I don't think there is a way to automatically keep it on unless you doing away with the auto and just keep it on. Not sure. It would seem that if it is always on, the vehicle will take longer to cool as you initially are just recirculating the extremely hot air that is much hotter than the outside air. I prefer to bring in the somewhat cooler outside air and then circulate to get the coldest air. Just my opinion.
I've been researching this, and so far, it seems GM perfers "fresh air" over "recirc". For example, recirc is disabled when either Floor or Defog/Defrost is commanded. And like dura_cell mentioned, it may be 100° outside, but can be 150°+ inside the vehicle, which makes it harder to cool than outside air. If I find anything definite, I'll certainly post.
1) I often park in the garage and it is cooler than outside, so I really want the recirculation always on.
2) I found that if recirculation is on, then A/C automatically turns on, but A/C light stays off. Very strange design/behavior.
3) OK, so I turn the key to start the car, then I have to remember to turn the day time lamps off (I like it this way), then I have to turn the recirculation on. If anything is connected to the power outlet, I have to turn it on as well (it is off, because the outlet doesn't go off). :banghead: :banghead: :banghead: Lots of buttons to be pressed!!! This piece of the car design really annoys me.
In the manual it states that there is always fresh air flow into the vehicle - even when recirculating. I would hazard to guess that it has something to do with the fact that the battery is in the passenger compartment to get rid of the battery gasses.
While the fresh air due to the battery is a good hypothisis, GM has put a hole about the size of a quarter in the recirculation door for years to prevent a carbon monoxide buildup in the interior. I think they were sued several decades ago and the hole has been there ever since. Recirc is not allowed by most manufacturers in defrost or floor due to moisture/fogging issues that can obscure your vision when using recirc in the winter or during defrost.