Marc Stefanon and colleagues from the Institut Pierre Simon Laplace, France, discovered that European heatwaves can be classified into six types according to their location. What's more, the physical mechanisms causing the hot spells also varied with position.

Heatwaves tend to occur when there is a persistent anticyclone over a high-temperature area. Previous analyses have examined only the 2003 event or have defined heatwaves from temperature anomalies averaged over the whole European continent, say the team.

In this study the researchers said that temperatures must exceed the local 95th centile; according to their criteria, there were 78 summer heatwaves in Europe between 1950 and 2009. The phenomena, which lasted a total of 643 days, could be split into six clusters: Russian, Western Europe, Eastern Europe, Iberian, North Sea and Scandinavian.

"We wanted to have a regional approach to assess and identify the geographical specifics of heatwave events," Stefanon told environmentalresearchweb. "We could identify heatwave patterns in the Euro-Mediterranean region with their typical size, a Rossby radius."

The clusters showed different characteristics. For example temperature anomalies ranged from 3 degrees above the mean for the Iberian cluster to more than 6 degrees above the mean for the North Sea cluster. The heatwaves typically lasted about one week, except in the Russian and Scandinavian clusters where hot spells tended to continue for nine days.

The Western European cluster tended to be caused by a European summer blocking high, and the Russian cluster by a Ural blocking high. The Iberian and Eastern Europe clusters, meanwhile, were associated with high pressure over central Europe and lows over the Atlantic, leading to southerly winds over the Iberian Peninsula.

The team also found that summer heatwaves in western and eastern Europe were linked to drought in southern Europe in the preceding winter and spring. The Iberian cluster tended to see local drought preceding the heatwave, which was generally caused by warm air advection from the south.

"[Our study] also brings some light on the role of droughts in heatwave preconditioning, which differs significantly between southern and central Europe – strong local and remote impact of droughts – and northern Europe (no impact of droughts)," said Stefanon. "These results impact communities working on water resources, and climate variability."

Now Stefanon plans to focus on the western Europe heatwave region and use the model to study different processes that could impact heatwave duration or amplitude. "For instance, an interacting plant phenology is quite important at the regional scale when evaluating the local expressiveness of an extreme event," he said. "For this purpose we developed a regional two-way soil-vegetation-atmosphere coupled system over the Mediterranean basin within the MORCE (Model of the Regional Coupled Earth system) platform."

The team reported the study in Environmental Research Letters (ERL).