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Due to their choked geometry, coastal lagoons can attenuate extreme water levels compared to the open sea. However, this protective property is expected to decrease due to sea‐level rise. By studying idealized lagoons in a non‐dimensional parameter space, this study describes non‐linear interactions between tides, storm surges, freshwater fluxes into the lagoon, and sea‐level rise. The non‐dimensional numbers include lagoon geometry and forcing scales. The main objective is to provide an overview of potentially affected lagoons and to highlight the importance of attenuation changes due to sea‐level rise. Tidal and storm surge induced maximum water levels inside lagoons rise faster than sea‐level rise for most of the parameter space. Maximum water levels due to freshwater fluxes rise slower than sea‐level rise for strongly choked lagoons. For compound events, the response between rising faster or slower than sea‐level rise depends strongly on the lagoon geometry. Plain Language Summary: Coastal lagoons are known to reduce extreme water levels compared to the open sea due to their narrow and shallow connections to the ocean. However, the rising mean sea level will decrease this protective property. In this study, we investigate lagoons using a simple box model to explore how combinations of tides, storm surges, river discharge, and sea‐level rise will change water levels inside lagoons. The study aims to provide an overview of potentially affected lagoons and to highlight the importance of these processes. Due to sea‐level rise, water from the ocean can more easily flow into the lagoon. Therefore, our results show that maximum water levels inside a lagoon can rise faster than sea‐level rise. On the other hand, river discharge can exit the lagoon more easily at the same time. For this case, water levels inside a lagoon rise slower than sea‐level rise. For events where tides, storm surges, and river discharge are all present, the maximum water level may rise either slower or faster than sea‐level rise, depending on the properties of the lagoon. Further studies are needed to understand the implications for coastal communities. Key Points: Tides, storm surges, freshwater fluxes, and sea‐level rise interact non‐linearly on the water level in choked coastal lagoonsMaximum water levels inside choked lagoons can increase faster than sea‐level rise due to non‐linear attenuation changesMaximum water levels due to freshwater fluxes increase slower than sea‐level rise [ABSTRACT FROM AUTHOR]

Extreme sea level events, such as storm surges, pose a threat to coastlines around the globe. Many tide gauges have been measuring the sea level and recording these extreme events for decades, some for over a century. The data from these gauges often serve as the basis for evaluating the extreme sea level statistics, which are used to extrapolate sea levels that serve as design values for coastal protection. Hydrodynamic models often have difficulty in correctly reproducing extreme sea levels and, consequently, extreme sea level statistics and trends. In this study, we generate a 13-member hindcast ensemble for the non-tidal Baltic Sea from 1979 to 2018 using the coastal ocean model GETM (General Estuarine Transport Model). In order to cope with mean biases in maximum water levels in the simulations, we include both simulations with and those without wind-speed adjustments in the ensemble. We evaluate the uncertainties in the extreme value statistics and recent trends of annual maximum sea levels. Although the ensemble mean shows good agreement with observations regarding return levels and trends, we still find large variability and uncertainty within the ensemble (95 % confidence levels up to 60 cm for the 30-year return level). We argue that biases and uncertainties in the atmospheric reanalyses, e.g. variability in the representation of storms, translate directly into uncertainty within the ensemble. The translation of the variability of the 99th percentile wind speeds into the sea level elevation is in the order of the variability of the ensemble spread of the modelled maximum sea levels. Our results emphasise that 13 members are insufficient and that regionally large ensembles should be created to minimise uncertainties. This should improve the ability of the models to correctly reproduce the underlying extreme value statistics and thus provide robust estimates of climate change-induced changes in the future. [ABSTRACT FROM AUTHOR]

Among the Baltic Sea littoral states, Germany is anticipated to endure considerable damage as a result of increased coastal flooding due to sea-level rise (SLR). Consequently, there is a growing demand for flood risk assessments, particularly at regional scales, which will improve the understanding of the impacts of SLR and assist adaptation planning. Existing studies on coastal flooding along the German Baltic Sea coast either use state-of-the-art hydrodynamic models but cover only a small fraction of the study region or assess potential flood extents for the entire region but rely on global topographic data sources and apply the simplified bathtub approach. In addition, the validation of produced flood extents is often not provided. Here we apply a fully validated hydrodynamic modelling framework covering the German Baltic Sea coast that includes the height of natural and anthropogenic coastal protection structures in the study region. Using this modelling framework, we extrapolate spatially explicit 200-year return water levels, which align with the design standard of state embankments in the region, and simulate associated coastal flooding. Specifically, we explore (1) how flood extents may change until 2100 if dike heights are not upgraded, by applying two high-end SLR scenarios (1 and 1.5 m); (2) hotspots of coastal flooding; and (3) the use of SAR imagery for validating the simulated flood extents. Our results confirm that the German Baltic coast is exposed to coastal flooding, with flood extent varying between 217 and 1016 km 2 for the 200-year event and a 200-year event with 1.5 m SLR, respectively. Most of the flooding occurs in the federal state of Mecklenburg-Western Pomerania, while extreme water levels are generally higher in Schleswig-Holstein. Our results emphasise the importance of current plans to update coastal protection schemes along the German Baltic Sea coast over the 21st century in order to prevent large-scale damage in the future. [ABSTRACT FROM AUTHOR]

