A Google search provides lots of links to outdoor graded speaker cable. I don't know much about it so I won't risk linking any.
Barring those, I'd say that I would probably use a very pliable in-wall rated cable. Because to get outside, you will have to run through a wall somewhere and your code may require that. Also, the outer sheathing will help keep the UV off of the real wire inside it. Pliable, so the sheathing doesn't crack with movement in the cold.
Also, I'd probably use spade clips or something to terminate with, rather than bare wire. Leave lots of spare so as the ends corrode, you can trim them back over the years.
