Hot weeks with scorching sun, then torrential rains again: the summer in Germany was an example of climate extremes becoming more frequent. A research team from Sweden has investigated why extreme weather events occur in mid-latitudes in summer.
Due to climate change, extreme weather events such as droughts and heavy rain seem to be increasing in temperate latitudes such as Europe. Using computer simulations, physicists have now discovered a mechanism that could explain this phenomenon. If the speeds of west-east winds fall below a certain threshold, they trigger reactions up into the higher atmosphere, which lead to a jet stream with particularly pronounced waves. This strong high-level wind in turn influences the trajectories of high and low pressure areas. The group around Woosok Moon from the Nordic Institute for Theoretical Physics in Stockholm (Sweden) describes the mechanism in the “Proceedings” of the US National Academy of Sciences (PNAS).
The polar jet stream is a band of strong winds at an altitude of nine to twelve kilometers between the 40th and 60th degrees of latitude – i.e. over most of Europe. It occurs because tropical warm air rises and moves poleward because the air pressure is lower there. The air is deflected by the Coriolis force resulting from the rotation of the earth – i.e. due to the fact that the rotation speed of the earth is much greater at the equator than near the poles – and the jet stream is created from west to east.
This air flow separates the warm, tropical air from the cold, polar air in the upper atmosphere. In addition to the Coriolis force, their decisive drive is the pressure difference between these air masses. Because the jet stream with the high and low pressure areas moves from west to east, it usually brings a mix of sun, clouds and rain to Central Europe. However, according to studies, especially in summer, high and low pressure areas often remain over a region for weeks – with heat waves on the one hand and constant rain on the other.
As a result of climate change, the earth in the polar regions is warming more than twice as fast as in the tropics, so that the pressure differences are becoming smaller. Exactly how this lower temperature gradient near the ground influences the jet stream in the higher atmosphere has not yet been finally clarified.
Moon and colleagues developed a theory and tested it using computer simulations. They found a threshold of about 36 kilometers per hour. Thus, when low-level west-east winds fall below this speed, it alters the currents in the atmosphere up to the level of the jet stream. This makes it slower and more wavy, showing more pronounced bulges to the north and south.
This happens especially in summer, when the temperature and pressure differences between the polar regions and the tropics are particularly small. As a result, the westerly winds react more strongly in summer to the different degrees of warming of land and sea surfaces, as Moon’s team writes. This reduces the frequency with which high and low pressure areas alternate, leading to long high phases with little or no rain and long low phases with permanent or heavy rain.
‘As mean flow velocities in westerly winds decrease, jet streams become more rippled and quasi-stationary high- and low-pressure blockage patterns form, causing severe floods and droughts,’ the researchers write.