As usual around this time of year, there is a lot of talk about Winter 2018-19 in the media saying it’s going to be the coldest Winter in decades or 6 years etc. This is nothing new, it happens just every other year and in the Daily Express newspaper’s case, it is the second earliest they’ve done such an article after 2015; see our catalogue showing every example of this Daily Express article back to 2012 up to 2018. In this post, I’m going to be discussing the current evolution of the background signals or teleconnections for Winter 2018-19 showing what they could mean for us. Before I get into it, I would like to give the disclaimer that everything I discuss here is based on averages and chances, no guarantees.
Quasi-Biennial Oscillation (QBO)
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. Nevertheless, 2018-19 is looking like a straightforward transition from easterly to westerly QBO. There are theories saying the QBO can affect the North Atlantic jet stream. The theory is that westerly QBO strengthens the power of the jet stream and in turn increases the chances of a mild, wet and windy Winter for Northern Europe. Meanwhile, easterly QBO weakens the power of the jet stream and in turn increases the chances of a cold Winter for Northern Europe. As I mentioned at the start of this post though, there is no guarantee. There have been cold Winters with westerly QBO and there have been mild Winters with easterly QBO.
The diagram below shows the Central England Temperature (CET) (x-axis) and QBO (y-axis) for every Winter from 1949-50 to 2017-18. Easterly QBO has QBO values below 0 whilst westerly QBO has QBO values above 0. It’s unfortunate how short the QBO record history is but from this diagram, we can see that we still have a good chance of achieving a cold Winter with a westerly QBO just as much as a mild Winter with easterly QBO. When transitioning, the QBO always has some sort of time lag before impacts are felt from it within the stratosphere.
One Winter that sticks out like a sore thumb in particular here is 1978-79 which was the infamous ‘Winter of Discontent’ in the UK. The Winter, which contained widespread strikes by trade unions demanding larger pay rises, was among the coldest on record. December 1978 was relatively chilly but very wet. January 1979 was the last subzero CET January up to 2018 whilst February was quite cold too. This was also the last time the UK had a blizzard occurring around the New Year period, up to 2018. Winter 1978-79 had weak westerly QBO.
This reconstructed QBO cross-section showed the exceptionally snowy and cold Winter of 1946-47 as a westerly QBO season. Grey areas indicate westerly QBO whilst the white areas indicate easterly QBO. The numbers on the Y-axis are of different layers of the stratosphere. 1946-47 was the second coldest Winter of the 20th century and is within at least the top 10 coldest Winters for the CET back to 1659.
Atlantic sea surface temperatures (SSTs)
One of the biggest talking points of 2018 has been the Atlantic sea surface temperatures. In May, there was a SST profile of cold-warm-cold in the North Atlantic with cold anomalies in the shape of a horseshoe from Greenland to Ireland down to Iberia and back into the tropics. There was a theory that this would promote blocking over the British Isles and or Scandinavia for Summer 2018, by the UK Met Office. That theory verified very well as Summer 2018 was the driest since 1995. This profile has changed somewhat since May with warmer anomalies appearing over a wider area in the central part of the North Atlantic stretching from the eastern seaboard of the US to western France. The chart below shows the latest global SST anomaly map from NOAA for 27 September 2018. You will notice a big blob of cold anomalies just to the south of Greenland and over towards Ireland whilst there is well above average sea surface temperatures near the US. This is a definitive positive North Atlantic Oscillation (NAO) signal for Winter with low pressure over Greenland and high pressure over Iberia and Europe from the Azores bringing in mild westerly winds. The NAO has been in already record breaking prolonged run of positivity since April. With this likely to continue (though plenty of time for change) going solely by the sea surface temperatures, it could be an unprecedented run of positive NAO.
