Oceanologia No. 66 (1) / 24


Foreword – ‘Assessing the Baltic Earth System’, Oceanologia 66 (2024) v-vi

Original research article

Original research article

The BALTEX/Baltic Earth program: Excursions and returns
Oceanologia 2024, 66(1), 1-8

Anders Omstedt1,*, Hans von Storch2
1 Department of Marine Sciences, University of Gothenburg, Sweden;
e-mail: anders.omstedt@marine.gu.se
*corresponding author
2 Institute of Coastal Systems, Helmholtz-Zentrum Hereon, Geesthacht, Germany

keywords: BALTEX, Baltic Earth, Water and energy cycles, Biogeochemistry, Carbon cycle, Climate and environmental research, Atmosphere-ocean-land surface modeling

Received 16 December 2022, Revised 1 June 2023, Accepted 7 June 2023, Available online 20 June 2023, Version of Record 12 April 2024.


The Baltic Sea Experiment (BALTEX) started in 1993 as part of the Global Energy and Water Cycle Experiment (GEWEX). It was later organized into three programs: BALTEX I, BALTEX II, and Baltic Earth. Here, we examine in a brief overview the overall BALTEX achievements, including program goals, risks encountered during the research journey, and knowledge development when finalizing the programs.

During three decades of climate and environmental studies of the Baltic Basin within the BALTEX/Baltic Earth programs, significant steps have been taken towards improved scientifically constructed knowledge and efforts to disseminate this knowledge to neighboring sciences and the public. These programs have illustrated the need to actively navigate the European research arena while remaining an independent science network. The well-organized International Baltic Earth Secretariat and many dedicated scientists made the research excursions safe and successful. The learning process relates to improved knowledge of the dynamics of the atmosphere–ocean–land climate system in the Baltic Sea region, the cycling of carbon and other substances, the region's anthropogenic climate and environmental changes, and how global warming and regional human activities can be detected outside natural variability.

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Environmental impact of water exchange blocking in a strait – a multidisciplinary study in the Baltic Sea
Oceanologia 2024, 66(1), 9-25

Taavi Liblik1,*, Fred Buschmann1, Enriko Siht1, Ivan Kuprijanov1, Germo Väli1, Maarja Lipp1, Ants Erm1, Jaan Laanemets1, Redik Eschbaum2, Aare Verliin2, Lauri Saks2, Ivar Zekker2
1Tallinn University of Technology, Tallinn, Estonia;
e-mail: Taavi.liblik@taltech.ee
2University of Tartu, Estonian Marine Institute, Tartu, Estonia e-mail: darius.jakimavicius@lei.lt, vytautas.akstinas@lei.lt
*corresponding author

keywords: Water exchange, Environmental impact of marine structures, Shallow sea, Fish migration, Baltic Sea

Received 20 October 2022, Revised 22 May 2023, Accepted 9 June 2023, Available online 22 June 2023, Version of Record 12 April 2024.


In this study, we report the environmental impact of water exchange blocking by a 3 km road dam built in 1896 in the shallow Väike Strait, north-eastern Baltic Sea. Using a multidisciplinary measurement campaign and numerical simulations, we show ecological conditions in the strait have considerably altered; the previously free-flowing strait now comprises two bays with separate circulation systems. Water exchange in the area close to the dam has decreased 10–12-fold. Since advection is weakened, exchange with the atmosphere and sediments has a relatively greater role in shaping water characteristics. Consequently, occasional very high sea surface temperature, salinity, and total nitrogen, and strong diurnal cycles in water temperature (>4°C) and dissolved oxygen (>4 mg l−1) were observed near the dam in summer. Oxygen levels are continuously below saturation in winter and concentration occasionally drops to hypoxic levels with ice cover. Nitrogen content in sediments near the dam is 3–4 times higher than in neighboring areas. The dam also modifies sea level, wind wave and suspended matter patterns in the strait. Sediments near the dam show elevated content of hazardous substances likely associated with traffic on the dam road. The phytobenthos community near the dam is dominated by annual green algae, which massively decompose during winter. The dam likely impedes fish migration between suitable feeding and spawning areas, also there have been fish kills caused by rapid fluctuations in sea levels, amplified by dam. The construction of new openings would alleviate negative impacts of the dam.
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Compound drought and heatwave events in the eastern part of the Baltic Sea region
Oceanologia 2024, 66(1), 26-36

