Author Archives: Simon Lee

About Simon Lee

PhD Meteorology student at the University of Reading.

10 FAQs on “Increased shear in the North Atlantic upper-level jet stream over the past four decades”

Lee, S. H., P. D. Williams, and T. H. A. Frame, 2019: Increased shear in the North Atlantic upper-level jet stream over the past four decades. Nature,

  1. What is the key result of the study?

Our study shows that the annual-mean vertical west-to-east (zonal) wind shear over the North Atlantic at aircraft cruising altitudes (within the jet stream) has increased by 15% (with a range of 11-17%) over the last four decades (1979-2017).

  1. What is vertical wind shear?

Vertical wind shear is the name given to the change in wind speed (or direction) with height. In our study, we consider the change in the speed of the west-to-east (“zonal”) wind with height.

  1. What data were used to get these results?

We used three reanalysis datasets (ERA-Interim, JRA-55, and NCEP/NCAR). These are our best estimates of the past evolution of the atmosphere, based on observations. We use data from 1979 onwards, as satellites have been routinely used to monitor the state of the atmosphere since then, increasing the reliability of the data.

  1. Why has wind shear increased?

On large scales like the North Atlantic, the vertical wind shear is driven by the difference in temperature as you move from the equator to the pole. When this is larger, the wind shear is larger. At aircraft cruising altitudes (around 34,000 feet), this temperature difference has increased, causing an increase in wind shear.

  1. Why has the temperature difference increased at aircraft cruising altitudes?

At low latitudes (less than about 50N), temperatures are warming at this level in response to increasing greenhouse gases. Poleward of 50N, temperatures are in fact cooling. At these latitudes, this level (34,000 feet) is in the stratosphere, not the troposphere where we live. Increasing greenhouse gas concentrations actually cause this layer of the atmosphere to cool. There is also a cooling effect from a decline in concentrations of ozone.

  1. Why should we care?

Wind shear is a key driver of clear-air turbulence (CAT), which is a significant aviation hazard as it is invisible and difficult to predict. Turbulence in general has been found to play a role in the majority of weather-related aviation incidents. Climate models project a large increase in CAT over the North Atlantic under future climate change (Storer et al. 2017). Our study shows observation-based evidence that a key driver has already increased by a large amount, supporting these projections.

  1. The study finds shear has increased. Have encounters with turbulence increased?

Unfortunately, it is very difficult to say this with any degree of confidence. Over the last four decades, airplane designs have changed, flight numbers have increased, and we are now able to identify regions of possible turbulence and avoid them – so it is difficult to produce any consistent record of turbulence observations. However, our study suggests it may have, as a key driver has increased by a large amount.

  1. So, is the jet stream strengthening or weakening?

We find that, in the lowermost ~5 km or so, the temperature difference between the equator and pole is weakening, thanks to the rapidly warming Arctic. This contrasts with the increasing difference at higher levels. We find that these effects are approximately equal and opposite, like a balanced tug-of-war. The overall speed of the jet stream aloft depends on the total effect (the vertical integral), so there has been no significant change to the speed of the jet stream at aircraft cruising altitudes. As our study only looks at the annual-mean, it is possible that different weakening or strengthening trends occur in different seasons, as other studies have found.

  1. Is the jet stream becoming wavier?

We do not look at this in our study. Some previous studies have suggested this, primarily as a result of the decreasing temperature difference in the lower part of the atmosphere. Our results neither prove nor disprove these findings.

  1. What does this mean for the future?

We focus on what has been observed to occur, rather than what is projected. However, most studies suggest the upper-level “tug” will win, leading to a strengthening jet stream, with impacts on transatlantic flight times (Williams 2016). As increasing greenhouse gas emissions continue to increase temperatures in the troposphere and cool those in the stratosphere, we expect shear to continue to increase at flight levels (though large interannual variability is likely), consistent with model projections of increasing turbulence.


Canadian, TX tornado – May 23 2019

May 23rd, 2019. Day 4 of our 2019 chasecation. Only a few days after the infamous and frustrating “high risk bust” in Oklahoma on May 19th (which remains the worst chase day I’ve had), we found ourselves in the Texas Panhandle under a Moderate risk – having driven all the way from Missouri the preceding day, where we had successfully chased the deadly Golden City tornado.

We based ourselves in Perryton, only a few miles south of the Oklahoma border, and in traditional chaser fashion took up residence in McDonald’s (cheap coffee, fast food, WiFi… heaven!).


McDonald’s in Perryton, TX, with a developing severe thunderstorm behind, 2:27 PM.

