Sound Channel Variability
The way in which sound speed changes with depth is not the same everywhere in the ocean because the ocean temperature and salinity profiles that determine sound speed can differ greatly from one location in the ocean to another.
In the deep, open ocean, between roughly 40°S and 40°N, temperature decreases with depth, and pressure increases with depth. Sound speed near the surface in this region decreases with increasing depth due to decreasing temperature. As the depth increases further, the water temperature gets colder and colder until it reaches a nearly constant value of about 2°C below water depths of roughly 1000 m. Where the temperature is nearly constant, the pressure of the water has the largest effect on sound speed. Because pressure increases with depth, sound speed increases with depth. Salinity has a much smaller effect on sound speed than temperature or pressure at most locations in the ocean.
Poleward of latitudes of about 40°, the ocean is almost uniformly cold from top to bottom. Pressure always increases with depth. Sound speed is therefore lowest at or near the surface and increases with increasing depth. The result is that the sound speed minimum sound channel axis is at or near the ocean surface. More generally, the depth of the sound-speed minimum varies in a complex way with location depending on the detailed structure of temperature and salinity.
A sound wave traveling through the ocean is refracted (bent) whenever it encounters changes in the speed of sound. Sound waves are continually refracted toward the region of lower sound speed. Differing sound-speed profiles therefore cause sound waves to travel on quite different paths.
In mid-latitudes sound that travels upward from a source at the sound speed minimum is bent back towards the minimum. Similarly, sound that travels down from the source is bent back up toward the minimum. The result is that sound can travel long distances, cycling above and below the sound speed minimum without hitting the seafloor or ocean surface.
At high latitudes all sound is bent back toward the sound speed minimum at the surface. The result is that sound waves loop down into the ocean before returning to the surface, where they are reflected and again loop down into the ocean. Whenever sound reflects from the rough ocean surface, some sound energy is scattered and lost. A sound wave that hits the ocean surface generally becomes weaker more quickly than one that does not. In the case of very low frequencies, where the wavelength of the sound is longer than the height of the surface waves, the ocean surface appears relatively smooth to the sound wave. Most of the sound energy is then reflected, and low-frequency sound can still travel long distances.
Additional Links on DOSITS
- History of the SOFAR Channel
- Sound Speed Minimum
- Sound Travel in the SOFAR Channel
- How fast does sound travel?
- How is sound used to measure global climate change?
- Sound Surveillance System (SOSUS)
- Classroom BATS: Sound in the Ocean – The SOFAR Channel.
- NAS Beyond Discovery – Sounding Out the Ocean’s Secrets.
- Munk, W., Worcester, P., & Wunsch, C. (1995). Ocean acoustic tomography. Cambridge, England: Cambridge University Press.
- Munk, W. H., & Forbes, A. M. G. (1989). Global ocean warming: An acoustic measure? Journal of Physical Oceanography, 19(11), 1765–1778. https://doi.org/10.1175/1520-0485(1989)019<1765:GOWAAM>2.0.CO;2