“Trans-atlantic” is a forced/learned accent that used to be more popular ~100 years ago. It was basically meant to represent people that were born in the US, but educated in the UK (or vice versa). Essentially, it was supposed to signify that you were wealthy enough to have connections on both sides of the Atlantic.

People intentionally learned to speak that way, though, and it became common on radio broadcasts (so you could also hear it referred to as an “old-timey announcer voice” or something similar).