In this work, we address the Black Sea setup of Nucleus for European Modelling of the Ocean (NEMO... more 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
The study addresses rotational motion of geophysical fluids in the horizontal and vertical planes... more The study addresses rotational motion of geophysical fluids in the horizontal and vertical planes. It is aimed mainly at tracing the development of high-resolution numerical modeling of the ocean, as well as at demonstrating new physical processes due to more correct consideration both of the tides in the eddy-resolving numerical models and sub-mesoscale dynamics in the models of the sea straits. Methods and Results. The ocean eddies and their interaction with tides are studied using numerical simulations by four NEMO models for the European NorthWest shelf with the resolutions ranging from 7 to 1.5 km. The vertical characteristics of motion in the Bosporus Strait were studied using numerical simulations with SCHISM, the unstructured grid model with the ultra-fine model resolution (less than 100 m). The barotropic tidal forcing resulted in substantial flattening of the slopes of the spectral curves. The most important difference between the spectral featurws of four models occurs in the motion rotational component. In the model with the 1.5 km resolution, the magnitude of the vorticity power spectral density at the scales ~70 km is by an order of magnitude higher than in the other three models. Although most of the tidal flattening is associated with the internal tides, beyond a certain horizontal resolution, the eddy dynamics become affected by the barotropic tides. The shelf of the Biscay Bay and the shallows around the Faroe Islands are the most sensitive areas to adding of the barotropic tides to the model forcing. Due to the grid ultra-fine resolution, new elements of physical motion emerged in the Bosporus region. The lateral circulation is dominated by the systems of multiple circulation cells with the scales ~ 1 km. In some areas, the lateral flow magnitude exceeds 0.5 m/s, which is comparable with the magnitude of the axial flow. This reveals importance of the helical elements of the strait circulation for overturning of water masses in the Bosporus. Conclusions. Without proper resolution, the models of tidal oceanic dynamics simulate the ocean general circulation, but do not describe correctly the energy cascades at the eddy scales including interaction between the tides and the mesoscale eddies. Absence of this sub-mesoscale dynamics in the models can largely affect their capability to simulate the two-layer inter-basin exchange.
This study addresses the impact of coupling between wind wave and circulation models on the quali... more 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 unstructured-grid model (FVCOM) coupled to a surface wave model (FVCOM-SWAVE) with two differe... more 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 and a storm situation in the southern North Sea. One setup covers the whole North Sea with moderately increased grid resolution at the coast while the other setup 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 results for the wave energy show big differences between moderate and storm conditions with time-averaged values up to 200 kW/m.
Here, we address the sediment dynamics in the Black Sea based on analysis of re-2 mote sensing da... more Here, we address the sediment dynamics in the Black Sea based on analysis of re-2 mote sensing data from MERIS satellite spectrometer and numerical simulations 3 with NEMO model. Boundary conditions consist of realistic meteorological forcing, 4 including significant wave height generated by WAM model. A number of sensitivity 5 runs was analysed with the aim to find the most suitable parameters governing sed-6 iment fluxes. The comparison between numerical simulations and remote sensing 7 data gives credibility to the quality of simulations. The combined effect of wind 8 waves and currents in the bed layer controls the sediment resuspension that appears 9 to be the major basin wide source of sediment. Sensitivity experiments included or 10 excluded different forcing terms, e. g. sediment flux from rivers enable to determine 11 the spatial extensions of different point-sources. It is concluded that wind-wave 12 forcing is manifested in the sediment dynamics through episodic high energy events 13 contributing to the increase of horizontal sediment fluxes over the northwestern 14 shelf. Both satellite images and numerical model simulations demonstrated that the 15 penetration of suspended sediment into the basin interior is governed by the dy-16 namics of coastal and open-ocean eddies. While fine sediment at sea surface could 17 cross the continental slope extending into the open ocean, coarser fractions follow 18 the bottom and their penetration into the open ocean is limited. The conclusion 19 is thus that the deposition patterns correlate with the specific shape of Black Sea 20 topography largest depositions are observed in the area of continental slope. Anal-21 ysis of numerical simulations could motivate further observations aiming to verify 22 the possibility identified in the numerical simulations that winter convection/mixing 23 could also affect the propagation of fine suspended sediment. 