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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.

This paper addresses the impact of atmospheric variability on ocean circulation in tidal and non-tidal basins. The data are generated by an unstructured-grid numerical model resolving the dynamics in the coastal area, as well as in the straits connecting the North Sea and Baltic Sea. The model response to atmospheric forcing in different frequency intervals is quantified. The results demonstrate that the effects of the two mechanical drivers, tides and wind, are not additive, yet non-linear interactions play an important role. There is a tendency for tidally and wind-driven circulations to be coupled, in particular in the coastal areas and straits. High-frequency atmospheric variability tends to amplify the mean circulation and modify the exchange between the North and the Baltic Sea. The ocean response to different frequency ranges in the wind forcing is area-selective depending on specific local dynamics. The work done by wind on the oceanic circulation depends strongly upon whether the regional circulation is tidally or predominantly wind-driven. It has been demonstrated that the atmospheric variability affects the spring-neap variability very strongly.

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.

This study addresses the impact of wind, waves, tidal forcing and baroclinicity on the sea level of the German Bight during extreme storm events. The role of wave-induced processes, tides and baroclinicity is quantified, and the results are compared with in situ measurements and satellite data. A coupled high-resolution modelling system is used to simulate wind waves, the water level and the three-dimensional hydrodynamics. The models used are the wave model WAM and the circulation model GETM. The two-way coupling is performed via the OASIS3-MCT coupler. The effects of wind waves on sea level variability are studied, accounting for wave-dependent stress, wave-breaking parameterization and wave-induced effects on vertical mixing. The analyses of the coupled model results reveal a closer match with observations than for the stand-alone circulation model, especially during the extreme storm Xaver in December 2013. The predicted surge of the coupled model is significantly enhanced during extreme storm events when considering wave–current interaction processes. This wave-dependent approach yields a contribution of more than 30 % in some coastal areas during extreme storm events. The contribution of a fully three-dimensional model compared with a two-dimensional barotropic model showed up to 20 % differences in the water level of the coastal areas of the German Bight during Xaver. The improved skill resulting from the new developments justifies further use of the coupled-wave and three-dimensional circulation models in coastal flooding predictions.

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.

This paper describes recent developments based on advances in coastal ocean forecasting in the fields of numerical modeling, data assimilation, and observational array design, exemplified by the Coastal Observing System for the North and Arctic Seas (COSYNA). The region of interest is the North and Baltic seas, and most of the coastal examples are for the German Bight. Several pre-operational applications are presented to demonstrate the outcome of using the best available science in coastal ocean predictions. The applications address the nonlinear behavior of the coastal ocean, which for the studied region is manifested by the tidal distortion and generation of shallow-water tides. Led by the motivation to maximize the benefits of the observations, this study focuses on the integration of observations and modeling using advanced statistical methods. Coastal and regional ocean forecasting systems do not operate in isolation but are linked, either weakly by using forcing data or interactively using two-way nesting or unstructured-grid models. Therefore, the problems of downscaling and upscaling are addressed, along with a discussion of the potential influence of the information from coastal observatories or coastal forecasting systems on the regional models. One example of coupling coarse-resolution regional models with a fine-resolution model interface in the area of straits connecting the North and Baltic seas using a two-way nesting method is presented. Illustrations from the assimilation of remote sensing, in situ and high-frequency (HF) radar data, the prediction of wind waves and storm surges, and possible applications to search and rescue operations are also presented. Concepts for seamless approaches to link coastal and regional forecasting systems are exemplified by the application of an unstructured-grid model for the Ems Estuary.

This study addresses the impact of coupling between wind wave and circulation models on the quality of coastal ocean predicting systems. This is exemplified for the German Bight and its coastal area known as the Wadden Sea. The latter is the area between the barrier islands and the coast. This topic reflects the increased interest in operational oceanography to reduce prediction errors of state estimates at coastal scales, which in many cases are due to unresolved nonlinear feedback between strong tidal currents and wind-waves. In this study we present analysis of wave and hydrographic observations, as well as results of numerical simulations. A nested-grid modelling system is used to producing reliable nowcasts and short-term forecasts of ocean state variables, including wind waves and hydrodynamics. The data base includes ADCP observations and continuous measurements from data stations. The individual and collective role of wind, waves and tidal forcing are quantified. The performance of the forecast system is illustrated for the cases of several extreme events. Effects of ocean waves on coastal circulation and sea level are investigated by considering the wave-dependent stress and wave breaking parameterization. Also the effects which the circulation exerts on the wind waves are tested for the coastal areas using different parameterizations. The improved skill of the coupled forecasts compared to the non-coupled ones, in particular during extreme events, justifies the further enhancements of coastal operational systems by including wind wave models.

