Depending on the time of year, in that part of the world thermal layers would be minimal to the point of not being effective. The cold current coming from the polar region would be as cold as the deep water there. Thus, no nice thermal layer like in the South Pacific where the sun heats up the surface water but the deep water remains cold. That's why thermal layers didn't really factor into the the North Atlantic sub warfare as much.
Even in warmer parts of the year the only significant thermal layer may be a meter or two deep, so detection devices would already be below the thermal layer themselves, thus no distortion from varying densities.
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