I spent 6 years as a sonarman in the US Navy. Over that time, I made countless Bathythermograph drops to predict ranges so I'll throw in some of my experience here. Thermal layers exist in any area where the weather warms the surface of the ocean. The actual layer depth will vary by time of day(shallower in early morning, deeper in late afternoon to early evening), as well as time of year and ocean area. Obviously spring & summer months as well as the closer you get to the tropics will increase the depth of the surface duct and make the layer deeper. Also, the warmer the temp, the greater the temp change at layer depth & the sharper the cutoff of sound. The cooler the surface temp, the weaker the layer.
As a general rule of thumb, below layer ranges for both active and passive will be approx. 1/5 of ranges in the surface duct. So if your above layer range is predicted as 5000 yds for active sonar, below layer range will be around 1000 yds or less.
Below the layer a shadow zone is created as the sound is bent two ways, up along the surface duct, and more vertical towards the ocean bottom. Changing a sonar's depression angle can work somewhat, but is not a cure all.
If you're attempting to model this in game, remember winter months and colder latitudes will most often give you a thermocline, a basically unchanged temp profile for depth and, therefore, no layer. I hope this helps you in your research.
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