Stratosphere watch 2021-22
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02-11-2021 2:55pm#1
It is that time again to look ahead, begin a new season of observing the stratosphere and see what it has in store with regards to impacting the weather lower down in the troposphere.
For those that are new, the earth's atmosphere is divided into different layers. The troposphere is where weather occurs and the stratosphere is just above the troposphere. Much like the weather around the earth, the stratospheric temperature varies by season with it reaching its lowest point in December on average. Each autumn, a phenomena known as the stratospheric polar vortex forms within the Arctic Circle. This is an upper-level area of low pressure formed by the temperature difference between the equator and the pole. The vortex weakens and strengthens from year to year via dynamic events like stratospheric warming or the annual summer hibernation where the SPV "goes to sleep" due to a lower temperature gradient between the equator and the pole. The SPV is well defined when it's strong with a single vortex lying within the Arctic Circle.
Normally, the troposphere and stratosphere are in sync with each other which results in downwelling. This is when planetary waves are propagated to either level of the atmosphere via the weather patterns that occur. For example, an Aleutian Low (low pressure over or around Alaska) / Scandinavian High (high pressure over Scandinavia) combination can be a sign of disrupting the Polar Vortex and a precursor to a SSW event as a result. Another example is that with a cold stratosphere and healthy SPV, the North Atlantic jet stream is powered up bringing mild and moist conditions to western Europe with low pressure centred over Iceland. Tropospheric patterns that can have an impact on the SPV are defined by zonal wavenumbers which basically refer to the amount of ridges and troughs within a full circle around the globe at a given latitude like zonal wavenumber-1 refers to 1 ridge and 1 trough for example.
There are different types of warming that occur and can have different degrees of impacts on the weather around the Northern Hemisphere if a tropospheric response occurs.
There is a minor stratospheric warming which involves the stratospheric temperature rising but less dramatically compared to a major warming and the zonal mean zonal winds do not reverse.
A major sudden stratospheric warming involves the zonal mean zonal winds at 60°N 10 hPa in the stratosphere to reverse from westerly to easterly. The SPV is completely disrupted and it will either be split into two or more vortices OR displaced from its normal location over the North Pole. Major SSW events are forced by tropospheric patterns disturbing the SPV but sometimes, these events can propagate back down into the troposphere and result in anomalously blocked patterns which gives a higher chance of colder weather for Europe. However, even if a SSW downwells successfully into the troposphere, the positioning of the blocking is a factor to consider also.
An important point to be raised is how the SPV tends to re-intensify following major warming events as there was some confusion last year on how it did so despite all the disruption through December and January 2020/21. To put it simply, major warming events cut off any wave activity to the stratosphere from the troposphere. Wave activity is what causes the stratosphere to warm up, SPV to be disrupted and in turn displace or split. With absence of wave activity, the stratosphere has no choice but to radiatively cool and the SPV intensifies simultaneously. The point in the season of which the warming event also occurs has a role here i.e. radiative cooling is strongest in mid-winter as compared to late autumn/early winter or late winter/early spring. This is the same mechanism for how the Indian Ocean Dipole (IOD) had a large play in the developments of the strong SPV winter of 2019-20.
Cohen and Jones (2011) did a good paper on tropospheric precursors and categorising past SSW events into splits and displacements here, it's free access to everybody.
Met Éireann highlighted previous Irish cold spells and if a SSW could have caused them in their Storm Emma paper here.
Other warming events that occur include final warming events which indicate the transition into stratospheric summer hibernation mode and typically happen during the months of April and May but have occurred in March before, and Canadian warming events which occur earlier in the winter.
Unlike 2019-20 (which can also be seen in the graph I have given below courtesy of NASA), the 2020-21 season was much more varied in terms of zonal mean zonal winds at 60°N 10 hPa. It began with a slow start to the developments of the SPV through October 2020 but it gained pace during November when it was stronger than average. Wave breaking occurred through December and would lead to a major SSW through early January 2021 as zonal mean zonal winds reversed to easterly on 5th January. There was plenty of uncertainty as to whether this SSW would constitute a displacement or a split, it was a displacement in the end. After a quiet period through the rest of January and early February, the SPV re-intensified as one would expect through February and into March following the major warming event (see above on why this is to be expected).
