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Stratosphere watch 2022-23
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18-01-2023 12:33am#1
This thread is quite late on arrival this year as I have not been bothered to make a new one but I think now is as good a time as any to do so to avoid flooding the FI thread with stratospheric charts and there is some ongoing interest with regards to what is likely in the stratosphere later in January.
First, I won't be doing my in-depth long explanation as I have been doing for the past few years. If you want to learn about how the stratosphere works and why it's of importance to our weather, please see last year's thread here.
Second before I get into the current outlook, I need to correct myself on a recent post in the FI thread with displacements vs splits. Recent analysis was done by James Peacock at Metswift (known as Singularity on Netweather). Initial results of 2m temperature during impact period following a PV displacement or split event showed results you would not expect which I outlined in the FI thread. I somehow missed that the impact period was recalculated subsequently and results were much more in line with what you expect, especially over North America. Both displacement and split events do create blocking in the Atlantic sector but displacements have a tendency more for this blocking to be over towards Canada (west based -NAO) with the storm track further northward into Europe whilst splits have a more well rounded blocking feature over Greenland that is conducive to widespread below average temperatures over Europe. As you'd expect there is a strong signal for colder than average temperatures in the eastern seaboard which could also blow up cyclogenesis and cause that storm track to be further northwards across Europe. Note how strong the cold signal is for Scandinavia following a displacement event though and northern UK is also favoured to be cold.
Meanwhile for the past week, the GFS has been hinting at a significant deceleration in zonal mean zonal winds at 60N 10hPa (which shall be referred to as U-60N from this point in the thread) in the stratosphere in the final week of January. The model has been consistent in showing this and the ensembles are starting to pick up on this somewhat too. Any sign of a major SSW (reversal of the U-60N) is way out into fantasy land towards the final few days of January or even the first days of February. At the moment this all looks largely wave-1 driven and a displacement which as shown above is unreliable for this part of the world if it's severe cold conditions you seek (provided the downwelling occurs in the first place).
The latest ECM ens from Monday also show a significant deceleration but no major SSW for the majority of ensembles. The ECM ens have been catching up with the GFS in this regard rather than the other way round, there was much more stronger than norm U-60N on previous EC46 runs. Still no apparent signal of a major SSW from Glosea but can only go by the Met Office longer range outlook on that as Glosea data is not in the public domain.
Now does the fact the U-60N achieve an arbitrary number of going easterly make a difference in the grand scheme of things? No, not to a great extent anyway. Tropospheric blocking positioning is key as always from other drivers such as the MJO regardless of the stratosphere. However, the more significant the U-60N reversal is, the higher likelihood it is to be of a wave-2 PV split event which as outlined holds the greatest potential for cold, blocked patterns in most of Europe.
Whatever the end result, it's always fascinating to watch the dynamics of the stratosphere unfold. Will it be a major SSW? Will the GFS' call finally be on the ball after numerous failed attempts this winter? Will it be a wave-2 split? Or will the GFS do a 180? Feels like I always end these posts with questions, questions, questions but we're always left second guessing with weather..
One to keep an eye on.
Photography site - https://sryanbruenphoto.com/
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I saw that post. I wonder did he look at the AO because that is the main way of classifying downward propagating (dSSW) and non downward propagation (nSSW) SSW events and if they were dSSW, we would have likely just found ourselves on the wrong side of the blocking as is typically the case. Judging by the fact he said there's been 12 February major SSW events since 1958, he seems to have used the NCEP dataset - various datasets have different major events due to differing reanalysed data on borderline events.
Doing a similar analysis to that of Simon Lee using D3-D30 MSLP anomalies following the central date of those 12 February major SSW events, I get the following results. NAO data is via NOAA/NCEP and PV classifications are via Cohen et al. The NOAA NAO data is based more on 500mb heights rather than MSLP so expect some divergence between the reanalysis and the NAO values. I used NAO here as it is more relevant as a means of signifying impact here than the AO. This dataset also tends to underestimate the strong nature of the NAO periods compared to other datasets which are primarily focused on MSLP.
