The basic story, as I glean from this article, is that the Pacific is in a positive phase of what is being termed the "North Pacific Mode". The warmest waters make a big horseshoe pattern, opening toward Japan. This warm water area shifts the position of the jet stream.
That's part of the story. The other, more contested part of the story is that melting Arctic sea ice -- something the denier crowd keeps saying is either a) not happening or b) not important -- may be involved, too.
"That study, also detailed in Geophysical Research Letters, suggests that while the Pacific heat set the atmospheric pattern in motion, Arctic sea ice loss in a particular region made the warm/cold difference so extreme, said Jennifer Francis of Rutgers University." (Francis wasn't an author, she was just commenting).
Abstract (of the second paper)
"Unprecedented atmospheric circulations with extreme weather were observed in the extratropical Northern Hemisphere during the winter of 2013–2014. The anomalous circulations were the manifestation of the Pacific pattern or the North Pacific Oscillation/Western Pacific pattern but with extremely large amplitude. Simulation results suggest that the anomalous atmospheric circulations were constructively induced by anomalous sea surface temperature in the tropical Pacific and extratropical North Pacific, as well as the low sea ice concentration in the Arctic. Natural variability played a major role in inducing the anomaly pattern, whereas the anomalously warm sea surface temperature and low Arctic sea ice concentration in the Bering Sea contributed to the intensity. If the anthropogenic warming has a significant impact on causing the synchronization of the aforementioned anomalies in sea surface temperature and sea ice concentration and this trend continues, severe winters similar to that in 2013–2014 may occur more frequently in the future."
Here's the pattern that's being caused, we just need agreement on what the cause(s) are.
Average February 2015 temperatures.
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