NOAA/NASA Goddard Rapid Response Team
The remnants of Hurricane Ophelia have struck the British Isles, causing widespread disruption and damage on Ireland. The cyclone, now downgraded to an extratropical storm, has reportedly led to three deaths.
It is unusual for a hurricane to reach western Europe while still at or near hurricane strength. The last comparable event was Hurricane Gordon in 2006, which had also weakened to a storm before it struck.
“The historical record only shows one hurricane reaching Ireland whilst still at hurricane strength: Debbie in 1961,” said Julian Heming of the UK Met Office. But in that case the data are sparse. “It is possible that, like Ophelia, Debbie transformed into an ‘extratropical cyclone’ some hours before it struck Ireland.”
However, hurricanes could be a big part of Britain and Europe’s future.
“There is evidence that hurricane-force storms hitting the UK, like Ophelia, will be enhanced in the future due to human-induced climate change,” said Dann Mitchell at the University of Bristol, UK.
Hurricanes form in the equatorial Atlantic, where the sea surface is warm enough to power them. Normally they head north-west towards North America, but some veer north-east and are carried to Europe by the jet stream. However, such a storm must first travel over cold north Atlantic waters, which saps its energy – so when it reaches Europe it is just a typical storm.
But that might not be how things play out in a warmer world, says Reindert Haarsma of the Royal Netherlands Meteorological Institute in De Bilt.
In a 2013 study he simulated the tracks of Atlantic hurricanes in the last decade of this century. Haarsma found that in this warmer world, hurricanes could form further east, because the sea there was now warm enough to support them. This meant many of the storms bypassed America entirely and instead struck Europe (Geophysical Research Letters, doi.org/kv2).
This conclusion has since been supported by a second group (Journal of Climate, doi.org/f96cf5).
In a follow-up published in 2015, Haarsma and his colleagues used higher-resolution models to show that ex-hurricanes could re-intensify as they approached Europe, even reaching hurricane status (Climate Dynamics, doi.org/f7hc2m).
That was because they were “hybrid storms”, a cross between a tropical hurricane and a northerly winter storm. These storms can be very powerful because they have two sources of energy.
They form as hurricanes, which are powered by heat. “Over warm sea water, water evaporates then condenses, and the heat released from condensation is transformed into kinetic energy and that’s the source of the hurricane,” says Haarsma. Some of this warm, moist air remains caught in the centre of the storm, supplying energy.
But when storms form in the north, they have a different power source. “The energy comes from the horizontal temperature difference between the north and south part of the storm,” says Haarsma. This is called “baroclinic instability”.
An ex-hurricane can sometimes keep its warm core even as it becomes an extratropical storm. “This storm Ophelia has now two sources of energy: the old source of the hurricane and the new source of baroclinic instability,” says Haarsma.
Is it happening?
However, Haarsma emphasises that so far the main evidence that more hurricanes will reach Europe comes from modelling.
In a study published in March this year, he and his colleagues examined 53 tropical storms that reached Europe between 1979 and 2013. They found no sign of an increase (Climate Dynamics, doi.org/cd75).
A 2015 study by another group looked at the years 1948-2014 and did find a slight increase in the number of hurricanes that turned north-east like Ophelia (Bulletin of the American Meteorological Society, doi.org/cd78).
Still, Haarsma remains cautious, saying that so few hurricanes go to Europe that the statistics are likely to be unreliable. “We still have to rely on models and maybe in the future we will know if we are correct or not,” he says. “We don’t know yet.”
Haarsma is part of a project called PRIMAVERA, which is using high-resolution models to simulate future hurricanes in detail. “Hurricanes have a small horizontal scale, so traditional models have difficulty simulating [them],” he says. It takes more fine-grained models to capture the detail of hurricanes’ behaviour.
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