Oceanic Turbulence Research Articles

Effect of zero-mean-shear turbulence on rise velocity of in-chain bubbles from marine natural seeps

This paper reports on a laboratory experiment that investigates the impact of ocean turbulence on the rise velocity of bubbles released from natural seeps. To simulate the turbulence conditions of ocean bottoms, we used an oscillating grid-stirred turbulence (OGT) tank to generate nearly homogeneous and isotropic turbulence (HIT) with a range of turbulence dissipation rates. By isolating the effect of zero-mean-shear turbulence on the bubble rise from the effect of cross flows, we found that the presence of turbulence reduced the bubble rise velocity by up to 19% across six different gas flow rates. Our data show a linear relationship between the normalized bubble rise velocity and the log-value of the normalized turbulence dissipation rate, with two different slopes. Our analysis suggests that turbulence increased the horizontal motion of the bubbles, causing them to scatter laterally, thereby reducing their rise velocity. Lastly, we propose a semi-empirical equation that can be used to calculate the rise velocity of in-chain bubbles in turbulent waters. These findings have important implications for our understanding of the role of turbulence in the transport and fate of seep bubbles in ocean waters.

BER analysis for PAM-based UWOC-NOMA system in oceanic turbulence environment

Pulse amplitude modulation (PAM) scheme and non-orthogonal multiple access (NOMA) are attractive technologies to achieve high rate transmission and massive connectivity in the future Ocean Internet of Things (OIoT). To investigate the performance of underwater wireless optical communication (UWOC) NOMA systems based on PAM modulation, this paper develops a joint mathematical model with ocean turbulence and underwater solar noise based on the Weibull distribution. Specifically, the resolution framework on the ground of constellation multi-boundary judgments is proposed to derive the closed-form BER expressions for the wireless optical NOMA systems under ocean turbulent channels using the Laguerre approximation, which is also applicable to the other NOMA-enabled systems under higher order modulations. The accuracy of the analytical solution is demonstrated by simulation results. In addition, simulation results show that the higher-order PAM modulation can effectively reduce the bit-error rate (BER) gap between NOMA users and balance the fairness of users within the NOMA group. Furthermore, we observe that turbulence is one of the primary factors influencing the BER of the NOMA system in the marine environment, and the tilt angle of NOMA nodes has varying degrees of influence on the BER performance varying turbulence intensities. Therefore, selecting the appropriate tilt angle for the receiving nodes in a NOMA system based on the marine environment is beneficial to lowering the BER of the system.

Monitoring Deep Sea Currents With Seafloor Distributed Acoustic Sensing

AbstractUnderwater fiber optic cables commonly traverse a variety of seafloor conditions, which leads to an uneven mechanical coupling between the cable and the ocean bottom. On rough seafloor bathymetry, some cable portions might be suspended and thus susceptible to vortex‐induced vibrations (VIV) driven by deep ocean currents. Here, we examine the potential of distributed acoustic sensing (DAS) to monitor deep‐sea currents along suspended sections of underwater telecom fiber optic cables undergoing VIV. Oscillations of a seafloor fiber optic cable located in southern France are recorded by DAS along cable sections presumably hanging. Their characteristic frequencies are lower than 1 Hz, at different ocean depths, and have an amplitude‐dependency consistent with the driving mechanism being VIV. Based on a theoretical proportionality between current speed and VIV frequencies, we derive ocean current speed time series at 2,390 m depth from the vortex shedding frequencies recorded by DAS. The DAS‐derived current speed time series is in agreement with recordings by a current meter located 3.75 km away from the hanging cable section (similar dominant period, high correlation after time shift). The DAS‐derived current speed time series displays features, such as characteristic periods and spectral decay, associated with the generation of internal gravity waves and weak oceanic turbulence in the Mediterranean Sea. The results demonstrate the potential of DAS along hanging segments of fiber optic cables to monitor a wide range of oceanography processes, at depths barely studied with current instrumentation.

Equalization equal gain combining for a single-input to multiple-output underwater wireless optical communication system under aGaussian beam.

