Early Thoughts On Winter 2017/2018 - A Colder Winter Than Recent Years?
It seems like summer was just yesterday and October was in the top ten warmest of all time, but we are only a month away from the start of meteorological winter. So, I think now maybe worth looking at longer range signals that may point towards an idea of what we can expect this coming winter. Seasonal forecasting is often based on current large-scale patterns that persist over months rather than weeks, such as La Nina/El Nino, QBO, Solar activity, etc. - rather than the teleconnections of AO (Arctic Oscillation), NAO (North Atlantic Oscillation) and MJO (Madden Julian Oscillation) – which are almost impossible to forecast more than a few weeks ahead.
However, seasonal forecasting is difficult and highly susceptible to go wrong, given there are so many factors or variables that control the weather day to day, week to week and over larger monthly or seasonal timescales which work in tandem, contradict or compete with each other. If one piece of the jigsaw that manifests in our weather isn’t accounted for, it can easily wreck attempts at seasonal forecasts. So, we can only really work on analogues of weather patterns that have occurred with particular long-range signals that appear likely to persist through winter 2017/18.
Technical/In-depth look at what may influence UK Winter 2017/18
To start off with, one of the main long-range drivers that can influence weather patterns across the northern hemisphere in winter is the ENSO (El Nino Southern Oscillation) state. This year we have seen a La Ninadeveloping, which is basically the cooling of waters in the Tropical Pacific. A weak to moderate La Nina is forecast to develop and persist basin-wide through the coming winter. Currently, the coldest anomalies are across the eastern side of the ENSO region. La Nina has been linked to a tendency to reduce the westerly winds which bring the UK mild air, particularly in early winter. And a La Nina phase of the ENSO has been linked to suppressed temperatures globally too, though this more likely during a strong event than a weak/moderate event as it predicted.
Another key long-term driver is the stratosphere. The tropical stratosphere is home to the QBO (Quasi-Biennial Oscillation) – a pattern of high altitude winds which alternate between westerly and easterly direction in cycles lasting around 27-29 months. This makes the QBO a regular and predictable tool, though, in early 2016, the QBO cycle had an unprecedented disruption. A band of easterly winds began to form above the westerlies, as expected, at the end of 2015, but it was cut off by a new band of westerly winds that appeared below it, keeping the QBO from completing a normal cycle. Although the QBO occurs in the stratosphere in the tropics, it can affect global teleconnections, by causing waves in the stratosphere that reach higher latitudes. Research shows that the QBO influences the North Atlantic Oscillation (NAO), a pattern of seesawing atmospheric pressures that dominate European weather. When the QBO winds are in a westerly/positive phase, pressure differences over the North Atlantic tend to be greater, which strengthens the jet stream which increases chances of mild and stormy winter weather. This winter, the QBO is expected to remain in an easterly/negative phase, which increases the likelihood of northern Europe having a good shot at a colder, drier winter.
Time-height section of monthly mean zonal winds (m/s) at equatorial stations. QBO is measured between 10hpa and 80hPa. Westerlies are shaded.
Furthermore, changes in sea surface temperature (SST) anomalies in the North Atlantic may have an impact on the NAO. Last year, there were below normal SSTs in the NW Atlantic, this year they have flipped to well above normal. This may increase the potential for low pressure systems to develop further south and increase high latitude blocking – resulting in a negative NAO signal.
Also, another key driver worth factoring in is the influence of Solar Cycle. Solar activity is currently falling to a minimum in the next few years for the first time since the winter of 2010/11, the last time the UK had a cold and below average winter. The combination of solar minimum and an easterly QBO can lead to a weaker and disturbed Polar Vortex, which leads to an increased chance of high latitude blocking, which allows cold polar/arctic air to escape south to Europe and North America more frequently.
One more factor to put in the melting pot of making some assumptions of the coming winter is snow growth in Siberia and its influence on the Polar Vortex. This subject is highly debated and needs more study, one of the leading authorities on this research is Judah Cohen who does regular updates on this subject. The idea is that above normal snow extent in Siberia leads to a weaker Polar Vortex, though there is uncertainty whether above normal snowfall here is a symptom of a weak Polar Vortex rather than a cause.
Daily Eurasian snow cover extent during the month of October from 2009-2017. Credit: aer.com
Conclusions toward preliminary pointers for Winter 2017/18
Putting all the above together, the combination of La Nina, easterly QBO and approach to solar minimum suggests to me a greater chance of colder and wintry weather at times this winter than recent winters. However, there are the caveats that more short-scale events in the weather patterns may hinder or prevent colder air reaching the UK for long periods. Some of the teleconnections can behave erratically and can scupper the underlying long-term signal for colder than average conditions over winter as a whole. The main one is the NAO, which in recent winters has spent long periods in a positive phase, even when its cousin the Arctic Oscillation (AO) has been negative. A -AO and -NAO often occur together and create a greater risk of colder weather in mid-latitudes. Whereas +NAO and +AO tend to occur in tandem too to bring mild and stormy weather. With this in mind, we may see bouts of a -NAO, but may still on average have a +NAO for longer throughout winter, despite the -QBO and solar minimum favouring a more -NAO signal.
So even though long-term drivers point to a colder winter overall than recent winters, certainly those since the last below average winter of 2010/11, don’t expect to be as cold or snowy as 2009/10 or 2010/11 winters. And with all long-range forecasts, none will ever be accurate and reliable all the time. Otherwise, long-range forecasters would be very rich!