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SMHI har analyserat hur havsisens utbredning och beskaffenhet i Bottenviken, Bottenhavet, Ålands hav och Norra Östersjön kan komma att förändras i ett perspektiv på 20 respektive 50 år, vilket motsvarar runt år 2040 och 2070. Analyserna utgår från sju indikatorer som beskriver olika aspekter av havsisens förändring. Indikatorerna är framtagna i samråd med Sjöfartsverket och valda både utifrån tillgång till data och med syfte att vara relevanta ur isbrytningsperspektiv.Som underlag för analyserna ligger historiska observationer från SMHI, det Finska Meteorologiska Institutet (FMI) och Sjöfartsverket, samt klimatscenarier framtagna i tidigare projekt.Klimatscenarier som representerar två olika utsläppsscenarier (RCP4,5 och RCP8,5) har analyserats från totalt tio olika klimatmodellsimuleringar. Klimatscenarier baserade på det lägre utsläppsscenariet (RCP2,6) saknas i underlaget eftersom befintliga klimatmodellsimuleringar för detta scenario bedömdes ha för låg kvalité. Tidsramen för denna utredning medgav inte heller framtagande av nya klimatscenarier.Resultatet av analyserna visar att framtidens isvintrar blir lindrigare med avseende på havsisens maximala utbredning jämfört med kontrollperioden (1975–2004). Issäsongens längd blir också kortare, med störst förändring i de södra delområdena. Inget scenario indikerar dock helt isfria vintrar, och åtminstone Bottenviken väntas i medel bli helt istäckt även i framtiden. I södra Bottenviken kommer is tjockare än 10 cm dock att försvinna för RCP8,5. På 20 års sikt är förändringen av den maximala isutbredningen mindre tydlig på grund av en fortsatt stor naturligt förekommande väderdriven mellanårsvariation. På 50 års sikt däremot är signalen tydligare och visar minskad isutbredning och en något mindre mellanårsvariabilitet.Istäcket förväntas bli tunnare i medel i samtliga delområden och utbredningen av tjock deformerad is förväntas minska. Modellerna saknar dock förmågan att simulera så kallade stampisvallar. Dessa vallar bildas då tunnare is pressas upp mot en landfast iskant eller land vid kraftiga vind- och vågförhållanden, och kan utgöra ett problem för sjöfarten även under lindriga isvintrar. Tunnare och glesare is kommer även att leda till ökade isdriftshastigheter i Bottenviken och Bottenhavet.Antalet dagar med utfärdade isrestriktioner till svenska hamnar förväntas minska i takt med att issäsongen blir kortare och isförhållandena lindrigare. Fördelningen av isrestriktioner förändras också, främst i Bottenviken där de högre isklasserna (1A/B) minskar till fördel för de lägre isklasserna (1C/II) som istället ökar.Förändringar i isutbredning, issäsongens längd och jämnisens medeltjocklek bedöms ha en låg osäkerhet eftersom resultatet styrks av både observationer bakåt i tiden och modellsimuleringarna som ligger förhållandevis nära observationerna. Förändringar i deformation, istjockleksfördelning och isdrift bedöms ha en hög osäkerhet eftersom det saknas eller finns väldigt få observationer som kan styrka resultatet från modellscenarierna.Utredningen begränsas delvis av att data för det lägre strålningsscenariot RCP2,6 och analys av eventuella förändringar i väder- och vindförhållanden saknas. En annan begränsande faktor som kan påverka resultatens tillförlitlighet är det låga antalet regionala klimatmodellsimuleringar med tillförlitliga isparametrar. SMHI has analysed how sea ice conditions in the Bothnian Bay, Bothnian Sea, Åland Sea and northern Baltic Proper may change in a 20 and 50 year perspective relative to 2020. The study is focused on seven indicators describing different aspects of sea ice change. The indicators were identified jointly with the Swedish Maritime Administration (SMA), and chosen based on available data and relevance to ice breaking.The study is based on historical observations from SMHI, the Finnish Meteorological Institute (FMI) and SMA, and climate scenario data from previous projects.Climate scenarios representing two different representative concentration pathways (RCP4.5 and RCP8.5) have been analysed based on a total of ten different climate model simulations. Scenarios based on the lower representative concentration pathway (RCP2.6) are absent because existing datasets for this pathway do not have sufficient quality for sea ice parameters. The time frame for this assignment did not allow for new climate scenario simulations to be produced.The results show that future winters will gradually, on average, have a smaller maximum ice extent compared to the control period (1975-2004). Ice seasons will also get shorter, with the largest differences in the southern areas. None of the scenarios yield ice free winters, and at least Bothnian Bay is expected to become fully ice covered on average, also during future winters. However, in the RCP8.5 scenario, ice with an average thickness of 10 cm or more disappears from the southern Bothnian Bay.In a 20-year perspective, changes in maximum ice extent are less distinct due to large inter-annual variations. In a 50-year perspective the change becomes more distinct and shows decreasing ice extents and smaller inter-annual variations.Level ice is expected to get thinner on average in all analysed areas, and the presence of heavily deformed ice is expected to