Ulf Gräwe

The future of the western Baltic Sea: two possible scenarios

Globally coupled climate models are generally capable of reproducing the observed trends in the globally averaged atmospheric temperature. However, the global models do not perform as well on regional scales. Here, we present results from four 100-year, high-resolution ocean model experiments (resolution less than 1 km) for the western Baltic Sea. The forcing is taken from a regional atmospheric model and a regional ocean model, imbedded into two global greenhouse gas emission scenarios, A1B and B1, for the period of 2000 to 2100 with each two realisations. Two control runs from 1960 to 2000 are used for validation. For both scenarios, the results show a warming with an increase of 0.5–2.5 K at the sea surface and 0.7–2.8 K below 40 m. The simulations further indicate a decrease in salinity by 1.5–2 practical salinity units. The increase in water temperature leads to a prolongation of heat waves based on present-day thresholds. This amounts to a doubling or even tripling of the heat wave duration. The simulations show a decrease in inflow events (barotropic/baroclinic), which will affect the deepwater generation and ventilation of the central Baltic Sea. The high spatial resolution allows us to diagnose the inflow events and the mechanism that will cause future changes. The reduction in barotropic inflow events correlates well with the increase in westerly winds. The changes in the baroclinic inflows can be consistently explained by the reduction of calm wind periods and thus a weakening of the necessary stratification in the western Baltic Sea and the Danish Straits.

Anatomizing one of the largest saltwater inflows into the Baltic Sea in December 2014

In December 2014, an exceptional inflow event into the Baltic Sea was observed, a so-called Major Baltic Inflow (MBI). Such inflow events are important for the deep water ventilation in the Baltic Sea and typically occur every 3–10 years. Based on first observational data sets, this inflow had been ranked as the third largest since 100 years. With the help of a multinested modeling system, reaching from the North Atlantic (8 km resolution) to the Western Baltic Sea (600 m resolution, which is baroclinic eddy resolving), this event is reproduced in detail. The model gave a slightly lower salt transport of 3.8 Gt, compared to the observational estimate of four Gt. Moreover, by using passive tracers to mark the different inflowing water masses, including an age tracer, the inflowing water masses could be tracked and their paths and timing through the different basins could be reproduced and investigated. The analysis is supported by the recently developed Total Exchange Flow (TEF) to quantify the volume transport in different salinity classes. To account for uncertainties in the modeled velocity and tracer fields, a Monte Carlo Analysis (MCA) is applied to correct possible biases and errors. With the help of the MCA, 95% confidence intervals are computed for the transport estimates. Based on the MCA, the “best guess” of the volume transport is 291.0 ± 13.65 km3 and 3.89 ± 0.18 Gt for the total salt transport.

Seasonal variability in M2 and M4 tidal constituents and its implications for the coastal residual sediment transport

We use an observational data set of tidal gauges in the North Sea to investigate the annual cycle of the M2 and M4 amplitudes and phases. The sea surface elevation amplitude of the M2 can vary by 8–10% and the M4 amplitude by 12–30% over the course of the year, with larger amplitudes in summer. The annual phase variations are in the range of 3–15?. The reason for these variations is the thermal structure of the North Sea: a well-developed thermocline in summer and well-mixed water column during winter. The interaction of the M2 and M4 tides is one of the main drivers of the residual sediment transport. Using an analytical model, the seasonal variability in residual sediment transport is estimated. This transport can vary by 10–50% over the course of the year. These variations are mainly related to the seasonal variability of the M2 and M4 amplitudes.

Numerical analysis of stratification and destratification processes in a tidally energetic inlet with an ebb tidal delta

Stratification and destratification processes in a tidally energetic, weakly stratified inlet in the Wadden Sea (south eastern North Sea) are investigated in this modeling study. Observations of current velocity and vertical density structure show strain-induced periodic stratification for the southern shoal of the tidal channel. In contrast to this, in the nearby central region of the channel, increased stratification is already observed directly after full flood. To investigate the processes leading to this different behavior, a nested model system using GETM is set up and successfully validated against field data. The simulated density development along a cross section that includes both stations shows that cross-channel stratification is strongly increasing during flood, such that available potential energy is released in the deeper part of the channel during flood. An analysis of the potential energy anomaly budget confirms that the early onset of vertical stratification during flood at the deeper station is mainly controlled by the stratifying cross-channel straining of the density field. In contrast to this, in the shallow part of the channel, the relatively weak cross-channel straining is balanced by along-channel straining and vertical mixing. An idealized analytical model confirms the following hypothesis: The laterally convergent flood current advecting laterally stratified water masses from the shallow and wide ebb tidal delta to the deep and narrow tidal channel has the tendency to substantially increase cross-channel density gradients in the tidal channel. This process leads to stratification during flood.

