Algal blooms are common in the Baltic Sea during the summer, where they pose a significant threat to coastal services and industries. Lagrangian Coherent Structures (LCS) have been shown to play an important role in driving the mixing and transport of water masses and tracers in other ocean basins, such as the Mediterranean Sea (Antivachis et al. 2023), and are thus expected to have a strong effect on transport processes and the development and spread of algal blooms in the Baltic Sea.In this work, we use trajectory-derived Largangian diagnostics to investigate the distribution and variability of LCS in the Baltic Sea, using a series of 10-day trajectory experiments during summer 2022. Finite Size Lyapunov Exponent (FSLE), Trajectory Rotation Angle (TRA) and related metrics areused to assess the impact of LCS on horizontal mixing and dispersion processes in the basin. The potential influence of LCS on the spread and impact of algal blooms by opening/closing off transport pathways and exposing/shielding coastal regions is investigated by relating the spatiotemporal distribution of LCS to surface cyanobacteria concentrations obtained from satellite observations. The LCS regime in the Baltic Sea is compared to the ones observed in the Mediterranean in the author’s previous work (Antivachis et al. 2023). This is the first study to map the LCSs of the Baltic Sea and investigate their impact on algal blooms in that basin.This work is part of the ongoing ALGOTL project, funded by the Swedish research council for sustainable development (FORMAS), aiming to develop a Lagrangian modelling and forecasting framework for algae growth and dispersion for assessing the risk posed by algal blooms. Particle advection is carried out using velocity fields from the Swedish Hydrological and Meteorological Institute (SMHI) NEMO-Nordic configuration (Hordoir et al. 2019) and the TRACMASS Lagrangian trajectory code (Aldama-Campino et al. 2020).ReferencesDimitrios Antivachis, Vassilios Vervatis, and Sarantis Sofianos. Lagrangian coherent structures in the mediterranean sea: Seasonality and basin regimes. Progress in Oceanography, 215:103051, 2023. https://doi.org/10.1016/j.pocean.2023.103051Hordoir, R., Axell, L., Höglund, A., Dieterich, C., Fransner, F., Gröger, M., Liu, Y., Pemberton, P., Schimanke, S., Andersson, H., Ljungemyr, P., Nygren, P., Falahat, S., Nord, A., Jönsson, A., Lake, I., Döös, K., Hieronymus, M., Dietze, H., Löptien, U., Kuznetsov, I., Westerlund, A., Tuomi, L., and Haapala, J.: Nemo-Nordic 1.0: a NEMO-based ocean model for the Baltic and North seas – research and operational applications, Geosci. Model Dev., 12, 363–386, https://doi.org/10.5194/gmd-12-363-2019, 2019. Aldama-Campino, Aitor, Döös, Kristofer, Kjellsson, Joakim, & Jönsson, Bror. (2020, December 17). TRACMASS: Formal release of version 7.0 (Version v7.0-beta). Zenodo. http://doi.org/10.5281/zenodo.433792

Lagrangian (parcel following) approach is a powerful method to diagnose the modelled flow and associated property changes in atmosphere and ocean and is used to investigate causal links between the property changes between the different regions. The salt in the saline sea water has traditionally been tracked as tracer property or a marker of sea water despite the seawater is constituted of both water and salt molecules. In the present study, we propose an new approach relying on tracking separately the mass of fresh water and salt in the ocean. As a study region we have chosen the Baltic Sea, a semi-enclosed sea characterised by a distinct estuarine circulation due to river runoff and deep salt water inflow from the North Sea. The salt was tracked by summing over both the advective and diffusive salt fluxes simulated by the circulation model NEMO. Salt and water trajectories were computed with the mass conserving TRACMASS model, where each trajectory tube is in units of m3/s of water flux or kg/s of salt flux. The preliminary results show a clear difference between salt and water trajectories, where e.g. the salt trajectories (red in the attached Figure) do not reach as far into the Baltic Sea as the (blue) water trajectories. Many diagnostics such as the residence time and age also differ, which opens up a completely new vision of the ocean circulation.Figure: Water mass (blue) and salt mass (red) trajectories entering the Baltic Sea through the Danish straits.

