Stratosphere watch 2022-23
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The U-60N reached -14.8 m/s today according to the GFS, this should be the "peak" of this warming spike and a recovery will take place to westerly by Wednesday but then quickly reverse back to easterly again from the Friday or Saturday. The latest GFS shows the U-60N getting down to -22.1 m/s at its peak on Wednesday, March 1st.
Here's the latest strat temp charts at 10hPa and 30hPa.
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The U-60N has reversed back to westerly today at 1.2 m/s and this is related to a spike downward in the stratospheric temperature as illustrated on the graph below.
However, as has been mentioned, the U-60N will reverse back to easterly again this weekend and will even do so as early as midnight tonight. Although, it is looking like there'll be another temporary westerly period on Sunday before this warming peaks on Tuesday, February 28th down to ~-18 m/s. This reversal could last until the second week of March before the U-60N reverts back to westerly.
Recent GFS runs have backed off downward propagation, today's NAM forecast is yet to update on stratobserve, the site I usually use, so here is yesterday's forecast. NAM looks to be around neutral on this forecast through the course of March, likely indicative of not much of a signal and the model is struggling to see what will come off of it. This is to be expected with further volatility and changes likely at least for a good week or so. It'd be very useful if there was an ECM equivalent to this.
Photography site - https://www.sryanbruenphoto.com/
Weather photo portfolio - https://sryanbruen.myportfolio.com/weather-1
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A short review of the February 2023 major SSW event. Provisional GEOS-FP data shows the major SSW peaking at ~-18 m/s U-60N which brought date records for the time of year and was very significant for a displacement event. Displacement events do not tend to get this strong and was more intense than both the January 2019 and January 2021 displacements.
We seen downward propagation occur during the first to second week of March with the NAO going negative and a transient Greenland block setting up by the 6th ushering in a northerly flow to Ireland. The block quickly weakened and numerous low pressure systems attacked from the southwest during the 8th-11th March bringing outbreaks of snow to many especially but not limited to higher ground. Low pressure successfully undercut by the 12th March and brought in a milder flow from the southwest with only occasional northwesterly cooler interludes. The longevity of the high latitude blocking was what you expect for a displacement event, you'd expect longer from a split type event although as seen in 2018, that can also mean extreme retrogression.
The GFS in recent days was showing perhaps another push of downward propagation from the major SSW into early April (see the second time series chart below from the 26th). It has backed off this in the very recent runs but we have to wait and see. Could change back or could disappear altogether.
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Weather photo portfolio - https://sryanbruen.myportfolio.com/weather-1
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On Thursday, April 20th, the final stratospheric warming (FSW) of the season was declared with the U-60N reversing to easterly and as far as the forecast period goes, there is no sign of reversal back to westerly for 15 consecutive days which marks the dissipation of the 2022-23 stratospheric polar vortex. This is a slightly later than normal FSW which is standard with winters containing mid-winter major stratospheric warming events but is not remarkably so relative to the average date which is April 15th.
The weak and slow descent nature of this FSW likely means it's radiative rather than dynamic which results in negligible impacts. Late FSW events are usually radiative with one major exception being 2018-19 where the FSW in spring 2019 had the effects of a mid-winter major SSW and May 2019 had record -NAO lasting into the summer of 2019 which was largely dominated by Greenland blocking. The current -NAO episode is purely coincidental rather than being the result of the FSW and is more likely to be a lasting effect related to the major SSW back in February along with favourable MJO.
From a stratospheric point of view, this was another winter of learning. We started off with a bottom up split in late November/early December which was last seen in the infamous late 2010 period. This meant a split of the polar vortex but from the troposphere to the lower stratosphere rather than the top layers of the strat increasing the risk of -NAO and indeed we got a notable -NAO period coinciding with the coldest December spell since 2010 in the country but due to various things like lack of sufficient instability and warm sea surface temperatures from the very warm spring-autumn period of 2022, there wasn't much snow to be seen for most. The effects of this bottom up split faded very quick by mid-December with the strong stratospheric polar vortex aloft resulting in a prolonged wet, zonal period to mid-January. There were numerous attempts of a SSW forecast by the GFS which had an "upgrade" on 30th November 2022 but its performance correlation throughout the winter showed it as more of a downgrade than anything else. Eventually, we did get a major SSW in late February in the form of a wave-1 displacement which ushered in a brief period of -NAO during March, brief as you'd expect given the nature of displacements. The second half of March had more of a +AO/+NAO setup generally and it was a mild month overall - the very warm Atlantic which reached record heights last September aided this.
Like 2020-21, another frustrating winter on the whole for winter enthusiasts of so much close shaves but least some managed to get something exciting and wintry out of the December, January and March spells which can't be said for every winter.
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Weather photo portfolio - https://sryanbruen.myportfolio.com/weather-1
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