Most (but not all) modern engines are flat-rated up to approx. ISA+15 - above that temp, thrust is dialed down to protect EGT.
And
ISA is a purely arbitrary definition - I don't see why longitude has anything to do with it. It dates back to an early definition of worldwide average SL air temperature (15C or 59F).
BREAKING NEWS!!! think the latest worldwide average temp promulgated is 14.9 C or thereabouts -