OS1.3

Mesoscale eddies play an increasingly recognized role on modulating turbulence levels and associated diapycnal fluxes in the ocean, in particular with increased dissipation rates found in anticyclones. In September 2017, the last cruise of the ProVoLo project in the Nordic Seas (https://www.uib.no/en/rg/fysos/97330/provolo) intensively surveyed an energetic mesoscale anticyclone (the permanent Lofoten Basin Eddy) to characterize turbulence of the upper layer and eventually quantify the resulting vertical fluxes nutrients caused by turbulence.

The sampling strategy combined ship-borne measurements and autonomous platforms. The vessel carried out a radial transect with stations spaced by 5 km near the center and 10-20 km outside the eddy with measurements of temperature and salinity (CTD), currents (lowered ADCP) and turbulence (Vertical Microstructure Profiler, VMP2000). Water samples were analyzed to estimate the concentration of the main nutrients (nitrate, phosphate and silicate). In addition, two autonomous oceanic gliders were used. A first glider profiling 0-1000 m deep was completing a 6-month mission. A second glider was specifically deployed during the cruise (5 days). This glider was equipped with a dissolved oxygen Aanderaa optode, a WET Labs FLNTU fluorescence and turbidity sensor and a Rockland Scientific Microrider sampling turbulence. It sampled the surface layer (0-300 m) at high temporal (~30 min) and spatial (~500 m) resolution from about 60 km to 5 km of the eddy center.

By combining those measurements, we characterized the turbulence dissipation rates, vertical diffusion and its associated fluxes across the different nutriclines from the center to the outside region area of the eddy, revealing significant contrasts. Below the thermocline, turbulent patches were observed within the core with dissipation rates elevated by one order of magnitude relative to the values outside. The higher levels of dissipation rates supported 10-fold stronger vertical diffusion coefficients, substantially increasing vertical turbulent fluxes through the nutriclines. The transition between the eddy tangential velocity maximum and the zero vorticity was characterized by a frontal region exhibiting important oscillations of the thermocline, manifesting important vertical exchanges.

This study is not only relevant in a local context, but also has global implications for the ocean energy budget and highlights the need for more high-resolution observations resolving scales from the mesoscale to the dissipation.

How to cite: Bosse, A. and Fer, I.: Contrasts in turbulent vertical fluxes of nutrients across the permanent Lofoten Basin Eddy in the Nordic Seas, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-5808, https://doi.org/10.5194/egusphere-egu22-5808, 2022.

Climatologies of the mixed layer depth have been provided using several definitions based on temperature/density thresholds or hybrid approaches. The upper ocean pycnocline (UOP) that sits below the mixed layer base, sometimes referred to as the transition layer or as the seasonal pycnocline, remains poorly characterised though it is an ubiquitous feature of the ocean surface layer. The UOP often consists in a rapid change in density with depth and enhanced vertical shear that connects the well-mixed surface layer to the stratified ocean interior. The UOP is important for the ventilation of the ocean as it represents a barrier to mixing between the upper ocean and the ocean interior.

Available hydrographic profiles (e.g., Argo, CTD on marine mammals) provide near-global coverage of the world's oceans and allow the characterisation of spatial and seasonal variations of the upper ocean vertical stratification, including the UOP. Based on these profiles, we estimate the depth, thickness and intensity of the UOP, and assess when and where the UOP can be considered as a layer with constant thickness. We provide monthly maps of the UOP complementing the available MLD climatologies and we compare the UOP characteristics with the depth and stratification of the mixed layer. We  aim at assessing the UOP intensity in winter and spring when the stratification is usually weak and submesoscale vertical motions can penetrate below the mixed layer base. During these seasons, the UOP intermittency must be taken into account because restratification may occur with intermittent events.

How to cite: Sérazin, G., Tréguier, A.-M., and de Boyer Montégut, C.: A seasonal climatology of the upper ocean pycnocline, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-11443, https://doi.org/10.5194/egusphere-egu22-11443, 2022.

