Darek J. Bogucki

Sediment Resuspension and Mixing by Resonantly Generated Internal Solitary Waves

The observation of internal solitary waves (ISWs) propagating upstream along a strongly stratified bottom layer on the California shelf is reported. An increased concentration of particulates in the water column accompanies the passage of these ISW packets. The estimated local Richardson number in the bottom vicinity is around 1/4, and a vertical coefficient of eddy diffusivity of order 1022 m2 s21 is associated with the upstream propagating leading ISW. The leading ISW gave rise to reversed flow in an 8-m layer above the bottom. It is argued that the upstream propagating ISWs were generated by resonant flow over bottom topography. Internal waves generated in this way seem to be frequent in the record of a month-long experiment. Model results suggest that the ISWs can carry up to 73% of such generated long wave energy. The ocean conditions at the site are similar to those of other coastal sites, which suggests that the phenomenon described here may be common.

Direct numerical simulations of passive scalars with Pr > 1 advected by turbulent flow

Direct numerical simulations of passive scalars, with Prandtl numbers Pr =3, 5, and 7, advected by turbulence at three low Reynolds numbers were performed. The energy spectra are self-similar under the Kolmogorov scaling and exhibit behaviour consistent with many other investigations: a short inertial range for the highest Reynolds number and the universal exponential form of the spectrum for all Reynolds numbers in the dissipation range. In all cases the passive scalar spectra collapse to a single self-similar curve under the Batchelor scaling and exhibit the k −1 range followed by an exponential fall-off. We attribute the applicability of the Batchelor scaling to our low-Reynolds-number flows to the universality of the energy dissipation spectra. The Batchelor range is observed for wavenumbers in general agreement with experimental observations but smaller than predicted by the classical estimates. The discrepancy is caused by the fact that the velocity scales responsible for the generation of the Batchelor range are in the vicinity of the wavenumber of the maximum energy dissipation, which is one order of magnitude less than the Kolmogorov wavenumber used in the classical theory. Two different functional forms of passive scalar spectra proposed by Batchelor and Kraichnan were fitted to the simulation results and it was found that the Kraichnan model agrees very well with the data while the Batchelor formula displays systematic deviations from the data. Implications of these differences for the experimental procedures to measure the energy and passive scalar dissipation rates in oceanographic flows are discussed.

Secondary circulation associated with a shelfbreak front

Evidence for secondary circulation associated with a shelfbreak front is obtained from a high-resolution, cross-shelf section of hydrographic, optical and velocity fields. Convergence in the bottom boundary layer on the inshore side of the front and subsequent upwelling into the interior is evident by a mid-water region of suspended bottom material emanating from the foot of the front and extending to within 35 m of the surface, 80 m above bottom. Downwelling on the offshore side of the front in the upper water column is inferred from a 20-m downward bend of the subsurface phytoplankton layer. These observations are in agreement with recent model predictions for secondary circulation near an idealized shelfbreak front. Convergence in measured cross-shelf velocity at the foot of the front is consistent with upwelling of bottom material detected there. An estimate of 9±2 m day−1 of upwelling on the inshore side of the shelfbreak front is obtained, implying a transit time from the bottom to the surface of 10–16 days.

A Simple Model for the Shear-induced Decay of an Internal Solitary Wave

Abstract Internal solitary waves (ISWs) are a common feature of the coastal zone and marginal seas, especially close to shelf breaks, and are observed to mix the water column at the depth of maximum density gradient. For a two-layer system separated by a thin interface with a finite density gradient, the Richardson number in the interface fails below 1/4 if, in the simplest case, the ISW amplitude exceeds 2(hH1)1/2, where h is the interface thickness and H1 the thickness of the upper layer. Assuming that mixing then thickens the interface and that the potential energy for this comes from the ISW, we derive formulas for the damping rate of the ISW. The model is generalized to allow for a stratified upper layer; a Richardson number of less than 1/4 now requires that the displacement of the base of the upper layer exceeds 0.82 times the thickness of the layer. The ISW damping rate is sensitive to the ratio of the mixing depths above and below the base of the upper layer but can be plausibly matched to field ...

