Entrainment and turbulence in saline underflow in Lake Ogawara

Under certain tidal conditions, a saline underflow originating in the Pacif ic Ocean moves into Lake Ogawara, Japan. The underflow consists of a unifor m saline bottom layer that is slightly warmer than the ambient and an inter facial shear layer in which the velocity and density are decreased. Within the experimental area the underflow is confined to a channel approximately 1 km wide and is essentially two-dimensional. The underflow had a bulk Rich ardson number, defined in terms of the mean properties, between 1 and 2. Th e rate of entrainment into the bottom layer was calculated using two distin ct methods. The first method used the change in the maximum salinity of the underflow measured at two stations along the path of the underflow to infe r the amount of ambient water entrained. The second method made direct meas urements of vertical mass fluxes with a profiler. The agreement between the two methods was excellent. The measured entrainment coefficients were cons istent with the derived entrainment law. The turbulent structure of the flo w was mapped for a 3 h quasisteady period of the flow. Turbulence is predom inantly generated on the bottom boundary and is transported vertically to t he density interface, where it leads to mixing.