Session OS4.2

OS4.2 | Eddies, waves, and instabilities: observing, modelling, and parameterizing oceanic energy transfers

Eddies, waves, and instabilities: observing, modelling, and parameterizing oceanic energy transfers

Convener: Manita ChoukseyECSECS | Co-conveners: Stephan Juricke, Nils Brüggemann, Knut Klingbeil, Friederike PollmannECSECS

Orals

| Wed, 17 Apr, 14:00–15:45 (CEST)

Room 1.34

Posters on site

| Attendance Tue, 16 Apr, 16:15–18:00 (CEST) | Display Tue, 16 Apr, 14:00–18:00

Hall X4

Energy conservation is a fundamental physical principle, yet it is generally not achieved in state-of-the-art models of geophysical flows owing to, for instance, the governing equations and their discretization, the coupling between model components, or the parameterization of unresolved processes. It is thus non-trivial to close the energy budget, which becomes even more challenging due to the multitude of oceanic processes that undergo nonlinear interactions and drive energy transfers across a range of scales: from (sub-)mesoscale eddies to internal waves to small-scale turbulence. This session is devoted to understanding these multi-scale interactions and associated energy transfers, which are ultimately crucial for developing energetically consistent models, confidently predict climatic changes, and quantify associated uncertainties, and thus improve our understanding of the climate system.

We invite contributions on oceanic energy pathways and their consistent representation in numerical models from theoretical, modeling, and observational perspectives. These include, but are not limited to, the processes involving (sub-)mesoscale eddies, internal gravity waves, instabilities, turbulence, small-scale mixing, and ocean-atmosphere coupling. Contributions on energy transfer processes and their quantification from in-situ measurements, (semi-)analytical approaches, and numerical models, as well as their parameterizations and spurious energy transfers associated with numerical discretizations, are also welcome along with interdisciplinary contributions such as novel applications in data science that diagnose, quantify, and minimize energetic inconsistencies and related uncertainties.
We particularly encourage early career researchers to participate in this session.

Orals: Wed, 17 Apr | Room 1.34

Chairpersons: Manita Chouksey, Stephan Juricke, Nils Brüggemann

14:00–14:05

5-minute convener introduction

14:05–14:25

EGU24-3494

solicited

On-site presentation

Prandtl Number Effects on Extreme Mixing Events in Forced Stratified Turbulence

Colm-cille P. Caulfield, Nicoloas Petropoulos, Miles M. P. Couchman, Steve de Bruyn Kops, and Ali Mashayek

`Strongly' stratified turbulent flows can self-organise into a `layered anisotropic stratified turbulence' (LAST) regime, characterised by relatively deep and well-mixed density `layers' separated by relatively thin `interfaces' of enhanced density gradient. Understanding the associated mixing dynamics is important for parameterising heat transport in the world's oceans. It is challenging to study `LAST' mixing, as it is associated with Reynolds numbers Re := UL/ν >> 1 and Froude numbers Fr :=(2πU)/(L N) << 1, (U and L being characteristic velocity and length scales, ν being the kinematic viscosity and N the buoyancy frequency). As a sufficiently large dynamic range (largely) unaffected by stratification and viscosity is still required, the buoyancy Reynolds number Re_b := ε/(ν N²) >> 1 where ε is the TKE dissipation rate. This requirement is exacerbated for oceanically relevant flows, as the Prandtl number Pr := ν /κ = O(10) in thermally-stratified water (where κ is the thermal diffusivity), thus leading (potentially) to even finer density field structures. We report here on four forced fully resolved direct numerical simulations of stratified turbulence at various Froude (Fr=0.5, 2) and Prandtl numbers (Pr=1, 7) forced so that Re_b=50, with resolutions up to 30240 x 30240 x 3780. We find that, as Pr increases, emergent `interfaces' become finer and their contribution to bulk mixing characteristics decreases at the expense of the small-scale density structures populating the well-mixed `layers'. Nevertheless, `extreme' mixing events (with elevated local destruction rates of buoyancy variance χ₀dominating the total mixing budget) are still preferentially found in strongly stratified interfaces, which has significant implications for parameterising diapycnal mixing in larger scale ocean models.

This project received funding from the European Union's Horizon 2020 research and innovation program under the Marie Sklodowska-Curie Grant Agreement No. 956457 and used resources of the Oak Ridge Leadership Computing Facility at the Oak Ridge National Laboratory, supported by the Office of Science of the U.S. Department of Energy under Contract No. DE-AC05-00OR22725. S.deB.K. was supported under U.S. Office of Naval Research Grant number N00014-19-1-2152.

How to cite: Caulfield, C.-P., Petropoulos, N., Couchman, M. M. P., de Bruyn Kops, S., and Mashayek, A.: Prandtl Number Effects on Extreme Mixing Events in Forced Stratified Turbulence, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-3494, https://doi.org/10.5194/egusphere-egu24-3494, 2024.

14:25–14:35

EGU24-13249

On-site presentation

Turbulent dynamics and energetics of anticyclonic submesoscale headland wakes

Tomás Chor and Jacob Wenegrat

Both submesoscale flows and interactions with topography have been posited to be important factors driving dissipation and mixing in the ocean. However, since their in-situ measurement is difficult and numerical approaches do not typically resolve the turbulent processes responsible for the irreversible mixing and dissipation, the dynamics and energetics of submesoscale flows generated by topography are not currently well-understood.

In this work we attempt to clarify the topic by investigating a series of realistic Large-Eddy simulations of submesoscale flows past a headland where the turbulence is fully resolved, allowing us to probe into the small-scale processes responsible for the energy cascade. Consistent with previous studies, we find strong evidence of submesoscale centrifugal-symmetric instabilities (CSIs) in the wake, with most of the wakes being energized mainly via horizontal shear production (i.e., centrifugal instabilities). We also find that the mixing efficiency (the fraction of total energy extracted from the flow spent mixing buoyancy) within CSI regions in the wake varies between approximately 0.1 and 0.3, consistent with previous studies that found similar variability in CSI mixing efficiency values.

