Science

Reason for extreme weather: Climate change changes airflow

cause of extreme weather
Climate change changes air currents

Hot weeks with a scorching sun, then again torrential rains: the summer in Germany has become an example of increasing climatic extremes. A research team from Sweden has found out why extreme weather events occur in mid-latitudes in the summer.

Due to climate change, extreme weather events such as droughts and heavy rains appear to have become more frequent in temperate latitudes such as Europe. Using computer simulations, physicists have discovered a mechanism that could explain this phenomenon. If the speed of the west-east winds falls below a certain threshold, they cause reactions in the upper atmosphere, which lead to a jet stream with especially pronounced waves. This strong wind at altitude, in turn, affects the trajectories of high and low pressure areas. The group around the moon Vusok from the Nordic Institute for Theoretical Physics in Stockholm (Sweden) describes the mechanism in 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 40 and 60 degrees latitude, i.e. over most of Europe. This is due to the fact that warm tropical air rises and moves towards the poles, because the atmospheric pressure is lower there. The air is deflected by the Coriolis force resulting from the rotation of the Earth, that is, due to the fact that the speed of the Earth’s rotation at the equator is much greater than near the poles, and a jet stream is created from west to east.

This air current separates the warm tropical air from the cold polar air in the upper atmosphere. Apart from the Coriolis force, their decisive driver is the pressure difference between these air masses. As the jet stream with areas of high and low pressure moves from west to east, it usually brings a mixture of sun, clouds and rain to Central Europe. However, according to studies, especially in summer, areas of high and low pressure often persist for weeks over the region – with heat waves, on the one hand, and constant rains, on the other.

The jet stream slows down and becomes more undulating

As a result of climate change, the land in the polar regions is warming more than twice as fast as in the tropics, so pressure drops are getting smaller. Exactly how this lower temperature gradient near the ground affects the jet stream in the upper atmosphere has not yet been fully elucidated.

Moon and his colleagues developed the theory and tested it with computer simulations. They found a threshold of about 36 kilometers per hour. Thus, when the low-level west-east wind falls below this speed, it changes the currents in the atmosphere to the level of a jet stream. This makes it slower and more undulating, showing more pronounced bulges in the north and south.

This especially happens in summer, when the difference in temperature and pressure between the polar regions and the tropics is especially small. As a result, westerly winds react more strongly in summer to varying degrees of land and sea surface warming, Moon’s team writes. This reduces the frequency with which high and low pressure areas alternate, resulting in long high pressure phases with little or no rain and long low pressure phases with constant or heavy rain.

“As the average flow velocity in the westerly wind decreases, the jet streams become more undulating and quasi-stationary high- and low-pressure blocking patterns form, causing severe flooding and drought,” the researchers write.

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