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| 2. |
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| 3. |
- Berglund, Sara, 1990-
(författare)
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Tracing pathways in the ocean circulation : A temperature and salinity perspective
- 2021
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- The ocean circulation plays an essential role in our climate system. It redistributes heat, salt, carbon and other tracers across the globe, making the climate of Earth more moderate. This thesis targets density differences that are driving the ocean circulation. These differences are caused by changes in temperature and salinity. The analysis is based on the usage of Lagrangian trajectories simulated with velocity fields from an Earth System Model. The Lagrangian approach opens up for the possibility to follow specific water paths and water masses. The results herein provide a new insight to specific circulation patterns in the ocean, and which regions that play an important role in controlling temperature and salinity changes.In the first two articles, the Lagrangian divergence is introduced. It shows the geographical distribution of heat and salt changes of a simulated water mass. Using this, we are able to show that the northward flowing water in the Atlantic Ocean cools and freshens in the North Atlantic Subtropical Gyre, the Gulf Stream and the North Atlantic Current. Similarly, we show that the water flowing from the Drake Passage, following the Antarctic Circumpolar Current (ACC) and moving northwards into the Atlantic, Pacific and Indian oceans, transforms from cold and fresh to warm and saline. This warming and salinification are a result of, not only air-sea fluxes, but also interior mixing.In the third study, we show that 70% of the water flowing northwards as part of the Atlantic Meridional Overturning Circulation circuits the North Atlantic Subtropical Gyre at least once before it continues northwards. In the gyre, the water spirals downwards as it gets denser, due to a combination of air-sea fluxes and interior mixing. These results bring a new perspective on the Subtropical Gyre's role to the circulation patterns of the Atlantic Meridional Overturning Circulation.In the last part of this thesis, the circulation in the North Atlantic Ocean is traced into four different pathways. The pathways are visualised geographically together with their change in temperature, salinity and density. With this, we are able to show that the northward flowing water in the Atlantic Ocean exchanges heat and salt with the colder and fresher waters circulating the Subpolar Gyre.
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| 4. |
- Bosi, Sofia, et al.
(författare)
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The Role of Stokes Drift in the Dispersal of North Atlantic Surface Marine Debris
- 2021
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Ingår i: Frontiers in Marine Science. - : Frontiers Media SA. - 2296-7745. ; 8
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Tidskriftsartikel (refereegranskat)abstract
- Understanding the physical mechanisms behind the transport and accumulation of floating objects in the ocean is crucial to efficiently tackle the issue of marine pollution. The main sinks of marine plastic are the coast and the bottom sediment. This study focuses on the former, investigating the timescales of dispersal from the ocean surface and onto coastal accumulation areas through a process called "beaching. " Previous studies found that the Stokes drift can reach the same magnitude as the Eulerian current speed and that it has a long-term effect on the trajectories of floating objects. Two particle tracking models (PTMs) are carried out and then compared, one with and one without Stokes drift, named PTM-SD and PTM-REF, respectively. Eulerian velocity and Stokes drift data from global reanalysis datasets are used for particle advection. Particles in the PTM-SD model are found to beach at a yearly rate that is double the rate observed in PTM-REF. The main coastal attractors are consistent with the direction of large-scale atmospheric circulation (Westerlies and Trade Winds). After 12 years (at the end of the run), the amount of beached particles is 20% larger in PTM-SD than in PTM-REF. Long-term predictions carried out with the aid of adjacency matrices found that after 100 years all particles have beached in PTM-SD, while 8% of the all seeded particles are still floating in PTM-REF. The results confirm the need to accurately represent the Stokes drift in particle models attempting to predict the behaviour of marine debris, in order to avoid overestimation of its residence time in the ocean and effectively guide policies toward prevention and removal.
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| 5. |
- Broström, Göran, et al.
(författare)
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A hazards approach to the biometric analysis of infant mortality
- 2023
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Rapport (övrigt vetenskapligt/konstnärligt)abstract
- A variation of the Bourgeois-Pichat biometric analysis of infant mortality is suggested. In the original model, cumulative mortality in the last eleven months of infancy is assumed to follow a uniform distribution given a log-cube transformation of age. Instead, we assume an exponential distribution. The difference is that while the denominator is constant in the Bourgeois-Pichat model, equal to the number of births, in our model, the denominator is the current population at risk. We argue that our assumption is more satisfactory from a theoretical point of view, since it focus on the conditional probability of dying. Our model gives different estimates of endogenous and exogenous mortality and, in addition, the model fit is slightly better, especially in cases with higher levels of infant mortality.
