Honestly, I think Elsweyr is the closest to the "Nirn Equator", or maybe even Pyandonea, which is covered in rainforests. It gets warmer as you go farther south, so I'm thinking it's probably in the Northern Hermisphere. Elsweyr's a desert, Black Marsh is too thick of swamps to get through easily, and Valenwood's rainforests.
They describe people needing less to survive the further north they go, in the book "The Children Of The Sky". Also the Falmer who originally inhabited Skyrim were bluish-white from a layer of ice over their skin.
Evidence points to Skyrim being pretty cold. There's also the Arena map of skyrim which is 99% likely to be pretty inaccurate, but it shows all of Skyrim except the Western side as completely frozen. People in Skyrim also build their homes in-ground to preserve heat, and from all the books I've read in-game they've talked about:
- Vampires that lurk under the frozen-over lakes.
- Visible steam coming off of a recently deceased werebear from the cold.
- Nords being able to survive the cold better than anybody.
There's also an Orc woman in Bruma who mentions that it's freezing in the Wrothgarian Mountains compared to Bruma when she says: "Yeah, you think Bruma's cold, try a winter in the Wrothgarians. Bruma's a day at the beach." So I'm assuming that also points to it getting colder as you go further north, since Bruma is pretty high up as it is.
I'm not saying it's near the North Pole, but it's still pretty high up there if Bruma, which is covered in snow year-round, isn't as cold as something as far north as the center of Skyrim.