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Just as CryoSat-2, Sentinel-3 embarks on board a radar altimeter (SRAL) with the novel Synthetic Aperture Radar (SAR) mode that enables higher resolution and more accurate altimeter-derived parameters in the coastal zone, thanks to the reduced along-track footprint. Exploiting the SAR data in the recent years, many researchers have already proven that the performance of SAR altimetry with specific coastal retrackers is superior to collocated Pseudo-Low Resolution Mode (PLRM) coastal altimetry algorithms but they also pointed out that residual errors due to land contamination are still present in the very proximity of the land (0–3 km). The objective of this work is to further improve these results by exploiting extra information provided by SAR altimeters, namely the so-called Range Integrated Power (RIP), the new waveform built by a simple integration of the Doppler beams in the range direction. The RIP characterizes the backscattering state of the ground cell, towards which all the Doppler beams have been steered. These developments lead to a new retracker, here coined SAMOSA++, in which the RIP, as computed from the L1B-S data, is converted into a surface backscattering profile and directly integrated in the SAMOSA retracker as part of the model formulation itself. In this way, the modified SAMOSA model is automatically and autonomously able to cope with the different return waveform shapes from different surface types: either diffusive or specular. The mean square slope computed from the RIP is also estimated, representing a new output of the retracker. The performance of this new retracker is here cross-compared against its previous version, SAMOSA+, and against the standard Sentinel-3 marine PDGS (Payload Data Ground Segment) SAR retracker (SAMOSA2) in both coastal zone and open ocean in order to ensure a seamless transition between these zones. The new retracker SAMOSA++ is validated in the North East Atlantic region, where appropriate in situ validation data are available. The retrievals from the new retracker are cross-compared against the network of tide gauges and buoys in the German Bight and versus the output of the GCOAST Helmholtz-Zentrum Geesthacht (HZG) regional circulation and wave model. In addition, sea level estimates derived with different ocean tide and wet path delay geophysical correction models are compared. Results indicate that in this region the best geophysical correction models are the FES2014b tide model and the GPD+ wet tropospheric correction that incorporates data from the Sentinel-3 on-board radiometer. Analyses show that both SAMOSA+ and SAMOSA++ ensure the continuity of the PDGS SAR Marine retracker in the open ocean, leading to clear improvements in the coastal zone, larger for SAMOSA++ than for SAMOSA+. In summary, the new SAMOSA++ retracker retrieves more accurate altimetric parameters in the coastal zone, with a better consistency with respect to regional ocean models and in situ data.

Reliable wave forecasts and hindcasts, together with long-term statistical analysis of extreme conditions, are of utmost importance for monitoring marine areas. Indeed, there is general consensus that high-quality predictions of extreme events during marine storms can substantially contribute to avoiding or minimizing human and material damage, especially in busy waterways such as the Mediterranean and Black Seas. So far, however, the wave climate characterization (average and anomaly relative to the average) has focused on the bulk characterization of the significant wave height Hs, and it has lacked a description of the individual waves, such as the maximum ones that may occur at a given location in the sea. To fill this gap, we provide the intensity and geographical distribution of the maximum waves in the Mediterranean and Black Seas over 27 years (1993-2019), by representing the average annual (1993-2018) and anomaly for 2019 relative to the average of the 99th percentile of the expected maximum wave height Hm and crest height Cm. The analysis combines wave model hindcasts available through CMEMS model setup and the wave model WAVEWATCH III®, both forced with ECMWF ERA5 reanalysis winds. Results show that in 2019 maximum waves were smaller than usual in the Black Sea (anomalies of Hm up to -1.5 m), while in the Mediterranean Sea a markedly positive anomaly (+2.5 m for Hm) was found in the southern part of the basin. The peculiar 2019 configuration seems to be caused by a widespread atmospheric stability over the Black Sea and by depressions that rapidly passed over the Mediterranean Sea.

