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Stratosphere watch 2021-22

  • 02-11-2021 2:55pm
    Registered Users Posts: 13,267 ✭✭✭✭

    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.

    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!



  • Registered Users Posts: 647 ✭✭✭TTLF
    save the trouble and jazz it up

    Thanks Sryan for making this thread for the upcoming stratosphere watch! Let's hope this year we have a little bit of "luck" on our side ;)

  • Registered Users Posts: 8,255 ✭✭✭cml387

    The very last thing we need this year with fuel prices and gas shortages the way they are is a prolonged cold spell. I shall be following this with interest.

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  • Registered Users Posts: 13,267 ✭✭✭✭sryanbruen

    EC46 is back to showing interest in terms of the stratosphere with a significant warm core over Canada in 2 weeks time at 10 hPa (a rare Canadian Warming maybe). Would be useful if there was a way to see the actual heights or temperatures from this to pick up on if there is a blocking high, as can only see the anomaly. There is not much signal on the zonal mean zonal wind front at the moment as they stay around average through the forecast period.

  • Registered Users Posts: 8,913 ✭✭✭Danno

    What are the known downstream effects (on us) of a Canadian based SSW if any? As you say they're rare!

    Thinking there was one in 1993?

  • Registered Users Posts: 8,219 ✭✭✭Gaoth Laidir

  • Registered Users Posts: 13,267 ✭✭✭✭sryanbruen

    Thanks GL, I had forgotten about that page! Doesn't quite extend to the EC46 range as it only shows the operational run but great to get a reminder.

  • Registered Users Posts: 13,267 ✭✭✭✭sryanbruen

    There was in December 1993 yes.

    The idea is that the PV is displaced towards Eurasia with downstream blocking over Canada which leads to a slow down in the jet stream. A vortex lobe around Canada/Greenland is almost always a guarantee for a westerly pattern - the rare exception is when strong Scandinavian blocking is able to hold back the Atlantic depressions. With the removal of this vortex lobe, the likelihood of heights rising in the Greenland region is increased. However, as is the case with minor and major SSW events, downwelling needs to occur to impact the tropospheric weather patterns.

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  • Registered Users Posts: 13,267 ✭✭✭✭sryanbruen

    The signs are still there on the latest GFS with a minor warming occurring over Siberia in FI around Christmas time.

  • Moderators, Science, Health & Environment Moderators Posts: 16,305 Mod ✭✭✭✭Gonzo

    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.

  • Moderators, Science, Health & Environment Moderators Posts: 16,305 Mod ✭✭✭✭Gonzo

    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.

  • Registered Users Posts: 10,504 ✭✭✭✭igCorcaigh

    Gav mentioning a splitting of the vortex towards the end of February?

  • Moderators, Science, Health & Environment Moderators Posts: 16,305 Mod ✭✭✭✭Gonzo

    A splitting might happen but it's too late now anyways to impact last 2 weeks of winter and most of March. At this stage the only interest I would have is a SSW sometime in March possibly leading to high pressure and warm easterlies during May perhaps. The ship has long sailed on an SSW doing a Beast From the East type scenario, we would have needed one of those back in January.

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  • Moderators, Science, Health & Environment Moderators Posts: 16,305 Mod ✭✭✭✭Gonzo

    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.

  • Moderators, Science, Health & Environment Moderators Posts: 16,305 Mod ✭✭✭✭Gonzo

    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...

  • Registered Users Posts: 15,681 ✭✭✭✭nacho libre

    Unless we get a major snowstorm, akin to something like the one in April 1933, out of it, I feel it's too late by Mid March onwards. As we saw with Storm Emma in Early March, the strength of the sun melts the snow, even if temperatures are at freezing point

  • Registered Users Posts: 15,681 ✭✭✭✭nacho libre

  • Moderators, Science, Health & Environment Moderators Posts: 16,305 Mod ✭✭✭✭Gonzo

    Yeah we are already too late to expect a freeze up. 3rd week of February is when the sun melting snow really gets going, even if temperatures are around 0C. We needed a SSW about 6 weeks ago. If we get a major warming in about 12 days time the best we can hope for in terms of cold and snow is a very quick turnaround on the warming affecting us with maybe a few cold northerlies or a last attempt at a cold easterly end of March. By then anything that falls will vanish not long after sunrise.

  • Moderators, Science, Health & Environment Moderators Posts: 16,305 Mod ✭✭✭✭Gonzo

    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 on

  • Registered Users Posts: 15,681 ✭✭✭✭nacho libre

    The ECM seems to be going for a split Vortex now too

  • Registered Users Posts: 1,236 ✭✭✭esposito

    If there is a split in the next two weeks what would the lag time be, 3-4 weeks ? I wonder will we see a repeat of late March 2013

  • Registered Users Posts: 15,681 ✭✭✭✭nacho libre

    I think sometimes the response can be quicker. If we were to get a cold pattern courtesy of a scandi high without a sudden warming, it's possible a split could flip the pattern to something milder. Although, according to Sryan, March is going to be very mild. So if a SSW does occur we might get something like March 2013, anything less at that time of year would result in cold rain or sleet instead of snow

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