Oceanic Velocity Microstructure Measurements in the 20th Century

The science of ocean turbulence was started more than 50 years ago by a small research group using a surplus mine-sweeping paravane to measure the velocity and temperature fluctuations in the ocean. The field has grown considerably and measurements are now conducted by researchers in many countries. A wide variety of sophisticated instrument systems are used to profile horizontally and vertically through the marine environment. Here we review the historical development of velocity micro-structure profiles over the past four decades and summarize the basic requirements for successful measurements. We highlight critical technological developments and glance briefly at some of the scientific discoveries made with these instruments. This revised version was published online in August 2006 with corrections to the Cover Date.     

Oceanic thermohaline intrusions: theory

This is a review of theories governing growth and evolution of thermohaline intrusive motions. We discuss theories based on eddy coefficients and salt finger flux ratios and also on molecular Fickian diffusion, drawing relationships and parallels where possible. We discuss linear theories of various physical configurations, effects of rotation and shear, and nonlinear theories. A key requirement for such theories to become quantitatively correct is the development and field testing of relationships between double-diffusive fluxes and average vertical gradients of temperature and salinity. While we have some ideas about the functional dependencies and rough observational constraints on the magnitudes of such flux/gradient relationships, many questions will not be answered until usable ‘flux laws’ exist. Furthermore, numerical experiments on double-diffusive intrusions are currently feasible, but will have more quantitative meaning when fluxes are parameterised with such laws. We conclude that more work needs to be done in at least two areas. Firstly, tests of linear theory against observations should continue, particularly to discover the extent to which linear theories actually explain the genesis of intrusions. Secondly, theoretical studies are needed on the nonlinear effects that control the evolution and finite amplitude state of intrusions, since these determine the lateral fluxes of salt, heat, and momentum.     

Turbulence and zooplankton production: insights from PROVESS

Zooplankton are directly influenced by turbulence in both a passive and an active manner. Passively, zooplankton are at the mercy of turbulence in how it affects their vertical mixing, encounter rate, detection abilities and feeding current efficiency. Many zooplankton species, however, are actively able to mitigate the effects of turbulence by modifying their behaviour, e.g. vertical migration, prey switching and habituation to hydromechanical stimuli. Both theoretical treatments of these processes and field observations from the northern North Sea are examined. Field observations show that some copepod species actively migrate to avoid high turbulence levels in surface waters. Furthermore, observations show a negative relationship between turbulence and zooplankton ingestion rates. This supports the paradigm of a dome-shaped response for zooplankton production with environmental turbulence. A theoretical treatment shows that the reaction distance, R, for an ambush-feeding copepod feeding on swimming organisms follows R ∝ ^−1/6 where is the turbulent dissipation rate, a result that shows close agreement with previously reported experimental results.     

Physical controls on mixing and transport within rising submarine hydrothermal plumes: A numerical simulation study

A computational fluid dynamics (CFD) model was developed to simulate the turbulent flow and species transport of deep-sea high temperature hydrothermal plumes. The model solves numerically the density weighted unsteady Reynolds-averaged Navier–Stokes equations and energy equation and the species transport equation. Turbulent entrainment and mixing is modeled by a k–ε turbulence closure model. The CFD model explicitly considers realistic vent chimney geometry, vent exit fluid temperature and velocity, and background stratification. The model uses field measurements as model inputs and has been validated by field data. These measurements and data, including vent temperature and plume physical structure, were made in the ABE hydrothermal field of the Eastern Lau Spreading Center. A parametric sensitivity study based on this CFD model was conducted to determine the relative importance of vent exit velocity, background stratification, and chimney height on the mixing of vent fluid and seawater. The CFD model was also used to derive several important scalings that are relevant to understanding plume impact on the ocean. These scalings include maximum plume rise height, neutrally buoyant plume height, maximum plume induced turbulent diffusivity, and total plume vertically transported water mass flux. These scaling relationships can be used for constructing simplified 1-dimensional models of geochemistry and microbial activity in hydrothermal plumes. Simulation results show that the classical entrainment assumptions, typically invoked to describe hydrothermal plume transport, only apply up to the vertical level of ~0.6 times the maximum plume rise height. Below that level, the entrainment coefficient remains relatively constant (~0.15). Above that level, the plume flow consists of a pronounced lateral spreading flow, two branches of inward flow immediately above and below the lateral spreading, and recirculation flanking the plume cap region. Both turbulent kinetic energy and turbulence dissipation rate reach their maximum near the vent; however, turbulent viscosity attains its maximum near the plume top, indicating strong turbulent mixing in that region. The parametric study shows that near vent physical conditions, including chimney height and fluid exit velocity, influence plume mixing from the vent orifice to a distance of ~10 times the vent orifice diameter. Thus, physical parameters place a strong kinetic constraint on the chemical reactions occurring in the initial particle-forming zone of hydrothermal plumes.     

