Thermocline
A thermocline (also known as the thermal layer or the metalimnion in lakes) is a distinct layer based on temperature within a large body of fluid (e.g. water, as in an ocean or lake) -source Thermocline wikipedia.
A thermocline is the transition layer between warmer mixed water at the ocean's surface and cooler deep water below. -source National Oceanic and Atmospheric Administration (NOAA).
Image Credit: NOAA.
When a submarine goes above or below thermal layer it can possibly hide from detection by other sonar systems in the area, in extremely simple terms. Its a proven tactic for submarines to stay above thermocline and deploy towed array sonar under the thermal layer, surface ships can deploy towed array above or below the layer as well.