By around 3:30 PM we were extremely close to a severe thunderstorm, but the visibility was terrible. Anyone who chased during this period will recall just how much haze and dust there was, and we were filled with the same frustration we felt back on the high risk day. Eventually, we gave up on this storm due to an inability to see anything noteworthy. It went on to be tornado-warned shortly after we abandoned it, so we thought we had made a huge mistake…

How close we were to the storm vs. the seemingly quiet grey skies we could see…

However, by 5:30 PM a new tornado-warned cell had developed further south, and we were ready and waiting – along with a huge number of other storm chasers which included the TORUS field campaign and Reed Timmer! We were in a perfect position and very hopeful, but yet again visibility was extremely poor due to the haze. Several possible wall clouds were noted amongst the murk, but the comparison between the NWS warning of a “very dangerous storm” and what we could actually see was stark.

The Dominator, and a healthy looking tornado-warned storm on radar – but not looking at all interesting in reality!

So, we gave up on this storm and headed a bit further south and east, now concerned that the storms would grow upscale into a linear mode – though we were far enough from the KAMA radar that there was quite a difference between the radar reflectivites and the ground truth (always worth remembering the beam height!).

At around 7 PM, we pulled over to look west and re-think our plan. At this point I’d pretty much lost hope, and I was chatting to a fellow frustrated storm chaser about the haze. Suddenly, one of my chase team (thanks Rounak!) suddenly cried out “THERE! ON THE HORIZON!” (make sure you read that in an Australian accent).

Our first view of the tornado (looking W from US Highway 83 N of Canadian) as it appeared on the horizon in the wilderness, and the accompanying radar presentation. Note that the storm was not tornado-warned whilst a large tornado was already in progress, highlighting the need for spotter observations.

Sure enough, a big wedge tornado had dropped to the ground and emerged out of the murk, and out of a region where there were so few roads it was almost impossible to actually be close enough to see the tornadogenesis. After grabbing a photo and quickly tweeting NWS Amarillo (important storm chaser duty), we all hit the road and bolted north to get close to the tornado – filled with awe and adrenaline that there was actually a big wedge on the ground and we were about to get really close to it! *cue classic excited girly noises*

By 7:09 PM we were just NW of Canadian on Highway 83, and now very close to the wedge. This is where the whole experience became surreal, and the reason I’m writing this blog. Here I was, standing under a kilometre away from what must have been, at that time, the most violent atmospheric phenomenon on Planet Earth. Wind and rain were lashing us, and right in front of us was this huge tornado churning along through the high Texan plains. It was making an incredible sound which I can’t quite describe, but I think at the time we described it as “eating”.

A collection of photos of the Canadian wedge tornado taken around 7:11 PM.

Some might have felt fear, others might have felt satisfaction or scientific curiosity.

I just remember feeling completely transfixed. I remember other members of my crew shouting that we should go because we were probably about to become too close, but finding it very difficult to tear myself away from it no matter how close it was becoming.

The tornado felt strangely personable. Now, I’m aware that’s a strange thing to say, and you might roll your eyes reading about a meteorologist having a moment like this. But in that moment I felt a transcendental connection to the atmosphere which I’m sure I won’t ever forget. I am now finding that it is difficult to put it into words.

Maybe it was a rush of knowing that the thing I love most – the atmosphere – was producing a deadly amount of vorticity right in front of me. Deadly, but in a peaceful way – tornadoes aren’t malicious, because the universe isn’t malicious (I’m sure this is almost a quote from Interstellar). Maybe it was the realisation of a life-long dream to get close to a wedge tornado.

It is moments like these why I chase storms. To be right there with the awesome power of nature, and to be all-encompassed by such a tremendous, indescribable feeling. I’m not religious, but I’d like to think this is what seeing God would feel like.

The tornado was rated EF2, but NWS noted that it may have been stronger during its time out in the open fields.

Trump on climate change: what he should have said

If I followed up every time Donald Trump opened his mouth on climate change with a blog post pointing out where he was wrong, I’d have no time left to do anything else – but this one is a bit more special. This week, Trump visited the U.K., and part of that visit involved a conversation with Prince Charles – an advocate of organic farming and fighting climate change. They couldn’t really have more opposing views, so what did POTUS have to say about climate change afterwards?

The following are quotes lifted from a BBC News article (as of 5 June 2019).

“I believe that there’s a change in weather and I think it changes both ways,” Mr Trump told Piers Morgan in an interview that aired on Wednesday.

This would make more sense if he said a “change in climate”, as weather changes all the time. This one sentence encapsulates the President’s inability to grasp the difference between weather and climate. He’s correct in what he says – weather changes both ways! Indeed, some places do have a cooling climate – e.g. the North Atlantic warming hole – whilst it’s possible that global warming may lead to all kinds of extreme weather including cold weather extremes due to changes to the jet stream.