Despite the ever-growing amount of ocean's data, the interior of the ocean remains poorly sampled, especially in regions of high variability such as the Gulf Stream. The use of neural networks to interpolate properties and understand ocean processes is highly relevant. We introduce OSnet (Ocean Stratification network), a new ocean reconstruction system aimed at providing a physically consistent analysis of the upper ocean stratification. The proposed scheme is a bootstrapped multilayer perceptron trained to predict simultaneously temperature and salinity (T-S) profiles down to 1000m and the Mixed Layer Depth (MLD) from satellite data covering 1993 to 2019. The inputs are sea surface temperature and sea level anomaly, complemented with mean dynamic topography, bathymetry, longitude, latitude and the day of the year. The in-situ profiles are from the CORA database and include Argo floats and ship-based profiles. The prediction of the MLD is used to adjust a posteriori the vertical gradients of predicted T-S profiles, thus increasing the accuracy of the solution and removing vertical density inversions. The root mean square error of the predictions compared to the observed in situ profiles is of 0.66 °C for temperature, 0.11 psu for salinity and 39 m for the MLD.
The prediction is generalized on a 1/4° daily grid, producing four-dimensional fields of temperature and salinity, with their associated confidence interval issued from the bootstrap. The maximum of uncertainty is located north of the Gulf Stream, between the shelf and the current, where the variability is large. To validate our results we compare them with the observation-based Armor3D weekly product and the physics-based ocean reanalysis Glorys12. The OSnet reconstructed field is coherent even in the pre-ARGO years, demonstrating the good generalization properties of the network. It reproduces the warming trend of surface temperature, the seasonal cycle of surface salinity and presents coherent patterns of temperature, salinity and MLD. While OSnet delivers an accurate interpolation of the ocean's stratification, it is also a tool to study how the interior of the ocean's behaviour reflects on the surface data. We can compute the relative importance of each input for each T-S prediction and analyse how the network learns which surface feature influences most which property and at which depth. Our results are promising and demonstrate the power of deep learning methods to improve the predictions of ocean interior properties from observations of the ocean surface.

How to cite: Pauthenet, E., Bachelot, L., Tréguier, A.-M., Balem, K., Maze, G., Roquet, F., Fablet, R., and Tandeo, P.: Four-dimensional temperature, salinity and mixed layer depth in the Gulf Stream, reconstructed from remote sensing with physics-informed deep learning., EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-833, https://doi.org/10.5194/egusphere-egu22-833, 2022.

How to cite: Babiker, O., Bjerkebæk, I., Xuan, A., Shen, L., and Ellingsen, S. Å.: Identifying and tracking surface-attached vortices in free-surface turbulence from above: a simple computer vision method, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-9776, https://doi.org/10.5194/egusphere-egu22-9776, 2022.

It has long been appreciated that Ekman transport and pumping velocities are modified through interactions with underlying geostrophic currents. Nonlinearity involving interaction of the Ekman flow with itself is, however, typically neglected. This nonlinearity occurs when the Rossby number based on the Ekman velocity and horizontal length scale approaches order one values. Such values are common, for example, in the ice-ocean stress field across sharp gradients such as leads in the sea ice cover. Recent work has shown strong asymmetry in the pumping velocities, with cyclonic forcing producing diffuse upwelling and anticyclonic forcing producing sharp downwelling fronts. To better understand this dynamics, we consider the steady response to a simple specified prescription of the stress. In the (x-z) plane perpendicular to the stress, dynamics are described by the 2-D Navier-Stokes equation, with a forcing term dependent on vertical shear of velocity in the y-hat direction, specified by a pressureless momentum equation. An expansion in an Ekman-velocity based Rossby number is used to solve the system and to better understand the asymmetry. Interactions with stratification and underlying geostrophic currents are also considered, and examples of where these effects might be important are given.

