Winter 2024/25 - General Discussion
Comments
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There was a major SSW in early 2013 but it occurred during the opening of January. This cross section of the NAM (an alternative name used for the Arctic Oscillation when discussing the stratosphere) shows how the January 2013 SSW could have had an effect on March via the lower stratosphere being very blocked for much of the winter from that point and never really recovering.
In fact there were a few interesting events or things happening through that winter.
- A decent period of negative AO conditions through late November and early December 2012. Posters from the time will remember this was an infamous period of model hype showing the possibility of a Beast from the East which ultimately failed hard. Even the UK Met Office mentioned this possibility and it all collapsed due to one shortwave.
- A temporary period of -AO conditions which occurred just about in the 9-14 day timeframe for a SSW response in mid-January 2013. There was easterly winds but not enough "oomph" to drive a severe cold flow to Ireland. This was a snowy period for the UK. Some parts of Ireland got snow, as usual the Wicklow Mountains pummelled, but many didn't.
- As mentioned the lower stratosphere between 30 and 100hPa remained blocked with positive heights through the rest of the winter 2012-13 season. This was important to determining what would become an exceptionally blocked month in March 2013. As naturally following an early SSW, radiative cooling occurs and strengthens the stratospheric polar vortex aloft. This is what happened in say early 2019 for example with the major SSW reversal occurring on New Year's Day January 1st but that time we did not have a blocked lower stratosphere, instead the strat and trop were coupled to bringing the exceptional mild in February 2019 and a westerly, mild March were to follow too. In 2013, the stratospheric polar vortex above 10hPa strengthened through February but did not impact our weather until mid-April when it finally became milder but April and May 2013 remained on the cooler than average side with more standard westerly conditions.
- The stronger than average stratospheric polar vortex through the spring with a late non-dynamic final warming was a good place to be going into the summer of 2013 as it discouraged the developments of -NAO conditions. The past 6 summers, 2007-2012, had all been dominated by -NAO which limited chances of heat and gave mostly unsettled, sometimes extremely wet and cool conditions. Summer 2013 was a big improvement with the warmest July on national record and it was the first +NAO summer since 2006.
The lower stratosphere was an important player in the role of March 2013 but there will have been other factors such as probably MJO timing and amplification that will have made it a historic event as the lower strat influence on its own wouldn't have made it as extreme as it was.
If you were to look solely at the synoptics, you will see a classic development for a cold spell in action. Mid-February 2013 mild with a southerly flow amplifies high pressure to our east. The anticyclone slowly retrogresses to Greenland finally reaching here by early March and the winds go easterly although there had already been some easterly winds for a while which gave flurries on the final weekend of February here in the east. Cold air builds to the northeast. Off we went into the cold on March 10th/11th. The blocking only got more and more intense - look at the final chart on the 20th March. The vortex is split from its roots with a strong block over the Arctic.
Photography site - https://sryanbruenphoto.com/
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