An observing system in the Black Sea combining remote sensing data such as sea level anomalies from altimetry, sea surface temperature from satellite radiometer and data from Argo floats has been analyzed with the aim to quantify the contribution of different information sources when reconstructing the ocean state. The main research questions are: (1) do Argo float measurements substantially impact the quality of estimates, (2) what is the dependence of this quality upon the data and sampling used, and (3) are there specific Black Sea issues? Numerical model output and statistical analysis were used for this purpose. It has been demonstrated that the statistical method performs in a consistent way reproducing known geophysical patterns. Maximum footprints of sea level, salinity and temperature were illustrated, most of them clearly connected with specific thermohaline conditions and the general circulation. Reduced analysis capabilities were identified as associated with a low level of dynamical coupling between the shelf and the open ocean, mesoscale dynamics and representation of diapycnic processes in the models. The accuracy of Argo pressure measurements appeared very important to resolve the extremely sharp stratification in the upper layers. The present-day number of Argo floats operating in the Black Sea of about 10, seems optimal for operational purposes.

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.

The Coastal Observing System for Northern and Arctic Seas (COSYNA) was established in order to better understand the complex interdisciplinary processes of northern seas and the Arctic coasts in a changing environment. Particular focus is given to the German Bight in the North Sea as a prime example of a heavily used coastal area, and Svalbard as an example of an Arctic coast that is under strong pressure due to global change.The COSYNA automated observing and modelling system is designed to monitor real-time conditions and provide short-term forecasts, data, and data products to help assess the impact of anthropogenically induced change. Observations are carried out by combining satellite and radar remote sensing with various in situ platforms. Novel sensors, instruments, and algorithms are developed to further improve the understanding of the interdisciplinary interactions between physics, biogeochemistry, and the ecology of coastal seas. New modelling and data assimilation techniques are used to integrate observations and models in a quasi-operational system providing descriptions and forecasts of key hydrographic variables. Data and data products are publicly available free of charge and in real time. They are used by multiple interest groups in science, agencies, politics, industry, and the public.

In the past years, the Helmholtz Zentrum Geesthacht put in place the Coastal Observing System for the North and Arctic Seas (COSYNA) in the frame of which different aspects of forecasting the marine environment have been developed. This paper describes these developments, which are based on recent advances in coastal ocean forecasting in the field of numerical modelling, data assimilation and observational array design. The region of interest is the North and Baltic Sea; most of the coastal examples discussed in the paper are for the German Bight. Several pre-operational applications are presented exemplifying the outcome of using the best available science in coastal ocean predictions. They help to identify new challenges; most of them are associated with resolving the non-linear behavior of coastal ocean, which for the studied region, is manifested by the tidal distortion and generation of shallow-water tides. Led by the motivation to maximize the benefit from observations, the authors focus on the integration of observations and modelling by using advanced statistical methods. The coastal and regional ocean forecasting systems do not run in isolation, but are linked, either weakly by just using forcing data, or interactively by using two-way nesting or unstructured-grid models. Therefore the problem of downscaling and upscaling, which currently attracts much attention, is also addressed. One example shown is the coupling of the coarse-resolution regional models by using a two-way nesting method with fine resolution in the area of connecting straits. The major part of the paper presents illustrations from assimilation of remote sensing, in situ and HF radar data, prediction of wind waves and storm surges, as well as possible applications to search and rescue operations, and modelling support for assessing the environmental impact of wind parks. Concepts for seamless approaches to link coastal and regional forecasting systems are also presented and the two examples given illustrate (1) an application of unstructured-grid model for the Ems Estuary, and (2) the potential influence of the information from coastal observatories or coastal forecasting systems on the regional models.

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.

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.

In this work, we address the Black Sea setup of Nucleus for European Modelling of the Ocean (NEMO), and in particular some model enhancements associated with the most important characteristic of ocean dynamics in this semi-enclosed basin, that is the sea-level variability and its relationship with water cycles and wind. Forcing data are presented in detail and compared with previously used coarser-resolution data. One emphasis in this paper is on the statistical analyses of forcing data and outputs from simulations with a focus on the sea level and its change. Numerical simulations are carried out as free run, and alternatively, altimeter data assimilation based on displacement of water properties in the pycnocline is used. Comparisons between the two runs identify the robustness of circulation driven by water balance and winter intensification. Problems in the model to replicate the redistribution of water properties between the two sub-basins in free-run mode are also discussed, which are observed during years with extreme climatic conditions.

