Historical Context for Recent Extreme

Following up on Saturday's post, I did a bit more work with the NCEP/NCAR global reanalysis data to get a better sense of just how unusual the recent circulation anomaly has been.  How many times has a comparable two-week anomaly been observed in the past, not just over Alaska but anywhere in the Northern Hemisphere?

I addressed this question with the following series of calculations:

(a) Obtain a smoothed daily climatology (normal) of 500mb heights for the 1958-2017 period.  Note that the NCEP/NCAR reanalysis starts in 1948, but the quality is lower before 1958 owing to the paucity of upper-air observations.

(b) Calculate mean height anomalies for 15-day periods ending on every day since 1958.

(c) Extract a smoothed daily climatology of the standard deviation of the 15-day anomalies.

(d) Find the daily maximum and minimum of standardized 15-day anomalies across the entire Northern Hemisphere.

Here are the results: the red line shows the daily hemispheric maxima of standardized 15-day height anomalies, and the blue line indicates the hemispheric minima.  (Click to enlarge)

Remarkably, the 15-day anomaly ending just 3 days ago at a grid point over the Bering Sea was the largest of any in the entire Northern Hemisphere history since 1958.  It was also larger in magnitude than any 15-day negative anomaly.  The closest competitor on the positive side was an extreme blocking ridge over northern Greenland in November 1965; here's a pair of maps showing a comparison of the two events:

Of course the long-term global rising trend in 500mb heights gives a slight "advantage" to the recent anomaly in comparison to a fixed climatology, so I re-did steps (b) - (d) above after removing the 1958-2017 linear trend (calculated for each day of the year and each grid point).  In this case the 1965 event moves into first place as the most extreme positive 15-day height anomaly at 500mb (in terms of standard deviations).

It would be interesting to see how other atmospheric variables like temperature and moisture reflect the nearly unprecedented nature of this month's anomaly, and it would also be worth looking at other, more modern data sets.

But based on these results, it seems safe to say that the recent high pressure ridge over the Bering Sea and western Alaska has been one of the most extreme 15-day weather anomalies in recent decades anywhere in the Northern Hemisphere.