Eddy Formation Research Articles

Prediction of wave reflection for quarter circle breakwaters using soft computing techniques

The modified form of the semi-circular breakwater is called Quarter-Circle Breakwater (QBW). It consists of a quarter-circular surface facing incident waves, a horizontal bottom, a rear wall, and is built on a rubble mound foundation. In general, QCB may be constructed as emerged, with and without perforations that may be on one side or either side based on the coastal designer. These perforations dissipate the energy due to the formation of eddies and turbulence created inside the hollow chamber. In the present study, experimental data obtained from Binumol, 2017 are fed as input to both the models. This data is used to predict the reflection coefficient of QBW by adopting the ANN system approach. The reliability of the Artificial Neural Network (ANN) approach is done with statistical parameters, namely Model Performance Analysis (MPA) viz., Correlation Coefficient (CC), Root Mean Square Error (RMSE), Nash-Sutcliffe Efficiency (NSE), and Scatter Index (SI). The results of the MPA indicate that the ANN is suited for predicting the reflection coefficient of QBW.

A CFD Analysis of Decommissioned Oil Platforms Jackets on the Brazilian Coast

Every facility reaches the last phase in its life cycle, which is decommissioning. Since the last decade, this subject has been gaining importance in Brazil’s offshore oil and gas companies. For jacket type rigs, one of the methods widely applied after idling the production is the conversion of these structures into artificial reefs (ARs). There are several critical aspects for choosing the best strategy for cutting and sinking a platform jacket, ensuring the success of an AR from a biological point of view. One of them is the influence of marine currents and their fluid-structure interaction which, by maximizing local upwelling and back vortex effects, favours the growth of aggregated flora and fauna. This study consists in the application of computational fluid dynamics (CFD) techniques for studying the marine flow around a disassembled and sunk jacket in the seabed for the purpose of converting it into an artificial reef. An FVM (Finite Volume Method) from a commercial software (most recent version of ANSYS FLUENT®) is applied with the upwind scheme. A k-ε turbulence model on steady state is chosen. Field data about Brazilian coastal currents are collected and analysed from the amount of information available on a Brazilian Navy's meteoceanographic program. Next, different combinations of cutting and sinking a jacket are studied, always keeping a minimum 55m free water column. The objective is to verify where the formation of local upwelling regions - that is, where the vertical velocity component reaches values equal or greater than 10% of the magnitude of the free flow velocity - is more significant, without decreasing back eddy formation. It is observed that the dismemberment of the jacket with the positioning of its parts in an increasing height sequence in the direction of the prevailing current is favourable to generate local upwelling while tipping the structure at 90° to the prevailing current results in the most voluminous back eddy region.

Formation of an Intrathermocline Eddy Triggered by the Coastal‐Trapped Wave in the Northern Bay of Bengal

AbstractThe present study investigates the characteristics and mechanism of intrathermocline eddy (ITE) formation in the northern Bay of Bengal (BoB) using observations from Research Moored Array for African–Asian–Australian Monsoon Analysis and Prediction (RAMA) buoys moored at 15°N, 12°N, and 8°N along 90°E and simulations from a high‐resolution numerical model. Among the RAMA locations, a relatively large number of anticyclonic ITEs are observed in the northern parts (15°N) compared to the other two (12°N and 8°N). The observed ITEs are characterized by a double‐convex shape of isotherms in the thermocline with a vertical extent of about 100–200 m and a positive sea level anomaly in the surface. Water mass properties show that Andaman Sea (AS) water is trapped inside the core of the ITEs. Trajectories of these ITEs (especially at 15°N) suggest that most of them originated in the vicinity of Preparis Channel, the northernmost passage between BoB and AS. Our study shows that vorticity developed downstream of Preparis Channel due to the strong subsurface flow (∼100 cm/s), which carries AS water into the BoB, results in the formation of observed anticyclonic ITE in the northern BoB. Further analysis revealed that this strong subsurface flow through the Preparis Channel is caused by the propagation of downwelling coastal‐trapped waves along the boundaries of BoB, which originates from the equatorial Indian Ocean. Our study suggests that ITEs could be an important pathway of water mass distribution in the intermediate layers in this region.

