Mesoscale and submesoscale features play a critical role in transporting heat and biogeochemical tracers from the surface ocean to depths below the mixed layer, by driving vertical motions across density gradients. In the winter of 2022, strong mesoscale and submesoscale features were observed in the Western Mediterranean Sea during the ONR CALYPSO oceanographic campaign. This multidisciplinary experiment combined multiplatform in-situ observations with high-resolution numerical simulations to observe and predict small-scale ocean variability. In particular, a mesoscale density front associated with a vortex dipole was observed using CALYPSO observations and satellite imagery. A 650m resolution model simulation is used here to understand the evolution of the front and the energy transfer to submesoscale cyclonic eddies. The simulation properly reproduces the intense, narrow, and elongated frontal convergence structure and a dense cyclonic ridge linked to the vortex dipole. The evolution of the front is characterized by: i) an intensification through frontogenesis, and ii) a decay due to favorable conditions for overturning instabilities during a down-front wind event. The frontogenesis and instabilities processes enhance vertical motion via an across-front ageostrophic secondary circulation and contribute to the restratifying effect. The front decayed within days, interacting with the mesoscale ridge to break down into smaller structures and generate submesoscale cyclonic eddies (SCEs) at its edges. The formation of SCEs is associated with the frontal decay, as well as centrifugal and gravitational instabilities, which transfer energy from the mesoscale front to the SCEs. The SCE structure reveals a 3D helical-spiral recirculation pattern that transports parcels vertically. Observations of oxygen and chlorophyll confirm the enhancement of the vertical transport of tracers from the surface to the ocean interior. Submesoscale eddy-induced frontogenesis mechanism and instability processes drove subduction along outcropping isopycnals at the periphery of the SCE.
How to cite: Garcia-Jove, M., Mourre, B., Zarokanellos, N., Haley Jr., P. J., Mirabito, C., Lermusiaux, P. F. J., Rudnick, D. L., and Tintoré, J.: Vertical Pathways Associated with the Evolution of a Mesoscale Front into Submesoscale Cyclonic Eddies, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-18677, https://doi.org/10.5194/egusphere-egu25-18677, 2025.
Oceanic fronts are regions of intense activity which play a crucial role in connecting surface flows to the deeper ocean. However, the complex dynamics of these systems remain challenging to observe and quantify. The CALYPSO program aims to uncover the pathways by which surface water is transported into the ocean interior, addressing fundamental questions about the evolution of oceanic fronts. Previous studies have identified frontal regions as hotspots of variability, where sharp gradients and small-scale structures can enhance mixing and drive significant transport of physical and biogeochemical properties.
Here, we focus on the characterization of a frontal system in the northern Balearic Sea through a multi-instrumental approach involving CTD (UCTD) and Acoustic Doppler Current Profiler (ADCP). The survey was conducted from 18 February to 12 March 2022 aboard the R/V Pelagia and consisted of 19 repetitions of a high-resolution, small-scale rectangular sampling box. UCTD profiles were collected at a mean horizontal resolution of 1km enabling detailed resolution of temperature, salinity, and density gradients across the front.
As part of the analysis, vorticity was calculated from the ADCP velocity field to estimate the role of submesoscale processes within the frontal system. To further investigate the structure of the velocity field, we applied a spectral Helmholtz decomposition technique to the ADCP data, which separates the one-dimensional observed velocities into their rotational and divergent components, providing detailed insights into the flow kinematics. A crucial step in this process is assessing the anisotropy of the flow to ensure the correct implementation of the method. The analysis used models tailored for both isotropic and anisotropic flows, enabling us to examine how flow anisotropy influences submesoscale dynamics. Additionally, the analysis revealed signatures of inertial-gravity waves, highlighting their role in the observed velocity field and their interaction with submesoscale processes. This approach offers a spectral view of the energy distribution, flow instabilities, and wave dynamics, improving our understanding of the interplay between different scales within frontal systems and the evaluation of the balanced and unbalanced flows. Preliminary results reveal a dominant contribution of rotational motions to the kinetic energy spectra, while the influence of internal waves becomes increasingly significant at deeper levels.
How to cite: Martí-Solana, C., Pascual, A., Johnston, T. M. S., Mahadevan, A., and Ruiz, S.: Wavenumber spectra of a submesoscale front from ADCP ship-track data, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-3021, https://doi.org/10.5194/egusphere-egu25-3021, 2025.
Diurnal warm layers (DWLs) develop under relatively weak winds and strong solar radiation and their associated stratified layer has important consequences for turbulent mixing and air‐sea interactions. In this paper we investigate DWLs during three consecutive days in the South Atlantic using observations from an underwater glider equipped with a turbulence microstructure package, a series of drifters at two different depths, and a 1-D turbulence model. The observations and modeling show that the DWLs create a near-surface stratification that partially decouples the surface current from the mixed layer below. However, we find that turbulent entrainment of momentum from below the DWL is important in the evolution of the surface current. We further derive buoyancy, potential and kinetic energy budgets, and identify the dominant terms. The upper ocean potential energy budget is dominated by the incoming solar radiation, with only a small contribution from turbulent mixing. Turbulent shear production, however, is found to be an important influence on the upper ocean mean kinetic energy, receiving a similar fraction of the wind work as the acceleration of the diurnal jet. The DWL evolution and energy budgets are corroborated with simulations from a freely-evolving one-D turbulence model, which additionally shows that the exchange of kinetic energy with the surface wave field is of minor importance to the development of the DWLs we observe.
How to cite: Miracca Lage, M., Ménesguen, C., Schmitt, M., Umlauf, L., Merckelbach, L., and R. Carpenter, J.: Turbulence Observations and Energetics of Diurnal Warm Layers, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-1265, https://doi.org/10.5194/egusphere-egu25-1265, 2025.
Under low wind conditions solar radiation is able to heat a thin layer near the ocean surface to create what is known as a diurnal warm layer (DWL). This layer exhibits relatively strong stratification that decouples it from the rest of the ocean mixed layer below, and leads to increased near-surface velocities within the DWL, called the diurnal jet. A number of recent studies have observed that the diurnal jet is highly turbulent, and that the entrainment and shear production that results is an important factor in the evolution of the jet momentum and kinetic energy budgets. Here we investigate shear instability of the stratified diurnal jet as a potential source of turbulence, highlighting a number of important considerations when considering the stability of such a flow. Based on measurements using turbulence microstructure equipped underwater gliders in the South Atlantic during the SONETT2 expedition, we perform a set of stability analyses of expected flow configurations. In particular, we find that despite the presence of an upper boundary there is no alteration of the stability criteria from the classic Miles-Howard result, namely the gradient Richardson number must be less than 1/4. This is in contrast to profiles with a sharp stratified region that is thin compared to the shear thickness, where the flow can be stabilized by the presence of a sufficiently close upper boundary (the sea surface). We examine the reasons for this difference in the stability criteria for different configurations of the shear and stratification and relate these potential changes in the stability to the observed behaviour of the diurnal jet. In addition, we discuss some important considerations that the analysis reveals on the propagation of internal waves in the presence of both an upper boundary and strong shears.
How to cite: Carpenter, J. and Miracca-Lage, M.: Shear instability of the ocean's diurnal warm layer, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-10461, https://doi.org/10.5194/egusphere-egu25-10461, 2025.