Finally, despite our simulations spanning a wide range of parameter space and at least three different dynamical regimes (namely regimes with detached eddy formation, attached boundary layers and tridimensional wake turbulence), we show that some quantities of interest can be predicted by simple scalings. As examples, the kinetic energy dissipation and buoyancy mixing rates scale with the Slope Burger number, and the vertical eddy diffusivity scales with the Rossby number times the Froude number.

How to cite: Chor, T. and Wenegrat, J.: Turbulent dynamics and energetics of anticyclonic submesoscale headland wakes, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-13249, https://doi.org/10.5194/egusphere-egu24-13249, 2024.

14:35–14:45

EGU24-7470

ECS

On-site presentation

Evolution from Agulhas Ring to Mode-Water Eddy?

Ria Oelerich, Maren Walter, Ralf Bachmayer, Christian Mertens, Lucas Merckelbach, and Jeff Carpenter

Agulhas Rings are anti-cyclonic warm-core eddies that originate from the interaction of the Agulhas current with the Antarctic Circumpolar Current (Agulhas Retroflection) at the southern tip of Africa. The Agulhas Rings are advected with the Benguela Current to the northwest and transport heat and salt into the South Atlantic Ocean and are thus affecting the ecosystem and the carbon cycle. The Walvis Ridge, which is located off the coast of Namibia, is a natural obstacle for the Agulhas Rings that are oftentimes unable to cross the ridge and thus remain in its vicinity until they dissolve. Due to the lack of long-term, high-resolution and sub-surface observations it is not well understood how the properties of these eddies evolve with time. We present long- and short-term observations from moorings (2022-2023) and two ocean glider campaigns (2022, 2023) as part of the SONETT I and SONETT II research expeditions near Walvis Bay. For the glider missions, automated adaptive sampling algorithms were developed to systematically improve the spatial and temporal resolution in the region of interest that is dynamically changing. Moreover, the gliders were equipped with microstructure probes for detailed energy dissipation measurements. In this study, we show how the eddy characteristics, such as temperature, salinity and oxygen, near Walvis Bay evolve with time and how these changes relate to the energy dissipation. Specifically in 2023, a deep glider (up to 1000 m) observed a unique eddy structure below the surface mixed layer that displayed subsurface eddy characteristics, but with a surface signal, indicating the characteristics of a mode-water eddy with a very distinct pattern of energy dissipation.

How to cite: Oelerich, R., Walter, M., Bachmayer, R., Mertens, C., Merckelbach, L., and Carpenter, J.: Evolution from Agulhas Ring to Mode-Water Eddy?, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-7470, https://doi.org/10.5194/egusphere-egu24-7470, 2024.

14:45–14:55

EGU24-5062

ECS

On-site presentation

Joint Effects of Winds and Tides on Near-Inertial Internal Waves in the Northern South China Sea: A Three-Dimensional Numerical Study

Yankun Gong

Joint effects of winds and tides on near-inertial internal waves (NIWs) are numerically investigated via a series of three-dimensional quasi-realistic simulations in the northern South China Sea (NSCS). Model results demonstrate that in the presence of wind-induced NIWs, more tidal energy is transferred to NIWs, while in the presence of tide-induced NIWs, the extreme wind (cyclone) would inject less near-inertial kinetic energy (NIKE). The interaction between wind-induced and tide-induced NIWs produces total NIKE more (or less) than a linear superposition of that generated by wind and tide forcing alone at different sites in the NSCS. Specifically, near the Luzon Strait, both tides and winds make positive contributions to the local near-inertial energy input, resulting in more than 30% enhancement of total NIKE (>0.5 kJ m^-2). However, in some deep-water regions along the cyclone paths, energy is transferred from cyclones to NIWs and also from NIWs to internal tides. Due to this “energy pipeline” effect, tide- and wind-induced NIWs contribute to weakening of total NIKE (~0.3 kJ m^-2 or 30%). Additionally, sensitivity experiments with varying initial tidal phases indicate that the interaction between wind-induced NIKE and tide-induced NIKE is robust in most model domain (over 80%) under different phase alignments between wind- and tide-induced NIWs. From the perspective of cyclones, tide-induced NIKE is comparable to wind-induced NIKE in the Luzon Strait before the arrival of cyclones, while tide-induced NIKE is two orders of magnitude smaller than wind-induced NIKE in most of the NSCS after the arrival of cyclones. Overall, our results highlight the joint effects of wind and tide forcing on the local NIW dynamics in the NSCS.

How to cite: Gong, Y.: Joint Effects of Winds and Tides on Near-Inertial Internal Waves in the Northern South China Sea: A Three-Dimensional Numerical Study, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-5062, https://doi.org/10.5194/egusphere-egu24-5062, 2024.

14:55–15:05

EGU24-9301

ECS

Virtual presentation

Data-driven and data-agnostic stochastic parametrization of unresolved processes

Francesco Tucciarone, Etienne Mémin, and Long Li

The simulation of planetary flows at all the scales that have a significant impact on the climate system is unachievable with nowadays computational resources. Large-scale simulations of the Ocean (as well as for Atmosphere) remains the primary tool of investigation while high resolution simulations can be obtained only for small geographical domains or short integration periods. The complex interdependence of mesoscale and sub-mesoscale dynamics, however, is lost in state-of-the-art simulations when performed at scales that are too large to capture these phenomena. Most of the modeling challenges arise from the representation of these effects in a parameterized manner. This work investigates the so called Location Uncertainty (LU) framework [1,2], that provides a solid theoretical background for the definition of a large-scale representation with an additional stochastic component representing the subgrid contribution, introducing new degrees of freedom to be exploited in the modeling of specific phenomena [2]. The model, that has been proven successful in several large-scale models for ocean dynamics [3,4,5,6], is implemented in the community ocean model NEMO (https://www.nemo-ocean.eu) in its hydrostatic primitive equation version, as outlined in [6] and already tested in [7]. An idealized double-gyre configuration is shown to be improved by the stochastic addition in both eddy permitting (~35km) and eddy resolving (~10km) regimes, under a variety of choices of the noise model including both data-driven and data-agnostic approaches.

