# Abstracts and Presentations/Posters

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### Oral Presentations

Alford, M. H.:
Turning upside down mixing upside down: observations of dye, turbulence and upwelling along a sloping boundary

Mixing is known to drive deep-ocean diapycnal velocity, but unknown details of near-bottom turbulence currently have us uncertain on its sign. Recent theory and inverse models have moved the discussion from energetics (“do we have enough Terawatts?”) to watermass transformation (“how does divergence of the buoyancy flux give rise to upwelling?”).  In this context, the observed bottom enhancement of ocean mixing gives divergent buoyancy flux and thus downwelling in a 1D interpretation.  The apparent paradox may be resolved by invoking zero buoyancy flux at the boundary, which leads to a narrow bottom boundary layer with a convergent buoyancy flux that allows upwelling.  However, the physics of this near-bottom region and the applicability of a 1D model are not known.  In the Boundary Layer Turbulence and Abyssal Recipes experiment, we are testing these ideas in a submarine canyon in the Rockall Trough off Ireland with i) tracer and fluorescein dye releases at the seafloor, ii) moorings directly measuring buoyancy flux and iii) a new specialized Epsilometer system allowing shear and temperature microstructure profiles from 5-400 meters above the bottom to be collected every 13 minutes.  Tracer and dye confirm near-bottom upwelling consistent with Eulerian velocity measurements, while the microstructure data reveal a highly complex and three-dimensional fields that conspire to drive the upwelling - however, crucially, differential advection of the internal tide rather than diffusive processes sets the vertical length scales.  This talk will provide an overview of the experiment and our current interpretation of how the complexity of the internal tide breaking process conspires to produce vigorous diapycnal upwelling and offshore-onshore exchange within the canyon.

Anselin, J., Reed, B., Jenkins, A., Green, J.A.M.:
The effect of tidally-induced turbulence on sub-ice shelf basal melting based on one-dimensional plume theory

Tidal currents are known to influence basal melting of Antarctic ice shelves through various mechanisms that take place both within the ice-ocean boundary layer and further away from the ice base, such as ice-ocean thermodynamic exchanges, vertical mixing, and tidal rectification. The separate effects of each of these processes are not well understood, limiting our ability to parameterize tide-ice interactions in numerical models. Here we focus on boundary layer processes and we use a one-dimensional plume model applied to a range of basal profiles representative of Antarctic ice shelves to study the sensitivity of basal melt rates to different representations of tide-driven turbulent mixing. In contrast to earlier findings, our simplified model predicts that the additional shear generated by tidal motion increases basal drag and reduces the speed of the buoyancy-driven plume, which in turn leads to a decrease in melt and freeze rates along the base of the ice shelf. Our results, which must be interpreted within the plume model framework, are highly dependent on how we choose to treat tidal currents within the plume model; contrasting assumptions regarding the way in which tides impact ambient water entrainment and boundary layer current speed may explain the divergence from findings reported by previous studies. These results highlight the need for direct observations of the boundary layer beneath ice shelves to improve the reliability of Antarctic ice shelf basal melt rate predictions, and hence sea level rise projections.

Baker, L.E., Mashayek, A.:
Interaction of bottom generated oceanic lee waves with vertically varying background flows and the ocean surface

Lee waves generated by stratified flow over rough bottom topography in the ocean extract momentum and energy from the geostrophic flow, causing drag and enhancing turbulence and mixing in the interior ocean when they break. Lee wave energy generation is generally parametrised using linear theory with a uniform background flow, and energy deposition due to wave breaking is assumed to occur in the bottom ~1 km of the ocean. However, the combined effect of changing background flow speed and stratification with depth and of reflection of waves from the ocean surface can lead to a departure from this regime. Here, we use a modified linear theory to demonstrate the effect of typical oceanic background flows on lee wave propagation. Lee waves are strongly affected by interactions with background flow structures such as shear layers and the thermocline, and with the ocean surface. Under a vertically increasing background flow, waves gain energy from the mean flow and can be expected to reflect from the ocean surface, resulting in constructive/destructive interference, enhanced upper ocean vertical velocities, and potential interaction with surface submesoscales at similar horizontal scales. Equally, breaking at critical levels due to wave interaction with a decreasing background flow may cause well mixed layers in the interior ocean.

Basdurak, N.B., Burchard, H., Schuttelaars, H.M.:
A local eddy viscosity parameterization for wind-driven estuarine exchange flow

A new parameterization for the estuarine turbulent eddy viscosity coefficient is developed considering the influence of wind forcing and feedback between stratification and shear. The emerging tidally averaged eddy viscosity profile Av is parameterized as parabolic under well mixed conditions, and is composed of a skewed-Gaussian-like form for the upper layer, and a parabolic form for the bottom layer under stratified conditions. The precise shape of the profiles depends parametrically on the bottom boundary layer thickness, the bulk Richardson number, and the Wedderburn number. The parameterized Av profiles show excellent agreement with profiles obtained from numerical models.