The island of Gotland located in the middle of the Baltic Sea is Sweden's largest island, and also a county and municipality with the population of about 60,000, employed mainly in agriculture and tourist sectors. Gotland is a very popular domestic tourist destination for mainland Swedes, reaching nearly 2 million ferry and plane passengers per year. Gotland experiences limited capacity in groundwater reservoirs combined with increased demand during the warm season when tourists visits peak leading to recurring water stress. Desalination of drinking water from the Baltic Sea is a promising alternative to complement municipal water supply. The operation of the two major desalination treatment plants becomes however disturbed by compound hazards due to extreme sea weather events (marine heatwaves, strong upwelling events) and related hydro-sedimentary and biological processes (macro- and microalgae blooms) that are predicted to intensify under the climate change. Developing an apt forecasting system for this "multi-hazard" to inform sustainable management of Gotland's water resources becomes thus a priority and is of broader relevance to other regions in Sweden. The ALGOTL project, funded by the Swedish research council for sustainable development (FORMAS), is a collaboration between Stockholm University, the Swedish Meteorological and Hydrological Institute (SMHI) and Region Gotland to develop a novel forecast framework for natural hazard impacts on management of water resources, both short term (early warning) and long term (climate scenarios). The project aims at development of Lagrangian- and risk modelling tools based on the operational ocean state forecast at SMHI. Our stakeholders on Gotland will provide input on adverse impacts, information required for management, and feedback on the forecast framework during the project. This contribution will summarize results from observational and modelling analysis of the oceanographic, hydrological and biological conditions due to the Hans storm event in August 2023 that led to the disturbance in operation of the desalination plants on Gotland, located at the Herrvik (eastern side) and Kvarnåkershamn (western side of the island). The storm triggered an upwelling event leading to sea temperature changes of 10 K (prompting the change from summer to winter operational mode) and high vertical and horizontal velocities and associated excessive transport of sediment and biological (algae) material disrupting the filtering process. We will also show results from Lagrangian backtracking of the source waters reaching the desalination plants and present prospects for developing of a forecast system for related events in the future. More information about the ALGOTL project: https://www.su.se/english/research/research-projects/algotl-forecast-framework-for-algae-blooms-to-secure-water-supply-on-gotland

The North Sea and the Baltic Sea still experience eutrophication and deoxygenation despite large international efforts to mitigate such environmental problems. Due to the highly different oceanographic frameworks of the two seas, existing modelling efforts have mainly focused on only one of the respective seas, making it difficult to study interbasin exchange of mass and energy. Here, we present NEMO–SCOBI, an ocean model (NEMO-Nordic) coupled to the Swedish Coastal and Ocean Biogeochemical model (SCOBI), that covers the North Sea, the Skagerrak–Kattegat transition zone and the Baltic Sea. We address its validity to further investigate biogeochemical changes in the North Sea–Baltic Sea system. The model reproduces the long-term temporal trends, the temporal variability, the yearly averages and the general spatial distribution of all of the assessed biogeochemical parameters. It is particularly suitable for use in future multi-stressor studies, such as the evaluation of combined climate and nutrient forcing scenarios. In particular, the model performance is best for oxygen and phosphate concentrations. However, there are important differences between model results and observations with respect to chlorophyll a and nitrate in coastal areas of the southeastern North Sea, the Skagerrak–Kattegat transition zone, the Gulf of Riga, the Gulf of Finland and the Gulf of Bothnia. These are partially linked to different local processes and biogeochemical forcing that lead to a general overestimation of nitrate. Our model results are validated for individual areas that are in agreement with policy management assessment areas, thereby providing added value with respect to better contributing to international programmes aiming to reduce eutrophication in the North Sea–Baltic Sea system.

