The UK is famous for its changeable weather so the fact that we have just basked in the sunniest, driest April since records began suggests something is afoot.
What held back the famous April showers? The answer is simple: a blocked weather pattern.
‘Boulder’ in the jet stream
Usually, weather systems cross the Atlantic from west to east as they go around the northern hemisphere, and bring a variety of conditions (in the UK, mostly rain and wind). That’s thanks to the jet stream, a current of wind about six miles above the Earth that moves “like a river of faster flowing air”, said the BBC, steering weather around the world. When that river speeds up or slows down, high- and low-pressure areas form, which “shape the changes in our day-to-day weather”.
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High-pressure systems mean the atmospheric pressure is higher than its surroundings, while low-pressure systems are the opposite. High pressure in the summer usually means hot, dry spells. In the winter, when clouds get trapped under high pressure, we end up with “unrelenting dull days”: the so-called “anticyclonic gloom”.
But for the past few months, the jet stream has been “amplified”, said the Met Office. That means it “meanders”, which can lead to areas of high and low pressure.
And for the last few weeks there has been “a stubborn area of high pressure” above the UK, said climate experts Simon H. Lee and Matthew Patterson, from the University of St Andrews, on The Conversation. That has “diverted the usual flow of mild, moist air from the North Atlantic like a boulder in a river”. This a “blocking weather system” – when the weather effectively gets “stuck”.
Contested role of climate change
A blocked high-pressure system is often linked to heatwaves and drought. The UK’s Environment Agency warned water companies last week to start safeguarding water as there is a “medium” risk of drought this summer.
Blocked weather was “a driver” of the UK’s 1976 summer drought, said Carbon Brief, as well as the “deadly” European summer heatwave of 2003, Siberia’s summer wildfires in 2013 and Australia’s “devastating” bushfires in the summer of 2019-20. But the important question is how global warming might be affecting the “frequency and severity” of blocking patterns.
Trends are extremely difficult to track because there is “no set definition” of a blocking event. But they do occur more frequently in the northern than the southern hemisphere – on the northern and eastern sides of the North Atlantic and the North Pacific region. They occur typically over Greenland and Europe, and blocking patterns over Greenland during summer have been increasing in frequency since the 1990s.
What’s not clear, said Lee and Patterson on The Conversation, is whether that’s down to “human-induced climate change“, or simply “down to chance”. Some researchers suggest that, because the Arctic is warming twice as quickly as the tropics, the jet stream is weakening and becoming more “wavy”, making blocking events more common. But this is “controversial among scientists”. It also runs contrary to climate models, most of which predict a decline in blocking events with climate change.
What we do know, said the BBC, is that global warming has made the weather caused by blocking patterns “more intense”, leading to more wildfires, droughts and, in low-pressure patterns, “devastating flooding”.