Laurynas Klimavičius*, Egidijus Rimkus
Institute of Geosciences, Vilnius University, Vilnius, Lithuania;
e-mail: laurynas.klimavicius@chgf.vu.lt
*corresponding author

keywords: Compound climate events, Drought, Heatwave

Received 28 February 2023, Revised 8 June 2023, Accepted 30 June 2023, Available online 11 July 2023, Version of Record 12 April 2024.


Droughts and heatwaves are natural phenomena that can cause severe damage to the economy, infrastructure, human health, and agriculture, among others. However, in recent years, it has been noted that their combined effect, known as compound drought and heatwave events (CDHE), often results in even greater harm. The main aim of this study was to identify CDHEs in this region during summers from 1950 to 2022 and assess the frequency and intensity of these events. To this end, the periods of droughts and heatwaves that occurred between 1950 and 2022 were determined, and the recurrence, extent, and intensity of these phenomena were evaluated. In this study, 1-month Standard Precipitation Index (SPI) values calculated for each summer day were used to identify droughts, while heatwaves were defined as a period of five or more consecutive days when the daily maximum air temperature (Tmax) was higher than the 90th percentile of Tmax. Precipitation and Tmax data (with a spatial resolution 0.25° x 0.25°) were obtained from the European Centre of Medium-Range Weather Forecast ERA-5 reanalysis dataset. The study showed that in most of the eastern part of the Baltic Sea region, the number of drought days had decreased from 1950 to 2022, while the number of heatwave days had increased significantly. In total, ten CDHEs were identified during the summers of 1950–2022. Eight of these events were recorded in 1994 or later. However, a statistically significant increase of CDHEs was found only in a small part of the study area.
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Seawater temperature changes in the southern Baltic Sea (1959–2019) forced by climate change
Oceanologia 2024, 66(1), 37-55

Tamara Zalewska1,*, Bartłomiej Wilman1, Bożena Łapeta2, Michał Marosz1, Dawid Biernacik1, Agnieszka Wochna1, Michał Saniewski1, Agnieszka Grajewska1, Michał Iwaniak1
1Institute of Meteorology and Water Management – National Research Institute, Gdynia, Poland;
e-mail: tamara.zalewska@imgw.pl
2Institute of Meteorology and Water Management – National Research Institute, Kraków, Poland
*corresponding author

keywords: Sea surface temperature, Water column temperature, Climate change, Long-term measurements, Satellite data, Reanalysis data

Received 19 February 2023, Revised 27 June 2023, Accepted 29 August 2023, Available online 19 September 2023, Version of Record 12 April 2024.


The study included the analysis of changes in sea surface and water column temperature and air temperature in the years 1959–2019 in the southern Baltic Sea based on in situ measurement (CTD probe), satellite data, and model data (ERA5). SST increased on average by 0.6°C per decade. Analyses at different depths showed that the highest temperature increase per decade at 0.60–0.65°C characterised the layers from 0 to 20 m. The smallest increase (0.11°C) was recorded at a depth of 70 m, below which the temperature change per decade increases again to 0.24°C. The results from satellite observations covering 1982–2019 were consistent with measurement data. The most intense water warming occured in the spring – summer (0.8–1°C per decade); in the winter, the change did not exceed 0.2°C. In the offshore area, in 1951–2020, air temperature increased by approx. 2°C, with an average increase of 0.37°C per decade. The average increase in seawater temperature in the coastal zone was 0.2°C per decade. The most intense warming characterised March to May (0.25–0.27°C). The average annual air temperature increase on the coast from 1951 to 2020 was 0.34°C per decade. The results represent an important contribution to research and prediction of changes in the marine environment caused by global climate change.
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Spatiotemporal variability of wave climate in the Gulf of Riga
Oceanologia 2024, 66(1), 56-77

