Final Stratospheric Warmings

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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 wxcharts.com.

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.

Record-strong #PolarVortex set to undergo significant weakening next 10 days with amplifying Aleutian ridge & vortex displacement towards Eurasia.

1 member of today’s 00Z GEFS reverses 10 hPa 60N zonal-mean zonal winds to easterly in next 16 days! pic.twitter.com/Qsjupq5VKf

— Simon Lee (@SimonLeeWx) March 14, 2019

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!

References

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. http://doi.org/10.1088/1748-9326/10/10/104007

Charlton, A. J., and L. M. Polvani, 2007: A New Look at Stratospheric Sudden Warmings. Part I: Climatology and Modelling Benchmarks. J. Climate. http://doi.org/10.1175/JCLI3996.1

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. https://doi.org/10.5194/acp-16-15371-2016

A “winter heatwave” in a warming world

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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 https://www.esrl.noaa.gov/psd/map/.

10 years later: the January 2009 SSW

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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 (http://p-martineau.com/ssw-animations/).

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.

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

References

February 2009 – Met Office: https://www.metoffice.gov.uk/climate/uk/summaries/2009/february

Hitchcock, P., T. G. Shepherd, and G. L. Manney, 2013: Statistical Characterization of Arctic Polar-Night Jet Oscillation Events. J. Climate., 26, 2096-2116, https://doi.org/10.1175/JCLI-D-12-00202.1.

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, https://doi.org/10.1002/qj.3017.

NASA MERRA-2 Annual Meteorological Statistics: https://acd-ext.gsfc.nasa.gov/Data_services/met/ann_data.html

NOAA CPC Stratosphere-Troposphere Monitoring: http://www.cpc.ncep.noaa.gov/products/stratosphere/strat-trop/

Polar vortex animation during Stratospheric Sudden Warming [Patrick Martineau]: http://p-martineau.com/ssw-animations/

Wetter3 UKMO surface chart archive: http://www1.wetter3.de/Archiv/archiv_ukmet.html

Not all SSWs were created equal

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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: http://www.geo.fu-berlin.de/en/met/ag/strat/produkte/winterdiagnostics/.

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 (www.stratobserve.com).

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: http://origin.cpc.ncep.noaa.gov/products/people/wwang/cfsv2fcst/.

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.

References

Baldwin, M. P., and T. Dunkerton, 2001: Stratospheric Harbingers of Anomalous Weather Regimes. Science, 294, 581-584, https://doi.org/10.1126/science.1063315.

The Stratosphere – why do we care?

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I study the stratosphere, the layer of atmosphere that extends above the troposphere from about 10-50 km. Friends and colleagues of mine often joke (I hope…) that “nobody cares about the stratosphere” *, primarily because it contains no real ‘weather’ – such as what happens in the troposphere. With little to no water vapour, it can’t be seen on visible satellite imagery – unlike the huge and beautiful weather systems in the troposphere. To visualise the stratosphere, we rely primarily on computer-generated graphics – and it’s not like you can walk outside and experience it, either. So, why do we care? What follows is a relatively simple (I hope!) explanation.

Weather forecasts, particularly on TV, often explain that our weather is “all down to the position of the jet stream” (the band of fast flowing air high in the troposphere that forms on the boundary between warmer and cooler airmasses). Now, that’s almost always true in the UK, but it’s particularly potent in winter – when the temperature contrasts either side of the jet become enhanced thanks to the Polar Night. One of the main driving factors behind the speed and position of the jet stream (particularly the Atlantic jet stream) in winter is… the stratosphere!

Rather like the jet streams we know and love/loathe in the troposphere that guide the development and evolution of weather systems, in the stratosphere there exists another jet stream – the Polar Night Jet (Figure 1). This encircles the Stratospheric Polar Vortex (SPV). Both of these form as the pole tilts away from the Sun in winter, leading to intense cooling. The strong temperature gradient then forms a jet stream and cyclonic vortex, which isolates the air within the vortex, and it cools further…etc. The Polar Vortex is a normal phenomenon which forms each winter – nothing sensational like some headlines suggest.

Figure 1: GFS zonal wind analysis from February 4th 2018. Reds indicate westerly winds. A strong Polar Night Jet exists in the stratosphere, associated with a strong tropospheric jet.

