Wolfgang Cyclone Landfall in October 2023: Extreme Sea Level and Erosion on the Southern Baltic Sea Coasts

Extremely high sea levels in the Baltic Sea and the resulting coastal flooding events are typically caused by the passage of extratropical cyclones (ETCs). ETCs raise the sea level by their strong winds and low atmospheric pressure. The role of individual ETCs for the extreme sea levels in the Baltic Sea has been studied in some case studies, but less attention has been paid to the serial clustering of ETCs (SCC, the passage of multiple cyclones within a short period of time). In particular, little research has been done on whether extremely high sea level events in the Baltic Sea are typically caused by just one extreme ETC or by the combined effect of several consecutive ETCs. In this study, the role of SCC for the extreme sea levels in the Baltic Sea is investigated.  We use objectively determined cyclone tracks from ERA5 reanalysis and sea level observations from four tide gauges: Kemi (Finland), Helsinki (Finland), Pärnu (Estonia) and Riga (Latvia). All ETCs from the October-March period in 1980-2019 that pass within 700 km of the tide gauge are included in the analysis. Clustering and non-clustering days are defined as days when the 7-day running sum of ETCs is ≥ 3 and 1, respectively. Then, SCC and single cyclone periods are defined by adding ±1 day to the dates of the first and last ETC around the clustering and non-clustering days. We find that SCC periods tend to produce on average 30 cm higher sea level than the climatology at the tide gauges, and about six days after the SCC onset dates. In addition, the daily maximum sea level peaks about 40 cm higher during SCC periods than during single cyclone periods. Thus, this result implies that the SCC periods are typically associated with higher sea level than the periods when only one intense ETC passes the tide gauge. However, when we look at the extreme sea level events at the tide gauges, we find that in Helsinki, Pärnu and Riga about 40 % of the events can be attributed to the SCC periods and about 15 % to the periods when only single ETC passes the tide gauge. Our results demonstrate that serial cyclone clustering is an important phenomenon for the occurrence of extreme sea levels in the Baltic Sea, and in fact relatively few extreme sea levels and the associated coastal flooding events in the Baltic Sea are caused by only one single ETC.

<p>Urbanized low-lying coastal areas become evidently increasingly vulnerable in the future climate, where higher mean sea level and more powerful storm surges are expected. Safe planning and operation particularly of critical coastal infrastructure, such as nuclear power plants, requires careful evaluation of external events due to extreme weather and sea level conditions. Finland is located in the northeast part of the Baltic Sea, where sea level variations are driven by short-lasting phenomena (storm surges, internal oscillations in the Baltic Sea, tides), long-term mean sea level change (global mean sea level, the post-glacial land uplift and the Baltic Sea water balance), and wind waves. In Finland, there are altogether five nuclear power plant units along the coast and approximately one third of electricity production in the country is grounded on nuclear energy.</p><p>The Finnish Meteorological Institute has studied weather, climate and sea level hazards potentially posing risks to nuclear power plants since 2007 in several research projects. In this presentation, we will introduce some preliminary results of studies on extreme sea levels on the Finnish coast, which are conducted in the PREDICT project (https://en.ilmatieteenlaitos.fi/predict). Their focus is on the short-term sea level variations, which might be several meters in the Baltic Sea, even if the tides in the region are mostly negligible.</p><p>In the first study, Bayesian hierarchical modelling is used to estimate return levels of annual sea level maximum on the Finnish coast and non-stationarity in the related sea level extremes. Our model setup enables sharing information on sea level extremes between the neighboring tide gauge stations. Additionally, it accounts temporal variations in the distribution parameters by incorporating climate indices such as North Atlantic oscillation (NAO) in the model. Preliminary results suggest that hierarchical approach reduces the range of uncertainty in the estimated parameters.</p><p>The second study tackles a question “What is the most severe flooding that could occur in the Baltic Sea coast in the present climate if the weather conditions are optimal?”. In this study, effects of low-pressure intensity, speed, direction and point of origin on the sea level extremes is examined by making simulations with numerical sea level model combined with the synthetic cyclones for the Finnish tide gauge locations. Tentative results indicate that the highest sea level extremes on the Finnish coast are caused by large and slowly propagating wind storms.</p>

Geographical diversity in the occurrence of extreme sea levels on the coasts of the Baltic Sea

