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Considerable effort has been made to parameterize turbulent kinetic energy (TKE) dissipation rate ε and mixing in buoyant plumes and stratified shear flows. Here, a parameterization based on Kunze et al. is examined, which estimates ε as the amount of energy contained in an unstable shear layer (Ri < Ric) that must be dissipated to increase the Richardson number Ri = N2/S2 to a critical value Ric within a turbulent decay time scale. Observations from the tidal Columbia River plume are used to quantitatively assess the relevant parameters controlling ε over a range of tidal and river discharge forcings. Observed ε is found to be characterized by Kunze et al.’s form within a factor of 2, while exhibiting slightly decreased skill near Ri = Ric. Observed dissipation rates are compared to estimates from a constant interfacial drag formulation that neglects the direct effects of stratification. This is found to be appropriate in energetic regimes when the bulk-averaged Richardson number Rib is less than Ric/4. However, when Rib > Ric/4, the effects of stratification must be included. Similarly, ε scaled by the bulk velocity and density differences over the plume displays a clear dependence on Rib, decreasing as Rib approaches Ric. The Kunze et al. ε parameterization is modified to form an expression for the nondimensional dissipation rate that is solely a function of Rib, displaying good agreement with the observations. It is suggested that this formulation is broadly applicable for unstable to marginally unstable stratified shear flows.

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.

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.

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.

The Air-Sea Interactions Regional Initiative (ASIRI) aims to understand vertical fluxes of momentum and heat across the surface layer in the Bay of Bengal. As the mesoscale and submesoscale eddies redistribute freshwater input over saline water of the bay, they influence the vertical distribution of salinity and thus impact air-sea fluxes. This study reports on numerical simulations performed to investigate processes that can lead to the observed vertical structure of stratification near the ocean surface. Processes are explored at multiple lateral scales, ranging from a few meters to tens of kilometers, to elucidate how the interplay among large-scale motion, submesoscale instabilities, and small-scale turbulent motion affects the surface layer.

Greenland fjords provide a pathway for the inflow of warm shelf waters to glacier termini and outflow of glacially modified waters to the coastal ocean. Characterizing the dominant modes of variability in fjord circulation, and how they vary over subannual and seasonal time scales, is critical for predicting ocean heat transport to the ice. Here we present a 2‐year hydrographic record from a suite of moorings in Davis Strait and two neighboring west Greenland fjords that exhibit contrasting fjord and glacier geometry (Kangerdlugssuaq Sermerssua and Rink Isbræ). Hydrographic variability above the sill exhibits clear seasonality, with a progressive cooling of near‐surface waters and shoaling of deep isotherms above the sill during winter to spring. Renewal of below‐sill waters coincides with the arrival of dense waters at the fjord mouth; warm, salty Atlantic‐origin water cascades into fjord basins from winter to midsummer. We then use Seaglider observations at Davis Strait, along with reanalysis of sea ice and wind stress in Baffin Bay, to explore the role of the West Greenland Current and local air‐sea forcing in driving fjord renewal. These results demonstrate the importance of both remote and local processes in driving renewal of near‐terminus waters, highlighting the need for sustained observations and improved ocean models that resolve the complete slope‐trough‐fjord‐ice system. Plain Language Summary Submarine melting of ice due to warm ocean waters has been implicated as a mechanism for the retreat and destabilization of marine‐terminating glaciers worldwide. In Greenland, fjords provide an important connection between marine‐terminating glaciers and warm subsurface waters located offshore. However, due to sparse ocean‐glacier observations in these ice‐choked systems, especially during winter months, we lack an understanding of how the large‐scale circulation along Greenland's periphery influences near‐glacier ocean temperatures. To address this, we present a 2‐year mooring record from Davis Strait and two neighboring west Greenland fjords. Above the sill, fjord temperatures exhibit a clear seasonal cycle that is inherited from the continental shelf and slope. Below the sill, warming of fjord waters occurs more intermittently and is initiated when salty Atlantic‐origin waters arrive at the fjord mouth. In summary, these novel observations allow for a better understanding of ocean‐glacier interactions in Greenland.

This is the publisher’s final pdf. The article is copyrighted by American Geophysical Union and published by John Wiley & Sons, Inc. It can be found at: http://agupubs.onlinelibrary.wiley.com/agu/journal/10.1002/%28ISSN%291944-8007/

Fjord-scale circulation forced by rising turbulent plumes of subglacial meltwater has been identified as one possible mechanism of oceanic heat transfer to marine-terminating outlet glaciers. This study uses buoyant plume theory and a nonhydrostatic, three-dimensional ocean–ice model of a typical outlet glacier fjord in west Greenland to investigate the sensitivity of meltwater plume dynamics and fjord-scale circulation to subglacial discharge rates, ambient stratification, turbulent diffusivity, and subglacial conduit geometry. The terminal level of a rising plume depends on the cumulative turbulent entrainment and ambient stratification. Plumes with large vertical velocities penetrate to the free surface near the ice face; however, midcolumn stratification maxima create a barrier that can trap plumes at depth as they flow downstream. Subglacial discharge is varied from 1–750 m3 s−1; large discharges result in plumes with positive temperature and salinity anomalies in the upper water column. For these flows, turbulent entrainment along the ice face acts as a mechanism to vertically transport heat and salt. These results suggest that plumes intruding into stratified outlet glacier fjords do not always retain the cold, fresh signature of meltwater but may appear as warm, salty anomalies. Fjord-scale circulation is sensitive to subglacial conduit geometry; multiple point source and line plumes result in stronger return flows of warm water toward the glacier. Classic plume theory provides a useful estimate of the plume’s outflow depth; however, more complex models are needed to resolve the fjord-scale circulation and melt rates at the ice face.

