Breaking Region Research Articles

The break phenomenon of self-pulsation for liquid-centered swirl coaxial injectors

Self-pulsation boundaries that spray switches between stable and self-pulsated behaviors are experimentally obtained under water and air flow conditions for a liquid-centered swirl coaxial injector configured with different recess lengths. The underlying mechanisms responsible for self-pulsation are investigated comprehensively. Results show that for the non-recessed injector, self-pulsation only occurs in a relatively narrower region, associated with weaker spray oscillation with the amplitude less than 2 mm2/Hz correspondingly. In particular, for the injector with larger recess length, a break region is generated with increasing the momentum flux ratio. It is indicated that spray transforms from the self-pulsated behavior to the stable behavior, and develops into self-pulsated behavior once again. The break phenomenon of self-pulsation reveals a destabilizing/stabilizing effect created by the violent gas-liquid flow dynamics. Self-pulsation occurring on either side of the break region is found to be caused by different mechanisms. As such, self-pulsation of the liquid-centered swirl coaxial injector is classified into two types. The first type occurs in the injectors configured with relatively small recess length, or with large recess length before the occurrence of the break phenomenon. This self-pulsation is caused by the periodic blockage of the conical liquid sheet. Whereas, the second type is attributed to the periodic squeezing influences of annular gas on the conical liquid sheet, corresponding to the self-pulsation occurring in the injectors with larger recess length and after the disappearance of the break phenomenon.

Discontinuous Galerkin methods for a dispersive wave hydro-sediment-morphodynamic model

A dispersive wave hydro-sediment-morphodynamic model developed by complementing the shallow water hydro-sediment-morphodynamic (SHSM) equations with the dispersive term from the Green–Naghdi equations is presented. A numerical solution algorithm for the model based on the second-order Strang operator splitting is presented. The model is partitioned into two parts, (1) the SHSM equations and (2) the dispersive correction part, which are discretized using discontinuous Galerkin finite element methods. This splitting technique provides a facility to select dynamically regions of a problem domain where the dispersive term is not applied, e.g. wave breaking regions where the dispersive wave model is no longer valid. Algorithms that can handle wetting–drying and detect wave breaking are provided and a number of numerical examples are presented to validate the developed numerical solution algorithm. The results of the simulations indicate that the model is capable of predicting sediment transport and bed morphodynamic processes correctly provided that the empirical models for the suspended and bed load transport are properly calibrated. Moreover, the developed model is able to accurately capture hydrodynamics and wave dispersion effects up to swash zones, and its application is justified for simulations where dispersive wave effects are prevalent.

Generation, propagation and dissipation of internal tides on the continental shelf and slope off the west coast of India

The present study focuses on the dynamics of M2 internal tides on the shelf and slope off the west coast of India (WCI) using a high-resolution (1/48∘) numerical ocean circulation model configured for the eastern Arabian Sea. The WCI is characterized by the presence of large internal tides where most of the internal tides generated on shelf-slope regions propagate offshore as beams. Baroclinic energy budget estimates show that about 2.4 GW of barotropic tides gets converted to baroclinic tides over the region. The regions of shelf breaks and upper slope around 16–18.5∘ N and the northern part of the Chagos-Laccadive Ridge (8–14∘ N) are the two most important regions of internal tide generation along the WCI, which together contribute to more than 50% of total area-integrated M2 conversion over the domain. The diapycnal diffusivity for internal tide dissipation exceeds 10−3m2/s in these regions. The central and northern slope regions off the WCI are dominated by locally generated internal tides, whereas part of internal tides in the southern region is characterized by the presence of remotely-generated internal tides coming from the Chagos-Laccadive Islands. Internal tides experience significant seasonal variation on the shelf owing to the variation in stratification, while their variations are negligible on the slope, where the seasonal variation in stratification is not significant.

Correlation matrix renormalization theory in multi-band lattice systems

An appropriate treatment of electronic correlation effects plays an important role in accurate descriptions of physical and chemical properties of real materials. The recently proposed correlation matrix renormalization theory with sum rule correction (CMR) [] for studying correlated-electron materials has shown good performance in molecular systems and a periodic hydrogen chain in comparison with various quantum chemistry and quantum Monte Carlo calculations []. This work gives a detailed formulation and computational code implementation of CMR in multi-band periodic lattice systems. This lattice CMR ab initio theory is highly efficient, has no material specific adjustable parameters, and has no double counting issues faced by the hybrid approaches like LDA + U, DFT + DMFT and DFT + GA type theories. Benchmark studies on materials with s and p orbitals in this study show that CMR in its current implementation consistently performs well for these systems as the electron correlation increases from the bonding region to the bond breaking region.

