Theoretical studies on the modulation of unidimensional regular waves over a flat bottom due to a current typically assign an asymmetry between the effects of opposing/following streams on the evolution of major sea variables, such as significant wave height. The significant wave height is expected to monotonically increase with opposing streams and to decrease with following streams. To some extent, observations on data sets containing a few thousand of waves or over a continuous series of about a day confirm this prediction. Here we show that in very broad-banded seas with high directional spread, the asymptotic behavior of sea variables over large data sets is highly non-trivial and does not follow the theoretical predictions, especially at high values of the ratio between tidal stream and group velocity. Furthermore, we analyze the anomalous statistics originating from both forward and opposing non-stationary currents. Despite the sea states being dominantly broad-banded and featuring a large directional spread, we found that anomalous statistics are of the same order of magnitude of those observed in unidirectional laboratory experiments and symmetrical in regard to the orientation of the tidal current.

How to cite: Mendes, S., Teutsch, I., and Kasparian, J.: Direction Symmetry of Wave Field Modulation by Tidal Current and its Consequences for Extreme Nonlinear Waves, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-3505, https://doi.org/10.5194/egusphere-egu24-3505, 2024.

Internal waves have a wide range of scales but are typically unresolved in climate or global models. With an unprecedented capability of observing and simulating these processes, they are becoming increasingly important to quantify the upscale effect of these processes. As the largest marginal sea in the western Pacific, the South China Sea has the most energetic and frequent internal waves around the world. These waves are also affected by multiscale processes, climate changes, and anthropogenic impacts. There have been considerable advances in exploring the generation and propagation of internal waves in recent years. However, the understanding of the formation and fate of internal solitary-like waves on the continental shelf is still very limited. It is widely accepted that these internal waves generally originate from the Luzon Strait. They usually have regular occurrence and are phase-locked to tidal forcing in the Luzon Strait. However, we present field measurements showing an irregular occurrence of nonlinear internal waves on the northern shelf of the South China Sea. This irregular occurrence is in striking contrast to the prominent predictability of internal waves originating from the Luzon Strait. We reveal that the intermittent nature of the occurrence is due to the local generation of nonlinear internal waves on the continental shelf, in addition to the fission of shoaling internal waves. The results reported here are expected to apply to other shelf regions of the world's oceans.

How to cite: Bai, X.: Intermittent Generation of Nonlinear Internal Waves on Continental Shelf, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-4294, https://doi.org/10.5194/egusphere-egu24-4294, 2024.

Well established energy-based methods of quantifying diapycnal mixing in process-study numerical models are often used to provide information about when mixing occurs, and how much mixing has occurred. However, describing how and where this mixing has taken place remains a challenge. Moreover, methods based on sorting the density field struggle with under resolution and uncertainty as to the definition of the reference density when bathymetry is present. Here, an alternative method of understanding mixing is proposed. Paired histograms of user selected variables (which we abbreviate USP) are employed to identify mixing fluid, and are then used to display regions of fluid in physical space that are undergoing mixing. Here, two case studies are presented to showcase this method: shoaling internal solitary waves and a shear instability in cold water influenced by the nonlinearity of the equation of state. For the first case, the USP method identifies differences in the mixing processes associated with different internal solitary wave breaking types, including differences in the horizontal extent and advection of mixed fluid. For the second case, the method is used to identify how density, and passive tracers are mixed within the core of the asymmetric cold-water Kelvin-Helmholtz instability.

How to cite: Hartharn-Evans, S., Stastna, M., and Carr, M.: A new approach to understanding fluid mixing in process-study models of stratified fluids, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-8014, https://doi.org/10.5194/egusphere-egu24-8014, 2024.