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 (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.

Internal oceanic waves are subsurface gravity waves that can be enormous and travel thousands of kilometres before breaking but they are difficult to study; here observations of such waves in the South China Sea reveal their formation mechanism, extreme turbulence, relationship to the Kuroshio Current and energy budget. Internal waves are the underwater version of more familiar surface waves. They can be enormous and travel thousands of kilometres before breaking. The South China Sea is known to be home to the largest internal waves in the world's oceans, but their size, generation mechanisms and role in the regional energy budget are unknown. Matthew Alford and colleagues now present the results from the IWISE observational campaign and reveal that internal waves more than 200 metres high break in the South China Sea and create turbulence that is orders of magnitude larger than in the open ocean, and that wave formation is influenced by the Kuroshio current. These results now allow for a complete energy budget of the South China Sea, and for a more accurate incorporation of internal waves into climate models. 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 photosynthesis1, sediment and pollutant transport2 and acoustic transmission3; they also pose hazards for man-made structures in the ocean4. Generated primarily by the wind and the tides, internal waves can travel thousands of kilometres from their sources before breaking5, making it challenging to observe them and to include them in numerical climate models, which are sensitive to their effects6,7. For over a decade, studies8,9,10,11 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.

Ocean turbulence (and turbulence in general) tends to be tremendously intermittent, events often dominating average values. Or, put another way, the distribution of turbulence tends to be highly skewed, requiring significant systematic observations to capture the important dynamics that control time and space averages. It is thus imperative to link large-scale processes (macroscale) to turbulence energetics (microscale) to characterize the dynamics of a particular regime and to develop a quantitative understanding of the role of turbulence in ocean momentum and scalar budgets.

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 spatiotemporal 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 5 years and under the auspices of U.S. Climate Variability and Predictability Program (CLIVAR), a National Science Foundation (NSF)- and National Oceanic and Atmospheric Administration (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.

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) m2 s−1 and above 1000-m depth is O(10−5) m2 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.

We use density and microstructure data to characterize the properties and physical setting of optical thin layers observed over the New Jersey shelf in the summer of 2006. Layers were differentiated into two types by their vertical position in the water column, fluorescence intensity, and possibly community composition or cell condition as indicated by the measured differences in the ratio of fluorescence to optical backscatter. Both layer types were associated with gradients in stratification; but, the turbulent mixing environment for the two layer types differed significantly. Shallow layers were located within the pycnocline near the maximum stratification and were exposed to strong turbulence within the surface mixed layer. Consequently, turbulent mixing of buoyancy may have been a key factor in maintaining shallow layers. In contrast, relatively weak density gradients and mixing indicate that turbulent processes may have been less critical for deep layers, which were located at the base of the pycnocline. More generally, both shallow and deep layers were geographically located in regions of low median and high mean turbulent kinetic energy dissipation, i.e., regions of rare yet intense mixing events. This work highlights the need to understand the detailed statistics of mixing at time and vertical scales relevant to thin layers, and more specifically, the need to discern the time history of mixing of the fluid that composes layers.

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.

Observations of the ocean surface boundary layer in the Northern Bay of Bengal were collected simultaneously from multiple platforms during the late summer of 2015. A spatial survey, consisting of a ∼ 4 km triangle, was repeated for 2 days by R/V Roger Revelle. The shipboard observations included profiles of temperature, salinity, velocity, and microstructure, and a towed bow-chain. Concurrently, an autonomous surface vessel and a drifting vertical profiler collected high resolution temperature, salinity, velocity and turbulence measurements nearby. An air-sea flux mooring provided continuous atmospheric and upper ocean data. The observed ocean surface boundary layer (SBL) was very shallow ( ∼ 10 m) and salinity stratified, with frequent observations of subsurface temperature maxima. Freshwater filaments strongly influenced SBL depth on horizontal scales of one to tens of kilometers. Our measurements showed a complex pattern in the strength and vertical structure of shear, stratification, and turbulent heat fluxes within and just below the SBL. SBL heat flux was impacted by surface buoyancy loss, shear at the SBL base from wind-driven near-inertial oscillations, and, at times, vertically spiraling Ekman currents. The phase of near-inertial currents displayed significant submesoscale lateral variability, as observed by the multiple high-resolution synoptic measurements, with horizontal differences in vertical turbulent fluxes in the SBL of the ocean varying by an order of magnitude over only a few kilometers. Integrated air-sea heat fluxes diverged by about 7–15 % over a few days within a series of one-dimensional simulations initialized with simultaneously observed SBL profiles only a few kilometers apart. Taken together, our results document the variability of ocean-atmosphere coupling on scales far smaller than those used in coupled ocean-atmosphere forecast models.

[1] The semidiurnal mode-1 internal tide receives 0.1–0.3 TW from the surface tide and is capable of propagating across ocean basins. The ultimate fate of mode-1 energy after long-distance propagation is poorly constrained by existing observations and numerical simulations. Here, global results from a two-dimensional semi-analytical model indicate that topographic scattering is inefficient at most locations deeper than 2500 m. Next, results from a one-dimensional linear model with realistic topography and stratification create a map of mode-1 scattering coefficients along the continental margins. On average, mode-1 internal tides lose about 60% of their energy upon impacting the continental margins: 20% transmits onto the continental shelf, 40% scatters to higher modes, and 40% reflects back to the ocean interior. These analyses indicate that the majority of mode-1 energy is likely lost at large topographic features (e.g., continental slopes, seamounts, and mid-ocean ridges), where it may drive elevated turbulent mixing.