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| 6. |
- Broström, Göran
(författare)
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Event history analysis with R
- 2021. - 2
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Bok (refereegranskat)abstract
- With an emphasis on social science applications, Event History Analysis with R, Second Edition, presents an introduction to survival and event history analysis using real-life examples. Since publication of the first edition, focus in the field has gradually shifted towards the analysis of large and complex datasets. This has led to new ways of tabulating and analysing tabulated data with the same precision and power as that of an analysis of the full data set. Tabulation also makes it possible to share sensitive data with others without violating integrity.The new edition extends on the content of the first by both improving on already given methods and introducing new methods. There are two new chapters, Explanatory Variables and Regression, and Register- Based Survival Data Models. The book has been restructured to improve the flow, and there are significant updates to the computing in the supporting R package.
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| 7. |
- Cabral dos Santos, Alex, 1990, et al.
(författare)
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Tidally driven porewater exchange and diel cycles control CO2 fluxes in mangroves on local and global scales
- 2024
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Ingår i: Geochimica et Cosmochimica Acta. - 0016-7037. ; 374, s. 121-135
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Tidskriftsartikel (refereegranskat)abstract
- Mangrove soils are highly enriched in organic carbon. Tidal pumping drives seawater and oxygen into mangrove soils during flood tide and releases carbon-rich porewater during ebb tides. Here, we resolve semi-diurnal (flood/ebb tides), diel (day/night) and weekly (neap/spring tides) drivers of porewater-derived CO2 fluxes in two mangroves and update global estimates of CO2 emissions building on earlier observations from other sites. Tidal pumping controlled pCO2 variability within the two mangrove creeks. The highest values of pCO2 (2,585–6,856 µatm) and 222Rn (2,315–6,159 dpm m−3) and lowest values of pH (6.8–7.1) and dissolved oxygen (1.7–3.7 mg L−1) at low tides were due to enhanced porewater export. 222Rn and pCO2 in mangrove porewater were 4–15 and 38–41 times greater than surface waters, respectively. pCO2 increased by 50 ± 30 % from high to low tide, 9 ± 22 % from day to night and 57 ± 5 % from neap to spring tide with clear changes in hourly, diel, and weekly time scales. Combining our new estimates with literature data, global porewater-derived (16 sites) and water-atmosphere (52 sites) CO2 fluxes in mangroves would upscale to 45 ± 12 and 41 ± 10 Tg C y−1, respectively. These fluxes represent 25 % of net primary production and are twice as high as the sediment carbon burial rates in global mangroves. Overall, our local observations and global compilation suggest that porewater-derived CO2 exchange is a major but often unaccounted source of CO2 in mangroves. The porewater-derived CO2 can be emitted to the atmosphere or laterally exported to the ocean and should be included in carbon budgets to solve global imbalances.
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| 8. |
- Edvinsson, Sören, 1953-, et al.
(författare)
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Is high social class always beneficial for survival? : a study of northern Sweden 1801-2013
- 2020
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Rapport (övrigt vetenskapligt/konstnärligt)abstract
- Focusing on two regions in northern Sweden 1801–2013, we challenge common notions of the assumed advantage in survival of belonging to a high social class. The issue is analysed according to gender and age group (adults and elderly) and in relation to the development of economic inequality. The results show that high social class is not always favourable for survival. Men in the elite category, particularly in working age, had higher mortality compared to others during a large part of the studied period; a male mortality class reversal appears at a surprisingly late date, while the social gradient among women conforms to the expected pattern. We suggest that health-related behaviour is decisive not only in later but earlier phases of the mortality transition as well. The results implicate that the association between social class and health is more complex than is assumed in many of the dominant theories in demography and epidemiology.
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| 9. |
- Elyouncha, Anis, 1978, et al.
