OS4.4 | Eddies, waves, and instabilities: observing, modelling, and parameterizing oceanic energy transfers
EDI
Eddies, waves, and instabilities: observing, modelling, and parameterizing oceanic energy transfers
Convener: Stephan Juricke | Co-conveners: Friederike PollmannECSECS, Nils Brüggemann, Manita ChoukseyECSECS, Knut Klingbeil
Orals
| Mon, 24 Apr, 16:15–18:00 (CEST)
 
Room 1.61/62, Tue, 25 Apr, 08:30–10:15 (CEST)
 
Room 1.61/62
Posters on site
| Attendance Tue, 25 Apr, 14:00–15:45 (CEST)
 
Hall X5
Posters virtual
| Attendance Tue, 25 Apr, 14:00–15:45 (CEST)
 
vHall CR/OS
Orals |
Mon, 16:15
Tue, 14:00
Tue, 14:00
Although it is a fundamental physical principle, energy conservation is generally not achieved in state-of-the-art ocean models. This can be mainly attributed to the model’s governing equations and their discretization, the coupling of different model components, or the parameterization of unresolved processes. This problem is not trivial, since from a theoretical and observational perspective the question of closing the energy budget poses several challenges: The energy is converted and propagated by a variety of processes such as eddies and waves acting on different spatial and temporal scales. In particular, instabilities and non-linear interactions among these processes transfer energy between the different dynamical regimes. For an appropriate description and understanding of our climate system, it is crucial to develop energetically consistent models to confidently predict climatic changes and quantify associated uncertainties.

This session invites contributions from theoreticians, modellers, and experimentalists with an aim to characterize oceanic energy pathways and their accurate representation in numerical models. This includes, but is not limited to, processes involving internal gravity waves,  (sub-)mesoscale turbulence, small-scale mixing, and ocean-atmosphere coupling. We welcome work that focuses on energy transfer processes and their quantification from in-situ measurements, (semi-)analytical approaches, and numerical models, as well as parameterizations of the key energy transfer processes, and spurious energy transfers associated with numerical discretizations. We also encourage cross-disciplinary contributions and presentations of novel approaches to data science that diagnose, quantify, and minimize energetic inconsistencies and related uncertainties.

Orals: Mon, 24 Apr | Room 1.61/62

Chairpersons: Stephan Juricke, Manita Chouksey, Nils Brüggemann
16:15–16:17
16:17–16:47
|
EGU23-17233
|
solicited
|
Highlight
|
Virtual presentation
Catherine Guiavarc'h, Helene Theresa Hewitt, Sophia Marie Moreton, Malcolm Roberts, and David Storkey

Using an explicit representation or a parametrisation of the ocean mesoscale affects not only the mean state of the ocean but also the climate variability. However, the choice of resolution is constrained by computational costs. Ocean models developed at the Met Office are used for a vast range of applications from short-range coupled NWP forecasts to Earth system models. Shorter range predictions can run with higher resolution models while climate models are very constrained. To support all applications, we developed a hierarchy of three ocean models: eddy parametrising (1°), eddy-present (1/4°) and eddy-rich (1/12°) resolution models. In the eddy parametrising configuration, mesoscale eddies are not resolved. In the eddy-present configuration, the resolutions allow some mesoscale eddies to be captured in the low and mid-latitudes. In the eddy-rich configuration, eddies are present at most latitudes.

In the 1° configuration, the mesoscale turbulence associated with eddies cannot be solved explicitly. The lateral turbulent fluxes are assumed to depend linearly on the lateral gradients of large-scale quantities requiring second order operators. A lateral diffusion of momentum on geopotential surfaces with a Laplacian viscosity is used. The effect on the large scale is represented using Gent and McWilliams (1990) proposed parameterisation of mesoscale eddy-induced turbulence. In the 1/4° and 1/12° configurations, the more scale selective biharmonic operator is used. It ensures the stability of the model while not interfering with the resolved mesoscale activity. To improve the circulation and biases in the Southern Ocean, a weak GM parametrisation is added in the eddy-present and eddy-rich models.

30-year forced integrations with the model hierarchy are assessed. Global and large-scale temperature and salinity biases are similar across the resolutions.  The largest differences occur in regions with strong mesoscale activity (Western boundary currents, Southern Ocean). Eddy-present and eddy-rich models significantly improve the representation of the Western boundary currents, both in position and strength. Improving the Western boundary currents has large impacts on temperature and salinity biases.

We review the results of Moreton et al (2020) on eddies in the Met Office hierarchy of models. The surface properties of eddies in eddy-present and eddy-rich coupled models are evaluated using an eddy tracking algorithm on SSH anomalies. Results show that relative to eddy-present, eddy-rich resolution simulates more (+60%) and longer-lasting (+23%) eddies, in better agreement with observations. The representation of eddies in Western Boundary Currents and the Southern Ocean compares well with observations at both resolutions.  However, a common deficiency in the models is the low eddy population in subtropical gyres. Despite a grid spacing larger than the Rossby radius of deformation at high-latitudes, eddy-present resolution only allows for eddy growth in these regions a lower rate than seen in observations and eddy-rich resolution. The westward displacement of eddies in eddy-rich model (mainly in the Agulhas region) is increased compared to the eddy-present model. The size of eddies is found to be dependent on model grid resolution.

How to cite: Guiavarc'h, C., Hewitt, H. T., Moreton, S. M., Roberts, M., and Storkey, D.: Representation of mesoscale in a hierarchy of Met Office ocean model configurations, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-17233, https://doi.org/10.5194/egusphere-egu23-17233, 2023.

16:47–16:57
|
EGU23-561
|
ECS
|
On-site presentation
Benjamin Storer, Michele Buzzicotti, Hemant Khatri, Stephen Griffies, and Hussein Aluie

Our understanding of the ocean’s spatial scales and their coupling has been derived mostly from Fourier analysis in small "representative" regions, typically a few hundred kilometers in size, that cannot capture the vast dynamic range at planetary scales. Using coarse-graining, we analyze a 1/12-degree reanalysis dataset on a range of spatial scales spanning more than three orders of magnitude, including both mesoscales and planetary scales. We present a truly global kinetic energy wavenumber spectrum, as well as the first measurements of the cascade across this entire range of scales. This provides us with the first estimates of the global amount of energy that is transferred by the KE cascade, as well as the scale-dependent depth structure of the oceanic KE spectrum and cascade. We find that within the mesoscales, the seasonal cycles of KE at larger length scales demonstrate a characteristic lag time relative to smaller length scales. The seasonal cycle of the inverse energy cascade exhibits the same lag time but is phase-shifted to earlier times, which suggests causality.

How to cite: Storer, B., Buzzicotti, M., Khatri, H., Griffies, S., and Aluie, H.: Energy Spectra and Cascades in the Global Ocean: Planetary Scales to Mesoscales, Surface to the Abyssal Ocean, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-561, https://doi.org/10.5194/egusphere-egu23-561, 2023.

16:57–17:07
|
EGU23-2773
|
ECS
|
On-site presentation
René Schubert and Jonathan Gula

The transfer of kinetic energy between currents of different horizontal scales shapes the structure of the global ocean circulation and associated heat, salt, nutrient, and oxygen fluxes, as well as atmosphere-ocean interactions. In particular the geostrophically balanced part of the flow has been shown to be associated with a net inverse cascade from currents of smaller scales to currents of larger scales. Here, we show with a submesoscale-permitting simulation of the Atlantic that the respective scale kinetic energy flux averaged over 5º x 5º boxes is linearly related to the product of quantities that are computable from along-track altimetry when they are averaged over the same region. This linear relationship is applied to JASON-3 along-track sea-surface height data to estimate for the first time the geostrophic kinetic energy cascade at scales between 60 and 200 km, as well as its regional distribution and seasonal cycle for large parts of the global ocean on the basis of observations. The results are consistent with previous findings based on regional observations, simulations, and indirect comparisons of spectral properties of satellite data.

How to cite: Schubert, R. and Gula, J.: Estimating the Oceanic Kinetic Energy Cascade from Satellite Along-Track Altimetry, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-2773, https://doi.org/10.5194/egusphere-egu23-2773, 2023.

