Sargassum has historically been found in the subtropical North Atlantic gyre where it provides important habitat for diverse marine species. However, since 2011 with the development of the Great Atlantic Sargassum Belt in the equatorial Atlantic, abundance has greatly increased, resulting in shorelines mass stranding. These coastal inundations of Sargassum have major impacts on the ecology, economies, and health of affected areas.
Understanding the Sargassum raft's motion is required to be able to predict the areas that could be affected by Sargassum. The motion of the rafts is fundamentally unlike Lagrangian (i.e., infinitesimally small, neutrally buoyant) particle motion since they represent finite-size, buoyant objects subjected to the action of ocean currents, wind, and waves. In this talk, we will present a Maxey-Riley model for the motion of Sargassum rafts that takes their inertial nature into account as well as the elastic interactions within a raft and physiological changes affecting the structure of the rafts.  This will be accompanied by a discussion of results from field and laboratory experiments used to validate the model. Joint work with F. J. Beron-Vera and G. Bonner.

How to cite: Olascoaga, M. J.: A Maxey-Riley modeling framework for Sargassum raft drift, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-6434, https://doi.org/10.5194/egusphere-egu24-6434, 2024.

Marine surface particle dispersal simulations are crucial for addressing societal issues such as plastic pollution, oil spills, biological connectivity, and recovery missions. However, the quality of these Lagrangian simulations depends on how well the underlying numerical model represents the prevalent ocean circulation features.

Here, we investigate how simulated surface particle dispersal changes, if the – often neglected or only approximated – impact of surface waves is included. Under the influence of surface waves, a particle not only moves with the Eulerian current velocity but also experiences a net drift in the direction of wave propagation, known as Stokes drift. Moreover, wave-current interactions result in wave-driven Eulerian currents. We use the output of a coupled ocean-wave model configuration for the Mediterranean Sea to answer the following questions: What is the relative impact of Stokes drift and wave-driven Eulerian currents? How well can the total wave impact be represented by the commonly used approximation consisting of the superposition of Eulerian currents and Stokes drift obtained from independenntly run ocean and wave models?

We find that Stokes drift as well as wave-driven Eulerian currents can have a non-negligible impact on surface particle dispersal. While both tend to act in opposing directions, they do not necessarily cancel each other out, due to different temporal and spatial variability. Our analyses suggest a seasonal dependency of the wave impact. For a major part of the Mediterranean Sea, ocean-wave coupling increases the simulated mean Lagrangian surface speed in winter through a dominant impact of Stokes drift and decreases it in summer through a dominant impact by wave-driven Eulerian currents. Yet, some regions also exhibit a dominance of either Stokes drift or wave-driven Eulerian current impact throughout the year. Consequently, applying the commonly used approximation is not always beneficial for surface particle simulations. The advantage or disadvantage of the approximation compared to neglecting any wave impact depends on the season, region, and Lagrangian measure of interest, and is difficult to estimate a priori. Hence, whenever possible, coupled ocean-wave models should be employed for surface particle dispersal simulations.

How to cite: Rühs, S., van Sebille, E., Moulin, A., Clementi, E., and van den Bremer, T.: Non-negligible impact of Stokes drift and wave-driven Eulerian currents on simulated surface particle dispersal in the Mediterranean Sea, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-13046, https://doi.org/10.5194/egusphere-egu24-13046, 2024.

We will report on two studies we have done related to Lagrangian transport of plastic particle in marine waters.

In the first study, we investigate the particle dynamics of tyre-wear microplastics that come from road traffic across two major bridges in Byfjorden, namely the Nordhordland Bridge and the Askøy Bridge. We employ a Lagrangian particle tracking framework, OpenDrift, with background horizontal velocities from Bergen Ocean Model (BOM), paired with a vertical sinking velocity obtained from Stokes law to track individual particle paths along the flow field until they reach the seafloor. The sinking velocity is picked from a distribution that is designed based on results from point source experiments, enabling us to cover the particle dynamics for a spectrum of sinking velocities. The basis of this study lies in using the variability in local currents, by conducting multiple experiments with distinct initial locations and release times to understand the similarities and differences in the footprint. In the particle simulation, the horizontal velocity experienced by individual particles depends on release time which is related to when in the tidal cycle the particle is released. We seek insights to discover potential aggregation zones and their corresponding gradients along the bottom of the fjord. We plan to shed light on ‘how particle dynamics change when we vary the sinking velocity’. These results could be applicable in identifying the mechanisms behind particle transport in fjords and can assist in designing sampling campaigns.

