Where a storm surge strikes along a stretch of UK coastline is decided by global climate patterns in the Atlantic, Arctic and even further afield – according to new research led by Cambridge University.
Studying 70 years of sea level data, the researchers found that global atmospheric conditions thousands of kilometres distant can determine which coastlines bear the brunt of extreme storm surges around the North Sea, English Channel and Irish Sea.
Scientists have long known that large-scale climate patterns determine storm tracks in northwestern Europe. But the new study shows that the same climate patterns also decide which stretch of coast may be more likely to be hit by a storm surge.
Storm surges are challenging to forecast, being caused by a complex interplay of wind, low surface pressure and coastal geography. Knowledge of why storm surges occur in certain locations could help forecasters provide warnings of flooding to coastal communities further in advance.
"If we can identify conditions emerging in, for example, the Atlantic that might result in a storm surge in East Anglia, then we can pinpoint parts of the coast that should prepare for flooding," said Lisanne Blok, lead author of the research from Cambridge Earth Sciences.
Storm surges happen when strong winds and low pressure combine to push a 'bulge' of seawater inland and overtop flood defences. When combined with high tides and extreme wave conditions, extreme sea levels can result, in turn causing coastal flooding.
In late January 1953, a deep area of low pressure moved down the North Sea, pushing a massive body of water along the east coast. Combined with a high spring tide, sea levels were three metres higher than expected – leading to devastating floods in Britain, the Netherlands and Belgium.
In 2013, another storm surge hit the east coast of Britain – this time with higher water levels than in 1953 – although modern flood defences averted disaster at the same scale.
The North Sea is particularly prone to extreme storm surges. Its narrow funnel shape, particularly between East Anglia and the Netherlands, pushes water inland and its shallow basin amplifies the surge height.
"The storm surges are very dynamic and complex in this region," said Blok. "There is also heightened risk, because lots of people live along the low-lying coastal regions bordering the North Sea."
The researchers analysed records of reconstructed historic sea level data from around northwestern Europe. They then isolated the storm-surge component of sea level, removing seasonal variations and other background fluctuations to focus on unusually large or 'extreme' surge events.
Using machine learning, they grouped different parts of the coastline into regions that experience extreme surge levels at the same time.
They then looked for relationships between the storm surges in each of these regions and larger scale climate patterns.
"We can link regional storm surge extremes to very distinct climate patterns," said Blok. "I didn't expect such clear relationships, especially across the large distances involved."
Blok found that the largest influence on storm surge location was the pattern of atmospheric pressure conditions across the Northern Hemisphere, and specifically the North Atlantic.
When atmospheric pressure is low over Iceland relative to the Azores, she found that storm surges were more likely to happen along northern and central UK coastlines. In this configuration, the jet stream moves toward higher latitudes, carrying storms along a more northerly track over the north of the UK. "As the storms shift to the north, they also deliver high seas to more northerly coasts," said Blok.
Conditions reverse when pressure is anomalously high over Iceland. With a less marked pressure contrast relative to the Azores, the jet stream slows and snakes southward – shunting storms along more southerly paths, for example to the south of the UK, in turn bringing high seas to southern coasts.
The researchers found that storms that drive extreme surges in southwest England tended to originate in the open Atlantic, off the coast of eastern Canada, thousands of kilometres away. Instead, the storms that drive surges in the North Sea and Scotland tend to come from further north, off the coast of Greenland.
The findings demonstrate the wider influence of northern hemisphere climate patterns in UK storm surges, with Arctic conditions particularly key. "Conditions in the Arctic have a big impact on UK weather," said Ali Mashayek, senior author of the study from Cambridge Earth Sciences. "That's something we've known for a while, although we haven't been able to link it to storm surge distribution before."
Mashayek explained that Arctic warming will reduce the pressure contrast that powers the jet stream, "that means it is likely we will see a shift in UK storm patterns, including where storm surges occur, in future," said Mashayek. But, he cautioned, "this is something we are still trying to understand." Mashayek is involved in a project exploring how Arctic warming, rising temperatures, and sea ice loss drive extreme UK weather events.
Knowing which conditions influence where storm surges happen is key information for forecasters, said Blok. "Computer models used to forecast risk of coastal flooding don't always account for large-scale climate modes, instead focusing on localised weather variables," she explained. "But what our findings tell us is that climate modes are really key."
It is time to view localised weather extremes in a global context, said Mashayek. "We now have more evidence that the extreme weather in our backyard may be driven by what happens in very distant parts of the world," he said. "This speaks to the broader issue that climate change and the extremes it induces are a truly global problem that need global solutions."
