Strong Currents Research Articles

Phytoplankton retention mechanisms in estuaries: a case study of the Elbe estuary

Abstract. Due to their role as primary producers, phytoplankton are essential to the productivity of estuarine ecosystems. However, it is important to understand how these nearly passive organisms are able to persist within estuaries when river inflow results in a net outflow to the ocean. Estuaries also represent challenging habitats due to a strong salinity gradient. Little is known about how phytoplankton are able to be retained within estuaries. We present a new individual-based Lagrangian model of the Elbe estuary which examines possible retention mechanisms for phytoplankton. Specifically, we investigated how reproduction, sinking and rising, and diel vertical migration may allow populations to persist within the estuary. We find that vertical migration, especially rising, favors retention, while fast sinking does not. We further provide first estimates of outwashing losses. Our simulations illustrate that riverbanks and tidal flats are essential for the long-term survival of phytoplankton populations, as they provide refuges from strong downstream currents. These results contribute to the understanding needed to advance the ecosystem-based management of estuaries.

Internal solitary waves observation and feature extraction based on wavelet transform by Sentinel-1A in Lombok Strait, Indonesia

Lombok Strait is an essential pathway in trans-oceanic water mass transport. Due to this water mass transport flowing over Indonesian waters, the current is known as Indonesian Throughflow (ITF), which plays a role in the transfer of warm water masses from the Pacific to the Indian Ocean annually. Lombok Strait has intensive characteristics of the internal waves (IWs) generation because of the strong current that passes through the complex bathymetry along the strait area. IWs with large amplitude and nonlinear properties are known as Internal Solitary Waves (ISW) that can be detected by Synthetic Aperture Radar (SAR) images. A Wavelet Transform method for ISW feature extraction was applied to SAR images by Sentinel-1A (C-band). ISW packet characteristics can be distinguished from other phenomena based on their geometrical structure and shape. ISW packet pattern consists of light and dark lines which decrease intensity from front to rear. SAR observation detected 5 parameters (phase speed, soliton numbers, wavelength, first crest length, and propagation direction). The arc-like ISW in the Lombok Strait propagated to the North of the sill with average phase speeds of about 2.13 m/s and was frequently detected during the northwest monsoon (NWM). The detected soliton number is less than 6 solitons per packet with a wavelength of about 1 – 4 km, and the first crest length varies from about 12.16 km to more than 100 km. ISW detected in Sentinel-1A images were located at the bathymetry about 800 meters around the Lombok Strait area.

Seismic stratigraphy and sediment cores reveal lake-level fluctuations in Lake Iznik (NW Turkey) over the past ∼70 ka

Our study aims to understand the palaeohydrological history of Lake Iznik and unravel the complex interplay between climatic, tectonic, and environmental factors that have shaped this Turkish basin. Through the analysis of seismic stratigraphy and sediment cores, we reveal a significant lowstand, indicating a lake level 60 m lower than today at ∼70 ka BP. Subsequently, a major phase of stepwise transgression is evidenced by 13 buried palaeoshorelines between ∼70 and 45 ka BP. From 45 to ∼10 ka cal BP, strong currents controlled the sedimentation in the lake, as evidenced by the occurrence of contourite drifts. Between ∼14 and 10 ka cal. BP, a major lowstand indicating a drier climate interrupted the current-controlled sedimentation regime. From ∼10 ka cal. BP, the subsequent increase in lake level occurred at the same time as the reconnection between the Mediterranean and Black seas. Archaeological evidence, including submerged structures of a basilica, establishes a link between lake-level changes and human settlement during the last millennium. The level of Lake Iznik has since continued to fluctuate due to climate change, tectonic events, and human activity.

Diel vertical migration and tidal influences on plankton densities in dynamic coastal systems

Recent increased application of optical imaging devices have facilitated efficient capture of plankton abundance and community composition, enabling the study of plankton distribution in situ and at a high spatio-temporal resolution. In this study, we aim to investigate how the abundances and distribution patterns of plankton taxa relate over 24-h periods, covering tidal and diel cycles, in the southern North Sea using data from a WP2 net and a Video Plankton Recorder. In the highly dynamic southern North Sea, we document diel vertical migration patterns in the pelagic zone of both pelagic and hyperbenthic taxa, including Calanoida (Copepoda), Amphipoda, Annelida, and Cumacea. In addition, the densities of plankton taxa showed significant small-scale geographical variation over a 24-h period for which tidal currents played an important role, a source of considerable variation that is typically not accounted for. This study adds to the current understanding of plankton distribution and behaviour, particularly in the context of coastal areas characterised by strong tidal cycles and currents, by using in situ imaging techniques.