(författare)
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Joint retrieval of ocean surface wind and current vectors from satellite SAR data using a Bayesian inversion method
- 2021
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Ingår i: Remote Sensing of Environment. - : Elsevier BV. - 0034-4257. ; 260
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Tidskriftsartikel (refereegranskat)abstract
- 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.
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| 10. |
- Fagerström, Vilhelm, et al.
(författare)
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Turbulence affects larval vertical swimming in the cold-water coral Lophelia pertusa
- 2022
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Ingår i: Frontiers in Marine Science. - : Frontiers Media SA. - 2296-7745. ; 9:1062884
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Tidskriftsartikel (refereegranskat)abstract
- Vertical migration of marine larvae may drastically affect their dispersal, especially if they are spawned in the deep sea. Previous studies have shown that the planktonic larvae of the cold-water coral Lophelia pertusa in still water swim upwards at a speed of ca. 0.5 mm s-1 during a pre-competency period of 3–5 weeks. This behavioral trait is thought to benefit dispersion of larvae as it promotes near surface drift in relatively strong currents. In the ocean however, larvae regularly encounter turbulent water movements potentially impeding their swimming ability. With no apparent stabilizing mechanism, it may be expected that the body orientation of these larvae, and consequently their directed swimming, is sensitive to perturbation by external forces. We investigated the effects of turbulence on vertical swimming of pre-competent L. pertusa larvae by exposing them to relevant turbulence intensities within a grid-stirred tank. Larval movement and water flow were simultaneously recorded, allowing for analysis of individual larval swimming velocities. We showed that the upwards directed swimming speed generally decreased with increasing turbulence, dropping to non-significant in turbulence levels occurring near ocean boundaries. Our results do however suggest that L. pertusa larvae maintain their upwards directed swimming, albeit at reduced speed, in a major part of the water column, thus allowing them to spend part of their planktonic phase in the uppermost ocean layer. This new insight into the behavior of L. pertusa larvae in their natural environment strengthens the notion of the species as one with strong potential for long-distance dispersal. Such information is important for the understanding of L. pertusa population connectivity, and vital when developing tools for modelling of larval transport.
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| 11. |
- Fredriksson, Sam, 1966, et al.
(författare)
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Modelling Deep Green tidal power plant using large eddy simulations and the actuator line method
- 2021
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Ingår i: Renewable Energy. - : Elsevier BV. - 0960-1481 .- 1879-0682. ; 179, s. 1140-1155
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Tidskriftsartikel (refereegranskat)abstract
- The Deep Green technique for tidal power generation is suitable for moderate flows which is attractive since larger areas for tidal energy generation hereby can be used. It operates typically at mid-depth and can be seen as a "flying" kite with a turbine and generator attached underneath. It moves in a lying figure-eight path almost perpendicular to the tidal flow. Large eddy simulations and an adaption of the actuator line method (in order to describe arbitrary paths) are used to study the turbulent flow with and without Deep Green for a specific site. This methodology can in later studies be used for e.g. array analysis that include Deep Green interaction. It is seen that Deep Green creates a unique wake composed of two velocity deficit zones with increased velocity in each wake core. The flow has a tendency to be directed downwards which results in locally increased bottom shear. The persistence of flow disturbances of Deep Green can be scaled with its horizontal path width, D-y, with a velocity deficit of 5% at approximately 8-10D(y) downstream of the power plant. The turbulence intensity and power deficit are approximately two times the undisturbed value and 10%, respectively, at 10D(y). (C) 2021 The Authors. Published by Elsevier Ltd.
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| 12. |
- Gustafsson, Malin, et al.
(författare)
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Unlocking the secret life of blue mussels: Exploring connectivity in the Skagerrak through biophysical modeling and population genomics
- 2024
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Ingår i: Evolutionary Applications. - 1752-4571. ; 17
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Tidskriftsartikel (refereegranskat)abstract
- Knowledge of functional dispersal barriers in the marine environment can be used to inform a wide variety of management actions, such as marine spatial planning, restoration efforts, fisheries regulations, and invasive species management. Locations and causes of dispersal barriers can be studied through various methods, including movement tracking, biophysical modeling, demographic models, and genetics. Combining methods illustrating potential dispersal, such as biophysical modeling, with realized dispersal through, e.g., genetic connectivity estimates, provides particularly useful information for teasing apart potential causes of observed barriers. In this study, we focus on blue mussels (Mytilus edulis) in the Skagerrak—a marginal sea connected to the North Sea in Northern Europe—and combine biophysical models of larval dispersal with genomic data to infer locations and causes of dispersal barriers in the area. Results from both methods agree; patterns of ocean currents are a major structuring factor in the area. We find a complex pattern of source-sink dynamics with several dispersal barriers and show that some areas can be isolated despite an overall high dispersal capability. Finally, we translate our finding into management advice that can be used to sustainably manage this ecologically and economically important species in the future.