[1] E. Mémin Fluid flow dynamics under location uncertainty,(2014), Geophysical & Astrophysical Fluid Dynamics, 108, 2, 119–146.

[2] G. Tissot, E. Mémin, Q. Jamet, (2023), Stochastic compressible Navier-Stokes equations under Location uncertainty, Stochastic Transport in Upper Ocean Dynamics, Springer.

[3] W. Bauer, P. Chandramouli, L. Li, and E. Mémin. Stochastic representation of mesoscale eddy effects in coarse-resolution barotropic models. Ocean Modelling, 151:101646, 2020.

[4] Rüdiger Brecht, Long Li, Werner Bauer and Etienne Mémin. Rotating Shallow Water Flow Under Location Uncertainty With a Structure-Preserving Discretization. Journal of Advances in Modeling Earth Systems, 13, 2021MS002492.

[5] V. Resseguier, L. Li, G. Jouan, P. Dérian, E. Mémin, B. Chapron, (2021), New trends in ensemble forecast strategy: uncertainty quantification for coarse-grid computational fluid dynamics, Archives of Computational Methods in Engineering.

[6] F.L. Tucciarone, E. Mémin, L. Li, (2022), Primitive Equations Under Location Uncertainty: Analytical Description and Model Development, Stochastic Transport in Upper Ocean Dynamics, Springer.

[7] F.L. Tucciarone, E. Mémin, L. Li, (2023), Data driven stochastic primitive equations with dynamic modes decomposition, Stochastic Transport in Upper Ocean Dynamics, Springer.

How to cite: Tucciarone, F., Mémin, E., and Li, L.: Data-driven and data-agnostic stochastic parametrization of unresolved processes, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-9301, https://doi.org/10.5194/egusphere-egu24-9301, 2024.

15:05–15:15

EGU24-4115

ECS

On-site presentation

Convective Mixing: an Energetically Consistent Non-Local Parameterization

Manolis Perrot and Florian Lemarié

During shallow and deep oceanic convection, eddy-diffusivity parameterizations are known to fail since advection by non-local plumes is the primary source of turbulent transport. Integration of the Mass-Flux concept to correct Eddy-Diffusivity approaches has long been studied and applied for parameterizing convection in atmospheric models, and very recently in ocean models. This closure involves breaking down vertical turbulent fluxes into two components:

a diffusion term that addresses local small-scale mixing in an near isotropic environment, which intensity is typically scaling with turbulent kinetic energy (TKE) ;
a mass-flux transport term, that accounts for the non-local transport due to vertically coherent plumes within the environment.

We expose an energetically consistent coupling of Eddy-Diffusivity Mass-Flux (EDMF) schemes with TKE schemes, in order to model oceanic convection.

To achieve such a goal, we reexamine PDE-based derivations from first principles relying on multi-fluid averaging techniques. This approach offers several key advantages. Firstly, it allows to establish fully consistent local and global energy budgets between resolved and subgrid scales, effectively rectifying energy biases present in prior EDMF schemes. Notably, it facilitates a clear separation of convective and turbulent small-scale energy reservoirs. This is a significant departure from traditional schemes used in ocean modeling to account for non-local effects (e.g. KPP). It also provides a lucid description of boundary terms in order to remedy double-counting errors.

Secondly, when compared to existing oceanic schemes, our model demonstrates performance in reproducing mean fields as well as higher-order moments such as TKE, vertical fluxes, and turbulent transport of TKE. It is validated against Large Eddy Simulation (LES) and observational data of oceanic convection.

Thirdly, during the theoretical development of the scheme, we maintain transparency regarding underlying assumptions, and systematically assess their validity in the light of LES data. Thus our framework exhibits the flexibility to formally relax scale assumptions, leading toward scale-awareness.

How to cite: Perrot, M. and Lemarié, F.: Convective Mixing: an Energetically Consistent Non-Local Parameterization, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-4115, https://doi.org/10.5194/egusphere-egu24-4115, 2024.

15:15–15:25

EGU24-349

ECS

On-site presentation

Breaking Internal Waves and Ocean Diapycnal Diffusivity in a High-Resolution Regional Ocean Model: Evidence of a Wave-turbulence Cascade

Kayhan Momeni, Yuchen Ma, William R. Peltier, Dimitris Menemenlis, Ritabrata Thakur, Yulin Pan, Brian K. Arbic, Joseph Skitka, and Matthew H. Alford

While the primary origin of ocean diapycnal diffusivity is commonly attributed to stratified turbulence induced by breaking internal waves (IWs), verifying diffusivity values in ocean circulation models within specific geographical regions remains challenging due to limited microstructure measurements. Recent analyses of a downscaled global ocean simulation into higher-resolution regional setups northeast of Hawaii, reveal a notably enhanced fit between simulated IW spectra and in-situ profiler measurements like the Garrett-Munk spectrum [Nelson et al. (2020), Pan et al. (2020), Thakur et al. (2022)].

In this study, we utilize this dynamically downscaled ocean simulation to scrutinize the dynamics of IW-breaking and the wave-turbulence cascade in this region explicitly. Employing a modified version of the Kappa Profile Parameterization (KPP), we infer the horizontally-averaged vertical profile of diapycnal diffusivity. Comparing this inferred profile to the background profile used in low-resolution coupled climate models—such as the Community Earth System Model (CESM) by the US National Center for Atmospheric Research (NCAR)—is a central aspect of our investigation.

Our exploration reveals that the wavefield in the high-resolution regional domain is dominated by a well-resolved spectrum of low-mode IWs, predictable through appropriate eigenvalue computations for stratified flow. Finally, we propose a novel tentative approach to enhance the KPP parameterization. This approach holds promise for refining our understanding of diapycnal diffusivity, offering valuable insights for improving ocean circulation models.