Abstract. There are a large number of geophysical processes affecting sea level dynamics and coastal erosion in the Baltic Sea region. These processes operate on a large range of spatial and temporal scales and are observed in many other coastal regions worldwide. This, along with the outstanding number of long data records, makes the Baltic Sea a unique laboratory for advancing our knowledge on interactions between processes steering sea level and erosion in a climate change context. Processes contributing to sea level dynamics and coastal erosion in the Baltic Sea include the still ongoing viscoelastic response of the Earth to the last deglaciation, contributions from global and North Atlantic mean sea level changes, or contributions from wind waves affecting erosion and sediment transport along the subsiding southern Baltic Sea coast. Other examples are storm surges, seiches, or meteotsunamis which primarily contribute to sea level extremes. Such processes have undergone considerable variation and change in the past. For example, over approximately the past 50 years, the Baltic absolute (geocentric) mean sea level has risen at a rate slightly larger than the global average. In the northern parts of the Baltic Sea, due to vertical land movements, relative mean sea level has decreased. Sea level extremes are strongly linked to variability and changes in large-scale atmospheric circulation. The patterns and mechanisms contributing to erosion and accretion strongly depend on hydrodynamic conditions and their variability. For large parts of the sedimentary shores of the Baltic Sea, the wave climate and the angle at which the waves approach the nearshore region are the dominant factors, and coastline changes are highly sensitive to even small variations in these driving forces. Consequently, processes contributing to Baltic sea level dynamics and coastline change are expected to vary and to change in the future, leaving their imprint on future Baltic sea level and coastline change and variability. Because of the large number of contributing processes, their relevance for understanding global figures, and the outstanding data availability, global sea level research and research on coastline changes may greatly benefit from research undertaken in the Baltic Sea.

<strong class="journal-contentHeaderColor">Abstract.</strong> There are a large number of geophysical processes affecting sea level dynamics and coastal erosion in the Baltic Sea region. These processes operate on a large range of spatial and temporal scales and are observed in many other coastal regions worldwide. This, along with the outstanding number of long data records, makes the Baltic Sea a unique laboratory for advancing our knowledge on interactions between processes steering sea level and erosion in a climate change context. Processes contributing to sea level dynamics and coastal erosion in the Baltic Sea include the still ongoing viscoelastic response of the Earth to the last deglaciation, contributions from global and North Atlantic mean sea level changes, or contributions from wind waves affecting erosion and sediment transport along the subsiding southern Baltic Sea coast. Other examples are storm surges, seiches, or meteotsunamis which primarily contribute to sea level extremes. Such processes have undergone considerable variation and change in the past. For example, over approximately the past 50Â years, the Baltic absolute (geocentric) mean sea level has risen at a rate slightly larger than the global average. In the northern parts of the Baltic Sea, due to vertical land movements, relative mean sea level has decreased. Sea level extremes are strongly linked to variability and changes in large-scale atmospheric circulation. The patterns and mechanisms contributing to erosion and accretion strongly depend on hydrodynamic conditions and their variability. For large parts of the sedimentary shores of the Baltic Sea, the wave climate and the angle at which the waves approach the nearshore region are the dominant factors, and coastline changes are highly sensitive to even small variations in these driving forces. Consequently, processes contributing to Baltic sea level dynamics and coastline change are expected to vary and to change in the future, leaving their imprint on future Baltic sea level and coastline change and variability. Because of the large number of contributing processes, their relevance for understanding global figures, and the outstanding data availability, global sea level research and research on coastline changes may greatly benefit from research undertaken in the Baltic Sea.

The study is dedicated to researching the storm surge Axel, the largest on the South Baltic coast in the 20th and 21st centuries. This unique event resulted in a very large erosion along the whole Polish Baltic Sea coast in January 2017 (max. HSL = 1.65 m, the average for the coast 1.36 m). Storm surge effects on the coast were followed based on field observations of dune retreat and analysis of hydrodynamic and meteorological parameters of the surge and its passage through the Baltic Sea. The material of dune erosion was collected based on cross-shore profiling of almost every 1 km, along the whole Polish sand barrier coast, before and after this storm. The work also studies the parameters of smaller storm surges from the end of 2016, which caused the lowering of beaches and dune erosion. A relationship was observed between erosion, and beach height and sea level (SL). The higher the beach, the lower the erosion that occurred. The average dune toe retreat was 5.1 m, and the largest exceeded 9–19 m (max. 42 m). The most important for dune erosion was the height of run-up, beach height and shore exposition for a surge. The largest dune erosion was observed during the heaviest SL with wave run-up higher than 3.8 m above mean sea level (AMSL). Each coast section was eroded, which also caused losses in infrastructure.

