Subsurface acoustic ducts in the Northern California current system

Area of Science:

• Oceanography
• Acoustics
• Physical Oceanography

Background:

• Acoustic ducts significantly affect underwater sound propagation by trapping acoustic energy.
• Subsurface sound channels can alter acoustic transmission loss, impacting sonar and acoustic monitoring.

Purpose of the Study:

• Investigate the seasonal occurrence and characteristics of a subsurface acoustic duct along the U.S. Pacific Northwest coast.
• Identify factors influencing the formation and location of this sound channel.
• Examine the relationship between acoustic duct formation and oceanographic conditions.

Main Methods:

• Analyzed archived mooring and glider data to obtain sound-speed profiles.
• Correlated acoustic duct presence with seasonal variations, geographical location (shelf vs. offshore), and water mass properties.
• Examined vertical profiles of temperature, salinity, and velocity.

Main Results:

• The subsurface sound channel is more prevalent during summer and fall, and offshore of the shelf break.
• Duct occurrence is linked to strong haloclines and weak thermoclines or temperature inversions.
• Acoustic ducts over the shelf slope correlate with vertically sheared along-slope currents.

Conclusions:

• Seasonal surface heat exchange and baroclinic advection are key mechanisms for acoustic duct formation.
• Surface heat exchange creates the upper duct boundary (downward refraction), while advection forms the lower boundary (upward refraction).
• Understanding these acoustic ducts is crucial for accurate acoustic modeling in the region.