Fjord to ocean connections

The fundamental difficulty with modeling exchange of water masses between fjords and the open ocean is the wide range of physical scales involved. Coastal topography is often complex, particularly along the coast of Norway with fjords connected to the open ocean via narrow straits requiring very high resolution of the model grid. In this project, we are comparing how two different model types cope with the range of scales, using one model with regular grid size in the whole model domain, and one with flexible grid size, which implies higher grid resolution in narrow straits. Tidal currents are dominating in these straits. In model runs with complete and realistic environmental forcing, varying wind will lead to large variability in the general flow field in a model domain. This type of variability is likely to behave similarly in regular and flexible grid models, and wind induced variability would thus make it difficult to detect modeled flow field differences that are caused by the different grid types. We have demonstrated the wind induced variability in regular grid model runs from eleven particular years (2002-2012), which have been produced during the project, covering the coastal zone from south of Vestfjorden to north of Andøya. The flow fields from these runs have been applied to simulate spreading of zooplankton nauplii. We present preliminary evaluation of the ability for numerical models to correctly simulate water exchange across a shelf break front. Observational studies on how important eddies are for such exchange is presented. Finally, a comparison of how structured and unstructured grid models simulate tidal induced flow through a narrow strait in a fjord of Northern Norway has been made, where consequences of differences in modeled flow fields for the spreading of particles following the flow field is shown.

The overall goal of the proposed project is to evaluate whether there is potential for significant improvement in numerical modelling of near-coastal ocean circulation and exchanges of water masses between fjords and the open ocean. The fundamental diffic ulty with modelling these exchange processes is the wide range of physical scales involved. Coastal topography is often complex, particularly along the coast of Norway with fjords connected to the open ocean via fjord openings and straits that can be much narrower than ocean models are able to resolve. The usual approach in model systems currently used in coastal management in Norway is to use so called nested modelling, where fine grid scale models are forced along open boundaries with model output from coarser grid scale models. An alternative method is now becoming increasingly in use in the modelling society; models with so called unstructured grid. The distance between the computed nodes in the model grid varies according to requirements for resolvin g either topographic features or important physical processes. Here we will compare the two approaches. State-of-the-art model systems representing the two approaches will be used for modelling the same coastal area; a part of the Lofoten-Vesterålen archi pelago. Model output of flow fields and water mass characteristics will be compared between the models and against observations, and there will be specific focus on possible implications from differences in flow field for the drift and spreading of biolog ical matter.