The volumetric star formation law for nearby galaxies

In the last decades, much effort has been put into finding the star formation law, which could unequivocally link the gas and the star formation rate (SFR) densities measured on a sub-kiloparsec scale in star-forming galaxies. The conventional approach of using the observed surface densities to infer star formation laws has however revealed a major and well-known issue, as such relations are valid for the high-density regions of galaxies but break down in low-density and HI-dominated environments. Recently, an empirical correlation between the total gas (HI+H2) and the SFR volume densities was obtained for a sample of nearby disc galaxies and for the Milky Way. This volumetric star formation (VSF) law is a single power-law with no break and a smaller intrinsic scatter with respect to the star formation laws based on the surface density. In this work, we explore the VSF law in the regime of dwarf galaxies in order to test its validity in HI-dominated, low-density, and low-metallicity environments. In addition, we assess this relation in the outskirts of spiral galaxies, which are low-density and HI-dominated regions similar to dwarf galaxies. Remarkably, we find that the VSF law, namelyρSFR∝ρgasαwithα ≈ 2, is valid for both these regimes. This result indicates that the VSF law, which holds unbroken for a wide range of gas (≈3 dex) and SFR (≈6 dex) volume densities, is the empirical relation with the smallest intrinsic scatter and is likely more fundamental than surface-based star formation laws.

Numerical Simulation of Mountain Waves over the Southern Andes. Part I: Mountain Wave and Secondary Wave Character, Evolutions, and Breaking

AbstractThis paper addresses the compressible nonlinear dynamics accompanying increasing mountain wave (MW) forcing over the southern Andes and propagation into the mesosphere and lower thermosphere (MLT) under winter conditions. A stretched grid provides very high resolution of the MW dynamics in a large computational domain. A slow increase of cross-mountain winds enables MWs to initially break in the mesosphere and extend to lower and higher altitudes thereafter. MW structure and breaking is strongly modulated by static mean and semidiurnal tide fields exhibiting a critical level at ~114 km for zonal MW propagation. Varying vertical group velocities for different zonal wavelengthsλxyield initial breaking in the lee of the major Andes peaks forλx~ 50 km, and extending significantly upstream for largerλxapproaching the critical level at later times. The localized extent of the Andes terrain in latitude leads to “ship wave” responses above the individual peaks at earlier times, and a much larger ship-wave response at 100 km and above as the larger-scale MWs achieve large amplitudes. Other responses above regions of MW breaking include large-scale secondary gravity waves and acoustic waves that achieve very large amplitudes extending well into the thermosphere. MW breaking also causes momentum deposition that yields local decelerations initially, which merge and extend horizontally thereafter and persist throughout the event. Companion papers examine the associated momentum fluxes, mean-flow evolution, gravity wave–tidal interactions, and the MW instability dynamics and sources of secondary gravity waves and acoustic waves.

IGUIDE Method for CRISPR Off-Target Detection.

With the advent of genome editing technologies, scientists have recognized that these technologies can be prone to nonspecific or off-target activity. As many areas of the genome are sensitive and can give rise to abnormalities if mutated, it is imperative that scientists identify regions of off-target activity in order to utilize these new technologies for medical benefits. GUIDE-seq and iGUIDE both use an oligo-based marker method to identify regions of DNA double-strand breaks in an unbiased manner. The repeated observation of these double-strand breaks across the genome in comparison with target sequences (such as guide RNAs) has allowed researchers to identify on- and off-target sites related to their targeted-nuclease technologies.

Experimental Determination of Mechanical Properties of the Anode Cell of an X-Ray Lithograph

We have prepared the anode cell of an X-ray lithograph in the form of a PolySi/Si3N4/SiO2 membrane structure using group technology. The design of the stand for determining mechanical properties of membranes has been modernized. The critical pressure of a membrane structure with a diameter 250 μm varies in the range from 0.484 to 0.56 MPa for 15 samples. The mechanical strength of the PolySi*/Si3N4/SiO2 structure is 3.13 GPa. The new model in the Comsol package shows good correlation between the experimental critical pressure and the theoretical mechanical strength of the membrane. The distribution of mechanical stresses over the membrane has been obtained by simulation and analytic calculation. It is proved that the structure breaking region is localized at the membrane/substrate interface.

