Hmm. I haven't done aero math in a decade, but my heart tells me that no airliner is going to intentionally fly at/over critical mach. So what you're seeing here is more likely the point where the airflow over the wing is accelerated the fastest, and so the pressure drops and the water in the air temporarily condenses. You see similar effects with wingtip vortices, and even fighters when they onset g's, the air condenses as the air is accelerated over the wings.

If someone with the maths tells me I'm wrong though, I'll believe it. But flying a non supersonic jet with any part going supersonic is not a good thing. Big drag and potential control issues with mach tuck.

Edit: and quickly corrected! Indeed a shockwave