Stratosphere watch 2021-22

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!