2018 is a year that will remain in our memories for a long time I think as it has produced some bizarre or very abnormal weather patterns for the UK including a record breaking late ‘Beast from the East’ type event at the end of February into the start of March bringing exceptionally cold days for so late in the season and heavy snowfalls followed by another easterly outbreak in mid-March and one of the most amazing Summers on record with persistent warmth and dry conditions. However, what could all these bizarre patterns mean for Winter 2018-19?
We did a Winter update a few weeks ago which gave mixed signals for Winter 2018-19. You can find the update in-depth here. We had a look at the following factors: the Quasi-Biennial Oscillation (QBO), Atlantic sea surface temperature anomaly profiles, the chance of a developing El Niño event and long range models.
For this update, we’re going to be delving again into the developing El Niño event and long range models but also include other things like the stratosphere.
El Niño-Southern Oscillation (ENSO)
To start off with what ENSO is (definition from the last update):
“ENSO is an irregular index with variation in winds and sea surface temperatures over the equatorial Pacific Ocean. The warming phase is known as El Niño whilst the cooling phase is known as La Niña. Southern Oscillation is the accompanying atmospheric component coupled with the sea temperature change. These phases of ENSO have various impacts on weather patterns around the world but the impacts from it on Europe are unknown due to event to event variation. However, it has been said that El Niño increases the chances of cold later in the Winter whilst La Niña does the opposite of increasing the chances of cold earlier in the Winter. Examples of El Niño Winters doing this include 2014-15 (where February was the coldest of the Winter with a frosty spell during the first half) and 1977-78 (where February produced a severe cold spell with one of the worst blizzards on record to hit the West Country). An example of a La Niña Winter doing this includes 2010-11 (where November/December produced some of the coldest weather on record to occur so early in the Winter including the coldest December on record for most). However there are many odd ones out. For instance, you only have to go back to last year for La Niña and February 2018 was the coldest of the Winter.”
The latest CFSv2 (shown below) still holds the prospects on this weak El Niño event for 2018-19. We are currently within the threshold for a weak El Niño (+0.5°C or more above average) as SST anomalies in the Equatorial Pacific Ocean are around +0.6 to +0.8°C above average. This is likely to continue into the Winter but as it is expected to be a weak ENSO event, it will not have much of an impact on the weather in northwestern Europe.
There has been talks of an El Niño Modoki for 2018-19 rather than a traditional El Niño event. Most of the time, warming occurs in ENSO regions 1+2 (see below) but sometimes, you can have another type of El Niño.
This type of El Niño is a “central based” El Niño or in Japanese, El Niño Modoki. With this type of El Niño, the warming is focused on the central part of the Equatorial Pacific i.e. ENSO region 3.4 (see diagram above). The diagram below compares the circulation of the traditional El Niño and El Niño Modoki.
El Niño Modoki Winters tend to be blocked as shown by the 500mb height anomaly reanalysis below which increases the chance of easterlies but mainly settled with high pressure just to the northwest of the UK blocking off the Atlantic. The UK would likely be on the periphery of very potent easterly spells, similar in vein to Winter 2005-06. Whether 2018-19 will be an El Niño Modoki or not is open to question. Time will tell.
The CFSv2 ensembles have been consistently pointing towards a sudden stratospheric warming event around the end of 2018 whether be it November, December or January though as time has gone on, it has tended to forward it keeping it in the very unreliable timeframe and not getting closer. What it hasn’t been keeping forward is the prospect of a disorganised Polar Vortex with weaker than average zonal wind speeds during November and December 2018.
The “zonal” pattern is what the UK has on a regular basis usually in terms of weather and gives the mild Winters to the country despite other countries on similar latitude being much much colder in Winter time. This zonal pattern is caused by temperature contrasts between the North Pole and the tropics in the North Atlantic which drives the jet stream to go crazy with low pressure systems being sent to the UK. The zonal wind speeds recently have gone to record highs for October in contrast to a record low observation in mid-September 2018 showing just how variable zonal wind speeds can be. Despite these record high zonal wind speeds at 10hPa in the stratosphere, the Polar Vortex remains disorganised and the North Atlantic jet stream remains weak leading to reoccurring blocking near or over the UK. This blocking has been consistently shown on recent short range model runs to retrogress into the Atlantic and possibly up to Greenland to give the first negative North Atlantic Oscillation (NAO)* since March this year.
“The North Atlantic Oscillation is an index that shows you the difference in mean sea level pressure between the Icelandic Low and the Azores High; two elements which make up the zonal pattern that give the UK and Ireland mild Winters and cool Summers. The more positive the NAO is, the deeper the Icelandic Low is and the stronger the Azores High is. The more negative the NAO is, the weaker the Icelandic Low and Azores High are – sometimes even can be swapped around which many cold Winters have.”
These continued unusual patterns could mean that nature is either:
a) getting the setup together for the Winter
b) teasing an early Winter
Tomorrow when writing this post is October 20th. The strongest zonal wind speed at 10hPa on record for October 20th was 1981 which was followed by a sudden stratospheric warming event in December that year. December 1981 was the coldest December of the 20th century with a Central England Temperature (CET) of just 0.3°C which is more than -2.5°C below average. This was the snowiest December since 1878 over the country with several blizzard events and two notable windstorms that left tragedy on those impacted including the Penlee Lifeboat Disaster. The end of December, just after Christmas, was milder and this continued into the first week of January before the extreme cold came back delivering the most severe cold in the UK since February 1895 (including the all-time UK record low of -27.2°C being equalled) and a notable blizzard to Wales. Scenes like those shown below in the photography weren’t uncommon during December 1981/January 1982.
Will we see something similar for Winter 2018-19? It’s possible but unlikely, those cold spells of 1981-82 were phenomenal in every right. Never say never! Everything can happen so suddenly with the snap of a finger.
Long range and seasonal models
To finish off this Winter update, let’s have a brief overview of the long range model output for Winter 2018-19.
- ECMWF: Still pointing towards the chance of a cold Winter especially in December with easterly potential but backtracked from its massive easterly domination in last month’s update of the seasonal model. Perhaps milder later on showing the chances of a front loaded Winter.
- CanSIPS: Backloaded Winter with a relatively mild, unsettled December, anticyclonic January and a very northeasterly February which would be cold.
- JAMSTEC: Colder than average Winter in the UK signalling northerly or northeasterly winds whilst the east of Europe is much milder.
- Méteo France: Cool in the north and west of the UK whilst close to average temperature and wet in the south and east.
- Glosea5: Mild and zonal with an increased risk of westerly winds.
- IRI: Warmer than average over Greenland indicating northern blocking here with no temperature deviation over the UK.
- CFSv2: Anticyclonic January but stormy December and February. Very mild.
This update has pointed more towards the cold side of things compared to the first one but not by a significant deviation. Model skill at this range is still very poor so be prepared for changes especially those that update regularly such as the CFSv2. We will be taking a look at other things again in future Winter updates that will certainly interest you readers, no doubt about that.