Turbulence-induced steady streaming in an oscillating boundary layer: On the reliability of turbulence closure models

The accuracy of several closure models of the Reynolds-Averaged Navier–Stokes Equations in predicting the characteristics of an oscillating turbulent wall boundary layer is analyzed. The analysis involves four low Reynolds number k − ε models and a k − ω model and it is carried out by comparing the model results both with experimental data and with data obtained by a Direct Numerical Simulation (DNS) of the Navier–Stokes equations. The boundary layer is generated by a spatially constant time-oscillating pressure gradient given by the sum of two harmonic components characterized by angular frequencies Ω^∗ and 2Ω^∗ respectively, which generates a steady streaming because of the asymmetry of turbulence intensity during the cycle. Thus the results are relevant to the boundary layer at the bottom of nonlinear sea waves. The attention is therefore focused on the accuracy of the models in reproducing the period averaged profiles of the hydrodynamic characteristics of the steady streaming. The instantaneous quantities, such as time development of the wall shear stress, profiles of the streamwise velocity, Reynolds stresses and turbulent kinetic energy are also considered and analyzed. The results shows that a model can be judged better or worse than other models depending on the specific flow characteristic under investigation. However, an approach has been adopted which allowed to rank the models according to their accuracy in predicting the values of the hydrodynamic quantities involved in the present study.     

High gray whale relative abundances associated with an oceanographic front in the south-central Chukchi Sea

We describe gray whale (Eschrichtius robustus) distribution in the south-central Chukchi Sea in relation to environmental factors during two 5-day surveys in June and September of 2003. Whale counts per 10-min scan (an index of relative abundance) ranged from 0 to 41 in June and from 0 to 28 in September. CTD data showed an ocean front around 67.8°N with strong horizontal gradients in temperature, salinity, chlorophyll-a concentration and water-column stability. Highest whale abundance indices occurred in or near the front in both periods. Preliminary qualitative assessment of biological communities in the study area suggests that infaunal clams, echinoderms, euphausids, chaetognaths and Arctic cod were common, while ampeliscid amphipods, the previously abundant infauna (and likely prey) in the nearby Chirikov Basin feeding area, were not dominant. Euphausids may be a prey for gray whales in this area. We suggest that frontal systems may play an important role in eastern North Pacific gray whale foraging grounds. Further study is needed to fully describe the role of frontal systems in gray whale foraging grounds.     

Estimates of Energy Dissipation Rates in the Three-Dimensional Deep Ocean Internal Wave Field

Using the “Eikonal Approach” (Henyey et al., 1986), we estimate energy dissipation rates in the three-dimensional Garrett-Munk internal wave field. The total energy dissipation rate within the undisturbed GM internal wave field is found to be 4.34 × 10⁻⁹ W kg⁻¹. This corresponds to a diapycnal diffusivity of about 0.3 × 10⁻⁴ m²s⁻¹, which is less than the value 10⁻⁴ m²s⁻¹ required to sustain the global ocean overturning circulation. Only when the high vertical wavenumber, near-inertial current shear is enhanced can diapycnal diffusivity reach ∼10⁻⁴ m²s⁻¹. It follows that the energy supplied at low vertical wavenumbers and low frequencies is efficiently transferred to high vertical wavenumbers and near-inertial frequencies in the mixing hotspots in the real ocean.     

