Winter Weather
Greetings from the Chair of Hunter College Geography
February 24, 2014

Dear Students, Staff, and Colleagues:

I hope it's not too late to welcome you all to the spring semester. I look forward to an enjoyable and productive semester for all. Mother Nature has obviously welcomed us all this year! And speaking of the weather...

Some of you may be following the discussion in various media outlets on two questions: (1) how extreme this winter's weather has been in our region, and (2) the relationship, if any, between this winter's weather and global climate change. I thought I would take this opportunity to give you my take on these questions, even though we still have some winter yet to come.

I am giving you three versions of this story. First, below, you will find just my conclusions. For those of you interested in seeing more detail, a longer discussion is pasted below the conclusions. Also, I have prepared a file with figures illustrating most of the points raised in the discussion, all of which are derived from publicly available sources (you can go online and make the figures yourself). The file also includes links to the data sources and some relevant articles:
http://www.geo.hunter.cuny.edu/~afrei/winter_weather_2013_2014_feb2014.pdf

 

CONCLUSIONS ONLY:
How extreme has this winter been in our region? The data shows that we have had some extremely cold days as well as an extremely snowy winter because of so many storms coming through, although no individual storm can be considered particularly extreme. While the frequency of precipitation (including both rain and snow events) has been unusually high across the Northeast, it has not been record setting. Neither monthly averaged temperature nor monthly precipitation totals in our region have in any way been extreme.

What, if any, relationship does global climate change have to do with this winter's weather? In my view, the small region of the globe (portions of North America) that has experienced these cold outbreaks this winter, viewed in the context of the rest of the globe being much warmer than normal, does not in any way detract from the concept of global warming. Furthermore, there exists a fairly recent thread of scientific research theorizing, based on physical principals, that global warming, by virtue of the more rapid warming in the Arctic compared to the rest of the Northern Hemisphere, may result in more persistent (i.e., longer lasting or slower moving) weather patterns, including more persistent cold outbreaks over the eastern US during winter. At this time there is strong empirical (i.e. observational) evidence supporting this theory, but many details have yet to be determined.

 

LONGER DISCUSSION

How extreme has this winter been in our region?

In some ways it has been quite extreme, but in other ways not so much; and, at least so far, it has not been in any way "the most extreme" or "record setting." First of all, SNOW, SNOW, SNOW! As of February 19th we have had about 56 inches of snowfall, compared to the average of around 25 inches per year. This puts us around the 7th or 8th snowiest winter since records began being kept in Central Park 120 years ago. It is very possible that we could move up in the ranks before this winter is out. Considered individually, none of the winter storms have been that big, it's just that they keep coming. We have had at least 17 days with snowfall between December 1 and February 19th. I have not run the stats yet to see how extreme the frequency of snowfall has been.

Second of all, COLD, COLD, COLD? We've had at least one day with the coldest temperature ever recorded for that calendar day, but we have not come close to the coldest temperatures ever recorded here, notwithstanding the invasion of the polar vortex! And when you look at average monthly temperatures, it has been nowhere near extreme levels. Based on "climate division" data, which divides the country spatially into regions, New York State regions 4 (NYC and Long Island) and 5 (Hudson River Valley) were warmer than average in December; in January they were about 3-4 degrees F below average, coming in at the 29th (Div 4) and 28th (Div 5) coldest out of 120 years of data. Cold, but not extreme. This should not be too surprising since the weather has bounced back and forth between below and above freezing.

Which brings us to RAIN, RAIN, RAIN! As a result of the frequent temperature swings in combination with the frequent storms and 17 days of snow, we have also had 14 days with rain since December 1st (that includes February 19th, because it began started raining as I wrote this sentence!). With regards to the total amount of precipitation (combining rain and snow), our region was slightly above normal in December, near normal in January, and certainly not extreme in either month.

Is the frequency of precipitation this winter considered extreme? About 20% of the Northeastern US has experienced an extremely high number of days with precipitation: this is the tenth highest percentage of the region with extremely frequent precipitation in the last 100 or so Januaries. No part of the Northeastern US had an extremely low number of days with precipitation in January: this has also happened only about ten times in the last 100 years. (This is a confusing statistic, you may have to think about it.) I have not had the time to run the statistics for NYC specifically. However, note that while we have experienced so many precipitation events, conditions over western North America have been warm and dry. These are related: both are caused by the unusual persistence of atmospheric circulation patterns this winter (discussed further below).

