***Update (11/12/2011): A version of this post is also available at the Earth Institute’s State of the Planet blog.  In a sad connection, the above post was also discussed at the State of the Planet blog in an ode to a beautiful tree on our campus that was one of the victims of the October 29th storm.

In the aftermath of the snow storm that hit the Eastern US this last Saturday (October 29th, 2011), it is worth revisiting the question of what caused the extreme winters of 2009-10 and 2010-11 in this region, how they stack up historically, and whether we should expect more.  We already reported on our paper describing the 2009-10 winter and how it measured up against winters over the last 600 years.  In a nutshell, that winter was attributed to a combination of an El Niño event in the equatorial Pacific and a strong and persistent negative phase of the North Atlantic Oscillation (NAO).  Note that the combined strength of these two phenomena during the 2009-10 winter turned out to trump everything since the 1400’s, making it truly anomalous against a long history of events.  But that was 2009-10.  What about 2010-11 and this coming winter?

The most important dynamical feature of the climate system regarding extreme winters in the Eastern US is the behavior of the NAO.  During negative phases of the NAO, the Eastern US will experience unusually snowy and cold conditions during the winter.  So the first thing to point out is that the 2010-11 winter, like 2009-10, was also marked by strong negative NAO anomalies for much of the season.  In fact, my colleagues and I have begun a preliminary analysis of the 2010-11 winter (D’Arrigo et al., in prep), and found that the NAO index showed the most negative Nov-Dec average dating back to the onset of instrumental observations in 1825 CE (Figure 1).  Note that the average NAO during the entire 2010-11 winter was only moderately negative, but the early part was indeed a whopper.  Not only was the Nov-Dec average unusual in the instrumental record, a monthly NAO paleoclimate reconstruction shows that the early-winter average was one of the most negative since 1658 CE (Figure 2).

The other interesting part of the story is that last year’s negative Nov-Dec NAO was joined by moderate-to-strong La Niña conditions in the tropical Pacific.  This is in contrast to the 2009-10 winter, during which a moderate El Niño occurred.  This difference contributed to a few contrasts between the two winters.  Firstly, the La Niña moved the storm tracks farther north than normal, thus bringing the extreme snowfall anomalies farther north during the 2010-11 winter (they were farther south in 2009-10).  La Niña also brings dry and warm conditions to much of the Southern US, and likely moderated the effects of the NAO in some regions by making them less cold than they otherwise would have been.  This is at least the general thinking currently, and leaves open a lot of interesting questions about how ENSO and the NAO combine to leave fingerprints of extreme winter conditions on the Eastern US, how they have done this in the past, and what they might do moving forward into the present century as the climate becomes warmer.

The last question is what we can expect for this coming winter.  We already know that this winter will include another La Niña.  The NAO is a little more tricky.  We can’t predict the oscillations in the NAO very well, but we do know it goes through persistent positive and negative phases.  The last time it was persistently negative was during the 1960’s and early 70’s.  At that time, there were multiple periods in which 3-5 years of persistent negative NAOs were observed.  Multiple years of persistently negative NAOs are therefore possible.  If it turns out that 2011-12 is another significantly negative NAO period, it would set the stage for three extreme winters in a row in the Eastern US.  We will have to wait and see, but last Saturday’s storm could be just the beginning.

Reference:

D’Arrigo, R., J. E. Smerdon, R. Seager and J. Luterbacher, The back-to-back extreme NAO-ENSO winters: 2009-10 and 2010-11, in preparation.