24 about 1,864,000 km 2 covers large parts of Europe and Asia (figure 1), providing a 30 total freshwater supply of 3 × 10 2 km 3 per year. This large amount of fresh water 31 flux along with the narrow opening in the strait of Bosporus make the Black Sea a 32 typical estuarine basin where the surface salinity is about half that of the Mediter-33 ranean's. The strong vertical (haline) stratification limits the vertical exchange and 34 creates a unique chemical and biological environment. 35 The circulation in the Black Sea is structured in two connected gyre systems called 36 Eastern and Western Gyres encompassing the basin (the Rim Current, figure 2). 37 This is a frontal current associated with a difference of ∼0.2 m between sea level 38 in the coastal and open sea. Its position is controlled by the bottom topography, 39 which is characterised by a gentle slope in the northwestern and western Black Sea 40 and abrupt one along the southern and eastern coasts (see the contrast between 41 light blue and dark blue areas in figure 1). Anticyclonic eddies are formed between 42 the Rim Current and the coast, the most prominent of them are the Batumi and 43 Sevastopol eddies (figure 2) 44 Annual production of organic mater of 818×10 6 t yr −1 exceeds substantially the total 45 sediment load from the rivers around the Black Sea periphery of about 150×10 6 t yr −1
The recent drop of sea level in the Aral Sea of about 0.6 m year À1 during the last 40 years repr... more The recent drop of sea level in the Aral Sea of about 0.6 m year À1 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.
We demonstrate in this paper that satellite altimeter data resolve the drop in the Aral Sea level... more We demonstrate in this paper that satellite altimeter data resolve the drop in the Aral Sea level during 1993-2000 of about 0.6 m per year resulting in a change of surface area from 35000 to 22000 km 2 and volume from 270 to 130 km 3. The sudden drop in the sea level of the Northern basin on 04.21.1999 resulting from dam break-up is also clearly resolved. The temporal and spatial variability of sea level reveals response patterns which are characteristic for friction dominated shallow sea dynamics. The combination of salinity and sea-level data enables to identify the major events of environmental transition, which are associated with the temporal variability in the total salt content: (1) a peak in 1993-1994, and (2) an increasing trend in the last decade. These events are indicative of an increase in the discharge of ground water, but could also reveal overestimated salinity estimates from recent observations.
Deep Sea Research Part I: Oceanographic Research Papers, 2004
Data from field observations and numerical model simulations are used to understand and quantify ... more Data from field observations and numerical model simulations are used to understand and quantify the pathways by which passive tracers penetrate into the Black Sea intermediate and deep layers. Chlorofluorocarbon (CFC) concentrations measured during the1988 R.V. Knorr cruise show strong decrease with increasing density in the Black Sea and illustrate the very slow rate of ventilation of deep water in this basin. We develop a 3D numerical model based on the Modular Ocean Model (MOM), and calibrate it in a way to produce consistent simulations of observed temperature, salinity and CFCs. One important feature is the implementation of a special parameterization for convection, which is an alternative of the convective adjustment in MOM and handles the penetration of the Bosporus plume into the halocline. The model forcing includes interannually variable wind, heat and water fluxes constructed from Comprehensive Ocean-Atmosphere Data Set and ECMWF atmospheric analysis data and river runoff data. The analysis of observations and simulated data are focused on correlations between thermohaline and tracer fields, dynamic control of ventilation, and the relative contributions of sources at the sea surface and outflow from the Bosporus Strait in the formation of intermediate and deep waters. A simple theory is developed which incorporates the outflow from the strait along with the vertical circulation (vertical turbulent mixing and Ekman upwelling) and reveals their mutual adjustment. The analyses of simulated and observed CFCs demonstrate that most of the CFC penetrating the deep layers has its source at the sea surface within the Black Sea rather than from the Marmara Sea via the Bosporus undercurrent. Under present-day conditions, the surface CFC signals have reached only the upper halocline. Intrusions below 600 m are not simulated. The major pathways of penetration of CFCs are associated with cold-water
