Spring 2012 vs. 2013

A lot of attention this spring in terms of weather and climate has been focused on the relatively cold weather observed in the central and southern United States.  This pattern started in February and has persisted right up to mid-April, with a broad region experiencing below to well-below normal temperatures and uncharacteristically heavy snowfall for this late in the season, especially when compared to previous years.  The map on the right shows the temperature departure from normal for March 2013.  Areas in green, blue, and purple reported below normal temperatures (most of the central and eastern U.S. with the exception of New England).  Some areas of the northern Plains were over 10 degrees below normal for the month.  Areas of the southwest and New England were slightly above normal for the month.

The current spate of cold weather can be partially attributed to the current phase of the North Atlantic Oscillation, or NAO.  The NAO is technically the difference in sea-level pressure between the Icelandic Low and Azores High (two semi-permanent weather systems over the North Atlantic).  When these two systems are relatively strong, the difference in pressure between the two is greater, and this pressure differential generates a strong west-to-east jet stream across the Atlantic (see image below-left, courtesy of NC State University).  When this jet is stronger than normal, it generally keeps cold Arctic air over or near the polar regions; this is called the positive phase of the NAO.  When the two systems are relatively weak, the pressure differential decreases, and in turn, so does the strength of the west-to-east jet stream.  This allows cold Arctic air to move southward into the United States and Europe, and is called the negative phase of the NAO.  The image below-right (NOAA-CDC) shows the state of the NAO so far for 2013.  Since February, the NAO has been in its negative phase, which fits with the colder than normal temperatures observed over the U.S. and Europe.  The red lines are model predictions for the NAO over the next few weeks, and show a dramatic change to the positive phase, so temperatures may finally warm up for the central and eastern U.S. later this month.

It was a cold March for much of the lower 48 to be sure, but it was far from record breaking cold compared to the historical record.  So, any claim that this colder weather is evidence against the existence of climate change is severely in error.  In fact, one need only look back to March 2012 when temperatures were indeed record breaking - but were much, much above normal.  The map on the right shows temperature departures from normal for March 2012.  Note the huge area of above normal readings in the same regions that currently are running below normal.  Much of the Midwest was 10 to even 15 degrees above normal for the month, which easily made March 2012 the warmest March on record for the U.S.  The NAO was in its positive phase at that time, which may explain a portion of the warmer weather.  However, the unparalleled warmth of March 2012 likely had some contribution from human forced climate change, as I talked about in a post last year.  This type of extreme weather is expected with climate change - record warm temperatures will become more common and more extreme (like March 2012), and while periods of below normal temperatures will still occur (like March 2013), they will be less common and less pronounced.