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The Bosphorus, located at the junction of Asia and Europe, controls the transports of water, material, and energy between the Black Sea (BS) and the Mediterranean Sea. The Canal Istanbul (CI), planned by Republic of Turkey (TR), is a gigantic project which bisects the current European side of Istanbul, thus forming a secondary waterway parallel to the existing natural channel of the Bosphorus. The environmental impact of this project has been a major concern since the very beginning. To address this concern, a 3D hydrodynamic modeling study is carried out to investigate the impact of CI on the hydrodynamics and salinity of the northern Sea of Marmara (SM) by using the route and cross-section (CS) of canal, which was recently made public by the TR. An extensive set of model calibration is performed in a model where only the Bosphorus Strait links the Black Sea and Marmara Sea. Once the calibration process is completed, the future canal is added into the grid and new simulations are performed to analyze the influence of CI on hydrodynamics and salinities in the SM. It was found that the shallow water depth of CI is the limiting factor constraining the two-layer flow. Thus the vertical current structure of CI appears mostly as one-layer flow from the BS to the SM. One-layer flow from the SM toward the BS is rarely observed. In such cases, strong northerly storms caused water level setup at the SM and set down at the BS. Adding the CI in the model is responsible for a salinity decrease of ~1 ppt in the surface layer of SM. The salinity difference between the two models decreases gradually with the depth and it diminishes at the depth of 25 m.

The Semi-implicit Cross-scale Hydroscience Integrated System Model (SCHISM), which uses unstructured grids, is set up for the North and Baltic Seas. With a resolution of ∼100 m in the narrow straits connecting the two basins, this model accurately resolves the inter-basin exchange. Validation against observations in the straits shows the model has good skill in simulating the transport and vertical profiles of temperature, salinity and currents. The timing and magnitude of the major inflow event in 2014–2015 is also realistically simulated. The analysis is focused on the two-layer exchange, its dependence on the atmospheric forcing, and dominant physical balances. The two-layer flows in the three connecting straits show different dependencies upon the net transport. The spatial variability of this dependence is also quite pronounced. The three-strait system developed specific dynamics, with time lags and differences between currents in the individual straits during inflow and outflow conditions. Analysis on the impact of resolution indicates that the performance of the model changes depending on whether the narrow parts of the straits are resolved with a resolution of 500 m or 100 m. With this ultra-fine resolution, gravity flows and variability of salinity in deep layers is generally more adequately simulated. This paper identifies the needs for more profound analysis of the coupled dynamics of Baltic and North Seas with a focus on the Danish straits.

The hydrodynamic response to morphodynamic variability in the coastal German Bight was analyzed via numerical experiments using time-referenced bathymetric data for the period 1982-2012. To this aim, time slice experiments were conducted for each year with the Semi-implicit Cross-scale Hydroscience Integrated System model (SCHISM). This is an unstructured grid model, which allows to resolve small-scale bathymetric features in the coastal zone, which are also resolved in the time-referenced bathymetric data with their fine horizontal resolution of 50\,m. The analysis of bathymetric data reveals continuous evolution of small-scale bathymetric features and, e.g., the migration of tidal channels and rather complex change of the depths of tidal flats in different periods. The almost linear relationship between the cross-sectional inlet areas and the tidal prisms of the intertidal basins in the East Frisian Wadden Sea demonstrates that these bathymetric data describe a consistent morphodynamic evolutionary trend. The results of numerical experiments are streamlined to explain the changes of hydrodynamics from 1982 to 2012. Although these changes were located mostly in a relatively small part of the model area, they resulted in substantial changes (exceeding 5\,cm) in the amplitudes of M2 tides. The hydrodynamic response to bathymetric changes exceeded largely the response to sea-level change. The tidal asymmetry appeared very sensitive to bathymetric changes, particularly between the southern tip of Sylt island and the Eider Estuary along the eastern coast. The peak current asymmetry weakened from 1982 to 1995 and even reversed in some of the tidal basins to become flood-dominant. This would suggest that the flushing trend in the 1980s was reduced or inverted in the second half of the period of bathymetric observations. Salinity also appeared sensitive to bathymetric changes; the deviations in the individual years reached ~2 psu in the tidal channels and tidal flats. One practical conclusion from the present numerical simulations is that wherever possible, the numerical modeling of near-coastal zones must employ time-referenced bathymetry.

More than 6,000 profiles from profiling floats in the Black Sea over the 2005–2020 period were used to study the ventilation of this basin and the mixing pathways along isopycnals. The layer of the minimum potential vorticity (PV), the Black Sea pycnostad, approximately follows the core of the cold intermediate layer, similar to the case of oceanic mode waters. However, unlike in the ocean, the horizontal patterns of PV are shaped by cyclonic gyre circulation. There is a principle difference in the probability distribution of the thermohaline properties presented in geopotential coordinates from those presented in density coordinates. In the latter case, several mixing pathways, which are not known from previous studies, dominate the ocean states. These formed after three intermittent events of cold water formation. The density ratio decreased three times during the last 15 years, revealing the decreasing role of temperature in the vertical layering of the Black Sea halocline. The basin‐wide distribution of PV above σθ = 16, which is where the maximum vertical density gradient appears, is opposite to the distribution below this depth. This finding suggests a complex change in the mesoscale dynamics in different layers. Comparisons of observations with data from the Copernicus Black Sea operational model demonstrate that the mixing parameterizations of models need further improvements., Journal of Geophysical Research: Oceans, 126 (2), ISSN:0148-0227, ISSN:2169-9275

Here, we revisit the existing concepts of the vertical structure of deep layers in the Black Sea using data from sensors deployed on profiling floats. The deep transition layer (DTL) between 700 and 1,700 m acts as an interface between the baroclinic layer and the largest bottom convective layer (BCL) of the world oceans. On top of DTL are the warm intermediate layer and deep cold intermediate layer. They both showed strong trends in the last 15 years due to warmer climate and intensification of warmer intrusions from Bosporus. A “salinity wave” was detected in 2005–2009 below ∼1,700 m, which evidenced for the first time the penetration of gravity flow from Bosporus down to the bottom. The layering of water masses was explained as resulting from the different distribution of sources of heat and salt, double diffusion, and balances between the geothermal and salinity flows in the BCL. © 2020. The Authors., Geophysical Research Letters, 48 (2), ISSN:0094-8276, ISSN:1944-8007

We present an application of generative adversarial networks (GANs) to reconstruct the sea level of the North Sea using a limited amount of data from tidal gauges (TGs). The application of this technique, which learns how to generate datasets with the same statistics as the training set, is explained in detail to ensure that interested scientists can implement it in similar or different oceanographic cases. Training is performed for all of 2016, and the model is validated on data from 3 months in 2017 and compared against reconstructions using the Kalman filter approach. Tests with datasets generated by an operational model (“true data”) demonstrated that using data from only 19 locations where TGs permanently operate is sufficient to generate an adequate reconstruction of the sea surface height (SSH) in the entire North Sea. The machine learning approach appeared successful when learning from different sources, which enabled us to feed the network with real observations from TGs and produce high‐quality reconstructions of the basin‐wide SSH. Individual reconstruction experiments using different combinations of training and target data during the training and validation process demonstrated similarities with data assimilation when errors in the data and model were not handled appropriately. The proposed method demonstrated good skill when analyzing both the full signal and the low‐frequency variability only. It was demonstrated that GANs are also skillful at learning and replicating processes with multiple time scales. The different skills in different areas of the North Sea are explained by the different signal‐to‐noise ratios associated with differences in regional dynamics., Plain Language Summary: The variability of sea level is one of the most important elements of the ocean dynamics. Basin‐wide observations are due to satellite altimeters, observations in coastal stations are provided by tidal gauges. The first are not very accurate in the coastal areas, the second do not provide basin‐wide coverage. The task in the present work is to use machine learning to reconstruct the sea‐level variability in the North Sea, which is an almost enclosed ocean region, using observations only. Using data from 19 coastal stations and data from numerical models as a representation of the true ocean (synthetic observations), we demonstrated that the generative adversarial networks reconstruct almost perfectly the sea level of the North Sea. The application of this technique, which learns how to generate datasets with the same statistics as the training set, is explained in detail to ensure that interested scientists can implement it in similar or different oceanographic cases., Key Points: Generative Adversarial Networks successfully reconstruct basin‐wide sea level in the North Sea using data from tidal gauges. Machine learning appeared successful when learning from different data sources. The proposed method is skillful at learning and replicating processes with multiple time scales.