How to cite: Rahnamaei, N. and Straub, D.: Intense Downwelling and Diffuse Upwelling in a Nonlinear Ekman Layer, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-10579, https://doi.org/10.5194/egusphere-egu22-10579, 2022.

Investigating energy injection mechanisms in stratified turbulent flows is critical to understand the multi-scale dynamics of the atmosphere and the oceans. Geophysical fluids are characterized by anisotropy, supporting the propagation of gravity waves. Classical paradigms of homogeneous isotropic turbulence may therefore not apply, the energy transfer in these frameworks being determined by the interplay of waves and turbulence as well as by the presence of structures emerging intermittently in space and time. In particular, it has been observed that stably stratified fluids can develop large-scale intermittent events in the form of extreme vertical velocity drafts, in a specific range of Froude numbers ([1]). These events were found to be associated with the enhancement of small-scale intermittency ([2]) and local dissipation ([3]). Here we verify the possibility that such extreme vertical drafts may release energy to the flow, affecting its overall dynamics and energetics. The analysis presented consists in the implementation of a space-filtering technique ([4]) applied to three-dimensional direct numerical simulations of the Boussinesq equations.

The strength of this approach relies on dealing with quantities (referred to as “sub-grid terms”) which are a reliable proxies of the classical Fourier flux terms but defined locally in the physical space, allowing for a scale analysis of the energy transfer at specific location of the domain flow. By investigating the correlation between values of the sub-grid terms and the presence of the extreme values of the vertical velocity, we found an increase in the energy transfer at intermediate scales that is likely to be associated with the development of vertical drafts in the flow. In the range of the governing parameters (namely the Froude and the Reynolds numbers) in which the extreme vertical drafts are detected in stratified turbulent flows, enhancement of the coupling between kinetic and potential energy modes is also observed, feeding in turn the scale-to-scale potential energy transfer.

[1] Feraco et al., EPL, 2018

[2] Feraco et al., EPL, 2021

[3] Marino et al., PRF, in review

[4] Camporeale et al., PRL, 2018

How to cite: Foldes, R., Cerri, S. S., Marino, R., Feraco, F., and Camporeale, E.: Local energy release by extreme vertical drafts in stratified geophysical flows, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-11160, https://doi.org/10.5194/egusphere-egu22-11160, 2022.

In fall 2020 and 2021, two field surveys examined the water column dynamics and surface mixing in a shallow lagoon, Szczecin (Stettin) Lagoon, located at the border between Germany and Poland. This was part of a larger experiment, looking into water column and air-sea interactions, and momentum fluxes, but this study is focused on how the presence of proposed Langmuir circulation affects the carbon and oxygen dynamics, and primary production in this shallow lagoon.

Measurements were collected from a station in Szczecin Lagoon, located near the Polish border, with water depth of about 4 meters. Measurements at and around the station were made using mobile FerryBox systems, or Pocket FerryBoxes, which measured almost continuously water temperature, salinity, dissolved oxygen, chlorophyll fluorescence, pH, turbidity, colored dissolved organic matter (CDOM) and in 2021 partial pressure of CO2 (pCO2). In addition, water column measurements of currents (ADCP) and water level were available, as well as surface drifters, and drone aerial measurements.

We found that during low wind conditions, the water column was well-mixed to a depth controlled by expected Langmuir cells, and bottom waters below this depth were quite different in most of the biogeochemical parameters measured. Therefore Langmuir circulation most likely controlled water column structure in large regions of the Szczecin Lagoon, consequently influencing the community, carbon and dissolved gas distributions in this shallow lagoon, and most likely the air-sea gas exchange rate. Only during short storm events, these conditions changed, and the water column structure and concentrations of biogeochemical parameters were altered.

How to cite: Voynova, Y. G., Buckley, M. P., Stresser, M., Cysewski, M., Bödewadt, J., Gehrung, M., and Horstmann, J.: Effect of Langmuir circulation on mixing and carbon dynamics in a shallow lagoon, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-8271, https://doi.org/10.5194/egusphere-egu22-8271, 2022.