Meanders of the West Greenland Current near Cape Farewell

The cyclonic boundary current that circumnavigates the Labrador Sea plays an important role in modulating the strength of wintertime convection in the interior basin, as well as restratifying the newly ventilated water in spring. Modeling studies indicate that meso and sub-mesoscale processes in the boundary current flux a significant amount of heat and freshwater offshore, although observations of this small-scale variability are lacking. Using four years of data from a mooring array west of Cape Farewell, Greenland, together with satellite altimetry and sea surface temperature measurements, we present the first observations of a meandering West Greenland Current. We describe the statistics, structure, characteristics, and formation mechanism of these features. The meanders occur roughly 30% of the time and are more prevalent in winter and early spring, with an increasing trend over the four-year record. It is shown that baroclinic instability of the boundary current is the cause of the meanders, triggered by seasonal steepening of the isopycnals between the interior basin and the boundary due to offshore convection. We argue that the meandering leads to the formation of small-scale eddies, and estimate the resulting seaward heat flux. Finally, possible connections between the meanders and the production of Irminger Rings are explored.

Role of mesoscale eddies in the sustenance of high biological productivity in North Eastern Arabian Sea during the winter-spring transition period

Convective mixing, mesoscale eddies and regenerated production sustain an above-average biological productivity in the North East Arabian Sea (NEAS) during the winter-spring transition period. Satellite-derived long-term data sets on Chlorophyll-a (Chl-a), Sea Surface Height Anomaly (SSHA), Sea Surface Temperature (SST) and Okubo-Weiss parameterization show existence of number of mesoscale eddies, propagating and non-propagating, that contribute to the regional production. The dominance of Eddy Kinetic Energy (EKE) over the Available Potential Energy (APE) in the core depth and the diameter (120km) of the observed eddy being wider than the Rossby Radius of Deformation (RRD, 55 km), it is suggested that the baroclinic instability is a possible mechanism for the eddy formation. Spatial variation in APE and its influences on the regional dynamics, including chemical and biological response are explained. In the non-eddy areas, where convective mixing is active, diatoms (96.74%) dominated than dinoflagellates (3.14%), and the Chl-a in the Cold Core Eddy (CCE) were two to three folds higher to non-eddy regions. The abundance increased from core (58,152 cells L−1) to periphery (5.95 × 105 cells L−1) where the water column is less dynamic. Extensive blooms of the dinoflagellate green Noctiluca (N. scintillans) contribute to the very high cell density in the periphery of the CCE, where the currents were comparatively weak, and water column was more stable. Active mixing is associated with diatom dominance, followed by Noctiluca when the mixing slackens, making use of the available nutrients and supported by regenerated production. The bloom dynamics is explained for pre-bloom, bloom and post-bloom conditions with measurements on nutrients and plankton assemblages. The Noctiluca bloom (mid-March) is succeeded by Trichodesmium (April–May), in the stratified nutrient depleted, abundant light environment and propagated southwards. Observed increasing trends in the SSHA over the period indicate strengthening of stratification and hence altered production patterns in the NEAS.

Analysis of Seasonal Energy Characteristics of the Marmara Sea Upper Layer Dynamics

Purpose. The aim of the work is to assess the role of the basic forces in formation of the dynamic structures of the upper layer in the Marmara Sea in different hydrological seasons. Methods and Results. The numerical model developed in Marine Hydrophysical Institute, Russian Academy of Sciences, was used to calculate the hydrodynamic and energy characteristics of the sea circulation. The horizontal spatial resolution was 1.22 × 0.83 km, 18 horizons were used vertically, and the time step was 0.5 min. The characteristics of the waters inflowing through the Bosporus and Dardanelles straits corresponded to the available observational data. On the sea surface, the daily average fields of tangential wind stress, heat fluxes, precipitation and evaporation for 2008 were preset; these parameters were calculated using the regional atmospheric model MM5. Spatial distributions of the vortex structures and the corresponding energy flows in the upper sea layer for different seasons were considered. Having been analyzed, the average seasonal spatial distributions of the current velocity fields and the components of the kinetic energy budget showed that during the year, the vortices in the upper sea layer were formed mainly by two mechanisms. In the central part of the sea, generation of a large-scale anticyclone and its seasonal variability are basically conditioned by the wind forcing, whereas formation and evolution of coastal cyclonic eddies are caused by the buoyancy force. In the fields where the buoyancy, pressure and friction forcing takes place, the zones of local extremes are distinguished. Two of them are the areas of water inflow through the Bosphorus and Dardanelles straits. The strongest variability is observed in the Bosporus region that is certainly conditioned by the fact that the inflowing Black Sea waters have a decisive influence on seasonal nature of the Marmara Sea circulation. The extreme values in the other zones are the result of the coastline structure heterogeneities, that leads to formation of the coastal eddies, the energy source for which is the available potential energy. Conclusions. Analysis of the results of the performed numerical experiment makes it possible to conclude that in the upper layer of the Marmara Sea, formation and variability of the central anticyclone are conditioned by the wind forcing, while those of the coastal cyclones – by the buoyancy effect.

