Observations of turbulence caused by a combination of tides and mean baroclinic flow over a fjord sill

Abstract

This study investigates the dissipation rates and flow conditions at the Drøbak Sill in the Oslofjord. The area was transected 13 times with a free-falling microstructure shear probe during 4 days in June 2011. At the same time, an ADCP was deployed inside the sill. During most tidal cycles, internal hydraulic jumps with high dissipation rates were found on the downstream side of the sill. However, the internal response varied strongly between different tidal cycles with similar barotropic forcing. In the beginning of the observational period, ebb tides had no hydraulic jumps, and in the end one of the flood tides did not have a hydraulic jump. During the same period, the mean baroclinic exchange flow changed from inflow to outflow in the bottom layer. The authors conclude that the conditions at the sill are on the edge of forming hydraulic jumps and that the mean baroclinic exchange may push the flow above or below the limit of a hydraulic jump depending on the situation. This conclusion is supported by two-layer hydraulic theory. The volume-integrated dissipation rates within 500 m from the sill crest compare well with estimates of energy loss in the lower layer calculated from the Bernoulli drop under the assumption of no energy loss in the upper layer. Finally, the mean dissipation rate at the sill was compared with the radiation of internal tidal energy away from the sill, and it was found that about 60%–90% of the total energy loss was dissipated locally.