A thermocline is
a distinct layer based on temperature within a large body of fluid with a high gradient of distinct temperature differences associated with depth. In the ocean, the thermocline divides the upper mixed layer from the calm deep water below.
Two moon jellyfish disturbing a thermocline in the top water layer of Gullmarn fjord, Sweden
Sonar is a technique that uses sound propagation to navigate, measure distances (ranging), communicate with or detect objects on or under the surface of the water, such as other vessels.
French F70 type frigates (here, La Motte-Picquet) are fitted with VDS (variable depth sonar) type DUBV43 or DUBV43C towed sonars.
Sonar image of the Soviet Navy minesweeper T-297, formerly the Latvian Virsaitis, which was shipwrecked on 3 December 1941 in the Gulf of Finland
Bubble clouds shown under the sea. From ref.
Variable depth sonar and its winch