Jennifer Francis: "Understanding the Jetstream".

The extreme weather trend in the Northern Hemisphere is  recent, so the science that can explain what is happening is still tentative. The first hypothesis blamed a slowing of the northern hemisphere’s polar jet stream. That sounded plausible, and was published in 2012 in Geophysical Letters. The paper “Evidence linking Arctic amplification to extreme weather in mid-latitudes”, was written by Jennifer Francis of Rutgers University and Stephen Vavrus of the University of Wisconsin-Madison. I think that their theory will turn out to be right. That is not good news....

Jennifer Francis - Understanding the Jetstream

The fact is that the Arctic has been warming faster than anywhere else on Earth, and the difference in temperature between the Arctic and the temperate zone has been shrinking. Since that difference in temperature is what drives the jet stream, a lower difference means a slower jet stream.

A fast jet stream travels in a straight line around the planet from west to east, just like a mountain stream goes  straight downhill. A slower jet stream, however, meanders like a river crossing a plain. The loops it makes extend much further south and north than when it was moving fast.

In a big southerly loop, you will have Arctic air much further south than usual, while there will be relatively warm air from the temperate air mass in a northerly loop that extends up into the Arctic. Moreover, the slower-moving jet stream tends to get “stuck”, so that a given kind of weather—snow or rain or heat—will stay longer over the same area.

Hence the “polar-vortex” winter in North America this year, the record snowfalls in Japan in 2012 and again this winter, the lethal heat waves in the eastern U.S. in 2012—and the floods in Britain this winter.

“They’ve been pummelled by storm after storm this winter [in Britain],” said Francis at the American Association for the Advancement of Science conference in Chicago last week. “It’s been amazing what’s going on, and it’s because the pattern this winter has been stuck in one place ever since early December.” There’s no particular reason to think that it will move on soon, either.