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.

Climatological variations of thermal and haline buoyancy fluxes are investigated in the Black Sea. Analyses are performed to determine whether or not thermal buoyancy flux due to net heat flux (or haline buoyancy flux due to freshwater flux) dominates net buoyancy flux in the Black Sea. The effect of the two types of buoyancy flux are examined using a ≈ 3.2 km resolution HYbrid Coordinate Ocean Model (HYCOM). In the Black Sea, salinity is increased by Bosphorus inflow and decreased by precipitation and the inflow from six major rivers. Thus, the monthly mean discharge values from six major rivers are used as additions to the precipitation field in the model. River discharges are obtained from four climatologies: (1) River DIScharge (RivDIS), (2) Perry, (3) University Corporation for Atmospheric Research (UCAR) and (4) Naval Research Laboratory (NRL). Statistical evaluations of climatological river discharges from these products result in similar annual mean values. However, there are differences in the seasonal cycle. In the case of Danube, which has the largest annual river discharge contribution of ≈ 6365 m3 s− l, RMS differences for river discharge values over the seasonal cycle are within ≈ 2% among all products. The Black Sea HYCOM simulation uses climatological monthly mean atmospheric forcing (wind and thermal forcing) from European Centre for Medium-range Weather Forecast (ECMWF) Re-analyses. Buoyancy flux fields obtained from the HYCOM simulation demonstrate that thermal buoyancy flux dominates haline buoyancy flux in all months except March when the basin-averaged absolute ratio of former to the latter is 0.5 in the Black Sea. On the contrary, large buoyancy ratio values of ≫ 1 in other months explain the buoyancy is much more sensitive to variations in heating. It is also found that near the Bosphorus Strait in the Black Sea the strongly concentrated source of salty water typically penetrates into the deeper layers as a plume. Further analyses of mean buoyancy fluxes reveal nonexistence of deep convection in the Black Sea on climatological time scales.

Kourafalou, V.H. et al., ecent advances in Coastal Ocean Forecasting Systems (COFS) are discussed. Emphasis is given to the integration of the observational and modeling components, each developed in the context of monitoring and forecasting in the coastal seas. These integrated systems must be linked to larger scale systems toward seamless data sets, nowcasts and forecasts (from the global ocean, through the continental shelf and to the nearshore regions). Emerging capabilities include: methods to optimize coastal/regional observational networks; and probabilistic approaches to address both science and applications related to COFS. International collaboration is essential to exchange best practices, achieve common frameworks and establish standards., V. Kourafalou acknowledges support from NOAA (NA13OAR4830224 and NA11NOS4780045). M. Herzfeld is thankful to the eReefs marine modeling team and partners (CSIRO: Commonwealth Industrial and Scientific Research Organization; SIEF: Science and Industry Endowment Fund; AIMS: Australian Institute of Marine Science). The system CGOFS_ECS (material contributed by X. Zhu) is supported by the National Natural Science Foundation of China (contract #41222038). E. V. Stanev acknowledges the input from COSYNA, which is supported by the Helmholtz Association of German Research Centers

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.

The recent drop of sea level in the Aral Sea of about 0.6 m year−1 during the last 40 years represents one of the most dramatic example globally about the possible consequences of man-induced environmental changes. This extremely strong signal, as well as the constantly changing hydrological and meteorological fluxes in this area are missing from the seaborne observations in the 1990s because the observational network developed by the Former Soviet Union has almost not been operating during one decade in the new independent states. Fortunately, the Aral Sea level has been regularly monitored from space, in particular by satellite altimetry. In this study, we present observations of the Aral Sea level and analyze the observed trends and shorter term variability based on TOPEX/Poseidon altimeter data. This data set (available since early 1993) is complemented by hydrometeorological data and gauge data (since 1950) allowing to quantify the evolving water balance of the Aral Sea. It is shown that even though the river runoff almost ceased recently, the rapid drawing of the Aral Sea is substantially reduced by the compensating discharge of ground water. The analysis of the available data makes it possible to address the changing salt balance and to identify the major control on this balance exerted by ground water discharge. The major event of ground water discharge is identified in the period 1993–1994 and resulted in a substantial increase of the salt content. The rapid drop of salt content thereafter could indicate an increase of salt precipitation.

