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Stratosphere watch 2023-24
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04-11-2023 12:00pm#1
It's that time of the year again to start watching the evolution of the stratospheric polar vortex for the coming 2023-24 winter season and have a look back at the preceding season reflecting on what happened as well as anything we have learned that can be applied to the future.
Get the basics out of the way... every autumn a ribbon of air called the stratospheric polar vortex (SPV) forms high above in the stratosphere due to the temperature contrast between the poles and the tropics as well as the coriolis effect. This drives the westerly flow across the Northern Hemisphere resulting in relatively mild winters to Ireland. Its climatological peak is through December and January as this is when the stratosphere is naturally at its coolest. As with the so often mentioned jet stream, the polar vortex moves west to east circulating around the arctic circle. A cold stratosphere has an intensified SPV whilst a relatively warm stratosphere has a perturbed or weakened SPV - on occasion the SPV may get displaced or split into two or more vortices via what's called a sudden stratospheric warming (SSW). There are numerous types of SSWs which can have various levels of impacts on tropospheric weather patterns.
1) A Canadian warming which happens early on in the season through November and December. This is where an Aleutian High moves eastward across Canada in the stratosphere and causes a modest increase in polar temperature. They do not result in a reversal of the zonal wind.
2) A minor warming which refers to all SSW events during the middle of winter where the zonal wind at 10hPa 60N does not reverse to easterly. It can be an intense warming but if the zonal wind does not reverse at the layer mentioned, then it's classified as a minor SSW. Minor warmings are usually only the result of displacements and tend to have temporary effects on tropospheric patterns but can be sometimes be significant.
3) A major mid-winter warming which refers to SSW events where the zonal wind at 10hPa 60N does reverse to easterly. This is the one you usually hear when people mention "SSW" even though it is technically incorrect to label only this as a SSW as SSW is a general term to describe all warming events of the stratosphere. Major warmings can be the result of displacements and the polar vortex splitting into two or more. Provided their effects downwell back down to the troposphere where weather takes place, they can have long-lasting effects. Polar vortex splits have a higher likelihood of tropospheric responses and tend to be less temporary compared to displacements.
4) The final warming which happens in April or May most of the time - can happen on exceptional occasions in March. This is where the stratosphere has its final warming before the polar vortex goes into hibernation for the incoming summer season and slowly wakes back up in late August but don't tend to see much impact until November. The wind reverses to easterly for a couple of months. Final warmings happen every year and don't tend to impact all that much but if it's a dynamic final warming where it dives off a cliff from a strong state then it can have the impacts of a major mid-winter warming.
To get a SSW, we need to see wave activity being upwelled to the stratosphere via blocking high pressure over the Urals and low pressure over the Aleutian Islands. Historically this is more likely during El Niño and easterly QBOs which we both have this year. However, the history of "major" SSW events doesn't show enough difference to definitively say major SSW events have a higher likelihood during El Niño compared to La Niña.
Here's a look back to last season, 2022-23. Pay attention to only the red line as the blue line is 2021-22.
Late November and early December 2022 gave a unique situation where the upper stratosphere didn't have much going on as evidence by the graph below of zonal winds at 10hPa. However, the lower stratosphere featured a split of the polar vortex and had an effect on the tropospheric weather patterns which aided the development of the cold, blocked pattern we had through the first half of December 2022. As any Ural blocking disappeared, the stratospheric polar vortex intensified through December and into January resulting in a prolonged period of stronger than climatological norm zonal winds.
Through February we started to see signs of the stratospheric polar vortex easing which naturally happens anyway closing in on spring. Late February brought a major SSW however with the zonal winds reversing to easterly. This would bring a blocked pattern into early March and the blocking would aid a wintry episode through the second week. This was only a displacement event and it wasn't long until we saw any blocking vanish with March turning milder too. The final stratospheric warming occurred around the climatological average of April 15th.
Not the most interesting season as far as the stratosphere is concerned but we've had worse.
With all that out of the way, what about the coming season? As with our weather, there is no way to tell what the stratosphere will hold for 2023-24. It has a forecasting limit too.
The extended ECM shows a weakening of the stratospheric polar vortex through December but very few are going for a major event (reversal) and this is also up from recently with more members veering towards staying on the strong side.
As far as the GEFS go, we can see they are going for an exceptionally strong SPV for the time of year getting to record-breaking territory through to mid-November. Sometimes this is not such a bad thing as very strong SPV patterns can lead to developments of blocking over the Urals down the line. Sounds counterintuitive I know but it happens. The saying "the bigger they are, the harder they fall" comes to mind. In fact, we often see a strong period of zonal winds prelude SSW events - two of the biggest ones January 2009 and February 2018 both featured one. January 2009 was especially remarkable.