Direct estimate of ocean surface motion sensed by the Doppler shift of the surface includes ocean surface current and a wind-wave induced artefact surface velocity (WASV). The Sentinel-1 (S1) C-band SAR mission includes direct ocean surface motion estimates as an operational Level-2 Ocean (OCN) Radial VeLocity (RVL) product. The existing operational RVL products suffer from significant uncorrected platform and instrument effects that presently prevent exploitation of the data. This paper proposes a simple method to calibrate and correct for these effects and evaluate the benefit of these corrections over 2.5 years S1A acquisition against ground truth measurements. A specific geometry for S1 has been chosen for S1-A over the HF radar (HFR) instrumented site in the German Bight. The 78 S1A snapshots end in 56 match-ups within 20 minutes of HFR measurements. HFR velocity fields were projected in the same radial direction as S1A. Land calibration corrects for constant snapshot biases of the operational products up to 2 m/s. Besides these constant biases there is persistent relative biases within snapshots between up to 0.4 m/s in addition to the TOPSAR uncorrected scalloping effect with an amplitude of 0.1 m/s. After calibration, corrected RVL are compared against HFR with various WASV correction. Applying WASV correction with a reduced 70% C-Dop model, gives the best results with a precision of 0.25 m/s and correlation in time of 0.9. This might be due to C-Dop amplitude in up/downwind being too strong for a coastal environment as encountered in the German Bight. Quadratic mean of all 78 S1A snapshots after all corrections applied exhibits coastal current jets in good agreement with bathymetry channels and is promising as a cheap way to infer local bathymetry channels.

Estuarine ecology suffers from both physical aspects of human influence, such as dredging, and biogeochemical aspects, such as eutrophication. Apart from being dredged, modern estuaries often manifest rectified geometries deprived of meanders or other nonlinear topographic features. This study has two overarching aims, a theoretical and a practical one. The theoretical objective is to establish an understanding of the effect of physical dynamics induced by channel meanders on the biogeochemical dynamics in a typical estuarine oxygen minimum zone. The practical aim is to clarify whether and how channel curvature can mitigate the consequences of human intervention, such as dredging and eutrophication. To answer these questions, a coupled hydrodynamic and water quality model is applied to a pair of idealized funnel-shaped topographies with dimensions and axial depth distribution similar to the Elbe Estuary, Germany, serving as the prototype estuary in this study. The first topography is symmetric about the channel axis (straight channel), while the second topography contains a small section of sinusoidal meanders in the dredged limnic reach of the estuary. The setups are forced by an M-2 tide and daily salinity and temperature data at the seaward open boundary. Atmospheric and river forcings are based on regional operational and observational data to impose seasonal temperature variability and biogeochemical cycles. The model simulates tidally driven estuarine physics, as well as seasonal estuarine cycles and axial gradients of nutrients, oxygen and plankton that characterize alluvial human-shaped and eutrophied estuaries. The channel meanders in the lower limnic reaches lead to locally enhanced ebb dominance, vertical overturning and increased levels of turbulent kinetic energy. The curvature-induced dynamics decrease turbidity levels by up to 12.5% and increase oxygen concentrations by up to 14% in the area of the oxygen minimum zone, improving the ecological status of the eutrophied estuary. Finally, we assess the sustainability of the ecological benefits of the channel meanders in the face of global warming by applying a simple 2 °C warming scenario to the straight and meandering channel cases. We demonstrate that channel meanders potentially improve estuarine ecology even under increased pressure due to climate change.

The mesoscale variability in the Mediterranean Sea is investigated through eddy detection techniques. The analysis is performed over 24 years (1993–2016) considering the three-dimensional (3D) fields from an ocean re-analysis of the Mediterranean Sea (MED-REA). The objective is to achieve a fit-for-purpose assessment of the 3D mesoscale eddy field. In particular, we focus on the contribution of eddy-driven anomalies to ocean dynamics and thermodynamics. The accuracy of the method used to disclose the 3D eddy contributions is assessed against pointwise in-situ measurements and observation-based data sets. Eddy lifetimes ≥ 2 weeks are representative of the 3D mesoscale field in the basin, showing a high probability (> 60%) of occurrence in the areas of the main quasi-stationary mesoscale features. The results show a dependence of the eddy size and thickness on polarity and lifetime: anticyclonic eddies (ACE) are significantly deeper than cyclonic eddies (CE), and their size tends to increase in long-lived structures which also show a seasonal variability. Mesoscale eddies result to be a significant contribution to the ocean dynamics in the Mediterranean Sea, as they account for a large portion of the sea-surface height variability at temporal scales longer than 1 month and for the kinetic energy (50–60%) both at the surface and at depth. Looking at the contributions to ocean thermodynamics, the results exhibit the existence of typical warm (cold) cores associated with ACEs (CEs) with exceptions in the Levantine basin (e.g., Shikmona gyre) where a structure close to a mode-water ACE eddy persists with a positive salinity anomaly. In this area, eddy-induced temperature anomalies can be affected by a strong summer stratification in the surface water, displaying an opposite sign of the anomaly whether looking at the surface or at depth. The results show also that temperature anomalies driven by long-lived eddies (≥ 4 weeks) can affect up to 15–25% of the monthly variability of the upper ocean heat content in the Mediterranean basin.