We examine the relative dispersion and the contribution of tides on the relative diffusivities of surface drifters in the North Sea. The drifters are released in two clusters, yielding 43 pairs, in the vicinity of a tidal mixing front in the German Bight, which is located in the southeastern area of the North Sea. Both clusters indicate decreasing dispersion when crossing the tidal mixing front, followed by exponentially increasing dispersion with e-folding times of 0.5 days for Cluster 1 and 0.3 days for Cluster 2. A transition of the dispersion regimes is observed at scales of the order of the Rossby radius of deformation (10 km). After that, the relative dispersion grows with a power-law dependency with a short period of ballistic dispersion (quadratic growth), followed by a Richardson regime (cubic growth) in the final phase. Scale-dependent metrics such as the relative diffusivities are consistent with these findings, while the analysis of the finite-scale Lyapunov exponents (FSLEs) shows contradictory results for the submesoscales. In summary, the analysis of various statistical Lagrangian metrics suggests that tracer stirring at the submesoscales is nonlocal and becomes local at separation scales larger than 10 km. The analysis of meridional and zonal dispersion components indicates anisotropic dispersion at the submesoscales, which changes into isotropic dispersion on the mesoscales. Spectral analysis of the relative diffusivity gives evidence that semidiurnal and shallow-water tides influence relative diffusivity at the mesoscales, especially for drifter separations above 50 km.

In this paper, we use the unstructured grid model SCHISM to simulate the thermohydrodynamics in a chain of baroclinic, interconnected basins. The model shows a good skill in simulating the horizontal circulation and vertical profiles of temperature, salinity, and currents. The magnitude and phases of the seasonal changes of circulation are consistent with earlier observations. Among the mesoscale and subbasin-scale circulation features that are realistically simulated are the anticyclonic coastal eddies, the Sebastopol and Batumi eddies, the Marmara Sea outflow around the southern coast of the Limnos Island, and the pathway of the cold water originating from the shelf. The superiority of the simulations compared to earlier numerical studies is demonstrated with the example of model capabilities to resolve the strait dynamics, gravity currents originating from the straits, high-salinity bottom layer on the shallow shelf, as well as the multiple intrusions from the Bosporus Strait down to 700 m depth. The warm temperature intrusions from the strait produce the warm water mass in the intermediate layers of the Black Sea. One novel result is that the seasonal intensification of circulation affects the interbasin exchange, thus allowing us to formulate the concept of circulation-controlled interbasin exchange. To the best of our knowledge, the present numerical simulations, for the first time, suggest that the sea level in the interior part of the Black Sea can be lower than the sea level in the Marmara Sea and even in some parts of the Aegean Sea. The comparison with observations shows that the timings and magnitude of exchange flows are also realistically simulated, along with the blocking events. The short-term variability of the strait transports is largely controlled by the anomalies of wind. The simulations demonstrate the crucial role of the narrow and shallow strait of Bosporus in separating the two pairs of basins: Aegean-Marmara Seas from one side and Azov-Black Seas from the other side. The straits of Kerch and Dardanelles provide sufficient interbasin connectivity that prevents large phase lags of the sea levels in the neighboring basins. The two-layer flows in the three straits considered here show different dependencies upon the net transport, and the spatial variability of this dependence is also quite pronounced. We show that the blocking of the surface flow can occur at different net transports, thus casting doubt on a previous approach of using simple relationships to prescribe (steady) outflow and inflow. Specific attention is paid to the role of synoptic atmospheric forcing for the basin-wide circulation and redistribution of mass in the Black Sea. An important controlling process is the propagation of coastal waves. One major conclusion from this research is that modeling the individual basins separately could result in large inaccuracies because of the critical importance of the cascading character of these interconnected basins.

The interplay between wind waves and currents in the coastal zone of the southern North Sea along with the resulting changes in the salinity distribution are quantified using simulations with the unstructured-grid ocean model SCHISM coupled with the wind wave model WWM III. Several sensitivity runs, which are carried out to estimate the individual contributions of different physical mechanisms and forcing, demonstrated that the density gradients in the coastal zone reduce tidal current by 18%, whereas the wind waves enhance the circulation in some cases. The latter happens when along-shore wind speed approaches ∼10 m s − 1 resulting in long-shore currents following the western Dutch coast and the German Wadden Sea islands. The wave-induced transport of salt leads to changes in the horizontal salinity distribution. These are most pronounced in front of barrier islands where coherent patterns caused by the coupling between tides, surface drift, and wind waves reveal salinity changes up to 0.5. The weak stratification of salinity in the coastal zone is mostly destroyed by wind waves. Thus, effects created by wind waves tend to substantially modify the estuarine circulation. An explanation of these important processes in the coastal zone has been given based on an analysis of the ratio between significant wave height and tidal range. This control-parameter, which is relatively small under mild weather conditions, can exceed unity under strong wind conditions in the coastal zone, thus mixing due to waves becomes dominant. The effect of fresh water fluxes from subterranean estuaries is relatively small and confined only in the vicinity of corresponding sources.

Observational evidence of turbulent wakes behind wind parks’ piles motivated a series of numerical experiments, aiming to identify the dynamic regimes associated with wakes’ generation in tidal basins. We demonstrate that the obstacles such as piles of wind parks give rise to vortices similar to the known Karman vortices which affect substantially the turbulent kinetic energy. The latter can be considered as the agent enhancing sediment remobilization from the ocean bottom, thus making wakes well visible in satellite data. The temporal and spatial variability of studied processes is analyzed under stationary and nonstationary conditions. The dependence of a vortex generation and evolution upon the environmental conditions is also studied, which demonstrates a large variety of appearances of turbulent wakes. The comparison between simulations using a suspended sediment model and satellite images demonstrated that the model is capable to realistically simulate sediment wakes observed in remote sensing data.

Observations using two kinds of drifters were carried out in the southern North Sea aiming to study the propagation pathways of marine litter. One drifter, which was driven by the upper layer currents, was equipped with Global Positioning System. Further 1600 wooden drifters, mostly driven by wind and Stokes drift, were released offshore in German waters. The detailed reports of stranded wooden drifters from members of the public, the majority of which are likely to be non-scientists, provided a valuable contribution to the drifter experiment demonstrating the usefulness of citizen science. In 2018, an extreme wind event reversed the circulation of North Sea for more than a month which resulted in a large number of wooden drifters being washed ashore on the British coast. Lagrangian numerical experiments, calibrated using data from the drifter observations, helped explain the anomalous transport and the reversal of the circulation at the sea surface and in deeper layers. The plausibility of similar events during past decades has also been estimated using data from atmospheric analyses. Events as strong as the one observed in 2018 occurred only four times in the last 40 years.

