Observations of acoustic propagation in the Canada Basin double-duct system from 2016 to 2017 using a 150-km radius tomographic array.

The Arctic Beaufort Sea has a unique double-duct sound-channel capped by seasonal ice cover. A roughly 90-m surface duct (SD) is formed by a river-driven halocline. Below the SD is the approximately 90-m to 250-m depth Beaufort Duct (BD) created by cold Pacific Winter Water sandwiched between warmer Pacific Summer Water and Atlantic Water. A yearlong record (2016-2017) of acoustic propagation measurements in this double-duct system was carried out using a 150-km radius, acoustic tomography array with broadband, 4-hourly transmissions at 175-m depth centered at 250 Hz. Double-duct signal analysis was carried out using a dense-vertical-receiving array spanning the BD. Observations reveal (1) consistent reverse geometric dispersion in the double-duct system with low modes faster than higher modes, (2) distinct first arrival and final cutoff times, and (3) normal dispersion for non-BD/SD modes causing the front to fold back upon itself after the final cutoff. A vertical-wave number spectrogram technique is used to decompose the pulses into an arrival time series for each wave number. Key observables are the first and final arrival travel times, dominant-vertical wave numbers, and signal intensities. Fluctuations are interpreted in terms of the varying stratification, ice cover, and implications for surface heat flux estimation.