A Simple Model of Fjord and Coastal Circulation Interaction

Abstract

The dynamical interaction of a narrow fjord with a wind-driven coastal regime is investigated using a linear, two-layer numerical model. The Coriolis acceleration is important in the coastal regime but assumed to he unimportant in the fjord dynamical because v = 0. For a wide variety of wind conditions, bottom topography and model parameters, the wind-forced coastal circulation, with its geostrophic alongshore currants, has a strong effect on the circulation within the fjord. These geostrophic currents control the free surface and pycnocline displacement at the fjord mouth, thereby strongly affecting fjord circulation. This mechanism is an alternative to the classical idea of hydraulic control at the mouth by sills or constrictions. Model simulators also show that the free surface slope is a baroclinic effect and that alongshore and across-shore winds affect the fjord differently. Alongshore winds produce flooding while up- and down-fjord winds simply sat up the surface. We find that offshore winds can produce large velocity shear in the fjord which can have a significant effect on turbulent intensity and diffusion. Data available from Norwegian lords, the Strait of Juan de Fuca and Alberni Inlet. British Columbia. support this idea of dynamic control of fjord circulation by offshore wind-driven coastal currents.