References:

AD Nelson, BK Arbic, D Menemenlis, WR Peltier, MH Alford, N Grisouard, and JM Klymak. Improved internal wave spectral continuum in a regional ocean model. Journal of Geophysical Research: Oceans, 125(5):e2019JC015974, 2020.

Yulin Pan, Brian K Arbic, Arin D Nelson, Dimitris Menemenlis, WR Peltier, Wentao Xu, and Ye Li. Numerical investigation of mechanisms underlying oceanic internal gravity wave power-law spectra. Journal of Physical Oceanography, 50(9):2713–2733, 2020.

Ritabrata Thakur, Brian K Arbic, Dimitris Menemenlis, Kayhan Momeni, Yulin Pan, W Richard Peltier, Joseph Skitka, Matthew H Alford, and Yuchen Ma. Impact of vertical mixing parameterizations on internal gravity wave spectra in regional ocean models. Geophysical Research Letters, 49(16): e2022GL099614, 2022.

How to cite: Momeni, K., Ma, Y., Peltier, W. R., Menemenlis, D., Thakur, R., Pan, Y., Arbic, B. K., Skitka, J., and Alford, M. H.: Breaking Internal Waves and Ocean Diapycnal Diffusivity in a High-Resolution Regional Ocean Model: Evidence of a Wave-turbulence Cascade, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-349, https://doi.org/10.5194/egusphere-egu24-349, 2024.

15:25–15:35

EGU24-15421

Highlight

On-site presentation

Constraining fiber-optic cable observations of internal waves with conventional oceanographic measurements

Mariona Claret, Arantza Ugalde, Kraig Winters, Anda Vladoiu, Ethan Williams, Joaquim Salvador, Nina Hoareau, Hugo Martins, Hugo Latorre, Pedro J. Vidal-Moreno, Miguel González-Herráez, and José Luís Pelegrí

Ocean mixing plays a crucial role in the Earth’s climate, however quantifying it is challenging because energy enters the ocean at basin-scale but it dissipates at cm-scale over the vast ocean. Internal waves play an important role in the cascade of energy toward dissipative scales. Energy transfers to the internal wave field are greatly enhanced through flow interactions with topography. Observations of wave-topography interactions are, however, scarce. Distributed Acoustic Sensing (DAS) has recently opened a new door for exploring near-bottom wave dynamics using fibre-optic cables at unprecedented spatio-temporal resolution (meters and seconds) over long spatio-temporal scales (kilometers and months). DAS is particularly attractive as it can use telecommunication cables already in place so that it could potentially be implemented at global scale.

Here we present repeated DAS observations on the continental slope east of Gran Canaria island complemented with contemporaneous hydrographic and velocity data collected with bottom moorings for the first time. Results show that upslope propagation of internal tides is a permanent feature at this site. DAS-inferred tidal temperature oscillations of 2 K magnitude agree with direct temperature observations. Preliminary results showing spectral peaks at the M₂ tidal frequency and its harmonics is suggestive of wave-wave interactions. Finally, the potential of DAS to estimate lateral diffusion coefficients is considered.

How to cite: Claret, M., Ugalde, A., Winters, K., Vladoiu, A., Williams, E., Salvador, J., Hoareau, N., Martins, H., Latorre, H., Vidal-Moreno, P. J., González-Herráez, M., and Pelegrí, J. L.: Constraining fiber-optic cable observations of internal waves with conventional oceanographic measurements, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-15421, https://doi.org/10.5194/egusphere-egu24-15421, 2024.

15:35–15:45

EGU24-13707

ECS

On-site presentation

A scale-dependent vertical structure for energy backscatter parameterizations

Wenda Zhang, Stephen Griffies, Elizabeth Yankovsky, Robert Hallberg, and Alistair Adcroft

Ocean mesoscale eddies constitute a crucial component of ocean energy cascade, engaging in energy exchange with large-scale circulations, submesoscale eddies, and internal waves. State-of-the-art ocean climate models, which partially resolve mesoscale eddies (i.e., eddy-permitting), often exhibit weaker and more surface-intensified eddy kinetic energy (EKE) than in higher-resolution simulations and observations. The energy backscatter scheme has been employed in eddy-permitting simulations to enhance the energy of mesoscale eddies by compensating for the excessive dissipation caused by the viscosity closure. In this scheme, a proper vertical structure for the backscatter coefficient is necessary to simulate a more realistic vertical distribution of kinetic energy.

Here we propose a parameterization for the vertical structure of subgrid EKE and implement it within the backscatter scheme in idealized eddy-permitting simulations of MOM6. The parameterization is grounded in the observation that the EKE is surface-intensified and decays faster with depth at smaller horizontal scales. The diagnosed vertical structure of EKE from eddy-resolving simulations is well-captured by surface quasi-geostrophic (SQG) modes, whose vertical structure depends on the eddy horizontal scale, Coriolis parameter, and stratification profile. Based on the SQG mode, we formulate a scale-aware parameterization of the vertical structure, accounting for the variation of subgrid eddy scale with the model horizontal grid spacing. This vertical structure is then applied to the energy backscatter coefficient used in 1/2° and 1/4° idealized simulations of basin-scale ocean circulations. The diagnostics of these eddy-permitting simulations are compared to those of a 1/32° reference simulation. The inclusion of the vertical structure in the backscatter improves the simulation of global kinetic energy distributions, large-scale circulation pathways, and isopycnal structures, compared with the eddy-permitting simulations without backscatter or with a depth-independent backscatter. Sensitivity tests show that a more surface-intensified backscatter tends to result in weaker total kinetic energy and more tilted isopycnals. This work provides insights into the parameterization of mesoscale energetics and its vertical variation in eddy-permitting simulations.

How to cite: Zhang, W., Griffies, S., Yankovsky, E., Hallberg, R., and Adcroft, A.: A scale-dependent vertical structure for energy backscatter parameterizations, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-13707, https://doi.org/10.5194/egusphere-egu24-13707, 2024.