17:07–17:17
|
EGU23-1533
|
ECS
|
On-site presentation
Romain Torres, Robin Waldman, Julian Mak, and Roland Séférian

Mesoscale eddies play a key role in the transport and mixing of ocean tracers such as heat and carbon. They widely contribute to the stratification of the ocean and form the highest reservoir of kinetic energy. However, although baroclinic instabilities are believed to be a central mechanism of eddy generation, little is known about their dissipation. Notably, how and where the kinetic energy flows out from the mesoscale reservoir remains uncertain. In the ocean, the mechanical energy is dissipated by a variety of processes but only a small part occurs in the top few hundred meters of the water column, adding some difficulty to their measurement. 

Here, a simplified equation of the mesoscale energy budget is used to get a global estimation of the eddy dissipation rate. We first validate this framework in a global ocean simulation using a parameterized eddy energy budget. With the ocean stratification as the main input, we then apply this framework to an observation-based density climatology and a global reconstruction of the eddy kinetic energy field. We find a global mesoscale dissipation rate of about 0.45 TW, in agreement with recent independent estimates. The results also show an intense dissipation near western boundary currents, where both large levels of energy and large baroclinic conversion rates occur.  The resulting dissipation map brings new insights for closing the ocean kinetic energy budget as well as constraining future mesoscale parameterizations and associated mixing processes.

How to cite: Torres, R., Waldman, R., Mak, J., and Séférian, R.: Global estimate of eddy dissipation from a diagnostic energy balance, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-1533, https://doi.org/10.5194/egusphere-egu23-1533, 2023.

17:17–17:27
|
EGU23-2884
|
Highlight
|
Virtual presentation
Thomas Meunier, Amy Bower, and Paula Perez Brunius

Quantifying and understanding the mixing and decay properties of oceanic mesoscale coherent eddies is crucial, as coupled climate models become eddy-permitting. Measuring experimentally the decay of coherent eddies is a difficult and expensive task. As of now, only a very limited number of repeated field surveys across the same eddy have been achieved. On the other hand, satellite altimetry offers a nearly synoptic two-dimensional view of the evolution of ocean vortices, and the record is now 30 years-long, allowing for the computation of solid statistics. But most coherent eddies are essentially baroclinic and knowledge of their vertical structure is crucial to understand the decay of their energy, or heat and salt contents. Here, we take advantage of the dense array of ARGO float profiles in the Gulf of Mexico (GoM) to reconstruct the three-dimensional structure of all Loop Current Rings (LCR) detached since 1993, using the Gravest Empirical Modes (GEM) method and a gridded altimetry product. The 3D reconstruction method was validated using independent glider observations and exhibits a striking accuracy in estimating the kinetic and available potential energy of LCRs, as well as their heat and salt contents. The decay of LCRs in terms of energy and thermohaline properties is then studied in details and it is shown that, despite their longevity of 6 to 15 months, they decay continuously with time at an inverse exponential rate, and have lost 80 % of their total energy as they reach the western GoM’s continental slope, where they were previously suspected to decay. We studied the impact of the wind and the feed back of the current on the wind on the energy decay of LCRs, and found that wind stress work (using relative wind stress), is responsible for half of the total energy loss during the eddy’s life time, while available potential energy decay is entirely driven by Ekman buoyancy fluxes and the barotropic vorticity decay is driven by wind stress curl. This suggests that wind forcing (when considering the feed-back of ocean currents on wind stress) is the leading-order mechanism in the decay of these coherent eddies.

How to cite: Meunier, T., Bower, A., and Perez Brunius, P.: Estimating and Understanding the Energy Decay of Coherent Eddies from In situ Observations and Satellite Altimetry., EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-2884, https://doi.org/10.5194/egusphere-egu23-2884, 2023.

17:27–17:37
|
EGU23-2789
|
ECS
|
On-site presentation
Marcela Contreras, Lionel Renault, and Patrick Marchesiello

The Gulf Stream (GS) is one of the strongest ocean currents on the planet. Eddy-rich resolution models are needed to properly represent the dynamics of the GS, however kinetic energy (KE) can be in excess in these models if not dissipated efficiently. The question of how and how much energy is dissipated and in particular how it flows through ocean scales thus remains an important and largely unanswered question. Using a high-resolution (2 km) ocean model (CROCO), we characterize the spatial and temporal distribution of turbulent cascades in the GS based on a coarse-grained method. We show that the balanced flow is associated with an inverse cascade while the forward cascade is explained by interactions between unbalanced and balanced motions. The forward cascade, which represents an interior route to dissipation, is compared to both numerical and boundary dissipation processes. The contribution of interior dissipation is an order of magnitude smaller than that of the other energy sinks.

How to cite: Contreras, M., Renault, L., and Marchesiello, P.: Understanding Energy Pathways in the Gulf Stream, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-2789, https://doi.org/10.5194/egusphere-egu23-2789, 2023.

17:37–17:47
|
EGU23-308
|
ECS
|
Highlight
|
Virtual presentation
Yue Wu, Eric Kunze, Amit Tandon, and Amala Mahadevan

Direct measurements of dissipation rates in the Southern Ocean unveiled a deficit of lee-wave dissipation compared to lee-wave generation (Brearley et al. 2013; Sheen et al. 2013; Waterman et al. 2013, 2014; Cusack et al. 2017, 2020), which is described as “suppression of turbulence” in Waterman et al. (2014). One possible explanation is the generation of freely propagating internal gravity waves (free waves with Eulerian frequency 𝜔𝐸 ≠ 0) that can radiate outside of the lee-wave critical layer and dissipate remotely. In a numerical simulation of lee waves generated by a localized, stable geostrophic current over sinusoidal topography, free waves are observed to emanate from the lee-wave critical layer. The escaped fraction of free-wave energy (the fraction that tunnels through the lee-wave critical layer and reaches the upper ocean) is 5%, while the majority remains trapped. This excludes remote dissipation by free waves as an explanation for the observed “suppression of turbulence”. Energy budget calculations show that ~50% of the bottom-generated lee-wave radiation is reabsorbed into the geostrophic current in vertical mean shear, ~10% is transferred to free waves as an indirect route to dissipation, and ~40% is lost through nonlinear wave-wave interactions leading to the increase of background potential energy. The total dissipative fraction (dissipation of lee waves plus indirect dissipation as free waves) is consistent with predictions by wave action conservation, regardless of the selected eddy viscosity and diffusivity. This study emphasizes wave-mean and wave-wave interactions in the ocean and will shed light on the choice of turbulent parameterization schemes for numerical modelers.

How to cite: Wu, Y., Kunze, E., Tandon, A., and Mahadevan, A.: Gravity-Wave Emission from Lee-Wave Critical Layers and Energy Budgets, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-308, https://doi.org/10.5194/egusphere-egu23-308, 2023.

17:47–17:57
|
EGU23-5050
|
On-site presentation
Xinyue Li, Qiang Wang, Nikolay Koldunov, Sergey Danilov, Thomas Jung, and Vasco Müller

The continuing retreat of sea ice affects the Arctic mesoscale eddies, and its future evolution will strongly influence air-sea-ice interactions. However, knowledge of eddy activity is limited to sparse observations and coarse resolution models. How future eddies and their effects will evolve remains uncertain. Here, we apply the global unstructured model FESOM2 for 143 years of 4.5 km-Arctic simulations up to 2100 and 1 km-Arctic simulations for 5 years from 2010; 2090 to reveal the interactions between eddies, winds, sea ice and the energy budget of eddy kinetic energy (EKE) in a high resolution view. We demonstrate a significant increase in future Arctic EKE from 0-200 m, which is stronger in summer when sea ice melts. The future abundance of EKE can be explained by an increase in winter eddy generation and a decrease in summer eddy dissipation. This also leads to an enhancement of the horizontal velocity field, thus filling the Arctic Ocean with eddies in the future.

How to cite: Li, X., Wang, Q., Koldunov, N., Danilov, S., Jung, T., and Müller, V.: Eddy-rich Arctic as future Sea ice disappears in high-resolution view, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-5050, https://doi.org/10.5194/egusphere-egu23-5050, 2023.

