Observations of internal tides and associated turbulent mixing in a shallow estuary

Observations of turbulent mixing and tidal currents in a highly stratified estuary, Ariake Sea in the western Japan are presented. Direct measurements of turbulent dissipation rate using a micro-structure profiler reveals frequent occurrence of strong turbulent mixing around the pycnocline (ε = 10^-7 - 10^-6 Wkg^-1), implying internal wave-induced shear instability. A set of current field measurement using bottom-mounted ADCPs during summer to early autumn shows vertical shear peaks at diurnal and semi-diurnal frequency bands. Separation tidal current profiles into barotropic and baroclinic components are made by CEOF analysis because the barotropic current in the sea is highly depth-dependent due to thick bottom boundary layer. The estimated baroclinic velocity exhibits a nature of diurnal internal tide. A comparison of divergence of the internal wave energy flux and dissipation rate indicates the diurnal internal tide is responsible for the strong turbulent mixing around the pycnocline.