Posters on site: Tue, 16 Apr, 16:15–18:00 | Hall X4

Display time: Tue, 16 Apr, 14:00–Tue, 16 Apr, 18:00

Chairpersons: Friederike Pollmann, Knut Klingbeil, Stephan Juricke

X4.26

EGU24-18340

ECS

Highlight

Transient Attracting Profiles in the Great Pacific Garbage Patch

Luca Kunz, Alexa Griesel, Carsten Eden, Rodrigo Duran, and Bruno Sainte-Rose

A major challenge for cleanup operations in the Great Pacific Garbage Patch is to predict day to day variabilities of plastic concentrations and to identify hotspots of marine debris. An ideal application would indicate material confluence at operational scales O(1 − 10km) but from mesoscale observations of sea surface height O(10 − 100km). The present study introduces the concept of TRansient Attracting Profiles (TRAPs, Serra et al. (2020), Serra and Haller (2016)) to this debate. TRAPs are computable from the instantaneous strain field on the ocean surface and act like short-term attractors for floating objects. Previous experiments have shown the potential of TRAPs to predict pathways of material transport and here we explore the occurrence of these profiles in the North Pacific subtropical gyre, a large-scale convergence zone that is known to entail the Great Pacific Garbage Patch. We compute TRAPs upon daily snapshots of near-surface geostrophic + Ekman current velocity and create a 20-years record of 4,076,065 TRAP objects. We identify 720,391 TRAP trajectories from this data and relate the propagation, persistence and attraction strength of TRAPs to detections of mesoscale eddies. We uncover a life cycle of long-living TRAPs and evaluate recurrent patterns in their vorticity environment. Our study culminates in an unprecedented statistical analysis of drifter motion around TRAPs. We highlight beneficial conditions for hyperbolic transport, estimate local retention times of drifters and find a preference of drifter visits during the formation stage of TRAPs. Our findings provide novel aspects towards mesoscale strain on the ocean surface and how it may benefit search operations at sea.

How to cite: Kunz, L., Griesel, A., Eden, C., Duran, R., and Sainte-Rose, B.: Transient Attracting Profiles in the Great Pacific Garbage Patch, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-18340, https://doi.org/10.5194/egusphere-egu24-18340, 2024.

X4.27

EGU24-16524

ECS

Decomposing Realistic Oceanic Flow in Balanced and Unbalanced Parts Using a Novel Balancing Approach

Silvano Rosenau, Manita Chouksey, and Carsten Eden

Decomposing oceanic flow fields into its slowly evolving geostrophic component and the fast wave mode is necessary to understand processes like mesoscale eddy dissipation and spontaneous wave emission. The application of existing decomposition methods, such as nonlinear normal mode decomposition or optimal balance is limited to idealized model settings that neither include topography nor a varying Coriolis parameter. To overcome these limitations, we propose a new approach that combines optimal balance with a time-averaging procedure. This approach eliminates the necessity for the Fourier transformation that is required in the original optimal balance method. We tested and compared the new variation of optimal balance with the original method in a scaled rotating shallow water model in various dynamical regimes, with Rossby numbers ranging from 0.03 to 0.5. In all tested configurations, the imbalances obtained with the new method converges towards the imbalances obtained with the original method. We further show that the convergence rate can be improved by doing multiple short time averages instead of a single long one. The new method is applicable to realistic ocean scenarios that include topography and a varying Coriolis parameter and shows promising results in decomposing complex flow fields.

How to cite: Rosenau, S., Chouksey, M., and Eden, C.: Decomposing Realistic Oceanic Flow in Balanced and Unbalanced Parts Using a Novel Balancing Approach, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-16524, https://doi.org/10.5194/egusphere-egu24-16524, 2024.

X4.28

EGU24-4023

Resolution of the bottom boundary layer in an eddy-rich model of double-gyre circulation

Si-Yuan (Sean) Chen and Olivier Marchal

The bottom boundary layer (BBL) is the portion of the water column which is directly affected by the drag of ocean currents on the seafloor. It is often assumed to coincide with the bottom mixed layer in which potential temperature is approximately uniform. Bottom mixed layers have an observed thickness of O(10 m), with maxima of O(100 m) in some basins. As a result, they are in general not represented in numerical models of large-scale circulation, which typically assume a vertical resolution of a few hundred meters near the bottom. The coarse vertical resolution near the bottom that is assumed in many numerical models implies that these models may not accurately represent the velocity shears near the bottom and the dissipation rates of kinetic energy by bottom drag, which depends on the near-bottom velocity. Here we present results from idealized numerical experiments of the circulation in a double (subtropical-subpolar) gyre, which are aimed at determining the effects of near-bottom vertical resolution on simulated ocean circulation and energetics. Results from experiments that resolve the BBL are compared to those from experiments that do not. In all experiments, the horizontal grid is fine enough to resolve the mesoscale eddy field. In our presentation, emphasis will be placed on energy dissipation by bottom drag in experiments with different near-bottom vertical resolutions. The implications of our results for the simulation of large-scale ocean circulation will then be clarified.

How to cite: Chen, S.-Y. (. and Marchal, O.: Resolution of the bottom boundary layer in an eddy-rich model of double-gyre circulation, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-4023, https://doi.org/10.5194/egusphere-egu24-4023, 2024.

X4.29

EGU24-2470

Impact of tides and eddies on ocean energy spectra in submesoscale resolving simulations of the South Atlantic

Moritz Epke and Nils Brüggemann

Comprehending how submesoscale dynamics and their potential interplay with tides affect climate models is challenging due to their small scales and high computational demands. To address this challenge, our approach integrates modelling and observational methods. In this study, we investigate the impact of internal tides, eddies and submesoscale currents on the frequency energy spectrum of the ocean. To this end, we apply a novel simulation with telescopic grid refinement to achieve a horizontal resolution finer than 600 m over large regions of the South Atlantic. This refined resolution allows us to accurately capture submesoscale turbulence and a relatively large part of the internal wave spectrum under realistic atmospheric conditions. By comparing simulations with and without tides, we find that without tidal forcing there is significantly less energy at the high frequency end of the spectrum. Validation with mooring and Pressure Inverted Echo Sounder data sets deployed over a two year period in the Walvis Ridge region indicates that the simulation with tides is more accurate in terms of high frequency energy levels. Using an eddy tracking algorithm allows us to differentiate energy spectra within the Agulhas rings from a ring-absent background state. Within these eddies, we observe a substantial shift towards higher power spectral densities of approximately one order of magnitude across both small and large scales.

