Observing Groups of Solitary Internal Waves and Turbulence with BATFISH and Echo-Sounder

Abstract

Acoustic backscatter from a 200 kHz echo-sounder and temperature fine-structure from a towed CTD (BAT-FISH) am compared prior to and during the passage of a group of large-amplitude internal waves on the continental shelf off Nova Scotia. The results indicate that a significant increase in acoustic backscatter and temperature gradient variance is observed in well-defined layers of approximately 5?15 m thickness and that these layers are generated by the passage of the group of internal waves. Consideration of the nature of the internal waves shows that the layers occur where shear instability is expected to be large and the local Richardson number is smaller than the critical value required for the generation of turbulence. Estimates of energy levels and rates of dissipation of energy in the internal waves provide a way, using standard turbulence theory, to calculate the expected acoustic scattering cross-sections. The measured and the calculated cross-sections are consistent. It is concluded that both acoustic backscatter and BATFISH temperature fine-structure are associated with active turbulence generated by the internal waves.