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Once again, strange US winter weather partly due to Arctic Oscillation

Katherine Leitzell
US Department of Agriculture photo

Last year, many scientists blamed the winter storms that blasted the Northeastern United States and Europe on the negative mode of a weather pattern called the Arctic Oscillation. This winter, the Arctic Oscillation started out in the opposite mode, which scientists connect to the warmer-than-average temperatures and unusually low snowfall over much of the U.S. The swings of the Arctic Oscillation also help control how sea ice moves in the Arctic Ocean, which is of great interest to climate scientists. Readers often write in to ask us about this powerful but mysterious climate phenomenon, and how it affects weather where they live. What is the Arctic Oscillation, and how does it affect Arctic sea ice and the rest of the Northern Hemisphere?

What is the AO?

Like El Niño and La Niña, the Arctic Oscillation (AO) is a big picture of atmospheric conditions that influence weather. The AO, which ranges between two distinct modes, describes how pressure patterns are distributed over the Arctic region and the middle latitudes of the Northern Hemisphere. NSIDC director Mark Serreze, an expert on Arctic climate and weather, said, “When the Arctic Oscillation flips from one mode to another, that represents a fundamental change in the circulation of the atmosphere, the way the winds blow.” The AO can persist in one phase from anywhere from days to months. When pressure is higher than normal over the Arctic, and lower than normal over middle latitudes, the AO is in its negative mode. When it is positive, air pressure is lower than normal over the Arctic and higher than normal over middle latitudes. Researchers look to the AO to better understand year-to-year variability in climate indicators like the Arctic sea ice cover.

AO and weather

The Arctic Oscillation also has big impacts on weather in the Arctic and beyond. Serreze said, “When the Arctic Oscillation is in its positive phase, the jet stream, which brings us much of our weather in middle latitudes, tends to shift to the north.” That means that in the positive phase of the AO, winter storms also shift north, leaving most of the U.S. warmer and drier than average. The positive AO helps explain why there were fewer snowstorms than usual in the first part of this winter. Serreze said, “The weather that we’ve had this winter, at least out here in Colorado, has been very consistent with a positive Arctic Oscillation.”

When the AO is negative, the jet stream moves south, bringing winter weather with it. Last winter, for example, the strong winter storms that hit the Eastern United States and Europe were influenced in part by a strongly negative phase of the AO. NSIDC scientist Walt Meier noted that the AO is not a sole actor—it works in concert with other large-scale patterns to influence the weather. He said, “The heavy snows the last couple years were related to the AO as well as La Niña.”

The Arctic Oscillation (AO) is a large-scale climate pattern that influences weather throughout the Northern Hemisphere. It alternates between a positive phase (left) and a negative phase (right). In its positive phase the AO tends to bring warmer weather to the middle latitudes, while in its negative phase, winter storms are more common in the Eastern United States and Europe. Credit: J. Wallace, University of Washington.

AO and sea ice

The impacts of AO on North American weather can affect our daily lives, but climate scientists want to know how the AO influences sea ice conditions in the Arctic Ocean. While over the long term, sea ice extent has been declining, during any particular winter extent can vary due to weather conditions. Meier said, “The Arctic Oscillation primarily affects sea ice through winds that cause changes in where the sea ice drifts.” When the Arctic Oscillation is in its negative mode, he said, the winds and ice tend to flow in a clockwise direction, generally keeping more of the older, thicker ice in the middle of the Arctic. In the positive phase, that old ice tends to get pushed out of the Arctic along the Greenland coast. Meier said, “This means that the sea ice tends to be younger and thinner and more prone to melt after a winter with a strong positive Arctic Oscillation.” The AO is just one of many weather wildcards that could spell the difference between a low sea ice year and a record low year. See the Icelights article, Climate Change or Variability: What Rules Sea Ice?

In the last few weeks, the AO switched from its positive phase back into a negative mode. Will that continue? Only time will tell. For more about how the AO is affecting sea ice this winter, read the latest update to Arctic Sea Ice News & Analysis.

For more information on the AO, read  Arctic Climatology and Meteorology: The Arctic Oscillation. To see the current graph of the AO phase, visit the NOAA National Weather Service Climate Prediction Center Web page.

This article originally appeared in the Icelights blog, produced by the National Snow and Ice Data Center, and is republished with permission.