Tide–topography interactions dominate the transfer of tidal energy from large to small scales. At present, it is poorly understood how low-mode internal tides reflect and scatter along the continental margins. Here, the coupling equations for linear tides model (CELT) are derived to determine the independent modal solutions to Laplace's Tidal Equations (LTE) over stepwise topography in one horizontal dimension. CELT is (i) applicable to arbitrary one-dimensional topography and realistic stratification without requiring numerically expensive simulations and (ii) formulated to quantify scattering because it implicitly separates incident and reflected waves. Energy fluxes and horizontal velocities obtained using CELT are shown to converge to analytical solutions, indicating that “flat bottom” modes, which evolve according to LTE, are also relevant in describing tides over sloping topography. The theoretical framework presented can then be used to quantify simultaneous incident and reflected energy fluxes in numerical simulations and observations of tidal flows that vary in one horizontal dimension. Thus, CELT can be used to diagnose internal-tide scattering on continental slopes. Here, semidiurnal mode-1 scattering is simulated on the Australian northwest, Brazil, and Oregon continental slopes. Energy-flux divergence and directional energy fluxes computed using CELT are shown to agree with results from a finite-volume model that is significantly more numerically expensive. Last, CELT is used to examine the dynamics of two-way surface–internal-tide coupling. Semidiurnal mode-1 internal tides are found to transmit about 5% of their incident energy flux to the surface tide where they impact the continental slope. It is hypothesized that this feedback may decrease the coherence of sea surface displacement on continental shelves.

As currents flow over rough topography, the pressure difference between the up- and downstream sides results in form drag—a force that opposes the flow. Measuring form drag is valuable because it can be used to estimate the loss of energy from currents as they interact with topography. An array of bottom pressure sensors was used to measure the tidal form drag on a sloping ridge in 200 m of water that forms a 1-km headland at the surface in Puget Sound, Washington. The form drag per unit length of the ridge reached 1 × 104 N m−1 during peak flood tides. The tidally averaged power removed from the tidal currents by form drag was 0.2 W m−2, which is 30 times larger than power losses to friction. Form drag is best parameterized by a linear wave drag law as opposed to a bluff body drag law because the flow is stratified and both internal waves and eddies are generated on the sloping topography. Maximum turbulent kinetic energy dissipation rates of 5 × 10−5 W kg−1 were measured with a microstructure profiler and are estimated to account for 25%–50% of energy lost from the tides. This study is among the first to measure form drag directly using bottom pressure sensors. The measurement and analysis techniques presented here are suitable for periodically reversing flows because they require the removal of a time-mean signal. The advantage of this technique is that it delivers a continuous record of form drag and is much less ship intensive compared to previous methods for estimation of the bottom pressure field.

Observations of breaking internal tides on the Oregon continental slope during a 40-day deployment of 5 moorings along 43°12′N are presented. Remotely generated internal tides shoal onto the slope, steepen, break, and form turbulent bores that propagate upslope independently of the internal tide. A high-resolution snapshot of a single bore is captured from lowered acoustic Doppler current profilers (LADCP)/CTD profiles in a 25-h time series at 1200 m. The bore is cold, salty, over 100 m tall, and has a turbulent head where instantaneous dissipation rates are enhanced (ε > 10−6 W kg−1) and sediment is resuspended. At the two deepest slope moorings (1452 and 1780 m), similar borelike phenomena are observed in near-bottom high-resolution temperature time series. Mean dissipation rates and diapycnal diffusivities increase by a factor of 2 when bores are present ( W kg−1 and m s−1) and observed internal tides are energetic enough to drive these enhanced dissipation rates. Globally, the authors estimate an average of 1.3 kW m−1 of internal tide energy flux is directed onto continental slopes. On the Oregon slope, internal tide fluxes are smaller, suggesting that it is a relatively weak internal tide sink. Mixing associated with the breaking of internal tides is therefore likely to be larger on other continental slopes.

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.

Packets of nonlinear internal waves (NLIWs) in a small area of the Mid-Atlantic Bight were 10 times more energetic during a local neap tide than during the preceding spring tide. This counterintuitive result cannot be explained if the waves are generated near the shelf break by the local barotropic tide since changes in shelfbreak stratification explain only a small fraction of the variability in barotropic to baroclinic conversion. Instead, this study suggests that the occurrence of strong NLIWs was caused by the shoaling of distantly generated internal tides with amplitudes that are uncorrelated with the local spring-neap cycle. An extensive set of moored observations show that NLIWs are correlated with the internal tide but uncorrelated with barotropic tide. Using harmonic analysis of a 40-day record, this study associates steady-phase motions at the shelf break with waves generated by the local barotropic tide and variable-phase motions with the shoaling of distantly generated internal tides. The dual sources of internal tide energy (local or remote) mean that shelf internal tides and NLIWs will be predictable with a local model only if the locally generated internal tides are significantly stronger than shoaling internal tides. Since the depth-integrated internal tide energy in the open ocean can greatly exceed that on the shelf, it is likely that shoaling internal tides control the energetics on shelves that are directly exposed to the open ocean.

Surface tides are the heartbeat of the ocean. Because they are controlled by Earth's motion relative to other astronomical objects in our solar system, surface tides act like clockwork and generate highly deterministic ebb and flow familiar to all mariners. In contrast, baroclinic motions at tidal frequencies are much more stochastic, owing to complexities in how these internal motions are generated and propagate. Here, we present analysis of current records from continental margins worldwide to illustrate that coastal internal tides are largely unpredictable. This conclusion has numerous implications for coastal processes, as across-shelf exchange and vertical mixing are, in many cases, strongly influenced by the internal wave field.

