Internal wave directional spectra using an acoustic Doppler current profiler

An acoustic Doppler current profiler was deployed through land‐fast ice near Resolute, Northwest Territories. It recorded a distinct train of high‐frequency (periods < 20.0 min) internal waves lasting 10 hours. Assuming linear dynamics, a modal decomposition of the three velocity channels led to the modal pressure anomaly taking a role analogous to that of the surface displacement sensed by a surface following heave, pitch, and roll buoy. Partitioning the data into frequency dependent cross‐spectral matrices of the modal horizontal pressure anomaly enabled the computation of high‐resolution, data‐adaptive directional spectra, recently presented for use with surface gravity waves. The mode 1 directional spectra were particularly sharp and indicated a cross‐channel point of origin of the waves to the northeast of the study site. The lowest two frequency bands contained considerable spectral power density that could not be accurately described by the modal decomposition. A direct examination of the cross‐channel velocity time series indicated substantial low‐frequency (periods > 30.0 min) horizontal flow without a corresponding coherent vertical velocity component, attributed to a density driven along‐isopycnal flow created by salt rejection emanating from a broad shelf to the northeast.