We investigate the properties of, and can carry out a stability analysis of a baroclinic current in, a stratified thermal rotating shallow-water model for the subinertial dynamics of the upper ocean, a key player in the global climate system where most of the ocean variability is concentrated affecting the lateral transport of floating material such as plastic garbage, oil, and Sargassum seaweed.  Unlike the standard thermal model, the model considered here includes linear buoyancy variation in the vertical, still maintaining its two-dimensional structure and that of the adabatic (constant density) model.  Like the standard thermal model, the stratified thermal model produces submesoscale circulations resembling those observed in satellite imagery, yet taking longer to manifest.  Our study is motivated by this numerical observation. The model possesses a Lie--Poisson Hamiltonian structure.  A particular aspect of the model is that it supports motion integrals which neither form the kernel of the corresponding bracket nor are related to any explicit symmetries via Noether's theorem.  Among other things, we investigate the role of these conservation laws in constraining the growth of finite-amplitude perturbations to a zonal flow with quadratic vertical shear. Joint work with Maria J. Olascoaga.

How to cite: Beron-Vera, F.: Properties and baroclinic instability of stratified thermal ocean flow, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-3654, https://doi.org/10.5194/egusphere-egu24-3654, 2024.

Internal solitary waves with large amplitude have long been observed in the Strait of Gibraltar (SoG). Beautiful satellite images depict the ISW observed at the entrance of the Alboran Sea, generated by the interaction between tides and the topography at Camarinal Sill, the most important topographic obstacle in the Strait. As the tidal current heads west, it becomes supercritical over the sill, leading to the development of an internal hydraulic jump. When the current weakens, the flow reverts to subcritical,  the internal hydraulic jump is released, leading to the eastward propagation of an internal bore. This bore progressively transforms into an ISW train due to non-hydrostatic dispersion and non-linear effects. While the main mechanism of generation is now well understood, there are still open questions about the intricate dynamics of these nonlinear internal waves and their evolution along the Strait. Previous studies show an important variability in the shape, intensity and arrival time of this internal solitary train. 

Recent field experiments have revealed a complex network of local hydraulic jumps forming near Camarinal Sill. From the same dataset, the potential generation of not just one but two ISW trains has been addressed.  Then, we have investigated the implication on the evolution of these trains during their journey in the Strait of Gibraltar. 

Our mooring data reveal the presence of two trains of ISWs with slightly different north-south fronts east of Camarinal Sill. We propose hypotheses to explain the tilting of the fronts based on differential mixing and meridional tidal variability . Moreover, these findings prompt us to reconsider our understanding of the physics responsible for the observation of non-rank ordered ISW trains in the eastern part of the strait. To deeply investigate the consequences of nonlinear wave-wave interaction in the disorganization of the train, we implemented a simplified 3D non-hydrostatic configuration. 

How to cite: Roustan, J.-B., Bordois, L., Dumas, F., Auclair, F., and Carton, X.: Analyzing the Dynamics of Multiple ISW Emissions in the Strait of Gibraltar, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-10281, https://doi.org/10.5194/egusphere-egu24-10281, 2024.

Isaac M. Held writes in his introduction of the book by Schneider and Sobel (2007) "A theory for the general circulation of the atmosphere has at its core a theory for the quasi-horizontal eddy fluxes of energy, angular momentum, and water vapor by the macro-turbulence of the troposphere." An analog of this atmospheric macro-turbulence can be studied by using data from the differentially heated rotating annulus laboratory experiment (e.g. Fowlis and Hide, 1965). From simultaneous measurements of surface temperature and horizontal flow it is possible to study elementary structures of Lagrangian tracer fluxes (Agaoglou, 2024) as well as eddy fluxes of heat and momentum. In a fluid layer, the three fluxes for PV, momentum M, and heat  T are connected via the Margules equation PV=∂M/∂y + f/H T. Here y is the north-south direction, f is the Coriolis parameter and H the layer depth. From our data we are able to compute all three fluxes and it is instructive to compare the results with fluxes from simplified models. E.g., in the Eady model T does not depend on z and we can thus obtain the total heat flux from this model using the surface data. Moreover, since M is zero for the Eady model, the PV flux is proportional to the heat flux. Using an even simpler model, the surface geostrophic approximation, we can deduce the flow from the temperature field alone. However, this model does not have the correct phase difference between the velocity and the temperature field and gives a wrong mean heat flux. Applying a phase difference such that the heat flux becomes comparable to the one from the Eady model allows to estimate the vertical flow structure from the temperature field alone. The result might be helpful for the construction of flow fields from satellite sea surface temperature data (LaCasce and Mahadevan, 2006).  