Strong philopatry in an estuarine-dependent fish.

Understanding fish movement is critical in determining the spatial scales in which to appropriately manage wild populations. Genetic markers provide a natural tagging approach to assess the degree of gene flow and population connectivity across a species distribution. We investigated the genetic structure of black bream Acanthopagrus butcheri across its entire distribution range in Australia, as well as regional scale gene flow across south-eastern Australia by undertaking a comprehensive analysis of the populations in estuaries across the region. We applied genome-wide sampling of single-nucleotide polymorphism (SNP) markers generated from restriction site-associated DNA sequencing. Genetic structure and potential gene flow was assessed using principal component analyses and admixture analyses (STRUCTURE). Using 33,493 SNPs, we detected broad scale genetic structuring, with limited gene flow among regional clusters (i.e. Western Australia, South Australia and western Victoria; and eastern Victoria, Tasmania and New South Wales). This is likely the result of unsuitable habitats, strong ocean currents (e.g. the Leeuwin Current and the East Australian Current), large water bodies (e.g. Bass Strait) and known biogeographical provinces across the continent. Local-scale genetic structuring was also identified across the south-eastern Australian estuaries sampled, reflecting that the coexistence of both migratory and resident individuals within populations (i.e. partial migration), and the movement of fish into coastal waters, still results in strong philopatry across the region. Instances of movement among estuaries at this spatial scale were primarily found between adjacent estuaries and were likely attributed to lone migrants utilising inshore coastal currents for movement beyond nearby habitats. Targeting SNP markers in A. butcheri at this continental scale highlighted how neither spatial proximity of estuaries nor black bream's ability to move into coastal waters reflects increased gene flow. Overall, our findings highlight the importance of location-specific management.

Numerical research on internal solitary waves in stratified fluid

Internal solitary waves are a common nonlinear large amplitude internal wave in nature, which can cause strong divergence and sudden strong currents in seawater during propagation, thus having great destructive effects on marine structures. The paper focuses on numerical wave generation based on three types of internal solitary wave theories. The waves are numerical simulated by RANS method in the paper. The numerical results of internal solitary wave theories are compared and analyzed. It shows that under the same numerical water tank scale and calculation conditions, the mKdV theory can obtain the largest wave amplitude, followed by the eKdV theory and the KdV theory. In addition, background currents will accelerate the propagation speed of internal solitary waves and increase their amplitude slightly.

Zoning Patterns of Mangrove Forests in Lampung Timur Regency

Mangrove forests form due to vegetative distribution along coastlines with varying growing conditions concerning substrate, freshwater intake, and oceanic currents. These disparities facilitate zoning and subsequent distinct growth patterns. The research objective was to identify the zoning structure of mangrove ecosystems in Lampung Timur Regency, Lampung Province, Indonesia. The degradation of mangrove forests in Lampung Timur Regency was thought to be caused by anthropogenic activities and inappropriate zoning patterns. Concerning data collection, the cluster sampling method was employed, which was selected based on the preliminary study's findings that the mangrove status in the study area exhibits a variety of substrates, freshwater supply, and strong ocean currents. There are five clusters on the study site, with each cluster containing sample plots measuring 10m x 10m. The sampling method employed a striped path starting from the outermost part of the mangrove ecosystem, with the number of plots in each cluster dependent on the thickness of the mangrove vegetation. A regional map and GIS applications were used for mangrove zonation mapping, and the factors influencing the zonation of mangrove ecosystems were analyzed descriptively. The research findings demonstrated that the mangrove ecosystem located in Lampung Timur comprises three distinct zones: the outer zone, the middle zone, and the terrestrial zone

