Graph of the Day: Arctic Oscillation and the Winter of Global Weirding
February 8, 2012
Weirdly warm and snowless in the US. Brutally cold and icy in Europe. What’s going on?
Note in the satellite map above, how cold air is shifted out of the arctic and on to the European land mass – while large polar areas are warmer than usual.
The weather pattern responsible for bringing the frigid air to Europe and Eurasia, and locking it in place, is being driven in part by a naturally-occurring pattern of climate variability known as the Arctic Oscillation. The Arctic Oscillation, or AO, is is a climate index that describes the characteristics of the atmospheric circulation over the Arctic, and a related index describes the circulation over the North Atlantic. Depending on whether it’s in a “positive” or “negative” phase, the Arctic Oscillation can bring warmer or cooler than average wintertime conditions to the U.S. and Europe.
Right now the Arctic Oscillation is in a negative phase, which tends to favor colder than average weather in Europe and the U.S. Scientists don’t fully understand what causes the Arctic Oscillation to switch from one phase to the other, which limits their ability to forecast these changes ahead of time beyond a week in advance.
Stu Ostro, a senior meteorologist at The Weather Channel who has been keeping close tabs on trends in weather and climate extremes in recent years, said the frigid weather in Europe is another in a series of weird winters that have been related to the Arctic Oscillation.
“It’s interesting that the winter started out the opposite of the previous two, with an exceptionally positive Arctic Oscillation and non-blocking pattern, and then it flipped,” he said via email.
“There has been extreme variability of the Arctic Oscillation in recent years, including a record negative monthly value for December in 2009 (the month of the so-called “Snowpocalypse” in the northeast U.S.), a record negative for any month in February 2010 (“Snowmageddon”), and a record positive one for April in 2011 (coinciding with the extraordinary number of tornadoes in the U.S. that month),” Ostro said. “And this has been occurring in the context of theprecipitous decline of Arctic sea ice volume.”
Also noteworthy is a very strong and persistent high pressure area that has been sprawled across Russia. The airflow around this stubborn high has been transporting Siberian air into Western Europe, as the maps below show. (Jeff Masters of WeatherUnderground discusses this in more detail in a blog post today.)
Jeff Knight of the U.K. Met Office also pointed to the importance of that high pressure area. “The current cold weather across Europe relates to the development of a large ‘blocking’ anticyclone over Scandinavia and north-western Russia. Easterly flow on the southern edge of this system has transported cold continental air westwards, displacing the more usual mild westerly influence from the Atlantic Ocean all the way to the British Isles,” he stated via email.
“One factor that may well have contributed to the onset of blocking this time is the weakening of the stratospheric jet. The high altitude (above 20 km) circumpolar winds weakened in early January. Research has shown that such weakening often produces a downward effect that subsequently impacts on the troposphere (the atmospheric layer below approximately 10 km where weather occurs). The result is weaker surface westerly winds (or even easterly winds) on average in the mid-latitude northern hemisphere – in other words a negative Arctic Oscillation.”Knight pointed to a cold snap in 2009 that seems to have had a stratospheric connection as well. Although only some cold European winter spells are the result of these stratospheric upheavals, other recent winters such as 2006 and 2010 have also shown clear examples of the effect.”As Ostro alluded to, in recent years there have been studies examining how the global warming-related loss of Arctic sea ice might affect winter weather patterns in the northern hemisphere. Some of this research shows that sea ice loss may favor winters with predominately negative phases of the Arctic Oscillation. One potential result of global warming, referred to as the “Arctic Paradox,” is that sea ice loss can help warm the Arctic during the winter, while setting in motion a chain reaction of events that make winters colder than they otherwise would be in Europe and the U.S..The weather pattern during the past two weeks bears some similarities to conditions last December, when the Arctic Oscillation was in an extremely negative phase, and the jet stream helped drive frigid air and winter storms into both Europe and the U.S. This year, though, only Europe and Eurasia have been unusually cold, as other factors have conspired to protect the Lower 48 from Old Man Winter’s wrath. At least for now, anyway, as there are signs this may change during the next few weeks.