Explain how do submarines operate using the thermocline region?

Submarines use the ocean's thermal layers to hide from surface ships and other submarines. The thermal layers in the operational area of any submarine is a key tactical consideration at all times for Sonar operations.

Sound travels at different speeds based on water temperature, pressure and salinity. Sound bends toward colder water - however, when sound reaches deeper water, pressure becomes the overriding variable and bends sound back toward the surface. Knowing where the thermal layers are lets Submarine Sonar Techs (like me) know where the thermal layers are, and from there how sound will propagate through the water. This is then used for tactical operations depending on the situation, allowing a boat to hide if necessary, or position itself at the best depth to detect surface or submerged contacts.

Sound speed and water temperature is constantly monitored and charted as the boat changes depth, and thermal layers can be charted from the surface to deep depths by a device known as an SSXBT (Submarine Launched Expendable Bathyothermograph) as situations warrant. An SSXBT is launched from the boat's countermeasures tube, and once it reaches the surface, it releases a weighted probe which sinks until the probe is crushed by sea pressure. The probe is connected by wire to the boat and a chart recorder and maps more precisely the thermal layers. They're only used when necessary, e.g., you know you're heading into a tactical scenario and you need a complete Sonar profile of the current oceanographic state of the surrounding area.

As an example of how sound speed is affected by water temperature, when passing through the Gulf Stream, if tracking a target on Sonar, the bearing will change by several degrees as a result of the temperature change when passing from normal ocean to extreme warm water. Also, if sound is being bent toward the bottom by colder water from say, 100' down, and the layer near the surface is warm, a boat coming to periscope depth might not detect a close-aboard contact until the boat passes through the thermal layer. Some accidents have happened in this manner in the past where submarines coming to PD have collided with surface ships that weren't picked up on Sonar until it was too late. Prior to coming to PD, a complete Sonar sweep is performed to alleviate such problems, but the reality is that technology can't always overcome nature.

If necessary, on the rare occasions when a boat might be detected by a helicopter dipping sonar or a passive sonobuoy from a P-3 or other aircraft, the thermocline regions, depth and speed capability allow the submarine to exit the area very quickly under cover of the ocean's natural thermal layers, making it extremely difficult to track.