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| 13. |
- Holm, H. H., et al.
(författare)
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Evaluation of Selected Finite-Difference and Finite-Volume Approaches to Rotational Shallow-Water Flow
- 2020
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Ingår i: Communications in Computational Physics. - 1815-2406. ; 27:4, s. 1234-1274
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Tidskriftsartikel (refereegranskat)abstract
- The shallow-water equations in a rotating frame of reference are important for capturing geophysical flows in the ocean. In this paper, we examine and compare two traditional finite-difference schemes and two modern finite-volume schemes for simulating these equations. We evaluate how well they capture the relevant physics for problems such as storm surge and drift trajectory modelling, and the schemes are put through a set of six test cases. The results are presented in a systematic manner through several tables, and we compare the qualitative and quantitative performance from a cost-benefit perspective. Of the four schemes, one of the traditional finite-difference schemes performs best in cases dominated by geostrophic balance, and one of the modern finite-volume schemes is superior for capturing gravity-driven motion. The traditional finite-difference schemes are significantly faster computationally than the modern finite-volume schemes.
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| 14. |
- Kukkonen, J., et al.
(författare)
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Towards a Comprehensive Evaluation of the Environmental and Health Impacts of Shipping Emissions
- 2022
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Ingår i: Springer Proceedings in Complexity. - Cham : Springer International Publishing. - 2213-8684 .- 2213-8692. ; , s. 329-336
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Konferensbidrag (refereegranskat)abstract
- We present a new concept for marine research, applied in the EU-funded project EMERGE, “Evaluation, control and Mitigation of the EnviRonmental impacts of shippinG Emissions” (2020–2024; https://emerge-h2020.eu/ ). For the first time, both the various marine and atmospheric impacts of the shipping sector have been and will be comprehensively analyzed, using a concerted modelling and measurements framework. The experimental part of the project focuses on five European geographical case studies in different ecologically vulnerable regions, and a mobile onboard case study. The EMERGE consortium has also developed a harmonised and integrated modelling framework to assess the combined impacts of shipping emissions, both (i) on the marine ecosystems and (ii) the atmospheric environment. The first results include substantial refinements of a range of models to be applied, especially those for the STEAM and OpenDrift models. In particular, the STEAM (Ship Traffic Emission Assessment Model) model has been extended to allow for the effects of atmospheric and oceanographic factors on the fuel consumption and emissions of the ships. The OpenDrift model has been improved to take into account the partitioning, degradation, and volatilization of pollutants in water. The predicted emission and discharge values have been used as input for both regional scale atmospheric dispersion models, such as WRF-CMAQ (Weather Research and Forecasting—Community Multiscale Air Quality Model) and SILAM (System for Integrated modeLling of Atmospheric composition), and water quality and circulation models, such as OpenDrift (Open source model for the drifting of substances in the ocean) and Delft3D (oceanographic model). The case study regions are Eastern Mediterranean, Northern Adriatic Sea, the Lagoon of Aveiro, the Solent Strait and the Öresund Strait. We have also conducted a substantial part of the experimental campaigns scheduled in the project. The final assessment will include the benefits and costs of control and mitigation options affecting water quality, air pollution exposure, health impacts, climate forcing, and ecotoxicological effects and bioaccumulation of pollutants in marine biota.
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| 15. |
- Melsom, A., et al.
