The thermocline--also known as 'the layer' and 'thermal layer'--is a transitory band of water that exists between relatively warm sea water near the surface and the constantly very cold water (0 degrees Celcius; sea water freezes at -2.3C) that exists beyond the first 330ft/100m. The movement of sound through water is influenced by temperature; It does not readily cross the thermocline. The thermocline acts almost like a mirror, bouncing sound waves off it rather than allowing them through it.
The way sound behaves underwater is heavily influenced by the temperature of the water. Due to heat from the sun being dispersed by waves, the first 330ft/100m or so is a uniform temperature that is relatively warm. In deeper water, the temperature remains relatively stable through the day/night cycle, and is usually not very far from 0 degrees Celsius (sea water freezes at -2.3C). The thermocline is the transition between these two relatively uniform bodies of water. It varies in depth, thickness and strength based on a variety of geographic and environmental factors, however in general it is strongest and thickest at the equator and weakest and thinnest at the poles.
You can get precise information on the thermocline in CMANO by looking at the information box. Here you will see the depth and strength of the layer.
When we talk about the layer from a submarines point of view, we can be above the layer, below the layer, or inside the layer. Above the layer we will be able to readily detect surface contacts and other submarines above the layer, below the layer we will only be able to hear very close or very noisy surface contacts—or other submarines below the layer. Inside the layer, we won’t be able to detect anything very well, but we will also be difficult to detect. These rules change a little when we’re using a towed array or facing off against variable depth sonar (VDS):
- Ships with towed arrays deploy them below the layer
- Ships with VDS treat these sensors as being both above and below the layer
- Submarines with towed arrays deploy their sensors based on depth
- At periscope and shallow depth the towed array is trailed above the layer
- At 'just over the layer' depth the towed array is 'dangled' below the layer (allowing a submarine to have its hull sonar above the layer and towed array below the layer)
- At 'just under the layer' and deeper the towed array is trailed below the layer