Subsurface Ocean Warming Hotspots and Potential Impacts on Marine Species: The Southwest South Atlantic Ocean Case Study

Fil: Franco, Barbara Cristie. Consejo Nacional de Investigaciones Cientificas y Tecnicas. Oficina de Coordinacion Administrativa Ciudad Universitaria. Centro de Investigaciones del Mar y la Atmosfera. Universidad de Buenos Aires. Facultad de Ciencias Exactas y Naturales. Centro de Investigaciones del Mar y la Atmosfera; Argentina. Instituto Franco-argentino sobre Estudios del Clima y sus Impactos; Argentina

Quantum information dynamics in a high-dimensional parity-time-symmetric system

Non-Hermitian systems with parity-time ($\mathcal{PT}$) symmetry give rise to exceptional points (EPs) with exceptional properties that arise due to the coalescence of eigenvectors. Such systems have been extensively explored in the classical domain, where second or higher order EPs have been proposed or realized. In contrast, quantum information studies of $\mathcal{PT}$-symmetric systems have been confined to systems with a two-dimensional Hilbert space. Here by using a single-photon interferometry setup, we simulate quantum dynamics of a four-dimensional $\mathcal{PT}$-symmetric system across a fourth-order exceptional point. By tracking the coherent, non-unitary evolution of the density matrix of the system in $\mathcal{PT}$-symmetry unbroken and broken regions, we observe the entropy dynamics for both the entire system, and the gain and loss subsystems. Our setup is scalable to the higher-dimensional $\mathcal{PT}$-symmetric systems, and our results point towards the rich dynamics and critical properties.

First Direct Observational Evidence for Secondary Gravity Waves Generated by Mountain Waves Over the Andes

AbstractA mountain wave with a significant brightness temperature amplitude and ~500 km horizontal wavelength was observed over the Andes on 24–25 July 2017 in Atmospheric Infrared Sounder (AIRS)/Aqua satellite data. In the Modern‐Era Retrospective Analysis for Research and Applications, version 2 (MERRA‐2), reanalysis data, the intense eastward wind flowed over the Andes. Visible/Infrared Imaging Radiometer Suite (VIIRS)/Suomi‐NPP (National Polar‐orbiting Partnership) did not detect the mountain waves; however, it observed concentric ring‐like waves in the nightglow emissions at ~87 km with ~100 km wavelengths on the same night over and leeward of the Southern Andes. A ray tracing analysis showed that the mountain waves propagated to the east of the Andes, where concentric ring‐like waves appeared above a region of mountain wave breaking. Therefore, the concentric ring‐like waves were likely secondary waves generated by momentum deposition that accompanied mountain wave breaking. These results provide the first direct evidence for secondary gravity waves generated by momentum deposition.

Precise Replacement of Saccharomyces cerevisiae Proteasome Genes with Human Orthologs by an Integrative Targeting Method.

Artificial induction of a chromosomal double-strand break in Saccharomyces cerevisiae enhances the frequency of integration of homologous DNA fragments into the broken region by up to several orders of magnitude. The process of homologous repair can be exploited to integrate, in principle, any foreign DNA into a target site, provided the introduced DNA is flanked at both the 5′ and 3′ ends by sequences homologous to the region surrounding the double-strand break. I have developed tools to precisely direct double-strand breaks to chromosomal target sites with the meganuclease I-SceI and select integration events at those sites. The method is validated in two different applications. First, the introduction of site-specific single-nucleotide phosphorylation site mutations into the S. cerevisiae gene SPO12. Second, the precise chromosomal replacement of eleven S. cerevisiae proteasome genes with their human orthologs. Placing the human genes under S. cerevisiae transcriptional control allowed us to update our understanding of cross-species functional gene replacement. This experience suggests that using native promoters may be a useful general strategy for the coordinated expression of foreign genes in S. cerevisiae. I provide an integrative targeting tool set that will facilitate a variety of precision genome engineering applications.