We present observational data from the mouth of the Connecticut River, a shallow salt-wedge estuary characterised by intense interfacial mixing. The motivation is to better understand, and ultimately predict, density-stratified turbulent mixing driven by shear instabilities at high Reynolds numbers (Re > 10^5). Such processes span an immense turbulent energy cascade across eight orders of magnitude, from coherent instabilities at kilometre scales to the smallest mixing eddies at micrometre scales. Using multi-beam echo-sounding imagery, we reveal the spatial structure and temporal evolution of turbulent mixing with unprecedented detail. During the flood tide, large-scale topography and hydraulics cause the pycnocline to slope, which triggers, through baroclinic forcing, primary shear instabilities in the form of long trains of Kelvin-Helmholtz billows. Our data demonstrate that at Re ~ 5x10^5, mixing occurs primarily by turbulence in the braids connecting the cores of the billows rather than within the cores themselves. This secondary 'braid turbulence' is continuously forced by the secondary baroclinic generation of shear within the sloping braids. This finding challenges the prevailing paradigm built upon direct numerical simulations (DNS) at lower Reynolds numbers (Re ~ 10^3-10^4), where mixing is thought to occur primarily by overturning in the billow cores. This distinction is a significant shift in understanding the high-Re turbulent cascade in mixing hotspots, with potential implications for mixing parameterisations in the coastal ocean.
How to cite: Lefauve, A., Bassett, C., Plotnick, D., and Geyer, R.: The structure of stratified mixing by shear instability in baroclinically forced shear flows, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-3555, https://doi.org/10.5194/egusphere-egu25-3555, 2025.
Mesoscale features and the corresponding submesoscale structures can vertically transport heat, freshwater, and biogeochemical tracers (i.e., phytoplankton, oxygen, and carbon) from the surface to the interior. These structures may grow, decay, and transfer energy through various processes. This study examines the small (~20 km) mesoscale eddy evolution and the associated energy transfers from a four-dimensional, three-month-long glider fleet survey in the Western Mediterranean Sea. The combined glider fleet covered nearly 15978 km over the ground, performing 704 glider days while doing over 4837 dives to as deep as 700 m, measuring physical and biochemical parameters. The sources of eddy kinetic energy are examined and compared with numerical eddy-resolving simulations (2-km grid-scale). The comparison allows us to identify whether the energy exchange is local or has a broader interaction between the mean and eddy flow. We study the redistribution of energy during the eddy merging and splitting processes, and how these processes relate to changes in the flow divergence and vertical velocities. During the eddy merging, the vertical velocity reaches up to 20 m/day. However, we observed a reduction in the areas where significant vertical motion occurs, which was associated with a decrease in frontogenesis in the periphery of the eddy and a redistribution of kinetic energy across the merging eddy. During the eddy splitting, the vertical velocity was significantly reduced (less than 10 m/day) by a frontolytic event in the northern eddy. Eddy splitting caused a significant reduction of the positive and negative divergence, and the energy of the two newly formed cyclonic eddies (CEs) decayed (vertical velocities decreased from ~20 m/day to ~10 m/day). The eddy merging event can be considered as a large-scale energy pump in the regions where an inverse energy cascade occurs. The observed imbalance in the transfer of EKE during eddy splitting suggests that the northern CE decays quicker and maintains less kinetic energy than the southern one. We examine the energy transfer terms of the baroclinic and barotropic components, taking into account both the horizontal and vertical energy transfer (baroclinic horizontal term, the baroclinic vertical term, and the barotropic term), which provides a better understanding of the instability processes responsible for the eddy formation.
How to cite: Zarokanellos, N. D., Rudnick, D. L., Mourre, B., Garcia-Jove, M., Lermusiaux, P. F. J., and Tintoré, J.: Eddy Dynamics and Energy Pathways from 4-Dimensional Glider Observations and Numerical Simulations., EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-15929, https://doi.org/10.5194/egusphere-egu25-15929, 2025.
Submesoscale dynamics play a key role in the oceanic energy cycle and drive material transport that shapes marine ecosystems. In this study, we present observational evidence of symmetric instabilities (SI) at the Mississippi River Plume front. The data was collected during the Submesoscales Under Near-Resonant Inertial Shear Experiment (SUNRISE), a project focused on exploring the interactions between wind-driven near-inertial oscillations, internal waves, and submesoscale dynamics in the energetically rich environment of the northern Gulf of Mexico. The observed SI occur during a transition from downwelling to upwelling winds. Downwelling winds initially push the front onshore. These winds introduce negative potential vorticity (PV), destabilizing the front with respect to submesoscale instabilities. Weak stratification and high mixing rates accompany the downwelling winds. Alternating bands of velocity and tracers suggest active SI during this period. As the winds weaken and shift to upwelling conditions, the system restratifies, yet the banded structures persist for about 36 hours. The instabilities are supported by negative PV input from the bottom boundary layer on the shoreward side of the front. The velocity bands associated with SI transport heat and oxygen along the sloping isopycnals, providing a pathway for exchange between surface and bottom waters. After approximately 36 hours, increasing upwelling winds cause the surface front to move offshore, leading to strong upper ocean stratification. These findings highlight SI as a mechanism for ventilating the bottom boundary layer, with potential impact for heat flux and oxygen transport even in the absence of direct wind forcing.
How to cite: Körner, M., Cusack, J., Nash, J., Shearman, K., Hsu, F., MacKinnon, J., Thomas, L., Liu, J., and Taylor, J.: Observation of symmetric instability at a bottom attached front, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-12530, https://doi.org/10.5194/egusphere-egu25-12530, 2025.
Internal tides, internal waves at tidal frequency, represent a considerable reservoir of kinetic energy within the broad spectrum of motions at frequencies higher than the inertial frequency f. Further, they are argued to play an important role in the modulation of energy pathways, such as the redistribution of energy toward smaller scales, promoting an energy transfer via the forward energy cascade.
New Caledonia, an archipelago in the southwestern tropical Pacific, has recently been identified as a hot spot for semidiurnal internal tides based on regional numerical modeling, with comparable energetics to those at Luzon Strait and the Hawaiian Ridge. Alongside strong internal tides, New Caledonia features elevated mesoscale-eddy activity, and is subject to eddy-internal tide interactions. A twin simulation without tides suggests that tidal forcing has a considerable impact on cross-scale energy exchanges by amplifying the forward energy cascade while modulating the transition scale between forward and inverse cascade toward larger wavelengths. Though, these modulations underlie strong seasonal variations. In turn, the eddy-internal tide interactions impact the internal-tide life cycle, from the barotropic-to-baroclinic tidal energy conversion to the energy propagation pathways. Specifically, mesoscale-eddy-induced stratification changes at the generation sites can considerably enhance/reduce the barotropic-to-barotropic conversion rate by more than 20% on monthly to intraseasonal time scales. In propagation direction, the tidal beams are primarily refracted by mesoscale currents and are characterized by overall increased phase variability and dispersion.
Important insights are gained by moorings deployed in the internal-tide generation hot spot and in the propagation direction south of New Caledonia, enhancing our understanding of eddy–internal tide interactions from an in-situ perspective. Mooring observations at fixed locations are further complemented by daily sea surface height (SSH) measurements provided by the Surface Water Ocean Topography (SWOT) satellite altimetry mission during its fast-sampling phase (1-day repeat orbit). Together, these datasets will help understand the governing dynamics of internal tides and their broader impact on oceanic energy pathways.
How to cite: Bendinger, A., Vic, C., Cravatte, S., and Gourdeau, L.: Eddy-internal tide interactions around New Caledonia: Insight from regional numerical modeling, in-situ observations, and SWOT, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-6480, https://doi.org/10.5194/egusphere-egu25-6480, 2025.
