Here’s an explanation for the unusually cold weather in parts of North America and Europe lately:
The folks who run the National Center for Atmospheric Research have a great rundown of the details of the AO Oscillation. In short, high pressure in the Arctic forces the jet stream south, and it drags cold air with it, chilling North American and northern Eurasia. In its opposite mode, those same regions tend to be much warmer. Right now, we’re in such an extreme high-pressure event that the readings have run off the scale of NOAA’s AO index. Fortunately for those hoping to warm up a bit, the AO is a weather event—it often changes states multiple times within a single season, and there’s no clear evidence linking its behavior to climate trends.
(Hat tip: Sera).