To explore the importance of vertically varying Av with regard to exchange processes, an analytical model is designed. This one dimensional model is based on a balance between frictional forces and pressure gradient. The resulting exchange flow is analyzed over the relevant parameter space that is associated with horizontal and vertical stratification through the bulk Richardson number, and the bi directional wind stress via the Wedderburn number. Down-estuary wind enhances theupestuary flow near the bottom and down-estuary flow near the surface driving an exchange flow pattern typicallyassociated with gravitational circulation. Up-estuary wind results in either a two-layer inverted circulation opposing the gravitational circulation, or a three-layer flow that is up-estuarine at the surface with classical two-layer circulation underneath. Three-layer flow emerges with a weak wind. With increasing runoff velocity, three-layer flow transitions to a single layer flow under weak stratification conditions.

Becherer, J., Carpenter, J., Merckelbach, L., Graewe, U., Burchard, H.:
The role of turbulence in fueling the subsurface chlorophyll maximum in tidally dominated shelf seas

Glider observations show a  subsurface chlorophyll maximum (SCM) at the base of the seasonal pycnocline (PCB) in the central North Sea during stable summer conditions. A co-located peak in the dissipation rate of turbulent kinetic energy suggests the presence of active turbulence that generates the nutrient fluxes  necessary to fuel the SCM.
A 1D-turbulence closure model is used to investigate the dynamics behind this local maximum in turbulent dissipation at the PCB as well as its associated nutrient fluxes. Based on a number of increasingly idealized forcing setups of the model, we are able to draw the following conclusions: (1) only turbulence generated inside the stratified PCB is able to entrain nutrients from the bottom mixed layer into the SCM region; (2) surface wind forcing only plays a secondary role during stable summe conditions; (3) interfacial shear from the tide accounts for the majority of turbulence production at the PCB;  (4) in  stable summer conditions the strength of the turbulent nutrient fluxes at the PCB is set by the strength of the anti-cyclonic component of the tidal currents.

Bethan Wynne-Cattanach, Matthew Alford, Gunnar Voet, Nicole Couto, Arnaud Le Boyer, Harry Allbrook, Henri Drake, Bieito Fernandez-Castro, Alexander Forryan, Eleanor Frajka-Williams, Jack Hughes, Catherine Kermabon, Yavor Kostov, Herle Mercier, Marie-Jose Messias, Alberto Naveira-Garabato, Kurt Polzin, Kaylim Reddy, Nile Ross, Xiaozhou Ruan, Carl Spingys, Raffaele Ferrari:
Diapycnal Upwelling in the Bottom Boundary Layer Observed by Dye Release

Observational evidence of bottom intensified mixing together with recent theoretical work suggests that upwelling of deep water is confined to a bottom boundary layer on the ocean's sloping boundaries. The transformation of dense water to lighter water is a vital part of the global overturning circulation. The Boundary Layer Turbulence and Abyssal Recipes (BLT Recipes) Program set out to observe diapycnal mixing close to the seafloor to investigate this view of upwelling. We present data from the fluorescein dye release experiment undertaken as part of BLT Recipes in July 2021. The experiment focused on one of many submarine canyons on the boundary of the Rockall Trough in the North Atlantic. Dye was released on the 3.76°C isotherm, approximately 1850m deep and 10m above the bottom at the center of the canyon. Fluorometer measurements of the dye concentration were accompanied by CTD data and estimates of the turbulence from microconductivity; all collected using the FastCTD profiler. The rapid cycling FastCTD allowed for high spatial and temporal resolution of the dye as it evolved. With the altimeter attached to the instrument, we were able to get within 10m of the seafloor. The dye was sampled for three days before concentrations dropped below the detection limit. We tracked the dye patch as it was advected up and down the canyon by tidal sloshing. Our observations provide the first direct evidence of very rapid diapycnal upwelling along the canyon at rates of O(100m/day). In this study, we examine the time evolution of the dye in the bottom boundary layer and investigate the mixing associated with the translation of the dye's center of mass to warmer water.

