Observation of strong turbulent mixing in the Australian North West Shelf

Abstract

The North West Shelf (NWS) of Australia is a region with strong barotropic tides up to 10 m, which provide a source for intense turbulent mixing. In the study, we used turbulent mixing measurements obtained on the NWS from October to November 2017 to investigate the mixing characteristics during the austral spring season. In the surface mixed layer, the averaged dissipation rate reached 1 × 10−5 W/kg. The dissipation rate decreased to 1 × 10−8 W/kg in the thermocline and increased to 7 × 10−8 W/kg near the bottom. The averaged diffusivity in the surface mixed layer reached 7 × 10−1 m2/s, decreased to 1 × 10−4 m2/s in the thermocline, and increased to 7 × 10−3 m2/s near the seafloor. These values were significantly larger than those in the open ocean. The study proposed a parameterization method that scaled the diffusivity in a function of the Richardson number, κρ=κ0+κm(Ri)−1.5, where κ0 and κm were derived from the observation data, to take into account the strong shear instability conditions in our observation. Applications of this method for other shelf regions need further validation.