In the second, we assess the amount of transboundary plastic coming along the coast of western Norway, employing a nested modelling approach. We utilize emissions data of buoyant plastics from major European rivers (Meijer et al., 2021), as an input to our Lagrangian particle tracking model simulated using OpenDrift. The background currents are provided by the nested model which includes surface currents from three grids: A 4km model of the North Atlantic - Nordic4K (Lein et al., 2013), An 800m model covering Norway’s coastline - Norkyst800 (Albertsen et al., 2011), and 160m hydrodynamical model - NorFjords160 (Dalsøren et al., 2020). As particles transit through these nested grids, we precisely track the plastic pathways into the western Norwegian fjords around the city of Bergen. Employing this nested grid setup addresses problems with boundary conditions and mass balance. We present the estimates for the fraction of plastic moving into the fjord with focus on relative influence of wind and ocean currents on the transboundary movement of plastic. This study sheds light on processes responsible for near and far field transport, providing valuable insights for agencies working on trans-national pollution laws and implementing ocean clean-up strategies.

How to cite: Alendal, G., Madduri, P., Oleynik, A., Avlesen, H., and R. Clark, J.: Lagrangian modelling of plastic transport in marine waters. , EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-20185, https://doi.org/10.5194/egusphere-egu24-20185, 2024.

Describing and tracking three-dimensional flow structures in an ocean setting may explain elemental and biodiversity pattern. A possible tool can be finite time coherent sets. These sets are Lagrangian Coherent Structures characterized by minimal leakage and minimal exchange with their surrounding environment.

In oceanic settings, they can be understood as separated waterbodies or eddies playing an important role for transport and mixing processes.

Due to limited interaction with their surroundings, they even influence biological processes by providing competitive advantages for some species, for example, optimal temperature or nutrient conditions.

In a case study of three-dimensional finite time coherent sets in the Western Baltic Sea in May and July 2018, we show some different impacts on biological processes:

• enhancement of phytoplankton growth in the set's surrounding,
• transport of cold nutrient rich water from shallower to deeper regions, and
• the formation of transient, moving dynamical niches with higher temperature inside the coherent set compared to its surrounding, prolonging the life of an existing phytoplankton bloom that is trapped during the formation of the coherent set.

Moreover, different dynamical patterns can be observed inside the finite time coherent sets during their travel and lifetime. Temporal stratification and mixing inside the coherent sets suppress or enhance growth temporally and locally.

In the coherent set’s surrounding, the formation of a “sticking” manifold supports the development of a local phytoplankton bloom in the upper water column.

Our case study in the Western Baltic Sea provides a first step towards understanding the impact of three-dimensional coherent sets on transport processes and phytoplankton growth in the Baltic Sea, as well as, the formation of dynamical pattern inside three-dimensional coherent sets.

How to cite: Vortmeyer-Kley, R., Cahill, B., Berthold, M., and Feudel, U.: Three-dimensional influencers in the Western Baltic Sea: finite time coherent sets and their role for biological processes, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-4656, https://doi.org/10.5194/egusphere-egu24-4656, 2024.

The methods used to identify coherent ocean eddies are either Eulerian or Lagrangian in nature, and nearly all existing eddy datasets are based on the Eulerian method. In this study, millions of Lagrangian particles are advected by satellite-derived surface geostrophic velocities over the period of 1993–2019. Using the method of Lagrangian-averaged vorticity deviation (LAVD), we present a global Lagrangian eddy dataset (GLED v1.0). This open-source dataset contains not only the general features (eddy center position, equivalent radius, rotation property, etc.) of eddies with lifetimes of 30, 90, and 180 days, but also the trajectories of particles trapped by coherent eddies over the lifetime. We present the statistical features of Lagrangian eddies and compare them with those of the most widely used sea surface height (SSH) eddies, focusing on generation sites, size, and propagation speed. A remarkable feature is that Lagrangian eddies are generally smaller than SSH eddies, with a radius ratio of about 0.5. Also, the validation using Argo floats indicates that coherent eddies from GLED v1.0 exist in the real ocean and have the ability to transport water parcels. Our eddy dataset provides an additional option for oceanographers to understand the interaction between coherent eddies and other physical or biochemical processes in the Earth system.