Engineering design and economic analysis of offshore seaweed farm

As global demand for sustainable biomass and need to mitigate global warming begin to rise, cultivation of seaweed has gained significant attention in recent years due to its potential for carbon recycling. However, limited availability of suitable coastal areas for large-scale seaweed cultivation has led to exploration of offshore environments as a viable alternative. The nature of many offshore environments often exposes seaweed farming systems to harsh environmental conditions, including strong waves, currents, and wind. These factors can lead to structural failures, kelp losses, and significant financial losses for seaweed farmers. The main objective of this study is to present a robust design and numerical analysis of an economically viable floating offshore kelp farm facility, and evaluate its stability and mooring system performance. A numerical method of preliminary designs of the offshore aquaculture systems were developed using the OrcaFlex software. The models were subjected to a series of dynamic environmental loading scenarios representing extreme events. These simulations aimed to forecast the overall dynamic response of an offshore kelp farm at a depth of 50m and to determine the best possible farm design with structural integrity for a selected offshore environment. Furthermore, to assess the economic feasibility of establishing offshore seaweed farms, a comprehensive capital expenses analysis was conducted. The results revealed that, in terms of the kelp farms with the same number of the kelp cultivating lines, the cost of building kelp farms will be strongly affected by the cost of mooring lines. The present study may help to understand the dynamic response and economic feasibility of offshore kelp farms.

Semidiurnal Internal Tides in a Shelf Sea South of Japan: Characteristics, Energetics, and Temporal variations

Tidal mixing in a shelf sea south of Japan (Bungo Channel) plays an important role in modulating the water exchange between the Seto Inland Sea and Pacific Ocean. In this study, based on moored observations and model results from the Japan Coastal Ocean Predictability Experiment—Tides (JCOPE-T), the generation, propagation, and dissipation of semidiurnal internal tides in the Bungo Channel are investigated. Observational results indicate that semidiurnal internal tides induce strong baroclinic currents reaching 0.3 m/s. Their energy shows obvious spring-neap tidal cycles, generally coinciding with the local barotropic tidal forcing. By conducting the empirical orthogonal function analysis, we find that the observed semidiurnal internal tides are mainly dominated by the first two baroclinic modes. The JCOPE-T results suggest two main generation sites for semidiurnal internal tides in the region: one is located at a narrow strait north of the Bungo Channel, while the other is at the shelf break south of the Bungo Channel. The latter makes a major contribution to the observed semidiurnal internal tides. Northward internal tides generated at the shelf break are superposed with those generated at the narrow strait, causing a complex interference pattern in the channel. The temporal variation of semidiurnal internal tides in the Bungo Channel is affected by several factors. The intraseasonal variation of semidiurnal internal tides can be modulated by the Kuroshio warm water intrusion (Kyucho) because the occurrence of Kyucho changes the stratification in the channel and hence affects the energy conversion. The seasonal variation of semidiurnal internal tides in the Bungo Channel is determined mainly by the seasonally varying stratification; while those generated at the shelf break are under the combined influence of seasonal stratification and background currents. Southward internal tides from the shelf break are refracted due to the spatially varying stratification and background currents. The varying Kuroshio path and strength modulate the refraction of internal tides.

Origin, evolution and significance of giant buried sediment mounds near the Sahara Slide Complex, North‐west African margin

AbstractMixed turbidite–contourite depositional systems result from interactions between down‐slope turbidity currents and along‐slope bottom currents, comprising excellent records of past oceanographic currents. Modern and ancient systems have been widely documented along the continental margins of the Atlantic Ocean. Yet, few examples have so far been identified on the North‐west African continental margin, limiting understanding of the sedimentary and palaeoceanographic evolution in this area. This work uses two‐dimensional seismic reflection profiles to report, for the first time, the presence of three giant sediment mounds beneath the headwall region of the Sahara Slide Complex. The sediment mounds are elongated and separated by two broad canyons, showing a north‐west/south‐east orientation that is roughly perpendicular to the continental margin. These mounds are 24 to 37 km long and 12 to 17 km wide, reaching a maximum height of ca 1000 m. Numerous slide scarps are observed within and along the flanks of the mounds, hinting at the occurrence of submarine landslides during their development. Based on their geometries, external shapes, internal seismic architecture and stratigraphic stacking patterns, it is proposed that these sediment mounds comprise down‐slope elongated mounded drifts formed in a mixed turbidite–contourite system during four evolutionary stages: onset, growth, maintenance and burial. The significance of this work is that it demonstrates the gradual transition from a turbidite system to a full mixed turbidite–contourite system to be associated, in the study area, with the establishment of strong ocean currents along north‐west Africa.

