Observations of ocean spice and isopycnal tilt sound-speed structures in the mixed layer and upper ocean and their impacts on acoustic propagation.

The article focuses on analyzing upper ocean acoustic propagation in the Northeast Pacific Ocean by decomposing a 970 km, 400-m deep conductivity, temperature, and depth (CTD) section into sound-speed variations caused by isopycnal tilt and ocean spice—density-compensated temperature and salinity variations. Using acoustic simulations at 400 and 1000 Hz, the study quantifies how these two dynamic fields differently affect sound propagation within the mixed layer acoustic duct (MLAD) and the transition layer (TRL). Results show that spice generally causes greater acoustic energy loss, while tilt can have a stabilizing effect on MLAD propagation, with discrete blocking features dominating at lower frequencies and diffuse scattering more important at higher frequencies. The decomposition highlights distinct spatial and spectral characteristics of tilt and spice, suggesting their separate oceanographic processes have significant and differing impacts on upper ocean acoustics.