Since our last Winter update in the second half of October, the forecasting scene has gradually turned more to the cold side for this incoming Winter. This will be the biggest update yet as it will also be our penultimate update before our final thoughts are released come next week.
For this update, we’ll be going over the El Niño-Southern Oscillation (ENSO), stratosphere, Quasi-Biennial Oscillation (QBO), Eurasian snow cover through October and Atlantic sea surface temperature (SST) anomaly profiles.
El Niño-Southern Oscillation (ENSO)
If unsure of what ENSO is, please note the following definition from our previous updates:
“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.”
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.
The chart below shows the daily sea surface temperature (SST) anomalies in the ENSO 3.4 region from mid-August up to November 19th 2018. Since mid-September, we have been on the verge between El Niño threshold and ENSO neutral most of the time though towards the end of October and start of November, we were well and truly into El Niño threshold with SST anomalies even going into moderate El Niño threshold. However, this has significantly reduced since with us again on the borderline between ENSO neutral and El Niño threshold.
The CFSv2 continues to forecast an El Niño for Winter 2018-19 with the model now showing the possibility of moderate El Niño conditions but this is kind of an outside chance at the moment as it will have been very late for such an ENSO event to occur and the model forecasts a slight drop off in early 2019 too into weak threshold.
The above chart is for the central part of the Pacific (ENSO region 3.4) whilst this chart is what the CFSv2 is forecasting for the eastern part of the Pacific (ENSO regions 1+2). The predicted SST anomalies aren’t well below the anomalies of those in ENSO 3.4 but they’re still lower than in that region. This shows the model is forecasting an El Niño Modoki event instead of a classic El Niño.
If you haven’t seen our previous updates or remember what the prospects of El Niño Modoki are for Winter in the UK. The overall reanalysis of such Winters in the past show above average heights over Greenland with below average heights to the southeast of us drawing in an easterly or northeasterly flow and is a clear cold signal. Mind you, this is no guarantee as there were some El Niño Modoki Winters that were mild and or wet such as 1994-95 but the majority were cold. The most recent example of an El Niño Modoki occurred in Winter 2009-10. Will we see a similar Winter for 2018-19? Very unlikely, that was the coldest Winter since 1978-79 after all but not impossible!
Quasi-Biennial Oscillation (QBO)
If you do not know what the QBO is, please note the following definition:
“The QBO is an index that reflects the variation of the zonal winds in the stratosphere above the equator. These winds travel in a belt around the planet and change direction approximately every 14 months. It is said to be among the most regular phenomena on earth but as 2016-17 showed, this is not always the case, when the QBO was meant to go into an easterly phase. There was an upward displacement of the westerly phase in early 2016 which cutoff the normal downward propagation of the easterly phase and 2016-17 was a second successful westerly QBO Winter. Back to the early 1950s, this was an unprecedented event and just showed that you cannot rely on the QBO to be as regular as it may showcase itself as.”
We are gradually transitioning to the westerly phase of the QBO which should increase the chances of a milder and wetter Winter but there have been cold Winters with westerly QBO before as much as mild Winters with easterly QBO. There is a time lag before the westerly QBO makes full impact on the weather conditions and one thing that can break this impact before arrival is the occurrence of a sudden stratospheric warming (SSW) event but again no guarantee.
A sudden stratospheric warming event is when stratospheric temperatures rise very suddenly with jumps of more than 30°C in a short period of time and the zonal winds in the stratosphere reversing (when speeds go below 0 m/s). This is what a SSW event looks like on charts:
We’re in the most favourable phase of the QBO for a SSW event to occur (when easterly QBO is transitioning into westerly QBO). The most recent example of this to occur was January 2013 which led to the coldest March since 1883. The above stratospheric warming chart was another example of a SSW taking place between the transitioning of the QBO from easterly to westerly in Winter 1984-85 which was a cold Winter especially in January.