The Semi-implicit Cross-scale Hydroscience Integrated System Model (SCHISM), which is an unstructured-grid model, was coupled with a 3D sediment model and established for the German Bight and its estuaries. The horizontal resolution of the model ranges from ~10 m in the estuaries to ~400 m in most of the open sea. Validation against tidal gauge data, fixed station data, and FerryBox data demonstrated that the model adequately simulated tidal dynamics in the entire area as well as the salinity fronts and estuarine turbidity maxima in the Ems, Weser and Elbe estuaries. Comparisons of model output with that of experiments with constant density allowed to estimate the role of density control, which appeared to be strongest in the landward reaches of salinity front. The increase of tidal range caused by density effects was most prominent in the Weser and Elbe estuaries, and relatively small in the Ems, where the runoff was also small. The magnitude of the density effect on the sea level oscillations was comparable to that of the M4 tide amplitude, demonstrating its importance in shaping the tidal asymmetry in the estuaries. Density effects not only reduced the dissipation of tidal energy; density stratification suppressed also the resuspension of sediment and contributed to a displacement of the position of the estuarine turbidity maximum upstream compared to the case of an homogeneous estuary. In the case of hyper-turbid Ems Estuary, density effects on stratification caused by high sediment concentrations resulted in a suppression of turbulence and further increase of concentration of suspended matter at the bottom. Although the three estuaries, which are only ~100 km apart, were driven by similar tidal and atmospheric forcings, they exhibited different extensions of both fronts and vertical stratification, mainly due to different river runoff conditions. At intra-tidal time scales, the dependencies between sea surface height and sea surface salinity varied considerably from estuary to estuary, and the largest flood asymmetry appeared in the Elbe Estuary. Wind acted as the dominant factor driving the longer-term estuarine variability; the correlation between zonal wind magnitude and sea surface height appeared to be very strong. The simulated suspended particulate matter dynamics and position of the estuarine turbidity maxima (ETM) were in agreement with observations. Secondary ETM appeared at different locations depending on the grain size, providing an illustration of sediment sorting.

The accumulation patterns of floating marine litter (FML) in the Black Sea and the stranding locations on coasts are studied by performing dedicated Lagrangian simulations using freely available ocean current and Stokes drift data from operational models. The low FML concentrations in the eastern and northern areas and the high concentrations along the western and southern coasts are due to the dominant northerlies and resulting Ekman and Stokes drift. No pronounced FML accumulation zones resembling the Great Pacific Garbage Patch are observed at time scales from months to a year. The ratio of circulation intensity (measured by the sea level slope) to the rate of the temporal variability of sea level determines whether FML will compact. This ratio is low in the Black Sea, which is prohibitive for FML accumulation. It is demonstrated that the strong temporal variability of the velocity field (ageostrophic motion) acts as a mixing mechanism that opposes another ageostrophic constituent of the velocity field (spatial variability in sea level slope, or frontogenesis), the latter promoting the accumulation of particles. The conclusion is that not all ageostrophic ocean processes lead to clustering. The short characteristic stranding time of ∼20 days in this small and almost enclosed basin explains the large variability in the total amount of FML and the low FML concentration in the open ocean. The predominant stranding areas are determined by the cyclonic general circulation. The simulated distribution of stranded objects is supported by available coastal and near-coastal observations. It is shown that the areas that were the most at risk extend from the Kerch Strait to the western coast.

The upscaling problem is investigated using the barotropic dynamics of the North Sea and the German Bight as an example. The impact of small scale perturbations of bathymetry, bottom roughness, wind forcing, and boundary forcing is quantified using a two-dimensional linear barotropic model for the entire North Sea with 5 km resolution. The model is solved in the spectral domain for the dominant M2 tide. Comparisons with results from a fully nonlinear 3D circulation model show that the main circulation features are well captured by the spectral model. The impact of different types of perturbations is estimated by inversion of the model using the perturbation covariance matrix as input. Case studies with white noise and fully correlated noise are presented. It is shown that the German Bight area stands out in its sensitivity with respect to small scale uncertainties of bathymetry. Small scale changes of bottom roughness have a particularly strong effect in the English Channel. Small scale wind perturbations have a significant local effect only in very shallow near coastal areas. It is shown that uncorrelated noise introduced along an open boundary around the German Bight only has a very local effect. Perturbations with long correlation length are shown to lead to significant far field effects along the east coast of England. It is demonstrated that this effect is related to the boundary conditions used for the North Sea model. In a next step a German Bight grid with 1 km resolution is nested into the North Sea grid and the spectral model is solved in a two way nested configuration. It is shown that there are some significant local and far field effects caused by the change of resolution in this coastal area. Finally, the potential impact of observations taken in coastal areas is investigated by evaluating the Kalman a posteriori distribution of analysis vectors based on different assumptions about model errors. The area of influence of a single tide gauge is quantified for the case where the model errors are dominated by boundary forcing errors. The results show a strong dependence on spatial correlation properties of the errors.

The response of the tidal system in the southern North Sea to morphodynamic changes was investigated in a modelling study using fine resolution bathymetric observations available for 1982–2011. The Semi-implicit Cross-scale Hydroscience Integrated System Model (SCHISM) was set up for the different sets of bathymetries. One set of bathymetry was compiled from a large number of bathymetric measurements over many years, while the other two reflected bathymetry state in the area of Wadden Sea during 2000 and 2011, respectively. The temporal and spatial evolution of bathymetry was dominated by migration of tidal channels. The M4 tide showed larger sensitivity to bathymetric change in the Wadden Sea than the M2 tide, whereas the structure of the latter remained rather robust. The largest change of the tidal wave due to the differences in bathymetries was located off the North Frisian Wadden Sea. Traces of changes were also found far away from the regions of their origin because the tidal waves in the North Sea propagate the local disturbances basin-wide. This illustrated an efficient physical mechanism of teleconnectivity, i.e. effecting the local responses to the larger-scale or remote change of ocean bottom caused by erosion and deposition. The tidal distortion resulting from the relatively small bathymetric changes was substantial, particularly in the coastal zone. This is a manifestation of the nonlinear tidal transformation in shallow oceans and is crucial for the sediment transport and the morphodynamic feedback, because of the altered tidal asymmetry.

A new unstructured-grid model and its application to the North Sea and Baltic Sea are described. The research focus is on the dynamics in the two basins and in the multiple straits connecting them and more specifically on how the model replicates the temporal and spatial variability of physical processes. The comparison against observed data indicates the realism in the simulations of the exchange flows. The simulations demonstrated that in contrast to the tidal variability which decreases in the strait, the role of the barotropic forcing due to weather systems increases. In this zone reversal of transport is well manifested by the increased difference between the surface and bottom salinity values. Small sub-basins like Arkona and Bornholm play the role of reservoirs for denser water which under specific conditions cascades on its way to the Gotland Deep. Unlike the intermediate and deep water salinity in the Baltic Sea, which is strongly affected by fluxes in the straits, the simulated winter-refill and evolution of cold intermediate water are rather driven by surface cooling and processes in the upper mixed layer.

In this paper, we explore the secondary flows in the Danish Straits using observations and numerical simulations performed with the unstructured-grid hydrodynamic model SCHISM covering the North Sea and Baltic Sea. The straits are resolved on scales of up to ∼ 100 m. Given that large-scale atmospheric variability dominates the transport in these straits, we focus on the processes with subtidal time scales. Similarities and differences between the in- and outflows in the straits and flood and ebb flows in estuaries are analyzed. Contrary to the tidal straining in estuaries, the Danish Straits feature substantial differences in the stratification stability during the outflow and inflow phases. With a resolution of ∼ 100 m, new transport and mixing pathways that were previously unresolved appear fundamental to the strait dynamics. The variety of the strait morphology leads to high variability in the appearance of secondary circulation. Helical cells, often with a horizontal extension of ∼ 1 km, develop in the deep parts of the channels. A comparison between the high-resolution simulation and a simulation with a coarse grid of ∼ 500 m in the straits suggests that the coarser resolution overestimates the stratification and misrepresents the transport balance; the axial velocities and transport through the Sound are underestimated by ∼ 12%. These differences are explained by the missing secondary circulation when the coarse resolution is used (approximately two grid-points per cell instead of ten grid-points per cell in the fine resolution model), along with the resulting changes in mixing along the straits. In conclusion, the use of ultrafine resolution grids is essential to adequately resolve secondary flow patterns and two-layer exchange. Thus, the problems caused by the failure to resolve the secondary circulation in straits appear similar to the problems caused by the failure to resolve mesoscale eddies in ocean models.

