Wednesday, September 19, 2018

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.

14 comments:

  1. Though there is less data and thus less reliable results, what does the southern hemisphere look like? I expect the minimum 500 heights to be super low due to Antarctica. But Australia can get some rather intense ridges.

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    1. I agree it would be interesting. I'll include a Southern Hemisphere chart when I provide an update in a few days.

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  2. I have never seen a two week september period anything like this one in Alberta in 33 years. This is unbelievable. We haven't hit an average high temp since the beginning of the second week of september and it doesn't look like its going to change anytime soon. We may end up with four or five weeks in a row of this pattern.

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  3. Fascinating - This event places it 5 solar minimums since the 1965 event and cycle 25 is just developing. It will be interesting to watch other emergent events from jet stream behaviour over the next few years as the minimum develops. https://www.lunarplanner.com/SolarCycles.html

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    1. Radio propagation is poor and will be in the next Solar Cycle according the Hathaway an others. Interesting paper via the Lunarplanner link. Living with change is a challenge.

      Gary

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    2. This is incredible...look at Calgary's 2 week forecast. The highs are around the average lows for a majority of it. I think we are witnessing modern day history. Who will get it next?

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  4. https://www.theweathernetwork.com/ca/14-day-weather-trend/alberta/calgary?from7day=1

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    1. Thanks - the cold in interior western Canada is certainly extreme! That's the downstream effect of a huge ridge.

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  5. I wonder if the smoke we were beat down with has anything to do with this new ridge and its seemingly inability to move out. can you comment?

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    1. By smoke you mean summer wildfire smoke? I don't think there would be any causal relationship, although of course the same underlying mechanisms could affect weather patterns for periods of months or even years.

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  6. Check this out. Calgary got 40cm of snow today and is supposed to get more. The 100 year daily record is under 5cm before today. I feel like something very big is being missed right now that is affecting the world's climate unless this is just climate change at its finest.

    https://www.theweathernetwork.com/photos/view/active-weather/40-cm-and-counting-at-530pm-tuesday/33431994

    Another full week ahead of temps much below average. They are calling for near or above average temps finally next weekend which would be almost 3 weeks of this? And that's not a guarantee as they have been calling for a warm up every single week and it hasn't happened.

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    1. Amazing weather - thanks - it illustrates the sheer enormity of the ridging anomaly to your northwest.

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    2. Hey Richard

      Not done yet. calgary could get another 20cm monday and tuesday next week. This past snowfall had some areas breaking all time snow records since 1888 as some areas were over 50cm. The official reading at the airport was just under 40cm which took the 7th spot all time.

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