Analiz sezonnykh energeticheskikh kharakteristik dinamiki verkhnego sloya vod Mramornogo morya

Purpose. The aim of the work is to assess the role of the basic forces in formation of the dynamic structures of the upper layer in the Marmara Sea in different hydrological seasons. Methods and Results. The numerical model developed in Marine Hydrophysical Institute, Russian Academy of Sciences, was used to calculate the hydrodynamic and energy characteristics of the sea circulation. The horizontal spatial resolution was 1.22 × 0.83 km, 18 horizons were used vertically, and the time step was 0.5 min. The characteristics of the waters inflowing through the Bosporus and Dardanelles straits corresponded to the available observational data. On the sea surface, the daily average fields of tangential wind stress, heat fluxes, precipitation and evaporation for 2008 were preset; these parameters were calculated using the regional atmospheric model MM5. Spatial distributions of the vortex structures and the corresponding energy flows in the upper sea layer for different seasons were considered. Having been analyzed, the average seasonal spatial distributions of the current velocity fields and the components of the kinetic energy budget showed that during the year, the vortices in the upper sea layer were formed mainly by two mechanisms. In the central part of the sea, generation of a large-scale anticyclone and its seasonal variability are basically conditioned by the wind forcing, whereas formation and evolution of coastal cyclonic eddies are caused by the buoyancy force. In the fields where the buoyancy, pressure and friction forcing takes place, the zones of local extremes are distinguished. Two of them are the areas of water inflow through the Bosphorus and Dardanelles straits. The strongest variability is observed in the Bosporus region that is certainly conditioned by the fact that the inflowing Black Sea waters have a decisive influence on seasonal nature of the Marmara Sea circulation. The extreme values in the other zones are the result of the coastline structure heterogeneities, that leads to formation of the coastal eddies, the energy source for which is the available potential energy. Conclusions. Analysis of the results of the performed numerical experiment makes it possible to conclude that in the upper layer of the Marmara Sea, formation and variability of the central anticyclone are conditioned by the wind forcing, while those of the coastal cyclones – by the buoyancy effect

Raschet gidrofizicheskikh poley s assimilyatsiyey dannykh nablyudeniy v severnoy chasti Chernogo morya v osenniy sezon 2016 goda

The paper analyzes the results of calculation of the circulation and thermohaline water structure based on a three-dimensional nonlinear hydrodynamic model and hydrological data from a survey, carried out in the northern part of the Black Sea in autumn 2016. We used a high spatial resolution (horizontal grid ~1.6 × 1.6 km and 27 vertical layers) model and realistic atmospheric forcing. The assimilation procedure of observational data was based on a modified Kalman filter taking into account the heterogeneity and anisotropy of estimation error of the temperature and salinity fields. The calculated fields of currents were characterized by eddies and jet currents. The following circulation features were observed in the northern part of the Black Sea: an intense jet of the Black Sea Rim Current along the Crimean coast, anticyclonic eddies with a radius of about 30 km near Sevastopol and in the western part of the region, a cyclonic eddy with a radius of about 40 km between 34.5 and 35.5 E, anticyclonic eddies with a radius of about 25 km along the Crimean coast. Eddy formations of different scales and different signs of rotation were obtained (in the Kalamitsky Bay, in the eastern part and along the eastern coast of the Crimea), when the current flowed around irregularities of the coastline and the bottom relief in the coastal zone. The correlation between the formation of zones of lower and higher temperature relative to adjacent waters and the location of features in the field of current (in particular, the correspondence of zones with warmer desalinated water to the anticyclonic formations) was noted from the analysis of the reconstructed thermohaline fields.