In this study, we examined the performance of an underwater wireless optical communication (UWOC) system employing a single-input to multiple-output (SIMO) scheme and proposed an equalization equal gain combining (EEGC) algorithm for it under Gaussian beam conditions. Furthermore, based on a Yue spectrum with the instability of oceanic water stratification and a finite outer scale, we derived the closed analytical formulas for the scintillation index and spatial coherence radius in weak oceanic turbulence for a Gaussian beam, from which we could obtain the threshold of the detector spacing and the strength of oceanic turbulence. We then derived the closed-form formula for the upper bound average bit error rate of the EEGC SIMO system with ON-OFF keying modulation by using the hyperbolic tangent distribution function. Our simulations demonstrate two issues if oceanic water stratification is treated as a steady state: the performance of the diversity receiver system will be significantly underestimated in salinity-dominated weak oceanic turbulence channels and will be significantly overestimated in temperature-dominated weak oceanic turbulence channels. Additionally, the SIMO system performance improvement using the proposed EEGC algorithm was more evident with increasing detector spacing, and the EEGC algorithm reduced the impact of the layout of the avalanche photodiode arrays on the UWOC system performance, in contrast to the equal gain combining algorithm.

Evaporation induced convection enhances mixing in the upper ocean

The upper ocean surface layer is directly affected by the air-sea fluxes. The diurnal variations in these fluxes also cause the upper ocean mixed layer turbulence and mixing to diurnally vary. The underlying thermohaline structure also varies accordingly throughout the day. Here we use large-eddy simulation to quantify the role of surface evaporation in modulating the diurnal mixed layer turbulence and mixing in the presence of wind forcing. During daytime, the upper ocean boundary layer becomes thermally stratified, and a salinity inversion layer is formed in the upper 10m, leading to double diffusive salt-fingering instability. During nighttime, the mixed layer undergoes convective deepening due to surface buoyancy loss redfrom both surface cooling and evaporation. We find that salinity makes a major contribution to the convective instability during both transitions between day and night. Overall surface evaporation increases the mixed layer depth and irreversible mixing through convection, both during nighttime and daytime, and leads to better prediction of the dynamical variables as sea surface salinity (SSS) and sea surface temperature (SST). Our findings can help improve the ocean parameterizations to improve the forecasts on a diurnal timescale.

Radially Phased-Locked Hermite–Gaussian Correlated Beam Array and Its Properties in Oceanic Turbulence

The descriptions of a radially phased-locked Hermite–Gaussian correlated beam array are introduced, the equation of this beam array in oceanic turbulence is derived, and the intensity profiles of this beam array are shown and analyzed. The results imply that the evolutions of the sub-beam of this beam array in free space are the same as the Hermite–Gaussian correlated beam, while the intensity of this beam array can be adjusted by controlling the initial beam radius R and the coherence length. The intensity profiles of this beam array in free space have multiple spots during propagation, while the same beam array in oceanic turbulence can become a beam spot due to the influences of R and oceanic turbulence. The beam array with smaller coherence length in oceanic turbulence retains the splitting properties better during propagation.

Numerical simulation of upper ocean responses to the passage of a submesoscale eddy

In this study, a large eddy simulation model (LES) is used to investigate the effect of an idealized westward propagating submesoscale eddy on ocean surface boundary layer turbulence. Large scale forcing (LSF) terms are introduced to the model to represent the effects of submesoscale eddies using the scale separation approach.Although re-stratification is observed at the arrival of the eddy center, consistent with previous studies, strong mixing and deepening is detected before/after the arrival of the eddy center. LES experiments are conducted to investigate the mechanism behind these dynamical responses, and explore the relative importance of the LSF terms. The interaction among these forcing terms is shown to be highly nonlinear, and the combined effects of buoyancy and momentum eddy forcing dominate the boundary layer response to the submesoscale eddy. While buoyancy flux is responsible for the enhanced mixing in the boundary layer, our analysis suggests that the momentum eddy forcing can significantly alter the timing of the enhanced mixing and boost its strength through generating strong mean current in the water column that accelerates westward density advection generated by the buoyancy eddy forcing. The lighter/heavier fluid intrusion brought by the mean flow leads to strong upwelling/downwelling, enhanced mixing and deepening of the mixed layer before the arrival of the eddy. Simulations using a general circulation model do not show similar response in the upper ocean because the K profile parameterization used in the model is a function of vertical shear only, and cannot represent the enhanced turbulence due to lateral mixing brought by the submesoscale eddy.Eddy distortion due to its asymmetric decay with time and northward Ekman transport generated by the wind stress also leads to substantial differences in boundary layer responses at different locations relative to the eddy center.