A Mechanism for Sediment Resuspension by Internal Solitary Waves

A mechanism whereby internal solitary waves can lead to enhanced resuspension in coastal regions is presented. The mechanism involves the creation of a local region of separated flow in the boundary layer under the footprint of a wave. This region is susceptible to a global instability which leads to a coherent dynamics in which gradients in the normal and tangential stresses acting on the bottom surface are increased tenfold. Such increased bottom stresses and the spatio-temporal character of the flow combine to produce conditions favorable for resuspension.

Light scattering on oceanic turbulence.

Turbulent inhomogeneities of fluid flow have the effect of scattering light in near-forward angles, thus providing an opportunity to use optics to quantify turbulence. Here we report measurements of the volume-scattering function in the range of 10(-7) to 10(-3) rad using a wave-front sensing technique. The total scattering coefficient b, due to scattering on turbulent inhomogeneities, is between 1 and 10 m(-1) under typical oceanographic conditions. The numerical calculations of turbulent volume-scattering functions compare well with the laboratory measurement. These results suggest that optical measurements at small angles are affected by turbulence-related scattering, and their effects can be well modeled with numerical calculations.

Monte Carlo simulation of propagation of a short light beam through turbulent oceanic flow

We use Monte Carlo time-dependent simulations of light pulse propagation through turbulent water laden with particles to investigate the application of Multiple Field Of View (MFOV) lidar to detect and characterize oceanic turbulence. Inhomogeneities in the refractive index induced by temperature fluctuations in turbulent ocean flows scatter light in near-forward angles, thus affecting the near-forward part of oceanic water scattering phase function. Our results show that the oceanic turbulent signal can be detected by analyzing the returns from a MFOV lidar, after re-scaling the particulate back scattering phase function.

Internal solitary waves in a structured thermocline with implications for resuspension and the formation of thin particle‐laden layers

A structured thermocline model is employed to elucidate the role of environmental factors in the characteristics of internal solitary waves, including the related implications for wave-induced resuspension and the formation of thin layers with enhanced concentration of passive particles. The model considers varying relative strengths between a thin seasonal thermocline and a broad permanent thermocline together with a sheared current in the upper mixed layer. The influence of these environmental factors on the stimulus of the benthic boundary layer under the footprint of solitary waves is assessed, particularly the potential for inducing boundary layer separation and lowering the near-bottom Richardson number. Also, the flow field associated with an internal solitary wave is shown not only to transport particles horizontally, but to create a tendency for passive particles located in the mixed layer to migrate vertically and concentrate in a thin layer. This latter effect is especially pronounced for particles that are heavier than the host fluid and have particle Reynolds numbers of order unity or greater. Although specific calculations for bubbles have not been included here, the net vertical displacement for light particles following the passage of a solitary wave may be quite large.

An Experiment in Two-Way Communication with a Multivariable Moored System in Coastal Waters

Abstract An experimental data acquisition system (telepack) was interfaced with a multivariable moored system in order to transmit physical and biooptical data from a coastal mooring site to a shore-based work station. The study site was located off the coast of Los Angeles, California, and the telepack provided two-way communication via the Los Angeles cellular telephone system with a work station at the University of Southern California, approximately 20 km away. Data were obtained from the system on a call-up basis at intervals of approximately 8 h. The present work demonstrates the utility of a relatively inexpensive communications system for near-real time data acquisition in coastal waters. In addition, this study represents an initial step toward development of a worldwide environmental data collection and distribution system that could exploit existing and planned communication resources, including satellites, to provide near-real time two-way, high data rate communications between researchers’ wo...

Preliminary and novel estimates of CO2 gas transfer using a satellite scatterometer during the 2001GasEx experiment

The ocean takes up approximately 30% of the annual anthropogenic emissions of CO2. However, the air–sea exchange of carbon dioxide varies by a factor of 2 depending on the formulation of the exchange process. This considerable uncertainty is due in part to the difficulty in parameterizing the gas transfer velocity, k 660, usually given as a function of wind speed. Recent field data showed that parametrization using the mean square slope of small scale surface waves provides a more robust strategy to estimate gas transfer (Frew et al. 2004). Here we present a preliminary estimation of the gas transfer velocity as a function of upwind Normalized Radar Cross-Section (NRCS) as measured by the scatterometer QuikSCAT. The gas transfer velocity calculated from upwind NRCS exhibits a quadratic-like dependence at low and intermediate wind speeds (≃6 ms–1 ). This approach represents a promising new tool to obtain global quasi-synoptic estimates of oceanic uptake of CO2.