Meteorological summer has begun in the Northern Hemisphere, but what is happening right now in the arctic could dramatically affect the weather you confront come December.

This past winter was the warmest in U.S. history whereas eastern Europe was stuck in a deadly deep freeze with snow piled up to the rooftops. The winter before, however, it was the U.S. that got clobbered. What's going on? What will happen this year?

We may finally have some answers.

A new analysis published today in Oceanography by atmospheric scientists Charles Greene and Bruce Monger at Cornell University traces the severity of winter to the extent of summer melting of arctic sea ice. An increase in melting is causing fundamental changes in the jet stream—the primary pressure gradient in the atmosphere that most affects winter weather in the middle latitudes across the Northern Hemisphere. Their conclusion: winter weirdness may become the norm.

Sea ice reflects sunlight. But as more of it melts, more ocean water is exposed. The water, much darker than ice, absorbs more of the sun’s heat and warms, which in turn melts even more of the ice, setting up a feedback loop. In the autumn the ocean releases the excess heat into the atmosphere. That decreases the difference (or gradient) in temperature between the arctic and middle latitudes, which in turn lessens the difference between the air-pressure fields in those regions. The pressure field from 70 degrees north latitude up to the North Pole is called the Arctic Oscillation. The pressure field from 70 degrees down to the subtropics is called the North Atlantic Oscillation.

Lessening the difference between the two oscillations, Greene says, alters how the jet stream behaves in winter—either socking us with extreme cold and snow from the arctic or allowing tropical heat to waft much farther north than usual. The new behavior, he says, can explain record European cold and the record U.S. warmth last winter, the "snowmageddon" that buried Washington, D.C., in 2010, and what is likely to happen this coming winter.

Cold European winter, 2011–12: The jet stream flows west to east across the northern latitudes of the U.S., Europe and Asia. It rarely looks like a straight horizontal line, however. As we often see on a typical TV weather forecast, one portion of the line usually bends gently southward and then bends gently northward again, looking somewhat like a sine wave as it circles Earth. Lessening the pressure gradient between the Arctic Oscillation and the North Atlantic Oscillation, however, lets arctic air drift southward into the jet stream. But here's the clincher: When the pressure gradient is weaker, the jet stream slows. That allows the big bends to reach even farther south and farther north than usual, and tends to lock in those extreme positions for more days than usual. Those circumstances put eastern Europe into the deep freeze.

Warm U.S. winter, 2011–12: So why did the U.S. see record warmth at the same time? Remember the sine wave. Over the eastern half of the U.S., the jet stream bent far north and stayed that way for a long time, allowing warm air from the subtropics to drift up to the Canadian border and remain there. This pattern was reinforced by a La Niña pressure pattern in the Pacific Ocean, which tends to turn the jet stream northward over the eastern U.S.

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."

The odds for a cold Northeast winter beginning this December got even higher on June 7, when the National Weather Service's Climate Prediction Center put the world on an El Niño watch for the season.