Synthesizing Sea Surface Temperature and Satellite Altimetry Observations Using Deep Learning Improves the Accuracy and Resolution of Gridded Sea Surface Height Anomalies

AbstractGridded sea surface height (SSH) maps estimated from satellite altimetry are widely used for estimating surface ocean geostrophic currents. Satellite altimeters observe SSH along one‐dimensional tracks widely spaced in space and time, making accurately reconstructing the two‐dimensional (2D) SSH field challenging. Traditionally, SSH is mapped using optimal interpolation (OI). However, OI artificially smooths the SSH field leading to high mapping errors in regions with rapidly‐evolving mesoscale features such as western boundary currents. Motivated by the dynamical relation between SSH and sea surface temperature (SST) and the notion that even the chaotic evolution of mesoscale ocean turbulence may contain repeating patterns, we outline a deep learning (DL) approach where a neural network is trained to reconstruct 2D SSH by synthesizing altimetry and SST observations. In the Gulf Stream Extension region, dominated by mesoscale variability, our DL method substantially improves the SSH reconstruction compared to existing methods. Our SSH map has 17% lower root‐mean‐square error and resolves spatial scales 30% smaller than OI compared against independent altimeter observations. Surface geostrophic currents calculated from our map are closer to surface drifter observations and appear qualitatively more realistic, with stronger currents, a clearer separation between the Gulf Stream and neighboring eddies, and the appearance of smaller coherent eddies missed by other methods. Our map yields significant re‐estimations of important dynamical quantities such as eddy kinetic energy, vorticity, and strain rate. Applying our DL method to produce a global SSH product may provide a more accurate and higher resolution product for studying mesoscale ocean turbulence.

Analysis of Optical Wireless Communication Links in Turbulent Underwater Channels With Wide Range of Water Parameters

In this study, the performance of underwater optical wireless communication links is investigated by taking into account turbulence, absorption and scattering effects. Weak turbulent channel is modeled using log-normal distribution while moderate and strong turbulence channels are modeled using gamma-gamma distribution. Rytov variance of Gaussian beam is derived analytically for oceanic turbulence optical power spectrum. Subsequently, scintillation index is calculated using the computed Rytov variance. Moreover, the closed-form expression of bit-error-rate (BER) for underwater wireless optical communication (UWOC) systems using intensity-modulated/direct detection (IM/DD) implementation and on-off-keying (OOK) modulation scheme is obtained. Results show that the performance of wireless optical communication link between two platforms in underwater medium is degraded significantly due to turbulence, absorption and scattering. In fact, as the turbulence level increases, its effect becomes quantitatively comparable to those of absorption and scattering effects. The variation of both scintillation index and BER performance are presented for various underwater medium and communication system parameters, such as chlorophyll concentration, average temperature, average salinity concentration, temperature and dissipation rates, wavelength, link length and receiver aperture size. Optical network and internet of underwater things (IoUT) applications, which are growing day by day and requiring high data rates, will benefit from the results of this study.

A Numerical Model Approach Toward a Settling Process and Feedback Loop of Ocean Microplastics Absorbed Into Phytoplankton Aggregates

