Innsamling av hydrografiske data i Oslofjorden – FFIs bidrag i prosjektet FjordOs II

The Norwegian Defence Research Establishment (FFI) is partner in the project FjordOs II, which is funded by the Oslofjordfondet. The goal of the project is to further develop, ensure the quality of and publish results from a high-resolution numerical ocean model for the Oslofjord. The model calculates currents and hydrography in the fjord. The project is owned by the Norwegian Coastal Administration, but administrated by the University of South-Eastern Norway (USN). Other partners in the project include the Norwegian Meteorological Institute, the Norwegian Institute for Water Research, the Østfold and Vestfold counties, Kongsberg Maritime and ESSO Norway. The Oslofjord is one of the areas in Norway with highest sea traffic density and has one of the highest risks for acute oil spill from shipping. Along the fjord there are 88 protected areas and two national parks, and the fjord is an important recreation area. A numerical model that calculates currents and hydrography is very important to increase the knowledge of spreading of oil spills and environmental toxins and the distribution of temperature and salinity in the Oslofjord. In 2017 FFI conducted four cruises along five transects in the Oslofjord from Færder in the south to Breiangen in the north. The cruises took place in January, May, August and December to cover different seasons. FFI’s research vessel H.U. Sverdrup II was used in the January and December cruises, while the coast guard ship Nornen was used in May and August. The data showed weaker salinity gradients in January and December than in May and August due to higher fresh water runoff during spring and summer. The vertical gradients were sharper at Breiangen than at Færder due to mixing caused by more windy conditions in the outer parts of the fjord. The surface layer was found to be thicker and less saline along the eastern shore than the western shore. This might be explained by wind direction prior to and during the cruises. FFI’s (and the Norwegian Navy’s) motivation to be a partner in FjordOs is because the project studies methods to better describe the vertical distribution of salinity and temperature in the ocean model. In ocean models a correct representation of vertical profiles and gradients is a challenge, and such profiles are therefore not sufficiently described to be used as input for acoustic modelling. The measurements described in this report will be of high value for validating the ocean model. Some of the methods are tested, and the results are promising.