17:57–18:00

Orals: Tue, 25 Apr | Room 1.61/62

Chairpersons: Stephan Juricke, Knut Klingbeil
08:30–08:40
|
EGU23-8066
|
ECS
|
On-site presentation
Adrien Bella, Noé Lahaye, and Gilles Tissot

Ocean internal tides are both ubiquitous and important for the transport of tracers and the meridional overturning circulation, because of the mixing in the deep ocean they cause when they break. They are specific kind of internal waves generated when the astronomical tide encounters topographic features, which can propagate over more than a thousand kilometres. This long range propagation leaves opportunities for these waves to interact with mesoscale flows and eddies, which can be of comparable length scale. These interactions are of importance since they impact both the wavelength and the phase of internal tides, making them difficult to map using satellite altimeter once they have lost their coherency with the astronomical forcing. These interactions may also impact the energy budget of the internal tide and the cascade of energy from the astronomical (barotropic) tide to shorter internal waves (down to scales under 1 km), down to 3D turbulence and dissipative scale. 

In this presentation, we will describe the energy life cycle of internal tides in the North Atlantic basin using outputs from the numerical simulation eNATL60. This simulation has an horizontal resolution of around 2 kilometres and 300 vertical levels. Using a vertical mode decomposition, we investigate the energy budget of the semi-diurnal internal tide and more precisely the exchanges of energy between modes triggered by the topography, the mesoscale flow and the variations of the ambient density field, as well as their time variability.

We will focus on two contrasted areas of the North Atlantic: the Azores Islands and the North mid Atlantic ridge with a weak mesoscale activity and strong topographic features, such as seamounts and a ridge, and the Gulf Stream area featuring a strong western boundary current as well as a continental shelf break.
In the vicinity of the Azores, topographic induced couplings are of leading order for mode 0 and 1 and induce a substantial transfer of energy from low to high modes. The fraction of energy transferred toward high modes by the topography is almost 100% of the total transfer, but advection by the balanced flow becomes significant in the energy budget for modes 2 or higher.
In contrast, in the Gulf stream region, interactions with the mesoscale balanced flow accounts for more than 35% of the energy transfer from low baroclinic modes to high modes, and the mesoscale plays an important role in the energy budget for all baroclinic modes. The most prominent contribution it is the advection by the mean flow: it accounts for 26% of this transfer toward high modes and dominate over topographic scattering for modes 1 to 10.

Advection of the internal tides is the dominant contribution of the interaction between internal tides and the mesoscale balanced flow in the two areas. We find that the internal tide only weakly extracts energy from the mesoscale flow and associated buoyancy field in the two areas.

How to cite: Bella, A., Lahaye, N., and Tissot, G.: Importance of the mesoscale circulation in the energetic of internal tides in two contrasted areas of the North Atlantic., EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-8066, https://doi.org/10.5194/egusphere-egu23-8066, 2023.

08:40–08:50
|
EGU23-15591
|
ECS
|
On-site presentation
|
Christopher Danek, Patrick Scholz, and Gerrit Lohmann

Small-scale eddies play an important role in preconditioning and restratifying the water column before and after mixing events, thereby affecting deep water formation variability. Results from a realistic eddy-resolving (~5 km local horizontal resolution) ocean model suggest that small-scale temperature fluxes due to turbulent potential to kinetic energy conversion are the main driver of mixed layer restratification during deep convection in the Labrador Sea interior and the West Greenland Current. This resupply of heat due to turbulent upward buoyancy fluxes exhibits a large interannual variability imposed by the atmospheric forcing. Eddy fluxes only become active in periods of strong buoyancy loss, while being quiescent otherwise. In a low-resolution (~20 km) control simulation the modeled turbulence is strongly reduced and the associated modeled and parameterized heat fluxes are too weak to increase stratification.

How to cite: Danek, C., Scholz, P., and Lohmann, G.: Decadal variability of eddy temperature fluxes in the Labrador Sea, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-15591, https://doi.org/10.5194/egusphere-egu23-15591, 2023.

08:50–09:00
|
EGU23-13166
|
ECS
|
On-site presentation
Nicolas Dettling and Martin Losch

The Weddell Sea is an Antarctic marginal sea featuring extensive continental shelf areas. Here, dense water is produced through interactions with ice shelves and sea ice and propagates down the continental slope where it forms the densest contribution to the Antarctic Bottom Water. The downflow of dense water creates an isopycnal connection between the continental shelf and slope allowing for the intrusion of warm Circumpolar Deep Water (CDW) onto the continental shelf. When reaching the ice shelf cavities, CDW has the potential to strongly increase the melt rate of the ice shelves with global implications such as sea level rise. Mesoscale eddies sourced from local baroclinic instability play a central role in the shoreward transport of CDW since they supply the momentum to overcome the vorticity gradient imposed by the continental slope. Capturing these eddies in ocean models is particularly challenging because the small Rossby radius of deformation at high latitudes requires a much higher horizontal resolution than currently available in state-of-the-art climate models. This invites the question as to how the shoreward heat flux can be parameterised at coarse resolution and motivates a process oriented modelling study. For this purpose we use the MIT general circulation model (MITgcm) in a configuration featuring idealised sloping topography and surface forcing and typical hydrographic fields representing the Weddell Sea continental shelf and slope. In this setup, a strong heat transport from the open ocean onto the continental shelf only emerges at a resolution of O(1km). At coarser resolution, shoreward heat transport is almost absent resulting in a cold bias on the continental shelf and the exported deep water. We then apply the classical schemes of Gent-McWilliams and Redi (GM/Redi) which parameterise the effect of eddies by introducing an advective tracer flux as a function of the isopycnal slopes and by aligning the diffusion operator with the local isopycnals. We show that using the GM/Redi parameterization shoreward heat transports can be represented so that the difference between the high and coarse resolution hydrographic fields strongly reduces. Advective heat transport dominates over the centre of the continental slope and is captured by the GM part of the parameterisation. The diffusive heat flux dominating over the continental shelf break on the other hand is reproduced by the Redi scheme. In light of potential future changes to the Weddell Sea system we further discuss different approaches to obtain the transfer coefficients needed for the GM/Redi parameterisation based on the resolved flow and sub-grid eddy kinetic energy.

How to cite: Dettling, N. and Losch, M.: Parameterising Eddy-Mediated Heat Transports Across the Weddell Sea Continental Slope, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-13166, https://doi.org/10.5194/egusphere-egu23-13166, 2023.

09:00–09:10
|
EGU23-8458
|
On-site presentation
Julia Draeger-Dietel, Alexa Griesel, Maren Walter, Jochen Horstmann, Ruben Carrasco Alvarez, and Jeff Carpenter
The characteristics of turbulence and  processes driving it in the regime connecting mesoscales (mainly upscale energy transfer rates) and classical microscales turbulence (downscale energy transfer) are not fully well-established.
Here we analyse two-point velocity data from 2 entangled near-simultaneous relases of two different kinds of surface drifters floating in different depth (50 cm and 15 m) of the ocean mixed layer in the Walvis Ridge Region in the South Atlantik. For the 'deep drifters' the compensated third order longitudinal velocity structure function  shows a positive plateau for inertial scales roughly between 15 km  and 150 km,
revealing evidenve of an inverse cascade similar to former findings in the Benguela region. In contrast the 'shallow drifter'
do not show a positive platau at these scales, but show evidence of a forward cascade (negative plateau) and Kolmogorov self-similarity on  spatial scale around 500 m.

How to cite: Draeger-Dietel, J., Griesel, A., Walter, M., Horstmann, J., Carrasco Alvarez, R., and Carpenter, J.: Evidence of a  dual kinetic energy cascade by surface drifter observation in the Walvis Ridge Region, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-8458, https://doi.org/10.5194/egusphere-egu23-8458, 2023.