Assessment of New High-resolution Regional Climatology in the East/Japan Sea

This study provides comprehensive assessment results for the most recent high-resolution regional climatology in the East/Japan Sea by comparing with the various existing climatologies. This new high-resolution climatology is generated based on the Optimal Interpolation (OI) method with individual profiles from the World Ocean Database and gridded World Ocean Atlas provided by the National Centers for Environmental Information (NCEI). It was generated from the recent previous study which had a primary focus to solve the abnormal horizontal gradient problem appearing in the other high-resolution climatology version of NCEI. This study showed that this new OI field simulates well the mesoscale features including closed-curve temperature spatial distribution associated with eddy formation. Quantitative spatial variability was compared to the other four different climatologies and significant variability at 160 km was presented through a wavelet spectrum analysis. In addition, the general improvement of the new OI field except for warm bias in the coastal area was confirmed from the comparison with serial observation data provided by the National Fisheries Research and Development Institute’s Korean Oceanic Data Center.

Detailed explanations for the occurrence and nature of interfacial phenomena in liquid–liquid systems on the basis of the theory of spontaneous surface convection

The occurrence of unidirectional ejection phenomena (both continuous & intermittent), during mass transfer in binary and ternary systems respectively; reported earlier, have been explained on the basis of the theory of spontaneous surface convection (SSC). Also, intermittent jerking of the interface followed by emulsification coupled with profuse streaming action in the extract phase of two ternary systems has been explained by the same theory. In addition, intermittent circulatory motion in the extract phase only, in a ternary system has been explained on the basis of vertical and/or angular ejections’ restrained motion due to high viscosity of the extract phase. The theory of reversible adsorption and desorption phenomena for the transfer of surface-active solutes in ternary systems have been taken to help in justifying the occurrence of SSC during initial contacting of two phase. In most of the ternary systems reported earlier, vigorous density convection in the form of mushroom type density eddies observed are due to larger unstable density gradient. This is caused by high rates of mass transfer during SSC. SSC in the form of continuous circulatory motion in the extract phase only has been observed during the transfer of a surface inactive solute. Such phenomena occurred in aqueous to organic direction of transfer. The circulatory motion is due to ejections being against gravity. Surface inactive solute’s transfer also caused oscillatory motion of the interface as a whole in a particular system. The nature of most of the phenomena observed in binary and ternary systems appears to be due to molecular motions. The theory of SSC states that if specific energy of driving force of mass transfer is greater than free energy of the interface, interfacial instability occurs. It is probably a simplified version of a concept suggested here that can be explained by the kinetic theory as follows. It states that chemical potential difference of a solute between the two phases, if greater than the interfacial barrier plus molecular energy of the solute (in raffinate phase) then interfacial instability occurs. Thus, cluster of solute molecules in the form of ejections of bulk mass have been found to occur from raffinate to extract phase indicating high rates of mass transfer.

Experimental and numerical study of turbulent flow and thermal behavior of automotive brake disc under repetitive braking

The frictional brake system is the most safety critical equipment to decelerate or stop a vehicle. Thermal performance of the frictional region parts, disc and pads, necessitates to evaluate precisely in the design and test steps. In this study, a brake test setup was designed and fabricated with exactly the same braking components used in a common passenger vehicle as disc, pads, rim, tire, and dust shield to simulate the sequential braking. The local temperature on the disc and pads and the brake fluid pressure were measured. In addition, a three dimensional numerical model was designed to simulate the aerodynamics and thermal performance of the braking in detail. Finite element method was employed to simulate the frictional heat between the brake disc and the pads. The results showed that although the velocity of mainstream airflow reduces significantly into the rim, turbulent flow develops in the form of eddies of swirling airflow. Additionally, transient temperature distribution on the braking components was predicted. The cooling vanes in the brake disc have considerably enhanced the convection heat transfer. The amount of convective heat transfer on the inner radial vanes was more than 58% of the total amount of convective heat transfer.