AbstractTo quantify the settling velocity of buoyant oceanic microplastics absorbed into phytoplankton (algae) aggregates heavier than seawater, a vertical two‐dimensional numerical particle tracking model (PTM) representing the motion of both particles and their interaction in oceanic turbulence was developed. The model incorporated “in silico phytoplankton,” and could generate an aggregate particle with phytoplankton and microplastic particles positioned close to the “aggregation radius.” The model included the dense phytoplankton concentrations typically observed in the spring/autumn boom or red tides, and reproduced sinking microplastics with settling velocities of Ο (10−5–10−4) m/s, which were comparable to the vertical velocities ubiquitously observed in oceans, by the absorption into phytoplankton aggregates. The model reproduced the observed vertical distribution of microplastic abundance, showing subsurface maxima and a rapid decrease in particles smaller than 1 mm in the water column, while only an equilibrium between rise velocity of buoyant microplastic particles and vertical diffusion by oceanic turbulence could not reproduce the observed abundance. Buoyant microplastics, therefore, are likely to settle to the deep layer by the absorption into phytoplankton aggregates in the actual oceans. A feedback loop was suggested in which the settling velocities of aggregates were also dependent on the abundance of microplastics in the ambient water. However, a further in‐depth examination is required to confirm the feedback by including ambient ocean circulations as well as oceanic turbulence, and by replacing the settling velocity with an alternative formula appropriate for aggregates absorbing lightweight microplastics, which might decrease the velocity more rapidly than expected.

Influence of Immersed Conductive Objects on Quasi-Steady Burning Behavior of Fuel Slick in Turbulent Waters

ABSTRACT In situ burning (ISB) is an effective technique for burning oil spills that occur in oceans. The need for enhancing the burning characteristics and advancing the technique of burning fuels on the water sublayer has been identified. Following this, the study analyzes the steady burning behavior of a fuel layer floating on a turbulent water surface. The turbulent water surface is generated using an experimental platform comprising an axisymmetric upward-pointing submerged jet, an approach used in studies of free-surface turbulence. The turbulence is isotropic in the horizontal plane and bulk-flow free, with a turbulence intensity equal to 1.7 cm/s comparable to water turbulence in a calm ocean. The turbulent boundary condition causes an increase in heat extraction from the bottom of the fuel layer, thereby reducing the mass burning rate by 45% compared to a case with no turbulence. To improve the burning under turbulent water sublayer condition, a 10 mm conductive object is immersed at different depths in the fuel. The copper rod collects the flame’s heat and transmits it into the fuel layer via conduction, thereby increasing the burning rate by more than 1.4 times. A heat transfer model shows that the case with a small rod immersion depth (~10 mm) in a 40 mm fuel layer is best at improving burning because the heat losses from the object to the water sublayer are the lowest. The model also shows that the heat from the flame transmitted by the rod is primarily distributed in a region close to the surface of the fuel, where nucleate boiling is observed around the rod surface. As more of the rod is immersed in the fuel layer, the heat dissipation to the turbulent water increases nearly five times, thereby reducing the bubbling around the rod surface and the burning rate. The implications of burning fuel slicks for fast, slick cleanup during in-situ burning in turbulent water conditions are espoused.

Mixing across stable density interfaces in forced stratified turbulence

Understanding how turbulence enhances irreversible scalar mixing in density-stratified fluids is a central problem in geophysical fluid dynamics. While isotropic overturning regions are commonly the focus of mixing analyses, we here investigate whether significant mixing may arise in anisotropic statically stable regions of the flow. Focusing on a single forced direct numerical simulation of stratified turbulence, we analyse spatial correlations between the vertical density gradient $\partial \rho /\partial z$ and the dissipation rates of kinetic energy $\epsilon$ and scalar variance $\chi$ , the latter quantifying scalar mixing. The domain is characterized by relatively well-mixed density layers separated by sharp stable interfaces that are correlated with high vertical shear. While static instability is most prevalent within the mixed layers, much of the scalar mixing is localized to the intervening interfaces, a phenomenon not apparent if considering local static instability or $\epsilon$ alone. While the majority of the domain is characterized by the canonical flux coefficient $\varGamma \equiv \chi /\epsilon =0.2$ , often assumed in ocean mixing parametrizations, extreme values of $\chi$ within the statically stable interfaces, associated with elevated $\varGamma$ , strongly skew the bulk statistics. Our findings suggest that current parametrizations of turbulent mixing may be biased by undersampling, such that the most common, but not necessarily the most significant, mixing events are overweighted. Having focused here on a single simulation of stratified turbulence, it is hoped that our results motivate a broader investigation into the role played by stable density interfaces in mixing, across a wider range of parameters and forcing schemes representative of ocean turbulence.

