Abstracts and Presentations/Posters

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Oral Presentations

Becker, Marius, Maushake, Christian, Winter, Christian:
Effects of mud-induced stratification on sediment transport in a hyperturbid tidal channel

In hyperturbid estuaries, the density structure is controlled by suspended sediment concentration. At this stage, understanding of sediment transport under hyperturbid conditions is still limited due to the lack of high resolution field data. Data used in this study show the intratidal variability of transport in different hydrodynamic conditions and fluid mud layer thickness. Stratification and velocities were measured in the tidal channel part of the Ems estuary, located at the border between The Netherlands and Germany. The study is based on five 13h data sets. Data were collected at the same location in the fluid mud reach.

In addition to differences in current velocity and mean velocity shear, differences are seen in the characteristics of stratification. However, despite a wide range in conditions, entrainment is substantially asymmetric in the same way, in all cases. Quasi-instantaneous entrainment and rapid vertical mixing occur at the beginning of the flood tide. During the ebb, transport is limited to the fluid mud layer surface and related to a gradual reduction in layer thickness.

At the beginning of the ebb transport phase, the upper part of the fluid mud layer is affected by a shear layer above the fluid mud surface, and transport is caused by drag of the uppermost part of the fluid mud layer. Subsequently, with increasing velocities, a stratified shear layer develops. Fluid mud is entrained and sediment concentrations increase above the fluid mud surface.

The local gradient Richardson number indicates reduced vertical mixing in the upper part of the water. Reduced mixing is spatially related to increased stratification by suspended sediments. Persisting throughout the ebb phase, this stratification delineates the upper boundary of a turbulent mixing layer. Consequently, ebb transport is restricted to a narrow band. Observations show that this band of transport covers the upper part of the mixing layer, and the lower part of the region of reduced mixing. During ebb, most transport occurs not in response to entrainment in the sense of upward mixing of sediment, but by continuous downward progression (deepening) of the shear layer. Only at the end of the ebb phase, the shear layer reaches the river bed.

To elucidate this spatial relation, the vertical distribution of the gradient Richardson number is used to relate the occurrence of high transport rates to regions characterized by increased or reduced mixing. To simplify, high transport rates, co-located with increased and decreased mixing, are described as turbulent and laminar transport, respectively. it is shown that transport exhibits a layered structure during the entire ebb phase. A layer of negligible transport (stationary suspension) exists directly above the bed. It is located below a layer of turbulent transport, which is followed by a laminar transport layer. Very low transport occurs in the upper part of the water column. The region of turbulent transport is located at the surface of the fluid mud layer. Laminar transport occurs in the region of significantly higher velocities but increased stratification, and reduced mixing. In the uppermost part of the water column, sediment concentrations are comparatively low, such as the transport rate, as a result of reduced mixing.

Despite the different hydrodynamic conditions, transport always showed the described characteristics during ebb, with a clear separation between laminar and turbulent transport layers. Previous analysis of mud transport in turbid estuaries discussed the tidal pumping of sediments and stressed the importance of strong flood currents. By contrast, this study indicates that net transport depends significantly on the ebb phase, which is controlled by a strong feedback of stratification on mixing.

Bluteau, C.E.*, Galbraith, P.S., Bourgault, D., Tremblay, J.-E., Villeneuve, V.:
Implications of turbulence mixing processes on supplying nutrients into a large Estuary during winter

We undertook a multi-disciplinary project in the St-Lawrence Estuary and Gulf during winter aboard a Canadian Coast Guard ice-breaker, the Amundsen. This vessel is typically stationed in the Arctic during summer for scientific research.  In 2018, however, the Coast Guard approved the presence of a science team to sample during their normal de-icing operations. Winter monitoring in the area is generally confined to conductivity-temperature vertical profiles and near-surface water samples collected from a helicopter. 

Aboard the ice-breaker, sampling included turbulence profiles, which were combined with oxygen and nutrients observations to estimate vertical fluxes of these two parameters. The first objective was to establish whether the Lower St-Lawrence Estuary, located downstream of the Saguenay Fjord, is better oxygenated in winter. The second objective was to assess nutrient transport pathways during winter before the spring bloom. 

In our winter observations, oxygenation was high at the head of the channel because of intense mixing from tidal upwelling processes  similar to those reported during summer. However, a box-analysis for nutrient inputs into the Lower Estuary shows that, contrary to summer, horizontal fluvial contributions are a more significant source than vertical mixing contributions from tidal upwelling at the head. The latter process mixed the sub-surface nutrient-poor water with the nutrient-rich surface water flowing from the Upper Estuary instead of transporting deep nutrient-rich water towards the surface. 

The talk will also present some challenges with estimating the vertical fluxes when selecting a model for computing the diapycnal mixing rate K. The model of Osborn-Cox, for instance, cannot be used since the water column was unstably stratified in temperature while stable in salinity. This density stratification is conducive to double-diffusive convective instabilities. The other commonly used model of Osborn, is hampered by doubts in the proper choice for the mixing efficiency.

Chrysagi, E., Holtermann, P., Umlauf, L., Burchard, H.:
Submesoscale Processes and Mixing in a Semi-Enclosed Basin: The Case of the Baltic Sea

Submesoscale processes have drawn the attention mainly due to their ability to extract energy from the large mesoscale flow and transfer it down-scale, thus contributing to the cascade of energy, but also, due to their important role on mixing and vertical transport of momentum and tracers. This research focuses on the quantification of mixing, caused by submesoscale fronts and filaments, and on the different types of instabilities that occur through loss of geostrophic balance. A realistic high-resolution numerical simulation, has been applied for the Baltic Sea, based on the General Estuarine and Transport Model. The Baltic Sea, which is used here as a natural laboratory, constitutes an ideal place for studying those processes since it is a non-tidal, brackish system with intense horizontal gradients in the surface. High-resolution shear microstructure transects measured in the Eastern Gotland Basin captured a variety of fronts and filaments, that were used to confirm the numerical simulations. The model has also been validated against satellite data, along with long-term mooring and research vessel data. The results show that during the winter period a strong and persistent lateral thermohaline gradient is found in the basin, created by a combination of upwelling favorable winds, at the western part and a warm coastal current in the East. This frontal structure favors the formation of cold elongated filaments. The filaments are characterized by O(1) Rossby and Richardson dynamics, indicating the prevalence of a submesoscale regime, which is accompanied by narrow regions of cyclonic vorticity, surface convergence, and strong downwelling. Localized areas where symmetric instability might occur have been identified. Although the submesoscale features cover part of the domain, the results show that they have a substantial contribution to the upper ocean average mixing, suggesting that submesoscales can be considered as another important mechanism for winter time mixing.