Looking at the CFSv2 forecast for November. Ideally would see lower heights over Alaska but a lot of blocking signalled for the Urals which would perturb the SPV down the line you would think to some extent.
Overall, no signs of a major SSW event but that's to be expected. There hasn't been a major SSW event in the month of December since 1998 and November since 1968. SSW events usually happen after the New Year. As far as possibilities down the line, we've been in worse situations before than 2023 in my opinion. A record-breaking strong SPV at this point in time isn't necessarily bad news and can be good compared to a weakened SPV where SSW events cut off wave activity and aid intensification after going into the winter which we have seen numerous autumns this past decade. The hemispheric pattern expected through November could be better in some ways but it's not terrible either.
Photography site - https://sryanbruenphoto.com/
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Yup. Longer range EC showing (and I don't use this word lightly) MASSIVE canadian warming through early to mid-December.
We haven't seen much canadian warmings occur this century, they seem to have been on the decline. This is excellent in two ways:
- It means we will have less of a worry about a Canadian vortex lobe blowing up explosive cyclogenesis across the North Atlantic.
- It can aid the developments of a PV split down the line.
We shall see 👀
Photography site - https://sryanbruenphoto.com/
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EC46 is still signalling the Canadian warming to occur around the same time as a previous post above.
Interesting how much the model ensemble mean (the thick blue line) is now going for a persistent weaker than average U-60N right the way to January. Maybe not totally realistic but interesting nonetheless. A significant minority starting to show a major SSW through the New Year of 2024 too - they're a minority and we're talking over a month away so just worth mentioning rather than taking into account at this stage.
GEFS continues to show a huge split as far out as they go with many going for a re-strengthening of the upper stratospheric polar vortex following a minor warming event or perhaps this is the same Canadian warming that the EC46 is seeing. The geopotential heights suggest it could well be. Keep in mind, the EC charts are anomalies and don't show the true temperature thus there is no true way of seeing where it places the Aleutian high that would be causing the warming or the distortion of the stratospheric polar vortex over Europe.
Photography site - https://sryanbruenphoto.com/
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Latest EC46 shows a very significant deceleration now in U-60N through December. More members also going for a major SSW (reversal) through early January 2024 but not a significant increase on yesterday. The main story is how many are now going for a weak stratospheric polar vortex through the month of December, look how far below the climatological mean (the thicker red line) the ensemble mean (the thicker blue line) goes.
The most recent available run of the extended GEFS is yesterday's 0z but even they start to show a sign of a deceleration through the month of December. Not as quick or as stubborn as the EC46 but definitely signs there.
Interesting times.
Photography site - https://sryanbruenphoto.com/
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Sryanbreun is the man for this but looks to me getting stretched towards Northern Europe.
Actually from what im reading a stretched PV can indicate a milder scenario for Europe.
But MJO predicted to be in colder phase 8-1 for early January.
Post edited by bazlers on1 -
A stretch of the stratospheric polar vortex is the start of its demise that is expected later on in December and into the New Year. A typical stretched SPV can result in mild weather for us because of the normal progression of minor SSW events where the warming starts over Siberia and displacing the PV towards western Europe which is favourable for explosive cyclogenesis in the North Atlantic. This brings mild and wet conditions into western Europe even though the NAO can go temporarily negative (infamously known as west based -NAO).
All these events have to begin somewhere. They take off via persistent Aleutian troughing and Ural blocking. This might mean the Atlantic can become active as a vortex lobe gets displaced over Greenland. We are expecting that anyway courtesy of the MJO cycle transitioning to phases 4/5. Phase 4 MJO in December during El Niño typically translates to an unsettled pattern for Ireland with deep Atlantic trough but running up against ridging to NE backing into the Urals - this combined with Aleutian troughing here is ideal for propagating wave activity upward to perturbing the SPV. Phase 5 is similar but can be less Atlantic dominated with more enhanced ridging from Europe, a very mild setup.
When the convection returns to the West Pacific and the MJO goes into a strongly amplified phase 8, this can lead to a major build in pressure northward from the south into a high latitude blocking feature over Scandinavia. The stratospheric polar vortex at that point in time would be probably primed for a major SSW event (reversal). Whether it would be just a displacement or a split is dependent on how much wave activity occurs.
The extended EC46 has been flip flopping a bit on its day to day runs (to be expected) on the strength of the signal for a deceleration in zonal winds but the latest is identical to what I've shared previously. The trend remains the same.