Extreme sea level events, such as storm surges, pose a threat to coastlines around the globe. Many tide gauges have been measuring sea level and recording these extreme events for decades, some for over a century. The data from these gauges often serve as the basis for evaluating the extreme sea level statistics, which are used to extrapolate sea levels that serve as design values for coastal protection. Hydrodynamic models often have difficulty in correctly reproducing extreme sea levels and, consequently, extreme sea level statistics and trends. In this study, we generate a 13-member hind-cast ensemble for the non-tidal Baltic Sea from 1979 to 2018 using the coastal ocean model GETM (General Estuarine Transport Model). In order to cope with mean biases in maximum water levels in the simulations, we include both simulations with and without wind speed adjustments in the ensemble. We evaluate the uncertainties in the extreme value statistics and recent trends of annual maximum sea levels. Although the ensemble mean shows good agreement with observations regarding return levels and trends, we still find large variability and uncertainty within the ensemble (95 % confidence levels up to 60 cm for the 30-year return level). We argue that biases and uncertainties in the atmospheric reanalyses, e.g. variability in the representation of storms, translate directly into uncertainty within the ensemble. The translation of the variability of the 99th percentile wind speeds into the sea level elevation is in the order of the variability of the ensemble spread of the modelled maximum sea levels. Our results emphasise that 13 members are insufficient and that regionally large ensembles should be created to minimise uncertainties. This should improve the ability of the models to correctly reproduce the underlying extreme value statistics and thus provide robust estimates of climate change-induced changes in the future. [ABSTRACT FROM AUTHOR]

Hydrodynamic models are increasingly being used in recent years to map coastal floodplains on local to continental scales. On regional scales, however, high computational costs and the need for high-resolution data limit their application. Additionally, model validation constitutes a major concern, as in-situ data are hardly available or limited in spatial coverage to small parts of the study region. Here we address these challenges by developing a modelling framework, which couples a hydrodynamic coastal inundation model covering the German Baltic Sea coast with a hydrodynamic coastal ocean model of the western Baltic Sea, to produce high resolution (50 m) regional scale flood maps for the entire German Baltic Sea coast. Using a LiDAR derived digital elevation model with 1 m horizontal resolution, we derive and validate the elevation of dikes and natural flood barriers such as dunes. Using this model setup, we simulate a storm surge event from January 2019, a surge with a return period of 200 years and two sea-level rise scenarios for the year 2100 (200-year event plus 1 m and 1.5 m). We validate the simulated flood extents by comparing them to inundation maps derived from Sentinel-1 SAR satellite imagery, acquired between 1.5 and 3.5 hours after the peak of the 2019 surge, covering a large part of the study region. Our results confirm that the German Baltic Sea coast is exposed to coastal flooding, with flood extent varying between 118 km² and 1016 km² for the 2019 storm surge and a 200-year return water level plus 1.5 m of sea-level rise, respectively. Hotspots of coastal flooding are mostly located in the federal state of Mecklenburg Western Pomerania. Our results emphasise the importance of current plans to update coastal protection schemes along the German Baltic Sea coast over the course of the 21st century in order to prevent large-scale damage in the future. [ABSTRACT FROM AUTHOR]

We analyzed changes in mean sea level (MSL) for the period 1950–2015 using a regional ocean model for the Baltic Sea. Sensitivity experiments allowed us to separate external from local drivers and to investigate individual forcing agents triggering basin-internal spatial variations. The model reveals a basin-average MSL rise (MSLR) of 2.08 ± 0.49 mm yr−1, a value that is slightly larger than the simultaneous global average of 1.63 ± 0.32 mm yr−1. This MSLR is, however, spatially highly nonuniform with lower than average increases in the southwestern part (1.71 ± 0.51 mm yr−1) and higher than average rates in the northeastern parts (2.34 ± 1.05 mm yr−1). While 75% of the basin-average MSL externally enters the Baltic basin as a mass signal from the adjacent North Sea, intensified westerly winds and a poleward shift of low pressure systems explain the majority of the spatial variations in the rates. Minor contributions stem from local changes in baroclinicity leading to a basin-internal redistribution of water masses. An observed increase in local ocean temperature further adds to the total basinwide MSLR through thermal expansion but has little effect on the spatial pattern. To test the robustness of these results, we further assessed the sensitivity to six different atmospheric surface forcing reanalysis products over their common period from 1980 to 2005. The ensemble runs indicated that there are significant differences between individual ensemble members increasing the total trend uncertainty for the basin average by 0.22 mm yr−1 (95% confidence intervals). Locally the uncertainty varies from 0.05 mm yr−1 in the central part to up to 0.4 mm yr−1 along the coasts. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