Burchard, Hans, Bolding, Karsten, Jenkins, Adrian, Losch, Martin, Reinert, Markus, Umlauf, Lars:
The vertical structure and entrainment of subglacial melt water plumes

Basal melting of marine-terminating glaciers is one of the major factors determining sea level rise in a world of global warming. Detailed quantitative understanding of dynamic and thermodynamic processes in melt-water plumes underneath the ice-ocean interface is essential for calculating the subglacial melt rate. The aim of this study is therefore to develop a numerical model of high resolution in space and processes to consistently reproduce the transports of heat and salt from the ambient water across the plume into the glacial ice. Based on boundary layer relations for momentum and tracers, stationary analytical solutions for the vertical structure of subglacial non-rotational plumes are derived, including entrainment at the plume base. These solutions are used to develop and test convergent numerical formulations for the momentum and tracer fluxes across the ice-ocean interface.
After implementation of these formulations into a water-column model coupled to a second-moment turbulence closure model, simulations of a transient rotational subglacial plume are performed. The simulated entrainment rate of ambient water entering the plume at its base is compared to existing entrainment parameterizations and results of vertically integrated plume models, both based on bulk properties of the plume.
A sensitivity study with variations of interfacial slope, interfacial roughness and ambient water temperature reveals substantial performance differences between these bulk models. An existing entrainment parameterization based on the Froude number and the Ekman number proves to have the highest predictive skill. Recalibration to subglacial plumes using a variable drag coefficient further improves its performance.

Carpenter, J.R., Buckley, M., Veron, F.:
Towards a quantification of wind-wave growth mechanisms

Despite much effort over the last 100 years devoted to the theoretical aspects of the problem, understanding the mechanisms by which wind produces ocean surface waves remains elusive.  This is due to the extreme difficulties of measuring the turbulent, small-scale, and rapidly varying structure of the airflow and pressure distribution at the wavy air-water interface.  These difficulties have been largely overcome in the laboratory through the use of PIV and LIF techniques, as well as a recent pressure reconstruction method from such measurements.  We describe the first results investigating the mechanisms of wind-wave growth.  Evidence for the dominance of the critical layer mechanism, first described by J.W. Miles in 1957, will be presented for low wind speeds and wave slopes.  However, at larger wind speeds and wave slopes airflow separation becomes more prevalent and other mechanisms are expected.

Dettling, N.:
Energy transfers in the Weddell Sea Bottom Water gravity current

On the continental shelf of the Weddell Sea, dense water is produced by surface cooling and sea ice formation. The dense water then propagates down the continental slope in the form of the Weddell Sea Bottom Water (WSBW) gravity current. The WSBW gravity current is known to be influenced by mesoscale eddies, tides and internal gravity waves. The small spatial and temporal scales of these dynamical processes as well as the complex nature of tidally generated internal gravity waves south of the critical latitude present a number of challenges for numerical ocean models. This motivates a process model study to determine the leading order processes shaping energy transfers and entrainment at the interfaces between gravity current, overlying Circumpolar Deep Water (CDW) and surface water masses. The energetics of the WSBW gravity current are therefore explored in an idealized setup of the Massachusetts Institute of Technology general circulation model (MITgcm), in which simplified topography and forcing is combined with realistic hydrography of the Weddell Sea. In this setup, the simulation already reproduces important dynamical structures such as mesoscale eddies over the continental shelf and slope as well as deep water export in a pronounced gravity current. Analysis shows an intricate dynamical balance, in which tides and mesoscale eddies generated by instabilities in the WSBW-CDW interface set the energy budget and hydrographic properties of the WSBW gravity current while also driving shoreward transport of CDW onto the continental shelf.

Eden, C. Olbers, D. and Eriksen, T.:
A closure for lee wave drag on the large-scale ocean circulation

A new, energetically and dynamically consistent closure for the lee wave drag on the large scale circulation is developed and tested in idealized and realistic ocean model simulations. The closure is based on the radiative transfer equation for internal gravity waves, integrated over wavenumber space, and consists of two lee wave energy compartments for up- and downward propagating waves, which can be co-integrated in an ocean model. Mean parameters for vertical propagation, mean-flow interaction, and the vertical wave momentum flux are calculated assuming that the lee waves stay close to the spectral shape given by linear theory of their generation. Idealized model simulations demonstrate how lee waves are generated and interact with the mean flow and contribute to mixing, and document parameter sensitivities. A realistic eddy-permitting global model at 1/10deg resolution coupled to the new closure yields a globally integrated energy flux of 0.27 TW into the lee wave field. The bottom lee wave stress on the mean flow can be locally as large as the surface wind stress and can reach into the surface layer. The interior energy transfers by the stress are directed from the mean flow to the waves, but this often reverses, for example in the Southern Ocean in case of shear reversal close to the bottom. The global integral of the interior energy transfers from mean flow to waves is 0.14 TW, while 0.04 TW is driving the mean flow, but this share depends on parameter choices for non-linear effects.