How to cite: Epke, M. and Brüggemann, N.: Impact of tides and eddies on ocean energy spectra in submesoscale resolving simulations of the South Atlantic, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-2470, https://doi.org/10.5194/egusphere-egu24-2470, 2024.

X4.30

EGU24-12074

ECS

Sensitivity Analysis of the Lorentz Energy Cycle in the ICON-O Model

Dominic Hillenkötter and Nils Brüggemann

X4.31

EGU24-4384

ECS

Estimating the Energy Flux of Internal Tides in the Northern South China Sea Using Underwater Gliders

Zhiyuan Gao, Zhaohui Chen, Xiaodong Huang, Haiyuan Yang, Yanhui Wang, Wei Ma, and Chenyi Luo

X4.32

EGU24-431

ECS

Spatial scales of kinetic energy and its transfer across large-meso scales in the Arctic Ocean

Caili Liu, Qiang Wang, Sergey Danilov, Nikolay Koldunov, Vasco Mueller, Xinyue Li, Dmitry Sidorenko, and Shaoqing Zhang

Despite the importance of the Arctic Ocean for the large-scale circulation and climate, there is still a knowledge gap in our understanding of the spatial characteristics of the Arctic Ocean circulation, especially for the mesoscale. This research diagoses the kinetic energy (KE) and its transfer features of a rich-eddy Arctic Ocean from eddy-resolved sea-ice model FESOM (Finite-Element/volumE Sea ice-Ocean Model) at about 1km in horizontal resolution, revealing the KE spectra at spatial scales and the seasonality. There are two peaks in the kinetic energy spectral density, one at the gyre scale of the Arctic boundary currents (centered at 1700-2000km), and the other associated with the mesoscale (centered at 60km), both of which exhibit a power-law scaling typical for large circulation and ocean eddies, respectively. Energy transfer diagnosis shows that there is inverse-cascade KE pathway at a hundred of kilometers to several killometers, of which the KE transfer value from larger scales to smaller scales is negative. The seasonality of the eddy KE spectrum and energy transfer in upper ocean is found to be highly consistent to sea ice loss trend, while eddy KE in deep ocean shows not significant seasonal variations.

How to cite: Liu, C., Wang, Q., Danilov, S., Koldunov, N., Mueller, V., Li, X., Sidorenko, D., and Zhang, S.: Spatial scales of kinetic energy and its transfer across large-meso scales in the Arctic Ocean, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-431, https://doi.org/10.5194/egusphere-egu24-431, 2024.

X4.33

EGU24-578

ECS

Modelling the seasonal and spatial variation of Turbulent Kinetic Energy budget in the north-western Indian Ocean

Rajesh Chauhan, Manasa Behera, and Sridhar Balasubramanian

North-western Indian Ocean (NIO), especially Arabian Sea (AS), experiences intense seasonally reversing winds and undergoes a seasonal change in the turbulence energetics. Given the importance of Arabian Sea dynamics on the Indian monsoon, a numerical investigation was carried out to study the energy budget using MOM 5. Model domain is between 32 to 118⁰ E and 28⁰ S to 30⁰ N and is forced with daily varying 10 year averaged atmospheric fluxes. Lateral open boundaries are prescribed with sea surface height anomaly (SSHA) in combination with radiation condition. Vertical profiles of temperature and salinity are also prescribed at lateral boundaries. Using the averaged forcing, model is run for 10 years as spin up and brought into equilibrium and then forced with 5 years inter-annually varying dataset, which is used for analysis. Though the model domain covers quite an extensive part of Indian Ocean, but our analysis is limited in the NIO between 8⁰S to 30⁰N and 32 to 82⁰E (hereafter called as Analysis domain). Model produced data is seasonally averaged into 4 seasons, DJF (December to February), MAM (March to May), JJAS (June to September), and ON (October-November). Currents are well represented by model and are validated with OSCAR currents. Ocean properties such as Sea Surface Temperature, Sea Surface Salinity and Mixed Layer Depth produced by model are within reasonable bias. The investigation of TKE indicates that it has seasonal and spatial preference. Depth averaged over 30m TKE, indicates that it is strongest in JJAS and DJF along the Somalia coast and equatorial region of NIO respectively. In MAM, TKE is strong in south-western AS, close to equator and in ON, a reminiscence of TKE is seen along the Western AS (WAS). Spatial average vertical profile of TKE for 3 regions, [1: 8⁰S to 30⁰N and 32 to 82⁰E (Analysis domain), 2: 4⁰S to 4⁰N and 56 to 82⁰E (Western Equatorial Indian Ocean, WEIO), and 3: 4 to 12⁰N and 45 to 55⁰E (WAS)] is shown. Analysis reveals that TKE is highest in upper 200m and decreases with depth, and in DJF, it is maximum in WEIO and least in WAS, but quite interestingly, after 150m till about 250m, TKE increases in WAS. This could indicate strong subsurface turbulent activity in WAS in DJF. In JJAS, TKE is almost 4 times that of DJF and is highest in WAS and least in WEIO. In WAS, turbulence produced due to buoyancy is suppressed by production flux being negative which indicates inverse energy cascade in DJF. In JJAS, production flux is strongly positive in WAS with negative production in the Great Whirl eddy and in the equatorial region. Turbulence is produced by buoyancy as well near horn of Africa, yet dissipation is weak in the said region, which could be due to strong positive transport. Production shows weak positive features in WAS and is strongly negative in equatorial region in MAM and ON. Buoyancy flux is negative in WAS in MAM and ON, indicative of stable stratification.

How to cite: Chauhan, R., Behera, M., and Balasubramanian, S.: Modelling the seasonal and spatial variation of Turbulent Kinetic Energy budget in the north-western Indian Ocean, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-578, https://doi.org/10.5194/egusphere-egu24-578, 2024.