The Baltic Sea is a semi-enclosed sea and consists of brackish water with a salinity of about 7 g/kg. It sporadically experiences salt water inflow events through the Danish straits with strong inflows occurring once per year on average. An increase in mean sea level, which is expected to rise similarly to the projected global mean sea level (GMSL), impacts the dynamics of these salt water inflow events. Therefore, we use a regional setup of the ocean circulation model GETM spanning over the western Baltic Sea and having a spatial resolution of 600 meter to investigate the response of GMSL rise to inflow events during the years 1980 to 2015. As case study, the processes of a recent inflow event in December 2014 are additionally simulated with a model setup, using a horizontal resolution of 200 meter. It is analyzed how transports of volume and salt would differ, if GMSL would be one meter higher than present day sea level. Hence, a simulation is performed with the same model setup as the reference simulation, only the bathymetry of the model setup is deepened by one meter.The results of the reference model simulations are evaluated by comparisons with observations. For example, salinity and temperature at two stations located at the Darss Sill and in the Arkona Basin agree well. In addition, the volume transport through the Öresund, computed from sea surface height in Viken and Klagshamn, have a correlation of 0.96 with observations during the years 2014 and 2015 (nudging of sea surface height is applied at the model boundaries). A detailed analysis of the change in the physical processes with one meter GMSL rise will be presented. However, first analyses of the inflow event in December 2014, simulated with the 600m GETM model setup, show that the duration of this event did not change. However, the volume of high saline water (> 17 g/kg) in the Arkona Basin increased by about 10.7% during the inflow event. In addition, results show similar relative changes in volume and salt transport through the Danish straits (at Darss Sill and Drodgen Sill), and a less than 2% higher relative salt transport than volume transport when considering only the transports where salinity is greater than 17 g/kg. These results suggest a quite linear increase in salt inflow with increasing volume transport through the Danish straits during this inflow event. [ABSTRACT FROM AUTHOR]

In this contribution we present a comprehensive analysis of sea level data around the Baltic Sea from 1777 to the present with a focus on the southwestern part, concentrating on quantifying and understanding long-term trends and the influence of inter-annual and decadal variability. Our analysis focuses on a new monthly mean sea level (MSL) dataset, generated from a first time available compilation of high-resolution sea level measurements combined with already existing monthly MSL time series. Our final MSL dataset consists of records at 139 sites in the Baltic Sea with a temporal resolution of at least 19 years. Additionally, we analyze outputs of barotropic and baroclinic reanalysis runs from a 3D hydrodynamic numerical ocean model to quantify different meteorological and climatological processes contributing to the local sea level budget. Throughout the 20th century (1900 to 2015), an average linear trend for the whole Baltic Sea of 1.66 ± 0.14 mm/yr could be determined (corrected for the in the region dominant vertical land motion signal from Glacial Isostatic Adjustment). This is in principal consistent with current GMSL trend estimates (1.3 to 2 mm/yr) for the same period (Church and White 2006, Hay et al., 2015; Dangendorf et al., 2017) within their specific error bars. However, within the basin large differences in trends from different stations are found, which reach from minimum values in the southwestern part (0.94 ± 0.15 mm/yr in Kolobrzeg) to maximum values in the eastern Gulf of Finland (2.21 ± 0.32 mm/yr in Kronstadt). These differences can primarily be explained by an intensification of westerly winds and a decrease of atmospheric pressure over the 20th century pushing lowering MSL rates in the west and increasing them in the east. With our study we outline major existing knowledge gaps in understanding sea level change and variability in the southwestern Baltic Sea. A better understanding of the past and present development of the MSL provides the basis for deriving more reliable future projections of sea-level rise. Church and White (2006): A 20th century acceleration in global sea-level rise. In: Geophysical Research Letters, Jg. 33, 1, n/a. doi: 10.1029/2005GL024826.Dangendorf et al. (2017): Reassessment of 20th century global mean sea level rise. In: Proceedings of the National Academy of Sciences of the United States of America, Jg. 114, 23, 5946-5951. doi: 10.1073/pnas.1616007114.Hay et al. (2015): Probabilistic reanalysis of twentieth-century sea-level rise. In: Nature, Jg. 517, 7535, 481-484. doi: 10.1038/nature14093. [ABSTRACT FROM AUTHOR]