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.

Atlantic Croaker
Annual average sound-speed profiles from 60°S to 55°N along 150°W in the Pacific Ocean. Sound speeds in the deep ocean all tend to be the same because of the nearly uniform temperature across latitude at depth. Adapted from Figure 2.2 of Munk et al., 1995.

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.

Atlantic Croaker
Depth of the sound channel axis. Adapted from Munk and Forbes, 1989.

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.

||Graph of temperate and polar ray diagrams.
Temperate and Polar sound-speed profiles and ray diagrams for on-axis sources, from Figure 2.3 of Munk et al., 1995.

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

References

  • Munk, W. H. and Forbes, A. M. G. 1989, "Global ocean warming: An acoustic measure?" Journal of Physical Oceanography, 19, 1765-1778. 
  • Munk, W., Worcester, P. and Wunsch, C. 1995, "Ocean Acoustic Tomography." Cambridge University Press. 
Additional Resources

  • "Classroom BATS: Sound in the Ocean - The SOFAR Channel." (Link)
  • "NAS Beyond Discovery - Sounding Out the Ocean's Secrets." (Link)