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 concepts of the formation of Cold Intermediate Water (CIW) in the Black Sea are revisited on the base of numerical simulations with modular ocean model (MOM) for the period July 1991–June 1995 and validated against the data from three basin-wide surveys carried out during the same period. The time of replenishment of Cold Intermediate Layer (CIL) estimated from simulations varies between 5 and 12 years. The regionalization of the rate of cold water mass formation demonstrates that the dynamical control is of utmost importance. The year-to-year variability of that rate could reach about the half of annual mean values, and the magnitude of the interannual signal of the temperature in the core of Cold Intermediate Layer is comparable with the magnitude of the seasonal one. That explains some of the problems with the earlier estimates of formation rates derived from the observation. It is not only the insufficient data coverage in the horizontal, which biased the estimates, but also the undersampling of the seasonal signal in time. Therefore the parallel analysis of model and survey data could give a more objective tool when estimating the temporal variability of water mass formation.

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.

The sensitivity of Black Sea level to variations in the global forcing is studied here using tide gauge and satellite altimeter data, as well as hydro-meteorological data for the fresh water flux components. The consistency between satellite and sea borne data is analyzed and the characteristics of variability with monthly to interannual time scales are revealed. The analysis of 6-year-long data series of TOPEX/Poseidon altimeter shows that the first EOF accounts for 85% of the total variance and is associated with the water cycle, the latter forced by the air–sea exchange, continental hydrological budgets and straits outflow. This result is a demonstration that the Black Sea level integrates the variations of global forcing over vast catchment area, thus making them quite distinguishable. The second EOF describes the seasonal variability of circulation. The third and higher EOFs describe synoptic and basin-oscillations, and the corresponding principal components are characterized by strong interannual variability. By analyzing the correlation of sea level and water balance in the last 70 years, we quantify the response to the external forcing. The mean sea level trend during 1993–1997, derived from the TOPEX/Poseidon data, of ∼12 cm is much lower than the largest trends of this type observed in the last 120 years, which are associated with interannual-to-decadal variability and reach ∼20–30 cm. The correlation between the sea level and NAO index, starting from 1870s, is well pronounced, suggesting that future variations of sea level could be predicted using global climate indices. It is shown that the long-term changes of water balance are strong enough to substantially affect the exchange between Black Sea and Mediterranean Sea. This in turn might result in changing the conditions of water mass formation in the Aegean Sea and motivates further studies on the prediction of extreme events of deep water mass formation in the Eastern Mediterranean Sea as function of the global and regional water cycles.

TOPEX/Poseidon (T/P) altimeter data for the period 1993–1997 and their correlation with available precipitation, evaporation and river runoff data are analysed with the aim to study the variability of sea level and water balance in the Black Sea. The difference between the time rate of change of basin volume estimated from altimeter and fresh water flux data is used as a measure of the vertically integrated transport through the Bosphorus Strait. The errors in the transport resulting from the use of tide gauge data when estimating the variations of basin volume are objectively quantified using sea level from T/P data as reference measurements. Then, from the long-term tide gauge and hydro-meteorological data series for the period 1923–1997, we derive the variability of basin volume and transport in the Bosphorus Strait. It is found that the main signal in the transport is the seasonal one, with maximum values in March–April and minimum in August. The amplitude of this signal is ∼2/3 of the amplitude of the net fresh water flux. In low frequency range, the oscillations with periods 10 and 4 years have significant amplitudes. However, the ratio between the magnitude of oscillations in the forcing (fresh water flux) and the response (transport in the strait) tends to unity. This indicates that the resistance of strait to climatic variability with interannual and decadal time scales is negligible. It is demonstrated that the available data present a valuable source of information when calibrating parameterizations of straits transport based on theoretical developments.

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 air–sea exchange in the Black Sea is revisited in the paper on the base of data provided from the ECMWF atmospheric re-analysis for the period 1979–1993. Climatological characteristics of analysis data are compared against available climatic data, demonstrating quite a good consistency. Several drawbacks are found: too low temperatures in the easternmost part of the sea, too low dew point temperature and slight underestimation of precipitation. However, the seasonal and inter-annual oscillations are resolved correctly, as well as the horizontal patterns of air–sea exchange. The shallow penetration of thermal signal in the Black Sea is well detected by the very small heat transports inferred from the ECMWF data. Thus, the conclusion is that the analysis ‘sees’ quite well the major thermal peculiarity of the Black Sea associated with its extremely small thermal inertia caused by the specific stratification.