The medium range GEFS continue to be less bullish on as significant a deceleration with little change on the wide variation by day 16. More have definitely veered towards the weaker side from previous updates but you would not be able to make a stance on where it sees the zonal wind going just by this forecast alone.
Here's the most recent available extended GEFS trends from the past 3 days. Initially we get the deceleration of the zonal winds via the expected Canadian warming. As always with Canadian warmings, the weakening is very modest as these warmings hardly make a difference to the placement of the SPV from the North Pole which this is based on (60N). The zonal winds make a recovery to average on the ensemble mean and show a deceleration again in the longer term. As the top left mentions, the GEFS have a strong vortex bias which must be considered. So the fact it's showing a deceleration at the long range is a good sign even if it ain't as bullish as the EC46.
Another way of seeing this is through the % of ensembles in the extended GEFS that go to particular strengths and percentiles. Below average passes the median % and goes over 70% by the end of 2023 with a small % going for record weak SPV. Above average or strong SPV is in the minority by then.
This is an excellent place to be in as far as the stratosphere is concerned. The SPV is primed to get perturbed a lot through the remainder of 2023 if the forecast verifies and we get persistent Aleutian troughing/Ural blocking. The active MJO being forecast by many models at the moment (not all mind, some have it much weaker) also favours disruption to the SPV. This is probably the most promising stratospheric outlook I've seen since 2017-18 but don't start getting excited yet, a lot of things can go wrong if you're not well used to that by now. We're talking about the risk of a major SSW event developing through early 2024 and we've only just began December.
Lots to talk about and keep an eye on anyway. Great to see.
Photography site - https://sryanbruenphoto.com/
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Certainly great to have Butler providing some thoughts as she's one of the best in this (stratospheric) field.
Also it continues, the EC46 not letting it go. Just getting more and more bullish. Has started to bring the (very low mind) chance of the major SSW earlier too.
Extended GEFS is also getting there, its most below climatological average trend yet. Reminder it does have a strong vortex bias at this range.
Photography site - https://sryanbruenphoto.com/
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No changes again.
If anything the medium range GEFS which I haven't showed for a bit (been showing the extended GEFS which lag a day behind and only run at 0z) have gone more towards the weakening side. Previously these had been showing a split between a re-strengthening and a weakening. Now more are going for a deceleration.
Note the CFS is going for a monsterous major SSW through January.
Photography site - https://sryanbruenphoto.com/
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No changes again in the medium range GEFS as per the previous post. Largely in line with the extended GEFS now.
No changes mostly on EC46 either so there's not much reason to update here though there is less members going for a reversal if you were to look at the smaller details.
Photography site - https://sryanbruenphoto.com/
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In fact just had a look at the GFS 12z operational. It showed U-60N getting down to 5 m/s by St Stephen's Day which in my opinion is too quick based on this current evidence. That's not far off getting to reverse levels and it's a displacement but it's displaced over towards Siberia so wouldn't be the worst in the world. Anyway, I suspect this is too hasty with its evolution.
Photography site - https://sryanbruenphoto.com/
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As I had expected, the operational runs have gotten more realistic since the above one with a slower evolution into a big deceleration.
However, yesterday's extended GEFS have massively upped the signal for a reversal into early January with as much as 48% of the ensemble members going for it by the second week. Nearly half the suite at such a lead time is nuts.
Photography site - https://sryanbruenphoto.com/
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This GEFS 500hPa anomaly from Cohen shows perfectly the pattern needed to drive a wave-2 attack on the stratosphere. If we manage this, there's little to suggest we won't get a major SSW in early 2024 but persistence would be required for a full on split.
Good heat flux for disruption signalled by the GFS (very basically a measure of how much wave activity is being propagated from the trop to the strat).
Photography site - https://sryanbruenphoto.com/
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Not as good trends today from both the extended GEFS and EC46 in terms of U-60N. Both have significantly levelled off on their deceleration, moreso the GEFS whilst the EC46 is more stark for the decrease in perturbations getting to reverse level. Not sure what this delves from so just gonna have to take them as they are for the moment waiting to see if it continues or is it another daily variation.
Just for the record, they both show a deceleration, just not as noteworthy as they had been showing. This decreases the risk of a major SSW naturally.
So there remains a moderate risk of the PV getting stretched by a significant minor SSW through the rest of December into the New Year. The likelihood of that evolving into a major event (reversal) has decreased judging by these runs. We shall see if that continues.