What he should have said: “I accept that there is solid scientific evidence for climate change, which, whilst it doesn’t necessarily rule-out some short-term increase in cold weather extremes, indicates there will be a long-term shift to overall more warmer weather.”

“He wants to make sure future generations have climate that is good climate as opposed to a disaster and I agree.”

Ah yes, “good climate vs. disaster climate”. He says he agrees… but does he? The following quote suggests he considers creating “good” climate to have nothing to do with greenhouse gases…

What he should have said: “Prince Charles, like many, is fighting to ensure future generations, and all plants and animals, can thrive on this planet even more than we do now. I want to join him in that.”

But Mr Trump once again placed the blame on other countries, namely China, India and Russia, for worsening air and water quality while claiming the US has one of “the cleanest climates there are”.

This is a great example of Trump seemingly getting confused between air quality, climate change, and greenhouse gas emissions. No one refers to a “clean” climate. That doesn’t make sense. If this statement was about greenhouse gases, it’s wrong – with US emissions placed 2nd to China globally.

What he should have said: “The US is one of the largest contributors to global greenhouse gas emissions, and we need to do something about that.”

“Don’t forget, it used to be called global warming, that wasn’t working, then it was called climate change, now it’s actually called extreme weather because with extreme weather you can’t miss,” the president said.

[insert WRONG! Trump GIF]

This is a fallacy which Trump keeps repeating. Global warming (the rise in Earth’s average temperature) drives climate change, which is defined over long time periods. It is manifest in an increased frequency of extreme or record-breaking weather events. I actually believe Trump doesn’t understand that, rather than repeating the falsehood for other reasons.

What he should have said: “Global warming leads to climate change, and now we’re starting to see the effects of this with extreme weather events around the world.”

Mr Trump pointed to past examples of weather disasters to refute the idea that “extreme weather” is becoming more common due to climate change.

“I don’t remember tornados in the United States to this extent but then when you look back 40 years ago we had the worst tornado binge we ever had. In the 1890s we had our worst hurricanes.”

Some serious cherrypicking here. And what’s a tornado binge? There’s also some implication in these words that people have suggested the recent US tornado outbreak is due to climate change, and that they have said so purely because of short-term memory. That’s a load of garbage. Moreover, tornadoes and hurricanes are among the more contentious when it comes to the effect of global warming on their frequency. Tornadoes and hurricanes are also very US-centric, suggesting the President doesn’t care about increases in severe weather in other parts of the world…

What he should have said: “I took an Advanced Statistics class at college, and I know that, in order to see whether there is a long-term change occurring, I need to perform many forms of statistical analysis – including significance tests on linear regression and Kolmogorov-Smirnov tests, to determine if my data is showing climate change, and not to randomly pick outliers in the past to prove my agenda. I also need to account for changes in data acquisition and homogeneity over time before making any conclusions. And a good example of this is the catastrophic loss of Arctic sea-ice since the late 1970s, when reliable records began.”

Final Stratospheric Warmings

The stratospheric Polar Vortex is currently at record-strong levels, based on the metrics of 10 hPa 60N zonal-mean zonal-wind and 60-90N average temperature. This is likely to be due to a combination of the timing and duration of the major Sudden Stratospheric Warming (SSW) in January: the duration of easterlies (most of January) shielded the vortex from vertically propagating wave activity for an extended period (these waves cannot propagate into a layer of mean easterly flow) allowing the vortex to redevelop following the SSW, whilst the early-season timing allowed strong radiative recovery in the absence of significant solar radiation. The associated positive stratospheric NAM is well coupled to a positive tropospheric NAM, which seems also to have enhanced North Atlantic storminess. It’s all a far cry from where 2019 began.


10 hPa winds per GFS 12Z analysis March 15 2019. With U1060 at 49.2 m/s, this fulfills the ‘strong vortex event’ criterion of Tripathi et al. 2015 (41.2 m/s). Image credit

However, and not for the first time this winter, our attentions have turned to stratospheric warming! Recent forecasts from the GFS ensemble have shown a large pulse of wave activity with an amplification of the Aleutian ridge stretching and displacing the vortex. Stratospheric temperatures are expected to rise rapidly, and some ensemble members are suggesting [U] 10 hPa 60N to turn easterly again in early April.

So, a rapid rise in stratospheric temperatures, accompanied by a displacement of the polar vortex and a reversal of zonal winds… is this then another major Sudden Stratospheric Warming?

Yes and no.