Abstract. A Baltic dataset covering 1990–2020 is reconstructed using a circulation model and data assimilation. Satellite observations of sea surface temperature and temperature and salinity (T/S) profiles are used to reduce model biases by a local Singular Evolutive Interpolated Kalman (SEIK) filter. The dataset is evaluated with assimilated T/S profiles and reprocessed grid observations, and the results demonstrate that the sea surface temperature, sea surface height, mixed layer depth, and vertical distribution of T/S are all reasonably reproduced. T/S trends at various depths in the Baltic sub-basins are analyzed from a reanalysis perspective, revealing a clear warming trend in recent decades, with a slight desalination trend in the northern Baltic Sea and a salination trend in the southern Baltic Sea. In particular, T/S trends of the Baltic Sea are larger in the south than in the north. In the Baltic Sea over the past 30 years, the temperature rises at a rate of 0.036 to 0.041 °C/year, with a larger warming trend below the thermocline than above it, while the salinity increases with a trend of -0.0036 to 0.049 PSU/year. In addition, seasonal variations are evident in the temperature at the surface, 60 m, and bottom, as well as in the surface salinity, whereas no clear seasonal variations are detected in the salinity below the surface and temperature at 100 m.

<p><span lang="en-US">The Orust-Tjörn fjord system is located </span><span lang="en-US">on</span><span lang="en-US"> the west coast of Sweden and consists of several fjords with many small islands. The fjord system has more than one connection to the open water, enabling a generally counterclockwise circulation through the fjords. </span><span lang="en-US">It</span><span lang="en-US"> is home to nature reservoirs, different industries on land and aquaculture farms. It is threatened for example </span><span lang="en-US">by </span><span lang="en-US">hypoxic areas, invasive species, water pollution </span><span lang="en-US">and</span><span lang="en-US"> algae blooms with the water quality strongly been influenced by the water exchange in the fjord system. Therefore it is important to understand the circulation, the state and the exchange of w­ater between the fjords system and the open water outside the fjord.</span></p> <p><span lang="en-US">Not much knowledge of the circulation inside the fjords and the drivers of the water exchange between the fjords and the open water exists. The reasons are that observations are spatially sparse and the resolution of regional ocean circulation models are generally to</span><span lang="en-US">o</span><span lang="en-US"> coarse to resolve this complex fjord system with its shallow and narrow straits between the different fjords. Therefore, we developed a setup of a coastal ocean circulation model with a horizontal resolution of 50 m to study the drivers of water exchange of the Orust-Tjörn fjord system. Within a sensitivity study a set of simulations are performed using (a) temporally constant wind forcing, (b) temporally and spatially constant sea level at the open boundaries (c) no tides, and (d) constant offshore density, i.e. temporally and spatially constant temperature and salinity profiles at the open boundaries. The simulation period is September 2016 – February 2017, which includes a high saline water inflow in October 2016. </span></p> <p><span lang="en-US">Model results compare well with observations </span><span lang="en-US">from moored high-frequency velocity, temperature and salinity instruments as well as regular monitoring data. </span><span lang="en-US">Results</span><span lang="en-US"> show for example that the density difference between the southern and northern entrance of the Orust-Tjörn fjord system highly influences the water exchange between the fjords and open water. This is because with constant offshore density applied, near surface velocities of the Baltic current as well as the circulation above sill level within the fjord system weakens. The timing and strength of the high-saline inflow event is most sensitive to </span><span lang="en-US">the</span><span lang="en-US"> cases of constant winds and constant offshore densities. In general, we will present a detailed analysis of the main drivers of water exchange above sill level. </span></p>

