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Climate Change and Coastal Processes in the Baltic Sea  

Tarmo Soomere

Various manifestations of climate change have led to complicated patterns of reactions of the Baltic Sea shores to varying hydrodynamic drivers. The northern and western bedrock and limestone coasts of this young water body experience postglacial uplift that is faster than the global sea-level rise. These coastal segments are thus insensitive with respect to changes in hydrodynamic forcing. Sedimentary and easily erodible coasts of the westernmost, southern, and eastern shores of this water body evolve under the impact of relative sea-level rise, changing wave properties and gradual loss of sea ice in conditions of chronic deficit of fine sediment. Several classic features of coastal processes, such as the cut-and-fill cycle of beaches, are substantially modified in many coastal sections. Waves approaching the shore systematically at large angles drive massive alongshore sediment transport in many coastal segments. This transport has led to the development of large sand spits and many relict lakes separated from the sea by coastal barriers. The concept of closure depth is reinterpreted because of frequent synchronization of strong waves and elevated water levels. The gradual loss of sea ice cover endangers most seriously coastal systems around the latitudes of the Gulf of Finland (about 60°N). The combined influence of climatically controlled sea-level rise and intense wave action leads to a gradual increase in eroding sections and the acceleration of coastal retreat on the southern downlifting shores of Poland and Germany. The bidirectional wind forcing has created a delicate balance of sediment on the shores of Latvia and Lithuania. This balance is vulnerable with respect to changes in strong wind directions. The sedimentary shores of Estonia host a number of small beaches that are geometrically protected against typical strong wind directions but are sensitive with respect to storms from unusual directions. Numerical analysis of sediment transport patterns along the eastern shores of the Baltic Sea has identified major changes in the wave directions in the Baltic Proper that can be attributed to manifestations of climate change.


Regional Sea Level  

Thomas Wahl and Sönke Dangendorf

Sea level rise leads to an increase in coastal flooding risk for coastal communities throughout the world. Changes in mean sea level are caused by a combination of human-induced global warming and natural variability and are not uniform throughout the world. The key processes leading to mean sea level rise and its variability in space and time are the melting of land-based ice and changes in the hydrological cycle; thermal expansion due to warming oceans; changes in winds, ocean currents, and atmospheric pressure; and, when focusing on the relative changes between the land and the ocean, any vertical motion of the land itself (subsidence or uplift). In addition to the change in mean sea level, which is the main climatic driver for changes in coastal flooding risk in most regions, additional changes in tides, storm surges, or waves can further exacerbate, or offset, the negative effects of mean sea level rise. Hence, it is important to analyze, understand, and ultimately project the changes in all of these sea level components individually and combined, including the complex interactions between them. Advances in sea level science in the 21st century along with new and extended observational records including in situ and remote sensing measurements have paved the path to being able to provide better and more localized information to stakeholders, particularly in the context of making decisions about coastal adaptation to protect the prosperity of coastal communities and ecosystems.


Atmospheric Blocking in Observation and Models  

Stefano Tibaldi and Franco Molteni

The atmospheric circulation in the mid-latitudes of both hemispheres is usually dominated by westerly winds and by planetary-scale and shorter-scale synoptic waves, moving mostly from west to east. A remarkable and frequent exception to this “usual” behavior is atmospheric blocking. Blocking occurs when the usual zonal flow is hindered by the establishment of a large-amplitude, quasi-stationary, high-pressure meridional circulation structure which “blocks” the flow of the westerlies and the progression of the atmospheric waves and disturbances embedded in them. Such blocking structures can have lifetimes varying from a few days to several weeks in the most extreme cases. Their presence can strongly affect the weather of large portions of the mid-latitudes, leading to the establishment of anomalous meteorological conditions. These can take the form of strong precipitation episodes or persistent anticyclonic regimes, leading in turn to floods, extreme cold spells, heat waves, or short-lived droughts. Even air quality can be strongly influenced by the establishment of atmospheric blocking, with episodes of high concentrations of low-level ozone in summer and of particulate matter and other air pollutants in winter, particularly in highly populated urban areas. Atmospheric blocking has the tendency to occur more often in winter and in certain longitudinal quadrants, notably the Euro-Atlantic and the Pacific sectors of the Northern Hemisphere. In the Southern Hemisphere, blocking episodes are generally less frequent, and the longitudinal localization is less pronounced than in the Northern Hemisphere. Blocking has aroused the interest of atmospheric scientists since the middle of the last century, with the pioneering observational works of Berggren, Bolin, Rossby, and Rex, and has become the subject of innumerable observational and theoretical studies. The purpose of such studies was originally to find a commonly accepted structural and phenomenological definition of atmospheric blocking. The investigations went on to study blocking climatology in terms of the geographical distribution of its frequency of occurrence and the associated seasonal and inter-annual variability. Well into the second half of the 20th century, a large number of theoretical dynamic works on blocking formation and maintenance started appearing in the literature. Such theoretical studies explored a wide range of possible dynamic mechanisms, including large-amplitude planetary-scale wave dynamics, including Rossby wave breaking, multiple equilibria circulation regimes, large-scale forcing of anticyclones by synoptic-scale eddies, finite-amplitude non-linear instability theory, and influence of sea surface temperature anomalies, to name but a few. However, to date no unique theoretical model of atmospheric blocking has been formulated that can account for all of its observational characteristics. When numerical, global short- and medium-range weather predictions started being produced operationally, and with the establishment, in the late 1970s and early 1980s, of the European Centre for Medium-Range Weather Forecasts, it quickly became of relevance to assess the capability of numerical models to predict blocking with the correct space-time characteristics (e.g., location, time of onset, life span, and decay). Early studies showed that models had difficulties in correctly representing blocking as well as in connection with their large systematic (mean) errors. Despite enormous improvements in the ability of numerical models to represent atmospheric dynamics, blocking remains a challenge for global weather prediction and climate simulation models. Such modeling deficiencies have negative consequences not only for our ability to represent the observed climate but also for the possibility of producing high-quality seasonal-to-decadal predictions. For such predictions, representing the correct space-time statistics of blocking occurrence is, especially for certain geographical areas, extremely important.


