Followers of The Sun, The Evening Standard, Sky News, The Independent, The Metro, The Sunday Times and The Daily Express will have noticed a stream of articles during the first week of September claiming that Britain is braced for a repeat of 2018's "Beast from the East". Allegedly, we're set to endure one of the coldest winters in the last 30 years in the upcoming 2018/2019 winter. Yet, at the same time, the Met Office is reluctant to back up this claim and in a statement put out to the Manchester Evening News a couple of days later, appears to disassociate itself from such a forecast. So what's going on, and what's the actual science behind this prediction? Bottom line: what should we believe?
The origin of the claim
The reason for the articles predicting this winter's weather is a paper put out by University College London at the at the end of August 2019, which predicted that temperatures will be colder than normal during January-February 2020. Delving into the science of the paper, their data points to a higher than normal likelihood that the North Atlantic Oscillation (NAO) will be negative, and that the central England temperature (CET) will be colder than normal for January-February 2020. Specifically, their weather data analysis points to an 87% probability that the NAO will be less than the 1981-2010 mean, and a 65% probability that the CET will be the colder than the 1981-2010 mean. To back up their data, they point to 1953-2019 weather data analysis, which shows that in nine of the ten years where these predictor fields had the same sign and similar magnitude to that in summer 2019 were followed in January-February by a negative NAO and by a CET "colder than the climatology".
What's a negative "North Atlantic Oscillation" and why does it matter anyway?
The NAO is a weather phenomenon in the North Atlantic Ocean of fluctuations in the difference of atmospheric pressure at sea level between the Icelandic Low and the Azores High. Through fluctuations in the strength of the Icelandic low and the Azores high, this oscillation controls the strength and direction of westerly winds and location of storm tracks across the North Atlantic, and therefore has a direct influence over the weather in the UK.
Westerly winds blowing across the Atlantic bring moist air into Europe. In years when westerlies are strong, summers are cool, winters are mild and rain is frequent. If westerlies are suppressed, the temperature is more extreme in summer and winter leading to heat waves, deep freezes and reduced rainfall. During the months of November to April, the NAO is responsible for much of the variability of weather in the North Atlantic region, affecting wind speed and wind direction changes, changes in temperature and moisture distribution and the intensity, number and track of storms.
A permanent low-pressure system over Iceland (the Icelandic Low) and a permanent high-pressure system over the Azores (the Azores High) control the direction and strength of westerly winds into Europe. The relative strengths and positions of these systems vary from year to year and this variation is known as the NAO. A large difference in the pressure at the two stations (positive NAO) leads to increased westerlies and, consequently, cool summers and mild and wet winters in Central Europe and its Atlantic facade. In contrast, if the index is low (negative NAO), westerlies are suppressed and northern European areas, such as the UK, suffer cold dry winters.
In the research published by University College London, the findings suggest that exactly this will happen: westerlies being suppressed. Specifically, the research alleges that the jet stream - the high- altitude wind that pushes weather systems across the Atlantic to Britain - will be deflected southwards for weeks in January and February, allowing freezing Arctic air to dominate the UK, making it very very cold indeed during those months in 2020.
Large scale weather system effects
A point of interest about the NAO is whilst this is strictly a theoretical meteorological phenomenon, observations suggest that meta phenomena such as this have a direct influence over ecological phenomena that are easily ascribed to other reasons. For example, the NAO has been in an overall more positive regime since the late 1970s, bringing colder conditions to the North-West Atlantic, which has been linked with the thriving populations of Labrador Sea snow crabs, which have a low temperature optimum.
This NAO+ warming of the North Sea has possibly influenced the survival of cod larvae which are at the upper limits of their temperature tolerance, as does the cooling in the Labrador Sea, where the cod larvae are at their lower temperature limits. Therefore the NAO+ peak in the early 1990s may well have contributed to the collapse of the Newfoundland cod fishery. On the East Coast of the United States an NAO+ caused warmer temperatures and increased rainfall, and thus warmer, less saline surface water. This prevented nutrient-rich upwelling which has reduced productivity. Georges Bank and the Gulf of Maine are still affected by this reduced cod catch today.
Is the harsh European winter of 2009–10 a point of proof?
The winter of 2009–10 in Europe was unusually cold. Called "The Big Freeze" of 2010 by British media, the Met Office reported that the UK, for example, experienced its coldest winter for 30 years... and this winter coincided with an exceptionally negative phase of the NAO. January 2010 was provisionally the coldest January since 1987 across the UK, and a persistent pattern of cold northerly and easterly winds brought cold, moist air to the United Kingdom with many snow showers, fronts and polar lows bringing snowy weather with it.
The most severe snowy weather began on 5 January in North West England and west Scotland with temperatures hitting a low of −17.6 °C (0.3 °F) in Greater Manchester, England. Snow spread to Southern England on 6 January, and by 7 January the United Kingdom was blanketed in snow. The winter weather brought widespread transport disruption, school closures, power failures, the postponement of sporting events and 25 deaths. A low of −22.3 °C (−8.1 °F) was recorded in Altnaharra, Scotland on 8 January 2010, and overall it was the coldest winter since 1978–79, with a mean temperature of 1.5 °C (34.7 °F).
Obviously this was a very cold winter, and indeed the NAO was unusually negative. There's also a 90% correlation between negative NAO and cold winters - which makes perfect sense, because, bottom line, when the Gulf Stream moves further south, it allows cold Arctic air to flow over the UK, and makes for a very cold winter.
So why did the Met Office disassociate itself from this prediction?
The answer is perfectly simple, and the Met Office and University College London are actually closer in agreement than might initially be concluded by reading the Met Office statement. The Met Office are in the business of forecasting, and a "forecast" will only cover the next few weeks. An "outlook" is different and can cover months ahead. So the Met Office won't recognise it as a forecast because in the Met Office lexicon, next January's weather is an "outlook", not a "forecast".
What if it's a cold winter?
Nobody obviously actually knows for certain if this winter is set to be especially cold or not, although the indicators would tend towards a very cold early part of the year. Slips, trips and falls in car parks and on footpaths are common in the wintry months, and should the weather deteriorate even further, you may find that business and operations grind to a halt if you're not prepared for inclement weather. As a business, school, facility or factory, you can cover yourself with a fixed budget for the entire season by speaking to one of the experts at GRITIT. We'll cover any claims against you and we'll always be there to grit your facility and clear away the snow.