Fatemeh Najafzadeh1,*, Mikolaj Z. Jankowski1, Andrea Giudici1, Rain Männikus1, Ülo Suursaar3, Maija Viška4, Tarmo Soomere1,2
1School of Science, Tallinn University of Technology, Tallinn, Estonia;
e-mail: fatemeh.najafzadeh@taltech.ee
2Estonian Academy of Sciences, Tallinn, Estonia
3Estonian Marine Institute, University of Tartu, Estonia
4Latvian Institute of Aquatic Ecology, Riga, Latvia
*corresponding author

keywords: Wave climate, Wave modelling, In situ measurements, SWAN, Baltic Sea

Received 28 March 2023, Revised 5 October 2023, Accepted 8 November 2023, Available online 24 November 2023, Version of Record 12 April 2024.


Basic properties of wind wave climate in the Gulf of Riga, the Baltic Sea, are evaluated based on modelled wave fields, instrumentally measured and historical visually observed wave properties. Third-generation spectral wave model SWAN is applied to the entire Baltic Sea for 1990–2021 with a spatial resolution of 3 nautical miles (nmi, about 5.5 km) forced by the wind field of ERA5, to the Gulf of Riga and its entrance area with a resolution of 1 nmi (about 1.85 km), and to nearshore areas of this gulf with a resolution of 0.32 nmi (about 600 m). The calculations are performed for an idealised ice-free climate. Wave properties are represented by 36 directional and 32 frequency bins. The simulations are complemented by five sessions of instrumental measurements in the 2000s and two sets of historical visual wave observations from the island of Ruhnu and the Sõrve Peninsula for 1954–2011. Predominantly representing fetch-limited windseas, the wave climate in the gulf is milder and more intermittent than in the open Baltic Sea. The average significant wave height is mostly in the range of 0.6–0.8 m and peaks at 0.82 m inside the gulf. Typical wave periods are shorter than in the Baltic proper. The spatial pattern of wave heights, with higher wave intensity in the northern and eastern parts of the basin, follows anisotropy in wind conditions. Interannual variations are highly synchronised in different parts of the gulf. Their magnitude is less than 10% of the long-term average wave height. No long-term trend has been found in significant wave height and no distinct decadal variation exists inside the gulf.
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Submesoscale processes in the surface layer of the central Baltic Sea: A high-resolution modelling study
Oceanologia 2024, 66(1), 78-90

Germo Väli1,*, H.E. Markus Meier2, Taavi Liblik1, Hagen Radtke2, Knut Klingbeil2, Ulf Gräwe2, Urmas Lips1
1Department of Marine Systems, Tallinn University of Technology, Tallinn, Estonia;
e-mail: germo.vali@taltech.ee
2Leibniz Institute for Baltic Sea Research Warnemünde (IOW), Rostock, Germany
*corresponding author

keywords: Ocean Modelling, Submesoscale, Kinetic energy, Rossby numbers, GETM, Baltic Sea

Received 25 March 2023, Revised 15 October 2023, Accepted 16 November 2023, Available online 30 November 2023, Version of Record 12 April 2024.


A high-resolution model with a horizontal resolution of 250 m was used to analyze the surface eddy fields and the distribution of kinetic energy in the Baltic Sea. The results indicate a close relationship between the wind speed and the kinetic energy at the surface and the vertically averaged kinetic energy in the sea, and a lagged correlation between the kinetic energy at the surface and the eddy field. The spatial patterns of kinetic energy indicate more energetic currents in the western and southern parts of the Baltic Sea. The distribution of vorticity is inhomogeneous and differs significantly between sea areas. Submesoscale features are also inhomogeneously distributed and occur more frequently in the Gdańsk Basin, the Gulf of Finland, and the western part of the northern Baltic proper.
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Changes in cloudiness contribute to changing seasonality in the Baltic Sea region
Oceanologia 2024, 66(1), 91-98

Piia Post1,*, Margit Aun2
1Institute of Physics, University of Tartu, Tartu, Estonia;
e-mail: piia.post@ut.ee
2Tartu Observatory, University of Tartu, Tõravere, Estonia
*corresponding author

keywords: Northern Europe, Cloud fraction, Seasonal shift, Shortwave irradiance, Baltic Sea

Received 3 April 2023, Revised 22 October 2023, Accepted 16 November 2023, Available online 8 December 2023, Version of Record 12 April 2024.


