According to the Baltic Earth Assessment Reports (https://baltic.earth/projects/bear), the climate of the Baltic Sea region is changing and will continue to change as a result of anthropogenic greenhouse gas emissions. However, the regional Earth system is influenced by many other factors as well, including climate forcings such as aerosol emissions, solar activity and natural climate variability, as well as local human activities such as agriculture and offshore wind farming. Disentangling these multiple causes and effects is challenging, but crucial for developing reliable decadal climate predictions and multi-decadal projections of the future state of the ecosystem (e.g. its oxygen dynamics).

Regional Earth system models are a key tool for this task, since global climate models with a resolution of 100 x 100 km² cannot reproduce the regional climate of coastal seas such as the Baltic Sea. Regional coupled atmosphere-ocean-land models with a resolution of a few km² or less enable a more accurate representation of parameters such as wind, air temperature, and ocean salinity. In addition to physical parameters, they can model changes in the ecosystem.

The working group 'Dynamics of Regional Climate Systems' develops and uses regional Earth system models to investigate long-term changes in the Baltic Sea region.

Main research topics are:

• Detection of past changes in the Earth system of the Baltic Sea region and attribution to anthropogenic drivers.
• Understanding internal climate variability on timescales ranging from decades to millennia.
• Developing a regional coupled atmosphere-ocean-land model, the IOW – Earth System Model (IOW-ESM).
• Developing multi-decadal future projections and decadal climate predictions
• Quantifying and understanding the causes of regional climate , particularly changes of climate extremes.

Long-term evolution of saltwater inflows

The Baltic Sea is a semi-enclosed brackish sea with high freshwater input (mainly from rivers) and saltwater input from the North Sea which is strongly limited due to the shallow and narrow Danish straits that connect the Baltic Sea to the North Sea. As salt water has a higher density than fresh water, saltwater inflows propagate along the bottom of the Baltic Sea. They are not only crucial to maintain the salt distribution in the Baltic Sea, but also provide oxygen to deep water layers which are otherwise only poorly ventilated. Previous research on saltwater inflows has mainly focused on the large (and relatively easy to observe) Major Baltic Inflows that happen mainly in winter. However, smaller inflows happen throughout the year and their effects on the deep basins of the Baltic Sea can be substantial. During the recent decades, small to medium-sized summer and autumn inflows caught attention, causing very high deep water temperatures in the Baltic Sea.

For the studies listed below, we created comprehensive time series of saltwater inflows, ranging from small to large and from winter to summer inflows, based on about 150 years long regional model simulations (from 1850 until 2008). We found that there has been a shift in inflow seasonality toward less winter and more summer and autumn inflows. This is linked to pronounced deep water temperature trends and enhanced oxygen depletion in the western Baltic Sea. The reason for the shift in inflow seasonality is not yet clear, but promising candidates are a change in river runoff seasonality and changes in wind patterns.

11-year running means of June–October (JJASO) salt import versus annual 
temperature maximum below the seasonal thermocline (below 40 m) at station 
BY5 in the Bornholm Basin (taken from Barghorn et al. 2023). 

Oxygen dynamics in the Bornholm Basin below 60 m before, during, and after 
warm inflow in autumn 1997. Top panel: Mean temperature and oxygen content. 
The yellow box displays the Pearson correlation coefficient between both 
time series. Bottom panel: Physical, biochemical water column, and 
biochemical sedimentary sources (positive values) and sinks (negative values) 
of oxygen (taken from Barghorn et al. 2025a). 

References:

Barghorn, L., H. E. M. Meier, H. Radtke, T. Neumann and L. Naumov (2025). Warm saltwater inflows strengthen oxygen depletion in the western Baltic Sea. Clim. Dyn. 63: 29, doi: 10.1007/s00382-024-07501-x

Barghorn, L., H. E. M. Meier and H. Radtke (2023). Changes in seasonality of saltwater inflows caused exceptional warming trends in the western Baltic Sea. Geophys. Res. Lett. 50: e2023GL103853, doi: 10.1029/2023GL103853

For further questions please contact Dr. Leonie Barghorn.

Impact of the North Atlantic region on European seas studied with large climate model ensembles

The European continent and its marginal seas (including the Baltic Sea) are affected by large-scale atmospheric and oceanic patterns over and in the North Atlantic. For instance, the sea surface temperature (SST) of the Baltic Sea is, on a multidecadal scale, correlated to the average SST of the North Atlantic. However, the exact physical links are difficult to decipher, one reason being that both SST time series are composed of different contributions: Responses to natural and anthropogenic external forcing (such as aerosols or greenhouse gases) as well as internal variability arising exclusively from the chaotic nature of the climate system. Such externally forced and internal components can be disentangled with the help of so-called Single Model Initial-condition Large Ensembles (SMILEs). They comprise multiple (~30-100) runs of the same (global) climate model which employ the same external forcing but all start from different initial conditions. Hence, they show the same forced response but different realizations of internal variability.

