The winter of 2009-2010: Could it have been colder?
Global warming may have tamed fierce winter
By Joe Casola, Special to Capital Weather Gang
This is the latest in a series of feature articles revisiting the historic winter of 2009-2010 leading up to Thursday's release of the 2010-2011 winter outlook.
The 2009-2010 winter in the DC metro area, and more generally the Southeast, was cold. To accompany our record-breaking snowfall, we experienced a relatively chilly winter, a rarity during the last few decades. However, some recent research suggests that our region, and the Northern Hemisphere as a whole, was actually "lucky" - it could have been substantially colder.
Previous CWG posts have shown that our frigid and snowy winter was produced by some relatively large and persistent wiggles in the atmospheric circulation. During most of the winter, the air that normally resides over the polar regions across the Northern Hemisphere migrated equatorward, invading our mild latitudes. The configuration of ridges and troughs that permitted this flow was largely a product of a strong El Niño and the negative phase of the Arctic Oscillation.
To complement the cold air, the jet stream and storm track were displaced to the south of their normal locations, improving the chances that impending storms could follow an ideal "snow track," and approach from our south/southeast where they can benefit from the Gulf Stream's moisture. While not a guarantee for cold and snow, the occurrence of an El Niño or the negative phase of the Arctic Oscillation can help tilt the odds to a colder or snowier winter. In our case, we experienced a "double-whammy."
Huug van den Dool of the National Weather Service's Climate Prediction Center has used a statistical technique called "constructed analogs" to examine similar occurrences of such a "double-whammy" circulation over the period 1961-1990.
Analogs of this circulation are generated by applying a statistical algorithm to historical records of the upper-level atmospheric flow (specifically, the 500-hPa stream function). Interestingly, comparison of the observed 2009-2010 winter to the analogs that have been constructed from the earlier period show a significant difference in temperature for our region and across much of the Northern Hemisphere.
In other words, if the atmospheric circulation that occurred during this past winter had occurred sometime during the 1960s-1990s, then it would have actually been colder by roughly 1°C-2°C (1.8°F-3.6°F).
When averaged for the Northern Hemisphere, this result was also true for the other winters between 1991 and 2010 - the observed winter conditions of the recent two decades were warmer than the analogs constructed from the 1961-1990 historical data by an average of 0.5°C (0.9°F).
So why were we "spared" from a colder winter?
By itself, the constructed analogs technique does not necessarily point to a particular reason or mechanism as to why the 2009-2010 winter was warmer than the analogs drawn from the historical data.
It's possible that our "luck" during the recent winter resulted from some combination of global warming and the long-term, decade-to-decade variability in the Atlantic Ocean. Although it can be difficult to tease apart these factors (and such work is an active area of research), this analysis provides evidence of some important differences between the winters of today and those occurring during the mid-20th century.
Joe Casola works as a consultant, helping natural resource managers use weather and climate information as part of their decision-making process. He holds a B.S. in Chemistry from Duke University. After discovering he was a klutz when mixing chemicals in the lab, Joe decided to follow his passion for looking at clouds and earned a PhD in Atmospheric Sciences from the University of Washington. He has also worked on weather and climate issues at the National Research Council.
| November 3, 2010; 12:30 PM ET
Categories: Climate Change, Winter Storms
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