> The speed of sound is around 660mph

true but the jetstream going US to EU is doing a lot of that lifting to get a 700mph readout on the SATNAV system..

700 mph 'across the ground' can also and at the same time be 550mph 'airspeed' when you surf the polar vortex going west to east.

Even though the aircraft is moving at 700mph across the ground, there is no sonic boom or breaking of the sound barrier which requires the aircraft to break 660mph in airspeed.

The polar jetstream tailwind is no joke. It's how/why the typical NY to London flight time is approx 7 hours and London - NY is 8 hours +

They take advantage on it going to London and try to avoid it as much as possible when going back to NY.