Fernández Castro, Bieito, Peña, Marian, Nogueira, Enrique, Gilcoto, Miguel, Broullón, Esperanza, Comesaña, Antonio, Bouffard, Damien, Naveira Garabato, Alberto C, Mouriño Carballido, Beatriz:
Intense ocean mixing by fish spawning aggregations

The contribution of marine biota to ocean mixing has been highly controversial since Walter Munk first suggested the idea in the 1960's. Laboratory and in situ studies have so far produced contradicting results, with field observations generally suggesting a negligible contribution, mainly attributed to the reduced mixing efficiency of biological turbulence. Here, we redress this balance by analysing 14-days of continuous turbulence microstructure measurements in a dynamic, productive coastal embayment. In every segment of nocturnal measurements we observed a 10-100 increase in turbulent energy dissipation and mixing rates associated with the presence of spawning fish aggregations. Under highly stratified conditions, biological turbulence occurred with a mixing efficiency comparable to that of geophysical turbulence. These results demonstrate that biologically-driven turbulence can be an effective mixing agent, and call for a re-examination of its impacts on productive upper-ocean regions.

Hasson, G., Støle-Hentschel, S., Toledo, Y.:
HF-radar remote sensing of ocean surface current shear using first order Bragg peaks

Among the various instrumentations available for measuring sea characteristics, high-frequency (HF) radars show some unique advantages. These shore-based remote-sensing systems enable over-the-horizon monitoring of surface currents, waves and winds. This allows for large real-time measurement coverage areas of tens to hundreds of kilometers. The main approach for surface current mapping utilizes the Bragg scattering from waves propagating toward the radar which are of wavelength of half of the transmitted electromagnetic wave. Similar waves that propagate away from the radar also create a similar scattering signal, but it is commonly neglected as it is assumed to provide redundant information. Through numerical and analytical solutions of an exact wave-shearing current model, we show that this signal contains significant information when the upper layer current has a vertical profile. We present a new methodology for measuring the vertical shear of the upper layer current using the waves propagating towards and away from a single HF radar. Exploiting this additional information reduces the complexity to estimate the vertical shear and can be leveraged to increase the knowledge of the upper layer momentum budget. The suggested method is examined through a field measurement campaign using HF and in-situ instrument on Israel’s Mediterranean continental shelf with good preliminary results.

Hetland, Robert:
Tracer variance mixing in simple box models

Tracer variance budgets can be used to estimate bulk mixing in a control volume. For example; simple, analytical, bulk formulations of salt mixing, defined here as the destruction of salinity variance, can be found for estuaries with a riverine source of fresh water and a 2-layer exchange flow at the mouth using salinity as a representative tracer. For a steady case, the bulk salt mixing, $M$, can be calculated as $M = s_{in} s_{out} Q_R$, where $s_{in}$ and $s_{out}$ are the representative salinities in the estuarine exchange flow, and $Q_R$ is the river transport. Here, a theory is offered that allows for a simple interpretation of multiple inputs and outputs into a control volume, as might be found in a complex estuarine network, in a region on a continental shelf, or any other control volume with multiple exchanges. For a steady state, first divide each input, proportionally by transport, among the outputs; this defines $N$ mixing pathways, $Q_i$. The integrated bulk mixing within the control volume is the sum of the mixing along each individual mixing pathway, $\sum Q_i \Delta S_i^2$, where $\Delta S_i$ is the salinity difference between input and output along the pathway associated with $Q_i$. Time dependent problems also require calculation of the integrated salinity variance, $S^2$, within the control volume to estimate total mixing. This approach allows for the estimation of mixing in a control volume for an arbitrary number of inputs and outputs, without first subtracting the mean salinity, and with each the relative contributions of each input and output to the bulk mixing quantified.

J. Pietrzak, S. Rijnsburger, R. Flores Audibert, A. Horner-Devine, A. Souza, K. Lamb, N. Jones:
Multiple tidal plume fronts, internal waves, mixing and sediment resuspension in a near to mid-field river plume

The freshwater discharge from the Rhine River forms a large Region of Freshwater Influence (ROFI). On every ebb tide it discharges significant freshwater into a strong tidal cross flow. Here we discuss the role of tidal plume fronts and tidal straining on mixing and re-circulation in the mid-field plume, and their consequences for near shore mixing and cross-shelf exchange. We present results from a multi-disciplinary field study; the STRAINS-II (STRAtification Impacts Near-shore Sediment) experiment which took place in 2014. In the STRAINS field experiment data sets were collected 10 km north of the river mouth, offshore from The Sand Engine. Temperature, salinity, velocity measurements, SPM and LISST data were collected. Current velocity was measured with an ADCP with 0.25 m resolution and a frequency of 1 Hz. Turbulent stresses were measured at the 12 m site with ADV’s at 0.25, 0.5 and 0.75 m above the bed.