Mesoscale eddies play a key role in the transport of physical and biogeochemical properties in the ocean. Prior numerical modelling studies have demonstrated that wind-eddy interactions can transform eddies into different types: anticyclones to Mode-Water Eddies (MWE) and cyclones to cyclonic “Thinnies”. However, there is a limited understanding of eddy transformations as direct observations are challenged by the lack of long-term time series within individual eddies. Here we report evidence of an eddy observed in 2005 transforming from a regular anticyclone to a MWE sampled by Argo floats and shipboard measurements while tracked via satellite altimetry data. Argo profiles of the inner core of the eddy (⪝30 km from eddy center) early in its lifetime are compared to climatologies from the World Ocean Atlas. Temperature profiles show a downward displacement of the main thermocline, between 250 and 500 meters, consistent with a regular anticyclone in January. Five months later in July, shipboard sampling revealed the Mode Water layer had thickened, with a notable upward displacement of the seasonal thermocline consistent with a MWE structure. Model reanalysis data (MERCATOR GLORYS12V1), which includes a wind-eddy interaction term, suggests a qualitatively similar result. After transforming into a MWE, the eddy was observed to have triggered a long-lasting diatom bloom as nutrients abundant in density layers over 26.0 kg/m3 were uplifted into the euphotic zone. Significant biological implications such as this are critical results of eddy transformations. This novel observation not only shows the potential transformation of eddies, but motivates a greater understanding of their features and frequency, and to what extent they impact the world’s oceans.
How to cite: Zhu, L., Bakker, R., Johnson, R., and McGillicuddy, D.: Observed Transformation of an Anticyclone into a Mode-Water Eddy in the Sargasso Sea, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-14950, https://doi.org/10.5194/egusphere-egu25-14950, 2025.
We report on novel observations of deep-reaching submesoscale eddy (SME). Open-ocean SMEs typically appear as mixed layer eddies (MLEs), with signatures rapidly decaying beneath the mixed layer depth. The observed eddy is, like MLEs, top-intensified, but its signature reaches to 300 m, over 10 times the mixed layer depth. Moreover, the eddy was detected in summer in shallow (20 m) mixed layer conditions, i.e., where relatively weak mixed layer instability is expected. Despite this, the eddy Rossby number and Richardson number are measured as O(1), and the eddy radius is 7 km, all in accordance with submesoscale flows. The vortex was detected in the East Mediterranean Sea at the foot of the continental slope offshore of Israel and was monitored via multiple in-situ platforms and via remote sensing for its full lifetime of three weeks. Based on satellite imagery, we attribute the anomalous deep-signature of the eddy to a different formation mechanism than mixed layer instability, namely formation from a meander in the regional boundary current, which has a similar depth signature. Finally, we note multiple similar events in the satellite record and discuss the border implications, including on cross-shelf material transport.
How to cite: Solodoch, A., Gildor, H., Toledo, Y., Barkan, R., Verma, V., Fadida, Y., and Lehahn, Y.: Full life-cycle observations of a deep-reaching summer submesoscale vortex, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-15966, https://doi.org/10.5194/egusphere-egu25-15966, 2025.
Atmospheric gravity waves (GWs) present a challenge to climate prediction since most of their spectrum is not resolved in global climate models and good observational constraints on GW activity do not exist. One of the long-standing approximations made in gravity wave parameterizations (GWPs) is the assumption of purely vertical propagation of these waves (no horizontal nonlocality). I will present recent developments in my group on using machine learning (ML) methods to aid the parameterization of the effects of breaking atmospheric gravity waves in global climate models. These efforts have advanced through two distinct approaches: a) replacing existing physics-based GW parameterizations with ML algorithms, and b) using ML methods to aid in the calibration of existing physics-based parameterizations. On a) I will describe ML-based GW parameterizations that incude various degrees of nonlocality, including a globally nonlocal scheme that is trained on high-resolution data. On b), I will present results on using methods including Ensemble Kalman methods and Bayesian methods to calibrate parameters in physics-based GWPs, as well as to estimate parametric uncertainty in climate projections.
How to cite: Sheshadri, A., Gupta, A., King, R., and Mansfield, L.: Using ML-based methods to improve the representation of atmospheric gravity waves in climate models, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-13526, https://doi.org/10.5194/egusphere-egu25-13526, 2025.
The largest source of internal waves in the ocean is tidal currents flowing across rough bottom topography, resulting in internal tides. When these waves break, they release some of their energy to the background ocean in the form of diapycnal mixing. The distribution of this internal tide-driven mixing has a significant impact on the ocean state. The parameterization of internal tide-driven mixing in climate models is based on estimates of the generation of internal tides, or tidal conversion. Semi-analytical methods based on linear wave theory have been used to compute the tidal conversion from the observed bottom topography, ocean stratification, and tidal currents. However, such linear calculations fail completely at steep continental slopes. Here, we construct a computationally inexpensive method to estimate the tidal conversion into vertical normal modes by continental slopes and shelves, and apply it at the global scale. It uses the usual observational data as inputs but relies on a reduced-physics numerical model rather than on linear theory to compute the tidal conversion. The method also resolves the onshore and offshore energy fluxes. The output is validated with the conversion diagnosed from a realistic simulation. The results are useful for parameterizing the subsequent propagation and breaking of the internal tides and their resulting diapycnal mixing.
How to cite: Geoffroy, G., Kelly, S. M., and Nycander, J.: Tidal conversion into vertical normal modes by continental margins, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-11506, https://doi.org/10.5194/egusphere-egu25-11506, 2025.
The spontaneous emission of internal waves (IWs) from balanced mesoscale eddies has been proposed as a source of oceanic IW kinetic energy (KE). This study investigates the mechanisms leading to the spontaneous radiation of spiral-shaped IWs from an anticyclonic eddy with an order-one Rossby number, using a high-resolution numerical simulation of a flat-bottomed, wind-forced, reentrant channel flow configured to resemble the Antarctic Circumpolar Current. It is shown that the IWs are spontaneously generated due to a loss of balance process that occurs at the edge of the edge and radiates radially outward. A 2D linear stability analysis of the eddy reveals that the spontaneous emission arises from a radiative instability, which involves an interaction between a vortex Rossby wave supported by the radial gradient of potential vorticity and an outgoing IW. This particular instability occurs when the perturbation frequency is superinertial. This finding is supported by a KE analysis of the unstable modes and the numerical solution, demonstrating that the horizontal shear production provides the source of perturbation KE. Additionally, the horizontal length scale and frequency of the most unstable mode from the stability analysis closely correspond to those of the spontaneously emitted IWs in the numerical solution.
How to cite: Kar, S., Barkan, R., McWilliams, J. C., and Molemaker, M. J.: Spontaneous emission of internal waves by a radiative instability, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-15215, https://doi.org/10.5194/egusphere-egu25-15215, 2025.
Agulhas Rings play a key role in the dynamics of the South Atlantic, especially through their interactions with the Walvis Ridge. However, the influence of mesoscale eddies, their interactions with bathymetry, and their interplay with internal waves on high frequency ocean dynamics remains unclear, especially in realistic setups. To address this issue, we combine high-resolution numerical simulations with telescopic grid refinement (achieving horizontal resolutions below 600 m over large regions of the South Atlantic) and observations from a dedicated field campaign, complemented by data from the recently launched SWOT mission. Our simulations, validated with mooring and Pressure Inverted Echo Sounder data from the Walvis Ridge region, show that tidal forcing and resolved submesoscale dynamics are critical to accurately capture high-frequency energy levels. Simulations without tides and submesoscale dynamics show significantly reduced energy at the high-frequency end of the spectrum. Using an eddy-tracking algorithm, we study the evolution of five anticyclones and three cyclones near the ridge. Hovmöller plots and bandpass-filtered fields show fluctuations at diurnal and semidiurnal frequencies, especially at the ridges and underneath large anticyclones. We provide new insights into mesoscale-bathymetry interactions and their role in shaping high-frequency ocean dynamics in the South Atlantic.
How to cite: Epke, M. and Brüggemann, N.: Impact of mesoscale eddies and bathymetry on high-frequent ocean dynamics in a submesoscale resolving simulation of the South Atlantic, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-9967, https://doi.org/10.5194/egusphere-egu25-9967, 2025.
