Autumn & Winter 2023---2024 Stratospheric Polar Vortex. Events, Analysis, Discussions AND Outlooks

[Andy8472]

ECM 10mb Zonal wind EPS

[lorenzo]

Well...

[lorenzo]

Any split chat is bs, this will be lucky to get to 0, watch the u10 ens on EC retract from here..

[bluearmy]

50 minutes ago, lorenzo said:

Any split chat is bs, this will be lucky to get to 0, watch the u10 ens on EC retract from here..

Agree that atm I don’t see where a a split comes from Tony 

but if the trop modelling has the Russian ridging incorrect then it will also have events higher up incorrect ????

it has to be noted that the last few days on the ec46 zonal flow for January has gradually increased that mean flow through the month. Still looks likely to be weaker than average but we are certainly in a momentum period where the step away from very weak to averagely weak is becoming apparent. 

 

 

 

 

 

[sebastiaan1973]

The importance of the polar vortex at 100 hPa by Simon Lee

The most commonly-used diagnostic of the strength of the stratospheric polar vortex is the zonal-mean zonal wind at 10 hPa (~30 km) and 60°N (U10-60), which is westerly during winter. It is an easy diagnostic to compute and understand, which probably helped drive its uptake. Reversals of U10-60 to easterlies indicate either a major sudden stratospheric warming (SSW) if they occur during midwinter, or the final stratospheric warming (and the transition to the summertime state) if they occur in spring. U10-60 has also been used to diagnose strong polar vortex events, sometimes taken to be when the winds exceed 40 m/s (this is not quite as strictly defined as SSWs, it must be said).

10 hPa is, however, about 20 km above the tropopause, while the polar vortex is very much a 3-dimensional phenomenon. Condensing it into a single diagnostic is rooted in solid dynamics, but is far from the only part of the story — particularly when it comes to interpreting the influence of the vortex strength on tropospheric weather patterns. Perhaps due to the availability of forecast plots, or the translation of information from academia to forecasters and the public* (which I try to contribute to as best I can), the importance of the lower stratosphere is often overlooked. That’s what this blog is about.

This year is the 20th anniversary (!) of the publication of “Stratospheric Memory and Skill of Extended-Range Weather Forecasts” in Science, by Mark Baldwin et al. It is one of the key stratosphere-troposphere coupling papers that helped establish the role of the stratospheric polar vortex in tropospheric weather and climate prediction. In the paper, Baldwin et al. compute the “e-folding timescale” of the Northern Annular Mode (NAM) as a function of pressure level and time of year. The NAM in the stratosphere is effectively the strength of the polar vortex and well-correlated with the zonal winds at 60°N. The e-folding timescale is just the time taken for the autocorrelation of the NAM index to decay to 1/e (about 0.37), which is a measure of the persistence of the NAM.

They showed that, during winter (mainly December to February), the timescale of the NAM maximises in the lower stratosphere at around 100-150 hPa. Its e-folding timescale peaks at over four weeks. In contrast, up at 10 hPa, the NAM timescale is much shorter during winter — two-to-three weeks. Above that, toward the stratopause, it’s even shorter (a few days; not shown in their paper). Baldwin et al. also demonstrated that the timescale of the tropospheric NAM peaks at the same time as the peak in the lower stratosphere, which would be expected from a downward influence. (They also showed that the lower-stratospheric NAM can predict the surface NAM better than the surface NAM predicts itself, confirming its utility.) Figure 1 here shows something similar to Figure 1a in the Baldwin et al. paper, but just for 100 and 1000 hPa. Note how the long timescales are not fully developed in November-December, which is important for thinking about any early-winter coupling.

Figure 1: Timescale of the NAM at 100 hPa (red) and 1000 hPa (black) following a similar method to Baldwin et al. 2003 (Science), but the NAM is here computed as EOF1 of zonal mean geopotential height poleward of 20°N (following Baldwin and Thompson 2009, QJ).

Given all that, it’s then perhaps not surprising that myriad subsequent studies have reported that the lower stratosphere plays a key role in whether or not an SSW strongly influences surface weather patterns. Back in 2009, Ed Gerber et al. stated:

Furthermore, not all SSW events are created equal;  a sharp reversal of the zonal winds at 10 hPa does not guarantee deep penetration through the stratosphere, and it is the lower stratosphere that appears to influence the troposphere.

More recently, Ian White et al. (2020) demonstrated a remarkably “generic” linear response of the troposphere to the 100 hPa circulation anomalies following SSWs. Hilla Afargan-Gerstman et al. (2022) also pointed out that spread in the lower-stratospheric anomalies post-SSW dominate spread in the Atlantic jet response. (This is not an exhaustive list of such studies.)