We compute rates of sediment removal from the Columbia River, USA plume near field using a control volume technique. By using data from repeated transect passes along the plume axis, we account for variations in plume width, depth, and velocity, and we construct a framework for determining the dominant terms in the complete sediment balance. The contribution due to turbulent sediment flux is estimated using collocated measurements of turbulent shear and suspended-sediment concentration. The maximum sediment clearance rate on the landward end of the transect, 4 km from the river mouth, is approximately 0.1 g m−2 s−1, which corresponds to an effective removal velocity of 10 mm s−1. This is significantly higher than the settling velocities expected for single-grain and aggregate particles observed in the estuary. The plume clearance rate decreases seaward, with values of 0.01 g m−2 s−1 6.5 km from the river mouth. We investigate several potential mechanisms to explain the along-axis variability in sediment removal, including gravitational settling of several particle-size distributions and removal by turbulent mixing. A simple model that accounts for the settling of the distribution of particles observed in the estuary with the addition of a very fine sand settling class best explains the observed magnitude and the seaward decrease in the sediment removal rate. Although it is unlikely that fine sands are retained in the plume from the estuary due to their high settling speeds, we hypothesize that they may be locally resuspended landward of the transect. The turbulent flux of sediment out of the plume is almost an order of magnitude smaller than the total removal rates observed on the landward end of the transect. However, the turbulent flux generally is greater than the removal rate predicted for unflocculated fine silt, indicating that it may be important for smaller particle size distributions or farther from shore.

The linear transfer of tidal energy from large to small scales is quantified for small tidal excursion over a near-critical continental slope. A theoretical framework for low-wavenumber energy transfer is derived from “flat bottom” vertical modes and evaluated with observations from the Oregon continental slope. To better understand the observations, local tidal dynamics are modeled with a superposition of two idealized numerical simulations, one forced by local surface-tide velocities and the other by an obliquely incident internal tide generated at the Mendocino Escarpment 315 km southwest of the study site. The simulations reproduce many aspects of the observed internal tide and verify the modal-energy balances. Observed transfer of tidal energy into high-mode internal tides is quantitatively consistent with observed turbulent kinetic energy (TKE) dissipation. Locally generated and incident simulated internal tides are superposed with varying phase shifts to mimic the effects of the temporally varying mesoscale. Altering the phase of the incident internal tide alters (i) internal-tide energy flux, (ii) internal-tide generation, and (iii) energy conversion to high modes, suggesting that tidally driven TKE dissipation may vary between 0 and 500 watts per meter of coastline on 3–5-day time scales. Comparison of observed in situ internal-tide generation and satellite-derived estimates of surface-tide energy loss is inconclusive.

Internal tide generation, propagation, and dissipation are investigated in Luzon Strait, a system of two quasi-parallel ridges situated between Taiwan and the Philippines. Two profiling moorings deployed for about 20 days and a set of nineteen 36-h lowered ADCP–CTD time series stations allowed separate measurement of diurnal and semidiurnal internal tide signals. Measurements were concentrated on a northern line, where the ridge spacing was approximately equal to the mode-1 wavelength for semidiurnal motions, and a southern line, where the spacing was approximately two-thirds that. The authors contrast the two sites to emphasize the potential importance of resonance between generation sites. Throughout Luzon Strait, baroclinic energy, energy fluxes, and turbulent dissipation were some of the strongest ever measured. Peak-to-peak baroclinic velocity and vertical displacements often exceeded 2 m s−1 and 300 m, respectively. Energy fluxes exceeding 60 kW m−1 were measured at spring tide at the western end of the southern line. On the northern line, where the western ridge generates appreciable eastward-moving signals, net energy flux between the ridges was much smaller, exhibiting a nearly standing wave pattern. Overturns tens to hundreds of meters high were observed at almost all stations. Associated dissipation was elevated in the bottom 500–1000 m but was strongest by far atop the western ridge on the northern line, where >500-m overturns resulted in dissipation exceeding 2 × 10−6 W kg−1 (implying diapycnal diffusivity Kρ > 0.2 m2 s−1). Integrated dissipation at this location is comparable to conversion and flux divergence terms in the energy budget. The authors speculate that resonance between the two ridges may partly explain the energetic motions and heightened dissipation.

A complex superposition of locally forced and shoaling remotely generated semidiurnal internal tides occurs on the Oregon continental slope. Presented here are observations from a zonal line of five profiling moorings deployed across the continental slope from 500 to 3000 m, a 24-h expendable current profiler (XCP) survey, and five 15–48-h lowered ADCP (LADCP)/CTD stations. The 40-day moored deployment spans three spring and two neap tides, during which the proportions of the locally and remotely forced internal tides vary. Baroclinic signals are strong throughout spring and neap tides, with 4–5-day-long bursts of strong cross-slope baroclinic semidiurnal velocity and vertical displacement . Energy fluxes exhibit complex spatial and temporal patterns throughout both tidal periods. During spring tides, local barotropic forcing is strongest and energy flux over the slope is predominantly offshore (westward). During neap tides, shoaling remotely generated internal tides dominate and energy flux is predominantly onshore (eastward). Shoaling internal tides do not exhibit a strong spring–neap cycle and are also observed during the first spring tide, indicating that they originate from multiple sources. The bulk of the remotely generated internal tide is hypothesized to be generated from south of the array (e.g., Mendocino Escarpment), because energy fluxes at the deep mooring 100 km offshore are always directed northward. However, fluxes on the slope suggest that the northbound internal tide is turned onshore, most likely by reflection from large-scale bathymetry. This is verified with a simple three-dimensional model of mode-1 internal tides propagating obliquely onto a near-critical slope, whose output conforms fairly well to observations, in spite of its simplicity.

