https://www.pcgamer.com/this-mod-turns-skyrim-into-a-tropical-rainforest/

There are countless mods that make Skyrim even colder and harsher than it already is, but Project Rainforest, a Skyrim Legendary Edition mod, does the opposite. Snowy tundras turn to tropical grasslands, frozen lakes to warm swamps, and icicles to lily pads. 

It changes the whole of Skyrim, Fort Dawnguard, Falmer Valley, and parts of Solstheim from the Dragonborn DLC. You'll need all of the game's DLCs installed to run it. Cave interiors are now green and temperate, and creator sa547 has gone to a lot of effort to tweak weather effects, swapping snow for rain storms, and adding tropical climates in certain areas.