The flux of kinetic energy between oceanic currents of different horizontal scales is of key importance for the oceans energy balance between wind-forcing on mainly large scales and dissipation on small scales. Oceanic eddies in quasi-geostrophic balance, including submesoscale mixed-layer eddies, are associated with an inverse cascade towards larger scales. In contrast, in regional simulations, internal gravity waves have been shown to reduce the inverse cascade by quasi-geostrophic eddies and to drive a strong forward cascade towards the small dissipative scales. The major forcing mechanisms of internal gravity waves are tides and high-frequency winds. In this study, we investigate the effect of both forcings on the cross-scale kinetic energy flux by comparing the latter in parallel submesoscale-permitting simulations of the full Atlantic i) with both forcings, ii) with only high-frequency wind forcing, and iii) without both forcings. We show that both internal gravity wave forcings contribute to an increase in the forward cascade, which is most pronounced in summer-time when balanced flows are weak. Both forcing effects are present at all investigated scales, but the tidal effect dominates at smaller scale (about 30 km), while at larger scales (about 100 km) the wind-effect dominates. By comparing fluxes from three-day- and hourly-mean velocities, we show that the forward cascade associated with both forcings is a result of high-frequency motions at time-scales less than three days. In spring, the high-frequency forward cascade is overcome by the inverse-cascade of mixed-layer eddies. The comparison of the (mainly inverse) fluxes from the three-day-mean flow between the parallel experiments show that the effect of tides on the low-frequency cascade is very small while the high-frequency winds are responsible for the reduction of the low-frequency inverse cascade. Finally, we show that geostrophic coarse-graining cross-scale kinetic energy fluxes can be computed from SWOT satellite observations despite the gap between the measurement swaths, by applying a SWOT simulator to the ocean model solutions. With this, it is possible to validate the submesoscale geostrophic cross-scale kinetic energy flux in ocean simulations.
How to cite: Schubert, R., Gula, J., Barkan, R., and Vergara, O.: The effect of tides and high-frequency winds on the oceanic cross-scale kinetic energy flux, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-8326, https://doi.org/10.5194/egusphere-egu25-8326, 2025.
The breaking of near-inertial wave (NIW) trapped in anticyclones after strong wind events is a well-known pathway for kinetic energy dissipation below the mixed layer in the ocean and one of the mechanisms by which the ocean responds to modified wind patterns under climate change. In the Mediterranean Sea, where turbulence is generally low far from topographic boundaries, NIW trapping has been documented only in few large (>100 km) and energetic mesoscale features. Whether NIW trapping is restricted to these few isolated and semi-permanent features or is a more widespread phenomenon remains a key open question, whose answer is hindered by the difficulty of tracking in space and time typical Mediterranean eddies of low energy and small radii.
Here we present an in-situ experiment conducted during the BioSWOT-Med cruise (doi.org/10.17600/18002392) that addressed this problem by surveying a moderately energetic small meander (<50 km, Ro ≈ 0.5 ~ Fr) of the North Balearic front assisted by the first high-resolution SSH images of the SWOT mission. We explore how the front modulates the evolution of the turbulence below the mixed layer after experiencing two consecutive strong wind events. We show that the turbulence remains low in the front and its cyclonic side while turbulence is greatly enhanced in the anticyclonic side. The latter side is dominated by a small anticyclone (~30 km of diameter) that trapped NIWs down to 300 m, generating intense shear and turbulence reaching up to several 10-8 W/kg. Estimations of vertical kinetic energy fluxes induced by NIWs are about one order of magnitude stronger than previous estimations outside anticyclones (8–10 mW/m2 vs 0.5–2.5 mW/m2) and about 3 times the estimation of Kunze et al. (1995) in a mesoscale anticyclone of the Gulf Stream (~3 mW/m2). More generally, these results suggest that moderately energetic fine-scale fronts and eddies are as important to structure the flows and the turbulence as strong fronts and eddies found in western boundary current and upwelling systems, addressing new challenges for their parameterization in Earth system models.
How to cite: Rolland, R., Bouruet-Aubertot, P., Cuypers, Y., Bosse, A., Petrenko, A., Berta, M., d'Ovidio, F., Grégori, G., and Doglioli, A.: Are we underestimating eddy-wave interactions in the Mediterranean Sea? Insights from the BioSWOT-Med 2023 cruise, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-10694, https://doi.org/10.5194/egusphere-egu25-10694, 2025.
In the ocean, internal tides (internal waves generated by the interaction of the barotropic tide and the irregular bathymetry) interact in various ways with the other types of motion, in particular the mesoscale currents.
These interactions disrupt the life cycle of the internal tide, from its generation to its dissipation.
These effects include the loss of coherence of the internal tide -- corresponding to a phase unlocking with the parent barotropic tide, which is essentially regular in the deep ocean -- and cross-scale energy transfers.
In this study, we investigate the effect of mesoscale flow on the semidiurnal internal tide propagation using outputs from a high-resolution realistic numerical simulation of the North Atlantic.
Using a vertical mode decomposition framework, we identify and quantify the mechanisms that affect the internal tide: in particular, we show that advection by the mean flow leads to an average transfer of energy from low modes to high modes -- hence large to small scales --, potentially contributing to the dissipation of the internal tide by "feeding" wave breaking.
We also quantify the loss of internal tide coherence due to these terms and discuss them in different regions with typical flow/internal tide configurations.
How to cite: Lahaye, N., Bella, A., and Tissot, G.: Impact of the mesoscale dynamics on the internal tide lifecycle in the North Atlantic, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-16633, https://doi.org/10.5194/egusphere-egu25-16633, 2025.
This study provides an analysis of the ocean Lorentz Energy Cycle (LEC) simulated with an ICON- O configuration of 5km horizontal resolution. Since most processes relevant for eddy dissipation cannot be resolved even with 5km horizontal resolution, parameterizations are required to dissipate the energy. Typical parameterizations used in ocean models are bottom friction, vertical viscous dissipation and horizontal biharmonic dissipation.
Here, we focus on all sources, sinks and fluxes of geostrophic eddy kinetic energy. To this end, we filter geostrophic turbulent eddies from internal waves by (a) approximating geostrophic pressure by a five day time average and (b) by using quasi-geostrophic equations to diagnose the geostrophic energetics. We find that the production of QG EKE is dominated by the baroclinic pathway. Horizontal and spatial energy pressure fluxes transport the produced energy to bottom and western margins. There it is dissipated mainly by horizontal friction, followed by bottom and vertical friction. Sensitivity simulations with altered friction parameters show, that dissipation changes marginally with respect to these alterations.
How to cite: Hillenkötter, D. and Brüggemann, N.: Dissipation of Eddy Kinetic Energy in the ICON-O Model, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-8164, https://doi.org/10.5194/egusphere-egu25-8164, 2025.
The oceanic Lorenz energy cycle and its baroclinic eddy production B is analysed in realistic global and idealized models. B shows a dipolar structure in spectral space, acting as source of eddy kinetic energy at smaller, and as sink (B<0) at larger scales, partly balancing the inverse kinetic energy cascade. Together with a forward potential energy cascade, this opens a potential pathway for small scale eddy dissipation. To understand this structure of B, its geographical relation to different dynamical regimes is examined.
In the realistic model, B<0 is found predominantly poleward of 30° latitude, where the zonal mean current U tends to be eastward. Simulations using idealized models also show a connection between B<0 and the sign of U<0 consistently appears for U>0 but diminishes for U<0. When the meridional gradient of planetary vorticity β is set to zero, B<0 disappears, suggesting that planetary Rossby waves are also essential for its existence.
Linear stability analysis can explain the findings: it shows no B<0 for β=0, but B<0 at large-scales when β ≠ 0. It also shows that the vertical structure of B<0 changes with the sign of U: for eastward currents, B<0 is located in the upper half of the water column, whereas for U<0, it shifts to the bottom. Since eddy energy and B is surface intensified, the near-bottom energy sink at large-scales by B for U<0 is damped and becomes unimportant, while the near-surface B<0 for U>0 is amplified, which we see in turn in the models.