14 pages, 13 figures, 1 table, Data collected during a 12-day cruise in July 2009 served to examine the structure of the nascent Mediterranean Outflow Water (MOW) immediately west of the Espartel Sill, the westernmost sill in the Strait of Gibraltar. The MOW is characterized by high salinities (>37.0 and reaching 38.3) and high velocities (exceeding 1 m s-1 at 100 m above the seafloor), and follows a submerged valley along a 30 km stretch, the natural western extension of the strait. It is approx. 150 m thick and 10 km wide, and experiences a substantial drop from 420 to 530 m over a distance of some 3 km between two relatively flat regions. Measurements indicate that the nascent MOW behaves as a gravity current with nearly maximal traveling speed; if this condition is maintained, then the maximum MOW velocity would decrease slowly with distance from the Espartel Sill, remaining significantly high until the gravity current excess density is only a small fraction of its original value. The sharp pycnocline between the Mediterranean and the overlying North Atlantic Central waters is dynamically unstable, particularly where the flow interacts with the 100 m decrease in bottom depth. Here, subcritical gradient Richardson numbers coincide with the development of large interfacial undulations and billows. The very energetic downslope flow is likely responsible for the development of a narrow V-shaped channel downstream of the seafloor drop along the axis of the submerged valley, this probably being the very first erosional scour produced by the nascent MOW. The coincidence of subcritical gradient Richardson numbers with relatively high turbidity values above the channel flanks suggests it may be undergoing upstream erosion, Funding for this work comes from the Ministerio de Ciencia e Innovación, Spain, through project MOC2 (CTM2008-06438-C02-01), project INGRES (CTM2006-02326), and two complementary actions (CTM2008-03422-E/MAR and CTM2009-05810-E/MAR); funding also comes from the US National Science Foundation (grants OCE-0825287 and OCE-0825297)

Extended measurements of temperature fluctuations that include the turbulence wavenumber band have now been made using rapidly sampled fast thermistors at multiple depths above the core of the Equatorial Undercurrent on the Tropical Atmosphere Ocean (TAO) mooring at 0°, 140°W. These measurements include the signature of narrowband oscillations as well as turbulence, from which the temperature variance dissipation rate χT and the turbulence energy dissipation rate εχ are estimated. The narrowband oscillations are characterized by the following:groupiness—packets consist of O(10) oscillations;spectral peaks of up to two orders of magnitude above background;a clear day–night cycle with more intensive activity at night;enhanced mixing rates;frequencies of 1–2 × 10−3 Hz, close to both the local buoyancy and shear frequencies, N/2π and S/2π, which vary slowly in time;high vertical coherence over at least 30-m scales; andabrupt vertical phase change (π/2 over

Narrowband oscillations observed in the upper equatorial Pacific are interpreted in terms of a random ensemble of shear instability events. Linear perturbation analysis is applied to hourly averaged profiles of velocity and density over a 54-day interval, yielding a total of 337 unstable modes. Composite profiles of mean states and eigenfunctions surrounding the critical levels suggest that the standard hyperbolic tangent model of Kelvin–Helmholtz (KH) instability is a reasonable approximation, but the symmetry of the composite perturbation is broken by the stratification and vorticity gradient of the underlying equatorial undercurrent. Unstable modes are found to occupy a range of frequencies with a peak near 1.4 mHz, consistent with the frequency content of the observed oscillations. A probabilistic theory of random instabilities predicts this peak frequency closely. An order of magnitude estimate suggests that the peak frequency is of order N, in accord with the observations. This results not from gravity wave physics but from the balance of shear and stratification that governs shear instability in geophysical flows. More generally, it is concluded that oscillatory signals with frequency bounded by N can result from a process that has nothing to do with gravity waves.

Several recent field efforts have revealed surprisingly complex and dynamic thermohaline structure in the upper ocean of the Beaufort Sea. Solitary and compact eddies with strong temperature contrasts and currents have been observed in multiple locations and are associated with vigorous mixing, staircase structure and intrusive feature formation. While many of the eddies are primarily found in the upper 300-m of the water column, rare deep eddies with cores near 500 to 1000-m depth have also been observed. Internal waves are generally weak with energies an order of magnitude less than mid-latitude values and they show marked dominance by near inertial waves, intermittency, spatial inhomogeneity, and deviations from the Garrett-Munk model. Strong intrusive structure, termed spice, is observed in the upper 150-m of the water column and is associated with the mixed layer and eddy activity. Acoustic implications of the associated sound speed structure will be discussed.

A tomography array was deployed in the Beaufort Sea for a year beginning in late summer 2016 during the Office of Naval Research sponsored project called the Canada Basin Acoustic Propagation Experiment (CANAPE.) This talk will look in detail at the propagation characteristics of broadband sound transmitted at 250 Hz over 108 km to 285 km ranges as measured on a 60 element vertical line array. Over the year, the heat content and vertical stratification of the ocean change as heat is gained and lost to the atmosphere and as the momentum transfer from the wind mixes the upper layers. The changes in heat content and stratification over the year affects the travel-time of the sound as well as its fluctuations. Examples will be shown of each. Sea-ice roughness adds scattering loss as the ice thickness increases. Thus, the seasonal ice-cover modulates the transmission loss of the acoustic paths. The losses on deep turning paths are different from the sound trapped in the surface duct. The ambient noise level also changes as the seasons progress.

