Subsurface acoustic ducts in the Northern California Current System

Abstract

A survey of sound speed profiles obtained from archived mooring and glider observations over the continental shelf, slope, and further offshore along the U.S. Pacific Northwest coastline reveals a shallow subsurface duct that appears at depths below the mixed layer. Sound propagation simulations using parabolic equation techniques demonstrate that the presence of the duct has a strong impact on sound propagation at 3.5 kHz. The duct is more prevalent in summer to fall than in winter to spring and extends to at least 200 km offshore from the shelf break. The depth of the minimum sound speed of the duct decreases onshore from between 80 m and 100 m offshore of the continental slope to less than 60 m over the shelf. The duct is also more prevalent offshore of the shelf break than over the shelf. Two processes potentially contribute to the formation of the subsurface duct: (1) differential advection of water masses driven by large-scale circulation within the northern California Current Systemand (2) wintertime surface cooling and convective deepening of the surface mixed layer. Data analysis conducted in this study suggests that the former is likely the dominant mechanism. [Work supported by the Office of Naval Research.]