M. Agaoglou and V. J. García-Garrido and U. Harlander and A. M. Mancho (2024) Building transport models from baroclinic wave experimental data, Physics of Fluids, in press.

W. W. Fowlis and R. Hide (1965) Thermal convection in a rotating annulus of liquid: effect of viscosity on the transition between axisymmetric and non-axisymmetric flow regimes, J. Atmos. Sci., 22, 541-558.
 
J. H. LaCasce and A. Mahadevan (2006) Estimating subsurface horizontal and vertical velocities from sea-surface temperature, Journal of Marine Research 64, 695–721.
    
T. Schneider and A. H. Sobel (Eds.) (2007) The Global Circulation of the Atmosphere, Princeton University Press.

How to cite: Harlander, U. and Mancho, A. M.: Transport and fluxes of atmospheric models deduced from laboratory data , EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-16445, https://doi.org/10.5194/egusphere-egu24-16445, 2024.

A complex system of alternating zonal jets has been observed in the low latitude Pacific ocean, both on the equator and extending away into the subtropics. The dynamics of the subtropical jets, which also seem to be associated with strong cross-jet gradients in chemical tracers, may be similar to those of much-studied beta-plane zonal jets, but the effect of longitudinal boundaries, which are present in oceanic cases, on such jets is still puzzling. The forcing mechanism behind this set of oceanic zonal jets and what determines its horizontal and vertical structure are also poorly understood. We consider a beta-plane two-dimensional model with rigid boundary conditions and apply stochastic forcing, which without rigid boundaries would generate zonal jets. The instantaneous flow field is strongly time-dependent, with a large component of stochastically forced basin modes. This seems to disrupt the formation of alternating zonal jets, which, in contrast to the case without rigid boundaries, are observable only in the time-mean field and much weaker than the instantaneous flow. There is some evidence that these apparent time-mean jets are primarily the signature of stochastically forced basin modes rather than genuinely persistent jet-like flows. Adding tracers to the model allows the investigation of the relation between jet structure and the transport and mixing of tracers, and act as an important diagnostic to verify the presence or absence of jets. The instantaneous tracer field in the case with rigid boundaries is highly unsteady and, unlike the doubly periodic case, the jet structure is not manifested in the tracer field. Two-dimensional simulations with simple rigid boundary geometry may overestimate the generation of basin modes relative to the real ocean. Non-uniform damping is applied as a model device to break the interaction between the flow and boundaries and test whether it can inhibit basin modes and hence allow generation of steady zonal jets. With this non-uniform damping, jets present in the zonal mean field become more persistent as our redefined zonostrophy parameter increases, but there exist caveats we need to examine further, such as small ratio of energy in the zonal flow to the total energy as compared to much-studied cases without boundaries.

How to cite: Jian, S.-W. and Haynes, P.: The dynamics of low-latitude sub-surface oceanic jets, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-17638, https://doi.org/10.5194/egusphere-egu24-17638, 2024.

The upper ocean is crowded with fronts of different intensities and extensions,  which are known to play a major role in the dynamics of the oceanic upper layers and contribute to set some properties such as the spectral slopes of Sea Surface Temperatures (SST), among others. Here, we show that the upper layers of the ocean can be modeled by the multifractal theory of turbulence and, then, we use this theory to predict the link between the most intense fronts and the scaling properties of the structure functions. This prediction is, finally, verified with a wide range of observations and numerical simulations.

In particular, we show that the behavior of thermal gradients at small enough scales can be characterized by singularity exponents. Then, we use the singularity exponents of thermal gradients as a measure of the intensity of thermal fronts; and the fractal dimension of the set of points with the same singularity exponent, known as the singularity spectrum, as a measure of their extension. This allows us to connect fronts with the structure functions of temperature using the multifractal formalism. Assuming that the turbulent cascade can be modeled with the log-Poisson model, we analytically shown that the anomalous scaling of the structure functions is a function of the intensity of the strongest front, i.e. the smallest singularity exponent. This prediction is verified using the SST provided by numerical simulations of an upwelling system; simulations of the global ocean; and satellite observations. Moreover, we show that the predicted relationship is also valid for other variables such as velocities.

