In the past several years we have occasionally mentioned our tracking of the areal extent of air colder than minus 5 degrees centigrade at about 1 mile above the surface of the Earth. This measurement has proven to be a very valuable addition to the collection of metrics of global warming. We have particularly commented on the wintertime (December, January and February) average of this extent measured over the entire Northern Hemisphere, noting that since 1948 the wintertime average extent has systematically decreased.
We have also been tracking this variable throughout the autumn over all these years and can report that this fall (Sept. through Nov. 30) recorded the smallest average areal extent of this cold air since at least 1948. That means we have just experienced the warmest Northern Hemisphere autumn in at least the past 77 years.
Naturally, one wonders if this warm start will carry on throughout the coming winter. This is not a simple question as it turns out. If one were to rank the 77 autumns since 1948 in a list from warmest to coldest, one would find that the warm autumn does not necessarily lead to warm winter. In fact, only 27 of the previous 76 seasons have seen rank changes of less than 10 places between autumn and winter. So, autumn does not really provide a reasonable forecast for winter.
A fairly recent example of this disconnect comes from September 2011 to February 2012. The autumn portion of that period ranks as the sixth-warmest autumn in the time series but the following winter was the 60th-warmest winter — that is a huge change in ranking.
So, we can’t make a reliable hemispheric prediction about winter based on the recent record-setting autumn.
Steve Ackerman and Jonathan Martin, professors in the UW-Madison department of atmospheric and oceanic sciences, are guests on WHA radio (970 AM) at noon the last Monday of each month. Send them your questions at stevea@ssec.wisc.edu or jemarti1@wisc.edu.