(författare)
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Exploring drift simulations from ocean circulation experiments: application to cod eggs and larval drift
- 2022
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Ingår i: Climate Research. - : Inter-Research Science Center. - 0936-577X .- 1616-1572. ; 86, s. 145-162
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Tidskriftsartikel (refereegranskat)abstract
- Drift models are commonly used to study the transport of early life stages of fish and other marine organisms. Various approaches may be applied to examine the distribution and variability of ocean trajectory pathways. In the present study, we compare results using passive Eulerian tracers and Lagrangian float trajectories that are embedded in numerical models. We supplement this analysis by applying an offline model for drift computations. The contrasts in the results from the various configurations are mainly due to differences in drift depth. Simulations were performed using horizontal resolutions of 4 and 0.8 km. The higher-resolution experiment gives somewhat more realistic results for the drift time from Lofoten to the Tromsoflaket bank at the southwestern entrance of the Barents Sea. Furthermore, differences in results between simulation years are much larger than the differences that arise from the choice of model configuration. Climate variability at high latitudes on a multi-decadal time scale is dominated by large interannual variability superimposed on an underlying moderate warming trend. We conclude that a properly configured offline drift model using hourly or 2-hourly results from a simulation with a horizontal resolution of 1 km or finer is the best approach for investigations of trajectory pathways. The flexibility of an offline drift model is also highly advantageous in biological contexts, as it easily allows for a variety of ways in which behavioural characteristics can be parameterized, including descriptions that are defined after the ocean circulation simulation has been executed.
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| 16. |
- Nylund, Amanda, 1989, et al.
(författare)
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Hydrographical implications of ship-induced turbulence in stratified waters, studied through field observations and CFD modelling
- 2023
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Ingår i: Frontiers in Marine Science. - 2296-7745. ; 10
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Tidskriftsartikel (refereegranskat)abstract
- Ship-related energy pollution has received increasing attention but almost exclusively regarding radiated underwater noise, while the effect of ship-induced turbulence is lacking in the literature. Here we present novel results regarding turbulent wake development, the interaction between ship-induced turbulence and stratification, and discuss the impact of turbulent ship wakes in the surface ocean, in areas with intense ship traffic. The turbulent wake development was studied in situ, using Acoustic Doppler Current Profilers (ADCP) and conductivity, temperature, depth (CTD) observations of stratification, and through computational fluid dynamics (CFD) modelling. Our results show that the turbulent wake interacts with natural hydrography by entraining water from below the pycnocline, and that stratification influences the turbulent wake development by dampening the vertical extent, resulting in the wake water spreading out along the pycnocline rather than at the surface. The depth and intensity of the turbulent wake represent an unnatural occurrence of turbulence in the surface ocean. The ship-induced turbulence can impact local hydrography, nutrient dynamics and increase plankton mortality due to physical disturbance, especially in areas with intense traffic. Therefore, sampling and modelling of e.g., contaminants in shipping lanes need to consider hydrographic conditions, as stratification may alter the depth and spread of the wake, which in turn governs dilution. Finally, the frequent ship traffic in estuarine and coastal areas, calls for consideration of ship-induced turbulence when studying hydrographic processes.
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| 17. |
- Nylund, Amanda, 1989, et al.