Characteristics of seasonal changes of the Baltic Sea extreme sea levels

Abstract. We have designed a machine learning method to predict the occurrence of daily extreme sea level at the Baltic Sea coast with lead times of a few days. The method is based on a random forest classifier. It uses spatially resolved fields of daily sea level pressure, surface wind, precipitation, and the pre-filling state of the Baltic Sea as predictors for daily sea level above the 95 % quantile at each of seven tide gauge stations representative of the Baltic coast. The method is purely data-driven and is trained with sea level data from the Global Extreme Sea Level Analysis (GESLA) dataset and from the meteorological reanalysis ERA5 of the European Centre for Medium-Range Weather Forecasts (ECMWF). Sea level extremes at lead times of up to 3 d are satisfactorily predicted by the method, and the relevant predictor and predictor regions are identified. The sensitivity, measured as the proportion of correctly predicted extremes, is, depending on the stations, on the order of 70 %. The precision of the model is typically around 25 % and, for some instances, higher. For lead times longer than 3 d, the predictive skill degrades; for 7 d, it is comparable to a random skill. The sensitivity of our model is higher than the one derived from a storm surge reanalysis with dynamical models that use available information of the predictors without any time lag, as done by Muis et al. (2016), but its precision is considerably lower. The importance of each predictor depends on the location of the tide gauge. Usually, the most relevant predictors are sea level pressure, surface wind, and pre-filling. Extreme sea levels at the meridionally oriented coastlines of the Baltic Sea are better predicted by meridional winds and surface pressure. In contrast, for stations located at zonally oriented coastlines, the most relevant predictors are surface pressure and the zonal wind component. Precipitation did not display consistent patterns or a high relevance predictor for most of the stations analysed. The random forest classifier is not required to have considerable complexity, and the computing time to issue predictions is typically a few minutes on a personal laptop. The method can, therefore, be used as a pre-warning system to trigger the application of more sophisticated algorithms that estimate the height of the ensuing extreme sea level or as a warning to run larger ensembles with physically based numerical models.

&amp;lt;p&amp;gt;Coastal areas are under rapid changes. Management to face flooding hazards in changing climate is of great significance due to the major impact of flooding events in densely populated coastal regions, where also important and vulnerable infrastructure is located. The sea level of the Baltic Sea is affected by internal fluctuations caused by wind, air pressure and seiche oscillations, and by variations of the water volume due to the water exchange between the Baltic Sea and the North Sea through the Danish Straits. The highest sea level extremes are caused by cyclones moving over the region. The most vulnerable locations are at the ends of the bays. St. Petersburg, located at the eastern end of the Gulf of Finland, has experienced major sea floods in 1777, 1824 and 1924.&amp;lt;/p&amp;gt;&amp;lt;p&amp;gt;In order to study the effects of the depths and tracks of cyclones on the extreme sea levels, we have developed a method to generate cyclones for numerical sea level studies. A cyclone is modelled as a two-dimensional Gaussian function with adjustable horizontal size and depth. The cyclone moves through the Baltic Sea region with given direction and velocity. The output of this method is the gridded data set of mean sea level pressure and wind components which are used as an input for the sea level model. The internal variations of the Baltic Sea are calculated with a numerical barotropic sea level model, and the water volume variations are evaluated using a statistical sea level model based on wind speeds near the Danish Straits. The sea level model simulations allow us to study extremely rare but physically plausible sea level events that have not occurred during the observation period at the Baltic Sea coast. The simulation results are used to investigate extreme sea levels that could occur at selected sites at the Finnish coastline.&amp;lt;/p&amp;gt;

Two of the most important topics in Sea Level Science are addressed in this paper. One is concerned with the evidence for the apparent acceleration in the rate of global sea level change between the nineteenth and twentieth centuries and, thereby, with the question of whether the twentieth century sea level rise was a consequence of an accelerated climate change of anthropogenic origin. An acceleration is indeed observed in both tide gauge and saltmarsh data at different locations around the world, yielding quadratic coefficients ‘c’ of order 0.005 mm/year2, and with the most rapid changes of rate of sea level rise occurring around the end of the nineteenth century. The second topic refers to whether there is evidence that extreme sea levels have increased in recent decades at rates significantly different from those in mean levels. Recent results, which suggest that at most locations rates of change of extreme and mean sea levels are comparable, are presented. In addition, a short review is given of recent work on extreme sea levels by other authors. This body of work, which is focused primarily on Europe and the Mediterranean, also tends to support mean and extreme sea levels changing at similar rates at most locations.

