Our heat waves in the summer months do not come from the Sahara

Last summer was long and hot. There was also unusually warm weather in India and Pakistan in 2022 and in South America in December. Climate researchers largely agree that such extreme events will become more common in the future.

Three known mechanisms are responsible for such heat waves. First, air from warmer regions can move into cooler regions, for example from the Sahara to Central Europe. Second, the air in a high-pressure area can sink and heat up due to compression, or pressure. In the third mechanism, the ground heats up considerably due to direct solar radiation. Then the air above it warms up more than usual.

But what process determines whether there will be a heat wave somewhere or not? To understand this, Matthias Röthlisberger and Lukas Papritz from ETH Zurich examined the heat wave in Canada, which saw temperatures approaching 50 degrees at the end of June 2021, and analyzed heat waves around the world from that. To do this, they used data from the European Center for Medium-Range Weather Forecasts (ECMWF) to filter out the hottest day in many places around the world over the last 40 years. And on those days they investigated which mechanism contributed how much to the heat wave.

The ETH researchers of the Atmospheric Dynamics professorship will publish the results on Monday in the journal Nature Geoscience. It turned out that the route taken by the hot air is crucial. If the air comes from a climatically warmer area, heat transport contributes substantially to the heat wave. If, on the other hand, the air comes from a climatically similar area, then it is more the factors of the soil warming and the high pressure area that push up the temperatures.

The regional differences are large. Each of the three factors dominates in certain parts of the world, but very often heat waves are caused by a complex interaction of all three mechanisms.

We are most interested in Central Europe, which is often under the influence of hot Saharan air. But surprisingly, this is not directly relevant to heat waves in Central Europe, says Röthlisberger. The air that is close to the ground during heat waves usually does not come from the south, but often from the Atlantic Ocean and from northern or eastern Europe. The air from the Atlantic is further heated at lower levels by surface heating and compression.

“However, it looks different when you look at the sky further up in the atmosphere, for example at the height of the Jungfraujoch,” explains Röthlisberger. “During heat waves in Central Europe, this air usually comes from southern regions and is therefore exceptionally warm.” This warm air at higher elevations also indirectly affects ground temperature by reducing the formation of thunderstorms. Due to the absence of thunderstorms, there will be a lack of cooling and precipitation at lower altitudes.

The insight gained into the components of the heat wave mechanisms could help to work out physically plausible processes of very strong heat waves in a warmer climate. “This understanding of the process will also help us better identify and subsequently improve existing weaknesses in climate and weather forecasting models,” says Röthlisberger. (bzbasel.ch)