Using these results, I get 5 of the 12 February major SSWE that were succeeded by a -NAO on the NOAA index. These years being 1980, 1984, 2001, 2010 and 2018. 2001, 2010 and 2018 were PV splits whilst 1980 and 1984 were PV displacements. If I'm able to guess, the 4 years that the NW user speaks of are 2018 (obviously), 2010, 2001 and maybe 1979? The first three I am certain on anyway. The NOAA index indicates a +NAO for 1979 but mid-March 1979 featured a severe cold easterly that coincided with St. Patrick's Day. Another year I would add is 1973 which featured an unusually cold and snowy northwest to northerly regime around Valentine's Day which I mentioned in the Irish Weather Statistics thread. March 1966 featured various cold snaps but nothing too remarkable. Mid-March 1980 had a very cold easterly with widespread snow showers between the 17th and 23rd so I would definitely include that one as another. March 1984 was just a cold, very cloudy month whilst 1989 was a nSSW following on from an exceptionally mild winter but April 1989 would later turn out to be notably cold. Early March 1999 featured northerly and easterly winds but it was not severely cold and the month later turned very mild to warm. Late February/early March 2001 brought one of the most wintry spells of the 2000s with a snow event for the east of Ireland and severe frost. 2007 and 2008 were nSSW - in fact, the February 2008 is officially the last nSSW that has occurred. Mid-February to mid-March 2010 was often cold, very frosty and light snowfalls at times during February whilst early March was dry and sunny. 2018 goes without saying so won't go there.
So from that I get 1979, 1980, 2001, 2010 and 2018 as having a notable easterly spell following a February major SSWE. 1973 is a strange and unique case. That's just under half the February events in the database so not the best odds indeed but who's to say this isn't just coincidence and our luck rather than anything deep? Interesting how, according to Cohen, most of them are splits and only 4 of them were displacements.
Photography site - https://sryanbruenphoto.com/
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There was a very delayed development of the stratospheric polar vortex in Nov/Dec 2000, see the U-60N below for 2000-01 indicated by the red line. There was no official major SSW at 10hPa as no reversal but it was a significant SSW nonetheless. Pretty much the entirety of December 2000 had slower than average U-60N and even at its strongest in meteorological winter through January 2001, it was just barely around average. This likely had a big hand in 2000-01 being a colder winter than most of the 1990s/2000s up to 2008.
This cross-section shows the NAM/AO from 1-1000hPa during the 2000-01 season, you can see the significant minor SSW that occurred in December 2000 around 10-14 days before the Christmas to New Year cold spell. There was clear downwelling from both this and the major SSW split in the following February which would result in that cold end to February/start of March.
Another one of those minor SSW events that had significant impacts for Europe much like the January 1991 event despite no reversal.
Photography site - https://sryanbruenphoto.com/
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Still looking like a minor wave-1 displacement is what is expected. No definitive sign of major (reversal) or a split.
The green lines below are the GEFS showing forecast U-60N on the 06z run this morning. Some go for a reversal, most do not but there is a significant group that do in the early days of February.
Photography site - https://sryanbruenphoto.com/
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Indeed.
Although perhaps the GFS OP is not as bad as I initially thought as NE Europe still has a lot of high pressure which would in theory send further planetary waves to give more disruption and an increased risk of wave-2 type warming. However, we'd be fast running out of time as spring would be upon us for earliest likelihood of tropospheric response unless a January 2009 scenario were to happen which is another extreme altogether. Gotta admit it gives me January 2014 vibes that 12z with the NE Europe ridging and active Atlantic.
As for the MJO phase 3 being blocked, composite for such shows strong signal for high pressure around Iceland/Greenland with a deep trough in the Atlantic. Looks cold but wouldn't be textbook snowy I'd have thought due to the alignment of the troughs, we would want that trough centred over the Low Countries. Then taking into account the displacement of the PV, would this blocking probably be further south?
Photography site - https://sryanbruenphoto.com/
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Some stunning nacreous or polar stratospheric clouds showing in Scotland where the stratosphere temperature is sub -78C which is required for visibility of these clouds. Ireland last seen them in 2016 to my knowledge with some great displays in early February 2016, could be sighting of some in the north today/tomorrow as the core of the coldest stratosphere temps gets displaced towards our shores. Shame there looks to be too much cloud cover with rain expected to approach later today, typical ain't it.