X4.34

EGU24-4161

ECS

Highlight

Responses of Phytoplankton Communities to Internal Waves inOligotrophic Oceans

wupeng xiao, lingqi ma, xiaolin bai, Edward Laws, cui guo, xin liu, Kuo-Ping Chiang, kunshan gao, and bangqin huang

Understanding the potential impacts of internal waves on phytoplankton communities in oligotrophic oceans remains an important research challenge. In this study, we elucidated the impact of internal waves on phytoplankton communities through a comprehensive 154-hr time-series of observations in the South China Sea (SCS). We identified distinctive variations in phytoplankton pigment biomass and composition across the upper, middle, and lower layers of the euphotic zone, which we attributed to the perturbations triggered by internal waves. Phytoplankton other than Prochlorococcus in the lower, nutrient-replete layer likely benefitted from allochthonous nutrients introduced by internal waves, but their growth rates were constrained by light limitation, and their pigment biomass was held in check by microzooplankton grazing. In contrast, in the upper, nutrient-depleted layer, the relative abundance of Prochlorococcus increased, likely because of the ammonium regenerated by zooplankton. The middle layer, characterized as the deep chlorophyll maximum layer, exhibited a dynamic equilibrium characterized by nutrient and light co-limitation. This equilibrium resulted in high nitrate assimilation and growth by phytoplankton. The balancing of those rates by significant grazing losses maintained total chlorophyll a concentrations at a high level. Based on these findings, we proposed a three-layer euphotic zone structure characterized by distinct physiological conditions, nutrient-light dynamics, grazing pressure, and phytoplankton responses to internal waves. This three-layer paradigm elucidated the intricate interplay between internal waves and phytoplankton communities and provided insights into the mechanisms that govern primary production and carbon cycling in oligotrophic oceanic ecosystems.

How to cite: xiao, W., ma, L., bai, X., Laws, E., guo, C., liu, X., Chiang, K.-P., gao, K., and huang, B.: Responses of Phytoplankton Communities to Internal Waves inOligotrophic Oceans, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-4161, https://doi.org/10.5194/egusphere-egu24-4161, 2024.

X4.35

EGU24-5641

Energy Transfer between Mesoscale Eddies and Near-inertial Waves from Surface Drifter Observations

Zhiwu Chen

X4.36

EGU24-6399

Implementation of implicit filter for spatial spectra extraction

Kacper Nowak and Sergey Danilov

Scale analysis based on coarse-graining has been proposed recently as an alternative to Fourier analysis. It is now broadly used to analyze energy spectra and energy transfers in eddy-resolving ocean simulations. However, it requires interpolation to a regular mesh for data from unstructured-mesh models. We present an alternative coarse-graining method which relies on implicit filters using discrete Laplacians. This method can work on arbitrary (structured or unstructured) meshes and is applicable to the direct output of unstructured-mesh ocean circulation models. We also present a high-performance Python implementation of this method. In this implementation computation has been split into two phases: preparation and solving. The first one is specific only to the mesh. This allows for auxiliary arrays that are then computed to be reused, significantly reducing computation time. The second part consists of sparse matrix algebra and solving linear system. Our implementation is accelerated by GPU to achieve unmatched performance and scalability. This results in processing data based on meshes with more than 10M surface vertices in a matter of seconds. At present, the method is used to compute energy or power spectra of ocean flows.

How to cite: Nowak, K. and Danilov, S.: Implementation of implicit filter for spatial spectra extraction, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-6399, https://doi.org/10.5194/egusphere-egu24-6399, 2024.

X4.37

EGU24-6907

Observations of different effects of an anti-cyclonic eddy on internal solitary waves in the South China Sea

Jiexin Xu

X4.38

EGU24-7177

ECS

Generation of the Equatorial Intermediate Current in the eastern Pacific Ocean

Yusuke Terada and Yukio Masumoto

It has been shown that the Equatorial Intermediate Current (EIC) in the Pacific Ocean, which is westward current along the equator at intermediated depth (from 500m to at least 2000 m), has basin wide zonally uniform structure. In addition to the EIC, intraseasonal variability (ISV) has also been observed near the equator at 1000 m depth with significant amplitude in the eastern Pacific Ocean. Although this intermediate ISV in the eastern basin is considered as an energy source for the EIC, the origin of the intermediate ISV and its relation to the EIC in the Pacific Ocean are still open questions. In this study, we use the 10-day mean horizontal velocity obtained from drift data of the Argo floats and the temperature data from Tropical Atmosphere-Ocean (TAO) moored array in the equatorial Pacific for a period from 2010 to 2022 to investigate the relationship among the EIC, the ISV at the depth of 1000 m, and the ISV in the upper layer.

The meridional component of eddy kinetic energy (V-EKE) at 1000 m depth depicts large intraseasonal variability with a period of about 30 days in the equatorial eastern Pacific, showing a significant seasonality and interannual amplitude modulation. Temperature time series above 500 m depth shows the large amplitude ISV along the equator with the meridionally anti-symmetric structure, whose phase (energy) propagates upward (downward). The surface ISV signal also indicates significant seasonal and interannual variation in its magnitude, which has the highest correlation with the ISV at 1000 m depth at a time lag of 3 months. These results suggest that the ISV at a depth of 1000 m is provided by the upper layer ISV through downward propagating Yanai wave, which takes about 3 months to reach 1000 m depth from the upper layer.

Argo-based zonal velocity in the equatorial Pacific Ocean is found to be westward during most of the high V-EKE season, and its magnitude varies on semiannual to interannual time scales. This suggests that intermittent Yanai wave propagation generates a westward flow at 1000 m depth, which could contribute to the generation of the EIC. Furthermore, comparison of results from two numerical simulations of idealized box ocean with or without eastern basin ISV indicates that downward propagating Yanai wave in the eastern Pacific Ocean generates westward flow locally at the intermediate depth. This may explain the observed basin-scale zonal extent of the EIC, which is unique to the Pacific Ocean. Influences of the realistic topography on the ISV and EIC at the intermediate depth will also be discussed.