Our results not only provide insight on the functioning of the upper ocean, but also provide a guide to develop and adjust numerical models. Indeed, our results imply that numerical models have to correctly model, or parametrize, those processes generating the most intense fronts, in order to properly reproduce some of the statistics of ocean temperatures.

How to cite: Isern-Fontanet, J., Turiel, A., González-Haro, C., and Gea, V. G.: On the contribution of ocean fronts to the anomalous scaling of the structure functions, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-13002, https://doi.org/10.5194/egusphere-egu24-13002, 2024.

Subaqueous sand waves are widely observed in the world′s oceans, but few in-situ observations have been performed to understand the dynamic processes of how they form and migrate. Large-amplitude subaqueous sand waves, 1.5 to 20 m in height and 55 to 510 m in length, are found on upper continental slope of the northern South China Sea, at water depths of ~ 150 to 800 m. Herein, high-resolution tripod observations were conducted in this sandwave field to understand the dynamic mechanism how sandy sediments are remobilized by oceanic dynamic processes, in particular internal solitary waves and internal tides. Our results indicate that near-critical reflection of diurnal internal tides at the continental slope can cause high suspended sediment concentration within the bottom water at a narrow belt further upslope to the observation site downslope locations. These sediments are transported downslope by the ebb tides to the observation site, forming the daily-recurring high suspended sediment concentration. The passage of episodic extreme internal solitary waves can result in much denser high sediment clouds with a thickness of up to ~ 40–50 m above the seafloor. These high suspended sediment concentration events are caused by in-situ resuspension of sediments from the seabed and upward transport of these sediments out of the boundary layer in response to passing of internal solitary waves. The two sub-processes of sediment resuspension are regulated by distinct dynamic mechanisms: incipient sediment resuspension from seafloor is controlled by current-induced strong bed shear stress, while the upward transport of sediments out of the bottom boundary layer is driven by the upwelling convergent currents at the rear of the internal solitary waves. Such results provide new insights into understanding the dynamic mechanism of the so-called ‘resuspension’ process in marine sedimentology. Our results also highlight the importance of internal solitary waves and internal tides in modulating sediment remobilization over subaqueous sandwave fields.

How to cite: Zhao, Y., Zhang, Y., Ma, P., Zhang, X., and Liu, Z.: Sediment remobilization over subaqueous sand waves: Insights from in-situ observation in the northern South China Sea, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-14146, https://doi.org/10.5194/egusphere-egu24-14146, 2024.

Numerous previous studies have been done to understand the physics of gravity currents. Some considered the propagation over smooth or rough horizontal surfaces (Tokyay et al. (2014), Zhou et al. (2017)), and others studied inclined surfaces without any roughness. What about the more complex situation of inclined rough surfaces? We have studied, experimentally, how a finite volume of heavy fluid (salted water with a clay suspension) rushing down a slope (20° & 30°) is affected by the multiple obstacles it counters on its way. Our setup is a classic volume release configuration in a 2D flume immersed in a 20 m³ water tank at INRAE Grenoble. The study tests (352 experiments per tilt) different initial conditions of the released flow (volume and density) and various surface conditions from smooth to rough:

• For the initial conditions: the experiments show that the front velocity is monotonically related to its initial volume and density. Although the initial mass of the flow is the product of its density times volume, the mass effect on the flow front velocity cannot come in replacement of both volume and density effects, as it was found that the front velocity is non-monotonically related to its initial mass.
• For the surface conditions: besides testing the smooth case, we have covered a wide range of roughness configurations using obstacles with different shapes, heights, and spacings. Walls or barriers blocking the whole width of the flume have been used (see Fig.1-a). Testing various heights and spacings shows that higher barriers decrease the flow front velocity, while non-monotonic relations were found when the spacing between successive barriers in the flow direction is changed. Flow propagation over and through an array of obstacles has also been studied with various obstacles arrangement (in-line and staggered) and different obstacles' cross-sections (rectangular and circular). For circular obstacles, the (x_𝑓-t) curve is no longer smooth but takes the shape of stairsteps, and they are found to be more efficient in decelerating the flow (see Fig.1-b).

