Why Europe is becoming a heatwave hot spot

NEW YORK (NYTIMES) – Two months ago, France experienced its hottest May on record, with record highs in some cities. Last month, France was blistered again, by a spring heatwave that also affected Spain, Italy and other countries. Then, this month, Poland and other parts of Eastern Europe suffered during a spell of extreme heat.

Now temperatures across Europe are soaring yet again, at or near triple digits from Spain to the British Isles and spreading east. Wildfires stoked by the heat are burning in many countries, and much of the continent is in the throes of a lengthy drought.

And there are still two months of summer left.

Scientists say the persistent extreme heat already this year is in keeping with a trend. Heatwaves in Europe, they say, are increasing in frequency and intensity at a faster rate than almost any other part of the planet, including the Western United States.

Global warming plays a role, as it does in heatwaves around the world, because temperatures are on average about 1.1 deg C higher than they were in the late 19th century, before emissions of carbon dioxide and other heat-trapping gases became widespread. So extreme heat takes off from a higher starting point.

But beyond that, there are other factors, some involving the circulation of the atmosphere and the ocean, that may make Europe a heatwave hot spot.

No two heatwaves are precisely the same. The current scorching temperatures that reached into England and Wales on Monday (July 18) were caused in part by a region of upper level low-pressure air that has been stalled off the coast of Portugal for days.

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It is known as a “cutoff low” in the parlance of atmospheric scientists, because it was cut off from a river of westerly winds, the mid-latitude jet stream, that circles the planet at high altitudes.

Low-pressure zones tend to draw air toward them. In this case, the low-pressure zone has been steadily drawing air from North Africa toward it and into Europe. “It’s pumping hot air northward,” said Dr Kai Kornhuber, a researcher at Lamont-Doherty Earth Observatory, part of Columbia University.

Dr Kornhuber contributed to a study published this month that found that heatwaves in Europe had increased in frequency and intensity over the past four decades, and linked the increase at least in part to changes in the jet stream.

The researchers found that many European heatwaves occurred when the jet stream had temporarily split in two, leaving an area of weak winds and high pressure air between the two branches that is conducive to the buildup of extreme heat.

Dr Efi Rousi, a senior scientist at Potsdam Institute for Climate Research in Germany and the lead author of the study, said the current heatwave appeared to be linked to such a “double jet”, which she said has been in place over Europe for the past two weeks.

This could have led to the creation of the cutoff low, Dr Rousi said, as well as to an area of weak winds over Europe that allowed the heat to persist.

“It seems this is really favoring the buildup of this heat wave,” she said.


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