Nonlinear internal tide dynamics on the Australian Northwest Shelf

The coastal shelf in Northwest Australia is a region of strong
nonlinear internal wave generation and dissipation that results
from strong tidal flows. Here we focus on moored observations
collected in 100 m of water 4-22 April 2012. The moored
instruments measured both mean vertical temperature and velocity
structure as well as turbulence quantities at two point locations, 7.5
and 20.5 m above the seabed. The nonlinear internal wave (NLIW)
packets were characterised by isotherm displacements of up to 60
m, occurred once per semi-diurnal tidal cycle and generally
persisted for 4 h, with periods ranging from 10- 30 minutes. The
NLIWs were not consistently phase-locked to either the surface
tide. The largest turbulent overturns observed coincided with the
downward isotherm movement of each of the NLIWs within a
packet. The dissipation of turbulent kinetic energy was highest in
the near-bed region during the passage of the largest amplitude
NLIWs, coinciding with a strong shoreward mean current and
pulse of cold water. Direct estimates of mixing from the moored
turbulence instruments showed that shear and overturns resulting
from the NLIW dynamics resulted in large increases in the
dissipation of turbulent kinetic energy and vertical mixing. Our
direct estimates of KT demonstrate that using the traditionally
employed Osborn model with a constant mixing efficiency can
both vastly over-predict and under-predict the mixing rate.