The main things that affect SONAR are quite simple, for example where i operate most of the time under the polar ice HF sonar is my greatest asset for navigation, the HF sonar picks up any ice bergs and polyna which i can surface through.
(polyna is a hole in the ice)
But due to the back ground noise the ice makes (due to constant movement) then i find that my SONAR equipment is not at its peak 100% level, and i cant detect submarines at far distances, unless i dive down to around 250 meters and sometimes that is not achievable.
There is a band of water there maybe 3 or more in one spot called a thermocline, this is where warm water and cold water meet to form an invisible cover so to speak, its difficault to detect a submarine if your above and its below unless you use my tricks.
Background noise such as large merchant ships or any ship for that matter can hamper detection whats even worse is if the enamy submarine is acctualy underneath that said vessel as the surface vessel will drown out any submarine noise.
But back ground noise in general can make you miss things on the sonar screen.
Another thing to consider is temprature of the water, sound travels faster in hot water than cold (unless its ice) this is because the monocules in the hot water are already vibrating alot and any sound is bounced along with the monocules therefore making detection easier in the tropics then in the bearents sea. (only small difference but it still can matter).
Water depth is also criticle, shallow water means slow speeds for many submarines, because the noise will bounce off the sea bed and go any where, this is generaly why littoral (shallow) water ops are left to the more suited SSK submarines such as the Kilo Type 209 and 212 or the Scorpene as they travel slower due to battery's and also thier powerplant makes less noise.
In littoral waters the use of the towed array is limited because at times you can wash it out due to the fact its acctualy dragging on the sea bed, so you have to rely on your hull mounted sensors, which again means your detection rate has droped by around 25% or more.
Thats just a low down on what you asked if you want more detail i could try and work it all out for you.
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