Fernández Castro, B., Sepúlveda Steiner, O., Knapp, D., Posch, T., Bouffard, D., Wüest, A.:
Extremely low mixing efficiency in a strongly seasonally stratified perialpine lake: implications for the persistence of a thin layer of harmful cyanobacteria

The filamentous harmful cyanobacterium Planktothrix rubescens has become the dominant phytoplanktonic species in Lake Zurich over the last 40 years. This success is linked to ongoing changes in the nutrient and thermal regime, including lake’s turnover rate and mixing depth. Thanks to their adaptation to low light and their capacity to control their vertical position via buoyancy regulation, P. rubescens accumulate in a thin layer in the thermocline that persists during the stratified season (May-October). Three surveys performed with microstructure profilers and a moored high-resolution current meter, temperature and fluorescence loggers were conducted in April, July and October 2018 in Lake Zurich. The goal was to characterize how the variations of the mixing regime affect the seasonal development and decline of the P. rubescens thin layer. In July, the lake showed a strong thermocline (1.5°C m-1 , N = 0.07 s-1) between 7-17 m, and a thin P. rubescens layer at its base (12-16 m). Turbulent kinetic energy dissipation from microstructure profiles peaked at the pycnocline (ε ~ 10-7 m2 s-3), but the Thorpe scale (LT ~ 10-3 m << LO ~ 10-2 m), Osborn-Cox diffusivity (KT < 10-7 m2 s-1), mixing efficiency (Γ < 0.02) and Cox number (Cz << 1) were extremely low, indicating that vertical turbulent overturns were overly small and infrequent to stimulate vertical diffusion above molecular levels. Eddy correlation heat fluxes from moored instruments and a seasonal heat budget were consistent with this result. High-precision current data also revealed seiching and internal-wave activity in the thermocline, but the velocity fluctuations remained isotropic (u’,v’ ~ 10-3 m s-1 vs. w’ ~ 10-4 m s-1 ) down to the smallest resolved scales. These observations suggest that strong stratification can inhibit vertical motion, overturning and mixing, thus explaining the long persistence of the thin plankton layer, which only starts to decline when the mixed-layer deepens as a result of convective instabilities during autumn.

Glenn Carter, Matthew Alford, Gunnar Voet, James Girton, Larry Pratt, Jesse Cusack, Kelly Pearson-Potts, Shuwen Tan:
Mixing coefficients in a weakly stratified abyssal passage

Mixing in the Samoan Passage plays an important role in setting the abyssal water properties of the North Pacific. The deepest water exiting the passage is 55 millidegrees Celsius warmer than the bottom water entering the southern end of the passage. As part of the Samoan Passage Abyssal Mixing Experiment 115 full-depth microstructure profiles with taken throughout the passage. Here we use velocity and temperature microstructure from these profiles to investigate the mixing coefficient in the weakly stratified water below 3500m depth. 

The approximately 99,000 epsilon / chi pairs have  Buoyancy Reynolds Numbers (Re_b) ranging from 3 to over 10^5. The analysis suggests four regimes:

Re_b < 10 where the mixing coefficient decreases with increasing with Re_b;

10 < Re_b < 100 where the mixing coefficient increases with increasing with Re_b;

100 < Re_b < 3000 where the mixing coefficient is fairly constant;

Re_b > 3000 where the mixing coefficient decreases with increasing Re_b.

Data taken close to a hydraulic jump had a steeper decrease with Re_b in the high Buoyancy Reynolds Number regime than the data as a whole. For this hydraulic jump data, chi/epsilon values where the stratification is statically unstable overlay chi/epsilon values with stable stratification when plotted as a function of Re_b.

Holtermann, P., Prien, R., Naumann, M., Umlauf, L.:
Lateral transport, not vertical mixing, is the most important oxygen transport process for the hypoxic transition zone in the Baltic Sea.

Semi‐enclosed marginal seas like the Baltic Sea are often characterized by permanently anoxic deep layers, and may therefore serve as important model systems to study the causes and consequences of the predicted global expansion of oxygen minimum zones. Here, we focus on the role of lateral intrusions in maintaining the “hypoxic transition zone” (HTZ) of the Baltic Sea, which characterizes the quasi‐permanent hypoxic region located between the oxygenized surface layer and the sulfidic deep‐water region. Based on long‐term deployments of an autonomous profiling system in the central Baltic Sea, we show that oxic mid‐water intrusions are ubiquitous features, providing the most important oxygen source for the HTZ, and largely control the vertical and lateral extent of the hypoxic areas. An oxygen budget for the HTZ suggests that oxygen turnover in the HTZ is, to first order, determined by a long‐term balance between sedimentary oxygen demand and oxygen import by intrusions. The downward mixing of oxygen into the HTZ is generally non‐negligible but unlikely to provide a first‐order contribution to the HTZ oxygen budget. On the long‐term average, mid‐water intrusions were shown to inject 30–60 Gmol of oxygen per year into the deep‐water region below the permanent halocline. This is approximately one order of magnitude larger than the average amount of oxygen imported during the massive deep‐water inflow events (Major Baltic Inflows) that occur on an approximately decadal time scale, highlighting the HTZ as a hotspot for biogeochemical turnover.

Karan Venayagamoorthy:
Turbulent mixing in oceanic flows: challenges and insights for improved prediction

The understanding and quantitative prediction of diapycnal (irreversible) mixing in the oceans remains an important ongoing challenge. From a practical perspective, there is a critical need to obtain accurate prediction of turbulent heat, mass and momentum fluxes using indirect measurements in the field. Indirect methods for estimating mixing rates typically rely on the inference of three key quantities namely: (i) the rate of dissipation of turbulent kinetic energy; (ii) the mixing efficiency, which is a measure of the amount of turbulent kinetic energy that is irreversibly converted into background potential energy; and (iii) the background density stratification, respectively. In this talk, an overview on how these quantities are typically inferred and/or parameterized will be presented. Some important challenges, ambiguities and new insights will also be presented with an eye toward improved prediction of ocean mixing.

Kokoszka, Florian, Conversano, Fabio, Iudicone, Daniele:
Microstructure observations of the water column's structure during a summer-to-winter destratification period in the coastal marine ecosystem of the Gulf of Naples.

The Long-Term Marechiara research program (LTER-MC), that began in 1984, is aimed at studying the structure and functioning of the coastal pelagic system of the Gulf of Naples and the relation with environmental changes. The weekly sampling occurs at a site located 2 miles off the city of Naples, on a depth of about 75 m, in a dynamical zone affected by the local river runoff and the advection of the open waters of the Southern Tyrrhenian Sea. Using a Rockland VMP250 profiler, we collected at this site 88 highly-resolved vertical profiles of shear, temperature, salinity, chlorophyll and turbidity that covered the de-stratification period, from July 2015 to January 2016, at a weekly frequency.

The vertical stratification shows a good agreement with the stratification inferred from the more classical CTD measurements. The mixed-layer depth (MLD) shows a clear deepening along the seasons while chlorophyll patches have a core below the MLD in August to September and above it from October to December. The turbidity profiles show patches that are generally co-located with chlorophyll, while a turbid bottom layer is observed along the cast in the 60-75m deep layer. High resolution shears show two layers with higher values, one within a 10-to-20m thick layer just below the MLD, followed by a second high-shear layer around 10m deeper. These layers keep they vertical distances from the MLD during its seasonal deepening, with the deeper layer causing a resuspension of sediments when it reaches the bottom. The resulting turbulent kinetic energy dissipation rates present patches/hotspots located below the MLD, where shears are intensified. Considering the whole dataset, the probability density function shows a distribution of dissipation that matches a right-tailed variety differing from the log-normal distribution.

Koszalka, Inga Monika:
Disentangling ocean mixing from Lagrangian trajectories

Drifting instruments and Lagrangian simulations forced by ocean model output are often employed to quantify lateral mixing in the ocean with eddy diffusivity coefficients, turbulent kinetic energy and characteristic eddy time and length scales estimated from the trajectories. Challenges related to this approach are the spatiotemporal variability of ocean turbulent flows and the non-local aspect of Lagrangian estimates of mixing parameters. Another unresolved aspect is applicability of Lagrangian diagnostics to mixing parameterizations applied in ocean models.