The coupling of models is a commonly used approach when addressing the complex interactions between different components of the Earth system. This study presents the development of a new, high -resolution, coupled ocean and wave model system for the North Sea and the Baltic Sea, which is part of the Geestacht COAstal model SysTem GCOAST. We focus on the nonlinear feedback between strong tidal currents and wind -waves, which can no longer be ignored, in particular in the coastal zone where its role seems to be dominant. The proposed coupling parameterisations account for the feedback between of the upper ocean on the atmospheric circulation by accounting for the effects of the sea level, and ocean temperature and salinity. A focus is given on the newly implemented parameterisations that consider the effect of non-liner contribution and the component transfer of the momentum and energy fluxes from the atmosphere to the ocean thought the waves interface. Sensitivity experiments are performed to estimate the role of different wave-ocean coupling components. The performance of the coupled modelling system is illustrated for the cases of several extreme events. For example, the inclusion of wave coupling changes sea surface temperature, the mixing and ocean circulation and the total sea level leading to better agreement with in -situ and satellite observations. The model comparisons with data from satellite altimeter and in-situ observations showed that the use of the fully coupled system reduces the errors, especially under severe storm conditions.

Ocean surface gravity waves are part of the climate system, regulating the exchange of energy and momentum at the atmosphere-ocean interface. Besides that, waves have a direct impact on coastal erosion and sediment transport, in ship routing and offshore/ship infrastructure design, as well as in coastal management. Most future wave climate projections point out to statistically significant changes in wave height conditions in the global ocean towards the end of the 21st century. In the present study, the near future impact of a warmer climate on the global ocean wave climate towards the mid-21st century (2031–2060) is investigated through a 4-member “coherent” ensemble of wave climate projections: single-model (wave model WAM), single-forcing (EC-Earth) and single-scenario (RCP8.5). The wave model WAM was forced by wind speed and ice coverage from four Coupled Model Intercomparison Project Phase 5 (CMIP5) EC-Earth projections. The ensemble historic climate reference period has been evaluated by comparison against wave reanalysis/hindcast data, and wave in-situ observations. Statistically significant increases in the global mean wind speed, wave height, wave period and wave energy flux are to be expected towards the mid-twenty first century, in less than 50 years, these changes being more striking in the mid-to-high latitudes of the Southern Hemisphere. Due to the propagation of swell to lower latitudes, statistically significant increases in wave heights, periods and energy fluxes are also projected to occur in equatorial areas, especially along the eastern side of the main ocean basins, during the Austral winter. Comparatively, the projected changes in wave heights and energy fluxes are expected to occur more intensely during the first half of the 21st century, in several areas of the global ocean.

Direct sensing of total ocean surface currents with microwave Doppler signals is a growing topic of interest for oceanography, with relevance to several new ocean mission concepts proposed in recent years. Among those, WaCM (Rodriguez et al., 2019), SKIM (Ardhuin et al., 2019) and the ESA Earth Explorer candidate Harmony (ESA, 2020) and SEASTAR (Gommenginger et al., 2019).Since 2014, the spaceborne C-band SAR instruments of the Copernicus Sentinel-1 (S1) mission routinely acquire microwave Doppler data, distributed to users through operational S1 Level-2 ocean radial velocity (L2 OCN RVL) products. S1 L2 RVL data could produce high-resolution maps of ocean surface currents that would benefit ocean observing and modelling, particularly in coastal regions. However, uncorrected platform effects and instrument anomalies continue to impact S1 RVL data and prevent direct exploitation.In this paper, a simple empirical method is proposed to calibrate and correct operational S1 L2 RVL products and retrieve two-dimensional maps of surface currents in the radar line-of-sight. The study focuses on the German Bight where wind, wave and current data from marine stations and an HF radar instrumented site provide comprehensive means to evaluate S1 retrieved currents. Analyses are deliberately limited to Sentinel-1A (S1A) ascending passes to focus on one single instrument and fixed SAR viewing geometry. The final dataset comprises 78 separate S1A acquisitions over 2.5 years, of which 56 are matched with collocated HF radar data. The empirical corrections bring significant improvements to S1A RVL data, producing higher quality estimates and much better agreement with HF radar radial currents.Comparative evaluation of S1A against HF radar currents for different WASV corrections reveal that best results are obtained in this region when computing the WASV with sea state rather than wind vector input. Accounting for sea state produces S1 radial currents with a precision (std of the difference) around 0.3 m/s at ~1 km resolution. Precision improves to ~0.24 m/s when averaging over 21 × 27 km2, with correlations with HF radar data reaching up to 0.93. Evidence of wind-current interactions when tides and wind align and short fetch conditions call for further research with more satellite data and other sites to better understand and correct the WASV in coastal regions.Finally, 1 km resolution maps of climatological S1A radial currents obtained over 2.5 years reveal strong coastal jets and fine scale details of the coastal circulation that closely match the known bathymetry and deep- water coastal channels in this region. The wealth of oceanographic information in corrected S1 RVL data is encouraging for Doppler oceanography from space and its application to observing small scale ocean dynamics, atmosphere and ocean vertical exchanges and marine ecosystem response to environmental change.