This year’s Atlantic SST profile is quite unique I must say though and even though it garners some similarities to 2015 (which was followed by the warmest Winter since 1868-69), it is still different. With the help of GavsWeatherVids, we can find two years that are somewhat similar to 2018. First example is 1946 (which we already mentioned above in terms of the QBO so won’t go delving into it). Like 2018, there is some anomalous warmth near the US whilst anomalous cooling over much of the North Atlantic in the shape of a horseshoe and some warmth over the Norwegian Sea too. Although, the cooling is more anomalous near Greenland in 2018 than 1946. You’ll never get a year with a perfect match and I think 1946 is a good close match.
Out of the two years, I think 1990 provides the greatest match to 2018. 1990 shows warmth over the Norwegian Sea, a large cold blob to the south of Greenland, anomalous warmth near the US and average temperatures over the tropics. Like 1946-47, 1990-91 was a westerly QBO Winter. It had a positive NAO but was a cold Winter despite this. December 1990 recorded a notable blizzard for the midlands of England on the weekend of the 8th/9th. January 1991 was anticyclonic and chilly after a stormy start. February 1991 began extremely cold with a “Beast from the East” type event at the end of the first week into the second week producing lake-effect snow and bitterly cold temperatures.
El Niño-Southern Oscillation (ENSO)
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 include 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.
Since the turn of La Niña to ENSO neutral took place during Spring, the models have pointed towards El Niño for Winter 2018-19. At first, this provided skepticism due to the fact that they showed the exact same for 2017-18 then it became a La Niña in the end despite models like the CFSv2 then consistently showing El Niño to occur. These signals went away by late Summer 2017 into early Autumn as the models came into line with how the ocean was actually reacting. Here in 2018, the El Niño signals have continued on the models with the CFSv2 in particular, unlike 2017. The chart below is of the latest CFSv2 for 2018-19 showing the predicted SST anomalies for the central part of the equatorial Pacific. The black dash line is the ensemble mean taking every member of the CFSv2 together into one prediction and the model is predicting weak El Niño for 2018-19 at the moment. Effects from ENSO tend to be felt if the event is moderate, strong or very strong. Therefore, weak ENSO conditions is not likely to lead to very much but there are exceptions.
Weak El Niño have mixed historical patterns but the 500mb height anomaly reanalysis (see below) combining these kinds of Winters shows well above average heights over Greenland with anomalous below average heights over France and south of the UK. This is a very cold and snowy reanalysis and is reminiscent of 2009-10 in some ways. In a future update, I’ll be talking about El Niño Modoki which is a rare form of El Niño that has a chance of happening in 2018-19.
Long range and seasonal models
The long range models have provided some interest this season so far with the ECM in particular. The chart below is the mean sea level pressure anomaly from the September update of the ECM seasonal model for Winter 2018-19 (December to February). There is anomalous northern blocking stretching from Greenland all the way to Siberia. The mean wind direction would be an easterly direction. This chart is among the coldest you could possibly get in terms of the setup of pressure systems. As the wind comes from an easterly, it draws in bitterly cold air from Europe into the UK. Once the cold air crosses the relatively warm seas we’re surrounded by, it causes lake-effect snow to occur which is what happened during the Beast from the East events in February/March 2018, February 1991 and January 1987.
Other models like the CanSIPS and Méteo-France show cold scenarios for 2018-19 too but not as severe as that of the ECM shown below. The CFSv2 as always shows a mild and wet scenario for 2018-19.
It’s a mixed bag all in all. QBO is expected to transition to its westerly phase by the end of 2018 which decreases the chances of a cold Winter as the theory states but no guarantee as there have been westerly QBOs with cold Winters (most notably 1978-79) and even late 2010 (November/December) was during westerly QBO. There will be a time lag on impact from the QBO.
Atlantic sea surface temperatures could increase the chance of blocking over Scandinavia which would bring cold easterly winds in Winter. However, they could also decrease the chance of a cold Winter as they support a positive NAO which is normally associated with mild and wet conditions.
A weak El Niño is likely to occur for 2018-19 as consistently shown by models such as the CFSv2. Historically, they can be very blocked with high pressure over Greenland but again there are exceptions.
Welcome aboard the Winter train. Choo choo!