In this contribution, I will first address the estimation of lateral mixing coefficients using single particle diffusivity and relative diffusivity diagnostics. The example applications will focus on surface flows in the Nordic Seas, in the Baltic Sea, and in the Agulhas Current. While single particle diffusivity estimates are intrinsicly difficult to relate to mixing rates used in ocean models, they have been successfully applied in stochastic Lagrangian parameterizations using higher order Markov schemes. The second diagnostics, relative diffusivity, encodes information about the spatial dependence of mixing rates and is by this is more promising regarding applications to ocean models. In the following, I will address the subsurface and three-dimensional aspects of ocean mixing viewed in Lagrangian frame. I will also consider heat transport estimates (eddy heat fluxes) as well as bio-physical interactions associated with stirring and mixing by ocean flows.

Lamb, K. G.:
Tide-Topography Interaction and Currents: Asymmetries in Internal Wave Breaking

Tide-topography interactions, the source of approximately half the internal wave energy and of most internal solitary waves in the oceans, have been the subject of many studies in recent decades. Many of these studies have considered the generation of internal waves over a two-dimensional symmetric ridge in the absence of steady background currents. In this situation waves propagating to the left and right of the ridge are the same. This symmetry is broken in the presence of steady background currents. In this talk the effects of a uni-directional steady background current confined to lie above a ridge will be discussed. Strong asymmetries in wave breaking can occur. Internal wave beams interacting with the background shear can result in overturning on the downstream side of the ridge. Lee waves on the upstream side of the ridge (with respect to the background current) are more prone to shear instabilities than those formed on the downstream side. Regions in parameter space where these types of asymmetric breaking occur will be discussed.

Leadbitter, Philip, Hall, Rob, Brearly, Alex:
Using Thorpe scaling to estimate turbulent kinetic energy dissipation rates with 512 Hz thermistor data from Seagliders

In the last 10 years, there have been efforts to increase the archive of turbulence measurements by fitting microstructure sensor systems to mechanically quiet autonomous platforms, including buoyancy-driven gliders equipped with shear probes and fast thermistors. Although the shear data is more commonly used to calculate turbulent kinetic energy (TKE) dissipation rates (ε), values can also be inferred from the temperature data by fitting the temperature gradient spectrum to a theoretical curve. Here we present an alternative, and complementary, approach using Thorpe scaling in which ε is empirically related to the physical size of resolved turbulent overturns.

Data was collected by a Seaglider in the Faroe-Shetland Channel in 2017. Seventeen dives greater than 500 m were used for this comparison. The Thorpe scale method is applied to conservative temperature data from both the FP07 fast thermistor (512 Hz) and the CT sail thermistor (0.2 Hz) as well as potential density calculated from the temperature and conductivity from the Seaglider’s CT sail (0.2 Hz). 

The 512 Hz conservative temperature data resolved Thorpe length-scales over 4 orders of magnitude (10-3 m to 101 m), which yields estimates of ε range over at least 6 orders of magnitude (10-10 to 10-4 W kg-1) The lower limits of these are at least 2 orders magnitude smaller than those seen in the 0.2 Hz data.  Identified overturns of a comparable size at the same depths show the same order of magnitude of calculated ε, across all the three variables. Estimated values of ε from 0.2 Hz potential density and conservative temperature are also consistent with previous estimations from other works carried out in the Faroe-Shetland Channel.Further work will determine the lower limit of ε that can be estimated 512 Hz conservative temperature data.  

Lenn Y D, Igor Polyakov, Laurie Padman, Andrew Pnyushkov, Rob Rember, Vladimir Ivanov:
Eastern Arctic Ocean Diapycnal Heat Fluxes through Large Double-Diffusive Steps

The diffusive layering (DL) form of double-diffusive convection cools the Atlantic Water (AW) as it circulates around the Arctic Ocean. Large DL steps, with heights of homogeneous layers often greater than 10 m, have been found above the AW core in the Eurasian Basin (EB) of the eastern Arctic. Within these DL staircases, heat and salt fluxes are determined by the mechanisms for vertical transport through the high-gradient regions (HGRs) between the homogeneous layers. These HGRs can be thick (up to 5 m and more) and are frequently complex, being composed of multiple small steps or continuous stratification. Microstructure data collected in the EB in 2007 and 2008 are used to estimate heat fluxes through large steps in three ways: using the measured dissipation rate in the large homogeneous layers; utilizing empirical flux laws based on the density ratio and temperature step across HGRs after scaling to account for the presence of multiple small DL interfaces within each HGR; and averaging estimates of heat fluxes computed separately for individual small interfaces (as laminar conductive fluxes), small convective layers (via dissipation rates within small DL layers), and turbulent patches (using dissipation rate and buoyancy) within each HGR. Diapycnal heat fluxes through HGRs evaluated by each method agree with each other and range from ~2 to ~8 W m−2, with an average flux of ~3–4 W m−2. These large fluxes confirm a critical role for the DL instability in cooling and thickening the AW layer as it circulates around the eastern Arctic Ocean.

Li, Qing:
Modeling the ocean surface boundary layer vertical mixing by Langmuir turbulence

Turbulent motions in the ocean surface boundary layer (OSBL) control the exchange of heat, momentum, and trace gases such as CO2 between the atmosphere and ocean, and thereby affect the weather and climate. But these turbulent motions are too small to be directly simulated in regional and global ocean models. Parameterization schemes representing the effect of these turbulent motions on OSBL vertical mixing, especially due to wind-driven shear and surface buoyancy flux-driven convection, are commonly adopted in large scale ocean models, with varying degree of success. There have been a growing number of studies in the recent literature assessing the effect of ocean surface waves on turbulent mixing in the OSBL, in particular via Langmuir turbulence, which was traditionally neglected. The lack of explicit representation of such effects in large scale ocean models may contribute to persistent biases in the simulated mixed layer depth, air-sea fluxes and temperature distribution and tracer concentrations in the upper ocean. Various parameterization schemes including the effect of Langmuir turbulence have been proposed. Yet in what conditions and to what extent do these different schemes agree or disagree are unknown.

In this presentation, different ideas to model the effect of Langmuir turbulence in OSBL vertical mixing schemes are reviewed. Results of a consistent and systematic comparison among six newly proposed Langmuir schemes and five traditional schemes without Langmuir turbulence are presented. These schemes are tested in scenarios versus matched large eddy simulations, across the globe with realistic forcing (JRA55-do, WAVEWATCH-III simulated waves) and initial conditions (Argo), and under realistic conditions as observed at ocean moorings. As expected, schemes with Langmuir turbulence generally produce stronger and deeper turbulent mixing than their non-Langmuir counterpart. However, large discrepancies among the six Langmuir turbulence schemes are also found, some resembling the disagreement among their non-Langmuir counterpart. These results highlight the uncertainties in our understanding and numerical implementation of the turbulent mixing in the upper ocean. Such discrepancies are quantified under different surface forcing conditions. Regions and seasons where the greatest discrepancies occur are also discussed to guide future research.

Lorenz, M., Klingbeil, K., Burchard, H.:
Numerical study of the exchange flow of the Persian Gulf using an extended Total Exchange Flow analysis framework

The Total Exchange Flow analysis framework computes consistent bulk values quantifying the estuarine exchange flow using salinity coordinates since salinity is the main contributor to density in estuaries and the salinity budget is entirely controlled by the exchange flow.
For deeper and larger estuaries temperature may contribute equally or even more to the density. That is why we included potential temperature as a second coordinate to the Total Exchange Flow analysis framework which allows to gain insights in the potential temperature-salinity structure of the exchange flow as well as to compute consistent bulk potential temperature and therefore heat exchange values with the ocean.
We applied this theory to the exchange flow of the Persian Gulf, a shallow, semi-enclosed marginal sea, where dominant evaporation leads to the formation of hyper-saline and dense Gulf water. This drives an inverse estuarine circulation which is analyzed with special interest on the seasonal cycle of the exchange flow. The exchange flow of the Persian Gulf is numerically simulated with the General Estuarine Transport Model (GETM) from 1993 to 2016 and validated against observations. Results show that a clear seasonal cycle exists with stronger exchange flow rates in the first half of the year. Furthermore, the composition of the outflowing water is investigated using passive tracers which mark different surface waters. The results show that in the first half of the year, most outflowing water comes from the southern coast, while in the second half most water originates from the north-western region.

