Observations of sound speed structure in the upper ocean and their effects on acoustic propagation at low and mid-frequencies

John Colosi, Naval Postgraduate School, Monterey, CA, United States, Luc Rainville, University of Washington, Seattle, United States and Daniel L Rudnick, Scripps Institution of Oceanography, La Jolla, CA, United States
Abstract:
Towed and underway CTD sections from the North Pacific and North Atlantic obtained in the spring time are used to characterize the scales of sound speed in the upper ocean (0-400m), and to explore the effects on acoustic propagation through both deterministic and stochastic modeling. The sections reveal complex thermohaline structure above the main thermocline giving rise to short spatial scale variations in mixed layer depth, mixed layer sound speed gradients, and sublayer gradients. Below the mixed layer but above the thermocline exists a strongly stratified and variable region associated with the remnant winter mixed layer. Acoustic modeling for 100 to 1000 Hz through the CTD sections reveal significant acoustical effects such as 1) strong diffusive leakage in and out of the surface duct, 2) abrupt duct blockage, and 3) phase and intensity randomization. A stochastic model of the sound-speed structure is proposed and utilized in a mode-based propagation model to predict low-order moments of the acoustical field. Implications for acoustic remote sensing of this region are addressed.