Saturday, February 18, 2017

Duration of Cold Spells - Part 2

A few weeks ago I showed some evidence that severely cold spells have become shorter in recent decades in Fairbanks - in other words, the coldest extremes of the modern climate (which are warmer than earlier cold extremes) have become less persistent as warmer conditions have tended to return more quickly.

This follow-up provides a quick look at the average 500mb height patterns associated with the most severe cold spells in two contrasting periods, 1931-1960 and 1981-2015.  To examine this I used data from the 20th Century Reanalysis, courtesy of NOAA/ESRL/PSD.  The 20th Century Reanalysis uses historical sea-level pressure and sea surface temperature observations to produce an evolving 3-D estimate of atmospheric conditions back to 1851.  Owing to the strong correlation between sea-level pressure and upper-level heights, the historical 500mb height estimates should be broadly correct back to the early 20th century or before; by 1925 there were already well over 1000 daily pressure observations north of 20°N (although not at all evenly distributed around the Northern Hemisphere).

The two sets of maps below show the mean height anomaly (departure from normal) for 1931-1960 cold spells on the left (as defined in the earlier post) and 1981-2015 on the right.  The first pair of maps is for 10 days prior to the first day exceeding the cold threshold, and then pairs of maps are shown at 5-day intervals until 15 days after the cold spell onset.  Click to enlarge the images.

-10 Days

-5 Days

0 Days

+5 Days

+10 Days

+15 Days

With respect to the main question of interest here, it's immediately clear why cold spells in the earlier period tended to last longer than more recent cold spells.  In 1931-1960 the average flow pattern remained quite similar from the date of onset through 10 days later, with high pressure over the Bering Sea and low pressure over western Canada; this pattern funnels cold air southward across the interior.  In contrast, the 1981-2015 pattern is completely different by +10 days, with low pressure over the Bering Sea and high pressure over western Canada; this pattern brings warmth to Fairbanks.

It's also interesting to note that the precursor flow patterns were quite similar between the two periods at a lead time of 10 days, with high pressure over and south of the Aleutians and low pressure over the Arctic Ocean.  However, at 5 days in advance, the modern pattern is much more amplified, with a very strong northerly flow anomaly over the Bering Sea (rather than northwesterly in 1931-1960).  It seems as if modern severe cold spells tend to arrive with greater ferocity, but the high-amplitude pattern continues to evolve rapidly so that the cold blast is followed quickly by warmth.  In the "old days" the cold-advection pattern gives the impression of being more stable and long-lived, at least as far as the Alaska-centric flow anomalies are concerned.

A possible next step would be to investigate historical sea surface temperature anomalies to see if we can identify a reason for the preferential patterns shown above.


  1. What's really remarkable to me is the trend toward ridging in western Canada in the modern regime. It looks like the +PNA pattern is firmly entrenched most of the time, and the severe cold snaps are minor blips compared to its dominance.

  2. Yes indeed. According to NOAA/CPC data, since 1981 only 19% of months in Dec-Feb have had a PNA index below -1. From 1950-1980 it was 42% of months. It would be very interesting to extend the calculation back to 1900 with the 20thCR data.

    The PNA is related to the PDO: since 1981 only 10% of Dec-Feb months have had a PDO index below -1, whereas it was 24% from 1950-1980.

    1. Is it safe to suggest that the +PNA ridge is just the downstream component of the enhanced +PDO Aleutian trough?

    2. Essentially yes. Compare the atmospheric and oceanic patterns below.

  3. Richard, as I compare the different figures, I can't help but wonder if the general Rossby wave speeds are different. Looking at an arctic-wide view of 250mb heights might show if this possible.

    1. Eric, it's indeed an interesting question as to how the entire extra-tropical circulation differed back then, and the 20thCR ought to be a valuable tool to find out. I'm sure others are looking at this too.

    2. I realized that my comment wasn't as clear as I thought it was. What I meant was look at a polar view of the 250 at stepped increments like you did in your analysis. The heights at 250 will be more distinct. Looking at the 5 day intervals will show the overall Rossby pattern and give an estimate of its speed. I suspect that similar and consistent wave patterns produce similar and consistent troughs and ridges. These in turn produce the common patterns seen in your analysis. I hope I'm clear in what I'm proposing.