Through a process known as stratosphere-troposphere coupling, the stratosphere and the troposphere beneath can ‘talk’ via the influence of planetary/Rossby waves. These very large waves in the mid-latitude westerly flow can propagate vertically from the troposphere into the stratosphere and influence the circulation there – a process known as wave-mean flow interaction. Sometimes, this is strong enough to strongly disrupt the SPV, and when that happens, the isolated reservoir of cold air is broken down and the temperature sky-rockets… by as much as 50C in only a few days. This is known as a Sudden Stratospheric Warming (SSW). Very strong SSWs – called major SSWs – occur in approximately 6 winters per decade, and result in a reversal of the Polar Night Jet to easterlies. The Polar Vortex is either displaced, split up, or destroyed (2018’s SSW is shown in Figure 2).

Figure 2: The February 2018 Major SSW, as told through daily analyses from the GFS of 10 hPa wind (filled) and geopotential height (contours). This is classified as a ‘split’ SSW, for obvious reasons.

This has implications for our weather, because anomalies in the strength and position of the SPV and the Polar Night Jet can propagate downwards and influence the tropospheric jet stream. A stronger than normal SPV is associated with a strengthened tropospheric jet stream – and for us in the UK, that means Atlantic westerlies and generally mild winter weather. In contrast, following a major SSW, the easterlies propagate downwards (Figures 3 and 4) – resulting in a reduction in strength of the Atlantic westerlies. Sometimes, there can be a complete reversal of circulation – this happened in March 2018 with the infamous ‘Beast from the East’, bringing cold and snowy weather.

Figure 3: As in Figure 1, but for February 17th, following the major SSW. Note the weaker tropospheric jet and surface easterlies as the ‘Beast from the East’ developed in response.

Thus, being able to predict the state of the Stratospheric Polar Vortex is a source of skill for wintertime forecasts. Moreover, because there tends to be some lag between the events in the stratosphere and their maximum impact at the surface (~2 weeks), stratospheric predictability can provide increased predictability on the sub-seasonal timeframe (~15-30 days). Additionally, anomalies associated with a major SSW tend to persist in the lower stratosphere for even longer – which again, is a source of skill.

Figure 4: Anomalies in geopotential height for January-March 2018. Note how anomalies associated with the major SSW (red blob in the centre) propagate downwards like ‘dripping paint’.

And that is why we care about the stratosphere!

Further reading:

Kidston et al., 2015: Stratospheric influence on tropospheric jet streams, storm tracks and surface weather. Nature Geoscience, 8, 433-440.

*A tongue-in-cheek quote from Reading Meteorology’s weekly ‘Weather and Climate Discussion’ a few years ago that stuck with me was “the stratosphere – nothing of interest lies therein”. I plan to use that in my thesis…

Careers ‘Advice’? Follow your dreams!

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Sara Thornton (co-owner of Weathertrending) recently shared an article highlighting how one particular careers adviser in her past told her to ‘give up on her dreams’ of presenting the weather on TV. Now, clearly that didn’t stop Sara! But it got me thinking about careers advice from my past – especially since meteorology is a relatively niche subject which extends far beyond what some assume it to be.

However, I am chuffed to be featured in today’s Metro! Never give up on your dreams folks. #makeyourownluck pic.twitter.com/RnwcJUuVDQ

— Sara Thornton (@SaraThornton1) November 1, 2018

I have pretty much always wanted to be a meteorologist – well, I can trace it back to about the age of 6. It didn’t take long before I started to find out for myself what you needed to do and be good at to do meteorology… it turned out that was Maths and Physics. I quickly found out that the Department of Meteorology at the University of Reading was one of the best in the world, and offered undergraduate courses in the subject. So, I had worked out what I needed to do to pursue my dream.

Naturally, you’d imagine that careers advisers and teaching staff during my time at school were helpful and encouraging – seeing as I had a definite dream in a very realistic career. Think again. The following is a genuine quote from a school careers adviser during a compulsory visit around the time I did GCSEs:

“Hmm. Have you ever considered becoming an actuary? They earn a lot of money.”

I have never forgotten that because the meeting had nothing to do with how I could pursue my dream or what I needed to or could do, but focused instead on the adviser rattling off as many careers as she could think of that weren’t in atmospheric science – including actuary because they earn a lot. Not because it might be something I wanted to do or had anything to do with my dreams.

It extends beyond that. These are quotes from teachers in my past:

“I checked, you don’t need Further Maths to do Meteorology. So you should do French!”

