Thursday, August 13, 2020

Reduced Temperature Variance

If residents of central Alaska have a sense that this summer has seen unusually steady temperatures, without extremes of either cold or heat, they are correct.  In fact, according to daily Fairbanks data, the standard deviation of daily mean temperatures since June 1st is the lowest on record.  Since at least 1930, there's never been a summer with less variability in daily temperatures through August 12.

The slight downward trend over the 90 years of data is not statistically significant, but the trend since 1990 actually is marginally significant at a 95% confidence level.

This summer's daily temperature variance is also the lowest on record - and by a larger margin - at McGrath, and it's the second lowest on record at Bettles.

Here are daily temperatures since May 1 in comparison to normal for the time of year in Fairbanks.  Lots of small deviations from normal.


What explanation might we propose for this summer's unusually steady conditions?  It would be interesting to look at seasonal climate drivers that tend to impart low variance, but for now the answer is rather simplistic: the general weather pattern has precluded the strong ridge/trough waves in the atmosphere that create larger swings in temperature from day to day and week to week.  In particular, it's the ridges that have been missing, as 500mb heights have been lower than normal over eastern and/or southern Alaska.  This seems to have been a relatively stable configuration, with few significant perturbations to bring much warmer air to Alaska's interior.

Here are 500mb anomaly maps from June, July, and August so far:

 

As for the downward trend in the past 30 years, it may well be the reduced frequency on the cold side of the climate distribution that explains the reduction in summer temperature variability.  In other words, as the Arctic has warmed far faster than the rest of the Northern Hemisphere in recent decades, air from the north will have warmed much more than air from the south.  It would be useful to look at this in more detail when time permits.

Update August 14:

It turns out there's no meaningful correlation between the mean and the variance of summer daily temperatures (after removing the long-term trend from the mean), so it seems we can't say that cool summer weather patterns (for Fairbanks) are inherently more stable.

 


7 comments:

  1. Lots of clouds this summer versus the no cloud days that create higher temps. Insolation has not had a chance to warm. And cold air aloft has moved overhead.

    Gary

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  2. And now today Fairbanks has warmth and no clouds of concern. Simple.

    Gary

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  3. Not on topic but hooray for the reestablishment of the Howard Pass RAWS site. Located in the northern Alaska Brooks Range it's a real breezy location with extremes of wind chill in winter. Hope it lasts but it often gets destroyed somehow.

    https://mesowest.utah.edu/cgi-bin/droman/meso_base_dyn.cgi?stn=HOWA2&unit=0&timetype=LOCAL

    Gary

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    1. I'll echo that sentiment, it's a great site to illustrate Brooks Range extremes. By my count it's been knocked out (at least the wind sensor) 5 of the last 7 winters.

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    2. If interested here's a book I've recently ordered relative to early human occupation of the Noatak River valley - Howard Pass RAWS. It's been referenced by several anthropologists

      https://www.amazon.com/gp/product/091200696X/ref=ppx_od_dt_b_asin_title_s00?ie=UTF8&psc=1

      Gary

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    3. This comment has been removed by the author.

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    4. Double post so deleted the 2nd. Howard Pass is cold and given to extreme chill factors. But can have lots of migrating game for food. I've been there year round flying and camping and even today there's not much for firewood in a shelter to stay warm. Back when they could have burned the bones of game as that works. That's what locals near Delta Junction did 14K years ago at Swan Point. As trees moved north later so did improved firewood sources.

      Gary

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