09:10–09:20
|
EGU23-10083
|
ECS
|
On-site presentation
Pablo Sebastia Saez, Carsten Eden, and Manita Chouksey

We investigate the interaction of internal gravity waves (IGW) with mesoscale eddies using the novel numerical Internal Wave Energy Model (IWEM). With IWEM, we integrate the radiative transfer equation to investigate the propagation and refraction of IGWs, and the energy exchange between IGWs and mean (eddying) flow. We evaluate the evolution of a typical IGW spectrum with energy density in physical and wavenumber space along a single column and over an eddy cross-section. We compare the simulations with the observations of a coherent mesoscale eddy in the Canary Current System. Results show that the changes in IGW energy are dominated by wave propagation effects, wave-mean flow interaction and wave breaking at critical layers, while wave capture effects are two orders of magnitude smaller. The wave propagation terms transport IGW energy from the eddy center to the rim. Energy gain by wave-mean flow interaction is dominated by low-frequency waves in the eddy center, while high-frequency waves are trapped in a cyclo-stationary up-/downward propagation cancelling out their gain or loss of energy. Energy loss by wave-mean flow interaction or wave breaking is largest at the eddy rim, where IGWs undergo a downscale energy transfer to small vertical scales and to the inertial frequency. Mooring observations agree with our model results on higher IGW energy values at the eddy center compared to the rim.  Following the Osborn-Cox relation, wave-breaking induced vertical diffusivities are found to be maximal at the eddy rim and range between κ≅10-7-10-5m2s-1, partly in range with the observed values in the ocean. The interaction of IGWs and mesoscale eddies is therefore a plausible process for explaining the near-surface enhanced mixing. 

How to cite: Sebastia Saez, P., Eden, C., and Chouksey, M.: Interaction of Internal Gravity Waves with Meso-Scale Eddies, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-10083, https://doi.org/10.5194/egusphere-egu23-10083, 2023.

09:20–09:30
|
EGU23-1042
|
On-site presentation
Zhiyu Liu, Chuanyin Wang, and Hongyang Lin

A long-standing challenge in dynamical oceanography is to distinguish nonlinearly intermingled dynamical regimes of oceanic flows. Conventional approaches focus on time-scale or space-scale decomposition. Here, we pursue a dynamics-based decomposition, where a mean flow is introduced to extend the classic theory of wavy and vortical modes. Mainly based on relative magnitudes of the relative vorticity and the modified horizontal divergence in spectral space, the full flow is decomposed into wavy and vortical motions. The proposed approach proves simple and efficient, and can be used particularly for online disentangling vortical and wavy motions of the simulated flows by ever-popular tide-resolving high-resolution numerical models. This dynamical approach, combined with conventional time-scale- or space-scale-based approaches, paves the way for online mixing parameterizations using model simulated vortical (for isopycnal mixing) and wavy (for diapycnal mixing) motions, and for understanding of multi-regime and multi-scale interactions of oceanic flows.

How to cite: Liu, Z., Wang, C., and Lin, H.: A Simple Approach for Disentangling Vortical and Wavy Motions of Oceanic Flows, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-1042, https://doi.org/10.5194/egusphere-egu23-1042, 2023.

09:30–09:40
|
EGU23-12504
|
On-site presentation
Remi Tailleux and Gabriel Wolf

By analogy with the case of a simple fluid, lateral stirring in the oceans has been traditionally envisioned as the notional form of stirring that minimally perturb the ocean stratification and its potential energy, as originally proposed over 80 years ago. If considered from the viewpoint of adiabatic and isohaline permutations of two fluid parcels, such a view leads to the idea that lateral stirring preferentially takes place on the ‘locally-referenced potential density surfaces’. To remedy the mathematically and physically ambiguous character of the latter, oceanographers then developed the concepts of potential density surfaces, patched potential density surfaces, and approximately neutral surfaces, which have been the cornerstone of isopycnal analysis for many decades. It has also provided the justification for constructing rotated diffusion tensors in terms of the directions parallel and perpendicular to the neutral directions. Nevertheless, while the concepts of neutral directions and neutral surfaces have been around for decades, their validity has never been really challenged nor confirmed experimentally. Worse, there has been little clue so far about how one might go about testing or refuting these concepts.

Part of the problem is that the current theory of quasi-neutral density variables is not currently formulated as a classical falsifiable (in Popper’s sense) physical theory capable of making testable predictions but more as unfalsifiable dogma. To improve on this situation, this work shows how to embed the theory of lateral stirring and lateral stirring surfaces into the APE-based study of the compressible Navier-Stokes equations for realistic seawater (APE standing as Available Potential Energy, as per Lorenz concept). Doing so succeeds in identifying the kind of models that can be studied to shed light on the issue while also making new predictions about lateral stirring that significantly depart from the prevailing view. A key new result is that isoneutral stirring must involve compensating work between buoyancy and thermobaric forces, which cast doubt on its physical realisability. Another key result is that the ‘true’ neutral directions are not those associated with the standard neutral vector, but rather with an APE-based form of the P vector previously identified by Nycander. Although the P-neutrality thus defined appears to coincide with the standard N-neutrality in most of the oceans, the two are found to significantly differ in the polar region and Gulf Stream area, where neutral rotated diffusion tensors are likely to be potentially a significant source of spurious diapycnal mixing. Evidence that this is the case and how to go about remedying the problem will be discussed.

How to cite: Tailleux, R. and Wolf, G.: A new paradigm for lateral stirring and lateral stirring surfaces in the oceans, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-12504, https://doi.org/10.5194/egusphere-egu23-12504, 2023.

09:40–09:50
|
EGU23-16324
|
ECS
|
On-site presentation
Anthony Bonfils, Dhrubaditya Mitra, Woosok Moon, and John Wettlaufer

Asymptotic analysis of surface waves interacting with a wind and a current

Following Miles (1957), surface waves are regarded as perturbations of the wind, modeled as an inviscid parallel shear flow; a water current can be included in the model. The linear stability analysis of the shear flow leads to an eigenvalue problem. The real part of the eigenvalue is the phase speed of the waves while the imaginary part times the wavenumber is the growth rate of the wave amplitude. The streamfunction of the perturbed flow, or eigenfunction, obeys the Rayleigh equation with coupled boundary conditions at the air-water interface. First, I will show how Miles simplified this problem using the small air-water density ratio. Next, for waves whose wavelength is much larger than the characteristic length scale of the shear, I will solve the Rayleigh equation asymptotically and infer the complex eigenvalue. Finally I will show that, in the strong wind limit, the fastest growing waves are those for which the aerodynamic pressure is in phase with the wave slope. 

References:

J. W. Miles, J. Fluid Mech., 3:185–204, 1957.

A. F. Bonfils, D. Mitra, W. Moon, and J. S. Wettlaufer, J. Fluid Mech., 944:A8, 2022.

A. F. Bonfils, D. Mitra, W. Moon, and J. S. Wettlaufer, arXiv:2211.02942.

How to cite: Bonfils, A., Mitra, D., Moon, W., and Wettlaufer, J.: Asymptotic analysis of surface waves interacting with a wind and a current, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-16324, https://doi.org/10.5194/egusphere-egu23-16324, 2023.

09:50–10:00
|
EGU23-15675
|
ECS
|
Highlight
|
On-site presentation
Janina Tenhaus, Marc Buckley, Silvia Matt, and Ivan Savelyev

Small-scale processes govern the transfer of energy and momentum at the coupled atmospheric and oceanic wave boundary layers. The physical wind energy input mechanisms by wave growth remain poorly understood (critical layer theory vs sheltering mechanism).

We conducted laboratory velocity measurements within the first millimeters to centimeters above and below surface waves. A high resolution 2D Particle Image Velocimetry (PIV) system was installed in a wind-wave tunnel at a fetch of approximately 10 m. In addition, wave field properties were captured by Laser-Induced Fluorescence (LIF). Experiments were run with wind waves and wind over mechanical swell. During the measurements, 10-m wind speeds of 5 to 10 m/s were observed, with peak wave ages (cp/u*) ranging from 1 to 7.

We will focus on the air phase and describe the modulations of the airflow structure. Furthermore, we will discuss the influence of peak wind-wave conditions (e.g., wave age, slope) on the dynamical role of the critical layer.