One thing I have pondered is what will happen if hypothetically we do get a major SSW event, split or displacement it doesn't matter, how much would it be able to force a change in the pattern without aid from the MJO for example which will be restarting its cycle in about 2 weeks after the recent forecast failure for an amplified phase 7/phase 8 that would have allowed pressure to build to the high latitudes. This just hasn't verified as the MJO struggles to transport from the Maritime Continent to the West Pacific. A weak stratospheric polar vortex (without major forcing from a huge event) on its own is unlikely to impact much as it's not terribly different to what it has been for a while now. One of the reasons why some SSW events hit hard and quick is because they succeed from strong PV events, February 2018 and January 2009 were just that.
But not worth talking about effects, let's see if we get an event happen in the first place and enjoy observing the stratosphere unfold above us.
Photography site - https://sryanbruenphoto.com/
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Better on the GFS this morning. Both the 0z and 06z OP runs got U-60N down under 10 m/s as far out as they go.
The 0z GEFS has a ridiculous spread generally between 0 and 40 m/s. It's only one single member doing it but the first medium range GEFS I have seen to show any perturbation going for a reversal.
Photography site - https://sryanbruenphoto.com/
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Terrible medium range GEFS this morning with a big uptrend in U-60N. Look at the difference in the number of ensembles going below 20 m/s for instance versus what I shared yesterday morning. Very few.
On the other hand, there could be the possibility of nacreous or polar stratospheric clouds over the north of Ireland on Wednesday and Thursday with the GFS showing some lobes of -80C from the displaced SPV.
Photography site - https://sryanbruenphoto.com/
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The afternoon's GEFS were a touch better but still quite poor on the stratospheric warming front.
Meanwhile, the 18z went off on one with a very significant warming over Siberia at the end which would put immense pressure on the SPV if we had more frames to go off of. The most intense warming shown by an operational run to date from what I've seen but this is still very much deep fantasy land.
Photography site - https://sryanbruenphoto.com/
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EC46 is the most bullish so far to suggest a big dip/deceleration in U-60N. This is a change from its recent trend towards less and less perturbations getting to reverse levels.
The most recent available ECM operational run only goes out to 29th December and at that point is showing a rather minor wave-1 warming over Siberia displacing the SPV towards eastern Europe. Besides that, there's not much else you can delve from this chart alone. Would need more frames but my best guess would be the possibility of a technical reversal of the U-60N through the first into the second week of January followed by a rather quick recovery as it would probably be a displacement event with how it looks here at t240.
The interesting part is where it is displaced. If it's displaced towards Scandi, that would open the doors for some very significant Atlantic ridging possibly if not sustained provided there would be downwelling. If a more major warming were to occur, the SSW would likely get displaced back to the North Atlantic which is not as interesting but also opens the door to the possibility of a split as the SPV would be more prone to getting stretched.
The medium range GEFS has seemed to have sort itself out after showing a poor trend the past week with an acceleration likely in U-60N through the final few days of December as the SPV returns temporarily to the North Pole after the recent displacement towards Europe. This is then succeeded by a deceleration through early January. Most keeping it within the 10-25m/s range as far out as they go on this latest 18z run.
I would mention the fact the GFS looks quite "meh" on the wave-2 amplitude front to give sufficient planetary wave activity to evolve into a split but the data we have to hand from Stratobserve... well.. the GFS goes the complete opposite to most of the modelling. Look at how cold it keeps the average stratosphere temperature for 60-90N 10hPa whilst the rest of the output warms things up. Laughable. Simultaneously as you'd expect this translates to an intensification in U-60N which is also the opposite to what the rest of the modelling goes for minus some minority ensemble members.
Some interest (food for thought if nothing else) shared by the World Climate Service. Using the top 10 analogs that had similar polar cap heights to the EC46 initialised on the 19th December, 8/10 of them seen a major SSW occur.
I'm pretty torn to be honest. More runs needed.
Photography site - https://sryanbruenphoto.com/
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Everything remains the same, nothing has changed as of the most recent post.
Only thing I'll update on is the GFS 12z is the first run that I've seen an evolution that would hint at the possibility of a split but it's at the very end. See where I've marked, you can see a second lobe over NA but note the absence of a secondary ridge.
The GFS continues to be different to the rest of the models with a slower progression and colder stratosphere.
Photography site - https://sryanbruenphoto.com/
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It's Christmas Day but I couldn't resist archiving this one from the latest GFS 06z.
It goes onto produce an absolute beast of a major SSW. A major wave-1 split with a reversal of -24.3 m/s at the very end. This is getting to the top percentile of major SSW events.
Fantasy land but it must be mentioned that both the GEFS and EPS are in favour of a strong wave-2 driven pattern which would split the vortex come the second week of January.
Anyway, I just wanted to bank this as it's a rare beauty. Merry Christmas to you all and enjoy the rest of your day best you can in spite of this mediocre weather!
Photography site - https://sryanbruenphoto.com/
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