In the first few weeks of 2012, we have certainly seen some strange occurrences throughout the U.S.: Snowmageddon and drenching rains for the Pacific Northwest, wildfires in Nevada and Texas, high temperatures across the central and Southern U.S. ranging between 15˚and 20˚F or more above normal, and snow amounts in Alaska as much as 160 inches above normal for this season.
All this makes me think Mother Nature is trying to take the "Extreme Weather of 2011" and rebrand it as the "Extreme Weather of 2012." Chicago saw one of the slowest starts to its snow season in the past 30 years, and the Northeast hasn’t really seen much snow at all, or even winter weather for that matter.
What in the world could be contributing to this unusually mild winter? There may be a few factors we have to consider: La Nina and the Arctic Oscillation.
First, let’s talk about the effects of La Nina, which NASA so affectionately referred to as “the diva of drought.” On January 18, NASA and the Jet Propulsion Laboratory released sea surface height and temperature data collected from their Jason-1 and -2 satellites. These are altimetric satellites, meaning they can detect sea-level heights and thus determine the temperatures of the water. How can sea surface heights tell us what the water temperatures are? Think about how air expands when it heats up. Water does the same thing, so when these satellites see higher sea levels, it means those waters are warmer.
Take a look at the satellite image above. As a word of caution, as cool as these images may appear, what they tell us may not be as exciting. The reds and yellows indicate warmer waters, and the blues and purples indicate cooler waters. Notice the blob of blues in the eastern Pacific? Those are the cooler waters that contribute to La Nina, and they’ve gotten slightly cooler, meaning La Nina has strengthened.
This La Nina was a more moderate event, but now that it has strengthened, that could mean continued stormy conditions across the Northwest this winter and spring. This could also lead to worsening wildfire conditions over the Southwest and Southern Plains, and scientists are now expressing concerns about conditions across Southern California and parts of the Southwest. The Colorado River basin has seen more dry years than wet years over the past decade, and Southern California has seen only two normal years of rainfall in the past six years. Experts fear that these conditions could lead to low water supplies if they do not see some improvements soon.
Although we know what La Nina can do to our weather patterns, there are other factors we have to consider. One particular climatological index that is affecting our weather is called the Arctic Oscillation. This is an index based on the differences in sea level pressure and the relative intensity of a semipermanent low-pressure area that is centered over the North Pole.
When this index is in a positive phase, that means the vortex is very strong and the upper-level winds restrict the arctic air to the North Pole, keeping it far away from North America, Europe and Asia. When the oscillation is in a negative phase, the circulation of this polar vortex is much weaker and allows the arctic air to spill into those same regions. It has been noted that this oscillation has been positive for the most part since the 1980s, but it has become slightly more positive over recent decades, to which scientists contribute the warmer conditions across much of the Northern Hemisphere.
In addition to this Arctic Oscillation, there is another climatological index called the North Atlantic Oscillation, which was actually responsible for the brutal winter of 2011 for the Eastern and Southeastern U.S. The North Atlantic Oscillation is an index representing differences in pressure across Iceland and the Azores. When this index is in a positive phase, it forces the colder arctic air to stay far to the north in Canada, but when it is negative, as it was through winter 2010-11, it allows that arctic air to dive into the Southeastern US.
This index is known to override the effects of La Nina, and unlike El Nino and La Nina, which change on the scale of months, the North Atlantic Oscillation can change phases within a matter of one or two weeks. Compare this with the current atmospheric conditions and how warm it has been throughout the eastern U.S. The oscillations have been in a positive phase, keeping the arctic air farther north, but one quick shift of the North Atlantic Oscillation, and we may not be so excited about the snow and bitter cold it could bring!
The Arctic Oscillation changes more gradually than its North Atlantic counterpart, and because the latter changes on a much shorter timescale, it is more difficult to predict. Current forecasts suggest that the North Atlantic Oscillation may become a little less positive than it is today, and that may cause some noticeable changes for the Southeast. Needless to say, some of us may have been enjoying the very mild conditions we’ve seen in 2012, thanks in part to La Nina and the oscillations. However, remember that everything comes with a price. Something tells me I should start saving for a snowy day ...