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.