The record breaking weather that defined half of the month of March, making it the warmest in the last 120 years is over. The first two weeks of April will be more seasonable and with the AO having attained a negative footing, we may actually see an occasional taste of winter. At this point however, most of us have mentally tuned out of winter either refocusing on NCAA basketball or perhaps gardening since the warmth has certainly caused many of those to spring to life.
In the pre-season extravaganza of 2010-2011 I devoted a chunk of time discussing the split personality of La Nina winters in Vermont. We had highlighted several winters going back over the last 50 years which had illustrated how La Nina can be either feast or famine in northern Vermont. This point was obviously underscored during the past two winters. 2010-2011, although we did miss a few storms, was generally a strong winter featuring consistent cold weather and deep snow persisting well into April. 2011-2012 by way of comparison was a complete abomination. I thought we might avoid the embarrassing distinction of having to live through the warmest November to March period in northern Vermont (we were actually slightly colder than 2001-2002 through February), but then we got a March heatwave and what amounted to some of the most anomalous weather ever recorded in Vermont from a temperature standpoint. By the end of the month, the winter of 2011-2012 as defined by the period beginning in November and ending in March was rather easily the warmest ever recorded. From a snowfall standpoint the winter was close to being one of the worst ever. For many ski areas, the combination of lack of snow and outbreaks of above-freezing temperatures made it the worst ski season ever. Only a handful of winters can rival this past winter for lack of snow. Mad River Glen recorded about 150 inches of snow, 100 less than average. 1990-91, 88-89, and 79-80 were all similar. The lack of snow in 1979-80 brought the region some notoriety with the 1980 Olympics being held in Lake Placid. I have some pictures somewhere of crews frantically spreading artificial snow in preparation for the various Nordic skiing events that year. This year would have been similar but in many ways worse because of the added effect of the invariable relative warmth.
The explanation behind the massive failure of the 2011-2012 can be boiled down to a few key points. For a La Nina winter to achieve some success in Vermont, it needs the support of a negative AO for key stretches of the winter. We got such support for about two weeks in late January and early February. The average daily AO index from November to March was roughly a positive 0.9. I would have to do some further analysis on this data but a rough glance indicates that the standard deviation of these results going back 100 years is probably about a 0.5 or so making this years number very statistically significant. The 2010-2011 winter was approximately negative 0.4 while the incredibly "blocked" winter of 2009-2010 was well below a negative 1 (My other weathergeek friend is currently telling me the 09-10 winter featured the most negative AO ever recorded over this period). We might have been able to endure the lack of high latitude blocking in the jet stream but repeatedly this was coupled with a very tight and vigorous jet stream across the mid-latitude Pacific Ocean. Though the jet tightening can be partially attributed to the positive AO there are other contributing factors. The more obvious would be the presence of La Nina since the tight Pacific Jet is one of its defining characteristics. The other factor I have heard mentioned is the MJO or Madden-Julian Oscillation. This is the term used to describe the cycle of convection in lower latitude regions ranging from the Indian Ocean to the Atlantic. It is complicated but phases of the MJO are determined by areas of enhanced and suppressed convection in these regions. In this particular winter we went through an extended period of time where it was difficult to determine a phase. Specifically there was a glaring lack of tropical convection in the Pacific and no mechanism to either divide or loosen the torrent of flow in the Pacific that largely killed our winter. This was very evident in the ensemble data where it seemed impossible to breakdown the prevailing ridge which dominated the lower latitudes of the Pacific.
Finally, it is inevitable that those in the field will be asked about "global warming", "climate change" or whatever other trendy term is being used to describe the warming global temperatures trend that seems to have matched the beginning of the industrial revolution. More specifically though is how to place this recent winter in the context of global warming. My answer and advice to this is to try and avoid it. Statistical noise and variability that is location specific, even spanning the course of a season, is insignificant in this conversation and proves very underwhelming when looking at trends that include the entire globe. Extreme weather events when spoken of individually and in one location should also not be part of the conversation. Temperatures across the eastern half of the U.S. between November and March were in many areas the warmest ever recorded over that period. Across the globe however temperatures in January and February of 2012 were very similar to and perhaps even colder than 2011. In other words, the warmth this year was location specific and was not part of a upward global temperature trend upward. In addition, there are much more ambient factors in a season that will dictate the outcome of a winter in a "globally warmed" winter and one that isn't. In other words, this winter if dominated by the same ambient forces would have also been mild and void of snow in 1912 just as it was in 2012.
If one is to frame the above-question a little differently however and ask the following. Would 2011-2012 been less warm without the impacts of global warming ? This I would have to answer a little differently. The data demands it does. It is estimated that winters in Vermont since 1990 are about 2-degrees warmer than winters in the 100-years prior to 1990. It is as clear as night and day. Yes, the data is still location specific and even periods spanning 20-year increments can feature statistical noise, but not that much statistical noise. Standard deviations for temperatures data spanning of 5-month cold season are between a degree and a degree and a half. This means more than half of the winters after 1990 are beyond a full standard deviation above the statistical mean. These things don't occur by accident, they don't occur because one anomalous winter such as the recent winter is skewing the data. It is in this case that Vermont is part of a larger global trend toward warmer temperatures. It means that yes the 2011-2012 winter would have been warm regardless but was likely warmer because of the effects of global warming. It means that the winter of 2010-2011 would have been colder without the effects of global warming. Fortunately, at least so far, the warming that has been prevalent the last 20 years is not translating to less snow. There is no evidence as of now, to suggest that the trend toward warmer temperatures is also part of a trend toward less snow. Snowfall in Vermont has remained relatively consistent.
Anyway, that is enough for this year. Thanks again to the Mad River Glen & Vermont skiing community for again being such a great audience to talk weather with. I couldn't motivate myself to do this without all of you. It has been a tough stretch thanks to Hurricane Irene and the warm snowless winter but better times are ahead. Have a great summer !!!