TOPEX/Poseidon (TIP) altimeter data in the Black Sea are analyzed for almost 5 years in parallel with available hydrological and meteorological data with the aim of studying the water balance and the dependency of sea level oscillations on meteorological and hydrological forcing. This forcing induces seasonal variations of mean sea level with oscillations of similar to 10-15 cm. The consistency between satellite and tidal gauge data is demonstrated in several coastal locations, and a mean ascending trend of similar to 3 cm yr(-1) is found in the two data sets. The variability in all components of water balance, including the Bosphorus outflow calculated as the difference between the fresh water flux and the time rate of sea level change estimated from altimeter data, is analyzed. The T/P data give very clear signals in the patterns of amplitudes of oscillations at intraannual, seasonal, and interannual timescales that help in understanding the variability of circulation. The intraannual variations are well pronounced on the continental slope and shelf and reach highest amplitudes in the areas of Sevastopol and Batumi quasi-permanent eddies. The dearest representation of oscillations with seasonal periodicity exists in the area of Batumi Eddy. This variability is associated with the transition between states with intense cyclonic circulation in winter and weaker (sometimes anticyclonic) circulation in summer-fall period, The Sevastopol Eddy is not clearly resolved in the seasonal variability. The interannual variability has the strongest signature in the area of western gyre and southeastern Black Sea. The analysis of satellite data supports some earlier studies on the circulation based on dynamic computations and numerical modeling. They make it possible to estimate the amount of water exchanged between coastal and open ocean areas caused by the time variability in the Ekman drift. The good quality of altimeter data and the high level of signals could ensure more accurate numerical simulations by means of data assimilation.

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.

The Black Sea general circulation is simulated by a primitive equation model with active free surface. The forcing is seasonally variable and is based on available climatic data. The model reproduces the main features of the Black Sea circulation, including the river discharge effects on the mean sea level and the Bosphorus outflow. Model results show that the simulated sea surface elevation increases in spring over the whole sea, reaching a maximum in the Danube delta area. In the same region, a minimum is observed in winter. The amplitude of the seasonal oscillations (about 8–12 cm over the whole basin) is of the same order of magnitude as the maximum horizontal variations (about 15–18 cm between the coastal areas and the basin interior). This strong signal formed mostly by river discharges, along with the seasonal variability in the other forcing functions and the local dynamics creates a well-pronounced interannual variability. The performance of the model in simulating the seasonal and interannual variability is critically analyzed, with a special attention on the cold intermediate water formation and the circulation in the upper 150 m. The simulations demonstrate that the source of intermediate waters is on the shelf, and that the water mass in the core of cold intermediate layer changes with time as a response to the periodic forcing at sea surface. This type of variability is characterized by pronounced interannual changes, proving that important differences could exist between water mass structure in different years, even when using identical atmospheric forcings each year.

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.

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.

A general circulation model for the Black Sea, based on the Bryan and Cox primitive equation model, is used to study the sea's response to atmospheric variability. The forcing is based on well-known bulk-formulas, model simulated sea surface temperatures and on atmospheric analysis data for temperature, relative humidity and winds. The climatology of the forcing and the model response are analyzed in different frequency ranges. Model data give strong indications that interannual variability can be easily traced in the behavior of the modelled averaged characteristics, particularly in the core of the intermediate layer, which is extremely sensitive to convection. Experiments with smoothed and non-smoothed atmospheric data show that short-period atmospheric variability, related to the atmospheric cyclones, is of paramount significance not only for the ocean convection, which has relatively short time scales, but also for some permanent and very important climatic characteristics, such as the water masses.

Based on an analysis of observations and one-dimensional coupled hydrophysical biogeochemical model, long-term variability of the physical and biogeochemical structure of oxic and suboxic layers in the Black Sea is studied here. The correlation between large-scale atmospheric forcing [2 m air temperature, surface level pressure, surface wind and North Atlantic oscillation (NAO) index] and local responses is the main point. The comparison of model performance with respect to spatial and temporal distribution of biogeochemical variables against observed vertical distribution patterns is quite good. It is demonstrated that during 1960–2000, the long-term variability of winter-mean-simulated SST in the Black Sea is reasonably well correlated with the variability of 2 m air temperature. Furthermore, it is demonstrated that the thermal state of the upper ocean impacts largely the variability of concentration of biogeochemical variables, such as oxygen and nitrate. The teleconnection between NAO and Black Sea biogeochemistry manifests differently for the periods 1960–2000. The corresponding regime shifts are also associated in a vital way with the large-scale forcing.

Surface winds are crucial for accurately modeling the surface circulation in the coastal ocean. In the present work, high-frequency radar surface currents are assimilated using an ensemble scheme which aims to obtain improved surface winds taking into account European Centre for Medium-Range Weather Forecasts winds as a first guess and surface current measurements. The objective of this study is to show that wind forcing can be improved using an approach similar to parameter estimation in ensemble data assimilation. Like variational assimilation schemes, the method provides an improved wind field based on surface current measurements. However, the technique does not require an adjoint, and it is thus easier to implement. In addition, it does not rely on a linearization of the model dynamics. The method is validated directly by comparing the analyzed wind speed to independent in situ measurements and indirectly by assessing the impact of the corrected winds on model sea surface temperature (SST) relative to satellite SST.