(författare)
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Modellering av omblandning i fartygs kölvatten, för ökad förståelse av belastning på havsmiljön till följd av utsläpp från fartyg
- 2020
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Rapport (övrigt vetenskapligt/konstnärligt)abstract
- Östersjön är ett av världens mest intensivt trafikerade områden och i vissa delar sker en fartygspassage i genomsnitt var tolfte minut. Fartygen kan betraktas som flytande industrier som släpper ut föroreningar, försurande och övergödande ämnen som produceras i olika ombordssystem, såsom svart-, grå-, ballast-, läns-, kyl- och skrubbervatten, giftiga båtbottenfärger och propellerhylsolja. De senaste årens ökade förståelse för att sjöfartens utsläpp inte är negligerbara för havsmiljön belyses i Miljömålsberedningens betänkande Havet och människan (2020), där det bland annat föreslås att Havs- och vattenmyndigheten bör få i uppdrag att integrera övervakning av sjöfartens påverkan på havsmiljön i sin verksamhet. För att kunna göra detta på ett effektivt sätt är det nödvändigt att förstå hur föroreningar som släpps ut från fartyg sprids vidare i havsmiljön. Med utgångspunkt från denna kunskap kan miljöövervakningsprogram utformas och i förlängningen åtgärder vidtas för att minska påverkan från sjöfart på havsmiljön, exempelvis genom förbud mot utsläpp eller omledning av farleder i känsliga områden. I befintliga modelleringar av utsläpp från fartyg görs antagandet att utsläppen sker i ytan, vilket får till följd att beräkningen av spridningen i miljön i stor utsträckning kommer att styras av rådande vind- och strömförhållanden i ytskiktet. Detta kan vara acceptabelt, exempelvis vid modellering av lättare oljeprodukter, men för andra ämnen är det nödvändigt att förstå hur omblandningen i kölvattnet sker för att mer korrekt kunna beräkna hur föroreningarna sprider sig i miljön; strömförhållanden kan variera avsevärt i både riktning och styrka på 5–10 m djup, jämfört med i ytan. Syftet med den här förstudien var att undersöka förutsättningarna att kombinera beräkningsströmningsdynamik (Computational Fluid Dynamics (CFD)) som vanligtvis används för att göra högupplösta 3D modelleringar av strömningen runt skrov och propeller på fartyg, med regionala oceanografiska modeller. Användningen av den högupplösta 3D Reynolds- Medelvärdesbildad Navier-Stokes (3D RANS) modellen är dock beräkningsintensiv och brukar endast användas för att modellera närvaken, upp till någon fartygslängd akter om fartyget. För att kunna brygga över skalorna i fjärrvaken (ca 0,8–3 km) akter om fartyget är det därför lämpligt att försöka förenkla beräkningarna, vilket kan göras genom semi-empirisk modellering och 2D modellering av ett tvärsnitt av vaken över tid, vilka i begränsad omfattning validerats mot 3D RANS resultaten. Vidare har in-situ mätningar med en Acoustic Doppler Current Profiler (ADCP) använts för att validera utbredningen av det turbulenta vakområdet. Resultaten av förstudien, både från modellering och fältmätningarna, visar tydligt att utsläpp i fartygsvaken kan spridas på olika djup och i olika riktning som en effekt av omblandningen som sker i vaken. Vidare är hydrografin viktig för hur och var spridningen i vaken sker och fältmätningarna indikerar att när det finns en stark skiktning kommer det omblandade vattnet i fartygsvaken att spridas i sidled, längs med skiktningen, istället för nedåt mot botten. Ur miljöövervakningsperspektiv är det därför viktigt att känna till skiktningen och djupet i relation till farledernas placering, då det får konsekvenser för spridningen av föroreningar från farleden och därmed även var provtagning bör ske. Slutsatsen från förstudien är att det föreslagna ramverket, kombinationen av olika teoretiska modeller och in-situ mätningar, har stor potential för att kunna leverera indata till regionala oceanografiska modeller, för att kunna modellera den storskaliga spridningen av föroreningar från fartyg och farleder. Denna information är också kritisk för utformning av övervakningsprogram, där det är avgörande att mätningar görs där föroreningar från fartyg kan förväntas att återfinnas.
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| 18. |
- Rohrs, J., et al.
(författare)
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Current shear and turbulence during a near-inertial wave
- 2023
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Ingår i: Frontiers in Marine Science. ; 10
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Tidskriftsartikel (refereegranskat)abstract
- Surface currents and turbulent mixing were observed during a near-inertial wave (NIW) using an accousting doppler current profiler (ADCP) and satellite-tracked drifters. Drifter trajectories sampled at three depth levels show characteristics of an Ekman solution superposed with the NIW. Velocity and dissipation estimates from the ADCP reveal strong shear with a distinct constant flux layer in between the roughness length and a critical depth at 4m. Below, a shear free slab layer performing an inertial oscillation is observed. Dissipation, as estimated from the vertical beam of the ADCP, peaks in the wave-enhanced friction layer when the current opposes the wind and wave direction. Below the constant flux layer, maximum turbulence is observed when the NIW is in a phase that is in opposite direction to the time-averaged current. During this phase, currents at various depths rapidly realign in the entire boundary layer.
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| 19. |
- Saravana Prabahar, Nimal Sudhan, et al.