The final warming took place on 26 April which was 11 days later than average and this was to be expected as January 2021 had a major SSW event - seasons with major SSW events tend to have later than average final warming events.
This season we have already had one minor stratospheric warming event through October and into early November which led to reduction in zonal mean zonal winds at 60°N 10 hPa. The forecast from the GEFS is for, not surprisingly following this event, an intensification in zonal mean zonal winds aloft.
You may notice at the very end of the ensemble there is a really slight downward trend in the GEFS but a fair bit of scatter. This initially would not be eye catching and there is clearly little signal of stratospheric warming events to occur, minor or major, in the foreseeable future from the GEFS. However, I mention as it's a period of interest from the ECM which has been playing with another minor warming event to occur not long after this re-intensification (see second image below of the latest EC 46 day forecast). That would be somewhat unusual even in the context of recent years which have tended to feature at least one significant minor warming event through the meteorological autumn season - most notably October/November 2016.
However, I must stress that every SSW event is unique. One cannot expect a 2018 repeat for every major SSW event that occurs. Posts like this are solely done for those interested in the stratosphere and the potential impacts its dynamic events may have on tropospheric weather patterns, whether through raging zonality or full on meridionality and blocked patterns.
Anyway, the rollercoaster is open for those who choose to ride! Have fun!
Photography site - https://www.sryanbruenphoto.com/
Weather photo portfolio - https://sryanbruen.myportfolio.com/weather-1
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No change in the forecast as per above post, the stratospheric polar vortex remains in a healthy state with stronger than normal zonal mean zonal winds at 60°N 10 hPa and is expected to continue throughout the forecast period of the GFS. There is no signal for any weakening any time soon and the EC46 day outlook has backed off the idea of a secondary minor warming event.
The forecast below is also of the GFS and showing the zonal mean zonal winds but at multiple layers of the atmosphere rather than specifically 10 hPa which may not be reflective of the weather patterns and is reflective of what's happening in the upper stratosphere. The one on the left shows the actual zonal mean zonal winds at 60°N from 1-1000 hPa whilst the right is the anomaly to average. The anomaly chart allows us to easily see the strong SPV in the upper stratosphere with no signs of weakening and even strengthening towards days 12-16, just confirming what I discussed above with the first chart. If we look further down towards the troposphere, we can see zonal mean zonal winds at 60°N are much more stable at the moment as the troposphere and stratosphere are in disconnection. This is commonplace this time of year, we've seen it occur pretty much most Novembers the past decade for instance before a reconnection through December. This is standard.
If we were to look at the northern hemispheric view for a week from now on the latest GFS, this disconnection is visually clear as there is a tropospheric polar vortex split with one lobe near Greenland and the other over Russia whilst there is a weak ridge over the pole. However, why is the weather pattern mild if there is a split polar vortex? Comes down to the fact that there is a vortex near Greenland which unless there's big amplification over Scandinavia usually means westerlies rule the roost. The ridging over the arctic is also weak and does not allow much cold air advection to Europe.
In conclusion, the stratospheric polar vortex remains in a strong state and is expected to continue throughout the foreseeable future with no sign of weakening. Despite this, the strat and trop are disconnected so the tropospheric polar vortex is split but its vortices are not in ideal positions to advecting cold air southward to mid-latitudes. Strat and trop disconnection are standard this time of year and it's only a matter of time before they reconnect again.
Photography site - https://www.sryanbruenphoto.com/
Weather photo portfolio - https://sryanbruen.myportfolio.com/weather-1
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This upper level profile from the ECM at day 10 on December 1st is the kind of pattern that would be conducive to SPV disruption with both an Ural ridge and an Aleutian low.