A winter upwelling event in the Northern Galician Rias: Frequency and oceanographic implications

A winter shelf-water upwelling evidence (February 2008) is described by first time in the Northern Galician Rias (NW Iberian Peninsula). On February 20, after 9 consecutive days of upwelling favorable conditions, inside the O Barqueiro Ria was observed the presence of seawater below 10 m depth which replaced the less saline water previously observed in January. This situation was in agreement with the analyzed Ekman transport close to the northern Galician coast. Salinity and temperature distribution revealed that the upwelled water inside the rias corresponds to shelf bottom seawater which is not associated with Eastern North Atlantic Central Water or the Iberian Poleward Current. In addition, TS diagram indicated a higher influence of upwelling eastward (Viveiro–Barqueiro–Ortigueira). Nutrient salts' concentrations also suggested the presence of seawater from subsurface origin with a small variation with regard to the winter mixing in this marine area. Plankton showed the existence of spring conditions related to solar radiation increase associated to upwelling favorable winds. Some species also corroborated the intrusion of shelf-water inside the ria. Results from the analysis of Ekman transport data from 1967 to 2007 revealed that this event cannot be considered an isolated episode. In fact, the number of days per month under upwelling favorable conditions in winter (January–March) was not negligible (8–10 days) showing that upwelling events along the northern Galician coast are also possible during winter.     

Energetics of wind-induced turbulent mixing in the ocean

The pattern and magnitude of the global ocean overturning circulation is believed to be strongly controlled by the distribution of diapycnal diffusivity below 1000 m depth. Although wind stress fluctuation is a candidate for the major energy sources of diapycnal mixing processes, the global distribution of wind-induced diapycnal diffusivity is still uncertain. It has been believed that internal waves generated by wind stress fluctuations at middle and high latitudes propagate equatorward until their frequency is twice the local inertial frequency and break down via parametric subharmonic instabilities, causing diapycnal mixing. In order to check the proposed scenario, we use a vertically two-dimensional primitive equation model to examine the spatial distribution of “mixing hotspots” caused by wind stress fluctuations. It is shown that most of the wind-induced energy fed into the ocean interior is dissipated within the top 1000 m depth in the wind-forced area and the energy dissipation rate at low latitudes is very small. Consequently, the energy supplied to diapycnal mixing processes below 1000 m depth falls short of the level required to sustain the global ocean overturning circulation.     

Winkler's method overestimates dissolved oxygen in seawater: Iodate interference and its oceanographic implications

Dissolved oxygen in seawater has been determined by using the Winkler's reaction scheme for decades. An interference in this reaction scheme that has been heretofore overlooked is the presence of naturally occurring iodate in seawater. Each mole of iodate can result in an apparent presence of 1.5 mol of dissolved oxygen. At the concentrations of iodate in the surface and deep open ocean, it can lead to an overestimation of 0.52 ± 0.15 and 0.63 ± 0.05 μmol kg^− 1 of oxygen in these waters respectively. In coastal and inshore waters, the effect is less predictable as the concentration of iodate is more variable. The solubility of oxygen in seawater was likely overestimated in data sources that were based on the Winkler's reaction scheme for the determination of oxygen. The solubility equation of García and Gordon [Garcia H.E., Gordon, L.I., 1992. Oxygen solubility in seawater: Better fitting equations. Limnol. Oceanogr. 37, 1307–1312] derived from the results of Benson and Krause [Benson, F.B., Krause, D. Jr., 1984. The concentration and isotopic fractionation of oxygen dissolved in freshwater and seawater in equilibrium with the atmosphere. Limnol. Oceanogr. 29, 620–632] is free from this source of error and is recommended for general use. By neglecting the presence of iodate, the average global super-saturation of oxygen in the surface oceans and the corresponding efflux of oxygen to the atmosphere both have been overestimated by about 8%. Regionally, in areas where the degree of super-saturation or under-saturation of oxygen in the surface water is small, such as in the tropical oceans, the net air–sea exchange flux can be grossly under- or overestimated. Even the estimated direction of the exchange can be reversed. Furthermore, the presence of iodate can lead to an overestimation of the saturation anomaly of oxygen in the upper ocean attributed to biological production by 0.23 ± 0.07%. AOU may have been underestimated by 0.52 ± 0.15 and 0.63 ± 0.05 μmol kg^− 1 in the surface mixed layer and deep water, while preformed phosphate and preformed nitrate may have been overestimated by 0.004 ± 0.001 and 0.06 ± 0.02 μmol kg^− 1 in the surface mixed layer, and 0.005 ± 0.0004 and 0.073 ± 0.006 μmol kg^− 1 in the deep water. These are small but not negligible corrections, especially in areas where the values of these parameters are small. At the increasing level of sophistication in the interpretation of oxygen data, this source of error should now be taken into account. Nevertheless, in order to avoid confusion, an internationally accepted standard needs to be adopted before these corrections can be applied.     