There continues to be some prospects of a SSW event to occur during December 2018 as shown by the CFS ensembles in the graph below. If one does occur, there should be increased potential of northern blocking to start appearing around the Arctic by late December which would increase the chances of easterly winds bringing cold air into the UK. This is rumoured to be a Polar Vortex split rather than a displacement which would favour the chance of easterly winds even further.
Atlantic sea surface temperature (SST) anomalies
The sea surface temperature anomalies in the Atlantic continue to be strange as they have been since Spring with colder than average conditions over much of the region with them forming the shape of a horseshoe, see chart below.
According to the UK Met Office, this should favour a positive NAO which usually leads to mild and wet conditions in Winter for the UK. However, the latest outlook shows the NAO going into negative territory for the rest of November and possibly into the first few days of December too which shows us there is high pressure around Greenland somewhere. If the rumoured SSW event were to verify too, this would mean negative NAO conditions are more favoured than positive NAO.
The Atlantic SST profile is a case of wait and see what it could mean for Winter because it’s in quite a unique and strange state, not seen since at least the 1970s.
Eurasian snow cover
Eurasian snow cover extent through October 2018 was slower than recent years but still relatively above average according to the chart below which shows Eurasian snow cover anomalies for every October from 1967 to 2018.
Long range and seasonal models
- ECMWF: Reversed from last month’s update with a backloaded Winter being hinted at as blocking builds to the northwest of us by Friday bringing a cold end to the season. The start looks zonal and fairly mild.
- CanSIPS: Lots of northern blocking throughout the Winter but perhaps too close to be delivering through cold conditions in December and January. February looks exceptionally cold however.
- JAMSTEC: Usually by this stage, this model cancels its cold bias and begins to show mild temperatures for the Winter. However, the model has continued to show colder than average conditions for the UK during Winter 2018-19.
- Méteo-France: Much like the CanSIPS, this model shows plentiful northern blocking throughout the Winter and even going into Spring 2019 with a southerly tracking jet stream. Certainly looks cold with easterly or northeasterly winds being the dominant feature.
- Glosea5: Well below average heights over Europe and above average heights over Greenland giving away to the possibility of frequent northeasterly/easterly winds. This is a cold Winter scenario and is a big flip on its previous updates.
- IRI: Very similar to its last update with well above average temperature probability over Greenland and no signal over the UK. Some parts of Europe going colder than average probability. This shows there is blocking up to the north bringing the winds in from an easterly direction. Again another cold Winter scenario.
- JMA: Above average heights over Greenland with below average heights over Europe bringing the wind in from the east or northeast. Would likely be a lot of cold and snow conditions.
- BCC: Above average heights stretching from Canada to Russia with below average heights in the Atlantic and parts of southern Europe. Mean wind direction would be easterly bringing in cold air from the continent.
- CPTEC (Brazillian): Below average heights over southwestern Europe with above average heights to the east of Europe and to the north. This should be a cold Winter pattern but with the above average heights to the east of Europe, think it would prove more as a battleground than a straightforward cold Winter with lots of volatility.
- CFSv2: Atlantic driven but not a clear cut mild signal as the jet stream is on somewhat of a southerly track with some northern blocking especially in January. Keep in mind that unlike the others named above which update monthly, this model updates daily so is prone to more chopping and changing.
So we have almost universal agreement from these long range models on Winter 2018-19 being blocked and cold, just slightly different positioning of the pressure systems which is key to how severe the season can get. Nevertheless, they’re variations on the same theme.
What can we take away from this update? Well it’s certainly a strong update for a cold and or wintry signal for the Winter now ain’t it? Lots of signals pointing towards such including from long range or seasonal models. It’s been a good while since we’ve seen such strong signals for cold conditions through Winter.
Does this mean Winter 2018-19 will be a cold Winter? No, there are still more factors to consider and there is plenty of time for signals to change too. However, one must admit that the signals are very compelling. Final thoughts and forecast will be released next week for Winter 2018-19. Stay tuned!
Want to take this discussion further? Head over to our Winter Model Discussion.