How to cite: Liu, T. and Abernathey, R.: A global Lagrangian eddy dataset based on satellite altimetry , EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-3997, https://doi.org/10.5194/egusphere-egu24-3997, 2024.

Odor transport in fluidic environments is a subject of great importance, holding implications for numerous scientific disciplines, including fluid dynamics, biological studies, and engineering disciplines. Odor tracking serves as the foundation for various natural processes, such as the navigation of marine organisms and the foraging behavior of insects. Turbulent fluctuations add another level of complexity to the problem of odor transport in fluidic environments. 

The odor emitted by swimmers is not only influenced by the environment but also by hydrodynamic fluctuations caused by their dynamics. This effect is evident in large swimmers, where the wakes caused by their swimming dynamics could potentially alter odor distribution. However, it is much less clear whether and how hydrodynamic interactions affect the odor distribution of mesoscale swimmers. 

In this work, we explore the coupling of chemical and mechanical signals from mesoscale swimmers (Reynolds number <= 50), immersed in a turbulent open channel flow. We use a model system comprising a collection of swimmers in an open channel flow to explore the propagation and interaction of these signals. Furthermore, we vary their Reynolds numbers and evaluate the consequential changes in odor distribution. We show that the velocity fluctuations due to the swimmers play a significant role in changing the range and distribution of odor signals by screening the intensity and fluctuations of odor distribution downstream. We found substantial differences in odor screening depending on whether the swimmers are 'pushers' or 'pullers', the latter being more effective in screening their odor from predators. Our findings provide valuable insights into the coupling of mechanical and chemical signals of mesoscale swimmers in turbulence with novel considerations regarding the evolutionary preferences of specific swimming modes. 

How to cite: James, M., Viola, F., and Seminara, A.: Impact of Swimmer Dynamics on Odor Transport by Mesoscale Swimmers in Turbulent Environments, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-7549, https://doi.org/10.5194/egusphere-egu24-7549, 2024.

The study aims to assess turbulent dispersion processes in the coastal regions of northeastern Vietnam, in order to meet the major challenge of monitoring the fate of particulate materials in this part of the East Vietnam Sea, home to the most famous tourist sites and economic sites. The study area is strongly influenced by the freshwater discharge of the Red River, which creates a large river plume, greatly affecting coastal circulation and turbulent dispersion. The monsoon wind profoundly alters the dynamics of the river plume, pushing light surface water seaward over long distances, in summer, and toward the coast, in winter. Sea surveys were organized for the first time in this region in 2022 and 2023 to better characterize the processes controlling coastal flow variability and turbulent dispersion in the plume region and surrounding waters. Surface drifters, released in the  plume region, were tracked during short periods of time, lfrom one to a few days. Current velocity profiling and CTD profiling have been also done. Estimates of the relative dispersion based on surface drifter measurements have revealed that the dispersion regime is local, mainly ballistic and Richardson, induced by turbulent eddies whose size does not exceed a few km. Local wind variability, combined with variations in bathymetry, considerably affects the transport pathways of real drifters and modifies the dispersion regime. A coastal circulation model was used to better assess dispersion processes over the entire study area and for a wide range of variability in the main forcing terms. Virtual surface drifters were tracked in the model velocity field during the surveying periods. The results revealed that, on scale of several days, the transport of passive tracers is considerably affected by irregularities in current velocity fields associated with zones of current convergence and divergence. The results also demonstrated that merging observations with model outputs significantly improves the representation of small scale features of current variability, turbulent mixing, and horizontal stirring of tracers in the plume region.

How to cite: Sentchev, A., Nguyen, D. T., Vu, D. V., and Berti, S.: Assessment of turbulent dispersion in the Red River plume region, northeast Vietnam, based on Lagrangian observations and modeling, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-8754, https://doi.org/10.5194/egusphere-egu24-8754, 2024.