Studying both 20° and 30°-flume tilts enables us to look through the slope effect. The analysis shows that, in general, increasing the slope results in higher front velocity values. Nevertheless, the degree of influence is dependent on diverse factors (volume, density, bed surface conditions). In addition, we have studied the effect of the initial flow parameters on the flow height just after the lock release (at an accurate predetermined distance from the lock chosen based on 252 experiments). This height depends only on the initial volume and density effect is negligible. Determination of this height is essential for our non-dimensional analysis: to study the temporal evolution of the non-dimensional front position (𝑥_𝑓−𝑥_o)/𝑥_o versus the non-dimensional time (t_𝑓/t_o). Indeed, it will enable us to avoid using the initial flow depth at the lock that is highly dependent on the inclination angle, or an estimated virtual height after the lock that would be less representative (see Fig.1-c).

Fig. 1: 

How to cite: Shehata, M., Rastello, M., Naaim, F., and Bellot, H.: Experimental study of gravity current propagation over rough tilted surfaces, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-1427, https://doi.org/10.5194/egusphere-egu24-1427, 2024.

Particle-laden gravity currents are extremely important in geophysical flow applications. They are the major pathway of sediments in subaqueous environments such as deep lakes and oceans and, to some extent, in the shallower seas of the continental shelves [1]. Various approaches have been developed to model these currents with different complexities ranging from box models, shallow water models, single-phase and two-phase flow models. The starting point of the present analysis is based on Gadal et al. [2] who investigated the role of non-dimensional parameters such as the bed slope (α), the Reynolds number (Re), the Stokes number (St), and the volume fraction (φ) on the dynamics of the front velocity at the early stage of the current propagation using both experimental and numerical approaches. The front velocity at short time scales as the square root of the reduced gravity times the initial lock-height. Overall, it is an increasing function of the bed slope and a decreasing function of the initial volume fraction (for φ>0.45). It is also shown that the duration of the initial constant velocity regime decreases with the particle settling velocity or Stokes number at small bed slope angles. The 2D two-fluid simulations performed with sedFOAM [3] have been shown to reproduce almost quantitatively these trends however a comprehensive description of the detailed underlying physical mechanisms is still missing. In this contribution, we propose to use the two-fluid model to address this question. To achieve this goal, 3D two-fluid simulations have been performed and the numerical results have been depth-averaged over the current shape. The mass balance is used to quantify the entrainment at the current interface and the various terms entering in the momentum balance are extracted from the simulation results. These analysis are used to understand the origin of the current dynamics attenuation such as fluid viscous and turbulent stresses, particle-particle interactions, and fluid-particle interactions.

References:

[1] Meiburg, E. and Kneller, B. (2010). Turbidity currents and their deposits. Annual Review of Fluid Mechanics, 42(1):135–156.

[2] Gadal C., Mercier M. J., Rastello M., and Lacaze L. (2023). “Slumping regime in lock-release turbidity currents”,. J. Fluid Mech., 974:A4,

[3] Chauchat, J., Cheng, Z., Nagel, T., Bonamy, C., and Hsu, T.-J. (2017). Sedfoam- 2.0: a 3-d two-phase flow numerical model for sediment transport.    Geoscientific Model Development, 10(12):4367–4392.

How to cite: Sharma, M. K., Bonamy, C., Rastello, M., Gadal, C., Lacaze, L., and Chauchat, J.: Investigating the dynamics of particle-laden gravity currents using two-fluid simulations, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-16782, https://doi.org/10.5194/egusphere-egu24-16782, 2024.

How to cite: Mercier, M., Schneider, J., Gadal, C., and Lacaze, L.: Influence of particle's buoyancy on turbidity currents from continuous influx, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-16920, https://doi.org/10.5194/egusphere-egu24-16920, 2024.

Hyperpycnal river inflows discharging into lakes or reservoirs will plunge and trigger gravity-driven underflows near the bed. Such underflows are called turbidity currents if the density excess is mainly caused by a high sediment concentration. These underflows can reach the bottom of the lake or, alternatively, detach from the bed and intrude horizontally into the lake waters to form an interflow if a layer of equal density is encountered. As underflows carry a number of constituents fed to them by the river or eroded from the bed, such as sediment, contaminants, nutrients and oxygen, their pathway and final destination have an impact on lake or reservoir water quality. The mixing processes in the plunging region are known to be of dominant importance for the dilution of the inflowing river water by entrainment of ambient water, thereby exerting a primary control on the final intrusion depth of underflows. Understanding and quantifying these processes is therefore key. Until now, the majority of estimations of the mixing extent in the plunging region were made within laterally confined laboratory experiments or via passive tracer methodologies. This study focuses on quantifying plunging mixing from flow velocity measurements in a laterally unconfined river inflow in the field and investigating its dependency on inflow conditions.