Therefore, perhaps the best way to think about it is that anomalous vortex states at 10 hPa can serve as a predictor of anomalous vortex states at 100 hPa, which themselves then “tickle the troposphere” (as Kushner and Polvani 2004 so delightfully described it) almost instantaneously.

Thus, a large fraction of instances when the stratosphere is purportedly in a different state to the troposphere, or somehow not influencing the troposphere, stem from looking at diagnostics 20 km above the troposphere. Rather, one should first consider whether large circulation anomalies are present in the lower stratosphere, below 10 hPa, for a more complete understanding. It is rare to see large circulation anomalies in the lower stratosphere opposing the tropospheric state: see, for example, the weather regime probabilities computed using 100 hPa 60°N zonal wind anomalies in Charlton-Perez et al. 2018, or my paper from the following year.

In fact, December 2022 (Figure 2) provides a nice example of when the bottom half of the vortex — below 10 hPa — was weak (negative NAM), while the top half was neutral or strong. A major SSW occurred on 16 February, but only coupled down to the troposphere once the vortex below 50 hPa finally weakened — which took place following a second burst of wave activity and deceleration in late February. The coupling was brief, as the vortex recovered quite quickly thereafter.

Figure 2: time-height cross-section of the NAM during winter 2022-2023. See here for more information.

All this brings me to where we are at present, on 23 November 2023. Up at 10 hPa, the vortex is currently unusually strong, with U10-60 close to 40 m/s (about 10 m/s above climatology). Figure 3 shows how this has evolved over the last 60 days. But, in addition to it being early in the season — when the vortex is not as well-developed in the lower stratosphere — the bottom half of the vortex is being bashed around by a surge in upward-propagating wave activity (Figure 4). Thus, in this case, the lower stratospheric vortex weakens first, as Figure 3 shows. The rapid loss of the strong vortex from the bottom half of the stratosphere is then favourable for Greenland blocking/negative tropospheric NAM regimes to develop, which is what forecasts are increasingly suggesting. This could be thought of more as a “feedback”, perhaps, than the downward propagation typically seen post-SSW.

Although the timescale at 100 hPa is long, a large contributor to that comes from events that encompass the entire vortex. In this case, if the upper stratospheric vortex remained strong, it would likely erode the weak vortex anomalies in the lower stratosphere. Some forecasts, such as ECMWF’s extended-range suggest the 10 hPa vortex will weaken (this system has been predicting a weak 10 hPa vortex by mid-December since late October). If that were to happen, to understand how that would influence the troposphere, we’d need to then be once again looking at the extent to which the anomaly reaches the lower stratosphere.

To summarise, (also, hello to everyone who skipped to the end), I am not suggesting one should abandon 10 hPa as a diagnostic for how the vortex is behaving. But, for interpreting the extent of coupling to the troposphere, one should first consider the state of the vortex just above the tropopause. Looking at 10 hPa and 1000 hPa and noting they are in apparent disagreement neglects the dynamics of the 30 km of atmosphere between them.

[sebastiaan1973]

The representation of the stratosphere and stratosphere–troposphere coupling processes is evaluated in the subseasonal Global Ensemble Forecast System, version 12 (GEFSv12), hindcasts. The GEFSv12 hindcasts develop systematic stratospheric biases with increasing lead time, including a too strong boreal wintertime stratospheric polar vortex. In the tropical stratosphere, the GEFSv12 winds and temperatures associated with the quasi-biennial oscillation (QBO) tend to decay with lead time such that they underestimate the observed amplitudes; consistently, the QBO-associated mean meridional circulation is too weak. The hindcasts predict extreme polar vortex events (including sudden stratospheric warmings and vortex intensifications) about 13–14 days in advance, and extreme lower-stratospheric eddy heat flux events about 6–10 days in advance. However, GEFSv12’s ability to predict these events is likely affected by its zonal-mean circulation biases, which increases the rates of false alarms and missed detections. Nevertheless, GEFSv12 shows stratosphere–troposphere coupling relationships that agree well with reanalysis and other subseasonal forecast systems. For instance, GEFSv12 reproduces reanalysis relationships between polar vortex strength and the Northern Annular Mode in the troposphere. It also exhibits enhanced weeks 3–5 prediction skill of the North Atlantic Oscillation index when initialized during strong and weak polar vortex states compared to neutral states. Furthermore, GEFSv12 shows significant differences in Madden–Julian oscillation (MJO) amplitudes and enhanced MJO predictive skill in week 4 during easterly versus westerly QBO phases, though these results are sensitive to the level used to define the QBO. Our results provide a baseline from which future GEFS updates may be measured. Evaluation of Processes Related to Stratosphere–Troposphere Coupling in GEFSv12 Subseasonal Hindcasts in: Monthly Weather Review Volume 151 Issue 7 (2023) (ametsoc.org)

https://journals.ametsoc.org/view/journals/mwre/151/7/MWR-D-22-0283.1.xml

 

