«Global» surface temperature more closely follow tropical ocean temperatures which are more strongly influenced by clouds and solar. (judithcurry.com)
There are other influences on the jet stream's behavior, and some scientists think that changes in tropical ocean temperatures, or the cyclical recurrence of El Niño, might have a bigger effect on the jet stream than changes in the Arctic. (insideclimatenews.org)
So a large factor in global average temperature is average tropical ocean temperature of around 30 C. Or one could divide the world between tropics of about 30 C and the other half of the world being about 0 C giving the average of around 15 C. (judithcurry.com)