The major SSWs we talk about are more technically “major midwinter warmings” (MMWs), which are followed by a recovery of the stratospheric Polar Vortex to westerlies for at least 10 consecutive days before April 30 (Charlton and Polvani, 2007). The upcoming warming event is likely to become what is known as a dynamical Final Stratospheric Warming (FSW), also known as a “major final sudden stratospheric warming (“major final warming”, MFW)” (Manney and Lawrence, 2016).

The “Final Warming” refers to the transition of the stratosphere to “summer mode” – when the polar vortex dissipates and easterlies develop which persist until late August/early September when the vortex reforms. This is most simply driven due to thermodynamics – a result of solar radiation returning and warming the polar region. Climatologically, the last day of zonal mean westerlies is April 12th.

However, sometimes a sudden warming driven by the same dynamical processes which cause MMWs can occur so late in the season that the vortex is unable to recover as the Sun returns to the pole. This then becomes a dynamical FSW.

There’s a couple of reasons why FSWs don’t get the same attention as MMWs. They are dynamically different in that the planetary wave behaviour following the event is different (since stratospheric easterlies remain). The late-season timing of the events means the typical cold-weather outbreaks don’t occur with quite such severity, and tropospheric responses are often lost in the seasonal transition (with tropospheric jet streams and storm tracks evolving from winter-mode to spring-mode). There’s also the consideration of the anomaly magnitude – stratospheric zonal winds are weak or easterly during April regardless of whether a dynamical FSW occurred, which is very different to mid-winter. But, largely, the differentiation is just based on statistics, and everything in the build-up and occurrence of the warming is the same – which makes them another source of information about the behaviour and predictability of the troposphere and stratosphere.

Some FSWs are rather special and are “SSWs in hiding”. March 5th 2016 saw a very strong stratospheric warming event which produced the second-strongest zonal-mean easterlies on record (at 10 hPa 60N), but is often forgotten because it became an FSW [interestingly, the 2016 event also decelerated a record-strong vortex, like 2019 is expected to do…]. The oft-mentioned statistic of the February 2018 SSW being the first since January 2013 is indeed true, but it means the March 2016 event – which was an MMW in all but a technicality – is forgotten. There was some lively debate on Twitter recently as Judah Cohen argued the March 2016 event deserved to be included in the SSW Compendium! Indeed, it did have some significant surface impacts with downward propagation of a negative NAM phase into the troposphere. Britain saw a cold and snowy Easter that year, for example.


MERRA-2 data for 2015-16 10 hPa 60N zonal-mean zonal wind. The event in March 2016 saw date-record strong easterlies which did not return to westerlies, and thus was a Final Warming. Image credit NASA.


Time-height cross-section of normalized 65-90N geopotential height anomalies. The March 2016 stratospheric warming event is clearly evident, with downward propagation into April. Image credit NOAA CPC.

You have to draw the line somewhere. Maybe we should have a Final Stratospheric Warming Compendium so these events are given equal treatment.

Returning to 2019, given that we currently have a strongly zonal state, it will be interesting to see whether the upcoming warming event has an effect on that – though discerning any transitions from those which would occur simply because of the seasonal cycle may be more challenging. And, just like sudden warmings, we face the question of whether or not the troposphere is even all that bothered by the stratosphere!


Tripathi, O. P., A. Charlton-Perez, M. Sigmond, and F. Vitart, 2015: Enhanced long-range forecast skill in boreal winter following stratospheric strong vortex conditions. Environ. Res. Lett.

Charlton, A. J., and L. M. Polvani, 2007: A New Look at Stratospheric Sudden Warmings. Part I: Climatology and Modelling Benchmarks. J. Climate.

Manney, G. L., and Z. D. Lawrence, 2016: The major stratospheric final warming in 2016: dispersal of vortex air and termination of Arctic chemical ozone loss. Atmos. Chem. Phys.

A “winter heatwave” in a warming world

The final week of February 2019 has been characterised by anomalously warm, record-setting conditions over NW Europe. The United Kingdom broke its all-time maximum record temperature for February on several occasions and at several stations – the previous record of 19.7C from 1998 was obliterated, replaced with a new record of 21.2C (a huge difference of 1.5C, which were it to be replicated in August would see the UK experience 40C). For the first time, the UK experienced 20C during a winter month, and this moved the date of the first recorded 20C forward from March 2nd to February 26th. This was by all counts a “winter heatwave”, in magnitude and duration, and widely produced temperatures which wouldn’t be out of place in summer.

At the University of Reading, we also saw a new all-time (since 1908) record maximum for February – the previous record of 17.4C (which was first tied on Feb 25th!) was replaced with 19.5C on Feb 26th.


