Time Series Observations Show Wind Influence on Fjord Circulation in Western Norway

Abstract:

The Storfjord system in Western Norway is a deep fjord system with several branches. Management of human activities that affect the fjord environment such as cruise ship tourism, aquaculture, and wastewater discharge require understanding of the physical oceanography of the region.

Time series of current velocity, water temperature and salinity, and wind speed and direction from seven oceanographic measurement buoys in the outer part of the fjord system are analysed. The aim is to identify the driving forces controlling the circulation and the water exchange between the fjord system and the coastal waters outside.

The mean circulation is classic fjord circulation with a fresh upper layer flowing out, and more saline coastal water flowing inward below it. We find strong variability at time scales characteristic of synoptic weather both in the near-surface layer and in the layer below. This suggests that wind is an important control on the fjord circulation, perhaps unsurprising since the fjord system is located in the region with the strongest winds in Scandinavia.

At shorter time scales, there is a peak in the spectral power of along-fjord current velocity at a period of 24 hours, distinct from the also present O1 diurnal tide peak at 25.8 hours. The daily signal varies in strength throughout the year and is most pronounced in winter. A land-sea breeze system would occur in summer rather than winter. Normally there is also less hydropeaking, changes in regulated river flow, in winter in the Nordic countries due to consistently high energy demand. The observed diurnal signal is likely tidal, but its properties remain to be investigated.

Here we present results from continuous measurements of currents, hydrography and wind at seven locations in the outer branches of the fjord system. The longest time series are almost four years.