The aim of this study is to investigate the potential of spaceborne synthetic aperture radar (SAR) to monitor the Baltic Sea inflow/outflow circulation through the Danish straits. The flow in the Danish straits is mainly driven by changes in atmospheric forcing and is dominated by irregular inflow and outflow events. SAR provides high spatial resolution observations of the sea surface, which are particularly relevant in coastal areas and shelf seas. During the last decade, a new application of SAR measurements based on the analysis of the Doppler shift has emerged. The SAR Doppler shift is directly related to the surface circulation, thus direct measurements of surface currents are possible. It is however a challenging problem in practice due to the wave contribution to the observed Doppler shift. The main limitation of spaceborne SAR for monitoring fast evolving ocean processes is the long revisit time. In order to overcome this limitation, data from three satellites are combined in this study, namely Sentinel-1A, Sentinel-1B and TanDEM-X. Sentinel-1 is a conventional single-antenna SAR, while TanDEM-X is an along-track interferometric SAR. In addition, the two systems differ in the operating frequency and in the imaging mode. In this study, two months of opportunistic data (June and July 2020) covering the Danish strait (Fehmarn Belt) are used. This time period is constrained by the availability of coincident (Sentinel-1 and TanDEM-X) data covering the area of interest. Since TanDEM-X is not an ocean-dedicated mission, acquisitions suitable for ocean current retrieval are sporadic. Comparison of the derived radial velocities shows a good agreement between Sentinel-1 and TanDEM-X, provided both datasets are calibrated over land and the time delay between acquisitions is below ~20 min. The residual difference is probably due to the wave-induced Doppler shift. The SAR derived velocities are compared to the Copernicus analysis product (BALTICSEA\_ANALYSIS\_FORECAST\_PHY\_003\_006) and in-situ measurements. A reasonable agreement is found, provided that the wave-induced Doppler shift is taken into account. The study also investigates the relationship between the surface current along the Fehmarn Belt, the sea surface wind and the sea level, as an attempt to understand the main drivers of the surface flow. First, a high variability in the duration of inflow/outflow is observed. The shortest and the longest durations are one day and 10 days, respectively. Second, it is found that the surface current is predominantly in the east-to-west direction (outflow). Third, the relationship between the local wind and the surface current is stronger in the outflow situation, whereas the relationship between the surface current and the sea level gradient is stronger in the inflow situation. Though these observations agree with previous studies, it is however difficult to draw firm conclusions on the driving force from these limited dataset, hence additional data are required to verify these results. However, the study clearly demonstrates the potential of SAR for monitoring sea surface flows.

We present Nemo-Nordic 2.0, the latest version of the operational marine forecasting model for the Baltic Sea used and developed in the Baltic Monitoring Forecasting Centre (BAL MFC) under the Copernicus Marine Environment Monitoring Service (CMEMS). The most notable differences between Nemo-Nordic 2.0 and its predecessor Nemo-Nordic 1.0 are the switch from NEMO 3.6 to NEMO 4.0 and an increase in horizontal resolution from 2 to 1 nautical mile. In addition, the model's bathymetry and bottom friction formulation have been updated. The model configuration was specially tuned to represent Major Baltic Inflow events. Focusing on a 2-year validation period from October 1, 2014, covering one Major Baltic Inflow event, Nemo-Nordic 2.0 simulates Sea Surface Height (SSH) well: centralized Root-Mean-Square Deviation (CRMSD) is within 10 cm for most stations outside the Inner Danish Waters. CRMSD is higher at some stations where small-scale topographical features cannot be correctly resolved. SSH variability tends to be overestimated in the Baltic Sea and underestimated in the Inner Danish Waters. Nemo-Nordic 2.0 represents Sea Surface Temperature (SST) and Salinity (SSS) well, although there is a negative bias around -0.5°C in SST. The 2014 Major Baltic Inflow event is well reproduced. The simulated salt pulse agrees well with observations in the Arkona basin and progresses into the Gotland basin in 3 to 4 months.

