Which stretch of British coastline is struck by an extreme storm surge is largely decided by atmospheric pressure patterns out over the North Atlantic, according to research from the University of Cambridge.
The study, published in the Journal of Geophysical Research: Oceans under the title "Spatiotemporal Variability and Large-Scale Drivers of Storm Surge Extremes on the Northwestern European Shelf", draws on 70 years of reconstructed sea level records for northwestern Europe. It was reported by Phys.org.
The method was to strip out the noise. Seasonal and other background variation was removed to isolate the storm-surge component of the record, and machine learning was then used to group the stretches of coastline that experience extreme surges at the same time.
The strongest single influence turned out to be the pressure contrast between Iceland and the Azores. When pressure over Iceland is low relative to the Azores, the jet stream shifts north and surges become more likely along northern and central British coasts.
When pressure over Iceland is high, the arrangement reverses. Storms track further south and the high seas arrive on southern coasts instead.
The storms themselves begin a long way off. Those that produce surges in southwest England mostly formed in the open Atlantic off eastern Canada, thousands of kilometres away.
Those that drive surges in the North Sea and along the Scottish coast mostly came from further north, off Greenland. The two groups are distinct, and the study can tell them apart.
"We can link regional storm surge extremes to very distinct climate patterns," said Lisanne Blok of Cambridge Earth Sciences, the study's lead author. "I didn't expect such clear relationships, especially across the large distances involved."
Senior author Ali Mashayek, also of Cambridge Earth Sciences, pointed further north still. "Conditions in the Arctic have a big impact on U.K. weather," he said, noting that Arctic warming and sea ice loss may weaken the pressure contrast that drives the jet stream.
"It is likely we will see a shift in U.K. storm patterns" as a result, he said, though "this is something we are still trying to understand".
Blok's practical point is that forecasting models tend to concentrate on local weather, and would do better to account for these large-scale climate modes. A surge in 2013 pushed water levels above the region's twentieth-century benchmark and modern flood defences held.
It follows that a British coastal authority's exposure is not, in any meaningful sense, a local matter. The decisive variable is a pressure difference between an island in the North Atlantic and an archipelago off Portugal, and the authority in question has no representation at either site.
The arrangement is best understood as a supply relationship. Southwest England is served from the open Atlantic off eastern Canada. The North Sea and the Scottish coast are served from off Greenland. The boundary between the two service areas runs somewhere through the middle of the ocean and is not marked.
Allocation between them is handled by the Iceland reading. Low pressure directs the service north; high pressure diverts it south. The coastline itself contributes nothing to the decision beyond being there when the decision is implemented.
The Department of Unnecessary Research has proposed that the pressure contrast be added to the statutory list of consultees for coastal planning, on the grounds that it is already the most influential party and the only one that cannot be written to. Cornwall is served from off eastern Canada. Aberdeenshire is served from off Greenland. Neither arrangement was put to either of them.