International audience; This work is dedicated to the study of the climate variability of the Mediterranean Sea, in particular the study of the Eastern Mediterranean Transient (EMT) which occurred in the early 1990s. Simulations of the 1961-2000 period have been carried out with an eddy-permitting Ocean General Circulation Model of the Mediterranean Sea, driven by realistic interannual high-resolution air-sea fluxes. Using different databases for the river runoff, Black Sea inflow, and Atlantic thermohaline characteristics at climatological or interannual scales, we assess the effects of the non-atmospheric hydrological forcings on the simulation of the interannual variations of the Mediterranean circulation. The evolution of the basin-scale heat content is in very good agreement with the observations (especially in the surface and intermediate layers), while the agreement is lower for the evolution of the salt content. Convection events in the Aegean Sea are noticed in the simulations between 1972 and 1976, in the late 1980s, and around the EMT period. The formation rates of Cretan Deep Water (CDW) are different during these periods, allowing or preventing the spreading of CDW into the eastern Mediterranean. The sequence of the EMT events is well reproduced: the high winter oceanic surface cooling and net evaporation over the Aegean Sea in the early 1990s, the high amount of dense CDW formed during these winters, and then the overflow and the spreading of this CDW in the eastern Mediterranean. Among the preconditioning processes suggested in the literature, we find that changes in the Levantine surface circulation, possibly induced by the presence in the Cretan Passage of anticyclonic eddies and a lasting period with reduced net precipitation over the eastern Mediterranean, lead to an increase of the salt content of the Aegean Sea. Changes in the Black Sea freshwater inflow or in the characteristic of the Atlantic Water entering at the Gibraltar Strait also modify the thermohaline state of the Aegean Sea before the EMT. But, as none of these preconditioning factors has a lasting impact on lowering the vertical stratification of the Aegean Sea, we conclude that concerning the EMT, the major triggering elements are the atmospheric fluxes and winds occurring in winters 1991-1992 and 1992-1993.

The Princeton OGCM is used to simulate the circulation in the Mediterranean Sea. A combination of prognostic and robust diagnostic techniques are applied to Levitus' (1982) data set in order to adjust the initial data to model physics and to restore to some extent the most general climatic features in salinity and temperature close to their climatic values. The simulated data are used to calculate water, salt and heat transport across sections in the Western Mediterranean Sea. Model estimates are compared with the estimates based on global water and salt budget analysis. It is shown that a strong correlation exists between vertical density anomaly and the baroclinic velocity when the two layer structure is well developed and in particular when the deep current direction is opposite to the direction of the surface current. For the Mediterranean Sea models, a formulation of open boundary conditions at the Straits is also discussed.

Bryan's (1989) model is used in this study to simulate the circulation in the Black Sea. Wind stress, heat exchange, precipitation and evaporation at sea surface are prescribed from climatic data. River inflow and the inflow from the Mediterranean Sea are taken from observations. Two types of models are discussed: GCM with horizontal resolutionΔλ = 1°, Δφ = 0.5° and 12 levels; and EGCM with horizontal resolutionΔλ = 20′,Δφ = 10′ on the same levels. GCM is designed to study the role of the wind and buoyancy fluxes for the establishment of circulation. Model data shows that in the baroclinic experiments kinetic energy is less than in the barotropic experiment. This is a consequence of the formation of a strong halocline in the upper layer, where a substantial part of the energy input transforms into potential energy. The specific vertical stratification in the Black Sea restricts the penetration of seasonal signal into deep layers, and preconditions the establishment of a cold intermediate layer. Model data from EGCM shows that eddies contribute to some change in the transport mechanisms, compared to those in GCM. Seasonal forcing affects substantially the variability of the main Black Sea gyre. Comparisons between model and observational data indicate that models resolve the main features of the Black Sea circulation. They illustrate correctly physical processes in strong stratified geophysical fluids, and particularly in semi-enclosed basins, where precipitation and runoff exceeds evaporation.