We present results showing the formation and evolution of the tidal plume fronts, in a shallow frictional system dominated by tidal straining. We present models and field data, to explore the interaction of the tidal plume fronts in the near to mid-field region, and their role on near shore mixing and sediment resuspension. We show that tidal plume fronts form on each ebb tide. As the fronts move onshore they increase turbulence, mixing, and sediment resuspension near shore. The field-data and radar images show tidal plume fronts propagating past the 12 m mooring towards the Dutch coast.

Additionally, we explore internal waves generated by multiple tidal plume fronts and their trapping in the mid-field plume of the Rhine river plume. The internal waves are released into a shallow frictional system, and their role on mixing, near shore sediment resuspension is examined. Using an idealised non-hydrostatic model we show that the fronts can generate high frequency internal waves as they propagate towards the coast, where they break and mix, and contribute to the resuspension of sediment. We explore the role of the different processes, and discuss how they influence cross-shelf exchange. We consider similarities and differences to other river plumes, and how we can apply our knowledge from highly sampled river plumes, to remote river plume systems.

Lorenz, M., Klingbeil, K., Burchard, H.:
Mixing in a marginal sea with dominant evaporation: the Persian Gulf

The Persian Gulf is located in a very arid region, resulting in a net freshwater loss due to a high evaporation rate that is compensated by a net inflow of ocean water from the Indian Ocean. Further, the evaporation leads to the formation of hyper-saline, dense bottom water that leaves the Gulf as a bottom current into the Gulf of Oman and finally back into the Indian Ocean. Still, non-negligible river discharge is entering the Gulf in the Northwest, creating large salinity gradients. In fact, the salinity of the Gulf ranges from water with a salinity close to 0 g/kg to salinities exceeding 50 g/kg. Both, surface freshwater fluxes and river discharge as well as the exchange flow with the Indian Ocean increase the salinity variance of the Gulf. This added variance is destroyed by turbulent mixing which transforms the entering fresh water and ocean water into the hyper-saline water mass of the outflow.

This presentation investigates the seasonal salt mixing and water mass transformation of the Gulf by evaluating numerical simulations. It is shown that the transformation and mixing follow a seasonal cycle that is associated with the build-up and destruction of seasonal stratification. In winter, the Gulf is in a well-mixed state. In spring and early summer, stratification is built up, mainly by surface heating and an increasing inflow of surface water with lower salinity. In summer, maximal stratification is reached. In fall, increasing winds, increasing evaporation, and heat losses lead to strong turbulent mixing that destroys the stratification and establishes the well-mixed state in early winter.

Manita Chouksey:
On the existence and role of an interface between the slow and fast manifolds

The balanced state in geophysical flows, such as the atmosphere and ocean, largely describes the dynamics and energetics of the low frequency flow. The unique depiction and diagnosis of this balanced state, or the so-called 'slow manifold', however remains conflicted. At the same time, the co-existing unbalanced high-frequency motions, or the 'fast manifold', also significantly influence the energetics of the flow. This co-existence of the balanced and unbalanced modes suggests the likelihood of the existence of an interface between the slow and fast manifolds, with a crucial role in energy transfers and exchanges that dictate the flow dynamics. However the identification, interpretation, and the interaction between these modes rely on the precise separation of the balanced and unbalanced modes, that remains complex and challenging.

Different flow decomposition methods are implemented in a numerical model of baroclinic instability, for a range of flow regimes characterized by the Rossby number. The separation of balanced and unbalanced modes is first achieved using non-linear modal decomposition along with initialization technique of Machenhauer (1977), that gives the balanced state accurate to the first order in Rossby number (Ro) (Chouksey et al. 2018). The diagnosed unbalanced state in this case however comprises largely of the slaved modes and is thus not precise. These slaved modes could be associated with the interface between the slow and fast manifold. The modal decomposition is further extended to higher orders, up to 4th order in Ro, and a more precise balanced state is achieved with increasing orders in Ro (Eden et al. 2019). Further, the balanced state obtained from this balancing procedure is compared to the balanced state obtained from the optimal balance procedure, independently implemented in a similar model setup. Both the methods produce a precise balanced state but the comparison reveals the dependence of the balanced state on the numerics. The results also suggest a weak importance of spontaneous loss of balance in comparison to convective or symmetric instabilities for internal wave generation. The results have implications for the generation and dissipation of the balanced and unbalanced modes, as well as for the interactions and energy transfers between them that form a key constituent of the flow's energy cycle. The question of the existence of an interface between the two manifolds continues to be intriguing and remains open for further exploration.