A comparison of marine Fe and Mn cycling: U.S. GEOTRACES GN01 Western Arctic case study

Dissolved iron (Fe) and manganese (Mn) share common sources and sinks in the global ocean. However, Fe and Mn also have different redox reactivity and speciation that can cause their distributions to become decoupled. The Arctic Ocean provides a unique opportunity to compare Fe and Mn distributions because the wide Arctic continental shelves provide significant margin fluxes of both elements, yet in situ vertical regeneration inputs that can complicate scavenging calculations are negligible under the ice of the Arctic Ocean, making it easier to interpret the fate of lateral gradients. We present here a large-scale case study demonstrating a three-step mechanism for Fe and Mn decoupling in the upper 400 m of the Western Arctic Ocean. Both Fe and Mn are released during diagenesis in porewaters of the Chukchi Shelf, but they become immediately decoupled when Fe is much more rapidly oxidized and re-precipitated than Mn in the oxic Chukchi Shelf water column, leading to Fe hosted primarily in the particulate phase and Mn in the dissolved phase. However, as these shelf fluxes are transported toward the shelf break and subducted into the subsurface halocline water mass, the loss rates of all species change significantly, causing further Fe and Mn decoupling. In the second decoupling step in the shelf break region, the dominant shelf species are removed rapidly via particle scavenging, with smallest soluble Fe (sFe < 0.02 µm) being least subject to loss, while colloidal Fe (0.02 µm < cFe < 0.2 µm), dissolved Mn (dMn), and non-lithogenic particulate Fe (pFexs) are all lost at similarly rapid rates. In the third decoupling step, once these species are swept >1000 km offshore with the prevailing current into the low-particle waters of the open Arctic, cFe and dMn appear conserved, while pFe, dFe, and sFe are very slowly removed with variable log-scale distances of transport: pFe ≪ dFe < sFe. To assess the role of physicochemical speciation on these trends, we observed that Fe(II) was a small (∼7%) fraction of total dFe in the upper 400 m of the Arctic, even over the shelf (∼2%). Also, colloidal contribution to dFe was very low (∼20%) in the open Arctic, in contrast to dFe in the North Atlantic, which is composed much more by colloids (≥50%). Throughout the Western Arctic Ocean, Fe and Mn are thus decoupled as a result of distinct oxidation kinetics and different scavenging rates within high- and low-particle regimes. As the “scavengers of the sea”, the relative distribution of particulate Fe and Mn phases across the Arctic Ocean shelf and slope, respectively, will play an important role in determining the distribution and ultimate sediment burial site for other scavenging-prone trace elements. Additionally, we suggest that the future effects of climate change, including loss of sea ice that could impact the formation of the halocline, might change distributions of Fe and Mn species in the future Western Arctic.

Search for correlation-induced adiabatic paths between distinct topological insulators

Correlations in topological states of matter provide a rich phenomenology, including a reduction in the topological classification of the interacting system compared to its non-interacting counterpart. This happens when two phases that are topologically distinct on the non-interacting level become adiabatically connected once interactions are included. We use a quantum Monte Carlo method to study such a reduction. We consider a 2D charge-conserving analog of the Levin-Gu superconductor whose classification is reduced from $\mathbb{Z}$ to $\mathbb{Z}_4$. We may expect any symmetry-preserving interaction that leads to a symmetric gapped ground state at strong coupling, and consequently a gapped symmetric surface, to be sufficient for such reduction. Here, we provide a counter example by considering an interaction which (i) leads to a symmetric gapped ground state at sufficient strength and (ii) does not allow for any adiabatic path connecting the trivial phase to the topological phase with $w=4$. The latter is established by numerically mapping the phase diagram as a function of the interaction strength and a parameter tuning the topological invariant. Instead of the adiabatic connection, the system exhibits an extended region of spontaneous symmetry breaking separating the topological sectors. Frustration reduces the size of this long-range ordered region until it gives way to a first order phase transition. Within the investigated range of parameters, there is no adiabatic path deforming the formerly distinct free fermion states into each other. We conclude that an interaction which trivializes the surface of a gapped topological phase is necessary but not sufficient to establish an adiabatic path within the reduced classification. In other words, the class of interactions which trivializes the surface is different from the class which establishes an adiabatic connection in the bulk.

