(-1) and above 1000-m depth is O(10(-5))m(2) s(-1). The compiled microstructure observations sample a wide range of internal wave power inputs and topographic roughness, providing a dataset with which to estimate a representative global-averaged dissipation rate and diffusivity. However, there is strong regional variability in the ratio between local internal wave generation and local dissipation. In some regions, the depth-integrated dissipation rate is comparable to the estimated power input into the local internal wave field. In a few cases, more internal wave power is dissipated than locally generated, suggesting remote internal wave sources. However, at most locations the total power lost through turbulent dissipation is less than the input into the local internal wave field. This suggests dissipation elsewhere, such as continental margins.

Terker, SR, Girton JB, Kunze E, Klymak JM, Pinkel R.  2014.  Observations of the internal tide on the California continental margin near Monterey Bay. Continental Shelf Research. 82:60-71.   10.1016/j.csr.2014.01.017   AbstractWebsite

Observations of the semidiurnal internal tide on the California continental margin between Monterey Bay and Point Sur confirm the existence of northward energy flux predicted by numerical models of the region. Both a short-duration tide-resolving survey with expendable profilers and a multi-week timeseries from FLIP measured northward flux in the mean, supporting the hypothesis that topographic features off Point Sur are the source of the strong internal tides observed in Monterey Canyon. However, the observed depth-integrated semidiurnal flux of 450 +/- 200 W m(-1) is approximately twice as large as the most directly-comparable model and FLIP results. Though dominated by low modes with O(100 km) horizontal wavelengths, a number of properties of the semidiurnal internal tide, including kinetic and potential energy, as well as energy flux, show lateral variability on O(5 km) scales. Potential causes of this spatial variability include interference of waves from multiple sources, the sharp delineation of beams generated by abrupt topography due to limited azimuthal extent, and local generation and scattering of the internal tide into higher modes by small-scale topography. A simple two-source model of a first-mode interference pattern reproduces some of the most striking aspects of the observations. (C) 2014 Elsevier Ltd. All rights reserved.

2013
Klymak, JM, Buijsman M, Legg S, Pinkel R.  2013.  Parameterizing surface and internal tide scattering and breaking on supercritical topography: The one- and two-ridge cases. Journal of Physical Oceanography. 43:1380-1397.   10.1175/jpo-d-12-061.1   AbstractWebsite

A parameterization is presented for turbulence dissipation due to internal tides generated at and impinging upon topography steep enough to be supercritical with respect to the tide. The parameterization requires knowledge of the topography, stratification, and the remote forcingeither barotropic or baroclinic. Internal modes that are arrested at the crest of the topography are assumed to dissipate, and faster modes assumed to propagate away. The energy flux into each mode is predicted using a knife-edge topography that allows linear numerical solutions. The parameterization is tested using high-resolution two-dimensional numerical models of barotropic and internal tides impinging on an isolated ridge, and for the generation problem on a two-ridge system. The recipe is seen to work well compared to numerical simulations of isolated ridges, so long as the ridge has a slope steeper than twice the critical steepness. For less steeply sloped ridges, near-critical generation becomes more dominant. For the two-ridge case, the recipe works well when compared to numerical model runs with very thin ridges. However, as the ridges are widened, even by a small amount, the recipe does poorly in an unspecified manner because the linear response at high modes becomes compromised as it interacts with the slopes.

Sun, OM, Pinkel R.  2013.  Subharmonic Energy Transfer from the Semidiurnal Internal Tide to Near-Diurnal Motions over Kaena Ridge, Hawaii. Journal of Physical Oceanography. 43:766-789.   10.1175/jpo-d-12-0141.1   AbstractWebsite