Gas transfer between ocean and atmosphere is largely governed by the turbulence in the topmost centimetres beneath the free surface. It has been frequently observed that areas of strong positive divergence of the surface-tangential velocity field correspond to efficient surface renewal and consequently increased transfer of gas across the interface. Patches of strong positive surface divergence occur through intermittent upwelling events visible as ``boils'' on the surface.

It has been qualitatively observed that surface-attached ``bathtub'' vortices tend to appear at the edges of upwelling boils, as well as sharp valleys, sometimes referred to as ``scars”. Surface-attached vortices and scars leave imprints on the surface which are particularly simple to detect: the vortices are circular and live for a long time, while scars are sharp and elongated structures.

From direct numerical simulations, we show that a very high correlation exists between the time-dependent number of surface-attached vortices and the mean square of the surface divergence. We use a newly developed method whereby the surface-attached vortices are identified with high precision and accuracy from their surface imprint only.

We also looked at the turbulent structures just beneath the surface-attached vortices and the scars, noting how far under the surface these structures propagate and, thus, how far into the flow subsurface features can be read from patterns on the surface only.

The main application is in remote sensing, as these patterns on the surface can be easily detected using camera footage, for example. These patterns would give estimates of the subsurface quantities without the need for expensive measurement.

How to cite: Babiker, O., Aanes, J., Xuan, A., Shen, L., and Ellingsen, S. A.: What information can be gathered from patterns in turbulent free-surface flows?, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-19490, https://doi.org/10.5194/egusphere-egu24-19490, 2024.

Flexpart (FLEXible PARTicle dispersion model) is a numerical model that simulates the dispersion of gases and aerosols in the atmosphere. In order for Flexpart to be used, it must be installed and run on a (super)computer. However, this is associated with obstacles, as not all scientists have access to a supercomputer and there are often technical problems during installation or execution. In this project, we therefore want to develop a Flexpart Web Service (FLEXWEB) in which Flexpart can be run via a website. We will show first results and details on the implementation of a test project for a potential operational service. Flexpart will be containerized and the service will be run in a Kubernetes cluster (in “the” cloud or on premises) to calculate trajectories and make these results easily accessible to users. As soon as the simulation is complete, the output files will be made available for download and displayed graphically. In this way, we hope to simplify access to Flexpart for scientists worldwide.

How to cite: Blaschek, M., Bakels, L., Dütsch, M., and Stohl, A.: FLEXWEB - A flexible particle dispersion model web interface, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-15051, https://doi.org/10.5194/egusphere-egu24-15051, 2024.

Atmospheric Blocking - also known as Quasi-Stationary Atmospheric States (QSAS) - exert a major influence on mid-latitude atmospheric circulation and are known to be associated with extreme weather events. Previous work has highlighted the importance of the origin of air parcels that define the blocking region, especially with respect to non-adiabatic processes such as moisture transport and latent heating. So far, an objective method for clustering the individual Lagrangian trajectories passing through the QSAS into larger and - more importantly - spatially coherent air streams has not been established, which is the focus of our current work.

    Coherent sets are regions in the phase space of dynamical systems that keep their geometric integrity to a large extent during temporal evolution. We extract a low-dimensional representation of the Lagrangian data via diffusion maps and cluster the trajectories in this representation to estimate coherent sets. Our implementation adapts the existing methodology to the non-Euclidean geometry of Earth's atmosphere and its challenging scaling properties. Several example cases are investigated. 

    The results confirm the existence of spatially coherent feeder airstreams differing with respect to their dynamical properties and, more specifically, their latent heating contribution. Air streams experiencing a considerable amount of latent heating occur mainly during the maturing and maintanence phases of the QSAS and contribute to its stability. In our example cases, trajectories also exhibit an increase in density when passing through the blocking region during its maintanence phase, which is in line with the common understanding of QSAS as regions of high stability. 

How to cite: Schoeller, H., Chemnitz, R., Pfahl, S., Koltai, P., and Engel, M.: Lagrangian Coherence in Atmospheric Blocking, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-35, https://doi.org/10.5194/egusphere-egu24-35, 2024.