What Causes the Subsurface Velocity Maximum of the East Australian Current?

Abstract The East Australian Current (EAC) system includes a poleward jet that flows adjacent to the continental shelf, a southward and eastward extension, and a complex eddy field. The EAC jet is often observed to be subsurface intensified. Here, we explain that there are two factors that cause the EAC to develop a subsurface maximum. First, the EAC flows as a narrow current, carrying low-density water from the Coral Sea into the denser waters of the Tasman Sea. This results in horizontal density gradients with a different sign on either side of the jet, negative onshore and positive offshore. According to the thermal wind relation, this produces vertical gradients in southward current that are surface intensified onshore and subsurface intensified offshore. Second, we show that the winds over the shelf are mostly downwelling favorable, drawing the surface EAC waters onshore. This aligns the region of positive horizontal density gradients with the EAC core, producing a subsurface velocity maximum. The presence of a subsurface maximum may produce baroclinic instabilities that play a role in eddy formation and EAC separation from the coast. Significance Statement Observations of the East Australian Current (EAC) show that the strongest currents are often below the surface at about 100-m depth. Two factors cause this subsurface maximum. First, because the EAC is a narrow jet, carrying warm water southward from the Coral Sea, the density gradient across the jet changes sign, causing surface-intensified currents onshore and subsurface-intensified currents offshore. Second, the wind field over the shelf often pulls the shallow waters shoreward, shifting the waters that cause subsurface intensification to align with the center of the jet, resulting in a subsurface maximum of the EAC. This process may be responsible for the generation of eddies in the Tasman Sea.

Reconnection along a Separator in Shock Turbulence

Numerous structures conducive to magnetic reconnection are frequently observed in the turbulent regions at quasi-parallel shocks. In this work, we use a particle-in-cell simulation to study 3D magnetic reconnection in shock turbulence. We identify and characterize magnetic null points, and focus on reconnection along the separator between them. We identify a reconnection region with strong parallel current, a finite parallel potential, and counterrotating electron flows. Electrons are shown to be accelerated by the parallel electric field before being scattered at the null.

The Influence of Freshwater Input on the Evolution of the 1995 Benguela Niño

AbstractBenguela Niño events are characterized by strong warm sea surface temperature (SST) anomalies off the Angolan and Namibian coasts. In 1995, the strongest event in the satellite era took place, impacting fish availability in both Angolan and Namibian waters. In this study, we use direct observations, satellite data, and reanalysis products to investigate the impact that the up‐until‐now unnoticed mechanism of freshwater input from Congo River discharge (CRD) and precipitation had on the evolution of the 1995 Benguela Niño. In the onset phase of the event, anomalous rainfall in November/December 1994 at around 6°S, combined with a high CRD, generated a low salinity plume. The plume was advected into the Angola‐Namibia region in the following February/March 1995 by an anomalously strong poleward surface current generated by the relaxation of the southerly winds and shifts in the coastal wind stress curl. The presence of this low surface salinity anomaly of about −2 psu increased ocean stability by generating barrier layers, thereby reducing the turbulent heat loss, since turbulent mixing acted on a weak vertical temperature gradient. A mixed layer heat budget analysis demonstrates that southward advection of Angolan waters drove the warming at the onset, while reduced mixing played the main role at the event's peak. We conclude that a freshwater input contributed to the SST increase in this exceptionally strong event and suggest that this input can influence the SST variability in Angola‐Namibia waters through a combination of high CRD, precipitation, and the presence of a strong poleward surface current.