Lueck, R. G., McMillan, J. M, Juteau, J. P., Wolk, F.:
Quality Control and Statistical Uncertainty of Turbulence Microstructure Measurements

The amount of turbulence data collected using shear probes mounted on autonomous platforms, such as underwater gliders, AUVs, and profiling floats, is quickly surpassing the total number of observations made over the last 50 years using traditional free-falling vertical profilers. This increase in data volume requires automated processing and rigorous quality control metrics to ensure that regions of enhanced mixing are identified correctly. In addition, estimates of statistical uncertainty on the computed dissipation rates are necessary to quantify confidence estimates.

The precise definition of these quantitative metrics is non-trivial because turbulence is a statistical process. The measurements are correlated over length scales that vary with time, implying that instantaneous measurements may not be statistically independent. In addition, the rate of dissipation of turbulent kinetic energy is estimated from the integration of a shear spectrum over a limited bandwidth, reducing the equivalent degrees of freedom.

In this presentation, shear probe measurements collected on a streamlined mooring in an unstratified, high Reynolds number tidal channel, are used to develop a method to compute associated confidence intervals on the dissipation rate. In particular, the equivalent degrees of freedom are estimated as a function of the decorrelation length scale. It is shown that dissipation rates are log-normally distributed for a wide range of averaging lengths -- from approximately 3 to 10000 times the Kolmogorov length scale -- and that the 95% confidence interval shrinks approximately inversely with the square-root of the averaging length. The standardization and automation of these methods will ensure that data processing keeps pace with the increased collection of turbulence data.

Malcherek, A., Schmidt, J., Naulin, M.:
Modelling hyperturbid estuaries using the k-omega-model in a holistic approach

Hyperturbid estuaries are characterized by very high concentrations of suspended particular matter (SPM). The SPM-induced periodic stratification influences flow structure and sediment transport. Different areas of flow states can be identified across the water column: (i) turbulent flow at the surface, (ii) damped flow at low SPM, (iii) laminar flow at high SPM, (iv) rheological flow in fluid mud and pre-consolidated mud, (v) no flow at consolidated mud. The flow regimes are subject to tidal variations as well as seasonal fluctuations.

It would be desirable to apply a holistic approach using one set of equations only that is capable to simulate the whole water column and its different flow states. Besides taking account of formulations for flocculation, hindered settling and consolidation as well as a rheological viscosity one major challenge remains the choice of a suitable turbulence model.

First results revealed that the k-omega model with customised boundary conditions is suitable for this purpose. For the lower boundary conditions, the turbulent kinetic energy is set to zero and omega is set to a high value at the consolidated bed. Therefore, omega is rather interpreted as a potential to dissipate turbulent kinetic energy instead of an actual dissipation rate.

The general ability of the holistic model approach has been successfully demonstrated by simulation results of a 1DV-model. However, multiple challenges remain for the 3D application. Preliminary results and future work steps using the k-omega turbulence model in a holistic model approach will be presented and discussed at the workshop.

Mironov, D., Machulskaya, E.:
Non-Realizability of Truncated Turbulence Closure Models: Origin of Ill Behaviour and Remedial Measures

Realizability of the second-order turbulence closure models (parameterization schemes) is addressed through the consideration of the so-called "stability functions'' (Mellor and Yamada 1974, 1982). The emphasis is on the turbulence kinetic energy - scalar variance (TKESV) closure scheme (Mironov and Machulskaya 2017, Machulskaya and Mironov 2013, 2019). The TKESV scheme (the level 3 scheme in the nomenclature of Mellor and Yamada) carries prognostic transport equations for the turbulence kinetic energy (TKE) and for the variances and covariance of quasi-conservative scalar quantities (e.g., liquid water potential temperature and total water specific humidity in the atmosphere, potential temperature and salinity in the ocean). These scalar (co)variances characterize the turbulence potential energy (TPE). Stability functions appear within the framework of truncated closure schemes, where (i) the Reynolds-stress and scalar-flux equations (and, within the framework of one-equation TKE schemes, also the equations for the scalar variances and covariance) are reduced to the diagnostic algebraic formulations by neglecting the substantial derivatives and the third-order transport terms, and (ii) simplified linear parameterizations of the pressure-scrambling terms are used. The stability functions are ill-behaved (tend to infinity or become negative) over a certain range of their governing parameters, e.g., mean velocity shear and buoyancy gradient. 
Using the approach of Helfand and Labraga (1988), we develop regularized stability functions for the TKESV scheme that reveal no pathological behaviour over their entire parameter space. The Helfand and Labraga regularization procedure is compared to some other realizability constraint techniques (e.g., weak non-equilibrium approximation), and physical interpretation of the various techniques is offered. Relation of the Helfand and Labraga approach to physically sound non-linear parameterizations of the pressure-scrambling terms are discussed. Advantages of the TKESV scheme, that treats the TKE and the TPE on equal terms, over one-equation TKE schemes are elucidated. The absence of critical Richardson number (beyond which turbulence cannot be sustained) within the framework of the TKESV scheme is highlighted. Finally, realizability of turbulence closures is considered within a more general framework of the moments problem of the probability theory. 
Helfand, H. M., and J. C. Labraga, 1988: Design of a nonsingular level 2.5 second-order closure model for the prediction of atmospheric turbulence. J. Atmos. Sci., 45,113-132. 
Machulskaya, E., and D. Mironov, 2013: Implementation of TKE--Scalar Variance mixing scheme into COSMO. COSMO Newsletter, No. 13, 25-33. (available from www.cosmo-model.org) 
Machulskaya, E. E., and D. V. Mironov, 2019: The so-called stability functions and realizability of the TKE - Scalar Variance closure for moist atmospheric boundary-layer turbulence. Submitted to Boundary-Layer Meteorol. 
Mellor, G. L., and T. Yamada, 1974: A hierarchy of turbulence closure models for planetary boundary layers. J. Atmos. Sci., 31, 1791-1806. 
Mellor, G. L., and T. Yamada, 1982: Development of a turbulence closure model for geophysical fluid problems. Rev. Geophys., 20, 851-875. 
Mironov, D. V., and E. E. Machulskaya, 2017: A turbulence kinetic energy  - scalar variance turbulence parameterization scheme. COSMO Technical Report, No. 30, Consortium for Small-Scale Modelling, 55 pp. (available from www.cosmo-model.org) doi:10.5676/DWD_pub/nwv/cosmo-tr_30

Olbers, D., Pollmann, F., Eden, C.:
On PSI interactions in internal gravity wave fields and the decay of baroclinic tides

Barotropic tidal oscillations over seafloor topography generate baroclinic tides which may be damped in turn via nonlinear triad interactions with internal gravity waves, fuelling the ambient wave field. We derive the kinetic equations for this tidal damping and the energy transfer to the ambient wave field and compute damping times and energy transfer rates for the M_2 tide and a Garrett-Munk-like ambient wave field. We show that parametric subharmonic instability (PSI) interactions are important, where the tide interacts resonantly with two background waves, each of half the tidal frequency. PSI is restricted to the latitude belt 28.8° N/S and yields under typical conditions damping times of about 20 days for tides with low vertical wavenumber. Damping times decrease with equivalent mode number j roughly as 1/j^2. Outside the critical latitudes PSI is not possible and damping times are  two orders of magnitude larger. The energy transfer to the ambient wave field is concentrated at half the tidal frequency omega at all latitudes within the critical latitude belt. Outside, the transfer is much smaller and  peaks at omega + f and N. An estimation of the tidal spectral transfer on the global scale is hampered by insufficient knowledge of the baroclinic energy distribution over the vertical modes. Using results from a numerical circulation model with tidal forcing, we find an energy transfer from the tide to the ambient wave field of 0.3 TW, about half of what is currently proposed for the conversion of barotropic to baroclinic energy.