How to cite: Tenhaus, J., Buckley, M., Matt, S., and Savelyev, I.: Wind-Wave Energy Flux Measurements using Particle Image Velocimetry, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-15675, https://doi.org/10.5194/egusphere-egu23-15675, 2023.

10:00–10:10
|
EGU23-15850
|
ECS
|
On-site presentation
Malte Loft, Thomas Rung, Niklas Kühl, Marc Buckley, Jeff Carpenter, Fabrice Veron, and Michael Hinze

The paper is devoted to two-phase flow simulations and investigates the ability of a diffusive interface Cahn-Hilliard Volume-of-Fluid model to capture the dynamics of the air-sea interface at geophysically relevant Reynolds numbers. It employs a hybrid filtered/averaging Improved Detached Eddy Simulation method to model turbulence, and utilizes a continuum model to account for surface tension if the diffuse interface is under-resolved by the grid. A numerical wind-wave tank is introduced to limit computational costs and results obtained for two wind-wave conditions are analyzed in comparison to experimental data at matched Reynolds numbers. The focus of the comparison is on both time-averaged and wave-coherent quantities, and includes pressure, velocity as well as modeled and resolved Reynolds stresses. In general, numerical predictions agree well with the experimental measurements and reproduce many wave-dependent flow features. Reynolds stresses near the water surface are found to be especially important in modulating the critical layer height. It is concluded that the diffusive interface approach proves to be a promising method for future studies of air-sea interface dynamics in geophysically relevant flows.

How to cite: Loft, M., Rung, T., Kühl, N., Buckley, M., Carpenter, J., Veron, F., and Hinze, M.: Two-Phase Flow Simulations of Surface Waves in Wind-Forced Conditions, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-15850, https://doi.org/10.5194/egusphere-egu23-15850, 2023.

10:10–10:15

Posters on site: Tue, 25 Apr, 14:00–15:45 | Hall X5

Chairpersons: Stephan Juricke, Nils Brüggemann, Knut Klingbeil
X5.368
|
EGU23-834
|
ECS
|
Rajka Juhrbandt, Stephan Juricke, Thomas Jung, and Peter Zaspel

Climate models are one of the most useful tools for predicting future climate states, which has become more important than ever in the ongoing climate crisis. However, due to their spatial and temporal resolutions, which are constrained by computing power and resources, climate models are not able to represent all processes in the ocean and atmosphere. Therefore, modelers need to estimate the effects unresolved processes have on the resolved processes.

One such structure is turbulent mesoscale eddies in the ocean. It is known from observations that eddies carry a large amount of kinetic energy and play a significant role in transport of tracers such as temperature and salinity as well as in heat uptake from the atmosphere. Therefore, it is crucial that eddies and their effects on the processes mentioned above are represented accurately in climate models.

To better estimate these effects in low-resolution simulations, high-resolution simulations can be used to constrain the parameters necessary for the estimates. However, tuning these parameters can be subjective and time-consuming. In this project, Machine Learning (ML) methods will be used to facilitate and speed up this process.

In my PhD project, high-resolution data from the FESOM2 ocean model will be used. At low resolution, which is insufficient to represent eddies, FESOM2 estimates the effects of the missing eddies using the Gent-McWilliams (GM) parameterization containing a GM coefficient. With the help of Bayesian Neural Networks, a framework will be developed to calculate a predictor for this parameterization as well as its variability. Using this framework, maps of the GM coefficient for multiple setups with increasing complexity and data volume will be created. The presentation includes a project outline as well as preliminary results.

How to cite: Juhrbandt, R., Juricke, S., Jung, T., and Zaspel, P.: Using neural networks and high-resolution simulations to improve mesoscale eddy representation in ocean models, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-834, https://doi.org/10.5194/egusphere-egu23-834, 2023.

X5.369
|
EGU23-962
|
ECS
Moritz Epke and Nils Brüggemann

Submesoscale Instabilities in the mixed layer can lead to ocean restratification and thus affect ocean-atmosphere feedbacks. In this study, a novel configuration of the ICON ocean model is applied which makes use of telescoping grid refinement such that a horizontal resolution finer than 600m is achieved over wide areas of the North Atlantic. The ability of the model to simulate mesoscale to submesoscale turbulence is validated by comparing spatial power spectra of sea surface temperature and sea surface hight with those of satellite products and a ICON simulation of 10km horizontal resolution (often referred to as "mesoscale eddy resolving").

We find more realistic variability in the simulation with the refined grid compared with coarser simulation over a wide range of scales that even includes the mesoscale eddy regime. Moreover, the high-resolution permits submesoscale baroclinic and symmetric instabilities. At single fronts, we observe strong overturning re-stratifying the fronts. Overturning rates are diagnosed regarding mean characteristics of the fronts like mean horizontal and vertical density gradients and mixed layer depth. Finally, the diagnosed overturning rates are compared to recent parameterizations introduced by Stone (1971) and Fox-Kemper (2008). It turns out that both parameterizations are roughly able to capture the bulk overturning along strong fronts but have problems in non-frontal regions.

How to cite: Epke, M. and Brüggemann, N.: Overturning of mixed layer eddies in a submesoscale resolving simulation of the North Atlantic, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-962, https://doi.org/10.5194/egusphere-egu23-962, 2023.

X5.370
|
EGU23-4096
|
ECS
|
Si-Yuan (Sean) Chen, Olivier Marchal, and Wilford Gardner

Benthic storms are episodes of strong bottom currents and sediment resuspension that occur at abyssal depths in the ocean. They are often observed in regions with strong, eddying surface currents, such as the western North Atlantic and the Argentine Basin. Deep cyclonic and anticyclonic eddies induced by surface current instabilities have been postulated to accelerate abyssal currents and generate benthic storms. Here, using a primitive-equation model with high vertical resolution, we conduct idealized numerical experiments of the unforced instability of a surface-concentrated, eastward-flowing jet in a zonal channel. We find that the jet, initially in strict thermal wind balance, becomes spontaneously unstable as a result of baroclinic instability, meanders, and eventually develops a complex eddy field with regions of strong ageostrophic flow. Associated with jet meandering, a train of cyclonic and anticyclonic eddies form along the jet and migrate in the same direction as the parent current but much slower. These eddies extend over the whole water column (4000 m depth), consistent with the tendency for eddy barotropization in geostrophic turbulence. They accelerate bottom currents, which are found to be more vigorous in a flat channel than in a channel with a meridional bottom slope. Over the course of the numerical integrations, bottom mixed layers with quasi-uniform potential temperature develop at various locations in the channel. These layers have thicknesses of O(100) m on a flat bottom and O(10) m on a sloping bottom. Our study yields preliminary insights into the fundamental role of surface current instabilities on near-bottom processes, calling for further investigations on downward energy transfer and near-bottom dissipation.

How to cite: Chen, S.-Y. (., Marchal, O., and Gardner, W.: Concurrent Development of Benthic Storms and Bottom Mixed Layers underneath an Eddying Surface-Concentrated Zonal Jet, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-4096, https://doi.org/10.5194/egusphere-egu23-4096, 2023.

X5.371
|
EGU23-4665
|
ECS
Wei-Chang Wu, Yiing-Jang Yang, and Ching-Ling Wei

Mesoscale eddy plays an important role in physical and biochemical processes in the upper ocean, it could redistribute seawater masses, heat, and biogeochemical properties. The region of the Subtropical Countercurrent (STCC) within the northwestern subtropical Pacific Ocean (NWSTP) is characterized by an abundance of mesoscale eddies caused by baroclinic instability. The NWSTP serves a good place for studying variations of dissolved oxygen (DO) concentration and temperature induced by eddies. Observed by two metocean moored buoys deployed in the NWSTP, there was one cyclonic eddy (CE) and one anti-cyclonic eddy (ACE) during the summer of 2018 to 2020. Time series dataset of hydrography, DO concentration and chlorophyll-a in the upper subsurface layer provided precious in-situ observations, which helps us better understand physical dynamics under the influence of eddies and improve numerical model forecasts. The results of analysis showed that there was a significant difference between two types of eddies in the upper layer ocean. During the CE period, DO concentration and temperature had significant fluctuations up to 30 μM and 5 ℃, respectively, at depth of 50 meter. In response to diurnal and semi-diurnal tidal processes, their variations gradually decreased toward deep ocean; whereas their amplitude narrowed to 5 μM and 1 ℃, respectively, during the ACE period. During the non-eddy period, there was a small amplitude of fluctuations in the tidal bands except at 150 meter. The phenomenon could be associated with modulated thermal structure under the influence of CE and ACE through upwelling and downwelling, respectively. This led to changes in the vertical structure of the internal tide amplitude and concentration gradient. The combination of these two factors consequently resulted in different amplitude of fluctuations in DO concentration and temperature during two types of eddies passing through. These results will be presented herein in detail.