(författare)
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Validation of Actuator Line Modeling and Large Eddy Simulations of Kite-Borne Tidal Stream Turbines against ADCP Observations
- 2023
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Ingår i: Energies. ; 16:16
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Tidskriftsartikel (refereegranskat)abstract
- The representation of tidal energy in future renewable energy systems is growing. Most of the current tidal turbine designs are limited by the minimum current velocity required for efficient operation. The Deep Green (DG) is a kite-borne tidal power plant designed to sustain efficient operation in tidal current velocities as low as 1.2 ms-1. This could increase the geographical areas suitable for large-scale tidal power arrays. Numerical modeling of the Deep Green was carried out in a previous study using large eddy simulations and the actuator line method. This numerical model is compared with acoustic Doppler current profiler (ADCP) measurements taken in the wake of a DG operating in a tidal flow under similar conditions. To be comparable, and since the ADCP measures current velocities using averages of beam components, the numerical model data were resampled using a virtual ADCP in the domain. The sensitivity of the wake observations to ADCP parameters such as pulse length, bin length, and orientation of the beams is studied using this virtual ADCP. After resampling with this virtual ADCP, the numerical model showed good agreement with the observations. Overall, the LES/ALM model predicted the flow features well compared to the observations, although the turbulence levels were underpredicted for an undisturbed tidal flow and overestimated in the DG wake 70 m downstream. The velocity deficit in the DG wake was weaker in the observations compared to the LES. The ALM/LES modeling of kite-borne tidal stream turbines is suitable for further studies of array optimization and wake propagation, etc.
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| 20. |
- Weber, J. E. H., et al.
(författare)
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Convection in conditionally unstable seawater
- 2021
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Ingår i: AIP Advances. - : AIP Publishing. - 2158-3226. ; 11:7
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Tidskriftsartikel (refereegranskat)abstract
- Buoyancy-driven convection in a conditionally unstable ocean is studied theoretically. Conditionally unstable conditions are related to supercooled seawater. The freezing point is depressed due to increasing pressure, and upward motion (reduced pressure) leads to the formation of ice crystals in the form of frazil ice and hence a reduced bulk density of the rising mass parcel. For downward motion (increasing pressure), freezing does not occur. To model this one-way process, we take that rising parcels become lighter as they follow the adiabatic density lapse rate due to freezing. Through the action of a unit step function, we can model the fact that frazil ice is added only in the upward motion. Supercooled seawater is observed in the proximity of ice shelf fronts. We consider an idealized ice shelf and take the front to be vertical. For this geometry, we present linear analytical solutions as well as numerical results for nonlinear two-dimensional steady conditional convection in the presence of a stable environmental density gradient. With a parallel to moist convection in the atmosphere, we find convection cells near the ice front with rising fluid in a narrow region and sinking fluid over a much broader region. © 2021 Author(s).
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| 21. |
- Ytreberg, Erik, 1980, et al.
(författare)
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EMERGE deliverable 6.1. Baltic and North Sea report
- 2023
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Rapport (övrigt vetenskapligt/konstnärligt)abstract
- Shipping is responsible for a range of different pressures affecting air quality, climate, and the marine environment. However, most social and economic analysis of shipping have focused on air pollution assessment and how shipping may impact climate change and human health. This risks policies to be biased towards air pollution and climate change, while trading off impacts on the marine environment. One example is the IMO’s global sulphur cap, which requires shipowners to use a compliant fuel with a sulphur content of 0.5% (0.1% in SECA regions) or use alternative compliance options (scrubbers) that are effective in reducing sulphur oxide (SOX) emissions to the atmosphere. The scrubber process results in large volumes of acidic discharge water. Although regulations primarily target SOX removal, other pollutants such as polycyclic aromatic hydrocarbons (PAHs) and metals are transferred from the exhausts to the wash water and subsequently discharged to the marine environment. The aim of this deliverable has therefore been to develop a holistic framework to evaluate the impacts of shipping emissions, particularly those related to scrubbers, on the marine environment, human health, climate, and economy. The structure of this deliverable follows the well-established