Field measurements of the plunging Rhône River entering Lake Geneva were conducted using a boat-towed ADCP along a grid of transects for six inflow conditions characterized by the inflow densimetric Froude number Fr_d. The plunging mixing coefficient E_p, which compares the underflow discharge immediately post-plunging to the initial river inflow discharge, was used to quantify the plunging mixing.

Results indicate that for larger Fr_d values (Fr_d > 4) E_p estimates align with laterally confined lab experiments (E_p = O(0.1)). Conversely, for smaller Fr_d values E_p estimates correlate with field tracer measurements of laterally unconfined inflows (E_p = O(1)). E_p decreases with increasing Fr_d, challenging existing numerical simulations predicting the opposite relationship.

This study offers key insights into turbulent mixing rates associated with hyperpycnal river inflows and highlights the need to incorporate realistic field conditions for accurate modeling of plunging river inflows and intrusion depth.

How to cite: Thorez, S., Lemmin, U., Barry, D. A., and Blanckaert, K.: Quantifying Turbulent Mixing in Plunging River Inflows: Insights from Field Measurements in Lake Geneva, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-17856, https://doi.org/10.5194/egusphere-egu24-17856, 2024.

We explore the dynamics of inclined temporal gravity currents using direct numerical simulation, and find that the current creates an environment in which the flux Richardson number, gradient Richardson number and turbulent flux coefficient are constant across a large portion of the depth of the outer layer. Changing the slope angle modifies these mixing parameters, and the flow approaches a maximum Richardson number of approx. 0.15 as the angle tends to zero, for which the entrainment coefficient E->0.

The turbulent Prandtl number remains O(1) for all slope angles, demonstrating that E->0 is not caused by a switch-off of the turbulent buoyancy flux. Instead, E->0 occurs as the result of the turbulence intensity going to zero as the angle tends to zero, due to the flow requiring larger and larger shear to maintain the same level of turbulence. We develop a conceptual model which is in excellent agreement with the DNS data.

How to cite: van Reeuwijk, M., Cui, L., and Hughes, G.: Mixing and entrainment in inclined gravity currents, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-22329, https://doi.org/10.5194/egusphere-egu24-22329, 2024.

The present work investigates the dynamical characteristics of a gravity current produced by a continuous and steady release of a dense fluid, with a source density ρ₀, into a lighter ambient fluid, with density ρ_a < ρ₀. A planar geometry is considered in which the ambient fluid is initially at rest inside a rectangular tank and the dense current is continuously released from a source located at the bottom-left corner of the tank.  The released current propagates along the horizontal bottom boundary of the domain displacing the ambient fluid. This configuration has been considered experimentally by Sher & Woods (2017). A numerical study has been carried out using the Direct Numerical Simulations (DNS) of the governing equations via the NEK5000 solver. Instantaneous three-dimensional velocity, pressure and density fields were extracted and two-dimensionalized by width-averaging. The state of the release is characterized by the source Froude number Fr₀ = u₀ / √(g^'₀ h₀ ) with u₀ being the velocity of the release, h₀ being the height at the inlet, and g^'₀ = g (ρ₀ - ρ_a)/ρ_a being the source buoyancy. Throughout the series of simulations, we control the state of the current at the source by only varying the source density ρ₀, resulting in a range of source Froude number between 0.6 < Fr₀ < 2.7, and we seek to record the effects of this variation on the dynamics. The source discharge Q₀ = u₀ h₀ and buoyancy flux B₀ = Q₀ g^'₀ are kept constant over time. The front speed, u_f, was shown to remain steady; a well-known feature of continuous gravity currents. A dimensionless parameter, λ = u_f/B₀^1/3, that characterizes the front speed was computed as a function of Fr₀ and the result shows a good agreement with the range recorded by Sher & Woods (2017). The entrainment of ambient fluid into the current is parametrized with two methods. First, we estimate the rate of change of the volume of the current, dV/dt, and we recorded the range 1.8Q₀ < dV/dt < 2.1Q₀ for the selected Fr₀  range. Secondly, the theory of inclined plumes introduced by Ellison & Turner (1959) was considered to estimate a local entrainment parameter, E, as a function of the local stratification represented by the local Richardson number Ri. The well-known relation, E proportional to Ri^-1, was held when Ri < 0.8; otherwise, the entrainment parameter tends to near-zero values.