How to cite: Dettmer, J. N.: Spectral resolution of oceanic baroclinic production: Exploring a novel eddy energy sink, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-4451, https://doi.org/10.5194/egusphere-egu25-4451, 2025.
Nonlocality originates when the energy extracted from the mean flow in one region does not sustain the energy growth of eddies in that region (or vice versa) but is redistributed and consequently used to support the energy growth of eddies outside that region. Quantifying the nonlocality of the eddy-mean flow interactions is crucial for improving our understanding of how energy is redistributed from the surface, where it is inputted, to the interior where it is dissipated. Furthermore, the characterisation of nonlocality is crucial in understanding how eddy-mean flow interactions, which are the primary mechanism for generating kinetic energy of the mesoscale variability, take place. We use eddy-resolving simulations generated using the ICON-O Model, with a 0.1° global horizontal resolution, to explain the modality of eddy-mean flow interactions. We also highlight the relevancy of eddy-mean flow interactions in vertical energy redistribution, an aspect that was previously neglected; previous studies focused on horizontal energy redistribution.
How to cite: Ssebandeke, J., von Storch, J.-S., and Brüggemann, N.: Role of nonlocal eddy-mean flow interactions in the redistribution of energy in the ocean, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-10163, https://doi.org/10.5194/egusphere-egu25-10163, 2025.
How the energy propagates between different oceanic scales is a fundamental problem that is still far from being completely understood. Not only is it important for theoretical considerations but it is also critical for improving the parameterization of oceanic models. Using the coarse-grain method, one can perform an energetic study on a turbulent flow such as the ocean that is local in both space and scale (Contreras et al. (2023)). Thus, through the application of a mathematical filter operation, the spatial and scaling properties of the local flux of energy between scales (Πr(x,t)) were investigated in oceanic simulations. To this end, this work exploits the behaviour of coarse-grained fields at small scales. When the filter scale is small, the coarse-grained velocity verifies the multifractal hypothesis:
The singularity exponents (h(x,t)) constitute a local measure of the degree of continuity of the underlying turbulent flow and define a multifractal decomposition into universality classes. From the multifractal hypothesis, a scaling law can be derived theoretically for Πr (see Isern-Fontanet and Turiel (2021) and references therein):
One month of data was analysed from a numerical simulation of the circulation in the North Atlantic Ocean generated using the Coastal and Regional Ocean COmmunity (CROCO) model, with a 6-7 km spatial resolution. Πr was computed through the application of a low-pass filter using a top-hat kernel. h(x,t) were extracted from the velocity field using the approach developed by Pont et al. (2013). The spatial analysis supports the existence of a connection between singularity and intensity of Πr. The scaling analysis found that Πr obeyed a scaling law governed by the exponent 2h+1 rather than the theoretical prediction of 3h+2. This finding is supported by the computation of the critical exponent in the singularity spectrum, h∞≈-0.5, under a numerical error of O(0.1). Such discrepancy in the representation of the scaling of Πr suggests that the Reynolds tensor scales as h+1 rather than 2h+2. Furthermore, it implies that the parameterization utilized by this model affects the representation of the turbulent energy cascade in the simulation and requires a compensation.
M. Contreras, L. Renault and P. Marchesiello, Understanding Energy Pathways in the Gulf Stream, Journal of Physical Oceanography, 53, pp 719-736, 2022.
J. Isern-Fontanet and A. Turiel, On the connection between intermittency and dissipation in ocean turbulence: A multifractal approach, Journal of Physical Oceanography, 51, pp. 2639–2653, 2021.
O. Pont , A. Turiel and H. Yahia, Singularity analysis of digital signals through the evaluation of their unpredictable point manifold, International Journal of Computer Mathematics, 90:8, 1693-1707, 2013.
How to cite: G. Gea, V., Isern-Fontanet, J., Renault, L., and Turiel, A.: The Multifractal Theory of Turbulence on the Oceanic Energy Flux Between Scales, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-17017, https://doi.org/10.5194/egusphere-egu25-17017, 2025.
A new closure, ROSSMIX 2.0, for the effect of meso-scale eddies in non-eddy-resolving ocean models is presented and evaluated. It combines aspects of several previous closures in a simplified approach: local linear stability analysis is used to predict the vertical and lateral shape of eddy correlations, while a wave energy equation co-integrated in the ocean model predicts their amplitudes. The new closure is implemented and evaluated with good success in an idealised channel model of vertical and lateral shear instability, and in a realistic quasi-global ocean model. The new closure enhances the meridional overturning circulation both globally and in the individual basins, with clearer connection of the large-scale overturning cells in the Southern Ocean. This comes along with enhanced northward heat transport and horizontal transports in better agreement with observations, and a reduced bias in watermasses.
How to cite: Eden, C. and Dettmer, J. N.: ROSSMIX 2.0: a simplified meso-scale eddy closure applied to a realistic ocean model, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-8715, https://doi.org/10.5194/egusphere-egu25-8715, 2025.
Mesoscale eddy parameterizations deployed in climate models are typically unable to produce eddy-driven topography-following flows which are known to dominate the flow structure in the Arctic. One theory for the development of topography-following flows is that they arise as a result of the cascades of energy and enstrophy in quasigeostrophic turbulence and the dissipation of enstrophy at small scales. Recent work in the field of eddy parameterizations has seen an emerging focus on developing energetically consistent parameterizations, but the same focus has not been applied as thoroughly to the enstrophy.
In this work, a parameterization of barotropic eddy potential vorticity fluxes is introduced which incorporates an energetic constraint and an additional enstrophetic constraint. The parameterization mixes potential vorticity in a manner that produces a net eddy-to-mean kinetic energy transfer and a net mean-to-eddy potential enstrophy transfer, consistent with the inverse kinetic energy cascade and direct potential enstrophy cascade typical of the barotropic mode of quasigeostrophic turbulence. The barotropic parameterization is integrated with the Gent and McWilliams (1990) parameterization of baroclinic eddies by providing a mechanism through which available potential energy, extracted by the baroclinic parameterization, can be converted to barotropic eddy kinetic energy.
The parameterization is tested in an Arctic configuration. We find that the parameterization is successful in driving a large-scale topography-following flow, broadly resembling the Arctic Circumpolar Boundary Current. We explain the evolution of this large-scale flow through a balance in the energy and potential enstrophy transfers.
How to cite: Eaves, R., Marshall, D., Maddison, J., and Waterman, S.: GM+PV: Testing a new mesoscale eddy parameterization in an Arctic configuration, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-9636, https://doi.org/10.5194/egusphere-egu25-9636, 2025.
The Kuroshio is well-known for its variability south of Japan, where it often meanders and forms stable eddies near the coast. In this study, we investigate the relevant process and physical mechanism using a well-validated high-resolution China Sea Multiscale Ocean Modeling System (CMOMS, https://odmp.hkust.edu.hk/cmoms/) and a process-oriented modeling. We found that the variability is chiefly governed by the dynamics of Kuroshio separation. The analysis shows that Kuroshio separates from the coast and forms eddy by inverse pressure gradient force due to both high nonlinearity of Kuroshio and the inverse wind-driven Ekman transport. Typically, the separation occurs near protruding capes where bottom pressure torque (BPT) injects inverse vorticity to the separated Kuroshio. Accompanying the separation, an accumulation of shear vorticity from shoreside/seaside of the separated Kuroshio forms a cyclonic/anticyclonic eddy dipole. During the course of active dynamics adjustment, increased barotropic and baroclinic instability strengthen the eddy which, in turn, stabilize and persist the separation. The study provides a new insight into the variability dynamics of western boundary current.
How to cite: Yongkang, Y. and Gan, J.: Dynamics of Kuroshio separation and associated eddy formation and instability off southern Japan, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-6590, https://doi.org/10.5194/egusphere-egu25-6590, 2025.