Observations of underway velocity profiles during complete spring and neap tidal cycles were used to determine whether the spatial structures of tidal and subtidal flows at a tidal inlet in a multiple-inlet embayment are consistent with those observed at single-inlet embayments. Measurements were obtained at the Otzumer Balje, one of the multiple inlets among the East Frisian Islands of the Wadden Sea. The 1.5 km-wide inlet displayed a bathymetric profile consisting of a channel ∼15 m deep flanked by

The dynamics in the transition zone between the North Sea and Baltic Sea are analyzed here using data from a 22-year-long climatic simulation with a focus on the periods 1992–1994 and 2001–2003 when two recent major inflow events occurred. Observations from gauges and in situ measurements are used to validate the model. Parameters, which cannot be easily measured, such as water and salt transports through straits, have been compared against similar previous estimates. The good performance of simulations is attributed to the finer resolution of the model compared to earlier set ups. The outflow in the Kattegat, which is an analogue of the tidal outflows, tends to propagate to the North over the shallows without showing a substantial deflection to the right due to the Earth's rotation. The inflow follows the topography. The different inflow and outflow pathways are explained as a consequence of the specific combination of bathymetry, axial and lateral processes. The circulation in Kattegat is persistently clockwise with an eastern intensification during inflow and a western one during outflow regimes. The tidal wave there propagates as Kelvin wave, keeping the coast on its right. The flows in the two main straits reveal very different responses to tides, which are also highly asymmetric during inflow and outflow conditions. The circulation has a typical two-layer structure, the correlation between salinity and velocity tends to increase the salt transport in the salinity conveyor belt. The transversal circulation in the entrance of the Sound enhances the vertical mixing of the saltier North Sea water. The long-term averaged ratio of the water transports through the Great Belt and the Sound is ∼2.6-2.7 but this number changes reaching lower values during the major inflow in 1993. The transports in the straits are asymmetric. During inflow events the repartition of water penetrating the Baltic Sea is strongly in favor of the pathway through the Sound, which provides a shorter connection between the Kattegat and Baltic proper. The wider Great Belt has a relatively larger role in exporting water from the Baltic into the North Sea. A demonstration is given that the ventilation of the Baltic Sea deep water is not only governed by the dynamics in the straits and the strong westerly winds enhancing the eastward propagation of North Sea water (a case in 1993), but also by the clockwise circulation in the Kattegat acting as a preconditioning factor for the flow-partitioning.

An unstructured-grid model (FVCOM) coupled to a surface wave model (FVCOM-SWAVE) with two different setups is used to investigate the hydrodynamic and wave energy conditions during a moderate wind and a storm situation in the southern North Sea. One setup covers the whole North Sea with moderately increased grid resolution at the coast, whereas the other is a very high-resolution Wadden Sea setup that is one-way coupled to the coarser North Sea model. The results of both model setups are validated, compared to each other and analysed with a focus on longshore currents and wave energy. The numerical results show that during storm conditions, strong wave-induced longshore currents occur in front of the East Frisian Wadden Sea islands with current speeds up to 1 m/s. The model setup with the higher resolution around the islands shows even stronger currents than the coarser setup. The wave-current interaction also influences the surface elevation by raising the water level in the tidal basins. The calculated wave energies show large differences between moderate wind and storm conditions with time-averaged values up to 200 kW/m.

Continuous observations during 3 years with a vertical resolution of 1 dbar from two Bio-Argo floats in the Black Sea that were equipped with oxygen optodes, chlorophyll fluorometers, and backscattering sensors are analyzed. The particle backscattering coefficient, b bp provides a proxy for the concentration of suspended particles. The observations clearly identify thermal and b bp intrusions down to ~700–800 m in the Bosporus inflow area. In this area, b bp is more than five times larger than elsewhere, which could indicate bacterial abundance and possible biological involvement in the precipitation of Mn-containing particles. The b bp anomalies become much shallower than the temperature anomalies with increasing distance to the east of the strait. Their maxima are located between the onset of the suboxic zone and the upper part of the anoxic layer. Unlike well-known intrusions that are caused by inflow, open ocean intrusions are shallower and often characterized by multiple layers of backscatter maxima with thicknesses of only 15–20 m. The ratio between backscattering coefficients measured at two wavelengths, which gives a proxy for particle size, shows that the relative amount of larger size particles in the anoxic layer increases with depth. The particle concentrations and their size distribution display different vertical variability, which indicates the complex transformation of biological matter. The lower concentration of particles and lower chlorophyll-a during the extremely warm 2016 reveals an overall positive correlation between the two properties. The trends in the particle backscattering coefficient in the suboxic zone during 2013–2016 could indirectly reveal a biogeochemical response to temperature changes.

A 3D unstructured-grid numerical model of the Ems Estuary is presented. The simulated hydrodynamics are compared against tidal gauge data and observations from research cruises. A comparison with an idealized test reveals the capability of the model to reproduce the secondary circulation patterns known from theoretical results. The simulations prove to be accurate and realistic, confirming and extending findings from earlier observations and modeling studies. The basic characteristics of dominant physical processes in the estuary such as tidal amplification, tidal damping, overtide generation, baroclinicity and internal mixing asymmetry are quantified. The model demonstrates an overall dominance of the flood currents in most of the studied area. However, the hypsometric control in the vicinity of Dollart Bay reverses this asymmetry, with the ebb currents stronger than the flood ones. Small-scale bathymetric characteristics and baroclinicity result in a very complex interplay between dominant physical mechanisms in different parts of the tidal channels and over the tidal flats. Residual flow reveals a clear overturning circulation in some parts of the estuary which is related to a mixing asymmetry between flood and ebb currents. We demonstrate that while areas close to the tidal river exhibit overall similarity with density controlled estuarine conditions, in large areas of the outer estuary barotropic forcing and complex bathymetry together with the density distribution affect substantially the horizontal circulation.

The temporal and spatial variability of the upper ocean hydrochemistry in the Black Sea is analysed using data originating from profiling floats with oxygen sensors and carried out with a coupled three-dimensional circulation-biogeochemical model including 24 biochemical state variables. Major focus is on the dynamics of suboxic zone which is the interface separating oxygenated and anoxic waters. The scatter of oxygen data seen when plotted in density coordinates is larger than those for temperature, salinity and passive tracers. This scatter is indicative of vigorous biogeochemical reactions in the suboxic zone, which acts as a boundary layer or internal sink for oxygen. This internal sink affects the mixing patterns of oxygen compared to the ones of conservative tracers. Two different regimes of ventilation of pycnocline were clearly identified: a gyre-dominated (cyclonic) regime in winter and a coastal boundary layer (anticyclonic eddy)-dominated regime in summer. These contrasting states are characterized by very different pathways of oxygen intrusions along the isopycnals and vertical oxygen conveyor belt organized in multiple-layered cells formed in each gyre. The contribution of the three-dimensional modelling to the understanding of the Black Sea hydro-chemistry, and in particular the coast-to-open-sea mixing, is also demonstrated. Evidence is given that the formation of oxic waters and of cold intermediate waters, although triggered by the same physical process, each follow a different evolution. The difference in the depths of the temperature minimum and the oxygen maximum indicates that the variability of oxygen is not only just a response to physical forcing and changes in the surface conditions, but undergoes its own evolution.

