Modelling the Orust fjord system on the Swedish west coast

Abstract

We use a numerical model to examine the dynamics controlling flushing of the basins of the Orust fjord system on the Swedish west coast over a period of 71days in the autumn of 2010. This fjord system is known for its seasonal and permanent hypoxic basins; Koljöfjord, Byfjord and Havstensfjord. It is shown that wind direction determines in- and outflows of the basins. Westerly winds result in surface currents towards the interior of the system, and counter currents at the bottom. In contrast, easterly winds cause the export of surface water, which is compensated by upwelling of Skagerrak water. Although these easterly winds cause renewal to take place several times a month in southern Havstensfjord, it occurs only once or twice in the northern part, while Koljöfjord and Byfjord are ventilated about every 3years.The reduction in density throughout the water column (i.e. weakening of the vertical stratification) is related to the amount of energy being supplied to the deepwater by internal waves, created by barotropic tides across the sills, and wind blowing across the surface. The model shows that, in the southern part of the system, tidal movement dominates the energy supply to the deepwater. Further into the system, the importance of wind energy increases. In Koljöfjord and Byfjord, wind accounts for about 40% of the supplied energy for deepwater mixing.Concurrent weekly measurements taken in each fjord basin reveal that the monthly monitoring programme currently in place cannot adequately resolve the dynamics of the fjords. Modelled and observed currents in Svanesund and Sunninge Strait show prominent baroclinic motion on time-scales between one day and one week, suggesting that monitoring programs that aim to support numerical modelling need to resolve these time scales.