Peng, J.-P., Holtermann, P., Umlauf, L.:
Frontal instability and energy dissipation in a submesoscale upwelling filament

In frontal regions of the ocean, submesoscale motions are frequently observed features in the surface layer. Due to their important role for surface-layer restratification, lateral dispersion, and the turbulent energy cascade, this type of motion has been extensively studied over the past decade. However, direct high-resolution turbulence observations inside submesoscale features (e.g., fronts and filaments), required to test the relevance of new theoretical concepts, are so far virtually lacking. Unlike previous studies focusing on persistent fronts (e.g., Gulf Stream and Kuroshio), here we present high-resolution turbulence microstructure and velocity data from a transient submesoscale upwelling filament, entrained between two mesoscale eddies in the Benguela upwelling system (South-East Atlantic). The focus of the study is a sharp submesoscale front at the edge of the filament, characterized by vigorous turbulence, down-front winds, and a pronounced low-PV layer near the surface.  Two distinct stability regimes are identified: (i) the main frontal region, where the cyclonic cross-front shear is strong enough to suppress symmetric instability, despite strong baroclinicity. Turbulence in this region is locally driven by Kelvin-Helmholtz instability; (ii) a neighboring region with a 30-35 m deep low-PV layer and strong horizontal density gradients, where down-front winds induce a vertical two-layer structure: the upper part of the low-PV layer is characterized by convective mixing due to the destabilizing cross-front Ekman transport, while in the lower part turbulence is driven by forced symmetric instability. These are the first direct field observations supporting the relevance of forced symmetric instability that has so far only been identified in theoretical and numerical investigations.

Pollmann, F., Eden, C., Olbers, D.:
Toward a spectral characterization of the global ocean’s internal wave field in observations and models

Small-scale turbulence in the ocean interior, which is mainly induced by breaking internal gravity
waves, is an important contributor to driving large-scale ocean dynamics but remains unresolved
in general circulation models. Consistent parameterizations are based on internal wave energetics,
which are influenced by a range of forcing processes at large scales, nonlinear wave-wave interac-
tions fluxing energy across the spectrum to smaller scales, and at high vertical wavenumbers the
breaking and energy transfer to turbulence. For the development, improvement and evaluation of
such parameterizations, a thorough understanding of what shapes the spectral characteristics of the
ocean’s internal wave field is thus prerequisite. To identify their global variability, vertical profiles
of temperature, salinity and pressure collected by the Argo program, are analyzed. The estimated
energy levels, spectral slopes and bandwidth parameters exhibit a clear geographic pattern and,
in some cases, also a pronounced seasonal cycle. Spectral transfers are investigated based on the
kinetic equation for wave action, showing that the energy transfer from internal tides to the in-
ternal wave continuum is dominated by parametric subharmonic instability and that the energy
dissipation scales with the spectral slope. These results will help to improve spectral internal wave
models like IDEMIX (Internal wave dissipation, energy and mixing) and also to refine standard
procedures for the analysis of observational data (finestructure method).

Reichl, B. G., Li, Q., Adcroft, A., Griffies, S., Hallberg, R., Fox-Kemper, B.:
A surface boundary layer parameterization with a constrained potential energy conversion rate for climate simulations

Accurate modeling of the ocean surface boundary layer (OSBL) is critical for many coupled ocean-atmosphere problems spanning from tropical cyclones to climate.  An approach to parameterize vertical mixing in this regime follows the Kraus-Turner-Niiler paradigm, which constrains OSBL entrainment using an integrated potential energy conversion rate.  The Kraus-Turner-Niiler method was first implemented in bulk (well-mixed) boundary layer models, but has been generalized in the energetic Planetary Boundary Layer (ePBL) framework to allow finite vertical mixing inside the OSBL.  We utilize a suite of Large Eddy Simulations to develop constraints for the total integrated potential energy conversion rate by vertical mixing for ePBL, including its variability in convective turbulence, wind-driven shear turbulence, and Langmuir turbulence.  A feature of the ePBL parameterization is that it is insensitive to ocean model design factors such as vertical resolution and model time-step, which is a critical feature for application to climate simulations. We utilize the new parameterization to investigate the impact of surface waves via Langmuir turbulence on coupled global ocean climate simulation.  Our results are generally consistent with previous studies, showing that Langmuir turbulence increases vertical mixing for wind-driven (Summer) mixed layers, but it has a muted relative impact on mixing in convective (Winter) mixed layers. This acts to reduce mixing biases common to previous generation ocean climate simulations.

Schulz, K., Janout, M.:
Turbulence, heat fluxes and the thermohaline structure along the eastern Arctic continental slopes in 2018

The continental slopes north of the Siberian shelf seas are dynamic regions characterized by fronts, sea ice formation and melt, and the progression of the Arctic Boundary Current (ABC). Increasing evidence points to a greater influence of Atlantic water in the eastern Nansen Basin with a hypothesized shift from a mainly year-round ice covered, quiescent, double diffusion-dominated and stratified system to an ocean that now features long open water seasons, enhanced turbulence and deeper mixed layers. In 2018, we revisited the region to carry out numerous hydrographic and microstructure shelf-to-basin transects along the eastern Kara, Laptev, and East Siberian Seas. The hydrography in summer 2018 was dominated by the Atlantic-derived waters in the ABC and a front separating the warm waters from the cold shelf waters. The temperature in the ABC progressively decreased eastwards between the Kara Sea and the East Siberian Sea, with indications for strong interleaving and mixing north of Severnaya Zemlya. Microstructure turbulence measurements highlight enhanced dissipation rates within the front above the continental slope, with maximum vertical heat fluxes of 10 W/m2 from the Atlantic water into the cold overlying water – an order of magnitude higher than typical heat fluxes associated with double diffusive processes. These heat fluxes will be contrasted with lateral heat loss to emphasize the contribution of both vertical and lateral exchange processes in controlling Atlantic water heat in the Arctic Ocean. With this contribution, we aim to present new observations from an Arctic key region that, like few others, suffers from atmospheric and oceanic warming trends and sea ice retreat. We will further contrast our findings with earlier observations and discuss our results in the context of “Atlantification”.

Tippenhauer, S., Fong,A., Torres-Valdes, S., Wulff, T., Janout, M.:
Frontal processes and extreme high chlorophyll a values investigated with an AUV

Physical processes as eddies, filaments and turbulence determine the conditions for growth of biomass in the ocean. Here we present a study on physical processes and ecological responses at a submesoscale front in Fram Strait at the interface between the Arctic Ocean and waters of lower latitude origin, investigated using an autonomous underwater vehicle (AUV). The AUV was equipped with several physical and biogeochemical sensors. High-resolution observations from the upper 50m of the water column show extremely large physical and biogeochemical gradients associated with the frontal system. Extraordinarily high chlorophyll avalues between 20m and 40m water depth were observed to be associated with polar surface water, which may recently have been raised above the compensation depth resulting in the subsurface bloom. A mooring in the vicinity showed that the chlorophyll a patch was present for a few days. Additional observations conducted from a Zodiac in parallel to the AUV survey show, that the observed patch had an along-front extent of more than 400m. This indicates that the observed feature was a dynamically active front, possibly experiencing wind-driven frontogenesis or the growth of mixed layer eddies.