How to cite: Wu, W.-C., Yang, Y.-J., and Wei, C.-L.: Internal-tide-induced fluctuations of hydrography and biogeochemical properties modulated by mesoscale eddies: Observation evidences, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-4665, https://doi.org/10.5194/egusphere-egu23-4665, 2023.

X5.372
|
EGU23-6254
|
ECS
Jan Niklas Dettmer

Eddy kinetic energy (EKE) and the conversion terms of the Lorenz energy cycle are estimated from an eddy-resolving global ocean model and resolved spectrally per horizontal wavenumber. The baroclinic conversion term (BC) exhibits a dipolar structure, where it is a source for EKE at scales close to the first baroclinic Rossby radius and a sink for EKE at larger scales close to the Rhines scale. The geographical and vertical distributions of the BC term are explored. It is found that in the ocean interior negative BC is limited to regions poleward of approximately 30° north and south. It is suggested that the cause for this distribution is the transfer of eddy energy to Rossby waves and zonal jets equatorward of 30°. It removes eddy energy before it cascades up to the scale where negative BC takes place. Equatorward of 30° the existence of a closed energy loop is suggested. Positive BC produces EKE which cascades upscale where it is converted to available eddy potential energy (EAPE) by negative BC, which cascades downscale again. The sink of EKE partly balances the EKE produced by baroclinic instability. The energy loop traps a certain amount of energy. Finally, the baroclinic conversion term is explored further in idealized model setups. The goal of the idealized setups is to test the robustness of the diagnostic methods and gain physical understanding of the negative baroclinic conversion.

How to cite: Dettmer, J. N.: Spectral Resolution of the Oceans Lorenz Energy Cycle, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-6254, https://doi.org/10.5194/egusphere-egu23-6254, 2023.

X5.373
|
EGU23-6525
|
ECS
Dinora Garcia Santacruz, Christian Mertens, Friederike Pollmann, and Dirk Olbers

The model ‘Internal Wave Dissipation, Energy and Mixing’ (IDEMIX) provides an energetically consistent representation for the diapycnal diffusivity induced by breaking of internal gravity waves in ocean and atmosphere circulation models. IDEMIX predicts the internal wave energy, dissipation rates, and diapycnal diffusivities. Such small-scale processes cannot be resolved but have to be parameterized due to their relevance for the large-scale circulation. The basic version of the model has been shown to be generally successful in ocean and atmosphere applications. However, in regions of strong forcing deviations from observational estimates were found. To evaluate the local performance of the model we analyzed the agreement with observational estimates of full-depth profiles of both stratification and horizontal velocity collected by several cruises around 47°N and 16°N in the Atlantic. The hydrographic profiles come from two dynamically different regions: the subpolar North Atlantic with energetic wind-induced near-inertial waves and the western subtropical Atlantic where the strong Deep Western Boundary Current interacts with the continental slope producing lee waves. Internal wave energy, dissipation rates and diapycnal diffusivities estimates are obtained using the finestructure method. These estimates can be calculated using shear and strain or strain only in lack of velocity data. In this study, both formulations have been calculated and contrasted between each other to evaluate the importance of shear information for a realistic energy budget. The results when comparing IDEMIX with observations show that the higher differences are close to the surface and over the rough topography where internal gravity waves are more predominant. The analysis of the observational data will increase our understanding of the spatiotemporal variability of the ocean’s internal gravity wavefield and will complement the model-based investigation of the responsible processes.

How to cite: Garcia Santacruz, D., Mertens, C., Pollmann, F., and Olbers, D.: Evaluating the Global Internal Wave Model IDEMIX with observations of both stratification and horizontal velocity, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-6525, https://doi.org/10.5194/egusphere-egu23-6525, 2023.

X5.374
|
EGU23-6546
|
ECS
Zoi Kourkouraidou and Jin-Song von Storch

Using the uncoupled model ICON-O with a high resolution of 5km, we aim to understand the working principles of interactions between low-mode internal tides and eddies such as their effect on internal mixing in the deep ocean. Such interactions can provide a direct energetic link between mesoscale processes and the internal wavefield. We focus the research on the Walvis Ridge region in the southeast Atlantic, since this is where energetic low-mode internal tides at the frequency of the principal lunar semidiurnal constituent (M2) are generated and propagate away from the ridge crossing the paths of eddies, which take the form of both Agulhas rings and other mesoscale features.
Concentrating on the stationary changes in the internal wave properties, we identify the eddies in the area of interest and then investigate the vertical structure of the internal tidal properties in the vicinity of an eddy as well as far away from it. Preliminary results indicating such wave-eddy interactions are shown and discussed on the poster.

How to cite: Kourkouraidou, Z. and von Storch, J.-S.: Interactions of low-mode internal tides with mesoscale eddies in the Walvis Ridge region, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-6546, https://doi.org/10.5194/egusphere-egu23-6546, 2023.

X5.375
|
EGU23-7686
|
ECS
|
Siva Kattamuri, André Paul, Friederike Pollmann, and Michael Schulz

The mid-Cretaceous (~90 Ma) was a natural case of the greenhouse climate state of the earth. Enhanced volcanic activity contributed to the higher atmospheric concentration of CO2. The continental configuration was different and the global mean sea level was around 160m higher compared to the present day.  Black shales from this period's marine sediments indicate a near-anoxic deep-ocean environment. This project hypothesizes that redistribution of tidal energy dissipation from the open ocean to the large continental shelf areas reduced the energy available for mixing in the deep ocean. The objective is to investigate the effect of climate parameters and geographical configuration on ocean circulation by using the energetically consistent internal wave mixing parameterization IDEMIX in the fully coupled Earth System Model iCESM1.2. As part of the second phase of the CRC - Transregio 181, this project aims to make the Earth system model energetically consistent for improved climate projections.

The paleo-topography/bathymetry of the mid-Cretaceous is adopted from Scotese and Wright’s PALEOMAP (2018) project. The paleo-vegetation distribution is taken from Sewall’s (2007) mid-Cretaceous boundary conditions. The necessary boundary conditions and the climate-forcing files for the individual component models are prepared by using the Paleoclimate ToolKit of the CESM documentation. This mid-Cretaceous setup is spun-up with present-day trace gas concentrations for over 1000 model years. Constant tidal mixing coefficients are used in this spin-up. The first results from this spin-up indicate deep-water formation in the mid-latitudes in the Northern Hemisphere. From the equilibrium state of the spin-up, trace gas concentrations of the mid-Cretaceous will be input in the model. The ocean circulation will then be simulated with the internal wave mixing schemes KPP and IDEMIX for a comparative study. We expect improved results for deep-water formation sites and deep-ocean oxygenation with the IDEMIX parameterization.

How to cite: Kattamuri, S., Paul, A., Pollmann, F., and Schulz, M.: Modelling the ocean circulation of the mid-Cretaceous using the Community Earth System Model (iCESM1.2) and the internal wave mixing parameterization IDEMIX, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-7686, https://doi.org/10.5194/egusphere-egu23-7686, 2023.