How to cite: Terada, Y. and Masumoto, Y.: Generation of the Equatorial Intermediate Current in the eastern Pacific Ocean, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-7177, https://doi.org/10.5194/egusphere-egu24-7177, 2024.

X4.39

EGU24-7446

Characterization of physical properties of a coastal upwelling filamentwith evidence of enhanced submesoscale activityand transition from balanced to unbalanced motions in the South Atlantic

(withdrawn)

Julia Draeger-Dietel, Ryan North, and Alexa Griesel

X4.40

EGU24-11635

ECS

Reducing spurious numerical mixing in simulations under strong tidal forcing : a case study

Adrien Garinet, Patrick Marsaleix, and Marine Herrmann

Tides, and more specifically the internal gravity wave field generated by the tidal current flowing over bathymetric features, play a significant role in the energy cascade and in setting the global state of the ocean by intensifying mixing. Accounting for these dynamics in numerical models is important for accurately depicting energy transfer across scales, and an increasing number of models use explicit tidal forcing. Yet, in fixed coordinates models, the strong vertical motions generated by internal waves trigger the tendency of advection to produce numerical errors, leading on the vertical to spurious numerical mixing that can easily be as high as the physical one. This can severely bias model output and impact the overall simulated dynamics by spuriously mixing water masses and modifying the stratification, which in turn impacts the propagation of internal waves. We propose a fresh way to look at advection schemes, along with a method to make their diffusivity more selective, and eventually protect the relevant vertical scales from being spuriously eroded. A case study on the South East Asian Seas using the Symphonie model is presented, since these seas are known for being the generation site of exceptionally strong internal tides.

How to cite: Garinet, A., Marsaleix, P., and Herrmann, M.: Reducing spurious numerical mixing in simulations under strong tidal forcing : a case study, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-11635, https://doi.org/10.5194/egusphere-egu24-11635, 2024.

X4.41

EGU24-12521

ECS

Interactions of internal gravity waves with other waves and mesoscale eddies

Pablo Sebastia Saez, Carsten Eden, and Manita Chouksey

X4.42

EGU24-17010

ECS

Observations of Eddy - Internal Wave Interactions in the Tropics

Marco Schulz, Florian Schütte, Marcus Dengler, and Tim Fischer

Based on an extensive dataset obtained from multiple individual ship-based samplings of mesoscale eddies and a long-term mooring (2006-2023), interactions between mesoscale eddies and internal waves are analyzed. Theory predicts that anticyclonic mesoscale eddies shift the effective Coriolis force for near-inertial waves (NIW) locally in regions of strong relative vorticity towards subinertial frequencies, leading to trapping of NIW in their core and accelerated downward propagation to a critical layer at the eddy base where mixing is eventually enhanced. In contrast, cyclones might expel NIW through the same but reverse effect. In both cases, and independent of their relative vorticity, increased mixing is expected in regions of strong vertical geostrophic shear at the rims due to critical layer processes. We are able to confirm these theoretical predictions in the observed dataset for several eddies (located mostly in the eddy-rich eastern boundary upwelling systems and waters surrounding the Cape Verde Archipelago). Velocity measurements in coherent anticyclonic eddies repeatedly show pronounced alternating current bands with amplitudes up to 15 cm/s, likely associated with convergence of downward propagated NIW. Microstructure measurements, supplemented by finescale parameterizations, show enhanced mixing in both, the base of anticyclones and slightly elevated dissipation rates at the rims of cyclones, where geostrophic shear is strong. Counterintuitive to the aforementioned theory, enhanced downward propagation was also observed below the center of cyclones. Frequency spectra from the mooring data indicate that the NIW frequency band is altered by the relative vorticity. Overall, a high variability of the internal wave field is observed and impacts of mesoscale eddies are readily apparent in the case of anticyclones, representing a path of energy into the deeper ocean and a key process for local mixing.

How to cite: Schulz, M., Schütte, F., Dengler, M., and Fischer, T.: Observations of Eddy - Internal Wave Interactions in the Tropics, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-17010, https://doi.org/10.5194/egusphere-egu24-17010, 2024.

X4.43

EGU24-18021

Cross-front wind forcing of a dense submesoscale filament

Lars Umlauf, Mira Schmitt, and Jen-Ping Peng

In this study, the effects of cross-front winds on a submesoscale dense filament are investigated using high-resolution turbulence and velocity observations, and idealized numerical simulations. Our study area is the Baltic Sea, which is characterized by strong frontal gradients and pronounced submesoscale dynamics. Embedded in a large-scale frontal region, our observations reveal the existence of a 3-4 km wide, dense filament with an asymmetric structure resulting from the interactions between cross-front winds and the two submesoscale fronts laterally bounding the filament. These two fronts are driven by either downgradient winds, directed from the lighter surrounding waters toward the dense center of the filament, or upgradient winds, directed in the opposite direction. While the effect of a wind stress that is aligned with the frontal jet has been investigated in numerous previous studies, especially field data focusing on the role of cross-front winds are largely lacking at the moment. We find that for downgradient winds, when the surface Ekman transport and the frontal jet are aligned, both the frontal jet and the cross-front secondary circulation are enhanced. The latter supports a tendency for frontal re-stratification, suppression of turbulence, and mixed-layer shoaling. For the front with upgradient wind forcing, the Ekman transport and the frontal jet nearly cancel, and also the cross-front secondary circulation is strongly suppressed. Restratification by the secondary circulation is weak in this case, and the well-mixed turbulent surface layer is approximately twice as deep compared to the other side of the filament with downgradient forcing. We show that these mechanisms are consistent with results from idealized numerical simulations of frontal regions with downgradient and upgradient wind forcing.

How to cite: Umlauf, L., Schmitt, M., and Peng, J.-P.: Cross-front wind forcing of a dense submesoscale filament, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-18021, https://doi.org/10.5194/egusphere-egu24-18021, 2024.