Thermocline Sea Power
See Thermocline Naval Warfare PMC Tactical.
You can see where the current thermal layer depth is by hovering mouse over "Keel" at the bottom left corner of the GUI as seen in example image below.
Image credit: Wolfpack345 youtube channel, SEA POWER The Barney Incident || New Naval Simulation video.
Submarines can try to hide easier from surface ships by staying under the thermal layer. PMCTODO