Publisher Summary This chapter presents a description of the major features of the large-scale Mediterranean Sea circulation and the most important changes that have been observed in the physical oceanography of the basin. It describes the basic circulation of the Mediterranean Sea that has long been recognized to be that of a concentration basin. The transformation of the inflowing Atlantic water to outflowing Mediterranean water is made through a thermohaline cell that involves the whole basin and leads to the formation of the Levantine Intermediate Water (LIW). The chapter discusses the oceanographic changes within the Mediterranean Basin concentrating mainly on the internal processes of the basin in relation to the regional forcing. It focuses on the large-scale thermohaline circulation of the Mediterranean and examines sub-basin processes when these are directly linked with deep water formation. It discusses two other key factors that are linked to the oceanic circulation, sea level variations and the wind–wave field.

Forcing mechanisms for sea level variability in the Black Sea are investigated in the context of an observed increase in the sea level of this basin by 2.5 mm/yr over the last 60 years. Temperature and salinity variations computed from the Mediterranean Data Archeology and Rescue (MEDAR) data set exhibit significant interdecadal variability. However, the corresponding steric height variation does not show a long-term increase and thus cannot account for the observed change in sea level. The impact of surface freshwater flux (P-E) changes is also investigated using two independent data sets. The first data set, which is based on measurements collected in the basin, can explain most of the sea level variability, with only 0.8 mm/yr remaining unexplained. The second data set, output from the National Centers for Environmental Prediction/National Center for Atmospheric Research (NCEP/NCAR) reanalysis, is unable to explain any of the observed trend. Potential contributions from changes in river runoff and surface pressure are quantified but found to be minor terms. By comparing the observed salinity changes with the sea level rise and the P-E variability in the first data set, we infer that the P-E variations are the primary cause for the observed sea level rise, while land movements are likely to partly contribute, too. The relationship of Black Sea temperature and salinity variability with corresponding variability in the connected Aegean Sea has also been explored. A significant correlation is found between the salinity of the upper water of the Aegean Sea and the layer between 50 and 300 m in the Black Sea, indicating that the latter layer is a product of the Mediterranean inflow.

Available climatic and atmospheric analysis data have been used to prepare forcing functions for the Black Sea numerical model, based on the Bryan-Semtner-Cox Modular Ocean Model and including parameterizations for the atmosphere-ocean exchange, inflow through the strait of Bosphorus and the Mediterranean plume. Atmospheric data from different sources are compared and the drawbacks of some of them illustrated. A new wind stress data set, based on ship observations, is prepared. Compared to the existing wind stress estimates, the present ones use additional data and more accurate parameterization of the boundary layer physics. The intercomparison between forcing data sets is focused on the heat flux and freshwater flux at the sea surface. The model simulates adequately vertical stratification, seasonal variability and horizontal patterns. Five data sets for heat flux, freshwater flux and wind stress are used in different combinations to study the model response. The large differences between the simulations, forced by different wind stress and identical thermohaline forcing, justify the computation of the new wind stress. Though the forcing data used are perhaps close to the best available at the moment for the Black Sea, the model simulations range in large intervals and some of them are very poor. The model responses to forcing functions of different origin give rough estimates on the possible errors in present-day simulations. Some inconsistencies give clear indications that further verifications, improvements of the forcing functions, and intercomparisons between the responses simulated by the ocean circulation models are needed., Des données climatiques et atmosphériques de la littérature ont été utilisées pour déterminer les fonctions de forçage dans une modélisation numérique de la mer Noire. Fondé sur le modèle modulaire de l'océan de Bryan-Semtner-Cox, le présent modèle prend en compte les échanges entre l'océan et l'atmosphère, l'apport par le détroit du Bosphore et le panache méditerranéen. Des données atmosphériques de différentes origines sont comparées et leurs défauts éventuels sont indiqués. Une représentation plus précise de la couche-limite est obtenue en considérant un nouveau paramètre, la tension du vent, fournie par des observations de navires. La comparaison des séries de données porte sur les flux de chaleur et d'eau douce à travers la surface libre. Le modèle représente correctement la stratification verticale, la variabilité saisonnière et les répartitions horizontales. Cinq séries de données, pour le flux de chaleur, le flux d'eau douce et la tension de vent, sont utilisées dans différentes combinaisons afin d'étudier la réponse du modèle. Avec plusieurs tensions de vent et un même forçage thermohalin, de grandes différences apparaissent entre les simulations, justifiant le calcul de la nouvelle tension de vent. Bien que les données utilisées pour le forçage soient probablement parmi les meilleures actuellement disponibles pour la mer Noire, les résultats se situent dans de larges intervalles et certains modèles sont plutôt médiocres. Le modèle donne une estimation des erreurs que l'on peut attendre dans le cadre des modèles forcés par des données actuelles. Des incohérences entre différences simulations montrent la nécessité de vérifications, améliorations des forçages et intercomparaisons de résultats