X5.376
|
EGU23-8948
|
ECS
Ilmar Leimann, Alexa Griesel, Maren Walter, Julia Dräger-Dietel, and Moritz Epke

The energy cycle in ocean models is still biased due to the large uncertainty regarding how processes in the mesoscale and submesoscale regimes are represented. Since mesoscale turbulence is largely geostrophic, it features an energy transfer towards larger scales. In contrast, submesoscale turbulence can contain both geostrophic and ageostrophic dynamics which makes the direction of the energy flux less clear. Measuring the kinetic energy spectrum in the ocean is challenging, since gridded satellite data are unable to resolve the subdeformation scales, and shipboard measurements are limited to a few regions. Lagrangian floats are globally available, can connect a range of scales from 10 m to 1000 km and hence are a unique source of information on meso- to submesoscale turbulence.
We estimate the kinetic energy spectral flux from SSH data which gives only the geostrophic part, compared to spectral fluxes from a submesoscale permitting ocean model with a focus on the North Atlantic.
The kinetic energy spectral fluxes are found to exhibit both inverse and forward cascade, with a higher inverse cascade in turbulent areas and maximum inverse wavenumber increasing with latitude.
In addition, we calculate velocity structure functions from the surface drifter data to bridge different scales, with the goal to compare and contrast the spectral estimates of different data sets. Velocity structure functions are the moments of velocity increments between two points and provide information about the properties of turbulent dynamics at different scales.

How to cite: Leimann, I., Griesel, A., Walter, M., Dräger-Dietel, J., and Epke, M.: Meso- to Submesoscale Turbulence in the Ocean, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-8948, https://doi.org/10.5194/egusphere-egu23-8948, 2023.

X5.377
|
EGU23-10399
|
ECS
|
Mauro Pinto, Oscar Pizarro, Ángel Rodriguez, Luis Valencia, and Osvaldo Ulloa

Recent studies have highlighted the importance of double diffusion instabilities in the diapycnal transport of dissolved substances in large regions of the oceans. Off central Chile (30-38°S), waters with very low dissolved oxygen are present at intermediate depths (between 50 m and 400 m depth) in a region where double diffusion instabilities can take place. This oxygen minimum layer (OML) is closely related to Equatorial Subsurface Waters (ESSW), a relatively salty and warm water mass that is transported poleward along the continental slope by the Peru-Chile Undercurrent (PCUC). Thus, this water mass –and so the OML– is delimited between two water masses of southern origin that are well-ventilated, relatively fresh, and cold, namely: the Eastern South Pacific Intermediate Water (ESPIW) and the Antarctic Intermediate Water (AAIW). In this study, we analyzed the role of diapycnal mixing in the dissolved oxygen fluxes in the upper and lower oxyclines that delimit the OMZ in the water column off central Chile (~36.5°S). Special emphasis is given to the evaluation of the contribution of salt fingers to these fluxes. We use a set of observations of fine structure (1-10 m) and microstructure (<1m) using CTD casts and a vertical microstructure turbulence profiler (VMP-250), respectively, along with current profiles obtained with ADCPs. The net diapycnal mixing is estimated using mixing models that allow us to separate the relative contributions of turbulent shear processes and instabilities associated with salt fingers. The thermohaline contrast in the ESSW-AAIW transition conditions the region for the development of double diffusion instabilities by salt fingers, which significantly contribute to the oxygen transport from the lower oxycline, thus favoring the ventilation of the OMZ from the AAIW.

How to cite: Pinto, M., Pizarro, O., Rodriguez, Á., Valencia, L., and Ulloa, O.: The importance of salt fingers in diapycnal mixing parametrization in Eastern Boundary Systems: Study Case in the Oxygen Minimum Zone off Central Chile, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-10399, https://doi.org/10.5194/egusphere-egu23-10399, 2023.

X5.378
|
EGU23-12417
|
ECS
|
Ole Pinner, Markus Janout, and Torsten Kanzow
The Weddell Sea is the largest contributor to deep water formation in the Southern Hemisphere. Dense and cold waters form during sea ice production on the continental shelves of the southern and western Weddell Sea, and are subsequently exported into the deep ocean via a dense near-bottom gravity current.  The current then propagates along the continental slope for several hundred kilometers. The gravity current is important for the global ocean circulation, although not all details are understood, as observations are sparse in this heavily ice-covered region. Furthermore, the current is likely modified by small-scale processes, which are generally unresolved by global ocean models. In this work, we use multi-year velocity measurements from 2017 to 2019 from moorings on the southern and northwestern Weddell Sea continental shelf and slope to quantify the relevant energy sources within the gravity current. Specifically, we investigate barotropic and baroclinic tidal energy, internal wave background and their dependence on location and time. Stronger internal waves up-slope coincide with the position of the gravity current main cores, which suggests that the bulk amount of mixing of the dense water with the ambient water occurs in shallower areas. Although the energy contained in waves with periods of several days varies throughout the year, the internal wave background on hourly time-scales seems to be largely unaffected. Our work is mainly aimed at the understanding of local energy levels within the dense gravity current, which may ultimately benefit a more accurate representation of dense water formation in global models.

How to cite: Pinner, O., Janout, M., and Kanzow, T.: Distribution of kinetic energy in the Weddell Sea gravity current, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-12417, https://doi.org/10.5194/egusphere-egu23-12417, 2023.

X5.379
|
EGU23-12892
|
ECS
|
Luca Kunz

Anthropogenic impact on the ocean and marine wildlife has never been as pointless as marine debris. Nevertheless, great efforts must be taken to estimate pathways and accumulation zones of marine litter in order to clean the most polluted areas of the sea. The present study intends to introduce a modern hydrodynamical concept to this debate. TRansient Attracting Profiles (TRAPs) act like short-term attractors on the ocean surface and have shown their potential to predict pathways of material transport in previous experiments. Here, I explore the occurrence of these profiles in the North Pacific subtropical gyre, a large-scale convergence zone that is known to entail a major accumulation region for floating debris, the Great Pacific Garbage Patch. There, I compute TRAPs upon daily snapshots of near-surface geostrophic velocity and create an unprecedented large dataset of approximately 4.5 million TRAP objects. With this record, I am able to provide a first characterisation of these structures at the mesoscale. I study their propagation, evaluate their persistency and uncover a driving mechanism behind the formation of TRAPs. Analysing their impact on nearby floating drifters allows me to reveal a signature of hyperbolic transport around these structures. I show that drogued drifters take around 7 days and undrogued ones around 5 days to pass by a TRAP, suggesting that these structures are most likely to organise transport if they are persistent. Throughout the thesis, I present a series of such findings that can be of particular interest to offshore cleanup operators.

How to cite: Kunz, L.: Transient Attracting Profiles in the Great Pacific Garbage Patch, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-12892, https://doi.org/10.5194/egusphere-egu23-12892, 2023.

X5.380
|
EGU23-14494
|
ECS
Ankitkumar Patel, Dirk Olbers, Friederike Pollmann, and Carsten Eden

Small-scale turbulent mixing plays an important role for the large-scale ocean circulation, but is unresolved in numerical ocean models. Since breaking internal gravity waves (IGWs) are a major source of this mixing, energetically consistent mixing parameterizations consider the internal wave energy balance. One such parameterization can be achieved with the aid of the IDEMIX (Internal Wave Dissipation, Energy and Mixing) model, which describes the generation, propagation and dissipation of internal wave energy and successfully reproduces energy and mixing estimates derived from Argo float observations. We extend the IDEMIX energy model to describe a coupled system of predictive equations for energy and bandwidth, where bandwidth is a shape parameter of the IGW energy spectrum fixing the number of excited vertical modes. The correlation between energy and bandwidth is a power law with an exponent given by the dynamical parameters. The power law agrees with energy and spectral shape estimates from finestructure observations by Argo floats. We present the coupled energy-bandwidth IDEMIX model in a stand-alone setup and preliminary results of how it affects vertical mixing and the ocean state in a global ocean model.

How to cite: Patel, A., Olbers, D., Pollmann, F., and Eden, C.: An energy-bandwidth basis for the internal wave model IDEMIX, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-14494, https://doi.org/10.5194/egusphere-egu23-14494, 2023.

