[1rudeboy]

I thought that this effect, whatever it is called, has nothing to do with the sound barrier?

[doorgunner69]

Prandtl Meyer Expansion

Supersonic flow, the aircraft is not supersonic, just near it so some areas of flow over it are.

[skeptoid]

I thought that this effect, whatever it is called, has nothing to do with the sound barrier?

It does and it doesn't

It it technically the Prandtl-Glauert singularity, and is not always present at Mach 1 transitions. It is merely water vapor which condenses as pressure drops; exactly the same mechanism that is quite commonly observed over wings and in the spiral vortexes from their tips as commercial jets approach landing.

Four "see alsos" at the link.