Interesting post.

It looks like having two sudden stratospheric warming events in a winter is kind of rare (about once every 10 years or so, on average).  So, questions:

* What did other years with two SSWs have in common?  That is, were there some atmospheric factors that happened during most or all of them that did not happen when there were not two? 

* Does this year have any of those in common with those?

Thanks for reading this, and I look forward to hearing from you.