In this work, we address the Black Sea setup of Nucleus for European Modelling of the Ocean (NEMO... more 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
The study addresses rotational motion of geophysical fluids in the horizontal and vertical planes... more The study addresses rotational motion of geophysical fluids in the horizontal and vertical planes. It is aimed mainly at tracing the development of high-resolution numerical modeling of the ocean, as well as at demonstrating new physical processes due to more correct consideration both of the tides in the eddy-resolving numerical models and sub-mesoscale dynamics in the models of the sea straits. Methods and Results. The ocean eddies and their interaction with tides are studied using numerical simulations by four NEMO models for the European NorthWest shelf with the resolutions ranging from 7 to 1.5 km. The vertical characteristics of motion in the Bosporus Strait were studied using numerical simulations with SCHISM, the unstructured grid model with the ultra-fine model resolution (less than 100 m). The barotropic tidal forcing resulted in substantial flattening of the slopes of the spectral curves. The most important difference between the spectral featurws of four models occurs in the motion rotational component. In the model with the 1.5 km resolution, the magnitude of the vorticity power spectral density at the scales ~70 km is by an order of magnitude higher than in the other three models. Although most of the tidal flattening is associated with the internal tides, beyond a certain horizontal resolution, the eddy dynamics become affected by the barotropic tides. The shelf of the Biscay Bay and the shallows around the Faroe Islands are the most sensitive areas to adding of the barotropic tides to the model forcing. Due to the grid ultra-fine resolution, new elements of physical motion emerged in the Bosporus region. The lateral circulation is dominated by the systems of multiple circulation cells with the scales ~ 1 km. In some areas, the lateral flow magnitude exceeds 0.5 m/s, which is comparable with the magnitude of the axial flow. This reveals importance of the helical elements of the strait circulation for overturning of water masses in the Bosporus. Conclusions. Without proper resolution, the models of tidal oceanic dynamics simulate the ocean general circulation, but do not describe correctly the energy cascades at the eddy scales including interaction between the tides and the mesoscale eddies. Absence of this sub-mesoscale dynamics in the models can largely affect their capability to simulate the two-layer inter-basin exchange.
This study addresses the impact of coupling between wind wave and circulation models on the quali... more 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 unstructured-grid model (FVCOM) coupled to a surface wave model (FVCOM-SWAVE) with two differe... more 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 and a storm situation in the southern North Sea. One setup covers the whole North Sea with moderately increased grid resolution at the coast while the other setup 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 results for the wave energy show big differences between moderate and storm conditions with time-averaged values up to 200 kW/m.
Here, we address the sediment dynamics in the Black Sea based on analysis of re-2 mote sensing da... more Here, we address the sediment dynamics in the Black Sea based on analysis of re-2 mote sensing data from MERIS satellite spectrometer and numerical simulations 3 with NEMO model. Boundary conditions consist of realistic meteorological forcing, 4 including significant wave height generated by WAM model. A number of sensitivity 5 runs was analysed with the aim to find the most suitable parameters governing sed-6 iment fluxes. The comparison between numerical simulations and remote sensing 7 data gives credibility to the quality of simulations. The combined effect of wind 8 waves and currents in the bed layer controls the sediment resuspension that appears 9 to be the major basin wide source of sediment. Sensitivity experiments included or 10 excluded different forcing terms, e. g. sediment flux from rivers enable to determine 11 the spatial extensions of different point-sources. It is concluded that wind-wave 12 forcing is manifested in the sediment dynamics through episodic high energy events 13 contributing to the increase of horizontal sediment fluxes over the northwestern 14 shelf. Both satellite images and numerical model simulations demonstrated that the 15 penetration of suspended sediment into the basin interior is governed by the dy-16 namics of coastal and open-ocean eddies. While fine sediment at sea surface could 17 cross the continental slope extending into the open ocean, coarser fractions follow 18 the bottom and their penetration into the open ocean is limited. The conclusion 19 is thus that the deposition patterns correlate with the specific shape of Black Sea 20 topography largest depositions are observed in the area of continental slope. Anal-21 ysis of numerical simulations could motivate further observations aiming to verify 22 the possibility identified in the numerical simulations that winter convection/mixing 23 could also affect the propagation of fine suspended sediment. 