There is great concern about rising sea levels in the coming century, particularly in terms of extreme sea levels and the increased likelihood of coastal flooding. This is especially true for the south-east coast of England where rising sea levels interact with a growing population and economy. This paper examines how extreme sea levels (excluding waves) have changed through the twentieth century at 16 sites around the English Channel. Extreme sea levels were found to have increased at all 16 sites, but at rates not statistically different from the observed rise in mean sea level. Potential future changes in extreme high sea levels throughout the twenty-first century are estimated for nine UK south coast sites using the 2009 projections from the UK Climate Impacts Programme. For the low, medium and high emissions scenarios (12, 40 and 81 cm total ocean rise, respectively), the exceedence frequency of extreme high sea levels along the south coast would on average increase over the twenty-first century by a factor of 10, 100 and about 1800, respectively. Finally these changes are considered in relation to a large recent surge event in March 2008, which caused significant flooding in the central Channel.

&amp;lt;p&amp;gt;Extremely high sea levels on the Finnish coast are typically caused by close passages of extratropical cyclones (ETCs), which raise the sea level with their associated extreme winds and lower air pressure. For coastal infrastructure, such as nuclear power plants, it is crucial to study physically possible sea level heights associated with ETCs. Such sea levels are not straightforward to determine from observational datasets only, because tide gauge records&amp;amp;#160; cover about 100 years and do not necessarily capture the most extreme cases having return periods longer than 100 years.&amp;lt;/p&amp;gt;&amp;lt;p&amp;gt;In this study, a method for generating an ensemble of synthetic low-pressure systems is being developed to investigate the extreme sea level heights on the Finnish coast of Baltic sea. As input parameters for the method, the point of origin, velocity of the center of the cyclone and depth of the pressure anomaly need to be given. Based on the input parameters, the method forms an idealized low-pressure system using a two-dimensional Gaussian function. In order to find extreme, but still reasonable values for the input parameters, cyclone tracks from ERA5 reanalysis data will be analysed.&amp;lt;/p&amp;gt;&amp;lt;p&amp;gt;The ensemble of synthetic low pressure systems (i.e. the wind and pressure data) is used as an input to a numerical sea level model. As a result, we have an ensemble of simulated sea levels, from which we can determine the properties of the ETCs that induce the highest sea levels on a given location on the coast. The preliminary simulation results show that this method works well, forming a basis for studies on extreme sea levels.&amp;amp;#160;&amp;lt;/p&amp;gt;&amp;lt;p&amp;gt;&amp;amp;#160;&amp;lt;/p&amp;gt;

Future extreme sea levels (ESLs) and flood risk along European coasts will be strongly impacted by global warming. Yet, comprehensive projections of ESL that include mean sea level (MSL), tides, waves, and storm surges do not exist. Here, we show changes in all components of ESLs until 2100 in view of climate change. We find that by the end of this century, the 100‐year ESL along Europe's coastlines is on average projected to increase by 57 cm for Representative Concentration Pathways (RCP)4.5 and 81 cm for RCP8.5. The North Sea region is projected to face the highest increase in ESLs, amounting to nearly 1 m under RCP8.5 by 2100, followed by the Baltic Sea and Atlantic coasts of the UK and Ireland. Relative sea level rise (RSLR) is shown to be the main driver of the projected rise in ESL, with increasing dominance toward the end of the century and for the high‐concentration pathway. Changes in storm surges and waves enhance the effects of RSLR along the majority of northern European coasts, locally with contributions up to 40%. In southern Europe, episodic extreme events tend to stay stable, except along the Portuguese coast and the Gulf of Cadiz where reductions in surge and wave extremes offset RSLR by 20–30%. By the end of this century, 5 million Europeans currently under threat of a 100‐year ESL could be annually at risk from coastal flooding under high‐end warming. The presented dataset is available through this link: http://data.jrc.ec.europa.eu/collection/LISCOAST.Plain Language SummaryFuture extreme sea levels and flood risk along European coasts will be strongly impacted by global warming. Here, we show changes in all acting components, i.e., sea level rise, tides, waves, and storm surges, until 2100 in view of climate change. We find that by the end of this century the 100‐year event along Europe will on average increase between 57 and 81 cm. The North Sea region is projected to face the highest increase, amounting to nearly 1 m under a high emission scenario by 2100, followed by the Baltic Sea and Atlantic coasts of the UK and Ireland. Sea level rise is the main driver of the changes, but intensified climate extremes along most of northern Europe can have significant local effects. Little changes in climate extremes are shown along southern Europe, with the exception of a projected decrease along the Portuguese coast and the Gulf of Cadiz, offseting sea level rise by 20–30%. By the end of this century, 5 million Europeans currently under threat of a 100‐year coastal flood event could be annually at risk from coastal flooding under high‐end warming.