Specificity of end resection pathways for double-strand break regions containing ribonucleotides and base lesions

DNA double-strand break repair by homologous recombination begins with nucleolytic resection of the 5’ DNA strand at the break ends. Long-range resection is catalyzed by EXO1 and BLM-DNA2, which likely have to navigate through ribonucleotides and damaged bases. Here, we show that a short stretch of ribonucleotides at the 5’ terminus stimulates resection by EXO1. Ribonucleotides within a 5’ flap are resistant to cleavage by DNA2, and extended RNA:DNA hybrids inhibit both strand separation by BLM and resection by EXO1. Moreover, 8-oxo-guanine impedes EXO1 but enhances resection by BLM-DNA2, and an apurinic/apyrimidinic site stimulates resection by BLM-DNA2 and DNA strand unwinding by BLM. Accordingly, depletion of OGG1 or APE1 leads to greater dependence of DNA resection on DNA2. Importantly, RNase H2A deficiency impairs resection overall, which we attribute to the accumulation of long RNA:DNA hybrids at DNA ends. Our results help explain why eukaryotic cells possess multiple resection nucleases.

Distribution and genetic variability of young-of-the-year greater amberjack (Seriola dumerili) in the East China Sea

We examined the distribution and genetic variability of the greater amberjack Seriola dumerili in the East China Sea (ECS) from the mitochondrial DNA (mtDNA) control region of young-of-the-year (YOY) individuals, which may reflect the genetic characteristics of the spawning population. In the years 2016 and 2017, a total of 165 YOY S. dumerili were collected from two different locations in the ECS: the southern part of the ECS, and western Japanese coast. The spawning period was estimated using otolith daily increments from a total of 67 larvae and juveniles collected in the ECS. Results from size and age distribution indicated that S. dumerili spawned near the shelf break region in the southern ECS from January to April. Phylogenetic analysis based on mtDNA sequence data showed no significant genetic differentiation among samples collected from different locations, implying S. dumerili in the ECS is mainly composed of one population. S. dumerili around the Japanese coast may be originated from the southern ECS.

Electromigration Study of Plasma Etched Copper Lines with Copper Oxide Capping Layers

