Why Was January So Warm?

Who can forget a year ago, when temperatures in the Southwest plunged well below freezing, bursting water pipes and devastating local vegetation (NOAA)? A little less than a year later in January, temperatures were well above average, not only in the Southwest but all over the contiguous U.S., where temperatures were the fourth warmest on record and the warmest since 2006. All of Arizona was at least 3 degrees F above normal for the month, and the highest positive departures were in the Rocky Mountains, with a large section of the Colorado Plateau experiencing temperatures as high as 6 degrees F above average (NOAA, CLIMAS). What’s going on? During both last winter and this winter there were La Niña conditions in the tropical Pacific, so why were the two winters so different? Why was this January so warm?

Welcome back our old friend the Arctic Oscillation (AO)—the main culprit of the extreme cold a year ago (see previous blog)—and its partner the North Atlantic Oscillation (NAO).The AO is a measure of the state of the atmospheric circulation over the Arctic. Last year around this time (and the winter before that as well) the AO was in a strong negative phase, meaning there was a high pressure system above the Arctic, and a cold, low pressure system over the U.S.  In contrast, this winter the AO has been in a strong positive phase (see data table of AO daily index values here), where strong winds have kept cold air over the Arctic in a low pressure system, leaving the U.S. abnormally warm.

The NAO also has a positive and negative phase, and as Jeff Masters so succinctly put in his WunderBlog, “can be thought of as the North Atlantic’s portion of the larger-scale AO”. The AO and NAO are two separate indices, but together they describe pressure differences in the northern latitudes and their ultimate effects on temperature and storm tracks throughout the region. This winter, the NAO has also been positive. In fact, December’s NAO index value was the most positive it has been during that month since records began in 1950 (see table). The positive AO and NAO were enough to overpower the effects of ENSO (El Niño and La Niña), keeping the jet stream and cold Arctic air further north and resulting in abnormally warm conditions in the Southwest during December and January. In mid-January, the AO became negative and remained strongly negative for the remainder of the month. You may be wondering why January was still so warm. The answer is because the NAO remained positive, keeping colder air to the north (see table).

So what can we expect to see in the next few weeks? Well, the AO returned to a positive phase in mid-February and is predicted to remain positive for at least the next 2 weeks, and the NAO is also projected to remain slightly positive for the next couple of weeks. With this in mind, it’s no wonder that the Climate Prediction Center from NOAA projects temperatures for the majority of the contiguous U.S., including the Southwest, to be well above average through at least March (Figure 1).

What can we expect to see in the next few decades? Jessica Conroy discussed in her blog last year how warmer temperatures and loss of sea ice in the Arctic due to increasing greenhouse gas concentrations have the potential to cause a more “wiggly” jet stream and bring cold Arctic air south to the U.S., resulting in more cold winters (also see February 2011 Climate Outlook from CLIMAS). Does this warm winter we are experiencing then discount this hypothesis? Not necessarily. One warm winter and positive AO doesn’t mean there isn’t an overall trend towards colder winters, just as one relatively cold year doesn’t mean the trend in global temperatures isn’t warming. The trend is what matters, and right now there just isn’t enough data to determine what will happen to the AO in the future. I guess we’ll just have to keep watching.