It is time for another crazy year of stratosphere watching yet again. I know this is quite unpopular after last year's SSW event that failed to propagate but the excitement lies in the journey in my opinion whether or not we do get a tropospheric response. Last year was fascinating from a stratospheric standpoint with a weak stratospheric polar vortex (SPV) during early autumn and record weak for a time in September (strengthened during November) to the SSW event during the New Year and then a big ramp up during February/March with daily records set for a strong SPV before a dramatic weakening for the final warming (FW) in April/May. I have illustrated each of these dynamic events on this MERRA2 graphic showing the daily zonal mean zonal winds at 60N 10hPa.
You can look at other years back to 1979 here for yourself if you so wish to.
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 I would like to direct you to GL's excellent post from last year's thread here as I couldn't have explained it better myself.
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 60N 10hPa 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 Canadian warming events which occur earlier in the winter.
Now with all that behind us, let's get on to the here, now and foreseeable future.
Currently, the forecast is for a readily strengthening SPV as zonal mean zonal winds at 60N 10hPa reach near record highs by this time next week on the GFS for the past few days. This should be an indication of a strong westerly flow across the North Atlantic but as it is right now, the troposphere and stratosphere are not in sync with one another as shown by the second chart below which shows the observed NAM (another name for the Arctic Oscillation basically) at different layers of the stratosphere with the troposphere at the bottom. The forecast after 1 November is by the GFS and it shows positive NAM within the stratosphere failing to downwell to the troposphere and the troposphere remains with negative NAM throughout the outlook. However, it's worth nothing that an earlier run of the GFS had the trop and strat coupling up with one another at the end of FI so even stratospheric forecasts are prone to wild changes on model runs as much as our own weather!
There is a minor warming on the latest GFS runs in FI associated with this negative NAM which forces a drop in the zonal mean zonal winds but I mean a drop from the record or near record levels is to be expected really. Nothing unusual here as far as I can see.
You can view updates on the NAM here.
Also always watch out for Cohen's Arctic Oscillation and Polar Vortex analysis updates: https://www.aer.com/science-research...c-oscillation/
To summarise for the reliable timeframe, the stratosphere is colder than average right now and is expected to get even colder as we progress through the first two weeks of November 2019. This results in a healthy stratospheric polar vortex but with the troposphere and stratosphere out of sync with one another, this does not look like downwelling any time soon to affect our weather patterns with the AO/NAO likely to remain either at negative or close to negative so blocking around the pole and the North Atlantic increasing the risk of colder than average conditions.