[sebastiaan1973]

Thanks to Gerhard. 

https://www.climate.gov/news-features/blogs/polar-vortex/welcome-polar-vortex-blog

We are excited to announce that NOAA Climate.gov, home of the highly popular ENSO Blog, is venturing into a colder, darker, and windier corner of the atmosphere with the new Polar Vortex Blog. We plan to explore various facets of the winds, climate, and chemistry within the fascinating region of the atmosphere known as the polar stratosphere, and explain how this region can sometimes drive big changes in our weather patterns!

While ENSO may be the seasoned celebrity in the seasonal forecasting world, in recent years the stratospheric polar vortex has become a rising star: constantly making headlines and being stalked by the paparazzi, but often misunderstood or misrepresented. We hope to clear up misconceptions, highlight new research, and discuss what the polar vortex is up to and how it may affect our winter’s weather. We expect there to be 1-2 posts per month between December and March, with the initial focus on the Northern Hemisphere polar vortex (yep, there’s one down south, too!).

So who’s on the team?

Amy Butler is a research scientist at the NOAA Chemical Sciences Laboratory and an expert on the stratosphere and its influence on weather;

Laura Ciasto is a meteorologist at the NOAA Climate Prediction Center. She leads the development of stratospheric and teleconnection forecast products, but is also a Week 3-4 forecaster (NOAA’s description for forecasts of weather conditions 3-4 weeks in the future);

The Climate.gov graphics and data visualization team and managing editor, Rebecca Lindsey, with the NOAA Climate Program Office.

While we [Amy & Laura] are the lead editors of the blog, we hope to have guest contributors who can share their own perspectives and research on the polar vortex and related topics. And of course, this blog will not succeed without active engagement from you, our readers. We are happy to hear your constructive feedback and suggestions, and are excited to engage with you on this topic!

After reading this introduction, the first question you might have is likely: What is the polar vortex? And so, that’s where we’ll begin!!

[Andy8472]

Zonal winds from the 8th vs the latest, marked drop plus the GFS Ext. 

[Mike Poole]

22 minutes ago, Andy8472 said:

Zonal winds from the 8th vs the latest, marked drop plus the GFS Ext. 

ECM 46 zonal winds back lower in the medium term with more reversals after yesterday’s blip.

[Andy8472]

3 minutes ago, Mike Poole said:

ECM 46 zonal winds back lower in the medium term with more reversals after yesterday’s blip.

Aye it moves around a bit & big spread later on as expected, ECM has a displacement atm with encroachment into the pole  

[northwestsnow]

4 minutes ago, Andy8472 said:

Aye it moves around a bit & big spread later on as expected, ECM has a displacement atm with encroachment into the pole  

Would that chart indicate the SPV displaced over the UK ?

[Andy8472]

2 minutes ago, northwestsnow said:

Would that chart indicate the SPV displaced over the UK ?

yup

[Mike Poole]

11 minutes ago, northwestsnow said:

Would that chart indicate the SPV displaced over the UK ?

Looks like it is centred over the Barents Sea to me (minimum of the height contours).

[Andy8472]

2 minutes ago, Mike Poole said:

Looks like it is centred over the Barents Sea to me (minimum of the height contours).

True, my bad 

[summer blizzard]

Interestingly it turns out that the two most likely periods to produce a SSW are 1st-10th January and 20th-28th February. 

GFS0z operational got as low as 8ms today but the GFS ensemble mean to 1st Jan was still 20ms.

37% of Euro ensembles do get a SSW between the 7th-29th January (only 17% during week 1 of Jan now) but as the mean is actually rising, I think that's noise. 

So my message is essentially that it looks like we don't get a SSW during early January. 

 

[frederiksen90]

Doesn't it look more like a split than a shift?