A classic “heatwave” sunset on February 27 from Whitley Wood Road, Reading, after temperatures reached 17.9C. A touch down on the previous day’s 19.5C, but still 0.4C above the old record!

Why was it so warm?

This is a difficult question to answer, but there’s several components which seem to have been required in order to get the atmospheric configuration such that high temperatures were possible over the UK. Here I present a few that I’ve noticed, but there’s likely other finer components, too (these are not necessarily in any meaningful order):

  • Rossby wave train: evident in Figure 1, there is a pattern in the 200 hPa height anomalies suggesting a Rossby wave train propagating out of East Asia and the Pacific has been evident for the last week. This provides the enhanced meridional flow associated with blocking weather regimes. Figure 2 also shows anomalously weak 250 hPa zonal flow in the mid-latitudes, suggesting reduced propagation speeds of weather systems allowing for (and associated with) extended blocking regimes.

Figure 1: 7-day (20-26 Feb) mean 200 hPa height anomalies from NCEP/NCAR Reanalysis. Apparent Rossby wave trains are shown with superimposed black arrows.


Figure 2: 250 hPa zonal-mean zonal wind anomalies from NCEP/NCAR Reanalysis for 20-26 Feb 2019. Note the anomalously weak zonal winds in the N Hemisphere mid-latitudes.

  • Extreme eastern USA jet streak & cyclogenesis: the record-setting jet stream winds seen on Tuesday 19th preceded the development of the blocking ridge. This may be associated through the downstream impacts of such extreme winds (Figure 3) – decelerating an unusually strong jet requires a very active jet exit region, leading to strong (anti)cyclogenesis. A series of deep cyclones (Figure 4) developed in the jet exit region, and when combined with other factors aiding their meridional track, the cyclones likely acted to build the downstream ridge, with positive feedbacks, helping to amplify the pattern. HYSPLIT trajectories also suggest some of the air over the UK originated within the extreme jet streak prior to undergoing strong descent, which may have been aided by its unusually strong nature driving unusually strong descent.

Figure 3: 250 hPa winds on Feb 19th showing a possible downstream impact of ridge amplification over NW Europe.



Figure 4: MODIS view of a 938 hPa cyclone in the central North Atlantic on Feb 20, 2019.

  • Strong adiabatic descent: HYSPLIT back-trajectories shown in Figure 5 reveal the airmass over the UK originated near the tropopause a few days prior, before descending through the depth of the troposphere. This not only adiabatically warms the air (on top of its warm source region), but also dries out the entire column, allowing for strong insolation needed for the sensible heating to generate strong surface warming.

Figure 5: Ensemble of 84 hour backwards trajectories for air at 1500 m AMSL over London at 12Z Feb 25th based on GFS 0.5 degree data.

  • Anomaly persistence: once established, the block lasted for several days. This allowed for further descent of air which also underwent diabatic warming thanks to the intense radiation under cloudless skies – recirculating around the anticyclone (a similar pattern existed during the summer 2018 European heatwave).

These are the weather components which contributed. They describe the prior and contemporary state of the atmosphere. To relate this to the climate, I’ll draw an analogy. You exeperience a car crash. Why? What I have presented so far would be equivalent to saying “You ran a stop sign”. Now we naturally ask, “what about climate change?”. In my analogy, this is asking, “Were you intoxicated?”. Being intoxicated doesn’t mean you will run a stop sign, and you certainly can do so without being drunk, but it will increase your risk of doing so.

There is no doubt that the configuration of the atmosphere during the last week has been extreme, and primed for producing these warm temperatures. However, in a stationary climate we do not expect to break records with the frequency that we are doing, especially given a lengthening record (e.g. Kendon 2014). Now that we have warmed the mean temperatures, an extreme dynamical perturbation to the mean state (e.g. a monster blocking ridge) will produce an even more extreme temperatures than we would have seen beforehand.

This mechanism is supported by looking more closely at the University of Reading’s weather data record (Figure 6 & Table 1). Similar events, even with similar sunshine, have historically produced cooler temperatures. The recent frequency of extremely warm February temperatures is also evident, and you can also see recent cases of very warm temperatures with much less sunshine than older cases that matched the temperatures but only with strong solar forcing – suggesting, as I mentioned earlier, that it doesn’t take as much of a ‘push’ to equal temperatures which were once close to a “theoretical maximum”, such that now we can obliterate those records with sufficiently unusual large-scale anomalies.


Figure 6: Data from Reading University Atmospheric Observatory 1957-2019 showing daily maximum temperatures above the monthly 95th percentile and associated sunshine hours. Red indicates February 2019, grey indicates pre-2000, black post-2000. The 2019 record is shown with a red star. Tmax exceeding 16C is selected for further analysis in Table 1.


