The Modulation of Internal Wave Propagation and Breaking in the Thermocline

It has been over 50 years since Bretherton, Garrett, and colleagues established the principles that govern internal wave propagation through the depth-changing density and velocity fields of the thermocline. Developing the experimental capabilityrequiredto verify this process remains one of the oldest New Frontiers in ocean research. In order to quantify the mixing efficiency of the breaking process, it is critical to understand where mixing events occur relative to perturbations in the background density gradients. Here integrated observations of the thermocline velocity and density fields from rapidly profiling CTDs and Doppler sonar are applied toward examining the sequence of events leading to density overturning in the thermocline. At sites where the baroclinic tide shoals and forward reflects, both the pattern of breaking and the buoyancy field are set by the non-sinusoidal waveform of the reflecting tide. Where the sea-floor slope has curvature, a first temporal harmonic of the tide is also forward reflected. This too can be associated with breaking. In the typical deep ocean thermocline, the propagation of small-scale wave packets is modulated by the shears associated with propagating near inertial waves (Broutman, 1984), by changes in the mean density gradient associated with low mode tides (Sun & Kunze, 1999), and by lateral variations in the vorticity and density fields associated with passing eddies (Thomas, 2017). Observations of density overturning show a mix of convective and shear instability, with the observational complication that convective instability is often triggered by small scale waves that have encountered adverse shear.