Nonlinear energy transfers from the semidiurnal internal tide to high-mode, near-diurnal motions are documented near Kaena Ridge, Hawaii, an energetic generation site for the baroclinic tide. Data were collected aboard the Research Floating Instrument Platform (FLIP) over a 35-day period during the fall of 2002, as part of the Hawaii Ocean Mixing Experiment (HOME) Nearfield program. Energy transfer terms for a PSI resonant interaction at midlatitude are identified and compared to those for near-inertial PSI close to the M-2 critical latitude. Bispectral techniques are used to demonstrate significant energy transfers in the Nearfield, between the low-mode M-2 internal tide and subharmonic waves with frequencies near M-2/2 and vertical wavelengths of O(120 m). A novel prefilter is used to test the PSI wavenumber resonance condition, which requires the subharmonic waves to propagate in opposite vertical directions. Depth-time maps of the interactions, formed by directly estimating the energy transfer terms, show that energy is transferred predominantly from the tide to subharmonic waves, but numerous reverse energy transfers are also found. A net forward energy transfer rate of 2x10(-9) W kg(-1) is found below 400 m. The suggestion is that the HOME observations of energy transfer from the tide to subharmonic waves represent a first step in the open-ocean energy cascade. Observed PSI transfer rates could account for a small but significant fraction of the turbulent dissipation of the tide within 60 km of Kaena Ridge. Further extrapolation suggests that integrated PSI energy transfers equatorward of the M-2 critical latitude may be comparable to PSI energy transfers previously observed near 28.8 degrees N.

MacKinnon, JA, Alford MH, Pinkel R, Klymak J, Zhao Z.  2013.  The latitudinal dependence of shear and mixing in the Pacific transiting the critical latitude for PSI. Journal of Physical Oceanography. 43:3-16.: American Meteorological Society   10.1175/JPO-D-11-0107.1   AbstractWebsite

Turbulent mixing rates are inferred from measurements spanning 25°–37°N in the Pacific Ocean. The observations were made as part of the Internal Waves Across the Pacific experiment, designed to investigate the long-range fate of the low-mode internal tide propagating north from Hawaii. Previous and companion results argue that, near a critical latitude of 29°N, the internal tide loses energy to high-mode near-inertial motions through parametric subharmonic instability. Here, the authors estimate mixing from several variations of the finescale shear–strain parameterization, as well as Thorpe-scale analysis of overturns. Though all estimated diffusivities are modest in magnitude, average diffusivity in the top kilometer shows a factor of 2–4 elevation near and equatorward of 29°N. However, given intrinsic uncertainty and the strong temporal variability of diffusivity observed in long mooring records, the meridional mixing pattern is found to be near the edge of statistical significance.

2012
Pinkel, R.  2012.  Velocity Imprecision in Finite-Beamwidth Shipboard Doppler Sonar: A. Journal of Atmospheric and Oceanic Technology. 29:1569-1580. AbstractWebsite

The finite angular width of Doppler sonar beams introduces errors into

Pinkel, R, Buijsman M, Klymak JM.  2012.  Breaking Topographic Lee Waves in a Tidal Channel in Luzon Strait. Oceanography. 25:160-165. AbstractWebsite

Barotropic tides generate energetic internal tides, smaller-scale waves, and turbulence as they flow through Luzon Strait, between Taiwan and the Philippines. Three-dimensional numerical simulations of this process suggest that small-scale lee waves will form and break preferentially in "outflow channels," trough-like depressions that descend the strait's flanks. In the simulations, these sites are the locations of the most intense dissipation in the eastern strait. To investigate this numerical prediction, an 11-day cruise on R/V Roger Revelle was devoted to exploring an outflow channel on the eastern slope of the strait, north of Batan Island. Using a rapidly profiling conductivity-temperature-depth sensor and shipboard Doppler sonars, observations of velocity and density fields were made at four sites in the channel. At Site III, approximately 4 km offshore the crest, the generated lee wave was found to occupy much of the water column. It expanded upward from the seafloor as an irregular disturbance with a dominant vertical scale of 250 m. Sea-surface horizontal currents exceeded 1.5 m s(-1) and were sufficient to cause surface waves to break at 1,300 m above the local topography. Widespread internal wave breaking appeared initially at the seafloor and spread to much of the water column during the outflow phase of the tide. Breaking was also seen to a lesser extent on the inflow phase, as Pacific waters were advected westward toward the crest. The average dissipation rate at Site III, 8 W m(-2), exceeds typical wind energy input rates by four orders of magnitude.

