A two-year time series of current velocity from moored buoys in a Western Norwegian fjord system
Jenny Ullgren

A wide sound - Breisundet - west of ≈lesund is the main inlet to the intricate Storfjorden fjord system which includes the World Heritage site Geiranger fjord. Outside the fjord mouth at Breisundet, a deep, narrow channel extends the fjord across the shallow continental shelf so that the effective sill is located well outside the opening. Environmental monitoring is done with regular measurements at some sites in the fjord system, a hydrodynamic spreading model is in operation to guide the sampling of salmon lice, and a numerical model (NorKyst-800) gives daily forecasts of surface currents. Still, we lack empirical answers to some fundamental questions about the flow regime of the Storfjorden system, for example regarding the relative strength of the estuarine circulation, the importance of intermediate circulation, and the time scales of deep water renewal. Not least, we need to validate the operational models for this unique and complex coastal region.

Reliable, up-to-date oceanographic data are also important for ship traffic. Environmental forces like wind, waves and surface currents will affect the speed, energy consumption and emissions of any moving ship. In coastal waters, local current conditions are often the most important unknown affecting ship operation. An ongoing research project led by Rolls-Royce Marine and funded by the Norwegian Research Council aims to develop voyage optimization methods for ships in coastal service by combining oceanographic forecast models and accurate ship-board measurements. To complement and verify the data from the model and the ship-board Acoustic Doppler Current Profiler (ADCP), we use time series measurements from five oceanographic buoys that are moored in different parts of the fjord system. The buoys are part of an environmental measurement programme financed by the Norwegian Public Roads Administration in order to prepare for a large bridge construction project.

A first, qualitative comparison between surface current velocity from the buoys and sample output from a run with a 160x160 m model shows relatively good agreement. Wind seems to have been the dominant forcing of the surface current field during the period in question. However, the two-year observational time series cover a range of conditions that are not represented in the model output sample of about a week, so this needs further analysis. We are also preparing to make additional in situ current measurements at higher horizontal resolution to complement the buoy, ship, and model data. At the symposium I will present some examples from our analysis of the high temporal resolution time series from the moored buoys.

2018-10-18, Jacopo Paglia