Fiber coupling efficiency in ocean with adaptive optics corrections

Underwater optical wireless communication (UOWC) is a very promising technology that enables high-speed data transfer through the use of laser beams in an oceanic turbulent medium. The high-tech fiber optical devices, which are already available in the market, can be integrated with the UOWC systems. When integration is achieved, oceanic turbulence, which distorts the wavefront of the propagating laser beam, plays an important role in reducing the fiber coupling efficiency (FCE), which in turn results in reducing the light power received from the fiber optical components. In this paper, we propose the use of the adaptive optics technique in a UOWC system to mitigate the effects of oceanic turbulence and boost the FCE. For this reason, the field correlation for a Gaussian laser beam is derived by using the Huygens–Fresnel principle. This way, the light power over the coupling lens and the light power accepted by the fiber core are formulated under the effect of adaptive optics corrections, which are represented by the number of Zernike modes. The results demonstrate that under the oceanic turbulence effect, the FCE of the UOWC system employing adaptive optics is always larger than that of the UOWC system employing no adaptive optics.

Do Submesoscales Affect the Large-Scale Structure of the Upper Ocean?

Abstract Submesoscale baroclinic instabilities have been shown to restratify the surface mixed layer and to seasonally energize submesoscale turbulence in the upper ocean. But do these instabilities also affect the large-scale circulation and stratification of the upper thermocline? This question is addressed for the North Atlantic Subtropical Mode Water region with a series of numerical simulations at varying horizontal grid spacings (16, 8, 4, and 2 km). These simulations are realistically forced and integrated long enough for the thermocline to adjust to the presence or absence of submesoscales. Linear stability analysis indicates that a 2-km grid spacing is sufficient to resolve the most unstable mode of the wintertime mixed layer instability. As the resolution is increased, spectral slopes of horizontal kinetic energy flatten and vertical velocities increase in magnitude, consistent with previous regional and short-time simulations. The equilibrium stratification of the thermocline changes drastically as the grid spacing is refined from 16 to 8 km and mesoscale eddies are fully resolved. The thermocline stratification remains largely unchanged, however, between the 8-, 4-, and 2-km runs. This robustness is argued to arise from a mesoscale constraint on the buoyancy variance budget. Once mesoscale processes are resolved, the rate of mesoscale variance production is largely fixed. This constrains the variance destruction by submesoscale vertical buoyancy fluxes, which thus remain invariant across resolutions. The bulk impact of mixed layer instabilities on upper-ocean stratification in the Subtropical Mode Water region through an enhanced vertical buoyancy flux is therefore captured at 8-km grid spacing, even though the instabilities are severely underresolved.

The diatom Fragilariopsis cylindrus: A model alga to understand cold-adapted life.

Diatoms are significant primary producers especially in cold, turbulent, and nutrient-rich surface oceans. Hence, they are abundant in polar oceans, but also underpin most of the polar food webs and related biogeochemical cycles. The cold-adapted pennate diatom Fragilariopsis cylindrus is considered a keystone species in polar oceans and sea ice because it can thrive under different environmental conditions if temperatures are low. In this perspective paper, we provide insights into the latest molecular work that has been done on F. cylindrus and discuss its role as a model alga to understand cold-adapted life.

Second-Order Statistics of Self-Splitting Structured Beams in Oceanic Turbulence

Free-space optical communication is restricted by random media-stimulated beam degradation. However, partially coherent structured beams modulated by the coherence structure can potentially mitigate this negative effect. By employing the extended Huygens–Fresnel integral, we provide an examination of the second-order statistical features of a common type of partly coherent structured beams, self-splitting structured beams, in a turbulent ocean. The implications of turbulence parameters relating to the ocean and beginning beam parameters corresponding to the progression of such beam propagation attributes are fully investigated. Our numerical outcomes show that, for turbulence with a low-dissipation kinetic energy rate per unit mass of fluid, small Kolmogorov inner scale, large relative strength of temperature to salinity undulations, and large dissipation rate of mean-square temperature has a greater negative effect on the structured beams. In addition, we suggest an effective approach, enhancing the order of the beam and reducing the coherence length of the beams, to lower the oceanic turbulence-induced negative effects, and thus have future extensive possibilities in free-space optical communication.