The South China Sea (SCS) is a large semi-enclosed marginal sea between East Asian continent and the West Pacific Ocean. Originating from the North Equatorial Current, the Kuroshio is the strongest western boundary current in the North Pacific. When it flows northward along the east Philippine coast, a branch of the Kuroshio intrudes northwestward into the SCS through Luzon Strait, significantly affecting the temperature, salinity, circulation, and eddy generation in the SCS. Previous studies have shown that Kuroshio intrusions are frequently accompanied by anticyclonic eddy shedding in the northern SCS. Based on satellite altimeter data and cruise observations, we explored the statistical characteristics and evolution of Kuroshio-shed eddies in the SCS. The Kuroshio eddy shedding events occur nearly annually, especially in boreal fall and winter. The shedding eddies propagated southwestward along the continental slope and generally dissipated around the Xisha Islands in spring, while some of them re-intensified east of Xisha Islands and survived until autumn. Further analysis found that eddy merging provides energy for re-intensification of long-lived Kuroshio-shed eddies in the SCS. Combined reanalysis data, we further investigated the evolution of eddy vertical structure. The Kuroshio anticyclonic eddies transited from surface-intensified eddies to subsurface-intensified eddies in the SCS with seasonal changes, exhibiting a unique surface cold-core and vertical lens-shaped structure. Changes of sea surface heat flux and eddy-induced Ekman pumping are conducive to the formation of lens-shaped structure.
How to cite: Wang, X.: Evolution of Kuroshio-shed anticyclonic eddies in the South China Sea, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-9758, https://doi.org/10.5194/egusphere-egu25-9758, 2025.
Previous studies have extensively described the coherent vortices at the ocean surface and shallow sub-surface at global and regional scales (from satellites, in-situ measurements, and numerical models). Few studies have investigated the dynamics of the deep coherent vortices (DCVs) below the mixed layer depth. This study focuses on DCVs in the Atlantic Ocean using a high-resolution numerical model. Since the properties of the water asses move along isopycnals, the detection and tracking of DCVs are performed along three isopycnal surfaces: 27.60 kg/m3, 27.80 kg/m3, and 27.86 kg/m3 with depths of 250-1700 m, 1200-2800 m, and 1900-3800 m, respectively. The quantification of the physical characteristics of the DCVs (population, radius, Rossby number, polarity between cyclones and anticyclones, and propagation in space and time) in different parts of the Atlantic Ocean (Mediterranean Water vortices, meddies and Mid-Atlantic Ridge, MAR). The dynamics involved in the generation and destruction of the DCVs throughout their life cycle are analyzed.There is an asymmetry between cyclonic DCVs and anticyclonic DCVs, as they propagate poleward and equatorward, respectively, due to the beta-effect. Cyclonic DCVs tend to be smaller and shorter lived than anticyclonic DCVs, so anticyclones dominate in terms of energetic, large, long-lived, and long-distance DCVs. The results also show that anticyclonic meddies, as well as other DCVs, can cross MAR. The region of the ocean west of the MAR is also characterised by a large number of shielded vortices and the formation of a group of transient shielded multiplets. The DCVs contribute to the transport of characteristic properties of different water masses from their source origin to the distance offshore.This sheds new light on the understanding of the formation, life cycle, physical and dynamical properties of the ocean interior in the Atlantic Ocean for the long-lived DCVs.
How to cite: Chouksey, A., Carton, X., and Gula, J.: Coherent Vortices in the Ocean interior of the Atlantic, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-892, https://doi.org/10.5194/egusphere-egu25-892, 2025.
A regional ocean model was used to study the influence of the Madagascar Ridge on the circulation and eddy variability in the Agulhas Current system. In the control experiment, the model was run with a realistic bathymetry, whereas in the idealized run the bathymetry was modified by flattening Madagascar Ridge. When the Ridge was suppressed, no obvious changes were observed in the large-scale circulation. However, integrated transports revealed an excess of about 10 Sv (1 Sv = 106m3 s−1) in the recirculation of the greater Agulhas Current system. Dynamic and statistically (p < 0.05) significant changes were observed at the mesoscale variability. Composite analysis of the radial distribution of the eddy azimuthal velocity, surface height, and relative vorticity within the Ridge domain in the experiments revealed that the presence of the Madagascar Ridge determines the emergence of a secondary class of large anticyclonic eddy types in the region, the ”Madagascar rings”.
How to cite: Halo, I., Raj, R., Penven, P., Lamont, T., Ansorge, I., and Johannessen, J.: Influence of the Madagascar Ridge on Eddy Variability in the Agulhas Current System: A Modelling Study, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-18402, https://doi.org/10.5194/egusphere-egu25-18402, 2025.
This study investigates the influence of wind curl on the zonal transport and vorticity of a barotropic flow over topography using an idealized quasigeostrophic model. While previous research focuses on how wind stress sets the zonal mean transport over a ridge using idealized models of the Southern Ocean, the interplay between wind curl and constant wind stress in determining zonal transport remains an open question. It is shown that the injection of vorticity through wind curl creates nonzero westward zonal transport, even when there is zero mean wind stress over the domain, which increases with wind curl. The existence of zonal transport is explained qualitatively through differences in zonal dynamics and further studied quantitatively through analytical solutions of governing equations and results obtained from constant wind stress simulations. Our findings additionally suggest that there are two distinct regimes where the zonal transport is determined by wind curl or the mean wind stress when both are present. The first regime is characterized by formation of gyres and Rossby waves whose strengths and amplitudes grow with increasing wind curl, whereas the second regime is described by zonal flow with formation of standing and transient eddies consistent with earlier studies. We determine that the zonal transport and vorticity of the flow are governed by both nonlocal and local mechanisms in the presence of wind stress curl and further explore the interaction between wind curl and topography across a range of ridge heights. The goal of this work is to shed new light on how wind curl influences the dynamics of the Antarctic Circumpolar Current and the eddy saturation regime.
How to cite: Dogan, S., Muller, C., Nadeau, L.-P., and Venaille, A.: Influence of Wind Stress Curl and Bottom Topography on the Transport of the Antarctic Circumpolar Current in a Barotropic Perspective, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-13499, https://doi.org/10.5194/egusphere-egu25-13499, 2025.
Posters on site: Fri, 2 May, 08:30–10:15 | Hall X4
This study addresses the challenges of uncertainty in wave simulations within complex and dynamic ocean environments by proposing a reinforcement learning-based model ensemble algorithm. The algorithm combines the predictions of multiple base models to achieve more accurate simulations of ocean variables. Utilizing the soft actor-critic reinforcement learning framework, the method dynamically adjusts the weights of each base model, enabling the model ensemble algorithm to effectively adapt to varying ocean conditions. The algorithm was validated using two SWAN models results for China’s coastal regions, with ERA5 reanalysis data serving as a reference. Results show that the ensemble model significantly outperforms the base models in terms of root mean square error, mean absolute error, and bias. Notable improvements were observed across different significant wave height ranges and in scenarios with large discrepancies between base model errors. The model ensemble algorithm effectively reduces systematic biases, improving both the stability and accuracy of wave predictions. These findings confirm the robustness and applicability of the proposed method for integrating multi-source data and handling complex ocean conditions, highlighting its potential for broader applications in ocean forecasting.
How to cite: Wu, X. and Huang, W.: Reinforcement learning-based multi-model ensemble for ocean waves forecasting, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-2124, https://doi.org/10.5194/egusphere-egu25-2124, 2025.