The mass-induced sea level variability and the net mass transport between Mediterranean Sea and Black Sea are derived for the interval between August 2002 and July 2008 from satellite-based observations and from model data. We construct in each basin two time series representing the basin mean mass signal in terms of equivalent water height. The first series is obtained from steric-corrected altimetry while the other is deduced from GRACE data corrected for the contamination by continental hydrology. The series show a good agreement in terms of annual and inter-annual signals, which is in line with earlier works, although different model corrections influence the consistency in terms of seasonal signal and trend. In the Mediterranean Sea, we obtain the best agreement using a steric correction from the regional oceanographic model MFSTEP and a continental hydrological leakage correction derived from the global continental hydrological model WaterGAP2. The inter-annual time series show a correlation of 0.85 and a root mean square (RMS) difference of 15 mm. The two estimates have similar accuracy and their annual amplitude and phase agree within 3 mm and 23 days respectively. The GRACE-derived mass-induced sea level variability yields an annual amplitude of 27 ± 5 mm peaking in December and a trend of 5.3 ± 1.9 mm/yr, which deviates within 3 mm/yr from the altimetry-derived estimate. In the Black Sea, the series are less consistent, with lower accuracy of the GRACE-derived estimate, but still show a promising agreement considering the smaller size of the basin. The best agreement is realized choosing the corrections from WaterGAP2 and from the regional oceanographic model NEMO. The inter-annual time series have a correlation and RMS differences of 0.68 and 55 mm, their annual amplitude and phase agree within 4 mm and 6 days respectively. The GRACE-derived seawater mass signal has an annual amplitude of 32 ± 4 mm peaking in April. On inter-annual time scales, the mass-induced sea level variability is stronger than in the Mediterranean Sea, with an increase from 2003 to 2005 followed by a decrease from 2006 to 2008. Based on mass conservation, the mass-induced sea level variations, river runoff and precipitation minus evaporation are combined to derive the strait flows between the basins and with the Atlantic Ocean. At the Gibraltar strait, the net inflow varies annually with an amplitude of 52 ± 10 × 10 −3 Sv peaking end of September (1 Sv = 10 6 m 3 s −1 ). The inflow through the Bosphorus strait displays an annual amplitude of 13 ± 3 ×10 −3 Sv peaking in the middle of March. Additionally, an increase of the Gibraltar net inflow (3.4 ± 0.8 × 10 −3 Sv/yr) is detected.

Here, we address the sediment dynamics in the Black Sea based on analysis of remote sensing data from the Medium Resolution Imaging Spectrometer and numerical simulations with Nucleus for European Modelling of the Ocean model. Boundary conditions consist of realistic meteorological forcing, including significant wave height generated by wave prediction model. A number of sensitivity runs was analysed with the aim to find the most suitable parameters governing sediment fluxes. The comparison between numerical simulations and remote sensing data gives credibility to the quality of simulations. The combined effect of wind waves and currents in the bed layer controls the sediment resuspension that appears to be the major basin-wide source of sediment. Sensitivity experiments included or excluded different forcing terms, e.g. sediment flux from rivers enable to determine the spatial extensions of different point sources. It is concluded that wind-wave forcing is manifested in the sediment dynamics through episodic high energy events contributing to the increase of horizontal sediment fluxes over the northwestern shelf. Both satellite images and numerical model simulations demonstrated that the penetration of suspended sediment into the basin interior was governed by the dynamics of coastal and open-ocean eddies. While fine sediment at sea surface could cross the continental slope propagating into the open ocean, coarser fractions follow the bottom and their penetration into the open ocean is limited. The conclusion is thus that the deposition patterns correlate with the specific shape of Black Sea topography, and the largest depositions are observed in the area of continental slope.

Tidal and wind-driven surface currents in the German Bight between shallow mudflats of the North Frisian islands and the island of Helgoland are studied using coastal high-frequency radar (HFR) observations and hindcasts from a primitive equation numerical model. The setup of the observational system is described, and estimates of expected measurement errors are given. A quantitative comparison of numerical model results and observations is performed. The dominant tidal components are extracted from the two data sources using tidal harmonic analysis and the corresponding tidal ellipses are defined. Results show that the spatial patterns of different tidal ellipse parameters are consistent in the two data sets. Model sensitivity studies with constant and variable salinity and temperature distributions are used to study density-related mechanisms of circulation. Furthermore, the role of the surface wind field in driving the German Bight circulation is investigated using the complex correlation between wind and surface current vectors. The observed change of the respective correlation patterns from the coastal to open ocean is shown to be due to a combination of density effects, the coastline and topography. The overall conclusion is that HFR observations resolve the small-scale and rapidly evolving characteristics of coastal currents well in the studied area and could present an important component for regional operational oceanography when combined with numerical modelling. Some unresolved issues associated with the complex circulation and large instability of circulation in front of the Elbe River Estuary justify further considerations of this area using dedicated surveys and modelling efforts.

High-Frequency (HF) radars measure the ocean surface currents at various spatial and temporal scales. These include tidal currents, wind-driven circulation, density-driven circulation and Stokes drift. Sequential assimilation methods updating the model state have been proven successful to correct the density-driven currents by assimilation of observations such as sea surface height, sea surface temperature and in-situ profiles. However, the situation is different for tides in coastal models since these are not generated within the domain, but are rather propagated inside the domain through the boundary conditions. For improving the modeled tidal variability it is therefore not sufficient to update the model state via data assimilation without updating the boundary conditions. The optimization of boundary conditions to match observations inside the domain is traditionally achieved through variational assimilation methods. In this work we present an ensemble smoother to improve the tidal boundary values so that the model represents more closely the observed currents. To create an ensemble of dynamically realistic boundary conditions, a cost function is formulated which is directly related to the probability of each boundary condition perturbation. This cost function ensures that the boundary condition perturbations are spatially smooth and that the structure of the perturbations satisfies approximately the harmonic linearized shallow water equations. Based on those perturbations an ensemble simulation is carried out using the full three-dimensional General Estuarine Ocean Model (GETM). Optimized boundary values are obtained by assimilating all observations using the covariances of the ensemble simulation.

This work deals with analysis of hydrographic observations and results of numerical simulations. The data base includes acoustic Doppler current profilers (ADCP) observations, continuous measurements on data stations and satellite data originating from the medium resolution imaging spectrometer (MERIS) onboard the European Space Agency (ESA) satellite ENVISAT with a spatial resolution of 300 m. Numerical simulations use nested models with horizontal resolutions ranging from 1 km in the German Bight to 200 m in the East Frisian Wadden Sea coupled with a suspended matter transport model. Modern satellite observations have now a comparable horizontal resolution with high-resolution numerical model of the entire area of the East Frisian Wadden Sea allowing to describe and validate new and so far unknown patterns of sediment distribution. The two data sets are consistent and reveal an oscillatory behaviour of sediment pools to the north of the back-barrier basins and clear propagation patterns of tidally driven suspended particulate matter outflow into the North Sea. The good agreement between observations and simulations is convincing evidence that the model simulates the basic dynamics and sediment transport processes, which motivates its further use in hindcasting, as well as in the initial steps towards forecasting circulation and sediment dynamics in the coastal zone.

This work deals with the analysis of simulations carried out with a primitive equation numerical model for the region of the East Frisian Wadden Sea. The model, with 200-m resolution, is forced by wind, air–sea heat, and water fluxes and river runoff and is nested in a German Bight 1-km-resolution numerical model, the latter providing tidal forcing for the fine resolution model. The analysis of numerical simulations is focused both on responses due to moderate conditions, as well as to extreme events, such as the storm surge Britta, for which the model demonstrates very good skills. The question addressed in this paper is how well the model output can be compressed with the help of empirical orthogonal function analysis. It is demonstrated that, for the short-time periods of the order of a spring–neap cycle, only a few modes are necessary to almost fully represent the circulation. This is just an illustration that the circulation in this region is subject to the dominating tidal forcing, creating clear and relatively simple response patterns. However, for longer periods of about several months, wind forcing is also very important, and correspondingly, the circulation patterns become much more complex. Possible applications of the results in hindcasting and forecasting of hydrodynamics and sediment dynamics in the coastal zone are considered.

