EOC analyses confirm that the number of extreme heat episodes in our latitudes has approximately doubled, perhaps even tripled, in the past 30 years. Extreme weather conditions have arisen in rapid succession in recent years, like the heat waves of 2018 and 2019, or the heavy rains in 2020 and 2021. Such extreme events are on the one hand related to rising temperatures on our planet. On the other hand there are also hints that the airstream patterns in our atmosphere are changing. EOC scientists are investigating how these processes interconnect.
The temporal trend of the episode frequency of extreme temperatures(exceeding nine days) in the mid-latitudes between 1979 and 2018. The red bars indicate heat waves, the blue bars cold waves. The horizontal black curve shows the temporal trend of the surface temperature. (Source: L. Küchelbacher, 2021)
The investigations focus on so-called “planetary waves”. These interact with a global belt of strong wind in the troposphere, the jet stream that decisively determines atmospheric circulation at our latitudes. This belt of strong wind, in which the wind blows at relatively high velocity from west to east, is a few kilometres (typically 7 to 12 km) above Earth’s surface and separates cold Arctic from warm subtropic air masses.
The jet stream is, however, disrupted by numerous effects, particularly from large-scale vortices, so-called planetary waves. These can spread out over the entire atmosphere at a wide range of altitudes and cause the jet stream to meander. Then the belt of strong wind no longer continues to ideally blow parallel to latitude circles but rather like an oscillating wave extending above and below a circle. As a result, a succession of warm or cold air masses is directed into our latitudes. In this way planetary waves influence the general weather situation.
Planetary waves in the atmosphere are constantly induced by a number of processes. Prominent sources can be large mountain chains like the Rocky Mountains, or land-sea-transitions that interrupt the airflow. Similar to the way a stone at the bottom of a brook generates a wave in the water stream, orographic barriers do the same in the atmosphere, to use a simplified analogy. Other sources for such waves are large-scale temperature and pressure differences, or areas with strong convection currents, like the Gulf of Mexico. In such ways a whole spectrum of waves with different wavelengths, oscillation periods and propagation velocities is constantly being generated. Given the right conditions these waves can overlap in a way that mutually intensifies them in a relatively short time, causing them to efficiently “bend” or deform the jet stream. The question of how the structure of planetary waves changes in the course of climate change is a current subject of research being addressed at many institutions worldwide.
In recent days interactions between planetary waves and the jet stream had an effect that led to Germany‘s most disastrous flood to date. The animation shows how the jet stream is deformed, producing an extreme bulge that on July 12 to 13 pinched off a cyclone that remained stationary over western Germany until July 17. The fatal complication was that air masses saturated with water vapor were continuously sucked in from the Mediterranean by this cyclone pinched off from the jet stream and then precipitated as rain over western Germany. The result was over 200 litres of rain within a short time, always over the same region.
This visualisation of the belt of strong wind shows how a cyclone is pinched off over western Germany on July 12 and remains there until July 17
The stationary cyclone over western Germany constantly draws in from the Mediterranean Sea air masses rich in water vapour that are released in the form of rain (Animation: Meteosat cloud channels in the visible and infrared spectral ranges)
EOC research so far indicates a high probability that planetary wave activity has changed already during the past 40 years. These changes are more clearly revealed in the higher altitudes of the atmosphere. So far ,they are not so easy to confirm in the troposphere because of the diversity of airstream patterns there. The researchers are assuming for the time being that such pinching off processes and the associated extreme weather situations are likely to increase in the future. One issue that they are investigating is to determine whether there are areas where such processes preferentially occur.