decrease. However, models lack the ability to simulate brash ice barriers, which are formed when thin ice is pressed against a thicker ice edge or land by wind and waves. These types of barriers can be problematic for ships even in mild winters, and are expected to occur also in the future. Thinner and less dense ice fields also lead to increased ice drift in the Bothnian Bay and Bothnian Sea.The number of days with ice class based traffic restrictions for Swedish harbours are expected to decrease as sea ice thickness become thinner and ice seasons become shorter. The distribution of restrictions will also change, mainly in the Bothnian Bay where days with heavier ice classes (1A/B) decrease and days with lighter ice classes (1C/II) increase.Changes in maximum ice extent, length of ice season and average level ice thickness are judged to have a low uncertainty as the results are supported by both historical observations, and by the fact that model simulations are relatively close to the observations during the historical period. Changes in ice deformation, ice thickness distribution, and ice drift are judged to have a higher degree of uncertainty as there are no or very few observations to support model results.The study is partly limited by the lack of data for the lower RCP2.6 and by lacking analyses of possible changes in meteorological conditions. Another limiting factor is the relatively low number of regional climate model simulations with reliable ice parameters used in the study.

This thesis explores freshwater-related processes and water mass transformation in the Arctic Ocean. Knowledge of these processes is important from both a local and a global perspective. Globally, because the export of cold and low saline water and sea ice might influence the North Atlantic and global meridional overturning circulation. Locally, because freshwater processes affect the vertical stratification and permit favorable conditions for the ice cover. Models of different complexity are the main tools of the present work. A part of the material considers how these models can be used to examine the key processes governing freshwater balance. Additionally, the freshwater budgets amongst 10 different ocean general circulation models (OGCMs) are compared and robust features and weaknesses identified. A large part considers the freshwater processes governing the stratification with an emphasis on the low saline upper parts. The interactions between freshwater sources and sinks are studied in an OGCM using passive tracers. It is found that the composition, pathways and shelf-basin exchange of low saline water primarily involve processes linked to Siberian runoff, Pacific water and sea-ice melting and formation. Motivated by observed changes and paleorecords the sensitivity of the stratification is further explored in freshwater perturbation experiments with an OGCM. The response yields a deeper halocline for decreasing freshwater input, in line with a theoretical model. The final part focuses on a new framework for analyzing water mass transformations. In the framework volume, heat and salt budgets are computed in salinity-temperature space. Using different OGCMs it is shown how surface and interior processes transform inflowing waters towards colder and fresher waters and how the halocline renewal rate can be estimated. Limiting cases for the water mass transformation balance are identified by separating contributions from surface, internal and boundary fluxes. At the time of the doctoral defense, the following papers were unpublished and had a status as follows: Paper 3: Submitted. Paper 4: Manuscript.

The Arctic freshwater (FW) has been the focus of many modeling studies, due to the potential impact of Arctic FW on the deep water formation in the North Atlantic. A comparison of the hindcasts from ten ocean-sea ice models shows that the simulation of the Arctic FW budget is quite different in the investigated models. While they agree on the general sink and source terms of the Arctic FW budget, the long-term means as well as the variability of the FW export vary among models. The best model-to-model agreement is found for the interannual and seasonal variability of the solid FW export and the solid FW storage, which also agree well with observations. For the interannual and seasonal variability of the liquid FW export, the agreement among models is better for the Canadian Arctic Archipelago (CAA) than for Fram Strait. The reason for this is that models are more consistent in simulating volume flux anomalies than salinity anomalies and volume-flux anomalies dominate the liquid FW export variability in the CAA but not in Fram Strait. The seasonal cycle of the liquid FW export generally shows a better agreement among models than the interannual variability, and compared to observations the models capture the seasonality of the liquid FW export rather well. In order to improve future simulations of the Arctic FW budget, the simulation of the salinity field needs to be improved, so that model results on the variability of the liquid FW export and storage become more robust. Citation: Jahn, A., et al. (2012), Arctic Ocean freshwater: How robust are model simulations?, J. Geophys. Res., 117, C00D16, doi: 10.1029/2012JC007907. AuthorCount:15