Emergence of threefold symmetric helical photocurrents in epitaxial low twinned Bi2Se3

We present evidence of a strong circular photon drag effect (PDE) in topological insulators (TIs) through the observation of helicity-dependent topological photocurrents with threefold rotational symmetry using THz spectroscopy in epitaxially-grown Bi2Se3 with reduced crystallographic twinning. We establish how twinned domains introduce competing nonlinear optical (NLO) responses inherent to the crystal structure that obscure geometry-sensitive optical processes through the introduction of a spurious mirror symmetry. Minimizing the twinning defect reveals strong NLO response currents whose magnitude and direction depend on the alignment of the excitation to the crystal axes and follow the threefold rotational symmetry of the crystal. Notably, photocurrents arising from helical light reverse direction for left/right circular polarizations and maintain a strong azimuthal dependence-a result uniquely attributable to the circular PDE, where the photon momentum acts as an applied in-plane field stationary in the laboratory frame. Our results demonstrate new levels of control over the magnitude and direction of photocurrents in TIs and that the study of single-domain films is crucial to reveal hidden phenomena that couple topological order and crystal symmetries.

Influences of tides on shelf circulation in the northeastern South China Sea during summer

Tides have significant effects on material and energy transports over the continental shelf. A mooring system located in the shelf region of the northeastern South China Sea observed strong and weak mean currents during the spring and neap tides in summer, respectively. The mechanism leading to this difference is investigated by using a three-dimensional circulation model. We find that the impact of tides (TI) on currents exhibited as current components opposing wind-driven circulation play an important role. The TI current components are stronger than the tidal residual currents in summer when vertical stratification enhances the tidal rectification. Tides can affect pressure gradient by modifying sea surface height on the shelf, while in shallow and shelf break regions, the non-linear advection and vertical diffusion become significant in momentum equations attributable to the interactions of tidal currents, bottom topography and water column stratification. Consequently, the along-shelf volume transport is reduced by about 21.6% due to the TI current components and only by about 6.1% due to the tidal residual currents. This study highlights the importance of both tidal mixing and tidal rectification in regulating shelf circulation in the northeastern South China Sea.

Archive for the East Australian Current: carbonate contourite depositional system on the Marion Plateau, Northeast Australia

The modern East Australian Current (EAC) sweeps across the Marion Plateau, forming a widespread contourite depositional system. The history of the EAC is recorded in the contourite drifts that have evolved through time as topographic, sea level, and oceanographic changes have influenced the current flow. During the Miocene, five isolated carbonate platforms on the Marion Plateau formed obstacles for the EAC, leading to the development of a complex contourite drift system in which drift types and non-depositional/erosional features transition laterally and temporally because of continuously evolving platform-current interactions. The significant topographic changes are caused by two episodes of platform drowning in the Middle and Late Miocene. We reinterpreted 2D seismic, well, and core data from ODP Legs 133 and 194 and updated the ages of the seismic sequences. The updated ages enable the reconstruction of the evolution of the EAC since the Miocene. The influence of the EAC in the study area began at 13.5 Ma, with five significant phases of change identified by shifts in contourite drifts' geometry and thickness. During the first phase (13.5 Ma – 11 Ma), strong currents carried sediment across platforms and deposited as periplatform drifts (Marion Drift B6), previously identified as a lowstand platform. In Phase 2 (11 Ma – 7 Ma), strong EAC caused the formation of Marion Drift C, characterized by thick, confined, and elongated mounded drifts separated by the wide moats and a marginal valley along the drowned Northern Marion Platform. At 7.5 Ma, a major re-arrangement of the contourite depositional system documents the influence of EAC across the drowned platforms. The subsequent Marion Drift D smoothed out some of the antecedent topography. However, the Marion Reef and Southern Marion Platform still presented obstacles to the EAC, leading to the deposition of multi-crested drifts. Phase 4 (5.7 Ma – 3.3 Ma) is marked by decreased sedimentation rates, possibly due to sea level fall at Mi-7 and the restriction of the Indonesian Throughflow. The youngest and currently active depositional phase began at around 3.3 Ma with a change in drift geometry from multi-crested to elongated mounded, coinciding with the onset of the Northern Hemisphere Glaciation and further reduction of the Indonesian Throughflow.