A new argument against cooling by convective air eddies formed above sunlit zebra stripes

There is a long-lasting debate about the possible functions of zebra stripes. According to one hypothesis, periodical convective air eddies form over sunlit zebra stripes which cool the body. However, the formation of such eddies has not been experimentally studied. Using schlieren imaging in the laboratory, we found: downwelling air streams do not form above the white stripes of light-heated smooth or hairy striped surfaces. The influence of stripes on the air stream formation (facilitating upwelling streams and hindering horizontal stream drift) is negligible higher than 1–2 cm above the surface. In calm weather, upwelling air streams might form above sunlit zebra stripes, however they are blown off by the weakest wind, or even by the slowest movement of the zebra. These results forcefully contradict the thermoregulation hypothesis involving air eddies.

Airflow-Assisted Impact of Drops of Various Viscosities: The Role of Viscous Dissipation, Normal Imposed Pressure, and Shear Flow of Air.

The role of liquid viscosity on the spreading for an airflow-assisted impact of drops on a surface is investigated. The spreading diameter is found to increase with the Reynolds number of the airflow (Reair) for a given viscosity and impact Weber number (We) compared to the still air. The increment is higher at a low We for viscous drops, whereas the effect of Reair dominates at the intermediate We as the viscosity decreases. Two extra forces, the normal imposed pressure and shear force of air, act on the drop and influence the spreading along with the viscous dissipation. The drop's curvature decreases depending on the viscosity and impact velocity while spreading. Large-scale eddies near the drop-surface region are observed due to the separation of the incident airflow. The formation of eddies signifies low-pressure zones, which extract the trapped air, causing the spreading diameter of the viscous drop to increase at a low We. With the increase in the We, the lamella thickness of low-viscosity drops decreases and is pushed out by the air shear causing the spreading factor to increase. The boundary layer thickness is estimated using the energy balance method to predict the maximum spreading factor. The prediction compares well with the experimental one for higher viscosities. The accuracy improves when the effect of low pressure is incorporated. To confirm, the experimental spreading is compared with that obtained from three existing models, and one, which considers the influence, is observed to provide a better prediction.

Tracing Glacial Meltwater From the Greenland Ice Sheet to the Ocean Using Gliders

AbstractThe Greenland Ice Sheet (GrIS) is experiencing significant mass loss and freshwater discharge at glacier fronts. The freshwater input from Greenland will impact the physical properties of adjacent coastal seas, including important regions of deep water formation and contribute to global sea level rise. However, the biogeochemical impact of increasing freshwater discharge from the GrIS is less well constrained. Here, we demonstrate the use of bio‐optical sensors on ocean gliders to track biogeochemical properties of meltwaters off southwest Greenland. Our results reveal that fresh, coastal waters, with an oxygen isotopic composition characteristic of glacial meltwater, are distinguished by a high optical backscatter and high levels of fluorescing dissolved organic matter (FDOM), representative of the overall colored dissolved organic matter pool. Reconstructions of geostrophic velocities are used to show that these particle and FDOM‐enriched coastal waters cross the strong boundary currents into the Labrador Sea. Meltwater input into the Labrador Sea is likely driven by mesoscale processes, such as eddy formation and local bathymetric steering, in addition to wind‐driven Ekman transport. Ocean gliders housing bio‐optical sensors can provide the high‐resolution observations of both dissolved and particulate glacially derived material that are needed to understand meltwater dispersal mechanisms and their sensitivity to future climatic change.

Submesoscale Vorticity and Divergence in the Alboran Sea: Scale and Depth Dependence