A numerical model for the entire Wadden Sea: Skill assessment and analysis of hydrodynamics

A baroclinic three-dimensional numerical model for the entire Wadden Sea of the German Bight in the southern North Sea is first assessed by comparison to field data for surface elevation, current velocity, temperature, and salinity at selected stations and then used to calculate fluxes of volume and salt inside the Wadden Sea and the exchange between the Wadden Sea and the adjacent North Sea through the major tidal inlets. The model is simulating the reference years 2009–2011. An overview of tidal prisms and residual volume fluxes of the main inlets and their variability is given. In addition, data from an intensive observational campaign in a tidal channel south of the island of Spiekeroog as well as satellite images and observations of sea surface properties from a ship of opportunity are used for the skill assessment. Finally, the intensity of estuarine overturning circulation and its variability in the tidal gullies are quantified and analyzed as function of gravitational and wind straining using various estimates including Total Exchange Flow (TEF). Regional differences between the gullies are assessed and drivers of the estuarine circulation are identified. For some inlets, the longitudinal buoyancy gradient dominates the exchange flow, for some others wind straining is more important. Also the intensity of tidal straining (scaled covariance of eddy viscosity and vertical shear) depends on buoyancy gradient and wind forcing in different ways, depending on local topography, orientation toward the main wind direction, and influence by freshwater run off inside or outside the tidal basin.

Escherichia coli pollution in a Baltic Sea lagoon: a model-based source and spatial risk assessment.

Tourism around the Oder (Szczecin) Lagoon, at the southern Baltic coast, has a long tradition, is an important source of income and shall be further developed. Insufficient bathing water quality and frequent beach closings, especially in the Oder river mouth, hamper tourism development. Monitoring data gives only an incomplete picture of Escherichia coli (E. coli) bacteria sources, spatial transport patterns, risks and does neither support an efficient bathing water quality management nor decision making. We apply a 3D ocean model and a Lagrangian particle tracking model to analyse pollution events and to obtain spatial E. coli pollution maps based on scenario simulations. Model results suggests that insufficient sewage treatment in the city of Szczecin is the major source of faecal pollution, even for beaches 20km downstream. E. coli mortality rate and emission intensity are key parameters for concentration levels downstream. Wind and river discharge play a modifying role. Prevailing southwestern wind conditions cause E. coli transport along the eastern coast and favour high concentration levels at the beaches. Our simulations indicate that beach closings in 2006 would not have been necessary according to the new EU-Bathing Water Quality Directive (2006/7/EC). The implementation of the new directive will, very likely, reduce the number of beach closings, but not the risk for summer tourists. Model results suggest, that a full sewage treatment in Szczecin would allow the establishment of new beaches closer to the city (north of Dabie lake).

Impact of the depth-to-width ratio of periodically stratified tidal channels on the estuarine circulation

The dependency of the estuarine circulation on the depth-to-width ratio of a periodically, weakly stratified tidal estuary is systematically investigated here for the first time. Currents, salinity, and other properties are simulated by means of the General Estuarine Transport Model (GETM) in cross-sectional slice mode, applying a symmetric Gaussian-shaped depth profile. The width is varied over four orders of magnitude. The individual along-channel circulation contributions from tidal straining, gravitation, advection, etc., are calculated and the impact of the depth-to-width ratio on their intensity is presented and elucidated. It is found that the estuarine circulation exhibits a distinct maximum in medium-wide channels (intermediate depth-towidth ratio depending on various parameters), which is caused by a maximum of the tidal straining contribution. This maximum is related to a strong tidal asymmetry of eddy viscosity and shear created by secondary strain-induced periodic stratification (2SIPS): in medium channels, transverse circulation generated by lateral density gradients due to laterally differential longitudinal advection induces stable stratification at the end of the flood phase, which is further increased during ebb by longitudinal straining (SIPS). Thus, eddy viscosity is low and shear is strong in the entire ebb phase. During flood, SIPS decreases the stratification so that eddy viscosity is high and shear is weak. The circulation resulting from this viscosity–shear correlation, the tidal straining circulation, is oriented like the classical, gravitational circulation, with riverine outflow at the surface and oceanic inflow close to the bottom. In medium channels, it is about 5 times as strong as in wide (quasi onedimensional) channels, in which 2SIPS is negligible.