Photography site - https://sryanbruenphoto.com/
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GFS yet again up to its old tricks right at the end of its run. Posting just for archival sakes. It shows a major displacement. We well know how the GFS goes by now with its new version since early December. Saying that, knowing our luck one will happen and we get a very blocked March. One hasn't happened since 2018..
Photography site - https://sryanbruenphoto.com/
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Another det run showing a major displacement and it looks prolonged too rather than a short affair.
The reversal starts on the 15th and ends the run with -19.9 m/s on the 18th.
Just realised the GEFS 06z had a significant cluster reversing too but the spread is large, as you'd expect at such a timeframe. The GEFS have been performing better than the OP for the stratosphere again this winter as you'd expect - though not necessarily so for our weather!
EC46 update to come this evening... Monday's update as a reminder was supportive of a significant weakening again in the second half of the month but not quite going to reverse like the GEFS above.
This possibility has more of a base to it than the previous two with more support but does that mean it's any more likely to occur? No but I'm probably going to be like a broken record saying this for numerous posts, it'll be typical for one to occur and result in a blocked March or mid-spring and sometimes the effects from a major event can continue into the final month too as U-60N take a deep dive through the spring season naturally and meridional patterns are much more likely.
Photography site - https://sryanbruenphoto.com/
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This evening's EC46 has brought forward the deceleration but again no reversal indicated by the ensemble mean. Interesting how much it keeps significantly under the average for a prolonged period of time though. More time and more runs needed. Talking like 2 weeks away after all for any shot of this. Health warnings and all that.
Photography site - https://sryanbruenphoto.com/
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The saga continues on the GFS today, no backtracking this morning. Reversal starts on 13th and still goes on to the end on the 19th.
A significant cluster of the GEFS 0z reversing again.
Photography site - https://sryanbruenphoto.com/
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Okay so here's the latest on what's happening.
The stratospheric polar vortex right now is weaker than average, the U-60N today was 13.2 m/s according to the GFS. It's only going to go down further in the next few days as warming of the stratosphere continues reversing to easterly (negative U-60N) by Wednesday afternoon which would likely peak by this time next week. The most recent GFS run shows a prolonged reversal where the U-60N continues to hover between 0 and -10 m/s up to day 13 then a secondary warming occurs bringing yet another descent in the U-60N back down into the double digits with a big ridge over the Arctic. If that were to occur, there is very little remnants of the stratospheric polar vortex to be seen anywhere. It is still a wave-1 major displacement as it's all one big ridge but the troughs are set up in a conducive position to bringing the North Atlantic Oscillation into negative territory and therefore, a heightened risk of Greenland or Iceland blocking high pressure. This is not your run of the mill displacement event if the GFS is correct. Generally speaking, it is said that the deeper the reversal and more prolonged it takes place, the higher the likelihood a shake up in the weather patterns occurs. Of course then comes the million questions as to where Ireland would find itself on which side of the jet stream to determine our weather.
Until the first spike (assuming the GFS is correct with forecasting a second spike in warming) occurs by later next week, I don't think there is much use in looking at if downward propagation takes place from the strat to the trop. Models will be playing lots of solutions. For the sake of it as I know some will not resist it, here's the latest GFS North Annular Mode (NAM) forecast anyway. The orange colours are indicative of a negative NAM (also known as a negative Arctic Oscillation (AO) which refers to blocking over the north pole and is associated with the major sudden stratospheric warming. The left side of the chart is the various layers of the atmosphere, if there's downward propagation visible you'll see those orange colours descending towards 1000hPa. At the moment, the GFS does not show this but note the model only takes us to final few days of February and if we are to see effects directly from this warming event, it'll likely be in the early days of March at earliest. Any earlier is probably down to other drivers such as the MJO.
After being inactive all season, University of Berlin have finally updated their ECM stratosphere charts though the latest run available is yesterday's 12z (10 February). The EC sees the U-60N reverse later Wednesday into Thursday, in line with the GFS. However, the warming is less dramatic on this model and does not reverse the U-60N quite as much. Details like that to be revealed though closer the time. It's variations on a theme.
In summary... A major SSW is happening, the U-60N will reverse mid-week next week and it waits to be seen whether there will be tropospheric impacts from this. Ignore the talks of beast from the east, it's usual media drivel. Let's wait for the event to happen first before thinking of the effects of our weather, shall we?
Photography site - https://sryanbruenphoto.com/
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