We investigate changes in the properties of Transient Coherent Mesoscale Eddies (TCMEs) in the Southern Ocean over the past three decades. Specifically, the study investigates whether eddy characteristics have changed in response to the strengthening and southward migration of circumpolar winds around Antarctica. Eddy data have been extracted from the Mesoscale Eddy Trajectory Atlas and AVISO altimetry from 1993 to 2019. The research area is the spherical segment between 30°S and 80°S, which includes the ACC. The eddy properties analysed are eddy number, amplitude, radius, lifetime, and eddy kinetic energy. We distinguish between the Total Eddy Kinetic Energy (TEKE), that is, the energy of all mesoscales turbulent motions and the Total Transient Coherent Mesoscale Eddy Kinetic Energy (TMKE), which is the energy contained in TCMEs. Both TEKE and TMKE increase by about 15% in the study period. TCME radius and amplitude also increase moderately by 2% and 3%, respectively, while eddy number goes down by 3%. We also study the connection between the first baroclinic Rossby Radius of Deformation (RRD), calculated from GLORYS ocean reanalysis data, and the altimetry-derived eddy radius. As expected, average TCME (RRD) radius increases from 44 km (7 km) south of the ACC, through 47 km (15 km) within the ACC, to 50 km (23 km) north of the ACC. We detect no significant trend in RRD in the Southern Ocean and so, the observed increase in TCME radius cannot be easily ascribed to changes in stratification. The eddy response to an increase in circumpolar winds in the Southern Ocean by about 3% during the last three decades has been used to explain the near insensitivity of the ACC to the changing winds via eddy saturation and eddy compensation mechanism. Our analysis, indeed, demonstrates that changes in eddy properties are commensurate, at least in relative magnitude, to coetaneous shifts in wind strength.
How to cite: Lo, D. Y.-J. and Morales Maqueda, M. Á.: Spatial-Temporal Analysis of Remotely Sensed Coherent Mesoscale Eddies in the Southern Ocean , EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-11923, https://doi.org/10.5194/egusphere-egu25-11923, 2025.
Oceanic dynamics, with their wide range of interacting scales, present challenges for accurate numerical modeling. This work focuses on improving the representation of mesoscale eddies at eddy-permitting resolutions by incorporating novel stochastic perturbations. Eddy viscosity, which leads to numerical over-dissipation near the grid scale, requires methods like dynamic backscatter to mitigate this effect. Stochastic perturbations help capture small-scale processes and uncertainties in ocean flows.
Using a double-gyre configuration, we apply linear inverse modeling with high-resolution reference simulations to generate stochastic perturbation patterns. We explore two stochastic forcing implementations based on different balanced flow constraints. Results show that stochastic forcing improves simulated dynamics, particularly heat distribution at intermediate and small scales, while previously implemented dynamic backscatter injects energy at larger scales. A scale analysis of production, advection, dissipation, and total energy supports these findings.
How to cite: Bagaeva, E., Juricke, S., Danilov, S., and Franzke, C. L. E.: Stochastic Ocean Energy Backscatter via Pressure and Momentum Perturbation, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-13129, https://doi.org/10.5194/egusphere-egu25-13129, 2025.
Starting from the well-known surface boundary conditions for the seawater continuum we derive the associated flux of squared salinity across the ocean surface. Based on concepts from Klingbeil and Henell (2023), we clarify the meaning of this flux by linking it to an instantaneous transformation between freshwater and seawater across virtual isohalines at the ocean surface. Finally, we demonstrate how mixing relations for estuaries need to be amended to account for this instantaneous mixing.
Klingbeil, K. and E. Henell (2023) A Rigorous Derivation of the Water Mass Transformation Framework, the Relation between Mixing and Diasurface Exchange Flow, and Links to Recent Theories in Estuarine Research. JPO. https://doi.org/10.1175/JPO-D-23-0130.1
How to cite: Klingbeil, K. and Lorenz, M.: On the instantaneous salt mixing due to freshwater boundary fluxes, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-13808, https://doi.org/10.5194/egusphere-egu25-13808, 2025.
It has been shown that the Equatorial Intermediate Current (EIC) in the Pacific Ocean, which is a time-mean westward current along the equator at intermediated depth (from 500m to at least 2000 m), has a nearly basin-wide structure, which is a unique feature of the Pacific EIC. In addition to the EIC, slowly varying vertically alternating eastward and westward currents called Equatorial Deep Jets (EDJs) are observed along the equator. Moreover, intra-seasonal waves have also been observed near the equator at 1000 m depth with significant amplitude in the eastern Pacific Ocean. Although these deep intra-seasonal waves in the eastern basin are considered an energy source for the EIC and EDJs, the relationship between them in the Pacific Ocean remains an open question. In this study, we conduct an idealized numerical simulation, which reproduces basic features of the nearly basin-wide Pacific EIC, EDJs, and deep intra-seasonal Yanai waves in the eastern Pacific Ocean. The momentum budget indicates that the Yanai waves provide westward momentum to the background field (sum of the EIC and EDJs) in the eastern part of the basin. Specifically, the vertical profile of the momentum convergence associated with the Yanai waves indicates that Yanai waves strengthen the westward background flow while the Yanai waves have little impact on the eastward background flow. This is attributed to the fact that the shear of the background zonal flows, the sum of the EICs and EDJs, is stronger (weaker) in the case of the westward (eastward) EDJs. Consequently, the acceleration by the Yanai waves does not cancel out in the time-mean field, indicating the westward momentum supply to the time-mean EIC. This westward acceleration is stronger than the previously reported eastward acceleration of the EIC by the EDJs in the eastern part of the basin, leading to the unique nearly basin-wide EIC in the Pacific Ocean.
How to cite: Terada, Y. and Masumoto, Y.: Generation of the deep zonal jets in the eastern equatorial Pacific Ocean, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-15601, https://doi.org/10.5194/egusphere-egu25-15601, 2025.
Submesoscale eddies play a pivotal role in upper-ocean dynamics by influencing turbulent mixing and the advection of heat, energy, and tracers. In this study, we examine critical aspects of the submesoscale energy cycle using a novel configuration of the ICON-O ocean model. This setup employs grid refinement technology to achieve sub-kilometer resolution across a broad region of the North Atlantic. We analyze and quantify the key processes driving the generation and dissipation of submesoscale energy. Our findings reveal that baroclinic instability is the primary mechanism for submesoscale energy generation in the upper ocean. Substantial dissipation of this energy occurs via horizontal friction, indicating a downscale energy transfer. Finally, we explore strategies for parameterizing essential energy transformations and dissipation processes, highlighting their potential applications in coarser-resolution models that cannot explicitly resolve submesoscale dynamics.
How to cite: Brüggemann, N., Linardakis, L., and Korn, P.: Sources and sinks of upper-ocean submesoscale turbulence, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-15877, https://doi.org/10.5194/egusphere-egu25-15877, 2025.
The variability of middle-layer water temperatures in the East Sea of Korea is a critical indicator of oceanic and ecological changes, complementing the more widely studied surface temperature trends. While surface water temperatures have shown an annual increase of approximately 0.05℃/yr, this study focused on analyzing temperature variability at depths of 300 m, 400 m, and 500 m, where seasonal fluctuations are minimal. Using CTD data from 64 stations collected by the Korea Oceanographic Data Center between 1995 and 2022, the study revealed annual temperature increases of 0.0126℃/yr at 300 m, 0.0085℃/yr at 400 m, and 0.0093℃/yr at 500 m. Spatial analysis indicated that the south of Ulleungdo exhibited relatively high temperature variability, correlating with the formation zone of the Ulleung Warm Eddy, which plays a pivotal role in regional heat and material transport.
Significantly, the observed warming below 300 m depth—beneath the permanent thermocline—suggests that deep-sea regions, previously thought to exhibit minimal thermal variation, are increasingly affected. This warming trend has potential implications for deep-sea ecological dynamics, oxygen concentration, and biogeochemical processes. Furthermore, these changes align with global patterns of ocean warming driven by anthropogenic climate change.
To better understand the long-term impacts on the East Sea’s thermal structure and its ecosystem, it is essential to extend monitoring efforts to deeper layers below 500 m. Such observations will provide insights into the interplay between regional oceanographic phenomena, such as the Ulleung Warm Eddy, and broader climate change influences.