Performance Analysis of MIMO-mQAM System with Pointing Errors and Beam Spreading in Underwater Málaga Turbulence Channel

Both the long-term beam spreading caused by ocean turbulence and the pointing errors induced by the jitter of transmitters and receivers degrade the performance of underwater wireless optical communication (UWOC) links. To effectively alleviate their effects, an in-depth study was carried out over the Málaga turbulence channel with pointing errors and beam spreading in multiple-input and multiple-output (MIMO) UWOC. First, we analyzed the long-term beam spreading and the received light power for the finite receiving aperture in the presence of pointing error displacements. Based on this, the relationship between beam spreading, pointing errors, and signal power was established. Second, the approximate expressions of the average bit error rate (BER) and the communication outage probability were derived theoretically for this MIMO system using maximal-ratio combining (MRC) diversity. Third, the effects of the pointing errors on the coding and the diversity gains were explored for the MIMO links. Finally, using the observed ocean data from the Global Ocean Argo gridded dataset, we numerically verified the combined effects of ocean turbulence strength, beam spreading, and pointing errors on the average BER and outage probability of this system. These results also proved that adjusting the size of the receiving aperture or the order of the multiple quadrature amplitude modulation (mQAM) could effectively mitigate their effects.

Vortex X-wave propagation through von Kármán oceanic turbulence with anisotropy.

Vortex X-waves with coupling effects of orbital angular momentum (OAM) and spatiotemporal invariance are introduced into the research of underwater wireless optical communication systems (UWOCSs). We establish the OAM probability density of vortex X-waves and channel capacity of UWOCS using Rytov approximation and correlation function. Furthermore, an in-depth analysis of OAM detection probability and channel capacity is performed on vortex X-waves carrying OAM in von Kármán oceanic turbulence with anisotropy. The results show that an increase in OAM quantum number results in a "hollow X" shape in the received plane, where the energy of vortex X-waves is injected into the lobes, reducing the received probability of the vortex X-waves transmitted to the receiving end. As the Bessel cone angle increases, the energy gradually concentrates toward the energy distribution center, and the vortex X-waves become more localized. Our research may trigger the development of UWOCS for bulk data transfer based on OAM encoding.

Propagation of hollow higher-order cosh-Gaussian beam in oceanic turbulence

The averaged received intensity of hollow higher-order cosh-Gaussian (HHOCG) beam propagating in oceanic turbulence is derived based on extended Huygens–Fresnel integral. In detail, the effect of beam parameters and oceanic turbulence parameters on the received intensity is analyzed. Interestingly, beam has a focusing nature along propagation. Our results indicate that received intensity distribution is not affected from the variation in source field parameters. Beam size at the receiver plane can vary according to the changes in turbulence nature. Accordingly, the provided results will contribute to the improvement of both underwater optical communication and imaging systems.

Ocean turbulent boundary-layer influence on ice crystal behaviour beneath fast ice in an Antarctic ice shelf water plume: The “dirty ice”

The oceanic connection between ice shelf cavities and sea ice influences sea ice development and persistence. One unique feature in regions near ice shelves is the potential for sea ice growth due to crystal accretion on its underside. Here we present observations of ocean boundary-layer processes and ice crystal behaviour in an Ice Shelf Water outflow region from the Ross/McMurdo Ice Shelves. From a fast ice field camp during the Spring of 2015, we captured the kinematics of free-floating relatively large (in some cases 10s of mm in scale) ice crystals that were advecting and then settling upwards in a depositional layer on the sea ice underside (SIPL, sub-ice platelet layer). Simultaneously, we measured the background oceanic temperature, salinity, currents and turbulence structure. At the camp location the total water depth was 536 m, with the uppermost 50 m of the water column being in-situ super-cooled. Tidal flow speeds had an amplitude of around 0.1 m s-1 with dissipation rates in the under-ice boundary layer measured to be up to ε=10-6 W kg-1. Acoustic sampling (200 kHz) identified backscatter from large, individually identifiable suspended crystals associated with crystal sizes larger than normally described as frazil. Measurement of crystals in the SIPL found dimensions of the range 5-200 mm with an average of 93-101 mm depending on the year. The existence and settlement of crystals has implications for understanding SIPL evolution, the structure of sea ice, as well as the fate of Ice Shelf Water.