The Winter of 2011-2012: Why was it so Warm?
By Matthew Crepon on March 18, 2012, 9:17am Last modified: January 7, 2013, 5:35pm
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As we approach the Vernal Equinox, I thought it would be a good time to go back and look at this past "winter" and examine some of the reasons it was so mild. Notice I put "winter" in quotations because the majority of the country, particularly the northern Plains and eastern part of the country, escaped relatively unscathed. This of course was a far cry from last winter, when many East Coast cities were buried with snow, including Bradley Airport's all-time snowiest month on record when a whopping 57" fell!
However, this winter sung a different tune. So you might be wondering, what changed in the weather pattern to produce such a dramatic shift between last year's snowy winter and this year's warm and much less snowy winter? There are a number of factors but I will focus on three of the most important to winter weather across eastern North America. These being the ENSO (El Nino Southern Oscillation), NAO (North Atlantic Oscillation) and EPO (Eastern Pacific Oscillation). ENSO refers to the oscillation of sea-surface temperature anomalies across the equatorial Pacific Ocean. When in its negative phase, temperatures are cooler than normal. We call this La Nina and it has implications on the overall weather pattern across the United States.
Interestingly enough, this past winter did not behave the way a typical La Nina would. This was very noticeable across the northern Plains where below normal temperatures are usually found during La Nina, we saw some of the largest positive departures in the U.S. This is where the other two oscillations come in to play. It's important to note that these indices, whether negative or positive, do not change very readily. Thus, long-range weather forecasters use these as tools to evaluate how a weather pattern will evolve for weeks or even months.
The EPO and NAO also have positive and negative phases. When both are negative, we see some of our coldest outbreaks of arctic air, particularly in the East. Given this fact, what phase do you suppose they were in during the Winter of '11-'12? You guessed it, positive!
This map is from the National Center of Environmental Prediction and it shows what are called height anomalies of the 500 millibar pressure surface. Typically, low height anomalies are associated with colder than normal temperatures and vice versa. This is because the height of the pressure surface is proportional to the temperature of the air column. As you can see, the continental United States had generally above normal heights (indicative of warmer than normal temperatures), especially in our neck of the woods.
This is thanks to the low height anomalies on the top left corner of the map across Alaska, a classic positive EPO pattern. Since flow around low pressure is counter-clockwise, air being moved via this circulation is traversing the ocean. As a result, this relatively mild Pacific air floods Canada, preventing cold air masses from building. The other piece to the puzzle is the North Atlantic Oscillation, or NAO. In its truest sense, the NAO is a measure of pressure differences between the Azores and Iceland. In its positive phase, as we saw this past winter, low pressure sets up over Iceland with high pressure over the Azores. This in effect bottles up the cold air across Iceland and Greenland, as shown on the map.
So there you have it, a look at some of the reasons the Winter of 2011-2012 was exceptionally mild. The gist of it is, when you see a persistent area of lower than normal heights across Alaska, chances are the rest of the United States will not see much in the way of wintry weather. This in tandem with a positive North Atlantic Oscillation gave our shovels and thermostats a much needed rest.
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