Klymak, JM, Legg S, Alford MH, Buijsman M, Pinkel R, Nash JD.  2012.  The Direct Breaking of Internal Waves at Steep Topography. Oceanography. 25:150-159. AbstractWebsite

Internal waves are often observed to break close to the seafloor topography that generates them, or from which they scatter. This breaking is often spectacular, with turbulent structures observed hundreds of meters above the seafloor, and driving turbulence dissipations and mixing up to 10,000 times open-ocean levels. This article provides an overview of efforts to observe and understand this turbulence, and to parameterize it near steep "supercritical" topography (i.e., topography that is steeper than internal wave energy characteristics). Using numerical models, we demonstrate that arrested lee waves are an important turbulence-producing phenomenon. Analogous to hydraulic jumps in water flowing over an obstacle in a stream, these waves are formed and then break during each tidal cycle. Similar lee waves are also observed in the atmosphere and in shallow fjords, but in those cases, their wavelengths are of similar scale to the topography, whereas in the ocean, they are small compared to the water depth and obstacle size. The simulations indicate that these nonlinear lee waves propagate against the generating flow (usually the tide) and are arrested because they have the same phase speed as the oncoming flow. This characteristic allows estimation of their size a priori and, using a linear model of internal tide generation, computation of how much energy they trap and turn into turbulence. This approach yields an accurate parameterization of mixing in numerical models, and these models are being used to guide a new generation of observations.

Smith, JA, Pinkel R, Goldin M, Sun O, Nguyen S, Hughen T, Bui M, Aja A.  2012.  Wirewalker Dynamics. Journal of Atmospheric and Oceanic Technology. 29:103-115.   10.1175/jtech-d-11-00049.1   AbstractWebsite

A wirewalker exploits the difference in vertical motion between a wire attached to a surface buoy and the water at the depth of a profiling body to provide the power to execute deep profiles: when the wire's relative motion is upward, the profiler lets go; when it is downward, the profiler clamps on, and the weight attached at depth pulls the wire down, dragging the profiler downward against its buoyancy. The difference between the upward wire and profiler motion has to exceed the buoyancy-driven upward acceleration of the profiler body for this to work. Because the relative motion of the wire and water decreases as the surface is approached, the profiler might get stuck near the surface, especially when it is calm. However, two things mitigate this: 1) the system has a damped resonant response (similar to 1.3 Hz), which induces relative motion between the buoy and water even at the surface; and 2) for waves too gentle to directly exceed the required acceleration, drag on the profiler can pull the clamped-together system down sufficiently that the buoy and wire without the profiler attached can suddenly release and bob upward faster than the profiler. For system parameters as estimated here, the latter requires submersion of less than 0.005 m below its equilibrium depth. Several such "bounces" can occur over a portion of the wave phase. These two effects explain why, in practice, the profiler does not stay long near the surface (although it does proceed downward a bit more slowly there).

Pinkel, R, Rainville L, Klymak J.  2012.  Semidiurnal Baroclinic Wave Momentum Fluxes at Kaena Ridge, Hawaii. Journal of Physical Oceanography. 42:1249-1269.   10.1175/jpo-d-11-0124.1   AbstractWebsite

Kaena Ridge, Hawaii, is a site of energetic conversion of the semidiurnal barotropic tide. Diffuse baroclinic wave beams emanate from the critical-slope regions near the ridge crest, directed upward and southward from the north flank of the ridge and upward and northward from the south flank. Here, the momentum fluxes associated with generation at the ridge are estimated. Continuous vertical profiles of density and velocity from 80 to 800 m were obtained from the Research Platform Floating Instrument Platform (FLIP) over the southern edge of the ridge, as an aspect of the Hawaii Ocean Mixing Experiment. Data are used to estimate the Reynolds stress, Eulerian buoyancy flux, and the combined Eliassen-Palm flux in the semidiurnal band. An upward-southward stress maximum of similar to 0.5 X 10(-4) m(2) s(-2) appears at depths of 300-500 m, generally consistent with beam-like behavior. A strong off-ridge buoyancy flux (similar to 0.3 X 10(-4) m(2) s(-3)) combines with large along-ridge Reynolds stresses to form an Eliassen-Palm flux whose along-ridge and across-ridge magnitudes are comparable. The stress azimuth rotates clockwise with increasing altitude above the ridge crest. The principal upward-southward beam is found to be at depths 100-300 m shallower than are predicted by an analytic two-dimensional (2D) model and a 3D numerical simulation. This discrepancy is consistent with previous observations of the baroclinic energy flux. If these observed tidal momentum fluxes were to diverge in a 100-m-thick near-surface layer, the forcing would be comparable to a moderate wind stress. Pronounced lateral gradients of baroclinic tidal stresses can be expected offshore of Hawaiian topography.