Copper (Cu) is a widely used interconnection material in integrated circuits (ICs) and large area thin film transistor (TFT) arrays. Chemical mechanical polishing (CMP) is the most popular production method in etching Cu fine lines [1,2]. Cu cannot be etched with a conventional plasma etching process at the room temperature because of the low volatility of copper halides [3,4]. Kuo’s group invented a plasma-based Cu etch method that includes two steps: the conversion of Cu into CuClx or CuBrx with a Cl- or Br-containing plasma and the subsequent removal of the halide by dipping in a dilute HCl solution [5-7]. The complete process, which can be carried out at room temperature, has been used to fabricate BiCMOS ICs and TFT LCDs [8,9]. The reliability of the Cu line can be evaluated using the electromigration (EM) method [10]. EM tests on Cu lines with different capping layers (e.g., TiW, Mo), which could prevent Cu oxidation, have been reported [11-13]. In this study, the influence of the copper oxide (CuOx) capping layer on the EM reliability is studied.The Mo/Cu stack was sputter deposited on a Corning glass substrate and etched into a 4-point pattern by the plasma-based etching method in a parallel-plate RIE reactor (PlasmaTherm 700C) at room temperature. The Cu chlorination reaction was conducted in CF4/HCl 5/20 sccm at 70 mTorr and 600 W for 2 minutes. Then, the CuClx layer was dissolved in a H2O:HCl (8:1 v/v) solution for 1 min. The Mo barrier layer was etched in CF4/O2 10/10 sccm at 60 mTorr and 600 W for 2 minutes. In order to form the CuOx capping layer, the Cu line was exposed to the O2 plasma at 200 mTorr and 100W for 3 minutes in the same PlasmaTherma reactor but under the plasma etching mode. For the comparison purpose, a control sample, i.e., the Mo/Cu line without the CuOx capping layer, was prepared and EM stressed. All EM tests were done at room temperature under the fixed current densities (J) condition.Figure 1 shows the resistance-time (R-t) and temperature-time (T-t) curves of Mo/Cu/CuOx lines stressed at different current densities. The resistance increases slightly with the stress time and jumps sharply at the line broken point. Also, the line broken time shortens with the increase of the current density. The line broken process starts from voids formation at grain boundaries followed by voids merging [11-13]. The T-t curves were calculated by the JEDEC standard guidelines of JESD33-B [14] modified with an iteration method. The change of the T-t curve follows that of the R-t curve since the temperature increase is caused by Joule heating. The extent of heat dissipation is dependent on the heat dissipation rate of the CuOx capping layer of which the composition changes during the EM stress.Figure 2 shows top views of a Mo/Cu/CuOx line at different stress times under J = 0.41×106 A/cm2. The original line has the aquamarine color. The color becomes lighter when stressed. Eventually, the broken region became dark. The line color change is a good indication of the change of the composition of the passivation layer, which is temperature dependent.The detailed EM failure mechanism with the CuOx composition change will be presented. Advantages of the CuOx passivation layer will also be discussed. Authors acknowledge the financial support of this work through NSF CMMI project 1633580. 1. A. Gladkikh and Y. Lereah, APL, 66(10), 1214-1215 (1995).2. C. S. Hau-Riege, et al., JAP, 96(10), 5792-5796 (2004).3. Y. Kuo and J.R. Crowe, JVST A, 8(3), 1529-1532 (1990).4. H. Miyazaki, et al., JVST B, 15(2), 237-240 (1997).5. Y. Kuo and S. Lee, JJAP, 39(3A), L188 (2000).6. S. Lee and Y. Kuo, JES, 148(9), G524-G529 (2001).7. Y. Kuo and S. Lee, APL, 78(7), 1002-1004 (2001).8. Y. Kuo and S. Lee, Vacuum, 74(3-4), 473-477 (2004).9. J. Yang, et al., ECS Trans., 16(9), 13 (2008).10. C. S. Hau-Riege, Microelectronics Reliability, 44(2), 195-205 (2004).11. M. Li, J. Q. Su and Y. Kuo, ECS Trans., 89(3), 87-92 (2019).12. J. Q. Su, M. Li and Y. Kuo, ECS Trans., 90(1), 65-72 (2019).13. J. Q. Su, M. Li, Y. Kuo and S. Hamaguchi, ECS Trans., 92(5), 39-46 (2019).14. EIA/JESD33-B, Standard Method of Measuring and Using the Temperature Coefficient of Resistance to Determine the Temperature of a Metallization Line (2004). Figure 1

Nutrients and particulate organic matter dynamics in the outer-shelf of the South Brazil Bight: Two distinct scenarios during summer 2013

The pool of particulate organic matter driven by primary production in western boundary continental margins is known to be smaller than the highly eutrophic upwelling systems of eastern margins. Its production and export, however, can be high compared to productive ocean environments due to inherent specific physical dynamics such as mesoscale instabilities driven by the boundary current. To investigate how such physical process influence nutrients, chlorophyll-a and particulate matter on the outer shelf of the South Brazil Bight, two oceanographic cruises were conducted three months apart (November 2012 and January 2013), both under wind conditions that favor intrusions of South Atlantic Central Water (SACW). Our results showed two distinct hydrographic scenarios derived by the relative positions of oceanographic fronts in the outer shelf and shelf break region, associated with the Brazil Current (BC). In November 2012, the oligotrophic Tropical Water occupied a larger area of the continental shelf. Low nutrients and chlorophyll-a concentrations were found, typical of a regenerated production system. In contrast, maximum concentration of particulate matter and particulate inorganic carbon occurred at deeper layers in this period, together with low POC:PON ratio — probably from sediment resuspension. In January 2013, a strong shelf break upwelling was detected that enhanced SACW intrusion, notably increasing nutrient availability in the euphotic zone and the chlorophyll-a concentration at deeper layers. These deep chlorophyll maximum layers contribute to the production and accumulation of particulate organic carbon and nitrogen under these conditions. BC mesoscale instabilities appear to act as a particulate matter transport and export agent to the deep waters throughout the South Brazil Bight outer shelf.

