Tidally-induced turbulent mixing in a sill fjord

Abstract

Internal waves are generated when water currents interact with bottom topography, as when tidal flow pass across a sill in a fjord. These internal waves carry energy that is made available for turbulent mixing when the waves break. By the use of field measurements and modeling it has been shown that the vertical mixing in different parts of the Oslofjord is caused by internal waves generated at the Drøbak Sill. The idea of a link between breaking internal waves and vertical mixing in the Oslofjord is almost 40 years old, but the details presented in this investigation are new. The amount of energy carried by the internal waves is gradually reduced as the waves propagate inward into the fjord. When they reach the innermost part, the Bunnefjord, very little energy is left for vertical mixing. As a result of this, the deep water in this basin is stagnant for longer periods than in basins closer to the Drøbak Sill. A dramatic consequence of these physical conditions are that the oxygen concentration in the depths of the inner basin is often depleted. Thus all higher forms of life disappears. At the time of writing (December 2015), the conditions in the Bunnefjord is on the brink of reaching anoxia. In the first paper the propagation and the loss of energy flux are described by field observations of the vertical stratification and current profiles. In the second paper details on the relation between dissipation in the vicinity of the Drøbak Sill and energy flux that radiates away from the sill, are studied with a drop sonde measuring vertical velocity shear. In the last paper energy flux is modelled with a high-resolution three-dimentional, hydrostatic model for the Oslofjord. The results clearly show the relationship between the loss of internal wave energy and vertical diffusivity.