In the Baltic Sea region, a significant increase in solar radiation has been detected during the past half-century. Changes in shortwave irradiance are associated with atmospheric transparency and cloudiness parameters like cloud fraction and albedo. One of the most important reasons for day-to-day changes in cloudiness is the synoptic-scale atmospheric circulation; thus, we look for reasons for solar radiation trends due to changes in atmospheric circulation. We analysed regional time series and trends from satellite-based cloud climate data record CLARA-A2 for the Baltic Sea region in 1982–2018. As the rise in solar radiation depends on the seasonally averaged values of total fractional cloud cover (CFC), surface incoming shortwave radiation (SIS) and occurrences of circulation types were analysed. We show that the shift in seasonality connected to the earlier accumulated sums of SIS is at least partly explained by the changes in synoptic-scale atmospheric circulation.
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The use of Argo floats as virtual moorings for monitoring the South Baltic Sea
Oceanologia 2024, 66(1), 99-110

Małgorzata Merchel*, Waldemar Walczowski, Daniel Rak, Piotr Wieczorek
Institute of Oceanology, Polish Academy of Sciences, Sopot, Poland;
e-mail: merchel@iopan.pl
*corresponding author

keywords: Argo floats, Virtual mooring, Baltic Sea, Temperature, Salinity

Received 24 February 2023, Revised 11 November 2023, Accepted 10 January 2024, Available online 2 February 2024, Version of Record 12 April 2024.


The Argo float measurement network is increasingly effectively covering internal seas and shelf areas. In this paper, a specific approach to using Argo floats as ‘virtual moorings’ within the conditions of the Baltic Sea is presented. Following a series of successful deployments with standard configurations, it was decided that the settings forcing the float to stay at the bottom between profiles should be tested. This significantly reduced the drift of the float and allowed measurements to be made in a limited water area for a longer time. The data obtained from Argo floats used as virtual mooring can be a valuable source for monitoring and analysing the hydrology of individual basins of the Baltic Sea. The results show that the temporal and spatial variability of the observed fields of temperature, salinity, and other properties of seawater is very high. More data are needed to correctly determine the mean properties of the basins and their temporal variability. Therefore, Argo floats can be a source of efficient and inexpensive hydrographic data for shallow seas such as the Baltic.
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Submarine groundwater discharge into a semi-enclosed coastal bay of the southern Baltic Sea: A multi-method approach
Oceanologia 2024, 66(1), 111-138

Cátia Milene Ehlert von Ahn1,*, Olaf Dellwig1, Beata Szymczycha2, Lech Kotwicki2, Jurjen Rooze1,10, Rudolf Endler3, Peter Escher1,4, Iris Schmiedinger1, Jürgen Sültenfuß5, Magdalena Diak2, Matthias Gehre6, Ulrich Struck7, Susan Vogler1, Michael Ernst Böttcher1,8,9
1Geochemistry & Isotope Biogeochemistry, Leibniz Institute for Baltic Sea Research (IOW), Warnemünde, Germany;
e-mail: merchel@iopan.pl, e-mail: michael.boettcher@io-warnemuende.de
2Institute of Oceanology, Polish Academy of Sciences (IO PAN), Sopot, Poland
3Marine Geophysics, Leibniz Institute for Baltic Sea Research (IOW), Warnemünde, Germany
4current address: Ecoandmore Freiburg, Germany
5Institute of Environmental Physics, University of Bremen, Bremen, Germany
6Department of Isotope Biogeochemistry, Helmholtz Centre for Environmental Research (UFZ), Leipzig-Halle, Germany
7Free University Museum for Natural History, Berlin, Germany
8Marine Geochemistry, University of Greifswald, Greifswald, Germany
9Interdisciplinary Faculty, University of Rostock, Rostock, Germany
10Department of Physical Oceanography and Instrumentation, Leibniz Institute for Baltic Sea Research (IOW), Warnemünde, Germany
*corresponding author

keywords: Stable isotopes, Radium isotopes, Acoustic survey, Diagenesis, Gulf of Gdańsk, Puck Bay

