Snowy U.S. winter, 2010–11: The year before, however, the weaker gradient between the Arctic Oscillation and the North Atlantic Oscillation caused the jet stream to dip deeply across the eastern U.S. and stay there. It also so happened that an El Niño event occurred in the Pacific, which tends to push the jet stream north across the western U.S., such that it bends southward across the Northeast, pulling down cold arctic air with it.
The lesson in all this is that the more that arctic sea ice melts in summer, the more extreme the jet stream's bends will become and the longer they will stay in place, making our winters either colder or warmer than usual. "The arctic climate system is changing so dynamically that the rules of the game are changing," Greene says. "This is not the same Arctic Ocean we've known. The Arctic and North Atlantic oscillations are changing in ways we hadn't anticipated." The interplay, which has always been fairly consistent, he says, has now become "a wild card" affecting our weather.
As a result, Greene says, an "average" winter will become less likely. Brace yourselves for extremely cold or extremely warm winters. Although climate scientists cannot predict in detail how the oscillations will behave beyond two weeks into the future, if arctic sea ice continues to melt extensively each summer, Greene says, "I would be willing to say that we should anticipate more outbreaks of cold arctic air and snow hitting the eastern seaboard [of the U.S.]. You couldn't bet on a specific year being that way, but if you bet over 10 years, you would probably make money."