Open the fridge door

How does a warming Arctic cause winter cold in Europe and Northern America? Or, in other words, if the globe is warming, why the cold freeze?

It's because the Arctic refrigerator door has been opened and the cold air spilled out at the bottom, according to John Cook of Skeptical Science.

The refrigerator door is the jet stream that marks the differences in atmospheric pressure in higher latitudes (20N) compared with mid-latitudes (37-45N). This difference in pressure is measured by the Arctic oscillation (AO)  index or Northern Annular Mode/Northern Hemisphere Annular Mode (NAM).

NASA climatologist Dr. James E. Hansen explains the mechanism by which the AO affects weather at points far from the Arctic:

The degree to which Arctic air penetrates into middle latitudes is related to the AO index, which is defined by surface atmospheric pressure patterns. When the AO index is positive, surface pressure is low in the polar region. This helps the middle latitude jet stream to blow strongly and consistently from west to east, thus keeping cold Arctic air locked in the polar region. When the AO index is negative, there tends to be high pressure in the polar region, weaker zonal winds, and greater movement of frigid polar air into middle latitudes.

So warmer temperatures in the Arctic mess with the air pressure and this tends to make the refrigerator door swing open. Sometimes it just chinks open a little bit, other times it's like a teenager came to raid the fridge. That's when Europe really freezes!