The University of Arizona

Weird Winter Weather

February 23, 2011
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February’s cold snap in Tucson left many sad-looking prickly-pear cacti and thousands of burst pipes across the city. The anomalous cold temperatures we experienced in Tucson extended across the Southwest, with dire consequences. Frozen pipelines cut off the natural gas supply to 19,000 Arizona customers and 32,000 New Mexico customers, causing NM Gov. Martinez to declare a state of emergency on Friday. And temperatures were far below normal as far east as Houston, which also experienced an ice storm.

As they were shivering, I had at least ten people ask me “Is this the end of global warming?” Short answer: no. If these two days of anomalously cold temperatures occurred numerous times throughout the winter, for every winter for the next several decades, and spread beyond the Southwest to include much of the world, then maybe we could start to entertain that question. But two days of cold weather in one small region just can’t negate over a century of continuously warming global average temperatures.

But, let’s explore why this cold snap happened. The winter weather has been weird elsewhere in the Northern Hemisphere, too. If you’ll recall, the news has also been filled with stories about the wet and cold Northeast U.S. And as we’ve been freezing in the Southwest, the Arctic has been experiencing warmer than average temperatures and a record monthly low extent of sea ice, according to the National Snow and Ice Data Center.

All of this weird weather in the U.S. is due to a distorted jet stream—instead of running nice and straight across North America, separating cold, arctic air to the north from warmer, midlatitude air to the south, it dipped down into the Southwest. This dip acted as a chute, sliding cold, arctic air into the Southwest. The jet then shot back up on the other side of the Rockies, and allowed warm moist air to move up into the Northeast, leading to lots of snowfall and the warmer temperatures in the Arctic.

These wobbles in the Northern Hemisphere jet stream are defined by the Arctic Oscillation, or AO for short. A negative AO indicates a wobbly jet stream that is shifted south, whereas a positive AO indicates a jet stream that stays north and flows in a relatively straight line. The last two winters have had strongly negative AO values—the monthly AO value for February, 2010 was the lowest recorded since 1950! (see data table here) And this past January and December also had strong negative average AO values.

The Arctic Oscillation (AO) defines the path of the jet stream, and thus is a major influence on the location of winter storms. NSIDC image.

Scientists have been trying to understand variability in the AO for the last few decades. Although there was a period of time from the late 1970s to the early 1990s when AO index values seemed to be rising, that trend hasn’t continued to present day. Yet global climate models forced with greenhouse gases do show a trend toward more positive AO values with increasing greenhouse gas concentrations (Yin, 2005; Miller et al., 2006). These model results seem to be due to the warming and poleward expansion of tropical air, which pushes the jet north.

However, the AO is starting to behave in a way we weren’t expecting, and melting Arctic sea ice may be the reason why.

Scientists speculate that less sea ice in the Arctic is causing the Arctic atmosphere to warm up, weakening the strong polar-midlatitude temperature gradient that keeps the jet stream in place. As sea ice retreats, warmer ocean water is exposed to the air, and releases its heat to the atmosphere. Ocean water is also darker than white ice, and absorbs more heat, thus less sea ice means more incoming solar radiation is absorbed rather than reflected. So an initial increase in temperatures from increased greenhouse gas concentrations results in less sea ice, leading to even warmer temperatures. This is a feedback process, and is one of the reasons why the Arctic has warmed so much more than the rest of the planet over the last century.

Now scientists think these warmer Arctic temperatures are also weakening the temperature gradient from the pole to the midlatitudes, making the jet wiggle and causing our crazy weather (Francis et al., 2009; Overland and Wang, 2010).

To sum up, even though we have a good handle on how global temperature has changed over the last century with increased greenhouse gas concentrations, scientists continue to be surprised about the impacts of temperature change on aspects of the climate system, like the jet stream.  As we continue to pump greenhouse gases into the atmosphere, performing the biggest experiment ever, the climate will continue to change, and new insights will be the norm. Stay tuned for more.