In this paper, we analyse the contribution of tidally induced drift in the surface layer to the overall dynamics of well-mixed tidal basins undergoing drying and flooding. The study area covers the East Frisian Wadden Sea (German Bight, Southern North Sea), which consists of seven tidal basins. The major interest is focused on the tidal basin behind the islands of Langeoog and Spiekeroog and the inlet connecting it with the North Sea. The comparison between theoretical concepts, results from direct observations, and simulations with a numerical model helps to understand the underlying physics controlling the tidal response. The data were collected during the period 1995–1998 and consist of cross-channel ADCP transects. The identification of the dominant spatial patterns and their temporal variability is facilitated by applying an EOF analysis to the data. The numerical simulations are based on the 3-D primitive equation General Estuarine Transport Model (GETM) with a horizontal resolution of 200 m and terrain-following vertical coordinates. We find distinct differences between the temporal variability of the transports near the surface and those in deeper layers of the tidal inlets. The near surface transport is dominated by the tidally induced drift (similar to the Stokes drift), whereas the deeper layer transport is dominated by asymmetries caused by the hypsometric properties of the intertidal basins. These transports, when averaged over a tidal period, have opposite directions and compensate each other. This explains the establishment of a vertical overturning cell: landward motion in the upper layers and seaward motion in the deeper parts of the tidal channels. This vertical circulation cell is also observable in our numerical simulations and shows a clear dependency of the temporal asymmetry in the transport patterns on the local depth. In deep tidal channels, the overall properties of the tidal signal show a clear ebb dominance, whereas in the shallow extensions of the channels the transports during flood are larger than during ebb. Although, our research area can be characterized as a well mixed estuary, baroclinicity associated with the fresh water flux from the coast can substantially affect vertical overturning.

An observation network operating three Wellen Radars (WERAs) in the German Bight, which are part of the Coastal Observing System for Northern and Arctic Seas (COSYNA), is presented in detail. Major consideration is given to expanding the patchy observations over the entire German Bight on a 1-km grid and producing state estimates at intratidal scales, and 6- and 12-h forecasts. This was achieved with the help of the proposed spatiotemporal optimal interpolation (STOI) method, which efficiently uses observations and simulations from a free model run within an analysis window of one or two tidal cycles. In this way the method maximizes the use of available observations and can be considered as a step toward the “best surface current estimate.” The performance of the analysis was investigated based on the achieved reduction of the misfit between model and observations. The complex dynamics of the study domain was illustrated based on the spatial and temporal changes of tidal ellipses for the M2 and M4 constituents from HF radar observations. It was demonstrated that blending observations and numerical modeling facilitates physical interpretation of processes such as the nonlinear distortion of the Kelvin wave in the coastal zone and in particular in front of the Elbe and Weser estuaries. Comparisons with in situ data acquired outside the area covered by the HF radar demonstrated that the analysis method is able to propagate the HF radar information to larger spatial scales.

The dynamics of the German Bight associated with river plumes and fresh water intrusions from tidal flats have been studied with numerical simulations. The horizontal and vertical patterns of the M2, M4 and M6 tides revealed complex distortions along the bathymetric channels connecting the coast and the open sea. A major focus was on the surface-to-bottom change in tidal asymmetries, which provides a major control on draining the tidal flats around the Elbe and Weser River mouths. Comparisons between baroclinic and barotropic experiments demonstrated that the estuarine gravitational circulation is responsible for pronounced differences in surface and bottom asymmetries. These differences could be considered as a basic control mechanism for sediment dynamics. The most prominent area of tidal distortions, manifested by a delay of the tidal wave, was located between the estuarine turbidity maximum and the estuarine mouth north of Cuxhaven. This area was characterized by the strongest periodic convergence and divergence of the flow and by the largest salinity gradients. The enhancement of the gravitational circulation occurred during the transition between spring and neap tides. The large-scale dynamics and small-scale topographic features could impact the sediment distribution as there was a marked interplay in the channels between stratification and turbulence. Also an explanation has been given for the mechanisms supporting the existence of a mud area (Schlickgebiet) south of Helgoland Island, associated with trapping suspended particular matter.

The paper addresses the individual and collective contribution of different forcing factors (tides, wind waves, and sea-level rise) to the dynamics of sediment in coastal areas. The results are obtained from simulations with the General Estuarine Transport Model coupled with a sediment transport model. The wave-induced bed shear stress is formulated using a simple model based on the concept that the turbulent kinetic energy (TKE) associated with wind waves is a function of orbital velocity, the latter depending on the wave height and water depth. A theory is presented explaining the controls of sediment dynamics by the TKE produced by tides and wind waves. Several scenarios were developed aiming at revealing possible trends resulting from realistic (observed or expected) changes in sea level and wave magnitude. The simulations demonstrate that these changes not only influence the concentration of sediment, which is very sensitive to the magnitude of the external forcing, but also the temporal variability patterns. The joint effect of tides and wave-induced bed shear stress revealed by the comparison between theoretical results and simulations is well pronounced. The intercomparison between different scenarios demonstrates that the spatial patterns of erosion and deposition are very sensitive to the magnitude of wind waves and sea-level rise. Under a changing climate, forcing the horizontal distribution of sediments adjusts mainly through a change in the balance of export and import of sediment from the intertidal basins. The strongest signal associated with this adjustment is simulated North of the barrier islands where the evolution of sedimentation gives an integrated picture of the processes in tidal basins.

In this paper we use a combination of nu- merical modeling and data analysis to gain a better understanding of the major characteristics of the circu- lation in the East Frisian Wadden Sea. In particular, we concentrate on the asymmetry of the tidal wave and its modulation in the coastal area, which results in a com- plex pattern of responses to the sea-level forcing from the North Sea. The numerical simulations are based on the 3-D primitive equation General Estuarine Transport Model (GETM) with a horizontal resolution of 200 m and terrain-following vertical coordinates. The model is forced at its open boundaries with sea-level data from an operational model for the German Bight (German Hydrographic Office). The validation data for our model simulations include time series of tidal gauge data and surface currents measured at a pile in the back-barrier basin of the Island Langeoog, as well as several ADCP transects in the Accumer Ee tidal inlet. Circulation and turbulence characteristics are inves- tigated for typical situations driven by spring and neap tides, and the analysis is focused on dominating tem- poral and spatial patterns. By investigating the response of five back-barrier basins with rather different mor- phologies to external forcing, an attempt is made to elucidate the dominating physical balances controlling the circulation in the individual sub-basins. It is dem- onstrated that the friction at the seabed tends to slow down the tidal signal in the shallow water. This leads to the establishment of flood dominance in the shallowsea north of the barrier islands. South of the islands, where the water volume of the channels at low tide is smaller than the tidal prism, the asymmetry of the tidal signal is shifted towards ebb dominance, a feature which is par- ticularly pronounced at spring tide. At the northern open boundary, the tidal wave propagating from west to east generates a sea-level dif- ference of � 1 m along the boundary, and thereby trig- gers vigorous alongshore currents. The frictional control in the model is located in the inlets, as well as along the northern boundary. The correlation between velocity and turbulent kinetic energy tends to the establishment of a net southward transport, giving theoretical support to the observed accumulation of sediments on the in- tertidal flats. Weak turbulence along the northern shores of the barrier islands and the small magnitude of the residual currents there promote accumulation of sus- pended matter in these areas, although wave action will generally counteract this effect.

