Met Office: Why 2020 saw a record-breaking dry and sunny spring across the UK

§ Record-breaking sunshine

This spring has been a season of record-breaking sun. The month of May 2020 was the sunniest calendar month since records began in 1929 – and April 2020 was found to be the sunniest April on record.

The maps below show how this year’s spring (left) and May (right) compare to the long-term average. The dark orange shading shows the parts of the UK that saw the largest amount of sunshine above average.

As might have been expected, these sunny months led to an exceptionally sunny spring season. Recording 626.2 hours of sunshine, the UK spring of March-May 2020 ranks as the sunniest spring on record, easily surpassing the previous record-holding spring of 1948, which saw 555 hours of sunshine. 

When focusing only on England, the numbers are even higher, with 695.5 sunshine hours recorded. This exceeds the previous record by more than 100 hours, amounting to an average of over one hour of extra sunshine every day for the three-month spring period.

This spring is also notable because it saw more sunshine hours than most UK summers, with only the summers of 1955, 1976 and 1989 seeing more sunshine. 

The graph below shows just how exceptional this spring was, as since records began in 1929, only nine springs have seen more than 500 hours of sunshine. In fact, spring “broke” the original y-axis on this graph, which needed to be amended to accommodate it.

One factor that has not played a role in the record levels of sunshine is the coronavirus lockdown across the UK (and much of the world). While the wide-ranging reductions in travel and industrial output have likely reduced emissions of aerosols – tiny particles and droplets – into the atmosphere, this has not caused the sunny weather.

As our colleague Prof Richard Betts explained in Carbon Brief’s recent webinar on the impact of coronavirus on global CO2 emissions:

“In the UK, we’ve been very lucky to have fine, dry, sunny weather for the past couple of months. This is nothing to do with the change in aerosol emissions. We’ve had beautiful blue skies and so on, but it’s just been high-pressure systems and the jet stream has been away from the UK. We’ve just been lucky with the weather. This is not an impact of the aerosol reductions.”

§ A warm and dry spring

As well as the exceptional sunshine seen this spring, we have also seen the eighth warmest spring in the UK and the fifth warmest in England and Wales.  

Seven of the Top 10 warmest springs on record in the UK have occurred since the year 2000. Daytime maximum temperatures were high, benefitting from the warming influence of the sun under clear skies, but minimum temperatures were closer to average, somewhat moderating the overall average temperatures.

The UK has seen very dry conditions throughout the spring, with lower-than-average precipitation in March, April and May. May was exceptionally dry, ranking as the driest May on record in England with 9.6mm average rainfall and the second driest in Wales with 14.3mm average rainfall.

The driest county this spring has been Northamptonshire, where only 1.7 mm of rain was recorded throughout the entire month of May. The driest location in the Met Office rain gauge network was RAF Benson, Oxfordshire, that recorded no measurable rain during May. A completely dry month like this is not common in the UK, but it does occur, most recently during the dry spell of June 2018.

In contrast with the rest of the UK, parts of north-western Scotland experienced above-average rainfall, with the Western Isles receiving 177% of the average May rainfall. 

The maps below show the regional variations in rainfall for March (left), April (middle) and May (right). The dark brown shading shows the areas with the lowest rainfall totals compared to average. For example, whilst May was very dry for the southwest of England, April was the driest month for northern England.

The lack of rainfall has impacted river flow and reservoir levels over spring, which had decreased by the end of May, with some sites classed as “exceptionally low”. 

As a result of below-average rainfall and increased temperatures, soil moisture levels have fallen significantly. This is impacting a range of sectors – most notably farming, in which the impact on rainfed grass and cereal crops is particularly concerning. Furthermore, the raised temperatures have led to an increasing water demand, leading to a call for efficient water usage.

At the end of May, temperatures rose significantly with maximum temperatures in the upper 20s Celsius for several days at the end of the month, reaching 28.3C at Cromdale (Highland) on 29 May. 

Some isolated locations in Wales, northwest England and Scotland even exceeded the new Met Office heatwave thresholds. Heatwaves in May are not unprecedented and the temperatures experienced this year were not record breaking, but May accounts for less than 1% of heatwave events based on the new thresholds. These relatively high temperatures in the second half of May offset cooler conditions, including some frosts, earlier in the month.

§ Large-scale circulation

The immediate explanation for the UK spring conditions lies with the jet stream – a core of strong winds blowing from west to east around five-to-seven miles above the Earth’s surface.

The jet stream is the most important driver of variations in UK weather. Its position changes throughout the year, resulting in pressure changes that can affect the weather over Europe. 

In winter, the jet stream is usually stronger and flows over the UK, bringing wetter weather. This was the case last winter, when a positive phase of the North Atlantic Oscillation led to a stronger than usual jet stream. This brought low-pressure systems over the UK, including storms Ciara, Dennis and Jorge.

This spring, the jet stream buckled and shifted northward, allowing successive areas of high pressure to dominate the UK. These “blocking” patterns of high pressure are the main reason for the extended periods of sunny, dry and relatively warm conditions that we have seen this spring.

The maps below show the shift in position of the jet stream from winter (left) to spring (right). The bring green, yellow and orange shading shows where the jet stream winds are at their strongest.

The location of high-pressure areas plays a crucial role in the variation of rainfall patterns across the country. In May, the centre of the high-pressure areas covered most of the south of England, resulting in dry and sunny conditions, whereas north-west parts of Scotland and the Western Isles were left outside of the high-pressure areas and exposed to weather fronts, bringing more rainfall. 

Similarly, when the high pressure slipped westwards, the UK was exposed to northerly winds, resulting in the colder night-time minimum temperatures that we experienced.

§ What can we expect for future springs in a warming climate?

Extended spells of dry sunny weather during spring – such as in 2020 – is primarily a consequence of large-scale circulation and the buckling of the jet stream, which allows the development of persistent high-pressure systems over or close to the UK. 

However, the increasing temperatures associated with climate change may have also played a role in the warm and very dry weather.

The contrast between a notably wet winter of 2019-20 and the dry spring of 2020 is striking and unprecedented. This is in line with UK Climate Projections 2018 (UKCP18) – the latest set of dedicated climate change projections for the UK, produced by the Met Office. UKCP18 suggests that future winters are likely to become wetter and warmer, and summers hotter and drier.

However, the remarkable statistics from spring 2020 are not necessarily a sign of things to come. UKCP18 suggests that although spring is likely to become sunnier in the future, a spring like 2020 is still likely to be considered extreme even by 2100. 

For example, a 2017 study by Met Office scientists investigating the record-breaking sunny winter of 2014-15 demonstrated that such an extreme was more than one-and-a-half times more likely to occur under the influence of human-caused climate change. For events like spring 2020, further analysis is required to quantify the potential role of climate change.