The paper addresses the physical mechanisms of eddy variability in the Black Sea and is based on analyses of simulation data. The model, used as a research tool, is the Bryan-Semtner-Cox model with 11 km horizontal resolution, forced by annual mean data. The study focuses on the basin oscillations, meanders, eddy formation and dissipation. Sub-basin scale eddies are spontaneously formed and propagate to the west. The dissipation is extremely strong in the western basin, where eddies slow down and their scales decrease. Simplistic models, corresponding to the Black Sea conditions, demonstrate the impact of different physical processes, basin shape, and forcing for the formation of the circulation. The bottom topography control has an utmost importance for enhancing baroclinic oscillations with interannual time periods. Model simulated phenomenology, time and length scales are compared against observations.

A hydrodynamical model of the general circulation in the Black Sea has been build up, using the GHER primitive equation model. In a first experiment, a model with 15 km horizontal resolution and 20 vertical levels is used to compute the typical seasonal cycle by forcing the model with climatological monthly mean fields of temperature, salinity and wind stress at the air sea interface. Furthermore, the corresponding river discharges of the Danube, Dnestr, Dnepr and Rioni are taken into account. The simulation results are presented in view of their later use in a 3D ecosystem model. An ecosystem model at basin scale is then defined by a nitrogen cycle, which is described by 5 state variables, i.e. NO 3 − , NH 4 + , Phytoplankton, Zooplankton and Dissolved organic matter. This model has been chosen as a first test for coupling the 3D hydrodynamical model and the ecosystem model. The ecosystem model is imbedded on-line into the 3D hydrodynamical model with a superimposed cycle for the light intensity. In this paper, the main findings of the hydrodynamic model are summarized stressing those aspects of the current and hydrological fields that affect the ecosystem. The ecosystem model is briefly described emphasizing the modifications made to adapt the equations to the Black Sea’s environmental conditions.

The response of the water masses to atmospheric forcing is studied using Bryan and Cox GCM, which is forced with twice daily atmospheric analysis data of the NMC. The model simulates, along with the other fields, Sea Surface Temperature (SST). Model simulated SST is regarded as synthetic observation data, and is assimilated in further experiments with the aim to improve model estimates in the case of imperfect atmospheric forcing. Model results reveal criteria for data sampling, which depend on the characteristic time scales of the convection events. It is shown that model simulated water mass characteristics could be substantially improved if data assimilation is in accord with the specific physical processes.

This paper describes the seasonal characteristics of the Mediterranean Sea general circulation as simulated by a primitive equation general circulation model. The forcing is composed of climatological monthly mean atmospheric parameters, which are used to compute the heat and momentum budgets at the air-sea interface of the model. This allows heat fluxes to be determined by a realistic air-sea interaction physics. The Strait of Gibraltar is open, and the model resolution is in the horizontal and 19 levels in the vertical. The results show the large seasonal cycle of the circulation and its transient characteristics. The heat budget at the surface is characterized by lateral boundary intensifications occurring in downwelling and up welling areas of the basin. The general circulation is composed of subbasin gyres, and cyclonic motion dominates the northern and anticyclonic motion the southern part of the basin. The Atlantic stream which enters from Gibraltar and assumes the form of different boundary current subsystems is a coherent structure at the surface. At depth it appears as current segments and jets around a vigorous gyre system. The seasonal variability is manifested not only by a change in amplitude and location of the gyres but also by the appearance of seasonally recurrent gyres in different parts of the basin. Distinct westward propagation of these gyres occurs, together with amplitude changes. For the first time a Mersa-Matruh Gyre is successfully simulated due to the introduction of our heat fluxes at the air-sea interface. The seasonal thermocline is formed each summer, and a deep winter mixed layer is produced in the region of Levantine intermediate water formation. Deep water renewal does not occur, probably due to the climatological forcing used.

The numerical results of a coarse resolution Black Sea circulation model with horizontal grid intervals Δλ= 1 deg and Δϕ = 0.5 deg and 12 levels are used to initialize the model with a fine resolution: Δλ = 20 min and Δϕ = 10 min on the same vertical levels. The boundary conditions include wind forcing, heat and salt fluxes on the sea surface and inflows through the Strait of Bosphorus and river inflow as well. Bryan's (1969) numerical model with Laplacian horizontal mixing is used to simulate the circulation. Seasonality in the model forcing contributes to some fundamental changes in eddy processes which can be better observed below the halocline. The results show a trend of better organization of the eddy energy patterns when the model is driven by seasonally variable boundary conditions. Comparison of the model results with some experimental ones is done.