How can I reduce the temp?
I think the 'official' response is that not every compound is suitable for every track... and so you should use a harder compound.
Barcelona is a track that absolutely roasts the tires. Its insane how hot you can get tires on that track. Medium and hard for the F1 cars seem to be the way to go, and it would probably be best to use hard for race sessions and medium for qualy sessions.
I think there is another problem that is perhaps a bit worse though. I generally use the default setup as a baseline to get accustomed to a track and get up to pace before tinkering. And while doing so, the tire pressures in the F1 car on medium compound tires and a cold psi of 17... end up hitting 40 psi just 2-3 laps later. Its completely crazy lol. Its rather comical just how astronomically high the tire pressures get. But hey, what are ya gonna do.
With a custom setup I usually have a gap of ~10 psi on the front tires just so the right front can get close to matching the left front when hot.
I wouldnt say Barcelona is the best track to test temps and pressures on, since it seems to have a huge affect on tires in AC with no real recourse from the player to be able to affect it.
You could try making a bug report on the official forums, but this isnt really a track specific issue, but rather a tire physics heating/pressure issue. And Kunos has been working on refining that for the better part of 2 yrs now.