DAPSIR (Driver-Activity-Pressure-State-Impact-Response) framework, under which information, findings and conclusions from previous work packages are synthesized and integrated, including experiments of direct emissions from shipping to the marine environment (WP2) and the atmosphere (WP3), assessment of marine environmental impacts (WP2, WP4 and WP6), as well as human health and climate change impacts (WP5 and WP6). Finally, this deliverable provides recommendations and guidance for stakeholders and policymakers. The assessment is performed using a baseline scenario (year 2018) and three future scenarios (for year 2050) based on different projected future developments of shipping transport volumes and considering the development of ships regarding fuel efficiency and ship size. In this deliverable, we focused primarily on two of the different future scenarios, scenario 3 (high scrubber pressure) and scenario 8 (high use of liquefied natural gas (LNG) and methanol). The marine environmental risk assessment, performed in the Öresund region for the baseline scenario (2018), showed unacceptable risks when ships in the area were using open loop scrubbers. In the assessment, modelled predicted environmental concentrations (PECs) of open loop scrubber discharge water exceeded the tolerable marine threshold value (predicted no-effect concentration, PNEC) in almost the entire Öresund region. The PEC value was derived based on ship activity and discharges of scrubber water in 2018, while the PNEC value was derived based on the ecotoxicological assays performed within the EMERGE project. Notably, the modelling of open loop scrubber discharge water was performed using the ship traffic activity in 2018 when less than 200 ships in the Baltic Sea used scrubbers, collectively releasing 192 million tonnes of discharge water. By 2022 there were approximately 800 ships equipped with scrubbers in the Baltic Sea. In the high scrubber future scenario (S3) in 2050 this led to an assumption of the considerably higher scrubber water discharge (1740 million tonnes), representing almost one order of magnitude higher compared to our baseline scenario in 2018. In addition, our impact assessment, following Marine Environment Protection Committee (MEPC) guidelines, shows that a ban on discharge water from scrubbers should be considered in the entire Baltic and North Sea region, since all sea basins in the region fail to reach good environmental status (GES) as defined by the EU Marine Strategy Framework Directive (Directive 2008/56/EC). However, the costs of such a measure for the shipping sector (banning discharges from scrubbers, i.e., in practice a ban on scrubbers) have been questioned within the International Maritime Organisation (IMO). Therefore, EMERGE also focused on analysing to what extent the global scrubber fleet has reached break-even on their scrubber installations and the potential monetary gain of using Heavy Fuel Oil (HFO) as compared to the more expensive Marine Gas Oil (MGO) or Very Low Sulphur Fuel Oil (VLSFO). Our results showed that 51% of the global scrubber fleet had reached break-even by the end of 2022, resulting in a summarised balance of 4.7 billion €2019. In addition, the marine ecotoxicity damage cost, by not restricting scrubbers in the Baltic Sea Area, accumulated to >680 million €2019 from 2015 to end of 2022. For air quality, both future scenarios showed a decrease in shipping contribution to PM2.5 exposure by a factor of 2 to 3 compared to our baseline scenario in 2018. Scenario 8 is somewhat more efficient in decreasing the shipping originated PM2.5 than scenario 3. Using the Greenhouse gas and Air pollution Interactions and Synergies (GAINS) model for human health impact assessment in scenario 3 revealed the loss of life expectancy in most areas around the Baltic Sea, when considering all sources, to be limited to two to four months. However, the differences in life shortening between Scenarios 3 and 8 are two to three orders of magnitude lower when compared to human health impacts resulting from all sources, indicating that scrubbers alone have a minor impact on human health in the Baltic region from air quality perspective. For Öresund case the shipping-related health impacts from PM2.5 represented approximately 10% of the total burden of air pollution, in 2050 scenario simulations this burden decreased to 7-9%. Important improvement of air quality in the scenario simulations come also from reduction of NO2 which is a criteria pollutant regulated by the Air Quality Directive, where the decrease is 3 to 5-fold. In relative terms the shipping contribution to NO2 concentration levels, however, maintains similar, approximately 25%, as the land emissions are also expected to decrease. The GAINS health impact assessment for the Baltic Sea was compared to the Solent region using a statistical technique. The latter study showed that a relatively small fraction of all premature deaths in Southampton, Portsmouth, Poole, Christchurch & Bournemouth are attributable to air pollution from shipping, corroborating the conclusion that the deployment scrubbers alone has a minor impact on human life shortening through atmospheric transport.
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