Relative dispersion in the Liguro-Provençal basin: From sub-mesoscale to mesoscale

Relative dispersion in the Liguro-Provençal basin (a subregion of the Mediterranean Sea) is investigated using clusters of surface drifters deployed during two Marine Rapid Environment Assessment (MREA) experiments covering different months in 2007 and 2008, respectively. The clusters have initial radii of less than 1 km, or an order of magnitude below a typical deformation radius (approximately 10-20 km). The data set consists of 45 original pairs and more than 50 total pairs (including chance ones) in the spatial range between 1 and 200 km. Relative dispersion is estimated using the mean square separation of particle pairs and the Finite Scale Lyapunov Exponents (FSLEs). The two metrics show broadly consistent results, indicating in particular a clear exponential behaviour with an e-folding time scale between 0.5 and 1 days, or Lyapunov exponent ?? in the range of 0.7-1 days⁻¹. The exponential phase extends for 4-7 days in time and between 1 and 10-20 km in separation space. To our knowledge, this is only the third time that an exponential regime is observed in the world ocean from drifter data. This result suggests that relative dispersion in the Liguro-Provençal basin is nonlocal, namely controlled mainly by mesoscale dynamics, and that the effects of the sub-mesoscale motions are negligible in comparison. NCOM model results are used to complement the data and to quantify errors arising from the sparse sampling in the observations.     

Longshore drift estimation using fluorescent tracers: New insights from an experiment at Comporta Beach, Portugal

A fluorescent sand-tracer experiment was performed at Comporta Beach (Portugal) with the aim of acquiring longshore sediment transport data on a reflective beach, the optimization of field and laboratory tracer procedures and the improvement of the conceptual model used to support tracer data interpretation.

The field experiment was performed on a mesotidal reflective beach face in low energetic conditions (significant wave height between 0.4 and 0.5 m). Two different colour tracers (orange and blue) were injected at low tide and sampled in the two subsequent low tides using a high resolution 3D grid extending 450 m alongshore and 30 m cross-shore. Marked sand was detected using an automatic digital image processing system developed in the scope of the present experiment.

Results for the two colour tracers show a remarkable coherence, with high recovery rates attesting data validity. Sand tracer displayed a high advection velocity, but with distinct vertical distribution patterns in the two tides: in the first tide there was a clear decrease in tracer advection velocity with depth while in the second tide, the tracer exhibited an almost uniform vertical velocity distribution. This differing behaviour suggests that, in the first tide, the tracer had not reached equilibrium within the transport system, pointing to a considerable time lag between injection and complete mixing. This issue has important implications for the interpretation of tracer data, indicating that short term tracer experiments tend to overestimate transport rates. In this work, therefore, longshore estimates were based on tracer results obtained during the second tide.

The estimated total longshore transport rate at Comporta Beach was 2 × 10^− 3 m³/s, more than four times larger than predicted using standard empirical longshore formulas. This discrepancy, which results from the unusually large active moving layer observed during the experiment, confirms the idea that most common longshore transport equations under-estimate total sediment transport in plunging/surging waves.     

基于一维扩散方程反演温度垂直扩散系数的变分方法

Diapycnal mixing is important in oceanic circulation. An inverse method in which a semi-explicit scheme is applied to discretize the one-dimensional temperature diffusion equation is established to estimate the vertical temperature diffusion coefficient based on the observed temperature profiles. The sensitivity of the inverse model in the idealized and actual conditions is tested in detail. It can be found that this inverse model has high feasibility under multiple situations ensuring the stability of the inverse model, and can be considered as an efficient way to estimate the temperature diffusion coefficient in the weak current regions of the ocean. Here, the hydrographic profiles from Argo floats are used to estimate the temporal and spatial distribution of the vertical mixing in the north central Pacific based on this inverse method. It is further found that the vertical mixing in the upper ocean displays a distinct seasonal variation with the amplitude decreasing with depth, and the vertical mixing over rough topography is stronger than that over smooth topography It is suggested that the high-resolution profiles from Argo floats and a more reasonable design of the inverse scheme will serve to understand mixing processes.     