Table 1: Data corresponding to the points within the box in Figure 6 plus the 2019 record value.

But sunny, warm weather in February is nice!

Indeed it is – it was my birthday on February 24th, and I never expected to be celebrating it sitting outside! This event didn’t have the same severe impacts as a summer heatwave, but to me it almost felt more disturbing – the knowledge of what this might mean should a similar extreme be generated in the summer months, and that climate change was “eating away” at winter’s very existence. Unusual late winter/spring temperatures mainly impact the natural world which is highly sensitive to temperature and sunshine at this time of year (e.g. Figure 7), and this is why we should care – this could have many wide-ranging impacts on the ecology which supports our existence.


Figure 7: Blackthorn blossom, complete with Honey Bee (if you look closely), on Feb 23rd in Reading. This blossom is more likely in March and April.

NCEP/NCAR anomaly plots credit

10 years later: the January 2009 SSW

January 24th, 2009. This was the ‘central date’ (defined as the day on which the 10 hPa 60N zonal-mean zonal wind reverses from westerly to easterly) of a remarkable, record-breaking major Sudden Stratospheric Warming event, and there are several reasons why this event is worth a revisit 10 years later.

The Jan 2009 event set a large number of significant records in the stratosphere – and it still holds almost all of these to this day, despite strong competition from warming events in 2016 and 2018.

The aspect of the event that I always recall is the monumental (you might say…stratospheric) deceleration of the 10 hPa 60N zonal wind, which you can see in Figure 1. In early January 2009, the stratospheric polar vortex was date-record strong, with westerly zonal-mean winds ~70 m/s. Only a few days later, they were all-time record-weak, with zonal-mean easterlies of ~30 m/s. Only the final stratospheric warming (FSW) event of March 2016 comes close to rivalling 2009’s easterlies – the event is in the clear in terms of SSWs that are ‘major mid-winter warmings’. The mean deceleration rate between the peak (Jan 8th) and trough (Jan 28th) was an astonishing 10 m/s/day!


Figure 1: 10 hPa 60N zonal-mean zonal winds for 2008-09 from MERRA-2 (via NASA Ozonewatch).

Associated with this rapid deceleration of the vortex was a rapid warming. This is the most wonderful example of why we call them “sudden warmings”. On January 12th, the mean 60-90N (polar-cap) 10 hPa temperature was 202K. On January 23rd, it was 253K – a rise of 51K in 11 days! The peak of 253K remains a satellite-era record for this region of the stratosphere, as you can see in Figure 2.


Figure 2: 10 hPa 60-90N average temperatures for 2008-09, according to MERRA-2 data.

Of course, to produce such a huge warming, you need a massive heat flux (which also indicates huge amounts of wave activity propagating into the stratosphere, through the Eliassen-Palm relation). The 45-75N heat flux at 10 hPa was another metric which hit outrageously high values that have never been matched. The peak of 564 K m/s on Jan 19th is 6.5 times larger than the daily-mean climatology (and 2.5 times larger than the 90th percentile!), which is shown in Figure 3. 


Figure 3: 45-75N 10 hPa eddy heat flux ([v*T*]) for 2008-09, according to MERRA-2 data.

This all came together to produce a textbook wave-2 vortex split. Figure 4 shows this fantastic evolution (thanks to Patrick Martineau for the excellent graphics) – you can really see the cold, strong vortex that existed beforehand, and the strong heat flux from the Atlantic sector. I think you could call this “catastrophic vortex failure”. It’s also worth comparing the location of the daughter vortices in this event versus what’s been happening so far in 2019.


Figure 4: Animation of 10 hPa geopotential height (left) and temperature (right) during the Jan 2009 SSW. Animation created by Patrick Martinaeau (

This SSW was also a Polar-night Jet Oscillation (PJO) event (Hitchcock et al. 2013), so had the associated impressive appearance on time-pressure plots of polar-cap geopotential height (Figure 5) – with a long-lasting signal in the lower-stratosphere. According to Karpechko et al. (2017)’s Table 1, 100% of the days 8-52 after the central date had a negative 150 hPa NAM (one of only 5 times that has occurred since 1979). I always think Jan 2009 looks like a side-on view of a foot stamping down on the troposphere! You can also see that the event affected the troposphere until late March – over 2 months after the initial warming.


Figure 5: 65-90N standardized geopotential height anomalies for JFM 2009 (credit: NOAA CPC).