24 about 1,864,000 km 2 covers large parts of Europe and Asia (figure 1), providing a 30 total freshwater supply of 3 × 10 2 km 3 per year. This large amount of fresh water 31 flux along with the narrow opening in the strait of Bosporus make the Black Sea a 32 typical estuarine basin where the surface salinity is about half that of the Mediter-33 ranean's. The strong vertical (haline) stratification limits the vertical exchange and 34 creates a unique chemical and biological environment. 35 The circulation in the Black Sea is structured in two connected gyre systems called 36 Eastern and Western Gyres encompassing the basin (the Rim Current, figure 2). 37 This is a frontal current associated with a difference of ∼0.2 m between sea level 38 in the coastal and open sea. Its position is controlled by the bottom topography, 39 which is characterised by a gentle slope in the northwestern and western Black Sea 40 and abrupt one along the southern and eastern coasts (see the contrast between 41 light blue and dark blue areas in figure 1). Anticyclonic eddies are formed between 42 the Rim Current and the coast, the most prominent of them are the Batumi and 43 Sevastopol eddies (figure 2) 44 Annual production of organic mater of 818×10 6 t yr −1 exceeds substantially the total 45 sediment load from the rivers around the Black Sea periphery of about 150×10 6 t yr −1
The recent drop of sea level in the Aral Sea of about 0.6 m year À1 during the last 40 years repr... more The recent drop of sea level in the Aral Sea of about 0.6 m year À1 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.
We demonstrate in this paper that satellite altimeter data resolve the drop in the Aral Sea level... more We demonstrate in this paper that satellite altimeter data resolve the drop in the Aral Sea level during 1993-2000 of about 0.6 m per year resulting in a change of surface area from 35000 to 22000 km 2 and volume from 270 to 130 km 3. The sudden drop in the sea level of the Northern basin on 04.21.1999 resulting from dam break-up is also clearly resolved. The temporal and spatial variability of sea level reveals response patterns which are characteristic for friction dominated shallow sea dynamics. The combination of salinity and sea-level data enables to identify the major events of environmental transition, which are associated with the temporal variability in the total salt content: (1) a peak in 1993-1994, and (2) an increasing trend in the last decade. These events are indicative of an increase in the discharge of ground water, but could also reveal overestimated salinity estimates from recent observations.
Deep Sea Research Part I: Oceanographic Research Papers, 2004
Data from field observations and numerical model simulations are used to understand and quantify ... more Data from field observations and numerical model simulations are used to understand and quantify the pathways by which passive tracers penetrate into the Black Sea intermediate and deep layers. Chlorofluorocarbon (CFC) concentrations measured during the1988 R.V. Knorr cruise show strong decrease with increasing density in the Black Sea and illustrate the very slow rate of ventilation of deep water in this basin. We develop a 3D numerical model based on the Modular Ocean Model (MOM), and calibrate it in a way to produce consistent simulations of observed temperature, salinity and CFCs. One important feature is the implementation of a special parameterization for convection, which is an alternative of the convective adjustment in MOM and handles the penetration of the Bosporus plume into the halocline. The model forcing includes interannually variable wind, heat and water fluxes constructed from Comprehensive Ocean-Atmosphere Data Set and ECMWF atmospheric analysis data and river runoff data. The analysis of observations and simulated data are focused on correlations between thermohaline and tracer fields, dynamic control of ventilation, and the relative contributions of sources at the sea surface and outflow from the Bosporus Strait in the formation of intermediate and deep waters. A simple theory is developed which incorporates the outflow from the strait along with the vertical circulation (vertical turbulent mixing and Ekman upwelling) and reveals their mutual adjustment. The analyses of simulated and observed CFCs demonstrate that most of the CFC penetrating the deep layers has its source at the sea surface within the Black Sea rather than from the Marmara Sea via the Bosporus undercurrent. Under present-day conditions, the surface CFC signals have reached only the upper halocline. Intrusions below 600 m are not simulated. The major pathways of penetration of CFCs are associated with cold-water
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Papers by Emil Stanev