In summary, we have had some extremely cold days and an extremely snowy winter, so far, because of so many storms coming through. However, no individual storm would be considered that extreme. Furthermore, the frequency of precipitation (rain and snow) has been unusually high, though not record setting, across the Northeast. Neither monthly averaged temperature or precipitation have been extreme.

What, if any, does this winter’s weather have to do with global climate change?

During extreme events people sometimes make unwarranted extrapolations. For example, during the cold outbreaks over eastern North America this winter I have heard derisive remarks about global warming. Also, there has been research indicating that on hot days more people believe that the earth is warming, but that's a whole other story. I believe that the confusion around this winter’s weather has been at least partly attributable to two difficulties: distinguishing local from global events and confusion about a recent branch of climatic research that has garnered media coverage over the last few years.

First, we must distinguish local from global events. Those who cite this winter's eastern US weather as evidence against the idea of global warming are ignoring news from across the continent and across the globe. The western US has had some of the most extreme heat, and the most prolonged drought, in history. January temperatures in Alaska were much warmer than in New York City.

If one looks across the globe, we find even more hot news! In December, below average temperatures for the mid-to-high latitudes of the Northern Hemisphere were confined to the upper mid-western US and Canada: the rest of the Arctic, northern Europe, and northern Asia (including Siberia) experienced temperatures well above average. In January, while the polar vortex put eastern North America 3-4 F below normal, and monthly average temperatures across Siberia were even colder (8-10F below normal), most of the rest of the Northern Hemisphere, including the Arctic Ocean and western North America, were warmer than usual. For example, across much of Alaska monthly mean January temperatures were 8-10F above average.

It turns out that for the global average, this year was the fourth warmest January since records begin in 1880! We have now had over 28 years in which every single month was above the 20th century average. I think that a small region (e.g. eastern North America) experiencing colder than usual temperatures, while most of the globe is warmer than usual, would be expected on a warming earth.

Again we ask, so what does this have to do with global climate change? A fairly recent branch of research, discussion of which has garnered attention in the mainstream media during the last few years, asserts that the unusually persistent atmospheric circulation patterns that have caused the warm-dry-west / cool-wet-east winter pattern over North America is caused by global climate change. But how?

Before I explain, let me say a word about the jet stream. The jet stream is a current of fast-moving air flowing from west to east in a wavy fashion approximately 8-12 kilometers (5-7 miles) above the surface around the Northern Hemisphere. It is known that the strength, location, and "waviness" of the jet stream (1) are determined largely by the meridional (i.e. north-south) temperature gradient (i.e. temperature difference) between the pole and the equator; and (2) influence the location and strength of high pressure systems (associated with dry conditions) and low pressure systems (associated with storms). Also, the jet stream is the boundary between colder air to the north and warmer air to the south. So, where the jet stream moves northward one typically finds warmer conditions, and where it moves southward one finds colder conditions.

The theory, based on atmospheric physics, states that since the Arctic region has been warming significantly more than the rest of the hemisphere, the average meridional temperature gradient has diminished, resulting in a weaker, more wavy jet stream; and that a weaker, more wavy jet stream should theoretically migrate more slowly around the planet. As a result of a more wavy, more slowly migrating jet stream, weather patterns that are influenced by the jet stream would be more persistent (i.e. would stay in the same place longer). In particular, some advocates of this theory predicted that the patterns that we have experienced this winter would be the mostly likely ones to occur. The theory, which is more complicated than described here, has been confirmed by a number of empirical studies. However, with only around ten years since the Arctic has really warmed more than the rest of the Northern Hemisphere, as time passes we will accumulate more data to make the empirical studies more precise. Also, the theoretical modeling community has its work cut out to make the models accurate enough to explain this theory more clearly. In any case, even if cold outbreaks are more frequent in the future, we would not expect that they will be colder in the future than they were in the past; in fact, the expectation would be that cold outbreaks in the future would on average be warmer than those in the past.

In summary, it is my view that with only a small region of the globe that has experienced these cold outbreaks this winter, viewed in the context of the rest of the globe being much warmer than normal, this winter’s weather does not in any way detract from the concept of global warming. Furthermore, the discussion of whether global warming, by virtue of the more rapid warming in the Arctic, may result in more persistent weather patterns, including more frequent cold outbreaks over the eastern US during winter, is an intriguing area of research for which there exists strong empirical evidence, but many details have not yet been worked out.