In our publication below, we utilized two large ensembles from the Coupled Model Intercomparison Project phase 6 (CMIP6) to study the links between European seas and the North Atlantic in the past and how they might evolve under future greenhouse gas emission scenarios. We found a high correlation between the externally forced SST variability in the European seas and the North Atlantic. In contrast, the direct impact of the internal SST fluctuations in the North Atlantic is only visible in the North Sea and the western part of the Mediterranean Sea while the internal SST variability in the Baltic Sea is more linked to the dominant atmospheric pattern over the North Atlantic. Those connections will persist under future climate change scenarios.

Cross correlations of internal Atlantic SST variability (iAMV) with internal 
Baltic SST (grey lines; black lines denote the ensemble means) and externally 
forced Atlantic SST variability (fAMV) with forced Baltic SST (red lines) for 
the historical period (1880–2020) and the Shared Socioeconomic Pathway 
scenarios (1950–2090) in the MPI-ESM ensemble. Positive lags indicate leading 
local SST and negative lags leading AMVs. The internal Baltic SST leads the iAMV 
by 7 years as high Baltic Sea temperatures are driven by a positive phase of 
the North Atlantic Oscillation which also drives positive AMV phases, but with 
a time lag of a few years (modified from Barghorn et al. 2025b). 

Reference:

Barghorn, L., F. Börgel, M. Gröger and H. E. M. Meier (2025). Atlantic multidecadal variability control on European sea surface temperatures is mainly externally forced. Environ. Res. Lett. 20: 034044, doi: 10.1088/1748-9326/adb6bf

For further questions please contact Dr. Leonie Barghorn.

Exploring the potential for regional decadal climate predictions

A set of hindcast simulations with altered initial conditions were performed for the period 1948–2018 to investigate the potential predictability of water temperature, salinity, and ice cover in the Baltic Sea on seasonal to decadal timescales. To address this research question, an ensemble of 10-year Baltic Sea model simulations using the ocean circulation model MOM6 were performed, combining different initial fields and different forcings from the reference simulation for 1948-2018. Various sub-sampling strategies applied to the entire ensemble were tested to optimise the predictability of the hindcast period with the ensemble mean of the sub-sampled ensemble members. The idea was to make use of both the memory imparted by the initial conditions and the potential of selected climate patterns of future atmospheric forcing to predict water temperature, salinity and sea ice cover over the following decade. The results highlight the Baltic Sea's potential for temperature predictions in years 1 and 2, as well as salinity predictions on decadal timescales.

Reference:

Wolff, M. (2025) The potential of the Baltic Sea for seasonal to decadal (S2D) predictability. Master Thesis, Universität Rostock, 21.10.2025, https://doi.org/10.18453/rosdok_id00005038)

For further questions please contact Marti Wolff, MSc.

Increasing frequency of marine heatwaves contributed to a rise in occurrences of seabed hypoxia in coastal zones

Analysing reanalysis data of the Baltic Sea, we found that since the 1990s marine heatwaves (MHWs), defined by a threshold of 20 °C, have occurred across the entire surface of the Baltic Sea, particularly numerous in the coastal zones of the southern and eastern Baltic Sea. On the seabed, MHWs defined by a 17 °C threshold are detected in areas shallower than 20 m. Moreover, we found a significant correlation between events of bottom MHWs and events of reduced oxygen concentration. The oxygen depletion can reach 1.7 mL L⁻¹ and increases the risk of episodic hypoxia in coastal zones.

Reference:

Safonova, K., Meier, H.E.M. & Gröger, M. (2024). Summer heatwaves on the Baltic Sea seabed contribute to oxygen deficiency in shallow areas. Commun Earth Environ 5, 106, https://doi.org/10.1038/s43247-024-01268-z

For further questions please contact Kseniia Safonova, MSc.

How marine heatwaves and climate change impact on western Baltic Sea herring habitats

The western Baltic Sea herring population is one of the most important economic fish stocks in the Baltic Sea. The population experienced a dramatic decrease in course of previous decades and consequently herring catches dropped from almost 200.000 t to roughly 25.000 t in recent years. It has been hypothesized that climate change, more precisely the anomalous warm winters contributed to this decline besides continuously ongoing high rates of industrial fishing. Recent field investigations from the Greifswald Bay, one of the most important spawning site for herring, revealed a clear statistical relationship between winter length, defined as the period in which shallow waters remain below 3.5 °C, and herring recruitment. Consequently, the timing of winter inception in autumn, and winter termination in spring plays a major role for the herring reproductive success. Projections of ocean climate were then used to elaborate how climate change under assumption of different climate targets from The United Nations Framework Convention on Climate Change (UNFCC) Paris 2015 Agreement will change the winter characteristic in future.