The study was conducted in Fram Strait but the results are most likely transferable to other, dynamically comparable frontal systems in the marginal ice zone that experience large horizontal density gradients, e.g. in the Southern Ocean.

Van Roekel, Luke:
A New Eddy Diffusive Assumed Distribution Closure for Ocean Models

Vertical turbulent fluxes of heat, salinity, and momentum in the ocean have a leading order impact on anthropogenic climate change, mediating over 90% of human induced heating between the atmosphere and deep ocean.  The current state of the science representations of vertical turbulent fluxes are subject to persistent biases stemming from missing physical processes (e.g., Langmuir turbulence, wave breaking) and poor inherent assumptions (e.g., lack of energetic constraints).  Here we discuss the first Assumed Distribution Higher Order Closure (ADC) for marine turbulence. By assuming a probability distribution function relationship between certain quantities (e.g., vertical velocity, w, potential temperature,, salinity S, etc.) we can construct all higher order moments, overcoming the classic turbulence closure problem.  The second moment equations (e.g., w’b’) are mathematically consistent with the Reynolds Averaged Navier Stokes equations. Thus the ADC parameterization has full energetic constraints, includes non-local convective turbulence, and can easily integrate other physical phenomena like Langmuir Turbulence (with appropriate modifications to the momentum equations and turbulent length scales).  We have tested the ADC scheme in a single column framework across a range of oceanographically relevant forcing scenarios using a combination of Large Eddy Simulation, GOTM, and KPP simulations under identical forcing as benchmarks. We find that the ADC scheme has little sensitivity to vertical resolution and timestep and compares well to the Large Eddy Simulation results. The ADC framework also allows for a natural implementation of an energetically consistent entrainment rate equation, where the entrainment is dependent on turbulent quantities at the boundary layer interface instead of boundary layer integrated quantities or diagnostic relationships.  Finally, the ADC scheme is also well suited to implementation on GPU systems allowing ocean models to be more amenable to emerging high performance computing architectures.

Voet, G., Alford, M., MacKinnon, J., Nash, J.:
Observations of large internal waves and strong turbulence over deep submarine ridges in the Western North Pacific

We observed large internal waves over submarine ridges, analogue to atmospheric mountain waves, at three different sites in the Western North Pacific along the Yap Ridge using shipboard towed CTD measurements. The waves were up to 300m tall at the deepest site where the ridge crest was at 3000m depth. Their local breaking led to enhanced levels of turbulent dissipation. Rates of turbulent dissipation as estimated both from density instabilities and fast sampling thermistors reached up to 10-6 W/kg in the vicinity of the ridges at all three sites. We compare the observations to various mixing parameterizations for the deep ocean.

W.D. Smyth:
Marginal instability and the efficiency of mixing

The efficiency with which stratified turbulence mixes the oceans and the atmosphere has been a topic of considerable interest in recent decades.  Ivey et al’s (2008) suggestion that the strength of mixing in the ocean was being seriously overestimated catalyzed a community-wide effort to understand how mixing efficiency varies. Less attention has been paid to the perhaps more interesting question of why. The Monin-Obukhov similarity scaling suggests that mixing efficiency is uniform in the interior but decreases in the strongly turbulent flow near boundaries. I will suggest an explanation for the uniformity of mixing far from boundaries based on the Miles-Howard theorem. Finally I will describe observations that conflict with this simple picture, including inefficient turbulence in the bottom boundary layer and also near more complex surfaces such as fish and trees.

Poster Presentations

Avsarkisov, V, Strelnikov, B:
On the stratified turbulence regime in the Mesosphere

In the present study we analyze the mesospheric measurements data from the recent WADIS-2 sounding rocket campaign in the buoyancy Reynolds number range of 10 < Reb < 1000 and the horizontal Froude number range of 0.0001 < Frh <0.007. From the analysis of the vertical potential energy spectra it is shown that there is a kz-3 scaling range in the region between the buoyancy and Ozmidov scales. This result indicates that the winter Mesosphere is in the strongly stratified turbulence regime.

Analysis of the spectra at high buoyancy Reynolds numbers (Reb = O(103)) and sub-critical horizontal Froude numbers (Frh = O(10-2)) suggests that a new scaling range with a kz-1 scaling dependency emerges in the region below the buoyancy scale and extends to the scales smaller than Ozmidov scale. This indicates that the new kz-1scaling range is a high buoyancy Reynolds number effect.

Results of the present study support the theory of the strongly stratified turbulence and suggest that Holmboe wave instability is a potentially important mechanism for mixing in the winter Mesosphere.

Burchard, H.:
A universal law of estuarine mixing

A universal law of estuarine mixing is derived here, combining the approaches of salinity coordinates, Knudsen relations, Total Exchange Flow, mixing definition as salinity variance loss, and the mixing - exchange flow relation. As a result, the long-term average mixing within an estuarine volume bounded by the isohaline of salinity S amounts to M(S)=S2 Qr, where Qr is the average river run-off into the estuary. Consequently, the mixing per salinity class is m(S)=dM(S)/dS = 2 S Qr, which can also be expressed as the product of the isohaline volume and the mixing averaged over the isohaline. The major differences between the new mixing lawand the recently developed mixing relation based on the Knudsen relations are threefold: (i) it does not depend on internal dynamics of the estuary determining inflow and outflow salinities (universality), (ii) it is exactly derived from conservation laws (accuracy) and (iii) it calculates mixing per salinity class (locality). The universal mixing law is demonstrated by means of an analytical stationary and one-dimensional and two-dimensional numerical test cases. Some possible consequences for the salinity distribution in real estuaries are briefly discussed. Since the mixing per salinity class only depends on the river run-off and the chosen salinity, and not on local processes at the isohaline, low-mixing estuaries must have large isohaline volumes and vice versa.

Can, S., Schaefer-Rolffs, U., Becker, E.:
Scale Invariant Diffusion Effects on a Mechanistic GCM

Observations by Nastrom&Gage in the upper troposphere showed the existence of continuous energy and enstrophy cascades of Kinetic Energy (KE) across the scales with regard to horizontal wavenumber, and as a result the well known spectral laws of -3 for synoptic and -5/3 for mesoscales were found. For strongly horizontal vortical motions in planetary and synoptic scales, quasi-geostrophic turbulence appears as a widely accepted framework. On the other hand a theoretical explanation for an atmosphere system resulting in a -5/3 slope under strong stratification for unbalanced regime remains inconclusive, unlike its balanced counterpart.

One theory to explain this Kolmogorov like spectral slope is Stratified Macro-Turbulence (SMT), where for both Available Potential Energy (APE) and KE are expected to have forward cascades. Unlike the idealized box simulations of Lindborg-2006, real atmosphere consists of vertical fluxes of energies. With consideration of Inertia Gravity Waves (IGW) dynamics for the mesoscale spectra energetics, it is under question to what extend SMT is valid for lower atmosphere.

In order to study the high wavenumber regime we use the high horizontal and vertical resolution Kühlungsborn Mechanistic general Circulation Model (KMCM) to obtain a realistic KE spectrum without employing any numerical filters or explicit hyperdiffusion. Instead, we parameterize horizontal momentum diffusion with an anisotropic version of the so-called Dynamic Smagorinsky Model (DSM). This scheme takes into consideration the hydrodynamic conservation laws and is also fully consistent with scale invariance.