The Arctic Ocean is undergoing dramatic changes in the ice cover and ocean structure. The 2016–2017 deep-water Canada Basin Acoustic Propagation Experiment (CANAPE) was designed to understand the effects of changing Arctic conditions on low-frequency, long-range propagation and ambient noise. Five acoustic transceivers were deployed in a pentagon with a sixth transceiver at the center, forming an ocean acoustic tomography array with a radius of 150 km in the central Beaufort Sea. The transceivers had broadband sources centered at approximately 250 Hz located at 175-m depth in the Beaufort Duct and 15 Hydrophone Modules spanning 135 m located above the sources. A Distributed Vertical Line Array receiver with 60 Hydrophone Modules spanning 540 m was embedded within the tomographic array to provide measurements of acoustic time fronts and their fluctuations. The tomographic array was largely in open water during summer, in the marginal ice zone as it transitioned across the array during the spring and autumn, and under complete ice cover during winter. The tomographic data, together with moored data from Sea-Bird MicroCATs, Acoustic Doppler Current Profilers, ice-profiling sonars, and precision temperature sensors, will help characterize the large-scale oceanographic variability throughout the year, aiding in the interpretation the acoustic data.

The energetics of large amplitude, high-frequency nonlinear internal waves (NLIWs) observed over the New Jersey continental shelf are summarized from ship and mooring data acquired in August 2006. NLIW energy was typically on the order of 105 Jm−1, and the wave dissipative loss was near 50 W m−1. However, wave energies (dissipations) were ~10 (~2) times greater than these values during a particular week-long period. In general, the leading waves in a packet grew in energy across the outer shelf, reached peak values near 40 km inshore of the shelf break, and then lost energy to turbulent mixing. Wave growth was attributed to the bore-like nature of the internal tide, as wave groups that exhibited larger long-term (lasting for a few hours) displacements of the pycnocline offshore typically had greater energy inshore. For ship-observed NLIWs, the average dissipative loss over the region of decay scaled with the peak energy in waves; extending this scaling to mooring data produces estimates of NLIW dissipative loss consistent with those made using the flux divergence of wave energy. The decay time scale of the NLIWs was approximately 12 h corresponding to a length scale of 35 km (O(100) wavelengths). Imposed on these larger scale energetic trends, were short, rapid exchanges associated with wave interactions and shoaling on a localized topographic rise. Both of these events resulted in the onset of shear instabilities and large energy loss to turbulent mixing.

Changes in the sea surface temperature of equatorial waters have critical effects on the large-scale atmospheric circulation. Shipboard measurements of turbulence kinetic-energy dissipation rate indicate that seasonal surface cooling in the central equatorial Pacific may be largely caused by mixing induced by tropical instability waves.

Observations off the New Jersey coast document the shoaling of three groups of nonlinear internal waves of depression over 35 km across the shelf. Each wave group experienced changing background conditions along its shoreward transit. Despite different wave environments, a clear pattern emerges. Nearly symmetric waves propagating into shallow water develop an asymmetric shape; in the wave reference frame, the leading edge accelerates causing the front face to broaden while the trailing face remains steep. This trend continues until the front edge and face of the leading depression wave become unidentifiable and a near-bottom elevation wave emerges, formed from the trailing face of the initial depression wave and the leading face of the following wave. The transition from depression to elevation waves is diagnosed by the integrated wave vorticity, which changes sign as the wave’s polarity changes sign. This transition is predicted by the sign change of the coefficient of the nonlinear term in the KdV equation, when evaluated using observed profiles of stratification and velocity.

Vaned, internally recording instruments that measure temperature fluctuations using FP07 thermistors, including fluctuations in the turbulence wavenumber band, have been built, tested, and deployed on a Tropical Atmosphere Ocean (TAO) mooring at 0°, 140°W. These were supplemented with motion packages that measure linear accelerations, from which an assessment of cable displacement and speed was made. Motions due to vortex-induced vibrations caused by interaction of the mean flow with the cable are small (rms < 0.15 cable diameters) and unlikely to affect estimates of the temperature variance dissipation rate χT. Surface wave–induced cable motions are significant, commonly resulting in vertical displacements of ±1 m and vertical speeds of ±0.5 m s−1 on 2–10-s periods. These motions produce an enhancement to the measurement of temperature gradient in the surface wave band herein that is equal to the product of the vertical cable speed and the vertical temperature gradient (i.e., dT/dt ∼ wcdT/dz). However, the temperature gradient spectrum is largely unaltered at higher and lower frequencies; in particular, there exists a clear scale separation between frequencies contaminated by surface waves and the turbulence subrange. The effect of cable motions on spectral estimates of χT is evaluated and determined to result in acceptably small uncertainties (< a factor of two 95% of the time, based on 60-s averages). Time series of χT and the inferred turbulent kinetic energy dissipation rate ε are consistent with historical data from the same equatorial location.

Highly resolved pressure measurements on the seafloor over New Jersey’s continental shelf reveal the pressure signature of nonlinear internal waves of depression as negative pressure perturbations. The sign of the perturbation is determined by the dominance of the internal hydrostatic pressure (p0Wh) due to isopycnal displacement over the contributions of external hydrostatic pressure (ρ0gηH; ηH is surface displacement) and nonhydrostatic pressure (p0nh), each of opposite sign to p0Wh. This measurement represents experimental confirmation of the wave-induced pressure signal inferred in a previous study by Moum and Smyth.