Received 28 February 2023, Revised 13 December 2023, Accepted 10 January 2024, Available online 2 February 2024, Version of Record 12 April 2024.


The present study aims to understand the impact of submarine groundwater discharge (SGD) on a coastal area with different lithology and degrees of SGD. Sampling campaigns took place in Puck Bay and the Gulf of Gdańsk, southern Baltic Sea encompassing years between 2009 and 2021. The methodological approach combined geophysical characterization of the surface sediments with detailed spatial and temporal (isotope) biogeochemical investigations of pore and surface waters, and was supported by nearshore groundwater and river surveys. Acoustic investigations identified areas of disturbance that may indicate zones of preferential SGD release. The composition of porewater and the differences in the bay's surface waters disclosed SGD as common phenomenon in the study area. Regional SGD was estimated through a radium mass balance. Local estimation of SGD, based on porewater profiles, revealed highest SGD fluxes at the sandy shoreline, but relatively low elemental fluxes. Though SGD was low at the muddy sites corresponding elemental fluxes of nutrients and dissolved carbon exceeded those determined at the sandy sites due to intense diagenesis in the top sediments. SGD appears to be sourced from different freshwater endmembers; however, diagenesis in surface sediments substantially modified the composition of the mixed solutions that are finally discharged to coastal waters. Overall, this study provides a better understanding of the SGD dynamics in the region by a multi-approach and emphasizes the need to understand the processes occurring at the sediment-water interface when estimating SGD.
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Continuing long-term expansion of low-oxygen conditions in the Eastern Gulf of Finland
Oceanologia 2024, 66(1), 139-152

Stella-Theresa Stoicescu1,*, Laura Hoikkala2, Vivi Fleming2, Urmas Lips1
1Tallinn University of Technology, Tallinn, Estonia;
e-mail: stella.stoicescu@taltech.ee
2Finnish Environment Institute (SYKE), Helsinki, Finland
*corresponding author

keywords: Dissolved oxygen, Baltic Sea, Eastern Gulf of Finland, Eutrophication, Climate change effects

Received 31 March 2023, Revised 21 December 2023, Accepted 19 February 2024, Available online 2 March 2024, Version of Record 12 April 2024.


To develop an oxygen indicator for the eastern part of the Gulf of Finland (EGOF), a dataset covering 1900–2021 was compiled. The analysis revealed a long-term declining trend in dissolved oxygen concentrations in the EGOF deep layer of 0.022 mg L–1 a–1 and multi-decadal variations associated with the observed changes in hydrographic conditions. About 27% of the decline in oxygen concentrations for 1900–2021 and 40% for 1990–2021 can be explained by the decrease in solubility due to the temperature increase and changes in hydrographic conditions. The water volume and bottom area under low oxygen conditions in 2016–2021, characterized by dissolved oxygen concentrations <= 6 mg L–1, have increased, compared to the selected reference period with almost no human impact in the 1920s–1950s, from 9.8 km3 to 78.0 km3 (from 2.6% to 20.9% of the EGOF total volume) and from 1190 km2 to 4950 km2 (from 13.4% to 56.0% of the EGOF total area), respectively. The environmental status of the EGOF was assessed as not good based on the introduced oxygen indicator. We conclude that, in the long-term, low oxygen conditions have expanded mostly due to the excess load and accumulation of nutrients in the system and temperature-related changes in biogeochemical processes and fluxes. However, on a decadal scale, changes in hydrographic conditions, i.e. stratification and mixing, can significantly impact the sub-surface oxygen conditions in the EGOF and similar estuarine basins.
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