Here we present a detailed spectral budget analyses for APE and KE for 9 Days January simulations with smooth orography and no latent-heat forcing for small resolved scales. Contributions to energy spectra and budgets from different terms in governing equations would be used to check which modes of motion are dominating the mesoscales. Spectral fluxes are further used to support provided explanation for conservative IGW dynamics.

Deschner, St.:
High Resolution Turbulence Measurements

Sea & Sun Technology develops and distributes Microstructure Probes (MSS) in coopeoration with ISW and IOW. The probes themselves are made of Titan accounting for robustness and reliability. The continuously growing scope of applications yields a wide range of different types of probes. In this overview we point out some highlights and new features of the MSS. We show two examples of tough measurement surroundings and a useful method gathering long time-range data. These examples nicely demonstrate the comfortable manageability of the MSS and hence the route to excellent scientific knowledge production.

Eden C., Chouksey M., Olbers D.:
Gravity wave emission from balanced flow by shear instability

Gravity wave emission by geostrophically balanced flow is diagnosed in numerical simulations of lateral and vertical shear instabilities. The diagnostic method in use allows for a separation of balanced flow and residual wave signal up to fourth order in the Rossby number Ro.
While evidence is found for a small but finite gravity wave emission from balanced flow in a single layer model with large lateral shear and large Ro,  a vertically resolved model with moderate velocity amplitudes appropriate
to the interior ocean hardly shows any wave emission.
Only when static instabilities generated by the shear instability of the balanced flow are allowed, a gravity wave signal similar to the ones reported in earlier studies  can be detected in the vertically resolved case.
This result suggests a relatively small role of spontaneous wave emission in the classical sense of Lighthill radiation, and emphasizes the role of convective or symmetric instabilities during frontogenesis for the generation of internal gravity waves in the ocean and atmosphere.

Eriksen, Thomas, Eden, Carsten, Olbers, Dirk:
Diapycnal diffusivity induced by the breaking of lee waves

A key component in setting the large scale ocean circulation is the process of diapycnal mixing, since this provides energy required to increase the potential energy of the ocean and thereby to close the meridional overturning circulation. Diapycnal mixing in the interior ocean is most commonly associated with the breaking of internal waves. Traditionally diapycnal mixing has been represented in ocean models by a diapycnal diffusivity either constant or exponentially decreasing with depth. This approach, however, does not take into account the actual physics behind the breaking of internal waves. The energetically consistent internal wave model IDEMIX (Internal wave Dissipation, Energetics and MIXing), on the other hand, computes diffusivity rates directly on the basis of internal wave energetics. One such type of internal wave is leewaves. These are generated and subsequently dissipated when geostrophic currents interact with bottom topography and are therefore believed to be a source of energy for deep ocean mixing. Furthermore, lee waves extract from or provide momentum to the mean flow through wave drag. The amount of energy contained in lee waves are largely dependent on the strength of the bottom flow and on the roughness of the topography. This amount is believed to be especially high in the Southern Ocean due to exactly these factors. In IDEMIX the energy flux into lee waves is calculated seperately and can be used as a bottom forcing term in numerical simulations of ocean models. As such this setup allows for a study of the direct impact on diffusivity due to breaking of lee waves. 

Klingbeil, K.:
New strategies to deal with spurious numerical mixing in ocean models

Despite the progress in new and advanced turbulence parameterizations, obtaining a realistic level of mixing in model simulations is still a challenge. Spurious numerical mixing can easily dominate the physical mixing from well-calibrated parameterizations. In this talk new strategies to diagnose and reduce numerical mixing in ocean models are presented.

Lange, X, Klingbeil, K and Burchard, H:
Inversions of estuarine circulation are frequent in a weakly tidal estuary with variable wind forcing and seaward salinity fluctuations

The hydrodynamics in estuaries is mainly governed by the competition between a horizontal density gradient, friction and wind stress. The sensitivity of the estuarine exchange flow to the wind stress increases in the absence of tides, which is investigated here using the example of the weakly tidal Warnow river estuary in the southwestern Baltic Sea - the mouth of which is characterized by strongly varying salinities of 8 to 20 g/kg. The interaction between a volatile salinity gradient and along-estuary wind forcing is found to cause temporary inversions of the estuarine circulation. Despite the highly dynamic conditions, the applicability of recent theories for isohaline mixing, using the framework of Total Exchange Flow, and the strength of the exchange flow, using a non-dimensional parameter space, could be confirmed. By analyzing salinity fluxes at the mouth of the estuary a mixing completeness of 84\% was calculated for the estuary. Furthermore, inversion of estuarine circulation was typically found for a local Wedderburn number (ratio of non-dimensional wind stress to non-dimensional horizontal density gradient) exceeding 0.33, indicating a high sensitivity to along-estuary wind.

Muchowski, J., Umlauf, L., Arneborg, L., Weidner, E.F., Holtermann, P.L., Peng, J.-P., Humborg, C., Stranne, C.:
Comparison of turbulence measurements from broadband echo sounder to microstructure profiles of stratified flow over steep seafloor topography in the Baltic Sea

Turbulence caused by the interaction of stratified flow with rough seafloor topography is often underestimated by in-situ measurements and models due to their limited spatial resolution. Here we use remote acoustic methods to improve our understanding of the spatial and temporal variations of turbulence and mixing in such flows.
We present a dataset of vigorously turbulent flow in the Sea of Åland in 2019, where we repeatedly measured a 2 km long transect across a steep sill that connects the central and northern parts of the Baltic Sea. We collected 150 microstructure profiles, serving as ground truth, and compared them to continuously acquired acoustic observations with a calibrated, state-of-the-art, broadband echo sounder (Simrad EK80). The echo sounder transmits a chirped pulse from 45-90 kHz with a ping rate of 2-5s, leading to a vertical/horizontal resolution of 0.1/1 m. With these data we were able to visualize the detailed structure of a highly energetic stratified overflow over the sill. The echograms show vertical overturns on the lee side of the sill with diameters of up to 100 m, lee waves, and Kelvin-Helmholtz instabilities. Preliminary results show good qualitative agreements between the acoustic observations of turbulence and the dissipation rates from the in-situ microstructure data, being among the highest ever observed in the deep layers of the Baltic Sea.

Pini, A., Zazzini,S., Monti, P., Leuzzi, G.:
Dispersion of microplastics in the Western Mediterranean Sea

Recently, the pollution of the marine environment has become a serious problem, so it is receiving significant interest by the scientific community. Plastics are of particular interest among the contaminants that may be dispersed in the sea, due to the progressive increase in their production. More specifically, microplastics (MP), i.e. debris with dimension smaller than 5 mm, were extensively studied in the last decade as their biological effects as well as the removal techniques are still under investigation. In particular, primary microplastics, the fraction that directly enter the oceans from land-based activities, are estimated to be around 15-30 percent of all plastic at sea. As primary MP are released from household and industrial products through unfiltered sewage systems, their production is strictly related to the population density. Furthermore, their presence has revealed to be ubiquitous, as from recent measurement campaigns, and they are found very far from the sources. Hence, it is fundamental to understand the phenomena involved in MP transport, dispersion and accumulation in marine environment as well as to estimate the amount of MP which enters the oceans from different sources.

In this work, the pathways of microplastic debris in the Western Mediterranean Sea were simulated by means of a Lagrangian Stochastic Model (LSM) developed by the authors. Long-term (7-year) simulations were conducted releasing more than 4·106 particles per year and using the hydrodynamic fields provided by the Copernicus Marine Environment Monitoring Service (CMEMS).

