Record-breaking heatwaves and droughts, and torrental rainfall and floods in the Northern Hemisphere last year are being attributed to a newly identified pattern of the ‘jet stream’ encircling the Earth. The jet stream traverses the globe about 10 km high up in the atmosphere from West to East, and is responsible for carrying the weather-altering western disturbances into North-West India.

These disturbances become active during the winter, but were conspicuous by their near absence in November-December 2018. They returned with a vengeance during the second half of the winter in January-March 2019.

While heatwaves and droughts had singed North America and West Europe during the summer, torrential rains and floods swamped South-East Europe and Japan in extreme events occurring almost simultaneously in June and July.

These extremes had something in common, a new study by an international team of climate researchers now finds: they were connected by a newly identified pattern of the jet stream.

The jet stream steers large scale weather systems from West to East around the globe, notes Kai Kornhuber from the University of Oxford and the Potsdam Institute for Climate Impact Research, lead author of the study.

The wind system can develop large meanders, or Rossby waves, and occasionally they stay in place for weeks. Under these conditions, warm sunny days can turn into a heat wave and drought and rainy days into a flood.

Stalled wave pattern

“Our study shows that the specific locations and timing of the 2018 summer extremes weren’t random but directly connected to the emergence of a re-occurring pattern in the jet stream,” says Kornhuber.

The jet stream formed a stalled wave pattern in the atmosphere, which made weather conditions more persistent and thus extreme in the affected regions.

The same pattern also occurred during European heat waves in 2015, 2006 and 2003, some of the most extreme heatwaves ever recorded. In recent years, the scientists observed a clear increase in these patterns.

“In the two decaces before 1999, there were no summers that saw a stalling wave pattern lasting for two weeks or more, but since then we have already seen seven such summers,” says co-author Dim Coumou from Vrije Universiteit Amsterdam and PIK.

The observed wave pattern is anticipated to re-occur more frequently in future because of climate change and human-caused global warming.