Mashayek, A.:
Mountains to climb: on the role of seamounts in upwelling of deep ocean waters

Ocean turbulent mixing exerts an important control on the rate and structure of the overturning circulation. Recent observational evidence suggests, however, that there could be a mismatch between the observed intensity of mixing integrated over basin or global scales, and the net mixing required to sustain the overturning's deep upwelling limb. Here, we investigate the hitherto largely overlooked role of tens of thousands of seamounts in resolving this discrepancy. Dynamical theory indicates that seamounts may stir and mix deep waters by generating lee waves and topographic wake vortices. At low latitudes, this is enhanced by a layered vortex regime in the wakes. We consider three case studies (in the equatorial zone, Southern Ocean and Gulf Stream) that are predicted by theory to be representative of, respectively, a layered vortex, barotropic wake, and hybrid regimes, and corroborate theoretical scalings of mixing in each case with a realistic regional ocean model. We then apply such scalings to a global seamount dataset and an ocean climatology to show that seamount-generated mixing makes a leading-order contribution to the global upwelling of deep waters. Our work thus brings seamounts to the fore of the deep-ocean mixing problem, and urges observational, theoretical and modelling efforts toward incorporating the seamounts' mixing effects in conceptual and numerical models of the ocean circulation.

Ole Pinner:
The role of boundary mixing for diapycnal oxygen fluxes in a stratified marine system

We were able to measure for the first time directly the seasonality of basin boundary mixing and oxygen fluxes in a strongly stratified shelf sea with deep water anoxia, namely the Baltic Sea. This was achieved by taking hundreds of microstructure profiles across different cruises and calculating the vertical turbulent oxygen flux from the oxygen gradient and turbulent diffusivity. We found that oxygen fluxes through the halocline are increased by an order of magnitude at the basin boundary and the boundary area contributes with 24% of its area to more than 50% of the central Baltic Sea oxygen transport.

Peng, J.-P., Umlauf, L.:
Submesoscale frontal dynamics and turbulence under theoretically unfavorable conditions for symmetric instability

Submesoscale fronts and filaments have been shown to play a critical role in surface boundary layer (SBL) dynamics and turbulence as well as the downscale transport of mesoscale energy. Recent studies have parameterized and implemented these processes into large-scale ocean models, focusing mostly on the relevance of symmetric instability under downfront winds conditions (i.e., the destabilizing Ekman buoyancy flux dominates). Based on high-resolution turbulence microstructure and near-surface velocity data from the Benguela upwelling system (South-East Atlantic) and the central Baltic Sea, we investigated the real-ocean relevance of frontal dynamics and instability and their relation to turbulence in two special cases: (a) the stabilizing atmospheric buoyancy flux dominates any potentially destabilizing Ekman buoyancy flux; (b) the winds are predominantly cross-winds and the atmospheric buoyancy is negligible. We found that during daytime, when the stabilizing atmospheric buoyancy flux associated with solar radiation is near its peak, the SBL quickly restratifies, the conditions for frontal instability are no longer fulfilled, and SBL turbulence collapses except for a thin wind-driven layer near the surface. However, during nighttime to early daytime conditions (when the stabilizing atmospheric buoyancy flux is still weak but sufficient to compensate the destabilizing Ekman buoyancy flux) and under cross-front winds conditions (when both Ekman and atmospheric fluxes are negligible), frontal turbulence is generated by a mix of symmetric instability and shear instability, indicating the occurrence of the downscale transport of large-scale energy. These are the first direct field observations supporting the relevance of frontal instability and its importance to SBL turbulence when the destabilizing Ekman buoyancy flux is less dominant, which has so far rarely been discussed in theoretical and numerical investigations.

Qing Li, Jorn Bruggeman, Hans Burchard, Knut Klingbeil, Lars Umlauf, Karsten Bolding:
Integrating CVMix into GOTM: a consistent framework for testing, comparing, and applying ocean mixing schemes

In this presentation I'm going to talk about a recent work of integrating the Community Vertical Mixing Project (CVMix) into the General Ocean Turbulence Model (GOTM). By doing so we are building towards a consistent framework for testing, comparing and applying ocean mixing schemes. In addition to bringing a set of newly developed ocean surface vertical mixing parameterizations of Langmuir turbulence into the library of turbulence closure models already in GOTM, we also implemented a Stokes drift module in GOTM to provide the necessary ocean surface waves information to the Langmuir turbulence parameterizations, as well as to facilitate future development and evaluation of new Langmuir turbulence parameterizations in the GOTM framework. I will also talk about a streamlined workﬂow running GOTM with Python and Jupyter notebooks, enabled by the newly developed and more ﬂexible conﬁguration capability of GOTM. These new capabilities of GOTM are tested in a variety of idealised and realistic test cases and the results will be discussed.