X5.381
|
EGU23-17252
|
ECS
|
Heves Pilatin, André Paul, Friederike Pollmann, and Michael Schulz

The Last Glacial Maximum (~21 ka) is the most recent glacial period when the ice sheet
coverage was at its greatest extent (8% of Earth's surface), and the atmospheric CO2
concentration was ~190 ppm. During this period, the continental shelves were exposed and
the global-mean sea level was lower by ~130 m compared to today. This project hypothesizes
that decrease in sea level caused tidal-energy dissipation to shift from the shelves to the open-
ocean resulting in enhanced vertical mixing in the deep ocean. The aim of this project is to
study the global ocean circulation and the marine biogeochemical processes of the LGM
climate state using an energetically consistent ocean mixing scheme: Internal Wave
Dissipation, Energy, and Mixing (IDEMIX) in the fully coupled isotope-enabled Earth
System Model (iCESM1.2). For this study, only the tidal-induced mixing is investigated.
Hence the tidal forcing is considered as the only source for generating internal waves in
IDEMIX parameterisation. The model uses the "KPP+IDEMIX" approach as the combined
vertical mixing parameterization. While the KPP is activated only in the mixed layer (up to ~
1 km), IDEMIX is applied only to the interior ocean, where the dissipation is generated by the
tidal forcing at the ocean floor. In our simulations, we use 2° resolution for the atmosphere
and 1° for the ocean, and we simulate LGM and pre-industrial climate states with and without
IDEMIX. The modal bandwidth tuning parameter (j) in IDEMIX determines the number of
excited vertical modes, which affects how fast the energy propagates from the bottom to the
upper ocean. We perform the sensitivity experiments by using different j values in our LGM
simulations and investigate its impact on the vertical mixing and the ocean state.

How to cite: Pilatin, H., Paul, A., Pollmann, F., and Schulz, M.: Modelling the ocean circulation and mixing processes of the Last Glacial Maximum using the Community Earth System Model (iCESM1.2) and the Internal Wave Mixing parameterization IDEMIX, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-17252, https://doi.org/10.5194/egusphere-egu23-17252, 2023.

Posters virtual: Tue, 25 Apr, 14:00–15:45 | vHall CR/OS

Chairpersons: Stephan Juricke, Knut Klingbeil, Nils Brüggemann
vCO.12
|
EGU23-9928
|
ECS
|
Yulin Pan and Yue Wu

The kinetic energy spectra of oceanic internal gravity waves (IGWs) from recent field measurements and wave turbulence theory exhibit large variability, deviating from the standard Garrett-Munk (GM) model. However, the current finescale parameterization of turbulent dissipation is based on the GM model, which does not consider general spectra. Thus an improved estimate of energy cascade across scales for different spectra is needed for better parameterization of ocean mixing for global circulation and climate models. In this work, we conduct direct calculation of energy transfer based on the kinetic equation, which describes the spectral evolution of IGWs due to wave-triad interactions. First, dominant mechanisms are identified, i.e., local and scale-separated interactions (including parametric subharmonic instability, elastic scattering and induced diffusion). Local interactions provide a forward cascade in frequency that were not understood before. Second, energy flux across a critical vertical wavenumber providing energy available for dissipation is calculated for different spectra. The importance of local interactions for such downscale cascade is emphasized. This will shed light on a new formulation of finescale parameterization incorporating varying spectral forms of IGWs and a realistic ocean environment.

How to cite: Pan, Y. and Wu, Y.: Numerical evaluation of the spectral evolution of internal gravity waves due to wave-wave interactions, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-9928, https://doi.org/10.5194/egusphere-egu23-9928, 2023.

vCO.13
|
EGU23-14321
|
ECS
|
Emelie Breunig, Dr. Alexa Griesel, Dr. Julia Dräger-Dietel, and Prof. Dr. Carsten Eden

Energy cannot be created nor destroyed; it can merely be transferred to different scales. Scott, Robert B., and Faming Wang (2005) showed that in the South Pacific, energy is transferred from mesoscale eddies to large ocean currents (inverse energy cascade). The regional dependencies of the inverse cascade are still not well understood and thus limit the correct parameterisation of energy transfers in climate models. The eastern South Atlantic (0-10°E, 30-35°S) offers a large dataset (satellite as well as drifter data) containing several high energetic processes, such as strong mesoscale eddies. The presence of such energetic mesoscale processes makes the question if the energy is transferred to larger scales, an especially interesting one. To investigate the spectral transitions of the inverse energy cascade, we calculate the energy flux similarly to Scott, Robert B., and Faming Wang (2005) using Sea Surface Height (SSH) satellite data and analyse the results for regional changes. Secondly, we use the same SSH dataset to calculate second- and third-order velocity structure functions (S2,S3). As S2,S3 are usually calculated from surface drifter datasets, this provides a new Eulerian insight into transition zones, making structure functions more widely applicable. 

How to cite: Breunig, E., Griesel, Dr. A., Dräger-Dietel, Dr. J., and Eden, P. Dr. C.: Diagnosing the Transition Zones and Regional Dependencies of the Inverse Energy Cascade in the Eastern South Atlantic using Satellite Data, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-14321, https://doi.org/10.5194/egusphere-egu23-14321, 2023.

vCO.14
|
EGU23-16509
|
ECS
|
Ingo Wagner, Carsten Eden, and Dirk Olbers

When an ocean current flows over uneven topography a specific kind of in-
ternal gravity wave called lee wave is emitted. These lee waves are propagating
through the water column and can interact with the ocean currents and other
waves. They are thought to play a role in the global ocean energy cycle and
can also affect the momentum balance in the interior. The waves extract energy
from the mean flow or eddies near the bottom and then dissipate this energy
somewhere in the water column.
However, the waves can not be resolved directly in global ocean models and in
particular their vertical propagation is still largely unknown. In order to study
these waves Eden and Olbers proposed a model of the lee wave energy. In this model
the radiative transfer equation is integrated over the wavenumber space which
yields a prognostic equation of the lee wave energy. This energy equation can
then be added to an ocean model. This model includes a term for the interac-
tion with the mean flow and a dissipation term parameterizing the interaction
with the background wave field.
In this work an additional term concerning the dissipation due to critical layers
is added to the energy equation. The critical layers can occur when the back-
ground current shift the wavelength to small scales so that the waves break. For
this critical layer parameterization the vertical refraction term in the radiative
transfer equation is integrated.
The energy equation is then added to the python ocean model (pyOM) and
simulations using a single column are conducted. The key results show that the
interaction with the background wave field typically dominates the other effects.
This leads to an exponential decay of the energy away from the ocean bottom.
If the waves reach a region with a vertical velocity shear the waves can also ex-
tract energy and momentum from the current. The leads to a slight downwards
deflection of the current and also enables some critical layer dissipation. Thus
in these conditions the lee waves also lead to some dissipation and mixing in
the ocean interior far away from the bottom. The maximum dissipation near
the bottom is found to be larger than 10-8m2/s3, which is in accordance with
other simulations and observations.

How to cite: Wagner, I., Eden, C., and Olbers, D.: Propagation And Dissipation Of Lee Wave Energy In A Single Column Model, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-16509, https://doi.org/10.5194/egusphere-egu23-16509, 2023.

vCO.15
|
EGU23-16596
Lars Czeschel and Joshua Pein

Symmetric instability (SI) plays an important role in the energy transfer from geostrophically balanced fronts to turbulent kinetic energy in the oceanic surface mixed layer.  SI can occur when the potential vorticity (PV) takes the opposite sign of the Coriolis parameter. Due to the “impermeability” of PV, the averaged PV  in a volume bounded by two outcropping isopycnals in the mixed layer can only be changed by PV fluxes through the surface or from the stratified interior.  Much attention is paid to PV fluxes at the surface caused by momentum and buoyancy fluxes. Here, we concentrate on the PV flux from the stratified interior which plays an important role in stabilising a symmetrically unstable front. Large Eddy simulations are used to study the impact of secondary Kelvin-Helmholtz instabilities  on the PV fluxes and the associated restratification of the mixed layer. Sensitivities to grid resolution show that a resolution of ~1m is needed to resolve the full influence of Kelvin-Helmholtz instabilities on the evolution of SI. Ideas for parameterising their effects will be discussed.

How to cite: Czeschel, L. and Pein, J.: On the role of secondary instabilities for symmetrically unstable fronts in the surface mixed layer, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-16596, https://doi.org/10.5194/egusphere-egu23-16596, 2023.