How to cite: Kim, S., Kim, B.-J., Kim, E., Kwon, S., and Kim, B.-N.: Annual Variability of Middle-Layer Water Temperature in the East Sea, Korea: Insights from Long-Term Observations (1995–2022), EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-16323, https://doi.org/10.5194/egusphere-egu25-16323, 2025.
Winds play a substantial role in the energetic balance of the ocean-atmosphere coupled system. They are known to largely influence ocean dynamics, namely by cooling the water surface, or inducing upper ocean turbulence. Another consequence of the passing of a wind event is the excitation of internal waves that oscillate at a frequency close to the inertial frequency (near inertial waves, NIWs). These waves carry energy into the different ocean layers and participate in their vertical mixing by generating shear instability.
Both observations and models show that wind energy input in the mixed layer is well dominated by strong wind events, such as midlatitude storms, tropical cyclones or hurricanes. While most of the energy is dissipated in the mixed layer, a portion is assumed to reach the interior ocean. For strong events, this energy input is locally comparable to -or even in some cases greater than – the contribution of internal tides, and therefore assumed to play a key role in maintaining abyssal stratification. However, observations and in-depth studies regarding the understanding and weight of this energy transfer are currently lacking. Such considerations lead to the following question: What are the necessary conditions for near inertial energy to become significant in the interior ocean?
Therefore, in this study, we focus on quantifying the propagation of near inertial wave energy below the mixed layer, and the associated mixing, using a time series derived from a mooring southwest of a seamount chain south of the Azores Islands at 30.49°N, 30.20°W. The dataset consists of vertically high-resolution measurements from May 18, 2018 to March 29, 2019. Hurricane Leslie, a category one hurricane, passed north of the mooring during the second half of October, 2018.
A comprehensive analysis of the observed kinetic energy below the mixed layer is conducted using two complementary methods. First, through rotary spectral analysis, kinetic energy is separated into downward and upward going components. Then, a normal mode analysis is employed to examine the contribution of different modes to the energy flux. We thus aim to determine the potential parameters influencing this complex energy distribution to later infer a parametric source model that will provide a more refined representation of the influence of extreme wind events in near inertial energy transfer into the interior ocean.
How to cite: Ramaherison, S., Walter, M., and Mertens, C.: Energy Transfer into the Interior Ocean Through Near-Inertial Waves After an Extreme Wind Event, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-17229, https://doi.org/10.5194/egusphere-egu25-17229, 2025.
Ocean eddies, which range in spatial scale from 10 to 250 km and can last from a few days to several months, play a critical role in regulating ocean heat balance, transporting energy and nutrients, and influencing global ocean circulation. Recent studies also show an increase in the number of eddies in the ocean. Therefore, it is important to understand the statistics, movement, and variability of ocean eddies. Altimetry satellite missions launched in 1993 provided the community with a great opportunity to understand ocean eddies. The Surface Water and Ocean Topography (SWOT) mission launched in 2022, measures the ocean surface at spatial scales of 15–25 km allowing us to understand even finer ocean structures. Meanwhile, the spatial resolution of ocean numerical models has been increasing to capture multi-scales of ocean features and has finer resolution than AVISO. The representation of ocean meso-scales eddies in numerical models is important for the ocean models to provide accurate ocean state estimates and the SWOT mission data allows the model community to understand the capability of representation of ocean eddies in ocean models.
The aim of this study is to analyse mesoscale eddies characteristics in satellite altimetry data (AVISO and SWOT) and ocean reanalyses at two different spatial resolutions to understand the impact of spatial resolution on ocean meso-scales in ocean models. These datasets were analyzed to identify eddies based on physical criteria, such as radius, amplitude, and contour area. Eddies are identified based on closed contours in absolute dynamic topography (ADT) fields. This helps distinguish eddies from oceanic features such as meanders. Preliminary results include a detailed statistical validation of eddy distributions by radius and spatial location, with datasets revealing consistent patterns for mesoscale eddies with radii between 25 and 250 km. The results show that the number of eddies between 25-250km is higher in the finer resolution of both ocean reanalyse data and altimetry data (1/4 vs 1/12°) as expected. However, the number of eddies in 1/12 ocean reanalyses is higher than AVISO satellite observations but lower than SWOT altimetry data. It indicates the importance of spatial resolution in numerical models to represent the finer scale of ocean features. The ongoing activities will explore in detail eddy characterise in different datasets and the outcome of this study will provide useful information for the community to use ocean models to investigate ocean eddies.
How to cite: Mauriello, P., Yang, C., and Storto, A.: Representation of ocean meso-scale eddies in ocean reanalyses, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-19195, https://doi.org/10.5194/egusphere-egu25-19195, 2025.
Submesoscale lateral dispersion in the ocean’s stratified interior is examined numerically in the context of linear internal-wave-driven processes vs. those associated with nonlinear waves and vortical mode. Simulations using a fully nonlinear three-dimensional Boussinesq model are initialized with a Garrett and Munk (GM) internal-wave spectrum, which, through nonlinear interactions, small-scale dissipation and wave breaking, leads to the formation of vortical mode. Lagrangian tracer and particles tracked in the model are used to diagnose isopycnal diffusivity at scales ranging from 1.0-10 km for GM background wave energy levels ranging from 0.01 to 1 times the canonical GM energy level observed in the mid-ocean pycnocline. Dispersion examined as a function of wave and vortical-mode energy level suggest that vortical mode, despite having much lower energy levels than internal waves in the ocean, is nearly as effective at lateral dispersion as internal waves. Furthermore, internal wave and vortical-mode driven dispersion appear to scale differently with energy level.
How to cite: Sundermeyer, M., Lelong, M.-P., Early, J., and Wortham, C.: Internal Waves, Vortical Mode and their Effects on Submesoscale Dispersion, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-20538, https://doi.org/10.5194/egusphere-egu25-20538, 2025.
Posters virtual: Wed, 30 Apr, 14:00–15:45 | vPoster spot 4
EGU25-8764 | Posters virtual | VPS18
Anisotropic internal tide forcing in the consistent internal wave mixing scheme IDEMIXWed, 30 Apr, 14:00–15:45 (CEST) | vP4.8
Breaking internal gravity waves cause small-scale turbulent mixing, which changes water mass properties, affects biogeochemical cycles, and contributes to driving the large-scale overturning circulation. Ocean general circulation models do not resolve this process and thus rely on a parameterization. The state-of-the-art IDEMIX (Internal wave Dissipation, Energy and MIXing) model predicts the propagation and dissipation of internal wave energy based on external forcing functions that represent the main generation mechanisms, notably the internal tide generation at the sea floor and the near-inertial wave generation at the sea surface. By linking small-scale mixing to internal wave energetics, IDEMIX allows the consistent parameterization of wave-induced mixing in ocean models. Its basic incarnation treats all internal waves as part of a horizontally homogeneous continuum and was shown to successfully reproduce observed turbulent kinetic energy dissipation rates and internal wave energy levels. In a newer configuration (IDEMIX2), the internal wave field is compartmentalized, distinguishing between a high-mode continuum on the one hand and low-mode near-inertial wave and internal tide compartments, whose horizontal propagation is explicitly resolved in wavenumber angle space, on the other hand. We present the evaluation of the IDEMIX2 model with a particular focus on the impact of applying an anisotropic internal tide forcing. So far, parameterizations of internal tide-driven mixing have not taken the strong anisotropy of the internal tide generation process into account. We demonstrate the need for doing so, showing a notable impact on the modeled internal wave energetics and predicted mixing when changing from the previous isotropic to the new anisotropic tidal forcing in IDEMIX2.
How to cite: Pollmann, F., Eden, C., Olbers, D., Nycander, J., and Zhao, Z.: Anisotropic internal tide forcing in the consistent internal wave mixing scheme IDEMIX, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-8764, https://doi.org/10.5194/egusphere-egu25-8764, 2025.