Consensus forecasting of intertidal seagrass habitat in the Wadden Sea

1. After the dramatic eutrophication-induced decline of intertidal seagrasses in the 1970s, theWadden Sea has shown diverging developments. In the northern Wadden Sea, seagrass bedshave expanded and become denser, while in the southern Wadden Sea, only small beds withlow shoot densities are found. A lack of documentation of historical distributions hampersconservation management. Yet, the recovery in the northern Wadden Sea provides opportunityto construct robust habitat suitability models to support management.2. We tuned habitat distribution models based on 17 years of seagrass surveys in the northernWadden Sea and high-resolution hydrodynamics and geomorphology for the entire WaddenSea using five machine learning approaches. To obtain geographically transferablemodels, hyperparameters were tuned on the basis of prediction accuracy assessed by non-random,spatial cross-validation. The spatial cross-validation methodology was combined with aconsensus modelling approach.3. The predicted suitability scores correlated amongst each other and with the hold-out observationsin the training area indicating that the models converged and were transferable acrossspace. Prediction accuracy was improved by averaging the predictions of the best models.4. We graphically examined the relationship between the consensus suitability score andindependent presence-only data from outside the training area using the area-adjusted seagrassfrequency per suitability class (continuous Boyce index). The Boyce index was positivelycorrelated with the suitability score indicating the adequacy of the prediction methodology.5. We used the plot of the continuous Boyce index against habitat suitability score to demarcatethree habitat classes – unsuitable, marginal and suitable – for the entire international WaddenSea. This information is valuable for habitat conservation and restoration management.6. Divergence between predicted suitability and actual distributions from the recent past indicatesthat unaccounted factors limit seagrass development in the southern Wadden Sea.7. Synthesis and applications. Our methodology and data enabled us to produce a robust andvalidated consensus habitat suitability model. We identified highly suitable areas where intertidalseagrass meadows may establish and persist. Our work provides scientific underpinningfor effective conservation planning in a dynamic landscape and sets monitoring priorities.

The Wadden Sea in transition - consequences of sea level rise

The impact of sea level rise (SLR) on the future morphological development of the Wadden Sea (North Sea) is investigated by means of extensive process-resolving numerical simulations. A new sediment and morphodynamic module was implemented in the well-established 3D circulation model GETM. A number of different validations are presented, ranging from an idealized 1D channel over a semi-idealized 2D Wadden Sea basin to a fully coupled realistic 40-year hindcast without morphological amplification of the Sylt-Rømøbight, a semi-enclosed subsystem of the Wadden Sea. Based on the results of the hindcast, four distinct future scenarios covering the period 2010–2100 are simulated. While these scenarios differ in the strength of SLR and wind forcing, they also account for an expected increase of tidal range over the coming century. The results of the future projections indicate a transition from a tidal-flat-dominated system toward a lagoon-like system, in which large fractions of the Sylt-Rømøbight will remain permanently covered by water. This has potentially dramatic implications for the unique ecosystem of the Wadden Sea. Although the simulations also predict an increased accumulation of sediment in the back-barrier basin, this accumulation is far too weak to compensate for the rise in mean sea level.

Regionalisation of Climate Scenarios for the Western Baltic Sea

Global coupled climate models are generally capable of reproducing the observed trends in the globally averaged atmospheric temperature. However, the global models do not perform as well on regional scales. Here, we present results from two 140-year, high-resolution regional ocean model experiments for the Western Baltic Sea. The forcing is taken from a regional atmospheric model and a medium scale ocean model. The model runs with two greenhouse gas emission scenarios (each for 100 years), A1B and B1, for the period 2000–2100. A control run (C20) from 1960 to 2000 is used for validation. For both scenarios, the results show the expected warming, with an increase of 0.5–2.5 K at the sea surface and 0.7–2.8 K below 40 m. The simulations further indicate a decrease in salinity, a change in stratification, and an increase of the return period of storm surges.