Choosing A-levels is hard enough. It can define your life before you’ve really worked out what you wanted to do. It’s even more difficult when teachers want you to do their subject instead of what’s right for you. It’s true, you don’t need Further Maths, but it has helped me far more than French ever would. Plus… I love maths!

“But I’m so disappointed you’re not doing Physics, you’re very good at that.”

The thing is I am doing Physics – it’s just all applied to the atmosphere and oceans! That’s something I could never seem to make this teacher, or many others, understand.

There were several other quotes which I can’t remember precisely enough to replicate. My overall point is that I had to stand my ground and just do what I wanted to do, and it often felt that this was going against the wishes of others. I’m not sure if this has something to do with a poor knowledge of what meteorology entails, or whether its snobbery towards the highest-ranked universities (instead of departments…which is what really matters, right?).

I don’t know whether I would have ended up where I am now if I didn’t have such a strong desire to go and study Meteorology at Reading, because I was never properly encouraged by those who should have done so. If you’re a young person who loves weather, go and study it. Words can’t express how happy my undergraduate degree, and now this PhD, has made me.

There’s never a dull day in weather.

“They get it wrong 90% of the time”

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I was on a long train journey a few days ago, and ended up in conversation with the person next to me. When I explained what I’m doing (a PhD project looking to improve sub-seasonal forecasting), I was greeted with the all-too-familiar response of “oh, that’s good because they get it wrong 90% of the time”. Doing my utmost to supress just how much that sentence annoyed me with its factual inaccuracy, I responded with a comment about how there is still so much to learn about the atmosphere-ocean system, and ‘things can only get better’ (perhaps that song by D:Ream should be the soundtrack to NWP – or is it still too tainted by Tony Blair?). I also recently saw a post on Reddit’s ‘showerthoughts’ saying “Forecasting the weather is the only job where you can get it wrong every day and still have a job” – I refrained from responding to that!

Of course, “they” don’t get it wrong 90% of the time (you could easily argue the reverse is true) and a forecaster won’t still be in the job if they’re getting it wrong every day.

In the last few days we have seen a spectacular example of what NWP is capable of with the forecast of Hurricane Florence. We knew days in advance that the storm would reach category 4 status. The track into the Carolinas is now without doubt (though the exact motion when the storm makes landfall remains uncertain). So how is it that a forecast of a huge, turbulent, dynamic vortex can be so incrediby accurate to an extent that even amazes meteorologists – and yet the general public can have such a different opinion about their experiences of weather forecasts? How is it that this perception is so ubiquitous?

I had noticed over the summer that there were noticeable issues with app forecasts – and noticeable failures – but there is often good reason. Showers are difficult to predict to postcode-level accuracy, and, sometimes, “shit happens”. A great example of the latter occurred earlier in 2018 in Reading, in perhaps the only time I’ve really experienced the forecast go completely wrong. The forecast: a dry, cloudy day. However, the inversion mixed out, resulting in clear, sunny skies. That in turn lead to unexpected solar heating and thus unexpected instability, which generated a heavy (but very isolated) shower over Reading (perhaps its localised nature was a response to additional urban heating – yet another complexity!). A wonderful non-linear response – and something which as a meteorologist made sense. However, to a member of the public, it was unexpected rain that may have left them irritatingly soaked – and perhaps fostered a resentment of weather forecasters.

Forecast accuracy has come on in leaps and bounds over the last 30 years, but it seems public’s trust has not increased accordingly. I think part of this is how quickly one adjusts to a ‘new normal’ – I can draw a parallel to Internet connection speeds (remember when 1 Mbps was fast, yet now seems painfully slow?). I think a large part comes down to a lack of understanding as to why a forecast may go wrong. Rather like a medical diagnosis, it may make logical sense as to why a Doctor misdiagnosed, but the patient’s response may be filled with anger and confusion. And part of that comes down to the human body seeming naiively simple (because we all have one!) in the same way forecasting the weather may seem simple (just some fancy graphics and looking at clouds, right?). Both are hugely complex, but its usually only the experts who truly comprehend that.

Thus, it’s my conclusion that the more meteorology we can get out there, the better the public will trust the forecasts we make.

Edit: after writing this post, I received a wonderful tweet which showed that, for some, the incredible accuracy of weather forecasts is understood.

The rest of us can't comprehend how amazing you meteorologists are for doing the science that makes it possible to make predictions with such remarkable accuracy. Respect.

— Ladybird Abroad (@ladybird_athome) September 12, 2018