The processes of cold intermediate water (CIW) formation in the Black Sea are quantified by analyzing the results of numerical simulations using the Modular Ocean Model (MOM) plus observations obtained from both hydrographic surveys and the TOPEX/POSEIDON altimeter. In the first phase of integration the model is forced with seasonally-variable atmospheric data. After reaching a quasi-periodic state, the integration is continued using forcing data for the period 1991-1994 provided by the United Kingdom Meteorological Office. The performance of the model is assessed in reproducing the dominant physical mechanisms that control the process and rate of CIW formation, viz., circulation, vertical stratification, preconditioning and convective cooling in winter. An analysis is presented of buoyancy fluxes through the sea surface, from rivers, and from the input of dense water through the Bosphorus Strait. The characteristics and rates of formation of the CIW are described, estimated from both the model and observations. The computations focus on the contribution of convective heat fluxes and the resulting re-supply of the upper pycnocline (at = 14.5-15.5) with cold water. A critical revision of existing theories is presented for the dynamic control and relative contributions to CIW formation of different regions of the Black Sea. These regions are: (i) the continental slope in the northwestern part of the sea; (ii) the interior deep basin (dominated by the Rim Current cyclonic circulation); (iii) the NW shelf; and (iv) the eastern basin. It is shown that the cold water mass is formed over the entire Black Sea, but with pronounced regional dependency in the ratio 42%, 28%, 20% and 10%, respectively. It is estimated that the cold intermediate layer is fully replenished every ∼5.5 yr.

The interaction between physical and biological processes in the areas of continental margins governs the variability of ecosystems. The complexity of processes in these areas requires detailed studies combining modelling and surveying efforts. One promising step in this direction was undertaken in the framework of the EROS 21 project, focusing on the shelf part of the north-western Black Sea. In the present paper, we focus on the results of physical studies aiming to improve the understanding of the fundamental exchange processes in the ocean margins, as well as to quantify some of them in the Black Sea. We illustrate the capabilities of circulation models to reproduce physical processes with different time- and space-scales: coastal waves, internal waves, baroclinic Rossby and topographic waves. Another class of important phenomena in the coastal zone is associated with convection. Sources at the sea surface and in the outflow areas give rise to plume dynamics that play a crucial role in the vertical mixing and provide the mechanism for water-mass formation. Most of the results are illustrated for the shelf part of the Black Sea. The verification of simulations is performed by comparison with survey data, altimeter data from the Topex/Poseidon mission and radiotracer observations. The latter, in combination with simulations from circulation models, are used to trace the penetration of tracers into the intermediate and deep layers. We show that although most 90Sr is introduced by river runoff, large amounts of this signal penetrate the halocline in the Bosphorus Straits area and along the southern coast. Another important fraction of the river water penetrates the intermediate layers at the shelf edge in the north-western Black Sea.

Mesoscale and submesoscale dynamics associated with tidal wave transformations were addressed in the German Bight using numerical simulations. Tidal gauge and velocity observations in several locations were used to validate the numerical model. A downscaling approach included analysis of simulations with horizontal resolutions of 1, 0.4, and 0.2 km. It was shown that the modified tidal wave lost most of its energy after reflection or refraction over the eastern part of the German Bight. Energy loss resulted in a pronounced change of the wave’s spectral composition and generation of overtides. Tidal oscillations were modified by mesoscale processes associated with bathymetric channels. Semidiurnal and quarterdiurnal tides revealed very different spatial patterns. The former were aligned with the bathymetric channels, while the latter were rather “patchy” and had about half the spatial scales. In numerous areas around the bathymetric channels, the major axis of the M4 ellipses was normal or at some angle with the major axis of the M2 ellipses. Thus, higher harmonics developed “orthogonal” patterns that drove secondary circulations. Moreover, the ratio between spring and neap tidal amplitudes was relatively low in the Wadden Sea, showing reduced sensitivity of this very shallow area to fortnightly tidal variations. It was demonstrated that simulated hydrodynamics patterns help explain the physical mechanism shaping the median grain size distribution in the German Bight.

In this paper we provide an overview of new knowledge on oxygen depletion (hypoxia) and related phenomena in aquatic systems resulting from the EU-FP7 project HYPOX ("In situ monitoring of oxygen depletion in hypoxic ecosystems of coastal and open seas, and landlocked water bodies", http://www.hypox.net). In view of the anticipated oxygen loss in aquatic systems due to eutrophication and climate change, HYPOX was set up to improve capacities to monitor hypoxia as well as to understand its causes and consequences. Temporal dynamics and spatial patterns of hypoxia were analyzed in field studies in various aquatic environments, including the Baltic Sea, the Black Sea, Scottish and Scandinavian fjords, Ionian Sea lagoons and embayments, and Swiss lakes. Examples of episodic and rapid (hours) occurrences of hypoxia, as well as seasonal changes in bottom-water oxygenation in stratified systems, are discussed. Geologically driven hypoxia caused by gas seepage is demonstrated. Using novel technologies, temporal and spatial patterns of water-column oxygenation, from basin-scale seasonal patterns to meter-scale sub-micromolar oxygen distributions, were resolved. Existing multidecadal monitoring data were used to demonstrate the imprint of climate change and eutrophication on long-term oxygen distributions. Organic and inorganic proxies were used to extend investigations on past oxygen conditions to centennial and even longer timescales that cannot be resolved by monitoring. The effects of hypoxia on faunal communities and biogeochemical processes were also addressed in the project. An investigation of benthic fauna is presented as an example of hypoxia-devastated benthic communities that slowly recover upon a reduction in eutrophication in a system where naturally occurring hypoxia overlaps with anthropogenic hypoxia. Biogeochemical investigations reveal that oxygen intrusions have a strong effect on the microbially mediated redox cycling of elements. Observations and modeling studies of the sediments demonstrate the effect of seasonally changing oxygen conditions on benthic mineralization pathways and fluxes. Data quality and access are crucial in hypoxia research. Technical issues are therefore also addressed, including the availability of suitable sensor technology to resolve the gradual changes in bottom-water oxygen in marine systems that can be expected as a result of climate change. Using cabled observatories as examples, we show how the benefit of continuous oxygen monitoring can be maximized by adopting proper quality control. Finally, we discuss strategies for state-of-the-art data archiving and dissemination in compliance with global standards, and how ocean observations can contribute to global earth observation attempts.

We present here results of numerical simulations with reduced gravity model of the Mediterranean plume intruding into the Black Sea. The model has horizontal resolution of 600 m. The scenarios analyzed in the paper aim at quantifying the sensitivity of the plume to the ambient stratification and the fluxes of mass, momentum and buoyancy through the Bosphorus Straits. The simulated plume characteristics are compared against observations. It is found that the mixing of Mediterranean and Black Sea water, as well as the termination depth of the plume, are very sensitive to specific combinations of the governing parameters. The behavior of gravity currents on the shelf and on the continental slope is also studied and the role of topographic control is demonstrated. The relatively large entrainment rate (∼10–12) compared to the one in the Atlantic ocean (∼2–3), shallow penetration and small deflection to the right caused by the earth rotation are explained as a result of the specific combination of governing parameters, topography routing and ambient stratification. A simple two-component chemical model for the interaction between H2S and O2 is coupled with the dynamical model in order to investigate the impact of the Bosphorus plume (rich in O2) on the oxidation of anoxic water.