This paper describes Nemo-Nordic 2.0, an operational marine model for the Baltic Sea. The model is used for both near-real-time forecasts and hindcast purposes. It provides estimates of sea surface height, water temperature, salinity, and velocity, as well as sea ice concentration and thickness. The model is based on the NEMO (Nucleus for European Modelling of the Ocean) circulation model and the previous Nemo-Nordic 1.0 configuration by Hordoir et al. (2019). The most notable updates include the switch from NEMO version 3.6 to 4.0, updated model bathymetry, and revised bottom friction formulation. The model domain covers the Baltic Sea and the North Sea with approximately 1 nmi resolution. Vertical grid resolution has been increased from 3 to 1 m in the surface layer. In addition, the numerical solver configuration has been revised to reduce artificial mixing to improve the representation of inflow events. Sea ice is modeled with the SI3 model instead of LIM3. The model is validated against sea level, water temperature, and salinity observations, as well as Baltic Sea ice chart data for a 2-year hindcast simulation (October 2014 to September 2016). Sea level root mean square deviation (RMSD) is typically within 10 cm throughout the Baltic basin. Seasonal sea surface temperature variation is well captured, although the model exhibits a negative bias of approximately −0.5 ∘C. Salinity RMSD is typically below 1.5 g kg−1. The model captures the 2014 major Baltic inflow event and its propagation to the Gotland Deep. The model assessment demonstrates that Nemo-Nordic 2.0 can reproduce the hydrographic features of the Baltic Sea.

The water exchange between the Orust-Tjörn fjord system (located on the Swedish west coast) and the Skagerrak depends on different factors such as winds, tides, the water mass properties and circulation in the Skagerrak, as well as the density gradients between the southern and northern openings of the fjord system. These processes are not yet well understood as observations in the area are spatially and temporally sparse and the existing regional ocean models for the North Sea and Baltic Sea area have a too coarse resolution to sufficiently resolve the complex structures of the fjord system, such as the narrow and shallow channels that connect the different fjords in the system. Therefore, we model the water exchange between the Orust-Tjörn fjord system and the Skagerrak using a NEMO3.6 model setup that has a horizontal resolution of 50 m. As validation, modelled temperature, salinity, velocity and sea surface height are compared with in-situ measurements. A detailed analysis of the modelled water flows in and out of the fjord system as well as between the different fjords will be presented. In addition, the different drivers of the modelled water exchange and their influence on the water properties above and below the sill depths in the fjords are investigated.

This paper presents a method for joint retrieval of the ocean surface wind and current vectors using the backscatter and the Doppler frequency shift measured by spaceborne single-beam single-polarization synthetic aperture radar (SAR). The retrieval method is based on the Bayesian approach with the a priori information provided by atmospheric and oceanic models for surface wind and currents, respectively. The backscatter and Doppler frequency shift are estimated from the along-track interferometric SAR system TanDEM-X data. The retrieval results are compared against in-situ measurements along the Swedish west coast. It is found that the wind retrieval reduces the atmospheric model bias compared to in-situ measurements by about 1 m/s for wind speed, while the bias reduction in the wind direction is minor as the wind direction provided by the model was accurate in the studied cases. The ocean model bias compared to in-situ measurements is reduced by about 0.04 m/s and 12 circle for current speed and direction, respectively. It is shown that blending SAR data with model data is particularly useful in complex situations such as atmospheric and oceanic fronts. This is demonstrated through two case studies in the Skagerrak Sea along the Swedish west coast. It is shown that the retrieval successfully introduces small scale circulation features detected by SAR that are unresolved by the models and preserves the large scale circulation imposed by the models.

