The complex interplay behind great heat

Unusually hot weather in India and Pakistan at the end of March; a long, warmer-than-average summer in central Europe; extreme December temperatures in northern Argentina, Uruguay and Paraguay: 2022 was definitely a year of heat waves. And there is currently a broad consensus among climate researchers that such extreme events will occur much more frequently in the future than they do today.

There are essentially three mechanisms that cause the thermometer to climb to unusually high values: air from warmer regions reaches cooler ones, for example from the Sahara to central Europe; the air in a high-pressure area sinks, warming up in the process due to compression; or the sun heats the ground with unusual intensity, so that the air above it is warmed up more than normal.

Which process dominates?

“These three processes are easy to understand and can be described well with physical formulas,” explains Matthias Röthlisberger, a scientist in the Atmospheric Dynamics Group at ETH Zurich. So he finds it all the more remarkable that there is still intense controversy surrounding the three. Indeed, experts disagree on the importance of each of these three processes and on which one ultimately determines whether or not a heatwave occurs in a given location.

This is not simply an academic question, Röthlisberger says: “It’s important to understand how big a role each of these mechanisms plays. Because that’s instrumental for assessing how reliable the climate model projections are.” Granted, these accurately predict the frequency and duration of heatwaves under current conditions. “But we don’t yet understand well enough whether or not these models are representing heatwaves correctly for the right physical reasons.”

Extensive dataset

Röthlisberger now wants to close this knowledge gap together with his colleague Lukas Papritz from the same group. To address this contentious issue, the researchers analysed heat extremes around the world, starting with a heatwave in Canada that measured temperatures of nearly 50 degrees Celsius in late June 2021.

For their analysis, the scientists used the latest dataset from the European Centre for Medium-Range Weather Forecasts (ECMWF), which contains three-dimensional global weather data at high temporal and spatial resolution. From this extensive dataset, they first filtered out the hottest day in each of the past 40 years at every location in the world. Next, they meticulously calculated the path taken by the air near the ground at each location over the preceding 15 days and the last point at which this air had had a normal temperature.

The path of the air is crucial, because it points to the main mechanism of heating. If the air originates from a region with a warmer climate, then heat transport is a major contributor to the heatwave. If, instead, the air comes from a climatically comparable region, then the other two factors must be the cause of anomalous temperatures.

Substantial regional differences

Röthlisberger and Papritz examined a total of 250 million air parcels for their study. An evaluation of the data shows that how the three factors interact varies extremely from region to region. Each of the factors dominates in certain regions of the world, but very often heatwaves result from a complex interplay of all three mechanisms.

In the case of the Canadian heatwave mentioned above, for example, the researchers were able to show that all three factors had a hand in the unusual weather situation. There are also regional differences within this single heatwave: in zones near the coast, it was primarily heat transport from the south and the sinking of the air that led to great heat, while further inland the extreme temperatures were a result of the dry and therefore heated ground warming the air.

For central Europe, meanwhile, the researchers find that hot Saharan air often has only an indirect influence. When hot air from Africa reaches Europe, rather than displacing the cooler air on the ground, it usually glides over it. “So the Saharan air ends up warming not the lower layers of the atmosphere but the middle and upper layers,” Papritz says. “In contrast, the hot air at ground level here mostly comes to us from the Atlantic and is then warmed up by the heated ground and compression.” Nevertheless, air from the Sahara does play a significant role: by heating up the higher layers, it prevents the thunderstorms that would normally develop in response to the warming near the ground, and would go on to provide urgently hoped-for cooling.