Baltic Sea Level: Past, Present, and Future  

Ralf Weisse and Birgit Hünicke

A multitude of geophysical processes contribute to and determine variations and changes in the height of the Baltic Sea water surface. These processes act on a broad range of characteristic spatial and timescales ranging from a few seconds to millennia. On very long timescales, the northern parts of the Baltic are uplifting due to the still ongoing visco-elastic response of the Earth to the last deglaciation, and mean sea level is decreasing in these regions. Over centuries, the Baltic Sea responds to changes in global and North Atlantic mean sea level. Processes affecting global mean sea level, such as warming of the world ocean or melting of glaciers and of polar ice sheets, do have an imprint on Baltic Sea levels. Over decades, variations and changes in atmospheric circulation affect transport through the Danish Straits connecting the Baltic and North seas. As a result, the amount of water in the Baltic Sea and the height of the sea level vary. Similarly, atmospheric variability on shorter timescales down to a few days cause shorter period variations of transport through the Danish Straits and Baltic Sea level. On even shorter timescales, the Danish Straits act as a low pass filter, and high frequency variations of the water surface within the Baltic Sea such as storm surges, wind waves, or seiches are solely caused internally. All such processes have undergone considerable variations and changes in the past. Similarly, they are expected to show variations and changes in the future and across a broad range of scales, leaving their imprint on observed and potential future Baltic Sea level and its variability.


Klaus Hasselmann: Recipient of the Nobel Prize in Physics 2021  

Hans von Storch and Patrick Heimbach

Klaus Hasselmann and Syukuro Manabe shared one half of the 2021 Nobel Prize in Physics for their achievements in “physical modelling of Earth’s climate, quantifying variability and reliably predicting global warming.” The Swedish Academy asserted: “Klaus Hasselmann created a model that links together weather and climate, thus answering the question of why climate models can be reliable despite weather being changeable and chaotic. He also developed methods for identifying specific signals, fingerprints, that both natural phenomena and human activities imprint in the climate. His methods have been used to prove that the increased temperature in the atmosphere is due to human emissions of carbon dioxide.” Klaus Hasselmann is best known for founding the Max Planck Institute for Meteorology in Hamburg, where he implemented his ideas on quantifying internal variability in the climate system and its components (“stochastic climate model”), and on devising a methodology to separate “noise,” that is, variability not provoked by external drivers, from a “signal” reflecting the impact of such external drivers. In this way, he introduced a paradigm shift from a deterministic view of the climate system to a genuinely stochastic one. This proved instrumental in detecting anthropogenic climate change beyond natural variability (“detection”) and in demonstrating that the ongoing change could not be explained without a dominant role of elevated atmospheric levels of greenhouse gases (“attribution”). Hasselmann and Manabe initiated the construction of two of the leading quasi-realistic climate models featuring not only an atmosphere and ocean but also the carbon cycle. These achievements were recognized by the Nobel Prize. The spectrum of themes where Klaus Hasselmann left significant footprints extends far beyond climate dynamics, covering a wide range of geophysical topics. By the time he entered the field of climate research, Hasselmann had already produced groundbreaking work on the modeling and predicting of ocean surface waves. He and his wife led the development of a third-generation wave model, versions of which are in operational use today at major numerical weather prediction centers around the world, including the European Centre for Medium-Range Weather Forecasts (ECMWF) in Europe and the National Oceanic and Atmospheric Administration (NOAA) in the United States. After his retirement, Hasselmann considered his contribution to geophysical issues of climate and climate change sufficient and chose to focus on two different topics. One concerned the coupling of societal decision making with the geophysical system. A second concerned Hasselmann’s interest in elementary particle physics, which dates back to his work in the 1960s when he described nonlinear resonant wave-wave interactions by means of Feynman diagrams. Following early ideas by Kaluza and Klein, Hasselmann pursued a deterministic, unified field theory of particles and fields, which he termed “metron theory.” It remains incomplete, and given Hasselmann’s age may never be completed by himself, but may have to await a smart young physicist to take on the challenge.