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Winter 2022-23 - General Discussion
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Rather than starting the (seemingly) annual stratospheric watch thread now, I thought I'd post it here instead as it is relevant to winter.
In case you've been living under a rock or you are new here, please find the information below which I have copied and pasted from previous posts because there is no better way I can explain it.
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.
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.
Currently as of writing this post on the 1st of October 2022, there is the prospect for a weakening of the SPV through the next week as indicated by the reduction in zonal mean zonal winds at 60N 10hPa below. However, the variation at this time of year is small and it is unlikely to cause much impacts on the weather patterns. We've seen significant disruptions to the SPV in autumn 2016 and recently October 2021.
Early significant stratospheric warming or disruption events "can" lead to blocking early on in winter BUT much of the time the significance is insignificant (pardon the pun) and in fact can allow the SPV to intensify by winter time. This is due to the fact that radiative cooling is strongest in winter and warming limits or cuts off further wave activity to occur from the trop to the strat. We saw this happen in both of the years I mentioned above - much of winter 2016-17 had a strong SPV with the one exception being late January when a significant warming did occur but wasn't quite a major warming as the zonal winds did not reverse at 10hPa 60N whilst 2021-22 had a consistently strong SPV throughout with very little disruption; similar to 2019-20 but not quite the same intensification as that infamous winter.
Just wanted to get that out there as I've seen some suggest this stratospheric warming that's likely to occur in the second week of October could affect our winter down the line in terms of blocking which as far as I'm concerned is pure hyperbolic nonsense.
Photography site - https://sryanbruenphoto.com/
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