A new parameterization for calculating the nonlinear near-bed wave orbital velocity in the shallow water was presented. The equations proposed by Isobe and Horikawa [1] were modified in order to achieve more accurate predictions of the peak orbital velocities. Based on field data from Egmond Beach in the Netherlands, the correction coefficient and maximum skewness were determined as functions of the Ursell number. The obtained equations were validated against measurements from Egmond Beach, and with laboratory data from small-scale wave flume experiments at Delft University of Technology and from large-scale wave flume experiments at Delft Hydraulics. Inter-comparisons with other previously developed parameterizations were also carried out. The model simulations by the present study were in good agreement with the measurements and have been improved compared to the previous ones. For Egmond Beach, the root-mean-square errors for the peak onshore (uc) and offshore (ut) orbital velocities were approximately 21%. The relative biases were small, approximately 0.013 for uc and &minus, 0.068 for ut. The coefficient of determination was in the range between 0.64 and 0.68. For laboratory experiments, the root-mean-square errors in a range of 7.2%&ndash, 24% for uc, and 7.9%&ndash, 15% for ut.

The effect of wind waves on water level and currents during two storms in the North Sea is investigated using a high-resolution Nucleus for European Modelling of the Ocean (NEMO) model forced with fluxes and fields from a high-resolution wave model. The additional terms accounting for wave-current interaction that are considered in this study are the Stokes-Coriolis force, the sea-state-dependent energy and momentum fluxes. The individual and collective role of these processes is quantified and the results are compared with a control run without wave effects as well as against current and water-level measurements from coastal stations. We find a better agreement with observations when the circulation model is forced by sea-state-dependent fluxes, especially in extreme events. The two extreme events, the storm Christian (25–27 October 2013), and about a month later, the storm Xaver (5–7 December 2013), induce different wave and surge conditions over the North Sea. Including the wave effects in the circulation model for the storm Xaver raises the modelled surge by more than 40 cm compared with the control run in the German Bight area. For the storm Christian, a difference of 20–30 cm in the surge level between the wave-forced and the stand-alone ocean model is found over the whole southern part of the North Sea. Moreover, the modelled vertical velocity profile fits the observations very well when the wave forcing is accounted for. The contribution of wave-induced forcing has been quantified indicating that this represents an important mechanism for improving water-level and current predictions.

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

A novel approach of data assimilation based on Neural Networks (NN’s) is presented and applied to wave modeling in the German Bight. The method takes advantage from the ability of NN’s to emulate models and to invert them. Combining forward and inverse model NN with the Levenberg–Marquardt algorithm provides boundary values or wind fields in agreement with measured wave integrated parameters. Synthesized HF-radar wave data are used to test the technique for two academic cases.

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

[1] Atmospheric boundary layer rolls and their impact on upper ocean circulation were investigated using a combination of two high-resolution data sources: (1) data from the Advanced Synthetic Aperture Radar (ASAR) onboard ENVISAT with a spatial sampling of approximately 500 m × 500 m and (2) continuous observations taken at the research platform FINO 1 with 2 min temporal sampling at eight heights between 33 and 100 m. The parallel analysis of instantaneous image data in combination with the FINO 1 time series enabled us to quantify both the spatial and temporal dynamics of mesoscale and submesoscale wind variations. The influence of these variations with different temporal and spatial scales on the hydrodynamics of the German Bight was addressed using outputs from a three-dimensional circulation model. It was demonstrated that while the coupling between wind and tidal forcing triggered substantial responses at mesoscales, the response of surface currents and sea surface temperature to the atmospheric boundary layer rolls appeared relatively weak. However, these ocean surface responses closely follow the surface footprint of the atmospheric boundary layer rolls, the signatures of which become more pronounced in the absence of strong tidal flows.

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

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

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

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

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

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.

