Danger from the Atlantic: As climate change continues, Europe could be hit more frequently by former hurricanes – cyclones that cross the Atlantic and become extratropical cyclones. The reason: global warming makes hurricanes more intense and steers them further north. There they can interact with the jet stream and thus receive a “second life” as a strong Atlantic storm, as researchers have determined.

Most European storm fronts come from the Atlantic – they are transported to us by the prevailing westerly winds and the jet stream. While these storms typically form over the North Atlantic, there are some that have a history. These are former hurricanes that swept north from the equatorial Atlantic and, after transforming into an extratropical storm, are moving northwest into Europe.

Only around ten percent of hurricanes make the leap across the Atlantic and reach Europe. “While ex-hurricanes are rare in Europe, they can be particularly destructive and deadly events,” says lead author Elliott Sainsbury of the University of Reading. “After becoming extratropical storms, they can bring extreme winds, rainfall and waves even to regions far from the tropics.” Such high-impact former hurricanes have included Debby in 1982, Katia in 2011 and Ophelia in 2017.

But what determines whether a hurricane can survive this transition to extratropical waters and even gain new strength? Sainsbury and his team investigated this using meteorological data on 180 former hurricanes over the past 40 years. “We wanted to know whether there are differences in the cyclones and their surroundings that determine who reaches Europe and who doesn’t,” the researchers explain.

In fact, there were several abnormalities in the Atlantic-crossing ex-hurricanes. The first concerns the strength of the initial storm: “Storms reaching Europe already have substantially higher intensities in their tropical storm phase,” report Sainsbury and his team. On average, the wind speeds of these ex-hurricanes are four to six meters per second higher than those of tropical storms that do not make it across the Atlantic.

That could mean more ex-hurricanes heading our way in the future: Studies show that warmer seas give tropical storms more energy. Driven by heat and abundant water vapor, hurricanes are therefore increasing in intensity as climate change progresses. “Because stronger hurricanes are becoming more frequent with climate change, we could also see more ex-hurricanes in Europe in the future,” says Sainsbury.

But the intensity of the hurricane is not the only factor: their timing also determines whether they are resilient enough to reach Europe. Hurricanes that develop towards the end of the hurricane season are therefore more likely to reach Europe than hurricanes that form at the beginning of the summer. “In June, this is only the case for 15 percent of the ex-hurricanes, in November it is 50 percent,” reports the team.

More importantly, however, is their track: as tropical storms move into cooler waters, they usually weaken quickly. However, they can gain new momentum if they migrate far enough north to meet the jet stream. Contact with the air pressure trough created by this wind highway gives new energy to the storms. “It is at this point that many of the post-tropical storms en route to Europe experience reintensification,” the researchers explain.

As a result of the reintensification, the ex-hurricanes are again expanding and their wind speeds are also increasing again, as the analyzes have shown. This increases their chance of crossing the Atlantic and reaching Europe despite adverse circumstances.

However: The jet stream and its location are also affected by climate change. Some studies suggest that this wind highway could shift further poleward in the future, others predict more pronounced north-south waves in this wind band. These shifts in the jet stream could therefore also influence whether an ex-hurricane experiences reintensification or not – and thus whether it reaches Europe.

“If the increase in tropical storm intensity outweighs the potential poleward shift in the jet stream, then more ex-tropical storms could reach Europe in the future,” Sainsbury said. Accordingly, it is important to break down the influencing factors for such “defects” even more precisely. (Monthly Weather Review, 2022; doi: 10.1175/MWR-D-22-0111.1)

Quelle: University of Reading

This article was written by Nadja Podbregar

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