How do temperature gradients in seawater affect sonar beams?

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Temperature gradients in seawater significantly influence sonar performance by altering the speed of sound, which typically increases by approximately 4.5 metres per second for every 1 °C rise in temperature.1

Because the speed of sound varies with temperature, sound waves traveling obliquely through layers of water with different thermal properties undergo refraction. Specifically, sound waves bend downward when the speed of sound decreases with depth—a common occurrence as temperature drops—and bend upward when the speed increases with depth.1

This refraction is critical for sonar operations because it dictates the actual path of the sound beam, which must be accurately calculated to determine the position of a target, such as a submarine. Furthermore, refraction can create "shadow zones" where sound waves do not penetrate due to their curvature, effectively hiding objects from detection. Understanding these gradients is essential, as they can also lead to the formation of the SOFAR channel, a zone of minimum sound speed that allows sound to travel over vast distances.1