We’re all keenly aware of the heat wave that is affecting the UK and beyond – but why might it be happening? Len Shaffrey, University of Reading Professor of Climate Science, explains all in a new post for The Conversation.
The UK and Ireland have been experiencing a prolonged hot and dry spell since June, with the first half of summer being the UK’s driest on record. The lack of rainfall has led to hosepipe bans in Northern Ireland and the north-west of England, while the weather is also playing havoc with farming. A shortage of lettuce and broccoli is expected in the next few months, and grass isn’t growing fast enough to feed Ireland’s sheep and cattle through the winter.
The hot and dry weather is associated with a high pressure weather system situated over the UK. The high pressure means that the storms the UK occasionally gets at this time of year are being steered much further northwards towards Iceland. While the UK and Ireland have been wilting in the sunshine, Reykjavík has recorded its wettest (May) and cloudiest (June) months on record.
This high pressure system is unusually persistent and has been building up over Europe throughout spring and early summer. In April it was over Central Europe and in May shifted northwards towards Scandinavia. Subsequently it was the hottest April and May in Germany and the hottest May in Denmark since observations began in the 1880s.
But why has it been so dry and warm? Here is a shortlist of candidates that could be playing a role – or not – in this unusual summer:
1. Climate change
Let’s start with the obvious: temperatures are increasing globally due to the burning of fossil fuels, which is increasing concentrations of atmospheric carbon dioxide. The global rise in temperatures means that heatwaves are occurring against a warmer background and so are more likely to become extreme.
The past few years have seen some record-breaking temperatures in Europe, for example the 2015 heatwave and the 2017 “Lucifer” heatwave in Central Europe. Unusually warm summer temperatures have also been recorded elsewhere, for example in Canada and Japan, and climate change is very likely to have played a role in the UK and Ireland as well.
2. North Atlantic temperatures
Temperatures over the North Atlantic Ocean can play a role in setting the position of a narrow band of strong wind in the upper atmosphere known as the jet stream. The position of the jet stream in turn has a profound impact on the weather experienced in the UK and Ireland.
This summer, ocean temperatures have been relatively warm between the Gulf of Mexico and North Africa while temperatures south of Greenland have been unusually cold. This is thought to have pushed the jet stream further northwards, sending bad weather towards Iceland while allowing areas of high pressure to linger over Europe.
3. La Niña
Every few years, ocean temperatures in the tropical Pacific swing between being relatively warm (known as El Niño) and cool (La Niña). Since October 2017 the area has been in a La Niña phase. This is sometimes associated with cold winters in north-western Europe (for example the winter of 2010-11 and the recent “Beast from the East” cold spell in March 2018). However, La Niña cannot really be blamed here – this year’s event had started to weaken around April and had almost gone by June when the UK’s current dry spell began.
4. It’s the weather
The above factors influence the type of weather the UK and Ireland will get but, within these broad possibilities, good or bad luck also plays a role. This is especially the case for very unusual weather such as the current hot and dry spell. This summer is no different and the hot and dry weather is partly due a combination of Atlantic temperatures, climate change and annual weather patters. Should weather patterns continue as they are then 2018 may turn out to be as hot and dry as the extreme summer of 1976.
This raises the question of how predictable it is. The science of forecasting the weather a few months ahead is still in its infancy, and such “seasonal forecasts” are subject to lots of uncertainty. However, the UK Met Office did predict back in early June that there was an increased probability of a drier and warmer summer. While meteorologists are not yet able to predict prolonged hot and dry spells months in advance there were useful indicators of an increased chance of extreme weather.
Advancing the science of seasonal forecasting requires a deeper understanding of the different factors than can influence the weather, from ocean temperatures and jet streams through to changes in Arctic sea ice. As part of the UK Drought and Water Scarcity programme research institutions in the UK are being funded to further our knowledge of these influences and to better understand how the UK can respond to and mitigate the impacts of prolonged dry weather.
Len Shaffrey is Professor of Climate Science at the National Centre for Atmospheric Science, within the University of Reading’s world-renowned Department of Meteorology. This post was first published on The Conversation, 25 July 2018.