How to cite: Harrouk, M., Mehaddi, R., Arcen, B., and Dossmann, Y.: The characterization of the dynamics of a continuous gravity current, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-10029, https://doi.org/10.5194/egusphere-egu24-10029, 2024.

We present a relatively recent laboratory experiment, the Stratified Inclined Duct (SID), that sustains a buoyancy-driven exchange flow and allows to accurately control and measure stratified sheared turbulence. The submesoscale turbulent mixing of momentum, heat, and salinity in the ocean have a leading-order but poorly constrained large-scale impact. We argue that SID may serve as a fruitful testbed for studying these processes, especially near boundaries, such as in estuaries.

First, we introduce SID, which consists of two large (400 litres) reservoirs containing salt solutions connected by a long rectangular duct. The long-lasting  exchange within the duct has a Reynolds number of order Re ~ 1,000-10,000. The apparatus can be tilted at a small angle θ with respect to the horizontal, which energises the flow and increases turbulence levels, due to the emergence of 'hydraulic control'.

Second, we present high-resolution experimental measurements of the three-dimensional velocity and density field within the duct which allow to delve into the energetics of stratified turbulence. SID uniquely allows the experimenter to control the level of turbulent kinetic energy dissipation, and to sweep through increasingly turbulent regimes by varying the key product Re*θ. The levels of turbulent intensity are comparable to those found in moderately turbulent patches in the ocean.

Third, we demonstrate that a data-driven analysis combining automated image analysis, data reduction and unsupervised clustering discovered previously unsuspected patterns in a large SID turbulence dataset. Multiple types of energetic turbulence were found, as well as intermittent turbulence that cycles between these types through distinct transition pathways. We argue that this data-driven identification of turbulence is a stepping stone towards better physics-based parameterizations.

How to cite: Lefauve, A., Couchman, M., and Linden, P.: The Stratified Inclined Duct: a new canonical laboratory experiment to study ocean turbulence and mixing, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-6404, https://doi.org/10.5194/egusphere-egu24-6404, 2024.

Gravity currents play a crucial role in the formation of deep waters in the ocean, contributing to the vorticity and energy transfers towards the ocean interior. We present results from an experimental study on downslope intruding and rotating gravity currents into an initially two-layer stably stratified ambient at high buoyancy Reynolds numbers. A new turbulent process of downslope transport, intermittent and localized, is identified, taking the form of cascades. The lifetime of cascades presents a power law relationship, and the related transport does not exhibit any characteristic length scale, suggesting self-organized criticality. Cascades reveal to be the main contributor to the vorticity and turbulence in the ocean interior, with a dependence on the Coriolis parameter and the density anomaly to the surrounding ambient. Vorticity is produced both by the spreading of the cascade into the interior, and by the meandering and the break up of the deep boundary current (formed from downward Ekman transport). When the intrusion spreads at the pycnocline only, anticyclonic eddies are formed in the intrusion and top layers, whereas for intrusions spreading through the full bottom layer, vortices of both signs are generated due to bottom friction. The turbulence in the receiving ambient reveals to be horizontally isotropic, non-stationary and non-homogeneous. In the intrusion area close to the slope, the turbulence is forced by energy injection at the penetration length scale through the cascades. The central area far from the boundaries is characterized, instead, by freely evolving two-dimensional turbulence, forced at large scales.

How to cite: Axel, T., Maria Eletta, N., and Achim, W.: On the turbulent structures generated by intruding downslope rotating gravity currents, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-5897, https://doi.org/10.5194/egusphere-egu24-5897, 2024.