Formation mechanism of drift-moat contourite systems revealed by in-situ observations in the South China Sea

Contourite drifts are ubiquitous sedimentary features in the world′s oceans, and their formation are usually ascribed to sedimentation from contour currents. However, in-situ observations of contour currents and their associated sedimentary processes were inadequate. Here, we present mooring observation results from a drift-moat contourite system in the South China Sea to gain insight into its sediment dynamics and formation mechanism. We found that quasi-persistent contour currents develop in the moat, on and below the drift, yet subject to diverse physical oceanographic processes. High suspended sediment concentrations observed on the drift mainly occur in winter, induced by high levels of sediments transported by contour currents from Taiwan. In the moat, however, high suspended sediment concentrations are mostly caused by erosion of the moat wall during periods of strong tidal currents and low shear variance. Such results underscore the complexity of current patterns and sedimentary processes across various topographic units in the drift-moat contourite system. A novel formation mechanism of the contourite drift is thereby proposed, wherein sediment transport by contour currents brings materials for constructing the drift, while its formation is driven by near-critical reflection and upslope transport of sediments by tidal currents.

A novel python-based floating offshore wind turbine simulation framework

The expansion of floating offshore wind brings the industry closer to achieving commercial viability. However, the challenging environment characterised by strong winds, waves, and currents, along with the growing size of wind turbines and the dynamic behaviour of floaters, introduces concerns about power production efficiency and system durability due to increased fatigue loads, which subsequently impacts overall costs. In an attempt to mitigate the financial implications coming from alterations made to control strategies and structural elements during the initial design phase, this paper propounds an all-encompassing simulation framework for offshore wind turbines. The current study thoroughly explores the various capabilities of the tool, with a focus on its simulation models. Importantly, the paper highlights the complex interactions between tool models and different controllers. Carefully designed, this tool offers users a variety of functions to enhance system design, fine-tune control strategies, and thoroughly assess performance metrics. The paper elaborates on these aspects, providing an explanation of the tool's capabilities and enhancing the dynamic comparison between the models.

The Influence of Seamounts on the Enrichment of Rare Earth Elements in Sediments—A Case Study of the Marcus-Wake Seamounts in the Western Pacific Ocean

Deep-sea sediments enriched in rare earth elements and yttrium (REY-rich sediments) are widely distributed on the deep-sea floor, and their formation mechanism remains elusive. Although studies have recognized the link between seamounts and REY-rich sediments, in-depth analysis of the specific roles and effects of seamounts in the formation of REY-rich sediments is lacking. In this study, we analyzed surface sediments from the Marcus-Wake Seamounts for grain size, geochemistry, and mineral composition and classified the samples into three types: samples with moderate REY content and dominated by terrestrial detritus; samples with high REY and authigenic mineral content; and samples rich in CaCO3 but poor in REY. The REY in the sediments of the study area partly originate from Asian dust input and partly from seawater and/or pore water, and are mainly enriched in REY carrier particles including bioapatite fossils and micronodules. The amount of REY carrier particles influences the REY content in the sediments. The current field, primary productivity, weathering process, and depositional environment around seamounts are different from those of abyssal plains, which are conducive to the formation of REY-rich sediments. Strong bottom currents may exist in the southeastern direction of some large seamounts (e.g., Niulang Guyot), leading to the selective accumulation of REY-rich bioapatite fossils and micronodules, resulting in the formation of REY-rich sediments.

Study on the geomorphological changes of deep troughs under the influence of reclamation in the Caofeidian

Reclamation projects generally change the hydrodynamics and thus affect the topography of the seabed, which is the main threat to the safety of marine engineering. The Caofeidian Reclamation Project, the largest reclamation project in China to date, involved the reclamation of coastal regions of the Hebei Province. However, the project also created several issues and flaws due to the size of the reclaimed area. Through the comparative analysis in the Caofeidian sea area of multi-phase bathymetric topography, shallow stratigraphic changes, and numerical simulation of sediment dynamics, the characteristics of surface sediments and spatial distribution of erodibility were summarized. New marine geological hazards were discovered, such as submarine landslides and erosion depressions. It was concluded that after the reclamation of the Caofeidian, the south of the deep trough was still in an erosion state, and the erosion rate increased slightly. The shear stress formed by strong tidal currents in the deep trough was greater than the critical erosion shear stress of surface sediments, which caused the erosion transport of sediments. It was the primary dynamic mechanism leading to the long-term erosion state of the deep trough.