The statistics of submesoscale divergence and vorticity (kinematic properties, KPs) in the Alboran Sea (Mediterranean Sea) are investigated, using data from drifters released during two experiments in June 2018 and April 2019 in the framework of the Coherent Lagrangian Pathways from the Surface Ocean to Interior (CALYPSO) project. Surface drifters sampling the first meter of water (CARTHE and CODE) and 15 m drifters (SVP) are considered. The area of interest is dominated by processes of strong frontogenesis and eddy formation as well as mixing, related to the high lateral gradients between Mediterranean and Atlantic waters. Drifter coverage and distribution allow to investigate the dependence of KPs on horizontal scales in a range between 1 and 16 km, that effectively bridges submesoscale and mesoscale processes, and at two depths, of 1 and 15 m. For both experiments, the surface flow is highly ageostrophic at 1 km scale, with positive vorticity skewness indicating the presence of submesoscale features. Surface divergence quickly decreases at increasing scales with a slope compatible with a turbulent process with broadband wavenumber spectrum, suggesting the influence of surface boundary layer processes such as wind effects, waves and Langmuir cells at the smaller scales. Vorticity, on the other hand, has a significantly slower decay, suggesting interaction between submesoscale and mesoscale dynamics. Results at 15 m are characterized by reduced ageostrophic dynamics with respect to the surface, especially for divergence. Submesoscale processes are present but appear attenuated in terms of KP magnitude and skewness. The results are generally consistent for the two experiments, despite the observed differences in the mixed layer stratification, suggesting that submesoscale instabilities occur mostly at surface fronts associated with filaments of Atlantic and Mediterranean waters that are present in both cases. The results are compared with previous literature results in other parts of the world ocean and a synthesis is provided. Good agreement with previous surface results is found, suggesting some general properties for divergence and vorticity scale dependence. The importance of further investigating very high resolution frontal processes at scales of tens of meters, as well as processes of interaction with high wind effects is highlighted.

Numerical study of water waves interacting with open comb-type caisson breakwaters

The waves interacting with open comb-type caisson breakwaters was numerically investigated based on a three-dimensional numerical model by compiling a modified k-ω turbulence model. The numerical model can significantly supress nonphysical overestimation of turbulence kinetic energy. The numerical model was established in a specific strip of breakwater by introducing a periodic boundary condition to improve computational efficiency. The numerical model was validated against analytical and experimental results. Effects of wave and structural parameters on hydrodynamic performances, including wave reflection, wave transmission, wave forces, and energy dissipation, were investigated. The numerical results suggested the significance of the relative chamber width on hydrodynamic quantities. The hydrodynamic interaction, including the wave run-up on the superstructure and the eddy formation process, was illustrated by the time instants of the velocity and dynamic pressure, turbulence kinetic energy field and streamline. The energy dissipation was mainly due to wave breaking and vortex shedding and hence effectively reduced the wave transmission but had little influence on wave reflection. More wave energy went to higher frequency component as incident wave period increased. The investigation regarding the phase shift of wave forces acting on rectangular caisson and the side plates proved the advantage in mechanical characteristics of comb-type structure.

Stronger Intraseasonal Variability Observed Below the Seasonal Thermocline in the Kuroshio East of Taiwan During 2014 and 2015

AbstractIntraseasonal variability in the Kuroshio east of Taiwan was studied using moored observations at 23°N, 122°E from November 17, 2014 to September 5, 2015, high‐resolution ocean model data, and altimetry data. It is found for the first time that the observed meridional velocity has a peak signal at 50–60 days with a significant maximum variation at 200–600 m depth. Model outputs show that the 50–60‐day variability at the mooring station is related to meandering of the Kuroshio at 23°N. Both surface eddies, which can extend downward to ∼1,000 m depth, and coastal cold water upwelling associated with cyclonic eddies from deep layers can greatly change the isopycnal slope below ∼200 m depth at 23°N, 122°E, and their joint effect results in the maximum variation at 200–600 m depth. Ocean eddies southeast of Taiwan propagate northward along the current axis of the Kuroshio and have a large impact on intraseasonal variability. Case analysis indicates that the formation of ocean eddies or upwelling near the coast may be related to the Lüdao and Lanyu islands southeast of Taiwan or Kuroshio transport along the southern tip of Taiwan. In addition, the evolution of subsurface meridional velocity from mooring observations is inconsistent with that of the surface but is closely related to the evolution of local temperature below the seasonal thermocline. The maximum intraseasonal variability below the seasonal thermocline highlights the complex dynamics in the Kuroshio east of Taiwan.

