Tuesday, December 18, 2018

Warmth Since June 2013

Long-time readers will be familiar with the steady drumbeat of unusual warmth that has affected Alaska in recent years; it has been quite relentless for more than five years now.  Of course there has been plenty of variability from day to day and month to month, but cold spells have been mostly brief and muted in comparison to the lengthy and often striking warm periods.

So for example, relatively cold weather has emerged across western and northern Alaska in the past few days, but so far it's a trivially small cold anomaly compared to the persistent and pronounced warmth since mid-September.  Four consecutive days below 0°F in Fairbanks may seem chilly, but this is normal: since 2001, there have been at least four such days in a row (and often many more) in November or December every year except 2014 and 2017.

When we look back at the evolution of the now multi-year warm spell, it is quite striking to note how suddenly it emerged in early summer of 2013.  Again, long-time readers may recall the extraordinary events of April and May 2013: one of the coldest Aprils on record, and then May transitioned from exceptional cold (with extremely late break-up and green-up) to remarkable warmth by the end of the month.  The following month, June 2013, produced the warmest week of record in Fairbanks.  The blog posts from back then make for an entertaining read, e.g.


Here's a chart of monthly mean temperature anomaly in Fairbanks since 2000, using the 1981-2010 normal as a baseline.  The absence of cold since June 2013 - with the exception of March 2017 - is very striking.

It's worth looking at standardized anomalies too, because the typical variance of temperature is so much smaller in summer than in winter.  The persistence of warmth since 2013 is perhaps a little less dramatic in standardized anomalies, implying that winter warm anomalies have contributed most in absolute terms, as expected.  Relative to the normal range of climate there have been a few notably cool summer months (e.g. June 2014 and June 2018), but nothing exceptional.

The chart below shows the monthly anomaly values for Alaska's statewide area-average mean temperature (from NCEI climate division data); the very sudden 2013 reversal from cold to warm is not quite as striking for the state-average temperature, but the absence of below-normal temperatures in the subsequent years certainly is remarkable.

In response to the persistent warm pattern, the 5-year running mean of monthly temperature anomalies has risen steadily since the 2013 change and has moved well above the previous record in the modern climate history - see below - and it's a very similar story for Fairbanks and for the state of Alaska as a whole.

An interesting aspect of this chart is that it reveals a very similar sustained rise in temperature in the late 1970s.  In that case the persistent warming really got under way with the exceptionally warm winter of 1976-77, coinciding with the "great Pacific climate shift" of the same time; this sudden change of climate has been studied extensively.

From the perspective of Alaska temperatures, the 2013 shift looks very analogous to that of 1976, and in fact there is a remarkable correspondence in the rate of rise of the 5-year running mean temperature anomaly.  Here's the linear trend in the 5-year running mean state-average temperature, for periods of equal length:

May 2013 - November 2018: +0.77°F/year
May 1976 - November 1981: +0.77°F/year

For Fairbanks, the trends for the two analogous periods are as follows:

May 2013 - November 2018: +0.67°F/year
May 1976 - November 1981: +0.59°F/year

Having noted the similarity of the trend, however, it's also worth noting that the prior warm spell beginning in 1976 did not have quite the same degree of exceptional month-to-month persistence that we've seen in recent years.  The chart below shows the monthly statewide anomalies relative to a contemporary normal.

Of course it will be very interesting to see if the analog continues to play out in terms of the duration of the period of rapid warming.  In the previous iteration, the 5-year mean rose rapidly for a little under 6 years; so perhaps Alaska will start to see a return to cooler conditions in the next year or so.  I wouldn't bank on it, however.

I'll follow up soon with another post on the large-scale changes that accompanied the 2013 shift; it would be nice to identify at least one or two candidate explanations for why such a dramatic change has occurred (beyond the obvious background long-term warming trend).

1 comment:

  1. Thanks for this very useful synthesis Richard. Looking forward to your forthcoming post exploring possible "why".