cleaning could help, but if the last time you did it was March, it's likely something else is going on...
I would check your case fans and the GPU fan carefully, make sure there's no dust-balls clogging them up or reducing their speed..
otherwise, you can probably expect a change on the order of 5C or so going from winter to summer, humidity can shift it too (water in the air absorbs heat!). 83C isn't that bad if it stabilizes, but I understand your concern
FYI, I'm in Boston so I'm pretty close to you. I've noticed a 4C raise in load temp on my sapphire 7970 dual-x compared to winter temps