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

A hydrodynamical modelling of the Black Sea is presented using the GHER 3-D primitive equation model. Results of a very high resolution model with 5 km grid size are analysed and the dominant features of general circulation in the Black Sea are highlighted. Compared with a coarse resolution model, forced with the same climatic monthly averaged atmospheric data, the high resolution model exhibits stronger variability, including frontal structures and coastal upwellings induced by baroclinic instabilities, in particular along the Turkish coast. A comparison of the shelf-open sea exchanges, with particular focus on the cold intermediate waters, shows that both models lead to a replenishment of the CIL in the basin interior by cold waters formed on the shelf. Due to the better representation of frontal structures, the high resolution model is an appropriate candidate for coupling with a biological model. Problems of calibration, interpretation and data availability typically arise from this kind of coupled 3-D model. In order to overcome such difficulties, the results of the 3-D hydrodynamical model are used to guide the development of an integrated 0-D box model capable of achieving the objectives of projects like EROS21, where the effect of changes in the Danube inflows on the shelf ecosystem is investigated. The 0-D model is thus designed to cover this region and is obtained through integration over an appropriate variable volume. The integration procedure shows where the weaknesses of an 0-D approach might lie. Diagnoses in the 3-D model of the integral quantities show the range of uncertainty one can expect in the exchange laws of the 0-D model.

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 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.

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

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

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

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

Ocean general circulation models which are constrained by altimetry data usually assimilate only temporal sea-surface height anomalies. It is known that this is not enough to correct the mean ocean state. Here we present first results of assimilating the full (absolute) dynamical topography into a steady state version of a finite element ocean model (FEOM) for the North Atlantic. This makes it possible to notably reduce the model-data misfit especially in the western part of the basin and in the southern Labrador Sea.

In the long term development of the research on wind waves and their modelling, in particular of the inner and coastal seas, the present situation is framed with a short look at the past, a critical analysis of the present capabilities and a foresight of where the field is likely to go. After a short introduction, Chapter 2 deals with the basic processes at work and their modelling aspects. Chapter 3 highlights the interaction with wind and currents. Chapter 4 stresses the need for a more complete, spectral, approach in data assimilation. Chapter 5 summarizes the situation with a discussion on the present status in wave modelling and a look at what we can expect in the future.

International Altimetry Team., In 2018 we celebrated 25 years of development of radar altimetry, and the progress achieved by this methodology in the fields of global and coastal oceanography, hydrology, geodesy and cryospheric sciences. Many symbolic major events have celebrated these developments, e.g., in Venice, Italy, the 15th (2006) and 20th (2012) years of progress and more recently, in 2018, in Ponta Delgada, Portugal, 25 Years of Progress in Radar Altimetry. On this latter occasion it was decided to collect contributions of scientists, engineers and managers involved in the worldwide altimetry community to depict the state of altimetry and propose recommendations for the altimetry of the future. This paper summarizes contributions and recommendations that were collected and provides guidance for future mission design, research activities, and sustainable operational radar altimetry data exploitation. Recommendations provided are fundamental for optimizing further scientific and operational advances of oceanographic observations by altimetry, including requirements for spatial and temporal resolution of altimetric measurements, their accuracy and continuity. There are also new challenges and new openings mentioned in the paper that are particularly crucial for observations at higher latitudes, for coastal oceanography, for cryospheric studies and for hydrology. The paper starts with a general introduction followed by a section on Earth System Science including Ocean Dynamics, Sea Level, the Coastal Ocean, Hydrology, the Cryosphere and Polar Oceans and the “Green” Ocean, extending the frontier from biogeochemistry to marine ecology. Applications are described in a subsequent section, which covers Operational Oceanography, Weather, Hurricane Wave and Wind Forecasting, Climate projection. Instruments’ development and satellite missions’ evolutions are described in a fourth section. A fifth section covers the key observations that altimeters provide and their potential complements, from other Earth observation measurements to in situ data. Section 6 identifies the data and methods and provides some accuracy and resolution requirements for the wet tropospheric correction, the orbit and other geodetic requirements, the Mean Sea Surface, Geoid and Mean Dynamic Topography, Calibration and Validation, data accuracy, data access and handling (including the DUACS system). Section 7 brings a transversal view on scales, integration, artificial intelligence, and capacity building (education and training). Section 8 reviews the programmatic issues followed by a conclusion., At the forefront of this support, we must obviously mention the space agencies CNES, ESA and NASA which have played and still play a decisive role in the development and launch of several prominent altimetry missions from the outset. Other agencies such as DLR, EUMETSAT, ISRO, NOAA, NSOAS and organizations such as CMEMS, also contribute significantly to developments in all forms of altimetry.