The erosive power of the Malvinas Current: Influence of bottom currents on morpho-sedimentary features along the northern Argentine margin (SW Atlantic Ocean)

Sediment deposits formed mainly under the influence of bottom currents (contourites) are widely used as high-resolution archives for reconstructing past ocean conditions. However, the driving processes of Contourite Depositional Systems (CDS) are not entirely understood. The aim of this study is to establish a clearer link between contourite features and the oceanographic processes that form them. The morphosedimentary characteristics of a large CDS were analysed together with the current dynamics along the northern Argentine continental margin. This study combines multibeam bathymetry, seismo-acoustic data, sediment samples, vessel-mounted Acoustic Doppler Current Profiler (VM-ADCP) data and numerical modelling of ocean currents.The contouritic features include large contourite terraces (La Plata Terrace, Ewing Terrace) and an abraded surface connecting the terraces, as well as smaller erosional and depositional features like moats, erosion surfaces on the Ewing Terrace, sediment waves and contourite drifts. Measured and modelled near-bottom currents are vigorous (up to 63 cm/s at 150–200 m above the seafloor) where abraded surfaces and moats are present, and relatively weak (below 30 cm/s) on the La Plata Terrace and the Ewing Terrace. Generally, bottom currents follow the upper and middle slope morphology. Decreasing velocity of water masses flowing northward leads to less erosion and finer sediment deposits. ADCP data and the hydrodynamic model show the formation of eddies near the seafloor which probably lead to the small erosion surfaces on the Ewing Terrace, even though it is mainly a depositional environment. Overall, this study contributes to a better understanding of the formation of CDS and can help future reconstructions of past ocean conditions based on sedimentary structures.

Modelling hydrodynamics of fast tidal stream around a promontory headland

Areas around headlands often have higher tidal flow, making them important foraging locations for seabirds and marine mammals, as well as of interest for tidal energy extraction. Using in situ field measurements and hydrodynamic modelling, this study investigated tidal features around the most northerly promontory headland on the United Kingdom mainland. An acoustic wave and current (AWAC) instrument and an acoustic Doppler current profiler (ADCP) were deployed for 109 days in upward-facing configurations. These data were used to calibrate a hydrodynamic model developed using the TELEMAC-MASCARET modelling suite. This study used a non-hydrostatic model with a vertical layer configuration to reproduce upwelling present in field data. Data showed complex hydrodynamics with occurrences of vertical water velocities of up to 0.219 m/s. Modelling suggests upwelling begins in association with headland eddy formation as a result of secondary circulation due to flow curvature, which occurs off the tip of the headland. Upwelling continues to develop as the eddy dissipates and forms an upwelling front which moves away from the headland as the tide starts to turn. The modelling work successfully reproduces upwelling recorded by field instruments. Models of sufficient complexity are required to resolve fine-scale hydrodynamics associated with such features, which is important for tidal stream renewable energy developments in these environments, as well as understanding foraging activity of seabirds and marine mammals.

Submesoscale Instability in the Straits of Florida

AbstractThe Florida Current (FC) flows in the Straits of Florida (SoF) and connects the Loop Current in the Gulf of Mexico to the Gulf Stream (GS) in the Western Atlantic Ocean. Its journey through the SoF is at time characterized by the formation and presence of mesoscale but mostly submesoscale frontal eddies on the cyclonic side of the current. The formation of those frontal eddies was investigated in a very high resolution two-way nested simulation using the Regional Oceanic Modeling System (ROMS). Frontal eddies were either locally formed or originated from outside the SoF. The northern front of the incoming eddies was susceptible to superinertial shear instability over the shelf slope when the eddies were pushed up against the slope by the FC. Otherwise, incoming eddies could be advected relatively unaffected by the current, when in the southern part of the straits. In absence of incoming eddies, submesoscale eddies were locally formed by the roll-up of superinertial barotropically unstable vorticity filaments when the FC was pushed up against the shelf slope. The vorticity filaments were intensified by the friction-induced bottom layer vorticity flux as previously demonstrated by Gula et al. (2015b) in the GS. When the FC retreated further south, negative vorticity West Florida Shelf waters overflowed into the SOF and led to the formation of submesocale eddies by baroclinic instability. The instability regimes, hence, the submesoscale frontal eddies formation appear to be controlled by the lateral ‘sloshing’ of the FC in the SoF.