Suspended sand concentration models under breaking waves: Evaluation of new and existing formulations

A total of 7 reference concentration (C0) models (6 existing and 1 newly proposed) were validated against 119 test cases from 4 recently published datasets collected under the LIP, CROSSTEX, SandT-Pro and SINBAD experimental studies. These models were evaluated for performance in different cross-shore regions: the shoaling zone, breaking (outer surf) zone and inner surf zone, under regular and irregular breaking wave conditions. In almost all existing C0 models, substantial under-prediction was found particularly around the wave plunging point (point at which breaking wave plunges and surface generated turbulent kinetic energy, TKE, is injected into the water column) where strong localised increases in C0 were observed. This strong increase in concentration was attributed to the large-scale breaking-generated turbulent vortices invading the wave bottom boundary layer (WBBL) and entraining dense clouds of sediment near the plunging point. Models that were directly or indirectly driven by local wave climate such as the local wave height (H), breaker height (Hb) or local water depth (d), were found to perform quite poorly in the breaking region under regular and irregular plunging breaker waves. Formulations that related C0 to the sand pickup rate (i.e. depending on exerted bed shear exceeding critical bed shear for entrainment) were adept in regions unaffected by external breaking-induced TKE (e.g. the shoaling zone) but could not account for the high levels of concentration observed at the plunging point. This is because these formulations were based on the implicit assumption that sediment entrainment is only induced by the local TKE generated by bed shear; not taking surface-generated breaking-induced TKE into account. This assumption was addressed in more recent studies, by including breaking-induced TKE into sediment pickup rate or reference concentration formulations. Though latest studies have shown promising relationships between near-bed TKE (kb) and reference concentration/sediment pickup, such formulations also face inherent limitations. These formulations are highly dependent on the accuracy of measured or modelled kb and are also sensitive to the magnitude of kb. For example, the magnitude of measured kb was found to vary by a factor of 1.1–1.3 between regular and irregular wave conditions, with kb being smaller under irregular wave conditions. This resulted in varied performance between datasets in kb-driven reference concentration formulations. The Froude-scaled TKE produced smaller deviations in magnitude of TKE between datasets, suggesting that it may be a more suitable driving parameter for reference concentration models than kb. A new reference concentration model, L19, was empirically derived from an inverse relationship observed between d and C0, and from the roller energy dissipation rate. The newly proposed L19 model shows good agreement with measured C0 (with RMSE ranging between 0.36 and 1.79 kg/m3 over the different datasets) in regular and irregular wave conditions, even at the plunging point where concentration is highest. The modified concentration profile [C(z)] equation also performs well, generally capturing the vertical concentration profile accurately throughout the whole water column.

Structure and dynamics of undercurrents in the western boundary current of the Bay of Bengal

The structure and variability of undercurrents in the East India Coastal Current (EICC), which is the western boundary current system in the Bay of Bengal (BoB), and the mechanisms of their formation are examined in this study. We used current data collected by Acoustic Doppler current profilers (ADCP) moored off Cuddalore (~ 12oN), Kakinada (~ 16.5oN), Visakhapatnam (~ 17.7oN), and Gopalpur (~ 19.4oN) and simulations for the period 2013–2014 from a high-resolution model configured for the BoB. The undercurrents were observed at all these locations, mainly during summer (June–August) and winter (October–December). Undercurrents were seen at relatively shallow depths (75 m), and their occurrences were more frequent off Cuddalore, whereas they were deep (100–150 m) and less frequent in the northern part of the east coast (off Visakhapatnam and Gopalpur). Numerical simulations showed that the interaction of the westward propagating anticyclonic eddies with the equatorward EICC weakened the strong surface flow and reversed the weak subsurface flow in the northern part of the western BoB. This interaction resulted in the formation of the poleward undercurrent here. Once these mesoscale eddies dissipated due to the interaction with the continental slope, the poleward undercurrents vanished and equatorward flow in the subsurface reappeared. The observed undercurrents near the shelf break region (75–200 m) in the southern part of the coast (off Cuddalore) were associated with small subsurface eddies (diameter of about 20–30 km), which developed due to large zonal gradient in the alongshore component of EICC. Subsurface anticyclonic circulations of larger spatial extent (diameter > 200 km) were responsible for the observed undercurrents in the deeper levels (deeper than 250 m) off Cuddalore. We further show that intraseasonal variability of undercurrents near the shelf break off Cuddalore was directly linked to intraseasonal variability in the strength of surface EICC itself. Results from this study suggest that the undercurrents observed below the EICC were not continuous poleward flow, but they were part of distinct anticyclonic eddies.