Global warming ;D ;D ;D ;D
Seriously, according to the Met Office (whom I believe to be a completely trustworthy lot), the current cold spell is being caused by the warming of the southern hemisphere's strataspheric conditions. This has also resulted in the hottest day ever in Australia today - 46 degrees centigrade.
I think its a good example of how the worlds varying temperatures interact with each other to give a colder location in one part of the world caused by a warmer one elsewhere in the world. Interesting, no?