A note on small organism diffusion around an attractive center: A mathematical model

A mathematical model of small organism diffusion around an attractive center is presented. The diffusion equation includes a forcing term which creates a virtual flow of organisms toward the center. Owing to the attractive force, the organisms tend to collect in swarms or patches notwithstanding the diffusive motion of surrounding water.Contribution No. 176 of the Chesapeake Bay Institute, Department of Earth and Planetary Sciences, The Johns Hopkins University. This work was supported by the Office of Naval Research Contract N00014-67-A-0163-0006, Research Project NR083-016, by the U.S. Atomic Energy Commission under Contract AT (30-1) 3109, Document COO-3109-2, and by National Science Foundation Grant, GA-16603.     

Longshore Current on an Equilibrium Beach

Natural beaches tend to be concave-up rather than planar and are reasonable to be modeled by an equilibrium beach profile. A governing equation for longshore current on an equilibrium beach is derived and its analytical solution is given in this paper. Through comparisons of the present solution and field data of longshore current for a step-type beach, the present solution is found to have fairly agreeable prediction to longshore current inside the surf zone. The effects of the shape of a concave-up beach and turbulent mixing stress on longshore current inside the surf zone are discussed in the present paper.     

Microstructure measurements in natural waters: Methodology and applications

Modern approaches to microstructure data processing, including wavelet denoising, are discussed. The wavelet procedure is applied to small-scale shear signals before estimating the dissipation rate ε and to the temperature/density profiles used to calculate Thorpe scales. Microstructure data obtained on the Mediterranean shelf of Catalonia are used to illustrate various approaches to the Thorpe displacement calculations. It is suggested that the Weibull probability function is an appropriate model for the Thorpe scale distribution. Microstructure measurements from the upper layer of the Boadella reservoir (Catalonia, Spain) support this finding.A new analytical approximation for the 1D Panchev–Kesich spectrum is deduced and the results of ε computation are compared with spectral fitting by the widely used Nasmyth spectrum. Applying the Kraichnan spectral model to compute ε from temperature spectra in the convective-viscous sub-range is examined as an alternative to the Batchelor spectrum. Microstructure measurements taken in Lake Banyoles (Catalonia, Spain) and in the North Atlantic were used for spectral calculations.Statistical analysis of eddy K_b and thermal K_θ diffusivities measured on a shallow shelf of the Black Sea shows the importance of process-orientated domain averaging of the diffusivities in obtaining good correspondence between K_b and K_θ in active turbulent regions. In weakly turbulent, stratified interior layers, the averaged K_b and K_θ differ significantly, which may point to the inapplicability of isotropic formulae used for ε and temperature dissipation χ_θ estimates, as well as to a dependence of the mixing efficiency γ on the Richardson number or in some cases on regions of fossil turbulence.     

Cross-Frontal Water Exchange on Georges Bank: Some Results from an U.S. GLOBEC/Georges Bank Program Model Study

This paper reviews recent progress on modeling cross-frontal water exchange on Georges Bank undertaken as part of the U.S. Global Ecosystem Northwest Atlantic/Georges Bank Study (U.S. GLOBEC/Georges Bank Program). A simple conceptual model is described first, followed by a discussion of four physical mechanisms associated with (1) strong nonlinear interaction, (2) asymmetric tidal mixing, (3) varying wind forcing, and (4) chaotic mixing. Some critical issues in modeling studies of fronts are also addressed. A new unstructured grid, finite-volume coastal ocean ecosystem model is introduced. This model combines the best of the finite-difference method for the simplest discrete computational efficiency and the finite-element method for geometric flexibility. Because the finite-volume method discretizes the integral form of the governing equations, this approach provides a better representation for the conservation laws of mass and momentum are satisfied, which is particularly important in the frontal regions. This revised version was published online in August 2006 with corrections to the Cover Date.     