This moves the discussion nicely onto the downward propagation of the event. It wasn’t as strong as some SSWs – only 69% of the days 8-52 after the event had a negative 1000 hPa NAM index, and therefore ranks nearer the bottom end of Karpechko et al.’s “downward-propagating” SSWs (dSSW) (and that’s not surprising looking at Figure 5). However, it was nevertheless a dSSW, and had various impacts across the N Hemisphere. I’m somewhat limited in my analysis by the current US government shutdown, so I’ll focus on the British Isles.

Figure 6 (a snippet from some work I’ve been doing on SSWs and easterly outbreaks in the UK) shows the British Isles were influenced by easterlies 7-13 days after the SSW – which was the only case of mean easterlies in the 45 days following the event. Using this metric… the 2009 SSW isn’t special at all, although does pass my semi-arbitrary threshold of 5 consecutive easterly days for a true “outbreak”. However, I still do find it incredible that such a huge, hemispheric phenomenon as a major SSW, involving massive planetary waves and propagation from 30-50 km above our heads, can have a detectable response in such a small area as the British Isles. That’s one of those “isn’t the atmosphere amazing?!” moments.


Figure 6: Average 850 hPa zonal winds across the British Isles in the 45 days following the January 2009 SSW, according to JRA-55 reanalysis.

This easterly spell brought with it colder-than-normal temperatures and snow. The Met Office’s monthly summary for Feb 2009 notes that “it was very cold during the first part of the month with snowfalls in many areas. This was the most widespread snowfall as a whole since February 1991”. Figure 7 shows the Met Office surface analysis for 18Z Feb 1st, with a negative NAO pattern and an easterly flow over NW Europe evident.


Figure 7: 18Z Met Office surface pressure and frontal analysis for Feb 1st 2009.

Finally, the January 2009 SSW will always be special to me on a personal level, as it was the first time I had heard of sudden stratospheric warming and its influence on the tropospheric weather patterns. A schematic posted by the Met Office in a press release (announcing an increased likelihood of cold weather in the next few weeks), showed wind reversals propagating down from the stratosphere to the troposphere and eventually the surface. This fascinated me, and it started a journey which, 10 years later, finds me doing a stratosphere-related PhD project.

met office ssw

Figure 8: Met Office diagram showing the downward propagation of zonal wind reversals associated with a major SSW.


February 2009 – Met Office:

Hitchcock, P., T. G. Shepherd, and G. L. Manney, 2013: Statistical Characterization of Arctic Polar-Night Jet Oscillation Events. J. Climate., 26, 2096-2116,

Karpechko, A. Y., P. Hitchcock, D. H. W. Peters, and A. Schneidereit, 2017: Predictability of downward propagation of major sudden stratospheric warmings. Quart. J. Roy. Meteor. Soc., 143, 1459-1470,

NASA MERRA-2 Annual Meteorological Statistics:

NOAA CPC Stratosphere-Troposphere Monitoring:

Polar vortex animation during Stratospheric Sudden Warming [Patrick Martineau]:

Wetter3 UKMO surface chart archive:

Not all SSWs were created equal

Non-downward propagating SSWs? 

Major stratospheric sudden warming events (SSWs) attract widespread attention because they are now known to have significant impacts on the tropospheric circulation (e.g. Baldwin and Dunkerton 2001, hereafter BD01). Anomalies in the stratospheric circulation (often expressed as the Northern Annual Mode (NAM) index, or polar cap geopotential height anomalies) propagate downwards through the stratosphere into the troposphere, rather like “dripping paint” (such as BD01 Fig. 2). A major SSW is associated with the development of a negative NAM in the stratosphere; the “typical” response is the development of a negative NAM (or the associated NAO/AO) in the troposphere ~10-14 days after the central date of the SSW (when the 10 hPa 60N zonal-mean zonal wind becomes easterly) which can persist for several months.

However, not all SSWs were created equal – and some SSWs do not strongly couple to the tropospheric circulation. A recent study by Karphechko et al. (2017) classified major SSWs as “downward propagating” (dSSW) or otherwise (nSSW) based on the 1000 hPa NAM index following the event, and found 43% were nSSW – i.e., not followed by a strong and persistently negative surface NAM. This is not a small fraction of SSWs, and the atmospheric evolution following the two types was found to be significantly different. 

Our perception of SSWs in recent years has been highly influenced by a relatively unusual clustering of vortex-split, downward-propagating events (Jan 2009, Feb 2010, Jan 2013 and Feb 2018) which all had similar tropospheric impacts (all 4 of those events were followed by an outbreak of snow/cold in the UK, for example). The most recent nSSW occurred in Feb 2008. Thus, the announcement of a major SSW – particularly on social media – has become synonymous with a specific weather pattern.

