March 23, 2013 — andyextance

Current once-in-a-century hurricane-force winds may become as much as 25 times as likely in parts of Western Europe at the end of the 21st century. That’s what Rein Haarsma and a team from the Royal Netherlands Meteorological Institute (KNMI) have shown using one of the highest-resolution climate models around today. Their findings spring from a change in where hurricanes will develop that could also affect western North America, though more research is needed to study this. “The statement that the wind climate in Western Europe will not change significantly is questionable,” Rein told me. “Significant changes in wind climate will have consequences for agriculture – the increased winds are during the autumn – infrastructure and coastal defence.”

With Europe so far from the tropical regions where warmth and unstable atmosphere spawns hurricanes, it rarely sees them today. But when hurricane conditions do happen, like the ‘Great Storm’ in 1987, or Hurricane Floyd in 1993, they live long in the memory. The hurricane remnants that sometimes reach Western Europe usually bring a lot of rain, Rein noted, and only occasionally hurricane-force winds.

The warming Arctic is reducing ocean temperature differences that help create Europe’s traditional storms, meaning they pose less of a threat. But recently findings have shown that a warmer atmosphere raises hurricane risks. “Many model simulations suggest that the strength of hurricanes will increase due to climate change,” Rein explains. “The area where hurricanes develop appears to move poleward and the moisture content in a warmer atmosphere will increase. These factors might alter the possibility that these remnants of hurricanes are still strong enough to produce hurricane-force winds.”

Hurricane-in-a-box

But studying this question needs climate models that work in extremely fine detail, using the most powerful computers to produce vast volumes of data. Such models work by dividing the world into three-dimensional grids, simulating processes happening in each box in the grid, and interactions between them. The boxes are usually 100 km long and wide, much bigger than a hurricane.

“You need a very high resolution model to simulate hurricanes, which have a small horizontal scale,” Rein explains. “There are special hurricane models, but these are only in specific areas. There are now only a few climate models that have a high resolution globally.” But Rein’s KNMI teammate Wilco Hazeleger leads one such model, the 25 km horizontal resolution EC-Earth. Rein therefore suggested using it to study the future of hurricanes in Western Europe.

In a paper published online in Geophysical Research Letters on Monday Rein explained how the team ran six simulations for each of two periods: 2002-2006 and 2094-2098. For the earlier runs, they used measured values for sea surface temperatures, greenhouse gas and aerosol concentrations. For the future simulations they used figures based around the RCP 4.5 scenario, where CO2 levels in the air reach roughly 650 parts per million (ppm), compared to almost 397 ppm today.

“We ran the model for present and future climate and analysed the differences,” Rein explained. “We also compared the results for the present climate with observations. We investigated the modelled days with hurricane-force winds in four target areas, North Sea, Gulf of Biscay, West UK, Norway, and traced back the origin of these storms.”

Moving the hurricane nursery

In the future, they found, hurricane winds become most likely in the autumn period from August to October, rather than the winter winds found today. The researchers saw two hurricane-force storms during this early autumn period in total across all of the six runs from 2002-2006, rising to 13 in the six 2094-2098 runs. The effect is clearest in the North Sea, where hurricance winds that happen once in a century today become four times as likely, and the Gulf of Biscay, where they become 25 times as likely. And though EC-Earth struggles to model the most severe hurricanes accurately, they happen four times as often in the team’s later runs as in their earlier ones.

Today, hurricanes generally start off in the western tropical Atlantic, where sea surface temperature is above the 27°C threshold they form at. However, the few hurricane-force storms reaching Western Europe in Rein’s team’s early 21st century simulations usually start off outside the tropics. But by the end of the century warming has raised sea temperatures in the eastern tropical Atlantic beyond the 27°C threshold. The models show the warmer eastern Atlantic becoming an important hurricane birthplace, with the overall number spawning in the western Atlantic appearing to fall. This could be important for North America, though Rein underlined that his team hasn’t fully investigated that point. Meanwhile hurricanes moving north from the eastern Atlantic are more likely to get caught by winds blowing towards Europe, making this the most common origin of the continent’s hurricanes in 2094-2098. The eastern-born hurricanes also have less far to travel to get to Europe, meaning they’re still strong when they arrive.

Rein concedes that this work is at an early stage, though he backs its conclusions. From here he and his team would like to try to understand the physics behind the changes, and call in other groups to check their work. “We have now contact with those few other groups in the world that have similar high resolution models, to test the robustness of our results,” he said.

Journal reference:

Haarsma, R., Hazeleger, W., Severijns, C., de Vries, H., Sterl, A., Bintanja, R., van Oldenborgh, G., & van den Brink, H. (2013). More hurricanes to hit Western Europe due to global warming Geophysical Research Letters DOI: 10.1002/grl.50360