Reconstruction of incomplete oceanographic data sets using empirical orthogonal functions: application to the Adriatic Sea surface temperature

A method for the reconstruction of missing data based on an EOF decomposition has been applied to a large data set, a test case of Sea Surface Temperature satellite images of the Adriatic Sea. The EOF decomposition is realised with a Lanczos method, which allows optimising computational time for large matrices. The results show that the reconstruction method leads to accurate reconstructions as well as a low cpu time when dealing with realistic cases. The method has been tested with different amounts of missing data, artificially adding clouds ranging from 40% to 80% of data loss, and then compared to the same data set with no missing data. A comparison with in situ data has also been made. These validation studies show that results are robust, even when the amount of missing data is very high. The reconstruction of the data from the Adriatic Sea shows realistic features and a reliable temperature distribution. In addition, the method is compared to an Optimal Interpolation reconstruction. The results obtained with both methods are very similar. The main difference is the computational time, which is reduced nearly 30 times with the method presented here. Once the reconstruction has been performed, the EOF decomposition is analysed to show the method’s reliability, and a cold event on the Albanian coast is studied. The reconstructed data reflect the effect of wind on the Albanian coast, that led to a cold-water episode in this zone for a 6-day period.     

Stable isotope analysis of Pacific salmon: insight into trophic status and oceanographic conditions over the last 30 years

Food web interactions and the response of Pacific salmon to physical processes in the North Pacific Ocean over interannual and interdecadal timescales are explored using naturally occurring stable isotope ratios of carbon (¹³C/¹²C) and nitrogen (¹⁵N/¹⁴N). Stable isotope analyses of five species of sexually mature North Pacific salmon from Alaska (Oncorhynchus spp.) cluster into three groups: chinook salmon (O. tshawytscha) have the highest values, followed by coho (O. kisutch), with chum (O. keta), sockeye (O. nerka), and pink (O. gorbuscha) together having the lowest values. Although detailed isotopic data on salmon prey are lacking, there are limited data on relevant prey items from areas in which they are found in high abundance. These data suggest that the characteristics of the sockeye, pink and chum we have analyzed are compatible with their diets including open ocean squid and zooplankton, which are in general agreement with stomach content analyses. Isotope relationships between muscle and scale show consistent relationships for both δ¹³C (R²=0.98) and δ¹⁵N (R²=0.90). Thus, scales, which have been routinely archived for many systems, can be used for retrospective analyses. Archived sockeye salmon scales spanning 1966–1999 from Red Lake, Kodiak Island, Alaska were analyzed for their stable isotope ratios of carbon and nitrogen. The δ¹⁵N record displays a decreasing trend of ˜3‰ from 1969–1982 and an increasing trend of ˜3‰ from 1982–1992, while the variations in δ¹³C are relatively minor. These trends may result from factors such as shifts in trophic level of feeding and/or feeding location, or may originate at the base of the food web via changes in processes such as nutrient cycling or primary productivity. Detailed studies on prey isotopic variability and its controls are needed to distinguish between these factors, and thus to improve the use of stable isotope analysis as a tool to learn more about present and past ecosystem change in the North Pacific and its relation to climatic change.     

Numerical modeling of hyperpycnal flows in an idealized river mouth

Numerical experiments in an idealized river mouth are conducted using a three-dimensional hydrodynamics model (EFDC model) to examine the impacts of suspended sediment concentration (SSC), settling velocity of sediment and tidal mixing on the formation and maintenance of estuarine hyperpycnal flows. The standard experiment presents an illustrative view of hyperpycnal flows that carry high-concentrated sediment and low-salinity water in the bottom layer (>1.0 m in thickness) along the subaqueous slope. The structure and intra-tidal variation of the simulated hyperpycnal flows are quite similar to those previously observed off the Huanghe (Yellow River) mouth. Results from the three control experiments show that SSC of river effluents is the most important parameter to the formation of hyperpycnal flows. High SSC will increase the bulk density of river effluents and thus offset the density difference between freshwater and seawater. Low SSC of river effluents will produce a surface river plume, as commonly observed in most large estuaries. Both the settling velocity of sediment particles and the tidal mixing play an important role in maintaining the hyperpycnal flows. Increasing settling velocity enhances the deposition of sediment from the hyperpycnal layer and thus accelerates the attenuation of hyperpycnal flows, whereas increasing tidal mixing destroys the stratification of water column and therefore makes the hyperpycnal flows less evident. Our results from numerical experiments are of importance to understand the initiation and maintenance of hyperpycnal flows in estuaries and provide a reference to the rapidly decaying hyperpycnal flows off the Huanghe river mouth due to climatic and anthropogenic forcing over the past several decades.