Following another summer of frequent downpours or thunderstorms leading to some very disruptive flash-flooding events, including the closure of multiple tube stations in London on 12th July, many in the UK will be wondering whether very wet days are becoming more common. Especially those suffering economic losses due to flooding, which for the UK was amounting to around £1.3 billion per year as of 2018.
To tackle this question, MetSwift has conducted in-house research into how the typical number of very wet days per year in the UK has changed since the 1970s, in each of the four meteorological seasons. As with so many things in weather and climate, the answer is not straightforward. For example, summer and autumn display drastically different trends… read on to discover more.
Winter: Decadal Oscillation
Starting with the coldest, darkest, and usually wettest season, we immediately run into a roller-coaster. In the graph below, the 1990s have higher very wet day frequencies across the board compared to 1973-1989. Yet 2000-2010 have lower frequencies up to 40+ mm, before 2011-2020 are higher again!
This has been seen despite temperatures trending markedly upward, increasing how much water vapour the air is able to contain (by about 7% per 1°C of warming). To understand why, we need to consider the weather patterns that were most frequent during each set of winters.
Both 1990-1999 and 2011-2020 saw more in the way of ‘zonal’ UK weather patterns than usual. Those are setups in which weather systems move quickly eastward from the North Atlantic. That means frequent bouts of wet weather in association with large low pressure systems (a.k.a. windstorms or depressions).
By contrast, during 2000-2010, the opposite sort of pattern was abnormally common; setups in which low pressure systems struggle to move eastward across Europe. Many (not all) instead headed south, becoming slow-moving over the mainland, typically sparing the UK the worst of the wet weather.
On these maps, blue shading broadly corresponds to areas seeing more low pressure systems than usual, and green less.
Teleconnections Outweigh Climate Trends
The clear impact on the frequency of heavy rain events tells us that on a decadal basis, predominance of certain weather patterns has a stronger influence than climate change during the UK’s winter season. The ‘favouring’ of some weather patterns over others is driven by large-scale ‘driving forces’ known as teleconnections. They’re a key ingredient used to determine the MetSwift Analogues, a set of most suitable years for providing guidance to conditions in the months and years ahead.
If we look longer-term, we can see that the 2000-2010 frequencies weren’t as low as 1973-1989s, so there’s an overall increasing trend between 1973 and 2020.
Spring: Also Bouncing Around
In spring, the days lengthen, and the sun strengthens a great deal, yet the rainfall frequency pattern is barely moved:
Compared to the winter season, the only significant difference is seen for frequencies of 20+ and 25+ mm daily rainfall. 2000-2010 is lower than 1973-1989 for those, while 2011-2020 is very close to the 1990s. This means we have no clear trend for 1973-2020.
…or do we? When considering what weather patterns were more common than usual, the story changes. The springs of 2010-2020 have seen the lowest frequency of low pressure systems affecting the UK. Less low pressure, yet similar very wet day frequency to the 1990s.
Low pressure was unusually frequent during the springs of 1973-1989, before a reversal in the 1990s. Then, 2000-2010 was very ‘normal’, before low pressure became unusually scarce again during 2011-2020.
This lends some credence to a widely held public opinion that ‘when it rains, it pours’ is truer now than it was decades ago.
Summer: Downpours on the Up
The season of hot (…when present) sunshine shakes things up. The lowest very wet day frequencies occurred in the 1990s and there’s been a strong increasing trend since then. The counts for 2011-2020 are very striking… and may not come as a surprise to those who’ve spent much summertime in the UK!
Is this a domination of climate trend for this season? Probably not, for there is also a distinct trend to the weather patterns since 2000.
In the summers of 1973-1989, low pressure located south of the UK more often than usual. After a well-balanced 1990s, a position close to the west of the UK has been unusually frequent since 2000.
When low pressure is close to the west of the UK, it moves warm, moisture-laden air northward across western Europe. This often starts interacting with cooler air near or over the UK. It’s a recipe for frequent outbreaks of heavy showers, sometimes thunderstorms. A recipe for more flash-flooding events, such as have blighted many areas in the UK and Europe during summer 2021. The UK is at increasing risk of ‘historic’ flood events such as Germany experienced in mid-July this year. According to Insurance Insider, insured losses relating to that have been estimated to reach as high as 5 billion euros ($5.9 bn).
For reasons that may have ties to Arctic sea ice loss, this setup has become increasingly common so far this century. It’s only a gradual trend, mind, so it’s unlikely to account for all the increase in very wet days. There’s likely to be a significant climate trend involved.
Autumn: Downpours Going Down
Yes, you read that right. In stark contrast to the summer, the UK has experienced a decreasing trend in very wet day frequency during 1973-2020. There’s just a faint suggestion that the trend may have begun to reverse for 10-15 mm days in the most recent decade.
As with the summer, this is connected to a prominent trend in weather pattern tendency. Throughout the 1970s to 2000s, the decadal averages were mostly close to the 30-year average. In the 2010s, however, there was a marked change: High pressure has become unusually frequent across most of Europe.
Note the mainly white shading across the UK in the first three maps. There were just slight variations in the typical location of low pressure systems (blue shading) and high pressure ones (green & yellow shading).
That open water supplies heat and moisture to the otherwise cold, dry atmosphere above. With climate change, typical Sep-Oct sea ice coverage has decreased by close to 2.5 million square km since the 1970s. That means a vast amount of extra heat and moisture entering the atmosphere. This in turn alters circulation patterns, which can have effects akin to what we’ve seen across Europe in the 2010s.
As I mentioned earlier, there could well be an Arctic connection affecting the summer season, too, but this is harder to be sure of. Reason being the change in open water coverage is still relatively small for the first half of the season (2011-2020 average is about 1 million below 1979-1990 average).
In the coming years, it should become more apparent whether European summers are being significantly altered as well.
Will Next Summer Bring More of the Same?
Speaking of coming years, you may be wondering if summer 2022 is likely to be ‘on trend’, with downpours aplenty. To answer that, natural variability must be considered. That’s variation on the scale of weeks or months that occurs on top of the underlying climate trend. It’s driven by those teleconnections I referred to earlier.
Next summer, I’m anticipating an important change to one of the most widely influential teleconnections. Currently, we have what’s known as a La Niña event in the tropical Pacific (explained here). This is expected to continue until early next year, by which time we’ll have gone 24 months without its counterpart; an El Niño event.
This statistically makes it more likely than not than an El Niño begins to take shape next summer. At the very least, the La Niña should cease, allowing weather patterns to sustainably shift.
While far from the only important influence on European summers, it does reduce the likelihood of it being dominated by low pressure systems ambling across western and central parts the continent.
It gives me hope that next summer will see a markedly lower frequency of flash-flooding events in those regions.
James Peacock MSc
Head Meteorologist at MetSwift
Cover Photo by ‘Beyond Survivor’ is licensed under CC BY. It shows a flooded suburban street scene from November 2012.