Just a possibility for now with the wacky nature of models this past week but it's one to keep an eye out for. The GFS earlier showed a fairly significant disruption too to the SPV in FI but was a huge outlier and was at complete odds with the rest of the modelling including its own ensembles.
Photography site - https://www.sryanbruenphoto.com/
Weather photo portfolio - https://sryanbruen.myportfolio.com/weather-1
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A minor warming currently taking place and signs of another 2 warmings between now and early in the New Year and the PV coming under quite a bit of strain. Another minor warming takes place just after Christmas.
This warming is forecast to move from Siberia and head towards the Canadian/US side crossing the pole as it does so and stretching the PV.
We finish up on January the 6th with a more significant area of warming taking place across Siberia which could get to SSW levels perhaps at a later stage but by this stage the PV may temporarily regain it's full strength.
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GFS still going with a fairly moderate warming 1st week of January and each run appears to be getting slightly warmer. We are still not at SSW levels but we're getting closer. This will definitely need watching as there is a chance this could upgrade to SSW levels over the next week, or it could get downgraded.
On this particular run we don't get to SSW levels but we get another warming appearing right at the end.
For an SSW event we need deep to dark reds getting into positive temperatures.
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Not much development in the Stratosphere, we are currently getting temperatures around -76C with the PV remaining fairly strong. If anything it becomes colder over the next week with temperatures going down to -80C or even lower. This could be enough to drive the westerlies even more and may end up finally getting rid of the high pressure dominance over us.
We're getting very late into the winter to expect a SSW to happen and deliver for us on the surface before the end of February. There are signs of another warming right at the end but once again this appears to be a minor warming and will not impact the PV to any great degree.
Perhaps we will finally see a weakening of the PV into February but we would need a warming much more significant than what is shown above to happen as soon as possible.
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The PV is really getting it's game face on for what remains of winter and this may also impact the first half of March. The NAO has pretty much been positive all winter except for the week before Christmas. It is due to become even more positive over the next few weeks. The AO is set to become very highly positive over the next few weeks, possibly getting close to the record breaking levels we had during winter 2019/2020. What this means is the Atlantic is about to go into hyper steroid mode and may well find ourselves going into a prolonged period of a conveyer belt of low pressures to see us through the rest of winter and perhaps early Spring. If you want a last chance of a cold end to February or a cold March this is not what you want to see happening.
The only chance we had at blocking was the week before Christmas, aside from that this winter has been a complete and utter write off for any chances/hints towards cold or snowy weather and we completely missed our one and only chance around Christmas.
The PV itself will remain intact and colder than average over the next few weeks, it should be starting to weaken naturally as we head into February but there is absolutely no sign of that happening so the jetstream will be very active over the next 3 to 4 weeks at least.
Current Pv:
2 weeks time PV:
We will have slight warmings but they will have absolutely no impact on the PV. If we do get an SSW it won't happen till Spring now and we've completely missed the boat on it doing anything of interest for us during late winter or early Spring. An SSW in March may not have an affect till April or even May. Maybe better luck next winter.
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The end to the PV may be in sight.
A significant warming looks possible around 1st week of March
This gets very close to a SSW but as the PV is weakening this late into the season we may not need one.
PV very much reduced end of 1st week of March.
If the above verifies we could have a very chilly end to March and April. Yesterday one of the GFS runs did actually split the PV in two.
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The GFS12z stops just shy of a SSW in the final day of February and first week of March. We are getting very close now with oranges and a tiny area of red appearing.
We end the first week of March with the PV displaced over Russia/Asia and very much weakened.
What impacts this will have, how soon etc remains to be seen but it would increase the chances of northern blocking as we head into the second half of March and April. Could we be facing another bitterly cold April...
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and there we have it, finally an SSW. Let's see if this will verify and if so will it affect us in any way. A best case scenario would be the final days of March or early April, leading to possibly a very blocked April and May. I wanted one of these late December/early January. Bit late now really.
It's nowhere to be seen on the GFS 12z.
Post edited by Gonzo on1 -
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