The Orust fjord system, located on the west coast of Sweden, has openings on both ends and consists of several fjords that are connected by narrow and shallow channels. The fjord system includes the islands Orust and Tjörn as well as various smaller islands. The water exchange between the Kattegat and the different fjords is influenced by different factors, such as winds, tides, and density gradients. However, advection between the open sea and the complex fjord system are not yet well understood as lower resolution ocean models cannot resolve the small scale structures of the fjords and their connections. In addition, observations are rather sparse. Therefore, the water exchange in the Orust fjord system is simulated using a high resolution (50 meter) NEMO3.6 ocean model setup, forced with the UERRA atmospheric reanalysis dataset. The lateral open boundary conditions for temperature, salinity, sea levels and velocities are provided by a low resolution (1.85 km) NEMO3.6 simulation, which spans the Baltic Sea and North Sea regions. The model results are validated by comparison of modelled temperature, salinity, velocities and sea surface height with in-situ measurements. A detailed analysis of the different drivers of modelled water exchange between the Kattegat and the fjord system as well as between the different basins is presented. In general, the modelled water properties of the near surface layer in the fjord system are influenced by the Skagerrak surface water, which is controlled by the prevailing northward flowing Baltic Sea water. However, the residence times of water masses below the sill level are longer than the ones of the surface water masses as dense inflows of Skagerrak water in the basins create a strong stratification leading to weak vertical exchange.

We present Nemo-Nordic, a Baltic and North Sea model based on the NEMO ocean engine. Surrounded by highly industrialized countries, the Baltic and North seas and their assets associated with shipping, fishing and tourism are vulnerable to anthropogenic pressure and climate change. Ocean models providing reliable forecasts and enabling climatic studies are important tools for the shipping infrastructure and to get a better understanding of the effects of climate change on the marine ecosystems. Nemo-Nordic is intended to be a tool for both short-term and long-term simulations and to be used for ocean forecasting as well as process and climatic studies. Here, the scientific and technical choices within Nemo-Nordic are introduced, and the reasons behind the design of the model and its domain and the inclusion of the two seas are explained. The model's ability to represent barotropic and baroclinic dynamics, as well as the vertical structure of the water column, is presented. Biases are shown and discussed. The short-term capabilities of the model are presented, especially its capabilities to represent sea level on an hourly timescale with a high degree of accuracy. We also show that the model can represent longer timescales, with a focus on the major Baltic inflows and the variability in deep-water salinity in the Baltic Sea.

In this study, a third-generation wave model is used to examine the wave power resource for the Baltic Sea region at an unprecedented one-kilometer-scale resolution for the years 1998 to 2013. Special focus is given to the evaluation and description of wave field characteristics for the Swedish Exclusive Economic Zone (SEEZ). It is carried out to provide a more detailed assessment of the potential of waves as a renewable energy resource for the region. The wave energy potential is largely controlled by the distance from the coast and the fetch associated with the prevailing dominant wave direction. The ice cover is also shown to significantly influence the wave power resource, especially in the most northern basins of the SEEZ. For the areas in focus here, the potential annual average wave energy flux reaches 45 MWh/m/year in the two sub-basins with the highest wave energies, but local variations are up to 65 MWh/m/year. The assessment provides the basis for a further detailed identification of potential sites for wave energy converters. An outlook is given for additional aspects studied within a broad multi-disciplinary project to assess the conditions for offshore wave energy conversion within the SEEZ.

An ensemble of regional climate change scenarios for the North Sea is validated and analyzed. Five Coupled Model Intercomparison Project Phase 5 (CMIP5) General Circulation Models (GCMs) using three different Representative Concentration Pathways (RCPs) have been downscaled with the coupled atmosphere–ice–ocean model RCA4-NEMO. Validation of sea surface temperature (SST) against different datasets suggests that the model results are well within the spread of observational datasets. The ensemble mean SST with a bias of less than 1 ∘ C is the solution that fits the observations best and underlines the importance of ensemble modeling. The exchange of momentum, heat, and freshwater between atmosphere and ocean in the regional, coupled model compares well with available datasets. The climatological seasonal cycles of these fluxes are within the 95% confidence limits of the datasets. Towards the end of the 21st century the projected North Sea SST increases by 1.5 ∘ C (RCP 2.6), 2 ∘ C (RCP 4.5), and 4 ∘ C (RCP 8.5), respectively. Under this change the North Sea develops a specific pattern of the climate change signal for the air–sea temperature difference and latent heat flux in the RCP 4.5 and 8.5 scenarios. In the RCP 8.5 scenario the amplitude of the spatial heat flux anomaly increases to 5 W/m 2 at the end of the century. Different hypotheses are discussed that could contribute to the spatially non-uniform change in air–sea interaction. The most likely cause for an increased latent heat loss in the central western North Sea is a drier atmosphere towards the end of the century. Drier air in the lee of the British Isles affects the balance of the surface heat budget of the North Sea. This effect is an example of how regional characteristics modulate global climate change. For climate change projections on regional scales it is important to resolve processes and feedbacks at regional scales.

