Multifrequency acoustical volume backscattering patterns in the Arabian Sea--265 kHz to 3 MHz

Area of Science:

• Oceanography
• Acoustics
• Marine Ecology

Background:

• Understanding acoustic backscattering is crucial for marine ecosystem monitoring.
• Seasonal wind patterns, like the monsoons, significantly influence Arabian Sea conditions.
• Previous studies have not fully characterized high-frequency acoustic backscattering across different monsoon periods.

Purpose of the Study:

• To investigate high-frequency acoustic volume backscattering in the Arabian Sea.
• To analyze backscattering variations across the Winter NE Monsoon, Summer SW Monsoon, and Fall Intermonsoon periods.
• To assess the influence of depth, coastal proximity, and diel cycles on acoustic backscattering.

Main Methods:

• Employed a multifrequency acoustic profiling system on a towed body.
• Measured acoustic backscattering strengths at six frequencies (265 kHz to 3.0 MHz).
• Collected data from near-surface to 250 m depth along transects parallel and perpendicular to the Omani coast.

Main Results:

• Volume backscattering strength decreased with depth across all frequencies.
• Contrary to expectations, Summer SW Monsoon backscattering was lower than Winter NE Monsoon backscattering.
• Coastal backscattering was higher during the Winter NE Monsoon but not significantly different offshore during the Summer SW Monsoon. Daily patterns of increased nighttime backscattering were observed at lower frequencies (265 and 420 kHz) in the upper 100 m.

Conclusions:

• Seasonal and regional variations in acoustic backscattering were smaller than anticipated, often masked by fine-scale spatial and temporal variability.
• The observed patterns challenge previous assumptions about monsoon influences on acoustic backscattering in the region.
• Diel vertical migration patterns may influence acoustic backscattering, particularly at lower frequencies.