Abstract. Sea level rise (SLR) is a major concern for Europe, where 30 million people live in the historical 1-in-100-year event flood coastal plains. The latest IPCC assessment reports provide a literature review on past and projected SLR, and their key findings are synthesized here with a focus on Europe. The present paper complements IPCC reports and contributes to the Knowledge Hub on SLR European Assessment Report. Here, the state of knowledge of observed and 21st century projected SLR and changes in extreme sea levels (ESLs) are documented with more regional information for European basins as scoped with stakeholders. In Europe, satellite altimetry shows that geocentric sea level trends are on average slightly above the global mean rate, with only a few areas showing no change or a slight decrease such as central parts of the Mediterranean Sea. The spatial pattern of geocentric SLR in European Seas is largely influenced by internal climate modes, especially the North Atlantic Oscillation, which varies on year-to-year to decadal timescales. In terms of relative sea level rise (RSLR), vertical land motions due to human-induced subsidence and glacial isostatic adjustment (GIA) are important for many coastal European regions, leading to lower or even negative RSLR in the Baltic Sea and to large rates of RSLR for subsiding coastlines. Projected 21st century local SLR for Europe is broadly in line with projections of global mean sea level rise (GMSLR) in most places. Some European coasts are projected to experience a RSLR by 2100 below the projected GMSLR, such as the Norwegian coast, the southern Baltic Sea, the northern part of the UK, and Ireland. A relative sea level fall is projected for the northern Baltic Sea. RSLR along other European coasts is projected to be slightly above the GMSLR, for instance the Atlantic coasts of Portugal, Spain, France, Belgium, and the Netherlands. Higher-resolution regionalized projections are needed to better resolve dynamic sea level changes especially in semi-enclosed basins, such as the Mediterranean Sea, North Sea, Baltic Sea, and Black Sea. In addition to ocean dynamics, GIA and Greenland ice mass loss and associated Earth gravity, rotation, and deformation effects are important drivers of spatial variations of projected European RSLR. High-end estimates of SLR in Europe are particularly sensitive to uncertainties arising from the estimates of the Antarctic ice mass loss. Regarding ESLs, the frequency of occurrence of the historical centennial-event level is projected to be amplified for most European coasts, except along the northern Baltic Sea coasts where a decreasing probability is projected because of relative sea level fall induced by GIA. The largest historical centennial-event amplification factors are projected for the southern European seas (Mediterranean and Iberian Peninsula coasts), while the smallest amplification factors are projected in macro-tidal regions exposed to storms and induced large surges such as the southeastern North Sea. Finally, emphasis is given to processes that are especially important for specific regions, such as waves and tides in the northeastern Atlantic; vertical land motion for the European Arctic and Baltic Sea; seiches, meteotsunamis, and medicanes in the Mediterranean Sea; and non-linear interactions between drivers of coastal sea level extremes in the shallow North Sea.

This article presents an analysis of time-series for hydrometeorological conditions determining the behavior of the natural environment in the South Baltic coastal zone of Poland. The analysis is based on monthly data for average air temperature, total atmospheric precipitation, and average sea level during the 50-year period from 1966–2015 for three coastal stations in Hel, Ustka, and Świnoujście. Time decomposition of these hydrometeorological conditions and formulation of short-term forecasts were carried out using ARIMA modelling. This study identifies the seasonal and non-seasonal parameters that determine both current and future hydrometeorological conditions. Moreover, it indicates the spatial differences among features of the analyzed time-series, estimated parameters of the selected models, and forecasts. The ARIMA models used for the Polish Baltic coastal zone are somewhat spatially homogenous. This is especially true of the models for average monthly air temperature, which are identical across the entire coastal zone (2,0,1)(2,1,0)12. Very similar are the models for average monthly sea level across the central and west coast (1,0,0)(1,1,0)12. The model for the east coast, however, was determined to be slightly different (2,0,2)(2,1,0)12. In contrast to those for air temperature and sea level, the models used for atmospheric precipitation were different for each site. Among the parameters modelled, the effect of AR(p) processes was greater than that of MA(q) processes. The monthly models for Ustka are an example of this: average air temperature (2,0,1)(2,1,0)12, atmospheric precipitation (0,0,3)(2,1,0)12, and average sea level (1,0,0)(1,1,0)12. Time decomposition of extreme hydrometeorological conditions has an important utilitarian significance. The climate of the Polish Baltic coastal zone is getting warmer, the sea level is rising, and the frequency of extreme hydrometeorological events is increasing. Time decomposition of hydrometeorological conditions based on monthly data did not reveal long-term trends. In the last half-century, hydrometeorological conditions have been conducive to erosion of coastal dunes and cliffs. These factors determine changes in the natural environment and limit the development potential of the coastal zone. The time decomposition, modelling, and forecasting of hydrometeorological conditions are thus very important for many areas of human activity, especially those related to management, protection, and development of the coast.