In the nSSW cases considered by Karpechko et al., the composite (their Fig. 1c) actually shows intermittently positive NAM in the troposphere following the SSW – with the sign of the NAM opposing between the lower stratosphere and the troposphere for ~40 days following the central date. This is very different to the picture of dripping -NAM anomalies into the troposphere that BD01 made famous (which is consistent with Karpechko et al.’s dSSW).

Composites of all major SSWs are influenced by the higher frequency of dSSW and the stronger circulation anomalies induced, but this work suggests we need to be wary of these stratospheric events which don’t strongly influence what happens beneath. However, forecast models often struggle to predict the downward propagation – so forecasting these events is troublesome. It also presents a communication problem, which current forecasts (see below!) suggest we may be about to run into: a major SSW could mean a significant reversal of the normal tropospheric circulation (with the potential for “Beast from the East”-type events in the UK), or it could mean very little (e.g. January 2002 following the non-downward propagating Dec 2001 SSW). Predicting these differences, and understanding the mechanisms involved, is an area of active research – and something I hope to address in my PhD work.

Do current forecasts suggest nSSW or dSSW?

As I write this, we’re in a tentative stage – the main stratospheric heat flux event has occurred, and the 60N zonal-mean zonal wind has reversed to easterlies in the upper stratosphere. However, at 10 hPa we’re still decelerating – with the event expected to become ‘major’ around Jan 1 (Fig. 1 & 2) if current forecasts are correct (inter-model agreement has substantially increased now the upper-stratospheric reversal is in the observations).  The event looks very likely to be first driven by a wave-1 displacement of the vortex towards Eurasia, with an increasing likelihoodo of a vortex split (wave-2) to then occur, with the dominant daughetr vortex over Eurasia and a smaller vortex over N America (interestingly, this is opposite to what happened in Feb 2018). However, agreement on the split evolution remains lower than the displacement.


Figure 1: Forecasts of the 10 hPa 60N zonal-mean zonal wind from 00Z December 27th. There is a good agreement between the GFS and its ensemble of a major SSW occurring around Jan 1st.


Figure 2: ECMWF operational forecast from 12Z December 26th for 12Z January 1st showing a major SSW. Source: 

So, predicting the tropospheric impacts is a challenge when the stratospheric forecasts don’t agree! The spread in the GEFS forecasts beyond 10 days is very large – with some members showing a quick return to stratospheric westerlies whilst others flirt with record-strong easterlies. There’s even some indication of bifurcation in the ensemble at longer ranges (perhaps relating to whether or not a split occurs), which may render the ensemble mean of less use.

Despite the uncertainty, one aspect that has been relatively persistent is the absence of a signal for downward propagation in the deterministic GFS (Fig. 3) and the longer-range models such as CFSv2 (Fig. 4). Comparing Fig. 3 here with the nSSW composite in the Karpechko paper is striking – there are many similarities, including the weak -NAM before the main event and the ~day 10 tropospheric +NAM development. On its own, this screams nSSW – but of course is just a single deterministic forecast from one model.


Figure 3: GFS NAM analyses and forecasts from 00Z December 26th. Source: Zac Lawrence’s website ( 

The CFSv2 initially trended strongly towards a -NAO for January 2019 as the SSW signal grew – but this has since decayed and transitioned more towards an Atlantic ridge pattern (Fig. 4). The model clearly picked up on a major SSW occurring – but, like all forecast systems this time, has struggled to predict the type of SSW. There is currently no indication (Fig. 5) from the CFSv2 forecasts of a widespread hemispheric cold outbreak (a “warm Arctic-cold continents” pattern).


Figure 4: CFSv2 forecasts from Dec 1 – Dec 27 for January 2019 700 hPa geopotential height anomalies. Note the initial trend away from a +NAO towards a strong -NAO, before trending towards an “Atlantic ridge” pattern.


Figure 5: CFSv2 2m tempertaure anomaly forecast for January 2019 from an ensemble of forecasts launched between Dec 16-25. Base period 1999-2010. Source: 

My advice would be not to hold your breath for a “Beast from the East 2019 Edition”. But as predictability typically increases once a major SSW has occurred, we should gain a much better picture in the first few days of 2019.

Takeaway message: the impacts of SSWs are more complex than whether it is a displacement or a split, and the mere reversal of the 10 hPa 60N zonal wind doesn’t mean you’ll be shovelling snow 2 weeks later.


Baldwin, M. P., and T. Dunkerton, 2001: Stratospheric Harbingers of Anomalous Weather Regimes. Science, 294, 581-584,

Karpechko, A. Y., P. Hitchcock, D. H. W. Peters, and A. Schneidereit, 2017: Predictability of downward propagation of major sudden stratospheric warmings. Quart. J. Roy. Meteor. Soc., 143, 1459-1470,