Schlichting, D., Hetland, R., Kobashi, D.:
Using salinity variance to quantify numerical mixing offline in a coastal ocean model

Numerical mixing, the mixing generated by the discretization of advection schemes, is often significant in estuarine and coastal models due to strong advection and sharp gradients. In this study, we define salt mixing as the rate of salinity variance dissipation within a control volume. We use two methods to quantify the volume integrated numerical mixing within a control volume using offline model output from a realistic simulation of the ocean state over the Texas-Louisiana continental shelf during summer when the water column is strongly stratified. We expand an analysis framework used to examine volume integrated tracer budgets and salt mixing in estuaries, total exchange flow (TEF), to the more complex circulation patterns and salinity distributions found in the coastal ocean to characterize the numerical mixing there. Based on previous studies of estuarine mixing, we use two methods to calculate the total mixing in a control volume on the shelf, here termed the ‘total’ method and the ‘residual’ method. The total method quantifies the numerical mixing as the difference between the integral of the diffusive salt flux in salinity coordinates and physical mixing. The residual method quantifies numerical mixing as the residual of the salinity variance budget. To investigate the sensitivity of numerical mixing to grid resolution, we apply both methods to a two-way nested child grid with five times the parent resolution. We find that the bulk numerical mixing is nearly halved for the child model relative to the parent. We believe this is due to newly resolved physical processes that cause the child model to grow more inertially unstable, which is evident in statistics of vertical vorticity, horizontal salinity gradient magnitude, and an instability metric. Additionally, while we find that the long-term mean numerical mixing derived from both methods is similar, the instantaneous estimates of numerical mixing can show strong differences over an inertial period; we discuss possible physical and numerical causes for these differences.

Schulz, K., Janout, M., Rippeth, T., Lenn, Y.-D., Hölemann, J., Büttner, S., Rogge, A., Lincoln, B., Povazhnyy, V., Alkire, M., Scannell, B., Torres-Valdés, S.:
Boundary layer and interior region exchange at high latitudes

The typically high levels of turbulence within the oceanic boundary layers foster the vertical transport of biogeochemical tracers. The pycnoclines bounding these mixed layers, however, inhibit exchange with the adjacent ocean, thereby limiting the resupply of nutrients to surface layer in the productive season, and confining the transport of suspended particles to the near bottom layer. Hence, turbulent transport mechanisms that facilitate the exchange between the boundary layers and the ocean interior play a key role for the basin-wide transport dynamics.

A prominent exchange mechanism in temperate regions is the breaking of internal waves at critical angles of the continental slope. North of the critical latitude for the M2 tidal frequency, where most of the Arctic continental slopes are located, this energy pathway is not permitted, and little is known about the exchange mechanisms in this inaccessible and severely undersampled region. Just recently, ship-based observations and data from a long-term mooring shed some light on the regional energy conversion mechanism from larger scale atmospheric and oceanic features to turbulent mixing.

In summer 2018, strongly enhanced mid-water dissipation rates were observed above the Siberian upper continental slope. This strong mixing homogenized the water column between a depth of 30 to 300m, resulted in the formation of an intermediate turbid layer that detached from the bottom boundary layer, and an upward transport of nutrients into the previously nutrient-depleted surface mixed layer, locally enhancing measured primary productivity. Mooring records reveal a prior barotropic current surge with an off-slope component, causing an isopycnal depression, and suggest that the enhanced dissipation rates are the results of the formation of an unsteady lee wave. While episodic in nature, these mixing events might significantly contribute both to the pan-Arctic sediment export and burial in the deep Arctic basins, and to the supply of nutrients to the surface. Conditions potentially leading to similar mixing events were mainly restricted to ice-free periods, suggesting that their frequency, and hence importance, will further increase with receding ice cover in the future.

Sutanu Sarkar:
The multiscale response of the turbulent ocean boundary layer to unsteady atmospheric forcing: LES case studies

The upper ocean response to an atmospheric event is determined by the nature of the atmospheric forcing and the initial oceanic state. The Bay of Bengal (BOB), with its voluminous freshwater inflow from the rivers, has a unique oceanic state and its response to the summer wind and rain of the southwest Indian monsoon is not well understood.  Indeed, uncertainty in mixed layer depth (MLD) and sea surface temperature (SST) predictions in coupled air-sea models is thought to play a major role in biases found in these model predictions. The freshwater input into the BOB often results in a shallow mixed layer, which is capped by a partially compensated, salinity-controlled barrier layer with unusually strong stratification.  We will start with LES results for a simpler problem: deepening of the ML into a stratified barrier layer under a constant wind stress.  We will then move on to a LES study whose initial ocean state and unsteady air-sea fluxes are based on shipboard observations obtained in 2018 during a cruise as part of a collaborative field experiment, MISO-BoB.   Results from the LES, which spanned a 2-week period, will be discussed to illustrate the multiscale -- from hourly to diurnal to inertial to intraseasonal – temporal variability of the turbulent boundary layer in the upper ocean.