A novel method to estimate the amount of primary microplastic discharged in the sea waters was carried out. This method identifies different kind of sources assigning an MP load proportional to the population density. In detail, rivers and coastal cities of the European countries facing the Western Mediterranean Sea, i.e. Italy, France and Spain, were considered as input of MP (214 sources in total).

This modelling analysis allows to determine high polluted and critical areas. Moreover, it was found that MP dispersion is highly influenced by the hydrodynamic characteristics of the basin. MP simulated concentrations were compared with measured values derived from different sampling campaigns realized in the same basin. Numerical and experimental values show a good agreement for different sampling locations.

Ramachandran,K. Gassmann,A.:
Horizontal Kinetic Energy spectrum and Spectral Fluxes Simulated by Non-Hydrostatic ICON-IAP

Recent studies indicate that the dynamics of mesoscales in free troposphere can be described by the concept of stratified macro-turbulence (SMT). The occurrence of forward cascade of kinetic energy and available potential energy where the power spectra obeys a -5/3 spectral slope with regards to horizontal wavenumber and an equivalent -3 slope in vertical wavenumber has been discussed by many. In this study the horizontal kinetic energy spectra and the spectral fluxes simulated by a new non-hydrostatic model (ICON-IAP: Icosahedral Non-hydrostatic model at Institute for Atmospheric Physics) are examined.  The kinetic energy spectra displays a downscale cascade as is consistent with the established theories. The processes that contribute to the spectral budget is made visible by transforming the governing equations wherein we remove all the terms which do not make any contributions. Using this transformed equations, the individual contribution to the spectral budgets due to various process such as horizontal advection, adiabatic conversion and vertical advection is studied.

The spectral budgets at two different pressure levels (220hPa and 518hPa) are analysed. The kinetic energy spectra calculated from the transformed and untransformed equations are compared. Differences between the rotational and divergent components of the spectra is smallest in the smallest scales where the contribution of both the components are roughly the same. The spectrum is dominated by the rotational component at all other scales. The results suggest that by transforming the equations, a better understanding of the momentum transport at different scales and the applicability of the Stratified Macro-Turbulence to explain energy cascades is obtained.

Umlauf, L., P. Holtermann, C. A. Gillner, R. Prien, L. Merckelbach, and J. R. Carpenter:
Diffusive convection under rapidly varying conditions

In most observations of diffusive convection in the ocean and in lakes, the characteristic diffusive staircases evolve over long time scales under quasi-stationary background conditions. In the Baltic Sea, however, diffusive staircases develop inside the flanks of intermittent intrusions that induce strong inverse temperature gradients over a vertical range of a few meters, varying on time scales of hours to days. Here, results are discussed from an extensive field campaign conducted in summer 2016 in the southern Baltic Sea, including temperature microstructure data from ocean gliders and an autonomous profiling platform. We find conditions favorable for diffusive instability in the vicinity of warm and cold intrusions with density ratios as small as Rρ = 1.3. The staircases evolving under these conditions are characterized by a small number of steps (typically 1–4) with order 0.1–1-m thickness, temperature differences exceeding 1 K across individual diffusive interfaces, and exceptionally large diffusive heat fluxes of order 10 W m−2. The standard heat flux parameterization of Kelley agrees within a factor of 2 with the directly observed interfacial heat fluxes, except for large fluxes at low Rρ, which are strongly overestimated. The glider surveys reveal a surprisingly small lateral coherency of order 100 m of the staircase patterns, and a spreading of the diffusively unstable intrusions across isopycnals.

van der Kaaden, A., Gerkema, T., Mohn, C., Soetaert, K., Rietkerk, M., van de Koppel, J., van Oevelen, D.:
The interplay between internal tides and coral mound growth

Mixing can be induced over mounds on the seafloor by breaking internal tides, stimulating transport of nutritious surface water towards the marine desert that is the deep-sea. Seafloor topography is generally assumed to be static on tidal time-scales, but on millennial time-scales cold-water corals can grow up to 400 m high mounds on the seafloor. As these coral mounds grow, the hydrodynamic processes around them change, which may affect mound formation because the corals rely on mixing for the delivery of fresh food particles from the ocean surface. We investigated this complex interplay between physical and biological processes using 3D hydrodynamic model simulations using Roms-Agrif (CROCO). To investigate the hydrodynamics-topography interaction during mound growth, we manipulated the bathymetry of a large mound from the south-eastern flank of the Rockall Bank (NE Atlantic).

The strength of internal tidal generation was expressed as the rate at which energy is converted from barotropic to baroclinic tides. We assume that this conversion rate is also indicative of the amount of mixing due to internal tides. At the mound top, the conversion rate decreased quickly towards zero with increasing mound height. Whereas at the flanks, conversion rates increased with increasing mound height until an optimum and then declined. Suggesting there is an optimum mound height for internal wave generation. Assuming that internal tides are the main process responsible for food delivery towards the corals, for all but the smallest mounds, flanks are expected to be a more favourable place than the mound top for reef growth. For coral mound formation this means the mounds may stabilise at a certain height where mixing is maximum.

Wünsche A., Maushake C.:
Investigating the Ems Estuary dynamics

The Ems River represents a tidal asymmetric river which is strongly characterized by density differences driven by salinity and sediments and a pronounced occurrence of fluid mud. The natural river has been altered due to multiple operations such as dredg-ing operations to preserve an economically fairway depth. The amount of sediments within the Ems River plays nowadays a key role. In fact, the Lower Ems River (river sec-tion between Herbrum and Pogum) is described by a very high concentration of sus-pended particulate matter (SPM), so called hyperturbid.
Observational data recorded since the 1950s show that the Lower Ems River and the Ems Estuary are becoming increasingly turbid, especially within the last 30 years (Jonge et al. 2014). In general, the SPM concentration in the Lower Ems River occurs to be high-er, on average by a factor of 1000, than in the Outer Ems (river section between Pogum and Borkum).
The reduction of the turbidity of the River Ems is of ecological and of economic interest. It requires enhanced knowledge of estuarine hydrodynamics, sediment dynamics and, furthermore, their interaction. For instance, the understanding of the mechanisms of the up-estuary sediment transport forms one of the main research questions.
The international campaign Ems Dollard Measurements (EDoM) aims to advance this knowledge by sampling data of comparable high temporal and spatial resolution within the River Ems. Within the scope of this campaign diverse measurement strategies have been carried out in August 2018 and in January 2019. The measurements span a time period of 3 days, each, and are embedded into a short-term monitoring of approximately 2 weeks. The study area is located between Pogum and Delfzjil and is subdivided to rep-resent five regions of different dynamical processes. Data covers primarily ship-board ADCP, bottom mounted ADCP, turbidity sensors, a MicroCTDturbulence sensor, multipa-rameter mooring chains and permanent monitoring provided by governmental insti-tutes.
Moreover, results from the analysed observational data can be used to provide a better representation of sediment-water interaction within model studies.
Drafts of preliminary results and possible future analyses using the EDoM measure-ments in combination of long-term measurements will be presented and discussed at the workshop.

Jonge, Victor N. de; Schuttelaars, Henk M.; van Beusekom, Justus E.E.; Talke, Stefan A.; Swart, Huib E. de (2014): The
influence of channel deepening on estuarine turbidity levels and dynamics, as exemplified by the Ems estuary. In:
Estuarine, Coastal and Shelf Science 139, S. 46–59. DOI: 10.1016/j.ecss.2013.12.030.