Oil spills are serious environmental issues that potentially can cause adverse effects on marine ecosystems. In some marine areas, like the Baltic Sea, there is a large number of wrecks from the first half of the 20th century, and recent monitoring and field work have revealed release of oil from some of these wrecks. The risk posed by a wreck is governed by its condition, hazardous substances contained in the wreck and the state of the surrounding environment. Therefore, there is a need for a common standard method for estimating the risks associated with different wrecks. In this work a state-of-the-art model is presented for spatial and stochastic risk assessment of oil spills from wrecks, enabling a structured approach to include the complex factors affecting the risk values. A unique feature of this model is its specific focus on uncertainty, facilitating probabilistic calculation of the total risk as the integral expected sum of many possible consequences. A case study is performed in Kattegat at the entrance region to the Baltic Sea to map the risk from a wreck near Sweden. The developed model can be used for oil spill risk assessment in the marine environment all over the world.

Introduction — s1 Chapter 1: Essential Variables — s4 1.1 Ocean temperature and salinity Sandrine Mulet, Bruno Buongiorno Nardelli, Simon Good, Andrea Pisano, Eric Greiner, Maeva Monier, Emmanuel...

An improved oil-in-ice parameterization has been implemented in the Seatrack Web oil spill model, tested using data from the Runner 4 oil spill accident, which occurred in high ice concentrations in the Gulf of Finland on 5 March 2006. The model is able to describe the observed transport and spreading of oil reasonably well. The sensitivity of the results towards oil-in-ice parameters and hydrodynamic forcing models has been investigated. Both the mean oil trajectory and the oil spreading are sensitive to a threshold velocity for the withdrawal of oil from below ice floes, and the oil spreading is highly sensitive to the size of the floes. The trajectories for the ice drift and for the water current drift set the limits of the oil drift, and these are in turn highly dependent on the hydrodynamic forcing and the assimilation of ice conditions. In future development of oil spill modeling in ice it is therefore important to focus not only on the ice parameterization but also on the ability to model ice drift, ice floe sizes, and the currents below the ice.

The Baltic Sea is a seasonally ice-covered marginal sea in northern Europe with intense wintertime ship traffic and a sensitive ecosystem. Understanding and modeling the evolution of the sea-ice pack is important for climate effect studies and forecasting purposes. Here we present and evaluate the sea-ice component of a new NEMO–LIM3.6-based ocean–sea-ice setup for the North Sea and Baltic Sea region (NEMO-Nordic). The setup includes a new depth-based fast-ice parametrization for the Baltic Sea. The evaluation focuses on long-term statistics, from a 45-year long hindcast, although short-term daily performance is also briefly evaluated. We show that NEMO-Nordic is well suited for simulating the mean sea-ice extent, concentration, and thickness as compared to the best available observational data set. The variability of the annual maximum Baltic Sea ice extent is well in line with the observations, but the 1961–2006 trend is underestimated. Capturing the correct ice thickness distribution is more challenging. Based on the simulated ice thickness distribution we estimate the undeformed and deformed ice thickness and concentration in the Baltic Sea, which compares reasonably well with observations.