Toledo, Yaron:
Surface waves propagation on irrotational, inviscid and turbulent currents - theory and measurement

Oceans, seas and other natural basins consist of a complicated flow behavior. Surface gravity waves mostly govern the flow in the thin layer that connects the ocean and the atmosphere, hence they have a significant effect on momentum, energy, heat and mass exchanges between the air and water layers. For water waves-on-currents context the most common approach for simplifying the flow problem exploits the disparity of scales. It separates the flow to equations describing propagation of fast/short surface waves in the slowly varying environment and equations for slow/long currents. The talk will combine a theoretical and a field measurement arcs to explore the connection between these two time/length scales. It will start with the fundamental potential solution of a monochromatic small-amplitude wave propagating on a constant current and show that even this simplistic solution has more than meets the eye. Water waves in natural basins almost always propagate on currents with a vertical structure, so the talk will continue with the wave-current model for inviscid rotational flow. It will explore some preliminary results accounting for the current’s vertical shear for extending radar and Acoustic Doppler Current Profiler’s measurement methodologies. The talk will conclude by presenting a new extension of the wave-current model to turbulent flows. Some interesting results showing bifurcation of the dispersion relation and the oscillatory flow’s vertical profile will be discussed.

Yin, Y., Gräwe, U., Klingbeil, K., Lorenz, M., Niehüser, S., Arns, A.:
Wave induced forces across through the German coasts

The surface waves are usually ignored in the ocean and coastal model due to the relative small spatial and temporal scale. However, it is very critical in the context of marine areas especially for the coastal regions and small scale process. The surface wave propagation could not only horizontally modify the surface and bottom layer currents, but also exert vertically extra energy and momentum flux into the water column so that changing the turbulence structure, thus affecting the consequent marine activities. However, the contributions of these wave effects are variously dependent on the specific environment. For example, for those locations with higher wave breaking probability, the wave energy dissipation could contribute more on the water column while for those areas with strong currents that are opposite to wave direction, wave-current interaction could increase the water elevations so called 'wave block'. The wave-induced set-up in the surf zone after breaking could contribute to the predicted water level so that most coastal inundations are underestimated. Therefore, in this study, we are going to comprehensively investigate the wave-induced forces that are easily neglected in the ocean modelling but could play critical role in the realistic context of coastal and ocean areas, with the combination effects from both wind-driven and swell waves. The sea states in both Wadden sea and south-west Baltic sea along the German coastline are investigated, associated with preliminary coupling effects between wave propagation and ocean circulation, under extreme circumstances. It is preliminarily found that surface stokes drift sometimes could be the same order as to the wind-driven surface current at most of the locations along the German coasts, where the partitioned stokes contribute differently. This could provide an insight that how the wave forces contribute variously to the context of German coastal areas for the continuous work on improving modelling skills.

Zazzini, S., Pini, A., Bello, P., Monti, P., Leuzzi, G.:
Lagrangian investigation of turbulent dispersion in the Ocean Surface Boundary Layer

This work explores the turbulent dispersion of pollutants in the marine environment with a 3D Lagrangian Stochastic Model, with particular attention on the Ocean Surface Boundary Layer (OSBL). The turbulent phenomena that affect physical and biological process in the OSBL are captured by a new parameterization of the vertical eddy diffusivity coefficient. The idea behind this parameterization is that through similarity laws based on quantities (such as the mixed layer depth and the friction velocity) calculated by operational oceanography models, it is possible to obtain information on turbulent diffusivity and therefore on the mixing of physical, chemical and biological quantities on the water column. The main actors of this parameterization are the experimental profile of the vertical velocity variance (Tseng & D’Asaro, 2004) and the theoretical formulation of the mixing length (Craig & Banner, 1994). The turbulent eddy diffusivity parameterization was implemented in a 3D Lagrangian Stochastic Model, developed by this research group. In this model, the pollutant particles trajectories are described by a Wiener process. The convenience of the Lagrangian approach is that enables to reproduce turbulent dispersion processes at sub-basin scales. The 3D dispersion of microplastics over the Mediterranean basin has been performed, so pathways and accumulation zones of microplastics in different periods of the year have been identified. The presence of plastics in the water column was noticed and it is the result of the fallout of small plastic fragments from the surface waters. A statistical analysis of the simulation results shows a characteristic behaviour of the pollutant plume in the OSBL: the concentration of particles is maximum at the sea surface, but the quantity spread into the water column is not negligible. This profile could be described by a similarity law.