The DieCAST ocean model is applied to a study of the circulation in the Black Sea, using 1/12° horizontal resolution and with 20 vertical layers. Boundary forcings are monthly wind stress, evaporation minus precipitation, air–sea heat flux, freshwater influx from 11 rivers and exchange with the Mediterranean Sea through the Bosphorus Strait. The model reproduces fundamental physical features of the Black Sea: seasonal fluctuations in the quasi-permanent cyclonic Rim Current, numerous anticyclonic meanders and eddies lying between the Rim Current and the coast, Rossby waves propagating westward across the basin, coastally trapped waves, and the annual cycle of vertical mixing. Model results shed light on the mechanisms affecting such features. These include interactions of the Rim Current with coastal bathymetry abutments, leading to recirculations that pinch off vortices as in island wakes, and possible baroclinic instability of the Rim Current; these are modulated by the large annual stratification cycle above a relatively shallow and strong pycnocline, as is the Rim Current itself. The resulting wake eddies often merge into major coastal circulation features such as the seasonal Batumi and Sevastopol eddies. These anticyclonic eddies play a fundamental role in coastal and open-sea exchange processes. Hydrographic data from sampling cruises and recent Topex–Poseiden (T/P) altimeter data strongly supports our analysis.

Mesoscale features in the Black Sea circulation obtained from fine resolution hydrographic survey data and TOPEX/Poseidon (T/P) altimeter are compared. Important scales, associated with pronounced eddy variability and diapycnal exchange at the depth of redox-zone, which are well resolved in the hydrographic survey data, are not sufficiently resolved in the altimeter data. However, the satellite data give clear signals associated with the variability of general circulation, as well as with the dynamics of larger eddies. The seasonal transitions in the circulation between the intense winter–spring state and the less intense summer–fall state is also well resolved. The propagation patterns of oscillations along the eastern coast detected from satellite altimetry support earlier concepts based on direct observations in this region. The correlation between two data types reveals the value of altimeter-derived signals as representative data source for describing the temporal changes of mesoscale ocean dynamics. Some estimates about the possible use of altimeter data for assimilation in numerical models are provided.

The heat exchange between the atmosphere and ocean (a function of the difference between the sea surface temperature and atmospheric temperature) is differently accounted for in numerical models, depending on their horizontal resolution. This could result in large differences in the rates of formation of water masses in coarse resolution and eddy resolving models. We address this issue in the present paper comparing simulations carried out with the modular ocean model (MOM), which is set-up with coarse (1/4°) and eddy (1/12°) resolution. Both models are forced by atmospheric analysis data. The Black Sea is used as a test area, since it has well constrained heat balance. The absence of large open boundaries, which is not usually the case in most ocean models, precludes the dependency of the results on poorly known open boundary conditions and makes possible to accurately evaluate the model heat fluxes associated with mesoscale processes. We find many indications that the transport and mixing associated with mesoscale eddies provide an important mechanism for the penetration of cold surface water into the pycnocline. Their contribution is illustrated on the example of heat exchange with the atmosphere and the resulting intermediate water mass formation. Two areas are investigated in more detail: the shelf edge/continental slope area, where the time averaged patterns of cooling are realistically simulated in eddy resolving models, and the area of anticyclonic circulation between the coast and the main current the latter following approximately the continental slope. It is shown that the winter convection is enhanced at the periphery of coastal anticyclones. Lateral intrusions of coastal waters into the pycnocline associated with eddies govern the characteristics of cold intermediate water and the amplitude of annual signal at these depths. Statistical characteristics quantifying the contribution of mean versus eddy part of the heat flux at sea surface in the two models are analyzed.

Model data from a 7-year integration of an eddy-resolving circulation model are analyzed to study the impact of sub-basin scale eddies on the transitions between different quasi-stable states in the Black Sea circulation. The data are produced using the Bryan–Semtner–Cox modular ocean model with grid intervals of 1/9°×1/12° in the longitude/latitude. The model is driven by realistic time variable wind and buoyancy forcing. The time variability simulated in the model is dominated by basin oscillations, which are strongly modified by baroclinicity and topography. The simulations support the concept that the anticyclonic circulation in the Black Sea is observed between the jet-current and the coast, where meanders and eddies with changing amplitudes are spontaneously formed. The baroclinic instability results in pronounced quasi-periodic changes of circulation. The meanders increase, and spectacular intrusions of coastal water into the open sea are simulated. In the extreme cases, the gyre breaks into sub-basin scale eddies. Altimeter data support the model simulations, particularly the characteristics of time variability. The baroclinic instability is strongly suppressed under coarse resolution, and the seasonal variability is characterized by almost constant amplitudes of the oscillations repeating every year. On the contrary, in the eddy-resolving model, the natural variability is well pronounced leading to interannual changes.

We address in this paper the planetary basin modes and their possible impact on the circulation of Black Sea. These oscillations could become important in semi-enclosed seas where the horizontal scales are smaller than the barotropic radius of deformation and the baroclinic radius of deformation is comparable with the basin scales. The physical balances studied here are based on theoretical results for the Rossby modes in rectangular basins and on model simulations carried out with Bryan–Cox ocean model with eddy resolution. Eddies with horizontal scales of 100–200 km spontaneously form and propagate to the west with phase velocity of few kilometers per day. External mode oscillations dominate the model solution and show close similarity with estimates based on simplistic theoretical models. In the baroclinic oceans these oscillations trigger active baroclinic variability and western propagation, but with strongly modified wave characteristics in comparison to the barotropic case. The comparisons between simulations and data from surveys and altimeters demonstrates that the planetary modes are a dominating form of wave motion in this sea and can explain the westward propagation of signals in its deep part. It is suggested that the excitation of rotational modes can be also important for the dispersion of tracers.

Strontium-90 data are used to verify the mixing and ventilation patterns simulated in a circulation model of the Black Sea. We trace physical processes using 90Sr which was delivered after the Chernobyl accident, primarily via riverine discharges in the Northwest shelf region. The results used for the verification of our model simulations were obtained from samples collected between 1986 and 1992. A three-dimensional circulation/transport model coupled with a tracer model simulates the transport of 90Sr and its penetration into the intermediate and deep layers. Though most of the 90Sr is introduced in the model at the sea surface by river runoff in the NW shelf area, the concentration patterns suggest that a large amount of this signal penetrates the halocline in the Bosphorus area and along the southern coast. This fundamental spatial characteristic of mixing in the Black Sea is associated with the entrainment of surface and intermediate waters in the vicinity of the Bosphorus strait, and with vertical exchanges along the southern coast resulting from the anticyclonic circulation patterns. Another important fraction of the river water penetrates intermediate layers at the shelf edge in the NW Black Sea. Higher model resolution and more elaborated forcing functions would be needed in order to more accurately define mesoscale features in this basin.

The enclosed boundaries and small scales of some seas lead to the formation of specific physical balances, which motivates the oceanographic interest in studying the dynamics of semienclosed ocean basins. The focus in the paper is on the specific appearances of eddy processes when the basin scales and the ones of the topographic features are comparable with the baroclinic radius of deformation. The Black Sea is used as a test basin. Eddy variability is analyzed using simulation results and compared with existing observations. The Bryan–Cox model with horizontal resolution Δφ = 1/10° and Δλ = 1/6° is forced with annual-mean wind stress data. Buoyancy flux at the sea surface is proportional to the deviation of the model density from the annual-mean climatological data. Sensitivity studies on different forcing and on the topographic control are carried out. Synoptic periods are estimated to be about 0.5 yr. Eddies form in the eastern Black Sea and propagate westward with a speed of about 3 cm s−1. T...

Numerical simulations reveal that variations in wind stress and heat fluxes can induce significant interannual fluctuations in the circulation of the upper layers of the Mediterranean. From January 1980 to November 1988, the atmosphere shows changes in the structure and magnitude of the surface winds and in the air temperatures which induce modifications in the upper ocean structure and currents. The model prediction of the interannual fluctuations of the Sicily Strait baroclinic westward volume transport is in agreement with observations and the variability is explained as a function of the wind curl forcing in the region. The current anomalies persist for many months after a Winter atmospheric anomalous disturbance has occurred over the basin. The Eastern Mediterranean basin is the area where the interannual ocean response is most pronounced.