Hence, in more and more spawning locations, winter length will successively decline and in the worst case winters suitable for successful reproduction won't occur anymore. In a global world 4 °C warmer compared to the preindustrial state, up to 60 % of local winter occurrences may be vanished. However, climate scenarios compliant with the Paris Agreement 2015 to limit global warming below 2.0°C or even 1.5 °C will mitigate the effect on winter climate considerably.

(Source: Gröger et al. 2024, Science of Total Environment)

Reference:

Gröger, M., F. Börgel, S. Karsten, H. E. M. Meier, K. Safonova, C. Dutheil, A. Receveur and P. Polte (2024). Future climate change and marine heatwaves - Projected impact on key habitats for herring reproduction. Sci. Total Environ. 951: 175756, doi: 10.1016/j.scitotenv.2024.175756. https://www.sciencedirect.com/science/article/pii/S0048969724059126

For further questions please contact Dr. Matthias Gröger.

Climate model calibration

One of the central questions in science is the estimate of uncertainties related to our methods and tools. This question is particularly relevant for regional climate models as they are used as a basis for real-world decisions with implications for entire communities, countries, or even continents. Thus, we pay special attention to the calibration of the IOW Earth System Model to deliver the best possible understanding and best possible estimates for local consequences of a globally changing climate. In particular, we treat model results as synthetic measurements of the Earth system and compare these with our colleagues’ continuous measurements of the Baltic Sea region. Systematic errors are then minimized through objective calibration techniques. Following the state-of-the-art in climate model tuning, we make use of the technique of perturbed physics ensembles, constructing surrogate models to represent the model’s sensitivity to parameter changes. With various strategies for such surrogate models, including quadratic inversions, polyharmonic spline inversions, and Gaussian process regression, we find the best estimates for the model settings. Moreover, the technique of history matching yields estimates for model parameter sensitivities and how these translate into uncertainties of model results.

Bias of the 2-meter temperture of the atmospheric model, CCLM, over the 
Baltic Sea before (top row) and after the objective calibration (bottom row).

For further questions please contact Dr. Georg Sebastian Völker.

Multi-decadal regional climate projections and natural variability

An ensemble of scenario simulations of 48 members applying a regional Earth system model was performed and used to investigate, inter alia, whether climate change will exacerbate hypoxia in the Baltic Sea and whether strategies to reduce nutrient loading could counteract this scenario. The projections incorporate various global sea level rise boundary conditions and were used to assess sources of uncertainty (Meier et al., 2021). The results showed that natural variability and model differences are great sources of uncertainty, preventing early detection of changes caused by global warming and nutrient load reductions. Due to differences in the assumptions made when designing the scenario simulations, the ensemble studied differs from earlier scenario simulations (Meier et al. 2022). For example, global sea-level rise was not explicitly taken into account in earlier simulations. Nevertheless, all model simulations suggest that a significant reduction in hypoxia can be achieved also in future climate through further reductions in nutrient loads in combination with existing measures.

To understand the causes of the pronounced multi-decadal fluctuations in the salinity of the Baltic Sea, sensitivity experiments were carried out for the historical climate (Meier et al., 2023). The model results reveal an internal feedback mechanism that amplifies salinity fluctuations with periods of around 30 years. Decades with greater precipitation over the catchment area and consequently greater freshwater inflow via the rivers reduce the salinity in the Baltic Sea and therefore also in the Danish Straits, meaning that, on average, the inflowing salt water has a lower salinity.

Mean salinity of the Baltic Sea. Low-pass filtered, spatially averaged salinity 
with a cut-off period of 12 years in five numerical experiments: the reference 
simulation (REF+, black line), the reference simulation without a sea level rise 
(REF, black dashed line), and the sensitivity experiments with a climatological 
mean river discharge and constant net precipitation with (RUNOFF+, red line) and 
without (RUNOFF, red dashed line) a sea level rise and with constant wind from 1904 
(WIND, cyan line). (Source: Meier et al., 2023)

References:

Meier, H. E. M., C. Dieterich, and M. Gröger (2021). Natural variability is a large source of uncertainty in future projections of hypoxia in the Baltic Sea. Commun. Earth Environ. 2, 50, https://doi.org/10.1038/s43247-021-00115-9

Meier, H. E. M., C. Dieterich, M. Gröger, C. Dutheil, F. Börgel, K. Safonova, O. B. Christensen, and E. Kjellström (2022). Oceanographic regional climate projections for the Baltic Sea until 2100. Earth Syst. Dynam., 13, 159–199, https://doi.org/10.5194/esd-13-159-2022

Meier, H. E. M., L. Barghorn, F. Börgel, M. Gröger, L. Naumov, and H. Radtke (2023). Multidecadal climate variability dominated past trends in the water balance of the Baltic Sea watershed. npj Climate and Atmospheric Science 6:58, https://doi.org/10.1038/s41612-023-00380-9.

For further questions please contact Prof. Dr. Markus Meier.