 

https://charts.ecmwf.int/streaming/20231216-2000/6f/webp-worker-commands-64f9dd744-p2f2h-6fe5cac1a363ec1525f54343b6cc9fd8-oflmUL.webp

 

And with strong wave 2 activity now, which is more of a precursor to a split than a dislocation:

 

https://www.stratobserve.com/plots/20231216/tseries/anoms/gfs_nh-hgt-w2-60n_20231216.png

Edited December 16, 2023 by frederiksen90

[Andy8472]

GFS mean way more progressive on any warming than the op at 240 

[bluearmy]

1 hour ago, Andy8472 said:

GFS mean way more progressive on any warming than the op at 240 

The ec op is similar to the gefs mean on temps 

the eps mean is similar to the eps mean at day 15

[bluearmy]

15 hours ago, frederiksen90 said:

Doesn't it look more like a split than a shift?

 

https://charts.ecmwf.int/streaming/20231216-2000/6f/webp-worker-commands-64f9dd744-p2f2h-6fe5cac1a363ec1525f54343b6cc9fd8-oflmUL.webp

 

And with strong wave 2 activity now, which is more of a precursor to a split than a dislocation:

 

https://www.stratobserve.com/plots/20231216/tseries/anoms/gfs_nh-hgt-w2-60n_20231216.png

I think all we have at the moment is a displacement to asia - I don’t see any warming on any modelling reaching levels we would expect ahead of an ssw at 10hpa 60N.  I absolute agree that I can see a way forwards to a split but that’s just wishcasting on my part for now.  I am becoming of a mindset that if we don’t see a full reversal modelled within the next week (so three weeks from now) then we won’t see one this side of feb.  

[Andy8472]

12 minutes ago, bluearmy said:

I think all we have at the moment is a displacement to asia - I don’t see any warming on any modelling reaching levels we would expect ahead of an ssw at 10hpa 60N.  I absolute agree that I can see a way forwards to a split but that’s just wishcasting on my part for now.  I am becoming of a mindset that if we don’t see a full reversal modelled within the next week (so three weeks from now) then we won’t see one this side of feb.  

i agree, a guy on americanwx says this

"So, looking ahead to the potential one for early Jan, we’re talking perhaps near Jan 1-3. Jan 1st is still 15 days out while Jan 3rd is 17 days out. If anything, the ensembles are and have been showing a somewhat higher % of members in very early Jan with a major SSW this time than were being shown 15 days prior to the 2/16/23 event. This tells me that we’re still likely ~3-5 days from put up or shut up time. So, I’m thinking we probably have til ~Wed-Fri of this week to finally consistently start seeing a forecasted notable 60N 10 mb wind collapse from, say, near half the members if there really is going to be one in the first few days of 2024. Along with that, perhaps we’d see some GFS runs with majors. If any actual event were to instead not be coming til, say, 1/5-7, we’d then probably have still another week or so (say ~12/25) til put up or shut up time"

[SqueakheartLW]

Bring those negative anomalies down to the trop please

Can't be too much to ask for surely

[summer blizzard]

3 hours ago, SqueakheartLW said:

Bring those negative anomalies down to the trop please

Can't be too much to ask for surely

SSW or not (not in this case), it's interesting to see that the weakness does actually propagate down.

Remember that Jan 12 failed to achieve an actual SSW but the hit was enough to give us the first half of Feb 12.

Edited December 17, 2023 by summer blizzard

[Andy8472]

The big 4 at 240, At the moment looks like a outside chance of a SSW early Jan, it's more likely a big reduction in zonal speeds instead 

[bluearmy]

9 hours ago, Andy8472 said:

i agree, a guy on americanwx says this

"So, looking ahead to the potential one for early Jan, we’re talking perhaps near Jan 1-3. Jan 1st is still 15 days out while Jan 3rd is 17 days out. If anything, the ensembles are and have been showing a somewhat higher % of members in very early Jan with a major SSW this time than were being shown 15 days prior to the 2/16/23 event. This tells me that we’re still likely ~3-5 days from put up or shut up time. So, I’m thinking we probably have til ~Wed-Fri of this week to finally consistently start seeing a forecasted notable 60N 10 mb wind collapse from, say, near half the members if there really is going to be one in the first few days of 2024. Along with that, perhaps we’d see some GFS runs with majors. If any actual event were to instead not be coming til, say, 1/5-7, we’d then probably have still another week or so (say ~12/25) til put up or shut up time"

Of course this was before the 18z gfs ……

[Cheshire Freeze]

4 minutes ago, bluearmy said:

Of course this was before the 18z gfs ……

Indeed

 

An op run finally backing the GEFS

Edited December 17, 2023 by Cheshire Freeze