Saturday, December 19, 2015

Winter Temperature Variance

I'll present this more or less without comment, as I'm feeling a bit under the weather this weekend (no pun intended); but I thought it would be interesting to examine the long-term changes in variance of temperature during winter in Fairbanks.  As we've noted before, there were some remarkable extremes in the 1930s; so has there been a long-term decrease in temperature variance?

The answer is yes.  The chart below shows the November-March standard deviation of daily, weekly, and 30-day mean temperature anomalies, with the anomalies calculated relative to contemporary normals (which have warmed substantially over time).  The variance has decreased at a similar percentage rate for each time scale from daily through 30-day mean temperatures.  We can conclude that the modern winter climate of Fairbanks has somewhat less variability of temperature than in earlier decades prior to about 1980.


What does the decreasing variance look like in practice?  The charts below show daily temperature anomalies for 1950-51, which had daily and 30-day variance very close to the 1930-1959 averages, and for 1993-94, which was very similar to the 1985-2014 average.  The decrease in variance is about 10% on both time scales, which is small but arguably just about perceptible on the charts.



The winter with the highest combined daily and 30-day variance was 1980, and that with the lowest was 1987 (interestingly the latter coming at the end of a strong El NiƱo episode).



6 comments:

  1. An important complement to this might be a time series of the skew. Then we could see how the cold tail has decreased as well.

    It is also worth noting that your 1980 and 1987 examples show the power of blocking regimes: warm gets warmer and cold gets colder when there is less variance.

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    1. Yes it would be interesting to look at skewness over time, although I'm not sure how useful the third moment is for a single winter. For any given year I think the skewness would generally take the sign of the overall mean anomaly.

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  2. I'm guessing that a positive PDO would limit variation. (Due to a more active Pacific moderation) Looking at the trends it almost looks as though there was a slight increase in variation 2000-2015 when the PDO turned negative. Hmmm

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    1. Yes, PDO and ENSO have major influences on variance. See here:

      http://ak-wx.blogspot.com/2015/09/enso-height-patterns.html

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  3. Off topic again but I came across this NWS online school for weather and find it friendly and informative. Nice and easy, especially for those of us unlikely to ever be offered an endowed chair position in this subject:

    http://www.srh.weather.gov/srh/jetstream/index.htm
    http://www.srh.weather.gov/srh/jetstream/matrix.htm

    Gary

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    Replies
    1. Very nice, Gary. Someone has put a lot of work into that. Thanks for the link.

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