Buoyant surface discharges into ambient water bodies can exhibit multiple complex flow processes, which cover the spatial range from the near field with initial jet mixing to the far field with passive ambient diffusion. Multiple flow phenomena can occur, such as buoyant collapse motions, bottom attachment, deflection by the ambient current, and dynamic shoreline interaction, in the near field, and lateral and/or upstream spreading motions and turbulent diffusion processes, in the far field. Efficient and reliable predictive techniques covering the whole range of these processes are needed for the design and prediction of wastewater effluents that are subject to water quality regulations that can apply in either near and/or far field. A new comprehensive classification framework distinguishes among ten hydrodynamically distinct flow classes within four major flow categories: free jets, shoreline-attached jets, wall jets, and upstream intruding plumes. A prediction methodology for these discharges has been presented that covers the entire spatial domain from the near to the far field. It is based on the linkage of separate predictive modules in form of the expert system CORMIX3. These hydrodynamic modules are implemented by specific flow protocols and transition criteria determine their spatial extent. The methodology, corroborated by numerous detailed laboratory and some field data sources, constitutes a simple and efficient, yet accurate and robust, tool with few data requirements for surface discharge design and mixing analysis.

Winter stratification on Oregon’s continental shelf often produces a near-bottom layer of dense fluid that acts as an internal waveguide upon which nonlinear internal waves propagate. Shipboard profiling and bottom lander observations capture disturbances that exhibit properties of internal solitary waves, bores, and gravity currents. Wavelike pulses are highly turbulent (instantaneous bed stresses are 1 N m−2), resuspending bottom sediments into the water column and raising them 30+ m above the seafloor. The wave cross-shelf transport of fluid often counters the time-averaged Ekman transport in the bottom boundary layer. In the nonlinear internal waves that were observed, the kinetic energy is roughly equal to the available potential energy and is O(0.1) megajoules per meter of coastline. The energy transported by these waves includes a nonlinear advection term 〈uE〉 that is negligible in linear internal waves. Unlike linear internal waves, the pressure–velocity energy flux 〈up〉 includes important contributions from nonhydrostatic effects and surface displacement. It is found that, statistically, 〈uE〉 ≃ 2〈up〉. Vertical profiles through these waves of elevation indicate that up(z) is more important in transporting energy near the seafloor while uE(z) dominates farther from the bottom. With the wave speed c estimated from weakly nonlinear wave theory, it is verified experimentally that the total energy transported by the waves is 〈up〉 + 〈uE〉 ≃ c〈E〉. The high but intermittent energy flux by the waves is, in an averaged sense, O(100) watts per meter of coastline. This is similar to independent estimates of the shoreward energy flux in the semidiurnal internal tide at the shelf break.

We review the physics of near-inertial waves (NIWs) in the ocean and the observations, theory, and models that have provided our present knowledge. NIWs appear nearly everywhere in the ocean as a spectral peak at and just above the local inertial period f, and the longest vertical wavelengths can propagate at least hundreds of kilometers toward the equator from their source regions; shorter vertical wavelengths do not travel as far and do not contain as much energy, but lead to turbulent mixing owing to their high shear. NIWs are generated by a variety of mechanisms, including the wind, nonlinear interactions with waves of other frequencies, lee waves over bottom topography, and geostrophic adjustment; the partition among these is not known, although the wind is likely the most important. NIWs likely interact strongly with mesoscale and submesoscale motions, in ways that are just beginning to be understood.

Lateral stirring is a basic oceanographic phenomenon affecting the distribution of physical, chemical, and biological fields. Eddy stirring at scales on the order of 100 km (the mesoscale) is fairly well understood and explicitly represented in modern eddy-resolving numerical models of global ocean circulation. The same cannot be said for smaller-scale stirring processes. Here, the authors describe a major oceanographic field experiment aimed at observing and understanding the processes responsible for stirring at scales of 0.1–10 km. Stirring processes of varying intensity were studied in the Sargasso Sea eddy field approximately 250 km southeast of Cape Hatteras. Lateral variability of water-mass properties, the distribution of microscale turbulence, and the evolution of several patches of inert dye were studied with an array of shipboard, autonomous, and airborne instruments. Observations were made at two sites, characterized by weak and moderate background mesoscale straining, to contrast different regimes of lateral stirring. Analyses to date suggest that, in both cases, the lateral dispersion of natural and deliberately released tracers was O(1) m2 s–1 as found elsewhere, which is faster than might be expected from traditional shear dispersion by persistent mesoscale flow and linear internal waves. These findings point to the possible importance of kilometer-scale stirring by submesoscale eddies and nonlinear internal-wave processes or the need to modify the traditional shear-dispersion paradigm to include higher-order effects. A unique aspect of the Scalable Lateral Mixing and Coherent Turbulence (LatMix) field experiment is the combination of direct measurements of dye dispersion with the concurrent multiscale hydrographic and turbulence observations, enabling evaluation of the underlying mechanisms responsible for the observed dispersion at a new level.

Repeat transects of full-depth density and velocity are used to quantify generation and radiation of the semidiurnal internal tide from Kaena Ridge, Hawaii. A 20-km-long transect was sampled every 3 h using expendable current profilers and the absolute velocity profiler. Phase and amplitude of the baroclinic velocity, pressure, and vertical displacement were computed, as was the energy flux. Large barotropically induced isopycnal heaving and strong baroclinic energy-flux divergence are observed on the steep flanks of the ridge where upward and downward beams radiate off ridge. Directly above Kaena Ridge, strong kinetic energy density and weak net energy flux are argued to be a horizontally standing wave. The phasing of velocity and vertical displacements is consistent with this interpretation. Results compare favorably with the Merrifield and Holloway model.

An integrated analysis of turbulence observations from four unique instrument platforms obtained over the Hawaiian Ridge leads to an assessment of the vertical, cross-ridge, and along-ridge structure of turbulence dissipation rate and diffusivity. The diffusivity near the seafloor was, on average, 15 times that in the midwater column. At 1000-m depth, the diffusivity atop the ridge was 30 times that 10 km off the ridge, decreasing to background oceanic values by 60 km. A weak (factor of 2) spring–neap variation in dissipation was observed. The observations also suggest a kinematic relationship between the energy in the semidiurnal internal tide (E) and the depth-integrated dissipation (D), such that D ∼ E1±0.5 at sites along the ridge. This kinematic relationship is supported by combining a simple knife-edge model to estimate internal tide generation, with wave–wave interaction time scales to estimate dissipation. The along-ridge kinematic relationship and the observed vertical and cross-ridge structures are used to extrapolate the relatively sparse observations along the length of the ridge, giving an estimate of 3 ± 1.5 GW of tidal energy lost to turbulence dissipation within 60 km of the ridge. This is roughly 15% of the energy estimated to be lost from the barotropic tide.

Energy flux is a fundamental quantity for understanding internal wave generation, propagation, and dissipation. In this paper, the estimation of internal wave energy fluxes 〈u′p′〉 from ocean observations that may be sparse in either time or depth are considered. Sampling must be sufficient in depth to allow for the estimation of the internal wave–induced pressure anomaly p′ using the hydrostatic balance, and sufficient in time to allow for phase averaging. Data limitations that are considered include profile time series with coarse temporal or vertical sampling, profiles missing near-surface or near-bottom information, moorings with sparse vertical sampling, and horizontal surveys with no coherent resampling in time. Methodologies, interpretation, and errors are described. For the specific case of the semidiurnal energy flux radiating from the Hawaiian ridge, errors of ∼10% are typical for estimates from six full-depth profiles spanning 15 h.

Direct numerical simulations are used to compare turbulent diffusivities of heat and salt during the growth and collapse of Kelvin–Helmholtz billows. The ratio of diffusivities is obtained as a function of buoyancy Reynolds number Reb and of the density ratio Rρ (the ratio of the contributions of heat and salt to the density stratification). The diffusivity ratio is generally less than unity (heat is mixed more effectively than salt), but it approaches unity with increasing Reb and also with increasing Rρ. Instantaneous diffusivity ratios near unity are achieved during the most turbulent phase of the event even when Reb is small; much of the Reb dependence results from the fact that, at higher Reb, the diffusivity ratio remains close to unity for a longer time after the turbulence decays. An explanation for this is proposed in terms of the Batchelor scaling for scalar fields. Results are interpreted in terms of the dynamics of turbulent Kelvin–Helmholtz billows, and are compared in detail with previous studies of differential diffusion in numerical, laboratory, and observational contexts. The overall picture suggests that the diffusivities become approximately equal when Reb exceeds O(102). The effect of Rρ is significant only when Reb is less than this value.

In this study, the energy flux and energy dissipation of deep ocean internal tides are examined. Properties of the internal tide from two distinct generation regions are contrasted: the Mid-Atlantic Ridge (MAR) and the Hawaiian Ridge. Considerable differences are noted for the baroclinic energy flux, hu 0 p 0 i; radiated from each site. Radiation from the MAR is relatively rich in high modes, with an energy flux spectral peak at mode 5 and modes 10 and greater accounting for 40% of the total flux. In contrast, Hawaiian Ridge radiation is dominantly composed of modes 1 and 2, with modes 10 and greater accounting for less than 5% of the total flux. Depth integrated energy flux levels are Oð1Þ kW m � 1 at the MAR site, and Oð10ÞkW m � 1 at the Hawaiian Ridge. Despite these differences, observed turbulent dissipation rates at these sites are similar in magnitude and depth dependence. Decay scales, estimated as L� ¼ð R 0 � H u 0 p 0 dzÞ=ð R 0 � H r0� dzÞ; range from Oð100Þkm to Oð1000Þ km: The mean decay scale based on the MAR data is 230 km, a factor of 3 smaller than at the Hawaiian Ridge site. We demonstrate that the dissipation level scales with the energy flux available in the high modes, which is comparable at both sites, rather than the total energy flux. r 2004 Elsevier Ltd. All rights reserved.

Observations of turbulence, internal waves, and subinertial flow were made over a steep, corrugated continental slope off Virginia during May–June 1998. At semidiurnal frequencies, a convergence of low-mode, onshore energy flux is approximately balanced by a divergence of high-wavenumber offshore energy flux. This conversion occurs in a region where the continental slope is nearly critical with respect to the semidiurnal tide. It is suggested that elevated near-bottom mixing (Kρ ∼ 10−3 m2 s−1) observed offshore of the supercritical continental slope arises from the reflection of a remotely generated, low-mode, M2 internal tide. Based on the observed turbulent kinetic energy dissipation rate ϵ, the high-wavenumber internal tide decays on time scales O(1 day). No evidence for internal lee wave generation by flow over the slope's corrugations or internal tide generation at the shelf break was found at this site.

Prediction of water column concentrations of suspended sediment is often necessary for environmental impact assessment of point source industrial discharges. For example, in ''flow lane'' or ''open water'' disposal, suction dredges discharge large volumes of suspended sediment into shallow water disposal locations. A sediment density current mixing model is presented here as part of the D-CORMIX expert system for hydrodynamic simulation of mixing zone behavior. This density current model extends the CORMIX decision support system to simulate continuous negatively buoyant discharges with or without suspended sediment loads on a sloping bottom with loss of suspended particles by sedimentation. Sedimentation is modeled using Stokes settling for five particle size classes. Density current width and depth, trajectory, total solids, tracer concentration, dilution, and particle size concentration are predicted. In addition, location and widths of sediment deposits, accretion rates, including particle size fractions within the spoils deposit, are predicted. The model results are in good overall agreement with available field and laboratory data.